Tunable Microstructure And Enhanced Photocatalytic Activity Of TiO₂-based Photocatalytic Materials

Energy and environmental issues are the biggest challenges in the21century. Semiconductor photocatalytic materials exhibit great potentials in environmental protection and solar energy conversion. TiO2nano-material is the most widely used material because of its wide potential application in waste water treatment, air purification, bacillus resistance and water splitting to generate hydrogen. However, owing to its low quantum efficiency and relatively wide band gap, which can only absorb the UV light, and thus greatly restricts its practical applications and commercial benefit. In this dissertation, our main ideas are tuning the electronic structure, surface property, band gap structure and photocatalytic performance of TiO2-based photocatalytic material, and developing highly activity semiconductor photocatalyst with modifying, doping and assembling to absorb visible light and to enhance the photocatalytic activity. The main points could be summarized as follows:Firstly, fabrication of TiO2photocatalysts with different morphology and comparison of their photocatalytic activity. Surface-fluorinated TiO2hollow microspheres and tabular-shaped anatase single mico-crystals with highly energetic (001) facets exposed were prepared by a one-step hydrothermal strategy using ammonium bifluoride (NH4HF2) as a morphology controlling agent. It was found that with increasing NH4HF2concentration, the average crystallite size and average pore size increase, whilst the specific surface area, pore volume and porosity steadily decrease. The TiO2hollow microsphere exhibits the highest photocatalytic activity for the photocatalytic decolorization of RhB in aqueous solution, and is higher than pure TiO2or P25due to the enhancement of crystallization, formation of hollow structures and surface fluorination. Subsequently, hierarchical flower-like TiO2superstructures (HFTS) self-assembled from anatase TiO2nanosheets with exposed {001} facets (up to87%) have been synthesized by a simple alcohothermal strategy in a HF-H2O-C2H5OH mixed solution using titanate nanotubes as precursor. The study shows that the activity of HFTS for photocatalytic oxidation decomposition of acetone in air and methyl orange (MO) in aqueous solution is greatly higher than that of commercial Degussa P25(P25) and tabular-shaped anatase TiO2obtained in pure water. A significant enhancement in the photocatalytic activity can be related to several factors, including hierarchical porous structure, exposed {001} facets, and the increased light-harvesting abilities.Secondly, we investage the effects of surface fluorination and exposed {001} facets on the photocatalytic decomposition of acetone in air and photocatalytic selectivity towards decomposition of azo dyes in water. Firstly, surface-fluorinated anatase TiO2nanosheets with dominant {001} facets were fabricated by a simple hydrothermal route in a Ti(OC4H9)4-HF-H2O mixed solution. All fluorinated TiO2nanosheets exhibit much higher photocatalytic activity than Degussa P-25TiO2(P25) and pure TiO2nanoparticles prepared in pure water. The fluorinated TiO2nanosheet with an optimal relative percentage of exposed anatase {001} facets exhibits the highest photocatalytic activity, and its photoactivity exceeds that of P25by a factor of more than9times due to the synergistic effect of surface fluorination and exposed {001} facets on the photoactivity of TiO2. Subsequently, the flower-like TiO2microspheres films were directly synthesized on the Ti foil in a dilute aqueous HF solution by a simple one-pot hydrothermal treatment. The photocatalytic selectivity of TiO2films towards decomposition of azo dyes in water can be tuned by modifying the surface of TiO2microspheres as well as by varying the degree of etching of {001} facets. In addition, the percentage of exposed {001} facets can be also controlled to some extent by adjusting the reaction time of the selective chemical etching. This is because that the selective photocatalytic degradation of charged contaminants can be tuned by altering the surface charge of TiO2depending on pH.Thirdly, microwave-hydrothermal preparation and visible-light photoactivity of plasmonic photocatalyst Ag-TiO2nanocomposite hollow spheres. Conventional TiO2photocatalyst possesses excellent activities and stabilities, but requires near-ultraviolet irradiation for effective photocatalysis thereby severely limiting its practical application. It is highly desirable to develop a photocatalyst that can use visible light in high efficiency under sunlight irradiation. Noble metal Ag nanoparticles can dramatically amplify the absorption of visible light and is therefore utilized to develop efficient visible-light-driven plasmonic photocatalysts. In this work, a visible-light-driven plasmonic photocatalyst Ag-TiO2nanocomposite hollow spheres was prepared using the template-free chemically-induced self-transformation method under microwave-hydrothermal condition, then reducing the Ag+ions on the surface of TiO2nanoparticles to Ag0species under xenon lamp irradiation. The