Dopant-Induced Band Tuning Determines Atmosphere-Sensitive Nucleation of Core–Shell Rh@RhCrO _<i>x</i> Cocatalysts for Photocatalytic Water Splitting

Cocatalyst structure during photodeposition is shaped not only by precursor chemistry but also by the semiconductor surface properties. However, the role of dopant-induced band edge shifts in governing cocatalyst nucleation remains elusive. In Al-doped SrTiO 3, Al-substitution upshifts the surface conduction band minimum by ∼0.43 eV, placing O 2 -driven reoxidation and Rh 3+ reduction in direct kinetic competition, a condition absent in undoped SrTiO 3 . Consequently, photodeposition under ambient conditions stabilizes Rh in a mixed-valence Rh 0 /Rh 3+ state, yielding ultrafine nanoparticles that anchor CrO x into a well-defined Rh@RhCrO x core–shell structure. In contrast, under evacuated conditions, Rh 3+ reduction proceeds more completely, forming aggregated metallic Rh 0 particles with poor interfacial contact. Time-resolved photoluminescence measurements confirm that the core–shell catalyst exhibits a dominant monoexponential decay with a lifetime of 1.26 ns, signifying suppressed trap-mediated carrier loss, whereas the sample prepared under evacuated conditions shows a multicomponent decay with a prominent slow component attributable to trap-mediated relaxation. The core–shell catalyst achieves H 2 and O 2 evolution rates of 2.98 and 1.47 mmol·g –1 ·h –1, approximately 2-fold higher than those of the sample prepared under evacuated conditions, with an apparent quantum efficiency of 77.9% at 365 nm. These results demonstrate that dopant-controlled band tuning determines the atmosphere sensitivity of cocatalyst nucleation, providing a practical strategy to tailor the oxidation state and interfacial structure of cocatalysts for highly efficient photocatalytic water splitting.