[show abstract][hide abstract] ABSTRACT: In this letter, we report, for the first time, the real-time in situ nucleation and growth of Ag filaments on α-Ag2WO4 crystals driven by an accelerated electron beam from an electronic microscope under high vacuum. We employed several techniques to characterise the material in depth. By using these techniques combined with first-principles modelling based on density functional theory, a mechanism for the Ag filament formation followed by a subsequent growth process from the nano- to micro-scale was proposed. In general, we have shown that an accelerated electron beam from an electronic microscope under high vacuum enables in situ visualisation of Ag filaments with subnanometer resolution and offers great potential for addressing many fundamental issues in materials science, chemistry, physics and other fields of science.
[show abstract][hide abstract] ABSTRACT: Lead molybdate (PbMoO4) crystals were synthesized by the co-precipitation method at room temperature and then processed in a conventional hydrothermal (CH) system at low temperature (70 °C for different times). These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman (MR) and Fourier transformed infrared (FT-IR) spectroscopies. Field emission scanning electron microscopy images were employed to observe the shape and monitor the crystal growth process. The optical properties were investigated by ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) measurements. XRD patterns and MR spectra indicate that these crystals have a scheelite-type tetragonal structure. Rietveld refinement data possibilities the evaluation of distortions in the tetrahedral [MoO4][MoO4] clusters. MR and FT-IR spectra exhibited a high mode νν1(Ag) ascribed to symmetric stretching vibrations as well as a large absorption band with two modes νν3(Eu and Au) related to anti-symmetric stretching vibrations in [MoO4][MoO4] clusters. Growth mechanisms were proposed to explain the stages involved for the formation of octahedron-like PbMoO4 crystals. UV–Vis absorption spectra indicate a reduction in optical band gap with an increase in the CH processing time. PL properties of PbMoO4 crystals have been elucidated using a model based on distortions of tetrahedral [MoO4][MoO4] clusters due to medium-range intrinsic defects and intermediary energy levels (deep and shallow holes) within the band gap.
[show abstract][hide abstract] ABSTRACT: In this paper, a combined theoretical and experimental study on the electronic structure and photoluminescence (PL) properties of beta zinc molybdate (b-ZnMoO4) microcrystals synthesized by the hydrothermal method has been employed. These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectroscopies. Their optical properties were investigated by ultraviolet–visible (UV–Vis) absorption spectroscopy and PL measurements. First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level have been carried out. XRD patterns, Rietveld refinement, FT-Raman and FT-IR spectra showed that these crystals have a wolframite-type monoclinic structure. The Raman and IR frequencies experimental results are in reasonable agreement with theoretically calculated results. UV–Vis absorption measurements shows an optical band gap value of 3.17 eV, while the calculated band structure has a value of 3.22 eV. The density of states indicate that the main orbitals involved in the electronic structure of b-ZnMoO4 crystals are (O 2p-valence band and Mo 4d-conduction band). Finally, PL properties of b-ZnMoO4 crystals are explained by means of distortions effects in octahedral [ZnO6] and [MoO6] clusters and inhomogeneous electronic distribution into the lattice with the electron density map.
[show abstract][hide abstract] ABSTRACT: Pure and Mn-doped TiO2 films have been deposited by sputtering technique onto SiO2 substrates. The films display a compact columnar morphology, as revealed by scanning electron microscopy. X-ray diffraction and Raman scattering results provide evidence that the pure TiO2 films are predominantly anatase phase, but the increase in Mn concentration favors the rutile phase. The optical characterization shows a systematic decrease in the value of band gap and an increase in the tail states with the increase in Mn concentration. Magnetization measurements display purely diamagnetic behavior in the undoped TiO2 film and substrate and paramagnetic behavior in the Mn-doped films. No indication of ferromagnetic signature has been evidenced. First-principle calculations based on density functional theory and periodic models were employed to calculate the band structure and the density of electronic states to investigate the influence of Mn incorporation in the electronic structure of TiO2. Both experimental data and electronic structure calculations evidence the fact that the presence of Mn produces important modifications in the electronic states, mainly related to the 3d Mn orbitals in the inside the gap and in the vicinity of the band edges.
