Revealing the Electrocatalytic Self-Assembly Route from Building Blocks into Giant Mo-Blue Clusters.

The assembly of single-core molybdate into hundreds of cores of giant molybdenum blue (Mo-blue) clusters has remained a long-standing unresolved scientific puzzle. To reveal this fascinating self-assembly behavior, we demonstrate an aqueous flowing in-operando Raman characterization system to capture the building blocks' evolution from the "black box" reaction process. We successfully visualized the sequential transformation of NaMoO into MoO ({Mo}), high nuclear MoO ({Mo}) cluster, and finally polymerization product of [HKMoO(SO)] ({Mo(SO)}) during the HSO acidification. Notably, the facile conversion of {Mo(SO)} back to the {Mo} cluster by simple dilution is also discovered. Furthermore, we identified {Mo} and {Mo(SO)} as exclusive precursors responsible for driving the electrochemical self-assembly of {Mo} and {Mo}, respectively. The study also unravels a pivotal intermediate, the pentagonal reduced state fragment [HMoMoO], originating from {Mo}, which catalyzes the autocatalytic self-assembly of {Mo} with electron and proton injection during electrochemical processes. Concurrently, {Mo(SO)} serves as the indispensable precursor for {Mo} formation, generating sulfation pentagon building blocks of [HNaO(HMoMoOSO)] that facilitate the consecutive assembly of giant {Mo} sphere clusters. As a result, a complete elucidation of the assembly pathway of giant Mo-blue clusters derived from single-core molybdate was obtained, and H/e redox couple is revealed to play a critical role in catalyzing the deassembly of the precursor, leading to the formation of thermodynamically stable intermediates essential for further self-assembly of reduced state giant clusters.