Effective management of heat transfer, such as conduction and radiation, through glass windows is one of the most challenging issues in smart window technology. In this work, reduced Graphene Oxide (rGO) thin films of varying thicknesses are fabricated onto Fluorine-doped Tin Oxide (FTO) glass via electrophoretic deposition technique.
Journal of Science: Advanced Materials and Devices (2019) 252e259 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Electrophoretic deposition of reduced graphene oxide thin films for reduction of cross-sectional heat diffusion in glass windows Loo Pin Yeo a, b, 1, Tam Duy Nguyen a, b, **, 1, Han Ling a, Ying Lee a, Daniel Mandler b, c, Shlomo Magdassi b, c, Alfred Iing Yoong Tok a, b, * a School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Singapore-HUJ Alliance for Research and Enterprise, NEW-CREATE Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore 138602 c Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel b a r t i c l e i n f o a b s t r a c t Article history: Received 15 February 2019 Received in revised form 29 March 2019 Accepted April 2019 Available online 26 April 2019 Effective management of heat transfer, such as conduction and radiation, through glass windows is one of the most challenging issues in smart window technology In this work, reduced Graphene Oxide (rGO) thin films of varying thicknesses are fabricated onto Fluorine-doped Tin Oxide (FTO) glass via electrophoretic deposition technique The sample thicknesses increase with increasing number of deposition cycles (5, 10, 20 cycles) It is hypothesized that such rGO thin films, which are well-known for their high thermal conductivities, can conduct heat away laterally towards heat sinks and reduce near-infrared (NIR) transmittance through them, thus effectively slowing down the temperature increment indoors The performance of rGO/FTO in reducing indoor temperatures is investigated with a solar simulator and a UV-Vis-NIR spectrophotometer The 20-cycles rGO thin films showed 30% more NIR blocked at 1000 nm as compared to clean FTO, as well as the least temperature increment of 0.57 C following 30 of solar irradiation Furthermore, the visible transmittance of the as-fabricated rGO films remain on par with commercial solar films, enabling up to 60% of visible light transmittance for optimal balance of transparency and heat reduction These results suggest that the rGO thin films have great potential in blocking heat transfer and are highly recommended for smart window applications © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Electrophoretic deposition Reduced graphene oxide Thin films Heat conduction Smart windows Introduction Global warming and rapid fossil fuel depletion are major issues that have continued to intensify over the years, yet remain without a clear resolution Governments have been prompted to source for renewable energy alternatives as well as methods to reduce their energy consumption The building industry, in particular, consumes a large percentage of energy each year, with room heating and cooling making up at least 32% of a building's total energy consumption [1,2] Modern building technology, in particular, is * Corresponding author School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 ** Corresponding author School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 E-mail addresses: tamnguyen@ntu.edu.sg (T.D Nguyen), miytok@ntu.edu.sg (A.I.Y Tok) Peer review under responsibility of Vietnam National University, Hanoi These authors contributed equally to this work typically assembled with many large scale window panels, thus causing the effect of heat and light transfer through windows to become increasingly significant Therefore, smart window technologies, which can modulate the transmittance of heat and light, are widely researched on due to their potential energy savings in lightings, heaters and air-conditioners Amongst smart window technologies, electrochromic devices, which are able to electrically modulate the transmittance of solar radiation, are one of the most widely investigated [3] However, heat can also be transferred in or out of the room through the glass window due to the conduction process, i.e by a temperature gradient The thermal conductivity of glass (with a typical thickness of mm) is approximately 0.9 W/mK [4] Depending on the temperature difference between the indoor and outdoor