Catalysis with TiO₂/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration

TLDR

Photoexcited semiconductor nanoparticles undergo charge equilibration when in contact with metal nanoparticles, shifting the composite Fermi level to more negative potentials. The study aims to determine the apparent Fermi level of the TiO₂–Au composite by equilibrating it with a C60/C60⁻ redox couple. Electron transfer to Au nanoparticles was probed by exciting TiO₂ under steady‑state and laser‑pulse conditions, and by equilibrating the composite with the C60/C60⁻ redox couple. The apparent Fermi level shifts 20 mV (8 nm), 40 mV (5 nm), and 60 mV (3 nm) Au nanoparticles, demonstrating that smaller Au particles enhance photoinduced charge separation, while isolated charge‑transfer steps reveal the metal’s role and size‑dependent catalytic activity.

Abstract

Photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials. The transfer of electrons to Au nanoparticles has now been probed by exciting TiO2 nanoparticles under steady-state and laser pulse excitation. Equilibration with the C60/C60- redox couple provides a means to determine the apparent Fermi level of the TiO2−Au composite system. The size-dependent shift in the apparent Fermi level of the TiO2−Au composite (20 mV for 8-nm diameter and 40 mV for 5-nm and 60 mV for 3-nm gold nanoparticles) shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation. Isolation of individual charge-transfer steps from UV-excited TiO2 → Au → C60 has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and size-dependent catalytic activity of metal−semiconductor nanocomposites.