Amine Capped Gold Colloids at Oxidic Supports: Their Electronic Interactions

Colloidal deposition of noble metal nanoparticles on oxidic supports is a recent approach for the fabrication of heterogeneous catalyst materials. We present studies on the interaction of different amine ligands with gold nanoparticles before and after deposition on several oxidic supports (titania, silica, alumina, magnesia or zinc oxide), using X-ray photoelectron and Auger spectroscopy, and high-resolution transmission electron microscopy. The adsorption of amines on thin gold films as well as on nanoparticles leads to a decrease in metal photoelectron binding energies. Usually, this is explained by donor-acceptor interactions via the amine group. By additional analysis of Auger signals, which are more sensitive to changes in the oxidation state than photoelectron spectra, we demonstrate that these shifts are due to a final state effect, namely, the increased photoelectron hole screening in presence of amine adsorbates. It will be shown, that this effect is not sensitive neither to the nanoparticle size nor the sterical properties of the capping amine. After deposition on oxide supports, the photoelectron binding energies are even further decreased. The presented findings exhibit that care has to be taken to interpret binding energy shifts simply with charging, which has impact on understanding the local electronic situation on the surface of metal-loaded oxides, crucial for heterogeneous catalysis.