Preparation of waterborne graphene paste with high electrical conductivity

Journal Pre-proofs Research paper Preparation of waterborne graphene paste with high electrical conductivity Jianing Wang, Huijun Tan, Ding Xiao, Rahul Navik, Motonobu Goto, Yaping Zhao PII: DOI: Reference:

S0009-2614(20)30013-0 https://doi.org/10.1016/j.cplett.2020.137098 CPLETT 137098

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

28 November 2019 6 January 2020 8 January 2020

Please cite this article as: J. Wang, H. Tan, D. Xiao, R. Navik, M. Goto, Y. Zhao, Preparation of waterborne graphene paste with high electrical conductivity, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/ j.cplett.2020.137098

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Preparation of waterborne graphene paste with high electrical conductivity Jianing Wang1, Huijun Tan1, Ding Xiao1, Rahul Navik1, Motonobu Goto2, Yaping Zhao1* 1School

of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China 2Department

of Materials Process Engineering, Nagoya University, Japan

*Corresponding author: E-mail:[email protected]

Abstract Waterborne graphene paste with high conductivity is highly desired because it can avoid using harmful organic solvents. Herein, the paste with both good conductivity and adhesion on substrates was prepared. Hydrophilic graphene was made firstly by oxidizing weakly pristine graphene using a plasma. Then, the obtained graphene, dipropylene glycol methyl ether, glycol, glycerol, the copolymer of N-vinyl-2pyrrolidone, and vinyl acetate (P(VP/VA)) were applied to fabricate the paste. The electrical conductivity of the paste-based patterns on polyimide substrates via screenprinting reached 1.34×104s/m, and its adhesion reached 4B level. The paste is expected to have wide applications in making flexible conductive films.

Keywords: plasma；waterborne graphene；conductivity；adhesion 1.Introduction The integration of functional electronic materials constitutes the core of the emerging field of printing electronics. The development of sensors[1,2], solar cells[3-5], flexible

electrodes[6,7], microelectronics[8], and radio frequency identification (RFID)[9,10] devices has drawn increasing attention to developing the conductive pastes for printing electronic

devices.

Metal

nanoparticles

or

nanowires[11,12],

conductive

polymers[13,14], and carbon nanomaterials were often selected as the conductive components in the paste. The metal-based paste could provide high electrical conductivity, like silver- and copper-based paste, but the former is high cost and has electromigration shortcoming limiting its widespread industrial applications[15-17], while the latter is easily oxidized damaging its conductive properties severely though it is cheap. The conductivity of the conductive polymer is not high enough for many applications though it is economical. Carbon nanomaterials, especially graphene, have become quite competitive candidates due to their high electrical conductivity, stability, and rheological properties[18]. However, solvents used for making graphene-based paste so far were environmental pollution and harmful to human health, such as Nmethyl-2-pyrrolidone(NMP) and N, N-dimethylformamide(DMF), resulting in its limited application in printing fields [19,20]. Del et al. [21] prepared a graphene ink with multiple graphene layers using a solvent exchange technique. The graphene was first exfoliated in DMF from graphite powders and stabilized using ethyl cellulose. Then, terpineol was applied to replace DMF to reduce solvent toxicity. Terpineol was a good substitute solvent for DMF/NMP due to its rheological property, but it still produced the serious environmental pollution itself [22,23]. Therefore, it is highly desired to make graphene-based pastes without using harmful organic solvents. However, few waterborne graphene pastes were reported until now. Andrea C. Ferrari

[24] reported a type of the graphene-based waterborne pastes formulated by using carboxymethylcellulose sodium salt. The electrical conductivity of the obtained printing pattern could reach 2×104S/m after annealing at 300℃ for 40 min, but the hightemperature post-treated process[25] was required in this method, limiting its wider application. The purpose of this paper is to prepare waterborne graphene paste cost-effectively without using toxic solvents and high-temperature post-treatment. The paste can be widely applied on different substrates largely by screen-printing technology. Oxygen plasma was studied to oxidize the graphene weakly to prepare hydrophilic graphene while maintaining its good conductivity. The influence of the plasma treatment time on the graphene in terms of hydrophilicity and conductivity was investigated. The ethylene glycol and propanetriol as an auxiliary dispersant were utilized to disperse the graphene. The water-soluble copolymer of N -vinyl-2-pyrrolidone and vinyl acetate (P(VP/VA)) was used as a binder. The orientated-screen printing formulation of the paste was investigated in terms of conductivity and adhesion. The electrical conductivity of the printed graphene film achieved 1.34X104 S/m, and its adhesion rating on PI substrate was 4B based on bagel knife test standard when the content of P(VP/VA) was 10% and the solvent composition of dipropylene glycol methyl ether（DPM）: glycol: glycerol was 1:1:1. The paste is expected to have wide applications in making flexible conductive films. 2. Experimental section 2.1 Preparation of weakly oxidized graphene by oxygen plasma

