Photoelectrochemical Performance for Water Oxidation Improved by Molecular Nickel Porphyrin-Integrated WO₃/TiO₂ Photoanode

A WO₃/TiO₂ heterojunction photoanode was prepared by in situ growth of WO₃ on a mesoporous TiO₂ electrode. The photoinduced charge-transfer properties and chargeseparation improvement in this kind of type-II heterojunction were characterized by transient surface photovoltage spectra. By using sulfite oxidation as a hole scavenger, we demonstrated that 72 % of the photo-generated holes are reaching the surface of the photoanode, but the efficiency of hole injection (η_ox) into the electrolyte was only 48 %. For the first time, a Ni^II meso-tetra(4-carboxyphenyl)porphyrin (NiTCPP) was incorporated as a water oxidation catalyst into the WO₃/TiO₂ heterojunction photoanode, which promoted the value of η_ox to 81 %. The maximum applied bias photon-to-current efficiency for the WO₃/TiO₂/NiTCPP photoanode was determined to be 0.2 % at 1.01 V vs. the reversible hydrogen electrode (RHE), under which condition a Faradic efficiency of 89 % for water oxidation was achieved (averaged over 1 h of photolysis).