Dissolution kinetics of oxidic nanoparticles: The observation of an unusual behaviour

The solubility of nanoparticles was measured in aqueous solution as a function of time, and oxides of aluminium, silicon, titanium, and zirconium were investigated. Our solubility results show a maximum at the beginning of the dissolution process, whereas over time, the solubility levels are shown to decrease. Depending on the special conditions the solubility maximum may exceed the long-time solubility of the nanoparticles by several orders of magnitude. This behaviour is called as kinetic size effect. The extent of the effect depends on the size, surface tension and mass of the particles exposed to dissolution. It will always be of practical interest if a larger quantity of nanoparticles is brought into contact with a solvent, even when the equilibrium solubility data appears negligible. A rigorous thermodynamic and kinetic analysis of a colloidal system, which includes nucleation, particle growth, Ostwald ripening, and dissolution of particles, shows at least a qualitative agreement between all the experimental results and model calculations.     

Synthesis of nanoscale titanium dioxide by precipitation using supercritical anti-solvent

The method of synthesis of nanoscale titanium dioxide from organic precursor using supercritical fluid technology was suggested. It was demonstrated that the obtained product consists of amorphous particles with a narrow size distribution. It possesses a high porosity and a large value of specific surface area. The effect of temperature, pressure, and concentration of the titanium(IV) isopropoxide precursor on the average size of TiO₂ nanoparticles has been studied. The method allows preparation of titanium dioxide free from precursor compounds. Complex physico-chemical study of the obtained product has been performed.     

Characterization of titanium dioxide nanoparticles using molecular dynamics simulations

Molecular dynamics simulations of titanium dioxide nanoparticles in the three commonly occurring phases (anatase, brookite, and rutile) are reported. The structural properties inferred by simulated X-ray diffraction patterns of the nanoparticles were investigated. The titanium-oxygen bond length as a function of size, phase, and temperature was determined and was found to be dependent on the coordination environment of the titanium and independent of phase and size. The equilibrium Ti-O bond length is 1.86 A for a four-coordinated titanium ion, 1.92 A for a five-coordinated titanium ion, and 1.94 A for an octahedral titanium ion. Smaller nanoparticles are characterized by a higher fraction of titanium ions that are four and five coordinated, due to the larger surface area-to-volume ratios. The surface energies for anatase, rutile, and brookite particles were reported. The surface energy of the nanoparticle increases and approaches a constant value as the particle gets bigger. The surface energies of small rutile particles are higher than that for anatase particles of a similar size, consistent with anatase being the more stable phase of nanocrystalline titanium dioxide.     

A Monte Carlo Simulation of the Development of Surface Roughness and Its Effect on the Dissolution Kinetics of SiO2 Aerogels

The development of surface roughness during dissolution of spherical particles is studied by the Monte Carlo method. The simulation results are used to analyze the dissolution kinetics of silicon dioxide aerogels in an aqueous solution of alkali (NaOH). The suggested model is shown to be suitable for describing the experimental dissolution curves obtained for aerogels with a small diameter of primary particles (3.5 and 2.9 nm). For aerogels with larger particles, a good agreement with the experiment can be achieved under the additional assumption that only part (p < 1) of the particle surface is originally active in dissolution; the best agreement is reached at p 0.5. In the kinetic regime of dissolution, the dissolution rate may be more than three times higher (owing to the formation of rough surfaces of primary particles with relatively large diameters, 40 atoms or more) than the rate calculated for the same parameters within the framework of the modified Delmon model, which does not make allowance for the development of roughness. Relatively small particles (with the diameter of less than 15 atoms) are dissolved before a significant roughness can be developed; therefore, the kinetic curves obtained for both models have virtually identical shapes in this case. The formation of roughness has an especially large effect on the dissolution of intermediate-size particles, whose dissolution time has the same order of magnitude as the time required for establishment of the steady-state roughness.     

