Crystallization time in ZnO: the role of surface OH groups in its photoactivity†
The use of ZnO as a photocatalyst for organics degradation has been reported in the literature, but a divergence of results suggests that surface groups play a critical role in its photoactivity. In this paper, we describe the influence of annealing time on the photoactivity of ZnO particles by analyzing their behavior against molecules with typically high surface adsorption, i.e., methylene blue and rhodamine B. ZnO prepared through the chemical precipitation route at room temperature has been calcined at mild temperatures to gradually eliminate surface –OH groups. Although the increase in annealing time had promoted the crystallite size to increase through the elimination of Zn(OH)2, the particle growth reduced the specific surface area and surface hydroxylation, both of which are critical features concerning photocatalysis. The probing of the main degradation mechanisms evinced that the generation of OH* radicals is the only relevant process for oxidation, which is closely related to the nature of –OH groups. After the complete ZnO crystallization, a balance of morphology, surface area, exposed surface, and surface hydroxylation is necessary to obtain the most efficient materials.