Synthesis of EuFeO<Subscript>3</Subscript> nanocrystals by glycine-nitrate combustion method

Powdered nanocrystalline europium orthoferrite was synthesized by the glycine-nitrate combustion method, and specific features of its formation were studied in relation to the redox composition of the starting reaction mixture. The products of glycine-nitrate combustion were characterized by X-ray fluorescence microanalysis, X-ray diffraction analysis, scanning electron microscopy, and helium pycnometry. It was found that, depending on the glycine-nitrate ratio, EuFeO₃ nanopowders with average crystallite size of 28 ± 3 to 46 ± 5 nm can be obtained. The crystallites form under the given conditions porous micrometer-size agglomerates with developed surface. Their morphology and characteristic size vary with the redox composition of the reaction mixture.     

Formation mechanism of nanocrystalline yttrium orthoferrite under heat treatment of the coprecipitated hydroxides

Formation of nanocrystalline yttrium orthoferrite of ～30 nm average crystallite size from coprecipitated iron(III) and yttrium hydroxides was studied by thermo-X-ray diffractometry and simultaneous thermal analysis over 25–900°C temperature range. A mechanism of physicochemical transformations leading to the formation of YFeO₃ nanoparticles was suggested.     

Influence of the synthesis conditions on the particle size and morphology of yttrium orthoferrite obtained from aqueous solutions

The size and morphology of yttrium orthoferrite nanoparticles were examined in relation to the synthesis conditions. The chemical composition of the crystalline phases resulted from heat treatment of the samples at 650°C varies with the procedure of coprecipitation of yttrium and iron(III) hydroxides. At the same time, heat treatment at 750°C leads to formation of yttrium orthoferrite solely. The size of the YFeO₃ crystals is weakly dependent on the initial composition preparation procedure, and the size and shape of their agglomerates are sensitive to the conditions of coprecipitation of the hydroxides.     

Production of Zinc-Doped Yttrium Ferrite Nanopowders by the Sol–Gel Method

Zinc-doped yttrium orthoferrite nanocrystals having the perovskite structure were prepared by coprecipitation of yttrium, zinc, and iron hydroxides. The limiting zinc doping level of the yttrium ferrite to yield a ZnFe₂O₄ spinel second phase was determined. The yttrium orthoferrite particle size was found to be a nonmonotone function of dopant concentration. The specific magnetization of yttrium ferrite nanocrystals increases with increasing zinc doping level from 0.242 A m²/kg (in undoped YFeO₃) to 1.327 A m²/kg (the ratio (1–x)YFeO₃: xZn (x = 0.4)) at Т = 300 K in 1250-kA/m field. A zinc ferrite impurity in samples enhances the ferromagnetism of the material.     

Formation of BiFeO<Subscript>3</Subscript> Nanoparticles Using Impinging Jets Microreactor

The influence of the reactants mixing in an impinging jets microreactor of the formation of singlephase nanocrystals of bismuth orthoferrite has been studied. The 30–100 nm amorphous particles are formed under the impinging jets microreactor conditions, which are converted in bismuth orthoferrite with mean crystallite size 17 nm at 420°С.     

Size-resolved kinetic measurements of aluminum nanoparticle oxidation with single particle mass spectrometry

Aluminum nanoparticles are being considered as a possible fuel in advanced energetic materials application. Of considerable interest therefore is a knowledge of just how reactive these materials are, and what the effect of size on reactivity is. In this paper we describe results of size resolved oxidation rate using a recently developed quantitative single particle mass spectrometer (SPMS). Aluminum nanoparticles used were either generated by DC Arc discharge or laser ablation, or by use of commercial aluminum nanopowders. These particles were oxidized in an aerosol flow reactor in air for specified various temperatures (25-1100 degrees C), and subsequently sampled by the SPMS. The mass spectra obtained were used to quantitatively determine the elemental composition of individual particles and their size. We found that the reactivity of aluminum nanoparticles is enhanced with decreasing primary particle size. Aluminum nanoparticles produced from the DC Arc, which produced the smallest primary particle size (approximately 19 nm), were found to be the most reactive (approximately 68% aluminum nanoparticles completely oxidized to aluminum oxide at 900 degrees C). In contrast, nanopowders with primary particle size greater than approximately 50 nm were not fully oxidized even at 1100 degrees C (approximately 4%). The absolute rates observed were found to be consistent with an oxide diffusion controlled rate-limiting step. We also determined the size-dependent diffusion-limited rate constants and Arrehenius parameters (activation energy and pre-exponential factor). We found that as the particle size decreases, the rate constant increases and the activation energy decreases. This work provides a quantification of the known pyrophoric nature of fine metal particles.     

