Puzzling Mechanism behind a Simple Synthesis of Cobalt and Cobalt Oxide Nanoparticles: In Situ Synchrotron X-ray Absorption and Diffraction Studies

Abstract

Here, we show a simple approach to synthesize cobalt and cobalt oxide nanoparticles in an organic solvent. We find that the cubic Co 3 O 4 nanoparticles can be easily obtained, even at temperatures as low as 80 °C. Moreover, exactly the same reaction at 180 °C leads to metallic Co nanoparticles. Thus, in addition to the synthetic efforts, we study the mechanism of occurrence of oxidation and reduction of a Co 2+ precursor in benzyl alcohol. Remarkably, the in situ X-ray absorption and diffraction measurements of the synthesis at 140 °C reveal that oxidation of Co 2+ to Co 3+/2+ and reduction of Co 2+ to Co 0 reactions take place simultaneously. It is followed by a rapid formation of Co 3 O 4 nanoparticles and its consecutive solid-state reduction to CoO. In parallel, metallic Co nanoparticles begin to grow. In addition, Multicomponent Curve Resolution−Alternating Least Squares (MCR-ALS) analysis of X-ray absorption spectroscopy (XAS) data efficiently reveals the nontrivial interdependence between four different reactions. Our strategy to control reduction and oxidation of Co-based nanoparticles as they grow opens up an elegant pathway for the one-pot-synthesis of the hybrid materials for energy-related applications.

Mechanistic Aspects in the Formation, Growth and Surface Functionalization of Metal Oxide Nanoparticles in Organic Solvents

“…Even though they just consisted of a precursor and a solvent, redox reactions occurring in parallel (rather than consecutively) resulting in different oxidation states of the metal species significantly complicated in situ analyses. An example in this direction is the formation of cobalt and cobalt oxide nanoparticles in benzyl alcohol . Starting from Co 2+ , these ions were simultaneously oxidized to Co 3+ and reduced to Co 0 .…”

“…Syntheses performed at low temperatures in open vessels are relatively straightforward to study, whereas harsh reaction conditions (high temperature, high pressure, corrosive environments,…) and closed containers cause some troubles. Solutions to these problems are offered by measuring cells that were specifically developed for such reaction systems …”

Matching the organic and inorganic counterparts during nucleation and growth of copper-based nanoparticles – in situ spectroscopic studies

“…5,12,[14][15][16] Remarkably, for some metal oxides with the occurrence of nanoparticles, the organic reactions do not terminate, and in fact, new reactions are initiated. [17][18][19][20][21] By far the most intriguing example is the formation of monolithic tungsten oxide-polybenzylene hybrids induced by the nucleation of tungsten oxide nanoparticles and their catalytic activity. 20 Moreover, in the last decade, the non-aqueous method has been further adopted to synthesize metal phosphates, -sulfides, -diimides and even metallic nanoparticles.…”

Trends and advances in Operando methodology

“…Newton has pioneered the combination of diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS), the former providing information about chemical bonds of surface species and the latter providing geometrical and electronic structures of active metals [23]. The advent of low-noise, high-resolution 2D detectors has enabled X-ray diffraction (XRD) to be combined with other techniques like XAS using the same capillary reactor cell [21,22,24,25]. Another combination gaining popularity recently using X-ray is XAS/X-ray emission spectroscopy (XES), which can yield a detailed picture of electronic states (unoccupied and occupied orbitals).…”