Forward and backward electron transfer on Pt loaded TiO₂ photocatalysts under visible-light illumination

The surface plasmon resonance (SPR) effects of metal nanoparticles prove versatile in terms of equipping wideband-gap photocatalysts, such as TiO₂, with visible-light responsiveness. In this regard, Au is the most frequently used material for its SPR, but alternative materials are also being actively developed to effectively utilize solar light from the visible to the near-infrared region. In this work, we found that Pt particles loaded on TiO₂ were also active in effecting SPR-induced photocatalysis. Time-resolved absorption measurements confirmed that visible-light irradiation induces electron transfer from Pt to TiO₂, and the efficiency of this electron injection increases as the wavelength of the incident light decreases from 660 to 450 nm. Observations of the peak shift of the vibrational frequency of adsorbed CO on Pt confirmed that the Fermi level of Pt decreases as the electron transfer from Pt to TiO₂ proceeds. These results imply that Pt nanoparticles can act as sensitizers to induce electron transfer from Pt to TiO₂, although Pt is a well-known cocatalyst that enhances H₂ evolution by collecting electrons from TiO₂. However, as the intensity of the irradiated light increased beyond 50 mW cm⁻², a portion of the Pt particles started to capture the injected electrons from TiO₂, suggesting that the electron transfer from one Pt particle to the other Pt particles via the conduction band of TiO₂ proceeds under visible light illumination. These opposing roles may be ascribed to the variation in the size of Pt particles as well as density of electrons injected into TiO₂.