The crystal structure of a material is essentially determined by the nature of its chemical bonding. Consequently, the atomic coordination intimately correlates with the degree of ionicity or covalency of the material. Based on this principle, materials with similar chemical compositions can be successfully categorized into different coordination groups. However, counterexamples have recently emerged in complex ternary compounds. For instance, covalent IB-IIIA-VIA2 compounds, such as AgInS2, prefer a tetrahedrally coordinated structure (TCS), while ionic IA-VA-VIA2 compounds, such as NaBiS2, would favor an octahedrally coordinated structure (OCS). One naturally expects that IB-VA-VIA2 compounds with intermediate ionicity or covalency, such as AgBiS2, should then have a mix-coordinated structure (MCS) consisting of covalent AgS4 tetrahedra and ionic BiS6 octahedra. Surprisingly, only the experimental presence of the OCS was observed for AgBiS2. To resolve this puzzle, we perform first-principles studies of the phase stabilities of ternary compounds at finite temperatures. We find that AgBiS2 indeed prefers MCS at the ground state, in agreement with the typical expectation, but under experimental synthesis conditions, disordered OCS becomes energetically more favorable because of its low mixing energy and high configurational entropy. Our work elucidates the critical role of configurational disorder in stabilizing chemically unfavorable coordination, providing a rigorous rationale for the anomalous coordination preference in IB-VA-VIA2 compounds.
Deep-ultraviolet (DUV) light sources are technologically highly important, but DUV light-emitting materials are extremely rare; AlN and its alloys are the only materials known so far, significantly limiting the chemical and structural spaces for materials design. Here, we perform a high-throughput computational search for DUV light emitters based on a set of carefully designed screening criteria relating to the sophisticated electronic structure. In this way, we successfully identify 5 promising material candidates that exhibit comparable or higher radiative recombination coefficients than AlN, including BeGeN2, Mg3NF3, KCaBr3, KHS, and RbHS. Further, we unveil the unique features in the atomic and electronic structures of DUV light emitters and elucidate the fundamental genetic reasons why DUV light emitters are extremely rare. Our study not only guides the design and synthesis of efficient DUV light emitters but also establishes the genetic nature of ultrawide-band-gap semiconductors in general.
Metal cation vacancies, a kind of structural defect, are viewed as a promising strategy for regulating the electronic properties to enhance the catalytic activity. However, the effective introduction of cation vacancies into electrocatalysts still remains a challenge. Herein, we present and elucidate a facile "fast reduction and in situ phase transformation" strategy at room temperature to simultaneously introduce abundant metal cation vacancies (cobalt vacancies and iron vacancies) into Co0.5Fe0.5OOH electrocatalysts. The incorporation of the Fe element could tailor the micrometer-sized ultrathin CoOOH platelets into nanometer-sized ultrathin Co0.5Fe0.5OOH platelets, and the tailoring process is accompanied with the generation of numerous cation vacancies. The defect degree of CoOOH could be effectively tuned by the incorporation of Fe, resulting in more active sites and lower energy barrier, and thereby the intrinsic catalytic activity of electrocatalysts was further enhanced. Compared to CoOOH, the optimized nanometer-sized ultrathin Co0.5Fe0.5OOH platelets (Co0.5Fe0.5OOH-NSUPs) require a smaller overpotential of 220 mV at a current density of 20 mA cm-2, lower Tafel slope of 38.2 mV dec-1, and better long-term durability without obvious decay for more than 200 h at a high current density of 40 mA cm-2. The electrochemical performances are equal to or better than that of the reported first-class electrocatalysts. More importantly, this work provides new perspective for designing and fabricating efficient multimetal electrocatalysts in large scale.
Capacity decay has been a well-known phenomenon in battery technology. V6O13 has been proved to be one of promising cathode materials for the lithium-metal polymer battery owing to high electrochemical capacity and electronic conductivity. However, these V6O13-based cathodes suffer from characteristic capacity decline under operating conditions, and it is also difficult to achieve the theoretical capacities of V6O13. Herein, we report, for the first time, the thermal instability between the components in the cathode composites using various analytical methods, such as in situ thermal gravimetric analysis: infrared spectroscopy, scanning electron microscopy, and X-ray diffraction techniques. This thermal instability is believed to be a chemical reaction between the binding material (polyalkylene glycols) and V6O13, which enables an improved understanding of the decay in the capacity of V6O13-based cathodes and initial capacities that are significantly below the theoretical value. The identification of the reaction between cathode and binding materials may trigger the further investigation of capacity decay of other cathode materials, paving the way to the design and development of high-capacity batteries.
