Yukiya Hakuta

Hydrothermal Synthesis of Metal Oxide Nanoparticles in Supercritical Water

This paper summarizes specific features of supercritical hydrothermal synthesis of metal oxide particles. Supercritical water allows control of the crystal phase, morphology, and particle size since the solvents properties, such as density of water, can be varied with temperature and pressure, both of which can affect the supersaturation and nucleation. In this review, we describe the advantages of fine particle formation using supercritical water and describe which future tasks need to be solved.

Continuous production of phosphor YAG:Tb nanoparticles by hydrothermal synthesis in supercritical water

Phosphor YAG:Tb ((Y{sub 2.7}Tb{sub 0.3})Al{sub 5}O{sub 12}) nano particles were synthesized by a hydrothermal method at supercritical conditions (400 deg. C and 30 MPa) using a flow reactor. Hydroxide sol solutions formed by stoichiometric aluminum nitrate, yttrium nitrate, terbium nitrate and potassium hydroxide solutions. The relationship between particle size and experimental variables including pH, concentration of coexistent ions and hydroxide sol were investigated. Particles were characterized by XRD, TEM and photo-luminescence measurements. Particle size of YAG:Tb became finer as pH was increased or potassium nitrate concentration of the starting metal salt solution was increased. By removing the coexisting ions (NO{sub 3}{sup -}, K{sup +}) from the metal salt solution, single phase YAG:Tb particles with 20 nm particle size were obtained. The emission spectra of YAG:Tb particles of 14 nm shows a blue shift.

Size-controlled synthesis of metal oxide nanoparticles with a flow-through supercritical water method

Hydrothermal synthesis of metal oxide (AlOOH/Al2O3, CuO, Fe2O3, NiO, ZrO2) 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 104 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.

Hydrothermal synthesis of potassium niobate photocatalysts under subcritical and supercritical water conditions

Hydrothermal synthesis of potassium niobate powders was carried out under various subcritical and supercritical water conditions using crystalline Nb2O5 powder as a starting material. A single phase of K4Nb6O17 was formed under subcritical water conditions, while mixed phases of K4Nb6O17 and KNbO3 were obtained under supercritical water conditions where KNbO3 was predominant over K4Nb6O17 as the heating duration was increased. Characterization of these hydrothermally synthesized potassium niobates by XRD, SEM, and TG-DTA analyses revealed that fine hydrated powders can be obtained under subcritical and supercritical water conditions. The hydrothermally synthesized potassium niobate powders were used for photocatalytic hydrogen evolution from water decomposition. The crystallinity is responsible for the high photocatalytic performance of the hydrothermally synthesized potassium niobate powders. The maximal hydrogen evolution rate was achieved for the potassium niobate hydrothermally synthesized at 400 °C for 4 hours. Besides, the hydrogen evolution rate was enhanced more than 10-fold by Ni loading for the hydrothermally synthesized potassium niobate powder which was much higher in comparison with the Ni loaded solid-state synthesized photocatalyst.

Dealing with the aftermath of Fukushima Daiichi nuclear accident: decontamination of radioactive cesium enriched ash.

Environmental radioactivity, mainly in the Tohoku and Kanto areas, due to the long living radioisotopes of cesium is an obstacle to speedy recovery from the impacts of the Fukushima Daiichi Nuclear Power Plant accident. Although incineration of the contaminated wastes is encouraged, safe disposal of the Cs enriched ash is the big challenge. To address this issue, safe incineration of contaminated wastes while restricting the release of volatile Cs to the atmosphere was studied. Detailed study on effective removal of Cs from ash samples generated from wood bark, household garbage, and municipal sewage sludge was performed. For wood ash and garbage ash, washing only with water at ambient conditions removed radioactivity due to (134)Cs and (137)Cs, retaining most of the components other than the alkali metals with the residue. However, removing Cs from sludge ash needed acid treatment at high temperature. This difference in Cs solubility is due to the presence of soil particle originated clay minerals in the sludge ash. Because only removing the contaminated vegetation is found to sharply decrease the environmental radioactivity, volume reduction of contaminated biomass by incineration makes great sense. In addition, need for a long-term leachate monitoring system in the landfill can be avoided by washing the ash with water. Once the Cs in solids is extracted to the solution, it can be loaded to Cs selective adsorbents such as Prussian blue and safely stored in a small volume.