Journal of Physical Chemistry C. 01/2012; 116(15):8753-8762.
[show abstract][hide abstract] ABSTRACT: PbZr0.40Ti0.60O3 (PZT40/60) thin films with ferroelectric and dielectric properties have been grown on Pt/Ti/SiO2/Si and LaAlO3 (100) substrates using the chemical solution deposition method. These films have been characterized by different techniques such as X-ray diffraction (XRD), Raman, infrared and optical transmittance measurements. The transmittance curve of the PZT40/60 thin films on a LaAlO3 (100) substrate showed an optical band gap of 4.03 and 3.10 eV for the direct and indirect transition processes, respectively. To complement experimental data, first principles calculations at the DFT-B3LYP level were performed on periodic model systems of PbTiO3 and PZT40/60 to provide an insight into structural, optical and electronic behavior. The band gap of the PZT40/60 system for PbO and ZrO2 terminations is in agreement with trends of experimental data and results in smaller values than the band gap calculated for the PbTiO3 system.
Journal of Materials Chemistry 01/2012; 22(14):6587-6596. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We analyze the behavior of the energy profile of the ring-closure process for the transformation of (3Z,5Z)-octa-1,3,5,7-tetraene 5 to (1Z,3Z,5Z)-cycloocta-1,3,5-triene 6 through a combination of electron localization function (ELF) and catastrophe theory (CT). From this analysis, concepts such as bond breaking/forming processes, formation/annihilation of lone pairs, and other electron pair rearrangements arise naturally through the reaction progress simply in terms of the different ways of pairing up the electrons. A relationship between the topology and the nature of the bond breaking/forming processes along this rearrangement is reported. The different domains of structural stability of the ELF occurring along the intrinsic reaction path have been identified. The reaction mechanism consists of six steps separated by fold and cusp catastrophes. The transition structure is observed in the third step, d(C1-C8) = 2.342 angstrom, where all bonds have topological signature of single bonds (C-C). The new C1?C8 single bond is not formed in transition state and respective catastrophe of the ELF field (cusp) is localized in the last step, d(C1?C8) approximate to 1.97 angstrom, where the two monosynaptic nonbonding basins V(C1) and V(C8) are joined into single disynaptic bonding basin V(C1,C8). The V(C1,C8) basin corresponds to classical picture of the C1?C8 bond in the Lewis formula. In cycloocta-1,3,5-triene 6 the single C1?C8 bond is characterized by relatively small basin population 1.72e, which is much smaller than other single bonds with 2.03 and 2.26e. (c) 2011 Wiley Periodicals, Inc. J Comput Chem, 2011
Journal of Computational Chemistry 12/2011; 33(7):748-756. · 3.84 Impact Factor
[show abstract][hide abstract] ABSTRACT: The structural and electronic properties of Zn2SnO4 under high pressure have been investigated using the density functional formalism under the nonlocal B3LYP approximation. We have determined its stability against decomposition to ZnO and SnO2 mixture, as well as toward recently observed orthorhombic phases, similar to CaTi2O4 (titanite-type) CaFe2O4 (ferrite-type) and Sr2PbO4 structures. Numerical and analytical fittings have been carried out to determine the equilibrium unit cell geometry and equation of state parameters for all the structures and compounds involved in the phase diagram. The pressure dependences of band structures, energy gap, and density of states are also investigated. For inverse and titanate-type structures, the vibrational frequencies and their pressure dependence have been calculated. A microscopic analysis in terms of polyhedral and bond compressibilities leads to identify the ionic displacements accompanying the phase transformations and to an appealing interpretation of the phase response to compression. The present results provide a deeper insight into the relative stability, structural, electronic, and vibrational properties of the different phases.
[show abstract][hide abstract] ABSTRACT: By the joint use of experimental techniques such as field emission scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HR-TEM) and ab initio theoretical calculations on the slabs surface energy with application of the Wulff construction was performed to study the shape of CaWO4 nanocrystals obtained by a microwave assisted hydrothermal (MAN) method. Under equilibrium conditions, the shape is a slightly truncated tetragonal bipyramid enclosed with eight isosceles trapezoidal surfaces of (101) (Ca-terminated) and two top squares of (001) (O-terminated). Calculated band structures and densities of states are analyzed to find the electronic structures of the different surfaces. The more pronounced elongation of the (001) facet of the experimental FIG with respect to the theoretical prediction results from the favorable interaction between hydroxyl anions and Ca exposed atoms of this surface.
