溶剂热法合成纯单斜和四方晶相氧化锆中的溶剂效应

在以水或甲醇为溶剂, 通过溶剂热反应合成纯单斜相或四方相氧化锆的前期工作基础上, 利用X射线衍射手段研究了硝酸氧锆(ZrO(NO3)2·2H2O)和尿素的溶剂热反应产物——水合ZrO2的物相结构在不同反应温度、反应时间及后处理温度等条件下的变化过程, 提出了不同晶相氧化锆的形成、转变以及稳定的可能机理. 不论在水还是甲醇溶液中, 最初水解得到的水合ZrO2沉淀即晶相氧化锆前体都具有四方对称性结构. 在水热反应条件下, 氧化锆沉淀物发生Ostwald熟化(溶解-沉淀)过程, 四方对称性结构转变为热力学稳定的单斜对称性结构. 而在甲醇热反应条件下, 氧化锆沉淀物不溶于甲醇, 从而Ostwald熟化过程被抑制, 使得四方对称性结构得以保持; 同时, 尿素与水合锆沉淀物反应脱除所含的结晶水, 形成更刚性的四方对称性结构, 这样使得高温热处理(400 ℃)只能促进其晶化过程, 但不改变其对称性. 因而, 不同溶剂对氧化锆沉淀物溶解性的差异以及造成的氧化锆沉淀物与尿素反应性能的差异可能是溶剂热反应合成单一晶相氧化锆的关键因素.

Solvent Effects on the Solvothermal Synthesis of Pure Monoclinic and Tetragonal Zirconia Nanoparticles

We have previously reported the solvothermal synthesis of pure monoclinic and tetragonal zirconia nanoparticles in water and methanol. The solvent effect on this synthesis was studied further for this work. The effects of reaction temperature, reaction time and thermal treatment temperature on the crystal phases of the as-prepared hydrous ZrO2 precipitates formed from the solvothermal reaction of hydrated zirconyl nitrate (ZrO(NO3)2·2H2O) and urea were characterized by X-ray diffraction. Plausible mechanisms for the formation of hydrous precipitates, their transformation and their stabilization were examined. Precipitates formed initially with a tetragonal structure irrespective of the solvent used. These tetragonal precipitates, in water, transform into thermodynamically stable monoclinic structures as a result of higher precipitate solubility under hydrothermal conditions. Dissolution and precipitation processes are thus favored. The tetragonal structure was, however, maintained under methanothermal conditions probably due to poor precipitate solubility in methanol. The removal of water from precipitates by reaction with urea in methanol may make the tetragonal structure more rigid. This contributes to the maintenance of the tetragonal structure after precipitate thermal treatment (at 400 ℃). Differences in precipitate solubility and their reaction with urea in water and methanol may dictate the synthetic pathway for pure monoclinic and tetragonal ZrO2 via the solvothermal method.