Weixin Zhang

Temperature dependence of field emission from cupric oxide nanobelt films

Films of aligned cupric oxide nanobelts have been prepared in an aqueous solution at room temperature. Field-emission characteristics, including emission-current–applied-field plot and emission site distribution, have been studied using the transparent anode technique. In addition, the temperature dependence of the field-emission characteristics has been studied from room temperature to 750 K. The threshold field for obtaining a current density of 10 μA/cm2 is ∼11 MV/m. This decreases with increasing temperature, and at 700 K it is ∼6 MV/m. At a fixed field of 10 MV/m, about a three-orders-of-magnitude increase of the emission current level has been observed. The results show that the cupric oxide nanobelt is a promising candidate for cathode material in a thermoelectric conversion device based on field emission.

Large-Scale Synthesis and Phase Transformation of CuSe, CuInSe2, and CuInSe2/CuInS2 Core/Shell Nanowire Bundles

Facile chemical approaches for the controllable synthesis of CuSe, CuInSe2 nanowire, and CuInSe2/CuInS2 core/shell nanocable bundles were developed. Hexagonal CuSe nanowire bundles with lengths up to hundreds of micrometers, consisting of many aligned nanowires with a diameter of about 10-15 nm, were prepared by reacting cubic Cu(2-x)Se nanowire bundles with a sodium citrate solution at room temperature. The CuSe nanowire bundles were then used as self-sacrificial templates for making bundles of tetragonal chalcopyrite CuInSe2 nanowires by reacting with InCl3 via a solvothermal process. Furthermore, bundles of CuInSe2/CuInS2 core/shell nanocables were obtained by adding sulfur to the reaction system, and the shell thickness of the polycrystalline CuInS2 in the nanocables increased with increasing S/Se molar ratios. It was found that the small radius of copper ions allows their fast outward diffusion from the interior to the surface of nanowires to react with sulfur atoms/anions and indium ions to form a CuInS2 shell. Enhanced optical absorption in the vis-NIR region of CuInSe2/CuInS2 core/shell nanocable bundles is demonstrated, which is considered beneficial for applications in optoelectronic devices and solar energy conversion.

Effects of light illumination on field emission from CuO nanobelt arrays

Effects of pulsed and continuous light illumination on field emission from CuO nanobelt arrays have been studied by using a transparent anode technique. It is found that, at low-field emission current level, the field emission current generally increases under a pulsed irradiation; at an emission current level of 1.1μA, a 19% increase in emission current was recorded. The photoinduced current increase is reduced with increasing emission current and is undetectable when the emission current is higher than 15μA. On the other hand, a long time illumination of the sample decreases the field emission current. Possible physical mechanisms behind the observed phenomena are discussed.

A General and Mild Approach to Controllable Preparation of Manganese‐Based Micro‐ and Nanostructured Bars for High Performance Lithium‐Ion Batteries

One-dimensional (1D) micro- and nanostructured electrode materials with controllable phase and composition are appealing materials for use in lithium-ion batteries with high energy and power densities, but they are challenging to prepare. Herein, a novel ethanol-water mediated co-precipitation method by a chimie douce route (synthesis conducted under mild conditions) has been exploited to selectively prepare an extensive series of manganese-based electrode materials, manifesting the considerable generalizability and efficacy of the method. Moreover, by simply tuning the mixed solvent and reagents, transition metal oxide bars with differing aspect ratios and compositions were prepared with an unprecedented uniformity. Application prospects are demonstrated by Li-rich 0.5 Li2 MnO3 ⋅0.5 LiNi1/3 Co1/3 Mn1/3 O2 bars, which demonstrate excellent reversible capacity and rate capability thanks to the steerable nature of the synthesis and material quality. This work opens a new route to 1D micro- and nanostructured materials by customizing the precipitating solvent to orchestrate the crystallization process.

Effect of structural parameter on field emission properties of semiconducting copper sulphide nanowire films

Films of aligned crystalline copper sulphide (Cu2S) nanowire arrays were grown by using a gas-solid reaction method. By controlling the preparation condition, copper sulphide nanowire arrays with different structural parameters were fabricated. Their field emission characteristics including the total current–applied field characteristics and the distribution of field emission sites are studied and compared using the transparent anode technique. It is found that the turn-on field of the copper sulphide nanowire film relates to the structural parameters of copper sulphide nanowire arrays. Nonlinearity in Fowler–Nordheim plot was observed from film consisting of sparse nanowire arrays with varied field enhancement factor. The physical mechanism accounting for the nonlinearity is discussed.

