Preparation of Au/TiO2 with Metal Cocatalysts Exhibiting Strong Surface Plasmon Resonance Effective for Photoinduced Hydrogen Formation under Irradiation of Visible Light

Titanium(IV) oxide (TiO2) having both small platinum (Pt) nanoparticles and large gold (Au) particles without alloying and nanoparticle coagulation was successfully prepared by the combination of traditional photodeposition of Pt in the presence of a hole scavenger (PH) and subsequent Au colloid photodeposition in the presence of a hole scavenger (CPH) onto TiO2-Pt. Au particles having an average diameter of 13 nm were fixed on both TiO2 and TiO2-Pt samples without change in the original size of Au particles, and the Au/TiO2 and Au/TiO2 Pt samples exhibited strong photoabsorption around 550 nm as a result of surface plasmon resonance (SPR) of Au to which the large size of Au particles was attributed. Bare TiO2, TiO2-Pt, Au/TiO2, and Au/TiO2-Pt samples were used for photoinduced hydrogen (H-2) formation from 2-propanol in aqueous solutions under irradiation of visible light. The first two samples yielded no H-2 because of no response to visible light, but the latter two formed H-2, indicating that SPR photoabsorption of supported Au particles contributed to the H-2 evolution under irradiation of visible light. The H-2 formation rate of the Au/TiO2-Pt sample was similar to 7-times larger than that of the Pt-free Au/TiO2 sample, indicating that Pt nanoparticles loaded on TiO2 acted effectively as a cocatalyst, that is, as reduction sites for H-2 evolution. The combination of the PH and CPH methods was effective for preparation of Au/TiO2 having other metal cocatalysts (M) including Au, that is, Au/TiO2 Au, and H-2 evolution rates decreased in the order of M; Pt > Pd > Ru > Rh > Au > Ag > Cu > Jr. An inverse correlation between the rate and the hydrogen overvoltage (HOV) of M, that is, it was observed that the higher the HOV, the more difficult it is to reduce protons by photogenerated electrons. Since the amounts of Au and M loaded on TiO2 were changed independently, the effects on photoabsorption and the rate of H-2 evolution were examined. A linear correlation was observed between rate and light absorption due to SPR, suggesting that SPR photoabsorption by Au particles was one of the important factors determining the rate of the H-2 evolution.