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Graphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications
Accepted Manuscript Graphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications
Tuan Sang Tran, Naba Kumar Dutta, Namita Roy Choudhury PII: DOI: Reference:
S0001-8686(18)30243-4 doi:10.1016/j.cis.2018.09.003 CIS 1911
To appear in:
Advances in Colloid and Interface Science
Please cite this article as: Tuan Sang Tran, Naba Kumar Dutta, Namita Roy Choudhury , Graphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications. Cis (2018), doi:10.1016/j.cis.2018.09.003
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ACCEPTED MANUSCRIPT Graphene inks for printed flexible electronics: graphene dispersions, ink formulations, printing techniques and applications Tuan Sang Trana, Naba Kumar Duttaa, Namita Roy Choudhury*a
Corresponding author. Email: [email protected]
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School of Engineering, RMIT University, Melbourne, Victoria 3000, Australia.
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a
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Abstract Graphene inks have recently enabled the dramatic improvement of printed flexible electronics due to their low cost, ease of processability, higher conductivity and flexibility. In this review, we discuss
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the state-of-the-art of the fundamental formulation of graphene inks and the current printing
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techniques used for inks deposition, followed by recent practical applications for printed flexible electronics. The progression of science and technology for the dispersion of graphene using variety
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of solvents and the characteristics of the resulting conductive inks have been highlighted, with
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specific emphasis focused on the challenges to be resolved. The printing techniques discussed here include screen printing, gravure printing, inkjet printing and other emerging printing technologies.
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Each approach’s pros and cons are discussed in correlation with the ink formulations and the operating principles. We also discuss the challenges and outlook of graphene ink for its future
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development in the world of printed flexible devices.
electronics.
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Keywords: Graphene dispersions, graphene inks, fluidic characteristics, printing techniques, flexible
ACCEPTED MANUSCRIPT Contents Abstract .................................................................................................................................................. 2 Graphical abstract ................................................................................ Error! Bookmark not defined. Contents ................................................................................................................................................. 3 1. Introduction ........................................................................................................................................ 4
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2. Formulation of graphene inks ............................................................................................................ 7
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2.1. Pristine graphene inks ................................................................................................................. 8
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2.2. Chemically-derived graphene inks ............................................................................................ 21 2.3. Graphene hybrid inks ................................................................................................................ 24
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2.4. Challenges on graphene inks formulation ................................................................................. 29 3. Printing techniques........................................................................................................................... 30
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3.1. Screen printing .......................................................................................................................... 30 3.2. Gravure printing ........................................................................................................................ 33
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3.3. Inkjet printing ............................................................................................................................ 35 3.4. Other printing techniques .......................................................................................................... 38
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4. Applications ..................................................................................................................................... 40 4.1. Flexible conductive circuits ...................................................................................................... 40 4.2. Energy devices .......................................................................................................................... 44
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4.3. Electrochemical sensors ............................................................................................................ 49 4.4. Other applications ..................................................................................................................... 51 5. Conclusion and outlook ................................................................................................................... 54 Acknowledgements .............................................................................................................................. 55 Conflict of Interest ............................................................................................................................... 55 Biographies .......................................................................................................................................... 56 References ............................................................................................................................................ 57
ACCEPTED MANUSCRIPT 1. Introduction For the last decade, the world of consumer electronics has experienced massive improvements in the manufacturing techniques towards the production of smaller, faster and better efficiency devices for everyday use. However, the use of traditional solid-state technology poses some limitation on the
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flexibility of the device, the environmental concerns and the processing cost. In recent years, a remarkable transition is taking place in the world of consumer electronics: devices are becoming
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thinner, flexible and wearable. Printing of flexible electronics has showed a promising alternative for
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the traditional fabrication of inorganic materials due to their numerous advantages [1-3]. By offering a low-cost, simple and scalable method for the production of devices with high flexibility and
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stretchability [4-6], printing of conductive inks on flexible substrates are facilitating to enable this
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transition.
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Until now, a number of mass printing techniques have been developed for the ultimate goal to
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achieve a fabrication process with high-performance, stable, low-cost, and zero-waste of materials. Such processing technologies are ink-jet [7], screen [8], and gravure printing [9]. These technologies are associated with liquid-phase inks with markedly different physical properties including the
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concentration of fillers present, viscosity and surface tension of the solution. The screen printing technique is compatible with a variety of inks, the gravure printing requires the use of a low shear viscosity ink suspension, while the ink-jet printing needs a high surface tension and diluted ink solution [10]. In the fabrication of printed electronics, d ifferent techniques possess their own pros and cons, but they all aim to provide rapid and efficient approaches in marking conductive traces on the flexible substrates. Printing technology is undergoing a rapid development and expected to bring a transformational change in the manufacturing of flexible electronics.
ACCEPTED MANUSCRIPT Multiple types of conductive inks have been developed for printed flexible circuits including metalbased inks [11-13], conductive polymers [7, 14], and carbon complexes [15-19]. Among them, metal-based inks particularly Ag and Cu [11, 20, 21] are widely used due to their high conductivity and their previous conversant use in solid-state electronics. Silver is an attractive material for conductive ink due to its excellent electrical properties [22]. However, silver is of high-cost and
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showed unstable performance by migrating into device layers [23, 24], making copper a good
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alternative for its abundance and fairly high conductivity. However, copper is facing oxidation issues
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under ambient condition, which can be facilitated by the high processing temperature, and reducing its electrical conductivity [25, 26]. Besides, the high sintering temperature of metal-based inks is
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also limiting their widespread use with papers and other flexible plastic substrates [27, 28]. It was also reported that the use of these metals is not environmentally friendly and might cause serious
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problems including water toxicity, cytotoxicity and genotoxicity [29]. Thus, there is a critical need for the further development of a low-cost, stable, and environmentally benign conductive ink, which can solve
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the above-mentioned disadvantages.
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Graphene, a two-dimensional carbon lattice, has received tremendous attention due to its excellent mechanical, thermal, and electrical properties [30-35]. With an exceptional carrier mobility of up to
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2×105 cm2 V-1 cm-2 [36-38], graphene has become a golden candidate for printed flexible electronics. However, the absence of a mass production method for high quality graphene prevents its practical application in inks for conductive patterns. Among the available graphene synthesis methods, oxidative-exfoliation of graphite can potentially be used for the production of large quantity of graphene [39-44]. This route basically allows for the oxidation of graphite into graphite oxide, which can be easily exfoliated as individual sheet of graphene oxide (GO). Subsequently, the exfoliated GO sheets can be subjected to a suitable reduction process to remove the oxygen-based functional groups to form reduced-graphene oxide (rGO) sheets, a graphene-like material [45-47]. However, the resulting graphene-like sheets are significantly damaged with holes and vacancies, which
ACCEPTED MANUSCRIPT compromise their outstanding electrical conductivity [48, 49]. For this reason, high quality graphene free from any defect is preferred for its use as conductive materials, especially as inks for printed flexible electronics. In recent years, research efforts have been focused on the direct exfoliation of pristine graphene in liquid media, which can be directly used for printing of conductive patterns [50-53]. The development of proper conducting inks for printing on flexible substrate are still facing certain
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challenges, which include the aggregation of graphene sheets in the suspension, the unsuitable
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viscosity and surface tension, and the lack of adhesion of the inks to the substrate. Enormous efforts
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have been devoted to overcome these issues, to bring graphene inks closer to practical applications. Currently, research on graphene inks has received intensive attention and entered an exploded
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growth phase (Fig. 1), where many applications and fundamental understanding of their formulations have been recently discovered. In the near future, it is no doubt that graphene inks can potentially
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replace the traditional solid-state-electronics and open up a whole new manufacturing process for
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Number of Publications
200
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low-cost, thinner, light weight, and flexible electronic devices.
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105
100
63
26
0
1
1
8
34
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2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
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Figure 1. The number of publications plotted against year obtained from the Web of Science by searching for the topic “graphene ink” (data acquired on September 2018).
In this review, we aim to guide the readers through recent advances in graphene inks for printed flexible electronics, specifically focusing on the synthesis of graphene-based inks and the techniques that have been developed for printing on flexible substrates, followed by several recent practical applications of graphene inks used in their realization.
ACCEPTED MANUSCRIPT 2. Formulation of graphene inks Graphene can be obtained using two distinct strategies, respectively (i) the bottom- up and (ii) the top-down [54, 55]. The bottom- up approach is based on the growth of carbon atoms into twodimensional carbon layers using chemical vapor deposition (CVD) [56, 57], which can produce very
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high quality graphene sheets on metal substrates. However, practical use of this approach is limited due to its high cost, complexity in the transfer process and difficult to scale- up. Alternatively, the
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top-down approaches involve the production of graphene from existing bulk carbon sources, by
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either exfoliation of graphite towards graphene, reduction of graphene oxide or carbonization of other materials [47, 54, 55, 58]. These methods are widely used for its numerous advantages in term
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of high yield, low cost, and solution processability, however the quality of graphene produced is of
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concern.
State-of-the-art techniques for printing of graphene are based on liquid-phase ink solutions, which
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composed of graphene (or its derivatives) fillers in stabilized solutions. For these reasons, top-down approaches are preferred for production of graphene-based conductive inks. The characteristics of
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the ink solutions are significant facets to the final mechanical and electrical properties of the conductive features. The resulting graphene inks should be stable against precipitation to provide a
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steady performance and uniform conductive patterns. It is important to optimize the composition of the graphene inks such as fillers, solvents and additives to be tailored for processing. A printable ink requires proper fluidic properties, with specific viscosity and surface tension, so it can easily be printed using the available instrumentation. An incompatible ink solution may cause failure of printing devices and resulting poor quality conductive traces. For its widespread use on substrates, the inks should have low volatilizing temperature and good adhesion to the layers, which allow the flexibility and stretchability characteristics of the printed items. A simple and low-cost production route with high yield is highly desirable for bringing graphene inks to its practical applications.
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2.1. Pristine graphene inks Alongside chemical vapor deposition (CVD) and mechanical exfoliation (Scotch tape method), liquid-phase exfoliation of graphite into graphene is considered as an effective route for production
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of high quality (pristine) graphene [33, 50, 51]. In bulk graphite, the parallel graphene layers are
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stacked like a pack of cards due to the van der Waals attraction, which needs to be overcome to achieve individual graphene sheets. Although the van der Waals force between the stacked graphene
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layers is weak enough to let them slide on each other, complete exfoliation into individual graphene
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sheet and stabilizing them against aggregation remain challenging [59]. So, the concept for liquidphase exfoliation is to reduce the intermolecular force of interaction between the adjacent graphene
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layers by liquid immersion. During ultra-sonication or under high shear rate, high energy fluctuations or high shear forces has the potential to peel off the adjacent layers on bulk graphite and induce
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exfoliation. After being exfoliated into individual sheets, graphene relies on the stabilization effect of the solvents and the surfactants used to minimize the intermolecular attraction, thus to improve the
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stability of graphene dispersion (Fig. 2).
Figure 2. Schematic illustration of the liquid-phase exfoliation process of graphite into graphene dispersions.
ACCEPTED MANUSCRIPT Significant research has been focused on the direct exfoliation of graphite into high quality graphene inks [60-69], where the raw graphite materials are directly exfoliated into graphene with the assistance of shear mixing or sonication in liquid media (Table 1). This approach has an advantage due to its low cost, solution processability, and the resulting footprints possess excellent
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conductivity.
Solvent or stabilizer
Resistance
Annealing
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Production method
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Table 1. Graphene based inks produced by liquid-phase exfoliation of graphite in various liquid media.
condition
High-shear mixing
40 Ω/sq
2% w/v ethyl cellulose in ethanol 60:40 NM P and vinyl acetate in isopropanol
Ultrasound-assisted
0.1% w/v ethyl cellulose/
supercritical CO 2
cyclohexanone
liquid phase exfoliation Probe sonication
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Ultrasonic-assisted
DM F, then exchanged to terpineol
1.5 mg polyvinylpyrrolidone (PVP) in 10 mL IPA solution
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Ultrasonication
acetone
30 Ω/sq
Ethyl cellulose in ethanol/toluene
Reference
procedure Inkjet printing
min
250 °C for 30 min Inkjet printing
M ajee et al., 2016 [60] Secor et al., 2013 [61]
100 °C for 5 min Screen printing
Arapov et al., 2015 [62]
810 ± 200 Ω/sq 300 °C for 30 min Inkjet printing Gao et al., 2014 [63]
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10 mg/mL of nitrocellulose in High-shear mixing
∼350 °C for 150
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Probe sonication
260 Ω/sq
NM P with additional ethyl cellulose
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Shear-exfoliation
Printing
∼100 Ω/sq ∼2×105 Ω/sq
Photonic annealing
Secor et al., 2017 3D printing
[64]
400 °C for few hours
Inkjet printing
-
400 °C for 30 min Inkjet printing
∼2×103 Ω/sq*
250 °C for 30 min Gravure printing
Li et al., 2013 [65]
Dodoo-Arhin et al., 2016 [66] Secor et al., 2014 [67]
85% cyclohexanone (with 15%
Vortex mixing
terpineol with additional ethyl
172.7 ± 33.3 Ω/sq
950 °C
Inkjet printing
He et al., 2017 [68]
cellulose stabilizer Blade coating
M icrofluidization of graphite
Carboxymethylcellulose sodium salt
∼2 Ω/sq
100 °C for 10 min
*Resistance is calculated based on the conductivity at the given thickness.
and screen printing
Karagiannidis et al., 2017 [69]
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2.1.1. Solvent based pristine graphene inks In the perspective on van der Waals interactions between the stacked graphene layers in graphite, the most simple and effective methods to reduce this intermolecular force is liquid immersion, where the
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potential energy between adjacent layers is significantly reduced in the presence of a liquid medium
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with proper interfacial tension [70, 71]. Previous studies have shown that interfacial tension plays an important role on dispensability, where the higher interfacial tension between solid and liquid results
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in the poorer stability of the dispersion [70]. In the case of graphitic flakes in liquid, if the interfacial
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tension is high, the energy required to detach two adjacent layers is also high and the flakes itself prone to aggregation, preventing the dispensability of graphitic flakes in colloidal dispersion. Recent
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study by Coleman and co-worker [50, 51, 72] indicated that the solvents with surface tension of ~ 40 mJ m-2 can minimize their interfacial tension with graphene. Such solvents include N-methyl-2-
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pyrrolidone (NMP, ~ 40.8 mJ m-2 ) and N,N-dimethylformamide (DMF, ~ 37.1 mJ m-2 ) have been
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established as the most effective liquids for preparing graphene dispersions [51, 52]. In 2016, Majee and colleagues [60] formulated a highly concentrated and stable graphene ink (3.2 mg/mL) composed of 4- layer graphene flakes with uniformly lateral size of ~160 nm, which was
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achieved by shear exfoliation of graphite in NMP using an L5M Silverson mixer. The graphene ink was then inkjet-printed on a glass substrate and annealed at ∼350 °C for 150 min, resulting in near transparent graphene conductive circuits with relatively low sheet resistance of 260 Ω/sq (ohms per square) at ~160 nm thickness. Li et al. [65] also prepared high concentration and stable graphene inks by ultrasonication of graphite in DMF, which have been demonstrated to be compatible to inkjet printing. The inks composed of grahene flakes with lateral dimension ranging from 100-500 nm were printed and annealed at 400 °C for few hours to achieve a sheet resistance of ∼200 kΩ/sq at a transmittance of about 90%.
ACCEPTED MANUSCRIPT However, the high boiling point solvents such as NMP and DMF (>150 °C) restricted the use of most plastic substrates, which require low treatment temperature. Therefore, dispersion of graphene in low-boiling point solvents is preferable. Still, common low-boiling point alternatives like acetone, isopropanol, and ethanol, usually come up with unsuitable surface tension (27.6 mJ m-2 , 23 mJ m-2 , and 22.1 mJ m-2 , respectively). The use of these solvents may lead to poor graphene dispersion,
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requiring a third added component to stabilize the exfoliated graphene flakes against aggregation.
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Chemical properties including surface tension, boiling point, chemical formula and structure of
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several common solvents used for direct liquid-phase exfoliation of graphene are given in Table 2.
Surface tension
N-M ethyl-2-pyrrolidone (NM P)
~ 40.8 mJ m-2
Boiling point
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Name
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Table 2. Chemical properties of common solvents use for liquid-phase exfoliation.
Formula
C5H 9NO
153 °C
C3H 7NO
~ 36.6 mJ m-2
181 °C
C6H 4Cl2
Cyclohexanol
~ 34.4 mJ m-2
162 °C
C6H 12O
Chlorobenzene
~ 33.6 mJ m-2
131 °C
C6H 5Cl
Toluene
~ 28.4 mJ m-2
111 °C
C7H8
Acetone
~ 27.6 mJ m-2
56 °C
C3H 6O
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1,2-Dichlorobenzene
~ 37.1 mJ m-2
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N,N-Dimethylformamide (DM F)
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202 °C
Structure
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~ 23 mJ m-2
83 °C
C3H 8O
Ethanol
~ 22.1 mJ m-2
78 °C
C2H 6O
Water
~ 72.8 mJ m-2
100 °C
H 2O
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Isopropanol
Noteworthy, in 2016 Arapov and co-worker [62] described an approach to prepare low-annealing
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temperature graphene dispersion for screen printing. In general, graphene dispersions were prepared
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by utilizing the intercalation of graphite and therma l expansion, followed by high-shear mixing of
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the expanded graphite in the presence of a polymeric binder. By mild heating, the gelation of graphene/polymer dispersion was triggered to form a highly concentrated and stable graphene paste,
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which showed excellent performance in screen printing with the line resolution of 40 µm in width (Fig. 3). In the final process, the printed patterns dried at 100 °C for only 5 min that exhibited a sheet
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resistance of 30 Ω/sq at 25 μm thickness. This work is important since it removed the need for long-
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to a myriad of substrates.
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time high temperature annealing and enabled high-volume roll-to-roll production, which compatible
Figure 3. a) Scheme of i) graphite intercalat ion, ii) thermal expansion, iii) preparat ion of graphene dispersion and subsequent gelation; b) image of graphene paste on a spatula; c) bent PET foil with a test pattern screen printed with graphene paste; f) SEM images of large-area prints at 5000× magnification. Reproduced with permission from ref. [62]. Copyright © 2016 Wiley.
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2.1.2. Surfactant assisted exfoliation of pristine graphene inks The use of such effective solvents like NMP and DMF is not ideal and suffers from some important drawbacks. These solvents are expensive and hazardous, restricting their practical application at
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industrial accessible scale. In addition, they tend to have high annealing temperature, making them inapplicable for printing onto most plastic substrates. Water, the most preferred solvent due to its low
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boiling point and non-toxic nature, has a surface tension that is too high (72.8 mJ m-2 ) to work on its
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own as an exfoliant for graphene. Moreover, the dispersion of graphene in water is challenging due to the hydrophobic nature of graphitic carbon. In order to achieve aqueous graphene dispersion, it is
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crucial to reduce water’s surface tension and suspend the exfoliated graphitic flakes without
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aggregation.
Recent studies have shown that the use of surfactants can promote exfoliation of graphite into
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graphene by lowering the interfacial energy between the two immiscible phases and forming colloidal systems [59, 73-75]. According to the Derjaguin, Landau, Verwey and Overbeek (DLVO)
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theory, the stability between two surfaces in suspension depends on the net sum of the electric double layer (EDL), steric repulsion (Steric) and van der Waals attraction (vdW). At the most basic
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level, aggregation is occured by van der Waals interactions. Electric double layer forms a net charge, and when two like-particles are in close proximity, the electric double layer repels the two. Steric stabilization provides a powerful tool to enhance the dispersion by preventing two particles from forming attractive van der Waals interactions. Attachment will occur if the objects are within the range such that the net sum of the interactions is attractive. The presence of a strong hydrophobic force contributes to the attractive forces, effectively increas ing the range at which attachment occurs (Fig. 4).
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Figure 4. Example interaction energy for the stable dispersion of nanoparticles in a liquid mediu m, according to the classical DLVO theory. Reproduced with permission fro m ref. [76]. Copyright © 2015 - Published by The Royal Society
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of Chemistry.
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Currently, a variety of surfactants including ionic, non- ionic, polymeric and bio-surfactants have been used to prepare graphene dispersions. Given in Table 3 are graphene dispersions that have been
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stabilized by some common surfactants.
