Carbon 148 (2019) 159e163
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Novel template-assisted microwave conversion of graphene oxide to graphene patterns: A reduction transfer mechanism Yajing Zhao a, b, Junhui He a, * a Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing, 100190, China b University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 February 2019 Received in revised form 19 March 2019 Accepted 24 March 2019 Available online 25 March 2019
Graphene patterned graphene oxide thin ﬁlms present a promising all-carbon material with potential in a wide range of applications. However, the direct conversion of designed domains of graphene oxide thin ﬁlms into those of graphene has proven difﬁcult. Herein, a template-assisted microwave conversion of graphene oxide to graphene patterns was developed, where microwave irradiation and chemical template of reduced graphene oxide were for the ﬁrst time applied to give conductive graphene patterns on an insulating graphene oxide thin ﬁlm through a reduction transfer mechanism. The characteristics of obtained graphene patterns were found to be closely related to those of templates used, which has, to our best knowledge, not been reported previously. The fast and easy conversion led to graphene patterns of tailorable shapes, clear boundaries and excellent electrical conductivity (36.5 U$sq 1), which may open a new avenue to graphene-based all-carbon electronic circuits. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Surface patterning, especially fabrication of conductive patterns on insulating substrates, is of great importance due to its wide potential applications , such as displays, sensors, batteries, microelectronics and microﬂuidics [2e7], etc. Graphene, the most promising conductive material, has advantages of atomic thickness, high electrical conductivity, superior thermal conductivity and excellent ﬂexibility [8e11], and its tailorable patterning on an insulating substrate would doubtlessly be highly desired. However, typical patterning methods usually need to carefully transfer preprepared graphene onto an insulating substrate followed by a sophisticated etching process  or grow desirable graphene patterns on a resist-masked copper foil by chemical vapor deposition (CVD) . In contrast, direct conversion of designed domains of graphene oxide (GO) into graphene would be conceptually much more straight forward, and may open a new avenue to all-carbon devices. Nevertheless, previous direct patterning strategies (laser, UV-irradiation) by reducing GO to graphene often suffer from ﬁlm structure damage, low conductivity, time-consuming procedure and high cost [14,15].
* Corresponding author. E-mail address: [email protected]
(J. He). https://doi.org/10.1016/j.carbon.2019.03.081 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
Recently, Voiry et al.  achieved high quality graphene (mobility value > 1000 cm2$V 1$s 1) by microwave reduction of solution-exfoliated graphene oxide. Very recently, Jiang et al.  reported preparation of high-quality graphene (sheet resistance: ~40 U$sq 1) using triggered microwave reduction under an air atmosphere. The microwave reductions are both fast and efﬁcient as compared with other methods. However, to our best knowledge, microwave approaches have never been reported so far for directly converting GO to graphene patterns. In the current work, we developed a simple, fast and efﬁcient approach to directly and selectively converting GO to graphene patterns under a joint effect of reduced graphene oxide (rGO) template and microwave irradiation. The obtained graphene patterns have tailorable shapes and high quality with a low sheet resistance of 36.5 U$sq 1, which may open a new avenue to free-standing and all-carbon based electronic circuits. 2. Experiments 2.1. Preparation of GO sheets GO was synthesized from natural graphite by a modiﬁed Hummers method. 1.8 g graphite, 3.0 g K2S2O8 and 3.0 g white phosphorus were added into 14.4 mL concentrated sulfuric acid
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(98%) in a 250 mL round-bottomed ﬂask. The mixture was heated to 80 C, and kept at this temperature for 4.5 h with stirring. After ﬁltering, the mixture was washed with deionized water and dried at 60 C. Then the obtained powder was mixed with 0.5 g sodium nitrate and 46 mL concentrated sulfuric acid (98%) in a 500 mL beaker, and stirred for 1 h in an ice bath. Subsequently, 6 g KMnO4 was added slowly under vigorous stirring to keep the mixture temperature below 20 C. After removal of the ice bath, the mixture was further stirred at 35 C for 2 h. Then 92 mL deionized water was added gradually, followed by additional 2 h stirring. Subsequently, 80 mL deionized water and 5 mL H2O2 (30%) were added into the mixture. After stirring for 0.5 h, the suspension was washed with lots of 5% HCl and deionized water, and dried at 60 C to obtain GO sheets. 2.2. Preparation of GO thin ﬁlms The as-prepared GO sheets were dispersed in deionized water to obtain 2 mg/mL GO suspension by sonication. Then 6 mL GO suspension was carefully casted on the surface of a polytetraﬂuoroethylene (PTFE) disc of 5 cm in diameter, and dried at 60 C in an oven. After peeled off from the PTFE disc, the GO ﬁlm was cut into square pieces with a side length of 1.1 cm. 2.3. Preparation of rGO templates The as-prepared GO sheets were dispersed in ethanol to obtain 0.7 mg/mL GO suspension by sonication. Subsequently, the GO suspension was cautiously drop-casted (75 mL/~1 cm2) on silicon nanowire (SiNW) arrays that had been prepared by chemical etching of silicon wafer . The SiNW arrays were then dried in an oven at 60 C for several minutes. After repeating the process of drop-casting and drying for 6 cycles, the SiNW arrays were dried overnight in air, then annealed for 3 h at 200, 600, and 1000 C, respectively, under an Ar (95%)-H2 (5%) ﬂow of 50 mL/min to form rGO thin ﬁlms. The rGO thin ﬁlms with different annealing temperatures were cut into squares with a side length of 0.5 cm, equilateral triangles with a side length of 0.7 cm, circles with a diameter of 0.6 cm and other different shapes for use as template.
