Bimetallic Pt−Au Clusters on TiO₂(110):  Growth, Surface Composition, and Metal−Support Interactions

Abstract

Au, Pt, and Au−Pt clusters were grown on TiO2(110) at room temperature and studied by scanning tunneling microscopy. For the same metal coverages, the deposition of pure Pt produces smaller clusters and higher cluster densities compared to pure Au because of the greater mobility of Au on the surface. Heating the surface causes greater sintering of the Au clusters compared to Pt; this behavior is explained by the stronger metal−metal bonds for Pt and the fact that atom detachment is the rate-limiting step in cluster sintering. For the deposition of 0.024 ML of Pt followed by 0.072 ML of Au, bimetallic clusters are formed from the nucleation of Au at existing Pt clusters, whereas the reverse order of deposition results in pure Pt clusters and pure Au clusters coexisting on the surface. The presence of Pt in the bimetallic Pt−Au clusters inhibits sintering, and the average size of the clusters after annealing decreases with increasing Pt composition. Low energy ion scattering experiments demonstrate that the deposition of Au on Pt does not produce core−shell structures with Au on top. Bulk thermodynamics predicts that the cluster surfaces should be pure Au, given that the Au surface free energy is lower than that of Pt, and Au and Pt are immiscible at the compositions studied here. However, surface compositions of the Au−Pt clusters are 10−30% richer in Pt compared to the overall compositions for total coverages of 0.10 ML and 25−75% Pt. These results demonstrate that Au and Pt atoms can intermix at room temperature and the surface properties of Au−Pt nanoclusters are different from those of the bulk. Grazing angle X-ray photoelectron spectroscopy experiments show that annealed Au−Pt clusters are covered by reduced titania. Annealing the Au−Pt clusters to temperatures above 600 K induces encapsulation of the clusters, but the presence of Au at the cluster surface decreases the extent of encapsulation compared to that of pure Pt clusters.

Point island models for nucleation and growth of supported nanoclusters during surface deposition

“…However, PIM can be applied directly to elucidate and interpret behavior in a host of specific systems, some of which are indicated above. An expanded list of examples includes: (a) homogenous nucleation and growth of metal NCs during deposition on metal, semiconductor, oxide substrates [8,9], with particular utility for Volmer-Weber growth of 3D islands on weakly binding substrates; (b) nucleation inhibited by attachment barriers as observed in metal (111) homoepitaxial systems which exhibit enhanced long-range adatom interactions with a "repulsive ring" due to surface states [38,49]; inhibited attachment was also recently suggested for Fe deposition on graphene [50]; (c) nucleation and growth with strongly anisotropic diffusion as observed for homoepitaxy on dimer-row reconstructed Si(100) surfaces [51]; (d) significant effects on nucleation of small mobile clusters as anticipated for homoepitaxy on metal (100) and (111) surfaces [52]; (e) growth inhibition in strained-layer heteroepitaxy with large mismatch [53]; (f) codeposition to form bimetallic NCs, e.g., of Pt and Au on TiO 2(110) [43], and Pt and Ru on graphene [44]; (g) exchange-mediated nucleation in Fe on Cu(100) [41], and Ni on Ag(111) [42] systems; (h) nucleation of metal NCs on graphite is often facilitated by sputtering to create surface damage and heterogeneous nucleation centers, but even a small fraction of Cu ions from an e-beam evaporator can create sufficient damage that heterogeneous nucleation dominates for Cu deposition on HOPG [46]; (i) deposition of metals on metal-supported graphene, with periodically rumpled morié structure due to lattice mismatch, often results in directed-assembly of 3D NCs [54], and the PIM framework is ideally suited to modeling of this complex process [48].…”

High catalytic activity of Au/CeO x /TiO 2 (110) controlled by the nature of the mixed-metal oxide at the nanometer level

“…Au NPs with a diameter as large as 5.9 nm and a height of 1.3 nm were seen, but smaller metal particles were also present on the CeO x /TiO 2 (110) surface. On this surface, the dispersion of the Au NPs was substantially larger than seen on a pure TiO 2 (110) surface where Au mainly binds to the steps (10,15).…”

“…Chemically etched W tips were used for imaging the surfaces. The TiO 2 (110) crystal was cleaned by several cycles of Ne ϩ sputtering (1 keV, 40 min) and annealing (950 K, 5 min), and XPS/AES studies confirmed that there were no surface contaminants after this treatment (15).…”

Structure and reactivity of Au–Rh bimetallic clusters on titanate nanowires, nanotubes and TiO2(110)

“…The presence of Pt in the bimetallic Pt-Au clusters inhibits sintering, and the average size of the clusters after annealing decreases with increasing Pt content. Based on LEIS and STM experiments, performed on TiO 2 (1 1 0), it was stated that the deposition of Au on Pt clusters results in the formation of bimetallic clusters due to the seeding of mobile Au atoms at existing Pt nuclei, but the deposition of Au on Pt does not produce core-shell structures with Au on top at small coverages [13]. Later on the formation of Pt core-Au shell structures was demonstrated at higher coverages (0.25-0.5 ML).…”

“…Low energy ion scattering spectroscopy (LEIS), besides other surface sensitive techniques, was successfully applied in the characterization of bimetallic nanoclusters on TiO 2 (1 1 0) [9][10][11]13,14], because it has very high sensitivity in the topmost layer. In this paper we use this technique with XPS and FTIR methods to investigate the effect of potassium in the arrangement of Au-Rh clusters on TiO 2 (1 1 0) surface and on titanate nanowire and nanotubes supports.…”