Tingwei Hu

Hydrothermal synthesis of 3D hierarchical MoSe2/NiSe2 composite nanowires on carbon fiber paper and their enhanced electrocatalytic activity for the hydrogen evolution reaction

A facile two-step hydrothermal approach is adopted to synthesize 3D hierarchical MoSe2/NiSe2 composite nanowires (NWs) on a carbon fiber paper (CFP) skeleton for the electro-catalytic hydrogen evolution reaction (HER) by water splitting. The MoSe2/NiSe2 NWs are composed of 2D nanosheets, and they are uniformly and tightly distributed on CFP. The electrochemical measurement suggests that only an overpotential of 249 mV (vs. the RHE) is required to deliver a current density of 100 mA cm−2 in 0.5 M H2SO4, which is lower by 56 and 96 mV as compared to MoSe2 (305 mV) and NiSe2 (345 mV). The Tafel slope of MoSe2/NiSe2 NWs is 46.9 mV dec−1 and is substantially smaller than those of MoSe2 (69.2 mV dec−1) and NiSe2 (68.8 mV dec−1). The MoSe2/NiSe2 composite NW arrays exhibit remarkably enhanced activity because their 3D hierarchical structure could effectively suppress the aggregation or re-stacking of nanosheets and expose more active sites to participate in the HER. Moreover, the highly conductive NiSe2 dispersed in nanosheets facilitates the transfer of electrons from the electrode to the active edges on MoSe2 and further improves the activity.

Preferred armchair edges of epitaxial graphene on 6H-SiC(0001) by thermal decomposition

Scanning tunneling microscopy is used to study the edge orientation of graphene fabricated by thermal decomposition of 6H-SiC. The exploration on the atomically resolved structures and the patterns in reciprocal space demonstrates that the armchair direction is always parallel to the basic vector of 6 × 6 reconstruction as well as the close-packed direction of 6H-SiC substrate. This can be used as the criterion to characterize the edge direction of graphene. With this method, it is found that armchair edges are preferred in both monolayer and bilayer regions. This special edge certainly will affect the electronic states and consequently the properties.

Electronic states in hybrid boron nitride and graphene structures

The energy bands and electronic states of hybrid boron nitride (BN) and graphene structures are studied by first principle calculations. The electronic states change from semi-metallic to insulating depending on the number of B and N atoms as well as domain symmetry. When there are unequal numbers of B and N atoms, mid-gap states usually appear around the Fermi level and the corresponding hybrid structure possesses magnetic and semi-metallic properties. However, when the numbers of B and N atoms are equal, a band gap exists indicative of a semiconducting or insulating nature which depends on the structural symmetry.

High-quality, single-layered epitaxial graphene fabricated on 6H-SiC (0001) by flash annealing in Pb atmosphere and mechanism

High-quality epitaxial graphene is produced on silicon carbide by flash annealing of 6H-SiC in a lead (Pb) atmosphere at ∼1400 °C for 30 s. Nearly three top bilayers of SiC are decomposed due to fast heating and cooling, and sublimation of Si atoms from SiC is retarded by the Pb atmosphere. The synergetic effects promote the growth of continuous single-layered graphene sheets on the SiC terraces, and a model is established to elucidate the effects and growth mechanism.

Evidence of atomically resolved 6×6 buffer layer with long-range order and short-range disorder during formation of graphene on 6H-SiC by thermal decomposition

Scanning tunneling microscopy (STM) is performed to study the formation mechanism of graphene on 6H-SiC by thermal decomposition in situ and the evolution of an atomically resolved 6×6 structure in the buffer layer is revealed. The long-range order of the 6×6 structure is maintained during growth, but the short-range arrangement changes with temperature. Based on STM images acquired at different voltages, a structure consisting of triangular silicon clusters with the 6×6 structure and filled by amorphous carbon atoms is proposed. The 6×6 silicon clusters serve as the template and amorphous carbon atoms provide the carbon source for graphene growth.