2.5. Characterization The samples were characterized using ﬁeld emission scanning electron microscopy (SEM) (Hitachi S-4800), atomic force microscopy (AFM) (MM8-SYS scanning probe microscope, Bruker AXR) on a mica substrate and Raman spectroscope (inVia-Reﬂex, Renishaw, U.K.) with the incident laser light of 532 nm. The sheet resistances of rGO templates obtained at varied annealing temperatures and corresponding graphene patterns were measured using the four-point probe method (Guangzhou Four Probes Tech Co.). X-ray photoelectron spectra (XPS) of samples were collected on an ESCALAB 250Xi X-ray photoelectron spectrometer. 3. Results and discussion The proposed approach is illustrated in Fig. 1 for converting graphene oxide to graphene patterns by microwave irradiation in combination with chemical template of rGO. A rGO template is placed on top of a GO thin ﬁlm, and then microwaved for a certain period of time under atmosphere. After removing the rGO template, a GO ﬁlm with graphene patterns is eventually obtained. The prepared GO ﬁlm and rGO templates, whose conductivity was tailored by varying their annealing temperature [18,19], were observed by scanning electron microscopy and naked eyes, and are shown in Fig. 2aed. The photographs and SEM images indicate that both the GO ﬁlm and rGO templates annealed at different temperatures had a good integrity and uniform morphology with a few folds. Moreover, the rGO templates demonstrated signiﬁcant metallic gloss in contrast to the GO ﬁlm.
2.4. Patterning of GO thin ﬁlms An as-prepared rGO template was placed on top of a GO thin ﬁlm in a glass vessel and microwaved (Panasonic microwave oven, 1000 W) for 4e5s under atmosphere. Arc discharge was observed at the end of reduction process. After the irradiation, the rGO template was removed and the GO ﬁlm with graphene patterns was obtained. Besides, preparation of graphene patterns assisted with copper template was also investigated by placing a copper template between a GO ﬁlm and a rGO ﬁlm under microwave irradiation.
Fig. 2. (aed) Top view SEM images of GO and rGO thin ﬁlms annealed at 200, 600 and 1000 C, respectively. The insets are photographs of GO and rGO thin ﬁlms. (A colour version of this ﬁgure can be viewed online.)
Fig. 1. Schematic illustration of template-assisted microwave conversion of graphene oxide to graphene patterns. (A colour version of this ﬁgure can be viewed online.)
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Fig. 3. (a) Schematic illustration of the patterning strategy by a conventional microwave oven. (bed) Photographs of obtained GO thin ﬁlms with simple graphene patterns. The upper parts of photographs are the rGO templates. (e) Sheet resistances of graphene patterns (Graphene Patterns) and rGO templates. (A colour version of this ﬁgure can be viewed online.)
