Moreover, the possible photocatalytic mechanism of the GO has been demonstrated for the better understanding of the CO2 photoreduction process. Chhowalla and Eda20 have reported that in GO isolated sp2 clusters with the oxygenated functional groups (C–OH or C–O–C) lead to localization of electron (e)– hole (h+) pairs on its basal plane. Similarly, we assumed in our modified GO with surplus oxygenated components on the basal plane stretching the bandgap energy that helps the electrons excite from the valence band to the conduction band. We propose that in the photocatalytic reduction, the photogenerated electrons (e) and holes (h+) would migrate to the GO surface and serve as the oxidizing and reducing sites, respectively, to react with absorbed reactants. The reduction potential of e in the graphene oxide (GO) conduction band was determined to be 0.79 V (vs. NHE), which is lower than the potential of CO2/CH3OH (0.38 V vs. NHE) and act as a donor. Whereas the oxidation potential of the h+ in the GO valance band was
estimated (with optical band potential) to be around 4 V (vs.NHE), which is higher than the potential of H2O/O2, H+ (E ¼ 0.82 V vs. NHE) and acts as an acceptor. This indicates that the
photogenerated electrons and holes on the irradiated GO can react with the adsorbed CO2 and H2O to produce CH3OH via six-electron reaction, as described in eqn (1)–(3) in Fig. 7. The conduction band position (0.79 V) of GO supports that the photocatalytic CO2 reduction on GO is less feasible for the one electron reduction process (E ¼ 1.90 V) as compared to a multi-electron process. In our CO2 photo-reduction process, only CH3OH was detected, although there are possibilities for the formation of other products such as CO, HCHO and CH4. The photocatalytic performance data, herein, potentially support our hypothetical interpretations of the CO2 reduction mechanism.