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Table 3. Graphene dispersions stabilized by some common surfactants.
Production method
Yield*
Stability
Reference
Ionic
M ild sonication for 430 h
6%
> 5 days
[77]
Ionic
Ultrasonication for 30 min
0.22 %
> 7 days
[78]
Sodium dodecylbenzenesulfonate (SDBS)
Ionic
Ultrasonication for 30 min
~ 3.57 %
~ 6 weeks
[73]
Pluronic F-127
Non-ionic
Ultrasonication for 30 min
3.1 %
-
[79]
Triton X-100
Non-ionic
Ultrasonication for 30 min
0.42 %
> 7 days
[78]
Poly(sodium-4-styrene sulfonate) (PSS)
Polymeric
Ultrasonication for 30 min
-
Several days
[80]
Flavin mononucleotide sodium salt (FM NS)
Bio
Ultrasonication for 5 h
~1 %
> 6 months
[81]
Sodium cholate (SC)
Type
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Surfactant
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Sodium dodecylsulfate (SDS)
*Yield is calculated by the final graphene concentration against the initial graphite concentration.
ACCEPTED MANUSCRIPT Ionic surfactants like sodium cholate (SC), sodium dodecylsulfate (SDS), and sodium dodecylbenzenesulfonate (SDBS) have been widely used to improve the dispersions of carbon nanotubes (CNTs) [82]. Owning to its good compatibility to carbon materials, ionic surfactants have gained much attention into graphene dispersion [83]. A typical study on sodium cholate, an ionic surfactant, for graphene dispersions was reported by Lotya and co-worker in 2010 [77]. By mild
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sonication for up to 430 h, graphene dispersions can be stabilized in water/sodium cholate medium at
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the concentrations up to 0.3 mg/mL. The graphene dispersions so produced contained 1-10 layer
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graphene flakes, with the average graphene flake consisted of ~4 layers and the lateral size of ~1 µm. The prepared dispersions were reported to be highly stable and can be easily cast into various
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substrates, which is ideal for formulation of graphene ink. In a subsequent study, a range of 12 different ionic and non- ionic surfactants were used to disperse graphene in water, reported by the
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same research group [78]. In all the cases, the quality of graphene dispersions (flake size and degree of exfoliation) was found to be comparable from surfactant to surfactant.
The dispersed
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concentration varied from 0.011 mg/mL for SDS to 0.026 mg/mL for SC.
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Non-ionic surfactants, such as Pluronic F-127 and Triton X-100 contain hydrophobic polypropylene oxide (PPO) and hydrophilic polyethylene oxide (PEO) blocks, can also be used to promote
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graphene exfoliation. A study by Seo et al [79] using a variety of Pluronic and Tetronic block copolymers showed that Pluronic F-127 can disperse pristine graphene at relatively high concentrations (0.064 mg/mL, 0.303 mg/mL, and 1.255 mg/mL at the centrifugation conditions of 15000 rpm for 5 min, 5000 rpm for 5 min, and 750 rpm for 10 min, respectively), enabling a new class of biocompatible dispersions for graphene inks. In an extended study, Smith et al., [78] have shown that by ultrasonication of graphite in aqueous solution with presence of additional Triton X100, graphene dispersions can be achieved with concentration of up to 0.021 mg/mL and high stability for more than 7 days.
ACCEPTED MANUSCRIPT Polymeric surfactants have previously been used to enhance the dispersion of CNTs in water and organic solvents [82, 84]. However, there is still a scarce of literature on their use for graphene dispersions. The only report to date on polymeric surfactants stabilizing graphene is provided by Stankovich and co-workers [80]. In this study, poly(sodium-4-styrene sulfonate) (PSS), a wellknown amphiphilic polymer, was used as surfactant for stabilizing reduced graphene oxide (rGO).
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Interestingly, PSS has shown exceptional effectiveness to keep the graphene dispersion stable against
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aggregation for over a year. Still, there is a lack of study on direct exfoliation of graphite into pristine
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graphene using polymeric surfactants. However, this approach is promising for stabilization and formulation of graphene inks.
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Bio-surfactants have recently emerged as a favorable candidate for preparing biocompatible aqueous
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graphene dispersions, which open the door for graphene to bio- medical applications. A recent report by Ayan-Varela and co-workers [81] has shown that a derivative of vitamin B2 , namely the sodium
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salt of flavin mononucleotide (FMNS), can be a highly efficient dispersant for the preparation of aqueous graphene dispersions. In general, aqueous solution of graphite powder and FMNS
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(concentrations of 30 and 1 mg mL−1 , respectively) was sonicated for 5 h, followed by centrifugation at 2300 rpm for 20 min to remove any unexfoliated graphite particles. A concentrated graphene
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dispersion of 0.3 mg mL−1 was achieved, implying a relatively high exfoliation yields (~1%), which are comparable to other efficient surfactants (Table 3). The stabilization mechanism for surfactant/graphene dispersions has not yet been fully explored due to the lack of fundamental understanding about the molecular interaction between surfactant molecules and graphene flakes. As the major challenge in formulating graphene inks is to overcome the van der Waals interactions between adjacent graphene layers, successful exfoliation into stable graphene dispersions means that surfactants have successfully minimized the graphene/water interfacial energy, which allows the extraction of individual graphene sheets under support of
ACCEPTED MANUSCRIPT external energy (e.g. ultrasonication, high shear force), and hinders these individual sheets from aggregation. Moreover, the presence of surfactants also renders hydrophilicity to graphitic carbon, making pristine graphene soluble in water in a similar manner with graphene oxide without disturbing their conjugated sp2 basal plane. The most widely accepted mechanism on surfactant/graphene stabilization to date is proposed by Smith and co-workers in 2010 [78]. They
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indicated that for ionic surfactants, graphene concentration is mainly controlled by the zeta potential
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of the surfactant-coated graphene sheet, which scales linearly with the repulsive electrostatic
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potential. This means that for ionic surfactants, graphene dispersions are mostly stabilized by electrostatic repulsion mechanism. On the other side, the non- ionic surfactants have hydrophobic tail
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adsorbed onto the graphene surface and hydrophilic segments extended into water, which creating a potential barrier between the flakes. Thus, the stabilizing mechanism of non- ionic surfactants tends
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to be based on steric stabilization.
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Surfactants have gained significant importance in promoting graphene exfoliation in water. However, there is one systematic problem preventing the use of surfactants in formulation of
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graphene inks. That is surfactant itself can become a residual component in the final product. Regardless of printing techniques, the printed composites are usually consisting of graphene flakes,
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surfactants and solvents. As surfactants do not possess any interesting electronic properties, it may cause negative effect onto the electrical properties of the printed patterns. Therefore, it is crucial to remove surfactants from the printed composites. Removal of alternating surfactants molecules between graphene layers, however, is nearly impossible without spoiling the printed patterns. Thus, water-based pristine graphene inks are still far from optimization, calling for further development.
2.1.3. Polymer assisted exfoliation of pristine graphene inks
ACCEPTED MANUSCRIPT Polymers have been established as an interesting alternative to surfactants for stabilizing graphene in liquid medium since they can form emulsion systems, hindering graphene flakes from aggregation [59, 85, 86]. A range of polymers such as polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), ethyl cellulose (EC), and many more has been used to prepare stable graphene dispersions in many different solvents, and even water. The combination of such
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different polymers and solvents has given a variety of options for formulation of graphene inks. Thus
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the search for a suitable polymer-solvent combination is still ongoing and attracts a great deal of
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attention. Chemical structures of typical polymers used as emulsifiers for liquid-phase exfoliation of
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graphite toward graphene are given in Fig 5.
Figure 5. Chemical structures of typical poly mers used as emulsifiers for liquid -phase exfo liat ion of graphite toward graphene.
Stable and high-quality graphene aqueous dispersions were successfully prepared with assistance of PVA stabilizer in a recent study by Paton and colleagues [87]. In general, a solution of 2 wt% of PVA in water was used as the mixing liquid for graphite exfoliation. Graphite was then added into the mixing solution and exfoliated using an L5M Silverson mixer. The resulting graphene dispersions are highly stable and can be achieved in liquid volumes from hundreds of millilitres up to
ACCEPTED MANUSCRIPT hundreds of litres and beyond, opening up an effective route for mass production of graphene inks toward practical applications. By using ethyl cellulose as stabilizing polymer, Secor et al. [61] has successfully developed a novel graphene ink based on ethanol, an environmentally benign solvent by solution-phase exfoliation of
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graphite (Fig. 6). At a concentration of 2.4 wt %, the prepared graphene ink has a surface tension of ∼33 mN/m and a viscosity of 10–12 mPas at 30 °C, which is compatible for inkjet printing. Printing
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was carried out at 30 °C using a Fujifilm Dimatix Materials Printer (DMP 2800) with a cartridge
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designed for a 10 pL nominal drop volume. Drop spacing for all printed features was maintained at 20 μm, yielding stable graphene line with the width of ~60 µm. The printed conductive features were
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annealed at 250 °C for 30 min and exhibited a low sheet resistance of 0.4 Ω·m at ~140 nm thickness
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with uniform morphology and excellent bending capability.
Figure 6. Schematic illustration of the ink preparation method. (a) Graphene is exfoliated fro m graphite powder in ethanol/EC by probe u ltrasonication. (b) centrifugation-based sedimentation to remove residual large graphite flakes and (c) salt-induced flocculation of graphene/EC. (d) An ink fo r inkjet printing is prepared by dispersion of the g raphene/EC powder in 85:15 cyclohexanone/terpineol. (e) Vial of the prepared graphene ink. Reprinted with permission from ref. [61]. Copyright © 2013 American Chemical Society.
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In 2014, Gao and colleagues [63] reported a new route for formulation of ethyl cellulose-stabilized pristine graphene ink. Graphene dispersions were prepared by direct exfoliation from graphite using ultrasound-assisted supercritical CO 2 . Under ultrasonic vibration, the CO 2 molecules accumulated in
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the graphitic interlayer absorbed energy, expanded the distance between the graphite layers and induced exfoliation (Fig. 7). The resulting graphene dispersions are highly stable with concentrations
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up to 1 mg/mL and compatible to inkjet printing. Graphene flakes in the ink dispersion were less
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than 1 nm thick and 30–100 nm laterally. The formulated graphene inks were inkjet printed and annealed at 300 °C for 30 min, which showed high conductivity of 9.24 × 103 S/m (0.81 ± 0.2 kΩ/sq)
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and excellent flexibility.
Figure 7. Schematic illustration of the preparation process of pristine graphene ink and its printed electrodes. (a) Layered graphite was immersed in supercritical CO2 . (b ) CO2 molecu les penetrated and intercalated in the interlayer of graphite, (c) forming single - o r few-layer-thick g raphene sheets. (d) Graphene sheets were stabilized by EC in cyclohexanone and (e) formed stable graphene ink. (f) Graphene electrodes were printed on PET and PI substrates. Reprinted with permission from ref. [63]. Copyright © 2014 American Chemical Society.
In an effort to get rid of the harsh annealing condition, Secor and co-workers [64] reported a highperformance and low-temperature processing route of graphene ink by rapid photonic annealing
ACCEPTED MANUSCRIPT using a Xenon lamp (Fig. 8). Graphene ink was produced by high shear mixing of graphite in a solution of nitrocellulose and acetone, followed by ethyl lactate-supported stabilization. This procedure yields a composite containing of ~35 wt % graphene flakes with a typical thickness of ~2 nm and lateral size of ~300 nm. The ink was 3D printed and annealed with a single millisecond light pulse. The resulting conductive traces showed superlative electrical conductivity with the resistance
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of ~100 Ω/sq at the thickness of ~1 µm, which is comparable to thermal annealing at 350 °C. This
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strategy enabled the widespread use of a myriad of substrates, includes thermally sensitive materials.
Figure 8. Process overview and p roof of concept. (a) Chemical structure of nit rocellulose (left) and graphene (right). (b)
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Differential scanning calorimetry of graphene/nitrocellulose showing the large exothermic reaction of nitrocellu lose at ∼200 °C. (c) Schematic illustration of photonic annealing, wh ich in itiates a nit rocellu lose propagating reaction. Cross -
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sectional scanning electron microscopy reveals the resulting porous graphene microstructure even on a thermally conductive silicon substrate. Reprinted with permission from ref. [64]. Copyright © 2017 American Chemical Society.
2.2. Chemically-derived graphene inks Graphene oxide (GO), also known as chemically modified graphene (CMG), contains a range of reactive oxygen functional groups, which can be reduced to yield graphene- like materials. Interestingly, the majority of studies on graphene and its application are not based on pristine graphene, but rather the reduced graphene oxide (rGO). This is because GO can be produced in an
ACCEPTED MANUSCRIPT industrially accessible scale in a cost-effective manner, and the functional groups of GO enable its hydrophilicity and solution processability, which allow functionalization to tailor its exceptional properties. One of the most simple and straightforward GO inks formulation in water was reported by Le et al.
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[88] in 2011. They found out that hydrophilic GO (average dimensions: 500 nm × 500 nm × 0.8 nm) dispersed in water was a stable ink for inkjet printing. The printed GO traces with the lateral spatial
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resolution of 50 μm were thermally reduced at 200 °C for 12 h under N2 air to form conductive
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graphene electrodes, which were used for supercapacitors. At room temperature, the viscosity and surface tension of the GO ink were 1.06 mPa s and 68 mN/m, respectively, which were outside of the
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ranges recommended by the manufacturer for normal operation of the printer (10–12 mPa s and 28–
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32 mN/m). Thus, the waveform function of the piezoelectric nozzles was tuned to form spherical ink droplets from the un-optimal water-based GO ink.
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Since the hydrophilicity of GO enables the use of the most preferable solvent, water, formulation of GO-based inks is more focused on their reduction route. Beside thermal reduction, which is
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undesirable for low- melting point substrates, chemical reduction has gained much attention. In a report in 2011, Shin et al. [89] demonstrated a simple and effective strategy to pattern graphene
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nanosheets on PET film using inkjet printing and low-temperature chemical-assisted reduction. The GO ink (with the viscosity of 2.2 mPa s and the surface tension of 72.8 mN/m) was printed onto the substrate as the designed images using the modified inkjet printer and the printed film was reduced in a chamber containing hydrazine and ammonia solution at 90 °C for 1 h. The printed graphene patterns consisted of a few layers of graphene sheets with surface resistance of approximately ∼65 Ω/sq. The use of hydrazine is not preferable due to its highly toxic nature. So research has turned to other alternatives such as vitamin C, trifluoroacetic acid, and hydroiodic acid as reducing agents. In 2010,
ACCEPTED MANUSCRIPT Dua et al. [90] described a flexible and lightweight chemiresistor made of a rGO thin film, which were inkjet-printed onto flexible plastic substrates using surfactant-assisted GO ink and vitamin C as reducing agent. The resulting film has electrical conductivity properties (σ ≈15 S cm−1 ) and has fewer defects compared to rGO films obtained by using hydrazine reduction. More recently, Overgaard and co-workers [91] reported that water-based graphene oxide ink could be screen-printed
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on flexible plastic substrates and subsequently reduced using a 1:1 mixture of trifluoroacetic acid and
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hydroiodic acid at low temperature (80 °C), creating a conductive circuit with low sheet resistance of
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327 Ω/sq. The resulting thin film was flexible and semitransparent with 37% transmittance (Fig. 9).
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Figure 9. (a) Photograph of screen-printed GO and b) rGO showing the printed circuits. c) A height profile taken across a grid line and the point of intersection between two grid lines are shown and d) the corresponding AFM image is shown. Reprinted with permission from ref. [91]. Copyright © 2017 Wiley.
Using polyvinylpyrrolidone (PVP) as rheology modifier and gelation inhibitor, Chang et al. [92] was able to form a highly concentrated ink solution with graphene oxide (GO/PVP ink), which is suitable for meniscus- guided printing (Fig. 10). The printing strategy is quite simple and effective since the GO/PVP ink was supplied continuously through the micronozzle simply by horizontal pulling of the
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electrical conductivity of ~529 S/m at room temperature.
Figure 10. (a) Schematic d iagram of the printing process based on the meniscus -guided printing; (b) FE-SEM image of
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the printed GO-PVP pattern; (c) FE-SEM image of the printed pattern after the thermal treat ment at 450 °C for 3 h in
2.3. Graphene hybrid inks
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vacuum. Reproduced with permission from ref. [92]. Copyright © 2017 Elsevier.
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Intensive research has been devoted on the design and synthesis of graphene hybrid complexes for enhancing their conductivity. By mixing graphene with conductive polymer or metal particles, the
applications.
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hybrid inks can take full advantage of the two materials to tailor their properties for different
Polyaniline, a typical conducting polymer, is favorable electrode material for supercapacitors due to its ease of synthesis and high pseudocapacitance. Xu et al. [93] prepared graphene/polyaniline inks for inkjet printing by probe-sonication of graphite powder and polyaniline in water, supported by SDBS surfactant. The printed films were annealed at 80 °C for 2 h that exhibited conductivity of 0.29 S cm−1 (846 Ω/sq). A supercapacitor cell was fabricated using the printed thin film electrodes and showed a long cycle life with high performance.
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Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic
acid),
(PEDOT:PSS),
an
important
conductive polythiophene derivative, is an attractive electrode material for electroanalysis due to its transparency, high conductivity, and stab ility under ambient conditions. Seekaew et al. [94]
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formulated graphene/PEDOT:PSS hybrid inks for ammonia gas sensor. The hybrid inks were synthesized by sonication-assisted mixing of chemically-derived graphene powder, PEDOT:PSS
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solution, dimethyl sulfoxide (DMSO), ethylene glycol, and Triton X-100. It was demonstrated that
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π–π interactions occurred between graphene and PEDOT:PSS. A flexible graphene/PEDOT:PSS gas
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sensor was printed and exhibited high sensing performance to NH3 .
A typical demonstration of graphene/PEDOT:PSS hybrid inks was recently reported by Liu et al. [95] in 2015. By spray-coating of graphene/PEDOT:PSS hybrid inks onto PET substrates, they have
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successfully fabricated transparent and highly conductive graphene films for ultrathin organic
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photodetectors (Fig. 11). The printed hybrid inks were processed at a relatively low temperature (90 °C) and displayed an average resistance of ~600 Ω/sq with 80% transmittance. The fabricated graphene films with excellent flexibility were applied as bottom electrodes in ultrathin organic
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photodetectors and exhibited comparable performance to that of the state-of-the-art Si-based inorganic photodetectors.
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Figure 11. a) Schematic illustration of spray-coating an graphene/PEDOT:PSS hybrid ink onto desired substrates. b) Transmittance spectrum of both graphene/PEDOT:PSS hybrid films and ITO on PET substrates. Inset shows the optical
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images of the graphene/PEDOT:PSS hybrid films on PET substrates with 90% and 80% transmittance, respectively. c) SEM image of spray-coated graphene/PEDOT:PSS hybrid film. d) A 4 paper-sized hybrid transparent conductive film
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obtained by spray-coating. Reprinted with permission from ref. [95]. Copyright © 2015 Wiley.
Recent research has been performed to enhance the conductivity of graphene-based inks by
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incorporation of additional metal fillers. Jabari and co-workers [96] developed a graphene/silver nanoparticles ink and deployed in an aerosol-jet printing system. Adding silver nanoparticles (Ag NPs) to the graphene ink resulted in a uniform microstructure and crack- free printed graphene/Ag NPs patterns, which facilitated the movement of the electrons. The Ag NPs also bridged the graphene flakes and prevented stacking between graphene layers, thus decreased the resistivity of the printed patterns. The printed graphene/Ag NPs features were annealed at 250 °C and displayed the enhanced electrical conductivity of 1.07 × 10-4 Ω·cm, higher than those of neat graphene and Ag inks. In a similar study, Wang et al. [97] prepared an annealed graphene nanosheet coupled with Ag organic complex ink with confirmed changes of graphene microstructure suitable for inkjet printing.
ACCEPTED MANUSCRIPT Graphene nanosheets were annealed at 2200 °C for 30 min to remedy its electrical conductivity and then dispersed with silver/organic complex by sonication and polymer stabilization. The prepared hybrid inks were inkjet-printed onto polyimide (PI) substrates and baked at 300 °C for 40 min, resulting in conductive patterns with the resistivity of 4.62 × 10−4 Ω·m and high flexibility.
