Effect of cations and anions on properties of zinc oxide particles synthesized in supercritical water

Hydrothermal synthesis of zinc oxide fine particles from zinc salt (Zn(CH₃COO)₂, ZnSO₄, Zn(NO₃)₂) and alkali metal hydroxide (LiOH, KOH) aqueous solution was carried out with a Ti alloy batch reactor in supercritical water. Particle size synthesized in LiOH solution was relatively smaller than that in KOH. Emission spectra of the particle produced from ZnSO₄ and LiOH aqueous solution shows the highest intensity among these systems. Hydrothermal synthesis of zinc oxide fine particles from Zn(NO₃)₂ and LiOH solution was also carried out with a flow-through apparatus for continuous production and rapid heating of the starting solution to supercritical states. Nanoparticles having an average particle diameter of 16 nm was produced at 659 K and 30 MPa.     

Investigating photocatalytic property of TiO2 nanorods fabricated hydrothermally at equal molar ratio of KOH and NaOH

TiO₂ photocatalysts were prepared by hydrothermal method at temperatures of 110, 155, and 200°C for 24 hours, using KOH (5 mol/L):NaOH (5 mol/L) solution and anatase TiO₂ powder with an average particle size of 0.13 μm as precursor and by calcining at two different temperatures of 450 and 700°C. An overall photocatalytic removal efficiency of 93% was achieved for Rhodamine B dye under UV light irradiation by the photocatalyst obtained at synthesis and calcination temperatures of 155 and 700°C, respectively, which indicated a nanorod morphology with a mean diameter of about 34 nm.     

Continuous hydrothermal synthesis of HT-LiCoO2 in supercritical water

A detailed investigation was made into the production of high temperature lithium cobalt oxide (HT-LiCoO₂) particles by continuous hydrothermal synthesis via the reaction of cobalt nitrate, lithium hydroxide, and hydrogen peroxide. The experiments were carried out in both subcritical and supercritical water, at temperatures ranging from 300 to 411 °C, with residence times less than 1 min in all instances. Although Co₃O₄ particles were synthesized in subcritical water at similar reaction conditions designed for comparison, well-ordered particles of HT-LiCoO₂ were obtained in supercritical water. In supercritical conditions, the variations in temperature and residence time did not have significant impacts on the average particle size, particle size distribution, or morphology of obtained HT-LiCoO₂. However, it was important to supply excessive lithium hydroxide and hydrogen peroxide in order to synthesize single-phased HT-LiCoO₂ particles without undesired by-products. The hydrothermal synthetic route for LiCoO₂, CoO, and Co₃O₄ in both subcritical and supercritical conditions was postulated.     

Low temperature deposition of titanium oxide containing thin films in trench features from titanium diisopropoxide bis(dipivaloylmethanate) in supercritical CO2

We studied supercritical carbon dioxide fluid deposition of titanium oxide (TiO₂) in trench features on Si substrates using a flow-type deposition apparatus from titanium diisopropoxide bis(dipivaloylmethanate), aiming at fabricating conformal films at a relatively low temperature. We investigated the deposition rate and step coverage under a fluid temperature from 40 to 60 °C, a pressure from 8.0 to 10.0 MPa, and a substrate temperature from 80 to 120 °C. They were dependent on the fluid density, indicating that the solubility difference between the bulk fluid and the neighborhood of the substrate surface plays a decisive role for the deposition. An excellent conformal filling of the trench features was achieved from the fluid of 60 °C under 8 MPa on the substrate kept at 80–100 °C. The XPS spectra of the deposited film suggested partial formation of TiO₂, and the XRD spectra showed the existence of some crystalline TiO₂ (anatase).     

