Ammonium Fluotitanate Route Synthesis And Photocatalytic Activities Of Titanium Dioxide Photocatalysts

Photocatalysis is one of the frontiers of photochemistry, which can oxidize and reduce many hazardous substance that can not be treated by normal redox reaction. Titania, due to its remarkable photocatalytic performance, absence of toxicity and long-term photostability, has always been the focus of photocatalysis research.Controllable synthesis of crystalline TiO2 is crucial. Previous work on TiO2 synthesis largely started with titanium(Ⅳ) compounds, mainly TiCl4 and alkoxides, which are highly sensitive to atmospheric moisture and therefore require special precautions. Besides, the reaction product is usually amorphous titania, which requires postannealing for crystallization. Postannealing, however, usually results in some unfavorable effects for cation- and anion-doped titania particles, such as surface dehydroxylation, hard aggregation, and segregation. Therefore, we select ammonium fluotitanate as the titanium source to prepare titania photocatalyst. This dissertation focuses on the preparation and photocatalytic properties of titania microspheres, titania nanoparticles and graphene/titania nanocomposites.A new route is designed to prepare anatase titania microspheres. By heating the mixture solution of ammonium fluotitanate and urea to 83-100℃and holding at that temperature for 30 min, anatase microspheres directly generated in solution. The microspheres obtained are hierarchically structured and are built up of rounded anatase nanocrystallites with a diameter of approximately 15-40 nm, which means the microsphere is formed by the aggregative growth mode. Prolonging reaction time leads to the collapse of the microsphere and the formation of small flowerlike particles composed of spindles along with some discrete spindle-like particles. A higher reaction temperature leads to enhanced growth of the nanocrystallites (primary particles) in solution and somewhat smaller anatase microspheres. Fluorine ions doped in titania lattice and adsorbed on titania surface. Ammonium group also adsorbed on titania surface. The as-prepared anatase microspheres nearly have no visible light absorption. Under ultraviolet light irradiation, anatase titania microspheres have good photocatalytic performance.High photocatalytic performance and easy recovery are two prerequisites for the application of titania photocatalyst in water cleaning. Nanosized anatase titania, due to its remarkable photocatalytic performance, has always been the focus of photocatalysis research. The recovery problem and potential biological toxicity of nanoparticles, however, hinder its practical application in water cleaning. Immobilization technology is preferred in liquid phase reactions to avoid a costly but necessary separation step after the photocatalytic reaction. However, the immobilization technique greatly decreases the photocatalytic efficiency of nano-sized anatase TiO2 for the largely reduced specific surface area. Therefore, we aim to develop easy recoverable photocatalyst with high photocatalytic performance. In addition, though anatase TiO2 doped with only iron or fluorine shows greater photoactivity than its undoped counterparts, it is still not clear whether fluorine- and iron-codoped anatase TiO2 has excellent photocatalytic performance. Based on these considerations, we dedicated to prepare fluorine- and iron-codoped submicron-sized anatase titania photocatalyst with nanoparticles-constructed hierarchical structure. By introducing ferric chloride into the mixture solution of ammonium fluotitanate and urea, highly dispersed fluorine- and iron-codoped anatase titania microspheres were prepared. The size of titania microspheres is in the range of 400-700 nm. The obtained titania powders with iron content less than or equal to 3.0 at.% are pure anatase titania, while the titania powders with iron content more that 3.0 at.% are the mixture of anatase titania and iron oxide. Fluorine- and iron-codoped anatase microspheres have strong absorption of visible light, and constantly enhanced visible absorptions are observed as the iron contents increased. Photocatalytic evaluation tests reveal that there are two effects induced by iron doping in titania. One is the separation of charge carriers, which tend to improve the photocatalytic performance. The other one is the formation of recombination centers, which would decrease the photocatalytic performance. The influence of iron doping on titania photocatalytic performance are therefore the joint actions of these two effects. Under 365 nm UV irradiation, the photocatalytic performance of titania improved as the iron content increased, and the fluorine- and 3.0 at.% iron-codoped anatase microspheres have excellent photocatalytic performance, which can nearly completely degrade methyl orange in 15 min. However, under visible light irradiation, the activity of the fluorine- and iron-codoped anatase microspheres decreased as the iron content increased. Fluorine-doped anatase microspheres have higher photodegradation ability than fluorine- and iron-codoped anatase microspheres under visible light irradiation.Through dropwise addition of alkaline solution (ammonium hydroxide, sodium hydroxide or tetramethyl ammonium hydroxide) to the magnetically stirred ammonium fluotitanate solution at 90℃, rice-shaped anatase titania nanoparticles were obtained. Rice-shaped anatase nanoparticles are mainly in the size range 100-150 nm and have high dispersibility. The as-prepared anatase nanoparticles have no visible light absorption though fluorine ions doped in titania lattice. Postannealing can not only remove the water content and ammonium group, but also improve the crystallinity of titania nanoparticles. The calcined anatase nanoparticles have visible light absorption. The property and dosage of precipitants have great influence on titania photocatalytic properties. Anatase nanoparticles prepared using sodium hydroxide as the precipitant have the best photocatalytic performace no matter under UV or visible light irradiation.Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes, which is quite different from most conventional three-dimensional materials and has excellent electronic properties. Graphene/titania nanocomposites may have wide application in solar cells, sensible devices and photocatalysis. Anatase nanoparticles, prepared via a urea-based homogeneous precipitation method using ammonium fluotitanate as the titanium source, have been successfully anchored on highly dispersed graphene oxide sheets to assemble the graphene oxide/anatase nanocomposite, which further transforms to graphene/anatase nanocomposite upon hydrazine reduction. Titania nanoparticles can stabilize graphene oxide and graphene sheets. Graphene oxide and graphene sheets all have the ability to promote the separation of charge carriers. Graphene has a high conductivity, and once graphene and anatase come in contact the Schottky barrier similar to that in Pt/TiO2 system would generate. The migration of excited electrons to graphene suppresses electron-hole recombination, and the hole is then free to diffuse to anatase surface where oxidation reaction can occur. Graphene oxide acts as an acceptor for the electrons from the UV-irradiated TiO2, prolonging the charge-carrier recombination time and thus enhancing the photocatalytic oxidation activity.