Solution Combustion Synthesis of ZnO Undoped and Doped with Fe, Co, Cu, and Mg Using Citric Acid as a Fuel for Photocatalytic Decomposition of Phenol

The solution combustion synthesis (SСS) was used to prepare ZnO from mixtures of solutions of zinc nitrate (oxidizer) and citric acid (fuel) with different fuel-to-oxidizer ratio, as well as for doping ZnO with one of the elements Fe, Co, Cu, and Mg whose concentration was 0.1, 0.3, 1, 3, 10, and 15 wt % when adding corresponding doping element nitrate to the reagent mixture. Combustion characteristics (ignition delay time, combustion duration, coefficient of product mass conservation), composition, and structure of combustion products were studied. It was shown that the content of carbon impurities in the combustion product can be reduced from 8–30 to 1 wt % as a result of calcination for 1 h at 650°C. Calcinated and attrition-ground ZnO powder consisting of individual highly dispersed (<1 µm) nano-sized and submicron ZnO particles with an average crystallite size of 40 nm and sintered porous agglomerates ranging in size from 0.2 to 100 µm was found to exhibit high photocatalytic activity in the decomposition of phenol under ultraviolet irradiation. Doping ZnO with elements Fe, Co, and Cu decreased the photocatalytic activity, and only doping with 1 wt % Mg markedly increased it. However, both undoped and doped ZnO were not effective in photocatalytic decomposition of phenol under visible light.