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The environmental pollution and the rapid depletion of fossil fuel caused by the rapid increase in industrial production became serious problems for humans. These issues have inspired many researchers to found eco-friendly materials, which can degrade pollutants and produce green energy. Titanium dioxide (TiO2) thin films are one of the important and promising semiconductor materials for environmental and energy applications because of their unique optical and electronic properties. In this chapter, an overview of the background of TiO2 structure and the different methods of synthesis TiO2 thin films were carried out. The photocatalytic water treatment and the water split for H2 production by TiO2 thin films were investigated. The strong influence on photocatalytic and water split efficiency of TiO2 thin films by crystal structure, surface area, crystalline structure, average particle size and porosity were summarized.

In the present work, pure BiFeO 3 and pure Bi 2 Fe 4 O 9 single phases were successfully synthesized by tailoring hydrothermal synthesis route. The structural and morphology analyses have been achieved by X-ray diffraction and by electron microscopy. Scanning and transmission electron microscopy images revealed a strong change in the morphology from agglomerated nanoparticles with sizes less than 10 nm for BiFeO 3 to plate-like particles with larger sizes (edges of 100 to 200 nm while a thickness of * 30 nm) for Bi 2 Fe 4 O 9 . Such a small size obtained for BFO nanoparticles has never been observed before for similar synthesis method without any added reagents, to the best of our knowledge. The photocatalytic activity of the as-prepared samples for degradation of methylene blue dye under sunlight irradiation shows a good efficiency. It reaches 61% and 83% after 3 h for BiFeO 3 and Bi 2 Fe 4 O 9 nanostructures, respectively. Despite a bigger size and higher bandgap, the better efficiency of Bi 2 Fe 4 O 9 compared to BiFeO 3 is explained by a much stronger absorption especially for light with energy higher than 2.65 eV.

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