Photocatalytic Application of Bismuth Based Semiconducting Nanoparticles and their Heterostructures towards Selective Organic Transformations and Degradation of Persistent Organic Pollutants

Abstract

In this thesis, Bi based complex oxides and sulphide semiconductor nanomaterials have been synthesized by combustion synthesis route. The Bi based materials have been coupled with low band gap metal sulphide materials to form heterojunction photocatalytic systems with improved visible light absorption and photon harvesting efficiency. The photocatalytic activity of the heterostructure systems have been studied for degradation of organic pollutants from aqueous sources and selective organic transformation reactions under visible light illumination.
Bismuth tungstate (Bi2WO6) nanoparticles were synthesized by combustion synthesis method using different N-containing organic compounds as fuels (urea, glycine, hexamethylenetetramine (HMTA), malonic acid dihydrazide (MDH)). Glycine as fuel was effective for synthesis of phase pure Bi2WO6 nanoparticles. Employing HMTA and MDH as fuel lead to mixed phase complex oxide system with Bi14W2O27 as minor (impurity) phase. The nature of the fuel significantly influences the particle size and morphology. The Bi2WO6 nanoparticles were used as an efficient photocatalyst for the visible light driven chemoselective oxidation of substituted thiophenols to disulfide using air as oxidant. Structurally diverse diphenyl disulfide moieties were obtained in high yield and excellent selectivity within a time span of 6 h of visible light irradiation. The Bi2WO6 nanoparticles were also synthesized by amorphous citrate process and subsequently modified by dispersing CuS to form CuS/Bi2WO6 (CuSBTA) heterojunction materials. XRD study indicated the existence of hexagonal covellite CuS and orthorhombic Russellite Bi2WO6 phase in the heterojunction materials. During hydrothermal treatment, the Cu2+ ions substituted for the W6+ ions in the Bi2WO6 lattice to form a substitutional solid solution (Bi2CuxW1-xO6-2x). The CuS and Bi2WO6 phases existed in distinct nanorod and nanosheet morphologies, respectively. During hydrothermal treatment, significant morphological reorganization of the Bi2WO6 phase took place leading to the formation of flower like hierarchical nanostructures. The CuSBTA materials possess characteristic features of a type-II heterojunction exhibiting narrow band gap, enhanced visible light absorption and efficient charge separation properties. The heterojunction materials were evaluated as visible light active photocatalyst for complete degradation of Congo red dye using H2O2 as oxidant. The CuSBTA materials exhibited higher apparent rate constant (Kapp) and greater efficiency for Congo red degradation compared to pure Bi2WO6 material.
A series of type-II heterojunction nanomaterials were synthesized by coupling Bi2W2O9 with CdS and CuS. Initially, phase pure Bi2W2O9 with orthorhombic crystalline structure was prepared by a facile combustion synthesis route using urea as a fuel. The CuS and CdS nanoparticles were dispersed over Bi2W2O9 matrix by using a hydrothermal route. The heterojunction materials were characterized using XRD, XPS, FTIR, UV-Vis-DRS, PL, FESEM and HRTEM study. Pure Bi2W2O9 exhibited micron-size plate-like particles. The occurrence of ultrafine CdS nanoparticles with diameter between 8-15 nm well dispersed over Bi2W2O9 plates is noticed for CdS/Bi2W2O9 materials. For CuS/ Bi2W2O9 materials, Cu2+ ions replaced partially W6+ ions in Bi2W2O9 lattice to form Bi2CuxW2-xO9-2x as a nonstoichiometric solid solution phase.

Under hydrothermal treatment, the desegregation of the Bi2W2O9 plates to nanosheets and the concurrent formation of CuS nanorods were noticed leading to their hierarchical reorganisation to microspherical structures. Both the heterojunction materials, exhibited improved visible light absorption, enhanced charge carrier separation and suitable band alignment characteristic of a type-II heterojunction. The CdS/Bi2W2O9 heterojunctions were evaluated as visible light active photocatalyst for aerobic oxidation of amines to imines. Structurally and functionally diverse amine molecules were oxidized to the corresponding imines with excellent selectivity in a short span of time. The CuS/Bi2W2O9 heterojunction materials were studied as an efficient photocatalyst for the degradation of diuron pesticide under visible light irradiation achieving 95% mineralization within 3 h. Mechanistic study indicated that the mineralization of diuron occurred in a cascade manner over the catalyst surface involving dechlorination, alkyl oxidation and oxidative ring-opening steps.
A visible light promoted photocatalytic route has been developed for mineralization of alachlor pesticide using type-II CuS/BiFeO3 heterojunction materials. The heterojunctions were synthesized by a two-step process involving synthesis of BiFeO3 by combustion route followed by deposition of CuS material by hydrothermal route. Microscopically, the heterojunction materials contained BiFeO3 nanoplates and CuS nanorods. Optical property study and photocurrent measurement suggested that these materials show excellent absorption in visible region with superior charge carrier mobility and separation compared to the individual components. The CuS/BiFeO3 materials showed high efficiency for mineralization of alachlor pesticide under visible light illumination achieving >95% degradation within 60 min. The mechanism of alachlor degradation over the catalyst surface was elucidated using GCMS and radical scavenger experiments.
A series of α-Fe2O3-Bi2S3 heterojunction materials were prepared by a one-step autocombustion method employing thiourea as fuel and characterized using XRD, UV–Vis-DRS, FTIR, PL, XPS, FESEM, TEM and HRTEM analytical techniques. XRD study indicated presence of rhombohedral α-Fe2O3 and orthorhombic Bi2S3 in the heterojunction materials. The heterojunctions displayed better optical absorption in the visible region. Microscopic studies indicated presence of well dispersed α-Fe2O3 nanorods in a continuous Bi2S3 matrix. The α-Fe2O3 nanorods were typically 30–50 nm in diameter and 120–150 nm in length growing isotopically in different direction from a single nucleation point. The calculated band positions of both components indicated a facile electrons transfer from the conduction band of α-Fe2O3 to Bi2S3 whereas migration of holes occurs in the reverse direction yielding a type-II heterojunction. The α-Fe2O3 -Bi2S3 heterojunctions materials were evaluated as selective and efficient photocatalyst for the hydrogen transfer reduction of nitroarenes under visible light illumination. Structurally diverse nitroarenes could be selectively reduced to the corresponding amines in high yield and purity using α-Fe2O3 -Bi2S3 as photocatalyst.