- Email: [email protected]
Efficient defect healing and ultralow sheet resistance of laser-assisted reduced graphene oxide at ambient conditions
Accepted Manuscript Efficient defect healing and ultralow sheet resistance of laser-assisted reduced graphene oxide at ambient conditions Aspasia Antonelou, Labrini Sygellou, Katerina Vrettos, Vasilios Georgakilas, Spyros N. Yannopoulos PII:
S0008-6223(18)30654-7
DOI:
10.1016/j.carbon.2018.07.012
Reference:
CARBON 13293
To appear in:
Carbon
Received Date: 1 June 2018 Revised Date:
29 June 2018
Accepted Date: 6 July 2018
Please cite this article as: A. Antonelou, L. Sygellou, K. Vrettos, V. Georgakilas, S.N. Yannopoulos, Efficient defect healing and ultralow sheet resistance of laser-assisted reduced graphene oxide at ambient conditions, Carbon (2018), doi: 10.1016/j.carbon.2018.07.012. 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.
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Raman Intensity [arb. units]
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Efficient defect healing and ultralow sheet resistance of laser-assisted reduced graphene oxide at ambient conditions Aspasia Antonelou1,2, Labrini Sygellou1, Katerina Vrettos2, Vasilios Georgakilas2, and 1
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Spyros N. Yannopoulos1,*
Foundation for Research and Technology Hellas – Institute of Chemical Engineering Sciences (FORTH/ICE-HT), P.O. Box 1414, GR-26504, Rio-Patras, Greece
Department of Materials Science, University of Patras, GR-26504, Rio-Patras, Greece
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Abstract
Reduction of graphene oxide (GO) to a low-resistance product is one of the most versatile routes to obtain large volume graphene-based materials. A number of chemical and thermal methods are being applied to achieve this goal, albeit each one is bound to certain disadvantages and limitations. Laser-assisted reduction has emerged as a promising method apt to overcome issues related to chemical and thermal reduction. Despite that a large number of efforts have been
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focused on laser-induced reduction of GO, its transformation to high quality reduced GO still remains a bottleneck. Here, it is shown that low-cost, millisecond lasers, widely used in the welding industry, achieve excellent reduction of GO to a product with the lowest sheet resistance yet reported by any laser-assisted method. For comparison, GO is reduced by chemical and
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thermal methods. Raman and x-ray photoelectron spectroscopies are applied to investigate the underlying structural changes providing evidence for the removal of oxygen-containing species
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and defect healing. Operation at ambient conditions, single pulse irradiation and a 2.60 mm wide focusing spot, demonstrate the high potential of this approach to the scalability of the reduction process towards producing large volumes of high-quality reduced graphene oxide at low cost.
_________________________ Corresponding author. E-mail: [email protected]
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1.
Introduction
Materials and composites based on graphene or graphene-like structures have received a great deal of attention over the last years on account of the unique properties such materials exhibit in a
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diverse set of applications including photonics, optoelectronics, energy conversion/storage, membranes and so on [1–3]. As the fundamental physics of graphene monolayer and the basic chemistry of graphene functionalization enabling adaption to proper matrices have been
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thoroughly explored, efforts are now focused on technologies for the optimization and low-cost upscaling of graphene production. This becomes imperative as the daily expansion of the global
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graphene market will be powered by novel production methods [1]. A vast number of different approaches for synthesizing graphene-based materials have been explored up until now, each one targeted to a particular application. However, none of them has ended up as a viable technology proper for large scale production of high quality graphene. On the other side, commercial
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applications based on the superb properties of the graphene monolayer are rather unfeasible to be achieved in view of the impediments leading to large scale monolayer preparation. Therefore, other graphene forms, such as graphene oxide and reduced graphene oxide (GO, rGO), and few
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or multilayer graphene, stand out as alternatives for advancing an assortment of applications. New technologies for large volume graphene production should be cost-effective, scalable, fast
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and environmentally friendly permitting in situ processing when necessary. In this direction, lasers have appeared so far, as a feasible technology for high-quality graphene production since can satisfy, in principle, most of the above criteria. In addition, laser light can produce graphene within the structure of a device offering flexibility and adaptability to current technological platforms. Lasers have up to now demonstrated capacity for high quality graphene growth by properly processing inorganic, e.g. SiC decomposition [4,5], and organic compounds [6].
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Over the past years a tremendous expansion of the research on graphene derivatives emerged aimed at less demanding applications [7–9]. This has led to the development of alternative pathways to produce graphene-like structures focusing in low cost and high yield procedures. The
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most intensively studied pathways are based on the oxidation of graphite to graphite oxide and its subsequent exfoliation to graphene oxide, which is followed by the reduction of the latter to graphene nanosheets. GO is atomically thin as graphene; however, it bears a semi-aromatic
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network structure of carbon atoms with sp2 and sp3 bonding, the latter arising from decoration with oxygen-containing groups[10]. The vast majority of these groups are hydroxyl (OH) and
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epoxy (C–O–C) species, while carbonyl (C=O) and carboxyl (COOH) functional units are also present. The presence of these groups suppresses dramatically the conductivity of the graphene layer. Partial removal of these species after application of different chemical or physical processes restores to a large extent the aromatic character resulting in rGO, a close analogue of
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graphene. Reduction of GO recovers a moderate conductivity [11–13]. Systematic efforts have been undertaken over the last ten years to prepare rGO, hence obtaining a graphene-like material. The main reduction method is the chemical route implemented by a
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number of reagents such as hydrazine, metal hydrides, hydroiodic acid [11,14]. Other reductive pathways were based on solvothermal heating, electrochemical, thermal or photocatalytic
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treatment [12,13]. Laser-assisted methods have gained ground owing to their versatility and the lack of use of chemicals [15,16]. A summary of laser-induced reduction works has recently been presented [17]. Two main requirements should be satisfied for a reduction method to be considered viable; obtaining enhanced electrical conductivity (partly related to the sp3 to sp2 transformation after removal oxygen-containing groups) and scalability of the process. We show here that using low-cost, commercially available lasers both the above requirements can be fulfilled. Notably, chemical methods are able to produce large volumes of rGO but fail in 3
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providing high conductivity and frequently have the disadvantage of using toxic or environmentally unfriendly reagents. On the contrary, laser-assisted methods have shown potential for preparing rGO with moderate to low resistance but have not up to now met the
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conditions for scalable production. The current work resolves this trade off by demonstrating that the use of low-cost lasers operating at the millisecond pulse-duration range with focusing spots of
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ca. 2 mm in diameter can provide superb quality graphene structures with ultra-low resistance.
Experimental
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GO synthesis: GO being prepared according to Staudenmaier's method is used as the starting material[18]. Graphite (2 g) was added to 150 mL of a cold mixture of H2SO4 and HNO3 (2:1) in an ice-water bath. KClO3 powder (50 g) was then added in small portions under continuous stirring and cooling. After 20 h, the reaction was quenched by pouring the mixture into distilled
dried at room temperature.
