Huaping Wu

Solvent-Assisted Oxygen Incorporation of Vertically Aligned MoS2 Ultrathin Nanosheets Decorated on Reduced Graphene Oxide for Improved Electrocatalytic Hydrogen Evolution.

Three-dimensional oxygen-incorporated MoS2 ultrathin nanosheets decorated on reduced graphene oxide (O-MoS2/rGO) had been successfully fabricated through a facile solvent-assisted hydrothermal method. The origin of the incorporated oxygen and its incorporation mechanism into MoS2 were carefully investigated. We found that the solvent N,N-dimethylformamide (DMF) was the key as the reducing agent and the oxygen donor, expanding interlayer spaces and improving intrinsic conductivity of MoS2 sheets (modulating its electronic structure and vertical edge sites). These O dopants, vertically aligned edges and decoration with rGO gave effectively improved double-layer capacitance and catalytic performance for hydrogen evolution reaction (HER) of MoS2. The prepared O-MoS2/rGO catalysts showed an exceptional small Tafel slope of 40 mV/decade, a high current density of 20 mA/cm(2) at the overpotential of 200 mV and remarkable stability even after 2000th continuous HER test in the acid media.

Hollow Au–Cu2O Core–Shell Nanoparticles with Geometry-Dependent Optical Properties as Efficient Plasmonic Photocatalysts under Visible Light

Hollow Au-Cu2O core-shell nanoparticles were synthesized by using hollow gold nanoparticles (HGNs) as the plasmon-tailorable cores to direct epitaxial growth of Cu2O nanoshells. The effective geometry control of hollow Au-Cu2O core-shell nanoparticles was achieved through adjusting the HGN core sizes, Cu2O shell thicknesses, and morphologies related to structure-directing agents. The morphology-dependent plasmonic band red-shifts across the visible and near-infrared spectral regions were observed from experimental extinction spectra and theoretical simulation based on the finite-difference time-domain method. Moreover, the hollow Au-Cu2O core-shell nanoparticles with synergistic optical properties exhibited higher photocatalytic performance in the photodegradation of methyl orange when compared to pristine Cu2O and solid Au-Cu2O core-shell nanoparticles under visible-light irradiation due to the efficient photoinduced charge separation, which could mainly be attributed to the Schottky barrier and plasmon-induced resonant energy transfer. Such optical tunability achieved through the hollow cores and structure-directed shells is of benefit to the performance optimization of metal-semiconductor nanoparticles for photonic, electronic, and photocatalytic applications.

Orientation dependence of dielectric behavior of ferroelectric bilayers and multilayers

A thermodynamic model is presented to describe the dielectric response of ferroelectric bilayers and multilayers with different orientations. Numerical analyses for PbTiO3/SrTiO3 epitaxially heteroepitaxial bilayer films on thick substrates show that complete polarization suppression and gigantic dielectric response occur at approximately 46%, 50%, and 63% of SrTiO3 fraction in the bilayer structures with (111), (110), and (001) orientations, respectively. Our study indicates that the ferroelectric and dielectric performances of ferroelectric bilayers and multilayers can be modified by changing the thicknesses and orientations of component layers, which is an effective approach for the optimization of microwave devices.

Three-dimensional graphene biointerface with extremely high sensitivity to single cancer cell monitoring

We developed a three-dimensional biointerface of graphene-based electrical impedance sensor for metastatic cancer diagnosis at single-cell resolution. Compared with traditional impedance sensor with two-dimensional interface, the graphene biointerface mimiced the topography and somatotype features of cancer cells, achieving more comprehensive and thorough single cell signals in the three-dimensional space. At the nodes of physiological behavior change of single cell, namely cell capture, adhesion, migration and proliferation, the collected electrical signals from graphene biointerface were about two times stronger than those from the two-dimensional gold interface due to the substantial increase in contact area and significant improvement of topographical interaction between cells and graphene electrode. Simultaneous CCD recording and electrical signal extraction from the entrapped single cell on the graphene biointerface enabled to investigate multidimensional cell-electrode interactions and predict cancerous stage and pathology.

Effect of crystal orientation on the phase diagrams, dielectric and piezoelectric properties of epitaxial BaTiO3 thin films

The influence of crystal orientations on the phase diagrams, dielectric and piezoelectric properties of epitaxial BaTiO3 thin films has been investigated using an expanded nonlinear thermodynamic theory. The calculations reveal that crystal orientation has significant influence on the phase stability and phase transitions in the misfit strain-temperature phase diagrams. In particular, the (110) orientation leads to a lower symmetry and more complicated phase transition than the (111) orientation in BaTiO3 films. The increase of compressive strain will dramatically enhance the Curie temperature TC of (110)-oriented BaTiO3 films, which matches well with previous experimental data. The polarization components experience a great change across the boundaries of different phases at room temperature in both (110)- and (111)-oriented films, which leads to the huge dielectric and piezoelectric responses. A good agreement is found between the present thermodynamics calculation and previous first-principles calculations. Our work provides an insight into how to use crystal orientation, epitaxial strain and temperature to tune the structure and properties of ferroelectrics.

