42 research outputs found
Transferable Classical Force Field for Pure and Mixed Metal Halide Perovskites Parameterized from First-Principles
Identificador de proyecto: PCI2019-111839-2 (SCALEUP)Many key features in photovoltaic perovskites occur
in relatively long time scales and involve mixed compositions. This
requires realistic but also numerically simple models. In this work
we present a transferable classical force field to describe the mixed
hybrid perovskite MAxFA1¿xPb(BryI1¿y)3 for variable composition
(¿x, y ¿ [0, 1]). The model includes Lennard-Jones and
Buckingham potentials to describe the interactions between the
atoms of the inorganic lattice and the organic molecule, and the
AMBER model to describe intramolecular atomic interactions.
Most of the parameters of the force field have been obtained by
means of a genetic algorithm previously developed to parametrize
the CsPb(BrxI1¿x)3 perovskite (Balestra et al. J. Mater. Chem. A.
2020, DOI: 10.1039/d0ta03200j). The algorithm finds the best parameter set that simultaneously fits the DFT energies obtained for
several crystalline structures with moderate degrees of distortion with respect to the equilibrium configuration. The resulting model
reproduces correctly the XRD patterns, the expansion of the lattice upon I/Br substitution, and the thermal expansion coefficients.
We use the model to run classical molecular dynamics simulations with up to 8600 atoms and simulation times of up to 40 ns. From
the simulations we have extracted the ion diffusion coefficient of the pure and mixed perovskites, presenting for the first time these
values obtained by a fully dynamical method using a transferable model fitted to first-principles calculations. The values here
reported can be considered as the theoretical upper limit, that is, without grain boundaries or other defects, for ion migration
dynamics induced by halide vacancies in photovoltaic perovskite devices under operational conditions.TU/
Practical Cluster Models for a Layered β-NiOOH Material
Due to the high oxygen evolution reaction (OER) activity, stability, and abundance of NiOx materials, they are found to be promising catalysts, competitive with expensive metal oxides such as IrO2 and RuO2. From a theoretical point of view, studies reported in the literature so far are mostly based on density functional theory using periodic slab models for the bulk and surface of β-NiOOH, one of the active NiOx phases. However, cluster models are a valid method to investigate many aspects about structure, charge carrier transport properties, and OER activity of β-NiOOH. Hence, here we present new cluster models for the surface of β-NiOOH, where the oxygen atoms are bonded to Mg effective core potentials (ECPs) mimicking neighboring atom cores. This cluster embedding procedure is superior to saturating the cluster with hydrogen atoms, and to using other atomic ECPs for β-NiOOH. We find that layered materials such as β-NiOOH are more vulnerable to geometrical rupture and therefore a cluster approach requires additional care in choosing the embedding approach. We evaluated the models by using them to calculate the energy required for water adsorption and deprotonation, which are essential ingredients for OER. Specifically, our results agree with previous slab models that the first deprotonation reaction step requires a large amount of energy. In addition, we find that water and hydroxyl groups have high adsorption energy and therefore the first deprotonation step is limiting the reaction efficiency
Hazardous Doping for Photo-Electrochemical Conversion: The Case of Nb-Doped Fe2O3 from First Principles
The challenge of improving the efficiency of photo-electrochemical devices is often addressed through doping. However, this strategy could harm performance. Specifically, as demonstrated in a recent experiment, doping one of the most widely used materials for water splitting, iron (III) oxide (Fe2O3), with niobium (Nb) can still result in limited efficiency. In order to better understand the hazardous effect of doping, we use Density Functional Theory (DFT)+U for the case of Nb-doped Fe2O3. We find a direct correlation between the charge of the dopant, the charge on surface of the Fe2O3 material, and the overpotential required for water oxidation reaction. We believe that this work contributes to advancing our understanding of how to select effective dopants for materials
Benchmarking Density Functional Theory Based Methods To Model NiOOH Material Properties: Hubbard and van der Waals Corrections vs Hybrid Functionals
NiOOH has recently
been used to catalyze water oxidation by way
of electrochemical water splitting. Few experimental data are available
to rationalize the successful catalytic capability of NiOOH. Thus,
theory has a distinctive role for studying its properties. However,
the unique layered structure of NiOOH is associated with the presence
of essential dispersion forces within the lattice. Hence, the choice
of an appropriate exchange-correlation functional within Density Functional
Theory (DFT) is not straightforward. In this work, we will show that
standard DFT is sufficient to evaluate the geometry, but DFT+U and
hybrid functionals are required to calculate the oxidation states.
