Cu₂O/TiO₂ heterostructures for CO₂ reduction through a direct Z-scheme: Protecting Cu₂O from photocorrosion

The development of artificial photosynthesis aims to solve the increasing energy demand and associated environmental problems. A model photosynthetic system employing a composite of semiconductors with a Z-scheme can potentially mimic the combined power of photosystems 1 and 2 to transfer electrons. In this work, octahedral cuprous oxide covered with titanium dioxide nanoparticles (Cu₂O/TiO₂) are synthesized by a solvothermal strategy that provides high morphological and crystallographic control. The formation of a p-n heterojunction and characterization of the Type II band alignment of the composite are performed by diffuse reflectance UV-visible (DRUV) spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). Upon UV-visible irradiation (λ ≥ 305 nm) of the composite in the presence of water vapor as the hole scavenger, the photoreduction of CO₂(g) proceeds selectively to generate CO(g). The production rate of CO by the composite, R_CO = 2.11 μmol g_cat⁻¹ h⁻¹, is 4-times larger than for pure Cu₂O under identical conditions. Contrasting XPS analyses of Cu₂O and Cu₂O/TiO₂, during photocatalysts operation and the detection of photogenerated hydroxyl radicals (HO[rad]) in the heterostructure at variance with the results obtained for pure Cu₂O are taken as evidences that TiO₂ protects Cu₂O from undergoing photocorrosion. These results provide direct evidence of an efficient Z-scheme as the main mechanism for harvesting energy during CO₂ reduction in the synthesized materials.