Template-free synthesis of BiVO4 nanostructures: I. Nanotubes with hexagonal cross sections by oriented attachment and their photocatalytic property for water splitting under visible light

Monoclinic structured BiVO4 nanotubes have been selectively prepared by a template-free and surfactant-free solvothermal process in an ethanol-H2O mixed solvent. Interestingly, the nanotubes obtained at V-ethanol:V-water=3:1 have hexagonal cross sections with long lengths of approx. 1.2 mu m, side lengths of approx. 200 nm and wall thicknesses of approx. 30 nm. A possible oriented attachment growth mechanism has been proposed based on the results of time-dependent experiments. Nanoplates were firstly formed by aggregation of primary nanocrystallites and then self-assembly converted to hexagonal-prismatic nanotubes via the corresponding oriented attachment mechanism. During this process, the presence of ethanol and the volume ratio of ethanol and water play important roles in the formation of the nanotubes with hexagonal cross sections. UV-vis spectroscopy was further employed to estimate the bandgap energy of the novel structure, which is larger (2.53 eV) than that for the bulk BiVO4. The valence band edge position for the as-synthesized material was estimated to be 2.8 eV, which is positive enough for water oxidation. The photocatalytic activity for O-2 evolution from water splitting over the samples under visible light (lambda>420 nm) irradiation was investigated by using AgNO3 as a sacrificial reagent. Experiment results indicate that the as-synthesized nanotubes exhibit higher photocatalytic activities for O-2 evolution than that of bulk BiVO4, which is attributed to the special tubular structure morphology.