Photoelectron Spectroscopy Study Of Au/TiO₂(110) Model System

It has been traditionally recognized that Au is one of the most inert metals until Haruta's work finding pronounced catalytic activity of Au nanoparticles highly dispersed on transition-metal oxides. Generally, it is observed that Au supported on reducible oxides, such as TiO₂, has a higher reactivity compared to Au supported on irreducible oxides, such as MgO and SiO₂. The catalytic activity of gold catalysts strongly depends on the size of gold particles, with a peak in activity for～3 nm wide Au particles.Charge transfer from the support was also considered to be an important factor for the enhanced catalytic activity of small gold particles, especially in the case of the reducible oxides.As a typical model system, Au/TiO₂(110) has been extensively investigated, theoretically and experimentally. Theoretical calculations have illustrated that the geometry and electron structure of Au cluster are influenced remarkably by the oxygen vacancies of rudeced TiO₂ surface when Au deposited on it. However, there are few experimental results evidencing charge transfer from the reduced TiO₂ substrate to the supported gold clusters.In our experiments, The growth of Au clusters on stoichiometric and reduced rutile TiO₂(110) surfaces were investigated by means of X-ray photoelectron spectroscopy.The shifts of Au4f binding energy with the increasing Au thickness illustrate that only particle size effect influence the electron structure of Au cluster on the stoichiometric surface ,however both the particle size effect and the charge transfer influence the electron structure of Au cluster on the reduced surface. Our results experimentally present direct evidence for the charge transfer from the reduced TiO₂(110) surface to Au clusters. The growth of Au clusters on a partially-reduced rutile TiO₂(110)-1×1 surface were investigated by high-resolution photoelectron spectroscopy using synchrotron-radiation-light. The valence-band photoelectron spectroscopy results demonstrate that the Ti³⁺ 3d feature attenuates quickly with the initial deposition of Au clusters, implying that Au clusters nucleate at the oxygen vacancy sites. The Au 4f core-level photoelectron spectroscopy results directly prove the existence of charge transfer from oxygen vacancies to Au clusters. These results experimentally present SR-PES evidence for the charge transfer .The thermal stability of Au clusters on the partially-reduced and stoichiometric TiO₂(110) surfaces was comparatively investigated by the annealing experiments. With the same film thickness, Au clusters are more thermally stable on the partially-reduced TiO₂(110) surface than on the stoichiometric TiO₂(110) surface. Meanwhile, large Au nanoparticles are more thermally stable than fine Au nanoparticles.The Au 4f core-level photoelectron spectroscopy results directly prove the existence of charge transfer from oxygen vacancies to Au clusters. The thermal stability of Au clusters on the partially-reduced and stoichiometric TiO₂(110) surfaces was also comparatively investigated by the annealing experiments. With the same film thickness, Au clusters are more thermally stable on the partially-reduced TiO₂(110) surface than on the stoichiometric TiO₂(110) surface. Meanwhile, large Au nanoparticles are more thermally stable than fine Au nanoparticles.