ambience, the direction of heat transfer will cause an increase or decrease in the indoor room temperature However, the management of heat conduction through the glass window has not been as widely investigated https://doi.org/10.1016/j.jsamd.2019.04.002 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) L.P Yeo et al / Journal of Science: Advanced Materials and Devices (2019) 252e259 An effective method for the management of heat conduction can be achieved by coating a superior thermally-conductive layer on the glass surface This layer, which works similarly to a double or triple-glazed window, can minimize heat diffusion through the glass [5,6] However, this approach can significantly reduce the weight and fabrication cost of the window pane The enhanced inplane thermal conductivities of such thin films enable their application in heat transfer management, in which the heat can be conducted away and collected at the edge of the window panels (Figure S1) Currently, carbon-based structures such as graphene, reduced graphene oxide (rGO) and carbon nanotubes (CNT) have been widely reported for their extremely high thermal conductivities Balandin et al reported a thermal conductivity as high as 4.84e5.30 kW/mK for a suspended single-layer graphene [7], while Seol et al reported 600 W/mK in supported graphene [8] Similarly, single-walled and multi-walled CNTs were reported to have high thermal conductivities of 3500 W/mK between 300 and 800 K and 3000 W/mK at room temperature respectively [9] As such, these carbon-based materials have huge potentials as thermal interface materials for heat removal, cooling systems for the electronic industry [10,11], anti-fogging devices and in heatable smart windows etc [12] This also gives rise to the possibility of carbon-based windows in cool or warm climates which can manage indoor temperatures by conducting heat inside or out respectively Current commercial solar films (e.g., V-Kool, Infratint, M) can block UV radiation, reflect or absorb infrared (IR) radiation and reduce glare with lower visible transmittance However, these films focus only on blocking wavelengths in the solar spectrum but neglect the consideration of the warming effect of conducted heat Carbon-based smart windows, on the other hand, allow management of both solar radiation and heat conduction, and thus could provide insights into novel applications in the field of smart windows Despite the high thermal conductivity of pristine graphene, its cost of fabrication through techniques such as epitaxial growth, chemical vapor deposition (CVD) and exfoliation is very high due to limited yield or expensive substrates and is thus not an ideal material when considering scale-up productions [13,14] Besides, graphene and CNTs also possess low visible transparency and has issues with homogenous, large-area surface grafting As such, rGO is considered the next best alternative for fabricating window coatings due to competitive thermal conductivities as high as 1043.5 W/mK [15] and 1390 W/mK [16] as previously reported Furthermore, rGO also has the advantages of ease of fabrication, mass producibility, strong infrared absorption and high transparency [17] In this study, rGO films of varying thicknesses were electrophoretically deposited onto FTO glass substrate and their effect on blocking heat entry were analyzed with a solar simulator The sample thicknesses increase with increasing number of deposition cycles (5, 10, 20 cycles) It is hypothesized that thermallyconductive rGO can conduct heat away and reduce heat transfer through it, thus effectively slowing down the temperature increment indoors Electrophoretic deposition (EPD) was selected for its potential to deposit homogenous coatings, control coating thickness and low cost [18] As-fabricated rGO thin films can be a promising coating material to manage the heat conduction through glass windows Materials and experimental methods The chemicals mentioned in this paper are obtained from SigmaeAldrich unless otherwise stated Fluorine-doped tin oxide (FTO) glass substrates are also obtained from Sigma Aldrich (Model: TEC-7, Pilkington™) 253 2.1 Fabrication of graphene oxide via modified Hummer's method Graphene Oxide (GO) was obtained from pure graphite flakes via modified Hummer's method [19,20] The fabrication procedure is illustrated in Fig Graphite powder (1 g) was added into 98% H2SO4 (40 ml) under continuous