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 Na₂MoO₄ into Mo₇O₂₄^6- ({Mo₇}), high nuclear Mo₃₆O₁₁₂^8- ({Mo₃₆}) cluster, and finally polymerization product of [H₆K₂Mo₃O₁₂(SO₄)] ({Mo₃(SO₄)}) during the H₂SO₄ acidification. Notably, the facile conversion of {Mo₃(SO₄)} back to the {Mo₃₆} cluster by simple dilution is also discovered. Furthermore, we identified {Mo₃₆} and {Mo₃(SO₄)} as exclusive precursors responsible for driving the electrochemical self-assembly of {Mo₁₅₄} and {Mo₁₀₂}, respectively. The study also unravels a pivotal intermediate, the pentagonal reduced state fragment [H₁₈Mo^VI₄Mo^VO₂₄]^-, originating from {Mo₃₆}, which catalyzes the autocatalytic self-assembly of {Mo₁₅₄} with electron and proton injection during electrochemical processes. Concurrently, {Mo₃(SO₄)}_n serves as the indispensable precursor for {Mo₁₀₂} formation, generating sulfation pentagon building blocks of [H₂Na₂O₂(H₄Mo^VMo^VI₄O₁₆SO₄)₄]^4- that facilitate the consecutive assembly of giant {Mo₁₀₂} 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.