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Facile deposition of reduced graphene oxide-based transparent conductive film with microwave assisted method
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Facile Deposition of Reduced Graphene Oxide-Based Transparent Conductive Film with Microwave Assisted Method Akfiny Hasdi Aimon , Rahmat Hidayat , Dina Rahmawati , Ronny Sutarto , Fitri Aulia Permatasari , Ferry Iskandar PII: DOI: Reference:
S0040-6090(19)30646-7 https://doi.org/10.1016/j.tsf.2019.137618 TSF 137618
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
20 December 2018 3 October 2019 4 October 2019
Please cite this article as: Akfiny Hasdi Aimon , Rahmat Hidayat , Dina Rahmawati , Ronny Sutarto , Fitri Aulia Permatasari , Ferry Iskandar , Facile Deposition of Reduced Graphene Oxide-Based Transparent Conductive Film with Microwave Assisted Method, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137618
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Highlight:
The reduced graphene oxide (rGO) films were deposited evenly on the substrate.
Microwave assisted method was used as thermal reduction to fabricate rGO film.
C-C bonding increased and C-O and C=O decreased in the radiated sample.
The electrical and optical properties of the rGO films were investigated.
1
Facile Deposition of Reduced Graphene Oxide-Based Transparent Conductive Film with Microwave Assisted Method Akfiny Hasdi Aimon1*, Rahmat Hidayat1, Dina Rahmawati1, Ronny Sutarto3, Fitri Aulia Permatasari1, and Ferry Iskandar1,2 1
Energy and Environmental Laboratory, Department of Physics, Institut Teknologi Bandung,
Jl. Ganesha 10 Bandung 40132, Indonesia 2
Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Jl.
Ganesha 10 Bandung 40132, Indonesia 3
Canadian Light Source, 44 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2V3,
Canada *
[email protected]
2
Abstract An alternative way to fabricate reduced graphene oxide-based transparent conductive films (rGO based TCF) using microwave assisted irradiation is introduced. This modified method offers lower cost, more efficiency and more convenience in thin-film fabrication. A high mode of conventional microwave irradiation was used in this research, with time variation at 30 and 60 minutes. For comparison, an unirradiated sample was prepared to see the direct effect of microwave radiation on the properties of the samples. The electrical and optical properties of the samples were characterized by sheet resistance measurement and UV-Vis spectrometry. Scanning electron microscopy was used to observe the morphology of the rGO thin film on the substrate. Microwave radiation at 30 minutes gave the best combination for electrical and optical properties, with the sheet resistance of 21.97 kΩ/sq and 30.37% transmittance, respectively. X-ray photoemission spectroscopy results confirmed the increased of C-C bonding and reduced C-O and C=O bonding in the sample subjected to 30 min of microwave radiation. This study provides a different perspective on the fabrication of rGO based TCF. Keywords: Transparent conductive film; Reduced graphene oxide; Microwave irradiation; thermal
reduction.
