Talgat M. Inerbaev

Dopant-mediated oxygen vacancy tuning in ceria nanoparticles

Ceria nanoparticles with 20 and 40 at.% RE (RE = Y, Sm, Gd, and Yb) dopants were synthesized through a microemulsion method. Independently of the dopant nature and concentration, nearly monodispersed nanoparticles of size 3-5 nm were observed in high resolution transmission electron microscopic analysis. The ceria lattice either expands or contracts depending on the dopant cation ionic radii, as indicated by x-ray diffraction studies. X-ray photoelectron and Raman spectroscopic studies were used to quantify the cerium oxidation state and oxygen vacancy concentration. The results show the tunability of the oxygen vacancy and Ce(3+) concentrations based on the dopant properties. First principles simulations using the free energy density functional theory method support the observed experimental trends. The reported results establish a relationship between the oxygen vacancies and oxidation states in doped ceria required for tailoring properties in catalytic and biomedical applications.

Quantum chemistry of quantum dots: Effects of ligands and oxidation

We report Gaussian basis set density functional theory (DFT) calculations of the structure and spectra of several colloidal quantum dots (QDs) with a (CdSe)(n) core (n=6,15,17), that are either passivated by trimethylphosphine oxide ligands, or unpassivated and oxidized. From the ground state geometry optimization results we conclude that trimethylphosphine oxide ligands preserve the wurtzite structure of the QDs. Evaporation of the ligands may lead to surface reconstruction. We found that the number of two-coordinated atoms on the nanoparticles surface is the critical parameter defining the optical absorption properties. For (CdSe)(15) wurtzite-derived QD this number is maximal among all considered QDs and the optical absorption spectrum is strongly redshifted compared to QDs with threefold coordinated surface atoms. According to the time-dependent DFT results, surface reconstruction is accompanied by a significant decrease in the linear absorption. Oxidation of QDs destroys the perfection of the QD surface, increases the number of two-coordinated atoms and results in the appearance of an infrared absorption peak close to 700 nm. The vacant orbitals responsible for this near infrared transition have strong Se-O antibonding character. Conclusions of this study may be used in optimization of engineered nanoparticles for photodetectors and photovoltaic devices.

Structural characterization combined with the first principles simulations of barium/strontium cobaltite/ferrite as promising material for solid oxide fuel cells cathodes and high-temperature oxygen permeation membranes.

Mixed ionic-electronic conducting perovskite type oxides with a general formula ABO(3) (where A = Ba, Sr, Ca and B = Co, Fe, Mn) often have high mobility of the oxygen vacancies and exhibit strong ionic conductivity. They are key materials that find use in several energy related applications, including solid oxide fuel cell (SOFC), sensors, oxygen separation membranes, and catalysts. Barium/strontium cobaltite/ferrite (BSCF) Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta) was recently identified as a promising candidate for cathode material in intermediate temperature SOFCs. In this work, we perform experimental and theoretical study of the local atomic structure of BSFC. Micro-Raman spectroscopy was performed to characterize the vibrational properties of BSCF. The Jahn-Teller distortion of octahedral coordination around Co(4+) cations was observed experimentally and explained theoretically. Different cations and oxygen vacancies ordering are examined using plane wave pseudopotential density functional theory. We find that cations are completely disordered, whereas oxygen vacancies exhibit a strong trend for aggregation in L-shaped trimer and square tetramer structure. On the basis of our results, we suggest a new explanation for BSCF phase stability. Instead of linear vacancy ordering, which must take place before the phase transition into brownmillerite structure, the oxygen vacancies in BSCF prefer to form the finite clusters and preserve the disordered cubic structure. This structural feature could be found only in the first-principles simulations and can not be explained by the effect of the ionic radii alone.

Tuning hydrated nanoceria surfaces: experimental/theoretical investigations of ion exchange and implications in organic and inorganic interactions.

Long-term stability and surface properties of colloidal nanoparticles have significance in many applications. Here, surface charge modified hydrated cerium oxide nanoparticles (CNPs, also known as nanoceria) are synthesized, and their dynamic ion exchange interactions with the surrounding medium are investigated in detail. Time-dependent zeta (zeta) potential (ZP) variations of CNPs are demonstrated as a useful characteristic for optimizing their surface properties. The surface charge reversal of CNPs observed with respect to time, concentration, temperature, and doping is correlated to the surface modification of CNPs in aqueous solution and the ion exchange reaction between the surface protons (H(+)) and the neighboring hydroxyls ions (OH(-)). Using density functional theory (DFT) calculations, we have demonstrated that the adsorption of H(+) ions on the CNP surface is kinetically more favorable while the adsorption of OH(-) ions on CNPs is thermodynamically more favorable. The importance of selecting CNPs with appropriate surface charges and the implications of dynamic surface charge variations are exemplified with applications in microelectronics and biomedical.

First Excited State Properties and Static Hyperpolarizability of Ruthenium(II) Ammine Complexes.

