Electrochemical and photoelectrochemical study on exfoliated Nb3O8 nanosheet

Transition metal oxide nanosheet of Nb3O8 was synthesized by delaminating a layered host compound of KNb3O8. Ultrathin two-dimensional morphology, about 1 nm thickness vs. submicrometer lateral size, was revealed by atomic force microscopy. The colloidal nanosheets were assembled layer-by-layer with polycation onto an indium-tin-oxide (ITO)-coated glass electrode via sequential adsorption technique. UV–vis absorption spectra in the assembly process showed the linear enhancement of absorbance at 270 nm per deposition cycle, indicating the successful formation of multilayer composite films, which can be converted into polymer free films upon exposure to UV light. The cyclic voltammogram of resulting Nb3O8 nanosheet film electrodes exhibited reduction/oxidation (Nb4+/Nb5+) peaks at −1.2 V (vs. Ag/Ag+) attributable to insertion and deintercalation of Li+ ions into and from the nanosheet galleries. The reaction (reduction/oxidation) ratio of the electroactive Nb atom in Nb3O8 nanosheet was calculated to be 6.3% by integrating a charge transfer in a sweep range of +1.0 to −1.9 V. The Nb3O8 nanosheet electrode generated anodic photocurrent in response to UV illumination. Analysis on the photocurrent action spectrum indicated that Nb3O8 nanosheet is an indirect transition-type semiconductor with bandgap energy of 3.58 eV. The flat-band potential was estimated to be −1.32 V vs. Ag/Ag+ from the applied potential dependence.