Understanding of crystal growth and reaction mechanism in hydrothermal synthesis at supercritical conditions

我们在超临界水中进行了金属氧化物细颗粒的水热合成。为了建立这些新技术，有必要阐明超临界水作为反应溶剂的特性和作用。在本研究中，首先，我们开发了平衡常数（溶解和解离反应）的简单估算模型和用于测量pH和金属氧化物在超临界条件下溶解度的流动装置。其次，利用流动式实验装置进行了几种金属氧化物的水热合成实验，考察了实验条件与纳米级微粒生成区域的关系，发现超临界水中的水热反应速率较高，金属氧化物的溶解度远低于亚临界水中。这两个事实导致产生更高的过饱和度。根据成核理论，成核速率由过饱和度和表面能的函数表示。因此，在超临界条件下可以预期极高的成核速率。反应动力学，成核和晶体生长的模拟将提供定量解释的温度依赖性的颗粒尺寸。尽管缺乏这样的定量信息，在这项研究中所示的结果表明，超临界水热合成法生产纳米晶体的具体功能。

We have conducted hydrothermal synthesis of metal oxide fine particles in supercritical water. To establish these new technologies, it is necessary to clarify characteristic and function of supercritical water as the reaction solvent. In this study, firstly, we developed the simple estimation model of equilibrium constant (dissolution and dissociation reaction) and the flow-through apparatus for measuring pH and metal oxide solubility at supercritical conditions. Secondary, we conducted the experiment of hydrothermal synthesis of several metal oxides with flow-through experimental apparatus and considered the relationship between experimental condition and generation region of nano size fine particle.The hydrothermal reaction rate in supercritical water is higher, and the solubility of metal oxides is much lower than that in subcritical water. Both facts lead to the generation of higher super-saturation degree. The nucleation rate is expressed by the function of super-saturation degree and the surface energy according to the nucleation theory. Thus, extremely high nucleation rate can be expected at supercritical conditions. Simulation of reaction kinetics, nucleation, and crystal growth would provide quantitative explanation for the temperature dependence of the particle size. In spite of the lack of such quantitative information, the results shown in this study suggests the specific features of supercritical water hydrothermal synthesis method for the production of nanocrystals.