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

Abstract

Monoclinic structured BiVO4 nanotubes have been selectively prepared by a template-free and surfactant-free solvothermal process in anethanol–H2O mixed solvent. Interestingly, the nanotubes obtained atVethanol:Vwater = 3:1 have hexagonal cross sections with long lengths ofapprox. 1.2 µm, side lengths of approx. 200 nm and wall thicknesses of approx. 30 nm. A possible orientedattachment growth mechanism has been proposed based on the results of time-dependentexperiments. Nanoplates were firstly formed by aggregation of primary nanocrystallites andthen self-assembly converted to hexagonal-prismatic nanotubes via the correspondingoriented attachment mechanism. During this process, the presence of ethanol and thevolume ratio of ethanol and water play important roles in the formation of the nanotubeswith hexagonal cross sections. UV–vis spectroscopy was further employed to estimate thebandgap energy of the novel structure, which is larger (2.53 eV) than that for the bulkBiVO4. 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 forO2 evolution from water splitting over the samples under visible light(λ>420 nm) irradiation wasinvestigated by using AgNO3 as a sacrificial reagent. Experiment results indicate that theas-synthesized nanotubes exhibit higher photocatalytic activities forO2 evolution thanthat of bulk BiVO4, which is attributed to the special tubular structure morphology.