Photophysical Characterization of Ru Nanoclusters on Nanostructured TiO 2 by Time-Resolved Photoluminescence Spectroscopy.

Despite the promising performance of Ru nanoparticles or nanoclusters on nanostructured TiO 2 in photocatalytic and photothermal reactions, a mechanistic understanding of the photophysics is limited. The aim of this study is to uncover the nature of light-induced processes in Ru/TiO 2 and the role of UV versus visible excitation by time-resolved photoluminescence (PL) spectroscopy. The PL at a 267 nm excitation is predominantly due to TiO 2 , with a minor contribution of the Ru nanoclusters. Relative to TiO 2 , the PL of Ru/TiO 2 following a 267 nm excitation is significantly blue-shifted, and the bathochromic shift with time is smaller. We show by global analysis of the spectrotemporal PL behavior that for both TiO 2 and Ru/TiO 2 the bathochromic shift with time is likely caused by the diffusion of electrons from the TiO 2 bulk toward the surface. During this directional motion, electrons may recombine (non)radiatively with relatively immobile hole polarons, causing the PL spectrum to red-shift with time following excitation. The blue-shifted PL spectra and smaller bathochromic shift with time for Ru/TiO 2 relative to TiO 2 indicate surface PL quenching, likely due to charge transfer from the TiO 2 surface into the Ru nanoclusters. When deposited on SiO 2 and excited at 532 nm, Ru shows a strong emission. The PL of Ru when deposited on TiO 2 is completely quenched, demonstrating interfacial charge separation following photoexcitation of the Ru nanoclusters with a close to unity quantum yield. The nature of the charge-transfer phenomena is discussed, and the obtained insights indicate that Ru nanoclusters should be deposited on semiconducting supports to enable highly effective photo(thermal)catalysis.