Evaluación teórica de rccc R-Pirogalo[4]arenos funcionalizados con metales como medio para el almacenamiento de Hidrógeno Molecular ensayos o artículos académicos

In the present study, a theoretical investigation of the potential of various metal-functionalized R-substituted pyrogallol[4]arenes (i.e., M-R-Pyg[4]arene; M = Li+, K+, Na+ and Mg2+; R = methyl and fluoroethyl) as media for molecular hydrogen (H2) storage is reported. Initially, the structural features of the metal-functionalized systems are obtained at the B3LYP/6-311G(d,p) level of theory. Subsequently, the interaction of a H2 molecule with the cations embedded in the cavity of the macrocyclic molecules is described with the B3LYP functional using two basis sets of different flexibility, namely BSA: 6-311G(d,p) for ell atoms, and BSB: 6-311G(d,p) and aug-cc-pVDZ for M-R-Pyg[4]arene and H2, respectively. Notably large BSSE-corrected binding energy values were obtained at the B3LYP/BSB level for the different H2/M-R-Pyg[4]arene complexes spanning the 1.3 – 17.0 kJ/mol range. The resulting values were further refined through two approaches: (i) by employing the functional B97D, which includes a Grimme´s type correction for describing dispersive forces and (ii) by performing MP2 calculations within the frame of the ONIOM approach. Binding energies refined at the MP2 level resulted in an average increment of about ~2.5 kJ/mol when considering all the complexes under investigation. On the other hand, B97D binding energies were found to be overestimated since too large increments (i.e., three- and fourfold with respect to B3LYP values for the case of Li- and Na-functionalized systems, respectively) were observed. For the specific case of the H2/Mg-fluoroethyl-Pyg[4]arene, an adsorption enthalpy (∆H°ads) of -17.6 kJ/mol was estimated by adding the zero point energy and thermal effects computed at 300 K from harmonic vibrational frequencies, obtained at the B3LYP/BSB level. This relatively high adsorption enthalpy suggests that Mg-functionalized R-Pyg[4]arenes can be envisaged as promising systems for molecular hydrogen storage.En el presente estudio se reporta la investigación teórica acerca del potencial de los pirogalol[4]arenos R-sustituidos funcioanlizados con varios metales (M-R-Pyg[4]arenos; M = Li+, K+, Na+ y Mg2+; R = meil y fluoretil) como medio para el almacenamiento de hidrógeno molecular (H2). Como punto de partida, las características estructurales de los sistemas funcionalizados con los metales fueron obtenidos al nivel de teoría B3LYP/6-311G(d,p). Subsecuentemente, la interacción de la molécula de hidrógeno con los cationes integrados en la cavidad de las moléculas macrocíclicas es descrita con el funcional B3LYP usando dos conjuntos base de diferente flexibilidad, BSA: 6-311G(d,p) para todos los átomos, y BSB: 6-311G(d,p) y aug-cc-pVDZ para M-R-Pyg[4]arenos e H2, respectivamente. Los valores obtenidos de las energías de amarre corregidas por el método BSSE usando el nivel de teoría B3LYP/BSB fueron notablemente más altas para los complejos H2/M-R-Pyg[4]areno abarcando el rango entre 1.3 y 17.0 kJ/mol. Estos resultados fueron posteriormente refinados mediante dos aproximaciones: (i) empleando el funcional B97D, el mismo que incluye una corrección de tipo Grimme para la descripción de las fuerzas de dispersión y (ii) realizando cálculos MP2 mediante la utilización del método ONIOM. Las energías de amarre resultantes, usando el nivel MP2, mostraron un incremento de aproximadamente 2.5 kJ/mol al analizar a todos los complejos. Por otra parte, se encontró que las energías de amarre obtenidas usando B97D muestran valores sobrestimados debido a que se evidenciaron incrementos considerablemente grandes (el triple y el cuádruple de os valores obtenidos mediante B3LYP para los casos de los sistemas funcionalizados con Li y Na, respectivamente). Para el caso específico del H2/fluoretil-Pyg[4]areno, la entalpía de adsorción estimada (∆H°ads) fue de -17.6 kJ/mol tmando en cuenta la energía del punto cero (ZPE) y los efectos térmicos calculados a 300 K a partir de las frecuencias armónicas vibracionales obtenidas al nivel de teoría B3LYP/BSB. Esta entalpía de adsorción alta sugiere que los R-Pyg[4]arenos funcioanlizados con Mg pueden ser tomados en cuenta como sistemas prometedores para el almacenamiento de hidrógeno molecular

