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Graphene Oxide as An Efficient Photocatalyst For Photocatalytic Reduction of CO2 Into Solar Fuel

Photocatalytic reduction of CO2 into solar fuel such as methane and methanol, is an attractive approach to simultaneously solve the energy crisis and global warming problem. Herein, comparative photocatalytic activity of graphene oxide nanosheets have been investigated for photocatalytic reduction of CO2 into methane and methanol in continuous gas and liquid phase photoreactor system. The graphene oxide sheets were prepared according to Tour’s method. The chemical composition and optical properties was evaluated through XPS and UV-vis spectroscopy. Graphene oxide nanosheets exhibited maximum amount of 224.87 μmol/g.h methanol and 14.8 μmol/g.h methane in liquid and gas phase system, respectively. Higher yield of methanol in liquid phase compared to methane in gaseous system can be attributed to dispersion of graphene oxide sheets in water. Hence, graphene oxide nanosheets are efficient photocatalyst for CO2 reduction into methanol. Nevertheless, further research is essential to improve the photostability of graphene oxide sheets for real application of photocatalytic CO2 reduction.


INTRODUCTION
Energy requirement of the world is increasing day by day due to growing population and advancements in scientific inventions. Fossil fuels are considered as primary source of energy and combustion of fossil fuel is polluting the environment particularly the global warming due to uncontrolled emission of carbon dioxide (CO2). Reduction of CO2 is essential to curb the global warming and save the environment [1][2][3]. Several processes have been used to reduce the level of CO2 such as carbon capture and storage, conventional absorption, biological, chemical and thermochemical reduction of CO2 [4][5][6][7][8]. Although, all processes are under development stages, they suffer from the higher cost and environmental limitations. Photocatalytic reduction of CO2 is the most economical and environmental friendly process using renewable sunlight energy [9][10][11][12].
Photocatalytic reduction of CO2 was pioneered by Inoue and Fujishima in 1979 [13]. Environmental friendly photocatalyst with novel properties that can enhance the process efficiency, have been the interest of researchers. Several semiconductors such as zirconium oxide [14], gallium oxide [15], magnesium oxide [16], zinc sulfides [17], cadmium sulfides [18], bismuth sulfide [19], graphitic carbon nitride [20] and titanium oxide (TiO2) [21] have been employed as photocatalyst. Among all, TiO2 has been the widely reported photocatalyst because of its unique properties, high stability, availability and non-toxicity [22][23][24][25]. However, photocatalytic activity of TiO2 is limited due to large bandgap energy and electrons-holes recombination problem. Lot of efforts have been made to improve the activity of TiO2 via coupling/doping with metals and non-metals [26,27]. However, photocatalytic efficiency of TiO2 is far from practical applications.
Graphene is a two dimensional (2D) single layered atomic sheet with hexagonal structures and zero bandgap material [28]. Meanwhile, GO which is oxygenated graphene sheets, comprise of covalently and non-covalently bonded oxygen functional groups such as epoxy, phenolic and carboxylic group [29]. In past few years, graphene based photocatalytic semiconductors have widely reported for CO2 reduction due to their excellent activity [30]. Distinguish feature of graphene base materials is fast transportation of charge carriers and lower charge recombination across photocatalyst [31,32]. The GO has wide bandgap energy and great potential to act as photocatalyst for CO2 reduction into useful product such as methane, methanol and formic acid.
Generally, GO has been synthesized by Brodie [33], Staudenmaier [34] and hummer's method [35]. Pros and cons of different methods has been enlisted by Lavin-Lopez et al. [36]. Most commonly used hummer's method involves oxidation of graphite using strong oxidizing agent such as H2SO4, KMnO4 and NaNO3 respectively. Incomplete oxidation and release of toxic gas are drawbacks of hummer's method. Recently, Tour et al. [37] have developed the safest approach as compared to hummer's method by replacing NaNO3 by H3PO4.
In the present study, GO was synthesized according to Tour's method as it gives higher yield and minimal basal defects in GO. According to our knowledge, there is limited study on direct use of GO in photocatalytic applications. Herein, comparative photocatalytic activity of GO has been investigated for liquid and gas phase CO2 reduction system.

