A controlled nucleation and growth of Si nanowires by using a TiN diffusion barrier layer for lithium-ion batteries.

Uniform size of Si nanowires (NWs) is highly desirable to enhance the performance of Si NW-based lithium-ion batteries. To achieve a narrow size distribution of Si NWs, the formation of bulk-like Si structures such as islands and chunks needs to be inhibited during nucleation and growth of Si NWs. We developed a simple approach to control the nucleation of Si NWs via interfacial energy tuning between metal catalysts and substrates by introducing a conductive diffusion barrier. Owing to the high interfacial energy between Au and TiN, agglomeration of Au nanoparticle catalysts was restrained on a TiN layer which induced the formation of small Au nanoparticle catalysts on TiN-coated substrates. The resulting Au catalysts led to the nucleation and growth of Si NWs on the TiN layer with higher number density and direct integration of the Si NWs onto current collectors without the formation of bulk-like Si structures. The lithium-ion battery anodes based on Si NWs grown on TiN-coated current collectors showed improved specific gravimetric capacities (>30%) for various charging rates and enhanced capacity retention up to 500 cycles of charging-discharging.

Recyclable Graphene Sheets as a Growth Template for Crystalline ZnO Nanowires.

Recent advances in nanoscience have opened ways of recycling substrates for nanomaterial growth. Novel materials, such as atomically thin materials, are highly desirable for the recycling substrates. In this work, we report recycling of monolayer graphene as a growth template for synthesis of single crystalline ZnO nanowires. Selective nucleation of ZnO nanowires on graphene was elucidated by scanning electron microscopy and density functional theory calculation. Growth and subsequent separation of ZnO nanowires was repeated up to seven times on the same monolayer graphene film. Raman analyses were also performed to investigate the quality of graphene structure along the recycling processes. The chemical robustness of graphene enables the repetitive ZnO nanowire growth without noticeable degradation of the graphene quality. This work presents a route for graphene as a multifunctional growth template for diverse nanomaterials such as nanocrystals, aligned nanowires, other two-dimensional materials, and semiconductor thin films.

Hierarchical assembly of ZnO nanowire trunks decorated with ZnO nanosheets for lithium ion battery anodes.

Hierarchical architectures composed of nanomaterials in different forms are essential to improve the performance of lithium-ion battery (LIB) anodes. Here, we systematically studied the effects of hierarchical ZnO nanostructures on the electrochemical performance of LIBs. ZnO nanowire (NW) trunks were decorated with ZnO NWs or ZnO nanosheets (NSs) by successive hydrothermal synthesis to create hierarchical three-dimensional nanostructures. The branched ZnO NSs on the ZnO NW trunk exhibited a two-fold higher specific gravimetric capacity compared to ZnO NWs and branched ZnO NWs on ZnO NW trunks after 100 cycles of charging-discharging at 0.2C (197.4 mA g-1). The improvement in battery anode performance is attributable to the increased interfacial area between the electrodes and electrolyte, and the void space of the branched NSs that facilitates lithium ion transport and volume changes during cycling.

Degradation of Si/Ge core/shell nanowire heterostructures during lithiation and delithiation at 0.8 and 20 A g-1.

Si/Ge core/shell nanowire heterostructures have been expected to provide high energy and power densities for lithium ion battery anodes due to the large capacity of Si and the high electrical and ionic conductivities of Ge. Although the battery anode performances of Si/Ge core/shell nanowire heterostructures have been characterized, the degradation of Si/Ge core/shell nanowire heterostructures has not been thoroughly investigated. Here we report the compositional and structural changes of the Si/Ge core/shell nanowire heterostructure over cycling of lithiation and delithiation at different charging rates. The Si/Ge core/shell nanowire heterostructure holds the core and shell structure at a charging rate of 0.8 A g-1 up to 50 cycles. On the other hand, compositional intermixing and loss of Si occur at a charging rate of 20 A g-1 within 50 cycles. The operation condition-dependent degradation provides a new aspect of materials research for the development of high performance lithium ion battery anodes with a long cycle life.

Strain-induced structural defects and their effects on the electrochemical performances of silicon core/germanium shell nanowire heterostructures.

We report on strain-induced structural defect formation in core Si nanowires of a Si/Ge core/shell nanowire heterostructure and the influence of the structural defects on the electrochemical performances in lithium-ion battery anodes based on Si/Ge core/shell nanowire heterostructures. The induced structural defects consisting of stacking faults and dislocations in the core Si nanowire were observed for the first time. The generation of stacking faults in the Si/Ge core/shell nanowire heterostructure is observed to prefer settling in either only the Ge shell region or in both the Ge shell and Si core regions and is associated with the increase of the shell volume fraction. The relaxation of the misfit strain in the [112] oriented core/shell nanowire heterostructure leads to subsequent gliding of Shockley partial dislocations, preferentially forming the twins. The observation of crossover of defect formation is of great importance for understanding heteroepitaxy in radial heterostructures at the nanoscale and for building three dimensional heterostructures for the various applications. Furthermore, the effect of the defect formation on the nanomaterial's functionality is investigated using electrochemical performance tests. The Si/Ge core/shell nanowire heterostructures enhance the gravimetric capacity of lithium ion battery anodes under fast charging/discharging rates compared to Si nanowires. However, the induced structural defects hamper lithiation of the Si/Ge core/shell nanowire heterostructure.

Polyoxometalate-mediated one-pot synthesis of Pd nanocrystals with controlled morphologies for efficient chemical and electrochemical catalysis.

