Chengxin Wang

Amorphous nickel hydroxide nanospheres with ultrahigh capacitance and energy density as electrochemical pseudocapacitor materials

Among numerous active electrode materials, nickel hydroxide is a promising electrode in electrochemical capacitors. Nickel hydroxide research has thus far focused on the crystalline rather than the amorphous phase, despite the impressive electrochemical properties of the latter, which includes an improved electrochemical efficiency due to disorder. Here we demonstrate high-performance electrochemical supercapacitors prepared from amorphous nickel hydroxide nanospheres synthesized via simple, green electrochemistry. The amorphous nickel hydroxide electrode exhibits high capacitance (2,188u2009Fu2009g−1), and the asymmetric pseudocapacitors of the amorphous nickel hydroxide exhibit high capacitance (153u2009Fu2009g−1), high energy density (35.7u2009Wu2009hu2009kg−1 at a power density of 490u2009Wu2009kg−1) and super-long cycle life (97% and 81% charge retentions after 5,000 and 10,000 cycles, respectively). The integrated electrochemical performance of the amorphous nickel hydroxide is commensurate with crystalline materials in supercapacitors. These findings promote the application of amorphous nanostructures as advanced electrochemical pseudocapacitor materials.

Directional Fano Resonance in a Silicon Nanosphere Dimer

Fano resonance arising from the interaction between a broad bright mode and a narrow dark mode has been widely investigated in symmetry-breaking structures made of noble metals such as plasmonic asymmetric oligomers or other well-designed nanostructures. However, Fano resonance in nanoscale all-dielectric dimers has not been experimentally demonstrated so far. We report the first experimental observation of directional Fano resonance in silicon nanosphere dimers (both homodimer and heterodimer) and clarify that the coupling between magnetic and electric dipole modes can easily generate Fano resonance in all-dielectric oligomers, distinctly differing from conventional Fano resonances based on electric responses or artificial optical magnetism. A silicon nanosphere dimer, exhibiting a strong magnetic response inside and an electric enhancement in the gap, is an excellent structure to support magnetic-based Fano scattering. Interactions between magnetic and electric dipoles can suppress backward scattering and enhance forward scattering at Fano wavelengths. This directional scattering is much more prominent than that from a single silicon sphere and shows promising applications in areas such as directional nanoantenna or optical switching, opening up avenues for developing all-dielectric low-loss metamaterials or nanophotonic devices at visible wavelengths.

Nucleation and growth kinetics of nanocrystals formed upon pulsed-laser ablation in liquid

To have a clear physical insight into the nanocrystal formation upon pulsed-laser ablation in liquid, we proposed a theoretical kinetic approach to elucidate the nucleation and growth of nanocrystals with respect to the capillary effect of the nanometer-sized curvature of crystalline nuclei. Taking the nanodiamond synthesis by pulsed-laser ablating a graphite target in water as an example, we predicted the nucleation time, growth velocity, and the grown size of nanodiamonds on the basis of the proposed kinetic model, and found that these theoretical results are in well agreement with our experiment cases. We expected that the kinetic approach is generally applicable to understanding the basic physics of nanocrystal formation in pulsed-laser ablation in liquid.

Electronic Reconstruction of α-Ag2WO4 Nanorods for Visible-Light Photocatalysis

α-Ag2WO4 (AWO) has been studied extensively due to its H2 evolution and organic pollution degradation ability under the irradiation of UV light. However, the band gap of AWO is theoretically calculated to be 3.55 eV, resulting in its sluggish reaction to visible light. Herein, we demonstrated that, by using the electronic reconstruction of AWO nanorods upon a unique process of laser irradiation in liquid, these nanorods performed good visible-light photocatalytic organics degradation and H2 evolution. Using commercial AWO powders as the starting materials, we achieved the electronic reconstruction of AWO by a recrystallization of the starting powders upon laser irradiation in liquid and synthesized AWO nanorods. Due to the weak bond energy of AWO and the far from thermodynamic equilibrium process created by laser irradiation in liquid, abundant cluster distortions, especially [WO6] cluster distortions, are introduced into the crystal lattice, the defect density increases by a factor of 2.75, and uneven intermediate energy levels are inset into the band gap, resulting in a 0.44 eV decrease of the band gap, which modified the AWO itself by electronic reconstruction to be sensitive to visible light without the addition of others. Further, the first-principles calculation was carried out to clarify the electronic reconstruction of AWO, and the theoretical results confirmed the deduction based on the experimental measurements.

A general strategy to fabricate simple polyoxometalate nanostructures: electrochemistry-assisted laser ablation in liquid.

