TiO₂Nanocrystalline Film From Micro-arc Oxidation:Synthesis, Characterizations And Photocatalytic Properties

Micro-arc oxidation (MAO) is a kind of "in-situ" formation of metal oxides nanocrystalline ceramic films on the surface of valve metals (Al, Mg, Ti et. al) by an electrochemical and plasma-chemical process, which occurs at potentials above the breakdown voltage of the anode oxidation. When Ti or Ti alloys is used as a substrate, an excellent adhesive photocatalytic TiO2film will be made, which will overcome the problems of recycling in TiO2powder. Herein, the MAO TiO2films have obtained widely applied in photocatalysis. The microstructures of the MAO films have great influence on their various properties. However, the current research mainly force on the microstructures in micro scale by the SEM observation. Moreover, the large band gad and high recombination of photo-generated electrons and holes also limit the photocatalytic activity of pristine MAO TiO2film.In this research, HRTEM were used to directly characterize the surface microstructure of nanocrystalline films by MAO in atom-nano scale. The influence of their microstructure on the photocatalytic activity and corrosion resistance was also studied. Meanwhile, the compounded TiO2films were prepared by adding semiconductor composite into electrolyte of MAO. The nonmetal doped TiO2films were synthesized based on thermochemical treatment plus MAO. Lastly, the mechanism of improved photocatalytic activity of hydrogenated TiO2, which has triggered extensive research interest recently, was study by positron annihilation lifetime spectrum (PALS).This dissertation is divided into seven chapters.The first chapter is the introduction, describes the research background, origin, meaning and importance of our work. The first part introduced the development and mechanism of MAO, the structure, properties and applications of MAO film. The research status and applications in photocatalysis of MAO TiO2film were also summarized. The structure of nano-TiO2, mechanism of photocatalysis, the preparation methods of TiO2nanoparticles and films, the improvement of TiO2photocatalytic activity and its applications were introduced in last part.The experimental methods, characterization methods and test equipment in the various parts of work are described in the second chapter. It includes several parts:the process of MAO film preparation on the substrate of Ti, Mg and Al, the synthesis method of MAO TiO2film compounded with semiconductors, the process of thermochemical treatment on the substrate of Ti and the hydrogenation of P25TiO2 nanoparticles. The characterization and measurement of MAO films and other samples by SEM, EDS, TEM, HRTEM, XRD, XPS, PL, Raman, UV-VIS absorbance spectrum, photo-generated current and photocatalytic activity were also introduced.In the third chapter, the compounded TiO2films were prepared by adding YAG: Ce3+and Eu2O3semiconductor particles into MAO electrolyte. The rare earth elements with incompletely occupied4f and empty5d orbit could restrain the growth of TiO2crystalline, inhibit the recombination of photo-generated electrons and holes and improve their photocatalytic property. It demonstrated the improvement of rare earth elements in TiO2photocatalyst and provided new approach to prepare compounded TiO2films.In the fourth chapter, a novel and valid process was introduced to prepare a high concentration substitutional nonmetal doped TiO2photocatalytic film. The process included:firstly the titanium substrate was thermochemical treated and then the nonmetal doped TiO2was synthesized on the surface of it by using MAO. The rapid oxidation of MAO restricts the doping elements escaping from the substrate, so high concentration substitutional nonmetal doped TiO2films were synthesized. The doped film exhibits a significant red-shift in the band gap transition, with a narrow band gap, higher photo-generated charge carrier density and more superior improved photocatalytic property. It also provides new approaches to prepare various doped TiO2films.In the fifth chapter, the microstructural variation of the MAO nanocrystalline film on the substrate of Ti, Mg, and A1was firstly found by using HRTEM and XRD:an amorphous layer with thickness around10nm on the surface and the crystallinity of the nanocrystalline in the film increasing with the depth. When the film was annealed, the amorphous layer was disappeared and the nanocrystalline in the film grows into bigger ones. For the MAO TiO2film, the crystalline of amorphous layer inhibits the recombination of photo-generated electrons and holes, hence increasing its photocatalytic activity. For the MAO MgO and Al2O3film, the dense microstructure in pristine film was destroyed by the disappeared of amorphous layer and the growth competition of nanocrystalline, hence debasing the corrosion resistance. It firstly revealed the microstructural variation of the MAO nanocrystalline film in atom-nanoscale and provided new approaches for the research on the properties of MAO films.Hydrogenation modified TiO2has triggered extensive research interest recently. In the sixth chapter, a sensitive approach for measuring vacancy-type defects, positron annihilation lifetime spectroscopy (PALS) was used to qualitatively characterize the oxygen-vacancy-induced defects in TiO2. It will be of significance to evaluate and understand the importance of oxygen vacancies, the associated with Ti3+sites in TiO2as electron and holes scavengers, which thus separated the charge carriers and remarkably improved the photocatalytic activity of TiO2. It provides novel approach to study the TiO2photocatalyst and explains the mechanism of hydrogenation modified TiO2.Chapter seven is a full summary.Finally, a brief introduction of published papers and patents, participated projects honors and awards, Curriculum Vitae and acknowledgements in the graduate were given.