Tunable surface: The surface structure of the Fe-Pt bimetallic catalyst can be reversibly modulated between the iron-oxide-rich Pt surface and the Pt-skin structure with subsurface Fe via alternating reduction and oxidation treatments (see figure). The regenerated active Pt-skin structure is active in reactions involving CO and/or O.

A bottom-up approach to produce a long-range ordered superlattice of monodisperse and isomorphic metal-oxide nanoparticles (NP) supported onto an oxide substrate is demonstrated. The synthetic strategy consists of self-assembling metallic NP on an ultrathin nanopatterned aluminum oxide template followed by a morphology-conserving oxidation process, and is exemplified in the case of Ni, but is generally applicable to a wide range of metallic systems. Both fully oxidized and core-shell metal-metal-oxide particles are synthesized, up to 3-4 nm in diameter, and characterized via spectroscopic and theoretical tools. This opens up a new avenue for probing unit and ensemble effects on the properties of oxide materials in the nanoscale regime. DOI: 10.1103/PhysRevLett.108.195507 PACS numbers: 81.16.Pr, 61.46.Df, 75.75.Cd, 81.16.Rf Metal-oxide nanoparticles (NP) find widespread interest in fundamental science and bear promise for high-level applications in many emerging fields of the advanced nanotechnologies, from nanospintronics and high-density magnetic storage devices [1][2][3] to chemical sensing [4,5] and nanocatalysis [6,7]. The size, shape, and mutual interactions among the particles play a key role in determining the novel properties of these nanosized oxide materials [8,9]. For example, oxide NP may display anomalous magnetic behavior such as ferromagnetism vs antiferromagnetism in the bulk [2,3] or size-dependent magnetic structures and unusual spin couplings [8,10]. Model systems of NP with atomic-level control of size, shape, chemical composition, and interparticle distance are therefore crucial to clarify the emergent phenomena in oxide materials at the nanometer scale, and to disentangle the effects of the various intra-, inter-and support-particle interactions. However, such systems are difficult to prepare especially for particles below 50-100 nm, which are not accessible to lithographic techniques [5], while the traditional wet chemical methods only allow moderate design variations [11]. Here we describe the fabrication of one such system: an ordered superlattice of monodisperse and isomorphic nickel oxide NP, and its characterization via spectroscopic and theoretical tools. Both fully oxidized and core-shell metal-metal-oxide particles (the latter of interest, e.g., for exchange bias effects [1]) are synthesized via a bottom-up template-directed self-assembly strategy, opening up a new avenue for probing unit and ensemble effects on the physical and chemical properties of oxide materials, from molecule-type cluster sizes to the solid state limit. Our strategy consists of self-assembling metallic NP on an ultrathin nanopatterned aluminum oxide template followed by a morphology-conserving oxidation process, thus producing a superlattice of oxide NP decoupled from a conducting metal substrate by an ultrathin oxide layer, with a very narrow size distribution and identical shapes. We illustrate the procedure for the case of NiO NP, but the method is general (it has been tested for CoO particles with e...

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.

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