To clarify the properties of the synthesized GO-based materials, UV-visible spectra (normalized) were plotted as shown in Fig. 1. The observed absorption near 200 nm suggests that all the synthesized GO samples possess an ordered graphitic structure with a p / p* transition of C]C. A small shoulder near 300 nm was observed in all GOs spectra, which is attributed to the n/p* transition of C]O. The optical band gaps of the three samples, GO-1, GO-2 and GO-3, were determined by using a Tauc plot with a linear extrapolation (Fig. 1). The amorphous GO sheet having a non-uniform oxidation level cannot exhibit sharp adsorption edges in the Tauc plot. We observed an approximate band gap ranges over 2.9–3.7, 3.1–3.9 and 3.2–4.4 for GO-1, GO-2 and GO-3 respectively. These observed approximate band gaps of all the GOs, showing an intrinsic semiconductor like absorption in the blue optical region, are adequately large to overcome the endothermic energy required for the CO2 reduction (0.38 V) under solar energy excitation.