Materials and Sample Preparation P25 TiO2 (purity: >99.5%) was purchased from Sigma-Aldrich (Sigma-Aldrich, St. Louis, MO). Au NPs were synthesized using chemical reduction of the 0.25 mM gold precursor HAuCl4 by 0.25 mM trisodium citrate at 100 o C for 20 min. In preparation for the P25 film, 1.5 grams of P25 powder were mixed and ground well with 3 ml ethylene glycol to form a smooth paste. Then, it was spread onto a Si or glass substrate with a blade in the space between transparent tape. The thickness of the tape determined the thickness of the P25 film. Then, the P25 films were dried overnight at 200 o C in the air for 2 hours and designated as-prepared P25 films. The asprepared P25 was immersed into the gold nanoparticle aqueous solution overnight to obtain the Au-decorated P25 film, which was washed with water to remove loosely attached Au NPs and then dried overnight at 75 o C.