Study on adsorption characteristics and thermodynamic behaviors of the molecules with sp2 hybrid orbitals on inorganic surfaces

Docto

Reversible phase transition of hydrogen sponge in VO2

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Reversible Phase Modulation and Hydrogen Storage in Multi-Valent VO2 Epitaxial Thin Films

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Reversible phase transition of hydrogen sponge in multi-valent VO2 epitaxial thin film

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Systematic Tuning of Hydrogen-induced Phase Transition in VO2 Epitaxial Thin Film

Phase transition by band filling control is one of the core concepts in correlated electronic systems. Unlike the substitutional dopants, hydrogen plays a key role in effectively filling significant amount of carriers in the empty narrow d band by reversibly adding it into interstitial sites and supplying carriers. Vanadium dioxide (VO2), typical correlated oxide with 3d 1 electronic configuration, can also reversibly incorporate hydrogen atoms into its interstitial sites and simultaneously occurs phase transition by its 3d band filling. 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 demonstrates two-step insulator (VO2) – metal (HxVO2) – insulator (HVO2) phase modulation during inter-integer d-band filling. Moreover, HVO2 can be thermodynamically stabilized regardless of facet direction of VO2 epilayer, but remarkable discrepancy in kinetics of phase modulation was clearly visualized depending on the crystal facet. The unprecedented insulating HVO2 with 3d 2 configuration is attributed to highly doped electrons via hydrogenation process in conjunction with huge lattice expansion. 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.2

Synchrotron x-ray study of hydrogen-induced phase transition in VO2 epitaxial thin films

Phase transition by band filling control is one of the core concepts in correlated electronic systems. Unlike the substitutional dopants, hydrogen, the smallest and the lightest atom, plays a key role in effectively filling significant amount of carriers in the empty narrow d band by reversibly adding it into interstitial sites and supplying carriers. Vanadium dioxide (VO2), typical correlated oxide with 3d1 electronic configuration, can also reversibly incorporate hydrogen atoms into its interstitial sites and simultaneously occurs phase transition by its 3d band filling. 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 due to the low temperature annealing process. This hydrogenation process demonstrates twostep insulator (VO2) – metal (HxVO2) – insulator (HVO2) phase modulation during inter-integer d-band filling. Moreover, HVO2 can be thermodynamically stabilized regardless of facet direction of VO2 epi-layer, but remarkable discrepancy in kinetics of phase modulation was clearly visualized depending on the crystal facet. Based on in situ XRD, XPS and NEXAFS in synchrotron, the unprecedented insulating HVO2 with 3d2 configuration is attributed to highly doped electrons via hydrogenation process in conjunction with huge lattice expansion. 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.2

Systematic Tuning of Hydrogen-Induced Phase Transition in VO2 Epitaxial Thin Films.

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Direct Probing of Oxygen Loss from the Surface Lattice of Correlated Oxides during Hydrogen Spillover

Hydrogen spillover is a catalytic process that occurs by surface reaction and subsequent diffusion to reversibly provide a massive amount of hydrogen dopants in correlated oxides, but the mechanism at the surface of correlated oxides with metal catalyst are not well understood. Here we show that a significant amount of oxygen is released from the surface of correlated VO2 films during hydrogen spillover, contrary to the well-established observation of the formation of hydrogen interstitials in the bulk part of VO2 films. By using ambient-pressure X-ray photoelectron spectroscopy, we prove that the formation of surface oxygen vacancies is a consequence of a favorable reaction for the generation of weakly adsorbed H2O from surface O atoms that have low coordination and weak binding strength. Our results reveal the importance of in situ characterization to prove the dynamic change during redox reaction and present an opportunity to control intrinsic defects at the surface.11Nsciescopu