Size-controlled liquid phase synthesis of colloidally stable Co3O4 nanoparticles

Abstract

Spinel cobalt(ii,iii) oxide (Co₃O₄) 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 Co₃O₄ 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 (i.e. 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 Co₃O₄ 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.

Fig. 1. Schematic illustration of the reaction scheme. (i) A solution of red Co(NO₃)₂·6H₂O in OLA is heated in a round-bottomed flask. (ii) Injection of a base (typically aqueous NaOH) induces the formation of green α-Co(OH)₂. (iii) Further heating leads to the precipitation of hydroxide precursor NPs. In contact with air, the mixture adopts a brown-green color presumably due to the presence of an oxyhydroxide oxidation product. (iv) The precursor particles are pseudomorphically converted into black Co₃O₄ NPs.

Fig. 2. Temperature-dependence of NP size and shape. TEM micrographs of Co₃O₄ nanoparticles synthesized with a precipitation step performed at temperatures of (a) 50 °C, (b) 60 °C, (c) 70 °C, (d) 80 °C, (e) 90 °C and (f) 100 °C (same scale bar). Trends in the temperature-dependent modulation of particle size and shape are indicated by grey arrows.

Fig. 3. Size and shape of NPs in the low nanometer regime. (a–f) TEM micrographs of NPs in the size range from 2.8 nm to 13.2 nm, ordered according to an increase in particle size. From a–f, samples S3 (50 °C), S4 (50 °C), S5 (50 °C), Cb8 (100 °C), Cb11 (80 °C), and Cb13 (80 °C) are displayed. (g) Combined, normalized size-distribution histograms obtained by TEM image analysis. The particles of each sample focus on a narrow size margin, despite a relatively broad basis. Due to the strong overlap between 3–5 nm, the count-size histogram of S4 was omitted.

Fig. 4. UV-vis/NIR spectra of Co₃O₄ NPs dispersed in heptane. (a) Absorption spectrum of OA-stabilized cuboidal Co₃O₄ NPs dispersed in heptane (Cb13) in the spectral range from 200–2000 nm (black line). For comparison, reference spectra of pure OA (blue line), as well as heptane (yellow line), are shown. (b) Series of absorption spectra for NPs with diameters between 2.8 and 10.9 nm showing size-dependent absorbance. Inset: photograph of cuvettes containing NP dispersions (left to right: increasing NP size from 2.8–10.9 nm). Intensification of colouration is observed with increasing NP size.

Fig. 5. Crystallographic characterization of Co₃O₄ NPs. (a) HR-TEM micrograph of an individual 7.2 nm-sized Co₃O₄ nanocube (Cb8). Two sets of lattice fringes with distances attributable to {022} and {004} lattice planes in the spinel-Co₃O₄ structure are highlighted in orange and yellow, respectively. (b) Electron diffractograms of samples S5 (green trace), Cb8 (yellow trace), and Cb13 (blue trace) obtained from SAED measurements. The peak profile of bulk Co₃O₄ is shown for comparison (red line). (c) Fast Fourier transformation corresponding to (a). (d–f) SAED patterns of NPs with 4.7 nm (S5), 7.2 nm (Cb8), and 13.2 nm (Cb13) size, respectively. The displayed data correspond to NPs in Fig. 3.

Fig. 6. Electrochemical characterization and OER performance of ligand-free cuboidal NPs. Inset: TEM micrograph of non-stabilized NPs (Cb13-2). The complete TEM micrograph is displayed in Fig. S15. Cyclic voltammograms of synthetic NPs (black curve) in comparison to a commercial Co₃O₄ nanopowder (red curve). Both samples were cycled 10 times at a scan rate of 50 mV s⁻¹. The blue trace shows the electrochemical response of a bare glassy carbon RDE. The vertical dashed line indicates the position of E_H2O/O2⁰ and the horizontal dashed line marks the current density of J = 10 mA cm⁻² at which the overpotentials η_{i = 10 mA cm⁻²} were determined.