Supercritical Hydrothermal Synthesis Research Articles

Impact of Mixing for the Production of CuO Nanoparticles in Supercritical Hydrothermal Synthesis

The mixing process of metal salt solutions and supercritical water is essential to supercritical water hydrothermal synthesis (SWHS) to produce nanoparticles. A computational fluid dynamics (CFD) model was developed for predicting mixing efficiency and crystallization kinetics in SWHS mixers. The mixing efficiency was calculated from the micromixing model, and the kinetics model of crystallization was built from the population balance equation. The effects of the fluid dynamics of mixing (Reynolds number and mixer diameter) on the mixing efficiency and crystallization kinetics (nucleation rate and growth rate) were investigated. The results showed that faster mixing can lead to a higher crystallization rate and the production of smaller particles with a narrower particle size distribution.

Continuous supercritical hydrothermal synthesis of dispersible zero-valent copper nanoparticles for ink applications in printed electronics

Surface-modified zero-valent copper nanoparticles (CuNPs) are of interest as conductive inks for applications in printed electronics. In this work, we report on the synthesis, stability and characterization of CuNPs formed with a continuous supercritical hydrothermal synthesis method. The precursor, copper formate, was fed as an aqueous solution with polyvinylpyrrolidone (PVP) surface modifier and mixed with an aqueous water and formic acid stream to have reaction conditions of 400°C, 30MPa and 1.1s mean residence time. The reaction pathway seemed to proceed step-wise as the hydrolysis of copper formate, followed by dehydration to oxide products and subsequent reduction by hydrogen derived from precursor and formic acid decomposition. The formed surface-modified zero-valent CuNPs had particle sizes of ca. 18nm, were spherical in shape and contained no oxide contaminants. The formed CuNPs were found to exhibit long-term (>1 year) stability in ethanol as evaluated by shifts in the surface plasmon resonance band of product solutions. Conductive films (0.33μm thickness) prepared with the CuNPs had a resistivity of 16μΩcm. The methods reported in this work show promise for producing conductive inks for use in practical printed electronics.

Supercritical Hydrothermal Synthesis of ZrO2/C Cathode Catalyst Without Non-Precious-Metal for PEFCs

not Available.

Supercritical hydrothermal synthesis of rod like Li2FeSiO4 particles for cathode application in lithium ion batteries

A cheap and rapid supercritical hydrothermal method has been adopted for the synthesis of Li2FeSiO4 cathode materials for the application in lithium ion battery. The as-synthesized Li2FeSiO4 particles at 380°C exhibited rod like morphology with 20–80nm in diameter and 500nm–1μm in length characterized by transmission electron microscopy (TEM). The Li2FeSiO4 material is systematically characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermo gravimetric analysis (TG), scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX). The electrochemical measurements were carried out after coating of Li2FeSiO4 particles with different weight percentage of conductive carbon. The discharge capacity of as-synthesized rod like Li2FeSiO4 with 30wt% conductive carbon coating exhibited 177mAhg−1 for the first cycle, which is more than that of as-synthesized and 20wt% conductive carbon coated rod like Li2FeSiO4 particles. The obtained discharge capacity is the highest for this material without high temperature heat treatment, which indicating that rod like morphology with diameter 20–80nm could be more active during electrochemical reactions.

Continuous synthesis of lithium iron phosphate (LiFePO4) nanoparticles in supercritical water: Effect of mixing tee

Continuous supercritical hydrothermal synthesis of olivine (LiFePO4) nanoparticles was carried out using mixing tees of three different geometries; a 90° tee (a conventional Swagelok® T-union), a 50° tee, and a swirling tee. The effects of mixing tee geometry and flow rates on the properties of the synthesized LiFePO4, including particle size, surface area, crystalline structure, morphology, and electrochemical performance, were examined. It was found that, when the flow rate increased, the particle size decreased; however, the discharge capacity of the particles synthesized at the high flow rate was lower due to the enhanced formation of Fe3+ impurities. The use of a swirling tee led to smaller-sized LiFePO4 particles with fewer impurities. As a result, a higher discharge capacity was observed with particles synthesized with a swirling tee when compared with discharge capacities of those synthesized using the 90° and 50° tees. After carbon coating, the order of initial discharge capacity of LiFePO4 at a current density of 17mA/g (0.1C) and at 25°C was swirling tee (149mAh/g)>50° tee (141mAh/g)>90° tee (135mAh/g). The carbon-coated LiFePO4 synthesized using the swirling tee delivered 85mAh/g at 20C-rate and at 55°C.

