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This work focuses on the size effects of Au nanoparticles (AuNPs) on the TiO 2 crystalline phase of nanocomposite AuNPs-embedded TiO 2 (Au-TiO 2 ) thin films, their adsorption ability, and photocatalytic activity. Au-TiO 2 films were synthesized through a solÀgel method using polyvinylpyrrolidone-protected AuNPs (AuNPs@PVP). The mean diameters of AuNPs@PVP dispersed in a sol solution were 2.0 ( 0.7 or 7.9 ( 3.1 nm and the heat-treatment temperature of the films was 400À900 °C. XRD and Ti L 3,2 -edge X-ray absorption nearedge structure (XANES) analysis revealed that AuNPs doping could suppress an anatase to rutile phase transformation. In addition, the larger size of AuNPs doped in TiO 2 film tended to prevent the transformation more effectively. The film doped with the smaller AuNPs@PVP and annealed at 500 °C showed the highest photocatalytic activity among the obtained films because it had the wellcrystallized anatase phase and the high adsorption ability, which was attributed to the existence of a five-coordinated Ti site that was revealed from Ti K-edge XANES measurements.