Synthesis of titanium oxide particles in supercritical CO2: Effect of operational variables in the characteristics of the final product

A new chemical precursor is proposed for the synthesis of TiO₂ anatase nanoparticles in supercritical CO₂: the organometallic diisopropoxititanium bis(acetylacetonate) (DIPBAT). DIPBAT thermohydrolysis in supercritical carbon dioxide (SC-CO₂) has been studied in the range 10.0–20.0 MPa and 200–300 °C, and compared with that of titanium tetraisopropoxide (TTIP). The proposed reaction mechanism is a thermohydrolysis, where the hydrolysis of acetylacetonate groups is the limiting step of the reaction rate. The addition of water directly to the reaction favours the growth of formed particles, whereas ethanol offers better results as hydrolysing reactant, leading to smaller particles. Experiments have been performed first in a batch process and secondly in a semi-continuous one, varying the residence time in the reactor from 30 s to 2 min. The effect of operational variables in the final product and their influence in the different steps of the process have been studied. Results have shown that product crystallinity is related with temperature, and temperatures higher than 250 °C are necessary to obtain well-crystallized TiO₂ anatase. In the same sense, area Brunauer–Emmett–Teller (BET) is connected with crystallinity, and amorphous product, Ti(OH)₄, shows the highest surface area. Particle size and particle size distribution (PSD) are controlled by instantaneously supersaturation degree, and precursor concentration together with pressure are the main responsible of particle size control. Operational conditions influence solubility of species, mass transfer, chemical reaction and nucleation and particle growth and they mark the final characteristics of the product and its application. In such a way, good crystallized TiO₂ anatase particles of about 200 nm in diameter have been obtained working at 300 °C, 20.0 MPa and residence time of 2 min, with a reaction medium composed by CO₂/ethanol (80/20, v/v). Such particles present good optical properties and specific surface area BET of around 150 m²/g. At lower working temperatures the obtained particles present worse crystallinity; however, their specific surface area increases to 350 m²/g and they are suitable as support of metal clusters in heterogeneous catalysis.     

Effect of KOH on the continuous synthesis of cobalt oxide and manganese oxide nanoparticles in supercritical water

The effects of KOH on the supercritical hydrothermal synthesis of cobalt oxide and manganese oxide particles are investigated using a continuous-flow reactor. Significant changes in morphology, particle size, and oxidation state are observed by adding KOH. The spinel Co₃O₄ phase is transformed to a rocksalt CoO phase and the pyrolusite MnO₂ phase is transformed to a hausmannite Mn₃O₄ phase in the presence of 0.5 M KOH. The average particle size of the metal oxides decreased with an addition of KOH. The OH⁻ ions of KOH may act as a reducing agent as well as a supersaturation enhancing agent under supercritical water conditions.     

Supercritical hydrothermal synthesis of nano-ZnO: Effects of key parameters and reaction mechanism

In this paper, ZnO particles with the particle size as low as 23 nm were successfully prepared by supercritical hydrothermal synthesis technique. The particle size of nano-ZnO decreased with the increase of temperature and pressure. Nano-ZnO particles in ZnSO₄ system were spherical with smaller particle size. The discrepancy in nano-ZnO produced by different precursor of Zn(NO₃)₂, Zn(CH₃COO)₂ or ZnSO₄ is attributed to the anion effects and supersaturation. The particles in the KOH system (29 nm) were smaller than those in the NaOH system (44 nm). For precursor concentration, intermediate Zn(OH)₂ was generated at lower concentration, while Zn₄SO₄(OH)₆·4H₂O was produced at higher concentration. For alkali concentration, as the gradual increase of KOH concentration, Zn(OH)₂ began to decrease and gradually transformed into Zn(OH)₃^– and [Zn(OH)₄]²⁻. When the KOH concentration reached a certain value, [Zn(OH)₄]²⁻ occupied the dominance in the mixed solution.     