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In a recent study, Deng and colleagues [98] demonstrated an in situ prepared silver nanoparticles decorated graphene conductive ink for printed flexible electronics (Fig. 12). The hybrid inks were
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obtained by liquid phase exfoliation of graphite and reduction of silver salt to nanosilver in the same
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system with ideally compatible fluidic characteristics for inkjet printing. After annealed at 400 °C for 30 min, the inkjet-printed graphene features attained low resistivity of 20 ± 1Ω/sq with
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uniform morphology and high compatibility to flexible substrates.
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Figure 12. The preparation process of the nanosilver decorated graphene conductive ink. Reprinted with permission from ref. [98]. Copyright © 2017 Springer.
More complex graphene hybrid inks were formulated by Li et al. [99] in 2017. In general, they assembled gold package silver nanocore@shell nanotriangle platelets (Ag@Au NTPs) on graphene oxide (GO) to formulate Ag@Au NTPs-GO hybrid inks, which are satisfactory for inkjet-printing. The hybrid inks were prepared by sonication of Ag and Au nanoparticles in water-based GO dispersions. It is found that the addition of a thin layer of Au coated on the surface of Ag can effectively reduce the surface energy of the dispersion and improve the stability of the material's
ACCEPTED MANUSCRIPT conductivity. The printed patterns were hydrazine-reduced at 110 °C for 3 h and maintained a stable low sheet resistance of ∼ 149.5 Ω/sq after 100 days, providing a promising route for fabrication of highly stable and transparent conductive circuits. A number of research have shown that the incorporation of gold nanoparticles (Au NPs) with
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graphene could represent a promising way towards significant improvements of the electrochemical properties of the sensing devices. Pan et al. [100] fabricated electrochemical biosensor for direct and
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rapid detection of bisphenol A using graphene/gold nanocomposites. The hybrid inks were prepared
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by sonication-assisted dispersing of graphene oxide (GO) and chloroauric acid (HAuCl4 ) in water, followed by microwave heated for 2 min and thermally baked at 80 °C for 20 min. The
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graphene/gold nanocomposites based biosensor was prepared by the simple classic casting method of
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the hybrid inks onto glassy carbon electrodes and exhibited excellent performance for detection of bisphenol A.
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So far, research on graphene hybrid inks were mostly focused on the use of additional conducting polymers and metal nanoparticles fillers to tailor the original properties of graphene. By adding high
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pseudocapacitance conducting polymers, high performance and flexible graphene-based energy storage devices can be printed. The use of metal fillers like Ag and Au nanoparticles also enhanced
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the electrical and electrochemical properties of graphene inks, therefore exploited the full advantage of these materials. Au filled graphene inks are promising for printing of flexible and high performance sensing devices, while Ag/graphene hybrid inks are favorable for low cost, flexible, and transparent conductive circuits.
ACCEPTED MANUSCRIPT 2.4. Challenges on graphene inks formulation High
quality
printing
requires
excellent
ink
formulation
and
properties
such
as
hydrophilic/hydrophobic balance of the ink for specific application, surface tension of the ink, surface energy of the substrate, etc. There is a direct relationship between the ink characteristics and
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the substrate properties. While during its maturation, graphene inks have shown its excellent capability and processability in advanced manufacturing of next generation flexible electronics,
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however, there are still significant challenges for its realization. The first shortcoming is the
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compatibility of the inks to the state-of-the-art printing techniques, especially for the widely used inkjet printing. The waveform function of the printing nozzles can be manipulated to form spherical
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ink droplets, but the fluid characteristics of the inks are far from optimal. Second, the high annealing
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temperature of the inks prevented its available use in a wide range of substrates, thus limited its practical applications for flexible devices. Third, current graphene inks formulations are not
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environmentally benign processes. It uses a large amount of organic solvents for liquid phase exfoliation and a number of strong oxidative reagents for production of graphene oxide. Post-
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treatment and hydrazine reduction are also not environmental friendly and use significant amount of highly toxic materials. Thus it calls for the development of an ecofriendly and sustainable route for
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production of graphene inks. And finally, to bring every invention to life, the benefits are required to outweigh the cost. Therefore, it still requires a great deal of effort to pioneer a feasible graphene based ink with low cost and high output.
ACCEPTED MANUSCRIPT 3. Printing techniques Printing technology is one of the foremost inventions for advanced lean manufacturing. Various printing techniques have been developed for fabrication of flexible electronics, which are associated with roll-to-roll processing. The printing techniques that will mainly be discussed in this review are
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mass printing techniques, which are feasible and cost-effective towards roll-to-roll fabrication.
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We can classify the available printing technologies into two distinct models, conventional and digital printing [101]. The conventional mass printing techniques described here are screen and gravure
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printing, while the only accessible digital printing technology is inkjet printing. Other printing
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techniques do exist, but we only take into consideration the potential printing strategies, which allow
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scaling- up for mass production in an industrially accessible scale.
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3.1. Screen printing
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Screen printing, a major type of conventional printing technologies, has been applied to the fabrication of printed circuit boards for decades [102]. Nowadays, screen printing is used for fabrication of solar cells, sensors, organic light-emitting devices, and thin film transistors [103-107].
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Among the available mass printing technologies, screen printing is considered as the most versatile and mature technique, which is simpler and faster in comparison to other printing tools [108]. The process of screen printing is illustrated in Fig. 13. The ink paste is filled onto the edge surface of the stencil, which was located on the desired substrate and secured in place by spacers. During printing, the squeegee moves against the stencil and pushes ink paste through the etched motifs, resulting in the desired patterns, which is transferred onto the substrate after a single pass of the squeegee [109]. The results from screen-printing can be uniformly reproduced by repeating the process on the new substrates with an optimal operating procedure.
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Figure 13. (b) Schematic illustration of the screen printing process ; (b) Cross-sectional illustration of the screen printing.
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Reprinted with permission from ref. [109]. Copyright © 2015 Wiley.
Screen printing can be fully adapted to roll-to-roll manufacturing by rotary screen printing [105]. Its
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principal design is composed of two opposite rotating cylinders, where the stencil is shaped as a hollow cylinder with a fixed internal squeegee and supported by a pressure roller, as shown in Fig
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14. As the roll- to-roll processes, the two cylinders rotate and contact with the flexible substrate. Under pressure of the squeegee, the ink paste inside the stencil cylinder is transferred to the substrate
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through the engraved pattern of the stencil. This technique is ideal for large-scale and continuous
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Figure 14. Schematic illustration of the roll-to-roll screen printing process.
Screen printing has advantages due to its simplicity, reproducibility and high compatibility with various inks and substrates; making it a cost effective approach for mass printing of flexible devices [108]. However, screen printing also have several drawbacks. It showed a relative low resolution
ACCEPTED MANUSCRIPT compared to digital printing, therefore inappropriate for printing of miniatured circuits with high precision. Another issue is the dry out of ink during the printing process, which may impair the devices as well as the printed features [10, 108]. The direct contact between stencil and substrates may also lead to undesirable scratches and creases on the substrates while squeegee stress is applied,
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thus affecting its transparency and flexibility. The quality of the screen-printed patterns are generally contingent on the ink rheology, printer
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hardware (stencil, squeegee, substrate), and the printing process [105, 108]. With a well‐defined
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silicon stencil, high‐quality graphene patterns as narrow as ~40 μm can be screen-printed, as hightlighted by Huyn et al. in 2014 [109]. While the hardwares of the screen-printer are standardized
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and the processing parameters can be controlled during printing, the rheological properties of the ink
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paste play an important role on the features of the printed specimens. The flow of ink through the mesh of the stencil is significant since it determines uniformity of the printed surface, an important
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factor of conductive circuits. Unsuitable ink characteristics may result in defects or short-circuits of the conductive patterns. Thus, current formulation of graphene inks for screen printing should take
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its rheological properties into consideration, apart from its electrical conductivity. Ink paste for screen printing is basically qualified by its flow properties (i.e., viscosity or shear stress
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as a function of shear rate) [110]. An ideal ink paste should have a high rest viscosity, a low viscosity at high shear rate, and a fast viscosity recovery time [108, 110]. The viscosity of a screen-printable ink can be varied from 0.05 to 5 Pa s [101]. To meet the viscosity requirements for a proper ink paste, current formulations of graphene inks for screen-printing are mostly focused on either gelation of graphene dispersions [62] or preparation of highly concentrated graphene pastes [109]. In 2015, Arapov and co-workers [62] formulated colloidally stable and highly concentrated (52 mg mL−1 ) graphene pastes by gelation of graphene in polymeric binder. The graphene pastes have excellent screen printability down to lines of ~40 μm in width and high conductivity of ~30 Ω/sq at 25 μm
ACCEPTED MANUSCRIPT thickness. In a different approach, Huyn et al. [109] formulated graphene pastes with much higher concentration of ~80 mg mL−1 with a shear viscosity ranging from 1-10 Pa s at a shear rate of 10 s−1 . The graphene pastes also showed excellent performance in screen printing with line width of ~40 μm and high conductivity of ~1.86 × 104 S/m.
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A significant drawback of screen printing when using graphene inks is the challenge on controlling the thickness of the printed pattern, as they are mainly determined by the thickness of the stencils.
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There is also a difficulty in producing highly viscous and concentrated dispersions of graphene with
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its inherent tendency toward aggregation.
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3.2. Gravure printing
Gravure printing is a common low-cost patterning process, which utilizes direct ink transfer through
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physical contact between the engraved cylinder and the substrate [105, 111, 112]. The gravure printing involves two opposite rolling cylinders, the first cylinder has an engraved pattern, which
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collects ink from container, while the second roller supports the imprint (Fig. 15). As the two cylinder rotates, the engraved pattern is filled with ink and doctored off to remove the excessive.
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Meanwhile, the substrate slides between the two cylinders and allows for transferring of ink from the engraved cylinder onto its surface, resulting imprinted feature on the substrate [113]. Gravure printing is fundamentally a roll-to-roll process that allows for high volume printing. Recent research reported that gravure printing can be effectively used for the fabrication of solar cells [114], organic light emitting diodes [115], sensors [116], and graphene based flexible electronics [67, 117].
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Figure 15. Schematic illustration of the gravure printing process.
Due to its roll- to-roll printability charateristic, gravure printing is industrially compatible [113].
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Hence, it offers a powerful approach for fabrication of flexible electronics in high volume in a costeffective manner. It is desirable to manufacture printed flexible electronics at very high speed, up to
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15 m/s, using gravure printing [105, 112]. However, this technique also has some downside. As
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gravure printing is a direct contact process, the pressure of the impression roller may create unexpected creases and scratches on the surface layers. The morphology of the imprint is dependent on the shape of the engraved patterns, therefore the printed features do not have very high resolution.
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Changing of patterns is costly and suffers from the fact that a whole new roller needs to be engraved and embedded. Thus, gravure printing is not a cost-effective route for low volume printing. The quality of the printed patterns is highly influenced by the ink rheology, so it is significant to optimize the inks fluid characteristics. Low shear viscosity inks (in the range of 0.05-0.2 Pa s) are often used to prevent ink bleed out from the gravure cells and allow better line resolution [101, 112, 118]. Thus, solvent‐exfoliated graphene is particularly well‐suited for this techniques. Surface tension, uniformity, drying rate are also important parameters defining the quality of the patterns on flexible substrates.
ACCEPTED MANUSCRIPT There is still a scarce of literature of graphene inks in gravure printing, which can be attributed to the difficulty in formulating suitable inks since graphene possesses poor dispersibility in common organic solvents. In 2014, Secor and co-workers [67] demonstrated gravure printing of large‐area graphene patterns on flexible substrates. By tailoring the ink properties and printing parameters, continuous graphene lines with resolution as fine as ~30 µm were printed over large areas with
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notable reliability and uniformity. The integration of graphene with gravure printing offers a rapid
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3.3. Inkjet printing
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Unlike conventional printing technologies, ink jet printing works without a physical printing mask that directly contacts the substrates [112]. Formulation of graphene inks for inkjet printing has attracted enormous attention. The operating principle of inkjet printing is illustrated in Fig. 16, where
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micro-sized ink droplets are continuously generated and sprayed onto the substrate to form desired
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patterns by a printing nozzle [119, 120]. The basic principle of inkjet printing is the accurate placement of ink droplets onto the desired spot (pixel), which is digitally controlled by a programmed computer [121]. The formation of the micro-droplets is based on either piezoelectric
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mechanism that produces a mechanical compression through a nozzle or by heating the ink to increase volume and pressure [10, 61, 121]. For the accurate positioning of inks droplets, inkjet printing can be classified into two distinct mode, continuous mode and drops-on-demand [101]. In continuous mode (Fig. 16a), ink droplets are continuously generated and electrostatically charged in an electrical field. Then, the charged droplets are selectively deflected by a special collector, allowing only proper droplets to be deposited in appropriate position. Drops on demand mode (Fig. 16b), in contrast, generate ink droplets only when needed. When the nozzle is located in proper spot, an electrical pulse is applied to form a single droplet and locate onto the desired area. Drops-on-
ACCEPTED MANUSCRIPT demand does not involve charging of the droplets and is the more widely used mode. As a result, the ink droplets are digitally sprayed onto the substrate, where the desired features are achieved by a
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droplet-to-pixel fashion.
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Figure 16. Schematic illustration of the (a) continuous mode and (b) drops-on-demand mode of inkjet printing.
Graphene oxide based inks (GO inks) have been earlier employed in inkjet printing which can be
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attributed to its hydrophilic nature since GO can be easily dispersed in water and common solvents. In 2011, Le et al. [88] formulated a novel GO ink in water and integrated to inkjet printing. A
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commercial Dimatix Material Printer (DMP 2800) inkjet printer was used for on-demand generation of 10 pL microscopic GO ink droplets, resulting GO patterns with the lateral spatial resolution of 50 μm. In a similar work, Huang and co-workers [122] used an office inkjet printer (HP Deskjet K7108) to print GO inks onto various plastic substrates, proved that inkjet printing of GO inks is simple and practical. In recent years, inkjet printing of pristine graphene has attracted tremendous attention. Secor et al. [61] formulated ethyl cellulose stabilized graphene inks suitable for inkjet printing. The graphene ink has a surface tension of ~33 mN/m and viscosity of 10–12 mPa·s at 30 °C, which has been printed
ACCEPTED MANUSCRIPT using a Fujifilm Dimatix Materials Printer (DMP-2800) equipped with a 10 pL drop cartridge. Drop spacing for all printed features was maintained at 20 µm, resulting in a line width of ~60 µm. More recently, Majee et al. [60] formulated a pristine graphene ink by shear exfoliation, which has a viscosity of ~11.2 cP at room temperature, suitable for inkjet printing.
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The fluid characteristic of the inks is crucial for the formation of ink droplets. The inks are generally required to have a low viscosity (4–30 cP) and a high surface tension (typically ~35 mN m−1 ) in
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order to generate a stream of droplets (these values may varies for different printing devices) [10, 61,
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88, 89]. In the context graphene inks of printed electronics, the lateral size of graphene sheets and its stability in the ink dispersion also needs to be controlled, otherwise the pin hole of the nozzle can be
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blocked during operation.
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Inkjet printing benefits from its high resolution and versatility. Being a drop-by-drop technique, inkjet printing is utilized to attain uniform images with high resolution, since each micro-droplet is a
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pixel [121]. The resolution of inkjet printing can easily reach 300 dpi, and may up to 1200 dpi without great difficulty [10, 121]. As it does not require a physical printing mask, inkjet printing
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allows for design renovation on-the-go, which may cost a large financial and time investment for conventional printing techniques. The non-contact printing manner also avoids strong impact to the
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substrate, minimizing contaminations, scratches and creases on the thin film materials. More importantly, inkjet printing allows for the deposition of a very thin graphene patterns, which is beneficial for thin-film applications. The drawback of inkjet printing is its lower throughput compared to roll- to-roll conventional printing techniques. To remediate the output, it is essential to employ more nozzles in a single printer, thus increases the possibility of malfunction and failure. The aggregation of graphene in the dispersed solution may also occur and can block the nozzle from jetting droplets.
ACCEPTED MANUSCRIPT 3.4. Other printing techniques Flexographic printing, a promising roll-to-roll fabrication technique, is similar to gravure printing except the fact that the printing roller is usually made from rubber or polymeric materials [8, 10, 105]. The typical flexography system is comprised of four rollers, as illustrated in Fig. 17. Ink is
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collected by the collecting cylinder and transferred to the Anilox roller (a cylinder with engraved patterns). The patterns on the Anilox roller are filled with ink and doctored off to remove the
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excessive ink. Then, the printed pattern is transferred intermediately to the substrate through the
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printing roller. This direct contact printing process benefits from the printing roller made of rubber (or polymer), which can be more pliable to reduce the impact of the rollers onto the thin film,
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minimizing the creases and scratches on the substrates. The process is simple and industrially
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scalable, which is highly applicable for printing of graphene inks.
Figure 17. Schematic illustration of the flexographic printing process.
Aerosol-jet printing is a digital and non-contact printing technique which allows deposition of various functional materials such as metal nanoparticles, paste, and liquid inks [123, 124]. The
ACCEPTED MANUSCRIPT printing process is illustrated in Fig. 18, where inks are atomized and sprayed by a focus jet stream onto the substrates [125]. Aerosol-jet printing differs from inkjet printing mainly in the fact that the inks are atomized instead of generating droplets, bringing numerous advantages such as higher print resolution, more applicable with various type of inks, and able to print smaller and thinner patterns [123, 125-127]. As the main challenge for formulation of graphene inks is tailoring its fluidic
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properties, aerosol-jet printing is capable of printing inks with a wide range of viscosity (1–1000 cP)
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[128]. Thus it is able to print graphene inks that include stabilizers, which make graphene inks more
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stable but also increase their viscosities. As inks are atomized during aerosol-jet printing, it can also crumple graphene flakes and prevent them from aggregation [96]. Since the technology is still in its
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early stage, aerosol-jet printing is a costly process. But it undergoes rapid development and is
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promising for digital printing of various types of graphene inks.
Figure 18. Schematic illustration of the aerosol-jet printing process. Reprinted with permission from ref. [125]. Copyright © 2015 Elsevier.
3D printing, a new type of digital printing, is a process of depositing materials layer by layer to make three-dimensional solid objects (Fig. 19) [129, 130]. It allows objects to be fabricated in a bottom- up fashion directly from digital designs without any milling or molding [131]. 3D printing technology
ACCEPTED MANUSCRIPT has found great potential in one-step manufacturing across diverse fields from medical implant to electronics [131-134]. Recent research has shown that graphene-based devices can be fabricated using 3D printing [135-137]. This advanced technology is still far from the ultimate goal of fabricating complex devices in-one- go, but 3D printing of graphene inks is promising for its future
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applications.
Figure 19. Schematic illustration of the 3D printing process and some 3D printed structures. Reprinted with permission
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Overall, it is likely that many of the above- mentioned techniques will ultimately be used for printing
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of graphene based electronics. For different ink formulations, different printing techniques will be optimal. Therefore, a mix-and- match printing route can be used based on requirements of conductive features, with the ultimate goal of maximizing the process output.
4. Applications 4.1. Flexible conductive circuits
ACCEPTED MANUSCRIPT Due to its outstanding electrical and mechanical properties, graphene ink is promising for printing of flexible conductive circuits. In 2013, Hyun et al. [138] prepared foldable paper-based electronic circuits by a simple selective transfer process with a pen. Graphene membranes with controlled thicknesses were prepared by vacuum filtration of graphene nanoplates dispersions (a type of chemically-derived graphene ink). Then, foldable electronic circuits were fabricated by selective
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transfer of graphene patterns onto a paper from the filter membrane without the need of a printing
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mask (Fig. 20). The resulting conductive circuits showed excellent folding stability with small
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decrease of conductance at any folding angles from -180° to 180°. A foldable circuit board for operating light-emitting diode (LED) chips array was also demonstrated. This approach could
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provide a simple patterning method for paper-based graphene conductive circuit with high foldability.
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Figure 20. Schematic illustration of the preparation of a graphene circuit on a paper substrate. (a) Filtration of graphene dispersion. (b) Drawing of graphene circuit by a pen. (c) Remov ing the memb rane filter fro m the paper substrate. (d) Photographs of a LED ch ips array on the folded graphene circuit. Reproduced with permission from ref. [138]. Copyright © 2013 Wiley.