Journal of Physical Chemistry C. 01/2011; 115(41):20113-20119.
[show abstract][hide abstract] ABSTRACT: A new cis-dioxomolybdenum complex MoO2(DMLA)(2) (DMLA = N,N-dimethyllactamide) has been synthesized and characterized by X-ray crystallography, H NMR and IR spectroscopies and electronic structure calculations at DFT/B3LYP level. This compound (chemical formula C10H20MoO6N2) crystallizes in the orthorhombic space group P2(1)2(1)2(1) with Z = 4, a = 6.9357(2) angstrom, b = 11.8761(4) angstrom, c = 17.7251(5), V = 1460.00(8) angstrom(3) and renders a slightly distorted octahedral structure with two long Mo-O bonds (2.253(3) angstrom and 2.257(3) angstrom) trans to each of the Mo=O groups and with two short Mo-O bonds of 1.942(3)4 angstrom cis to them. The Mo=O bond length are 1.715(3) and 1.704(3) angstrom). Each lactamide ligand is bidentate; they are coordinated in their deprotonated form with the carbonyl oxygen occupying a position trans to the Mo=O moiety while the deprotonated hydroxyl oxygen is located cis to them. Structural characterization is complemented by DFT/B3LYP calculations. (C) 2011 Elsevier B.V. All rights reserved.
[show abstract][hide abstract] ABSTRACT: SrTiO3 nanoparticles were synthesized for the first time via a modified polymeric precursor method. The samples were characterized by thermogravimetry, X-ray diffraction (XRD), BET surface area, micro-Raman spectroscopy, field emission scanning and transmission electron microscopy (FE-SEM and FE-STEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence measurements. It is found that calcination atmosphere (air, nitrogen and oxygen) plays an important role of both crystal size and photolumiscence behavior of the SrTiO3 nanocrystallites. Results show that the powders obtained in nitrogen/oxygen atmosphere possess controllable particles size of approximately 11nm presenting the highest photoluminescence emission.
Materials Chemistry and Physics 01/2011; 125(1–2):168-173. · 2.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: By combining experimental techniques such as x-ray diffraction, Fourier transform Raman, ultraviolet-visible, x-ray absorption near edge structure, extended x-ray absorption fine structure spectroscopy, and theoretical models, a general approach to understand the relationship among photoluminescence (PL) emissions and excited electronic states in CaWO4 crystals is presented. First-principles calculations of model systems point out that the presence of stable electronic excited states (singlet) allow us to propose one specific way in which PL behavior can be achieved. In light of this result, we reexamine prior experiments on PL emissions of CaWO4. (C) 2011 American Institute of Physics. [doi:10.1063/1.3615948]
Journal of Applied Physics 01/2011; 110(4). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Density functional theory was used to study the mechanism for the oxidation of methanol to formaldehyde. A vanadium oxide cluster O=V(OH)(3) has been utilized to represent the catalytic system under hydrated conditions, i.e., in the presence of V-OH hydroxyl groups. Two types of methoxy-intermediates have been considered: a penta-coordinate methoxy-intermediate (OH)(4)V(OCH(3)) and a tetrahedral methoxy-intermediate (OH)(2)VO(OCH(3))(H(2)O). The most plausible reaction pathway corresponds to the process involving first the formation of the tetrahedral methoxide, and a subsequent rate-limiting step where hydrogen is transferred from the methoxy groups toward the oxygen atom of the vanadyl V=O site. The reaction mechanism is a typical two-state reactivity process due to a change of the multiplicity (reactive singlet --> product triplet) along the reaction coordinate accompanied by a reduction of the vanadium center from V(V) (d(0)) to V(III) (d(2)). Minimum energy crossing points were localized and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate approach to find the most favorable reaction pathways. The hydration effect is found to be mainly the destabilization of the methoxy intermediates. An alternative reaction pathway with a lower apparent barrier is presented.
