Fabrication And Photocatalytic Performance Of Regularly Morphological Porous BiVO₄and Fluorine-Doped BiVO₄

Phtotocatalysis is an emerging technology for the purification of waste water andphotocatalyst is a key issue in the photocatalytic process. As a non-titania basedvisible-light-driven semiconductor photocatalyst, BiVO₄has recently attracted muchattention due to its excellent photocatalytic performance. BiVO₄has three crystalphases of tetragonal zircon, monoclinic scheelite, and tetragonal scheelite, amongwhich the monoclinic scheelite BiVO₄with a bandgap energy of2.4eV is the mostactive photocatalytically under visible-light irradiation. In the thesis, monoclinicBiVO₄and fluorine-doped BiVO₄with regular morphologies and/or porousstructures were fabricated by adopting various methods. Physicochemical propertiesof the as-prepared materials were characterized by means of techniques, such asX-ray diffraction (XRD), thermogavimetric analysis (TGA), differential scanningcalorimetry (DSC), laser Raman spectroscopy (Raman), Fourier transform infraredspectroscopy (FT-IR),, scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), selected-area electron diffraction (SAED), N2adsorption-desorption (BET), X-ray photoelectron spectroscopy (XPS), X-rayfluorescence spectroscopy (XRF), and ultraviolet-visible diffuse reflectancespectroscopy (UV-vis DRS). Photocatalytic activities of the as-fabricated sampleswere evaluated for the degration of phenol, methylene blue (MB), and methylorange (MO) under visible-light illumination. The main results obtained in thepresent investigations are as follows:(1) Bismuth vanadates with multiple morphologies and/or porous structures wereprepared using the alcoho-hydrothermal strategy with bismuth nitrate andammonium metavanadate as metal source, NaOH as pH adjustor, ethanol andethylene glycol as solvent, and/or dodecylamine (DA), oleylamine (OL) or oleicacid (OA) as surfactant. It is found that the surfactant and pH value had asignificant influence on the particle morphology and even the crystalline structureof the product. Porous olive-like monoclinic BiVO₄samples could be preparedwith DA, OL or OA as surfactant at pH=1.5or3.0and alcoho-hydrothermaltemperature=100oC. With DA as surfactant at an alcoho-hydrothermaltemperature of100oC, short-rod-like monoclinic BiVO₄and porous layeredspherical orthorhombic Bi₄V₂O₁₁were obtained when the pH value of theprecursor solution was raised to7.0and11.0, respectively. Among the BiVO₄samples, the porous olive-like one with a surface area of12.7m²/g exhibited thebest visible-light-driven photocatalytic performance for phenol degradation. It isconcluded that the excellent photocatalytic activity of the porous olive-like BiVO₄sample was associated with its higher surface area and surface oxygen vacancydensity, porous structure, lower bandgap energy, and unique morphology. (2) Monoclinic BiVO₄single-crystallites with a polyhedral, porous spherical orporous octapod-like morphology were selectively prepared using the triblockcopolymer P123(HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H)-assistedhydrothermal method with bismuth nitrate and ammonium metavanadate as metalsource and various bases as pH adjustor. The pH value of the precursor solution,surfactant, and hydrothermal temperature had an important impact on particlearchitecture of the BiVO₄product. The introduction of P123favored thegeneration of BiVO₄with porous structures. The BiVO₄derived hydrothermallywith P123at pH=3or6possessed good optical absorption performance both inUV-and visible-light regions and hence showed excellent photocatalytic activitiesfor the degradation of MB and phenol. It is concluded that the highvisible-light-driven catalytic performance of the porous octapod-like BiVO₄single-crystallites is associated with the higher surface area, porous structure,lower band gap energy, and unique particle morphology.(3) Monoclinic BiVO₄with multiple morphologies and/or porous structures werefabricated using the hydrothermal strategy. It is observed that the pH value andsurfactant exerted a great effect on the morphology and pore structure of theBiVO₄product. Spherical BiVO₄with porous structures, flower-cluster-likeBiVO₄, and flower-bundle-like BiVO₄were generated hydrothermally at100oCwith poly(vinyl pyrrolidone)(PVP) and urea (pH=2) and at160oC with NaHCO₃(pH=7and8), respectively. The PVP-derived BiVO₄showed much highersurface areas (5.08.4m²/g) and narrower bandgap energies (2.452.49eV). Thebest photocatalytic performance of the spherical BiVO₄material (surface area=8.4m²/g) in the degradation of MO under visible-light irridiation was associatedwith its higher surface area, narrower bandgap energy, higher surface oxygenvacancy density, and unique porous architecture.(4) Monoclinic BiVO₄with multiple morphologies were fabricated using thealcoho-hydrothermal strategy with bismuth nitrate and ammonium metavanadateas inorganic source, NaOH as pH adjustor, and/or the triblock copolymer P123assurfactant. The surfactant and pH value had a significant influence on the particlemorphology of the BiVO₄product. Porous spherical, flower-like, and sheet-likeBiVO₄were fabricated at alcoho-hydrothermal temperature=180oC and pH=2,7or10, respectively, whereas the rod-like BiVO₄was obtained in the presence ofP123at alcoho-hydrothermal temperature=180oC and pH=2. The difference inparticle morphology of BiVO₄led to the changes in surface area, surface oxygenvacancy density, and (040) crystal plane exposure. Among the four BiVO₄samples,the rod-like one showed the highest surface area, surface oxygen vacancy density,and (040) crystal plane exposure, and the lowest bandgap energy, rendering it to exhibit the best photocatalytic activity for MO photodegradation. We believe thatthere was the presence of a morphological effect on the photocatalyticperformance of the BiVO₄material and the rod-like morphology seems to befavorable for the enhancement in photocatalytic performance.(5) Fluorine-doped BiVO₄materials with different fluorine concentrations werefabricated by adopting the post-treatment of the hydrothermally derived BiVO₄with NH4F. The doping of fluorine did not change the crystal structure of BiVO₄.Compared to the undoped BiVO₄, the fluorine-doped BiVO₄samples showedhigher crystallinity and surface oxygen vacancy density, better optical absorbanceperformance, and lower bandgap energy. The spherical porous BiVO₄sample witha surface area of14.6m²/g and a bandgap energy of2.42eV (the real F/Bi molarratio was0.29) exhibited the excellent photocatalytic activity for the degradationof phenol in the presence of a small amount of H₂O2under visible-light irradiation.Such an excellent photocatalytic performance is attributed to the higher surfacearea and surface oxygen vacancy density, lower band gap energy, stronger opticalabsorbance performance, lower bandgap energy, and unique particle morphology.