Firstly, the graphene, a conductive phase of the paste, was prepared by exfoliating graphite using supercritical CO2 associated with the ultrasonic approach in a similar process to our previously published papers[26,27]. Then, the exfoliated graphene was treated by the plasma (A parallel-plate, 40 kHz radio-frequency (rf) plasma system). The radio-frequency power was maintained at 55 W (corresponding to a~55Mw cm−2 power density) and the chamber pressure at 100Kpa. The different treated time was carried out. 2.2. Preparation of graphene paste 0.5g plasma-treated graphene was added in a 600 ml glass cylinder containing 500 ml deionized water and 4.4 ml auxiliary dispersant—glycol, and glycerol (1:1,v/v) to form the mixture, and then the mixture was subjected to high-shear mixing using a homogeneous dispersion apparatus (Ystral GmbH, Germany) consisted of a 35 mm diameter internal stator and a 25 mm diameter internal rotor at 2000 rpm for 2 h. Finally, 0.05g of a 7:3 copolymer of N -vinyl-2-pyrrolidone and vinyl acetate (PlasdoneS-630, Ashland Inc., USA) were added to the above solution(graphene:P(VP/VA) weight ratio of 10:1) and ultrasound treated for 2 hours. The graphene loading amount in the paste could be adjusted by evaporating the solvent using a rotary evaporator (Heidolph Instruments GmbH, Germany). In a typical experiment, the graphene dispersion was transferred into a round-bottom flask followed by the addition of 2.3 mL of dipropylene glycol methyl ether (Dowanol DPM, Sinopharm Group Chemical Reagent, China). The dispersion was then evaporated at 70 °C and 0.1 MPa until no distillate came out, in which the graphene content was 75mg/ml in the paste.The obtained paste was

transferred to the container by spoon for later use. 2.3. Fabrication of graphene patterns and their properties The graphene patterns on the substrate were fabricated by a manual screen printer (ZYS-3024, Dongguan Electromechanical Technology Co., Ltd). Through a stencil screen with an aperture of 150 µm mesh with a designed pattern, the paste was transferred to the polyimide (PI, DuPont Kapton, 125μm) substrate. The wet graphene pattern was dried at 70 °C for an hour. Finally, the film (pattern) was mechanically pressed by a desktop powder tablet machine (FY-40, Tianjin Sichuang Jingshi technology development co., Ltd.). The sheet resistance of the obtained film was measured by a four-point probe with silver paint electrodes (SX1944, Suzhou Baisheng Technology Co., Ltd., Suzhou, China). The morphology of the film was characterized by FE-SEM operated at 12 kV and 5 μA. The bagel knife test was applied to evaluate the adhesion of the film, which was divided into five grades according to the American Society for Testing Material (ASTM ). 3. Results and discussion 3.1.

Characterization of plasma-treated graphene

3.1.1. Confirmation of the oxygen-containing group of the treated graphene In order to prepare the hydrophilic graphene with maintaining high electrical conductivity, we adopted controllable oxygen plasma to oxidize pristine graphene weakly. The FT-IR spectrometer (Spectrum 100, Perkin Elmer, Inc., USA) was utilized to characterize whether the oxygen group was generated on the graphene. Comparing with the initial graphene, we can know that the oxygen-containing group was generated

on the plasma-treated graphene. The FTIR pattern of the initial graphene did not have significant characteristic absorption peak of the oxygen-containing groups as shown in black line of Fig.1, while in the treated graphene, many new peaks appeared at about 3410cm-1 and 1334cm-1, which respent the characteristic absorption peak of -OH, and at around 1735cm-1, which respent the characteristic absorption peak of -COOH, as shown in red line of Fig.1. It suggests that the hydrophilic groups such as hydroxyl group and carboxyl group were successfully generated on the plasma-treated graphene.