Notably, the benefit of DFT with van der Waals correction is marginal.
Furthermore, only hybrid functionals succeed in opening a bandgap,
and such methods are necessary to study NiOOH electronic structure.
In this work, we expect to give guidelines to theoreticians dealing
with this material and to present a rational approach in the choice
of the DFT method of calculation
Water Oxidation Catalysis for NiOOH by a Metropolis Monte Carlo Algorithm
Understanding catalytic
mechanisms is important for discovering
better catalysts, particularly for water splitting reactions that
are of great interest to the renewable energy field. One of the best
performing catalysts for water oxidation is nickel oxyhydroxide (NiOOH).
However, only one mechanism has been adopted so far for modeling catalysis
of the active plane: β-NiOOH(01̅5). In order to understand
how a second reaction mechanism affects catalysis, we perform Density
Functional Theory + U (DFT+U) calculations of a second mechanism for
water oxidation reaction of NiOOH. Then, we use a Metropolis Monte
Carlo algorithm to calculate how many catalytic cycles are completed
when two reaction mechanisms are competing. We find that within the
Metropolis algorithm, the second mechanism has a higher overpotential
and is therefore not active even for large applied biases
Dual Mechanisms: Hydrogen Transfer during Water Oxidation Catalysis of Pure and Fe-Doped Nickel Oxyhydroxide
Metal
oxides are often used as catalysts for the oxygen evolution
reaction which is of significant importance for water splitting as
an alternative energy source. One of the best performing catalysts
reported to date for water oxidation under alkaline conditions is
nickel oxyhydroxide (NiOOH) doped with iron. However, NiOOH has hydrogen
atoms whose positions are not known and may transfer through the material.
In order to understand how hydrogen transfer affects catalytic efficiency,
we use density functional theory+<i>U</i> (DFT+<i>U</i>) calculations that model oxygen evolution reaction catalysis for
pure and Fe-doped NiOOH. Our calculations reveal that hydrogen transfer
is possible in the Fe-doped case but is less probable in the pure
case. The duality of proton and charge transfer at the surface of
reactive materials provides further evidence to the effectiveness
of doping for improving catalysis
Novel High-Throughput Screening Approach for Functional Metal/Oxide Interfaces
Metal/oxide interfaces have long
been studied for their fundamental
importance in material microstructure as well as their broad applicability
in electronic devices. However, the challenge involved in characterizing
the relation between structure and electron transport of a large number
of metal/oxide combinations inhibits the search for interfaces with
improved functionality. Therefore, we develop a novel high-throughput
screening approach that combines computational and theoretical techniques.
We use a Density Functional Theory + U (DFT+U) quantum mechanical
formalism to produce effective Schrödinger equations, which
are solved by wave packet propagation to simulate charge transport
across the metal/oxide interface. We demonstrate this method on α-Fe<sub>2</sub>O<sub>3</sub>/Mt interfaces, for Mt = Ag, Al, Au, Ir, Pd,
or Pt metals. We use this novel method to screen for binary alloys
of these metals at the α-Fe<sub>2</sub>O<sub>3</sub>/Mt interface
and perform a successful validation test of the methodology. Finally,
we correlate the interface potential energy and the charge transport
permeability through the interface. Counterintuitively, among the
interfaces studied, we find that higher mismatch interfaces have better
charge transport permeability. We anticipate that this method will
be useful as a computationally tractable strategy to perform high-throughput
screening for new metal/oxide interfaces