Hydrogen Diffusion and Stabilization in Single-crystal VO2 Micro/nanobeams by Direct Atomic Hydrogenation

TL;DR

This study measures atomic hydrogen diffusion in single-crystal VO2 micro/nanobeams, revealing rapid diffusion along the rutile phase c-axis and stabilization of the metallic phase at low temperatures, with implications for hydrogen transport applications.

Contribution

It provides the first direct measurement of hydrogen diffusion constants in VO2 micro/nanobeams and compares diffusion kinetics between rutile and monoclinic phases, supported by ab initio calculations.

Findings

Hydrogen diffusion constant along rutile c-axis is 6.7 x 10^{-10} cm^2/s at 373 K.

Diffusion along rutile c-axis exceeds that along monoclinic a-axis by over three orders of magnitude.

Atomic hydrogen stabilizes the metallic phase of VO2 down to 2 K.

Abstract

We report measurements of the diffusion of atomic hydrogen in single crystalline VO2 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 VO2 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 VO2 micro/nanobeams down to 2 K, the lowest point we could reach in our measurement setup. Based on observing the movement of the hydrogen diffusion front in single crystalline VO2 beams, we estimate the diffusion constant for hydrogen along the c-axis of the rutile phase to be 6.7 x 10^{-10} cm^2/s at approximately 373 K, exceeding the value in isostructural TiO2 by ~ 38x. Moreover, we find that the diffusion constant along the c-axis of the rutile phase exceeds that along the equivalent a-axis of the monoclinic phase by at least three orders of magnitude. This remarkable change in kinetics must originate from the distortion of the "channels" when the unit cell doubles along this direction upon cooling into the monoclinic structure. Ab initio calculation results are in good agreement with the experimental trends in the relative kinetics of the two phases. This raises the possibility of a switchable membrane for hydrogen transport.