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

[Image: see text] 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.