Platinum-based alloys have been extensively shown to be effective catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Most of these catalysts are nanoparticles without shape control. Recently, extended Pt(3)Ni(111) surfaces prepared in ultrahigh vacuum were demonstrated to possess enhanced ORR catalytic activity as compared to the state-of-the-art carbon supported Pt (Pt/C) nanoparticle catalysts. How and whether this promising surface can be transformed into practical nanoscale electrocatalysts used in PEMFCs remain a challenge. We report a new wet-chemical approach of preparing monodisperse Pt(3)Ni nanoctahedra and nanocubes terminated with {111} and {100} facets, respectively. We further show that the ORR activity on the Pt(3)Ni nanoctahedra is approximately 5-fold higher than that of nanocubes with a similar size. Comparison of ORR activity between carbon-supported Pt(3)Ni nanoctahedra and commercial Pt/C reveals that the Pt(3)Ni nanoctahedra are highly active electrocatalysts. This synthetic strategy may be extended to the preparation of other shape-controlled fuel cell electrocatalysts.

The formation of novel and complex structures with specific morphologies from nanocrystals via a direct assembly of atoms or ions remains challenging. In recent years, researchers have focused their attention on nanocrystals of noble metals and their controlled synthesis, characterization, and potential applications. Although the synthesis of various noble metal nanocrystals with different morphologies has been reported, most studies are limited to low-index facet-terminated nanocrystals. High-index facets, denoted by a set of Miller indices {hkl} with at least one index greater than unity, possess a high density of low-coordinated atoms, steps, edges, and kinks within these structures and serve as more active catalytic sites. With the potential for enhanced catalytic performance, researchers have used the insights from shape-controlled nanocrystal synthesis to construct noble metal nanocrystals bounded with high-index facets. Since the report of Pt tetrahexahedral nanocrystals, researchers have achieved significant progress and have prepared nanocrystals with various high-index facets. Because of the general order of surface energy for noble metals, high-index facets typically vanish faster in a crystal growth stage and are difficult to preserve on the surface of the final nanocrystals. Therefore researchers have had limited opportunities to examine high-indexed noble metal nanocrystals with a controlled morphology and investigate their resultant behaviors in depth. In this Account, we thoroughly discuss the basic concepts and state-of-the-art morphology control of some noble metal nanocrystals enclosed with high-index facets. We briefly introduce high-index facets from both crystallographic and geometrical points of view, both of which serve as methods to classify these high-index facets. Then, we summarize various typical noble metal nanocrystals terminated by different types of high-index facets, including {hk0} (h > k > 0), {hhl} (h > l > 0), {hkk} (h > k > 0), and {hkl} (h > k > l > 0). In each type, we describe several distinct morphologies including convex, concave, and other irregular shapes in detail. Based on these remarks, we discuss key factors that may induce the variations of Miller indices in each class, such as organic capping ligands and metallic cationic species. In a look at applications, we review several typical high-indexed noble metal nanocrystals showing enhanced electrocatalytic or chemical catalytic activities.

A facile, reliable, general, and robust synthetic method for preparation of high-quality, (100)-terminated Pt(3)M nanocubes (M = Pt or 3d-transition metals Co, Fe, and Ni) has been developed. It was identified that addition of W(CO)(6) is crucial for control of the nucleation process when the metallic precursors are reduced, whereas an optimized ratio of the solvent pair, oleylamine and oleic acid, is the key to enabling the lowest total surface energy on {100} facets in order to develop cubic nanocrystals in the present system. The resultant monodisperse nanocubes, in which Pt is partially substituted, are expected to exhibit unusual electrocatalytic characteristics, providing an alternative for developing high-performance electrocatalysts for use in fuel cells.

Spherical SiO2 particles have been successfully coated with YVO4:Eu3+ phosphor layers through a Pechini sol−gel process. The resulted YVO4:Eu3+@SiO2 core−shell phosphors were characterized by X-ray diffraction (XRD), Fourier-transform IR spectroscopy, scanning electron microscopy, X-ray photoelectron spectra, transmission electron microscopy, UV/vis absorption spectra, general and time-resolved photoluminescence spectra, as well as kinetic decays. The XRD results demonstrate that the YVO4:Eu3+ layers begin to crystallize on the SiO2 particles after annealing at 400 °C, and the crystallinity increases with raising the annealing temperature. The obtained core−shell phosphors have perfect spherical shape with narrow size distribution (average size ca. 500 nm), nonagglomeration, and smooth surface. The thickness of the YVO4:Eu3+ shells on SiO2 cores could be easily tailored by varying the number of deposition cycles (60 nm for two deposition cycles). The Eu3+ shows a strong photoluminescence (PL) (dominated by 5D0−7F2 red emission at 617 nm) due to an efficient energy transfer from vanadate groups to Eu3+. The energy transfer process was further studied by the time-resolved emission spectra as well as kinetic decay curves of Eu3+ upon excitation into the VO4 3- ion. The PL intensity of Eu3+ increases with raising the annealing temperature and the number of coating cycles, and optimum polyethylene glycol concentration in the precursor solution was determined to be 0.08 g/mL for obtaining the strongest emission of Eu3+.