Hydrothermal Synthesis of Euhedral Co3O4Nanocrystals via Nutrient-Assisted Topotactic Transformation of the Layered Co(OH)2Precursor under Anoxic Conditions: Insights into Intricate Routes Leading to Spinel Phase Development and Shape Perfection

Abstract

Euhedral cobalt spinel cubes, octahedra, and cuboctahedra with narrow size dispersions have been synthesized in a one-pot hydrothermal reaction, using cobalt(II) nitrate and sodium hydroxide at variable concentrations as the only reagents, while their ratio was kept constant at cCo2+/cOH- = 2.7. Three main reaction stages, including parent reactive template (PRT) formation, nutrient-mediated topotactic nucleation (NTN), and morphogenesis of nanocrystals (MNC), were distinguished. In the NTN step, the primary spinel grains development occurs with the [100] and [111] directions of the Co3O4 facets inheriting the [1-11] direction of the elongated PRT plates (formation of cubes) or the [001] direction of the hexagonal PRT plates (formation of octahedra). In an anoxic environment, the excess nitrate anions play a critical role as the Co2+ to Co3+ oxidants and oxygen donors required to attain the Co3O4 stoichiometry. The nucleated Co3O4 primary nanocrystals are spontaneously assembled into sub-micrometer spinel mesocrystals via imperfectly oriented attachments and then consolidated into euhedral bulk nanocrystals by a hydrothermal treatment (nanocubes) or via dissolution and reentrant recrystallization processes (octahedra and cuboctahedra).