Formation of nanocrystalline BiFeO<Subscript>3</Subscript> under hydrothermal conditions

The formation of bismuth orthoferrite under hydrothermal conditions at temperature 160, 180, or 200°С and pressure 100 MPa in aqueous solution of potassium hydroxide has been studied. The determined composition and structure of polycrystalline phase with sillenite structure have evidenced its formation at the interface of the crystallites of amorphous iron oxide. It has been shown that the formation of polycrystalline round-shaped BiFeO₃ particles with size about 20 μm occurs via aggregation of perovskite-type phase crystallites (38–70 nm). Pycnometric density of BiFeO₃ and the amorphous phase has been determined, and Mossbauer spectra reflecting the state of iron in the phases coexisting during the formation of BiFeO₃ have been analyzed.     

The role of pre-nucleus states in formation of nanocrystalline yttrium orthoferrite

Formation of hexagonal and orthorhombic YFeO₃ nanocrystals from an amorphous phase upon heat treatment of glycine–nitrate combustion products has been studied. The initial X-ray amorphous precursor has been shown to have pre-nucleus species of two types. Rapid formation of h-YFeO₃ nanocrystals is explained by the presence of structurally similar pre-nucleus species in the precursor, while o-YFeO₃ nanocrystals are formed much more slowly through recrystallization of the hexagonal and amorphous yttrium orthoferrite phases.     

Sol-gel formation and properties of nanocrystals of solid solutions Y1 − x Ca x FeO3

Nanopowders of ferrites Y_{1 ? x} Ca _x FeO₃ (x = 0.1, 0.2, and 0.3) were prepared by chemical coprecipitation of cations Y³⁺, Ca²⁺, and Fe³⁺ by an aqueous sodium carbonate solution. It was found that an increase in the calcium content leads to a decrease in the size of nanocrystals, the average size of which is 25–50 nm. Doping of yttrium orthoferrite with the doubly charged calcium ion enhances magnetization and decreases coercivity in samples.     

Preparation and study of colloid-chemical and optical properties of the nanocrystals of zinc sulfide modified with amino acids

The effect of some amino acids: cysteine, methionine, glycine, lysine, and aspartic acid, on the formation of nanoparticles of zinc sulfide in aqueous solutions at pH 5.5–10.0 was investigated. A method of obtaining stable sols of ZnS particles of 2–4 nm size with narrow distribution of the particle size was developed. The investigated nanoparticles are shown to be sphalerite, the cubic modification of zinc sulfide. The ZnS sols modified with methionine and glycine show intense luminescence at 415–425 nm.     

Tungsten Carbide and Vanadium Carbide Nanopowders Synthesis in DC Plasma Reactor

Air–methane and nitrogen–hydrogen DC thermal plasma confined flows were used to synthesize tungsten carbide and vanadium carbide nanopowders. The influence of input process parameters such as C/W and C/V molar ratio, plasma jet chemical composition, plasma jet enthalpy, and reactants flow rates on the average nanoparticle size, chemical and crystallographic phase compositions were investigated. During post heat treatment, the synthesized MeC_1?x nanopowders were fully carburized to monocarbides WC and VC with particles size less than 80 and 40 nm correspondently.     

Raman study of the variation in anatase structure of TiO<Subscript>2</Subscript> nanopowders due to the changes of sol–gel synthesis conditions

TiO₂ nanopowders were produced by sol–gel technique under different synthesis conditions. XRD results have shown that obtained nanopowders are in anatase phase, with the presence of a small amount of highly disordered brookite phase, whereas nanocrystallite size and amount of brookite slightly depend on sol–gel synthesis conditions. Raman measurements confirm these results. The analyses of the shift and width of the most intensive anatase E _g Raman mode by phonon confinement model suggest that anatase crystallite size should be in the range between 11 and 15 nm, what is in excellent correlation with XRD results. Obtained results have shown that Raman spectroscopy is a highly sensitive method for the estimation of anatase crystallite size as well as brookite content in TiO₂ nanopowders synthesized by variable sol–gel synthesis conditions.     

