Monoclinic structured BiVO(4) nanotubes have been selectively prepared by a template-free and surfactant-free solvothermal process in an ethanol-H(2)O mixed solvent. Interestingly, the nanotubes obtained at V(ethanol):V(water) = 3:1 have hexagonal cross sections with long lengths of approx. 1.2 microm, 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 BiVO(4). 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 AgNO(3) as a sacrificial reagent. Experiment results indicate that the as-synthesized nanotubes exhibit higher photocatalytic activities for O(2) evolution than that of bulk BiVO(4), which is attributed to the special tubular structure morphology.
The shape-controlled synthesis of nanostructured materials has opened up new possibilities to improve their physical and chemical properties. In this work, new types of monoclinic structured BiVO4 with complex morphologies, namely flowerlike, disclike, tubelike and platelike shapes, have been synthesized in a binary green solvent (water and ethanol) through controlling reaction conditions such as solvent, pH value, concentration of precursors and reaction temperature. The morphology of BiVO4 can transform from three-dimensional (3D) flowerlike superstructures and hexagonal-prismatic nanotubes to two-dimensional (2D) platelike and disclike structures. UV-vis absorption spectra show that all of the prepared nano- and microstructures can respond to visible light and the optical properties of BiVO4 samples are relevant to their structures. More importantly, the photocatalytic activities of various BiVO4 samples are strongly dependent on their morphology for the degradation of rhodamine B (RhB) under visible-light irradiation. The 2D (disclike and platelike) BiVO4 demonstrates better photocatalytic activity than 3D and bulk BiVO4. Among the nano- and microstructures, the nanoplate BiVO4 exhibit the highest photocatalytic activity for degradation of organic pollutants. Additionally, it is found that the different microstructure of BiVO4 leads to the different degradation route for organic compounds of RhB. The reasons for the differences in the photocatalytic behavior for these BiVO4 nanostructures are further discussed. The relationship between the microstructure and the photocatalytic activity for BiVO4 may give clues for the preparation of photocatalysts with high activity based on material morphology design. Moreover, the prepared 2D BiVO4 can be a good photocatalyst used in environmental pollution control.
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