Kinetic and mechanistic investigations of the light induced formation of gold nanoparticles on the surface of TiO2

The kinetics of the formation of gold nanoparticles on the surface of pre-illuminated TiO(2) have been investigated using stopped-flow technique and steady state UV/Vis spectroscopy. Excess electrons were loaded on the employed nanosized titanium dioxide particles by UV-A photolysis in the presence of methanol serving as hole scavenger, stored on them in the absence of oxygen and subsequently used for the reduction of Au(III) ions. The formation of gold nanoparticles with an average diameter of 5 nm was confirmed after mixing of the TiO(2) nanoparticles loaded with electrons with aqueous solution of tetrachloroaureate (HAuCl(4)) by their surface plasmon absorbance band at 530 nm, as well as by XRD and HRTEM measurements. The rate of formation of the gold nanoparticles was found to be a function of the concentration of the gold ions and the concentration of the stored electrons, respectively. The effect of PVA as a stabilizer of the gold nanoclusters was also studied. The observed kinetic behavior suggests that the formation of the gold nanoparticles on the TiO(2) surface is an autocatalytic process comprising of two main steps: 1) Reduction of the gold ions by the stored electrons on TiO(2) forming gold atoms that turn into gold nuclei. 2) Growth of the metal nuclei on the surface of TiO(2) forming the gold particles. Interestingly, at higher TiO(2) electron loading the excess electrons are subsequently transferred to the deposited gold metal particles resulting in "bleaching" of their surface plasmon band. This bleaching in the surface plasmon band is explained by the Fermi level equilibration of the Au/TiO(2) nanocomposites. Finally, the reduction of water resulting in the evolution of molecular hydrogen initiated by the excess electrons that have been transferred to the previously formed gold particles has also been observed. The mechanism of the underlying multistep electron-transfer process has been discussed in detail.     

Enhancement of titanium dioxide photocatalysis by water-soluble fullerenes

Fullerenes are known for their unique electronic properties including high electron affinity. Although use of fullerenes for scavenging photo-generated electrons from titanium dioxide particles has been demonstrated, no attempts have been made to utilize the unique properties of fullerenes to increase the efficacy of photocatalysis. The present study has demonstrated that a mixture of water-soluble polyhydroxy fullerenes (PHF) and titanium dioxide (anatase polymorph) enhances photocatalytic degradation of organic dye. The PHF molecules adsorbed to the surface of titanium dioxide due to electrostatic forces, with adsorption density being higher at lower pH values. The surface coverage of titanium dioxide nanoparticles by PHF molecules determined the extent of enhancement, with an optimum dosed weight ratio of PHF to titanium dioxide at 0.001. Hydroxylation and concomitant solubilization of fullerenes allow their unique electronic properties to be harnessed for photocatalysis.     

Preparation of pickering-emulsion-based capsules with shells composed of titanium dioxide nanoparticles and polyelectrolyte layers

A procedure has been proposed for the preparation of capsules with shells composed of titanium dioxide nanoparticles and polymers via the formation of Pickering emulsions (colloidosomes) followed by layer-by-layer deposition of polyelectrolytes. The feasibility of stabilizing oil droplets of emulsions by spontaneous adsorption of partly hydrophobized nanoparticles of anatase-form titanium dioxide at the oil/water interface has been studied. Conditions have been determined for the formation of stable colloidosomes and the subsequent layer-by-layer deposition of oppositely charged polyelectrolytes onto their surfaces. It has been shown that hydrophobic dyes may be encapsulated using the procedure developed for the preparation of the capsules. The photocatalytic activity of TiO₂ particles occurring in the shell has been demonstrated by the example of degradation of Nile red which is incorporated in the oil core of a capsule.     