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water. GO was isolated by filtration and purified extensively with water (until the pH ∼ 6) and
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Graphite Oxide Reduction: Reduction of GO took place using three different procedures, i.e. (i) chemical (C-rGO), (ii) thermal (T-rGO) and (iii) laser-induced (L-rGO). In brief, the three
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reduced forms of r-GO were prepared as follows. (i) GO (10 mg) was dispersed in an excess of NaBH4 (100 mM) in water and heated to 80 oC for 4 hours. The color of the reaction mixture changed from brown to almost black indicating the successful transformation of GO to rGO. The product was purified extensively with distilled water and collected by filtration in the form of a free standing thin film [11]. (ii) A three-zone horizontal tube furnace was used for the thermal reduction. The tube was purged with high purity Ar gas for 1 h at a flow of 120 sccm to remove
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oxygen traces. Subsequently, the temperature was raised at 800 °C, in about 20 min, and kept at this temperature for 30 min, while the same gas flow was maintained during the reduction process. (iii) Reduction of GO by laser irradiation took place utilized a Rofin (Basel Lasertech)
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Desktop 4007 Nd-YAG Laser (pulse width in the range of ms) with an excitation wavelength of 1064 nm. The sample was prepared by cutting a piece of GO film 1×1 cm2 and mounted on a mechanically operated XY stage. The procedure was performed at ambient conditions; no inert
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gas was purging during irradiation. Overlapping steps comparable to the radius of the focused
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spot was used to ensure homogeneous irradiation due to the Gaussian beam profile of the beam.
Characterization methods: Electron microscopy images were recorded with a high resolution field-emission scanning electron microscope (FE-SEM) instrument (Zeiss, SUPRA 35VP) operating at 15 kV. Raman spectra was obtained with a Kimon He-Cd Laser apparatus at a laser
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excitation wavelength 441.6 nm The scattered light is analyzed by a LabRam HR800 (Jobin Yvon) micro-Raman spectrometer at a spectral resolution of about 2.0 cm−1. A microscope objective with 50× magnification is used to focus the light onto a spot of about 3 µm in diameter.
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Low laser intensities were used (∼0.27 mW on the sample) to avoid spectral changes due to heatinduced effects. The Raman shift was calibrated using the 520 cm−1 Raman band of crystalline Si.
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Electrical measurements were performed by using the four-probe technique and a Keithley 2401 SourceMeter. The cr-GO and tr-GO samples were deposited on a glass substrate and dried carefully.
The surface analysis studies are performed in a UHV chamber (P<10−9 mbar) equipped with a SPECS LHS-10 hemispherical electron analyzer. The XPS measurements are carried out using unmonochromatized Al Kα radiation at 1486.6 eV under conditions optimized for maximum signal (constant ∆Ε mode with pass energy of 36 eV, giving a full width at half maximum 5
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(FWHM) of 0.9 eV for the Au 4f7/2 peak). The analyzed area is an ellipsoid with dimensions 2.5×4.5 mm2. The XPS core level spectra are analyzed using a fitting. A routine, which allows the decomposition of each spectrum into individual mixed Gaussian–Lorentzian components
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after a Shirley background subtraction. Errors in our quantitative data are found in the range of about 10% (peak areas) while the accuracy for BE assignments is about 0.1 eV. The UPS spectra were obtained using HeI irradiation with hν=21.22 eV produced by a UV source (model UVS
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10/35) and the analyzer was working at the Constant Retarding Ratio (CRR) mode, with CRR=10. A bias of −12.28 V was applied to the samples in order to avoid interference of the
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spectrometer threshold in the UPS spectra. The high and low binding energy cutoff positions were assigned by fitting straight lines on the high and low energy cutoffs of the spectra and determining their intersections with the binding energy axis. Regarding measurement errors, it should be noted that an error of ±0.1 eV is assigned to the absolute values for ionization energies,
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work function and other UP-spectra cutoff features where the error margin is significant, due to the process of fitting straight lines. For the XPS-UPS measurements the C-rGO and T-rGO samples were drop casted on oxygen plasma cleaned ITO substrates, whereas the GO and L-rGO
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were in the form of powder.
Results and Discussion
3.1
Raman spectroscopy
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3.
Raman scattering is presumably one of the most useful techniques to monitor fast and consistently the structure of the graphene-based materials and hence reveal the extent of reduction of GO. Various band parameters such as the relative intensity ratio of the D, G and 2D bands (i.e. ID/IG, I2D/IG), the width of the G band, the spectral shape of the 2D band and so on, are frequently employed as reliable indicators of the graphene quality, structure and sp3 to sp2 6
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relative fraction. Figure 1 shows representative Raman spectra of laser-assisted reduced GO recorded for various irradiation parameters as described in the caption. For all irradiation
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conditions used the spectra display significant reduction of the GO, despite the minor or
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1.2 J
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1500
2.6 J 2.9 J 3.1 J 3.4 J
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Raman Intensity [arb. units]
0.7 J
GO 2000
2500
3000
3500
-1
Raman Shift [cm ]
Figure 1: Representative Raman spectra of pristine (bottom curve) and irradiated GO at ambient conditions for various laser parameters. The laser beam focus diameter is 1 mm for the energy range 3.4-2.6 J and 2.6 mm for the two lowest energies, 0.7 and 1.2 J.
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moderate differences they exhibit. It evident that in all cases laser irradiation results in a number of spectral changes. These include, appreciable decrease of the ID/IG ratio, narrowing of the D and G bands, and increase of the I2D/IG ratio. Low energy and large focal spot improve the reduction
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process, as is demonstrated by the two topmost spectra displayed in Fig. 1. The observed changes
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T-rGO
C-rGO
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Raman Intensity [arb. units]
L-rGO
1500
GO 2000
2500
3000
3500
-1
Raman Shift [cm ]
Figure 2: Raman spectra of pristine and reduced GO by chemical (C-rGO), thermal (T-rGO) and laser-assisted (L-rGO) methods.
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testify not only the removal of a large fraction of the O-containing groups, but also an efficient transformation of bonding from sp3 to sp2. It is worth-noting that in the vast majority of respective laser-assisted reduction attempts of GO in the literature - using ns, ps or fs laser
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sources - the corresponding spectral changes although to the same direction are only moderate[15,16]. Indeed, although in those studies the removal of oxygen species was typically demonstrated by a number of techniques, the spectral shape of the D, G band did not show
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appreciable changes after laser reduction. This is associated to the degree of restoration of the carbon lattice. Thus, despite that oxygen containing species can be removed to a large extent, in
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previous works, the healing of structural defects did not seem to take place, and needed special further treatment. Eliminating structural defects is of utmost importance since their presence affects drastically the electrical conductivity, thus being detrimental for device applications [19,20].
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Figure 2 displays Raman spectra of GO samples reduced by three different methods, namely, chemical (C-rGO), thermal (T-rGO) and laser-assisted (L-rGO). The comparison reveals the superior ability of the particular laser-assisted method against the two classical reduction
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methods. The high frequency region of the L-rGO spectrum has been fitted by Lorentz-type lines revealing the single Lorentzian character of the 2D band. Table 1 compiles selected spectral
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details to provide a more quantitative description of the structural changes of reduced GO. It is worth-noting that the area ratio of 2D over the G band for the L-rGO reaches the value of I2D / IG ≈ 1.67, which, to our knowledge, is the highest reported ever for laser or chemical reduction. The Raman spectrum of the thermally reduced GO, in Fig. 2, could be interpreted as heating produces a material with less sp2 bonded C atoms in comparison to pristine GO, as the D and G bands become wider that those of the pristine GO. Despite that oxygen elimination takes place
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even at this typically low annealing temperature, the mechanism of defects healing is not yet activated. Indeed, to achieve this, thermal annealing at very high temperatures, up to 2500 oC, is needed[21]. These authors showed that heating up to 1500 oC leads to complete removal of
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oxygen groups generating at the same time atomic vacancies. Heating to higher temperatures, 1800–2700 °C, causes defect healing (vacancy annihilation) and coalescence of adjacent overlapping layers to produce larger crystals[21]. Although using this two-step procedure
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extensive repair of defects can be achieved, the bonding of adjacent layers leads eventually to large fragments reminiscent of graphite, hence discarding the graphene properties.
