The effect of lattice strain on the catalytic properties of Pd nanoparticles is systematically studied. Synthetic strategies for the preparation of a series of shape-controlled Pd nanocrystals with lattice strain generated from different sources has been developed. All of these nanocrystals were created with the same capping agent under similar reaction conditions. First, a series of Pd nanoparticles was synthesized that were enclosed in {111} surfaces: Single-crystalline Pd octahedra, single-crystalline AuPd core-shell octahedra, and twinned Pd icosahedra. Next, various {100}-terminated particles were synthesized: Single-crystalline Pd cubes and single-crystalline AuPd core-shell cubes. Different extents of lattice strain were evident by comparing the X-ray diffraction patterns of these particles. During electrocatalysis, decreased potentials for CO stripping and increased current densities for formic-acid oxidation were observed for the strained nanoparticles. In the gas-phase hydrogenation of ethylene, the activities of the strained nanoparticles were lower than those of the single-crystalline Pd nanoparticles, perhaps owing to a larger amount of cetyl trimethylammonium bromide on the surface.

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Submicron-sized mesoporous nickel ferrite (NiFe 2 O 4 ) spheres were prepared by an aerosol spray pyrolysis method using Pluronic F127 as a structure-directing agent, and their photocatalytic performance for hydrogen (H 2 ) evolution was examined in an aqueous MeOH solution by visible light irradiation (λ > 420 nm). The structure of the spherical mesoporous nickel ferrites was studied by transmission electron microscopy, powder X-ray diffraction, and N 2 adsorption−desorption isotherm measurements. Mesoporous NiFe 2 O 4 spheres of high specific surface area (278 m 2 g −1 ) with a highly crystalline framework were prepared by adjusting the amount of structure-directing agent and the calcining condition. High photocatalytic activity of mesoporous NiFe 2 O 4 for H 2 evolution from water with methanol was achieved due to the combination of high surface area and high crystallinity of the nickel ferrites. ■ INTRODUCTIONMetal oxides are of great interest for a variety of applications due to their unique catalytic, photocatalytic, electronic, and optical properties. 1,2 Chemical composition, crystallinity, and surface area are all primary determinants for the performance of metal oxides in catalysis and photocatalysis. 3,4 In particular, multicomponent metal oxides with high surface area could have great catalytic performance due to the synergetic composite effect attributed to the multicomponent structure and the larger number of active sites available through the high surface area. 5−8 Spinel nickel ferrite (NiFe 2 O 4 ), a multicomponent metal oxide composed of only earth-abundant metals, is an attractive material due to its promising catalytic applications in various reactions such as sulfuric acid decomposition, 9,10 selective oxidation of CO, 11 thermochemical water splitting, 12−14 and electrocatalytic hydrogen (H 2 ) evolution. 15 Recently, photocatalytic H 2 evolution utilizing solar energy has attracted much attention for realizing an energy sustainable society. 16−22 Nickel ferrite has been reported to exhibit photocatalytic activity for visible light-driven H 2 evolution 23,24 and water oxidation. 25 In addition, the ferromagnetic properties of nickel ferrite have allowed the easy separation of its particles from a reaction solution. 25 Nickel ferrite has been prepared by several different methods for morphological control. 26−32 For example, a hydrothermal method in a basic solution was reported to prepare micronsized octahedral nickel ferrite crystals. 28 Nickel ferrite nanoparticles with a size of 7.1 nm were prepared by a solvothermal method in hexanol. 29 Sol−gel combustion can also produce nano-sized nickel ferrites by using citric acid as a capping agent. 30 Monodispersed nickel ferrite was reported to be prepared by sol−gel coprecipitation of nickel and iron chloride in ethylene glycol. 31 To increase the surface area for catalysis applications, Shi et al. have demonstrated the first mesoporous nickel ferrite synthesis by calcining NiFe 2 (C 2 O 4 ) 3 as a precursor. 32 In their work, the surface area...

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