Using inkjet printing, Gao et al. [63] prepared highly flexible conductive circuits with excellent uniformity by printing of pristine graphene ink. The graphene circuits showed excellent conductivity of 0.81 ± 0.2 kΩ sq-1 after 30 printing passes, and a transmittance of approximately 60% after being
ACCEPTED MANUSCRIPT annealed at 300 °C for 30 min. The resistances of the printed graphene circuits as a function of flexibility were also investigated, which were slightly increased despite 1000 times of bending or folding cycles.
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Huang and co-workers [122] also described a simple and practical inkjet printing method of
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graphene oxide based inks (GO inks) for a series of flexible electric circuits and chemical sensors (Fig. 21). The printed GO patterns were thermally treated to restore its electrical properties, resulting
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in printed graphene features with high conductivity (up to 874 S/m) and excellent flexibility. Flexible
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conductive circuits and high sensitive electrochemical H2 O2 sensors were also fabricated based on
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the printed graphene patterns.
Figure 21. Patterns printed on various substrates by high resolution inkjet printing of GO inks. Reprinted with permission from ref. [122]. Copyright © 2011 Springer.
Graphene flexible circuits can also be fabricated using 3D printing, as demonstrated by Zhang and co-workers in a recent report [139]. In this study, graphene composite was prepared by melt mixing
ACCEPTED MANUSCRIPT of thermally reduced graphene oxide (rGO) and polylactic acid (PLA) thread, which was then extruded into filament with the diameter of 1.75 mm and used for the 3D printer. On various different substrates, 2D and 3D flexible circuits were rapidly printed and exhibited outstanding conductivity of up to 4.76 S/cm at 6 wt% rGO. The demonstrated 3D printing of graphene filament
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is promising for the future of all-printed flexible electronic devices.
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can be used for fabrication of not only flexible circuits, but also for any complex 3D structure, which
ACCEPTED MANUSCRIPT 4.2. Energy devices Another promising application of printable graphene ink is in the field of newable energy devices. The modern energy devices may be broadly classified into two categories: (i) energy storage and (ii) energy harvesting devices. The available printed flexible energy storage devices described here are
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batteries and supercapacitors, while energy harvesting devices are mostly focused on printed solar cells and nanogenerators. In the following section, our intention is to discuss about recent advances
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in graphene based inks for printed energy storage and energy harvesting devices.
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Rechargeable lithium batteries are an important class of energy storage devices, where lithium ions
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are transferred between the electrodes during the charge and discharge reactions. Wei et al. [140] demonstrated a scalable and versatile printing process for solid-state and flexible lithium batteries
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based on graphene inks. The batteries are comprised of polymer-based gel electrolyte sandwiched between a lithium foil (anode) and graphene hybrid inks printed on current collector (cathode). It has
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been demonstrated that the graphene hybrid ink, formulated by polystyreneslufonate (PSS) doped graphene, lithium salt and TiO 2 nanoparticles, can be either printed or drop-casted onto the current
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collector for fabrication of high performance cathode in the lithium batteries. The assembled solidstate and flexible batteries showed excellent performance with specific capacity of up to 582 mA h
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g−1 . In this work, not all of the components are printed except the cathode, calls for further development of feasible fully-printed batteries.
In 2016, Fu and colleagues [141] created fully printed lithium- ion batteries by 3D printing technology using GO-based hybrid inks. As shown in (Fig. 22), the anode and cathode inks were prepared separately and stored in different syringes, which were printed layer-by- layer and bridged by a barrier of 3D printed solid-state electrolyte. The all-component 3D-printed batteries showed a
ACCEPTED MANUSCRIPT reasonable performance with good cycling stability. This work demonstrated the potential of 3D
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printing in one-step fabrication of advanced electronics.
Figure 22. Schematic illustration of the fabrication of all-component 3D-printed lithium-ion batteries using 3D printing.
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Reprinted with permission from ref. [141]. Copyright © 2016 Wiley.
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Printing of graphene based supercapacitors, an emerging class of energy storage devices, differing from batteries as it physically stores energy on its separated electrodes rather than based on chemical
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reactions, has also been advanced. The supercapacitors benefit from its fast charge/discharge rate, high power density and ultralong lifetime. One of the very first studies on printing of graphene based
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supercapacitors was reported by Xu et al. [142] in 2013. They have formulated and screen-printed graphene/polyaniline hybrid inks to prepare active electrodes for supercapacitors. The fabricated
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supercapacitor device exhibited high specific capacitance of 269 F g−1 and power density of 454 kW kg−1 with excellent cycling stability. More research effort has been paid to micro-supercapacitor, a miniaturized design of supercapacitors. Micro-supercapacitor is advantageous due to its thinness, high flexibility, and ease of fabrication (i.e. printing). Recently, Hyun et al. [143] has reported a high-throughput inkjet printing process for manufacturing of flexible graphene micro-supercapacitors based on self-aligned capillarity-assisted lithography for electronics (SCALE) method, followed by photonically annealing by a high- intensity pulsed Xenon lamp (Fig. 23). The printed micro-supercapacitors, comprised of pristine graphene
ACCEPTED MANUSCRIPT electrodes and ion gel electrolyte, exhibited a fairly high aerial specific capacitance of 268 µF cm−2
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with high flexibility and excellent charge/discharge stability.
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Figure 23. Schematic illustration of fabrication steps for micro-supercapacitors using the SCALE process. Reprinted
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with permission from ref. [143]. Copyright © 2017 Wiley.
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Another approach for fabrication of micro-supercapacitors was demonstrated by Liu et al. [144] in 2016. In this report, in-plane micro-supercapacitors were direct-printed by spray deposition and
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inkjet printing using graphene/conductive-polymer hybrid inks. The printed ultrathin microsupercapacitors offered superior areal specific capacitance and excellent rate capability with an
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ultrahigh flexibility, making it appropriate for next-generation flexible energy storage microdevices.
More recently, Li and colleagues [145] developed a simple and scalable inkjet-printing process for massive fabrication of graphene micro-supercapacitors. Graphene inks and polyelectrolyte inks were subsequently inkjet-printed onto various substrates for assembling of fully printed microsupercapacitor cells, which attained the highest areal capacitance of ∼0.7 mF/cm2 . This work provided a promising solution to compact and on-chip power sources for future wearable electronics.
ACCEPTED MANUSCRIPT Using 3D printing, Zhu and co-workers [146] reported the fabrication of three-dimensional graphene aerogels for supercapacitors (Fig. 24). Graphene composite inks were prepared by sol-gel mixing of graphene oxide, graphene nanoplatelets, and silica fillers to form a thixotropic and highly viscous inks, which were suitable for 3D printing. The printing was conducted in an organic solvent (2,2,4trimethylpentane) and the 3D periodic graphene aerogel was formed after supercritical drying. The
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fabricated supercapacitors used the 3D-printed electrodes showed excellent electrochemical
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properties and remarkable stability. More importantly, the deve loped 3D printing technique can be
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used to build unique electrode and device structures with irregular shapes and well-controlled
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internal structural.
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Figure 24. Schematic illustration of fabrication process. (a) Preparation of graphene based inks, (b) 3D printing of threedimensional graphene composite, and (c) formation of 3D graphene aerogels. Reprinted with permission from ref. [146]. Copyright © 2016 American Chemical Society.
Printed graphene has also been advanced for photovoltaic devices, which is a typical energy harvesting device that converts photons from the sun (solar light) into electricity. Among the available photovoltaic devices, dye-sensitized solar cells are advantageous due to its low cost and high efficiency [147]. For fabrication of dye-sensitized solar cells, Dodoo-Arhin et al. [66]
ACCEPTED MANUSCRIPT formulated a stable graphene ink by liquid phase exfoliation of pristine graphite in isopropyl alcohol with a polymer stabilizer and repeatedly inkjet-printed onto FTO/glass substrate. With a ∼3.0% conversion efficiency, the printed graphene electrodes exhibited a comparable performance to
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platinum counter electrodes with only ∼2.7% cost.
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Importantly, Casaluci et al. [148] demonstrated a viable method for large-area fabrication of graphene-based dye-sensitized solar cells by spray coating of graphene ink (Fig. 25). The printed
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graphene counter electrodes (44% transmittance) were integrated in a solar cell modules and
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achieved conversion efficiency of 3.5%, similar than those of platinum counter electrodes. This work
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paved the way to all-printed, flexible, and transparent graphene solar cells.
Figure 25. (a) Spray coating graphene ink. (b) Optical image of graphene-based counter electrode. (c) Optical transmittance of FTO (grey curve) and graphene-coated FTO substrate (black curve). Reprinted with permission from ref. [148]. Copyright © 2016 Royal Society of Chemistry.
Nanogenerator is a set energy harvesting device that converts thermal or mechanical energy into electricity. Nanogenerators can be classified into three different types: piezoelectric, triboelectric,
ACCEPTED MANUSCRIPT and pyroelectric nanogenerators [149]. While piezoelectric and triboelectric nanogenerators can be used to harvest mechanical energy, pyroelectric nanogenerators can convert thermal energy into electricity [149-151]. By screen-printing of graphene nanoplatelets inks onto a PVDF film (a novel pyroelectric material), Zabek et al. [152] has successfully fabricated graphene-based pyroelectric nanogenerators (Fig. 26). The use of printed interconnected graphene layer minimized thermal
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reflection, enhanced thermal radiation absorption and increased the electrical conductivity, thereby
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improved the energy harvesting potential. The printed graphene-based nanogenerators showed a
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significantly improved thermal energy harvesting efficiency, which is 25 times higher than those of aluminum-based systems. The presented technology provided a new approach for powering wearable
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and Internet of Things (IoT) devices.
Figure 26. Thermal rad iation heating princip le for (a) a reflective pyroelectric alu minu m/PVDF electrode device and (b)
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an absorbent asymmetric graphene/PVDF in k pyroelectric device. Rep rinted with permission fro m ref. [152]. Copyright © 2017 American Chemical Society.
4.3. Electrochemical sensors
The electrochemical sensors are detectors that work based on electrochemical reactions, where the target chemicals/reagents are oxidized or reduced at the electrodes resulting in change of the electrical current [153-155]. Recent research has emerged graphene and its derivatives as one of the most popular platforms for designing of new high-performance sensing devices [156-158]. However, designing and manufacturing of graphene-based electrochemical sensors in a cost-effective manner
ACCEPTED MANUSCRIPT remain a major challenge. Printing of graphene inks is dawning as an efficient route for mass production of high quality electrochemical sensors and to solve the problem thereof. For detection of metabolites, Labroo et al. [159] reported a simple and inexpensive printing process of graphene-ink biosensor arrays on a microfluidic paper. The sensor arrays based on enzyme-
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immobilized graphene hybrid inks were printed and showed its capability to detect multiple
pharmaceuticals, food science, and personal health monitoring.
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metabolites rapidly with high sensitivity. This approach paved the way to a variety of applications in
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In a report by Seekaew and co-workers [94], simple and low-cost ammonia gas sensors were
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fabricated by inkjet-printing of graphene/PEDOT:PSS hybrid inks (Fig. 27). The formulated hybrid inks were inkjet-printed onto a prepared interdigitated silver electrodes using an office inkjet printer.
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As a result, the printed flexible graphene/PEDOT:PSS gas sensor exhibited high sensitivity to NH3 a low concentration range of 25 to 1000 ppm at room temperature. More importantly, the printed gas
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sensor benefits from its flexibility, where gas response increased with increasing the bending angle.
Figure 27. Schematic diagram for the fabrication of flexib le graphene/PEDOT:PSS gas sensor. Reprinted with permission from ref. [94]. Copyright © 2014 Elsevier.
ACCEPTED MANUSCRIPT By printing of graphene based inks, Kanso et al. [160] integrated a new screen-printed graphene electrode in one channel flow-cell for enzymatic sensors, with an improved analytical response and enhanced electroactive area of up to 388%. In another study, An and colleagues [161] printed a highly conductive graphene aerogel with controllable 3D porous nanostructure via micro extrusion printing, which is appropriate for a multi-recognition flexible wearable electric sensor for analyzing
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complicated perception of movements.
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In 2015, Santra and co-workers [162] has successfully integrated functional graphene inks with
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CMOS-MEMS (complementary metal-oxide-semiconductor)-(micro-electro- mechanical systems) technology for fabrication of a resistive humidity sensor with high response and reproducibility. This
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based sensor systems for a variety of applications.
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work is important since it opens up an exciting opportunity for mass production of low cost CMOS
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4.4. Other applications
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The development of advanced printing techniques and ink formulations allows for more and more graphene-based printed devices. Huang et al. [163] reported a low cost, highly conductive and
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flexible printed graphene film for wireless wearable communications applications. The printed graphene patterns were experimentally used to transmit/receive radio frequency (RF) signals through wires and wirelessly. Benefited from a simple and low-cost patterning process, combined with other advantages in flexible, lightweight, and high conductivity, printed graphene is prospected for wireless wearable communications in the near future. In another study, Huang et al. [164] also demonstrated a rolling compression method for improving the conductivity of screen-printed graphene laminates, which were experimentally used as dipole antenna to effectively radiate RF power (Fig. 28). Using the similar strategy, Shin and co-workers [89] inkjet-printed graphene lines
ACCEPTED MANUSCRIPT with high resolution and sustained electrical conductivity onto flexible PET films, which was used as
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electrodes for a wideband dipole antenna and exhibited 96.7% transmittance efficiency.
Figure 28. (a) Geometric dimension of the dipole antenna, (b) photo of the printed graphene laminate dipole antenna, and
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(c) graphene laminate antenna connected with a SubMiniature version A (SMA) connector for measurement. Reprinted
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with permission from ref. [164]. Copyright © 2015 AIP Publishing.
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Flexible thin- film transistors (TFTs) are mostly relied on organic semiconductors. Howerver, the limited carrier velocity in polymers and molecular films prevents their use at high frequencies
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beyond megahertz [165]. In parallel, graphene is promising to deliver high-speed performance while maintain the flexibility. In a report by Sire and co-workers [166] in 2011, a flexible and high frequency TFT was developed employing solution-based single- layer graphene in 2% w/v aqueous sodium cholate solution. The printed TFTs showed low resistance, extrinsic and intrinsic current gain cutoff frequency of 2.2 and 8.7 GHz, respectively, with excellent performance up to high frequencies. A graphene embedded with Indium- Gallium- Zinc-Oxide (IGZO) thin- film transistor was fully printed by Secor et al. [167]. In this study, the authors used inkjet printing for fabrication of a highperformance graphene/IGZO thin- film transistor based on IGZO and pristine graphene inks.
ACCEPTED MANUSCRIPT Graphene source and drain electrodes were alternative printed and embedded between two consecutive IGZO printing passes, provided a chemically stable electrode-channel interface (Fig. 29). The printed TFT device achieved high-performance with an electron mobility of ∼6 cm2 /V.s and current on/off ratio of ∼105 , higher than those of silver/IGZO TFT. Furthermore, the printed graphene/IGZO TFT exhibited robust stability to aging and excellent resilience to thermal stress,
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thereby offered promising platform for advanced fabrication of future high-performance and stable
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printed electronics.
Figure 29. Schematic illustration of the fabrication of inkjet-printed IGZO TFTs with silver and graphene source/drain
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electrodes. Reprinted with permission from ref. [167]. Copyright © 2017 American Chemical Society.
Flexible all-carbon-based field effect transistor (FET) was also successfully fabricated using aerosoljet printing. Reported in 2013, Liu and colleagues [168] formulated three different types of inks and
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subsequently printed them onto flexible PET substrates for fabrication of the FET chips. Highly conductive rGO was used as electrodes and channel, while isolating GO acted as the gate dielectrics, and multi- walled carbon nanotubes (MWCNTs) were printed as the gate electrode. This flexible allcarbon-based FET is fabricated entirely through a low cost and environment-friendly printing process, promising for high throughput fabrication of electronic devices.
ACCEPTED MANUSCRIPT 5. Conclusion and future outlook In this report, most recent work on the formulation of graphene inks, their printing techniques and applications for printed flexible electronic devices have been reviewed. The preparation of proper graphene inks, which allows for mass printing in a cost-effective manner is desired for its
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commercial adoption. It has been clearly demonstrated that pristine graphene inks are highly conductive, but they show poor solubility in common solvents. It appears that the chemically-derived
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graphene (GO) inks can be easily formulated into inks due to its hydrophilicity, however, they are
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not electrically conductive, requiring additional treatment to restore the conductivity after deposition. The graphene hybrid inks hold significant promise to enhance the electrical properties of the inks,
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but they are costly and the fluid characteristics still far from optimal. Moreover, the high annealing
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temperature of such inks also obstructed their available use in a wide range of substrates. Therefore, the main challenges still remained on formulation of a low cost and printable graphene ink with
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appropriate characteristics in an ecofriendly fashion. It can also be noticed that current studies are still at the initial stage with main focus on basic demonstration of printability and conductivity, while
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much less attention is paid to the correlation of viscoelastic and the physicochemical properties of the inks with printing factors. It is crucial to provide a holistic evaluation approach to determine the
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printability and performance of the inks. Formulation which meets the specifications of the industry standards will hold the key to the future. Further development of feasible graphene inks along with improvement of a facile and efficient deposition process will be key to realize many applications in graphene-based flexible electronics. The ink formulations should focus on the selection of a variety of solvents and surfactants to find the best combination for direct exfoliation of pristine graphene inks, which enable high yield, volatile, and proper fluidity. The use of fluid-modifiers may be a solution toward formulating a printable ink with genuine characteristics. Other cordial and rapid annealing mechanisms need to be examined in
ACCEPTED MANUSCRIPT correlation with the volatile solvents. Poly (ionic liquid) has recently emerged as a versatile and effective stabilizer for preventing aggregation of carbon nanostructures to form homogeneous dispersions in water. Moreover, poly (ionic liquid)s are more environmentally friendly and electrically conductive itself. Therefore, their emulsions with graphene are also expected to be
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advantageous for engineering feasible and sustainable graphene inks. Interestingly, the mass of graphene is usually account for less than 1 wt% of the ink in the current
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formulations, while the dominant portion of the inks is water and/or solvent based. This will
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certainly incur an increased costs for storage and transportation. Therefore, the development of redispersible powder-phase graphene inks is also a promising direction for future development. The
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current printing technologies of graphene inks are mainly focused on ink-jet printing, whose
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compatibility with a variety of inks is limited. Further development of viable aerosol-jet printing and 3D printing technologies are trendy toward the future of printed electronic devices.
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Acknowledgements
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research hub funding.
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The research has been supported by the Australian Research Council (ARC) Industry Transformation
Conflict of Interest
The authors declare no conflict of interest.
ACCEPTED MANUSCRIPT Biographies
Tuan Sang Tran obtained his Bachelor’s degree in Chemical Engineering fro m Hochiminh University of Industry, Vietnam, in 2014, and the Master’s degree in Nanoscience from Gachon University, South Korea, in 2016. Mr. Tran is currently a PhD student at the Department of
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Chemical Engineering, RMIT University, Australia. His research interests include carbon nanomaterials for energy applications and graphene-based inks for printed electronics.
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Naba Kumar Dutta is a Professor at School of Engineering, RMIT University. He performs
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research at the interface cutting across the disciplines of Nanomaterials Engineering, Structural Biology and Chemical Engineering. He has wide-ranging research interests and published widely
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in the areas of Poly mers, Advanced materials for energy, Carbon-nanomaterias and structureproperty relationship in mult i-co mponent hetero-structures with specific focus on the role of the
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interfaces.
Nami ta Roy Choudhury is a Professor at School of Engineering, RMIT University. She
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received her PhD fro m IIT, Kharagpur and subsequently did her post-doctoral research at CNRS, Mulhouse, France. Choudhury’s research interest spans fro m Hybrid poly mers to Bio mimetic
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polymers to Graphene based inks for renewable energy and advanced manufacturing.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights An insight into the fundamental formulation of stable graphene dispersions.
State-of-the-art formulation of graphene inks utilising solution processable materials.
Conventional and digital printing techniques for deposition of graphene inks.
Inks formulation and devices fabrication are inextricably linked.
Challenges and outlook of formulating graphene ink for its future development.