Adjustable plasmonic optical properties of hollow gold nanospheres monolayers and LSPR-dependent surface-enhanced Raman scattering of hollow gold nanosphere/graphene oxide hybrids

Colloidal hollow gold nanospheres (HGNs) with adjustable localized surface plasmon resonance (LSPR) properties were synthesized and self-assembled into HGNs monolayers as a proper substrate for investigation of LSPR-dependent surface enhanced Raman scattering (SERS) behavior with Rhodamine B as the probe molecule. The SPR peaks of colloidal HGNs could be tunable over a wavelength region from 540 nm to 630 nm by carefully tailoring HGNs sizes and wall thicknesses. The broadening and red shift of SPR peaks of self-assembled HGNs monolayers were attributed to the electromagnetic coupling of adjacent HGN plasmons and the change of dielectric constant around HGNs when compared to those of colloidal HGNs. The relative SPR peak shift decayed nearly exponentially with the ratio of gap separation to diameter of HGNs, which fitted well with the “plasmon ruler” behavior of solid gold nanospheres and HGN dimeric reported previously. Our results indicated that the SERS performance of HGNs monolayers was highly LSPR-dependent and an obvious enhancement in the Raman signals was collected when the laser excitation line matched with the peak position of LSPR band of HGNs monolayers. Furthermore, the sandwich-structured HGN/graphene oxide/HGN (HGN/GO/HGN) hybrid was constructed through layer-by-layer method with spin-coated GO nanosheets as the interlayer. The cladding structures had better SERS activity than the HGNs monolayer due to the chemical enhancement of GO and coupled electromagnetic enhancement of double HGNs in the horizontal and vertical directions. The designed HGN/GO/HGN hybrid configuration with LSPR-dependent SERS performance exhibited a potential application in chemical sensors, environmental monitoring, disease controlling, and food safety fields.

One-step hydrothermal synthesis of NiS/MoS2-rGO composites and their application as catalysts for hydrogen evolution reaction

The composites of sulphide and reduced graphene oxide (NiS/MoS2-rGO) were synthesized through a facile solvent-assisted hydrothermal method. The introduction of NiS was paramount not only in enhancing the conductivity of whole catalysts but also in modulating the layer structures of MoS2 with additional active sites. Moreover, the NiS and rGO functioned together in controlling the morphology of as-prepared composites, resulting in uniformly distributed NiS/MoS2 nanosheets perpendicular to rGO scaffold. This further contributed to the excellent hydrogen evolution performance of the composites with a small onset overpotential of 80mV and Tafel slope as low as 65mV/decade.

Dielectric tunability of vertically aligned ferroelectric-metal oxide nanocomposite films controlled by out-of-plane misfit strain

A nonlinear thermodynamic model based on the vertically aligned nanocomposite (VAN) thin films of ferroelectric-metal oxide system has been developed to investigate the physical properties of the epitaxial Ba0.6Sr0.4TiO3 (BST) films containing vertical Sm2O3 (SmO) nanopillar arrays on the SrTiO3 substrate. The phase diagrams of out-of-plane lattice mismatch vs. volume fraction of SmO are calculated by minimizing the total free energy. It is found that the phase transformation and dielectric response of BST-SmO VAN systems are extremely dependent on the in-plane misfit strain, the out-of-plane lattice mismatch, the volume fraction of SmO phase, and the external electric field applied to the nanocomposite films at room temperature. In particular, the BST-SmO VAN systems exhibit higher dielectric properties than pure BST films. Giant dielectric response and maximum tunability are obtained near the lattice mismatch where the phase transition occurs. Under the in-plane misfit strain of umf=0.3% and the out-of-...

Effect of texture dispersion on the effective biaxial modulus of fiber-textured hexagonal, tetragonal, and orthorhombic films

The effective biaxial modulus (Meff) of fiber-textured hexagonal, tetragonal, and orthorhombic films is estimated by using the Voigt–Reuss–Hill and Vook–Witt grain-interaction models. The orientation distribution function with Gaussian distributions of the two Euler angles θ and ϕ is adopted to analyze the effect of texture dispersion degree on Meff. Numerical results that are based on ZnO, BaTiO3, and yttrium barium copper oxide (YBCO) materials show that the Vook–Witt average of Meff is identical to the Voigt–Reuss–Hill average of Meff for the (001) plane of ideally fiber-textured hexagonal and tetragonal films. The ϕ distribution has no influence on Meff of the (hkl)-fiber-textured hexagonal film at any θ distribution in terms of the isotropy in the plane perpendicular to the [001] direction. Comparably, tetragonal and orthorhombic films represent considerable actions of ϕ dispersion on Meff, and the effect of ϕ dispersion on Meff of a (001)-fiber-textured YBCO film is smaller than that for a (001)-fib...