Using Photoionization Efficiency Spectroscopy and Density Functional Theory to Investigate Charge Transfer...

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 Ce₃O (n = 0-4) and AuCe₃O (n = 0-3) clusters were recorded and compared to spectral simulations based on DFT calculations. Calculated structures and PIE spectra for the Ce₃O_5,6 and AuCe₃O_4-6 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 AuCe₃O (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 Ce₃O 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 AuCe₃O (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 Ce₃O and AuCe₃O clusters. The low energy fragmentation calculated for the Ce₃O_5,6 and AuCe₃O_4-6 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.