Direct Z-Scheme Heterostructure of In Situ Planted ZnO Nanorods on g-C₃N₄ Thin Sheets Sprayed on TiO₂ Layer: A Strategy for Ternary-Photoanode Engineering toward Enhanced Photoelectrochemical Water Splitting

In this study, we developed an approach to enhance the separation and transfer of charge carriers for photoelectrochemical water splitting in solar-driven hydrogen production. We achieved this by designing a highly efficient Z-scheme TiO₂/g-C₃N₄/ZnO photoanode. The process involved electrodepositing a thin TiO₂ layer on FTO and optimizing the in situ ZnO implantation onto g-C₃N₄. These composites were confirmed by XRD, SEM, EDX, and TEM measurements. The growth of ZnO on g-C₃N₄ resulted in strong chemical adhesion between the interface of ZnO and g-C₃N₄, as supported by XPS data, and increased active surface area, as demonstrated by BET. The composition of ZnO and g-C₃N₄ facilitated rapid charge separation and retarded change recombination through directional charge migration and decreased charge resistance, as evidenced by PEIS and TRPL measurements. Our airbrushing procedure for fabricating the g-C₃N₄/ZnO composite on TiO₂ also enhanced the charge collection efficiency, enabling us to construct a high-performance photoanode. The Z-scheme-type charge migration route was verified by EPR spectroscopy by trapping the radicals generated by charges and holes. PEC-WS measurements showed that TiO₂/g-C₃N₄/ZnO heterostructure improved the produced photocurrent by about 160-, 40-, 20-, 8-, 2-, and 2-fold, relative to pristine g-C₃N₄, pristine ZnO nanorods, ZnO/g-C₃N₄ composite, pristine TiO₂, TiO₂/ZnO, and TiO₂/g-C₃N₄, respectively, versus reversible hydrogen electrode (RHE) at 1.23 V. The charge carriers’ separation and injection measurements showed that the fabrication of this ternary photoanode remarkably improved the PEC-WS performance. DFT results contributed to a deeper understanding of the mechanism of the photocatalytic process and confirmed that the as-fabricated ternary heterojunction promoted the separation/transfer efficiency of the photogenerated charge carriers, thereby promoting the activity of the photocatalytic process. This work could pave the way for better fabrication of ternary-based photoanodes.