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1. Introduction Transparent conductive films (TCF) with excellent optical transmittance and high electrical conductivity are essential in optoelectronic technology and devices. Up until now, the most widely used TCF are based on indium tin oxide and fluorine tin oxide. However, the use of this metal oxide for TCF faces obstacles because of the scarcity of the raw material on earth. Several attempts have been made to find a replacement material for TCF, such as metallic nanowire, conductive polymers, and carbon-based materials [1–3]. Graphene is a carbon-based material that has been widely developed. This allotrof of carbon materials consists of a two-dimensional hexagonal carbon lattice with sp2 bonds and has excellent mechanical, electrical, thermal, and optical properties [4,5]. Application of graphene is found in optoelectronics, transistors, biosensors, energy storage devices, display media, and electrochemical systems [6–11]. Majee et al. has developed pristine graphene for flexible TCF [12]. However, the most effective method for large-scale and low-cost preparation of graphene is the reduction of graphene oxide (GO) by wet chemical process [13,14]. GO has hydrophilic characteristics, which allows it to be dissolved in demineralized (DI) water. This permits deposition onto various substrates to form a thin film due to its stable, superhydrophilic properties, and facile fabrication of TCF. The reduction process is necessary to remove oxygen containing groups that decorate the GO. Although reduction of GO does not produce pure graphene, it does yield properties close to pristine graphene. The most well-known reduction techniques are chemical reduction and thermal reduction. Chemical reduction uses a chemical agent, such as borohydride (NaBH4) or hydrazine, to cut the oxygen bonds in the GO basal plane [15]. Thermal reduction uses heat energy to eliminate the oxygen groups and restore the sp2 bonds. Widely used thermal
4
reduction methods are based on joule heating and furnace annealing [16,17]. However, both methods consume a large amount of energy because they require power to be converted into thermal energy and it also takes some time to heat up and cool down the heater. Microwave irradiation is an alternative strategy in the thermal reduction process because it consumes less energy and time. Microwaves can produce heat uniformly and rapidly. Furthermore, microwave irradiation is a facile and fast heating method. In our previous studies, a commercial microwave was used in the reduction of GO to produce reduced graphene oxide (rGO) powder [18,19]. The rGO powder that was obtained using this technique had an electrical conductivity of 1180 S m-1 after 20 minutes of heating process [18]. The microwave method was also used for reduction of GO under nitrogen atmosphere, improving the electrical conductivity value up to 1810 S m-1 after 3 minutes of radiation [19]. However, using microwaves for thermal reduction in the fabrication of rGO based TCF has not been reported yet. In this work, a commercial microwave was used in thermal reduction to fabricate rGO film. Variation of time was done to study the effect of microwave irradiation on film quality. Scanning Electron Microscopy (SEM) imagery showed rGO flakes deposited on the substrate. Further analysis of the X-ray Photoemission Spectroscopy (XPS) spectra showed the alteration of the bonding structure of the sample caused by microwave irradiation. The electrical properties indicate a decrease in sheet resistance after irradiation. It can be concluded that microwave irradiation is a facile, cheap and effective reduction method for the fabrication of rGO based TCF. This study offers a different outlook on the fabrication of rGO based TCF by making use of a commercial microwave for thermal reduction process.
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2. Experimental Setup Synthesis of graphite oxide was conducted using a modified Marcano method developed by our group [18]. The resulted graphite oxide was dispersed into 150 ml DI water and then exfoliated using ultraturrax and sonication for 2 hours to produce GO. Subsequently, the mixture was centrifuged at 4000 rpm for 5 min to separate the precipitate from the supernatant. The supernatant was a homogeneous colloidal suspension, which was then used for the deposition of graphene thin films. In an ice bath, 1 ml of prepared supernatant was cooled until the temperature reached 4 °C. 8% hydrazine with a volume of 200 µl was then added to the mixture, followed by stirring for 15 minutes. A glass slide substrate was prepared in a piranha solution for cleaning and increasing the hydrophilicity of the glass. The GO-hydrazine mixture was subsequently dropped onto the substrate with a deposition volume of 50 µl / cm2 and a deposition temperature of 60 °C. The substrate where the film grows was isolated using a funnel cup to control the evaporation process and prevent contamination from outside air. Microwave irradiation treatment, as thermal reduction, was performed to improve film quality. Three samples were prepared with different microwave irradiation treatment times: (a) 0 minute (without microwave irradiation), (b) 30 minutes, and (c) 60 minutes. The rGO film was characterized using the following instrumental methods. The electrical properties were measured by determining the sheet resistance of the film using a two-probe technique. An insulating current of 10-100 kA (Advantest R6240A) was injected and the resulted voltage was measured (Keithley 2100). The optical properties were characterized using a UV-Vis spectrometer (HR2000 UV-NIR Series High-Resolution Fiber Optic Spectrometer) by observing the transmittance at wavelength 550 nm. The morphology of the
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resulted film was observed using a SEM (Hitachi SU3500) operated at 15 kV and 5 kV. Analysis of the surface chemical composition was carried out using XPS at the Surface Science Facility of the REIXS beamline at the Canadian Light Source (CLS). An Omicron monochromatized Al-K source and a Sphera EA125 hemispherical electron energy analyzer were used and the overall energy resolution was set to 0.35 eV.
