Heterometallic {Re₄Mo₂Qⁱ₈} cluster-based building blocks: towards the rational nanoarchitectonics of optimized photoelectrodes for solar cells and water splitting

Metal atom clusters are well-defined nanoscale objects containing a precise number of metal atoms and ligands. Face-capped cluster units of the type [{M6L8i}L '(a)(6)] (M = Mo, Re, L = S, Se or I, L ' = Cl, Br, I, CN or H2O) exhibit unique optical and electronic properties that make them relevant building blocks for the rational design of nanomaterials using nanoarchitectonic concepts. Photoelectrodes based on Mo-6 and Re-6 clusters with various compositions obtained by deposition of uniform layers of those building blocks onto semiconducting surfaces were recently reported. Remarkably, high quality interfaces were formed not only between building blocks but also between the building blocks and the semiconducting surfaces. On the one hand, layers based on active {Mo6I8i} cluster cores exhibit an ambipolar behavior like carbon nanotube, graphene and transition metal chalcogenides. On the other hand, mixing the two types of {Re6S8i} and {Re6Se8i}-based building blocks enables the creation of micro-(p-n) junctions with enhanced photogenerated current intensity. Herein, we report new advances in the design of photoelectrodes using heterometallic Re4Mo2 cluster-based building blocks. The association of Mo and Re in {Re(4)Mo(2)Q(8)(i)} cluster cores (Q = S and Se) leads to electronic properties and absorption properties significantly different from those of homometallic {Mo6I8i} and {Re(6)Q(8)(i)}. Indeed, beyond different molecular orbital diagrams, the {Re(4)Mo(2)Q(8)(i)} cluster-based units exhibit 22 valence electrons per cluster (VEC) whereas the VEC value for {Mo6I8i} and {Re(6)Q(8)(i)} cluster units is 24. The mixing of rhenium and molybdenum within the same heterometallic cluster enables not only the optical and transport properties of the active layers to be optimized but it also enables the position of the energy levels to be tuned. This appears very appealing for band alignment engineering in order to design optimized photoelectrodes for solar energy conversion. We show herein that the energy levels of the photoelectrodes built on {Re(4)Mo(2)Q(8)} cluster-based layers immobilized on FTO surfaces are compatible with the photoelectrochemical water splitting.