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The above results demonstrate that using a low cost laser operating at ambient conditions and irradiating mm-wide spots onto GO, very high quality graphene structures can be prepared. Notably, this process is able not only to remove oxygen-containing species but to achieve at the Table 1: Raman band parameters for the pristine and the three reduces GO samples obtained by
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spectra fitting. I denotes integrated intensity; FWHM: full-width at half maximum. ω(D)
D(FWHM)
ω(G)
G(FWHM)
ID/IG
ω(2D)
2D(FWHM)
GO
1366
112
1588
70
1.24
2719
194.35
C-rGO
1367
46
1578
39
1.12
2728
89.5
T-rGO
1387
233
1590
96
2.31
-
-
1583
35
0.56
2719
68
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L-rGO
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Sample
1362
48
same time restoration of the sp2 bonding. To alleviate the issue of high defect concentration in rGO some researchers have attempted defect healing by conducting CVD to provide extra C atoms[22].
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3.2
X-Ray and Ultraviolet Photoelectron Spectroscopies
Photoelectron spectroscopy was employed to figure out the relation between the chemical structure of GO / rGO and their physical properties. Figure 3 shows the C1s deconvoluted peaks
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of the pristine GO and the rGO samples reduced by the three methods. For all samples (except GO), the peak is analyzed into five components assigned to C-C sp2 and sp3 configurations, to C-O
L-rGO
C-C sp
C=O
292
290 288 286 284 Binding Energy EB [eV]
282
π-π*
O-C=O
280
294
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294
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C-C sp2
3
C-C sp2
XPS AlKα C1s
292
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C-O
π-π*
294
292
290 288 286 284 282 Binding Energy EB [eV] C-C sp2
C=O O-C=O
π-π*
282
280
XPS AlKα C1s
C-O
C-C sp3
290 288 286 284 Binding Energy EB [eV]
XPS AlKα C1s
C-C sp3
C-O C=O
T-rGO
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C-rGO
C-C sp2
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GO
XPS AlKα C1s
280
294
292
C=O O-C=O
290 288 286 284 Binding Energy EB [eV]
C-C sp3
282
280
Figure 3: Deconvoluted C1s XP spectra of GO, L-rGO, C-rGO and T-rGO samples.
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epoxides / hydroxyls C-O-C and C-O(H), carbonyls C=O and carboxyls O=C-O(H) at binding energies 284.5±0.1 eV, 285.4±0.1 eV, 286.6±0.1 eV, 287.8±0.1 eV and 289.5±0.1 eV, respectively. Additionally, the π-π* satellite peak at ~292 eV is observed in the reduced
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samples[23]. The fractions of the above mentioned chemical species and of the delocalized π-π* bonds for each sample are compiled in Table 2. Comparing the fractions of the reduced samples, it is evident that the reduction is more effective in the case of L-rGO since the relative fraction of
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sp2 and delocalized π-π* bonds are appreciably higher than the corresponding fractions of C-rGO and T-rGO. In addition, the carbon-oxygen components of L-rGO is lower than those of C-rGO
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and T-rGO. The findings of the XPS analysis are in line with the Raman results, demonstrating that the laser reduction method is the most effective for GO reduction than the other two methods leading to elimination of carbonyl and carboxyl groups. Moreover, the higher percentage of sp2 hybridization and π-π* suggests that the delocalized π conjugation, a characteristic of aromatic C
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structure, is to some extent restored in L-rGO samples. Based on the sp3 relative concentration it follows that the most defected film is the chemically reduced graphene oxide in agreement with
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other studies[24].
Table 2: Relative atomic percentages of sp2, sp3, epoxides-hydroxides (C-O-C, C-O(H)),
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carbonyl, carboxyls, π-π* bonding and C/O ratio as derived from C1s XPS spectrum deconvolution. Sheet resistance (Rs) values are also shown for comparison. Sample
% sp2
% sp3
GO
38.2
L-rGO
C=O
O-C=OH
π-π*
4.3
% C-O-C, C-O(H) 54.8
-
C/O (±0.1) 1.4
Rs (kΩ sq-1) >103
2.7
-
75.8
10.2
6.9
1.4
1.8
3.9
8.4
4×10-2
C-rGO
64.3
14.9
11.6
3.5
3.5
2.1
5.2
1.2
T-rGO
63.0
15.0
15.0
3.8
2.4
0.9
8.6
2.0
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The valence band spectra as well as the work function (WF) were measured with UPS. Figure 4(i) shows the valence band region near the Fermi level, EF, while Fig. 4(ii) illustrates the high binding energy cutoffs, used to determine the work function of the pristine and reduced GO. The
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work function was determined, based on the fitting procedure reported elsewhere[24], by the equation WF=hν-∆Ε, where hν is the photon energy and ∆Ε is the energy difference between ΕF and Ecutoff. The work function of the GO is estimated at 5.4 eV whereas in the reduced samples its
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value is 4.3±0.1eV in agreement with previous work[25]. No differences between the WF values
UPS HeI
(i)
GO C-rGO L-rGO T-rGO
(ii)
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Intensity [arb. units]
GO C-rGO L-rGO T-rGO
2pπ
Intensity [arb. units]
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of reduced samples is observed, as expected, However, the peak at ~3 eV in the valence band
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EF
5
4
3 2 1 Binding Energy [eV]
Ecutoff 0
18.0
17.5
17.0
16.5
16.0
15.5
15.0
14.5
Binding Energy [eV]
Figure 4: (i) Valence band and (ii) high binding energy cut-off where the work function
determined from UPS measurements determined from the secondary electron onset.
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region appears more intense in the L-rGO. This peak is formed by 2pz orbitals and is attributed to pπ electrons (sp2 hybridization) [26], while it is absent in the spectrum of GO. Since the presence of sp3 C-H defects in graphene reduce significantly the delocalized π-electrons, these differences
above results are in agreement with XPS and Raman results.
Morphological changes by SEM and sheet resistance of r-GO
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in the valence band indicate that the L-rGO sample presents the lowest density of defects. The
Figure 5 illustrates typical FE-SEM images of the pristine and modified GO using the various
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reduction methods. The electron microscopy images reveal that the initially smooth surface and
(b)
(c)
(d)
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(a)
Figure 5: FE-SEM images of (a) pristine, (b) laser-assisted reduced, (c) chemically reduced and (d) thermally reduced, graphene oxide.
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compact texture of the GO (a) material is transformed to a more open or porous structure in the case of the photo-processed GO. Several edges are created implying also increase of the surface area of the irradiated area (b). Such structural change giving rise to partial exfoliation of the
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layers; the increased free-standing layers could presumably be responsible for the reduced resistance of the laser-assisted reduced GO in comparison to pristine GO. The chemically (c) and thermally (d) reduced GO show rougher structure in comparison to the pristine GO, whereas they
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are less porous or expanded in relation to the photoreduced GO.
