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

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.