One-step synthesis of fine SrTiO3 particles using SrSO4 ore under alkaline hydrothermal conditions

The employment of mineral SrSO₄ crystals and powders for preparing SrTiO₃ compound was investigated, with coexistence of Ti(OH)₄·4.5H₂O gel under hydrothermal conditions, at various temperatures (150–250 °C) for different reaction intervals (0.08–96 h) in KOH solutions with different concentrations. The complete dissolution of the SrSO₄ crystal occurred at 250 °C for 96 h in a 5 M KOH solution, resulting in the synthesis of SrTiO₃ particles with two different shapes (peanut-like and cubic). In contrast, very fine SrTiO₃ pseudospherical particles were crystallized when SrSO₄ powders were employed as precursor. Variations on the SrTiO₃ particle shape and size were found to be caused by the differences in the dissolution rate of the SrSO₄ phase in the alkaline KOH solution. The crystallization of SrTiO₃ particles was achieved by a bulk dissolution–precipitation mechanism of the raw precursors, and this mechanism was further accelerated by increasing the reaction temperature and concentration of the alkaline media. Kinetic data depicted that the activation energy required for the formation of SrTiO₃ powders from the complete consumption of a SrSO₄ single crystal plate under hydrothermal conditions, is 27.9 kJ mol⁻¹. In contrast, when SrSO₄ powders were employed (28–38 μm), the formation of SrTiO₃ powder proceeded very fast even for a short reaction interval of 3 h at 250 °C in a 5 M KOH solution.     

Physical properties and enzymatic hydrolysis of poly(L‐lactide)–TiO2 composites

Amorphous poly(L ‐lactide) (PLLA) composite films with titanium dioxide (TiO₂) particles were prepared by solution‐casting using methylene chloride as a solvent, followed by quenching from the melt. The effects of surface treatment, volume fraction, size, and crystalline type of the TiO₂ particles on the mechanical properties and enzymatic hydrolysis of the composite films were investigated. The tensile strength of the PLLA composite films containing TiO₂ particles except for anatase‐type ones with a mean particle size of 0.3–0.5 μm was lowered and the Young's modulus became higher with increasing the content of TiO₂ particles. The tensile strength of the composite films containing anatase‐type TiO₂ with a mean particle size of 0.3–0.5 μm at contents of 20 wt % or less was almost the same as that of the pure PLLA film. The enzymatic hydrolysis of PLLA matrix was accelerated by the addition of the hydrophilic anatase‐type TiO₂ particles (nontreated or Al₂O₃ treated) with a mean particle size of 0.3–0.5 μm at relatively high contents such as 20 wt %. On the other hand, the enzymatic hydrolysis of PLLA matrix was inhibited by composite formation with the hydrophobic rutile‐type TiO₂ particles (Al₂O₃‐stearic acid treated, or ZrO₂‐Al₂O₃‐stearic acid treated). These results suggest that the mechanical properties and enzymatic hydrolyzability of the PLLA can be controlled by the kind and amount of the added TiO₂ particles. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 190–199, 2005     

Formation of deagglomerated PLGA particles and PLGA-coated ultra fine powders by rapid expansion of supercritical solution with ethanol cosolvent

Rapid expansion of supercritical solution (RESS) was used for preparing polymer particles and polymer coating of ultra fine powders. The polymer of pharmaceutical interest was Poly(lactic-co-glycolic acid) (PLGA with PLA: PGA ratio of 85: 15 and MW of 50,000–75,000) and the simulated core particles were 1.4-μm SiO₂ and 70-nm TiO₂ particles. The supercritical solution was prepared by dissolving PLGA in supercritical carbon dioxide with ethanol as a cosolvent. Supercritical solution of CO₂-PLGA was sprayed through capillary nozzles to ambient conditions, resulting in formation of submicron PLGA particles. Similarly, rapid expansion of supercritical solution of CO₂-PLGA suspended with the core particles could provide solvent evaporation and deposition of submicron PLGA particles on the surface of the core particles, resulting in the formation of coating films on dispersed particles of SiO₂ and agglomerates of TiO₂. The influences of the core particle size, spray nozzle diameter as well as powder-to-polymer weight ratio were also investigated and discussed with respect to the coating performance.     

Facile preparation of Na-free anatase TiO2 film with highly photocatalytic activity on soda-lime glass

Na-free anatase TiO₂ film was prepared on soda-lime glass (SL-glass) from a TiF₄ aqueous solution upon addition of boric acid at 60 °C. It was found that the as-prepared TiO₂ film before calcination showed a higher photocatalytic activity than the calcined sample (500 °C). This could be attributed to the fact that the calcined TiO₂ film contained decent Na⁺ ions, which was diffused from the SL-glass substrate into the TiO₂ film during calcination, resulting in the decrease of photocatalytic activity.     

