DOI: 10.1002/adma.201400336 substrate, which makes the Co 3 O 4 more easily oxidized. [ 6b ] Our electrochemical study shows that our Au@Co 3 O 4 NCs have an OER activity 7 times as high as a mixture of Au and Co 3 O 4 NCs or Co 3 O 4 NCs alone, and 55 times as high as Au NCs, most likely due to a strong synergistic effect between the core and the shell, and this effect does not exist between the physically mixed NCs. Some Au and Co 3 O 4 hybrid NCs have been reported, [ 10 ] however, none of them have well-defi ned core–shell structures and uniform sizes. High-quality Au@Co 3 O 4 core–shell NCs are still desired. In our experiment, a three-step approach ( Figure 1 a) was adopted to synthesize Au@Co 3 O 4 NCs, comprising synthesis of the Au NC, growth of the Co shell, and oxidation of Co to Co 3 O 4 . First, Au NCs were prepared by reducing HAuCl 4 with tertbutylamine borane (TBAB) in the presence of oleylamine (OAm) as the ligand, following the procedure described in a previous study by Peng et al. [ 11 ] Second, Co shells were grown on the Au NC cores to prepare Au@Co NCs by using Co(acac) 2 , where acac is acetylacetonate, as the cobalt source and TBAB as the reducing agent. OAm and oleic acid (OA) were introduced to control the shape and uniformity. Third, the Au@Co NCs were loaded on carbon and then the Co shells were oxidized to Co 3 O 4 by calcination in air. The experimental details are described in the Supporting Information. Figure 1 b shows the transmission electron microscopy (TEM) image of the as-obtained Au NCs. They have a narrow size distribution with a diameter of 3.6 ± 0.5 nm. Five-fold symmetry is found in the high-resolution TEM (HRTEM) image (Figure S1, Supporting Information), which is in agreement with the literature, indicating the multiple-twinned structure of the Au NCs. [ 11 ] Figure 1 c shows a TEM image of Au@Co core–shell NCs that were synthesized by growing Co shells on the pre-synthesized Au cores with 0.5 mmol OA. A dark core corresponding to Au can clearly be seen located at the center of the NC, and a uniform lighter Co shell caps around it. The Au@Co NCs are nearly monodisperse with an overall diameter of 8.1 ± 0.7 nm. The thickness of the Co shell is ca. 2 nm. The Au@Co NCs are highly uniform so that they can assemble into an ordered structure (Figure 1 d). The energy dispersive spectrometry (EDS) spectra (Figure S2a, Supporting Information) show the signals of Au and Co (atomic ratio 1:4), which confi rms the hybrid Au@Co composition. The Co shell seems to be amorphous because no clear lattice fringe can be seen in the HRTEM image (Figure 1 e). This may be due to the lattice mismatch between Au (face-centered cubic, fcc) and Co (hexagonal close packed, hcp). The multiple twinned nature of the Au core may also infl uence the crystallinity of the Co shell. It is noted that Co NCs cannot be synthesized under the same condition The hydrogen economy can provide an effi cient energy system that is free from environmental issues related to the combustion of coal, oil, and natural gas. [ 1 ] However, such a system requires a clean and sustainable source of hydrogen, which can be provided by splitting of water either electrochemically or photoelectrochemically. [ 2 ] One of the key problems in splitting water is the kinetically sluggish anode reaction, i.e., oxygen evolution reaction (OER, 4OH − → 2H 2 O + 4e − + O 2 in base). An overpotential of several hundred millivolts is often required to achieve a current density of 10 A g catalyst −1 . [ 3 ] Recent studies have shown that spinel-type Co 3 O 4 has relatively good OER activities. [ 2e , 3c , 4 ] Hybrid materials have been proposed to further promote the OER activity of Co 3 O 4 , such as doping Co 3 O 4 with other metals to make substituted cobaltites [ 5 ] or growing Co 3 O 4 on a special substrate. [ 6 ]

Synthesis of Monodispere Au@Co3O4 Core‐Shell Nanocrystals and Their Enhanced Catalytic Activity for Oxygen Evolution Reaction

Hybrid nanoparticles combine two or more disparate materials on the same nanosystem and represent a powerful approach for achieving advanced materials with multiple functionalities stemming from the unusual materials combinations. This review focuses on recent advances in the area of semiconductor–metal hybrid nanoparticles. Synthesis approaches offering high degree of control over the number of components, their compositions, shapes, and interfacial characteristics are discussed, including examples of advanced architectures. Progress in hybrid nanoscale inorganic cage structures prepared by a selective edge growth mechanism of the metal onto the semiconductor nanocrystal is also presented. The combined and often synergistic properties of the hybrid nanoparticles are described with emphasis on optical properties, electronic structure, electrical characteristics, and light induced charge separation effects. Progress toward the application of hybrid nanoparticles in photocatalysis is overviewed. We conclude...

