Synthesis and characterization of silica—titania core—shell particles

Abstract Nearly monodispersed particles of silica were prepared and coated with uniform layers of titanium dioxide in anatase phase by hydrolysis and condensation of titanium butoxide. The coating thickness could be altered by adjusting the concentration of reactants (titanium butoxide and water) and the amount of added silica particles. Different coating thicknesses were deposited and studied using optical absorption spectroscopy, electron microscopy and Fourier transform infra-red spectroscopy. It was found that silica particles of size 170 ±5 nm were coated with 23±5 nm thick layer of titanium dioxide. Alternatively titania particles of size 340±5 nm were synthesized by controlled hydrolysis of titanium ethoxide in the presence of sodium chloride. These particles were further coated with 135±5 nm thick layer of silica to investigate changes in properties after changing the shell material     

<Emphasis Type="Italic">In Situ</Emphasis> Synthesis of Field-responsive Nanocrystalline BaTiO<Subscript>3</Subscript> Particles Modified with Functional Organics

Nanocrystalline BaTiO₃ particles modified with polarizable ligands were synthesized through hydrolysis of modified metal–organics below 100°C. BaTiO₃ precursor was synthesized from barium metal, titanium isopropoxide and polarizable ligands in a mixture of ethanol and 2-ethoxyethanol. The modified Ba–Ti complex alkoxide was hydrolyzed yielding BaTiO₃ particles modified with organics. The crystallinty of nanometer-sized particles was dependent upon the hydrolysis conditions, and increased with increasing water amount and time. The nanocrystalline particles were identified to be BaTiO₃ by X-ray diffraction (XRD) and electron diffraction. A fluid consisting of modified BaTiO₃ particles and silicone oil revealed a typical electrorheological (ER) behavior on applying DC field. The ER behavior was found to depend upon the kind of ligand. 4-Fluorobenzyloxy modifier revealed the highest yield stress in the ER measurement among 4-substitued benzyloxy ligands examined.     

Modeling and simulation of titanium dioxide nanoparticle synthesis with finite-rate sintering in planar jets

Numerical simulations of titanium dioxide nanoparticle synthesis in planar, non-premixed diffusion flames are performed. Titania is produced by the oxidation of titanium tetrachloride using a methane–air flame. The flow field is obtained using the two-dimensional Navier–Stokes equations. The methane–air flame and oxidation of titanium tetrachloride are modeled via one-step reactions. Evolution of the particle field is obtained via a nodal method which accounts for nucleation, condensation, coagulation, and coalescence with finite-rate sintering. The modeling of finite-rate sintering is accomplished via the use of uniform primary-particle size distribution. Simulations are performed at two different jet-to-co-flow velocity ratios as well as with finite-rate and instantaneous sintering models. In doing so we elucidate the effect of fluid mixing and finite-rate sintering on the particle field. Results show that highly agglomerated particles are found on the periphery of the eddies, where the collisions leading to nanoparticle coagulation occur faster than nanoparticle coalescence.     

Effect of reaction atmosphere on particle morphology of TiO2 produced by thermal decomposition of titanium tetraisopropoxide

Thermal decomposition of titanium tetraisopropoxide (TTIP) was carried out in varying reaction atmospheres: nitrogen, oxygen, and nitrogen plus water vapor. The effect of reaction atmosphere on the morphology, size, and crystalline structure of produced TiO₂ particles was studied. The reactor used was similar to the microreactor proposed earlier by Park et al. (2001, J. Nanopart. Res., 3, 309–319), but for a modification in the precursor evaporator. The reactor temperature was varied from 300 to 700°C and the TTIP concentration in the evaporator from 1.0 to 7.0 mol%, holding the reactor residence time at 0.7 s. The primary-particle size was in the range 25–250 nm, varying with operating condition. The crystalline structure was amorphous in nitrogen, a mixture of rutile and anatase in nitrogen plus water vapor, and anatase in oxygen atmospheres. In nitrogen, agglomerates composed of very small particles whose individual boundaries are not clearly distinguished were produced. In oxygen, the particles composing an agglomerate became larger and were clearly spherical. As the atmosphere was varied to the nitrogen plus water vapor, the particle size increased further. The variation of primary particle size with reaction atmosphere was discussed in comparison with previous experimental data.     