Time-Resolved in Situ Neutron Diffraction under Supercritical Hydrothermal Conditions: A Study of the Synthesis of KTiOPO4

In the first in situ neutron powder diffraction study of a supercritical hydrothermal synthesis, the crystallization of KTiOPO(4) (KTP) at 450 °C and 380 bar has been investigated. The time-resolved diffraction data suggest that the crystallization of KTP occurs by the reaction between dissolved K(+)(aq), PO(4)(3-)(aq), and [Ti(OH)(x)]((4-x)+)(aq) species.

Synthesis of Ca0.8Sr0.2Ti0.7Fe0.3O3 − δ thin film membranes and its application to the partial oxidation of methane

We propose the preparation of thin films on the porous substrate using one component Ca0.8Sr0.2Ti0.7Fe0.3O3−δ (CTO) perovskite-type oxide with the same composition, prepared by using different procedures. An asymmetric membrane with a 20μm thickness was prepared by using CTO synthesized by the supercritical hydrothermal synthesis method. This asymmetric membrane was used as a one component membrane reactor (OCMR) comprising the membrane and the partial oxidation catalyst (Ni supported on Ca0.8Sr0.2Ti0.9Fe0.1O3−δ) to produce synthesis gas from methane. The performance of OCMR has been investigated for the partial oxidation of methane in synthesis gas in natural gas conversion process. For this OCMR, the conversions of methane and selectivity to CO were 82.3% and 99.8% at 1173K, respectively, and were significantly improved compared with the unsupported solid disk type membrane with a 500μm thickness. Moreover, the activation energy of the oxygen permeation process and the relationship between oxygen permeation and film thickness were confirmed to investigate oxygen permeation properties of the CTO thin film under OCMR.

Carbon coating on lithium iron phosphate (LiFePO4): Comparison between continuous supercritical hydrothermal method and solid-state method

Carbon coating on lithium iron phosphate (LiFePO4) plays a crucial role in determining its electrochemical performance. This study investigates the effect of carbon coating on lithium iron phosphate particles synthesized using a continuous supercritical hydrothermal synthesis (SHS) method and a conventional solid-state (SS) method, with sucrose as a carbon precursor. The carbon content, carbon structure, morphology, electronic conductivity, and electrochemical performance of the carbon-coated LiFePO4 (C-LiFePO4) are characterized as a function of the following coating conditions: sucrose concentration, calcination temperature, and calcination time. The particles produced using supercritical water have a smaller size (400–1000nm), larger BET surface area of 7.3m2/g, and lower degree of particle aggregation compared with those produced via solid-state synthesis (particle size: 3–15μm; BET surface area: 2.4m2/g). The differences in the particle size and particle morphology of the LiFePO4 prepared using the two synthetic methods cause a significant difference in the uniformity of the carbon coating, carbon structure, and electronic conductivity. A more uniform carbon layer coating and greater amount of graphitic carbon are found in the LiFePO4 particles produced via the SS method. This leads to a higher discharge capacity of 147mAh/g at a current density of 17mA/g (0.1 C) after 30 cycles when compared with the C-LiFePO4 produced by the SHS method (135mAh/g). No obvious capacity fading was observed. At a high current of 1700mA/g (10 C), the delivered capacities of the C-LiFePO4 particles produced via the SS and the SHS methods are 55% and 52% of the theoretical value, respectively, at a carbon content of 6wt.%. The carbon-coated samples prepared using the SHS and SS methods exhibit similar discharge capacity trends for the carbon content. As the carbon content increased to 6wt.%, the discharge capacity increased, while a further increase in the carbon content to 10wt.% resulted in a decrease in the discharge capacity. Thus, the carbon content and particle properties need to be carefully optimized to enhance the electrochemical performance of C-LiFePO4.

超臨界水熱プロセスによるナノ粒子の合成とハイブリッド化—粒子表面改質による相反機能材料開発—

超臨界水熱プロセスによるナノ粒子の合成とハイブリッド化<br/>&mdash;粒子表面改質による相反機能材料開発&mdash;

Neutron radiography on tubular flow reactor for hydrothermal synthesis: In situ monitoring of mixing behavior of supercritical water and room-temperature water

Neutrons are effectively scattered by hydrogen atoms and have high permeability in heavier elements including Fe, Cr, and Ni. Therefore, neutron radiography should enable the detection of differences in water density in a stainless-steel reactor. To test this, we performed neutron radiography on a tubular flow reactor for supercritical hydrothermal synthesis and visualized the mixing behavior of supercritical water and room-temperature water at a T-junction. The results showed that the difference in density between supercritical water and room-temperature water, as well as how the density changed during mixing, was clearly visualized. The partitioned flow in the side tube was also visualized while feeding room-temperature water. The results indicated the importance of buoyancy forces, as discussed by others in previous reports.

