Electric field changes on Au nanoparticles on semiconductor supports - The molecular voltmeter and other methods to observe adsorbate-induced charge-transfer effects in Au/TiO₂ Nanocatalysts

Infrared (IR) studies of Au/TiO₂ catalyst particles indicate that charge transfer from van der Waals-bound donor or acceptor molecules on TiO₂ to or from Au occurs via transport of charge carriers in the semiconductor TiO₂ support. The δν_CO on Au is shown to be proportional to the polarizability of the TiO₂ support fully covered with donor or acceptor molecules, producing a proportional frequency shift in ν_CO. Charge transfer through TiO₂ is associated with the population of electron trap sites in the bandgap of TiO₂ and can be independently followed by changes in photoluminescence intensity and by shifts in the broad IR absorbance region for electron trap sites, which is also proportional to the polarizability of donors by IR excitation. Density functional theory calculations show that electron transfer from the donor molecules to TiO₂ and to supported Au particles produces a negative charge on the Au, whereas the transfer from the Au particles to the TiO₂ support into acceptor molecules results in a positive charge on the Au. These changes along with the magnitudes of the shifts are consistent with the Stark effect. A number of experiments show that the ∼3 nm Au particles act as molecular voltmeters in influencing δν_CO. Insulator particles, such as SiO₂, do not display electron-transfer effects to Au particles on their surface. These studies are preliminary to doping studies of semiconductor-oxide particles by metal ions which modify Lewis acid/base oxide properties and possibly strongly modify the electron-transfer and catalytic activity of supported metal catalyst particles.