First principles calculations were used to study the electronic excitation energies (E), transition dipole moments (μ), and difference of dipole moments between ground and excited states (Δμ) for low-lying singlets of the series of ruthenium(II) ammine complexes. Both cases of the gas phase and the acetonitrile solution were investigated in order to explain the discrepancy between the recent experimental and theoretical results and to develop the optimal way of estimation for the first static hyperpolarizability in the framework of a two-state model introduced by Oudar and Chemla. The present calculations reveal that the effect of solvent on the electronic properties of investigated compounds is not only the change of the excitation energy but also the increasing of ground-state molecular polarization and intensification of metal-to-ligand intramolecular charge transfer for electronic excitations. These effects lead to increasing of the values of Δμ and ground-state dipole moment μg in solution as compared with the gas-phase ones. The proposed theoretical approach gives good agreement with experiment and allows one to apply it for designing a new perspective nonlinear optical active organometallics.

Thermal expansion and lattice distortion of clathrate hydrates of cubic structures I and II

Abstract Thermal expansion of clathrate hydrates of argon, krypton, and propane with cubic structure II (CS-II), methane and xenon hydrates of cubic structure I (CS-I) and empty lattices of CS-I and CS-II at zero pressure have been investigated within the framework of lattice dynamics approach in quasiharmonic approximation. For all hydrates a good agreement with experiment for lattice parameters at some fixed temperatures have been obtained. In the case of the CS-II, it is found that inclusion of sufficiently small molecules such as argon and krypton into the water framework results in effective compression of empty hydrate lattice. In the case of large propane molecules included only in the large cavities the lattice is expanded relative to the empty lattice. The thermal expansion coefficients of hydrates with large enclathrated molecules are less than for hydrates formed by small guest molecules and the smallest value of thermal expansion coefficient is obtained for the empty lattice. By comparison of the data obtained for xenon and methane hydrates of CS-I and the empty lattice of CS-I it is found that the same behavior is observed also in the case of hydrates of CS-I. The effect of lattice stretching due to guest size on the reference chemical potential between the empty lattices of CS-I and ice Ih and empty lattice of CS-II and ice Ih is calculated too.

Terahertz Vibrational Modes of Crystalline Salicylic Acid by Numerical Model Using Periodic Density Functional Theory

The terahertz vibrational modes of crystalline salicylic acid at 1–6 THz were investigated using a vibrational calculation method in which the diagonalization of force constant matrix was estimated by periodic density functional calculations. The result proved sufficient to enable the terahertz vibrational modes to be assigned to lattice modes at 32.2–64.9 cm-1 coupled with translational and rotational modes, intermolecular bending modes at 67.0–132.1 cm-1 including the torsion of carboxyl groups, and out-of-plane intramolecular bending modes at 162.0–175.4 cm-1. The theoretical model presented allows for the interpretation of the terahertz spectra of organic crystals with a hydrogen-bonded dimer lacking the molecular vibrations of a large amplitude, by including effects arising from crystal periodicity.

First Principles Calculation of Terahertz Vibrational Modes of a Disaccharide Monohydrate Crystal of Lactose

First-principles calculations of the crystalline vibrations of a lactose monohydrate crystal in the terahertz (THz) region were performed using periodic density functional theory calculations. The calculated vibrational modes in the THz region were derived from group motions with different sizes: molecules of lactose and crystal water, pyranose rings, and intramolecular frames. The intermolecular modes with large vibrational amplitude of lactose of 17.5–100.6 cm-1 and of crystal-water of 136.1–237.7 cm-1 were clearly separated. This article especially refers to the intermolecular vibrational modes of crystal water with the THz absorption, which provide detectable spectral features of hydrated crystals.

Density functional study of oxygen vacancy formation and spin density distribution in octahedral ceria nanoparticles

AbstractWe report plane wave basis density functional theory (DFT) calculations of the oxygen vacancies formation energy in nanocrystalline CeO2-x in comparison with corresponding results for bulk and (111) CeO2 surface. Effects of strong electronic correlation of Ce4f states are taken into account through the use of an effective on-site Coulomb repulsive interaction within DFT+U approach. Different combinations of exchange-correlation functionals and corresponding U values reported in the literature are tested and the obtained results compared with experimental data. We found that both absolute values and trends in oxygen vacancy formation energy depend on the value of U and associated with degree of localization of Ce4f states. Effect of oxygen vacancy and geometry optimization method on spatial spin distribution in model ceria nanoparticles is also discussed. FigureSpin density in Ce44O80 nanocrystal reflects redistribution of localized f-electorns after the oxygen vacancy is introduced: all Ce cations at corner sites (except one) increase their magnetic moment to 0.97μB, while the edge opposite to the facet with vacancy has two Ce atoms with magnetic moment equal to 0.57μB, sharing f-electron.

Excited State Dynamics of Ru10 Cluster Interfacing Anatase TiO2(101) Surface and Liquid Water

Charge transfer dynamics at the interface of supported metal nanocluster and liquid water by GGA+U calculations combined with density matrix formalism is considered. The Ru10 cluster introduces new states into the band gap of TiO2 surface, narrows the band gap of TiO2, and enhances the absorption strength. The H2O adsorption significantly enhances the intensity of photon absorption, which is due to the formation of Ti-O(water) and Ru-O(water) bonds at the interfaces. The Ru10 cluster promotes the dissociation of water, facilitates charge transfer, and increases the relaxation rates of holes and electrons. We expect that our results are helpful in understanding basic processes contributing to photoelectrochemical water splitting.