Using Photoionization Efficiency Spectroscopy and Density Functional Theory to Investigate Charge Transfer Interactions in AuCe3On Clusters.

ABSTRACT

The characteristics of small cerium oxide and gold-cerium oxide clusters were investigated as models for gold attachment to various defect sites on a ceria surface. Photoionization efficiency (PIE) spectra of gas phase Ce3On (n = 0-4) and AuCe3On (n = 0-3) clusters were recorded and compared to spectral simulations based on DFT calculations. Calculated structures and PIE spectra for the Ce3O5,6 and AuCe3O4-6 clusters are also presented; however, these species were not detected during photoionization experiments. Addition of an Au atom to Ce3 was found to increase the energy of the ionization onset by ∼0.4 eV, whereas addition of one or more oxygen atoms decreases the onset by ∼0.25 eV. The optimized AuCe3On (n = 0-4) cluster geometries correlate with Au atoms adsorbed to oxygen vacancy sites while the AuCe3O5 and AuCe3O6 clusters are consistent with Au adsorption to CeO3 and CeO2 vacancies, respectively. The interactions between the cerium oxide cluster surface and the adsorbed Au atom were found to strongly depend on the nature the of the adsorption site. Au adsorbed to O vacancies are negatively charged with a Ce → Au charge transfer, whereas Au adsorbed to CeO2 and CeO3 vacancies have a reversed Au → Ce charge transfer, resulting in a positively charged Au atom. Au adsorption to the Ce3On clusters has the effect of (i) reducing the differences in the HOMO energies of the AuCe3O4, AuCe3O5, and AuCe3O6 clusters and (ii) lowering the binding energy of oxygen atoms for all AuCe3On (n = 1-6) clusters. Au adsorption appears to have a minimal effect on CeO2 vacancy formation, although CeO2 vacancies were calculated to form more readily than O vacancies on both the Ce3On and AuCe3On clusters. The low energy fragmentation calculated for the Ce3O5,6 and AuCe3O4-6 clusters, via loss of either Au, O, or CeO2, could potentially make photoionization experiments unfeasible since these clusters may simply dissociate when exposed to high energy photons above the ionization threshold.