Electronic Band Structure of Exfoliated Titanium- and/or Niobium-Based Oxide Nanosheets Probed by Electrochemical and Photoelectrochemical Measurements

Exfoliated two-dimensional (2D) unilamellar nanosheets of Ca2Nb3O10-, TiNbO5-, Ti2NbO7-, and Ti5NbO143- were deposited layer-by-layer to produce multilayer films on indium-tin-oxide (ITO)-coated glass electrodes, and their electrochemical and photoelectrochemical properties were explored. The layer-by-layer assembly process via sequential adsorption with counter polycations was monitored by UV-visible absorption spectra and X-ray diffraction measurements, which confirmed the successful growth of films, where nanosheets and polycations are alternately stacked at a separation of 1.6-2.4 nm. Exposure to UV light totally removed polycations, producing inorganic films. Cyclic voltammetry on Ti and/or Nb oxide nanosheet electrodes thus fabricated showed reduction/oxidation (Ti3+/Ti4+ and Nb4+/Nb5+) peaks associated with insertion/extraction of Li+ ions into/from intersheet galleries of the films. The extent of the redox reaction is found to be governed by the cation density in the nanosheet gallery. Anodic photocurrents of the oxide nanosheet electrodes were observed under UV light irradiation. These action spectra showed close resemblance to optical absorption profiles of the colloidal nanosheets, indicating that the photocurrent was generated from the nanosheets. Their analysis indicates that the nanosheets of Ca2Nb3O10-, TiNbO5-, Ti2NbO7-, and Ti5NbO143- are all indirect transition-type wide-gap semiconductors with bandgap energies of 3.44, 3.68, 3.64, and 3.53 eV, respectively. These values are larger than those for corresponding parent layered oxide compounds before delamination, suggesting confinement effects into 2D nanosheet structure. Furthermore, the value was invariable for the films with a different number of nanosheet layers, indicating that quantized nanosheets were electronically isolated with each other. In addition, photocurrent generation was measured as a function of applied electrode potential, and the flatband potential was estimated from the photocurrent onset values as -1.12, -1.33, -1.30, and -1.29 V vs Ag/Ag+, for Ca2Nb3O10-, TiNbO5-, Ti2NbO7-, and Ti5NbO143- nanosheets, respectively, providing a diagram of electronic band structure for the nanosheets.