Fig. 3a schematically shows the set-up for the proposed template-assisted patterning by a conventional microwave oven. In order to pattern a GO thin ﬁlm, a glass slide, a rGO template, a GO ﬁlm and a piece of paper were stacked one on another. The glass slide was used to press the template and the GO ﬁlm to make them ﬁt closely with each other, and the paper was used to disperse the heat and force generated by the reaction. RGOs obtained by annealing at 200 C, 600 C and 1000 C were respectively used as template to pattern GO thin ﬁlms by microwave reduction for 4e5 s under air conditions in a conventional microwave oven. Fig. 3bed shows top views of the obtained GO thin ﬁlms with a graphene pattern after removing the template. Clearly, the color of templatecovered areas (dashed squares) of GO ﬁlms turned darker. Furthermore, the graphene pattern covered by the rGO template annealed at 200 C (denoted as G-rGO200 C) has the most complete shape and the cleanest border, as compared to the patterns covered by the other templates (denoted as G-rGO600 C and GrGO1000 C). In contrast, photographs of patterning processes using square templates of other different materials (aluminum foil, copper, silicon, paper) (Fig. S2) show no conversion of graphene oxide to graphene patterns, proving that the rGO template played a key role as chemical template in the patterning process. Sheet resistance measurements (Fig. 3e) demonstrate the covered part turned from insulating to conductive, conﬁrming the fabrication of insulating GO ﬁlms with conductive graphene patterns. While the sheet resistance of rGO template decreases with increase of the annealing temperature, the sheet resistance of graphene pattern surprisingly decreases with decrease of the annealing temperature of rGO template, i.e., a converse trend is observed. Particularly, when the rGO template was annealed at 200 C, the sheet resistance of obtained graphene pattern dropped to 36.5 U$sq 1. The sheet resistance is one of the lowest values of sheet resistances of rGO ﬁlms reported up to now, as shown in Table 1. The possible conversion mechanism of graphene oxide to
graphene patterns is proposed on the basis of experimental observations and previous literature, and is illustrated in Fig. 1. Before microwave reduction, GO contains a number of oxygen-containing groups attached to the carbon skeleton with partial carbon atoms missing, which results in the destruction of the graphene electronic structure . RGO was reported to have good microwave absorption performance [23e25]. Thus, in the microwave reduction process, a large number of delocalized p electrons accumulated between the rGO template and the GO ﬁlm, causing rapid heating of the covered GO region and strong arcs emitting inside the template boundary [16,17]. The intensive energy release induced desorption of oxygen-containing groups covalently bonded to the carbon network and rearrangement of carbon atoms. Due to the poor microwave absorption, thermal conductivity and electrical conductivity of GO, the strong energy could not be transmitted and diffused in time over the area outside the template coverage. The microwave irradiation for patterning was terminated when the spark ﬁrst appeared, preventing the additional reduction caused by the obtained graphene edge and preserving the size of template. All these effects ultimately made the GO be reduced at an extremely fast manner, giving a pattern consistent with the used rGO template in shape. As a result, the overall conversion process of graphene oxide to graphene patterns can be considered as a reduction transfer process from the rGO template to the GO thin ﬁlm through the combined action of microwave irradiation and rGO chemical template. The reason why the template annealed at 200 C led to better reduction might be that such a rGO template had more defects and oxygen-containing groups, thereby absorbing more microwave energy. XPS survey spectra (Fig. 4a) indicates that the in-plane oxygen concentration of graphene patterns decreases signiﬁcantly as compared with the GO. The amounts of oxygen-containing groups in the G-rGO200 C area decreases by nearly 44%, 70% and over 77% as compared with G-rGO600 C, G-rGO1000 C and the rGO ﬁlm obtained by annealing the GO ﬁlm at 200 C (Fig. S3). Elemental
Table 1 Comparison of the sheet resistance of rGO ﬁlms. Preparation strategy
Sheet resistance (U$sq
Graphene patterns in GO ﬁlms obtained by rGO template under microwave irradiation GO ﬁlm reduced by laser scribing Graphene ﬁlm patterned by using a photoresist GO ﬁlm reduced by UV-irradiation GO ﬁlm reduced by annealing at 1000 C under AreH2 atmosphere GO ﬁlm reduced by rGO trigger with microwave irradiation GO ﬁlm reduced by dipping into NaeNH3 solution GO ﬁlm reduced via hydrazine vapor and annealing at 200 C under N2 atmosphere
36.5 80 444 47000a 21.2 40 350 43000
The unit is U/cm.
✓ ✓ ✓ ✓ 7 7 7 7
This work       
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Fig. 4. (a) Survey XPS spectra of GO thin ﬁlm and graphene patterns, C1s XPS spectra of (b) G-rGO200 C, (c) G-rGO600 C, (d) G-rGO1000 C patterns and (e) GO thin ﬁlm, and their Raman spectra (f). (A colour version of this ﬁgure can be viewed online.)
