No abstract

A facile chemical vapor deposition method to prepare single-crystalline VS2 nanosheets for the hydrogen evolution reaction is reported. The electrocatalytic hydrogen evolution reaction (HER) activities of VS2 show an extremely low overpotential of -68 mV at 10 mA cm(-2), small Tafel slopes of ≈34 mV decade(-1), as well as high stability, demonstrating its potential as a candidate non-noble-metal catalyst for the HER.

Vanadium dioxide (VO(2)) is a strongly correlated transition metal oxide with a dramatic metal-insulator transition at 67 °C. Researchers have long been interested in manipulating this transition via the field effect. Here we report attempts to modulate this transition in single-crystal VO(2) nanowires via electrochemical gating using ionic liquids. Stray water contamination in the ionic liquid leads to large, slow, hysteretic conductance responses to changes in the gate potential, allowing tuning of the activation energy of the conductance in the insulating state. We suggest that these changes are the result of electrochemical doping via hydrogen. In the absence of this chemical effect, gate response is minimal, suggesting that significant field-effect modulation of the metal-insulator transition is not possible, at least along the crystallographic directions relevant in these nanowires.

Controlling electronic population through chemical doping is one way to tip the balance between competing phases in materials with strong electronic correlations. Vanadium dioxide exhibits a first-order phase transition at around 338 K between a high-temperature, tetragonal, metallic state (T) and a low-temperature, monoclinic, insulating state (M1), driven by electron-electron and electron-lattice interactions. Intercalation of VO2 with atomic hydrogen has been demonstrated, with evidence that this doping suppresses the transition. However, the detailed effects of intercalated H on the crystal and electronic structure of the resulting hydride have not been previously reported. Here we present synchrotron and neutron diffraction studies of this material system, mapping out the structural phase diagram as a function of temperature and hydrogen content. In addition to the original T and M1 phases, we find two orthorhombic phases, O1 and O2, which are stabilized at higher hydrogen content. We present density functional calculations that confirm the metallicity of these states and discuss the physical basis by which hydrogen stabilizes conducting phases, in the context of the metal-insulator transition.

We report measurements of the diffusion of atomic hydrogen in single crystalline VO 2 micro/nanobeams by direct exposure to atomic hydrogen, without catalyst. The atomic hydrogen is generated by a hot filament, and the doping process takes place at moderate temperature (373 K). Undoped VO 2 has a metal-to-insulator phase transition at ~340 K between a high -temperature, rutile, metallic phase and a low-temperature, monoclinic, insulating phase with a resistance exhibiting a semiconductor-like temperature dependence.Atomic hydrogenation results in stabilization of the metallic phase of VO 2 micro/nanobeam down to 2 K, the lowest point we could reach in our measurement setup. Optical characterization shows that hydrogen atoms prefer to diffuse along the c-axis of rutile (a-axis of monoclinic) VO 2 , along the oxygen "channels". Based on observing the movement of the hydrogen diffusion front in single crystalline VO 2 beams, we estimate the diffusion constant