Supercritical hydrothermal synthesis (SHS) is a new hydrothermal synthesis method and a promising technology. This method mainly utilizes the outstanding properties of supercritical water (T > 374 oC, P > 22.1 MPa) with much lower dielectric constant than subcritical water (T < 374 oC). The rate of hydration (hydrothermal reaction) of metal salts increases with decreasing dielectric constants of water, while the solubility of metal oxides decreases. Therefore, fine metal oxide particles can be formed in supercritical water. SHS is a promising method for preparation of nanosize materials. SHS method requires neither sophisticated processing nor high processing temperature in comparison with other methods. At the same time, SHS method has rapid reaction rate and the products do not require post-treatment, such as calcinations and grinding.
Although SHS method is at the start stage in nanosize materials preparation, it has received considerable attention in recent years. SHS method has many potential applications in nanosize materials preparation. The mechanism of SHS is not clear. This work is therefore concerned with the basic research of nanosize materials preparation by SHS method. The applications of SHS method in nanosize materials preparation were investigated. Many nanosize materials have been synthesized, such as monomer oxides, binary spinel oxides, multi-cations spinel oxides and composite. SHS method can also be used to modification of nanoparticles. The synthesis mechanism of nanosize materials in supercritical water is also discussed.
CeO2 nanoparticles with high crystallization were prepared from Ce(NO3)3•6H2O solution by SHS method in a batch reaction autoclave. It was found that the characteristics of products depended on the pH value, reaction temperature and reactant concentration (C0). The reaction time and coexisting cations (Li+, Na+ and K+) had little effect on the size and morphology of CeO2 particles. Uniform spheric CeO2 nanoparticles with diameter of about 5 nm were synthesized at 390 oC, pH = 9 and C0 = 0.06 M. The mechanism for batch SHS of CeO2 nanoparticles is discussed. The reaction process could be divided into three stages. The first stage was the formation of fog-like small particles in the temperature range of RT to 200 oC. The second stage was the formation of particles with a non-uniform size distribution from 200 oC to 350 oC. The third stage was the formation of small particles with a diameter of about 5 nm and a uniform size distribution at 390 oC.
CoFe2O4 nanoparticles with high crystallization were prepared from Co(NO3)2•6H2O and Fe(NO3)3•9H2O solutions by SHS method in a batch reaction autoclave. It was found that the characteristic of products depended on pH, temperature and mole ratio (r) of Co2+ to Fe3+ in reactant solutions. The reaction time and the coexisting cations (Na+ and K+) had little effect on the size and morphology of CoFe2O4 particles. Pure elliptic CoFe2O4 nanoparticles with diameter of about 5 nm were synthesized at 390 oC, pH = 12 and r = 0.5. The maximum coercivity and saturation magnetization of CoFe2O4 prepared by the present method were 340.6 Oe and 68.9 emu/g, respectively. The mechanism for batch supercritical hydrothermal synthesis of CoFe2O4 nanoparticles is believed to be a homogeneous phase reaction of Co(NO3)2•6H2O and Fe(NO3)3•9H2O aqueous solution.
Zn2+, Mn2+ or Al3+ substituted cobalt ferrite have been synthesized by batch SHS method. X-ray diffraction studies of the nanoparticles of ZnxCo1-xFe2O4 (x = 0.5，1) and CoAlxFe2-xO4 (x =1，2) shown that the products possessed pure spinel structure. However, the MnxCo1-xFe2O4 (x = 0.5，1) had impurity Fe2O3 except spinel structure. The particle size of substituted cobalt ferrite was smaller than that of binary spinel. Ce3+ or Nd3+ substituted cobalt ferrite could not be synthesized by this method. The probably reason is believed to be related to radius of cations. The radius of Ce3+ and Nd3+ is beyond the critical value for interspacial fill in AB2O4. Therefore, Ce3+ and Nd3+ can not be filled in interspace of spinel structure.
By SHS method, ceria nanoparticles with diameters of 3-8 nm were successfully supported on multi-wall carbon nanotubes (MWNTs) homogeneously. It was found that the amount and distribution of ceria nanoparticles supported on the MWNTs depended on the pH of reaction mixture. The largest amount of CeO2 particles and the best homogeneity were observed on the MWNTs treated by nitric acid at pH=9. The amount of CeO2 particles supported on the MWNTs treated by nitric acid was much more than that supported on the untreated MWNTs. The MWNTs treated by nitric acid had much more functional groups than the untreated MWNTs. These functional groups were believed to be beneficial to the adhesion of the particles on the walls of the MWNTs.
Surface modification of ceria (CeO2) nanoparticles in supercritical water was examined by adding CH3(CH2)4CHO as organic modifier reagent to the reactant. It was found that the surface modification had little effect on the size and morphology of CeO2 particles. Due to the formation of chemical bonding between the surface of CeO2 nanoparticles and modifier, the CeO2 nanoparticles were changed from hydrophilic to hydrophobic by the addition of modifier reagent. The organic surface modification of CeO2 nanoparticles provides a basic for its application in organic materials field.
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