Quantum tunneling injection of hot electrons in Au/TiO₂ plasmonic photocatalysts

Visible light absorption of plasmonic Au nanoparticles supported on semiconductor TiO₂ leads to injection of their photoactivated "hot electrons (e_hot^-)" into the TiO₂ conduction band. This charge separation facilitates several oxidation and reduction reactions. These plasmonic systems, however, suffer from low quantum yields because the Schottky barrier created at the Au-TiO₂ interface suppresses e_hot^- injection. Here we report that Au nanoparticles supported on the anatase particles isolated from Degussa (Evonik) P25 TiO₂ promote e_hot^- injection with much higher efficiency than those supported on other commercially-available TiO₂ and catalyze aerobic oxidation with very high quantum yield (7.7% at 550 nm). Photoelectrochemical and spectroscopic analysis revealed that the number of Ti⁴⁺ atoms located at the Au-TiO₂ interface is the crucial factor. These Ti⁴⁺ atoms neutralize the negative charge of the Au particles and create a Schottky barrier with narrower depletion layer. This facilitates efficient e_hot^- injection by "quantum tunneling" through the Schottky barrier without overbarrier energy. The e_hot^- injection depends on several factors, and loading of 2 wt% Au particles with 3.5-4 nm diameters at around room temperature exhibits the highest activity of plasmonic photocatalysis.