The closed-edge structure of graphite and the effect of electrostatic charging

The properties of graphite, and of few-layer graphene, can be strongly influenced by the edge structure of the graphene planes, but there is still much that we do not understand about the geometry and stability of these edges. We present an experimental and theoretical study of the closed edges of graphite crystals, and of the effect of an electric field on their structure. High-resolution transmission electron microscopy is used to image the edge structure of fresh graphite and of graphite that has been exposed to an electric field, which experiences a separation of the graphene layers. Computer simulations based on density functional theory are used to rationalise and quantify the preference for the formation of multiple concentric loops at the edges. A model is also presented to explain how the application of an electric field leads to the separation of the folded edges

Density Functional Theory Investigation of Carbon- and Porphyrin-based Nanostructures

The present doctoral thesis examines the properties of carbon-based, porphyrin-based and hybrid carbon-porphyrin nanostructures as promising candidate materials for catalysis (including photocatalysis) applications. I use density functional theory simulations, together with experimental insights from collaborators, to both explain known behaviour and suggest ways in which these materials can be modified for improved catalytic efficiency. Since the catalytic activity of graphitic materials is concentrated on the edges, I investigate their properties in several ways. First, I attempt to understand the properties of folded edges of graphitic nanostructures and quantify their thermodynamic stability, and explain how the application of an electric field leads to their opening. Edge folding can reduce catalytic activity by allowing bond saturation at the edge, but at the same time they provide a way to achieve highly porous carbon-based materials, which could be very useful for catalytic applications. My calculations rationalise the experimental observations about these folded edges. Additionally, I investigate catalysts based on carbon- and iron-based nanostructures, in collaboration with experimentalists. I present models for N-doped graphitic/ferrihydrite nanocatalysts for CO2 reduction, and for Fe-N active sites in graphite-based catalysts. In contrast with carbon nanostructures, porphyrin nanostructures exhibit a welldefined band gap which makes them more useful in photocatalytic applications. In this thesis I explore possible routes to engineer the electronic properties of two types of porphyrin-based materials. The first type consists of fully-organic porphyrin nanostructures with various dimensionalities, and we show how the length of the linkers between porphyrin can be used to engineer their electronic band structures. The second type consists of two-dimensional (2D) porphyrin-based metal-organic frameworks, where we explored different strategies to optimise the photocatalytic behaviour, by changing metal centres, partially reducing the porphyrins or changing the bridges between the porphyrin units. Finally, I consider mixed graphitic/porphyrinic structures, based on the idea that such composites could combine the advantages of both types of structures, leading to superior photocatalytic behaviour. I discuss the adsorption of porphyrins on the surfaces or edges of graphene nanoribbon, and how the interaction affects the electronic properties of the combined structures. Overall, the thesis shows how computer simulation approaches can be used to understand, and also to design and optimise the electronic properties of carbon and porphyrin-based nanostructures to be applied in catalysis and photocatalysis

Theoretical investigation of the lattice thermal conductivities of II-IV-V2 pnictide semiconductors