stirring Subsequently, KMnO4 (6 g) was gradually added into the mixture The oxidation of graphite to graphite oxide in this step is highly exothermic, thus small portions of KMnO4 was added in 5e10 interval Deionized (DI) water (50 ml) was slowly added to minimize heat generation and the mixture was stirred for h 30% H2O2 (10 ml) was then added to the mixture and further stirred for 10 to remove excess KMnO4 Next, the resultant mixture was centrifuged (Thermo Scientific Sorvall Legend X1R) at 8000 rpm for 10 The residue was washed with 6% HCl and DI water before being centrifuged again The washing step was repeated for at least times The residue was then mixed with DI water (200 ml) Lastly, the graphite oxide was exfoliated with a probe sonicator (SONICS Vibra-Cell) at 70% amplitude for h to obtain a homogenous GO suspension 2.2 Reduction of GO via electrophoretic deposition The GO suspension was first diluted to mg/ml using Phosphate-buffered Saline (PBS) solution and subsequently, its pH was adjusted to 10 with NaOH The electrophoretic deposition (EPD) procedure of GO was carried out in a three-electrode set-up consisting of clean FTO as the working electrode, platinum sheet as the counter electrode and Ag/AgCl as the reference electrode (Figure S2) [21] Before EPD, N2 gas was bubbled into the GO suspension under continuous stirring throughout the EPD procedure, starting from 30 before the actual deposition A potential range of ỵ0.6 to 1.5 V was used to deposit reduced graphene oxide (rGO) onto the FTO substrate to obtain samples of 5, 10 and 20 cycles 2.3 Characterization Surface morphology of the deposited rGO thin films was analyzed with a Field Emission Scanning Electron Microscope (FESEM, JEOL JSM-7600F) and Atomic Force Microscopy (AFM, Park Systems NX10) Thickness of the film samples were measured with the surface profiler (Alpha-Step IQ) X-ray Diffractometer (XRD, Panalytical X'Pert Pro), equipped with Cu-Ka radiation, was carried out to analyze the crystallographic structure of the samples Fourier Transform Infrared Spectroscopy (FTIR, PerkinElmer Frontier) was carried out to observe the changes in molecular bonding following the reduction of GO The surface chemistry of as-fabricated rGO thin films was characterized by X-ray Photoelectron Spectrometer (XPS, Kratos AXIS Supra) The UV-Vis-NIR Spectroscopy (Agilent Cary 5000) was carried out to measure the visible light and NIR transmittance across the rGO/FTO samples The performance of the different rGO films in blocking heat was analyzed with a solar simulator (Class 150 W XES-40S2-CE) equipped with a xenon lamp The set-up prepared for the solar simulation is depicted in Fig The set-up shown was enclosed in an opaque acrylic box to prevent entry of external lighting With an irradiance of 1000 W/m2, the equipment simulates the sun, allowing the effect of rGO on blocking heat entry into the box to be analyzed Thermocouples were attached in the box to measure the increase in temperature in the internal environment during solar irradiation The samples were irradiated for 30 and the internal temperature of the box was recorded at every minute The initial internal temperature of the box was maintained between 24.75 and 24.79 C at the start of each analysis Irradiance of air and plain FTO glass substrate were also measured as references 254 L.P Yeo et al / Journal of Science: Advanced Materials and Devices (2019) 252e259 Fig Illustration of the GO fabrication procedure by modified Hummer's Method Fig Illustration of the solar simulation experimental set-up Results and discussion 3.1 Morphology, crystal and chemical structures of electrophoretically-deposited rGO thin films Fig a-c present the FESEM images of electrophoreticallydeposited rGO film (5, 10, 20-cycles) on FTO glass substrate The formation of rGO thin films after the electrophoretic deposition process can be clearly observed The deposited rGO films are relatively large, in the range of several micrometres They consist of multiple, thin layers of overlapping rGO due to the layer-bylayer alignment typical of EPD Irregular folds can be observed on the films which become more prominent as the number of deposition cycles increases The thicknesses of the rGO films were also measured with a surface profiler and the average thickness is 0.374 mm, 0.578 mm and 1.759 mm for 5, 10 and 20 cycles rGO films respectively The view of the rGO thin films