Effect of Pd and Cu co-catalyst on the charge carrier trapping, recombination and transfer during photocatalytic hydrogen evolution over WO₃–TiO₂ heterojunction

Co-catalysts are well known for improving the charge carrier’s separation and transfer to species in solution, and hence, the photocatalytic hydrogen production. Thus, in this work, the effect of loading Cu and Pd species over the WO₃–TiO₂ structure was evaluated. The structure of WO₃–TiO₂ was obtained by direct hydrolysis of titanium isopropoxide (sol–gel method) in previously synthesized WO₃ nanoparticles (6 mol% of WO₃), forming a composite that provided direct contact between WO₃ and TiO₂ nanoparticles. Subsequently, 0.5 wt% of copper or 0.5 wt% of palladium loadings was deposited onto WO₃–TiO₂. The photocatalytic hydrogen production results show that the activity increased with the presence of Cu and Pd species, reaching hydrogen production rates of 1496 μmol g⁻¹ h⁻¹ and 5427.07 μmol g⁻¹ h⁻¹ for Cu/WT and Pd/WT, respectively, as compared to WT structure (770.10 μmol g⁻¹ h⁻¹). To understand this behavior, semiconducting properties of the synthesized materials were characterized by (photo)electrochemical techniques. The presence of Cu and Pd in the structure moved the flatband position, increased the photocurrent and modified the open circuit potential under illumination toward less negative values, indicating the formation of energy states in the interface between WO₃–TiO₂ and the co-catalysts. These energy states at the heterojunction allow the transfer of photogenerated electrons toward co-catalysts, preventing the recombination of photogenerated charge carriers.