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Tuan Sang Tran, Naba Kumar Dutta, Namita Roy Choudhury PII: DOI: Reference:
S0001-8686(18)30243-4 doi:10.1016/j.cis.2018.09.003 CIS 1911
To appear in:
Advances in Colloid and Interface Science
Please cite this article as: Tuan Sang Tran, Naba Kumar Dutta, Namita Roy Choudhury , Graphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications. Cis (2018), doi:10.1016/j.cis.2018.09.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Graphene inks for printed flexible electronics: graphene dispersions, ink formulations, printing techniques and applications Tuan Sang Trana, Naba Kumar Duttaa, Namita Roy Choudhury*a
Corresponding author. Email: [email protected]
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School of Engineering, RMIT University, Melbourne, Victoria 3000, Australia.
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ACCEPTED MANUSCRIPT
Abstract Graphene inks have recently enabled the dramatic improvement of printed flexible electronics due to their low cost, ease of processability, higher conductivity and flexibility. In this review, we discuss
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the state-of-the-art of the fundamental formulation of graphene inks and the current printing
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techniques used for inks deposition, followed by recent practical applications for printed flexible electronics. The progression of science and technology for the dispersion of graphene using variety
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of solvents and the characteristics of the resulting conductive inks have been highlighted, with
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specific emphasis focused on the challenges to be resolved. The printing techniques discussed here include screen printing, gravure printing, inkjet printing and other emerging printing technologies.
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Each approach’s pros and cons are discussed in correlation with the ink formulations and the operating principles. We also discuss the challenges and outlook of graphene ink for its future
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development in the world of printed flexible devices.
electronics.
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Keywords: Graphene dispersions, graphene inks, fluidic characteristics, printing techniques, flexible
ACCEPTED MANUSCRIPT Contents Abstract .................................................................................................................................................. 2 Graphical abstract ................................................................................ Error! Bookmark not defined. Contents ................................................................................................................................................. 3 1. Introduction ........................................................................................................................................ 4
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2. Formulation of graphene inks ............................................................................................................ 7
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2.1. Pristine graphene inks ................................................................................................................. 8
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2.2. Chemically-derived graphene inks ............................................................................................ 21 2.3. Graphene hybrid inks ................................................................................................................ 24
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2.4. Challenges on graphene inks formulation ................................................................................. 29 3. Printing techniques........................................................................................................................... 30
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3.1. Screen printing .......................................................................................................................... 30 3.2. Gravure printing ........................................................................................................................ 33
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3.3. Inkjet printing ............................................................................................................................ 35 3.4. Other printing techniques .......................................................................................................... 38
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4. Applications ..................................................................................................................................... 40 4.1. Flexible conductive circuits ...................................................................................................... 40 4.2. Energy devices .......................................................................................................................... 44
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4.3. Electrochemical sensors ............................................................................................................ 49 4.4. Other applications ..................................................................................................................... 51 5. Conclusion and outlook ................................................................................................................... 54 Acknowledgements .............................................................................................................................. 55 Conflict of Interest ............................................................................................................................... 55 Biographies .......................................................................................................................................... 56 References ............................................................................................................................................ 57
ACCEPTED MANUSCRIPT 1. Introduction For the last decade, the world of consumer electronics has experienced massive improvements in the manufacturing techniques towards the production of smaller, faster and better efficiency devices for everyday use. However, the use of traditional solid-state technology poses some limitation on the
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flexibility of the device, the environmental concerns and the processing cost. In recent years, a remarkable transition is taking place in the world of consumer electronics: devices are becoming
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thinner, flexible and wearable. Printing of flexible electronics has showed a promising alternative for
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the traditional fabrication of inorganic materials due to their numerous advantages [1-3]. By offering a low-cost, simple and scalable method for the production of devices with high flexibility and
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stretchability [4-6], printing of conductive inks on flexible substrates are facilitating to enable this
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transition.
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Until now, a number of mass printing techniques have been developed for the ultimate goal to
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achieve a fabrication process with high-performance, stable, low-cost, and zero-waste of materials. Such processing technologies are ink-jet [7], screen [8], and gravure printing [9]. These technologies are associated with liquid-phase inks with markedly different physical properties including the
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concentration of fillers present, viscosity and surface tension of the solution. The screen printing technique is compatible with a variety of inks, the gravure printing requires the use of a low shear viscosity ink suspension, while the ink-jet printing needs a high surface tension and diluted ink solution [10]. In the fabrication of printed electronics, d ifferent techniques possess their own pros and cons, but they all aim to provide rapid and efficient approaches in marking conductive traces on the flexible substrates. Printing technology is undergoing a rapid development and expected to bring a transformational change in the manufacturing of flexible electronics.
ACCEPTED MANUSCRIPT Multiple types of conductive inks have been developed for printed flexible circuits including metalbased inks [11-13], conductive polymers [7, 14], and carbon complexes [15-19]. Among them, metal-based inks particularly Ag and Cu [11, 20, 21] are widely used due to their high conductivity and their previous conversant use in solid-state electronics. Silver is an attractive material for conductive ink due to its excellent electrical properties [22]. However, silver is of high-cost and
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showed unstable performance by migrating into device layers [23, 24], making copper a good
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alternative for its abundance and fairly high conductivity. However, copper is facing oxidation issues
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under ambient condition, which can be facilitated by the high processing temperature, and reducing its electrical conductivity [25, 26]. Besides, the high sintering temperature of metal-based inks is
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also limiting their widespread use with papers and other flexible plastic substrates [27, 28]. It was also reported that the use of these metals is not environmentally friendly and might cause serious
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problems including water toxicity, cytotoxicity and genotoxicity [29]. Thus, there is a critical need for the further development of a low-cost, stable, and environmentally benign conductive ink, which can solve
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the above-mentioned disadvantages.
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Graphene, a two-dimensional carbon lattice, has received tremendous attention due to its excellent mechanical, thermal, and electrical properties [30-35]. With an exceptional carrier mobility of up to
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2×105 cm2 V-1 cm-2 [36-38], graphene has become a golden candidate for printed flexible electronics. However, the absence of a mass production method for high quality graphene prevents its practical application in inks for conductive patterns. Among the available graphene synthesis methods, oxidative-exfoliation of graphite can potentially be used for the production of large quantity of graphene [39-44]. This route basically allows for the oxidation of graphite into graphite oxide, which can be easily exfoliated as individual sheet of graphene oxide (GO). Subsequently, the exfoliated GO sheets can be subjected to a suitable reduction process to remove the oxygen-based functional groups to form reduced-graphene oxide (rGO) sheets, a graphene-like material [45-47]. However, the resulting graphene-like sheets are significantly damaged with holes and vacancies, which
ACCEPTED MANUSCRIPT compromise their outstanding electrical conductivity [48, 49]. For this reason, high quality graphene free from any defect is preferred for its use as conductive materials, especially as inks for printed flexible electronics. In recent years, research efforts have been focused on the direct exfoliation of pristine graphene in liquid media, which can be directly used for printing of conductive patterns [50-53]. The development of proper conducting inks for printing on flexible substrate are still facing certain
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challenges, which include the aggregation of graphene sheets in the suspension, the unsuitable
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viscosity and surface tension, and the lack of adhesion of the inks to the substrate. Enormous efforts
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have been devoted to overcome these issues, to bring graphene inks closer to practical applications. Currently, research on graphene inks has received intensive attention and entered an exploded
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growth phase (Fig. 1), where many applications and fundamental understanding of their formulations have been recently discovered. In the near future, it is no doubt that graphene inks can potentially
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replace the traditional solid-state-electronics and open up a whole new manufacturing process for
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158
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Number of Publications
200
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low-cost, thinner, light weight, and flexible electronic devices.
159
105
100
63
26
0
1
1
8
34
14
2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
Year
Figure 1. The number of publications plotted against year obtained from the Web of Science by searching for the topic “graphene ink” (data acquired on September 2018).
In this review, we aim to guide the readers through recent advances in graphene inks for printed flexible electronics, specifically focusing on the synthesis of graphene-based inks and the techniques that have been developed for printing on flexible substrates, followed by several recent practical applications of graphene inks used in their realization.
ACCEPTED MANUSCRIPT 2. Formulation of graphene inks Graphene can be obtained using two distinct strategies, respectively (i) the bottom- up and (ii) the top-down [54, 55]. The bottom- up approach is based on the growth of carbon atoms into twodimensional carbon layers using chemical vapor deposition (CVD) [56, 57], which can produce very
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high quality graphene sheets on metal substrates. However, practical use of this approach is limited due to its high cost, complexity in the transfer process and difficult to scale- up. Alternatively, the
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top-down approaches involve the production of graphene from existing bulk carbon sources, by
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either exfoliation of graphite towards graphene, reduction of graphene oxide or carbonization of other materials [47, 54, 55, 58]. These methods are widely used for its numerous advantages in term
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of high yield, low cost, and solution processability, however the quality of graphene produced is of
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concern.
State-of-the-art techniques for printing of graphene are based on liquid-phase ink solutions, which
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composed of graphene (or its derivatives) fillers in stabilized solutions. For these reasons, top-down approaches are preferred for production of graphene-based conductive inks. The characteristics of
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the ink solutions are significant facets to the final mechanical and electrical properties of the conductive features. The resulting graphene inks should be stable against precipitation to provide a
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steady performance and uniform conductive patterns. It is important to optimize the composition of the graphene inks such as fillers, solvents and additives to be tailored for processing. A printable ink requires proper fluidic properties, with specific viscosity and surface tension, so it can easily be printed using the available instrumentation. An incompatible ink solution may cause failure of printing devices and resulting poor quality conductive traces. For its widespread use on substrates, the inks should have low volatilizing temperature and good adhesion to the layers, which allow the flexibility and stretchability characteristics of the printed items. A simple and low-cost production route with high yield is highly desirable for bringing graphene inks to its practical applications.
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2.1. Pristine graphene inks Alongside chemical vapor deposition (CVD) and mechanical exfoliation (Scotch tape method), liquid-phase exfoliation of graphite into graphene is considered as an effective route for production
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of high quality (pristine) graphene [33, 50, 51]. In bulk graphite, the parallel graphene layers are
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stacked like a pack of cards due to the van der Waals attraction, which needs to be overcome to achieve individual graphene sheets. Although the van der Waals force between the stacked graphene
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layers is weak enough to let them slide on each other, complete exfoliation into individual graphene
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sheet and stabilizing them against aggregation remain challenging [59]. So, the concept for liquidphase exfoliation is to reduce the intermolecular force of interaction between the adjacent graphene
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layers by liquid immersion. During ultra-sonication or under high shear rate, high energy fluctuations or high shear forces has the potential to peel off the adjacent layers on bulk graphite and induce
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exfoliation. After being exfoliated into individual sheets, graphene relies on the stabilization effect of the solvents and the surfactants used to minimize the intermolecular attraction, thus to improve the
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stability of graphene dispersion (Fig. 2).
Figure 2. Schematic illustration of the liquid-phase exfoliation process of graphite into graphene dispersions.
ACCEPTED MANUSCRIPT Significant research has been focused on the direct exfoliation of graphite into high quality graphene inks [60-69], where the raw graphite materials are directly exfoliated into graphene with the assistance of shear mixing or sonication in liquid media (Table 1). This approach has an advantage due to its low cost, solution processability, and the resulting footprints possess excellent
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conductivity.
Solvent or stabilizer
Resistance
Annealing
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Production method
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Table 1. Graphene based inks produced by liquid-phase exfoliation of graphite in various liquid media.
condition
High-shear mixing
40 Ω/sq
2% w/v ethyl cellulose in ethanol 60:40 NM P and vinyl acetate in isopropanol
Ultrasound-assisted
0.1% w/v ethyl cellulose/
supercritical CO 2
cyclohexanone
liquid phase exfoliation Probe sonication
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Ultrasonic-assisted
DM F, then exchanged to terpineol
1.5 mg polyvinylpyrrolidone (PVP) in 10 mL IPA solution
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Ultrasonication
acetone
30 Ω/sq
Ethyl cellulose in ethanol/toluene
Reference
procedure Inkjet printing
min
250 °C for 30 min Inkjet printing
M ajee et al., 2016 [60] Secor et al., 2013 [61]
100 °C for 5 min Screen printing
Arapov et al., 2015 [62]
810 ± 200 Ω/sq 300 °C for 30 min Inkjet printing Gao et al., 2014 [63]
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10 mg/mL of nitrocellulose in High-shear mixing
∼350 °C for 150
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Probe sonication
260 Ω/sq
NM P with additional ethyl cellulose
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Shear-exfoliation
Printing
∼100 Ω/sq ∼2×105 Ω/sq
Photonic annealing
Secor et al., 2017 3D printing
[64]
400 °C for few hours
Inkjet printing
-
400 °C for 30 min Inkjet printing
∼2×103 Ω/sq*
250 °C for 30 min Gravure printing
Li et al., 2013 [65]
Dodoo-Arhin et al., 2016 [66] Secor et al., 2014 [67]
85% cyclohexanone (with 15%
Vortex mixing
terpineol with additional ethyl
172.7 ± 33.3 Ω/sq
950 °C
Inkjet printing
He et al., 2017 [68]
cellulose stabilizer Blade coating
M icrofluidization of graphite
Carboxymethylcellulose sodium salt
∼2 Ω/sq
100 °C for 10 min
*Resistance is calculated based on the conductivity at the given thickness.
and screen printing
Karagiannidis et al., 2017 [69]
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2.1.1. Solvent based pristine graphene inks In the perspective on van der Waals interactions between the stacked graphene layers in graphite, the most simple and effective methods to reduce this intermolecular force is liquid immersion, where the
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potential energy between adjacent layers is significantly reduced in the presence of a liquid medium
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with proper interfacial tension [70, 71]. Previous studies have shown that interfacial tension plays an important role on dispensability, where the higher interfacial tension between solid and liquid results
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in the poorer stability of the dispersion [70]. In the case of graphitic flakes in liquid, if the interfacial
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tension is high, the energy required to detach two adjacent layers is also high and the flakes itself prone to aggregation, preventing the dispensability of graphitic flakes in colloidal dispersion. Recent
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study by Coleman and co-worker [50, 51, 72] indicated that the solvents with surface tension of ~ 40 mJ m-2 can minimize their interfacial tension with graphene. Such solvents include N-methyl-2-
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pyrrolidone (NMP, ~ 40.8 mJ m-2 ) and N,N-dimethylformamide (DMF, ~ 37.1 mJ m-2 ) have been
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established as the most effective liquids for preparing graphene dispersions [51, 52]. In 2016, Majee and colleagues [60] formulated a highly concentrated and stable graphene ink (3.2 mg/mL) composed of 4- layer graphene flakes with uniformly lateral size of ~160 nm, which was
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achieved by shear exfoliation of graphite in NMP using an L5M Silverson mixer. The graphene ink was then inkjet-printed on a glass substrate and annealed at ∼350 °C for 150 min, resulting in near transparent graphene conductive circuits with relatively low sheet resistance of 260 Ω/sq (ohms per square) at ~160 nm thickness. Li et al. [65] also prepared high concentration and stable graphene inks by ultrasonication of graphite in DMF, which have been demonstrated to be compatible to inkjet printing. The inks composed of grahene flakes with lateral dimension ranging from 100-500 nm were printed and annealed at 400 °C for few hours to achieve a sheet resistance of ∼200 kΩ/sq at a transmittance of about 90%.
ACCEPTED MANUSCRIPT However, the high boiling point solvents such as NMP and DMF (>150 °C) restricted the use of most plastic substrates, which require low treatment temperature. Therefore, dispersion of graphene in low-boiling point solvents is preferable. Still, common low-boiling point alternatives like acetone, isopropanol, and ethanol, usually come up with unsuitable surface tension (27.6 mJ m-2 , 23 mJ m-2 , and 22.1 mJ m-2 , respectively). The use of these solvents may lead to poor graphene dispersion,
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requiring a third added component to stabilize the exfoliated graphene flakes against aggregation.
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Chemical properties including surface tension, boiling point, chemical formula and structure of
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several common solvents used for direct liquid-phase exfoliation of graphene are given in Table 2.
Surface tension
N-M ethyl-2-pyrrolidone (NM P)
~ 40.8 mJ m-2
Boiling point
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Name
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Table 2. Chemical properties of common solvents use for liquid-phase exfoliation.
Formula
C5H 9NO
153 °C
C3H 7NO
~ 36.6 mJ m-2
181 °C
C6H 4Cl2
Cyclohexanol
~ 34.4 mJ m-2
162 °C
C6H 12O
Chlorobenzene
~ 33.6 mJ m-2
131 °C
C6H 5Cl
Toluene
~ 28.4 mJ m-2
111 °C
C7H8
Acetone
~ 27.6 mJ m-2
56 °C
C3H 6O
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1,2-Dichlorobenzene
~ 37.1 mJ m-2
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N,N-Dimethylformamide (DM F)
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202 °C
Structure
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~ 23 mJ m-2
83 °C
C3H 8O
Ethanol
~ 22.1 mJ m-2
78 °C
C2H 6O
Water
~ 72.8 mJ m-2
100 °C
H 2O
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Isopropanol
Noteworthy, in 2016 Arapov and co-worker [62] described an approach to prepare low-annealing
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temperature graphene dispersion for screen printing. In general, graphene dispersions were prepared
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by utilizing the intercalation of graphite and therma l expansion, followed by high-shear mixing of
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the expanded graphite in the presence of a polymeric binder. By mild heating, the gelation of graphene/polymer dispersion was triggered to form a highly concentrated and stable graphene paste,
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which showed excellent performance in screen printing with the line resolution of 40 µm in width (Fig. 3). In the final process, the printed patterns dried at 100 °C for only 5 min that exhibited a sheet
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resistance of 30 Ω/sq at 25 μm thickness. This work is important since it removed the need for long-
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to a myriad of substrates.
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time high temperature annealing and enabled high-volume roll-to-roll production, which compatible
Figure 3. a) Scheme of i) graphite intercalat ion, ii) thermal expansion, iii) preparat ion of graphene dispersion and subsequent gelation; b) image of graphene paste on a spatula; c) bent PET foil with a test pattern screen printed with graphene paste; f) SEM images of large-area prints at 5000× magnification. Reproduced with permission from ref. [62]. Copyright © 2016 Wiley.
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2.1.2. Surfactant assisted exfoliation of pristine graphene inks The use of such effective solvents like NMP and DMF is not ideal and suffers from some important drawbacks. These solvents are expensive and hazardous, restricting their practical application at
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industrial accessible scale. In addition, they tend to have high annealing temperature, making them inapplicable for printing onto most plastic substrates. Water, the most preferred solvent due to its low
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boiling point and non-toxic nature, has a surface tension that is too high (72.8 mJ m-2 ) to work on its
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own as an exfoliant for graphene. Moreover, the dispersion of graphene in water is challenging due to the hydrophobic nature of graphitic carbon. In order to achieve aqueous graphene dispersion, it is
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crucial to reduce water’s surface tension and suspend the exfoliated graphitic flakes without
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aggregation.
Recent studies have shown that the use of surfactants can promote exfoliation of graphite into
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graphene by lowering the interfacial energy between the two immiscible phases and forming colloidal systems [59, 73-75]. According to the Derjaguin, Landau, Verwey and Overbeek (DLVO)
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theory, the stability between two surfaces in suspension depends on the net sum of the electric double layer (EDL), steric repulsion (Steric) and van der Waals attraction (vdW). At the most basic
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level, aggregation is occured by van der Waals interactions. Electric double layer forms a net charge, and when two like-particles are in close proximity, the electric double layer repels the two. Steric stabilization provides a powerful tool to enhance the dispersion by preventing two particles from forming attractive van der Waals interactions. Attachment will occur if the objects are within the range such that the net sum of the interactions is attractive. The presence of a strong hydrophobic force contributes to the attractive forces, effectively increas ing the range at which attachment occurs (Fig. 4).
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Figure 4. Example interaction energy for the stable dispersion of nanoparticles in a liquid mediu m, according to the classical DLVO theory. Reproduced with permission fro m ref. [76]. Copyright © 2015 - Published by The Royal Society
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of Chemistry.
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Currently, a variety of surfactants including ionic, non- ionic, polymeric and bio-surfactants have been used to prepare graphene dispersions. Given in Table 3 are graphene dispersions that have been
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stabilized by some common surfactants.
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Table 3. Graphene dispersions stabilized by some common surfactants.
