Surface Anchoring and Active Sites of [Mo3S13]2-Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo3S13]2-clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo3S13]2-to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S22-groups, upon which Mo-O-Tibonds with the hydroxylated TiO2surface are established. Theheterogenized [Mo3S13]2-clusters are active and stable co-catalystsfor the light-driven hydrogen evolution reaction (HER) withperformance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate tothe accessibility of the TiO2surface required for efficient hole scavenging. We further elucidate the active HER sites by applyingthermal post-treatments in air and N2. Our data demonstrate the importance of the trinuclear core of the [Mo3S13]2-cluster andsuggest bridging S22-and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime examplefor the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive toexplore other thio(oxo)metalates.