Photocatalytic functional coatings of TiO2 thin films on polymer substrate by plasma enhanced atomic layer deposition

We prepared photocatalytic TiO₂ thin films which exhibited relatively high growth rate and low impurity on polymer substrate by plasma enhanced atomic layer deposition (PE-ALD) from Ti(NMe₂)₄ [tetrakis (dimethylamido) Ti, TDMAT] and O₂ plasma to show the self-cleaning effect. The TiO₂ thin films with anatase phase and bandgap energy about 3.3 eV were deposited at growth temperature of 250 °C and the photocatalytic effects were compared with commercial Activ glass. From contact angles measurement of water droplet and photo-induced degradation test of organic liquid, TiO₂ thin films with anatase phases showed superhydrophilic phenomena and decomposed organic liquid after UV irradiation. The anatase TiO₂ thin film on polymer substrate showed highest photocatalytic efficiency after 5 h UV irradiation. We attribute the highest photocatalytic efficiency of TiO₂ thin film with anatase structure to the formation of suitable crystalline phase and large surface area.     

Engineering study of continuous supercritical hydrothermal method using a T-shaped mixer: Experimental synthesis of NiO nanoparticles and CFD simulation

In the synthesis of metal oxide fine particles by continuous supercritical hydrothermal method, rapid mixing of starting solution with supercritical water is a key factor for producing nanoparticles that have a narrow size distribution. In this paper, continuous hydrothermal synthesis of NiO nanoparticles from Ni(NO₃)₂ aqueous solution at 400 °C and 30 MPa was carried out with T-shaped mixers and the effect of inner diameter, flow rate, and mixing directions on the particle size was examined. The computational fluid dynamics (CFD) simulation of the mixers was performed to evaluate the heating rate of the starting solution. When the inner diameter of the T-shaped mixer was decreased from 2.3 to 0.3 mm and the flow rate was increased from 30 to 60 g/min, the produced NiO particle size decreased remarkably from 54.3 to 20.1 nm. This trend of the decrease in particle size could be described as a function of the heating rate. The experimental and CFD results showed the detail regions of local heating that correlated with the NiO nanoparticle size.     

Polymeric coating of fluidizing nano-curcumin via anti-solvent supercritical method for sustained release

Nano-curcumin was coated by poly(lactic-co-glycolic acid) (PLGA) using a novel fluidization assisted supercritical anti-solvent procedure. PLGA solution was sprayed into supercritical CO₂ media, in which nano-curcumin particles were fluidized by ultrasonic vibration. The influences of process parameters, such as solvent types, solution concentrations, CO₂ flow rates, the ratio of PLGA to curcumin, and ultrasonic power on the particles size and the curcumin loading were investigated. Scanning electron microscopy, laser particle size analyzer, and differential scanning calorimetry were used to characterize as-produced samples in terms of the structure, morphology and particle size distribution. The PLGA-curcumin nano-capsules were obtained with the average size of 63 nm and the loading of 38%, under the ultrasonic power of 210 W, and with the average size of 40 nm and 36% loading, at the ultrasonic power of 350 W. In vitro studies prove that proposed method is successful in preparing sustained release systems.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Selective synthesis of TiO2 nanopowders

Nanosized anatase, rutile, brookite, and mixtures of these materials taken in different ratios are synthesized using the detonation method with variations in the densities and ratios of explosives composed of the TiO₂ precursor, NH₄NO₃, and C₃H₆N₃(NO₂)₃. It is shown that the phase composition, the phase content, and the average particle size of TiO₂ nanopowders depend on the composition of the explosive mixtures and their densities. When the weight ratio between the C₃H₆N₃(NO₂)₃ compound and the TiO₂ precursor lies in the range 0.695–1.270, the average size of rutile particles is larger than that of anatase particles by a factor of approximately two.     