Raman spectroscopy studies of structural changes in hydrated titanium and tin dioxide gels under drying

Raman spectroscopy is used in this work to reveal an increase in the size of anatase crystallites in the process of drying of hydrated titanium dioxide gel and ordering of the structure of primary particles of hydrated tin dioxide gel under drying. Raman spectra are interpreted with account of dimensional effects. It is found that mechanical stresses that arise in the materials under drying have a great effect on the structural changes found there. TERMOXIDE Research and Production Company, 624051 Zarechnyi, Sverdlovsk Region, P/O Box 94, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 266–268, March–April, 1997.     

Synthesis and characterization of carbon coated nanoparticles produced by a continuous low-pressure plasma process

Core–shell nanoparticles coated with carbon have been synthesized in a single chamber using a continuous and entirely low-pressure plasma-based process. Nanoparticles are formed in an argon plasma using iron pentacarbonyl Fe(CO)₅ as a precursor. These particles are trapped in a pure argon plasma by shutting off the precursor and then coated with carbon by passing acetylene along with argon as the main background gas. Characterization of the particles was carried out using TEM for morphology, XPS for elemental composition and PPMS for magnetic properties. Iron nanoparticles obtained were a mixture of FeO and Fe₃O₄. TEM analysis shows an average size of 7–14 nm for uncoated particles and 15–24 nm for coated particles. The effect of the carbon coating on magnetic properties of the nanoparticles is studied in detail.     

Characteristics of electrohydrodynamically prepared titanium dioxide films

A titanium dioxide precursor sol flowing through a needle at a flow rate of 10^-10 m³ s^-1 was subjected to an electric field of 4.5 kV to generate droplets in the size range 0.3–6 μm. The droplets were collected on a silicon substrate to form uniformly thick, dense films. Raman spectroscopy, X-ray diffraction, field emission scanning electron microscopy and UV/Vis spectroscopy were used to characterize as-deposited and annealed films. Raman spectra show the annealed films were anatase phase with annealing converting it to the rutile phase. The energy bandgap of the titanium dioxide film annealed to 500 °C shows an indirect bandgap energy of 3.50 eV and a direct bandgap energy of 3.95 eV. PACS 81.15.Rs; 81.07.-b; 78.20.-e; 78.30.-j; 78.67.-n; 78.70.ck     

Microstructure of TiN coatings synthesized by direct pulsed Nd:YAG laser nitriding of titanium: Development of grain size,microstrain, and grain orientation

Pure titanium was irradiated by pulsed Nd:YAG laser irradiation in nitrogen atmosphere. As a result, nitrogen uptake and diffusion occurred and a TiN layer was synthesized at the titanium surface. These TiN coatings were analyzed by X-ray diffraction and the diffraction patterns were investigated in detail, in order to obtain more information about the physical processes during the coating formation. The diffraction peaks were fitted by Pearson VII profiles and the grain size and the microstrain were determined by the analysis of line broadening and peak shifts, using the Williamson–Hall and the Warren–Averbach formalisms. Additional single-line analyses were performed by means of the method of Langford and Keijser to obtain information about the preferred grain orientation and the texture development. The maximum grain size was about 100 nm and a corresponding average lattice strain of 0.002 was found. A relation between the treatment parameters and the coating properties, such as grain size and microstrain, can be shown. Thus, it was possible to determine optimal scan parameters for material processing and to establish the physical limits of the coating properties. PACS 81.65.Lp; 81.15.Fg; 61.10.Nz; 68.55.Jk     

Formation of localized silver centers on the surface of titanium dioxide films using a scanning tunneling microscope

A scanning tunneling microscope (STM) is used to create changes in the surface relief of titanium dioxide films containing adsorbed silver ions. Structures measuring ∼10 nm, which presumably consist of silver particles, form on the film surface as a result of the application of short pulses with an amplitude ⩾15 V to an STM probe operating in the tunneling-current regime. Zh. Tekh. Fiz. 67, 72–76 (June 1997)     

Characterization of TiO2 Smoke Prepared Using Gas‐Phase Hydrolysis of TiCl4

The formation of submicron TiO₂ smoke (a gas‐phase suspension) from titanium tetrachloride in a low‐pressure hydrolysis reaction in a simple reactor configuration has been studied. Particle size distribution, particle morphology and degree of crystallinity have been characterized as a function of reaction conditions. Highly crystalline anatase TiO₂ particles with narrow size distribution and smaller particle size were formed at high reactor temperature, while larger, amorphous particles were found at lower reactor temperatures. These results are consistent with literature studies. At 817 °C, small (450 nm), spherical, unagglomerated particles could be produced. A gas‐phase dispersion of these particles is intended for use as seeds in subsequent kinetic studies of titanium dioxide formation reactions involving a rapid compression methodology.     