Polyoxometalate nanostructures have attracted much attention because of significant technical demands in applications such as catalysts, sensors, and smart windows. Therefore, researchers have recently developed many methods for the synthesis of these nanomaterials. However, these techniques have many visible flaws such as high temperatures or high pressure environments, various templates or additives, demanding and complicated synthesis procedures as well as the presence of impurities in the final products. We therefore propose a general strategy for the fabrication of particular polyoxometalate nanostructures by electrochemically assisted laser ablation in liquid (ECLAL). These polyoxometalates are usually simple as they typically contain two metals and are not soluble in water. This approach is a green, simple, and catalyst-free approach under an ambient environment. Apart from these merits, this novel technique allows researchers to choose and design interesting solid targets and to use an electrochemical approach toward the fabrication of polyoxometalate nanostructures for the purpose of fundamental research and for potential applications. Using the synthesis of Cu(3)Mo(2)O(9) nanorods as an example, we substantiate the validity of the proposed strategy. For the fabrication of Cu(3)Mo(2)O(9) nanostructures, we chose molybdenum as a solid target for laser ablation in liquid copper electrodes for the electrochemical reaction and water as a solvent for the ECLAL synthesis. We successfully fabricated Cu(3)(OH)(2)(MoO(4))(2) nanorods with magnetic properties. Interestingly, we obtained well-defined Cu(3)Mo(2)O(9) nanorods by annealing the Cu(3)(OH)(2)(MoO(4))(2) nanostructures at 500 °C. Additionally, the basic physics and chemistry involved in the ECLAL fabrication of nanostructures are discussed.

Magnetically induced forward scattering at visible wavelengths in silicon nanosphere oligomers

Electromagnetically induced transparency is a type of quantum interference that induces near-zero reflection and near-perfect transmission. As a classical analogy, metal nanostructure plasmonic ‘molecules produce plasmon-induced transparency conventionally. Herein, an electromagnetically induced transparency interaction is demonstrated in silicon nanosphere oligomers, wherein the strong magnetic resonance couples with the electric gap mode effectively to markedly suppress reflection. As a result, a narrow-band transparency window created at visible wavelengths, called magnetically induced transparency, is easily realized in nearly touching silicon nanospheres, exhibiting low dependence on the number of spheres and aggregate states compared with plasmon induced transparency. A hybridization mechanism between magnetic and electric modes is proposed to pursue the physical origin, which is crucial to build all-dielectric metamaterials. Remarkably, magnetic induced transparency effect exhibiting near-zero reflection and near-perfect transmission causes light to propagate with no extra phase change. This makes silicon nanosphere oligomers promising as a unit cell in epsilon-near-zero metamaterials.

Origin of Self-Limiting Oxidation of Si Nanowires

A new kinetic model is suggested to describe the self-limiting oxidation of Si nanowires by only considering the diffusion step with the influence of stress due to the two-dimension nonuniform deformation of the oxide but not including any rate-limiting step for interfacial reaction. It is assumed the stress results in the change of distribution of diffusion activation energy in the high density region which rises monotonically along with the oxidation, and may be the main physical origin of the self-limiting oxidation behavior of SiNWs. Moreover, the present kinetic model can excellently describe the experimental results for the wide initial diameter over the range of self-limiting oxidation temperature.

Carbyne with finite length: The one-dimensional sp carbon

Carbyne with one-dimensional sp-hybridized carbon atoms is synthesized under ambient conditions in the laboratory. Carbyne is the one-dimensional allotrope of carbon composed of sp-hybridized carbon atoms. Definitive evidence for carbyne has remained elusive despite its synthesis and preparation in the laboratory. Given the remarkable technological breakthroughs offered by other allotropes of carbon, including diamond, graphite, fullerenes, carbon nanotubes, and graphene, interest in carbyne and its unusual potential properties remains intense. We report the first synthesis of carbyne with finite length, which is clearly composed of alternating single bonds and triple bonds, using a novel process involving laser ablation in liquid. Spectroscopic analyses confirm that the product is the structure of sp hybridization with alternating carbon-carbon single bonds and triple bonds and capped by hydrogen. We observe purple-blue fluorescence emissions from the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of carbyne. Condensed-phase carbyne crystals have a hexagonal lattice and resemble the white crystalline powder produced by drying a carbyne solution. We also establish that the combination of gold and alcohol is crucial to carbyne formation because carbon-hydrogen bonds can be cleaved with the help of gold catalysts under the favorable thermodynamic environment provided by laser ablation in liquid and because the unique configuration of two carbon atoms in an alcohol molecule matches the elementary entity of carbyne. This laboratory synthesis of carbyne will enable the exploration of its properties and applications.

A microfibre assembly of an iron-carbon composite with giant magnetisation.

Iron carbide is among the oldest known materials. The utility of this ancient advanced material is greatly extended in its nanostructured forms. We demonstrate for the first time that one-dimensional iron carbide microfibres can be assembled in liquid using strong magnetic field-assisted laser ablation. The giant saturation magnetisation of these particles was measured a 261u2005emu/g at room temperature, which is the best value reported to date for iron nitride and carbide nanostructures, is 5.5 times greater than the 47u2005emu/g reported for Fe3C nanoparticles, and exceeds the 212u2005emu/g for bulk Fe. The magnetic field-induced dipolar interactions of the magnetic nanospheres and the nanochains played a key role in determining the shape of the product. These findings lead to a variety of promising applications for this unique nanostructure including its use as a magnetically guided transporter for biomedicine and as a magnetic recording material.

Anomalous interfacial diffusion in immiscible metallic multilayers: a size-dependent kinetic approach

In order to gain a better understanding of the interfacial alloying in binary immiscible metallic mutilayers, we established a size-dependent kinetic model to address the diffusion that takes place at the interface in the immiscible metallic bilayer on nanoscale, and found an abnormal interfacial diffusion behavior when the diffusion length less than 2 nm. The nanosize-dependent diffusion coefficients of binary immiscible metals in bilayers were predicted on the basis of the proposed model. Taking the Cu–Ta multilayers as an example, we showed that these theoretical predictions are in agreement with experiments and computational simulations.