A. F. Craievich

SAXS study of the kinetics of formation of ZnO colloidal suspensions

We have investigated, by in situ small-angle X-ray scattering (SAXS), the kinetics of formation of zinc oxide colloidal suspensions obtained after refluxing alcoholic solution of zinc acetate and catalysed by lithium hydroxide. The experimental results demonstrate that the suspensions are composed of colloidal spheroidal particles with a multimodal size distribution. The average radius of the main mode, approximately 2 nm, is invariant but the number of these basic particles continuously increases for increasing hydrolysis reaction time. The other two modes correspond to particles with average radii close to 6 and 10 nm, respectively. The larger particles are formed by coagulation of the smaller ones.

A two-crystal monochromator for the UVX ring of LNLS

A two‐crystal x‐ray monochromator was built at LNLS. It is based on the Golovchenko [J. A. Golovchenko, R. A. Levesque, and P. L. Cowan, Rev. Sci. Instrum. 52, 51 (1981)] geometry with one rotation and two translations in order to tune the wavelength and keep constant the position of the monochromatic beam, respectively. The Bragg angle can vary from 10° to 80° using a standard high precision external goniometer whose minimum step is 0.001°. The two translations are based on a leaf spring device, with a maximum wobbling of ±2 arcsec along 50 mm. Fine tuning between the two crystals and detuning for harmonic rejection are produced by elastic devices using the magnetic force between a permanent magnet and a solenoid acting on a weak link of the second crystal. The monochromator housing is vacuum compatible. The equipment was tested using a conventional x‐ray source and a standard control system. It will be used associated with classical extended x‐ray absorption fine structure (EXAFS) and the high precision...

Formation of colloidal particles of hydrous iron oxide by forced hydrolysis

The mechanism of formation and growth of hydrous iron oxide (FeOOH) during the initial stages of forced hydrolyses of ferric chloride aqueous solution was studied by small angle X-ray scattering (SAXS). The effect of the hydrolysis temperature (60°C, 70°C and 80°C) and of the addition of urea on the formation of colloidal particles under isothermal conditions were investigated. Based on the experimental scattering functions in the Guinier range, we suggest the presence of elongated colloidal particles. The particle diameter and length, and their variation with time, were determined by fitting the form factor of prolate ellipsoids to the experimental scattering functions. We have assumed that our solutions are in a dilute state and that all colloidal particles are approximately of the same size. The colloidal particles have geometrical shapes similar to those of the subcrystals that build up the superstructure of β-FeOOH crystals, indicating that the formation of this hydrous iron oxide is governed by an aggregation process. Otherwise, the addition of urea hinders the growth and yields smaller particles, with a reduction in size greater than 50%.

Small-angle X-ray and nuclear-magnetic resonance study of siloxane-PMMA hybrids prepared by the sol-gel process

Transparent siloxane-polymethylmethacrylate (PMMA) hybrids were synthesized by the sol-gel process through hydrolysis of methacryloxyproyltrimethoxysilane (TMSM), tetramethoxysilane (TMOS) and polymerization of methylmethacrylate (MMA) using benzol peroxide (BPO) as catalyst. These composites have a good chemical stability due to the presence of covalent bonds between the inorganic (siloxane) and organic (PMMA) phases. The effects of siloxane content, pH of the initial sol and BPO content on the structure of the dried gels were analyzed by small-angle X-ray scattering (SAXS). SAXS results revealed the presence of an interference (or correlation) peak at medium q-range for all compositions, suggesting that siloxane groups located at the ends of PMMA chains form isolated clusters that are spatially correlated. The average intercluster distance - estimated from the q-value corresponding to the maximum in SAXS spectra - decreases for samples prepared with increasing amount of TMSM-TMOS. This effect was assigned to the expected increase in the number density of siloxane groups for progressively higher siloxane content. The increase of BPO content promotes a more efficient polymerization of MMA monomers but has no noticeable effect on the average intercluster distance. High pH favors polycondensation reactions between silicon species of both TMOS and TMSM silicon alcoxides, leading to a structure in which all siloxane clusters are bonded to PMMA chains. This effect was confirmed by 29Si nuclear-magnetic resonance (NMR) measurements.

CdTe quantum dots by melt heat treatment in borosilicate glasses

CdTe quantu m dots in borosilicate glasses were produced and their growth kinetics, the effects of quantum confinement for electrons and phonons and the time resolved optical transmission were studied. Quantum dots in the range of 10 to 25 with a 5% size dispersion were grown. The growth kinetics were studied by small angle X-ray scattering using synchrotron light. A spherical k .p model for the electron confinement explains absorption data and selection rules, and a dielectric continuum model for the phonon confinement could explain the resonant longitudinal and surface optical phonons, as well as their overtone combinations. The time response of absorption was studied by a pump-probe measurement on the femtosecond time scale. For samples showing a large amount of surface traps the recovery time can be as fast as 300 fs, while for an almost surface trap free sample it remains on the 1-3 ps time scale.