One-step flame method for the synthesis of coated composite nanoparticles

A simple in situ flame coating method has been developed by designing a new type of coflow diffusion flame burner having a sliding unit. The sliding unit was shown to be very effective in finding a right position where the precursor for coating layer should meet with core particles. SiO₂-coated TiO₂ nanoparticles were first prepared and whether most surfaces of particles were coated was examined by both direct observation of particles through a transmission electron microscope and Zeta potential measurements. Mean core sizes varied from 28 to 109 nm and mean coating thickness was about 2.4 nm for silica-coated titania particles. By simply changing chemical precursors, we demonstrated that SiO₂-coated SnO₂, SnO₂-coated TiO₂, SiO₂–SnO₂-coated TiO₂ nanoparticles could be also synthesized.     

The Formation of Nano-sized Selenium–titanium Dioxide Composite Semiconductors by Photocatalysis

Nano-sized selenium (Se) particles were deposited onto titanium dioxide (TiO₂) by the photocatalytic reduction of selenate (Se(VI)) and selenite (Se(IV)) ions. Se particles deposition on TiO₂ was only observed in the presence of formic acid, which acted as the organic hole scavenger. The Se particles formed were crystalline. Se particles of different size could be formed onto the TiO₂ particles by manipulating experimental parameters such as pH and the Se precursor used. When Se(VI) ions were used as the precursor, the Se particles formed on TiO₂ were found to be spherical in shape, up to 6 times bigger than the TiO₂ particles (up to 145nm) and discretely formed on the TiO₂ particles. The growth and sphericity of the Se particles were explained in terms of electron transfer across the p–n junctions formed by the p-type Se and n-type TiO₂ semiconductors under illumination and the adsorption of the Se(VI) ions. The size of the Se particles were found to be dependent on the amount of Se(VI) photoreduced. When Se(IV) ions were used as the precursor for Se particles formation, the particles formed were much smaller than that of TiO₂ crystals (less than 25nm) and also more evenly dispersed on the TiO₂ particles.     

Zinc oxide films prepared by sol–gel spin coating technique

Zinc oxide (ZnO) thin films and micro- and nanostructures are very promising candidates for novel applications in emerging thin-film transistors, solar cells, sensors and optoelectronic devices. In this paper, a low-cost sol–gel spin coating technique was used to fabricate ZnO films on glass substrates. The sol–gel fabrication process of the ZnO films is described. The influence of precursor concentration on the material properties of the ZnO films was investigated. Atomic force microscopy and X-ray diffractometry were employed to examine the structural properties of the ZnO films. The optical properties of the ZnO films were characterized with ultraviolet–visible spectroscopy. The experimental results reveal that the precursor concentration in the sol–gel spin coating process exerts a strong influence on the properties of the ZnO films. The effects of the precursor concentration are discussed.     

Silica-based composite and mixed-oxide nanoparticles from atmospheric pressure flame synthesis

Binary TiO₂/SiO₂ and SnO₂/SiO₂ nanoparticles have been synthesized by feeding evaporated precursor mixtures into an atmospheric pressure diffusion flame. Particles with controlled Si:Ti and Si:Sn ratios were produced at various flow rates of oxygen and the resulting powders were characterized by BET (Brunauer–Emmett–Teller) surface area analysis, XRD, TEM and Raman spectroscopy. In the Si–O–Ti system, mixed oxide composite particles exhibiting anatase segregation formed when the Si:Ti ratio exceeded 9.8:1, while at lower concentrations only mixed oxide single phase particles were found. Arrangement of the species and phases within the particles correspond to an intermediate equilibrium state at elevated temperature. This can be explained by rapid quenching of the particles in the flame and is in accordance with liquid phase solubility data of Ti in SiO₂. In contrast, only composite particles formed in the Sn–O–Si system, with SnO₂ nanoparticles predominantly found adhering to the surface of SiO₂ substrate nanoparticles. Differences in the arrangement of phases and constituents within the particles were observed at constant precursor mixture concentration and the size of the resultant segregated phase was influenced by varying the flow rate of the oxidant. The above effect is due to the variation of the residence time and quenching rate experienced by the binary oxide nanoparticles when varying the oxygen flow rate and shows the flexibility of diffusion flame aerosol reactors.     

