Optimization of electron transfer pathways in atomically precise metal nanoclusters: catalyzing a leap in solar water oxidation.

Atomically precise metal nanoclusters (NCs, <2 nm) with precise numbers of metal atoms and ligands have attracted enormous attention as highly promising photosensitizers owing to their peculiar atom-stacking mode, quantum confinement effect, enriched active sites and discrete molecular-like energy band structures. Despite these merits, metal NCs still suffer from inherent drawbacks such as ultrafast photogenerated carrier recombination, poor photostability and difficulties in regulating the charge transport pathway, which severely hinder their applications in photocatalysis. Herein, we strategically constructed spatially hierarchically ordered MOs/(PDDA/MQDs/PDDA/Ag ) heterostructure artificial photosystems by layer-by-layer assembly of oppositely charged poly(diallyldimethylammonium chloride) (PDDA), MXene quantum dot (MQD) and Ag NC [Ag @GSH, Ag(GSH), and Ag(GSH)] building blocks on a metal oxide (MO) substrate under ambient conditions for customizing the directional charge migration/separation pathway over atomically precise metal NCs. In these on-demand artificial photosystems, Ag NCs serve as highly efficient photosensitizers that markedly enhance the visible light absorption capacity, while the synergistic and concurrent electron-withdrawing roles of PDDA (as an electron extraction medium) and MQDs (as electron acceptors) contribute to the long-range tandem charge transport chain, synergistically driving the directional electron transfer from the metal NCs to the MO framework. This significantly accelerates the charge separation/transfer of metal NCs and markedly improves the solar water oxidation performances of the MOs/(PDDA/MQDs/PDDA/Ag ) heterostructures. The essential roles of each building block are specifically explored with the photoelectrochemical mechanism clearly elucidated. Our work offers an accessible and universal route to strategically construct metal NC-based heterostructure photosystems and unveils the multi-functional synergy in regulating the electron migration pathway of atomically precise metal NCs towards solar energy conversion.