Polyoxometalates (POMs), as inorganic ligands, can endow metal nanocrystals (NCs) with unique reactivities on account of their characteristic redox properties. In the present work, we present a facile POM-mediated one-pot aqueous synthesis method for the production of single-crystalline Pd NCs with controlled shapes and sizes. The POMs could function as both reducing and stabilizing agents in the formation of NCs, and thus gave a fine control over the nucleation and growth kinetics of NCs. The prepared POM-stabilized Pd NCs exhibited excellent catalytic activity and stability for electrocatalytic (formic acid oxidation) and catalytic (Suzuki coupling) reactions compared to Pd NCs prepared without the POMs. This shows that the POMs play a pivotal role in determining the catalytic performance, as well as the growth, of NCs. We envision that the present approach can offer a convenient way to develop efficient NC-based catalyst systems.

Anodized pore structural evolution of focused ion beam patterned Al: direct analysis of branched nanopores and nanosacks.

In this work we describe three different trends of pore growth for anodic aluminum oxide nanopores based on their dependence on prepatterned interpore distances. Nanopatterned hexagonal concave arrays were formed by focused ion beam (FIB) lithography on aluminum foil with interpore distances in the range of 100 to 240 nm and the Al foil was anodized under the standard conditions known to result in a 100 nm interpore distance. This method allowed a systematic investigation of pore formation under the non-equilibrium conditions created by the FIB prepatterning. The pore diameter and the pore growth direction, which are affected by the interpore distance, were measured by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis with ion milling. When the interpore distance increases from 100 to 140 nm, the pore diameter becomes larger and nanopores are slightly tilted but maintained the interpore distance and straightness. As the interpore distance increases from 150 to 180 nm, the pore diameter becomes smaller and each nanopore starts to split into two nanopores. At interpore distances of over 190 nm, prepatterned concaves are developed into round flask-shaped nanosacks instead of one-dimensional tubes, and then these are split into three more sub-nanopores. The fundamental characteristics of anodic aluminum oxidation are discussed in accordance with various prepatterned concaves in the nanopore growth processes, providing a rational theory for the design of various complex 3-D AAO structures that can be controlled by prepatterning.

Kinetically controlled growth of polyhedral bimetallic alloy nanocrystals exclusively bound by high-index facets: Au-Pd hexoctahedra.

Au-Pd alloy nanocrystals (NCs) with a hexoctahedral structure, enclosed exclusively by high-index {541} facets, are prepared via the simultaneous reduction of Au and Pd precursors without added seeds or additional metal ions as structure-regulating agents. Manipulating the NC growth kinetics via control of the relative amount of reductant is the key synthetic lever for controlling the morphology of the Au-Pd NCs. The hexoctahedral Au-Pd NCs exhibit higher catalytic performance toward the electro-oxidation of ethanol than low-index-faceted Au-Pd NCs.

Reshaping nanocrystals for tunable plasmonic substrates.

Plasmonic nanostructures with tunable optical properties and their designed spatial arrangements can facilitate a variety of application ranging from plasmonics to biosensors with unprecedented sensitivity. Here we describe a facile and versatile method for fabricating tunable plasmonic substrates based on the reshaping of metal nanocrystals. Anisotropic etching and redeposition of Ag atoms mediated by halide ions transformed Ag nanoprisms deposited on two- or three-dimensional surfaces or in solution into nanostructures with an oblate spheroidal shape, and corresponding localized surface plasmon resonances features could be tuned. The reshaping nanocrystal strategy can even facilitate the preparation of new classes of plasmonic substrates with gradient or patterned plasmonic properties, which cannot be realized easily using existing lithographic techniques. The substrates with gradient plasmonic properties can serve as platforms for tunable surface-enhanced Raman scattering.

Synthesis and photocatalytic properties of Cu2S-Pd4S hybrid nanoplates.

Hex appeal! Cu(2)S-Pd(4)S hybrid nanocrystals with a novel hexagonal nanoplate structure were prepared in high yield by a simple one-pot synthetic method (see figure). Successful synthesis of this unique structure was achieved through a consecutive thermolysis process. The Cu(2)S-Pd(4)S hybrid nanoplates exhibited substantially higher photocatalytic activities than pure Cu(2)S nanoplates.

Controlled synthesis of Pd-Pt alloy hollow nanostructures with enhanced catalytic activities for oxygen reduction.

Pd-Pt alloy nanocrystals (NCs) with hollow structures such as nanocages with porous walls and dendritic hollow structures and Pd@Pt core-shell dendritic NCs could be selectively synthesized by a galvanic replacement method with uniform Pd octahedral and cubic NCs as sacrificial templates. Fine control over the degree of galvanic replacement of Pd with Pt allowed the production of Pd-Pt NCs with distinctly different morphologies. The synthesized hollow NCs exhibited considerably enhanced oxygen reduction activities compared to those of Pd@Pt core-shell NCs and a commercial Pt/C catalyst, and their electrocatalytic activities were highly dependent on their morphologies. The Pd-Pt nanocages prepared from octahedral Pd NC templates exhibited the largest improvement in catalytic performance. We expect that the present work will provide a promising strategy for the development of efficient oxygen reduction electrocatalysts and can also be extended to the preparation of other hybrid or hetero-nanostructures with desirable morphologies and functions.

Controlled synthesis and characterization of the enhanced local field of octahedral Au nanocrystals.

Octahedral Au nanocrystals with localized surface plasmon-assisted enhancing optical properties can be prepared in aqueous solution via the forced reduction of Au ions by ascorbic acid through the addition of NaOH.