Production method
Yield*
Stability
Reference
Ionic
M ild sonication for 430 h
6%
> 5 days
[77]
Ionic
Ultrasonication for 30 min
0.22 %
> 7 days
[78]
Sodium dodecylbenzenesulfonate (SDBS)
Ionic
Ultrasonication for 30 min
~ 3.57 %
~ 6 weeks
[73]
Pluronic F-127
Non-ionic
Ultrasonication for 30 min
3.1 %
-
[79]
Triton X-100
Non-ionic
Ultrasonication for 30 min
0.42 %
> 7 days
[78]
Poly(sodium-4-styrene sulfonate) (PSS)
Polymeric
Ultrasonication for 30 min
-
Several days
[80]
Flavin mononucleotide sodium salt (FM NS)
Bio
Ultrasonication for 5 h
~1 %
> 6 months
[81]
Sodium cholate (SC)
Type
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Surfactant
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Sodium dodecylsulfate (SDS)
*Yield is calculated by the final graphene concentration against the initial graphite concentration.
ACCEPTED MANUSCRIPT Ionic surfactants like sodium cholate (SC), sodium dodecylsulfate (SDS), and sodium dodecylbenzenesulfonate (SDBS) have been widely used to improve the dispersions of carbon nanotubes (CNTs) [82]. Owning to its good compatibility to carbon materials, ionic surfactants have gained much attention into graphene dispersion [83]. A typical study on sodium cholate, an ionic surfactant, for graphene dispersions was reported by Lotya and co-worker in 2010 [77]. By mild
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sonication for up to 430 h, graphene dispersions can be stabilized in water/sodium cholate medium at
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the concentrations up to 0.3 mg/mL. The graphene dispersions so produced contained 1-10 layer
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graphene flakes, with the average graphene flake consisted of ~4 layers and the lateral size of ~1 µm. The prepared dispersions were reported to be highly stable and can be easily cast into various
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substrates, which is ideal for formulation of graphene ink. In a subsequent study, a range of 12 different ionic and non- ionic surfactants were used to disperse graphene in water, reported by the
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same research group [78]. In all the cases, the quality of graphene dispersions (flake size and degree of exfoliation) was found to be comparable from surfactant to surfactant.
The dispersed
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concentration varied from 0.011 mg/mL for SDS to 0.026 mg/mL for SC.
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Non-ionic surfactants, such as Pluronic F-127 and Triton X-100 contain hydrophobic polypropylene oxide (PPO) and hydrophilic polyethylene oxide (PEO) blocks, can also be used to promote
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graphene exfoliation. A study by Seo et al [79] using a variety of Pluronic and Tetronic block copolymers showed that Pluronic F-127 can disperse pristine graphene at relatively high concentrations (0.064 mg/mL, 0.303 mg/mL, and 1.255 mg/mL at the centrifugation conditions of 15000 rpm for 5 min, 5000 rpm for 5 min, and 750 rpm for 10 min, respectively), enabling a new class of biocompatible dispersions for graphene inks. In an extended study, Smith et al., [78] have shown that by ultrasonication of graphite in aqueous solution with presence of additional Triton X100, graphene dispersions can be achieved with concentration of up to 0.021 mg/mL and high stability for more than 7 days.
ACCEPTED MANUSCRIPT Polymeric surfactants have previously been used to enhance the dispersion of CNTs in water and organic solvents [82, 84]. However, there is still a scarce of literature on their use for graphene dispersions. The only report to date on polymeric surfactants stabilizing graphene is provided by Stankovich and co-workers [80]. In this study, poly(sodium-4-styrene sulfonate) (PSS), a wellknown amphiphilic polymer, was used as surfactant for stabilizing reduced graphene oxide (rGO).
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Interestingly, PSS has shown exceptional effectiveness to keep the graphene dispersion stable against
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aggregation for over a year. Still, there is a lack of study on direct exfoliation of graphite into pristine
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graphene using polymeric surfactants. However, this approach is promising for stabilization and formulation of graphene inks.
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Bio-surfactants have recently emerged as a favorable candidate for preparing biocompatible aqueous
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graphene dispersions, which open the door for graphene to bio- medical applications. A recent report by Ayan-Varela and co-workers [81] has shown that a derivative of vitamin B2 , namely the sodium
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salt of flavin mononucleotide (FMNS), can be a highly efficient dispersant for the preparation of aqueous graphene dispersions. In general, aqueous solution of graphite powder and FMNS
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(concentrations of 30 and 1 mg mL−1 , respectively) was sonicated for 5 h, followed by centrifugation at 2300 rpm for 20 min to remove any unexfoliated graphite particles. A concentrated graphene
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dispersion of 0.3 mg mL−1 was achieved, implying a relatively high exfoliation yields (~1%), which are comparable to other efficient surfactants (Table 3). The stabilization mechanism for surfactant/graphene dispersions has not yet been fully explored due to the lack of fundamental understanding about the molecular interaction between surfactant molecules and graphene flakes. As the major challenge in formulating graphene inks is to overcome the van der Waals interactions between adjacent graphene layers, successful exfoliation into stable graphene dispersions means that surfactants have successfully minimized the graphene/water interfacial energy, which allows the extraction of individual graphene sheets under support of
ACCEPTED MANUSCRIPT external energy (e.g. ultrasonication, high shear force), and hinders these individual sheets from aggregation. Moreover, the presence of surfactants also renders hydrophilicity to graphitic carbon, making pristine graphene soluble in water in a similar manner with graphene oxide without disturbing their conjugated sp2 basal plane. The most widely accepted mechanism on surfactant/graphene stabilization to date is proposed by Smith and co-workers in 2010 [78]. They
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indicated that for ionic surfactants, graphene concentration is mainly controlled by the zeta potential
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of the surfactant-coated graphene sheet, which scales linearly with the repulsive electrostatic
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potential. This means that for ionic surfactants, graphene dispersions are mostly stabilized by electrostatic repulsion mechanism. On the other side, the non- ionic surfactants have hydrophobic tail
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adsorbed onto the graphene surface and hydrophilic segments extended into water, which creating a potential barrier between the flakes. Thus, the stabilizing mechanism of non- ionic surfactants tends
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to be based on steric stabilization.
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Surfactants have gained significant importance in promoting graphene exfoliation in water. However, there is one systematic problem preventing the use of surfactants in formulation of
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graphene inks. That is surfactant itself can become a residual component in the final product. Regardless of printing techniques, the printed composites are usually consisting of graphene flakes,
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surfactants and solvents. As surfactants do not possess any interesting electronic properties, it may cause negative effect onto the electrical properties of the printed patterns. Therefore, it is crucial to remove surfactants from the printed composites. Removal of alternating surfactants molecules between graphene layers, however, is nearly impossible without spoiling the printed patterns. Thus, water-based pristine graphene inks are still far from optimization, calling for further development.
2.1.3. Polymer assisted exfoliation of pristine graphene inks
ACCEPTED MANUSCRIPT Polymers have been established as an interesting alternative to surfactants for stabilizing graphene in liquid medium since they can form emulsion systems, hindering graphene flakes from aggregation [59, 85, 86]. A range of polymers such as polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), ethyl cellulose (EC), and many more has been used to prepare stable graphene dispersions in many different solvents, and even water. The combination of such
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different polymers and solvents has given a variety of options for formulation of graphene inks. Thus
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the search for a suitable polymer-solvent combination is still ongoing and attracts a great deal of
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attention. Chemical structures of typical polymers used as emulsifiers for liquid-phase exfoliation of
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graphite toward graphene are given in Fig 5.
Figure 5. Chemical structures of typical poly mers used as emulsifiers for liquid -phase exfo liat ion of graphite toward graphene.
Stable and high-quality graphene aqueous dispersions were successfully prepared with assistance of PVA stabilizer in a recent study by Paton and colleagues [87]. In general, a solution of 2 wt% of PVA in water was used as the mixing liquid for graphite exfoliation. Graphite was then added into the mixing solution and exfoliated using an L5M Silverson mixer. The resulting graphene dispersions are highly stable and can be achieved in liquid volumes from hundreds of millilitres up to
ACCEPTED MANUSCRIPT hundreds of litres and beyond, opening up an effective route for mass production of graphene inks toward practical applications. By using ethyl cellulose as stabilizing polymer, Secor et al. [61] has successfully developed a novel graphene ink based on ethanol, an environmentally benign solvent by solution-phase exfoliation of
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graphite (Fig. 6). At a concentration of 2.4 wt %, the prepared graphene ink has a surface tension of ∼33 mN/m and a viscosity of 10–12 mPas at 30 °C, which is compatible for inkjet printing. Printing
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was carried out at 30 °C using a Fujifilm Dimatix Materials Printer (DMP 2800) with a cartridge
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designed for a 10 pL nominal drop volume. Drop spacing for all printed features was maintained at 20 μm, yielding stable graphene line with the width of ~60 µm. The printed conductive features were
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annealed at 250 °C for 30 min and exhibited a low sheet resistance of 0.4 Ω·m at ~140 nm thickness
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with uniform morphology and excellent bending capability.
Figure 6. Schematic illustration of the ink preparation method. (a) Graphene is exfoliated fro m graphite powder in ethanol/EC by probe u ltrasonication. (b) centrifugation-based sedimentation to remove residual large graphite flakes and (c) salt-induced flocculation of graphene/EC. (d) An ink fo r inkjet printing is prepared by dispersion of the g raphene/EC powder in 85:15 cyclohexanone/terpineol. (e) Vial of the prepared graphene ink. Reprinted with permission from ref. [61]. Copyright © 2013 American Chemical Society.
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In 2014, Gao and colleagues [63] reported a new route for formulation of ethyl cellulose-stabilized pristine graphene ink. Graphene dispersions were prepared by direct exfoliation from graphite using ultrasound-assisted supercritical CO 2 . Under ultrasonic vibration, the CO 2 molecules accumulated in
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the graphitic interlayer absorbed energy, expanded the distance between the graphite layers and induced exfoliation (Fig. 7). The resulting graphene dispersions are highly stable with concentrations
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up to 1 mg/mL and compatible to inkjet printing. Graphene flakes in the ink dispersion were less
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than 1 nm thick and 30–100 nm laterally. The formulated graphene inks were inkjet printed and annealed at 300 °C for 30 min, which showed high conductivity of 9.24 × 103 S/m (0.81 ± 0.2 kΩ/sq)
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and excellent flexibility.
Figure 7. Schematic illustration of the preparation process of pristine graphene ink and its printed electrodes. (a) Layered graphite was immersed in supercritical CO2 . (b ) CO2 molecu les penetrated and intercalated in the interlayer of graphite, (c) forming single - o r few-layer-thick g raphene sheets. (d) Graphene sheets were stabilized by EC in cyclohexanone and (e) formed stable graphene ink. (f) Graphene electrodes were printed on PET and PI substrates. Reprinted with permission from ref. [63]. Copyright © 2014 American Chemical Society.
In an effort to get rid of the harsh annealing condition, Secor and co-workers [64] reported a highperformance and low-temperature processing route of graphene ink by rapid photonic annealing
ACCEPTED MANUSCRIPT using a Xenon lamp (Fig. 8). Graphene ink was produced by high shear mixing of graphite in a solution of nitrocellulose and acetone, followed by ethyl lactate-supported stabilization. This procedure yields a composite containing of ~35 wt % graphene flakes with a typical thickness of ~2 nm and lateral size of ~300 nm. The ink was 3D printed and annealed with a single millisecond light pulse. The resulting conductive traces showed superlative electrical conductivity with the resistance
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of ~100 Ω/sq at the thickness of ~1 µm, which is comparable to thermal annealing at 350 °C. This
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strategy enabled the widespread use of a myriad of substrates, includes thermally sensitive materials.
Figure 8. Process overview and p roof of concept. (a) Chemical structure of nit rocellulose (left) and graphene (right). (b)
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Differential scanning calorimetry of graphene/nitrocellulose showing the large exothermic reaction of nitrocellu lose at ∼200 °C. (c) Schematic illustration of photonic annealing, wh ich in itiates a nit rocellu lose propagating reaction. Cross -
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sectional scanning electron microscopy reveals the resulting porous graphene microstructure even on a thermally conductive silicon substrate. Reprinted with permission from ref. [64]. Copyright © 2017 American Chemical Society.
2.2. Chemically-derived graphene inks Graphene oxide (GO), also known as chemically modified graphene (CMG), contains a range of reactive oxygen functional groups, which can be reduced to yield graphene- like materials. Interestingly, the majority of studies on graphene and its application are not based on pristine graphene, but rather the reduced graphene oxide (rGO). This is because GO can be produced in an
ACCEPTED MANUSCRIPT industrially accessible scale in a cost-effective manner, and the functional groups of GO enable its hydrophilicity and solution processability, which allow functionalization to tailor its exceptional properties. One of the most simple and straightforward GO inks formulation in water was reported by Le et al.
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[88] in 2011. They found out that hydrophilic GO (average dimensions: 500 nm × 500 nm × 0.8 nm) dispersed in water was a stable ink for inkjet printing. The printed GO traces with the lateral spatial
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resolution of 50 μm were thermally reduced at 200 °C for 12 h under N2 air to form conductive
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graphene electrodes, which were used for supercapacitors. At room temperature, the viscosity and surface tension of the GO ink were 1.06 mPa s and 68 mN/m, respectively, which were outside of the
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ranges recommended by the manufacturer for normal operation of the printer (10–12 mPa s and 28–
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32 mN/m). Thus, the waveform function of the piezoelectric nozzles was tuned to form spherical ink droplets from the un-optimal water-based GO ink.
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Since the hydrophilicity of GO enables the use of the most preferable solvent, water, formulation of GO-based inks is more focused on their reduction route. Beside thermal reduction, which is
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undesirable for low- melting point substrates, chemical reduction has gained much attention. In a report in 2011, Shin et al. [89] demonstrated a simple and effective strategy to pattern graphene
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nanosheets on PET film using inkjet printing and low-temperature chemical-assisted reduction. The GO ink (with the viscosity of 2.2 mPa s and the surface tension of 72.8 mN/m) was printed onto the substrate as the designed images using the modified inkjet printer and the printed film was reduced in a chamber containing hydrazine and ammonia solution at 90 °C for 1 h. The printed graphene patterns consisted of a few layers of graphene sheets with surface resistance of approximately ∼65 Ω/sq. The use of hydrazine is not preferable due to its highly toxic nature. So research has turned to other alternatives such as vitamin C, trifluoroacetic acid, and hydroiodic acid as reducing agents. In 2010,
ACCEPTED MANUSCRIPT Dua et al. [90] described a flexible and lightweight chemiresistor made of a rGO thin film, which were inkjet-printed onto flexible plastic substrates using surfactant-assisted GO ink and vitamin C as reducing agent. The resulting film has electrical conductivity properties (σ ≈15 S cm−1 ) and has fewer defects compared to rGO films obtained by using hydrazine reduction. More recently, Overgaard and co-workers [91] reported that water-based graphene oxide ink could be screen-printed
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on flexible plastic substrates and subsequently reduced using a 1:1 mixture of trifluoroacetic acid and
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hydroiodic acid at low temperature (80 °C), creating a conductive circuit with low sheet resistance of
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327 Ω/sq. The resulting thin film was flexible and semitransparent with 37% transmittance (Fig. 9).
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Figure 9. (a) Photograph of screen-printed GO and b) rGO showing the printed circuits. c) A height profile taken across a grid line and the point of intersection between two grid lines are shown and d) the corresponding AFM image is shown. Reprinted with permission from ref. [91]. Copyright © 2017 Wiley.
Using polyvinylpyrrolidone (PVP) as rheology modifier and gelation inhibitor, Chang et al. [92] was able to form a highly concentrated ink solution with graphene oxide (GO/PVP ink), which is suitable for meniscus- guided printing (Fig. 10). The printing strategy is quite simple and effective since the GO/PVP ink was supplied continuously through the micronozzle simply by horizontal pulling of the
ACCEPTED MANUSCRIPT nozzle without any applied pressure. Then, the printed wafers were thermally treated at 450 °C for 3 h under vacuum for reduction of GO and removal of PVP. The resulting rGO patterns had a good
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electrical conductivity of ~529 S/m at room temperature.
Figure 10. (a) Schematic d iagram of the printing process based on the meniscus -guided printing; (b) FE-SEM image of
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the printed GO-PVP pattern; (c) FE-SEM image of the printed pattern after the thermal treat ment at 450 °C for 3 h in
2.3. Graphene hybrid inks
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vacuum. Reproduced with permission from ref. [92]. Copyright © 2017 Elsevier.
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Intensive research has been devoted on the design and synthesis of graphene hybrid complexes for enhancing their conductivity. By mixing graphene with conductive polymer or metal particles, the
applications.
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hybrid inks can take full advantage of the two materials to tailor their properties for different
Polyaniline, a typical conducting polymer, is favorable electrode material for supercapacitors due to its ease of synthesis and high pseudocapacitance. Xu et al. [93] prepared graphene/polyaniline inks for inkjet printing by probe-sonication of graphite powder and polyaniline in water, supported by SDBS surfactant. The printed films were annealed at 80 °C for 2 h that exhibited conductivity of 0.29 S cm−1 (846 Ω/sq). A supercapacitor cell was fabricated using the printed thin film electrodes and showed a long cycle life with high performance.
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Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic
acid),
(PEDOT:PSS),
an
important
conductive polythiophene derivative, is an attractive electrode material for electroanalysis due to its transparency, high conductivity, and stab ility under ambient conditions. Seekaew et al. [94]
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formulated graphene/PEDOT:PSS hybrid inks for ammonia gas sensor. The hybrid inks were synthesized by sonication-assisted mixing of chemically-derived graphene powder, PEDOT:PSS
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solution, dimethyl sulfoxide (DMSO), ethylene glycol, and Triton X-100. It was demonstrated that
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π–π interactions occurred between graphene and PEDOT:PSS. A flexible graphene/PEDOT:PSS gas
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sensor was printed and exhibited high sensing performance to NH3 .
A typical demonstration of graphene/PEDOT:PSS hybrid inks was recently reported by Liu et al. [95] in 2015. By spray-coating of graphene/PEDOT:PSS hybrid inks onto PET substrates, they have
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successfully fabricated transparent and highly conductive graphene films for ultrathin organic
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photodetectors (Fig. 11). The printed hybrid inks were processed at a relatively low temperature (90 °C) and displayed an average resistance of ~600 Ω/sq with 80% transmittance. The fabricated graphene films with excellent flexibility were applied as bottom electrodes in ultrathin organic
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photodetectors and exhibited comparable performance to that of the state-of-the-art Si-based inorganic photodetectors.
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Figure 11. a) Schematic illustration of spray-coating an graphene/PEDOT:PSS hybrid ink onto desired substrates. b) Transmittance spectrum of both graphene/PEDOT:PSS hybrid films and ITO on PET substrates. Inset shows the optical
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images of the graphene/PEDOT:PSS hybrid films on PET substrates with 90% and 80% transmittance, respectively. c) SEM image of spray-coated graphene/PEDOT:PSS hybrid film. d) A 4 paper-sized hybrid transparent conductive film
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obtained by spray-coating. Reprinted with permission from ref. [95]. Copyright © 2015 Wiley.
Recent research has been performed to enhance the conductivity of graphene-based inks by
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incorporation of additional metal fillers. Jabari and co-workers [96] developed a graphene/silver nanoparticles ink and deployed in an aerosol-jet printing system. Adding silver nanoparticles (Ag NPs) to the graphene ink resulted in a uniform microstructure and crack- free printed graphene/Ag NPs patterns, which facilitated the movement of the electrons. The Ag NPs also bridged the graphene flakes and prevented stacking between graphene layers, thus decreased the resistivity of the printed patterns. The printed graphene/Ag NPs features were annealed at 250 °C and displayed the enhanced electrical conductivity of 1.07 × 10-4 Ω·cm, higher than those of neat graphene and Ag inks. In a similar study, Wang et al. [97] prepared an annealed graphene nanosheet coupled with Ag organic complex ink with confirmed changes of graphene microstructure suitable for inkjet printing.
ACCEPTED MANUSCRIPT Graphene nanosheets were annealed at 2200 °C for 30 min to remedy its electrical conductivity and then dispersed with silver/organic complex by sonication and polymer stabilization. The prepared hybrid inks were inkjet-printed onto polyimide (PI) substrates and baked at 300 °C for 40 min, resulting in conductive patterns with the resistivity of 4.62 × 10−4 Ω·m and high flexibility.