The kilogram-scale fabrication of V6O13 cathode materials has been notably assisted by in situ thermal gravimetric analysis (TGA)-infrared spectroscopy (IR) technology. This technology successfully identified a residue of ammonium metavanadate in commercial V6O13, which is consistent with the X-ray photoelectron spectroscopy result. Samples of V6O13 materials have been fabricated and characterized by TGA-IR, scanning electron microscopy, and X-ray diffraction. The initial testing results at 125 °C have shown that test cells containing the sample prepared at 500 °C show up to a 10% increase in the initial specific capacity in comparison with commercial V6O13.
A facile and efficient seeded growth approach was used to fabricate single-crystal Pt multipods nanocrystals, which were intensively characterized by TEM, ED, HRTEM, XRD, EDX, and XPS. The size and shape of Pt multipod nanocrystals can be easily controlled by varying the ratio of Pt seeds to H2PtCl6. The catalytic performance of these nanocrystals as heterogeneous catalysts was examined using the hydrogenation of cyclohexene as a model reaction in a biphasic system. These Pt nanocrystals should have potential applications as catalysts in organic synthesis, electronics, sensors, and other devices.
Crystallization/methods , Nanostructures/chemistry , Nanostructures/ultrastructure , Nanotechnology/methods , Platinum/chemistry , Macromolecular Substances/chemistry , Materials Testing , Molecular Conformation , Particle Size , Surface Properties
A polyol process was developed to synthesize Co3O4 with controllable superstructures. By tuning the reaction conditions, the prepared Co3O4 were readily regulated in its morphologies, which could vary from nanosphere to two-dimensional (2D) nanoplates and 3D hierarchical structures, and finally to microspheres. The growth kinetics of such a process was also studied. The synthesized Co3O4 exhibited good sensitivity, remarkable selectivity, and high stability as an alcohol sensor material.
A self-assembly technique assisted with surfactant is developed to fabricate one-dimensional (1D) nanostructure of zinc meso-tetra (4-pyridyl) porphyrin. The so-prepared nanostructure appears in a shape of hollow hexagonal nanoprism with uniform size. The length and aspect ratio of the nanoprisms is easily tunable by controlling the stoichiometric ratio of porphyrin over surfactant. The internal structure of the nanoprisms is well described by XRD. Furthermore, as a result of dispersivity and regular geometric shape, these nanoprisms can readily self-organize into an ordered, smectic three-dimensional (3D) architecture through simple evaporation of the solvent. The results should be significant in porphyrin crystallization and porphyrin application in optoelectronic device, catalysis, drug delivery, and molecular filtration.
Highly ordered Ni-Pt multilayered nanowire arrays have been fabricated using a porous anodic aluminum oxide (AAO) template by pulse electrodeposition. The cylindrical Ni nanoparticles with different lengths and diameters in these arrays were characterized by transmission electron microscope (TEM) and alternating-gradient magnetometer (AGM) measurements. Magnetization measurements revealed that an array of such nanowires with 20-nm diameters has an enhanced coercivity (ca. 1169 Oe) and a high remanence ratio (ca. 0.96).
Uniform gold hollow nanospheres with tunable interior-cavity sizes were fabricated by using Co nanoparticles as sacrificial templates and varying the stoichiometric ratio of starting material HAuCl4 over the reductants. The formation of these hollow nanostructures is attributed to two subsequent reduction reactions: the initial reduction of HAuCl4 by Co nanoparticles, followed by the reduction by NaBH4. In addition, a thick layer of silica was successfully coated onto the gold hollow nanospheres. These nanostructures are extensively characterized by TEM, XRD, HRTEM, SEM, electron diffraction, energy-dispersive X-ray analysis, and UV-visible absorption spectroscopy. It is evident that the SPR peak locations corresponding to these hollow nanospheres are shifted over a region of more than 100 nm wavelength due to changes of shell thickness, which make these optically active nanostructures of great interest in both fundamental research and practical applications.
We describe a facile procedure for one step, large-scale synthesis of AuPt bimetallic hollow tube-like 1-D nanomaterials and hollow nanospheres, which can be easily manipulated by merely changing the concentration of citric acid.
The electrical property of a self-assembled monolayer (SAM) of mercaptohexadecanoic acid (HS-C15H30-COOH) has been investigated with a tunneling junction confined in a capillary. This capillary method can shun the interference of vacuum gap. The contact area can be determined according to the diameter of capillary. The nonlinear current-voltage curve and dl/dV curve are measured in the potential range of +/-0.8 V. The slight asymmetry of I-V curve and dl/dV curve is attributed to the different coupling between the metal and the end-groups of molecule. This method will supply a simple way to measure the property of single molecules and monolayers.
Crystallization/methods , Electrochemistry/methods , Electrodes , Gold/chemistry , Nanostructures/chemistry , Nanotechnology/methods , Palmitic Acids/chemistry , Semiconductors , Capillary Action , Electric Conductivity , Electronics , Materials Testing , Nanostructures/analysis , Nanostructures/ultrastructure , Nanotechnology/instrumentation , Particle Size , Surface Properties
