We show that the noncrystalline-to-crystalline transition of supported Pt nanoparticles (NPs) in the subnanometer to nanometer size range is statistical in nature, and strongly affected by particle size, support, and adsorbates (here we use H2). Unlike in the bulk, a noncrystalline phase exists and is stable in small NPs, reflecting a general mesoscopic feature. Observations of >3000 particles by high-resolution transmission electron microscopy show a noncrystalline-to-crystalline transition zone that is nonabrupt; there is a size regime where disordered and ordered NPs coexist. The NP size at which this transition occurs is strongly dependent on both the adsorbate and the support, and this effect is general for late 5d transition metals. All results are reconciled via a statistical description of particle-support-adsorbate interactions.

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We have studied the structural, morphological, and electronic properties of CuO/CeO 2 and Ce 1Àx Cu x O 2 nanocatalysts during reduction/oxidation cycles using H 2 and O 2 as chemical probes. Time-resolved in situ characterization was performed by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) as well as aberration-corrected environmental transmission electron microscopy (ETEM). We have found that both types of nanocatalysts reduce to a Cu/CeO 2 biphase system with significant oxygen vacancies in CeO 2 . Important variations are seen in the Cu particle size and metal dispersion depending on the initial state of the copper oxi-deÀceria systems. During subsequent in situ oxygen annealing, the Cu precipitated from the CuO/CeO 2 system reoxidized to form CuO through a Cu 2 O intermediate phase as expected. However, the Cu precipitated from the Ce 0.8 Cu 0.2 O 2 solid solution behaved rather differently under oxidizing conditions, and neither oxidized to form CuO nor fully returned to a bulk Ce 0.8 Cu 0.2 O 2 phase in solid solution. We found that ∼50% of the Cu returned to a Ce 1Àx Cu x O 2 solid solution, while the remainder was observed by in situ ETEM to form an amorphous copper oxide phase with a Cu oxidation state similar to Ce 1Àx Cu x O 2 , but with a local bonding environment similar to CuO. The behavior of the reduced Ce 0.8 Cu 0.2 O 2 reflects strong interactions between Cu and the ceria matrix and illustrates the advantages of working with solid solutions of mixed oxides.

Although the (GaN)(1-x)(ZnO)x solid solution is one of the most effective systems for driving overall solar water splitting with visible light, its quantum yield for overall water splitting using visible light photons has not yet reached ten percent. Understanding and controlling the nanoscale morphology of this system may allow its overall conversion efficiency to be raised to technologically relevant levels. We describe the use a Ga2O3(ZnO)16 precursor phase in the synthesis of this phase which naturally results in the production of arrays of nanorods with favorable diameters (∼100 nm) and band gaps (∼2.5 eV). Substantial absorption within the band gap is observed, part of which is found to follow the E(-3) scaling characteristic of free carriers scattered by ionized impurity sites. Compositional analysis suggests that a substantial quantity of cation vacancies (∼3%) may be present in some samples. The typical nanorod growth direction and dominant {1011} facet for powders in this system have been identified through electron microscopy methods, leading to the conclusion that polarity may play an important role in the high photoactivity of this family of wurtzite semiconductors.