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In a recent study, Deng and colleagues [98] demonstrated an in situ prepared silver nanoparticles decorated graphene conductive ink for printed flexible electronics (Fig. 12). The hybrid inks were
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obtained by liquid phase exfoliation of graphite and reduction of silver salt to nanosilver in the same
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system with ideally compatible fluidic characteristics for inkjet printing. After annealed at 400 °C for 30 min, the inkjet-printed graphene features attained low resistivity of 20 ± 1Ω/sq with
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uniform morphology and high compatibility to flexible substrates.
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Figure 12. The preparation process of the nanosilver decorated graphene conductive ink. Reprinted with permission from ref. [98]. Copyright © 2017 Springer.
More complex graphene hybrid inks were formulated by Li et al. [99] in 2017. In general, they assembled gold package silver nanocore@shell nanotriangle platelets (Ag@Au NTPs) on graphene oxide (GO) to formulate Ag@Au NTPs-GO hybrid inks, which are satisfactory for inkjet-printing. The hybrid inks were prepared by sonication of Ag and Au nanoparticles in water-based GO dispersions. It is found that the addition of a thin layer of Au coated on the surface of Ag can effectively reduce the surface energy of the dispersion and improve the stability of the material's
ACCEPTED MANUSCRIPT conductivity. The printed patterns were hydrazine-reduced at 110 °C for 3 h and maintained a stable low sheet resistance of ∼ 149.5 Ω/sq after 100 days, providing a promising route for fabrication of highly stable and transparent conductive circuits. A number of research have shown that the incorporation of gold nanoparticles (Au NPs) with
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graphene could represent a promising way towards significant improvements of the electrochemical properties of the sensing devices. Pan et al. [100] fabricated electrochemical biosensor for direct and
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rapid detection of bisphenol A using graphene/gold nanocomposites. The hybrid inks were prepared
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by sonication-assisted dispersing of graphene oxide (GO) and chloroauric acid (HAuCl4 ) in water, followed by microwave heated for 2 min and thermally baked at 80 °C for 20 min. The
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graphene/gold nanocomposites based biosensor was prepared by the simple classic casting method of
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the hybrid inks onto glassy carbon electrodes and exhibited excellent performance for detection of bisphenol A.
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So far, research on graphene hybrid inks were mostly focused on the use of additional conducting polymers and metal nanoparticles fillers to tailor the original properties of graphene. By adding high
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pseudocapacitance conducting polymers, high performance and flexible graphene-based energy storage devices can be printed. The use of metal fillers like Ag and Au nanoparticles also enhanced
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the electrical and electrochemical properties of graphene inks, therefore exploited the full advantage of these materials. Au filled graphene inks are promising for printing of flexible and high performance sensing devices, while Ag/graphene hybrid inks are favorable for low cost, flexible, and transparent conductive circuits.
ACCEPTED MANUSCRIPT 2.4. Challenges on graphene inks formulation High
quality
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requires
excellent
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formulation
and
properties
such
as
hydrophilic/hydrophobic balance of the ink for specific application, surface tension of the ink, surface energy of the substrate, etc. There is a direct relationship between the ink characteristics and
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the substrate properties. While during its maturation, graphene inks have shown its excellent capability and processability in advanced manufacturing of next generation flexible electronics,
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however, there are still significant challenges for its realization. The first shortcoming is the
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compatibility of the inks to the state-of-the-art printing techniques, especially for the widely used inkjet printing. The waveform function of the printing nozzles can be manipulated to form spherical
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ink droplets, but the fluid characteristics of the inks are far from optimal. Second, the high annealing
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temperature of the inks prevented its available use in a wide range of substrates, thus limited its practical applications for flexible devices. Third, current graphene inks formulations are not
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environmentally benign processes. It uses a large amount of organic solvents for liquid phase exfoliation and a number of strong oxidative reagents for production of graphene oxide. Post-
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treatment and hydrazine reduction are also not environmental friendly and use significant amount of highly toxic materials. Thus it calls for the development of an ecofriendly and sustainable route for
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production of graphene inks. And finally, to bring every invention to life, the benefits are required to outweigh the cost. Therefore, it still requires a great deal of effort to pioneer a feasible graphene based ink with low cost and high output.
ACCEPTED MANUSCRIPT 3. Printing techniques Printing technology is one of the foremost inventions for advanced lean manufacturing. Various printing techniques have been developed for fabrication of flexible electronics, which are associated with roll-to-roll processing. The printing techniques that will mainly be discussed in this review are
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mass printing techniques, which are feasible and cost-effective towards roll-to-roll fabrication.
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We can classify the available printing technologies into two distinct models, conventional and digital printing [101]. The conventional mass printing techniques described here are screen and gravure
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printing, while the only accessible digital printing technology is inkjet printing. Other printing
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techniques do exist, but we only take into consideration the potential printing strategies, which allow
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scaling- up for mass production in an industrially accessible scale.
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3.1. Screen printing
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Screen printing, a major type of conventional printing technologies, has been applied to the fabrication of printed circuit boards for decades [102]. Nowadays, screen printing is used for fabrication of solar cells, sensors, organic light-emitting devices, and thin film transistors [103-107].
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Among the available mass printing technologies, screen printing is considered as the most versatile and mature technique, which is simpler and faster in comparison to other printing tools [108]. The process of screen printing is illustrated in Fig. 13. The ink paste is filled onto the edge surface of the stencil, which was located on the desired substrate and secured in place by spacers. During printing, the squeegee moves against the stencil and pushes ink paste through the etched motifs, resulting in the desired patterns, which is transferred onto the substrate after a single pass of the squeegee [109]. The results from screen-printing can be uniformly reproduced by repeating the process on the new substrates with an optimal operating procedure.
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Figure 13. (b) Schematic illustration of the screen printing process ; (b) Cross-sectional illustration of the screen printing.
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Reprinted with permission from ref. [109]. Copyright © 2015 Wiley.
Screen printing can be fully adapted to roll-to-roll manufacturing by rotary screen printing [105]. Its
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principal design is composed of two opposite rotating cylinders, where the stencil is shaped as a hollow cylinder with a fixed internal squeegee and supported by a pressure roller, as shown in Fig
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14. As the roll- to-roll processes, the two cylinders rotate and contact with the flexible substrate. Under pressure of the squeegee, the ink paste inside the stencil cylinder is transferred to the substrate
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through the engraved pattern of the stencil. This technique is ideal for large-scale and continuous
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Figure 14. Schematic illustration of the roll-to-roll screen printing process.
Screen printing has advantages due to its simplicity, reproducibility and high compatibility with various inks and substrates; making it a cost effective approach for mass printing of flexible devices [108]. However, screen printing also have several drawbacks. It showed a relative low resolution
ACCEPTED MANUSCRIPT compared to digital printing, therefore inappropriate for printing of miniatured circuits with high precision. Another issue is the dry out of ink during the printing process, which may impair the devices as well as the printed features [10, 108]. The direct contact between stencil and substrates may also lead to undesirable scratches and creases on the substrates while squeegee stress is applied,
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thus affecting its transparency and flexibility. The quality of the screen-printed patterns are generally contingent on the ink rheology, printer
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hardware (stencil, squeegee, substrate), and the printing process [105, 108]. With a well‐defined
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silicon stencil, high‐quality graphene patterns as narrow as ~40 μm can be screen-printed, as hightlighted by Huyn et al. in 2014 [109]. While the hardwares of the screen-printer are standardized
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and the processing parameters can be controlled during printing, the rheological properties of the ink
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paste play an important role on the features of the printed specimens. The flow of ink through the mesh of the stencil is significant since it determines uniformity of the printed surface, an important
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factor of conductive circuits. Unsuitable ink characteristics may result in defects or short-circuits of the conductive patterns. Thus, current formulation of graphene inks for screen printing should take
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its rheological properties into consideration, apart from its electrical conductivity. Ink paste for screen printing is basically qualified by its flow properties (i.e., viscosity or shear stress
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as a function of shear rate) [110]. An ideal ink paste should have a high rest viscosity, a low viscosity at high shear rate, and a fast viscosity recovery time [108, 110]. The viscosity of a screen-printable ink can be varied from 0.05 to 5 Pa s [101]. To meet the viscosity requirements for a proper ink paste, current formulations of graphene inks for screen-printing are mostly focused on either gelation of graphene dispersions [62] or preparation of highly concentrated graphene pastes [109]. In 2015, Arapov and co-workers [62] formulated colloidally stable and highly concentrated (52 mg mL−1 ) graphene pastes by gelation of graphene in polymeric binder. The graphene pastes have excellent screen printability down to lines of ~40 μm in width and high conductivity of ~30 Ω/sq at 25 μm
ACCEPTED MANUSCRIPT thickness. In a different approach, Huyn et al. [109] formulated graphene pastes with much higher concentration of ~80 mg mL−1 with a shear viscosity ranging from 1-10 Pa s at a shear rate of 10 s−1 . The graphene pastes also showed excellent performance in screen printing with line width of ~40 μm and high conductivity of ~1.86 × 104 S/m.
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A significant drawback of screen printing when using graphene inks is the challenge on controlling the thickness of the printed pattern, as they are mainly determined by the thickness of the stencils.
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There is also a difficulty in producing highly viscous and concentrated dispersions of graphene with
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its inherent tendency toward aggregation.
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3.2. Gravure printing
Gravure printing is a common low-cost patterning process, which utilizes direct ink transfer through
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physical contact between the engraved cylinder and the substrate [105, 111, 112]. The gravure printing involves two opposite rolling cylinders, the first cylinder has an engraved pattern, which
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collects ink from container, while the second roller supports the imprint (Fig. 15). As the two cylinder rotates, the engraved pattern is filled with ink and doctored off to remove the excessive.
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Meanwhile, the substrate slides between the two cylinders and allows for transferring of ink from the engraved cylinder onto its surface, resulting imprinted feature on the substrate [113]. Gravure printing is fundamentally a roll-to-roll process that allows for high volume printing. Recent research reported that gravure printing can be effectively used for the fabrication of solar cells [114], organic light emitting diodes [115], sensors [116], and graphene based flexible electronics [67, 117].
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Figure 15. Schematic illustration of the gravure printing process.
Due to its roll- to-roll printability charateristic, gravure printing is industrially compatible [113].
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Hence, it offers a powerful approach for fabrication of flexible electronics in high volume in a costeffective manner. It is desirable to manufacture printed flexible electronics at very high speed, up to
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15 m/s, using gravure printing [105, 112]. However, this technique also has some downside. As
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gravure printing is a direct contact process, the pressure of the impression roller may create unexpected creases and scratches on the surface layers. The morphology of the imprint is dependent on the shape of the engraved patterns, therefore the printed features do not have very high resolution.
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Changing of patterns is costly and suffers from the fact that a whole new roller needs to be engraved and embedded. Thus, gravure printing is not a cost-effective route for low volume printing. The quality of the printed patterns is highly influenced by the ink rheology, so it is significant to optimize the inks fluid characteristics. Low shear viscosity inks (in the range of 0.05-0.2 Pa s) are often used to prevent ink bleed out from the gravure cells and allow better line resolution [101, 112, 118]. Thus, solvent‐exfoliated graphene is particularly well‐suited for this techniques. Surface tension, uniformity, drying rate are also important parameters defining the quality of the patterns on flexible substrates.
ACCEPTED MANUSCRIPT There is still a scarce of literature of graphene inks in gravure printing, which can be attributed to the difficulty in formulating suitable inks since graphene possesses poor dispersibility in common organic solvents. In 2014, Secor and co-workers [67] demonstrated gravure printing of large‐area graphene patterns on flexible substrates. By tailoring the ink properties and printing parameters, continuous graphene lines with resolution as fine as ~30 µm were printed over large areas with
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notable reliability and uniformity. The integration of graphene with gravure printing offers a rapid
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3.3. Inkjet printing
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Unlike conventional printing technologies, ink jet printing works without a physical printing mask that directly contacts the substrates [112]. Formulation of graphene inks for inkjet printing has attracted enormous attention. The operating principle of inkjet printing is illustrated in Fig. 16, where
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micro-sized ink droplets are continuously generated and sprayed onto the substrate to form desired
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patterns by a printing nozzle [119, 120]. The basic principle of inkjet printing is the accurate placement of ink droplets onto the desired spot (pixel), which is digitally controlled by a programmed computer [121]. The formation of the micro-droplets is based on either piezoelectric
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mechanism that produces a mechanical compression through a nozzle or by heating the ink to increase volume and pressure [10, 61, 121]. For the accurate positioning of inks droplets, inkjet printing can be classified into two distinct mode, continuous mode and drops-on-demand [101]. In continuous mode (Fig. 16a), ink droplets are continuously generated and electrostatically charged in an electrical field. Then, the charged droplets are selectively deflected by a special collector, allowing only proper droplets to be deposited in appropriate position. Drops on demand mode (Fig. 16b), in contrast, generate ink droplets only when needed. When the nozzle is located in proper spot, an electrical pulse is applied to form a single droplet and locate onto the desired area. Drops-on-
ACCEPTED MANUSCRIPT demand does not involve charging of the droplets and is the more widely used mode. As a result, the ink droplets are digitally sprayed onto the substrate, where the desired features are achieved by a
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droplet-to-pixel fashion.
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Figure 16. Schematic illustration of the (a) continuous mode and (b) drops-on-demand mode of inkjet printing.
Graphene oxide based inks (GO inks) have been earlier employed in inkjet printing which can be
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attributed to its hydrophilic nature since GO can be easily dispersed in water and common solvents. In 2011, Le et al. [88] formulated a novel GO ink in water and integrated to inkjet printing. A
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commercial Dimatix Material Printer (DMP 2800) inkjet printer was used for on-demand generation of 10 pL microscopic GO ink droplets, resulting GO patterns with the lateral spatial resolution of 50 μm. In a similar work, Huang and co-workers [122] used an office inkjet printer (HP Deskjet K7108) to print GO inks onto various plastic substrates, proved that inkjet printing of GO inks is simple and practical. In recent years, inkjet printing of pristine graphene has attracted tremendous attention. Secor et al. [61] formulated ethyl cellulose stabilized graphene inks suitable for inkjet printing. The graphene ink has a surface tension of ~33 mN/m and viscosity of 10–12 mPa·s at 30 °C, which has been printed
ACCEPTED MANUSCRIPT using a Fujifilm Dimatix Materials Printer (DMP-2800) equipped with a 10 pL drop cartridge. Drop spacing for all printed features was maintained at 20 µm, resulting in a line width of ~60 µm. More recently, Majee et al. [60] formulated a pristine graphene ink by shear exfoliation, which has a viscosity of ~11.2 cP at room temperature, suitable for inkjet printing.
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The fluid characteristic of the inks is crucial for the formation of ink droplets. The inks are generally required to have a low viscosity (4–30 cP) and a high surface tension (typically ~35 mN m−1 ) in
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order to generate a stream of droplets (these values may varies for different printing devices) [10, 61,
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88, 89]. In the context graphene inks of printed electronics, the lateral size of graphene sheets and its stability in the ink dispersion also needs to be controlled, otherwise the pin hole of the nozzle can be
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blocked during operation.
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Inkjet printing benefits from its high resolution and versatility. Being a drop-by-drop technique, inkjet printing is utilized to attain uniform images with high resolution, since each micro-droplet is a
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pixel [121]. The resolution of inkjet printing can easily reach 300 dpi, and may up to 1200 dpi without great difficulty [10, 121]. As it does not require a physical printing mask, inkjet printing
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allows for design renovation on-the-go, which may cost a large financial and time investment for conventional printing techniques. The non-contact printing manner also avoids strong impact to the
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substrate, minimizing contaminations, scratches and creases on the thin film materials. More importantly, inkjet printing allows for the deposition of a very thin graphene patterns, which is beneficial for thin-film applications. The drawback of inkjet printing is its lower throughput compared to roll- to-roll conventional printing techniques. To remediate the output, it is essential to employ more nozzles in a single printer, thus increases the possibility of malfunction and failure. The aggregation of graphene in the dispersed solution may also occur and can block the nozzle from jetting droplets.
ACCEPTED MANUSCRIPT 3.4. Other printing techniques Flexographic printing, a promising roll-to-roll fabrication technique, is similar to gravure printing except the fact that the printing roller is usually made from rubber or polymeric materials [8, 10, 105]. The typical flexography system is comprised of four rollers, as illustrated in Fig. 17. Ink is
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collected by the collecting cylinder and transferred to the Anilox roller (a cylinder with engraved patterns). The patterns on the Anilox roller are filled with ink and doctored off to remove the
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excessive ink. Then, the printed pattern is transferred intermediately to the substrate through the
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printing roller. This direct contact printing process benefits from the printing roller made of rubber (or polymer), which can be more pliable to reduce the impact of the rollers onto the thin film,
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minimizing the creases and scratches on the substrates. The process is simple and industrially
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scalable, which is highly applicable for printing of graphene inks.
Figure 17. Schematic illustration of the flexographic printing process.
Aerosol-jet printing is a digital and non-contact printing technique which allows deposition of various functional materials such as metal nanoparticles, paste, and liquid inks [123, 124]. The
ACCEPTED MANUSCRIPT printing process is illustrated in Fig. 18, where inks are atomized and sprayed by a focus jet stream onto the substrates [125]. Aerosol-jet printing differs from inkjet printing mainly in the fact that the inks are atomized instead of generating droplets, bringing numerous advantages such as higher print resolution, more applicable with various type of inks, and able to print smaller and thinner patterns [123, 125-127]. As the main challenge for formulation of graphene inks is tailoring its fluidic
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properties, aerosol-jet printing is capable of printing inks with a wide range of viscosity (1–1000 cP)
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[128]. Thus it is able to print graphene inks that include stabilizers, which make graphene inks more
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stable but also increase their viscosities. As inks are atomized during aerosol-jet printing, it can also crumple graphene flakes and prevent them from aggregation [96]. Since the technology is still in its
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early stage, aerosol-jet printing is a costly process. But it undergoes rapid development and is
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promising for digital printing of various types of graphene inks.
Figure 18. Schematic illustration of the aerosol-jet printing process. Reprinted with permission from ref. [125]. Copyright © 2015 Elsevier.
3D printing, a new type of digital printing, is a process of depositing materials layer by layer to make three-dimensional solid objects (Fig. 19) [129, 130]. It allows objects to be fabricated in a bottom- up fashion directly from digital designs without any milling or molding [131]. 3D printing technology
ACCEPTED MANUSCRIPT has found great potential in one-step manufacturing across diverse fields from medical implant to electronics [131-134]. Recent research has shown that graphene-based devices can be fabricated using 3D printing [135-137]. This advanced technology is still far from the ultimate goal of fabricating complex devices in-one- go, but 3D printing of graphene inks is promising for its future
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applications.
Figure 19. Schematic illustration of the 3D printing process and some 3D printed structures. Reprinted with permission
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from ref. [130]. Copyright © 2017 Elsevier.
Overall, it is likely that many of the above- mentioned techniques will ultimately be used for printing
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of graphene based electronics. For different ink formulations, different printing techniques will be optimal. Therefore, a mix-and- match printing route can be used based on requirements of conductive features, with the ultimate goal of maximizing the process output.
4. Applications 4.1. Flexible conductive circuits
ACCEPTED MANUSCRIPT Due to its outstanding electrical and mechanical properties, graphene ink is promising for printing of flexible conductive circuits. In 2013, Hyun et al. [138] prepared foldable paper-based electronic circuits by a simple selective transfer process with a pen. Graphene membranes with controlled thicknesses were prepared by vacuum filtration of graphene nanoplates dispersions (a type of chemically-derived graphene ink). Then, foldable electronic circuits were fabricated by selective
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transfer of graphene patterns onto a paper from the filter membrane without the need of a printing
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mask (Fig. 20). The resulting conductive circuits showed excellent folding stability with small
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decrease of conductance at any folding angles from -180° to 180°. A foldable circuit board for operating light-emitting diode (LED) chips array was also demonstrated. This approach could
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provide a simple patterning method for paper-based graphene conductive circuit with high foldability.
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Figure 20. Schematic illustration of the preparation of a graphene circuit on a paper substrate. (a) Filtration of graphene dispersion. (b) Drawing of graphene circuit by a pen. (c) Remov ing the memb rane filter fro m the paper substrate. (d) Photographs of a LED ch ips array on the folded graphene circuit. Reproduced with permission from ref. [138]. Copyright © 2013 Wiley.