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3. Result and Discussion The morphology of the rGO based TCF was analyzed using SEM imagery. Figure 1(a) shows that the rGO sheets covered the substrate homogeneously. This is evidence that the rGO films were deposited on the glass slide substrate. The oxygen functional groups in GO play a role in making bonds between the rGO flakes on the substrate. Figure 1(b) shows a cross sectional SEM image of the sample. It can be seen that the rGO with 95 nm thickness was deposited well on top of the glass slide substrate. XPS measurements were performed to determine the elemental composition and chemical bonding of the samples. Survey scans were collected to get an overview of the chemical composition of the samples, as shown in figure 2(a). There were two dominant peaks that correspond to the presence of carbon (C 1s) at a binding energy of 284.5 eV and oxygen (O 1s) at 533.36 eV, which were the main constituents of the sample. The functional groups containing carbon and oxygen are analyzed below. Nitrogen (N 1s) was also detected but at very low intensity. It likely exists due to a byproduct of the reduction process from the chemical hydrazine in the form of N2 and cis-diazene (cis-H2H2) [19]. Detailed scans of the C 1s and O 1s photoemission spectra were made to study the C and O bonding in the rGO based TCF sample. Figure 2(b) shows the C 1s photoemission spectra. The C 1s spectra from the unirradiated sample showed a dominant peak around binding energy 284.4 eV and a shoulder peak around binding energy 286.6 eV. Meanwhile, the samples with microwave irradiation treatment showed a diminished shoulder peak around 286.6 eV, indicating the removal of some constituents from the samples. These peaks agree well with the O1s spectra, as shown in figure 2(c). After microwave irradiation treatment, the
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O1s peak was reduced significantly. These results suggest that the microwave treatment increased the percentage of C atoms in the sample and reduced the oxygen bonds. Further analysis was done by deconvoluting the C1s spectrum into five peaks using Casa XPS. As can be seen in figure 2(b), which shows a breakdown of the C 1s spectrum of the sample, two major peaks are visible at ~284.5 eV and ~286 eV, accompanied by three minor peaks at ~284.9 eV, 287.6 eV and ~288.4 eV. The major peak at binding energy ~284.5 eV corresponds to C-C bonds and the one at ~286 eV corresponds to C-O bonds. Meanwhile, the minor peak at ~284.9 eV is related to C-N bonds and the one at ~287.6 eV and ~288.4 eV corresponds to C=O and C(O)O bonds [20,21]. The peaks at 286 eV correspond to the hydroxyl group and those at 287.6 eV and 288.4 eV are related to the carbonyl group in the basal plane and carboxylic acid on the edges of the graphene sheet [22,23]. The chemical composition of the sample was analyzed based on the normalized area from the deconvoluted spectra with respect to the non-irradiated sample as shown in table 1. The effect of microwave irradiation was deduced from the change in the amount of organic bonding in the sample by defining the non-radiated sample as normalizer. Based on table 1, after microwave irradiation, the intensity of the organic bonding changed. In the sample prepared with 30 minutes microwave irradiation, the C-C bonds increased by 13%, while the C-N, C-O and C=O bond ratio decreased. This result indicates restoration of C-C bonds and oxygen group release caused by the microwave treatment. After microwave treatment, the C-N ratio also decreased, which indicates elimination of N bonds from the sample [24]. The same goes for the sample with 60 minutes of microwave treatment. This result indicates a tendency of increased C-C bonding and decreased oxygen groups as well as C-N bonding at longer microwave irradiation time.