Flame-retardancy and photocatalytic properties of cellulosic fabric coated by nano-sized titanium dioxide

We have investigated the effect of titanium dioxide as a durable finish on the flammability and photocatalytic self-cleaning of cellulosic fabric. Nano-sized titanium dioxide particles were successfully synthesized and deposited onto cellulosic fibers with good compatibility, significant photocatalytic self-cleaning activity, and flame-retardancy properties using the sol–gel process at low temperature. The photocatalytic activity was tested by measuring the photodegradation of methylene blue under ultraviolet–visible illumination, and also flame-retardancy effect was tested by flammability tester. The samples have been characterized by several techniques such as scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, X-ray diffraction, and thermogravimetric analysis. The titanium dioxide nanoparticles with 10–20 nm in size have been found to form a homogeneous thin film on the fiber surface which shows efficient photocatalytic and flame-retardancy properties. This preparation technique can also be applied to new fabrics to create self-cleaning and flame-retardancy properties in them.     

Preparation of TiO<Subscript>2</Subscript> nanoparticles by sonochemical method,isotherm, thermodynamic and kinetic studies on the sorption of strontium

The objective of this study is to evaluate the use of titanium dioxide nanoparticles which were prepared by novel sonochemical method as an ion exchange material for the removal of Sr from aqueous solution. The pH effect on the Sr²⁺ sorption was investigated. The data obtained have been correlated with Freundlich, Temkin and Dubinin–Radushkevich (D–R) isotherm models. Thermodynamic parameters fort he sorption system have been determined at four temperatures. Simple kinetic models have been applied to the rate and isotherm sorption data and the relevant kinetic parameters were determined from the graphical presentation of these models at 298°K. Results explained that the pseudo second-order sorption mechanism is predominant and the overall rate constant of sorption process appears to be controlled by chemical sorption process. The value of sorption energy E = 13 kJ/mol at 298°K and the value of Gibbs free energy ∆G° = 3,222 kJ/mol at 298°K prove that the sorption of strontium on titanium dioxide nanoparticles is an endothermic and non-spontaneous process.     

In-Situ Formation of Silver Nanoparticles in PEI

Nanocomposites consisting of spherical particles of Ag were prepared using a single step in situ method whereby Ag is introduced into the dissolved polymer via dissolution as the organometallic complex Ag TFA in the same solvent as the polymer. The kinetic rate of formation of the particles is determined using WAXS and SAXS measurements. Nanoparticle formation is found to depend on reducing the solvent/polymer ratio, which leads to de-solvating the metal complex. This destabilizes the metal precursor complex, causing it to degrade and the metal particles to phase separate by a thermodynamic driving force. The size of the nanoparticles varies with the cure temperature and the conditions affecting molecular mobility.     

Aqueous Long-Term Solubility of Titania Nanoparticles and Titanium(IV) Hydrolysis in a Sodium Chloride System Studied by Adsorptive Stripping Voltammetry

The solubility of industrially produced titanium dioxide nanoparticles has been studied in aqueous sodium chloride media in the pH range 1 to 13 at 25 °C by using adsorptive stripping voltammetry (AdSV). Kinetic dissolution curves have been obtained as well as long-term solubilities that provide an approximation of the equilibrium solubilities. The titania nanoparticles used in the dissolution experiments have been characterized by nitrogen sorption measurements, XRD and colloid titration. The equilibrium solubilities and titanium(IV) speciation and their dependences on pH have been modelled by assuming the formation of the mononuclear titanium hydroxo complexes [Ti(OH) _n ]⁽⁴⁻ⁿ⁾⁺ (n=2 to 5) to be the only titanium species present. The solubility product of titanium dioxide and equilibrium constants for titanium(IV) hydrolysis, calculated from the AdSV solubility data, are presented.     

A Study of the Influence of Nanoparticles on the Properties of Drilling Fluids

The effect of nanoparticles with different compositions and sizes on the rheological properties, filtration losses, and lubricating ability of drilling fluids has been experimentally studied. Nanoparticles of silicon, aluminum, and titanium oxides have been examined, while an aqueous bentonite suspension with a solid phase mass fraction of 5% has been used as a basic model of a drilling fluid. The concentrations and sizes of nanoparticles in the drilling fluids have been varied from 0.25 to 2 wt % and from 5 to 100 nm, respectively. It has been shown that the addition of nanoparticles substantially changes the properties of the drilling fluids. In contrast to suspensions of particles with macro- and microscopic sizes, the rheological parameters, filtration losses, and lubricating and sticking abilities of the suspensions containing nanoparticles depend on the size and nature of the latter and vary markedly already at low nanoparticle concentrations.     

