Interaction of Pt Clusters with the Anatase TiO₂(101) Surface:  A First Principles Study

The adsorption of Pt_n (n = 1−3) clusters on the defect-free anatase TiO₂(101) surface has been studied using total energy pseudopotential calculations based on density functional theory. The defect-free anatase TiO₂(101) surface has a stepped structure with a step width of two O−Ti bond distances in the (100) plane along the [10] direction and the edge of the step is formed by 2-fold-coordinated oxygen atoms along the [010] direction. For a single Pt adatom, three adsorption sites were found to be stable. Energetically, the Pt adatom prefers the bridge site formed by 2 2-fold-coordinated oxygen atoms with an adsorption energy of 2.84 eV. Electronic structure analysis showed that the Pt−O bonds formed upon Pt adsorption are covalent. Among six stable Pt₂ adsorption configurations examined, Pt₂ was found to energetically favor the O−O bridge sites on the step edge along [010] with the Pt−Pt bond axis perpendicular to [010]. In these configurations, one of the Pt atoms occupies the same O−O bridge site as for a single Pt adatom and the other one either binds a different 2-fold-coordinated oxygen atom on the upper step or a 5-fold-coordinated Ti atom on the lower terrace. Three triangular and three open Pt₃ structures were determined as minima for Pt₃ adsorption on the surface. Platinum trimers adsorbed in triangular structures are more stable than in open structures. In the most stable configuration, Pt₃ occupies the edge O−O site with the Pt₃ plane being upright and almost perpendicular to the [001] terrace. The preference of Pt_n to the coordinately unsaturated 2-fold-coordinated oxygen sites indicates that these sites may serve as nucleation centers for the growth of metal clusters on the oxide surface. The increase in clustering energy with increasing size of the adsorbed Pt clusters indicates that the growth of Pt on this surface will lead to the formation of three-dimensional particles.