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Peak deconvolution of the O 1s XPS spectra was also conducted, as shown in figure 2(c), by using Casa XPS and defining two deconvoluted spectra. The peak emerging at binding energy 531.7 eV corresponds to C=O bonding in the carbonyl and carboxyl groups [25] and the C-O peak at binding energy 532.8 eV is related to epoxy and hydroxyl [26]. C-O bonds are the major constituent of the O 1s spectra. The oxygen functional groups should be eliminated from the sample in the reduction process, but there were still some residual oxygen groups left. For the sample irradiated 30 and 60 minutes, the intensity of the C=O bond decreased to 0.70 and 0.65, respectively, and the intensity of the C-O bond decreased to 0.71 and 0.70, respectively. These results tell us that the oxygen functional groups decreased significantly after the microwave irradiation. Microwave heating was performed to increase the quality of the rGO thin film. The sheet resistance was observed to indicate the electrical properties of the rGO based TCF. The graph in figure 3(a) shows the relationship between microwave irradiation time and sheet resistance. In the unirradiated sample, the sheet resistance obtained was 1430 kΩ/sq. This result is governed by the reduction process through reaction self-assembly, which uses a chemical reducing agent. However, this value is not sufficient to be applied as TCF material. After microwave irradiation, the sheet resistance decreased significantly. The sample irradiated for 30 minutes had a sheet resistance of 21.97 kΩ/sq. This treatment reveals a correlation between microwave irradiation and the electrical properties. The energy released by the microwaves causes oxygen to be released from the sample and carbon to be restored. C-C bonding allows electrons to move freely in the sample, which causes the sample to be more conductive. In other words, microwave irradiation causes further reduction of the sample. This result is in accordance with information gathered from XPS deconvolution in figure 2(b) and 2(c), which
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show C1s increased and O1s decreased after the microwave irradiation treatment. This study showed that microwave irradiation can reduce rGO powder as well as rGO film. Furthermore, in the sample irradiated for 60 minutes the sheet resistance produced was 7.84 kΩ/sq. This result was better than for the sample treated with microwave irradiation for 30 minutes. Based on this tendency, it seems that the electrical properties are better for longer irradiation times. At this point we conclude that microwave irradiation is an alternative method for fabrication of rGO based TCF. Information on the transparency of the produced sample is needed in order to be able to apply the sample as a transparent electrode. UV-Vis measurement was done to analyze the transparency, as shown in figure 3(b). The unirradiated sample had 28.32% transmittance at 550 nm. After microwave treatment for 30 minutes, the transmittance value increased to 30.37%. The reduction process increased the charge carrier concentration and mobility in the sample and improved the reflection of light, which made the rGO film more transparent [15]. However, in the sample treated with microwave irradiation for 60 minutes, the resulted transmittance value was 27.63%. A longer irradiation time produces lower sheet resistance but the optimum time to produce the best transparency was 30 minutes. This can be seen from the sample irradiated for 60 minutes, where the transmittance decreased again along with the decrease of sheet resistance.
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Conclusion In this paper, an alternative way of fabricating rGO based TCF was reported, using microwave irradiation to reduce the oxygen functional groups of GO. The effects of irradiation on the electrical and optical properties of the rGO based TCF were investigated. Compared to the unirradiated sample, the samples that were treated with microwave irradiation were found to have better electrical and optical properties. This result was confirmed by XPS measurement, which exhibited the decrease of the oxygen peaks in the irradiated samples. SEM images showed that the rGO was deposited evenly on the substrate. This study enriches alternative synthesis routes for fabricating rGO based TCF using microwave irradiation.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors would like to thank RISTEKDIKTI for their research funding for the year 2018. Part of the research described in this paper was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan.
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Figure 1. Surface (a) and cross section (b) SEM images of deposited rGO based TCF.
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Figure 2. (a) XPS wide scan of deposited rGO based TCF (data are offset for clarity), (b) C 1s XPS spectra and the deconvoluted spectra, and (c) O 1s XPS spectra and the deconvoluted spectra.
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Figure 3. (a) Graph of sheet resistance versus microwave irradiation time, and (b) transmittance spectra of rGO based TCF.
Table 1. Normalized organic functional groups extracted from the C 1s and O 1s XPS spectra.