Cobalt particle size effects in the Fischer-Tropsch reaction studied with carbon nanofiber supported catalysts

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.     

Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting

Laser-induced breakdown spectroscopy of particles ejected by optical catapulting is discussed for the first time. For this purpose, materials deposited on a substrate were ejected and transported from the surface in the form of a solid aerosol by optical catapulting using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at 1064 nm. A Q-switched Nd:YAG laser at 532 nm was used for chemical characterization of the particles by laser-induced breakdown spectroscopy. Both lasers were synchronized in order to perform suitable spectral detection. The optical catapulting was optimized and evaluated using aluminum silicate particles, nickel spheres, and quartz and stainless steel particles. Experimental parameters such as the interpulse delay time, the sampling distance, the laser fluence, the sampling rate and the particle size have been studied. A correlation between these parameters and the particle size is reported and discussed.     

On the measurement of gold nanoparticle sizes by the dynamic light scattering method

The application of the dynamic light scattering (DLS) method for determining the size distribution of colloidal gold nanoparticles in a range of 1–100 nm is discussed. It is shown that rotational diffusion of nonspherical strongly scattering particles with sizes of larger than 30–40 nm results in the appearance of a false peak in a size range of about 5–10 nm. In this case, the uncritical application of the DLS method may yield particle volume or number size distributions different from those obtained by transmission electron microscopy. For weakly scattering particles with diameters of smaller that 20 nm, the DLS method demonstrates an additional peak of intensity distribution in the region of large sizes that is related to particle aggregates or byproduct particles rather than individual nanoparticles. Practical methods for solving the problem of false peaks are discussed. It is established that the width of the DLS distribution does not correspond to transmission electron microscopy data and is overestimated. The advantages and drawbacks of the methods are compared and it is noted that, at present, the DLS method is the only instrument suitable for nonperturbative and sensitive diagnostics of relatively slow aggregation processes with characteristic times on the order of 1 min. In particular, this method can be used to diagnose gold nanoparticle conjugate aggregation initiated by biospecific interactions on their surface.     

Electrical surface charge and potential of hematite/yttrium oxide core–shell colloidal particles

Interest in the synthesis of composite colloidal particles consisting of a core and shell with different compositions stems from the fact that such particles can be useful in processes where the properties of both core (e.g., size and shape homogeneity, ease of preparation in large amounts, magnetic characteristics, etc.) and shell (interfacial properties, porosity, chemical stability, etc.) might be of interest. However, the applicability must be based on a proper characterization of those properties. In this work, colloidal spheres of hematite (α-Fe₂O₃) were used as nuclei of mixed particles where the shell is yttrium oxide. The electrical properties of the aqueous interface are compared to those of the pure oxides by means of potentiometric titration of their surface charge and potential against pH, as a function of indifferent electrolyte concentration. It is found that the mixed particles efficiently mimic yttrium oxide, since the behavior of their surface electrical characteristics closely resembles that of the latter compound. Differences are found, however, that can be ascribed to an incomplete or porous coverage, but such divergences are of little significance when an overall comparison is carried out. Received: 30 January 2001 Accepted: 11 July 2001     

Control over structural-dimensional characteristics of tungsten disulfide particles in aerosol-assisted chemical vapor deposition

Solutions of ammonium thiotungstate in dimethylformamide were used to synthesize spherical tungsten disulfide particles with average radius of 500–100 nm by the method of aerosol-assisted chemical vapor deposition. Nanoparticles with composition close to stoichiometric tungsten disulfide are formed at pyrolysis temperatures not lower than 800°C. It was found that the average particle radius linearly decreases as the reagent concentration in solution becomes lower, and the nebulizer power has no effect within the range under study on the size characteristics and structure of the particles obtained. It was demonstrated that the particles have a layered structure that is formed in all probability by S–W–S packets, which must provide high antifriction properties of the material in its use as a high-temperature solid lubricant. The results obtained indicate that the size of tungsten disulfide particles can be controlled in a wide range in the course of the aerosol-assisted chemical vapor deposition. This may be of interest for developing a technology for creating high-temperature wear-resistant antifriction coatings.     