Characterization of the porosity developed in a new titania-alumina catalyst support prepared by the sol gel route

Titanium oxide (TiO 2 ) is a good candidate for support of hydrotreating catalysts but has the disadvantage of presenting a low surface area and a poor thermal stability when compared with Al 2 O 3 . A mixed TiO 2 -Al 2 O 3 support was proposed as an alternative that is expected to be free from these drawbacks. The variation during firing of the nanoporous texture of supports composed of TiO 2 -Al 2 O 3 , TiO 2 and Al 2 O 3 was studied by small angle X-ray scattering (SAXS). The supports were prepared by the sol-gel route using Ti and Al isopropoxides. We have particularly analyzed the effects of acid and basic hydrolysis on the nanostructural features of catalyst supports fired at different temperatures. The nanopore radius distribution functions were determined from SAXS results assuming a simple model of spherical nanopores embedded in a homogeneous solid matrix. The modal pore radius in both pure TiO 2 and pure Al 2 O 3 supports grows from 1.3 to 2.2 nm as the firing temperature increases from 673 to 973 K. On the other hand, the modal pore radius in the mixed TiO 2 -Al 2 O 3 support remains below 1.2 nm over the same range of firing temperatures. These results demonstrate the good thermal stability of the nanoporous texture of mixed TiO 2 -Al 2 O 3 supports.

Nucleation and growth of CdTe1-xSx nanocrystals embedded in a borosilicate glass. Effects of sulfur content and two-step thermal annealing

Abstract Nucleation and growth of CdTe1−xSx nanocrystals embedded in a borosilicate glass matrix and submitted to isothermal annealing were studied by small-angle X-ray scattering (SAXS). Two different sulfur contents (x=0.3 and x=0.7) were investigated. The formation and growth of the nanocrystals was studied in situ, maintaining the samples at a constant temperature (560 °C) inside a high-temperature cell. The effect of a nucleation pretreatment at 460 °C on the characteristics of nanocrystal formation and growth was also studied. The experimental results demonstrate that, in composites with high sulfur content (x=0.7), nanocrystals grow during the isothermal annealing by coarsening of preformed small crystals. In glasses with low sulfur content (x=0.3), nanocrystals grow by progressive diffusion of Cd, Te and S atoms initially dispersed in the glass matrix.

Formation and Growth of Semiconductor PbTe Nanocrystals in a Borosilicate Glass Matrix

Pb- and Te-doped borosilicate glasses are transformed by appropriate heat treatment into a composite material consisting of a vitreous matrix in which semiconductor PbTe nanocrystals are embedded. This composite exhibits interesting non-linear optical properties in the infrared region, in the range 10-20 000 A. The shape and size distribution of the nanocrystals and the kinetics of their growth were studied by small-angle X-ray scattering (SAXS) during in situ isothermal treatment at 923 K. The experimental results indicate that nanocrystals are nearly spherical and have an average radius increasing from 16 to 33 A after 2 h at 923 K, the relative size dispersion being time-invariant and approximately equal to 8%. This investigation demonstrates that the kinetics of nanocrystal growth are governed by the classic mechanism of atomic diffusion. The radius of nanocrystals, deduced by applying the simple Efros & Efros [Sov. Phys. Semicond. (1982), 16, 772-775] model using the energy values corresponding to the exciton peaks of optical absorption spectra, does not agree with the average radius determined by SAXS.

Nanostructure and properties of ZnO films produced by the pyrosol process

Indium doped ZnO films were deposited by the pyrosol process on glass substrates at different temperatures from solutions containing In/Zn molar ratios up to 10%. The nanostructure of the films was investigated using grazing-incidence small-angle X-ray scattering (GISAXS). The mass density was determined by X-ray reflectivity and the composition by X-ray photoelectron spectroscopy. The GISAXS measurements revealed an anisotropic pattern for films deposited at 573 and 623 K and a isotropic one for those deposited at higher temperatures. The anisotropic patterns indicate the presence of elongated nanopores with their long axes perpendicular to the film surface. In contrast, the isotropic nature of GISAXS patterns of films grown at high temperatures (673 and 723 K) suggests the presence of spherical voids. The pore size distribution function determined from the isotropic patterns indicates a multimodal size distribution. On the other hand, the measured mass density of the doped films with isotropic nanotexture is higher than that of the anisotropic films while the electric resistivity is significantly lower. This is in agreement with the detected strong reduction of the void density and specific surface area at approximately constant pore size.

Thermostimulated Sol–Gel Transition in Suspensions of Sulfate-Zirconium Oxychloride

The sol-gel transition of an aqueous solution of zirconium oxychloride modified by sulfuric acid (Zr:S 3:1) has been studied by small-angle X-ray scattering (SAXS) performed in situ during one cycle of heating and cooling between 298 and 358 K. The experimental SAXS curves exhibit three regions, at small, medium and high q values, characteristic of the Guinier, fractal and Porod regimes, respectively. The value of 5.5 A for the radii of the primary particles, obtained from the cross over of the Porod and fractal regimes, is consistent with the size of the inner core of the polynuclear Zr18O4(OH)38.8(SO4)12.6.33H2O molecule. These molecules aggregate as small clusters (31 A) of fractal structure, with a dimensionality D = 2.16 in the sol. The value of the fractal dimensionality, which is characteristic of ideal branched polymers, decreases during heating, attaining a value of 2.0, characteristic of swollen polymers. Above the critical temperature (329 K), the average size of aggregates increases abruptly to 200 A and the fractal dimensionality decreases to D = 1.75, this value being characteristic of a cluster-cluster aggregation process.