Coupling of Engineered High Entropy Alloys with Semiconducting TiO2 Nanofilms for Scalable and Ultrafast Alkaline Hydrogen Evolution Reaction.

High entropy alloys (HEAs) are a promising class of electrocatalysts because of their high reactivity. However, the development of scalable synthesis strategies and fundamental understanding of their interfacial synergy with metal oxides remains underexplored. Herein, a new approach is reported for the fabrication of hybrid photoelectrocatalysts combining PtFeCoNiCu HEA structures with titanium dioxide (TiO₂) nanofilms via sequential anodization and electrodeposition. The TiO₂ nanofilms function as both a photoactive semiconducting framework and nanostructured substrate, enabling controlled nucleation and growth of HEA nanoparticles through a Volmer-Weber mechanism. The resulting hybrid photoelectrocatalysts exhibit outstanding hydrogen evolution reaction (HER) performance, achieving an ultralow overpotential of -11 mV at 10 mA cm^-2 under simultaneous illumination and elevated electrolyte temperature. Mechanistic studies combining in situ Raman spectroscopy and density functional theory simulations reveal that HER occurs through three distinct stages, during which the TiO₂ support undergoes dynamic structural and electronic evolution - from a passive scaffold to an electron-buffering layer. This process involves Ti⁴⁺ reduction, hydrogen intercalation, and accelerated turnover of OH^* intermediates, which collectively enhance interfacial charge transfer and broaden active-site availability. These findings provide new insights into the dynamic interplay between HEAs-semiconducting metal oxide substrates, enabling a generalizable design strategy for scalable, high-performance photoelectrocatalysts.