Photoelectrocatalytic performance of nanostructured p-n junction NtTiO2/NsCuO electrode in the selective conversion of CO2 to methanol at low bias potentials

Aiming a selective reduction of CO2 to methanol, a p-n junction semiconductor was constructed based on CuO nanospheres (NsCuO) deposited at TiO2 nanotubes (NtTiO2). The NtTiO2/NsCuO material demonstrated smaller charge transfer resistance, smaller flat band potential and wider optical absorption when compared with NtTiO2 and/or Ti/TiO2 nanoparticles coated by higher size particles of CuO (Ti/TiO2/CuO). The selective reduction of dissolved CO2 to methanol was promoted at lower potential of +0.2 V and UV–vis irradiation in 0.1 mol L−1 K2SO4 electrolyte pH 8 with 57% of faradaic efficiency. Even though the performance of the nanostructured material NtTiO2/NsCuO was similar to the non-completely nanostructured material Ti/TiO2/CuO (0.1 mmol L−1 methanol), the conversion to methanol has been significantly increased when hydroxyls (0.62 mmol L−1) and holes scavengers (0.71 mmol L−1), such as p-nitrosodimethylaniline (RNO) or glucose, respectively, were added in the supporting electrolyte. It indicates that photogenerated electron/hole pairs are spatially separated on p-n junction electrodes, which produces effective electrons and long-life holes, influencing the products formed in the reaction. A schematic representation of the heterojunction effect on the photoelectrocatalytic CO2 reduction is proposed under the semiconductor and each supporting electrolyte, which improves the knowledge about the subject.