86 research outputs found
Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy
The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm-1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm-1 below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti4+ sites. At a higher density of defects, adsorption on Ti3+ sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti4+ sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm-1 . These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.Fil: Lustemberg, Pablo German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Quimica Fisica; ArgentinaFil: Scherlis Perel, Damian Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de Los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Quimica Fisica; Argentin
A unified electrostatic and cavitation model for first-principles molecular dynamics in solution
The electrostatic continuum solvent model developed by Fattebert and Gygi is
combined with a first-principles formulation of the cavitation energy based on
a natural quantum-mechanical definition for the surface of a solute. Despite
its simplicity, the cavitation contribution calculated by this approach is
found to be in remarkable agreement with that obtained by more complex
algorithms relying on a large set of parameters. Our model allows for very
efficient Car-Parrinello simulations of finite or extended systems in solution,
and demonstrates a level of accuracy as good as that of established
quantum-chemistry continuum solvent methods. We apply this approach to the
study of tetracyanoethylene dimers in dichloromethane, providing valuable
structural and dynamical insights on the dimerization phenomenon
First-principles molecular dynamics simulations at solid-liquid interfaces with a continuum solvent
Continuum solvent models have become a standard technique in the context of
electronic structure calculations, yet, no implementations have been reported
capable to perform molecular dynamics at solid-liquid interfaces. We propose
here such a continuum approach in a DFT framework, using plane-waves basis sets
and periodic boundary conditions. Our work stems from a recent model designed
for Car-Parrinello simulations of quantum solutes in a dielectric medium [J.
Chem. Phys. 124, 74103 (2006)], for which the permittivity of the solvent is
defined as a function of the electronic density of the solute. This strategy
turns out to be inadequate for systems extended in two dimensions, by
introducing new term in the Kohn-Sham potential which becomes unphysically
large at the interfacial region, seriously affecting the convergence. If the
dielectric medium is properly redefined as a function of the atomic
coordinates, a good convergence is obtained and the constant of motion is
conserved during the molecular dynamics simulations. Moreover, a significant
gain in efficiency can be achieved if the simulation box is partitioned in two,
solving the Poisson problem separately for the "dry" region using fast Fourier
transforms, and for the solvated or "wet" region using a multigrid method.
Eventually both solutions are combined in a self-consistent procedure, and in
this way Car-Parrinello molecular dynamics simulations of solid-liquid
interfaces can be performed at a very moderate computational cost. This scheme
is employed to investigate the acid-base equilibrium at the TiO2-water
interface.Comment: 36 pages, 7 figure
Structure and dynamics of nanoconfined water and aqueous solutions
This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed
Driven Liouville−von Neumann Equation for Quantum Transport and Multiple-Probe Green’s Functions
The so called Driven Liouville-von Neumann equation is a dynamical
formulation to simulate a voltage bias across a molecular system and to model a
time-dependent current in a grand-canonical framework. This approach introduces
a damping term in the equation of motion that drives the charge to a reference,
out of equilibrium density. Originally proposed by Horsfield and co-workers,
further work on this scheme has led to different coexisting versions of this
equation. On the other hand, the multiple-probe scheme devised by Todorov and
collaborators, known as the hairy-probes method, is a formal treatment based on
Green's functions that allows to fix the electrochemical potentials in two
regions of an open quantum system. In this article, the equations of motion of
the hairy probes formalism are rewritten to show that, under certain
conditions, they can assume the same algebraic structure as the Driven
Liouville-von Neumann equation in the form proposed by Morzan et al. [J. Chem.
Phys. 2017, 146, 044110]. In this way, a new formal ground is provided for the
latter, identifying the origin of every term. The performance of the different
methods are explored using tight-binding time-dependent simulations in three
trial structures, designated as ballistic, disordered, and resonant models. In
the context of first-principles Hamiltonians the Driven Liouville-von Neumann
approach is of special interest, because it does not require the calculation of
Green's functions. Hence, the effects of replacing the reference density based
on the Green's function by one obtained from an applied field are investigated,
to gain a deeper understanding of the limitations and the range of
applicability of the Driven Liouville-von Neumann equation.Comment: 40 pages, 15 figure
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