Journal
NANO LETTERS
Volume 14, Issue 9, Pages 5445-5451Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nl5030694
Keywords
VO2; Atomic Hydrogenation; Hydrogen Diffusion; MIT
Categories
Funding
- U. S. DOE Office of Science/Basic Energy Science [DE-FG02-06ER46337]
- Robert A. Welch Foundation [C-1818]
- AFOSR MURI [FA9550-12-1-0035]
- Sandia National Laboratory [1100745]
- Lockheed Martin Corporation through the LANCER IV Program
- ONR MURI program [00006766, N00014-09-1-1066]
- China Scholarship Council
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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. Optical characterization shows that hydrogen atoms prefer to diffuse along the c axis of rutile (a axis of monoclinic) VO2, along the oxygen channels. 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 1010 cm2/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 3 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.
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