Method
The GO was synthesized according to Tour's method [37] with modified parameters. In simple, 3 g of graphite powder was slowly added to 400 ml of mixed acid (H2SO4/H3PO4:9/1) in an ice bath to maintain the temperature at 10 o C along with mechanically stirring at 300 rpm. In addition, 18 g of KMnO4 was added slowly and mixture was shifted into oil bath at the temperature of 50 o C and stirred for 24 h. Then, 400 mL (Deionized water) ice was gradually poured into mixture at room temperature followed by dropwise addition of 5 mL of H2O2. The color of mixture was turned to yellow indicating the complete oxidation of graphite to graphite oxide. The obtained suspension was then pass through polyester fiber and further centrifuged. Graphite oxide was washed with 10 % HCl (1 litre) and absolute ethanol (500 ml) followed by washing with deionized water. Washing with deionized water continued until pH became neutral (7). Then, recovered graphite oxide was oven-dried at 35 o C for two days followed by grinding. Dried graphite oxide was again dispersed into deionized water and sonicated for 1 h to exfoliate it to GO sheets and centrifuged at 1000 rpm (20 min) to remove unoxidized graphite particles. The GO sheets were separated from suspension at high centrifugation speed of 4000 rpm (for 4 h) followed by drying at 35 o C.
Characterization X-ray photoelectron spectroscopy was performed to investigate the surface chemical composition. It was equipped with Al K-α radiations with corrected calibration binding energies against the C1 of carbon fixed at 284.6 eV. XPS spectra was recorded on pass energy of 50-200 eV. UV-visible spectroscopy was used to measure the absorption profile of synthesized photocatalyst. The spectrum measurements were performed using Agilent Cary technologies 100 UV-vis spectrometer Model G9821A with wavelength range of 200-800 nm. The band gap energies of the photocatalyst was measured from the Kubelka-Munk (KM) function.

Photocatalytic Reduction of CO2
Photocatalytic reduction of CO2 was carried out in liquid and gas phase continuous flow photoreactor. The gas phase photoreactor was comprise of cylindrical stainless-steel chamber with quartz window on top and 500 W Xenon (Xe) lamp as source of light. Typically, 0.1 g of photocatalyst was uniformly dispersed in photoreactor. Helium (He) gas was passed through reactor for certain time to purge the gaseous impurities from system. Then, CO2 was bubbled through water bubbler to have mixture of CO2 and H2O vapours and passed through reactor for half hour to establish equilibrium in system. Photoreactor was fully covered with aluminium foil to avoid the interference of light from surrounding. The CO2 flowrate was set at 20 mL/min and lamp was switched on to start the photoreaction. Product was analysed through online GC-TCD&FID. Moreover, FID was coupled with a HP-PLOT Q capillary column (Agilent, length 30 m, ID 0.53 mm, film 40 µm) for the segregation of C1-C6 paraffin and olefin hydrocarbon, alcohols and other oxygenated compounds. TCD was attached to UCW982, DC-200, Porapak Q and Mol Sieve 13X columns. In addition, liquid phase photoreactor was comprising of quartz glass vessel with 500W Xe lamp as source of light. Typically, 0.1 g of photocatalyst was suspended in 100 mL H2O with continuous stirring. The sodium hydroxide (NaOH) was added into 100 mL H2O to ensure the higher absorption of CO2. Prior to start, system was purged with CO2 for half hour and then CO2 flow rate was adjusted to 20 mL/min. System was fully covered with aluminium foil to stop the interference of light from surrounding. Lamp was switched on after 30 min so that system should be at steady state and equilibrium condition. Samples were collected after every 30 min and analysed through GC-FID to measure quantity of photoproduct.

XPS Analysis
XPS analysis indicated the chemical structure of prepared GO and nature of surface moieties. Figure 1(a) and (b) show the XPS spectrum of C1s and O1s along with deconvoluted peaks. In XPS spectra of C1s, there are four deconvoluted peaks at 284. 28 [38][39][40]. In O1s spectra, major peak at 533.48 eV is attributed to aromatic group (C-O) [41]. Percentage area of peak for C=O (287.48 eV) is 38.17 % of total C1s spectra which indicates that most of GO sheets are crowded with carbonyl groups. In addition, O1s spectra exhibited higher percentage of phenolic groups, C-O (40.36 %), at 533.48 eV. Peaks of oxygen functional group along with carbon indicate the oxidation of graphite into GO and would enhance adsorption and photocatalytic reduction of CO2 [42].

