Facile Synthesis and Photocatalytic Application of Bi-Based Binary/Ternary Heterostructure Nanomaterials Towards Degradation of Emerging Pharmaceutical and Agrochemical Contaminants

Abstract

In this thesis, the fabrication and photocatalytic application of bismuth based novel binary/ternary heterostructure materials has been described for mineralization of selected pesticides and emerging pharmaceutical contaminants. Initially, morphology controlled synthesis of Bi2O2CO3 was performed by using a modified hydrothermal route. The Bi2O2CO3 was subsequently used as a precursor for synthesis of phase pure tetragonal -Bi2O3 material. The Bi2O2CO3 and -Bi2O3 had been used as base materials for construction of visible light active binary/ternary heterostructure photocatalysts. In a novel approach, facile one pot synthesis routes were also developed for fabrication of CdS/BiOBr/Bi2O2CO3 and Bi2S3/β-Bi2O3/ZnIn2S4 ternary heterostructure materials with improved charge carrier separation and enhanced photocatalytic activity.

Bi2O2CO3 nanoplates with high aspect ratio and hierarchical nanostructures were prepared by a hydrothermal technique using urea/hexamethylenetetraamine as hydrolysing agent and KCl/KBr as additive in different solvent systems. The relative molar proportion of urea and KCl was crucial for phase purity as well as thickness and planar dimension of the BSC plates. The BSC nanoplates were used as substrates to prepare CuS/Bi2O2CO3 binary heterojunction systems. The presence of crystalline tetragonal BSC and hexagonal covellite CuS phase was inferred from XRD study. Morphologically, the CuS/BSC material contained CuS nanorods and BSC nanoplates. An HRTEM study suggested microscopic close contact between the CuS nanorods and BSC nanoplates. A study of optical properties revealed improvement in visible light absorption and enhanced separation of excitons. The CuS/BSC materials were used as photocatalyst for chlorpyrifos (CP) pesticide degradation under visible light irradiation. The heterojunction materials were highly active achieving >90% degradation within 3 h of reaction. The degradation pathway of CP and photocatalytic mechanism was studied in detail by using radical trapping experiments and analysis of intermediate products by GC-Ms analysis. In order to further explore the potential of Bi2O2CO3 based semiconductor heterostructure towards pesticide degradation, binary BiOBr/Bi2O2CO3 material was prepared by optimizing the molar ratio of Bi(NO3)3.5H2O precursor salt, urea and KBr using same hydrothermal protocol. In addition, a one-step hydrothermal method was developed for morphology controlled synthesis of CdS/BiOBr/Bi2O2CO3 ternary heterostructure materials. The ternary system contained well dispersed CdS nanoparticles (50-80 nm) anchored over ultrathin BiOBr and Bi2O2CO3 nanoplates with high interfacial contact. A significant enhancement in visible light absorption, prolonged life time decay and improved charge carrier separation and migration property accounted for the excellent photocatalytic activity of the ternary heterostructure towards atrazine herbicide degradation in a short span of time (>95% in 30 min). An MTT assay study revealed that the photo-catalytically treated atrazine solution showed significant reduction in cytotoxicity.
The Bi2O2CO3 was used further as a precursor to prepare metastable -Bi2O3 by using a low temperature calcination route. The -Bi2O3 was subsequently modified with metal chalcogenide (NiS, Bi2S3, and ZnIn2S4) to prepare binary/ternary heterostructure systems. The visible light driven photocatalytic activity of the heterostructure materials was explored for photodegradation of pharmaceutical contaminants from aqueous sources. Initially, a facile method was also developed for synthesis of ultrathin α-NiS nanosheets under mild conditions using hexamethylenetetramine as hydrolyzing agent and Na2S2O3 as sulfur source. The α-NiS nanosheets were subsequently hybridized with -Bi2O3 to prepare a series of α-NiS/-Bi2O3 composite nanomaterials. The α-NiS/-Bi2O3 composite materials were thoroughly characterized using a variety of techniques to understand their structural, optical, electrochemical, microstructural and morphological attributes. The α-NiS/-Bi2O3 materials exhibited improved visible light absorption, enhanced charge carrier separation and photo-electrochemical properties. The microscopic close contact between the two semiconductor phases was established from the morphological studies. The α-NiS/-Bi2O3 ¬system was highly active for tramadol degradation from aqueous solution achieving 94% degradation in 3 h. The effect of various reaction parameters and the photocatalytic mechanism was investigated thoroughly. The photocatalytic potential of -Bi2O3 based heterostructure materials was further explored by rational design of a Bi2S3/β-Bi2O3/ZnIn2S4 ternary heterostructure for degradation of tetracycline antibiotic and microbial disinfection. The co-assembly of Bi2S3 and ZnIn2S4 with β-Bi2O3 was achieved by a tailor made in situ reflux route using thioacetamide as sulfur source. Comprehensive characterization of the ternary composite revealed close microscopic contact between the semiconductors, fast electron channelization, enhanced charge carrier separation and a prolonged life time (11.25 ns) of the excited state. The ternary composite exhibits excellent visible light assisted photocatalytic activity for aqueous phase tetracycline (TCN) degradation achieving 96.3% (kapp= 0.0868 min-1) degradation in a short reaction span of 40 min. A bacterial inactivation study conducted using Enterobacter cloacae suggested good visible light assisted bacterial degradation activity of the coupled semiconductor system. MTT assay study confirmed the non-cytotoxic nature of the treated TCN solutions. Finally, the occurrence of Z-scheme electron transfer mechanism in the above mentioned binary/ternary heterojunction was deduced from radical trapping experiments and ESR study which accounted for the excellent photocatalytic activity of the ternary composite material.