Electronic properties and charge transfer phenomena in Pt nanoparticles on γ-Al2O3: size, shape, support, and adsorbate effects.

This study presents a systematic detailed experimental and theoretical investigation of the electronic properties of size-controlled free and γ-Al(2)O(3)-supported Pt nanoparticles (NPs) and their evolution with decreasing NP size and adsorbate (H(2)) coverage. A combination of in situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculations revealed changes in the electronic characteristics of the NPs due to size, shape, NP-adsorbate (H(2)) and NP-support interactions. A correlation between the NP size, number of surface atoms and coordination of such atoms, and the maximum hydrogen coverage stabilized at a given temperature is established, with H/Pt ratios exceeding the 1 : 1 ratio previously reported for bulk Pt surfaces.

Nanoepitaxy using micellar nanoparticles.

The shape of platinum and gold nanoparticles (NPs) synthesized via inverse micelle encapsulation and supported on TiO2(110) has been resolved by scanning tunneling microscopy. Annealing these systems at high temperature (∼1000 °C) and subsequent cooling to room temperature produced ordered arrays of well-separated three-dimensional faceted NPs in their equilibrium state. The observed shapes differ from the kinetically limited shapes of conventional physical vapor deposited NPs, which normally form two-dimensional flat islands upon annealing at elevated temperatures. The initial NP volume was found to provide a means to control the final NP shape. Despite the liquid-phase ex situ synthesis of the micellar particles, the in situ removal of the encapsulating ligands and subsequent annealing consistently lead to the development of a well-defined epitaxial relationship of the metal NPs with the oxide support. The observed epitaxial relationships could be explained in terms of the best overlap between the interfacial Pt (or Au) and TiO2 lattices. In most cases, the ratio of {100}/{111} facets obtained for the NP shapes resolved clearly deviates from that of conventional bulklike Wulff structures.

Size-dependent adhesion energy of shape-selected Pd and Pt nanoparticles.

Thermodynamically stable shape-selected Pt and Pd nanoparticles (NPs) were synthesized via inverse micelle encapsulation and a subsequent thermal treatment in vacuum above 1000 °C. The majority of the Pd NPs imaged via scanning tunneling microscopy (STM) had a truncated octahedron shape with (111) top and interfacial facets, while the Pt NPs were found to adopt a variety of shapes. For NPs of identical shape for both material systems, the NP-support adhesion energy calculated based on STM data was found to be size-dependent, with large NPs (e.g. ∼6 nm) having lower adhesion energies than smaller NPs (e.g. ∼1 nm). This phenomenon was rationalized based on support-induced strain that for larger NPs favors the formation of lattice dislocations at the interface rather than a lattice distortion that may propagate through the smaller NPs. In addition, identically prepared Pt NPs of the same shape were found to display a lower adhesion energy compared to Pd NPs. While in both cases, a transition from a lattice distortion to interface dislocations is expected to occur with increasing NP size, the higher elastic energy in Pt leads to a lower transition size, which in turn lowers the adhesion energy of Pt NPs compared to Pd.

Nano Pinstripes: TiO2 Nanostripe Formation by Nanoparticle-Mediated Pinning of Step Edges.

The present scanning tunneling microscopy study describes the high-temperature growth of TiO2 nanostripes with tunable width, orientation, and spacing, mediated by thermally stable micellar Pt and Au NPs deposited on TiO2(110). This phenomenon could not be explained by spillover effects but is based on the preferential stabilization of [11̅0] step edges on TiO2(110) by the metal NPs. Contrary to the behavior of physical-vapor-deposited NPs, which are known to move toward step edges upon annealing, our micellar NPs remain immobile up to 1000 °C. Instead, the mobility of TiO2 step edges toward the micellar NPs, where they become stabilized, is observed. Our findings are relevant to the technological application of nanostructured materials in the fields of catalysis, molecular electronics, and plasmonics.

Size- and support-dependent electronic and catalytic properties of Au0/Au3+ nanoparticles synthesized from block copolymer micelles.

Supported Au nanoclusters synthesized from diblock copolymer micelles can be reliably prepared with well-controlled sizes and dispersions. For particles with diameters between approximately 1 and 6 nm, the particle size and the support were found to strongly influence the oxygen reactivity, the formation and stabilization of a metal-oxide, and the catalytic activity for electrooxidation of carbon monoxide. The smallest particles studied (1.5 nm) were the most active for electrooxidation of CO and had the largest fraction of oxygen associated with gold at the surface as measured by the Au(3+)/Au(0) X-ray photoemission intensities. Conducting and semiconducting substrates, ITO-coated glass and TiO(2), respectively, were associated with greater stabilization of Au(3+) oxide as compared to insulating, SiO(2), substrates.