When the Au NPs were attached to the P25 thin film, the SEM image (Figure ) showed that the Au NPs were distributed uniformly but randomly on the TiO2 porous film. Here, the Au and P25 NPs correspond to the brighter and darker particles. The estimated Au nanoparticle density from SEM images was about 600/μm 2 (Figure ). The elemental composition image of the P25-Au film was obtained using the HAADF-STEM (Figure ), showing the composition of the P25-Au mixture. The elemental composition from the EDS map verified that the actual particles, Au (Figure ) and Ti (Figure ), from TiO2 NPs present in the mixture match the spatial distribution of TiO2 and Au NPs from the HAADF image.