Surface or Bulk? Mechanistic Insights into Ni²⁺-Doped Brookite TiO₂ Photocatalysts

Solar energy, as an alternative source to catalyze chemical reactions, has been rapidly utilized and developed over the past few decades, particularly with TiO₂-based semiconductor photocatalysts. Regulating the carrier dynamics under photoexcitation and controlling the interfacial reaction kinetics have been emphasized as fundamental approaches to increase the quantum yield of photocatalytic systems. Transition-metal-ion doping is a promising strategy to address these issues, although the precise roles and optimal spatial distribution of dopants remain unclear. In this systematic study, we designed surface-only, bulk-only, and surface-bulk-doped brookite TiO₂ nanoparticles using Ni²⁺ as dopants and evaluated the photocatalytic performance of these doped samples based on the apparent reaction rate constants. It is demonstrated that the crystal structure, morphology, and surface composition did not change significantly after doping, and the observed enhancement in photocatalysis can be correlated to the doping positions. Continuous doping from the bulk to surface, forming the trap-to-transfer centers to mediate interfacial electron transfer, proves to be the most effective pathway. This proof-of-concept work offers a unique perspective on the transition-metal-ion-induced photocatalysis mechanism of brookite TiO₂ nanoparticles and will help us design more efficient photocatalytic systems.