Shape-Controlled Synthesis Of M-BiVO₄ Crystals And Their Photocatalytic Properties

With band gap of 2.4 eV, monoclinic bismuth vanadate (BiVO4) has drawn great attention for its excellent performance under visible light illumination. The synthesis technology of nanocrystals (NCs), as well as the synthesis parameters on NCs morphology and photocatalytic activity has been studied in this paper. Nonpolar-solvothermal and mixed-solvothermal technology based on hydrothermal technique have been developed for the synthesis of m-BiVO4, by using Bi(NO3)3·5H2O and NaVO3 as raw materials, and oleylamine (OM) as surfactant. The phase structure, morphology and photocatalytic properties of as-prepared BiVO4 particles were characterized by X-ray diffraction (XRD), field scanning electron microscope (FESEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-Ray spectroscopy (EDS), UV-vis absorption spectra, BET special surface and the degradation of methylene blue, respectively. The relationships of synthesis technology and synthesis parameters with the phase structure, morphology and photocatalytic activity were dicussed. The experimental results are as follows:1. At the presence of non-polar organic solvent toluene, the reaction activity of inorganic ions groups of nonpolar-solvothermal in the synthetic system could keep relatively lower. Nonpolar-solvothermal technique was applied to fabricate flower-shaped and spherical m-BiV04 NCs with small and uniform grain size (400-500 nm). The crystallinity increased and the morphology transformed from flower-shaped into sphere with temperature. Meanwhile, the higher temperature, the higher photocatalytic activity. But when the reaction temperature was more than 180℃, impurities of Bi appeared in the products.2. Non-polar organic solvent toluene and polar solvent water were chosen as reaction medium of mixed-solvothermal, which led the lower reaction activity of the inorganic ions groups (between hydrothermal and solvothermal). The morphology of BiVO4 NCs could be easily controlled by mixed-solvothermal. Furthermore, the as-fabricated BiVO4 NCs were monoclinic scheelite and showed higher photocatalytic activity than those synthesized by nonpolar-solvothermal.3. When pH=7 and the W/T values were adjusted from 30/30 to 40/20,50/10,20/40, and 10/50, the morphology of as-obtained BiVO4 by mixed-solvothermal transformed from sphere into ellipse, needle, and bone-like. And the size of BiVO4 NCs decreased with the increase of W/T value. The concentration of OM relative to the amount of oil precursor increased with the increase of W/T value, which could induce the NCs growth of one-dimensional and the formation of needle shape. With smaller particle size, larger BET surface areas and special particle morphology, sphere-like (W/T=30/30) and needle-like (W/T=50/10) m-BiVO4 NCs showed excellent photocatalytic performance.4. When the W/T values were kept at 30/30 and the pH values were adjusted from 7 to 12,3 and 0.5, the morphology of as-obtained BiVO4 by mixed-solvothermal transformed from sphere to coarse rod, short nanorod and cluster. And the size of primary nanoparticle synthesized at pH<7 was bigger than those synthesized at pH>7. When pH=12 and the temperature increased to 180℃, the coarse rod changed into cuboid with better crystallinity and smooth surface. Sphere-like (pH=7) m-BiVO4 NCs showed a better photocatalytic performance than others.5. With different volumes of surfactant OM, the morphology of as-obtained BiVO4 by mixed-solvothermal transformed from sphere to irregular shape, short cuboid and nanorod. The results showed that OM contributed to the shape-controlling of NCs and the increasing of volumes of OM induces the NCs growth of one-dimensional, which was corresponding to the conclusion 3.Compared with the conventional hydrothermal, the inorganic ions groups could keep lower reaction activity in the nonpolar-solvothermal and mixed-solvothermal synthetic system, which could be beneficial to synthesis of NCs. Meanwhile, phase transition adopted to synthesize precursor in the synthetic process could reduce the impurities of the products. Overall, the high purity, different morphologies and high photocatalytic activity of BiVO4 NCs were synthesized by nonpolar-solvothermal and mixed-solvothermal technology. And the possible mechanisms for the shape self-assembly and the photocatalytic performance of BiVO4 were generally discussed.