The measurement of sheet resistance (Rs) is a characteristic feature for the efficiency of the
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reduction of GO since it is highly connected with the aromatic character of graphene nanosheets. In general, the electrical insulator GO becomes partly conductive after chemical reduction. According to current literature data the sheet resistance of rGO obtained by chemical methods or by combined chemical/thermal procedures falls in the range MΩ sq-1 to kΩ sq-1 [11]. The
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treatment of GO films with the laser beam induced a remarkable increase in the electrical conductivity as revealed by the measured sheet resistance about 40 Ω sq-1. For comparison, films of C-rGO and T-rGO samples of the current work showed sheet resistant about 1.2 kΩ sq-1 and 20
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kΩ sq-1, respectively as shown in Table 2. Taking into consideration the spectroscopic results, the low resistivity of the L-rGO sample is correlated with: (a) the higher relative fraction of sp2 as
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showed in Table 2 from the XPS measurements, (b) the presence of the peak at ~3 eV (sp2 hybridization) in the UPS spectra and (c) the value of the I2D/IG ratio obtained from the Raman analysis. This means that laser reduction is the most effective method for restoration of the πconjugated structure and leads to a low resistivity reduced graphene oxide sheets. It is known that the presence of sp3 bonds disrupts the transport of carriers thus the mobility and the conductivity of reduced GO films is limited [20]. This is in agreement with the results summarized in Table 2, where T-rGO exhibits the higher resistivity among the various rGO samples. Indeed, the fraction 15
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of carbon-oxygen bonds is higher for this material, whilst the fraction of the π-π* transition and the value of the I2D/IG ratio are the lowest. It would be instructive to compare the resistivity values obtained in the current work with the best
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corresponding results in the literature that have been obtained by laser-assisted reduction. In general, reduction by lasers results in low to moderate improvement of the very high resistance of pristine GO. Only in a few studies values of the sheet resistance below ~104 Ω sq-1 have been
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reported. The most successful reduction procedures of GO using laser beams has been reported[27–29]. Sokolov et al.[27] investigated the role of continuous (532 nm) and pulsed (532
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and 355 nm; 9 ns) on the reduction of GO in air and under N2 atmosphere. The bandwidth of the 2D Raman peak of their work is similar to the one we found here; however the I2D/IG ratio in our case is much higher than that in Ref.[27]. The same group explored further the reduction of GO using an excimer laser (248 nm; 25 ns) under high vacuum or N2 atmosphere. The highest I2D/IG
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ratio was ~0.5. The lowest sheet resistance measured was ~491 Ω sq-1. Only for GO samples that were first subjected to thermal reduction at 150 oC for 24 h, the lowest sheet resistance measured after irradiation reached ~100 Ω sq-1. Guan et al.[29] used a pulsed laser (1064 nm, 10 ps) to
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reduce GO at atmospheres of gas and liquid N2. Raman spectra showed successful reduction with adequate defect healing and a 2D band width comparable to the current work. However, the
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I2D/IG ratio still remains appreciably lower than our findings. Best sheet resistance values reported by Guan et al.[29] are 104 and 50-60 Ω sq-1 for gas and liquids N2, respectively. In a recent study, Arul et al.[30] reported a comparison of GO photo-reduction using cw, ns and ps laser sources, irradiating GO at ambient condition. The ns laser resulted in the more efficient reduction. However, even in that case, the 2D band intensity seems not to exceed 1/3 of the G band intensity, while the D band is for all cases the most dominant in the Raman spectra.
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Various mechanisms have been proposed for the laser-assisted reduction of GO which consider the effect as photothermal [6], photochemical [31] or a combination of both [30]. Ultrashort pulses, i.e. emerging from a fs laser, are the prerequisite for an athermal reduction of GO.
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However, although at the first stages following the interaction of the pulse with GO and the athermal rupture of C-O bonds, thermal effects inevitably intervene after i.e. 100 ps, due to recombination of photoexcited electrons and holes [17]. The long pulse width used in the current
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study clearly support a photothermal origin of GO reduction. The process of simultaneous oxygen removal and defect healing at ambient conditions using laser-assisted GO reduction has
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not yet been reported. Typically, laser reduction of GO leads to efficient removal of oxygencontaining species; however, leaving a large fraction of sp3 defects. The atomic scale mechanism leading to improved crystallinity, which the current approach offers is still elusive. In a recent study [32] we have shown using the same laser irradiation conditions that elemental carbon
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powders (known commercially as Vulcan and Norit) with high sp3 content can successfully be transformed to structures with high sp2 bonding. The high temperature of the irradiated volume might be the main factor for this structural transformation.
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The above comparison indicates that the current approach is superior to any of the previous attempts which employ laser beams to selectively reduce GO. The merits of our method are
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summarized as follows. In comparison to chemical methods the avoidance of using chemicals renders laser-assisted GO reduction an environmentally friendly, green approach. Moreover, the current method provides much lower Rs values than current approaches using chemical reduction. In relation to other laser-based methods our approach excels for the perspectives of production cost and scalability. The use of low-cost industrial-type lasers working under literally ambient conditions outweighs methods using expensive equipment such as pico- and femto-second lasers operating at laboratory specifications under either high vacuum or inert atmosphere conditions. A 17
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focusing spot of 2.6 mm in diameter implies a GO reduction processing speed exceeding by several orders of magnitude the speed of other approaches, operating at focusing points of few tens in microns in diameter.
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Closing this discussion it would be instructive to mention that recent heat-induced GO reduction procedures have shown lowering of the resistance up to ~1 Ω sq-1[33]. However, the method followed to achieve this ultralow value of the resistance necessitates the process of Joule heating
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a GO foil at 2750 K under vacuum, implying that the process is proper for substrateless samples such as free-standing GO foils. Notably, attempts to thermally reduce GO pre-deposited on soda
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lime glass substrates in an effort to prepare transparent/conducting electrodes, resulted in values of the sheet resistance in the regime Rs > 2×104 Ω sq-1 for prolonged annealing at 500 oC, i.e. the maximum temperature that the substrate can withstand [34]. The above demonstrate the merit of laser-assisted methods, which can offer the opportunity of selectively in situ processing of GO
4.
Conclusions
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pre-deposited on any substrate, being thus fully compatible to current technological platforms.
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In summary, we present a systematic exploration of the merits of various reduction methods of GO to rGO, including chemical and thermal, with emphasis on laser-assisted approaches. The
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realization of the reduction was quantified by the extent of oxygen-containing species removal and more importantly by the success of each approach in healing defects created after reduction. Comparing structural and resistivity data it turns out that the current laser-induced reduction of GO produces high quality graphene-like structures with resistivity lower than that of the chemical and thermal routes by almost two orders of magnitude. Notably, the resistance value measured for the L-rGO in the present study is not only lower than any other corresponding measurement for laser-assisted reduction, but also among the lowest reported in the literature i.e. those
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necessitating heating GO foils up to 3000 K under vacuum. Extended spectroscopic and morphological characterization was performed in order to rationalize these findings in terms of structural or chemical changes taking place during the reduction process. For the first time, the
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2D Raman band of the L-rGO has been found to be stronger than the G band, indicating the predominance of graphene-like structures with low defect concentration. The results demonstrate that the particular irradiation conditions employed in the current work are able to accomplish
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simultaneously both steps needed for a successful GO reduction, namely, almost complete removal of oxygen-containing species and exceptional healing of defects. This achievement has
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not been reported previously in laser-assisted GO reduction methods. Additional merits of the present approach over other laser-induced reduction methods include the ultrahigh speed of the process. A single laser pulse, 1 mm wide, and a focus beam of 2.60 mm in diameter can provide a laser writing speed (i.e. ca 1 cm2 per second) more than two orders of magnitude faster than that
Acknowledgements
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in previous work where typically the focused beam profile is in the range of few tens of microns.
