A theoretical analysis on electronic structure of the (110) surface of TiO2-SnO2 mixed oxide

Mixed oxide compounds, such as TiO2-SnO2 system are widely used as gas sensors and should also provide varistor properties modifying the TiO2 surface. Therefore, a theoretical investigation has been carried out characterizing the effect of SnO2 on TiO2 addition on the electronic structure by means of ab initio SCF-LCAO calculations using all electrons. In order to take into account the finite size of the cluster, we have used the point charge model for the (TiO2)(15) cluster to study the effect on electronic structure of doping the TiO2 (110) Surface. The contracted basis set for titanium (4322/42/3), oxygen (33/3) and tin (43333/4333/43) atoms were used. The charge distributions, dipole moments, and density of states of doping TiO2 and vacancy formation are reported and analysed. (C) 2003 Elsevier B.V. All rights reserved.

Small-angle X-ray scattering study of gelation and aging of Eu3+-doped sol-gel-derived siloxane-poly(oxyethylene) nanocomposites

The aggregation, gelation, and aging of urea-cross-linked siloxane-poly(oxyethylene) nanohybrids [(U600)-n] containing two different amounts of europium triflate initially dissolved in an ethanol-water mixture were investigated by in situ small-angle X-ray scattering (SAXS). For both low (n = [O]/[Eu] = 80) and high (n = 25) europium contents, the SAXS intensity was attributed to the formation of siloxane clusters of about 8-11 Angstrom in size. Siloxane cluster formation and growth is a rapid process in hybrids with low Eu contents and slow in Eu-rich hybrids. An additional contribution to the scattering intensity at very low angles was attributed to the formation of a coarse structure level. At this secondary level, the structure can be described as a set of dense domains containing siloxane clusters embedded in a depleted matrix composed of unfolded polymer chains and solvent. By fitting a theoretical function for this model to the experimental SAXS curves, relevant structural parameters were determined as functions of time during the sol-gel transition and gel aging. For hybrids with low europium contents (n = 80), the size of the siloxane clusters remains essentially invariant, whereas the dense segregation domains progressively grow. In hybrids with high doping contents (n = 25), the preponderant structure variation during the first stages of the sol-gel transformation is the slow growth of siloxane clusters. For these hybrids, the segregation of siloxane clusters forming dense domains occurs only during advanced stages of the process.

A theoretical analysis of the TiO2/Sn doped (110) surface properties

We have used the periodic quantum-mechanical method with density functional theory at the B3LYP level in order to study TiO2/Sn doped (1 1 0) surfaces and have investigated the structural, electronic and energy band properties of these oxides. Our calculated relaxation directions for TiO2 is the experimental one and is also in agreement with other theoretical results. We also observe for the doped systems relaxation of lattice positions of the atoms. Modification of Sri, O and Ti charges depend on the planes and positions of the substituted atoms. Doping can modify the Fermi levels, energy gaps as well as the localization and composition of both valence and conduction band main components. Doping can also modify the chemical, electronic and optical properties of these oxides surfaces increasing their suitability for use as gas sensors and optoelectronic devices. (c) 2005 Elsevier B.V. All rights reserved.

Theoretical analysis on TiO(2)(110)/V surface

Theoretical analysis based on the Hartree-Fock method were performed in order to study the stoichiometric TiO(2) (110) surface and the vanadium substituted system. The Pople with polarization 3-21G* basis set level was used. The TiO(2) (110) surface was modeled using a (TiO(2))(15) cluster model. In order to take into account the finite size of the cluster, we have studied two different models: the point charge and the hydrogen saturated methodologies. The charge values used in the point charge calculations were optimized. The density of states, orbital self-consistend field (SCF) energies, and Mulliken charge values were analyzed. The method and model's dependence on the analyzed results are discussed. The theoretical results are compared with available experimental data. (C) 2001 John Wiley & Sons, Inc.

Study of the most suitable new glass laser to incorporate ytterbium: alkali niobium tellurite, lead fluorborate or heavy metal oxide

Diode-pumped Yb-doped glass lasers have received considerable attention for applications such as high-power beam production or femtosecond pulses generation. In this paper, we evaluate the laser potential of three different glass families doped with Yb3+ : alkali lead fluorborate (PbO-PbF2-B2O3), heavy metal oxide (Bi2O3-PbO-Ga2O3) and niobium tellurite (TeO2-Nb2O5-K2O-Li2O). Spectroscopic properties were studied for the samples and calculations of the minimum laser pump intensity (I-min), saturation fluence (U-sat) and the theoretical limit of peak power (P-max) are also presented. A comparison of laser properties of these three different glasses and their importance is shown and analyzed. (C) 2002 Elsevier B.V. B.V. All rights reserved.

