Photocatalytic oxidation is a promising technique for environmental pollutants remediation. TiO2 is widely used as a photocatalyst due to its superior characteristics such as high photoactivity, chemical stability and low cost. This dissertation focuses on the TiO2 photocatalyst and its modification for environmental applications, through both experimental and theoretical methods. The experimental method is by the atomic layer deposition (ALD) technique, including two major parts. One is to deposit TiO2 ALD thin films on magnetic core particles to create a magnetic photocatalyst. This novel composite material is both magnetic and photocatalytically active, providing for a promising strategy for environmental treatment. The other is to deposit Pt nanoparticles on TiO2 photocatalyst to increase the photoactivity. The Pt/TiO2 photocatalyst with high Pt surface dispersion was synthesized by optimizing the Pt ALD deposition temperature and the Pt loading, resulting in a significant improvement on the TiO2 photoactivity. One ALD cycle of Pt deposited TiO2 has been demonstrated to increase the photoactivity of TiO2 nanoparticles by threefold for decomposition of methylene blue with the Pt loading of 0.64 wt%. The theoretical work is via density functional theory (DFT), involving the study of the Pt particle growth on TiO2 surfaces and the effect of the Pt deposition on the photoactivity of TiO2 . The Pt was found to prefer 3D rather than 2D growth, due to the large cohesive energy of Pt. Large particles are thermodynamically favorable, however the low mobility of Pt over the TiO2 surface coupled with the high desorption barrier for Pt detaching from particles limits the large particle growth. This results in a large number of small particles during Pt ALD processing. The role of the Pt deposition on the photoactivity of TiO2 was explored. The initial increase and subsequent decrease in TiO2 's photoactivity with increasing Pt loading was explained by the competition between increased O2 adsorption induced by Pt and the electron-hole recombination in Pt. For Pt loadings above the optimum level, the increased electron-hole recombination outweighs the increased O2 adsorption, which is limited by its concentration and diffusion in solution.