Standalone photoconversion of CO₂ using Ti and TiO_x-sandwiched heterojunction photocatalyst of CuO and CuFeO₂ films

Artificial photoconversion of carbon dioxide to value-added chemicals remains a challenge. In this work, we synthesize heterojunction copper oxide and copper iron oxide (CuO/CuFeO₂; CFO) films on transparent conducting substrates with metallic Ti layers, and deposit disordered TiO_x particles on the CFO films. The Ti/CFO/TiO_x samples convert CO₂ into formate with ∼100% selectivity and drive O₂ evolution under simulated sunlight in the absence of any electrical and chemical biases, achieving the overall solar-to-chemical energy conversion efficiency of ∼5%. The primary roles of the Ti underlayer are to create a robust contact between CFO and substrates and facilitate hole transfer. The TiO_x (with unsaturated Ti atoms) delocalizes the electron density of the CFO and enhances electron transfer to the adsorbed CO₂. Density functional theory calculations reveal that Ti/CFO/TiO_x is the most suitable among the bare and modified CFO samples for efficient CO₂ adsorption, formiloxyl intermediate formation, and formate desorption.