Facet-Dependent Photocatalytic Deposition of Metal Nanoparticles Onto Rutile TiO 2

Deposition of noble metal nanoparticles onto semiconductor such as TiO2 is important for improving photocatalytic activities with metallic co-catalysts and for achieving plasmon-induced charge separation. In that context, facet-selective deposition of metal nanoparticles is attracting attention. There are many reports on the facet-selective deposition, and the selectivity has been explained in terms of different work functions depending on facets. 1,2 However, it is likely that factors other than work function also contribute to the selectivity. Therefore, in this work, we focus also on the number of electrons involved in the reactions and the surface and interfacial energies of the semiconductor and metal, so as to understand the mechanisms of the selective deposition and to control the deposition sites and morphologies of the metal nanoparticles. Here, we employed rutile TiO2 particles with exposed crystal planes as a semiconductor material and deposited Au, Pt, and Ag nanoparticles onto the TiO2 particles by photocatalytic means. Rutile TiO2 particles with exposed crystal planes were synthesized by a flux method. Anatase TiO2 as a precursor was heated at 1000 °C in a NaCl molten salt in air. The TiO2 particles thus obtained (50 mg) was added to 50 vol% aqueous ethanol (5 mL). The solution was purged with nitrogen and a metal precursor, HAuCl4, H2PtCl6, or AgNO3, was added to the solution, followed by irradiating with a UV LED lamp (365 nm) for 10 min, for deposition of Au, Pt, or Ag, respectively, onto the TiO2 particles. From XRD measurements and SEM observation, it was revealed that the obtained TiO2 particles have rutile structure and (110) and (111) planes. The average exposed area ratio between (110) and (111) planes was estimated to be around 7:3. After photocatalytic deposition of Au, we found 87% of the Au nanoparticles were deposited on (110) planes. Even higher selectivity was observed for Pt nanoparticles; 94% of them were deposited on (110) planes. In contrast, interestingly, 75% of Ag nanoparticles were found at (111) planes. Other Ag nanoparticles were found on the edges in between (110) planes. Namely, facet selectivity was different for Au, Pt, and Ag, and the observed difference was difficult to explain solely in terms of work function. We therefore focused on difference in the number of electrons required for the metal deposition reactions. In the case of Au and Pt, these metals are deposited by multi-electron reduction reactions; [AuC