Chemical equilibria and particle morphology of boehmite (AlOOH) in sub and supercritical water

In a recent study, we proposed a supercritical water crystallization method for production of metal oxide particles. Around the critical point, the morphology of boehmite (AlOOH) particles varied greatly with the reaction temperature, pressure and concentration of aqueous aluminum nitrate solution. In this study, the relationship between the morphologies of particles obtained and the chemical species in solution is discussed. For estimation of chemical species concentrations, evaluation of equilibrium constants of the hydrothermal reactions around the critical point is required. For this, a model based on the Gibbs energy change by temperature, solvent effects and ion–ion interactions is employed. The solvent effect was calculated by the Born equation. The effect of ion–ion interaction was calculated by the extended Debye–Huckel equation. Using this model, the distribution of chemical species for the AlOOH system (Al3+, Al(OH)2+, Al(OH)2+, Al(OH)3, Al(OH)4−, NO3−) in subcritical (350°C, 30 MPa) and supercritical water (400°C, 30 MPa) was estimated. The particle morphology seems to be determined by selective adsorption of positive charged species, Al(OH)2+, on the negatively charged faces of AlOOH crystal.

Size and Form Control of Titanylphthalocyanine Microcrystals by Supercritical Fluid Crystallization Method

Abstract Supercritical fluid crystallization (SCFC) method is proposed for preparing microcrystals of π-conjugated organic compounds as a development of conventional reprecipitation method. In SCFC method, both size and form of titanylphthalocyanine microcrystals could be controlled by changing experimental conditions such as temperature and type of solvent. We have been able to fabricate γ-form of titanylphthalocyanine microcrystals with 50 nm in size, which is expected to be promising material for organic photoconductors.

Hydrothermal synthesis of zirconia nanocrystals in supercritical water

Zirconia nanocrystals were prepared by hydrothermal reaction of 0.05 M zirconyl nitrate and zirconyl acetate solutions at supercritical conditions of 400 °C and30 MPa for 1.8 s reaction time. Characterization of products were performed byx-ray diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller measurements. The product particles were compared with zirconia particles prepared by conventional hydrothermal synthesis routes and precipitation-calcination. From the results, zirconia powders prepared in supercritical water had higher crystallinity than those obtained by other methods. Product particles with tetragonal crystal structure with a mean diameter of 6.8 nm could be formed from 0.05 M zirconyl acetate solution in the presence of 0.1 M potassium hydroxide at supercritical conditions.

Continuous production of LiCoO2 fine crystals for lithium batteries by hydrothermal synthesis under supercritical condition

Abstract Specific features of supercritical hydrothermal crystallization synthesis (SHCS) by a rapid heating flow type method are reviewed. The attractive features of the method are (i) quantitative production of ultrafine particles, and (u) control of particle morphology or crystal structure by varying the pressure, temperature or reaction atmosphere (reducing or oxidizing). In this study, we use this method to continuously produce LiCoOz fine crystals that are used as for the cathode materials of rechargeable lithium ion batteries. LiOH and Co(NO& were chosen as starting materials. For oxidizing Co(II) to Co(III), O2 was introduced into the reactor after decomposing HzOz aqueous solution in a preheating tubing. LiCoO2 particles were formed in a single phase at supercritical conditions. scanning electron micrographs showed that the particle size was in the range of 500 to 1,000 nm in diameter. Electrochemical characterization was performed by a constant current discharge and charge test. Little decrease in the discharge capacity suggests high stability of the LiCoOz crystals prepared by this method.

Recovery of metals from simulated high-level liquid waste with hydrothermal crystallization

Abstract Separation of metals from high-level radioactive liquid waste (HLLW) streams is a major problem in the nuclear power industry and usually requires additional solvents or adsorption media. In this work, we examined the separation of metals from simulated HLLW mixtures with thermal energy which completely avoids additional components. The 21 element simulated HLLW mixture consisted of nitrates of Cs, Sr, Pd, Na, Ba, Ag, Cd, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Zr, Cr, Mo, Mn, Fe and Ni in 1.6 M nitric acid. Reactive crystallizations were carried out at temperatures up to 723 K and at pressures up to 30 MPa. Remarkably, many of the elements precipitated out over a narrow range of temperatures with high recoveries. In general, most of the precipitates were highly amorphous, stable solids. Elements that could be precipitated in a solid phase with high recoveries (> 95%) were Mo, Zr, Fe, Pd, Cd and Ce. Other metals that could be separated with moderate recoveries (50–91%) were Cr, Pr, Mn and Ni. The elements Sr, Cs and Na were found to remain in the liquid phase. Single-element studies were made on Pd, Ce, Mn, Ru and Rh. It was found that hydrothermal treatment at temperatures higher than 623 K led to recoveries approaching 100% for these elements, except for Mn, in which recoveries were 18–59%. An empirical model provided an excellent fit of the data in terms of the maximum recovery, RMAX, a one-half recovery temperature, T 1 2 , and a sharpness of separation index, S. The model was found to be useful for characterizing the metal separation temperatures. For the simulated stream studied, more than 35 wt.% of the total metals can be separated by elevating the stream to supercritical conditions. Hydrothermal crystallization is an effective technique for recovering metals from HLLW mixtures.