Hydrothermal synthesis of nano-carbons

Water is an inevitable component of hydrothermal systems and both temperature and pressure have a great influence on the resulting properties of water. When water attains supercritical state, its surface tension approaches nearly zero. At this point, the distinction between gas and liquid phase breaks down and water effectively occupies the pores of the material and facilitates the hydrothermal process [1]. Hydrothermal processing can be defined as “any homogeneous (nanoparticles) or heterogeneous (bulk materials) reaction in the presence of aqueous solvents or mineralizers under high pressure (above 1 atmospheric) and temperature (above a room temperature) conditions to dissolve and recrystallize (recover) materials that are relatively insoluble under ordinary conditions” [1, p.1]. Hydrothermal synthesis of materials indeed has a longstanding history. Schafthaul was the first person who used hydrothermal treatment to prepare fine particles of quartz in a Papin’s digester during 1845 [2]. In the early 1900s, more than 150 mineral species, including diamond, were synthesized by hydrothermal methods [3]. Since 1940s, the early stage of hydrothermal research was conducted by several groups in the United States, Europe, and Japan. They mostly focused on crystal growth of artificial Zeolite and Quartz as indicated in a review by Somiya [4]. During the years between 1950s and 1970s, hydrothermal processes have become a facile method mostly used in the area of geology and mineralogy [5-7]. However, ever since the late 1970s, material scientists adopted hydrothermal processes to prepare various compounds with controlled size, shape, and composition.