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Authors from FORTH/ICE-HT acknowledge support of this work by the project “ΑΕΝΑΟ – Materials and processes for energy and environmental applications” (MIS 5002556) which is implemented under the “Action for the Strategic Development on the Research and Sector”,
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Technological
funded
by
the
Operational
Programme
"Competitiveness,
Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).
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DOI:
10.1016/j.carbon.2018.07.012
Reference:
CARBON 13293
To appear in:
Carbon
Received Date: 1 June 2018 Revised Date:
29 June 2018
Accepted Date: 6 July 2018
Please cite this article as: A. Antonelou, L. Sygellou, K. Vrettos, V. Georgakilas, S.N. Yannopoulos, Efficient defect healing and ultralow sheet resistance of laser-assisted reduced graphene oxide at ambient conditions, Carbon (2018), doi: 10.1016/j.carbon.2018.07.012. 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.
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Raman Intensity [arb. units]
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Efficient defect healing and ultralow sheet resistance of laser-assisted reduced graphene oxide at ambient conditions Aspasia Antonelou1,2, Labrini Sygellou1, Katerina Vrettos2, Vasilios Georgakilas2, and 1
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Spyros N. Yannopoulos1,*
Foundation for Research and Technology Hellas – Institute of Chemical Engineering Sciences (FORTH/ICE-HT), P.O. Box 1414, GR-26504, Rio-Patras, Greece
Department of Materials Science, University of Patras, GR-26504, Rio-Patras, Greece
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Abstract
Reduction of graphene oxide (GO) to a low-resistance product is one of the most versatile routes to obtain large volume graphene-based materials. A number of chemical and thermal methods are being applied to achieve this goal, albeit each one is bound to certain disadvantages and limitations. Laser-assisted reduction has emerged as a promising method apt to overcome issues related to chemical and thermal reduction. Despite that a large number of efforts have been
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focused on laser-induced reduction of GO, its transformation to high quality reduced GO still remains a bottleneck. Here, it is shown that low-cost, millisecond lasers, widely used in the welding industry, achieve excellent reduction of GO to a product with the lowest sheet resistance yet reported by any laser-assisted method. For comparison, GO is reduced by chemical and
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thermal methods. Raman and x-ray photoelectron spectroscopies are applied to investigate the underlying structural changes providing evidence for the removal of oxygen-containing species
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and defect healing. Operation at ambient conditions, single pulse irradiation and a 2.60 mm wide focusing spot, demonstrate the high potential of this approach to the scalability of the reduction process towards producing large volumes of high-quality reduced graphene oxide at low cost.
_________________________ Corresponding author. E-mail: [email protected]
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1.
Introduction
Materials and composites based on graphene or graphene-like structures have received a great deal of attention over the last years on account of the unique properties such materials exhibit in a
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diverse set of applications including photonics, optoelectronics, energy conversion/storage, membranes and so on [1–3]. As the fundamental physics of graphene monolayer and the basic chemistry of graphene functionalization enabling adaption to proper matrices have been
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thoroughly explored, efforts are now focused on technologies for the optimization and low-cost upscaling of graphene production. This becomes imperative as the daily expansion of the global
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graphene market will be powered by novel production methods [1]. A vast number of different approaches for synthesizing graphene-based materials have been explored up until now, each one targeted to a particular application. However, none of them has ended up as a viable technology proper for large scale production of high quality graphene. On the other side, commercial
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applications based on the superb properties of the graphene monolayer are rather unfeasible to be achieved in view of the impediments leading to large scale monolayer preparation. Therefore, other graphene forms, such as graphene oxide and reduced graphene oxide (GO, rGO), and few
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or multilayer graphene, stand out as alternatives for advancing an assortment of applications. New technologies for large volume graphene production should be cost-effective, scalable, fast
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and environmentally friendly permitting in situ processing when necessary. In this direction, lasers have appeared so far, as a feasible technology for high-quality graphene production since can satisfy, in principle, most of the above criteria. In addition, laser light can produce graphene within the structure of a device offering flexibility and adaptability to current technological platforms. Lasers have up to now demonstrated capacity for high quality graphene growth by properly processing inorganic, e.g. SiC decomposition [4,5], and organic compounds [6].
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Over the past years a tremendous expansion of the research on graphene derivatives emerged aimed at less demanding applications [7–9]. This has led to the development of alternative pathways to produce graphene-like structures focusing in low cost and high yield procedures. The
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most intensively studied pathways are based on the oxidation of graphite to graphite oxide and its subsequent exfoliation to graphene oxide, which is followed by the reduction of the latter to graphene nanosheets. GO is atomically thin as graphene; however, it bears a semi-aromatic
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network structure of carbon atoms with sp2 and sp3 bonding, the latter arising from decoration with oxygen-containing groups[10]. The vast majority of these groups are hydroxyl (OH) and
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epoxy (C–O–C) species, while carbonyl (C=O) and carboxyl (COOH) functional units are also present. The presence of these groups suppresses dramatically the conductivity of the graphene layer. Partial removal of these species after application of different chemical or physical processes restores to a large extent the aromatic character resulting in rGO, a close analogue of
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graphene. Reduction of GO recovers a moderate conductivity [11–13]. Systematic efforts have been undertaken over the last ten years to prepare rGO, hence obtaining a graphene-like material. The main reduction method is the chemical route implemented by a
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number of reagents such as hydrazine, metal hydrides, hydroiodic acid [11,14]. Other reductive pathways were based on solvothermal heating, electrochemical, thermal or photocatalytic
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treatment [12,13]. Laser-assisted methods have gained ground owing to their versatility and the lack of use of chemicals [15,16]. A summary of laser-induced reduction works has recently been presented [17]. Two main requirements should be satisfied for a reduction method to be considered viable; obtaining enhanced electrical conductivity (partly related to the sp3 to sp2 transformation after removal oxygen-containing groups) and scalability of the process. We show here that using low-cost, commercially available lasers both the above requirements can be fulfilled. Notably, chemical methods are able to produce large volumes of rGO but fail in 3
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providing high conductivity and frequently have the disadvantage of using toxic or environmentally unfriendly reagents. On the contrary, laser-assisted methods have shown potential for preparing rGO with moderate to low resistance but have not up to now met the
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conditions for scalable production. The current work resolves this trade off by demonstrating that the use of low-cost lasers operating at the millisecond pulse-duration range with focusing spots of
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ca. 2 mm in diameter can provide superb quality graphene structures with ultra-low resistance.
Experimental
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GO synthesis: GO being prepared according to Staudenmaier's method is used as the starting material[18]. Graphite (2 g) was added to 150 mL of a cold mixture of H2SO4 and HNO3 (2:1) in an ice-water bath. KClO3 powder (50 g) was then added in small portions under continuous stirring and cooling. After 20 h, the reaction was quenched by pouring the mixture into distilled
dried at room temperature.
