Nucleation and growth on defect sites: experiment–theory comparison for Pd/MgO(001)

Abstract

It is well established that nucleation of metal clusters on oxide and halide surfaces istypically dominated by defect sites. Rate equation models of defect nucleation have beendeveloped and applied to these systems. By comparing the models with nucleation densityexperiments, energies for defect trapping, adsorption, surface diffusion and pair bindinghave been deduced in favourable cases, notably for Pd deposited on Ar-cleaved MgO(001).However, the defects responsible remain largely unknown. More recently, several types ofab initio calculation have been presented of these energies for Pd and relatedmetals on MgO(001) containing several types of surface defect; these calculatedvalues are surveyed, and some are widely divergent. New rate equation nucleationdensity predictions are presented using the calculated values. Some calculations, forsome defect types, are much closer to experiment than others; the singly chargedFs+ centre and the neutral divacancy emerge as candidate defects. In these two cases, thePd/MgO(001) nucleation density predictions agree well with experiment, and thecorresponding surface defects deserve to be taken seriously. Energy and entropy values arediscussed in the light of differences in calculated charge redistribution between the metalatoms, clusters and (charged) surface defects, and (assumed or calculated) clustergeometries.