Counter-ion influence on the mechanism of HMTA-mediated ZnO formation†
Crystalline materials are often formed via transient phases. Here we focus on ZnO as a widely used and investigated material for technological applications. Although the literature for the wet chemical synthesis of ZnO is extensive, its formation pathway using these strategies has gained limited attention so far and is poorly understood. To gain insight into these pathways, a HMTA-mediated ZnO synthesis protocol with a variety of zinc salts was employed using in situ pH measurements combined with discrete cryoTEM and SEM sampling studies, in addition to more typical pXRD and SEM product analysis. These results indicate a significant counter-ion effect on the reaction product. Using acetate, nitrate, chloride and sulphate as counter-ions all result first in the formation of a layered zinc hydroxy salt (LZHS), the exact composition of which depends on the counter-ion. Rather stable LZHSs are formed using chloride and sulphate, preventing the eventual formation of ZnO. Only acetate and nitrate result in the formation of ZnO. For acetate, ZnO is preferably grown in-dispersion, while for nitrate it is formed on exposed solid interfaces to the reaction medium (on-surfaces). For the latter the nucleation of its LZHS precursor requires an additional incubation time, resulting in heterogeneous nucleation instead.