Ternary pnictides semiconductors with II-IV-V2 stoichiometry hold potential as cost effective thermoelectric materials with suitable electronic transport properties, but their lattice thermal conductivities ($\kappa$) are typically too high. Gaining insight into their vibrational properties is therefore crucial to finding strategies to reduce $\kappa$ and achieve improved thermoelectric performance. We present a theoretical exploration of the lattice thermal conductivities for a set of pnictide semiconductors with ABX2 composition (A = Zn, Cd; B = Si, Ge, Sn; and X = P, As), using machine-learning based regression algorithms to extract force constants from a reduced number of density functional theory simulations, and then solving the Boltzmann transport equation for phonons. Our results align well available experimental data, decreasing the mean absolute error by ~3 Wm-1K-1 with respect to the best previous set of theoretical predictions. Zn-based ternary pnictides have, on average, more than double the thermal conductivity of the Cd-based compounds. Anisotropic behaviour increases with the mass difference between A and B cations, but while the nature of the anion does not affect the structural anisotropy, the thermal conductivity anisotropy is typically higher for arsenides than for phosphides. We identify compounds, like CdGeAs2, for which nanostructuring to an affordable range of particle sizes could lead to values low enough for thermoelectric applications.Comment: 24 pages, 8 figure

Band structures of periodic porphyrin nanostructures

Recent progress in the synthesis of π-conjugated porphyrin arrays of different shapes and dimensionalities motivates us to examine the band structures of infinite (periodic) porphyrin nanostructures. We use screened hybrid density functional theory simulations and Wannier function interpolation to obtain accurate band structures of linear chains, 2D nanosheets and nanotubes made of zinc porphyrins. Porphyrin units are connected by butadiyne (C4) or ethyne (C2) linkers, or “fused” (C0), i.e. with no linker. The electronic properties exhibit strong variations with the number of linking carbon atoms (C0/C2/C4). For example, all C0 nanostructures exhibit gapless or metallic band structures, whereas band gaps open for the C2 or C4 structures. The reciprocal space point at which the gaps are observed also show fluctuations with the length of the linkers. We discuss the evolution of the electronic structure of finite porphyrin tubes, made of a few stacked six-porphyrin rings, towards the behavior of the infinite nanotube. Our results suggest approaches for engineering porphyrin-based nanostructures to achieve target electronic properties

Theoretical investigation of the lattice thermal conductivities of II–IV–V₂ pnictide semiconductors

Ternary pnictide semiconductors with II–IV–V2 stoichiometry hold potential as cost-effective thermoelectric materials with suitable electronic transport properties, but their lattice thermal conductivities (κ) are typically too high. Insights into their vibrational properties are therefore crucial to finding strategies to reduce κ and achieve improved thermoelectric performance. We present a theoretical exploration of the lattice thermal conductivities for a set of pnictide semiconductors with ABX2 composition (A = Zn, Cd; B = Si, Ge, Sn; and X = P, As) using machine-learning-based regression algorithms to extract force constants from a reduced number of density functional theory simulations and then solving the Boltzmann transport equation for phonons. Our results align well with available experimental data, decreasing the mean absolute error by ∼3 W m–1 K–1 with respect to the best previous set of theoretical predictions. Zn-based ternary pnictides have, on average, more than double the thermal conductivity of the Cd-based compounds. Anisotropic behavior increases with the mass difference between A and B cations, but while the nature of the anion does not affect the structural anisotropy, the thermal conductivity anisotropy is typically higher for arsenides than for phosphides. We identify compounds such as CdGeAs2, for which nanostructuring to an affordable range of particle sizes could lead to κ values low enough for thermoelectric applications

Engineering the electronic and optical properties of 2D porphyrin paddlewheel metal-organic frameworks

