Adsorption of Pt and Bimetallic PtAu Clusters on the Partially Reduced Rutile (110) TiO2 Surface: A First-Principles Study

An extensive study of the adsorption of small Ptn (n = 1−8) and bimetallic Pt2Aum (m = 1−5) clusters on the partially reduced rutile (110) TiO2 surface has been performed via total energy pseudopotential calculations based on density functional theory. Structures, energetics, and electronic properties of adsorbed Ptn and Pt2Aum clusters have been determined. The surface oxygen vacancy site has been found to be the nucleation center for the growth of Pt clusters. These small Pt clusters strongly interact with the partially reduced surface and prefer to form planar structures for n = 1−6 since the cluster−substrate interaction governs the cluster growth at low Pt coverage. We found a planar-to-threedimensional structural transition at n = 7 for the formation of Ptn clusters on the reduced TiO2 surface. GGA+U calculations have also been performed to get a reasonable description of the reduced oxide surface. We observed significant band gap narrowing upon surface−Ptn cluster interaction which leads to the formation of gap localized Pt states. In the case of bimetallic Pt−Au clusters, Aum clusters have been grown on the Pt2−TiO2 surface. The previously adsorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. The presence of the Pt dimer remarkably enhances the binding energy and limits the migration of Au atoms on the titania surface. The charge state of both individual atoms and clusters has been obtained from the Bader charge analysis, and it has been found that charge transfer among the Pt atoms of Ptn clusters and the metal oxide surface is stronger compared to that of Au clusters and the Pt2−TiO2 system. ■ INTRODUCTION TiO2 is a versatile material and extensively used as both catalysts and catalyst supports due to its several advantages including high activity, low cost, chemical and mechanical stabilities in different conditions, environmental compatibility, and stability under illumination. Pt−TiO2 and Au−TiO2 are the most active systems studied both theoretically and experimentally. Au nanoparticles supported on the TiO2 surfaces exhibit unusual catalytic activity for CO oxidation at low temperatures, while the bulk form of Au is known as a chemically inert material. Similarly, Pt−TiO2 is considered as the prototype of strong-metal−support-interaction (SMSI) which is a strong interaction observed between the small metal clusters and TiO2 surface and influences the catalytic activity over the surface. Surface defects such as oxygen vacancies are generally believed to play an important role in the growth of metal clusters. Metal adatoms can be easily trapped in the defect sites. These defect sites behave as nucleation centers for the adatoms diffusing on the surface. Furthermore, it is contemplated that O vacancies influence the catalysis over the metal oxide surfaces. There are several studies reporting that small gold clusters adsorbed on an oxygen vacancy of the TiO2 surface become negatively charged, and this charging causes the unusual catalytic activity of this small Au cluster. Because of the important scientific and technological applications of Pt− and Au−TiO2 systems, a fundamental study of the interaction between these metal atoms and defectfree as well as partially reduced rutile (110) surface is crucial, and it will contribute to our understanding about photocatalytic applications of TiO2 surfaces. In this study, we have presented a complete picture for the interaction of Pt and bimetallic PtAu clusters with the partially reduced TiO2 surface within the density functional theory. The growth behaviors of such small clusters help to get more insight into the early stage nucleation of the Pt and bimetallic Pt−Au nanoparticles dispersed on the titania surfaces. Bimetallic PtAu clusters adsorbed on the TiO2 surfaces promise a great potential to produce special catalysts which have high activity, selectivity, and stability in photocatalysis through tuning of their compositions. We believe that our results are crucial for further research on these metal− titania systems to reveal their properties for practical