analyses of graphene patterns (Table S1) indicate that the GO ﬁlms with graphene patterns are all-carbon. The deconvolution of C1s spectra (Fig. 4bee) show the height of peaks at 285.6 eV (CeO), 287.2 eV (C]O) and 288.8eV (O]CeO) were markedly lowered after microwave treatment, suggesting that the oxygen-containing groups of the covered GO areas were signiﬁcantly removed. Moreover, the full width at the half maximum of G-rGO200 C is apparently smaller than those of G-rGO600 C and G-rGO1000 C, indicating it has less defects . Fig. 4f shows the Raman spectra of G-rGO200 C, G-rGO600 C, G-rGO1000 C and the GO. The intensity ratio of two peaks, ID/IG, decreases from 0.880 to 0.753 with decrease in the heat-treatment temperature of rGO template, and they are all lower than that of the GO. The ID/IG value of GrGO200 C is also lower than that (0.832) of the thermally reduced rGO at 200 C (Fig. S3). Meanwhile, a clearer and sharper 2D peak of G-rGO200 C further suggests a more ordered structure and a larger graphene domain size. RGO templates of different shapes were designed (Fig. 5aed) and obtained by annealing the GO at 200 C. They were used as template to pattern GO thin ﬁlms. Fig. 5eeh shows that the obtained graphene patterns ﬁt perfectly with the shapes of templates. Less perfect results were obtained using templates of varied shapes annealed at 600 C and 1000 C, respectively (Fig. 6). In addition,
Fig. 5. (aed) Models of rGO templates with varied shapes. (e-h, k) Photographs of GO thin ﬁlms with graphene patterns after microwave treatment. (i) SEM image of boundary between graphene domain and unreduced GO domain after microwave treatment. (j) Magniﬁed SEM image of the boundary. The inset is SEM image of graphene patterns. Scale bars, 5 mm. (l) Cross-sectional SEM image of boundary between graphene domain and unreduced GO domain after microwave treatment. The insets are magniﬁed views of the junction and obtained graphene. Scale bars, 5 mm. (A colour version of this ﬁgure can be viewed online.)
Fig. 5k demonstrates that this method could achieve more complex patterns. As shown in Fig. 5i and j, the boundary between the unreduced GO domain and graphene domain is clearly deﬁned, and the inserted SEM image of Fig. 5j shows that the surface of graphene has a uniform morphology with a few wrinkles. From the cross-sectional view (Fig. 5l) of the two-domain junction, graphene had a signiﬁcant inter-layer expansion compared to the GO. Thus, the multi-layer pattern is ridged and has a thickness of ca. 50 mm. To evaluate the performance of GO ﬁlms with graphene patterns, they were designed as touch switches in circuits, and both single(Fig. 7a) and multi-line (Fig. 7bed) patterns could ﬁnely control the LED. Due to the uneasy implementation of complex patterns based on the rGO template method, a more complicated pattern was obtained by means of a copper template that is easy to obtain, handle and can shield the microwave absorption of rGO. A rGO ﬁlm, a copper template and a GO ﬁlm were stacked one on top of another, and exposed to microwave irradiation (Fig. 8e). The patterns obtained after microwave irradiation (Fig. 8aed) prove that this approach is feasible.
Fig. 6. (aec) Photographs of patterning processes assisted with different rGO ﬁlm templates annealed at 600 C. (def) Photographs of pattern processes assisted with different rGO ﬁlm templates annealed at 1000 C. (A colour version of this ﬁgure can be viewed online.)
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Fig. 7. (a) Photograph of single-line patterned ﬁlm-based electronic circuit when it was turned on. Photographs of multi-line patterned ﬁlm-based electronic circuit when it was turned (b) off and (c) on, respectively. (d) Schematic diagram of the circuit. (A colour version of this ﬁgure can be viewed online.)
 Fig. 8. Photographs of (a) solid copper template, (b) hollow copper template, (c, d) GO thin ﬁlms with graphene patterns. (e) Schematic of sample placement method. (A colour version of this ﬁgure can be viewed online.)
4. Conclusions In summary, we ﬁrstly developed a facile, rapid, efﬁcient method for in-plane and region-speciﬁc patterning of GO ﬁlms with controllable graphene patterns, by subtly utilizing the microwave absorption property of rGO templates. The obtained graphene patterns have clear boundaries, excellent electrical conductivity and controllable shape ﬁtting well with the template. The characteristics of obtained graphene patterns were found to be closely related to those of templates used, which has, to our best knowledge, not been reported previously. In addition, the patterned GO ﬁlms are free-standing, all-carbon, and can be achieved using an ordinary household microwave oven under atmospheric conditions, which may open a new avenue to graphenebased electronic circuits towards electronic skin, wearable devices, chem/biosensors, energy devices and so forth. Acknowledgements This work was supported by the National Natural Science Foundation of China (21571182), the National Key Research and Development Program of China (2017YFA0207102), and the Science and Technology Commission of Beijing Municipality (Z151100003315018). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.03.081. References  S. Xu, L. Ren, B. Liu, J. Wang, B. Tang, W. Zhou, et al., Single-step selective metallization on insulating substrates by laser-induced molten transfer, Appl. Surf. Sci. 454 (2018) 16e22.  A. Basu, K. Roy, N. Sharma, S. Nandi, R. Vaidhyanathan, S. Rane, et al., CO2 laser direct written MOF-based metal-decorated and heteroatom-doped porous graphene for ﬂexible all-solid-state microsupercapacitor with extremely high cycling stability, ACS Appl. Mater. Interfaces 8 (46) (2016) 31841e31848.  S. Hong, J. Yeo, G. Kim, D. Kim, H. Lee, J. Kwon, et al., Nonvacuum, maskless
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