TiO2 nanoparticles modified with ZnIn2S4 nanosheets and Co-Pi groups: Type II heterojunction and cocatalysts coexisted photoanode for efficient photoelectrochemical water splitting

The optimization of photoelectrode is the key issue for the efficient photoelectrochemical water splitting process. In this work, the TiO2 photoanode is synthesized and modified with ZnIn2S4 nanosheets and Co-Pi cocatalyst (TiO2/ZnIn2S4/Co-Pi) for a favorable photoelectrochemical performance. The synthesis and modification process of the TiO2 photoanode are optimized. The physical and chemical characterizations indicate that the TiO2 has a nano-cauliflower-like structure and rutile crystal form modified with a network hexagonal ZnIn2S4 nanosheets and amorphous Co-Pi groups. After optimization of the hydrothermal and annealing process, the optimized TiO2 photoanode manifests a photocurrent density of 1.82 mA cm−2, 1.73-fold of the pristine TiO2 photoanode (1.05 mA cm−2). With the surficial ZnIn2S4 and Co-Pi modification, the photocurrent density of the TiO2/ZnIn2S4/Co-Pi photoanode is raised to 5.05 mA cm−2, 5.32-fold of the optimized TiO2 photoanode (1.82 mA cm−2). The applied bias photon-to-current efficiency, the charge separation and injection efficiencies of the TiO2/ZnIn2S4/Co-Pi photoanodes are 8.79, 3.40, and 1.64-folds of the optimized TiO2 photoanode. Combined the Tauc plot, valence band XPS spectra, EIS and Mott-Schottky analysis, the PEC water splitting mechanism could be that: (i) the type II heterojunction formed by the TiO2 and ZnIn2S4 semiconductors improves the charge separation/injection efficiencies; (ii) the Co-Pi groups facilitate the oxygen evolution kinetics; (iii) the Co-Pi groups and 2D ZnIn2S4 nanosheets synergistically enhance the charge separation efficiency. This investigation could offer a prospect of practical implementation for photoelectrochemical water splitting.