Application Of Bismuth Titanate And Bismuth Ferrite Based Binary/Ternary Heterostructure Nanomaterials Towards Photocatalytic Degradation Of Agrochemical Contaminants

Abstract

In this thesis, the facile synthesis and photocatalytic application of bismuth titanate and bismuth ferrite based binary and ternary heterostructure materials has been presented for visible light assisted degradation of agrochemical contaminants with strong endocrine disrupting properties. Initially, suitable combustion synthesis routes were designed for phase pure synthesis of Bi4Ti3O12, Bi2Fe4O9 and Bi20TiO32/Bi4Ti3O12 composites. The synthesized complex oxides were integrated with metal sulfide nanoparticles to prepare binary/ternary p-n and Z-scheme heterostructure materials with improved optical absorption and enhanced photoelectochemical features. The photocatalytic application of the synthesized heterostructure materials were evaluated for photodegradation of pesticides/herbicides/insecticides and water pathogens under visible light irradiation. The detailed mechanistic study and evaluation of degradation pathway was carried out to understand the mechanism of photocatalytic action. Bi4Ti3O12MCPA degradationDiazinon degradationEndosulfan degradationBi2Fe4O9Sulfamethoxazole degradationPhenyl urea herbicides degradationBacterial Inactivation A series of CuS/Bi4Ti3O12 p-n heterojunction materials were synthesized by a two-step process. Initially, the Aurivillius phase Bi4Ti3O12 (BT) was synthesized by a facile combustion route using urea as a fuel. The Bi4Ti3O12 was subsequently modified by deposition of CuS (5-20 wt%) using a hydrothermal route to prepare the heterojunction materials. The methods of synthesis and calcination temperature were important factors which influenced the morphology, particle size and phase purity of Bi4Ti3O12 material. The heterostructure materials exhibited hierarchical flower like structure consisting of ultrathin CuS nanosheets and BT nanoflakes. HRTEM and microstructural study revealed microscopic close interaction between the two phases. The optical and photoelectrochemical measurement study suggested significant improvement in visible light absorption (400-800 nm) and charge carrier separation due to heterojunction formation. The CuS/Bi4Ti3O12 materials showed excellent photocatalytic activity for aqueous phase degradation of 2-methyl-4-chlorophenoxyacetic acid (MCPA) herbicide under visible light (> 95 % degradation in 3 h). The rate constant for CuS/Bi4Ti3O12 materials was 4.5 times higher than the pure BT material towards MCPA degradation. The OH and O2– radicals were identified as the reactive species, the formation of which was confirmed by spectrometric method using terephthalic acid and nitroblue tetrazolium as molecular probes. Further, the combustion synthesized Bi4Ti3O12 material was hybridized with SnS2 to prepare Z-scheme SnS2/Bi4Ti3O12 heterostructure photocatalyst materials. The SnS2 nanoparticles (10-20 nm) were decorated over BT surface using a hydrothermal method. Structural and morphological characterization suggested the presence of orthorhombic BT and hexagonal SnS2 crystalline phases with high interfacial contact and strong interaction at microscopic level. Optical and photoelectrochemical (PEC) measurements revealed improved visible light absorption and enhanced charge carrier separation and migration properties. The SnS2/Bi4Ti3O12 materials displayed excellent photocatalytic activity for degradation of diazinon insecticide (> 90% with kapp ~ 0.019 min-1) in aqueous media under visible light illumination. The valence band (VB) h+ and •OH radicals were identified as major transient species responsible for photodegradation of diazinon. A Z-scheme electron transfer pathway has been proposed to explain adequately the generation of these radicals which was deduced from PEC measurements, scavenger and radical trapping experiments. Mechanistic study revealed that diazinon degradation occurred in a series of steps to produce 2-isopropyl-6-methyl-pyrimidin-4-ol as a major intermediate which was further mineralized over the photocatalyst surface. The process potential of the developed photocatalytic method has been tested for different water matrices and interfering anionic species with radical quenching activity. A series of ternary multi-heterojunction CdS/Bi20TiO32/Bi4Ti3O12 (CdSxBTC) photocatalysts were prepared by hydrothermal deposition of CdS nanoparticles (15-25 nm) over one pot combustion synthesized Bi20TiO32/Bi4Ti3O12 (BTC) nanostructures. Comprehensive characterization of the ternary composites revealed enhanced optical absorption, high interfacial contact, fast electron channelization and a prolonged excited state life time in comparison to pure components. The CdSxBTC composite materials displayed enhanced photocatalytic activity for endosulfan degradation (kapp value 6-12 times greater than pure semiconductors). The cell viability assay study disclosed non-cytotoxic nature of the treated endosulfan solution. A synergistic Type-I bridged coupled Z-scheme electron migration process accounted for robust radical generation ability (•O2− and •OH) and photocatalytic activity of the ternary composites. The facile fabrication of In2S3/Bi2Fe4O9 (ISxBFO) binary heterostructure was performed by hydrothermal deposition of In2S3 nanoparticles (20-40 nm) over combustion synthesized Bi2Fe4O9 nanocuboids. In depth characterization of the composite revealed broad spectrum UV-Vis response, large interfacial contact, facile charge carrier separation and mobility and a prolonged life time of excited state photoelectrons. The ISxBFO heterostructure material exhibited enhanced photocatalytic efficiency for aqueous phase degradation of sulfamethoxazole antibiotics (kapp=0.06 min-1) and phenyl urea herbicides (kapp= 0.028 min-1) with reaction rates 5-8 times higher than the pure components. The cell viability study confirmed non-cytotoxic nature of treated sulfamethoxazole and diuron solutions. The composite materials also showed convincing antibacterial behavior towards toxigenic Vibrio cholerae pathogen. Haemagglutination assay study revealed excellent biocompatibility of the binary composite up to a concentration of 200 mg/L. A Z-scheme electron migration mechanism accounted for the robust radical generation ability (•OH and O2–) and multidimensional photocatalytic activity of the ISxBFO heterostructure material.