Synthesis of Microwave Assisted Transition Metal Dichalcogenides for the Catalytic Degradation of Organic Pollutants

Abstract

Chapter 1: A brief introduction about hazardous pollutants and wastewater treatment methods is given with a precise discussion on advanced oxidation processes including semiconductor photocatalysis. This section also illustrates the photocatalytic mechanism in detail using semiconductors. Furthermore, an extensive discussion on transition metal dichalcogenides (TMDCs) is provided with description of their unique photo or electrocatalytic properties like high surface-to-volume ratio, excellent charge transfer capacity, mechanical strength, and low bandgap energy. The chapter also highlights the advantages of microwave-assisted methods for synthesis of different TMDC composites and the requirement for the formation of heterojunction composites to overcome charge recombination. In addition, the research gaps, research objectives, and characterization approaches have been combined with specifications.

Chapter 2: The photodegradation of toxic pollutants is a promising way to deal with water-pollution. In this regard, MoS2/g-C3N4 (MSC) composites with varying weight-ratios were prepared via fast one step microwave-assisted method. The material was characterized by XRD, XPS, EDS, FESEM and HRTEM which validate the successful formation of catalyst having the flower-like and sheet-like morphologies of MoS2 and g-C3N4, respectively. The PL and UV-vis DRS spectra exhibit low recombination-rate and band-gap (1.7 eV) which is appropriate for efficacious visible-light degradation. The photocatalytic performance of catalysts was then analyzed by the degradation of a model dye methylene blue and pesticide fipronil. Best results were obtained by 5:1 MSC (98.7% degradation efficacy; rate constant 0.0261 min-1) in 80 min under sunlight. The effects of solution pH, catalyst-dose, scavengers and illumination-area were also explored. The catalyst is highly-reusable as confirmed by the characterization and degradation studies (~82% efficiency) after 5-cycles. The photocatalytic treatment of real industrial-wastewater was also conducted. The electrochemical degradation of methylene blue was also investigated using glassy carbon electrode modified with different MSC-ratios. The electrode modified with 5:1 MSC at pH 7 manifested maximum peak current. The plausible mechanisms for photocatalytic and electrochemical degradation were proposed. The study therefore reveals that synthesized nanocomposites have a remarkable potential for wastewater treatment.

Chapter 3: Two-dimensional (2D) heterojunctions with layered structures give high flexibility in varying their photocatalytic/electrocatalytic properties. Herein, 2D/2D heterostructures of MoS2/MoSe2 with different weight-ratios (1:1, 1:3, and 3:1) have been prepared by simple one-step microwave-assisted technique. The characterization studies confirm formation of crystalline MoS2/MoSe2 nanospheres with high surface area (60 m2/g) and porous structure. High synergistic-effect (1.73) and narrow bandgap (~1.89 eV) of the composites shows enhanced photo-degradation efficiency towards methylene blue dye (94%) and fipronil pesticide (80%) with high rate constant (0.33 min-1 and 0.016 min-1 respectively) under visible light. The effect of pH, catalyst dose, and illumination area on photodegradation has been optimized. Photodegradation of real-industrial wastewater shows 65% COD and 51.5% TOC removal. Trapping experiments confirm that holes are mainly responsible for degradation. The composites were highly reusable as showing 75% degradation after 5-cycles. MoS2/MoSe2 composites show excellent electrochemical water-splitting efficacy through hydrogen-evolution-reaction (HER) exhibiting stable high current density of –19.4 mAcm-2 after 2500 cyclic-voltammetry (CV) cycles. The CV-plots reveals high capacitance activity (Cdl value ~607 µFcm-2) with a great % capacitance retention (>90 %). The as-prepared 2D/2D-catalysts are highly active in sunlight and beneficial for long-time physico-chemical wastewater treatment. Moreover, the electrochemical studies confirm that these composites are potential materials for HER activity and energy-storage applications. Chapter 4: Visible light-driven photocatalytic degradation is one of the promising ways to deal with the major problem of water pollution. This study involves one-step, fast, in-situ preparation of BiOCl/MoS2 (2D/2D) heterostructure via microwave irradiations. The photocatalytic performance of BiOCl/MoS2 composite with different weight ratios (1:1, 3:1, and 1:3) have been evaluated for degradation of organic (methylene blue dye, fipronil pesticide) and inorganic pollutants (Cr(VI)) along with real industrial wastewater under visible light irradiation. XRD, XPS, FESEM, HRTEM, and SAED analysis indicate the formation of single-crystalline nanorods with a high aspect ratio of 1:10. The composites possess high surface area (~40m2/g) with lower charge recombination and high visible light absorption tendency due to lower band gap (2.30eV) energy. The high synergistic interaction (~2.29) between BiOCl and MoS2 results in enhancement of degradation activity which shows 94%, 89%, and 90% removal of MB, fipronil, and Cr(VI) metal, respectively by BiOCl/MoS2 (1:3) catalyst. The optimum conditions to get maximum degradation efficiency were determined by varying different reaction parameters like pH, catalyst dose, and illumination area. Radical trapping experiments indicate that holes and hydroxyl radicals had a dominant role in the degradation process. The catalyst is highly stable and reusable as confirmed by the reusability studies. The photocatalytic treatment of industrial wastewater without any physico-chemical pretreatment showed 75% COD and 63% TOC removal under visible light, which indicates high practical efficiency of the catalyst. The current study validates that the BiOCl/MoS2 composites with superior characteristic properties can be efficiently used for wastewater treatment under natural light.

Chapter 5: The alarming situation of water pollution by human overuse of medications and pesticides can be efficiently dealt with by using visible-light active, efficient photocatalysts. Herein, different weight ratios (1:1, 1:3, and 3:1) of MoS2/GO composites decorated with Ag nanoparticles (named as MAG) have been prepared by the microwave-assisted route. XRD and XPS investigations indicated the crystallinity of the catalyst and the oxidation states of the elements, while EDS and color mapping proved the uniform dispersion of elements in the catalyst. FESEM and HRTEM analyses revealed the presence of small MoS2 nanopetals scattered on GO sheets with Ag nanoparticles dispersed on the surface whereas BET-analysis disclosed its excellent surface area (~88 m2/g). The optical properties of MAG catalysts revealed that they were highly visible-light active, with a bandgap of 2.15 eV and a lower charge recombination rate. The photocatalytic performance of the as-prepared catalysts has been evaluated for degradation of antibiotic (tetracycline (TC)), pesticide (fipronil (FIP)), and real industrial wastewater. The effects of catalyst amount, pH, and effective area of illumination on degradation were investigated. Excellent efficiency was observed for TC (90.7%; 0.0186 min-1) and FIP-degradation (85.2%; 0.0177 min-1) with 4mg MAG (3:1) catalyst at neutral pH under visible-light irradiation owing to the high synergistic interaction (~2.21) in the composite. The high reusable nature of the catalyst (65% (TC) and 58% (FIP) efficiency after 5 cycles) was supported by post-photocatalytic characterization studies. The scavenging experiments revealed that hydroxyl radicals and holes played an important part in the process of degradation. High COD (70.4%) and TOC (55.1%) removal rates were achieved in the photo-mineralization of real wastewater used without any pre-treatment. The current investigation, combined with comparative literature, illustrates the real-world potential of the MAG catalysts for the eradication of resistant pollutants.