Controlled vapor-phase synthesis of cobalt oxide nanomaterials with tuned composition and spatial organization

Abstract

Cobalt oxide nanostructures are deposited by Chemical Vapor Deposition (CVD) on Si(100) substrates at temperatures between 300 and 550 °C, using for the first time a novel Co(II) adduct as molecular precursor [Co(hfa)₂·TMEDA; hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate, TMEDA = N,N,N′,N′-tetramethylethylenediamine]. The preparation is conducted either under dry (O₂) or wet (O₂ + H₂O) oxygen atmospheres, at total pressures of 3.0 or 10.0 mbar. The obtained results evidence that, upon dry O₂ at 10.0 mbar, the initial nucleation of CoO occurs, followed by its progressive oxidation to Co₃O₄ during the subsequent growth stages. In a different way, cobalt monoxide can be selectively obtained at 3.0 mbar. In all cases, water vapor acts as an oxidant towards cobalt, favoring the formation of Co₃O₄ phases with a more pronounced {111} and {110}-type faceting. Structural, compositional and morphological characterization evidences the possibility of obtaining high purity CoO/Co₃O₄ systems with tailored morphological features, from films to columnar nanostructures, thus highlighting the potential and versatility of the proposed synthetic strategy.