Super Hybrid Materials

Various composite materials have been developed, but in many cases problems arise due to the combined materials such as fabrication becoming difficult because of the significant increase in viscosity, and transparency of the polymer is sacrificed. These issues can be overcome by controlling the nanointerface; however, this is considered as a difficult task since nanoparticles (NPs) easily aggregate in polymer matrices because of their high surface energy. Organic functionalization of inorganic NPs is required to increase affinity between NPs and polymers. For fabricating multi-functional materials, we proposed a new method to synthesize organic modified NPs by using supercritical water. Because organic molecules and metal salt aqueous solutions are miscible in supercritical water and water molecules serve as acid/base catalysts for the reactions, hybrid organic/inorganic NPs can be synthesized under the supercritical condition. The hybrid NPs show high affinity for the organic solvent and the polymer matrix, which leads to the fabrication of these super hybrid NPs. How to release the heat from the devices is the bottle neck of developing the future power devices, and thus nanohybrid materials of polymer and ceramics are required to achieve both high thermal conductivity and easy thin film flexible fabrication, namely trade-off functions. Surface modification of the BN particles via supercritical hydrothermal synthesis improves the affinity between BN and the polymers. This increases the BN loading ratio in the polymers, thus resulting in high thermal conductivity. Transparent dispersion of high refractive index NPs, such as TiO2 and ZrO2, in the polymers is required to fabricate optical materials. By adjusting the affinity between NPs and the polymers, we could fabricate super hybrid nanomaterials, which have flexiblility and high refractive index and transparency.

ChemInform Abstract: Green Materials Synthesis with Supercritical Water

AbstractReview: 68 refs.

Batch Supercritical Hydrothermal Synthesis of CeO&lt;sub&gt;2&lt;/sub&gt; Nanoparticles

CeO2 nanoparticles with diameter of about 5 nm were prepared by batch supercritical hydrothermal synthesis method at 390 °C without additional treatment. It was found that the characteristics of products depended on the pH value, reactant concentration (C0), and reaction temperature. The reaction time and coexisting cations (Li+, Na+ and K+) had little effect on the size and morphology of CeO2 particles. Uniform CeO2 nanoparticles were synthesized at 390 °C, pH = 9 and C0 = 0.06 M. The mechanism for batch supercritical hydrothermal synthesis of CeO2 nanoparticles is discussed.

Size-dependent electrocatalytic activities of printed Co 3O 4 films for a monolithic photovoltaic-electrolytic hydrogen generation system

As an electrocatalyst in a monolithic PV-electrolytic cell for water splitting hydrogen generation Co 3O 4 films were prepared by a paste coating method using Co 3O 4 particles. Different sized Co 3O 4 particles with average diameters of 145.9, 63.3 and 36.5 nm were prepared using a supercritical hydrothermal synthesis method. Electrochemical properties with respect to the particle size in the film were investigated by evaluating overpotential, charge transfer resistance (R ct), and number of active sites (q∗). The relation between overpotential in water oxidation at 5 mA/cm 2 and BET surface area showed a slope of −73.8 ± 6.6, implying strong particle size dependence on electrocatalytic activity. Moreover, the R ct and q ∗ values and the actual hydrogen evolution rate indicated that the electrocatalytic activity of Co 3O 4 is attributed to physical properties (e.g. particle size) of the film. Whereas, intrinsic single site activity of the film was not significantly changed with respect to the particle size in the film.

Supercritical hydrothermal synthesis of metallic cobalt nanoparticles and its thermodynamic analysis

Metallic cobalt nanoparticles could be synthesized via supercritical hydrothermal reduction process using decomposition of formic acid (340 °C to 420 °C, 22.1 MPa and 10 min). To obtain metallic cobalt nanoparticles, several series of experiments were conducted with changing amount of formic acid. The produced cobalt nanoparticles were characterized with XRD, TEM and SEM. To estimate the required amount of H 2, EOSs were employed (ideal gas law, SRK EOS and PSRK EOS). The results of PSRK EOS were well matched with experimental data and gave good explanation for formation of metallic cobalt nanoparticles. Important thing is that required amount of H 2 was much smaller than estimated by using ideal gas law. This result suggests that around the critical point of water, fugacity of H 2 increased drastically and this leads to reduce the required amount of H 2 for the synthesis of cobalt nanoparticles.