with varying electrophoretically deposited cycles is included in Figure S3 The cyclic-voltammetry (CV) curves recorded during the electrophoretic deposition and reduction of GO are shown in Figure S4 A large reduction peak was recorded for 5-cycles reduction at À1.2 V, with a starting potential of e 0.85 V; 10-cycles reduction at À1.32 V, with a starting potential of À0.9 V; 20-cycles reduction at À1.06 V, with a starting potential of À0.4 V The large reduction peaks observed are due to the removal of oxygen functional groups in GO to form rGO [22] Fig FESEM images of 5-cycles (a), 10-cycles (b), and 20-cycles (c) rGO thin films deposited on FTO glass observed at 10 K magnification L.P Yeo et al / Journal of Science: Advanced Materials and Devices (2019) 252e259 Fig a-c present the AFM images of rGO thin films of different thicknesses The average surface roughness of 5, 10 and 20-cycles rGO are estimated to be about 33.3, 39.8 and 58.3 nm, respectively, as presented in Fig 4d As expected, there is an increase in the film surface roughness when an increasing amount of rGO was deposited Despite the presence of folds observed in the FESEM images, the surface roughness remains relatively low This suggests that the current electrophoretic deposition and reduction method is capable of producing relatively smooth films regardless of the deposited thickness A study by Chen et al reported that cluttering in the arrangement of graphene atoms could reduce the speed of thermal phonon propagation and thus decrease the thermal conductivity of graphene [23] As such, a smooth rGO film is highly desirable for quick heat transfer across the film Fig presents the FTIR spectra of rGO and drop-casted graphene oxide (DC-GO) films to confirm the successful reduction of GO to rGO with the electrophoretic deposition method The procedure for the fabrication of DC-GO is illustrated in Figure S4 From the DC-GO spectra, it can be observed that GO was successfully oxidized from graphite due to the presence of oxygen-containing functional groups [24] The GO spectrum consists of a broad peak centered at 3435 cmÀ1 which originates from the stretching mode of OeH group, while the peak at 1642 cmÀ1 is associated with aromatic C]C ring stretching, the broad peak at 1186-1458 cmÀ1 and the peak at 1085 cmÀ1 is related to stretching of epoxy CeO groups [25e27] Following reduction into rGO, it can be observed that peak intensities associated with the alkoxy and hydroxyl groups are significantly reduced or have disappeared, suggesting successful reduction of GO Fig presents the XRD patterns of the rGO films As the films were deposited onto an FTO glass substrate, characteristic peaks of FTO were observed at 2q ¼ 26.6 , 33.9 , 37.9 , 51.7, 61.8 and 66.0 255 Fig FTIR spectra of RGO and drop-casted graphene oxide (DC-GO) which corresponds to the (110), (101), (200), (211), (310) and (301) planes of SnO2 (ICDD 01-070-4176), respectively The XRD pattern of drop-cast graphene oxide (DC-GO) obtained from modified Hummer's method produced a peak at 2q ¼ 9.3 with an average dspacing of 0.95 nm, which corresponds to the (001) plane [28e30] Following reduction of GO, the GO peak disappears, indicating the successful removal of oxygen-containing functional groups Although there is an overlap in peaks, the diffraction peak characteristic of rGO is also located at 2q ¼ 26.6 in the rGO films, which Fig AFM images of 5-cycles (a), 10-cycles (b), and 20-cycles (c) rGO thin films and (d) their corresponding surface roughness 256 L.P Yeo et al / Journal of Science: Advanced Materials and Devices (2019) 252e259 Fig XRD patterns of rGO thin films fabricated with different deposition cycles (5cycles, 10-cycles, 20-cycles) corresponds to the (002) plane [29] The d-spacing of rGO was observed to be approximately 0.34 nm, with negligible differences amongst the 5, 10 and 20-cycle samples 3.2 Surface chemistry and UV-Vis eNIR absorption of rGO thin films Fig shows the XPS spectra of as-fabricated rGO thin films The wide scan spectra (Fig 7a) of all samples indicate the presence of C1s and O1s, which again confirms the formation of rGO thin films The Sn3d peaks are present due to the FTO substrate, with the Sn3d5/2 and Sn3d3/2 components located at approximately 487 and 496 eV, respectively There are small energy shifts observed as compared to typical SnO2 XPS spectra (where Sn3d5/2 and Sn3d3/2 components are located