Fig.1. Characterization of plasma-treated graphene: (a) FTIR curves of the initial graphene and plasma-treated graphene; (b) Raman spectra of the plasma-treated graphene and initial graphene;(c) SEM image of the initial graphene; (d) SEM of the plasma-treated graphene for 900s Raman spectroscopy is a good way to identify the structure of materials. The newly

formed compound[28], structural defect, functional groups of graphene could be identified. The Raman spectroscopy of the initial pristine graphene and the plasmatreated graphene are shown in Fig.1(b). Compared with the initial graphene, the G peak at ~1571cm-1, D peak at ~2710cm-1, G at ~1580cm-1, and 2D at ~2718cm-1 of the treated graphene shifted to the right a little, and an acropate D' was observed at ~1610cm-1 too. The existence of D' can be attributed to the disorder-induced double resonance Raman scattering process, which indicates the collapse of the k selection rule [29]. This phenomenon illustrates the defects in the surface structure of the plasma-treated graphene. ID/IG represents the intensity ratio of non-sp2 bonding to sp2 bonding. Compared with ID/IG =0.139 of the initial graphene, the ID/IG of the treated graphene increased to 0.203, which indicates the defect of the treated-graphene increased, which might be caused by the hydrophilic group (oxygen-containing group) on the treated graphene. It is an agreement with the FTIR characterization results. Also, it can be seen from Fig.1(c,d) that the surface of the treated graphene had become rough compared with the initial graphene. 3.1.2. Influence of plasma treatment time on the dispersibility of graphene in water Plasma treatment time is critical to breaking chemical bonds along with forming new chemical bonds. Different plasma treatment time would produce different content of oxygen-containing groups in graphene, which directly affects its dispersibility in water. The relationship between treatment time and oxygen content in the graphene was analyzed by EDX method. It can be seen from Fig.2a that the oxygen content increased almost linearly with the increase of treatment time until 900s, but tended to be slow

when the treatment time exceeded 900s. It might be ascribed to that the numbers of chemical bond which could be broken by the energy provided by the plasma at the experimental conditions had been completed. The water contact angles of the treated samples with a period of 0s, 180s, 450s, and 900s were exhibited in Fig.2b, respectively. It can be seen that the contact angle of the initial pristine graphene was 138 °. When the treatment time was 180s, the water contact angle decreased to 67 °. It indicates that the graphene had changed from hydrophobicity to hydrophilicity after plasma treatment. When the treatment time increased to 450s, the water contact angle reduced to 23°. It suggests that the hydrophilicity of the graphene increased further [30]. When the treating time increased to 900s, the water contact angle approached to 0°. It means the graphene could be dispersed in water completely. In other words, the 900s of the plasma treatment time was enough to transfer the hydrophobic graphene to hydrophilic one at the radio-frequency power of 55w. The big change of the water contact angle confirms the oxygen-containing group had been produced on the treated-graphene after plasma treatment, which is consistent with the analysis results of FTIR and Raman spectroscopy. The hydrophilicity of the graphene is a prerequisite to making the waterborne graphene paste.

Fig.2. Oxygen contents of the treated graphene and its dispersion stability. Influences of plasma treatment time on oxygen content (a) and water contact angle (b); Digital photos of the suspension of the treated graphene at different times (c) and the corresponding suspension settled in 30 days (d) Also, we examined the dispersion of the graphene in water by observing the color change of the graphene suspension in the water. As shown in Fig.2c, the color of the graphene dispersion became darker when the treatment time increased, and there were scarcely sediments of the graphene for the 450s- and 900s-treated samples and the suspension was uniform confirming the graphene had excellent dispersity. Also, the dispersion had good stability. The dispersion of the 900s-treated graphene was still very stable after it was kept for 30 days at room temperature, as shown in Fig.2d. It can be attributed to the more oxygen-containing groups produced in the graphene when plasma treating time increased. These results are an agreement with the results of FTIR and Raman characterization and the contacting angle test.

3.2. The constituent ratio of the mixed solvent Generally, the paste for screen-printing should have high viscosity and low fluidity. Usually, the viscosity of the paste could be adjusted by changing the viscosity of the mixed solvent. In order to match the requirement of screen printing, we scrutinized the common solvents. A slightly less volatile, moderate evaporation rate of solvents, such as dipropylene glycol methyl ether (DPM), was selected. The viscosity of the mixed solvents of DPM, ethylene glycol, and glycerol formulated at different proportions are listed in Table1. It was found that the constituent of No.4 of Table 1 (DPM:glycol:glycerol=1:1:1,v/v/v) was the best one after testing in terms of the screenprinting when the graphene concentration was fixed at 75%, in which the paste had high viscosity and low fluidity, which was consistent with the viscosity value between 1k~4k mPa·s required for the screen-printing [31]. Accordingly, the appropriate viscosity of the mixed solvent should be in the range of 35~65mPa·s to get the paste with the loading of 75% graphene. Therefore, the constituent ratio of the mixed solvent was determined as No.4 in Table 1, in which the viscosity was 46.5mPa·s. Table 1 Viscosity of mixed solvents Mixture solvent

DPM(%)

glycol(%)

glycerol(%)

Viscosity(mPa·s)