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water. GO was isolated by filtration and purified extensively with water (until the pH ∼ 6) and
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Graphite Oxide Reduction: Reduction of GO took place using three different procedures, i.e. (i) chemical (C-rGO), (ii) thermal (T-rGO) and (iii) laser-induced (L-rGO). In brief, the three
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reduced forms of r-GO were prepared as follows. (i) GO (10 mg) was dispersed in an excess of NaBH4 (100 mM) in water and heated to 80 oC for 4 hours. The color of the reaction mixture changed from brown to almost black indicating the successful transformation of GO to rGO. The product was purified extensively with distilled water and collected by filtration in the form of a free standing thin film [11]. (ii) A three-zone horizontal tube furnace was used for the thermal reduction. The tube was purged with high purity Ar gas for 1 h at a flow of 120 sccm to remove
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oxygen traces. Subsequently, the temperature was raised at 800 °C, in about 20 min, and kept at this temperature for 30 min, while the same gas flow was maintained during the reduction process. (iii) Reduction of GO by laser irradiation took place utilized a Rofin (Basel Lasertech)
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Desktop 4007 Nd-YAG Laser (pulse width in the range of ms) with an excitation wavelength of 1064 nm. The sample was prepared by cutting a piece of GO film 1×1 cm2 and mounted on a mechanically operated XY stage. The procedure was performed at ambient conditions; no inert
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gas was purging during irradiation. Overlapping steps comparable to the radius of the focused
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spot was used to ensure homogeneous irradiation due to the Gaussian beam profile of the beam.
Characterization methods: Electron microscopy images were recorded with a high resolution field-emission scanning electron microscope (FE-SEM) instrument (Zeiss, SUPRA 35VP) operating at 15 kV. Raman spectra was obtained with a Kimon He-Cd Laser apparatus at a laser
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excitation wavelength 441.6 nm The scattered light is analyzed by a LabRam HR800 (Jobin Yvon) micro-Raman spectrometer at a spectral resolution of about 2.0 cm−1. A microscope objective with 50× magnification is used to focus the light onto a spot of about 3 µm in diameter.
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Low laser intensities were used (∼0.27 mW on the sample) to avoid spectral changes due to heatinduced effects. The Raman shift was calibrated using the 520 cm−1 Raman band of crystalline Si.
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Electrical measurements were performed by using the four-probe technique and a Keithley 2401 SourceMeter. The cr-GO and tr-GO samples were deposited on a glass substrate and dried carefully.
The surface analysis studies are performed in a UHV chamber (P<10−9 mbar) equipped with a SPECS LHS-10 hemispherical electron analyzer. The XPS measurements are carried out using unmonochromatized Al Kα radiation at 1486.6 eV under conditions optimized for maximum signal (constant ∆Ε mode with pass energy of 36 eV, giving a full width at half maximum 5
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(FWHM) of 0.9 eV for the Au 4f7/2 peak). The analyzed area is an ellipsoid with dimensions 2.5×4.5 mm2. The XPS core level spectra are analyzed using a fitting. A routine, which allows the decomposition of each spectrum into individual mixed Gaussian–Lorentzian components
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after a Shirley background subtraction. Errors in our quantitative data are found in the range of about 10% (peak areas) while the accuracy for BE assignments is about 0.1 eV. The UPS spectra were obtained using HeI irradiation with hν=21.22 eV produced by a UV source (model UVS
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10/35) and the analyzer was working at the Constant Retarding Ratio (CRR) mode, with CRR=10. A bias of −12.28 V was applied to the samples in order to avoid interference of the
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spectrometer threshold in the UPS spectra. The high and low binding energy cutoff positions were assigned by fitting straight lines on the high and low energy cutoffs of the spectra and determining their intersections with the binding energy axis. Regarding measurement errors, it should be noted that an error of ±0.1 eV is assigned to the absolute values for ionization energies,
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work function and other UP-spectra cutoff features where the error margin is significant, due to the process of fitting straight lines. For the XPS-UPS measurements the C-rGO and T-rGO samples were drop casted on oxygen plasma cleaned ITO substrates, whereas the GO and L-rGO
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were in the form of powder.
Results and Discussion
3.1
Raman spectroscopy
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Raman scattering is presumably one of the most useful techniques to monitor fast and consistently the structure of the graphene-based materials and hence reveal the extent of reduction of GO. Various band parameters such as the relative intensity ratio of the D, G and 2D bands (i.e. ID/IG, I2D/IG), the width of the G band, the spectral shape of the 2D band and so on, are frequently employed as reliable indicators of the graphene quality, structure and sp3 to sp2 6
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relative fraction. Figure 1 shows representative Raman spectra of laser-assisted reduced GO recorded for various irradiation parameters as described in the caption. For all irradiation
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conditions used the spectra display significant reduction of the GO, despite the minor or
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1.2 J
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2.6 J 2.9 J 3.1 J 3.4 J
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0.7 J
GO 2000
2500
3000
3500
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Raman Shift [cm ]
Figure 1: Representative Raman spectra of pristine (bottom curve) and irradiated GO at ambient conditions for various laser parameters. The laser beam focus diameter is 1 mm for the energy range 3.4-2.6 J and 2.6 mm for the two lowest energies, 0.7 and 1.2 J.
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moderate differences they exhibit. It evident that in all cases laser irradiation results in a number of spectral changes. These include, appreciable decrease of the ID/IG ratio, narrowing of the D and G bands, and increase of the I2D/IG ratio. Low energy and large focal spot improve the reduction
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process, as is demonstrated by the two topmost spectra displayed in Fig. 1. The observed changes
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C-rGO
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GO 2000
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3000
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Figure 2: Raman spectra of pristine and reduced GO by chemical (C-rGO), thermal (T-rGO) and laser-assisted (L-rGO) methods.
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testify not only the removal of a large fraction of the O-containing groups, but also an efficient transformation of bonding from sp3 to sp2. It is worth-noting that in the vast majority of respective laser-assisted reduction attempts of GO in the literature - using ns, ps or fs laser
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sources - the corresponding spectral changes although to the same direction are only moderate[15,16]. Indeed, although in those studies the removal of oxygen species was typically demonstrated by a number of techniques, the spectral shape of the D, G band did not show
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appreciable changes after laser reduction. This is associated to the degree of restoration of the carbon lattice. Thus, despite that oxygen containing species can be removed to a large extent, in
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previous works, the healing of structural defects did not seem to take place, and needed special further treatment. Eliminating structural defects is of utmost importance since their presence affects drastically the electrical conductivity, thus being detrimental for device applications [19,20].
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Figure 2 displays Raman spectra of GO samples reduced by three different methods, namely, chemical (C-rGO), thermal (T-rGO) and laser-assisted (L-rGO). The comparison reveals the superior ability of the particular laser-assisted method against the two classical reduction
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methods. The high frequency region of the L-rGO spectrum has been fitted by Lorentz-type lines revealing the single Lorentzian character of the 2D band. Table 1 compiles selected spectral
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details to provide a more quantitative description of the structural changes of reduced GO. It is worth-noting that the area ratio of 2D over the G band for the L-rGO reaches the value of I2D / IG ≈ 1.67, which, to our knowledge, is the highest reported ever for laser or chemical reduction. The Raman spectrum of the thermally reduced GO, in Fig. 2, could be interpreted as heating produces a material with less sp2 bonded C atoms in comparison to pristine GO, as the D and G bands become wider that those of the pristine GO. Despite that oxygen elimination takes place
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even at this typically low annealing temperature, the mechanism of defects healing is not yet activated. Indeed, to achieve this, thermal annealing at very high temperatures, up to 2500 oC, is needed[21]. These authors showed that heating up to 1500 oC leads to complete removal of
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oxygen groups generating at the same time atomic vacancies. Heating to higher temperatures, 1800–2700 °C, causes defect healing (vacancy annihilation) and coalescence of adjacent overlapping layers to produce larger crystals[21]. Although using this two-step procedure
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extensive repair of defects can be achieved, the bonding of adjacent layers leads eventually to large fragments reminiscent of graphite, hence discarding the graphene properties.