CO2 utilization as gas antisolvent for the pharmaceutical micro and nanoparticle production: A review

A reduction in particle size improves the solubility and bioavailability of pharmaceuticals. The traditional methods utilized in this regard are associated with problems so the use of supercritical fluid has been highlighted in recent decades. To prepare nanoparticles by employing the gas antisolvent (GAS) technique, a specific amount of solution (solute dissolved in organic solvent) was loaded into a cell in the oven. The supercritical carbon dioxide was injected and dissolved into the organic solvent. Therefore, volume expansion occurred and the solute was precipitated with a new particle size distribution on the filter at the end of the cell. This technique exhibits advantages such as particle size control, solvent-free product, and low-temperature process. Many experimental and modeling research has been conducted to synthesize nano- and microparticles based on the GAS process. The present study seeks to review the effective factors and literature on the GAS technique. All parameters affecting the GAS process including pressure, temperature, antisolvent addition rate, initial soluble concentration, and solvent were investigated. Volume expansion, thermodynamic modeling, and kinetic modeling of the GAS process were reviewed. A comparison was conducted between the advantages and disadvantages of this method with other methods of producing nanoparticles with supercritical fluid.     

Synthesis of uniform anatase TiO2 nanoparticles by gel-sol method. 1. Solution chemistry of Ti(OH)(4-n)+(n) complexes

The mole fractions of hydroxo complexes of titanium(IV) ion in an aqueous solution with 0.10 mol dm(-3) NaClO4 at 25 degrees C have been determined as a function of pH by a newly developed analytical procedure based on UV spectrophotometry, using a metastable homogeneous solution of 1.25 x 10(-4) mol dm(-30 in total concentration of Ti(IV). Also, the total concentration of the hydroxo complexes in equilibrium with Ti(OH)4 solid, or the solubility of Ti(OH)4 solid in an inhomogeneous system, has been obtained by ICP measurement for the solution phase. A combination of these data yielded the absolute concentration of each complex species in equilibrium with Ti(OH)4 solid. Finally, Ti(OH)+3 complex has been assigned to the precursor for the formation of anatase TiO2 nanoparticles transformed from Ti(OH)4 gel from a comparison between the above equilibrium data and a kinetic study on the formation rate of the anatase TiO2 particles in the gel-sol system.     

Titanium Dioxide/Electrolyte Solution Interface: Electron Transfer Phenomena

Electron transfer between a titanium dioxide/electrolyte solution interface has been studied. As found by other researchers of similar interfaces (TiO(2)- and ZnO-electrolyte solution), a slow consumption of OH(-) ions takes place in this type of interface. A theoretical model has been developed for calculating the change in the Fermi energy of both electrolyte solution and semiconductor, showing that ion consumption from the solution is favoured by the decrease of the difference between their Fermi energies. A kinetic constant (upsilon) is found to characterise the consumption process, its value increasing with electrolyte and semiconductor mass concentrations. Furthermore, this process may be used to estimate the point of zero charge of a titanium dioxide colloidal dispersion. Copyright 2000 Academic Press.     

