Plasma-solution synthesis of cobalt oxides

Abstract

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. 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 ~(0.2-6)10-1 s-1, the energy yields were ~(0.2-0.5) ions per 100 eV, and the degrees of conversion were ~(0.1-0.5). The resulting powders were analyzed by DSC, XRD, and EDS. The sizes of the powder aggregates were estimated from the results of SEM, 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 ~500 nm. The specific surface area of the powders, determined by the BET method, was ~53 m2/g. The average pore volume and their size was ~17 cm3/g and ~240 A0. The advantages of the proposed method over other methods are high process rates (process time ~10 minutes) and the absence of any additional reagents.