Synthesis and Characterization of Surface-modified FePt Nanocrystals by Supercritical Hydrothermal Method

Abstract We have synthesized single nanosized FePt nanocrystals (NCs) by supercritical (sc-) hydrothermal synthesis. The synthesized FePt NCs were characterized by X-ray diffraction (XRD), transmission electron micrography (TEM), electron diffraction (ED), superconducting quantum interference device magnetometry (SQUID), and Fourier transform infrared spectroscopy (FTIR). Surface modification with oleic acid was also successful, and it gave spherical FePt NCs that had average particle size (APS) of 5.0 ± 1.5 nm. In addition, the surface-modified FePt NCs showed good dispersition into organic solvents. A dispersion in tetrahydrofuran (THF) was stable for at least 3 days. The dried surface-modified FePt NCs were easily oxidized in the atmosphere. This high reactivity to oxygen makes the characterization difficult and, at the same time, may indicate that quite a small amount of Fe NC is included in the reaction products.

Facile synthesis of nanosized Li 4Ti 5O 12 in supercritical water

The high rate capability of lithium titanate (Li 4Ti 5O 12, LTO) is prepared using supercritical hydrothermal synthesis (SHS). The particle size, morphology, crystalline structure and electrochemical properties are analyzed and compared to the properties of LTO particles prepared using a typical solid-state method (SS). Nanosized LTO particles having a high crystallinity are produced in a very short reaction time (15 min) and with subsequent calcination at a low temperature (700 °C) or it can be produced in 6 h without calcination. The size of SHS particles is much smaller (20–200 nm, BET surface area of 10–38 m 2 g −1) than that of the SS particles (micron size, BET surface area of 5.4 m 2 g −1). The SHS LTO particles are highly phase-pure while the SS particles have impurity phases such as anatase TiO 2, rutile TiO 2 and Li 2TiO 3. The SHS LTO show higher initial discharge capacity (212 mAhg −1) and better cycling stability compared to those of the SS LTO particles (149 mAhg −1).

Continuous supercritical hydrothermal synthesis: lithium secondary ion battery applications

Nanosized lithium iron phosphate (LiFePO4) and transition metal oxide (MO, where M is Cu, Ni, Mn, Co, and Fe) particles are synthesized continuously in supercritical water at 25–30 MPa and 400°C under various conditions for active material application in lithium secondary ion batteries. The properties of the nanoparticles, including crystallinity, particle size, surface area, and electrochemical performance, are characterized in detail. The discharge capacity of LiFePO4 was enhanced up to 140 mAh/g using a simple carbon coating method. The LiFePO4 particles prepared using supercritical hydrothermal synthesis (SHS) deliver the reversible and stable capacity at a current density of 0.1 C rate during ten cycles. The initial discharge capacity of the MO is in the range of 800–1,100 mAh/g, values much higher than that of graphite. However, rapid capacity fading is observed after the first few cycles. The continuous SHS can be a promising method to produce nanosized cathode and anode materials.

Green materials synthesis with supercritical water

This paper describes the chemistry of green materials synthesized with supercritical fluids. First, the properties and some specific features of supercritical water are summarized. Then, supercritical hydrothermal synthesis of nanoparticles is explained, and various applications of green materials are described. The surface control of nanoparticles in supercritical water is also explained. Green processes involving chemical recycling of waste polymers and a combination of hydrothermal synthesis and supercritical water oxidation are also discussed. Finally, commercialization of supercritical water processes is discussed.

Small capacity decay of lithium iron phosphate (LiFePO4) synthesized continuously in supercritical water: Comparison with solid-state method

Nanosize lithium iron phosphate (LiFePO4) particles are synthesized using a continuous supercritical hydrothermal synthesis method at 25MPa and 400°C under various flow rates. The properties of LiFePO4 synthesized in supercritical water including purity, crystallinity, atomic composition, particle size, surface area and thermal stability are compared with those of particles synthesized using a conventional solid-state method. Smaller size particles ranging 200–800nm, higher BET surface area ranging 6.3–15.9m2g−1 and higher crystallinity are produced in supercritical water compared to those of the solid-state synthesized particles (3–15μm; 2.4m2g−1). LiFePO4 synthesized in supercritical water exhibit higher discharge capacity of 70–80mAhg−1 at 0.1C after 30 cycles than that of the solid-state synthesized LiFePO4 (60mAhg−1), which is attributed to the smaller size particles and the higher crystallinity. Smaller capacity decay at from 135 to 125mAhg−1 is observed during the 30 cycles in carbon-coated LiFePO4 synthesized using supercritical water while rapid capacity decay from 158 to 140mAhg−1 is observed in the carbon-coated LiFePO4 synthesized using the solid-state method.