Growth and structure of Cu, Ag and Au clusters on -Fe₂O₃(0001): A comparative density functional study

Transition metals supported on metal oxide surfaces have broad applications in heterogeneous catalysis, microelectronics, and gas detection. For many uses, it is critical to control the dispersion of the supported metals to obtain either atomically dispersed systems or isolated particles of controlled size. The morphology of the designed surfaces is mainly governed by the relative intensities of the metal-metal and metal-oxide interactions. Here, we have investigated the adsorption of small Mn clusters (M = Cu, Ag and Au, with n = 1-5) on the Fe-terminated (0001) surface of alpha-Fe2O3 (hematite) by using the density functional theory, including an on-site Coulomb term (DFT + U). The M-M and M-oxide interactions were quantified by computing the association and adhesion energies, respectively. Whereas the former is defined in relation to an ideal atom-by-atom nucleation mechanism, the latter is described considering the particle adsorption as a whole. In the nucleation process, the magnitude of the association energy follows the order Au > Ag greater than or similar to Cu. The M-oxide strength interaction (adhesion) was found to follow a different trend: Cu > Ag greater than or similar to Au. In other words, Cu, which presents the highest strength interaction with the surface, shows the lowest tendency to grow by nucleation; Au presents the opposite behavior: a relative weak interaction with the surface and a strong trend to grow by nucleation. Ag exhibits an intermediate behavior being similar to Cu in nucleation and to Au in adhesion. Thus, for the formation of supported clusters using evaporation/deposition techniques, the present results suggest that whereas Cu shows a tendency to disperse over the hematite surface, Au tends to nucleate and to form large aggregates.