Sonochemical coating of magnetite nanoparticles with silica

Magnetite nanoparticles were coated with silica through the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) under ultrasonic irradiation. The ultrasonic irradiation was used to prevent the agglomeration of the magnetite particles and accelerate the hydrolysis and condensation of TEOS. TEM, DLS, XRF, VSM, TG and sedimentation test were used to characterize the silica-coated magnetite particles. The dispersibility of silica-coated magnetite particles in aqueous solution was improved significantly and the agglomerate particle size was decreased to 110 nm. It was found that the agglomerate particle size of silica-coated magnetite particles was mainly decided by the coating temperature and the pH value in the silica-coating process. The weight ratio of silica in silica-coated magnetite particles was mainly decided by the pH value in the silica-coating process. The dispersibility of silica-coated magnetite particles was mainly decided by the agglomerate particle size of the suspension. The oxidation of magnetite particles in air was limited through the coated silica. The magnetism of silica-coated magnetite particles decreased slightly after silica-coating.     

Structure and properties of the Ti-Si-N nanocrystalline coatings synthesized in vacuum by the electroarc method

Phase composition, defect substructure, and mechanical properties of the Ti-Si-N coating deposited on metal and ceramic-metal substrates by electroarc sputtering of the Ti-Si composite cathode in an ionized nitrogen atmosphere are investigated by the methods of modern materials science. It is established that coatings so formed with a thickness of ∼1–3 μm are superhard (H_v ∼ 50 GPa), and have the nanocrystalline structure (with crystalline sizes D = 7 nm) based on titanium nitride δ-TiN. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 46–51, February, 2007.     

A novel one-step electrochemical method to obtain crystalline titanium dioxide films at low temperature

A crystalline titanium dioxide (TiO₂) thin film on various metal substrates such as hot-dip-galvanized (HDG), electrochemically galvanized (EG) and cold rolled steel (CRS) has been successfully prepared for the first time by simple one-step electrodeposition method from alkaline aqueous solution containing potassium titanium oxalate and hydroxylamine. The as-grown titanium dioxide films are composed of a mixture of anatase, rutile and brookite structures and have a good crystalline state. The present investigation reveals that the electrochemical deposition of crystalline titanium dioxide on different metal substrates from single electrochemical bath is possible and very promising as a preparation method for industrial applications.     

Microwave-assisted fractional precipitation of magnetite nanoparticles using designed experiments

In this study, highly pure magnetite nanoparticle dispersed in water and an organic solvent (n-hexane) and its powder form were prepared in laboratory scale by the fractional precipitation using ammonium hydroxide and microwave heating in the presence of linoleic acid as capping agent. In order to overcome the oxidation of Fe²⁺ during magnetite formation ferrous ammonium sulfate, sodium azide, and fractional precipitation technique were used. The Fe₃O₄ products were investigated by XRD, LLS, EDX, TEM, viscosity measurements, and chemical analysis. The effects of seven main factors on the average diameter of magnetite particles were studied by a screening design. The analysis of the samples showed that this new modified method is able to produce pure magnetite particles in the range of 1–15 nm. The most important factors on the particle size reduction of magnetite were found to be the capping agent used and the pH of solution at the end of precipitation process. Data analysis was performed using Qualitek-4 and Minitab softwares.     

Biotinylated chitosan-based SPIONs with potential in blood-contacting applications

Haemocompatible biotinylated superparamagnetic nanoparticles (size range 300–700 nm) have been obtained by coating magnetite through ionic gelation with a mixture of chitosan and sodium tripolyphosphate, followed by subsequent functionalisation with biotin. The evaluations of their magnetic properties together with haemocompatibility tests have shown that these nanoparticles exhibit the prerequisite behaviour for use in magnetic field–assisted separations within biological systems.     

Synthesis and characterization of silicon-doped gallium oxide nanowires for optoelectronic UV applications

Silicon-doped gallium oxide nanowires have been synthesized by thermal methods using either a mixture of gallium oxide and silicon powders or metallic gallium with silicon powder as precursor materials. The growth mechanism has been found to be a vapour–liquid–solid (VLS) or vapour–solid (VS) process, respectively, depending on the precursor used. In the former case, silicon oxide droplets at the end of the nanowires have been observed. Their possible role during the growth of the nanostructures is discussed. Structural and morphological characterization of the doped nanowires has been performed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show a high crystalline quality and a uniform distribution of silicon along the nanowires. Room temperature cathodoluminescence (CL) in the SEM shows that slight variations in the composed UV–blue emission band appear due to the influence of Si impurities in the oxygen vacancy defect structure.