Tuning Optical and Photoelectrochemical Properties of TiO2/WOx Heterostructures by Reactive Sputtering: Thickness-Dependent Insights

Metal-oxide heterostructures represent an effective strategy to overcome the limitations of pristine TiO2, including its ultraviolet-only light absorption and rapid electron–hole recombination, which hinder its performance in solar-driven applications. Among various configurations, coupling TiO2 with tungsten oxide (WOx) forms a favorable type-II band alignment that enhances charge separation. However, a comprehensive understanding of how WOx overlayer thickness affects the optical and photoelectrochemical (PEC) behavior of device-grade thin films remains limited. In this study, bilayer TiO2/WOx heterostructures were fabricated via reactive DC magnetron sputtering, with controlled variation in WOx thickness to systematically investigate its influence on the structural, optical, and PEC properties. Adjusting the WOx deposition time enabled precise tuning of light absorption, interfacial charge transfer, and donor density, resulting in markedly distinct PEC responses. The heterostructure obtained with 30 min of WOx deposition demonstrated a significant enhancement in photocurrent density under AM 1.5G illumination, along with reduced charge-transfer resistance and improved capacitive behavior, indicating efficient charge separation and enhanced charge storage at the electrode–electrolyte interface. These findings underscore the potential of sputtered TiO2/WOx bilayers as advanced photoanodes for solar-driven hydrogen generation and light-assisted energy storage applications.