Interface-driven energy-independent charge extraction in GaN photocatalysts.

Ultrafast charge transfer dynamics are key to photocatalytic efficiency, governing energy relaxation and surface reactivity. However, the temporal evolution of carrier energy landscapes following photoexcitation, particularly at complex metal/semiconductor interfaces, remains poorly understood. Here, we present a surface- and energy-resolved investigation of ultrafast electron dynamics across bare and Pt-modified gallium nitride (GaN) surfaces using time-resolved two-photon photoemission spectroscopy. We show that photogenerated electrons rapidly thermalize to the conduction band minimum and undergo sub-picosecond trapping in nitrogen-vacancy-related surface states. Surface modification with Pt suppresses these trapping channels and introduces an energy-independent ultrafast electron transfer pathway (~50 fs) from GaN into Pt. By disentangling interfacial charge transfer from intrinsic relaxation mechanisms through tailored pump-probe configurations, we demonstrate that Pt facilitates picosecond-scale electron transport from the bulk to the surface by photoinduced dynamic band flattening. Modulating these ultrafast dynamics through interfacial engineering significantly enhances charge separation and photoelectrochemical performance. This study deepens the understanding of interface-dependent relaxation and transfer processes of photocarriers and provides valuable guidance for rational design of advanced photocatalytic systems.