Reversible phase modulation and hydrogen storage in multivalent VO2 epitaxial thin films

Journal name:
Nature Materials
Volume:
15,
Pages:
1113–1119
Year published:
DOI:
doi:10.1038/nmat4692
Received
Accepted
Published online

Abstract

Hydrogen, the smallest and the lightest atomic element, is reversibly incorporated into interstitial sites in vanadium dioxide (VO2), a correlated oxide with a 3d1 electronic configuration, and induces electronic phase modulation. It is widely reported that low hydrogen concentrations stabilize the metallic phase, but the understanding of hydrogen in the high doping regime is limited. Here, we demonstrate that as many as two hydrogen atoms can be incorporated into each VO2 unit cell, and that hydrogen is reversibly absorbed into, and released from, VO2 without destroying its lattice framework. This hydrogenation process allows us to elucidate electronic phase modulation of vanadium oxyhydride, demonstrating two-step insulator (VO2)–metal (HxVO2)–insulator (HVO2) phase modulation during inter-integer d-band filling. Our finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO2 and opens up the potential application in proton-based Mottronics and novel hydrogen storage.

At a glance

Figures

  1. Low-temperature hydrogenation of VO2.
    Figure 1: Low-temperature hydrogenation of VO2.

    ac-axis empty channel in the rutile VO2 lattice. b, Hydrogenation process of VO2 thin films using the hydrogen spillover method. Nano-sized and disconnected Pt islands were formed on epitaxial VO2 layers. For hydrogen intercalation, the Pt/VO2 samples were annealed under forming gas containing 5% hydrogen gas at low temperature below 200°C. c,d, HAADF-STEM (c) and ABF-STEM (d) image of the (100) fully hydrogenated HVO2 epitaxial film grown on (100) TiO2 substrates with a [001]R zone axis. Yellow dashed line: interface between VO2 and TiO2. Owing to the sensitivity of HAADF to high-weight atoms, vanadium and titanium atoms appear bright; in contrast, owing to the sensitivity of the ABF technique to light-weight atoms, hydrogen atoms (white arrows) are observed only along the [001]R channels in the VO2 lattice. No contrast is detectable in the [001]R channels in TiO2, which strongly indicates that the contrast (white arrows) in VO2 is not artefact contrast.

  2. Structural phase modulation by hydrogenation.
    Figure 2: Structural phase modulation by hydrogenation.

    a,b, Symmetrical X-ray scan of pristine (blue line) and fully hydrogenated (orange line) HVO2 films on (0001) Al2O3 (a) and (100) TiO2 (b) substrates. Out-of-plane lattice parameters were significantly expanded in all VO2 films after hydrogenation regardless of the underlying substrates. c,d, Reciprocal space mapping around the (101) and (110) reflection of (100)R-oriented VO2 (c) and HVO2 (d) films on (100) TiO2 substrates. These results confirm significant expansion of the out-of-plane (a axis in rutile coordinates) lattice constant and relatively negligible change of in-plane (b axis and c axis in rutile coordinates) lattice constants, and giant volume expansion after hydrogenation. e, Change of unit-cell volume of HxVO2 as a function of hydrogen content x. Owing to the relationship between hydrogen content and unit-cell volume from bulk HxVO2 (black circles) (ref. 23), the hydrogen content of our VO2 (orange diamond) and HxVO2 (red diamond) films is accurately determined from the unit-cell volume, and the result is consistent with the estimate of the hydrogen content obtained using RBS measurement combined with ERD (Supplementary Fig. 17).

  3. Dynamic resistivity modulation of HxVO2 by hydrogenation.
    Figure 3: Dynamic resistivity modulation of HxVO2 by hydrogenation.

    a,bIn situ resistivity of (100)R-VO2 films on (0001) Al2O3 during hydrogenation and dehydrogenation as a function of time at 70°C (a) and 120°C (b). The inset in a represents the initial metallization by low hydrogen doping. Note the clear observation of pristine insulator–hydrogenated metal–hydrogenated insulator phase modulation during hydrogenation at 70°C and reversible resistivity change of up to 105 during hydrogenation/dehydrogenation at 120°C. c, Temperature-dependent sheet resistance in HxVO2 thin films from plot I to plot IV with increasing hydrogen content x.

  4. X-ray photoemission spectroscopy and NEXAFS of pristine VO2 and fully hydrogenated insulating HVO2.
    Figure 4: X-ray photoemission spectroscopy and NEXAFS of pristine VO2 and fully hydrogenated insulating HVO2.

    a, V 2p and O 1s core-level spectra of pristine VO2 and fully hydrogenated insulating HVO2, confirming the V valence change and the existence of O–H bonding after hydrogenation. b,c, V L-edge (b) and O K-edge (c) NEXAFS of pristine VO2 and fully hydrogenated insulating HVO2. The chemical shift of the V L-edge and the suppression of the t2g band peak in O K-edge are clearly observed in fully hydrogenated HVO2 films.

  5. Calculated structural and electronic modulation in HVO2.
    Figure 5: Calculated structural and electronic modulation in HVO2.

    a,b, Calculated total energy of the VO2 phase with different a-axis lattices by adding up to two electrons (a) and two hydrogens (b) into the unit cell that corresponds to the HVO2. Atomic positions were fully relaxed in the calculations. Note that the crystal structure with the elongated a axis became energetically stable with increases in both electron doping and hydrogen doping, that is, ~16% expansion and ~10% expansion (green circles) for two electrons per unit cell and for two hydrogens per unit cell, respectively. ce, Calculated density of states (DOS) of undoped rutile VO2 (c) and the heavily electron-doped VO2 (d), that is, two electrons doping per unit cell, and heavily hydrogen-doped VO2 (e), that is, two hydrogens per unit cell, with the experimental lattice constant of hydrogenated insulating HVO2. Note that the t2g components (red line) shift downward in partial DOS and a bandgap appears in both d and e.

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Author information

  1. These authors contributed equally to this work.

    • Hyojin Yoon &
    • Minseok Choi

Affiliations

  1. Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

    • Hyojin Yoon,
    • Hyunah Kwon,
    • Jong Kyu Kim &
    • Junwoo Son
  2. Materials Modeling and Characterization Department, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea

    • Minseok Choi,
    • Tae-Won Lim &
    • Si-Young Choi
  3. Department of Physics, Inha University, Incheon 22212, Republic of Korea

    • Minseok Choi
  4. Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea

    • Kyuwook Ihm

Contributions

M.C. and J.S. conceived the idea and designed the study. H.Y. performed the film growth, X-ray diffraction and synchrotron measurement under the supervision of J.S. H.Y. and H.K. carried out the in situ electrical measurement under the guidance of J.S. and J.-K.K. M.C. performed the electronic structure calculations. T.-W.L. and S.-Y.C. characterized the samples with scanning transmission electron microscopy. J.S. directed the research. H.Y., M.C. and J.S. wrote the manuscript and all authors commented on it.

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The authors declare no competing financial interests.

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