Facile Low Temperature Synthesis of Bismuth Molybdate (Bi2MoO6) Based Heterostructure Materials for Photocatalytic Micropollutant Remediation and Reduction Reactions

Abstract

In this thesis, various types of nano-heterostructure materials involving Bi2MoO6 as base semiconductor have been designed for water decontamination and activation of atmospheric molecules. Initially, Bi2MoO6 was synthesised by various low temperature methods. To improve the photoexcited charge carrier separation and photocatalytic activity, Bi2MoO6 was subsequently coupled with spinel metal ferrites and metal vanadates semiconductors to construct binary and ternary heterostructure materials. Further, to facilitate rapid formation of the composites, in many cases, one pot synthesis techniques were used while in some other cases, in situ synthesis method was successfully employed. In addition, defects such as doping and oxygen vacancy were introduced into the crystal structure of the composites to tailor the band positions and improve their spectral response. The prepared materials were characterized by XRD, Raman, FTIR, XPS, EPR, BET, FESEM, TEM, UV-vis DRS, PL, water contact angle and electrochemical techniques to investigate their structural, morphological, surface, photon absorption and charge separation properties. The photocatalytic activity of the materials were evaluated for decontamination of Bisphenol A, Cr(VI) and ciprofloxacin from aqueous sources and activation of atmospheric molecules like H2O, O2 and N2 to produce H2, H2O2 and NH3, respectively. In addition, numerous optimization experiments such as catalyst dosage, pollutant concentration, pH, h+ scavengers, water matrices and intervening anions were carried out. Degradation pathway of Bisphenol A and ciprofloxacin were analysed by identifying the reaction intermediates by HRMS analysis. The reactive radicals responsible for the photo-degradation reactions were identified by scavenger experiments. Further, various radical trapping studies were carried out to confirm the generation of reactive radicals such as •OH and •O2− in aqueous illuminated suspension of the photocatalysts. The band gap, band position, and type of semiconductivity of the individual semiconductors were estimated by Tauc plots, Mott-Schottky plots, and valence band spectra. Finally, by considering the band positions, XPS peak shifting, work function and radicals quenching studies, the mechanism of photoinduced charge carriers’ migration are rationally deduced for each heterostructure material to explain their photocatalytic activity. A mild reflux route was designed for facile synthesis of Bi-self doped Bi2MoO6 (BMO-A) with nanoplate morphology. Microstructural study revealed substitution of Bi5+ ions in the molybdate layer leading to partial reduction of Mo6+ to Mo5+ ions and creation of Mo vacancy. The defect engineered BMO-A exhibited improved optical and photoelectrochemical properties compared to its undoped analogue. The BMO-A material was subsequently used as host lattice for in situ construction of CaFe2O4/Bi2MoO6 0D-2D p-n heterojunctions. Well dispersed CaFe2O4 quantum dots over BMO-A nanoplates provided a strong interfacial contact conducive for fast charge mobilization. The CaFe2O4/Bi2MoO6 composites displayed improved photocatalytic performance for bisphenol A (BPA) degradation and Cr(VI) reduction with rates 5–9 times higher than pure components. The rapid production of •OH and •O2− radicals, construction of an interfacial p-n heterojunction with double charge migration mechanism accounted for the improved photocatalytic efficacy of the composite. A mild CTAB assisted one pot reflux synthesis route is designed for in situ integration of metal organic framework (MOF)-derived NiFe2O4 with tetragonal-BiVO4 and γ-Bi2MoO6 to prepare NiFe2O4/t- BiVO4/Bi2MoO6 ternary composites. Morphologically, fine dispersion of NiFe2O4 (NFO) quantum dots over Bi2MoO6 (BMO) and t-BiVO4 (BVO) nanoplates lead to microscopic heterojunction formation among BMO-BVO, BVO-NFO and BMO-NFO phases. The ternary composites displayed high surface area, strong optical absorption and superior charge mobility that accounted for its improved photocatalytic activity for ciprofloxacin (CIP) degradation (>99% in 90 min) and H2 evolution (1.11 mmolh-1g−1, photon conversion efficiency 18.5%). Kinetics study revealed 12–55 higher CIP degradation activity and 31–41 times higher H2 evolution rate in comparison to the pure semiconductors. A conjugated S-scheme charge transfer mechanism has been deduced from comprehensive band position analysis and radical trapping study to explain the enhanced photocatalytic activity. A series of Bi2MoO6/InVO4/CeVO4 ternary heterostructures were constructed by in situ deposition of Bi2MoO6 nanoplates over one pot synthesized InVO4/CeVO4 using a facile oil bath heating method. A distinct morphology consisting of Bi2MoO6 nanoplates, CeVO4 nanosheets and InVO4 nanorods was noted. The significant intergrowth among the constituent phases led to the construction of tight interfacial microscopic junctions. The ternary materials displayed intense absorption in UV–visible region, drastic decrease in charge recombination and higher excited state lifetime. Both InVO4 and CeVO4 individually as well as the ternary heterostructure contained surface oxygen vacancies that further promoted space charge separation. The optimised ternary photocatalyst displayed 2314 μmol/g/h H2 generation and 1700 μM/g/h H2O2 production which are 12–86 and 11–27 times higher than the pure materials, respectively. Band position assessment and radical trapping study suggested the occurrence of a dual S-scheme charge transfer mechanism that rationally accounted for the improved photocatalytic performance. A novel CeVO4/Bi/Bi2MoO6 ternary heterostructure was fabricated by in situ deposition of CeVO4 over one-pot-synthesized Bi/Bi2MoO6 binary composite. The effect of the salt precursor and reaction duration on the morphology and crystal structure of Bi/Bi2MoO6 was studied in detail. The initial formation of Bi2MoO6 nanoplates and their subsequent disintegration to nanorods upon prolonged reaction time was observed due to concurrent leaching and reduction of Bi3+ ions to plasmonic Bi0 metal. The CeVO4/Bi/Bi2MoO6 ternary heterostructure demonstrated a uniform deposition of CeVO4 nanoparticles (10–20 nm) over Bi2MoO6 nanorods that are embedded with ultrasmall Bi0 nanodots (2–5 nm). The ternary composites displayed improved optoelectronic features which have been ascribed to the creation of surface oxygen vacancies and plasmonic nature of Bi nanodots. The optimized ternary photocatalyst exhibited encouraging photocatalytic activity for H2O2 generation (953 μM/g/h) and NH4+ production (131 μmol/g/h) with reaction kinetics 7–20 and 4–5 times greater than those of pure semiconductors and CeVO4/Bi2MoO6 binary heterostructure. Based on experimental evidences, a switching of charge migration route from Type-I in CeVO4/Bi2MoO6 to Bi0-mediated all-solid-state Z-scheme for the CeVO4/Bi/Bi2MoO6 composite is proposed, which accounted for its improved photocatalytic activity.