surface plasmon absorption band of the silver clusters supported on the TiO2hollow spheres is observed. The prepared plasmonic photocatalyst exhibits a highly visible-light photocatalytic activity for the photocatalytic degradation of RhB aqueous solution. This is due to the fact that the photoexcited electrons at the silver nanoparticles are injected into the TiO2conduction band, and the injected electrons can be transferred to the ubiquitously present molecular oxygen to form first the superoxide radical anions, O2-, then on protonation yields the HOO radicals, and HOO· radicals and the trapped electrons combine to produce H2O2, finally forming HO·radicals. These active species will result in the degradation and mineralization of RhB. This study may provide new insight into design and preparation of advanced visible-light photocatalytic materials.Fourthly, enhanced photocatalytic H2-production activity of graphene-modified titania. Firstly, graphene-modified TiO2nanosheets with exposed (001) facets were prepared by microwave-hydrothermal treatment of graphene oxide and the hydrothermally-synthesized TiO2nanosheets with exposed (001) facets in an ethanol-water solvent. These nanocomposite samples showed high photocatalytic H2-production activity in aqueous solutions containing methanol as sacrificial reagent even without Pt co-catalyst. The potential of graphene/graphene-are deemed to be less negative than the conduction band of TiO2and more negative than H+/H2potential, which favors the electron transfer from CB of TiO2to graphene and the reduction of H+, thus enhancing photocatalytic H2-production activity. Subsequently, the proposed two-step hydrothermal synthesis of titania-based composite photocatalysts with layered MoS2/graphene co-catalyst afforded an effective photocatalyst for H2production. The TiO2/MoS2/graphene composite photocatalysts showed high photocatalytic H2-production activity with the rate as high as165.3μmol h-1for the sample having0.5%of co-catalyst that consisted of MoS2(95%) and of graphene (5.0%). The corresponding apparent quantum efficiency reached9.7%at365nm even without noble metal co-catalyst. At last, a novel and facile solvothermal route for the preparation of visible-light responsive nitrogen self-doped TiO2nanosheets with exposed{001} facet is proposed by treating TiN in a HNO3-HF ethanol solution. Due to nitrogen doping, the presence of highly reactive {001} facets and large specific surface area, nitrogen self-doped TiO2nanosheets with exposed {001} facets exhibited much higher visible-light photocatalytic H2-production activity than nitrogen doped TiO2microcrystallites with exposed{001} facets (ca.60%).Fifthly, we investigate graphene-based semiconductor photocatalysts. Graphene, a single layer of graphite, possesses a unique two-dimensional structure, high conductivity, superior electron mobility and extremely high specific surface area, and can be produced on a large scale at low cost. Thus, it has been regarded as an important component for making various functional composite materials. Especially, graphene-based semiconductor photocatalysts have attracted extensive attention because of their usefulness in environmental and energy applications. Firstly, graphene and graphitic carbon nitride (g-C3N4) composite photocatalysts were prepared by a combined impregnation-chemical reduction strategy involving polymerization of melamine in the presence of graphene oxide (precursors) and hydrazine hydrate (reducing agent) followed by thermal treatment at550℃under flowing nitrogen. Graphene sheets act as electronic conductive channels to efficiently separate the photogenerated charge carriers, and consequently, to enhance the visible-light photocatalytic H2-production activity of g-C3N4. Subsequently, we summarize the recent progress in the design and fabrication of graphene-based semiconductor photocatalysts via various strategies including in situ growth, solution mixing, hydrothermal and/or solvothermal methods. Furthermore, the photocatalytic properties of the resulting graphene-based composite systems are also discussed in relation to the environmental and energy applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation and photocatalytic disinfection. This chapter ends with a summary and some perspectives on the challenges and new directions in this emerging area of research.Sixthly, quantitative characterization of hydroxyl radicals produced by various photocatalysts. The·OH produced on various semiconductor photocatalysts in aqueous solution under Xenon lamp irradiation was quantitatively investigated by the photoluminescence technique using coumarin as a probe molecule. The formation rates of·OH on anatase TiO2and P25were much higher than that of·OH on the other semiconductors (such as rutile TiO2, ZnO, WO3, CdS, Bi2WO4and BiOCl, etc.). A new "OH-index" was introduced by comparing the relative-OH formation rate of the target photocatalyst to that of P25, which was proposed to compare oxidation activity of various photocatalysts. This study would provide new insights and understanding on the photocatalytic mechanism.