Photophysical Characterization of Ru Nanoclusters on Nanostructured TiO₂ by Time-Resolved Photoluminescence Spectroscopy

Despite the promisingperformance of Ru nanoparticles or nanoclusterson nanostructured TiO2 in photocatalytic and photothermalreactions, a mechanistic understanding of the photophysics is limited.The aim of this study is to uncover the nature of light-induced processesin Ru/TiO2 and the role of UV versus visible excitationby time-resolved photoluminescence (PL) spectroscopy. The PL at a267 nm excitation is predominantly due to TiO2, with aminor contribution of the Ru nanoclusters. Relative to TiO2, the PL of Ru/TiO2 following a 267 nm excitation is significantlyblue-shifted, and the bathochromic shift with time is smaller. Weshow by global analysis of the spectrotemporal PL behavior that forboth TiO2 and Ru/TiO2 the bathochromic shiftwith time is likely caused by the diffusion of electrons from theTiO(2) bulk toward the surface. During this directional motion,electrons may recombine (non)radiatively with relatively immobilehole polarons, causing the PL spectrum to red-shift with time followingexcitation. The blue-shifted PL spectra and smaller bathochromic shiftwith time for Ru/TiO2 relative to TiO2 indicatesurface PL quenching, likely due to charge transfer from the TiO2 surface into the Ru nanoclusters. When deposited on SiO2 and excited at 532 nm, Ru shows a strong emission. The PLof Ru when deposited on TiO2 is completely quenched, demonstratinginterfacial charge separation following photoexcitation of the Runanoclusters with a close to unity quantum yield. The nature of thecharge-transfer phenomena is discussed, and the obtained insightsindicate that Ru nanoclusters should be deposited on semiconductingsupports to enable highly effective photo(thermal)catalysis.