at 485 and 494 eV, respectively) due to fluorine doping in FTO glass substrate The fine XPS spectra of C1s (b), O1s (c) and Sn3d (d) of 5-cycles rGO thin film are presented in Fig b-d By curve-fitting analysis (Gauss*Lorentz Algorithm), the C1s core-level spectrum of 5-cycles rGO thin films was deconvoluted to obtain main peaks: CeC (~284.4 eV), CeO (~285.8 eV), and C]O (~288 eV) The CeC peak is attributed to carbon with sp2 and sp3 hybridization, with some shoulders at higher binding energies due to the oxygen linkages (CeO, C]O) The O1s core-level spectrum was deconvoluted into three peaks: C]O (~531.1 eV), CeO (~533.7 eV), C(O)OH (~535.7 eV) [31] The fine XPS spectra of 10 and 20-cycles rGO thin films are also presented in Figure S5 The detailed elemental composition of the various rGO thin films is presented in Table In general, the atomic ratio of C/O is about 7:3 for all three rGO samples Interestingly, in the C1s deconvolution, while the CeC and CeO components remain unchanged, the C]O component mostly decreases with increasing number of deposition cycles For O1s deconvolution, the C]O component and carboxylic groups reduce while the CeO component increases with increasing number of deposition cycles This may imply the presence of less eCOOH and eCOH functional groups in the electrophoreticallydeposited rGO thin films with increasing film thickness This is also in agreement with the observation from the FTIR and XRD analyses Fig presents the UV-Vis-NIR spectra of the rGO films with different number of deposition cycles (5, 10, 20-cycles) within the range of 300e1600 nm [32] Wavelengths above 1600 nm were not included as the FTO glass substrate itself allows less than 30% infrared transmittance above 1600 nm [33] The effect of increasing rGO thickness on NIR transmittance thus cannot be observed clearly in that wavelength range and was subsequently Fig Wide scan XPS spectra (a) of various rGO thin films The fine XPS spectra of C1s (b), O1s (c) and Sn3d (d) of 5-cycles rGO thin films L.P Yeo et al / Journal of Science: Advanced Materials and Devices (2019) 252e259 257 Table Elemental composition of rGO thin films determined by XPS Sample 5-cycles rGO 10-cycles rGO 20-cycles rGO Elemental composition (at.%) C1s deconvolution (at.%) C O CeC CeO C¼O O1s deconvolution (at.%) C¼O CeO C(O)OH 70.6 70.2 73.4 29.4 29.8 26.6 55.6 55.3 60.4 31.9 26.5 34.8 12.5 18.2 4.8 42.2 31.3 25.1 34.4 51.5 73.2 23.4 17.2 1.7 further increasing rGO deposition cycles to reduce the NIR blockage indefinitely Nonetheless, the visible transmittance of the as-fabricated rGO films remains on par with commercial solar films [34,35], enabling up to 60% of visible light transmittance for optimal balance of transparency and heat reduction 3.3 Reduction of heat transfer through glass using rGO thin films Fig UV-Vis-NIR transmittance spectra of rGO films (5-cycles, 10-cycles, 20-cycles) over the wavelength range of 300e1600 nm omitted With an increasing thickness of rGO films deposited, the transmittance of Vis and NIR wavelengths sees a general decreasing trend The NIR transmittance of clean FTO, 5, 10 and 20cycles rGO thin films is in the range of 16.75e80.14%, 22.52e60.07% 16.47e45.19% respectively By increasing from to 10 deposition cycles, there is approximately 12.63% more NIR blocked at 1000 nm Increasing from 10 to 20 cycles shows negligible difference in NIR blockage However, a 20-cycle rGO thin film still enables 30% more NIR blocked at 1000 nm as compared to clean FTO In the visible range, there is a consistent reduction in transmittance as the number of rGO film thickness increases The visible transmittance decreases from 50.61 e 60.07% to 36.30e45.19% to 24.27e40.40% when the number of cycles increased from to 20 cycles In order for the rGO thin films to be used in windows, the visible transparency has to be sufficiently high Hence, these results suggest that there are limitations in Fig presents the results from the solar simulation test As previously mentioned, the rGO samples (5, 10, 20-cycles) were irradiated with artificial light for a period of 30 and the internal temperature of the box was recorded every minute Air and plain FTO glass substrate were also irradiated under the same parameters as references As shown in Fig 9a, following 30 irradiation, the FTO samples coated with rGO films showed an increase in temperature of only 0.76 C, 0.68 C and 0.57 C for 5, 10 