The Role of TiO₂ Powder Concentration in Spin-Coated WO₃ Photoanodes

Photoactive tungsten oxide-based coatings were fabricated via a liquid-phase spin-coating method, with the addition of commercial TiO₂ powder to make a composite. TiO₂-added WO₃ thin-film photoanodes (WO₃-TiO₂ composite) were investigated to identify the optimal concentration level for an enhanced photoelectrochemical performance. In the composite sample WO₃-TiO₂-(20%), TiO₂ integrates into WO₃, forming a heterojunction network that preserves crystallinity, improves charge separation, and accelerates charge transport. Further increasing the TiO₂ concentration promotes particle agglomeration, increases the contribution of the wide band gap TiO₂ phase, and consequently leads to band gap widening, reduced visible-light absorption, and less efficient charge transfer. The WO₃-TiO₂(20%) composite exhibits the best energy alignment, fastest interfacial kinetics, and highest photocurrent. This work demonstrates a novel, facile, solution-processed, and reproducible spin-coating route to explore the critical TiO₂ concentration threshold in the WO₃-TiO₂ heterojunctions.