20
Facile Deposition of Reduced Graphene Oxide-Based Transparent Conductive Film with Microwave Assisted Method Akfiny Hasdi Aimon , Rahmat Hidayat , Dina Rahmawati , Ronny Sutarto , Fitri Aulia Permatasari , Ferry Iskandar PII: DOI: Reference:
S0040-6090(19)30646-7 https://doi.org/10.1016/j.tsf.2019.137618 TSF 137618
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
20 December 2018 3 October 2019 4 October 2019
Please cite this article as: Akfiny Hasdi Aimon , Rahmat Hidayat , Dina Rahmawati , Ronny Sutarto , Fitri Aulia Permatasari , Ferry Iskandar , Facile Deposition of Reduced Graphene Oxide-Based Transparent Conductive Film with Microwave Assisted Method, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137618
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Highlight:
The reduced graphene oxide (rGO) films were deposited evenly on the substrate.
Microwave assisted method was used as thermal reduction to fabricate rGO film.
C-C bonding increased and C-O and C=O decreased in the radiated sample.
The electrical and optical properties of the rGO films were investigated.
1
Facile Deposition of Reduced Graphene Oxide-Based Transparent Conductive Film with Microwave Assisted Method Akfiny Hasdi Aimon1*, Rahmat Hidayat1, Dina Rahmawati1, Ronny Sutarto3, Fitri Aulia Permatasari1, and Ferry Iskandar1,2 1
Energy and Environmental Laboratory, Department of Physics, Institut Teknologi Bandung,
Jl. Ganesha 10 Bandung 40132, Indonesia 2
Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Jl.
Ganesha 10 Bandung 40132, Indonesia 3
Canadian Light Source, 44 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2V3,
Canada *
[email protected]
2
Abstract An alternative way to fabricate reduced graphene oxide-based transparent conductive films (rGO based TCF) using microwave assisted irradiation is introduced. This modified method offers lower cost, more efficiency and more convenience in thin-film fabrication. A high mode of conventional microwave irradiation was used in this research, with time variation at 30 and 60 minutes. For comparison, an unirradiated sample was prepared to see the direct effect of microwave radiation on the properties of the samples. The electrical and optical properties of the samples were characterized by sheet resistance measurement and UV-Vis spectrometry. Scanning electron microscopy was used to observe the morphology of the rGO thin film on the substrate. Microwave radiation at 30 minutes gave the best combination for electrical and optical properties, with the sheet resistance of 21.97 kΩ/sq and 30.37% transmittance, respectively. X-ray photoemission spectroscopy results confirmed the increased of C-C bonding and reduced C-O and C=O bonding in the sample subjected to 30 min of microwave radiation. This study provides a different perspective on the fabrication of rGO based TCF. Keywords: Transparent conductive film; Reduced graphene oxide; Microwave irradiation; thermal
reduction.
