General Route to Colloidally Stable, Low-Dispersity Manganese-Based Ternary Spinel Oxide Nanocrystals

While certain ternary spinel oxideshave been well-exploredwithcolloidal nanochemistry, notably the ferrite spinel family, ternarymanganese (Mn)-based spinel oxides have not been tamed. A key compositionis cobalt (Co)-Mn oxide (CMO) spinel, Co x Mn3-x O4, that, despiteexemplary performance in multiple electrochemical applications, hasfew reports in the colloidal literature. Of these reports, most showaggregated and polydisperse products. Here, we describe a syntheticmethod for small, colloidally stable CMO spinel nanocrystals withtunable composition and low dispersity. By reacting 2+ metal-acetylacetonate(M(acac)(2)) precursors in an amine solvent under an oxidizingenvironment, we developed a pathway that avoids the highly reducingconditions of typical colloidal synthesis reactions; these reducingconditions typically push the system toward a monoxide impurity phase.Through surface chemistry studies, we identify organic byproductsand their formation mechanism, enabling us to engineer the surfaceand obtain colloidally stable nanocrystals with low organic loading.We report a CMO/carbon composite with low organic contents that performsthe oxygen reduction reaction (ORR) with a half-wave potential (E (1/2)) of 0.87 V vs RHE in 1.0 M potassium hydroxideat 1600 rpm, rivaling previous reports for the highest activity ofthis material in ORR electrocatalysis. We extend the general applicabilityof this procedure to other Mn-based spinel nanocrystals such as Zn-Mn-O,Fe-Mn-O, Ni-Mn-O, and Cu-Mn-O. Finally, we show the scalability ofthis method by producing inorganic nanocrystals at the gram scale.