Selective CO₂ Photoreduction with Cu-Doped TiO₂ Photocatalyst: Delineating the Crucial Role of Cu-Oxidation State and Oxygen Vacancies

Abstract

Cu-doped TiO2 photocatalysts [Cu-(1 to 10) atom %] synthesized by the sol–gel method and thoroughly characterized using several techniques were evaluated for photocatalytic reduction of CO2 under ambient conditions. These photocatalysts converted the reactant mixture of CO2 and moisture selectively into methane and oxygen under visible irradiation without the use of any sacrificial agent. The Cu dopant considerably lowered the bandgap of TiO2, thereby making it feasible for doped TiO2 to absorb light in the visible region. Beyond 1% Cu doping the Cu-doped TiO2 materials were far superior as compared to anatase TiO2 for the reduction of CO2 to CH4, and the lowest doping (Cu-1%) concentration in TiO2 lattice shows the maximum production of CH4 (1081 μL/h/g), whereas the highest doping (Cu-10%) shows the least activity (200 μL/h/g). The photoreduction activity was found to decrease with increasing Cu concentration; under similar conditions without use of any cocatalysts. X-ray photoelectron spectroscopy (XPS) results along with cyclic voltammetry data (CV) indicate that the doped Cu exists in the two stable oxidation states of Cu1+ and Cu2+. The electron paramagnetic resonance (EPR) results show that there are two distinct types of Cu2+ species in the doped system and clustering of the Cu2+ states at higher doping concentrations may be detrimental for the photoactivity. The electronegative surface Cu1+ species along with O-vacancies seem to play the most important role in the photocatalytic reduction of CO2 to CH4.