3
1. Introduction Transparent conductive films (TCF) with excellent optical transmittance and high electrical conductivity are essential in optoelectronic technology and devices. Up until now, the most widely used TCF are based on indium tin oxide and fluorine tin oxide. However, the use of this metal oxide for TCF faces obstacles because of the scarcity of the raw material on earth. Several attempts have been made to find a replacement material for TCF, such as metallic nanowire, conductive polymers, and carbon-based materials [1–3]. Graphene is a carbon-based material that has been widely developed. This allotrof of carbon materials consists of a two-dimensional hexagonal carbon lattice with sp2 bonds and has excellent mechanical, electrical, thermal, and optical properties [4,5]. Application of graphene is found in optoelectronics, transistors, biosensors, energy storage devices, display media, and electrochemical systems [6–11]. Majee et al. has developed pristine graphene for flexible TCF [12]. However, the most effective method for large-scale and low-cost preparation of graphene is the reduction of graphene oxide (GO) by wet chemical process [13,14]. GO has hydrophilic characteristics, which allows it to be dissolved in demineralized (DI) water. This permits deposition onto various substrates to form a thin film due to its stable, superhydrophilic properties, and facile fabrication of TCF. The reduction process is necessary to remove oxygen containing groups that decorate the GO. Although reduction of GO does not produce pure graphene, it does yield properties close to pristine graphene. The most well-known reduction techniques are chemical reduction and thermal reduction. Chemical reduction uses a chemical agent, such as borohydride (NaBH4) or hydrazine, to cut the oxygen bonds in the GO basal plane [15]. Thermal reduction uses heat energy to eliminate the oxygen groups and restore the sp2 bonds. Widely used thermal
4
reduction methods are based on joule heating and furnace annealing [16,17]. However, both methods consume a large amount of energy because they require power to be converted into thermal energy and it also takes some time to heat up and cool down the heater. Microwave irradiation is an alternative strategy in the thermal reduction process because it consumes less energy and time. Microwaves can produce heat uniformly and rapidly. Furthermore, microwave irradiation is a facile and fast heating method. In our previous studies, a commercial microwave was used in the reduction of GO to produce reduced graphene oxide (rGO) powder [18,19]. The rGO powder that was obtained using this technique had an electrical conductivity of 1180 S m-1 after 20 minutes of heating process [18]. The microwave method was also used for reduction of GO under nitrogen atmosphere, improving the electrical conductivity value up to 1810 S m-1 after 3 minutes of radiation [19]. However, using microwaves for thermal reduction in the fabrication of rGO based TCF has not been reported yet. In this work, a commercial microwave was used in thermal reduction to fabricate rGO film. Variation of time was done to study the effect of microwave irradiation on film quality. Scanning Electron Microscopy (SEM) imagery showed rGO flakes deposited on the substrate. Further analysis of the X-ray Photoemission Spectroscopy (XPS) spectra showed the alteration of the bonding structure of the sample caused by microwave irradiation. The electrical properties indicate a decrease in sheet resistance after irradiation. It can be concluded that microwave irradiation is a facile, cheap and effective reduction method for the fabrication of rGO based TCF. This study offers a different outlook on the fabrication of rGO based TCF by making use of a commercial microwave for thermal reduction process.
5
2. Experimental Setup Synthesis of graphite oxide was conducted using a modified Marcano method developed by our group [18]. The resulted graphite oxide was dispersed into 150 ml DI water and then exfoliated using ultraturrax and sonication for 2 hours to produce GO. Subsequently, the mixture was centrifuged at 4000 rpm for 5 min to separate the precipitate from the supernatant. The supernatant was a homogeneous colloidal suspension, which was then used for the deposition of graphene thin films. In an ice bath, 1 ml of prepared supernatant was cooled until the temperature reached 4 °C. 8% hydrazine with a volume of 200 µl was then added to the mixture, followed by stirring for 15 minutes. A glass slide substrate was prepared in a piranha solution for cleaning and increasing the hydrophilicity of the glass. The GO-hydrazine mixture was subsequently dropped onto the substrate with a deposition volume of 50 µl / cm2 and a deposition temperature of 60 °C. The substrate where the film grows was isolated using a funnel cup to control the evaporation process and prevent contamination from outside air. Microwave irradiation treatment, as thermal reduction, was performed to improve film quality. Three samples were prepared with different microwave irradiation treatment times: (a) 0 minute (without microwave irradiation), (b) 30 minutes, and (c) 60 minutes. The rGO film was characterized using the following instrumental methods. The electrical properties were measured by determining the sheet resistance of the film using a two-probe technique. An insulating current of 10-100 kA (Advantest R6240A) was injected and the resulted voltage was measured (Keithley 2100). The optical properties were characterized using a UV-Vis spectrometer (HR2000 UV-NIR Series High-Resolution Fiber Optic Spectrometer) by observing the transmittance at wavelength 550 nm. The morphology of the
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resulted film was observed using a SEM (Hitachi SU3500) operated at 15 kV and 5 kV. Analysis of the surface chemical composition was carried out using XPS at the Surface Science Facility of the REIXS beamline at the Canadian Light Source (CLS). An Omicron monochromatized Al-K source and a Sphera EA125 hemispherical electron energy analyzer were used and the overall energy resolution was set to 0.35 eV.
