Size-controlled liquid phase synthesis of colloidally stable CoO nanoparticles.

Spinel cobalt(ii,iii) oxide (CoO) represents a p-type semiconductor exhibiting promising functional properties in view of applications in a broad range of technological fields including magnetic materials and gas sensors as well as sustainable energy conversion systems based on photo- and electrocatalytic water splitting. Due to their high specific surface area, nanoparticle-based structures appear particularly promising for such applications. However, precise control over the diameter and the particle size distribution is required to achieve reproducible size-dependent properties. We herein introduce a synthetic strategy based on the decomposition of hydroxide precursors for the size-controlled preparation of purified CoO nanoparticles with narrow size distributions adjustable in the range between 3-13 nm. The particles exhibit excellent colloidal stability. Their dispersibility in diverse organic solvents further facilitates processing ( ligand exchange) and opens exciting perspectives for controlled self-assembly of the largely isometric primary particles into mesoscale structures. In view of potential applications, functional properties including absorption characteristics and electrocatalytic activity were probed by UV-Vis spectroscopy and cyclic voltammetry, respectively. In these experiments, low amounts of dispersed CoO particles demonstrate strong light absorbance across the entire visible range and immobilized nanoparticles exhibit a comparably low overpotential towards the oxygen evolution reaction in electrocatalytic water splitting.