Selective growth of Pb islands on graphene/SiC buffer layers

Graphene is fabricated by thermal decomposition of silicon carbide (SiC) and Pb islands are deposited by Pb flux in molecular beam epitaxy chamber. It is found that graphene domains and SiC buffer layer coexist. Selective growth of Pb islands on SiC buffer layer rather than on graphene domains is observed. It can be ascribed to the higher adsorption energy of Pb atoms on the 63 reconstruction of SiC. However, once Pb islands nucleate on graphene domains, they will grow very large owing to the lower diffusion barrier of Pb atoms on graphene. The results are consistent with first-principle calculations. Since Pb atoms on graphene are nearly free-standing, Pb islands grow in even-number mode.

Microstructural characteristics and biocompatibility of a Type-B carbonated hydroxyapatite coating deposited on NiTi shape memory alloy

Microstructural characteristics and biocompatibility of a Type-B carbonated hydroxyapatite (HA) coating prepared on NiTi SMA by biomimetic deposition were characterized using XRD, SEM, XPS, FTIR and in vitro studies including hemolysis test, MTT cytotoxicity test and fibroblasts cytocompatibility test. It is found CO(3)(2-) groups were present as substitution of PO(4)(3-) anions in HA crystal lattice due to Type-B carbonate. The growth of Type-B carbonated HA coating in SBF containing HCO(3)(-) ions is stable during all periods of biomimetic deposition. The carbonated HA coating has better blood compatibility than the chemically-polished NiTi SMA. There was a good cell adhesion to this HA coating surface and cell proliferation in the vicinity of the coating was better than that for the chemically-polished NiTi SMA. Thus biomimetic deposition of this carbonated HA coating is a promising way to improve the biocompatibility of NiTi SMA for implant applications.

Direct and diffuse reflection of electron waves at armchair edges of epitaxial graphene

Scanning tunneling microscopy (STM) is adopted to characterize the commonly formed armchair edge structure of epitaxial graphene prepared by thermal decomposition of 6H-SiC. At the smooth armchair edges, patterns are usually observed as a result of quantum interference (QI) of the incident and directly reflected Bloch waves of electrons at the Fermi level and, the distinct morphologies, such as, dumbbell- and horseshoes-shaped ones can be ascribed to the phase shift of the reflected Bloch wave. However, atomically-resolved graphene lattice instead of QI patterns is imaged near the ridged armchair edges. On the analogy of light interference, when the electron waves encounter such a “rough” edge, diffuse reflection occurs and coherent conditions between the incident and reflected waves cannot be satisfied and so no QI pattern is observed. This provides a unified model describing the electronic states on graphene edges and facilitates optimal design of graphene-based electronics.

Effects of loading mode and orientation on deformation mechanism of graphene nano-ribbons

Molecular dynamics simulation is performed to analyze the deformation mechanism of graphene nanoribbons. When the load is applied along the zigzag orientation, tensile stress yields brittle fracture and compressive stress results in lattice shearing and hexagonal-to-orthorhombic phase transformation. Along the armchair direction, tensile stress produces lattice shearing and phase transformation, but compressive stress leads to a large bonding force. The phase transformation induced by lattice shearing is reversible for 17% and 30% strain in compressive loading along the zigzag direction and tensile loading along the armchair direction. The energy dissipation is less than 10% and resulting pseudo-elasticity enhances the ductility.

Enhanced retained dose uniformity in NiTi spinal correction rod treated by three-dimensional mesh-assisted nitrogen plasma immersion ion implantation

Owing to the nonconformal plasma sheath in plasma immersion ion implantation of a rod sample, the retained dose can vary significantly. The authors propose to improve the implant uniformity by introducing a metal mesh. The depth profiles obtained with and without the mesh are compared and the implantation temperature at various locations is evaluated indirectly by differential scanning calorimeter. Our results reveal that by using the metal mesh, the retained dose uniformity along the length is greatly improved and the effects of the implantation temperature on the localized mechanical properties of the implanted NiTi shape memory alloy rod are nearly negligible.