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3. Result and Discussion The morphology of the rGO based TCF was analyzed using SEM imagery. Figure 1(a) shows that the rGO sheets covered the substrate homogeneously. This is evidence that the rGO films were deposited on the glass slide substrate. The oxygen functional groups in GO play a role in making bonds between the rGO flakes on the substrate. Figure 1(b) shows a cross sectional SEM image of the sample. It can be seen that the rGO with 95 nm thickness was deposited well on top of the glass slide substrate. XPS measurements were performed to determine the elemental composition and chemical bonding of the samples. Survey scans were collected to get an overview of the chemical composition of the samples, as shown in figure 2(a). There were two dominant peaks that correspond to the presence of carbon (C 1s) at a binding energy of 284.5 eV and oxygen (O 1s) at 533.36 eV, which were the main constituents of the sample. The functional groups containing carbon and oxygen are analyzed below. Nitrogen (N 1s) was also detected but at very low intensity. It likely exists due to a byproduct of the reduction process from the chemical hydrazine in the form of N2 and cis-diazene (cis-H2H2) [19]. Detailed scans of the C 1s and O 1s photoemission spectra were made to study the C and O bonding in the rGO based TCF sample. Figure 2(b) shows the C 1s photoemission spectra. The C 1s spectra from the unirradiated sample showed a dominant peak around binding energy 284.4 eV and a shoulder peak around binding energy 286.6 eV. Meanwhile, the samples with microwave irradiation treatment showed a diminished shoulder peak around 286.6 eV, indicating the removal of some constituents from the samples. These peaks agree well with the O1s spectra, as shown in figure 2(c). After microwave irradiation treatment, the
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O1s peak was reduced significantly. These results suggest that the microwave treatment increased the percentage of C atoms in the sample and reduced the oxygen bonds. Further analysis was done by deconvoluting the C1s spectrum into five peaks using Casa XPS. As can be seen in figure 2(b), which shows a breakdown of the C 1s spectrum of the sample, two major peaks are visible at ~284.5 eV and ~286 eV, accompanied by three minor peaks at ~284.9 eV, 287.6 eV and ~288.4 eV. The major peak at binding energy ~284.5 eV corresponds to C-C bonds and the one at ~286 eV corresponds to C-O bonds. Meanwhile, the minor peak at ~284.9 eV is related to C-N bonds and the one at ~287.6 eV and ~288.4 eV corresponds to C=O and C(O)O bonds [20,21]. The peaks at 286 eV correspond to the hydroxyl group and those at 287.6 eV and 288.4 eV are related to the carbonyl group in the basal plane and carboxylic acid on the edges of the graphene sheet [22,23]. The chemical composition of the sample was analyzed based on the normalized area from the deconvoluted spectra with respect to the non-irradiated sample as shown in table 1. The effect of microwave irradiation was deduced from the change in the amount of organic bonding in the sample by defining the non-radiated sample as normalizer. Based on table 1, after microwave irradiation, the intensity of the organic bonding changed. In the sample prepared with 30 minutes microwave irradiation, the C-C bonds increased by 13%, while the C-N, C-O and C=O bond ratio decreased. This result indicates restoration of C-C bonds and oxygen group release caused by the microwave treatment. After microwave treatment, the C-N ratio also decreased, which indicates elimination of N bonds from the sample [24]. The same goes for the sample with 60 minutes of microwave treatment. This result indicates a tendency of increased C-C bonding and decreased oxygen groups as well as C-N bonding at longer microwave irradiation time.