Metal organic frameworks (MOFs) are promising photocatalytic materials due to their high surface area and tuneability of their electronic structure. We discuss here how to engineer the band structures and optical properties of a family of two-dimensional (2D) porphyrin-based MOFs, consisting of M tetrakis(4 carboxyphenyl) porphyrin structures (M TCPP, where M = Zn or Co) and metal (Co, Ni, Cu or Zn) paddlewheel clusters, with the aim of optimising their photocatalytic behaviour in solar fuel synthesis reactions (water splitting and/or CO2 reduction). Based on density functional theory (DFT) and time-dependent DFT simulations with a hybrid functional, we studied three types of composition/structural modifications: a) varying the metal centre at the paddlewheel or at the porphyrin centre to modify the band alignment; b) partially reducing the porphyrin unit to chlorin, which leads to stronger absorption of visible light; and c) substituting the benzene bridging between the porphyrin and paddlewheel, by ethyne or butadiyne bridges, with the aim of modifying the linker to metal charge transfer behaviour. Our work offers new insights on how to improve the photocatalytic behaviour of porphyrin- and paddlewheel-based MOFs

Harnessing the unusually strong improvement of thermoelectric performance of AgInTe2 with nanostructuring

Nanostructuring is a well-established approach to improve the thermoelectric behavior of materials.However, its effectiveness is restricted if excessively small particle sizes are necessary to considerably decrease the lattice thermal conductivity. Furthermore, if the electrical conductivity is unfavorably affected by the nanostructuring, it could cancel out the advantages of this approach. Computer simulations predict that silver indium telluride, AgInTe2, is unique among chalcopyrite structured chalcogenides in requiring only a mild reduction of particle size to achieve a substantial reduction in lattice thermal conductivity. Here, ab-initio calculations and machine learning are combined to systematically chart the thermoelectric properties of nanostructured AgInTe2, in comparison with its Cu-based counterpart, CuInTe2. In addition to temperature and doping carrier concentration dependence, ZT is calculated for both materials as functions of the polycrystalline average grain size, taking into account the effect of nanostructuring on both phonon and electron transport. It is shown that the different order of magnitude between the mean free path of electrons and phonons disentangles the connection between the power factor and lattice thermal conductivity when reducing the crystal size. ZT values up to 2 are predicted for p-type AgInTe2 at 700 K when the average grain size is in the affordable 10-100 nm range

Charting the lattice thermal conductivities of I-III-VI2 chalcopyrite semiconductors

Chalcopyrite-structured semiconductors have promising potential as low-cost thermoelectric materials, but their thermoelectric figures of merit must be increased for practical applications. Understanding their thermal properties is important to engineer their thermal conductivities and achieve better thermoelectric behavior. We present here a theoretical investigation of the lattice thermal conductivities of 20 chalcopyrite semiconductors with composition ABX2 (I-III-VI2), with A=Cu, Ag; B=Al, Ga, In, Tl; and X=S, Se, Te. To afford accurate predictions across this large family of compounds, we solve the Boltzmann transport equation with force constants derived from density functional theory calculations and machine-learning-based regression algorithms, reducing between one and two orders of magnitude the computational cost with respect to conventional approaches of the same accuracy. The results are in good agreement with available experimental data and allow us to rationalize the role of chemical composition, temperature and nanostructuring on the thermal conductivities across this important family of semiconductors

Fast, accurate and non-empirical determination of the lattice thermal conductivities of I-III-VI2 chalcopyrite semiconductors

The use of computer simulation to predict the lattice thermal conductivity of materials has the potential to accelerate the discovery of new thermoelectric materials. However, the accurate prediction of this property from first principles, without input from experiment, is very computationally demanding, which limits the use of high-throughput strategies in thermoelectric materials design. We present here an accurate, fast, and non-empirical determination of the lattice thermal conductivities of a large family of semiconductors, with composition ABX2 (I-III-VI2), with A=Cu, Ag; B=Al, Ga, In, Tl; and X=S, Se, Te. We solve the Boltzmann transport equation with force constants derived from density functional theory calculations and machine-learning-based regression algorithms, reducing between one and two orders of magnitude the computational cost with respect to conventional approaches of the same accuracy. The results are in good agreement with available experimental data and allow us to rationalize the role of chemical composition, temperature and nanostructuring on the thermal conductivities across this important family of semiconductors