1

100

0

0

12.1

2

50

50

0

72.4

3

50

0

50

130.2

4

33

33

33

46.5

3.3. Influence of binder on adhesion of graphene patterns on the substrate

Binder is important to make the paste to combine the substate tightly. However, it would introduce negative performance in terms of conductivity. Also, it would affect the viscosity, disperse stability, and the rheological property of the paste. Therefore, the selection of binders should be trade-off by adhesion and conductivity performance. Considering copolymer P(VP/VA)[32] has a high thermal decomposition temperature, being above 350℃, as shown in Fig. 3a, we selected it as a binder and examined its influence on the adhesion and conductivity of the paste. As shown in Fig.3b, the square resistance increased with the increase of the binder content. In particular, when the binder content reached 12.5% (graphene:P(VP/VA)=8:1,w/w), the square resistance increased sharply. It might be attributed to the block of the conductive path caused by the high content of the binder. When the binder content was 9.1%, the square resistance of the printed film with 25 µm thickness was only 1.4 Ω/.

Fig.3. The influences of binder on performance of the graphene paste: (a)TGA curves of plasma-treated graphene 、 P(VP/VA) and composite paste; (b) The influence of

binder content on conductivity of the graphene paste; (c) The influence of binder content on adhesion of graphene paste; (d) Digital photograph of the graphene pattern with 10% binder content after the bagel knife test; (e) and (f) Optical microscopy images of the graphene pattern after the bagel knife test On the other hand, when the binder content decreased, the adhesion of the paste on the PI substrate weakened. According to the ASTM grade of testing adhesion [33], we examined the adhesion of the printed graphene pattern of the PI substrate. Higher ASTM levels mean stronger adhesion. It can be seen from Fig.3c that the adhesion of the printed pattern decreased from the highest grade 5B to 3B with the reduction of the binder content. However, when the binder content reduced to 9.1%, the adhesion level was still 4B, showing excellent adhesion to the substrate. The patterns were intact without tearing off except small damage at the edges of the lattice as shown in Fig.3 (df). Thus, 9.1% of binder in the paste is the best one in terms of high conductivity and and strong adhesion. 3.4. Influence of printing times on the conductivity For the screen-printing process, printing time is one of the important factors that affect the conductivity of graphene pattern because it directly affects the thickness of the pattern and the tightness of the graphene stack and thus affects the integrity of the conductive network. Generally, the conductivity of the graphene pattern increases with the printing times because the graphene sheets in the paste could contact each other tightly, forming a shorter distance of conductive pathway. It can be seen from Fig. 4a that the resistivity sharply decreased from 25×10−5 to 6.9×10−5Ω·m and gradually

leveled off when the screen-printing times increased from 1 to 4 times. Since after four printing times, the graphene sheets in the paste had formed a complete and tight electrical network. Therefore, the change in conductivity tended to slow down even though the printing times continue to extend. The surface of the four-times printing graphene is much flatter and had fewer defects than that of the one-time printing, as shown in Fig.4 (b,c). It illustrates that the graphene sheets had been stacked tightly and formed a complete electrical network after printing in four times.

Fig.4 The effect of printing times on resistivity (a); SEM images of printed films after printing in one-time (b) and in four times (c) 4. Conclusions The hydrophilic graphene was prepared from treating pristine graphene by oxygen plasma and had excellent conductivity and dispersion in water. The waterborne graphene paste was formulated by determining a suitable ratio of the mixed solvent of dipropylene glycol methyl ether, glycol, and glycerol, and the binder of P(VP/VA) when the hydrophilic graphene was fixed 75wt %. The paste-based pattern on the PI substrates was made by screen-printing, and it had high conductivity(~1.34×10-4s/m) and strong adhesion(4B) when the graphene and P(VP/VA) content was 10 to 1 after

printing in four times. The obtained graphene paste is expected to have important applications in manufacturing flexible electrical conductive films Declaration of Interest Statement The authors declare no competing financial interest Acknowledgments This work was supported by the National Natural Science Foundation of China (21576165) and the Sino-Japanese Joint Research Platform on Energy and Environmental Industry Grant (2017YFE0127100, China), and JST SICORP Grant (JPMJSC18H1, Japan).

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I, standing for co-authors, claim that the manuscript is original and no submitted to other journals

Conflict of interest Authors declare no financial conflicts of interest

Graphical abstract

Highlights 

Hydrophilic graphene was obtained via oxidizing weakly pristine graphene by a plasma



Waterborne graphene paste was formulated using hydrophilic graphene



The conductive pattern of 1.34×104 S/m was printed from the paste by screenprinting



The adhesion of the pattern on the PI substrate reached a 4B level