Dependence of the textural properties and surface species of ZnO photocatalytic materials on the type of precipitating agent used in the hydrothermal synthesis

Three different precipitating agents (NaOH, NH4(H)CO3 and CO(NH2)2) have been applied for the hydrothermal synthesis of ZnO powder materials, aiming at obtaining various types of porosity and surface species on ZnO.
The synthesis procedures were carried out in the presence and in the absence of tri-block copolymer Pluronic (P123, EO20PO70EO20). These materials were characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM)–energy-dispersive X-ray spectroscopy (EDX), BET method and TG–differential thermal analysis (DTA) method, and their photocatalytic activities were tested in the removal of azo dye Reactive Black 5 (RB5). The urea precipitant yields spongy-like surface forms and the greatest share of mesopores. It has the highest specific surface area, the highest degree of crystallinity of wurtzite ZnO phase and largest content of surface OH
groups in comparison with the other two precipitants. The zinc hydroxycarbonate intermediate phase is missing in the case of NaOH as a precipitating agent; therefore, it manifests poorer textural characteristics. The morphology of P123-modified sample is different and consists of needle-shaped particles. The urea-precipitated samples display superior performance in the photocatalytic oxidation reaction, compared with the other precipitated samples. The other two precipitating agents are
inferior in regard to their photocatalytic activity due to a greater share of micropores (not well-illuminated inner surface) and different surface morphologies.