RESUMO
An efficient, template-free solution-chemical route to nanostructured ZnO sponges is presented: A mixture of Zn(NO3)2·6H2O, Zn(OAc)2·2H2O, and triethanolamine in methanol was evaporated to a highly viscous liquid and rapidly heated to >200 °C for 1-3 min to achieve highly porous, nanocrystalline sponges of ZnO. The viscous precursor concentrate obtained on evaporation in air was characterized by TG, DSC, and IR spectroscopy, and the product ZnO sponges by XRD, SEM, TEM, and IR spectroscopy. The fast reaction forming ZnO started at 140 °C and finished within a few seconds. Scherrer analysis of the XRD peak broadening showed average crystallite sizes of 8 to 11 nm for ZnO prepared by annealing at 200-450 °C (3 min), while grain growth to 134 nm was observed from 500 to 900 °C (3 min). The ZnO powders obtained at 200-900 °C had cell dimensions of a = 3.25 Å and b = 5.21 Å, matching the ZnO literature data well. SEM and TEM analyses showed highly porous, bread-like 3D nanostructures built by ca. 30-70 nm thick walls of ZnO crystallites of the approximate average sizes given by the XRD Scherrer analysis. It seems that the crystal growth above 450 °C takes place within the ZnO 3D structure obtained at lower temperatures without much sintering of the larger porous structure.