Microscopic Mechanism for Precise Control of the Sizes of Molybdenum Blue Nanorings

Molybdenum blue (MB) nanorings with monodispersed, well-defined structures can be feasibly prepared via treatment of molybdate solutions with reducing agents at acidic conditions. However, the hidden constraints that control their ring sizes have been unknown for decades. Herein, the formation of 3.4 and 4.1 nm nanorings, {Mo₁₅₄} and {Mo₁₇₆}, is monitored using small-angle X-ray scattering (SAXS) with systematically varied synthetic conditions. Results suggest that {Mo₁₅₄} forms quickly in solution and then undergoes slow transformation into {Mo₁₇₆} when reducing agents are used, while {Mo₁₇₆} can form directly with MoCl₅ supplied as reduced Mo^V. These are attributed to {Mo₁₅₄} being the kinetic product catalyzed by the template-effect while {Mo₁₇₆} is the thermodynamic product due to its low ring tension. Our findings unravel one of the long-standing mysteries in MB and point out effective routes for the precise synthesis of nanorings.