A Study on Titanium Oxide-based Composite Photocatalysts for the Removal of Organic Dyes in Aqueous Media

Abstract

The speed of industrialization and modernization has been increasing rapidly, resulting in serious consequences of environmental pollution. The work presented in this dissertation dealt with the organic dyes contaminants that released to waste water from textual industry by using titanium oxide-based composite photocatalysts. The main statement is to produce photocatalysts which can be applied under normal visible light irradiation. In Part II related to improve titanium dioxide/reduce graphene oxide (TiO2/rGO) photocatalysts by introducing transition metals, i.e. doping copper-Cu into the lattice of TiO2 and incorporating nickel-Ni. The Cu-doped TiO2/RGO film photocatalysts showed better performance in the photodegradation of methylene blue than an undoped TiO2/RGO film. Doping Cu could narrow the bandgap of TiO2, thereby influencing the reduction of graphene oxide and enhancing the hydrophilicity of the materials. The synergetic interaction of Cu with the photocatalyst system improved its photocatalytic activity in the decolorization of methylene blue (MB). Relationship between wettability and photocatalytic activity has been clarified as the more hydrophilic film surface, the higher photoreduction upon MB induced and reversed. In the other hand, Ni-incorporated titanium dioxide (TiO2)/graphene oxide composite photocatalysts were prepared by anchoring the TiO2 and Ni onto the surface of graphene oxide (GO) sheets by a straightforward microwave-assisted with one-pot method (N1) and two-steps (N2) method. The as-prepared composite photocatalysts N1 with high Ni content (40-50 wt%) showed good adsorption capacity in the dark and high reaction rate constants under visible illumination while the composite photocatalysts with low Ni content (5-10 wt%) exhibited weak activity. Increasing the Ni content up to 40 and 50 wt% induced not only a structural change affording high porosity but also a narrowing of the band gap to 2.51 eV. Meanwhile, the presence of GO in the composite photocatalysts inhibited the agglomeration of Ni particles regardless of the Ni content. GO also played an important role as an adsorbent as well as a charge separator. The incorporated Ni existed mainly in Ni metal and a small amount of NiTiO3 was formed only at high Ni content, which provided a favorable photocatalytic activity under visible light irradiation. As for the N2 photocatalyst, the role of nickel loading and the interactions between Ni, TiO2 and graphene oxide on the materials were investigated as a function of the Ni content. The interactions between Ni-Ti and graphene oxide at high Ni content in resulted in a novel photocatalyst with fine adsorbility and high photocatalytic activity under visible irradiation. Due to the presence of larger amount of ilmenite NiTiO3, which is a well-known structure for visible light-driven photocatalytic reactions, the photocatalytic activity of N2 composites are calculated to be higher than that in N1 catalysts. In Part III studied of the NiTiO3 and g-C3N4, where NiTiO3 was replaced TiO2 based on its visible-driven ability and g-C3N4 was applied as a semiconductor to support more efficient charge transfer model. Several steps to improve the composites were conducted including (i) control the crystallite size and reduce recombination rate of NiTiO3 by doping molybdenum-Mo, (ii) monitor the thermal polymerization of dicyandiamide (DCDA) to graphitic carbon nitride (g-C3N4) in the presence of nickel titanium trioxide (NiTiO3), (iii) g-NiTiO3 precursors selection for composites and the last step is (iv) synthesize the final Mo-doped NiTiO3/g-C3N4 composites (CMNT) and study of their photocatalytic reactions. The highly crystalline MNT catalysts in step (i) were successfully synthesized via modified Pechini methods to enhance the properties of pure NiTiO3. The role of Mo doped into NiTiO3 materials was to not only enhance optical properties by increasing absorption rates and inhibiting recombination processes, but to also create defects within the lattice structure, leading to a decrease in grain sizes and an improvement in porosity. In step (ii), CNT photocatalysts were investigated to understand the influence of the presence of NiTiO3 on the thermal formation of g-C3N4 layers from DCDA and to find an optimal processing temperature for fabricating CNT photocatalysts at T = 500 ℃ under N2 flow environment. The NiTiO3 inorganic phase in the composite photocatalysts acted as a catalyst to accelerate the thermal polymerization to form the g-C3N4 structure and as a promoter to increase the photocatalytic activity during photodegradation. Step (iii) related to synthesizing CNT composites using different type of nitrogen rich precursors (DCDA, melamine, urea and thiourea). Among the precursors, melamine and urea are unfavorable to involve in thermal polymerization with the presence of NiTiO3 due to phase segregation consequence and early decomposition behavior, respectively. In contrast, DCDA and thiourea exhibit good interaction between components in the as-prepared catalysts by the formation of Ti-N and Ti-S bonding during thermal treatment process. Final products of CNT using DCDA and thiourea in composite processing could overcome the disadvantage in fast recombination rate of pristine g-C3N4 and NiTiO3 and applicable in producing active species for photocatalytic reactions. Finally, CMNT contributed in step (iv) are composite of MNT in step (i) and g-C3N4 from DCDA (step (ii) and (iii)). CMNT composites have higher photocatalytic activity than both g-C3N4/TiO2 (CT) and CNT composites, implying the more effective charge transfer system following Z-scheme model. The performance of CMNT photocatalyst are controlled by several parameters such as particle size of metal oxide, the thin layer of g-C3N4, the interaction between components, the arrangement of band positions, the charge transfer model, etc.