High-surface-area catalyst design: Synthesis, characterization, and reaction studies of platinum nanoparticles in mesoporous SBA-15 silica

Platinum nanoparticles in the size range of 1.7-7.1 nm were produced by alcohol reduction methods. A polymer (poly(vinylpyrrolidone), PVP) was used to stabilize the particles by capping them in aqueous solution. The particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations demonstrate that the particles have a narrow size distribution. Mesoporous SBA-15 silica with 9-nm pores was synthesized by a hydrothermal process and used as a catalyst support. After incorporation into mesoporous SBA-15 silica using low-power sonication, the catalysts were calcined to remove the stabilizing polymer from the nanoparticle surface and reduced by H2. Pt particle sizes determined from selective gas adsorption measurements are larger than those determined by bulk techniques such as XRD and TEM. Room-temperature ethylene hydrogenation was chosen as a model reaction to probe the activity of the Pt/SBA-15 materials. The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm. The hydrogenolysis of ethane on Pt particles from 1.7 to 7.1 nm was weakly structure sensitive with smaller particles demonstrating higher specific activity. Turnover rates for ethane hydrogenolysis increased monotonically with increasing metal dispersion, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C2H6 hydrogenolysis than the low index planes that dominate in large particles. An explanation for the structure sensitivity is suggested, and the potential applications of these novel supported nanocatalysts for further studies of structure-activity and structure-selectivity relationships are discussed.     

New preparation method of gold nanoparticles on SiO2

It is shown that adsorption of the [Au(en)(2)](3+) cationic complex can be successfully employed for the deposition of gold nanoparticles (1.5 to 3 nm) onto SiO(2) with high metal loading, good dispersion, and small Au particle size. When the solution pH increases (from 3.8 to 10.5), the Au loading in the Au/SiO(2) samples increases proportionally (from 0.2 to 5.5 wt %), and the average gold particle size also increases (from 1.5 to 2.4 nm). These effects are explained by the increase in the amount of negatively charged sites present on the SiO(2) surface, namely, when the solution pH increases, a higher number of [Au(en)(2)](3+) species can be adsorbed. Extending the adsorption time from 2 to 16 h gives rise to an increase in the gold loading from 3.3 to 4.0 wt % and in the average particle size from 1.8 to 2.9 nm. Different morphologies of gold nanoparticles are present as a function of the particle size. Particles with a size of 3-5 nm show defective structure, some of them having a multiple twinning particle (MTP) structure. At the same time, nanoparticles with an average size of ca. 2 nm exhibit defect-free structure with well-distinguishable {111} family planes. TEM and HAADF observations revealed that Au particles do not agglomerate on the SiO(2) support: gold is present on the surface of SiO(2) only as small particles. Density functional theory calculations were employed to study the mechanisms of [Au(en)(2)](3+) adsorption, where neutral and negatively charged silica surfaces were simulated by neutral cluster Si(4)O(10)H(4) and negatively charged cluster Si(4)O(10)H(3), respectively. The calculation results are totally consistent with the suggestion that the deposition of gold takes place according to a cationic adsorption mechanism.     

Agglomeration of alumina submicronparticles by silica nanoparticles: application to processing spheres by colloidal route

In aqueous media, heterocoagulation between submicronic alumina (400 nm) and nanometric silica (25 nm) leads to the adsorption of silica on the alumina surface. By controlling the coverage rate of alumina particles, this adsorption destabilizes the suspension that leads to a very porous network of agglomerated particles. This work shows that the structure is all the more open as the density of charge carried by the two oxides is high and the ionic strength in the suspension low. From such a flocculated suspension, a new colloidal process to fabricate ceramic spheres is proposed which is based on a size increase of agglomerates. Under a controlled rotation of the vessel, electrostatic attraction between the surface charges of opposite polarity induces a size increase of agglomerates until the formation of spheres occurs. It has been shown that the mechanism of growth is poisoned by species adsorbed such as ions. Nevertheless, this new process proves very promising because it leads to a narrow size distribution of spheres by colloidal way, which can be subsequently consolidated by sintering, with a smooth surface.