Journal of Computational Chemistry 10/2010; 31(13):2493-501. · 3.84 Impact Factor
[show abstract][hide abstract] ABSTRACT: This Letter deals with the assignment of photoluminescence (PL) emission spectra of SrTiO3. Ideal and distorted structures of SrTiO3 model systems have been selected to localize and characterize at DFT calculation level both ground (singlet) and excited (triplet and singlet) electronic states. This study provides clear evidence of the role carried out by stable electronically excited states, and their relationship with order–disorder effects as a key component of the PL behaviour.
Chemical Physics Letters 06/2010; 493:141-146. · 2.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: Barium molybdate (BaMoO4) powders were synthesized by the co-precipitation method and processed in microwave-hydrothermal at 140 °C for different times. These powders were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR), ultraviolet–visible (UV–vis) absorption spectroscopies and photoluminescence (PL) measurements. XRD patterns and FT-Raman spectra showed that these powders present a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a large absorption band situated at around 850.4 cm−1, which is associated to the Mo–O antisymmetric stretching vibrations into the [MoO4] clusters. UV–vis absorption spectra indicated a reduction in the intermediary energy levels within band gap with the processing time evolution. First-principles quantum mechanical calculations based on the density functional theory were employed in order to understand the electronic structure (band structure and density of states) of this material. The powders when excited with different wavelengths (350 nm and 488 nm) presented variations. This phenomenon was explained through a model based in the presence of intermediary energy levels (deep and shallow holes) within the band gap.
Current Applied Physics 03/2010; 10(2):614-624. · 1.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: The chorismate to prephenate enzyme catalyzed reaction has been used in this review as the conduit to show different theoretical approaches that have been used over the years in our laboratory to explain its molecular mechanism. This pericyclic reaction has the advantage that other protein scaffolds such as catalytic antibodies or some promiscuous enzymes present certain chorismate mutase activity. The obtained results on all these protein environments, by comparison with the uncatalyzed reaction in solution, have been used to propose, as a general conclusion, that the origin of enzyme catalysis is in the relative electrostatic stabilization of the transition state with respect to the Michaelis complex. This feature implies that reactants of catalyzed reaction were closer to the transition state than those of the non-catalyzed reaction. From this hypothesis, and considering the features of the wild type chorismate mutases as the optimal catalyst for the reaction, some mutations on both kinds of alternative proteins have been proposed which would presumably enhance the rate constant of the chemical step.The studies presented in this paper demonstrate that the improvements and developments of the methods and techniques of theoretical and computational chemistry are now mature enough to model physic-chemical properties of biological systems with good accuracy. The combination of a potent computational protocol with molecular engineering techniques can be a promising methodology to develop novel enzymes with new or more efficient catalytic functions.
Interdisciplinary Sciences Computational Life Sciences 03/2010; 2(1):115-31.
[show abstract][hide abstract] ABSTRACT: A computational study based on B3LYP calculations was carried out to investigate the kinetic and mechanistic aspects of the selective oxidation of methanol to formaldehyde using titania-supported vanadate as a catalyst model. A complete picture of the possible mechanisms to obtain formaldehyde is given. Statistical mechanics as well as transition state theory (TST) were utilized to determine the rate coefficients and equilibrium constants of the most plausible mechanism. A tetrahedral vanadia containing a methoxy species is found to be the most stable intermediate. The rate-limiting step in the most commonly accepted mechanism is the hydrogen transfer from the tetrahedral methoxy intermediate to the catalyst sites V-O-Ti (46.4 kcal/mol) or V=O (41.0 kcal/mol) via a spin-crossing process. The transition states associated to these steps are biradicaloid. The simultaneous formation of H(2) and formaldehyde can be discarded because it proceeds with a higher energetic barrier of 57.0 kcal/mol. The plausibility of a more reactive site involving fivefold coordinated vanadium species along a H-transfer process with a energetic barrier of 20.1 kcal/mol is discussed. Finally, the dependence of the calculated values of energy barriers for the rate-limiting step on the functional used is analyzed.
Journal of Physical Chemistry C. 01/2010; 114(13):6039-6046.