Hydrothermal synthesis of metal oxide (AlOOH/Al 2 O 3 , CuO, Fe 2 O 3 , NiO, ZrO 2 ) nanoparticles from metal nitrate aqueous solution was carried out at 673 K and pressures ranging from 25 MPa to 37.5 MPa with a flow-through supercritical water method. Size, phase and crystallinity of the obtained particles were characterized by TEM, XRD and TG, respectively. Effect of the difference of the metals in starting materials, pressures and concentrations on particle size and crystallinity was analyzed on the basis of supersaturation, which was evaluated by estimated metal oxide solubility. The result suggests that supersaturation should be set to higher than around 10 4 in this method to obtain particles under 10 nm in diameter. Further, crystallinity of the obtained particles was evaluated as weight loss through TG analysis. It was found that higher supersaturation decreased the crystallinity. This result can be explained that high supersaturation led to the inclusion of water molecules during the formation of particles.

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Various chemical reactions using supercritical fluids have been investigated as a breakthrough that reduces environmental problems. Especially, water (Tc = 374˚C, Pc = 22.1 MPa, and ρc = 0.323 g cm -3 ) is well known as an environmentally benign solvent, and it has been reported that the various inorganic and organic reactions, such as the synthesis of metal oxide nanoparticles, 1-3 the dehydration of alcohols, 4-6 the hydrolysis of ethers, 4,5,7,8 Friedel-Crafts alkylation 9 and the acylation 10 of aromatics occur without a catalyst in supercritical water. These reactions are expected to be controlled by the temperature and pressure (or density), 11,12 since drastic changes in the solvent properties, especially the polarity 13,14 and hydrogen-bonding ability 15 of solvents, cause changes in chemical potential of the reactants, intermediates and products, and thus the activation energy. The solvent properties, which influence the reactions, are mainly the local level around the solute. Therefore, it is necessary to understand the local solvent properties around the solute, especially near to the critical region, in which the local density is different from the bulk. In addition, the hydrogenbonding ability of the solvent also has an influence on the reaction. 42 In the case of nucleophilic reactions, because the nucleophilic species are hydrated by solvent molecules, the reaction rate decreases along with an increase in the hydrogen bonding ability of the solvent. 42 However, hydrogen bonding between the solute and solvent molecules in supercritical water has not yet been clarified.UV-vis spectroscopy has been used to estimate the hydrogen bonding through the spectral shift of a probe molecule. [45][46][47][48] The frequency of the maximum absorbance, νmax, can be empirically represented by the Kamlet-Taft equation 42-48 based on linear solvation energy relationships (LSER), as follows:where the first term, A, is the frequency of the maximum absorbance under the condition with no solvent effect, such as in a gaseous state or a vacuum. The second term, sπ*, is for interactions between the solute and solvent polarities. The third term, aα, and fourth term, bβ, are for interactions between the hydrogen-bond acceptor ability (HBA) of the solute and the hydrogen-bond donor ability (HBD) of the solvent and the interactions between HBD of the solute and HBA of the solvent, respectively. The solvent parameters (π*, α, and β), which are obtained from the spectral shift, have found widespread use in describing many solvation effects observed for physicochemical properties, such as rate constants and equilibrium constants based on LSER. 42-48In addition, the spectral shift under conditions without the effect of hydrogen bonding can be theoretically expressed by the physical properties of the solvent, which are the refractive The local density around pyridazine was evaluated by examining the UV-vis spectral shift of pyridazine in a highpressure liquid state and supercritical water from 25 to 450˚C and from 20 to 45 MPa. Au...

Synthesis of Ni nanoparticles having an average diameter of 8.1 nm by reduction of NiO nanoparticles was carried out at 673 K. HCOOH was used as a reductant, which decomposes to CO and/or H2 at higher temperatures. NiO nanoparticles having an average diameter of 3.8 nm were obtained from Ni(NO3)2 + KOH aqueous solution by a flow-through supercritical water method at 673 K and 30 MPa. Effect of HCOOH concentration on particle size and saturation magnetization was investigated.