Optical And Photocatalytic Properties Of TiO₂ And BiFeO₃ Films

Nanometer-sized titanium dioxide (TiO₂) has received considerable attention during the recent decade in view of its application for photocatalyst. Practically, it's self-cleaning, water-splitting and antibacterial properties have been applied in many fields. In this thesis, TiO₂ films were prepared by a reactive pulsed laser deposition (PLD) with a 248 nm wavelength KrF excimer laser using a Ti target in O₂ ambient. In this method, TiO₂ film is synthesized via the chemical reaction between the ejected species ablated from the Ti target and the ambient O₂ gas. However, the process window for the growth of anatase TiO₂ film is proved to be very narrow. And, to our knowledgably, the optical properties and photocatalytic activities of the films have not been reported in relation to the deposition parameters.The microstructure along with optical and photocatalytic properties of the deposited films were systematically studied by changing the deposition parameters, including O₂ gas pressure, substrate temperature and laser fluence. It was found that TiO₂ films having nearly pure anatase crystal structure grew effectively in O₂ atmosphere. When the films were fabricated at a substrate temperature of 400℃, the crystal structure, surface morphology, optical properties and photocatalytic activities of the TiO₂ films were greatly affected by the O₂ gas pressure. Nearly pure anatase phase TiO₂ film with typical (101) and (004) peaks can be obtained under an O₂ pressure of 15 Pa, which had smallest grain size among the films deposited under various O₂ gas pressure from 5 to 30 Pa. This film also showed good optical transmittance between the wavelength of 200 and 800 nm and high photocatalytic efficiency. For the deposition at 700℃, the crystal structure of the TiO₂ films exhibited a strong anatase (004) peak and was inert to the oxygen pressures. Two modes, namely, substrate-temperature-controlled mode and oxygen-pressure-controlled mode, were considered for the growth of the anatase TiO₂ films under different substrate temperatures. In addition, the optical properties and photocatalytic activities were found to be sensitive to both the microstructure and the grain sizes of the TiO₂ films. It was found that the film deposited at 700℃and 2.4J/cm² had the best photocatalytic property for the decomposition of methylene blue in an aqueous solution. Thus the fabrication parameters are very crucial for the reactive PLD anatase TiO₂ films.In recently, a multiferroic material, namely, Bismuth ferric oxide (BiFeO₃), has attracted a great deal of attention duo to the potential applications for photocatalyst, which can response to the visible light irradiation. However, the study about the optical properties of the BiFeO₃ films was lack. In this thesis, BiFeO₃ films were grown on quartz substrates prepared by a simple chemical solution deposition. The optical constants (refractive index n, extinction coefficient k) were calculated. The influence of the processing parameters on microstructure and optical properties, especially the band gap, were studied. Relatively pure phase film was obtained at an anneal temperature of 600℃, which had the largest grain size about 60 nm. The study of optical properties revealed that the films had a high refractive index with a direct band gap of 2.7eV, and the band gap was greatly affected by the microstructure. The investigation of the optical properties provides a reference for further research into photocatalytic properties.