Hybrid Semiconductor–Metal Nanoparticles: From Architecture to Function

Conversion chemistry is a rapidly maturing field, where chemical conversion of template nanoparticles (NPs) into new compositions is often accompanied by morphological changes, such as void formation. The principles and examples of three major classes of conversion chemical reactions are reviewed: the Kirkendall effect for metal NPs, galvanic exchange, and anion exchange, each of which can result in void formation in NPs. These reactions can be used to obtain complex structures that may not be attainable by other methods. During each kind of conversion chemical reaction, NPs undergo distinct chemical and morphological changes, and insights into the mechanisms of these reactions will allow for improved fine control and prediction of the structures of intermediates and products. Conversion of metal NPs into oxides, phosphides, sulphides, and selenides often occurs through the Kirkendall effect, where outward diffusion of metal atoms from the core is faster than inward diffusion of reactive species, resulting in void formation. In galvanic exchange reactions, metal NPs react with noble metal salts, where a redox reaction favours reduction and deposition of the noble metal (alloying) and oxidation and dissolution of the template metal (dealloying). In anion exchange reactions, addition of certain kinds of anions to solutions containing metal compound NPs drives anion exchange, which often results in significant morphological changes due to the large size of anions compared to cations. Conversion chemistry thus allows for the formation of NPs with complex compositions and structures, for which numerous applications are anticipated arising from their novel catalytic, electronic, optical, magnetic, and electrochemical properties.

Nanoparticle conversion chemistry: Kirkendall effect, galvanic exchange, and anion exchange

Compartmentalization is ubiquitous to many biological and artificial systems, be it for the separate storage of incompatible matter or to isolate transport processes. Advancements in the synthesis of sequential block copolymers offer a variety of tools to replicate natural design principles with tailor-made soft matter for the precise spatial separation of functionalities on multiple length scales. Here, we review recent trends in the self-assembly of amphiphilic block copolymers to multicompartment nanostructures (MCNs) under (semi-)dilute conditions, with special emphasis on ABC triblock terpolymers. The intrinsic immiscibility of connected blocks induces short-range repulsion into discrete nano-domains stabilized by a third, soluble block or molecular additive. Polymer blocks can be synthesized from an arsenal of functional monomers directing self-assembly through packing frustration or response to various fields. The mobility in solution further allows the manipulation of self-assembly processes into specific directions by clever choice of environmental conditions. This review focuses on practical concepts that direct self-assembly into predictable nanostructures, while narrowing particle dispersity with respect to size, shape and internal morphology. The growing understanding of underlying self-assembly mechanisms expands the number of experimental concepts providing the means to target and manipulate progressively complex superstructures.

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Self-assembly concepts for multicompartment nanostructures

A simple yet efficient visible-light-driven CdS nanowires-carbon nanotube 1D–1D nanocomposite photocatalyst

A methodology providing access to dumbbell-tipped, metal–semiconductor and metal oxide–semiconductor heterostructured nanorods has been developed. The synthesis and characterization of CdSe@CdS nanorods incorporating ferromagnetic cobalt nanoinclusions at both nanorod termini (i.e., dumbbell morphology) are presented. The key step in the synthesis of these heterostructured nanorods was the decoration of CdSe@CdS nanorods with platinum nanoparticle tips, which promoted the deposition of metallic CoNPs onto Pt-tipped CdSe@CdS nanorods. Cobalt nanoparticle tips were then selectively oxidized to afford CdSe@CdS nanorods with cobalt oxide domains at both termini. In the case of longer cobalt-tipped nanorods, heterostructured nanorods were observed to self-organize into complex dipolar assemblies, which formed as a consequence of magnetic associations of terminal CoNP tips. Colloidal polymerization of these cobalt-tipped nanorods afforded fused nanorod assemblies from the oxidation of cobalt nanoparticle tips a...

Directing the deposition of ferromagnetic cobalt onto Pt-tipped CdSe@CdS nanorods: synthetic and mechanistic insights.

We report a novel methodology to prepare hierarchical structured Au–Co3O4 materials composed of Co3O4 nanowires decorated with Au peripheral inclusions. The electrochemical activity of these materials was also demonstrated by fabrication into negative electrodes for rechargeable Li-batteries.

Sean E. Derosa

Papers

Directing the deposition of ferromagnetic cobalt onto Pt-tipped CdSe@CdS nanorods: synthetic and mechanistic insights.

Morphological conversion of dipolar core–shell Au–Co nanoparticles into beaded Au–Co3O4 nanowires