ESR STUDY OF CUCL2 ADSORBED ON A CHEMICALLY-MODIFIED SILICA-GEL SURFACE WITH BENZIMIDAZOLE MOLECULE

The ESR spectrum of CuCl2 adsorbed onto a silica gel surface chemically modified with the benzimidazole molecule showed that the surface complex has an octahedral symmetry with tetragonal distortion. The measured ESR parameters were g(parallel to) = 2.287, g(perpendicular to) = 2.062, A(parallel to) = 153 G and superhyperfine splitting A(N) = 15 G. The fit of the theoretical expressions to the experimental data was very reasonable. The effective spin orbit coupling constant for Cu2+ was reduced from its normal free ion value of lambda = -828 cm(-1) by as much as 30%. This reduction of lambda is normal in the solid state and in frozen solution complexes.

Thermoanalytical study of some anti-inflammatory analgesic agents

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Density functional theory study on the structural and electronic properties of low index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems

The present study is concerned with the structural and electronic properties of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems. Periodic quantum mechanical method with density functional theory at the B3LYP level has been carried out. Relaxed surface energies, structural characteristics and electronic properties of the (I 10), (0 10), (10 1) and (00) low-index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 models are studied. For, comparison purposes, the bare rutile TiO2 and SnO2 structures are also analyzed and compared with previous theoretical and experimental data. The calculated surface energy for both rutile TiO2 and SnO2 surfaces follows the sequence (110) < (010) < (101) < (001) and the energy increases as (010) < (101) < (110) < (001) and (010) approximate to (110) < (101) < (001) for SnO2/TiO2/SnO2 and TiO2/SnO2/TiO2 composite systems, respectively. SnO2/TiO2/SnO2 presents larger values of surface energy than the individual SnO2 and TiO2 metal oxides and the TiO2/SnO2/TiO2 system renders surface energy values of the same order that the TiO2 and lower than the SnO2. An analysis of the electronic structure of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 systems shows that the main characteristics of the upper part of the valence bands for all the studied surfaces are dominated by the external layers, i.e., by the TiO2 and the SnO2, respectively, and the topology of the lower part of the conduction bands looks like the core layers. There is an energy stabilization of both valence band top and conduction band bottom for (110) and (010) surfaces of the SnO2/TiO2/SnO2 composite system in relation to their core TiO2, whereas an opposite trend is found for the same surfaces of the TiO2/SnO2/TiO2 composite system in relation to the bare SnO2. The present theoretical results may explain the growth of TiO2@SnO2 bimorph composite nanotape.

Synthesis, thermal properties and spectroscopic study of solid mandelate of light trivalent lanthanides

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Intense and broad photoluminescence at room temperature in structurally disordered Ba[Zr0.25Ti0.75]O-3 powders: An experimental/theoretical correlation

Intense and broad photoluminescence (PL) emission at room temperature was observed on structurally disordered Ba[Zr0.25Ti0.75]O-3 (BZT) powders synthesized by the polymeric precursor method. BZT powders were annealed at 573 K for different times and at 973 K for 2 h in oxygen atmosphere. The single-phase cubic perovskite structure of the powder annealed at 973 K for 2 It was identified by X-ray diffraction and Fourier transform Raman techniques. PL emission increased with the increase of annealing time, which reached its maximum value in the powder annealed at 573 K for 192 h. First principles quantum mechanical calculations based on density functional theory (B3LYP level) were employed to study the electronic structure of ordered and disordered models. The theoretical calculations and experimental measurements of Ultraviolet-visible absorption spectroscopy indicate that the presence of intermediary energy levels in the band gap is favorable for the intense and broad PL emission at room temperature in disordered BZT powders. The PL behavior is probably due the existence of a charge gradient on the disordered structure, denoted by means of a charge transfer process from [TiO5]-[ZrO6] or [TiO6]-[ZrO5] clusters to [TiO6]-[ZrO6] clusters. (C) 2008 Elsevier Ltd. All rights reserved.