Synthesis of Titanium Dioxide Nanoparticles for Photocatalytic Degradation of Cyanide in Wastewater

Cyanide is highly toxic, and although various approaches have been employed for its remediation, these methods do not have the required performance. This report describes the synthesis of titanium dioxide nanoparticles with maximum photocatalytic properties and their application for the treatment of cyanide in wastewater. Three types of crystalline titanium nanoparticles were synthesized: anatase, rutile, and a biphasic mix of rutile and anatase. The phase and microscopic properties were characterized by X-ray diffraction and scanning electron microscopy. The specific surfaces of the particles were determined, and their photocatalytic activity was studied under the ultraviolet irradiation for cyanide degradation. The results indicate that an increase in sulfate ion leads to changes in the phase ratios of rutile to anatase. The diameter of the nanoparticles was between 18 and 22 nm, and they displayed high photocatalytic properties. The biphasic form of the titanium dioxide nanoparticles demonstrated the highest removal of cyanide. Complete degradation of cyanide was observed in a pilot scale experiment.     

聚丙烯酸在纳米TiO2表面吸附行为的研究

讨论了聚丙烯酸在纳米TiO₂水悬浮体系中的吸附行为.红外光谱分析和吸附实验结果表明,纳米TiO₂通过氢键吸附PAA.PAA吸附量随着浓度的升高而增大直至饱和吸附量,且分子量越大,饱和吸附量越大.pH值增大,则饱和吸附量减小.在相同条件下,表面吸附层的厚度随PAA分子量、浓度和pH值增大而增大.这是由PAA在颗粒表面构型的变化所致.吸附PAA后的纳米TiO₂的表面电荷密度和ζ电位发生变化,pHiep值向低值方向移动.表面吸附自由能的计算结果说明,PAA在纳米TiO₂表面的吸附是自发过程.     

The influence of ionic and nonionic surfactants on aggregative stability and electrical surface properties of aqueous suspensions of titanium dioxide

The influence of concentration of nonionic TRITON X-100 and anionic ATLAS G-3300 surfactants, and pH of medium on the size and zeta-potential of TiO2 particles in the water suspensions has been studied. Suspensions have been prepared by mixing of the titanium dioxide in the suitable mediums at 10 min and 6 h correspondingly. It was established, that the duration of mixing of the suspensions has an essential influence on the dependence of zeta-potential and size of particles versus concentration of surfactant. However, the duration of mixing does not influence the dependence of electrical conductivity and pH of the suspensions on concentration of surfactant. It is shown that anionic ATLAS G-3300 surfactant is more effective stabilizator of aqueous suspensions of titanium dioxide, than nonionic surfactants of TRITON X-100. It is found that hydrophobic interaction has important role in the processes of stabilization of suspensions for nonionic surfactant, and for anionic surfactant--moving of psi(delta)-planes into solution's depth.     

Leaching of Celestite in Sodium Hydroxide Solutions and Kinetic Modelling

In this study, the dissolution kinetics of celestite in solutions of sodium hydroxide was investigated by batch process. The results showed that the parameters which had the greatest effect on the dissolution of celestite in sodium hydroxide solutions were reaction temperature, the concentration of sodium hydroxide and stirring speed. It was determined that the dissolution rate increased with increased stirring speed, sodium hydroxide concentration, reaction time and temperature and decreased with increasing particle size and solid-liquid ratio. The leaching process fitted the shrinking core model with diffusion through the product layer model as the rate-determining step. The activation energy of the dissolution of celestite was calculated as 62.24?kJ/mol. A semi-empirical kinetic model was obtained for dissolution of celestite in sodium hydroxide solution.     

The leaching kinetics of brannerite ore in sulfate solutions with iron(III)

A kinetic study of the leaching of powdered brannerite ore by sulfuric acid has been investigated. The effects of stirring speed ranging from 100 to 1,000?min^?1, particle size ranging from 20 to 120???m, concentration of Fe(III) ranging from 0.0025 to 0.20?M, acid concentration ranging from 0.1 to 2.0?M and temperature ranging from 15 to 90?°C on uranium dissolution are reported. The dissolution rate was founded to be significantly influenced by the temperature and concentration of the acid in solution. The experimental data for the dissolution rates of uranium have been analyzed with the shrinking-core model for reaction control. The apparent activation energy for the dissolution of uranium has been evaluated using the Arrhenius expression.