Controlled Synthesis Of BiVO₄Photocatalyst With The Assistance Of L-cysteine And Its Photocatalytic O₂Evolution Under Visible Light Irradiation

Photocatalytic oxidation of water ivolves a four electron transfer process and requiresadditional high energy (2H2Oâ†'4H++O2, ΔG=1.23eV,<1000nm), which is thereforethe rate-determining step in the photocatalytic decomposition of water and make it verydifficult to split water into H2under solar energy. As a narrow band gap semiconductor (Eg=2.4eV), BiVO4photocatalyst has also received a great deal of attention because it exhibitsstrong absorption in the visible range and shows excellent performance for photocatalyticwater oxidation to O2under visible light irradiation. As is known, L-cysteine contains-SH,-NH2,-COOH functional groups and unique spatial structure with nanometer scale units,which helps to the nucleation, growth and assembly of micro/nano-sized materials.Nevertheless, to the best of our knowledge, the synthesis of BiVO4photocatalyst in presenceof L-cysteine as the structure-directing agent to date remains very limited. The thesis focuseson the controlled synthesis of micro/nano-scaled BiVO4photocatalyt by anL-cysteine-assisted hydrothermal method and the photocatalytic performance for wateroxidation under visible light illumination. The main results are summarized as follows:A series of monoclinic BiVO4photocatalyts with a variety of special morphologies havebeen fabricated hydrothermally in presence of Bi(NO3)3·5H2O as bismuth source, NH4VO3asvanadium source and L-cysteine as the structure-directing agent by employing HNO3andNaOH solutions to regulate the pH value of the solution. In the experiments, the effects ofexperimental parameters (pH, temperature, time, the molar ratio of Bi(NO3)3·5H2O toL-cysteine) on the crystalline phase, morphology, size and photocatalytic O2-evolutionactivity of the products are also investigated. The results indicate that the phase andmorphology of BiVO4photocatalyts can be readily controlled by modulating the pH value ofthe solution. At pH=1.5, ellipsoidal BiVO4ca.2.5-3.0μm assembled by a number ofnanoparticles with the diameter of200nm are observed. At pH=2.0, the peanut-like productswith with the length of2-5μm are obtained. At pH=2.5, the spercial products (3.0-3.5μm)assembled by the nanoparticles (150nm) are obtanied. At pH=3.5, the products arecomposed of apple-like particles with the diameter of4μm. At pH=6.5, the products areround pie-like with the diameter of3.5-4.0μm and the thickness of1.5-2.0μm. At pH=8.0,the BiVO4flowers (6-8μm) composed by leafy particles (2.5μm) are demonstrated. Whenthe pH value is further elevated to10.0, the products are composed of rhombus particles with the size of2.5-3.0μm. The growth mechanism of monoclinic BiVO4photocatalyts with theassistance of L-cysteine is still underway.In addition, the amount of L-cysteine plays an important role in regulating the growthdirection of monoclinic BiVO4. When the molar ratio of Bi(NO3)3·5H2O/L-cysteine equals to2:1, the (040) diffraction peak becomes the strongest. The optimum activity of photocatalyticwater oxidation to O2in the prsence of AgNO3as sacrificial agents is530mol/h/g undervisible light (>420nm).