Adsorption of small Au clusters on MgO and MgO/Mo:: the role of oxygen vacancies and the Mo-support

We report a systematic density functional theory investigation of adsorption of small Au-n (n = 1-6) clusters on ideal and defected MgO(100) single crystal surfaces and Mo(100) supported thin MgO(100) films. As a model defect, we consider a neutral surface oxygen vacancy (F-s). Optimal adsorption geometries and energies, cluster formation energies and cluster charges are discussed and compared in detail over four different substrates. For a given cluster size, the adsorption energy among these substrates increases in the order MgO, F-s/MgO, MgO/Mo and F-s/MgO/Mo. While cluster growth by association of atoms from gas phase is exothermic on all the substrates, cluster growth by diffusion and aggregation of pre-adsorbed Au atoms is an endothermic process for Au-1 -> Au-2, Au-3 -> Au-4 and Au-5 -> Au-6 on MgO/Mo and Au-2 -> Au-3 and Au-5 -> Au-6 on F-s/MgO/Mo. The adsorbed clusters are close to neutral on MgO, but adopt a significant anionic charge on other supports with the increasing order: MgO/Mo, Fs/MgO and F-s/MgO/Mo. The adsorption strength thus correlates with the amount of negative charge transferred from the substrate to gold.