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The above results demonstrate that using a low cost laser operating at ambient conditions and irradiating mm-wide spots onto GO, very high quality graphene structures can be prepared. Notably, this process is able not only to remove oxygen-containing species but to achieve at the Table 1: Raman band parameters for the pristine and the three reduces GO samples obtained by
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spectra fitting. I denotes integrated intensity; FWHM: full-width at half maximum. ω(D)
D(FWHM)
ω(G)
G(FWHM)
ID/IG
ω(2D)
2D(FWHM)
GO
1366
112
1588
70
1.24
2719
194.35
C-rGO
1367
46
1578
39
1.12
2728
89.5
T-rGO
1387
233
1590
96
2.31
-
-
1583
35
0.56
2719
68
AC C
L-rGO
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Sample
1362
48
same time restoration of the sp2 bonding. To alleviate the issue of high defect concentration in rGO some researchers have attempted defect healing by conducting CVD to provide extra C atoms[22].
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3.2
X-Ray and Ultraviolet Photoelectron Spectroscopies
Photoelectron spectroscopy was employed to figure out the relation between the chemical structure of GO / rGO and their physical properties. Figure 3 shows the C1s deconvoluted peaks
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of the pristine GO and the rGO samples reduced by the three methods. For all samples (except GO), the peak is analyzed into five components assigned to C-C sp2 and sp3 configurations, to C-O
L-rGO
C-C sp
C=O
292
290 288 286 284 Binding Energy EB [eV]
282
π-π*
O-C=O
280
294
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294
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C-C sp2
3
C-C sp2
XPS AlKα C1s
292
AC C
C-O
π-π*
294
292
290 288 286 284 282 Binding Energy EB [eV] C-C sp2
C=O O-C=O
π-π*
282
280
XPS AlKα C1s
C-O
C-C sp3
290 288 286 284 Binding Energy EB [eV]
XPS AlKα C1s
C-C sp3
C-O C=O
T-rGO
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C-rGO
C-C sp2
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GO
XPS AlKα C1s
280
294
292
C=O O-C=O
290 288 286 284 Binding Energy EB [eV]
C-C sp3
282
280
Figure 3: Deconvoluted C1s XP spectra of GO, L-rGO, C-rGO and T-rGO samples.
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epoxides / hydroxyls C-O-C and C-O(H), carbonyls C=O and carboxyls O=C-O(H) at binding energies 284.5±0.1 eV, 285.4±0.1 eV, 286.6±0.1 eV, 287.8±0.1 eV and 289.5±0.1 eV, respectively. Additionally, the π-π* satellite peak at ~292 eV is observed in the reduced
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samples[23]. The fractions of the above mentioned chemical species and of the delocalized π-π* bonds for each sample are compiled in Table 2. Comparing the fractions of the reduced samples, it is evident that the reduction is more effective in the case of L-rGO since the relative fraction of
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sp2 and delocalized π-π* bonds are appreciably higher than the corresponding fractions of C-rGO and T-rGO. In addition, the carbon-oxygen components of L-rGO is lower than those of C-rGO
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and T-rGO. The findings of the XPS analysis are in line with the Raman results, demonstrating that the laser reduction method is the most effective for GO reduction than the other two methods leading to elimination of carbonyl and carboxyl groups. Moreover, the higher percentage of sp2 hybridization and π-π* suggests that the delocalized π conjugation, a characteristic of aromatic C
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structure, is to some extent restored in L-rGO samples. Based on the sp3 relative concentration it follows that the most defected film is the chemically reduced graphene oxide in agreement with
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other studies[24].
Table 2: Relative atomic percentages of sp2, sp3, epoxides-hydroxides (C-O-C, C-O(H)),
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carbonyl, carboxyls, π-π* bonding and C/O ratio as derived from C1s XPS spectrum deconvolution. Sheet resistance (Rs) values are also shown for comparison. Sample
% sp2
% sp3
GO
38.2
L-rGO
C=O
O-C=OH
π-π*
4.3
% C-O-C, C-O(H) 54.8
-
C/O (±0.1) 1.4
Rs (kΩ sq-1) >103
2.7
-
75.8
10.2
6.9
1.4
1.8
3.9
8.4
4×10-2
C-rGO
64.3
14.9
11.6
3.5
3.5
2.1
5.2
1.2
T-rGO
63.0
15.0
15.0
3.8
2.4
0.9
8.6
2.0
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The valence band spectra as well as the work function (WF) were measured with UPS. Figure 4(i) shows the valence band region near the Fermi level, EF, while Fig. 4(ii) illustrates the high binding energy cutoffs, used to determine the work function of the pristine and reduced GO. The
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work function was determined, based on the fitting procedure reported elsewhere[24], by the equation WF=hν-∆Ε, where hν is the photon energy and ∆Ε is the energy difference between ΕF and Ecutoff. The work function of the GO is estimated at 5.4 eV whereas in the reduced samples its
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value is 4.3±0.1eV in agreement with previous work[25]. No differences between the WF values
UPS HeI
(i)
GO C-rGO L-rGO T-rGO
(ii)
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Intensity [arb. units]
GO C-rGO L-rGO T-rGO
2pπ
Intensity [arb. units]
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of reduced samples is observed, as expected, However, the peak at ~3 eV in the valence band
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EF
5
4
3 2 1 Binding Energy [eV]
Ecutoff 0
18.0
17.5
17.0
16.5
16.0
15.5
15.0
14.5
Binding Energy [eV]
Figure 4: (i) Valence band and (ii) high binding energy cut-off where the work function
determined from UPS measurements determined from the secondary electron onset.
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region appears more intense in the L-rGO. This peak is formed by 2pz orbitals and is attributed to pπ electrons (sp2 hybridization) [26], while it is absent in the spectrum of GO. Since the presence of sp3 C-H defects in graphene reduce significantly the delocalized π-electrons, these differences
above results are in agreement with XPS and Raman results.
Morphological changes by SEM and sheet resistance of r-GO
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3.3
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in the valence band indicate that the L-rGO sample presents the lowest density of defects. The
Figure 5 illustrates typical FE-SEM images of the pristine and modified GO using the various
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reduction methods. The electron microscopy images reveal that the initially smooth surface and
(b)
(c)
(d)
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(a)
Figure 5: FE-SEM images of (a) pristine, (b) laser-assisted reduced, (c) chemically reduced and (d) thermally reduced, graphene oxide.
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compact texture of the GO (a) material is transformed to a more open or porous structure in the case of the photo-processed GO. Several edges are created implying also increase of the surface area of the irradiated area (b). Such structural change giving rise to partial exfoliation of the
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layers; the increased free-standing layers could presumably be responsible for the reduced resistance of the laser-assisted reduced GO in comparison to pristine GO. The chemically (c) and thermally (d) reduced GO show rougher structure in comparison to the pristine GO, whereas they
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are less porous or expanded in relation to the photoreduced GO.