Synthesis of TiO<Subscript>2</Subscript> Nanoparticles by Microemulsion/Heat Treated Method and Photodegradation of Methylene Blue

In this work, TiO₂ nanoparticles were prepared by microemulsion (ME)/heat treated method and its photodecomposition property of methylene blue. Microemulsion (ME) consisted of water, cyclohexane and an anionic surfactant such as bis (2-ethylhexyl) sodium sulfosuccinate (AOT). Titanium tetraisopropoxide (TTIP) was dropped into the ME solution and then then TiO₂ nanoparticles were formed by the hydrolysis reaction between TTIP in the organic solvent and the water in the core of ME. The smallest diameter of the particles was 20 nm in the system of cyclohexane with surfactant when the molar ratio of water to surfactant was 2. The effect of the process parameters (water/surfactant ratio, different temperatures) on the final characteristics has been investigated, in terms of structural phase and particle size. The TiO₂ nanoparticles were characterized by means of X-ray diffraction, Transmission and scanning electron microscopy, Fourier-Transformed infrared and differential thermal analysis. TiO₂ nanoparticles prepared in this condition were collected as amorphous powder, and converted to anatase phase at less than 350 °C, which is lower than the ordinal phase transition temperature. The crystallite size and crystallinity increase with an increase of heat treated s temperature. The particles are shown to have a spherical shape and have a uniform size distribution. The size of nanoparticles raises with an increase of water/surfactant ratio. In the photocatalytic decomposition of methylene blue, the photocatalytic activity is mainly determined by the crystallinity of TiO₂. In addition, the TiO₂ heat treated at 350 °C shows the highest activity on the photocatalytic decomposition of methylene blue (k = 1.7 × 10⁻² min⁻¹).     

Role of Surface Area, Primary Particle Size, and Crystal Phase on Titanium Dioxide Nanoparticle Dispersion Properties

Characterizing nanoparticle dispersions and understanding the effect of parameters that alter dispersion properties are important for both environmental applications and toxicity investigations. The role of particle surface area, primary particle size, and crystal phase on TiO₂ nanoparticle dispersion properties is reported. Hydrodynamic size, zeta potential, and isoelectric point (IEP) of ten laboratory synthesized TiO₂ samples, and one commercial Degussa TiO₂ sample (P25) dispersed in different solutions were characterized. Solution ionic strength and pH affect titania dispersion properties. The effect of monovalent (NaCl) and divalent (MgCl₂) inert electrolytes on dispersion properties was quantified through their contribution to ionic strength. Increasing titania particle surface area resulted in a decrease in solution pH. At fixed pH, increasing the particle surface area enhanced the collision frequency between particles and led to a higher degree of agglomeration. In addition to the synthesis method, TiO₂ isoelectric point was found to be dependent on particle size. As anatase TiO₂ primary particle size increased from 6 nm to 104 nm, its IEP decreased from 6.0 to 3.8 that also results in changes in dispersion zeta potential and hydrodynamic size. In contrast to particle size, TiO₂ nanoparticle IEP was found to be insensitive to particle crystal structure.     

The relationship between photo-catalytic performance and optical property over Si-incorporated TiO2

This study focuses on the relationship between photo-catalytic performance and optical property over Si-incorporated TiO₂. The Si-incorporated TiO₂ particles exhibited a pure structure of anatase having a particle size of less than 20 nm and surface area of more than 190 m²/g. The absorbance did not shift to a higher wavelength in spite of the incorporation of the Si ions, but the intensity of the photoluminescence (PL) curve was the smallest in the case of the 2.0 mol% Si-TiO₂, which was related to the recombination between the excited electrons and holes. Based on these results, the photodecomposition of methyl orange in the liquid reaction was enhanced over the 2.0 mol% Si-incorporated TiO₂ compared with that over pure TiO₂: Methyl orange at 10.0 ppm was completely decomposed after 100 min when 1.0 g of the 2.0 mol% Si-incorporated TiO₂ was used.     

Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase,photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.