Controlled Synthesis Of Micro/Nano Structured Tungsten Trioxide And Its Composites Towards Enhanced Optical And Photocatalytic Properties

Transition metal oxides are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties. Among them, tungsten trioxide (WO3), a transition metal oxide semiconductor with distinguished physical and chemical properties has received great attention because of its potential applications in the field of renewable energy and other related technological applications. The micro/nano structured WO3 photocatalysts are thus considered as one of the potential candidates for water-splitting and photocatalytic decontamination of organic pollutants. The studies in this thesis have been focused on the fabrication and characterization of novel WO3 as well as its composites like WO3/g-C3N4, WO3/CdWO4 and WO3/Fe2(WO4)3.10.7H2O to explore their tremendous optical and photocatalytic properties. Facile routs (hydrothermal and CVD) were adopted to synthesize these novel photocatalysts and advanced techniques including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-Vis absorption, photoluminescence (PL), and Fourier transform infrared (FTIR) were employed to characterize the as-synthesized materials.In the start, the synthesis of novel structures of WO3 and their photocatalytic properties along with optical properties were focused. A novel three-dimensional octahedral structure of WO3 was prepared via simple surfactant/catalyst-free by using sodium tungstate dehydrate and sodium chloride as precursors. The size of the as-synthesized sample was estimated by FESEM analysis which was in the range of ca.1 to 5 μm. The optical properties such as UV-Vis absorption spectrum, photoluminescence (PL) spectrum and Fourier transform infrared (FTIR) spectra of the product were studied in detail. The synthesized material was utilized as a photocatalyst for the degradation of methylene blue under visible light irradiation and it was observed that the{120} side facet of the exposed surface of the resulting product with large surface area (15.26 m2 g-1), effective crystallinity and increased number of surface active sites exhibited excellent photocatalytic efficiency with a higher rate constant 0.03254 min-1. The apparent high photocatalytic activity of the photocatalyst could be ascribed to large surface area and a highly reactive{120} facet of the exposed surface of the WO3 octahedra.In the next portion, the fabrication of WO3-based composites or heterostructure photocatalysts was focused. By adopting a very simple heating and mixing method, a facile, scaled up, efficient and morphology-based novel WO3/g-C3N4 photocatalyst was developed with different mass ratios of WO3 and g-C3N4. The fabricated material was utilized for the photodegradation of Rhodamine B (RhB) under visible light irradiation which exhibited excellent enhanced photocatalytic efficiency as compared to that of pure g-C3N4 and WO3. The apparent photocatalytic efficiency of the as-prepared composite/hybrid was 3.65 times greater than pure WO3 and 3.72 times than pure g-C3N4 respectively, and it was found that investigated performance is much higher than the previously reported ones. Further, the optical properties of composite samples were evaluated. The bandgap of composite samples lies between 2.3 to 2.5 eV which was benificial for photodegradation process. The possible mechanism for enhanced photocatalytic efficiency of the as-synthesized WO3/g-C3N4 photocatalyst was discussed in detail. It was found that the enhanced performance is due to synergistic effect between WO3 and g-C3N4 interface, improved optical absorption in visible region and suitable band positions of WO3-C3N4 composites.Next, a novel Z-scheme WO3/CdWO4 photocatalyst was synthesized by hybridizing the sheetslike tungsten trioxide (WO3) with rodslike cadmium tungstate (CdWO4) via hydrothermal and chemisorptions method. The as-prepared WO3/CdWO4 heterostructured photocatalyst showed enhanced photocatalytic performance for the degradation of different prganic dyes under visible light irradiation. It was noticed that the photocatalytic performance of the composite WO3/CdWO4 was much higher than that of either bare WO3 or CdW04 for the degradation of each organic pollutant. The photocatalyst exhibited highest activity for the degradation of MB which was about 7 times greater than pure CdW04 and 2.3 times than that of pure WO3. The enhanced performance of the photocatalyst was mainly ascribed to the increased specific surface area and introduction of WO3 into the composite sample, which causes to induce the higher adsorption activity for prganic dyes and increased electron-hole separation at the interface between two semiconductors due to the formation of an inner electric field.Finally, highly efficient visible-light driven one-step hybrid/composite photocatalyst WO3/Fe2(WO4)3.10.7H2O was synthesized by hydrothermal method using sodium tungstate dihydrate and iron chloride as precursors. The FESEM and TEM analysis showed that the as-prepared material possesses porous microplate structure of size about 2 μm and thickness in the range 100 to 120 nm. The as-synthesized photocatalyst maintained a high specific surface area (96.906 m2/g) and exhibited remarkable photocatalytic activity (k=0.06771 min-1) for the decomposition of methyl orange (MO) under visible light radiation. The improved photocatalytic activity was mainly attributed to enhanced optical absorption in the visible region, large specific surface area, effective generation of OH radicals and efficient separation of photogenerated charge carriers at the heterojunction interface between two materials.