The measurement of sheet resistance (Rs) is a characteristic feature for the efficiency of the
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reduction of GO since it is highly connected with the aromatic character of graphene nanosheets. In general, the electrical insulator GO becomes partly conductive after chemical reduction. According to current literature data the sheet resistance of rGO obtained by chemical methods or by combined chemical/thermal procedures falls in the range MΩ sq-1 to kΩ sq-1 [11]. The
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treatment of GO films with the laser beam induced a remarkable increase in the electrical conductivity as revealed by the measured sheet resistance about 40 Ω sq-1. For comparison, films of C-rGO and T-rGO samples of the current work showed sheet resistant about 1.2 kΩ sq-1 and 20
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kΩ sq-1, respectively as shown in Table 2. Taking into consideration the spectroscopic results, the low resistivity of the L-rGO sample is correlated with: (a) the higher relative fraction of sp2 as
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showed in Table 2 from the XPS measurements, (b) the presence of the peak at ~3 eV (sp2 hybridization) in the UPS spectra and (c) the value of the I2D/IG ratio obtained from the Raman analysis. This means that laser reduction is the most effective method for restoration of the πconjugated structure and leads to a low resistivity reduced graphene oxide sheets. It is known that the presence of sp3 bonds disrupts the transport of carriers thus the mobility and the conductivity of reduced GO films is limited [20]. This is in agreement with the results summarized in Table 2, where T-rGO exhibits the higher resistivity among the various rGO samples. Indeed, the fraction 15
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of carbon-oxygen bonds is higher for this material, whilst the fraction of the π-π* transition and the value of the I2D/IG ratio are the lowest. It would be instructive to compare the resistivity values obtained in the current work with the best
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corresponding results in the literature that have been obtained by laser-assisted reduction. In general, reduction by lasers results in low to moderate improvement of the very high resistance of pristine GO. Only in a few studies values of the sheet resistance below ~104 Ω sq-1 have been
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reported. The most successful reduction procedures of GO using laser beams has been reported[27–29]. Sokolov et al.[27] investigated the role of continuous (532 nm) and pulsed (532
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and 355 nm; 9 ns) on the reduction of GO in air and under N2 atmosphere. The bandwidth of the 2D Raman peak of their work is similar to the one we found here; however the I2D/IG ratio in our case is much higher than that in Ref.[27]. The same group explored further the reduction of GO using an excimer laser (248 nm; 25 ns) under high vacuum or N2 atmosphere. The highest I2D/IG
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ratio was ~0.5. The lowest sheet resistance measured was ~491 Ω sq-1. Only for GO samples that were first subjected to thermal reduction at 150 oC for 24 h, the lowest sheet resistance measured after irradiation reached ~100 Ω sq-1. Guan et al.[29] used a pulsed laser (1064 nm, 10 ps) to
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reduce GO at atmospheres of gas and liquid N2. Raman spectra showed successful reduction with adequate defect healing and a 2D band width comparable to the current work. However, the
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I2D/IG ratio still remains appreciably lower than our findings. Best sheet resistance values reported by Guan et al.[29] are 104 and 50-60 Ω sq-1 for gas and liquids N2, respectively. In a recent study, Arul et al.[30] reported a comparison of GO photo-reduction using cw, ns and ps laser sources, irradiating GO at ambient condition. The ns laser resulted in the more efficient reduction. However, even in that case, the 2D band intensity seems not to exceed 1/3 of the G band intensity, while the D band is for all cases the most dominant in the Raman spectra.
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Various mechanisms have been proposed for the laser-assisted reduction of GO which consider the effect as photothermal [6], photochemical [31] or a combination of both [30]. Ultrashort pulses, i.e. emerging from a fs laser, are the prerequisite for an athermal reduction of GO.
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However, although at the first stages following the interaction of the pulse with GO and the athermal rupture of C-O bonds, thermal effects inevitably intervene after i.e. 100 ps, due to recombination of photoexcited electrons and holes [17]. The long pulse width used in the current
SC
study clearly support a photothermal origin of GO reduction. The process of simultaneous oxygen removal and defect healing at ambient conditions using laser-assisted GO reduction has
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not yet been reported. Typically, laser reduction of GO leads to efficient removal of oxygencontaining species; however, leaving a large fraction of sp3 defects. The atomic scale mechanism leading to improved crystallinity, which the current approach offers is still elusive. In a recent study [32] we have shown using the same laser irradiation conditions that elemental carbon
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powders (known commercially as Vulcan and Norit) with high sp3 content can successfully be transformed to structures with high sp2 bonding. The high temperature of the irradiated volume might be the main factor for this structural transformation.
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The above comparison indicates that the current approach is superior to any of the previous attempts which employ laser beams to selectively reduce GO. The merits of our method are
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summarized as follows. In comparison to chemical methods the avoidance of using chemicals renders laser-assisted GO reduction an environmentally friendly, green approach. Moreover, the current method provides much lower Rs values than current approaches using chemical reduction. In relation to other laser-based methods our approach excels for the perspectives of production cost and scalability. The use of low-cost industrial-type lasers working under literally ambient conditions outweighs methods using expensive equipment such as pico- and femto-second lasers operating at laboratory specifications under either high vacuum or inert atmosphere conditions. A 17
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focusing spot of 2.6 mm in diameter implies a GO reduction processing speed exceeding by several orders of magnitude the speed of other approaches, operating at focusing points of few tens in microns in diameter.
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Closing this discussion it would be instructive to mention that recent heat-induced GO reduction procedures have shown lowering of the resistance up to ~1 Ω sq-1[33]. However, the method followed to achieve this ultralow value of the resistance necessitates the process of Joule heating
SC
a GO foil at 2750 K under vacuum, implying that the process is proper for substrateless samples such as free-standing GO foils. Notably, attempts to thermally reduce GO pre-deposited on soda
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lime glass substrates in an effort to prepare transparent/conducting electrodes, resulted in values of the sheet resistance in the regime Rs > 2×104 Ω sq-1 for prolonged annealing at 500 oC, i.e. the maximum temperature that the substrate can withstand [34]. The above demonstrate the merit of laser-assisted methods, which can offer the opportunity of selectively in situ processing of GO
4.
Conclusions
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pre-deposited on any substrate, being thus fully compatible to current technological platforms.
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In summary, we present a systematic exploration of the merits of various reduction methods of GO to rGO, including chemical and thermal, with emphasis on laser-assisted approaches. The
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realization of the reduction was quantified by the extent of oxygen-containing species removal and more importantly by the success of each approach in healing defects created after reduction. Comparing structural and resistivity data it turns out that the current laser-induced reduction of GO produces high quality graphene-like structures with resistivity lower than that of the chemical and thermal routes by almost two orders of magnitude. Notably, the resistance value measured for the L-rGO in the present study is not only lower than any other corresponding measurement for laser-assisted reduction, but also among the lowest reported in the literature i.e. those
18
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necessitating heating GO foils up to 3000 K under vacuum. Extended spectroscopic and morphological characterization was performed in order to rationalize these findings in terms of structural or chemical changes taking place during the reduction process. For the first time, the
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2D Raman band of the L-rGO has been found to be stronger than the G band, indicating the predominance of graphene-like structures with low defect concentration. The results demonstrate that the particular irradiation conditions employed in the current work are able to accomplish
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simultaneously both steps needed for a successful GO reduction, namely, almost complete removal of oxygen-containing species and exceptional healing of defects. This achievement has
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not been reported previously in laser-assisted GO reduction methods. Additional merits of the present approach over other laser-induced reduction methods include the ultrahigh speed of the process. A single laser pulse, 1 mm wide, and a focus beam of 2.60 mm in diameter can provide a laser writing speed (i.e. ca 1 cm2 per second) more than two orders of magnitude faster than that
Acknowledgements
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in previous work where typically the focused beam profile is in the range of few tens of microns.
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Authors from FORTH/ICE-HT acknowledge support of this work by the project “ΑΕΝΑΟ – Materials and processes for energy and environmental applications” (MIS 5002556) which is implemented under the “Action for the Strategic Development on the Research and Sector”,
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Technological
funded
by
the
Operational
Programme
"Competitiveness,
Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).
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