Exfoliated two-dimensional (2D) unilamellar nanosheets
of Ca2Nb3O10
–,
TiNbO5
–, Ti2NbO7
–, and Ti5NbO14
3– 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 Ti5NbO14
3– 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 Ti5NbO14
3– nanosheets,
respectively, providing a diagram of electronic band structure for
the nanosheets.