Surface charge accumulation and electrochemical protonation of transition metal oxides using water-infiltrated nanoporous glass

<jats:title>Abstract</jats:title> <jats:p> Water, composed of two strong electro-chemically active agents of proton (H <jats:sup>+</jats:sup> ) and hydroxyl ion (OH <jats:sup>–</jats:sup> ), is expected to be a strong reductant as well as oxidant for transition metal oxides (TMOs). We have investigated the applicability of water for the optical, electronic, and magnetic property modification of TMOs using three-terminal thin-film transistor (TFT) structure with <jats:italic>water-infiltrated nanoporous glass</jats:italic> of amorphous 12CaO · 7Al <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> as the gate insulator. In this paper, we review the electronic property modulation of TMOs using our developed TFT structure with water-infiltrated nanoporous glass and discuss the different operation mechanism using the examples of SrTiO <jats:sub>3</jats:sub> single crystal and VO <jats:sub>2</jats:sub> epitaxial film as the TMO channels. Electronic properties of the TMOs can be modulated in two ways depending on the magnitude relationship between the energy level of conduction band minimum ( <jats:italic>E</jats:italic> <jats:sub>CBM</jats:sub> ) of TMOs and hydrogen generation potential ( <jats:italic>E</jats:italic> <jats:sub>H2</jats:sub> ). When <jats:italic>E</jats:italic> <jats:sub>CBM</jats:sub> is higher than <jats:italic>E</jats:italic> <jats:sub>H2</jats:sub> in the case of SrTiO <jats:sub>3,</jats:sub> two-dimensional electron gas layer is formed at the TMO surface by the electrostatic charge accumulation and subsequent redox reaction. When <jats:italic>E</jats:italic> <jats:sub>CBM</jats:sub> is lower than <jats:italic>E</jats:italic> <jats:sub>H2</jats:sub> in the case of VO <jats:sub>2</jats:sub> , protonation driven metal–insulator conversion of TMOs occurs by the penetration of H <jats:sup>+</jats:sup> into the bulk region. The former approach may accelerate the development of nanostructures of high performance thermoelectric materials and the latter is applicable for the development of electrochromic device with non-volatile operation. </jats:p>