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Peak deconvolution of the O 1s XPS spectra was also conducted, as shown in figure 2(c), by using Casa XPS and defining two deconvoluted spectra. The peak emerging at binding energy 531.7 eV corresponds to C=O bonding in the carbonyl and carboxyl groups [25] and the C-O peak at binding energy 532.8 eV is related to epoxy and hydroxyl [26]. C-O bonds are the major constituent of the O 1s spectra. The oxygen functional groups should be eliminated from the sample in the reduction process, but there were still some residual oxygen groups left. For the sample irradiated 30 and 60 minutes, the intensity of the C=O bond decreased to 0.70 and 0.65, respectively, and the intensity of the C-O bond decreased to 0.71 and 0.70, respectively. These results tell us that the oxygen functional groups decreased significantly after the microwave irradiation. Microwave heating was performed to increase the quality of the rGO thin film. The sheet resistance was observed to indicate the electrical properties of the rGO based TCF. The graph in figure 3(a) shows the relationship between microwave irradiation time and sheet resistance. In the unirradiated sample, the sheet resistance obtained was 1430 kΩ/sq. This result is governed by the reduction process through reaction self-assembly, which uses a chemical reducing agent. However, this value is not sufficient to be applied as TCF material. After microwave irradiation, the sheet resistance decreased significantly. The sample irradiated for 30 minutes had a sheet resistance of 21.97 kΩ/sq. This treatment reveals a correlation between microwave irradiation and the electrical properties. The energy released by the microwaves causes oxygen to be released from the sample and carbon to be restored. C-C bonding allows electrons to move freely in the sample, which causes the sample to be more conductive. In other words, microwave irradiation causes further reduction of the sample. This result is in accordance with information gathered from XPS deconvolution in figure 2(b) and 2(c), which
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show C1s increased and O1s decreased after the microwave irradiation treatment. This study showed that microwave irradiation can reduce rGO powder as well as rGO film. Furthermore, in the sample irradiated for 60 minutes the sheet resistance produced was 7.84 kΩ/sq. This result was better than for the sample treated with microwave irradiation for 30 minutes. Based on this tendency, it seems that the electrical properties are better for longer irradiation times. At this point we conclude that microwave irradiation is an alternative method for fabrication of rGO based TCF. Information on the transparency of the produced sample is needed in order to be able to apply the sample as a transparent electrode. UV-Vis measurement was done to analyze the transparency, as shown in figure 3(b). The unirradiated sample had 28.32% transmittance at 550 nm. After microwave treatment for 30 minutes, the transmittance value increased to 30.37%. The reduction process increased the charge carrier concentration and mobility in the sample and improved the reflection of light, which made the rGO film more transparent [15]. However, in the sample treated with microwave irradiation for 60 minutes, the resulted transmittance value was 27.63%. A longer irradiation time produces lower sheet resistance but the optimum time to produce the best transparency was 30 minutes. This can be seen from the sample irradiated for 60 minutes, where the transmittance decreased again along with the decrease of sheet resistance.
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Conclusion In this paper, an alternative way of fabricating rGO based TCF was reported, using microwave irradiation to reduce the oxygen functional groups of GO. The effects of irradiation on the electrical and optical properties of the rGO based TCF were investigated. Compared to the unirradiated sample, the samples that were treated with microwave irradiation were found to have better electrical and optical properties. This result was confirmed by XPS measurement, which exhibited the decrease of the oxygen peaks in the irradiated samples. SEM images showed that the rGO was deposited evenly on the substrate. This study enriches alternative synthesis routes for fabricating rGO based TCF using microwave irradiation.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors would like to thank RISTEKDIKTI for their research funding for the year 2018. Part of the research described in this paper was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan.
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Figure 1. Surface (a) and cross section (b) SEM images of deposited rGO based TCF.
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Figure 2. (a) XPS wide scan of deposited rGO based TCF (data are offset for clarity), (b) C 1s XPS spectra and the deconvoluted spectra, and (c) O 1s XPS spectra and the deconvoluted spectra.
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Figure 3. (a) Graph of sheet resistance versus microwave irradiation time, and (b) transmittance spectra of rGO based TCF.
Table 1. Normalized organic functional groups extracted from the C 1s and O 1s XPS spectra.
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