Facile Synthesis and Photocatalytic Application of MOF and MOF Derived Heterostructures for Activation of Atmospheric Molecules and Environmental Remediation

Abstract

In this thesis, the synthesis and photocatalytic applications of metal organic framework (MOF) based binary and ternary heterostructure materials have been presented for visible light induced photodegradation of agrochemical contaminants and activation of atmospheric molecules to renewable energy and value-added chemicals. Initially, Zr-based MOF namely UiO-66 and amine functionalized UiO-66 are prepared by solvothermal and low temperature reflux route, respectively. The MOF materials are subsequently integrated with ternary metal sulphide nanostructures to prepare novel binary/ternary S-scheme heterostructure materials with improved optical absorption and enhanced photoelectochemical features. In an alternate approach, the MIL-68(In) MOF is also used as a sacrificial template to prepare hollow tubular In2O3 nanorods which are subsequently hybridized with mixed metal sulfides to prepare binary heterostructure materials. The comprehensive characterization of the composite materials is carried out to understand their structural, morphological, optical and photoelectrochemical properties. The photocatalytic application of the synthesized heterostructure materials is evaluated for photodegradation of agrochemical contaminants and activation of atmospheric molecules. The detailed mechanistic study and band position analysis of the composites are performed to understand the mechanism of photocatalytic action. A facile hydrothermal route has been developed for the preparation of a series of novel UiO-66/CdIn2S4 heterojunction materials containing finely dispersed UiO-66 spherical nanoparticles (20−40 nm) anchored over high aspect ratio CdIn2S4 nanosheets. The hierarchical materials exhibited unique structural, compositional as well as opto-electrical properties, which include high crystallinity, surface exposed reactive site, nanosized interfacial contact, strong absorption in visible region, rapid migration of charge carriers and high resistance to recombination. The optimal 30UiO-66/CIS photocatalyst exhibited excellent photocatalytic performances towards triclosan degradation (> 92 %) as well as H2 production (1.95 mmolg−1 h−1 with apparent conversion efficiency of 20.8%) with kapp value six and fourteen times higher than that of pure CIS, respectively. Radical trapping experiments suggested vigorous formation of reactive •O2− and •OH radicals in aqueous suspension of the UiO-66/CIS heterostructure. The pivotal role of e−, •O2− and •OH radicals has been described towards degradation of triclosan. Further, the UiO-66 MOF has been functionalized with amine chromophore to improve its visible light absorption characteristics. The UiO-66(-NH2) is used as a base semiconductor for construction of sulphur (S)-vacancy rich hierarchical UiO-66(–NH2)/CdIn2S4/CaIn2S4 ternary heterostructure materials by in-situ preferential growth of ultrathin CaIn2S4 nanosheets and CdIn2S4 nanorods in presence of UiO-66(–NH2) (UN) spherical nanoparticles. The photocatalytic efficacy of UiO-66(–NH2)/CdIn2S4/CaIn2S4 ternary hybrid material is explored for photocatalytic H2 evolution reaction and decontamination of asulam (ASM) herbicide. The development of S-vacancy facilitated interfacial charge carrier migration and reduced the recombination rate. The enhanced photocatalytic H2 production and ASM degradation could be ascribed to staggered band alignment between UiO−66(–NH2) (UN), CdIn2S4 (CDS) and CaIn2S4 (CAS), which support S-scheme charge channelization in the heterojunction. Due to its physio-chemical advantages, the optimal 15UN/CDS/CAS30 hybrid photocatalyst exhibited highest photocatalytic H2 evolution rate of 4931 μmolg−1h−1 with apparent conversion efficiency of 31.5% and ASM degradation > 93% (k = 0.02 min−1) under irradiation of visible light. The photocatalytic potential of amine functionalized UiO-66 MOF is further extended by construction of sulphur defect rich hierarchical UiO-66(-NH2)/CuInS2 n-p heterojunction system. Morphologically, the composite contained well dispersed UiO-66(-NH2) nanoparticles over the surface of hierarchical CuInS2 nanosheets. The hierarchical CuInS2 component showed surface sulfur defect leading to creation of more surface exposed active sites with improved absorption of visible light and augmented diffusion of charge carriers. Photocatalytic performance of prepared UiO-66(-NH2)/CuInS2 heterojunction materials has been explored for N2 fixation and O2 reduction reactions (ORR). The optimal UN66/CIS20 heterostructure photocatalyst exhibited outstanding N2 fixation and O2 reduction performances with yield of 398 and 4073 mol g-1 h-1 under visible light illumination, respectively. An S scheme charge migration pathway coupled with improved radical generation ability accounted for the superior N2 fixation and H2O2 production activity. The synthesis of hollow tubular In2O3 nanorods is accomplished by using MIL-68(In) metal-organic framework (MOF) as sacrificial template. The HT-In2O3 is subsequently integrated with MIIIn2S4 (MII: Ca, Mn, and Zn) nanosheets to prepare HT In2O3/MIIIn2S4 binary heterojunction materials. The targeted growth of ternary metal sulfide nanosheets on In2O3 is performed using a facile low-temperature reflux route. The efficiency of HT-In2O3/MIIIn2S4 binary heterojunctions has been studied for the photocatalytic H2, NH3, and H2O2 production. The ultrafast interfacial charge migration, better photoexcited charge separation, and significantly improved photocatalytic activity could be attributed to the narrow band gap energy of the MIIIn2S4 and S-scheme electron migration mechanism between In2O3 and MIIIn2S4 components in the heterostructures. Among the HT-In2O3/MIIIn2S4 (1:1) hetrostructures, the HT-In2O3/ZnIn2S4 (1:1) photocatalyst displayed the highest H2, NH3, and H2O2 generation (5331, 870, and 5716 μmol g−1 h−1) with conversion efficiency of 34%, 6.5%, and 0.291%, respectively. This study provides a comprehensive analysis on how the coupling of three different d0, d5, and d10 ternary metal chalcogenides MIIIn2S4 with HT-In2O3 affects the photocatalytic H2, NH3, and H2O2 production.