Optimization Study on Formation and Decomposition of Zinc Hydroxynitrates to Pure Zinc Oxide Nanoparticles in Supercritical Water

Abstract

In the present study, synthesis and decomposition of zinc hydroxynitrates, i.e., Zn(NO3)(OH)·H2O, Zn5(NO3)2(OH)8·2H2O, and Zn3(OH)4(NO3)2 into ZnO nanoparticles were investigated at supercritical conditions by combination of Taguchi experiment design method, analysis of variance (ANOVA), and complementary verifying experiments. First, the effect of several important parameters such as temperature, residence time, and initial nitrate salt concentration and its pH on “purity” of synthesis particles (e.g., ZnO) was studied using an orthogonal experiment design methodology coupled with instrumental analysis such as powder X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, and thermo-gravimetric analysis. Then ANOVA was performed on the data and the best conditions for the synthesis of pure ZnO nanoparticles were attained. The second aim was the elucidation of formation and decomposition mechanism of hydroxinitrates by combining the statistical results and additional experiments which influential parameters were investigated independently. It was found that the initial pH of zinc nitrate alongside temperature had the most effects on formation of Zn5(NO3)2(OH)8·2H2O as the first product of the formation reaction, while at higher temperatures it was converted to Zn3(OH)4(NO3)2 which in turn decomposed to ZnO nanoparticles as an ultimate product.

An investigation into the formation and conversion of metal complexes to metal oxide nanoparticles in supercritical water

“…Therefore, one can conclude that the metal nitrate salts are good candidates to be used as precursors. On the other hand, our findings indicated that the formation of some nitrate, hydroxide, or hydoxinitrates complexes as the byproducts of metal oxide formation reaction is unavoidable [16,29]. The existence of such…”

“…In our previous works the formation and decomposition of copper and zinc nitrate complexes were investigated [16,29]. In the current study, we extend this investigation to a wide range of metal oxide nitrate salts.…”

“…There are some studies in the literature that propose a comprehensive mechanism for some metal oxide synthesis in SCW medium. For instance, Outokesh et al [29] and Ahmadi et al [16] developed two detailed mechanisms for copper oxide and zinc oxide, respectively. According to our pervious works, the temperature has the most significant impact on the quality of the produced nanoparticles.…”

“…This method is environmentally more benign than the other synthesis techniques because no organic solvents are used during the process [15]. In contrast to the conventional hydrothermal method, the most important advantages of SCW are faster kinetics, and smaller obtained particle size due to the high super-saturations resulted from low metal oxide solubility in SCW [16]. The dielectric constant (ε), which changes with temperature, plays a significant role in the formation of nanoparticles especially after the critical point (374°C and 22.1 MPa) [15].…”

The role of supercritical water on the rapid formation of ZSM-5 nanocatalyst

“…Due to reduction in dielectric constant of water (ε), the solubility of nonpolar species increases whereas that of ionic and polar compounds decreases [22,23]. Under this condition, the thermodynamic and transport properties of water remarkably change in such a way that the reaction rate of formation accelerates and crystal growth decreases [24,25]. Therefore, the synthesis of nano-zeolites in this situation may have some advantages including lower reaction duration, higher reaction yield, smaller size and narrower particle distribution compared crystals are produced in low temperature hydrothermal (LTH) method [26,27].…”