To confirm the chemical stability of GOs we conducted XPS analysis of GO-1, GO-2 and GO-3 aer photocatalytic reaction as shown in Fig. S4.† XPS spectra of GOs showed negligible changes in the oxygenated functional groups as a result of irradiation. It was observed that all GO C1s spectra contain three components corresponding to sp2-hybridized (C]C), C–O and C]O functional groups. During the photocatalytic process, the oxygenated –COOH groups were removed from the GO
edges, narrowing the bandgap. The correlation between the relative peak intensities of the C–O and C]O components (normalized with sp2), as illustrated in Fig. S4d,† shows negligible changes in the GO samples aer photocatalytic reaction as compared with the pristine samples before the photocatalytic reaction. Moreover, major parts of the oxygenated functional groups remain chemically stable during irradiation to sustain the critical bandgap of the GO for the photocatalytic reaction. Under these circumstances, the loss of a small number of oxygenated functional groups from the GO during irradiation only affect initial (upto 2 h) methanol formation, after that the catalyst became stable during the photocatalytic reaction.
Additionally, to eliminate the possibility of GO as one of the major methanol source, we conducted the isotope tracer analyses of GOs with 13CO2 as described below.