UV-vis Spectroscopy
Optical response of GO was evaluated through UV-vis spectrometer as shown in Figure  2(a). GO exhibited strong light absorption spectra in visible light region and light absorption start to decrease at wavelength (λ) < 400 nm. Higher absorption of visible light compared to UV light indicate that GO is more active under visible light irradiations.
Response of GO in λ > 400 nm can be attributed to n→π* transition of C=O [29]. Optical band gap of GO calculated by Kubelka-Munk function using Tauc plot was 1.3 eV as shown in Figure 2(b). Bandgap energy of GO depends on oxygen function groups density and high oxygen density will generate larger bandgap [43]. Low bandgap of 1.3 eV suggest that GO should produce high yield of solar fuel. Huang et al. reported that bandgap energy of GO can be tuned from 2.7 to 1.15 eV depending on density of oxygen functional groups [44]. Thus, Low bandgap energy GO could be efficient photocatalyst for CO2 reduction to solar fuel under visible light irradiation.

Photocatalytic Activity
To investigate the photocatalytic performance of GO, CO2 reduction experiments were carried out in continuous liquid and gas phase photoreactors. Figure 3(a) and (b) show the yield of solar fuels produced in gas and liquid phase, respectively. In gas phase system, CH4 was detected as main product with maximum yield of 14.8 µmol/g.h.
Initially, yield of CH4 increased till 90 min and then started to decrease. It could be due to instability of photocatalyst caused by accumulation of holes on the surface of GO [30]. In addition, there was no CO in our system, which indicate that GO photocatalyst promote the conversion of CO2 into CH4. Formation of CH4 needs 8 electrons which required high surface electron density and GO exhibited the multielectron reduction process [45]. Quantity of methane was higher as compared to reported value by Tan et al. [46]. Although, significant amount of CH4 was produced by GO, it suffered from photo stability problem as activity decreased with time [30]. In liquid phase photocatalytic CO2 reduction, CH3OH was detected as main product. Significantly, large quantity of 224.87 µmol/g.h of CH3OH was observed at 60 min but yield decreased sharply with time like gas phase system. According to literature, most of graphene base photocatalyst exhibited the higher production of CH3OH in liquid phase and CH4 in gas phase CO2 reduction system [30]. Decrease in activity of GO in both liquid and gas phase system reflects the photostability problem of material [47]. However, activity was almost similar and constant for 90 min and 120 min which indicate that GO sustained the activity with little decrease of yield in liquid phase system. Most of graphene base photocatalyst produced CH4 and CH3OH in gas phase and liquid phase photocatalytic CO2 reduction [48]. Production of CH4 and CH3OH suggest that reduction took place through multielectrons reduction path rather single electron reduction path.
Higher yield of CH3OH could be attributed to efficient dispersion of GO in water. Moreover, conduction band potential of GO is -0.79 V (vs. Normal Hydrogen Electrode (NHE)) which is more negative than reduction potential of CO2/CH4 (-0.24 V) and CO2/CH3OH (-0.38 V), respectively. Valence band potential of GO is 4 V (vs. NHE) which is higher than oxidation potential of H2O/O2, H + (-0.82) [29]. Although, GO can be potential photocatalyst for the CO2 reduction into CH4 and CH3OH, stability of photocatalyst is to be improved.

CONCLUSION
GO nanosheets were successfully synthesized according to Tour's method through acid oxidation of graphite flakes. Photocatalytic activity of GO nanosheets was evaluated for the reduction of CO2 into CH4 and CH3OH using gas and liquid phase photoreactor system, respectively. Activity of GO nanosheets was higher in liquid phase system with maximum production 224.87 µmol/g.h of CH3OH compared to 14.8 µmol/g.h of CH4 in gas phase system. However, GO sheets were instable due to oxygen groups, and their activity decreased with time in both liquid and gas phase CO2 reduction system. Further study need to be conducted to improve the photostability and photoactivity of GO nanosheets.