Electric Field Changes on Au Nanoparticles on Semiconductor Supports – The Molecular Voltmeter and Other Methods to Observe Adsorbate-Induced Charge-Transfer Effects in Au/TiO₂Nanocatalysts

Abstract

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 ΔνCO 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 νCO. 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 ΔνCO. 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.

Detection of electron tunneling across plasmonic nanoparticle–film junctions using nitrile vibrations

“…The VSE occurs when the local electric field perturbs the electronic environment of a chemical bond and results in a change in its vibrational energy, which can be directly measured with infrared or Raman spectroscopy. 33 Due to these advantages, the VSE has been used to quantify local electric fields in a diverse range of chemical environments including organic solvents 34, 35 , ionic liquids 36, 37 , catalysts 31, 38 , proteins 39, 40 and biomembranes 41 .…”

Effects of electron transfer in model catalysts composed of Pt nanoparticles on CeO2(1 1 1) surface

“…Yet, relatively little is known regarding atomic and electronic mechanism of electronic metal-support interactions [17][18][19]. In particular, it is largely unexplored how metal-support electron transfer may affect the catalyst [20,21]. Partially, this is because the magnitude of the electron transfer is hard to measure and control both in experiments and simulations.…”

Alkyl-nitrile adlayers as probes of plasmonically induced electric fields

“… 30 Measurement of the change in vibrational frequency of the bond can thus be used to determine the magnitude of the electric field. The vibrational Stark effect has been used to measure electric fields in a variety of chemical systems including the electrochemical double layer, 31 – 33 catalysts, 34 proteins, 30 , 35 – 37 biomembranes, 38 and the energy levels of molecules. 39 – 41 …”