Self‐Sustained Cycle of Hydrolysis and Etching at Solution/Solid Interfaces: A General Strategy To Prepare Metal Oxide Micro‐/Nanostructured Arrays for High‐Performance Electrodes

Assembling micro-/nanostructured arrays on conducting substrates allows the integration of multiple functionalities into modern electronic devices. Herein, a novel self-sustained cycle of hydrolysis and etching (SCHE) is exploited to selectively synthesize an extensive series of metal oxide micro-/nanostructured arrays on a wide range of metal substrates, establishing the generality and efficacy of the strategy. To demonstrate the potential application of this method, the as-prepared NiO porous nanobelt array was directly used as the anode for lithium-ion batteries, exhibiting excellent capacity and rate capability. Conclusively, the SCHE strategy offers a systematic approach to design metal oxide micro-/nanostructured arrays on metal substrates, which are valuable not only for lithium-ion batteries but also for other energy conversion and storage systems and electronic devices at large.

Cosurfactant-mediated microemulsion to free-standing hierarchical CuO arrays on copper substrates as anodes for lithium-ion batteries

A cosurfactant-mediated microemulsion synthesis of free-standing CuO arrays with hierarchical micro-cog architectures on copper substrates has been successfully established. The CuO cog-array films directly employed as anode electrodes derive from thermal dehydration of Cu(OH)2 arrays grown from copper substrates in the presence of AOT–n-butanol–isooctane–water microemulsions. Introducing n-butanol as a cosurfactant into the ternary AOT–isooctane–water system increases the rigidity of the reverse micelles and it can be selectively adsorbed on particular crystal faces, leading to well-aligned arrays as well as enlarged aspect ratios with average heights of over 6 μm and diameters of 1–2 μm. This result sharply contrasts with the multilayer film of micro-cog particles with a shortened aspect ratio prepared in the absence of n-butanol. The CuO film electrodes of free-standing micro-cog-arrays exhibit excellent electrochemical performance, including a long cycling life (with capacity retention of 91.6% at 1 C over 300 cycles) and outstanding rate capability even at high current rates (about 466 and 418 mA h g−1 at high rates of 12 and 15 C) in lithium ion batteries.

Chemical replacement route to Cu2−xSe-coated CuO nanotube array anode for enhanced performance in lithium ion batteries

The utilization of well-aligned hybrid one-dimensional hollow nanostructured arrays is a promising strategy for the development of transition metal oxides as high-cycle-life stability and high-rate performance electrode materials for lithium ion batteries. Here we report a chemical replacement route to prepare well-aligned Cu2−xSe-coated CuO nanotube arrays with diameters of 400 nm and length of several micrometers, based on Cu(OH)2 nanotube arrays grown on a copper substrate as precursors. As an integrated anode for lithium ion batteries, the Cu2−xSe-coated CuO nanotube array on a copper substrate is capable of delivering a high cycling capacity of 764 mA h g−1 after 100 cycles at a current density of 0.08 mA cm−2 (0.1 C), and retains a discharge capacity of 382.5 mA h g−1 and 94.5 mA h g−1 at current densities of 10 mA cm−2 (12.5 C) and 20 mA cm−2 (25 C), respectively, exhibiting superior performance to the bare CuO nanotube array film. The synergistic effect of the successful integration of the CuO nanotubes with the Cu2−xSe semiconducting coating layer significantly contributes to the enhanced electrochemical properties of the Cu2−xSe-coated CuO nanotube array anode.

Photoelectrochemistry of pure and core/sheath nanowire arrays of Cu2S directly grown on copper electrodes

Photoelectrochemistry (PEC) of straight and isolated Cu 2 S nanowires arrayed on a copper foil is studied. A cathodic photocurrent is observed, which increases with the increasing negative bias of the film electrode, conforming to the p-type semiconducting nature of Cu 2 S. PEC studies on the core/sheath nanowires of Cu 2 S (Cu 2 S nanowires coated with other nanoparticles such as CdS, polypyrrole, and Au) have revealed distinctly different characteristics. Cu 2 S/CdS nanowires exhibit a slightly higher photocurrent response due to the charge transfer between the p-type Cu 2 S core and n-type CdS sheath, whereas the photocurrent response obtained for Cu 2 S/PPy nanowires is generally an order of magnitude smaller than that for pure Cu 2 S nanowires under the same bias voltage. For Cu 2 S/Au nanowires, however, Au nanoparticles can readily accept the photogenerated electrons from the conduction band of Cu 2 S; this redirects the electron transfer pathway and results in the anodic photocurrent response. On the basis of these findings, different photocurrent generation mechanisms are proposed.

Oriented attachment growth of ultra-long Ag2Se crystalline nanowires via water evaporation-induced self-assembly

Ultra-long Ag2Se crystalline nanowires with lengths up to several hundred micrometers and diameters of 100–300 nm have been successfully fabricated by water evaporation-induced growth method at 120 °C for 6 h. Experiments have revealed that the growth of Ag2Se crystalline nanowires is dominated by oriented attachment mechanism. The continuous water evaporation plays an important role for the formation of long nanowires, since it can cause high overall supersaturation and crystallite–solution interfacial supersaturation in the system, which can induce secondary nucleation, and thus promote the oriented attachment growth of long crystalline nanowires. Therefore, it is promising that the water evaporation-induced growth method could be developed for the synthesis of other 1D inorganic nanomaterials.