and 20-cycles samples, respectively On the other hand, irradiating air and plain FTO glass substrates caused an increase by 4.40 C and 1.80 C, respectively Fig 9b shows the % increment in temperature with different samples A magnified graph of the DT of 5, 10, 20-cycles films is also included A steep decline in temperature increment can be observed once the rGO thin films were utilized The 5-cycles rGO thin films alone shows approximately 5.8 times and 2.4 times reduction in interior temperature increment as compared to air and FTO glass substrate, respectively The interior temperature increment showed a 13% reduction when the rGO film thickness is increased from to 10-cycles and a further 15.5% reduction when the film thickness increased from 10 to 20-cycles Increasing the film thickness show, as expected, lower increment in interior temperature There are possible reasons for this explanation: DQ =Dt ¼ Àk  ðDT=LÞ (1) Firstly, according to the heat transfer equation - where DQ/Dt is the rate of heat conduction, Кis the thermal conductivity of the material per unit thickness, A is the area of the material, L is the layer thickness and DT is the temperature difference across the material - increasing film thickness would result in a slower heat transfer through the rGO film by conduction, due to the presence Fig The temperatureetime plots (a) and temperature change analysis (b) of the solar simulation results for different samples 258 L.P Yeo et al / Journal of Science: Advanced Materials and Devices (2019) 252e259 of more conduction pathways in the thicker film Furthermore, increasing thickness of the rGO film gives rise to denser films with better connectivity, thus increasing the ease of in-plane thermal phonon propagation [36] Within the testing period, most of the heat was likely conducted laterally across the rGO thin film instead of through the sample, resulting in a postponement of perpendicular heat entry Secondly, as previously observed in Fig 8, increasing the rGO film thickness has the effect of reducing NIR transmittance across the thin film The reduced entry of radiant heat into the room also contributed to the lower room temperature increment However, since there is a limit to how much NIR transmittance can be blocked by increasing rGO thickness, this suggests that the further reduction in interior temperature increment between the 10 and 20 cycle film is likely due to the preferred propagation of the thermal phonons through other conduction pathways Conclusion In this study, the rGO thin films were investigated for its potential to be incorporated into smart windows to block heat transfer The GO suspension was fabricated via modified Hummer's method and was reduced and deposited onto FTO substrates using electrophoretic deposition technique Three different rGO thin films of varying number of deposition cycles (5, 10, 20-cycles) were fabricated and tested in a UV-Vis-NIR spectrophotometer and a solar simulator to determine their ability to block NIR wavelengths and reduce indoor temperatures The 20-cycles rGO thin films showed an NIR transmittance of 18.8e40.4%, which is 30% more NIR blocked at 1000 nm as compared to clean FTO It also showed the least temperature increment of 0.57 C following 30 of solar irradiation While maintaining excellent heat transfer reduction, the visible transmittance of the films was also on par with commercial solar films, enabling up to 60% of visible light transmittance for optimal balance of transparency and heat reduction It is suggested that the excellent heat blocking results of rGO thin films are due to a combination of good heat conduction and reduced NIR transmittance and as such, possesses great potential in heatblocking window technologies Declaration of interest statement The authors declare no conflict of interest Acknowledgements This research is supported by grants from the National Research Foundation, Prime Minister's Office, Singapore under its Campus of Research Excellence and Technological Enterprise (CREATE) Program Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.04.002 References [1] D Perera, N.-O Skeie, Comparison of space heating energy consumption of residential buildings based on traditional and model-based techniques, Buildings (2) (2017) 27 https://doi.org/10.3390/buildings7020027 [2] D Ürge-Vorsatz, L.F Cabeza, S Serrano, C 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