Intercalation processes and diffusion paths of lithium ions in spinel-type structured Li(1+x)Ti(2)O(4): Density functional theory study

Intercalation processes and corresponding diffusion paths of Li ions into spinel-type structured Li(1+x)Ti(2)O(4) (0 <= x <= 0.375) are systematically studied by means of periodic density functional theory calculations for different compositions and arrangements. An analysis of the site preference for intercalation processes is carried out, while energy barriers for the diffusion paths have been computed in detail. Our results indicate that the Li insertion is thermodynamically favorable at octahedral sites 16c in the studied composition range, and Li migration from tetrahedral sites 8a to octahedral sites 16c stabilizes the structure and becomes favorable for compositions x >= 0.25. Diffusion paths from less stable arrangements involving Li migrations between tetrahedral and octahedral sites exhibit the lowest energy barrier since the corresponding trajectories and energy profiles take place across a triangle made by three neighboring oxygen anions without structural modification. Theoretical and experimental diffusion coefficients are in reasonable agreement.

Effects of HCl on the ultrasound catalyzed TEOS hydrolysis as determined by a calorimetric study

The acid catalyzed and ultrasound stimulated hydrolysis of solventless tetraethoxysilane-water mixtures was studied at 39°C as a function of HCl added to the mixtures (log[HCl]-1 ranged from 0.8 to 2.0), The reaction was carried out in a specially designed device, in which a steady state heat flow is maintained, while sonication is taking place, if no reaction is expected to occur. The exothermal hydrolysis reaction causes an increasing temperature (ΔTt) as a function of the reaction time, t. The isothermal hydrolysis rate constant, k, has been evaluated from the experimental ΔTt versus t data, after corrections for the increasing temperature effects, by using a method resulting from our theoretical modeling based on a dissolution and reaction mechanism. The hydrolysis rate constant fits closely a k α [H+] law as expected for this kind of hydrogen-ion catalyzed reaction.

Theoretical approaches to forensic entomology: II. Mathematical model of larval development

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A joint theoretical and kinetic investigation on the fragmentation of (N-halo)-2-amino cycloalkanecarboxylates

A combined theoretical and experimental study to elucidate the molecular mechanism for the Grob fragmentation of different (N-halo)-2-amino cyclocarboxylates with the nitrogen atom in exocyclic position: (N-Cl)-2-amino cyclopropanecarboxylate (1), (N-Cl)-2-amino cyclobutanecarboxylate (2), (N-Cl)-2-amino cyclopentanecarboxylate (3) and (N-Cl)-2-amino cyclohexanecarboxylate (4), and the corresponding acyclic compounds, (N-Cl)-2-amino isobutyric acid (A), (N-Cl)-2-amino butyric acid (B), has been carried out. The kinetics of decomposition for these compounds and related bromine derivatives were experimentally determined by conventional and stopped-flow UV spectrophotometry. The reaction products have been analyzed by GC and spectrophotometry. Theoretical analysis is based in the localization of stationary points (reactants and transition structures) on the potential energy surface. Calculations were carried out at B3LYP/6-31+G* and MP2/6-31+G* computing methods in the gas phase, while solvent effects have been included by means the self-consistent reaction field theory, PCM continuum model, at MP2/6-31+G* and MP4/6-31+G*//MP2/6-31+G* calculation levels. Based on both experimental and theoretical results, the different Grob fragmentation processes show a global synchronicity index close to 0.9, corresponding to a nearly concerted process. At the TSs, the N-Cl bond breaking is more advanced than the C-C cleavage process. An antiperiplanar configuration of these bonds is reached at the TSs, and this geometrical arrangement is the key factor governing the decomposition. In the case of 1 and 2 the ring strain prevents this spatial disposition, leading to a larger value of the activation barrier. Natural population analysis shows that the polarization of the N-Cl and C-C bonds along the bond-breaking process can be considered the driving force for the decomposition and that a negative charge flows from the carboxylate group to the chlorine atom to assist the reaction pathway. A comparison of theoretical and experimental results shows the relevance of calculation level and the inclusion of solvent effects for determining accurate unimolecular rate coefficients for the decomposition process. © 2002 Published by Elsevier Science B.V.

A comparative study between two relaxed LMI-based fuzzy control designs

A comparative study, with theoretical analysis and digital simulations, of two conditions based on LMI for the quadratic stability of nonlinear continuous-time dynamic systems, described by Takagi-Sugeno fuzzy models, are presented. This paper shows that the methods proposed by Teixeira et. al. in 2003 provide better or at least the same results of a recent method presented in the literature. © 2005 IEEE.