A Short Review on the Electron Transfer at the Interface Metal/Semiconductor During Hydrogen Ions Reduction to H₂...

Electron transfer at metal/oxide interfaces is central to photocatalytic hydrogen formation, yet direct mechanistic studies remain limited by the complexity of practical photocatalysts. This review summarizes experimental evidence for charge transfer and H₂ evolution in well-defined Au/TiO₂ model systems, with emphasis on rutile TiO₂(110) single crystals in ultrahigh-vacuum and investigated by scanning tunneling microscopy (STM), in situ X-ray photoelectron spectroscopy (XPS), and online mass spectrometry. Comparison of the catalytic activity is also made with polycrystalline semiconductor photocatalysts. Observations relevant to electron transfer are highlighted. For example, XPS measurements reveal the formation of reduced Ti³⁺ in TiO₂ after photocatalytic reaction, yet Au when present suppresses this route. Moreover, online mass spectrometry demonstrates that H₂ formation increases nonlinearly with Au coverage before saturation at a very low loading <0.05 monolayer equivalent. Further studies show that this is due to the effect of dispersion, where increasing the coverage hinders the reaction. The situation is more complex because pump-probe transient absorption spectroscopy results indicate that electron transfer between Au and the semiconductor occurs both ways depending on light energy. These observations establish quantitative links between three parameters needed for the design of metal/semiconductor photocatalysts: interface electronic states, cluster density and spacing, and hydrogen production kinetics.