Size-dependence of Fermi energy of gold nanoparticles loaded on titanium(IV) dioxide at photostationary state

TiO2 particle-supported Au nanoparticles (NPs) with varying sizes and good contact (Au/TiO2) were prepared under a constant loading amount by the deposition-precipitation method. The Fermi energy of Au NPs loaded on TiO2 at the photostationary state (E-F') was determined in water by the use of S/S-2-having specific interaction with Au as a redox probe. The E-F' value goes up as the mean size of Au NPs (d) increases at 3.0 <= d <= 13 nm. The photocatalytic activities for the Au/TiO2-photocatalyzed reductions of 2,2'-dipyridyl disulfide and nitrobenzene increase with increasing d. The photoluminescence spectra for Au/TiO2 and their dynamic analysis indicated that the efficiency of the interfacial electron transfer from TiO2 to Au NPs increases as the result of an increase in d. Density functional theoretical calculations for (Au)(10) and (Au)(22) model clusters showed that negatively charged Au clusters are greatly stabilized by water solvation, of which energy is smaller for (Au)(22). The d-dependence of E-F' could be rationalized in terms of both the increase in the efficiency of the photoinduced electron transfer and the strong solvation of charged Au NPs by water. The parallel correlation between the E-F' and photocatalytic activity proves for E-F' to be a very important parameter for designing metal NP-loaded semiconductor photocatalysts.