Using inkjet printing, Gao et al. [63] prepared highly flexible conductive circuits with excellent uniformity by printing of pristine graphene ink. The graphene circuits showed excellent conductivity of 0.81 ± 0.2 kΩ sq-1 after 30 printing passes, and a transmittance of approximately 60% after being
ACCEPTED MANUSCRIPT annealed at 300 °C for 30 min. The resistances of the printed graphene circuits as a function of flexibility were also investigated, which were slightly increased despite 1000 times of bending or folding cycles.
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Huang and co-workers [122] also described a simple and practical inkjet printing method of
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graphene oxide based inks (GO inks) for a series of flexible electric circuits and chemical sensors (Fig. 21). The printed GO patterns were thermally treated to restore its electrical properties, resulting
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in printed graphene features with high conductivity (up to 874 S/m) and excellent flexibility. Flexible
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conductive circuits and high sensitive electrochemical H2 O2 sensors were also fabricated based on
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the printed graphene patterns.
Figure 21. Patterns printed on various substrates by high resolution inkjet printing of GO inks. Reprinted with permission from ref. [122]. Copyright © 2011 Springer.
Graphene flexible circuits can also be fabricated using 3D printing, as demonstrated by Zhang and co-workers in a recent report [139]. In this study, graphene composite was prepared by melt mixing
ACCEPTED MANUSCRIPT of thermally reduced graphene oxide (rGO) and polylactic acid (PLA) thread, which was then extruded into filament with the diameter of 1.75 mm and used for the 3D printer. On various different substrates, 2D and 3D flexible circuits were rapidly printed and exhibited outstanding conductivity of up to 4.76 S/cm at 6 wt% rGO. The demonstrated 3D printing of graphene filament
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is promising for the future of all-printed flexible electronic devices.
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can be used for fabrication of not only flexible circuits, but also for any complex 3D structure, which
ACCEPTED MANUSCRIPT 4.2. Energy devices Another promising application of printable graphene ink is in the field of newable energy devices. The modern energy devices may be broadly classified into two categories: (i) energy storage and (ii) energy harvesting devices. The available printed flexible energy storage devices described here are
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batteries and supercapacitors, while energy harvesting devices are mostly focused on printed solar cells and nanogenerators. In the following section, our intention is to discuss about recent advances
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in graphene based inks for printed energy storage and energy harvesting devices.
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Rechargeable lithium batteries are an important class of energy storage devices, where lithium ions
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are transferred between the electrodes during the charge and discharge reactions. Wei et al. [140] demonstrated a scalable and versatile printing process for solid-state and flexible lithium batteries
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based on graphene inks. The batteries are comprised of polymer-based gel electrolyte sandwiched between a lithium foil (anode) and graphene hybrid inks printed on current collector (cathode). It has
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been demonstrated that the graphene hybrid ink, formulated by polystyreneslufonate (PSS) doped graphene, lithium salt and TiO 2 nanoparticles, can be either printed or drop-casted onto the current
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collector for fabrication of high performance cathode in the lithium batteries. The assembled solidstate and flexible batteries showed excellent performance with specific capacity of up to 582 mA h
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g−1 . In this work, not all of the components are printed except the cathode, calls for further development of feasible fully-printed batteries.
In 2016, Fu and colleagues [141] created fully printed lithium- ion batteries by 3D printing technology using GO-based hybrid inks. As shown in (Fig. 22), the anode and cathode inks were prepared separately and stored in different syringes, which were printed layer-by- layer and bridged by a barrier of 3D printed solid-state electrolyte. The all-component 3D-printed batteries showed a
ACCEPTED MANUSCRIPT reasonable performance with good cycling stability. This work demonstrated the potential of 3D
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printing in one-step fabrication of advanced electronics.
Figure 22. Schematic illustration of the fabrication of all-component 3D-printed lithium-ion batteries using 3D printing.
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Reprinted with permission from ref. [141]. Copyright © 2016 Wiley.
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Printing of graphene based supercapacitors, an emerging class of energy storage devices, differing from batteries as it physically stores energy on its separated electrodes rather than based on chemical
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reactions, has also been advanced. The supercapacitors benefit from its fast charge/discharge rate, high power density and ultralong lifetime. One of the very first studies on printing of graphene based
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supercapacitors was reported by Xu et al. [142] in 2013. They have formulated and screen-printed graphene/polyaniline hybrid inks to prepare active electrodes for supercapacitors. The fabricated
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supercapacitor device exhibited high specific capacitance of 269 F g−1 and power density of 454 kW kg−1 with excellent cycling stability. More research effort has been paid to micro-supercapacitor, a miniaturized design of supercapacitors. Micro-supercapacitor is advantageous due to its thinness, high flexibility, and ease of fabrication (i.e. printing). Recently, Hyun et al. [143] has reported a high-throughput inkjet printing process for manufacturing of flexible graphene micro-supercapacitors based on self-aligned capillarity-assisted lithography for electronics (SCALE) method, followed by photonically annealing by a high- intensity pulsed Xenon lamp (Fig. 23). The printed micro-supercapacitors, comprised of pristine graphene
ACCEPTED MANUSCRIPT electrodes and ion gel electrolyte, exhibited a fairly high aerial specific capacitance of 268 µF cm−2
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with high flexibility and excellent charge/discharge stability.
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Figure 23. Schematic illustration of fabrication steps for micro-supercapacitors using the SCALE process. Reprinted
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with permission from ref. [143]. Copyright © 2017 Wiley.
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Another approach for fabrication of micro-supercapacitors was demonstrated by Liu et al. [144] in 2016. In this report, in-plane micro-supercapacitors were direct-printed by spray deposition and
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inkjet printing using graphene/conductive-polymer hybrid inks. The printed ultrathin microsupercapacitors offered superior areal specific capacitance and excellent rate capability with an
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ultrahigh flexibility, making it appropriate for next-generation flexible energy storage microdevices.
More recently, Li and colleagues [145] developed a simple and scalable inkjet-printing process for massive fabrication of graphene micro-supercapacitors. Graphene inks and polyelectrolyte inks were subsequently inkjet-printed onto various substrates for assembling of fully printed microsupercapacitor cells, which attained the highest areal capacitance of ∼0.7 mF/cm2 . This work provided a promising solution to compact and on-chip power sources for future wearable electronics.
ACCEPTED MANUSCRIPT Using 3D printing, Zhu and co-workers [146] reported the fabrication of three-dimensional graphene aerogels for supercapacitors (Fig. 24). Graphene composite inks were prepared by sol-gel mixing of graphene oxide, graphene nanoplatelets, and silica fillers to form a thixotropic and highly viscous inks, which were suitable for 3D printing. The printing was conducted in an organic solvent (2,2,4trimethylpentane) and the 3D periodic graphene aerogel was formed after supercritical drying. The
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fabricated supercapacitors used the 3D-printed electrodes showed excellent electrochemical
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properties and remarkable stability. More importantly, the deve loped 3D printing technique can be
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used to build unique electrode and device structures with irregular shapes and well-controlled
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internal structural.
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Figure 24. Schematic illustration of fabrication process. (a) Preparation of graphene based inks, (b) 3D printing of threedimensional graphene composite, and (c) formation of 3D graphene aerogels. Reprinted with permission from ref. [146]. Copyright © 2016 American Chemical Society.
Printed graphene has also been advanced for photovoltaic devices, which is a typical energy harvesting device that converts photons from the sun (solar light) into electricity. Among the available photovoltaic devices, dye-sensitized solar cells are advantageous due to its low cost and high efficiency [147]. For fabrication of dye-sensitized solar cells, Dodoo-Arhin et al. [66]
ACCEPTED MANUSCRIPT formulated a stable graphene ink by liquid phase exfoliation of pristine graphite in isopropyl alcohol with a polymer stabilizer and repeatedly inkjet-printed onto FTO/glass substrate. With a ∼3.0% conversion efficiency, the printed graphene electrodes exhibited a comparable performance to
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platinum counter electrodes with only ∼2.7% cost.
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Importantly, Casaluci et al. [148] demonstrated a viable method for large-area fabrication of graphene-based dye-sensitized solar cells by spray coating of graphene ink (Fig. 25). The printed
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graphene counter electrodes (44% transmittance) were integrated in a solar cell modules and
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achieved conversion efficiency of 3.5%, similar than those of platinum counter electrodes. This work
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paved the way to all-printed, flexible, and transparent graphene solar cells.
Figure 25. (a) Spray coating graphene ink. (b) Optical image of graphene-based counter electrode. (c) Optical transmittance of FTO (grey curve) and graphene-coated FTO substrate (black curve). Reprinted with permission from ref. [148]. Copyright © 2016 Royal Society of Chemistry.
Nanogenerator is a set energy harvesting device that converts thermal or mechanical energy into electricity. Nanogenerators can be classified into three different types: piezoelectric, triboelectric,
ACCEPTED MANUSCRIPT and pyroelectric nanogenerators [149]. While piezoelectric and triboelectric nanogenerators can be used to harvest mechanical energy, pyroelectric nanogenerators can convert thermal energy into electricity [149-151]. By screen-printing of graphene nanoplatelets inks onto a PVDF film (a novel pyroelectric material), Zabek et al. [152] has successfully fabricated graphene-based pyroelectric nanogenerators (Fig. 26). The use of printed interconnected graphene layer minimized thermal
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reflection, enhanced thermal radiation absorption and increased the electrical conductivity, thereby
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improved the energy harvesting potential. The printed graphene-based nanogenerators showed a
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significantly improved thermal energy harvesting efficiency, which is 25 times higher than those of aluminum-based systems. The presented technology provided a new approach for powering wearable
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and Internet of Things (IoT) devices.
Figure 26. Thermal rad iation heating princip le for (a) a reflective pyroelectric alu minu m/PVDF electrode device and (b)
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an absorbent asymmetric graphene/PVDF in k pyroelectric device. Rep rinted with permission fro m ref. [152]. Copyright © 2017 American Chemical Society.
4.3. Electrochemical sensors
The electrochemical sensors are detectors that work based on electrochemical reactions, where the target chemicals/reagents are oxidized or reduced at the electrodes resulting in change of the electrical current [153-155]. Recent research has emerged graphene and its derivatives as one of the most popular platforms for designing of new high-performance sensing devices [156-158]. However, designing and manufacturing of graphene-based electrochemical sensors in a cost-effective manner
ACCEPTED MANUSCRIPT remain a major challenge. Printing of graphene inks is dawning as an efficient route for mass production of high quality electrochemical sensors and to solve the problem thereof. For detection of metabolites, Labroo et al. [159] reported a simple and inexpensive printing process of graphene-ink biosensor arrays on a microfluidic paper. The sensor arrays based on enzyme-
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immobilized graphene hybrid inks were printed and showed its capability to detect multiple
pharmaceuticals, food science, and personal health monitoring.
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metabolites rapidly with high sensitivity. This approach paved the way to a variety of applications in
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In a report by Seekaew and co-workers [94], simple and low-cost ammonia gas sensors were
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fabricated by inkjet-printing of graphene/PEDOT:PSS hybrid inks (Fig. 27). The formulated hybrid inks were inkjet-printed onto a prepared interdigitated silver electrodes using an office inkjet printer.
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As a result, the printed flexible graphene/PEDOT:PSS gas sensor exhibited high sensitivity to NH3 a low concentration range of 25 to 1000 ppm at room temperature. More importantly, the printed gas
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sensor benefits from its flexibility, where gas response increased with increasing the bending angle.
Figure 27. Schematic diagram for the fabrication of flexib le graphene/PEDOT:PSS gas sensor. Reprinted with permission from ref. [94]. Copyright © 2014 Elsevier.
ACCEPTED MANUSCRIPT By printing of graphene based inks, Kanso et al. [160] integrated a new screen-printed graphene electrode in one channel flow-cell for enzymatic sensors, with an improved analytical response and enhanced electroactive area of up to 388%. In another study, An and colleagues [161] printed a highly conductive graphene aerogel with controllable 3D porous nanostructure via micro extrusion printing, which is appropriate for a multi-recognition flexible wearable electric sensor for analyzing
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complicated perception of movements.
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In 2015, Santra and co-workers [162] has successfully integrated functional graphene inks with
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CMOS-MEMS (complementary metal-oxide-semiconductor)-(micro-electro- mechanical systems) technology for fabrication of a resistive humidity sensor with high response and reproducibility. This
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based sensor systems for a variety of applications.
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work is important since it opens up an exciting opportunity for mass production of low cost CMOS
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4.4. Other applications
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The development of advanced printing techniques and ink formulations allows for more and more graphene-based printed devices. Huang et al. [163] reported a low cost, highly conductive and
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flexible printed graphene film for wireless wearable communications applications. The printed graphene patterns were experimentally used to transmit/receive radio frequency (RF) signals through wires and wirelessly. Benefited from a simple and low-cost patterning process, combined with other advantages in flexible, lightweight, and high conductivity, printed graphene is prospected for wireless wearable communications in the near future. In another study, Huang et al. [164] also demonstrated a rolling compression method for improving the conductivity of screen-printed graphene laminates, which were experimentally used as dipole antenna to effectively radiate RF power (Fig. 28). Using the similar strategy, Shin and co-workers [89] inkjet-printed graphene lines
ACCEPTED MANUSCRIPT with high resolution and sustained electrical conductivity onto flexible PET films, which was used as
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electrodes for a wideband dipole antenna and exhibited 96.7% transmittance efficiency.
Figure 28. (a) Geometric dimension of the dipole antenna, (b) photo of the printed graphene laminate dipole antenna, and
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(c) graphene laminate antenna connected with a SubMiniature version A (SMA) connector for measurement. Reprinted
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with permission from ref. [164]. Copyright © 2015 AIP Publishing.
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Flexible thin- film transistors (TFTs) are mostly relied on organic semiconductors. Howerver, the limited carrier velocity in polymers and molecular films prevents their use at high frequencies
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beyond megahertz [165]. In parallel, graphene is promising to deliver high-speed performance while maintain the flexibility. In a report by Sire and co-workers [166] in 2011, a flexible and high frequency TFT was developed employing solution-based single- layer graphene in 2% w/v aqueous sodium cholate solution. The printed TFTs showed low resistance, extrinsic and intrinsic current gain cutoff frequency of 2.2 and 8.7 GHz, respectively, with excellent performance up to high frequencies. A graphene embedded with Indium- Gallium- Zinc-Oxide (IGZO) thin- film transistor was fully printed by Secor et al. [167]. In this study, the authors used inkjet printing for fabrication of a highperformance graphene/IGZO thin- film transistor based on IGZO and pristine graphene inks.
ACCEPTED MANUSCRIPT Graphene source and drain electrodes were alternative printed and embedded between two consecutive IGZO printing passes, provided a chemically stable electrode-channel interface (Fig. 29). The printed TFT device achieved high-performance with an electron mobility of ∼6 cm2 /V.s and current on/off ratio of ∼105 , higher than those of silver/IGZO TFT. Furthermore, the printed graphene/IGZO TFT exhibited robust stability to aging and excellent resilience to thermal stress,
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thereby offered promising platform for advanced fabrication of future high-performance and stable
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printed electronics.
Figure 29. Schematic illustration of the fabrication of inkjet-printed IGZO TFTs with silver and graphene source/drain
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electrodes. Reprinted with permission from ref. [167]. Copyright © 2017 American Chemical Society.
Flexible all-carbon-based field effect transistor (FET) was also successfully fabricated using aerosoljet printing. Reported in 2013, Liu and colleagues [168] formulated three different types of inks and
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subsequently printed them onto flexible PET substrates for fabrication of the FET chips. Highly conductive rGO was used as electrodes and channel, while isolating GO acted as the gate dielectrics, and multi- walled carbon nanotubes (MWCNTs) were printed as the gate electrode. This flexible allcarbon-based FET is fabricated entirely through a low cost and environment-friendly printing process, promising for high throughput fabrication of electronic devices.
ACCEPTED MANUSCRIPT 5. Conclusion and future outlook In this report, most recent work on the formulation of graphene inks, their printing techniques and applications for printed flexible electronic devices have been reviewed. The preparation of proper graphene inks, which allows for mass printing in a cost-effective manner is desired for its
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commercial adoption. It has been clearly demonstrated that pristine graphene inks are highly conductive, but they show poor solubility in common solvents. It appears that the chemically-derived
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graphene (GO) inks can be easily formulated into inks due to its hydrophilicity, however, they are
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not electrically conductive, requiring additional treatment to restore the conductivity after deposition. The graphene hybrid inks hold significant promise to enhance the electrical properties of the inks,
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but they are costly and the fluid characteristics still far from optimal. Moreover, the high annealing
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temperature of such inks also obstructed their available use in a wide range of substrates. Therefore, the main challenges still remained on formulation of a low cost and printable graphene ink with
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appropriate characteristics in an ecofriendly fashion. It can also be noticed that current studies are still at the initial stage with main focus on basic demonstration of printability and conductivity, while
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much less attention is paid to the correlation of viscoelastic and the physicochemical properties of the inks with printing factors. It is crucial to provide a holistic evaluation approach to determine the
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printability and performance of the inks. Formulation which meets the specifications of the industry standards will hold the key to the future. Further development of feasible graphene inks along with improvement of a facile and efficient deposition process will be key to realize many applications in graphene-based flexible electronics. The ink formulations should focus on the selection of a variety of solvents and surfactants to find the best combination for direct exfoliation of pristine graphene inks, which enable high yield, volatile, and proper fluidity. The use of fluid-modifiers may be a solution toward formulating a printable ink with genuine characteristics. Other cordial and rapid annealing mechanisms need to be examined in
ACCEPTED MANUSCRIPT correlation with the volatile solvents. Poly (ionic liquid) has recently emerged as a versatile and effective stabilizer for preventing aggregation of carbon nanostructures to form homogeneous dispersions in water. Moreover, poly (ionic liquid)s are more environmentally friendly and electrically conductive itself. Therefore, their emulsions with graphene are also expected to be
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advantageous for engineering feasible and sustainable graphene inks. Interestingly, the mass of graphene is usually account for less than 1 wt% of the ink in the current
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formulations, while the dominant portion of the inks is water and/or solvent based. This will
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certainly incur an increased costs for storage and transportation. Therefore, the development of redispersible powder-phase graphene inks is also a promising direction for future development. The
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current printing technologies of graphene inks are mainly focused on ink-jet printing, whose
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compatibility with a variety of inks is limited. Further development of viable aerosol-jet printing and 3D printing technologies are trendy toward the future of printed electronic devices.
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Acknowledgements
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research hub funding.
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The research has been supported by the Australian Research Council (ARC) Industry Transformation
Conflict of Interest
The authors declare no conflict of interest.
ACCEPTED MANUSCRIPT Biographies
Tuan Sang Tran obtained his Bachelor’s degree in Chemical Engineering fro m Hochiminh University of Industry, Vietnam, in 2014, and the Master’s degree in Nanoscience from Gachon University, South Korea, in 2016. Mr. Tran is currently a PhD student at the Department of
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Chemical Engineering, RMIT University, Australia. His research interests include carbon nanomaterials for energy applications and graphene-based inks for printed electronics.
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Naba Kumar Dutta is a Professor at School of Engineering, RMIT University. He performs
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research at the interface cutting across the disciplines of Nanomaterials Engineering, Structural Biology and Chemical Engineering. He has wide-ranging research interests and published widely
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in the areas of Poly mers, Advanced materials for energy, Carbon-nanomaterias and structureproperty relationship in mult i-co mponent hetero-structures with specific focus on the role of the
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interfaces.
Nami ta Roy Choudhury is a Professor at School of Engineering, RMIT University. She
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received her PhD fro m IIT, Kharagpur and subsequently did her post-doctoral research at CNRS, Mulhouse, France. Choudhury’s research interest spans fro m Hybrid poly mers to Bio mimetic
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polymers to Graphene based inks for renewable energy and advanced manufacturing.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights An insight into the fundamental formulation of stable graphene dispersions.
State-of-the-art formulation of graphene inks utilising solution processable materials.
Conventional and digital printing techniques for deposition of graphene inks.
Inks formulation and devices fabrication are inextricably linked.
Challenges and outlook of formulating graphene ink for its future development.
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