Plasma-solution synthesis of cobalt oxides

The production of nanosized oxide materials is an important practical problem. This is due to the fact that many of these substances are intensively used as catalysts, microelectronic devices, medical applications, etc. Among the various methods for their production, methods based on the use of gas-discharge plasma are the least studied. But these methods, compared to others, have a number of significant advantages. Among them, the main ones are high process rates and the absence of additional reagents. Therefore, in this work the regularities of the processes of formation of insoluble compounds under the action of a direct current discharge of atmospheric pressure in air on aqueous solutions of cobalt (II) nitrate have been studied. The solutions served as the cathode and anode of the discharge. The discharge was excited by applying a high voltage to two pointed titanium electrodes placed above the liquid anode and liquid cathode in the H-shaped cell. The range of discharge currents was (20-80) mA, and the range of concentrations was (20-60) mmol l-1. It was discovered that when a discharge acts on a liquid anode, a colloidal solution is formed in it, the destruction of which leads to the formation of precipitates. Based on measurements of the kinetics of consumption of Co2+ ions (spectrophotometric method) and the power inputted in the discharge, the rates of this process, effective rate constants, as well as the degree of conversion of Co2+ ions and energy yields of conversion were determined. These parameters depended on the discharge current and the initial concentration of the solution. The values of the constants were similar to(0.2-6) x 10-1 s-1, the energy yields were similar to(0.2-0.5) ions per 100 eV, and the degrees of conversion were similar to(0.1-0.5). The resulting powders were analyzed by differential scanning calorimetry (DSC), x-ray diffraction (XRD), and energy-dispersive x-ray spectroscopy (EDS). The sizes of the powder aggregates were estimated from the results of scanning electron microscopy (SEM), transmission electron microscope (TEM) and Scherrer's relation. It turned out that the resulting precipitates were a mixture of amorphous Co2+ and Co3+ hydroxides. Their calcination in air led, depending on the temperature, to the formation of predominantly cubic either CoO or Co3O4. The size of the aggregates of the calcined samples was similar to 500 nm. The specific surface area of the powders, determined by the Brunauer-Emmett-Teller (BET) method, was similar to 53 m2 g-1. The average pore volume and their size was similar to 17 cm3 g-1 and similar to 240 & Aring;. The advantages of the proposed method over other methods are high process rates (process time similar to 10 min) and the absence of any additional reagents.