Ligand-Mediated Proton-Coupled Electron Injection into Reactive Cores of Soluble Macroanion-Like Complexes of Titanium Dioxide

Identifier:  imarina:9462618

Handle: https://hdl.handle.net/20.500.11797/imarina9462618

Authors:  Raula, Manoj; Avnaim, Sapir Dabush; Kar, Aranya; Samin, Meital; Roy, Shubasis; Baranov, Mark; Leffler, Nitai; Lang, Zhong-Ling; Poblet, Josep M; Weinstock, Ira A

Abstract:
Although widely utilized as reactive components of devices and functional materials, the harnessing of metal-oxide nanocrystals (NCs) as ligand-tunable reaction centers of soluble catalysts remains an ongoing challenge. We now report that readily modifiable, oxidatively inert polyoxometalate (POM) ligands amplify and rationally control rates of photocatalytic H2 evolution by anatase-TiO2 NCs. This was achieved using a series of four POM-derived oxo-donor ligands, [(Xn+W11O39)TiIV-O-](10-n)-, Xn+ = Al3+, Si4+, P5+, and [(P2W17O61)TiIV-O-]8- (written here as oxo-anion donors) coordinated via bridging-oxo linkages to Ti atoms at the surfaces of 7 +/- 1.6 nm anatase-TiO2 NCs, wherein the four respective macroanion-like complexes are each ligated, on average, by 90 +/- 15 POM anions. Under UV-vis light irradiation in water (10% MeOH), rates of H2 evolution are a linear function of the quantum yield for photocatalytic oxidation of MeOH by the Ti-substituted POM ligands. This rate-limiting ligand reduction is followed by rapid visible-light driven proton-coupled electron injection into TiO2, which proceeds until rates of H2 evolution achieve parity with those of POM reduction, resulting finally in a dynamic steady state. Here, the NC core of the most reactive, [(P5+W11O39)TiIV-O-]5--ligated complex, 3, is populated by 440 electrons and 440 protons, representing an effective H+ concentration of 10 M (per nm3 of TiO2). The findings show that metal-oxide NCs can function as the reactive centers of ligand-tunable catalysts and, more generally, provide fundamental mechanistic understanding that establishes new structure-reactivity relationships for the design of rationally tunable modular components of functional materials.