Computational studies of magnetite Fe₃O₄ and related spinel-structured materials

This thesis presents the results of ab initio based simulation studies of magnetite (Fe₃O₄) and related FeM₂X₄ (thio)spinels with M = Cr, Mn, Fe, Co and Ni and X = O and S. Using density functional theory with long-range dispersion correction and on-site Coulomb interactions (DFT + U – D2), we have investigated a number of properties of these materials. Firstly, we present a study of the inversion degree and its relevance in the electronic structure and magnetic properties of the spin filter candidates FeM₂X₄, which are one of the key devices in spintronic applications. We also analyze the role played by the size of the ions and by the crystal field stabilization effects in determining the equilibrium inversion degree. Secondly, we present the calculations of the elastic constants and other macroscopic mechanical properties by applying elastic strains on the unit cell of Fe₃O₄, which is the main component in different types of catalysts used in myriad of industrial processes. Thirdly, we calculate the geometries and surface free energies of a number of Fe₃O₄ surfaces at different compositions, including the non-dipolar stoichiometric plane, and those with a deficiency or excess of oxygen atoms. We propose a morphology in thermodynamic equilibrium conditions for the nanocrystals of this compound. We also present the simulated scanning tunnelling microscopy images of the different terminations of the surfaces shown on the Fe₃O₄ morphology. Finally, we investigate the initial oxidation stages of the greigite (Fe₃S₄) (001) surface induced by water. Fe₃S₄ is a mineral widely identified in anoxic aquatic environments and certain soils, which can be oxidised by these environments producing and extremely acid solution of sulfur-rich wastewater called acid mine drainage (AMD). We propose a number of mechanisms involving one or two water molecules and one OH group to explain the replacement of one sulfur by one oxygen atom in this mineral. The findings presented in this thesis provides a theoretical insight into various bulk and surface properties of this group of compounds

Propiedades de Calidad de Servicio en el Descubrimiento de Recursos Grid

Uno de los problemas abiertos en el contexto de las Arquitecturas Orientadas a Servicios es del descubrimiento de recursos y/o servicios adecuados para llevar a cabo una tarea determinada. Los proveedores de información Grid básicamente ofrecen información funcional sobre los recursos Grid que monitorizan, por lo que los modelos de información Grid básicamente representan esta información sintáctica, y los consumidores de información Grid usan normalmente dichas propiedades funcionales para seleccionar recursos. En la práctica, muchos trabajos se reinician debido a fallos en los recursos, aunque existen iniciativas que tratan de usar técnicas aisladas para manejar algunas propiedades de calidad de servicio. En el presente artículo se propone un nuevo enfoque para modelar recursos Grid junto con propiedades de calidad de servicio. Por un lado, este modelo está basado en una ontología desarrollada para integrar los modelos existentes tanto a nivel de representación de información Grid como de calidad de servicio en general. Por otro lado, también propone la creación de un sistema de medida - actualmente en desarrollo - para algunas propiedades de calidad de servicio (disponibilidad, rendimiento y fiabilidad)

Early Oxidation Processes on the Greigite Fe₃S⁴(001) Surface by Water: A Density Functional Theory Study

Greigite (Fe3S4), the sulfide counterpart of the spinel-structured oxide material magnetite (Fe3O4), is a mineral widely identified in anoxic aquatic environments and certain soils, which can be oxidized, thereby producing extremely acid solutions of sulfur-rich wastewaters, so-called acid mine drainage (AMD) or acid rock drainage (ARD). Here we report a computational study of the partial replacement of sulfur (forming H2S) by oxygen (from H2O) in the Fe3S4(001) surface, derived from density functional theory calculations with on-site Coulomb approach and long-range dispersion corrections (DFT+U–D2). We have proposed three pathways for the oxidation of the surface as a function of H2O coverage and pH. Different pathways give different intermediates, some of which are followed by a solid-state diffusion of the O atom. Low levels of H2O coverage, and especially basic conditions, seem to be essential, leading to the most favorable energetic landscape for the oxidation of the Fe3S4(001) surface. We have derived the thermodynamic and kinetic profile for each mechanism and plotted the concentration of H2S and protons in aqueous solution and thermodynamic equilibrium with the stoichiometric and partially oxidized Fe3S4(001) surface as a function of the temperature. Changes in the calculated vibrational frequencies of the adsorbed intermediates are used as a means to characterize their transformation. We have taken into account statistical entropies for H2S and H2O and other experimental parameters, showing that this mineral may well be among those responsible for the generation of AMD

A DFT+U study of the oxidation of cobalt nanoparticles: Implications for biomedical applications

Nanomaterials – magnetic nanoparticles in particular have been shown to have significant potential in cancer theranostics, where iron oxides are commonly the materials of choice. While biocompatibility presents an advantage, the low magnetisation is a barrier to their widespread use. As a result, highly magnetic cobalt nanoparticles are attracting increasing attention as a promising alternative. Precise control of the physiochemical properties of such magnetic systems used in biomedicine is crucial, however, it is difficult to test their behaviour in vivo. In the present work, density functional theory calculations with the Dudarev approach (DFT+U) have been used to model the adsorption of oxygen on low Miller index surfaces of the hexagonal phase of cobalt. In vivo conditions of temperature and oxygen partial pressure in the blood have been considered, and the effects of oxidation on the overall properties of cobalt nanoparticles are described. It is shown that oxygen adsorbs spontaneously on all surfaces with the formation of non-magnetic cobalt tetroxide, Co3O4, at body temperature, confirming that, despite their promising magnetic properties, bare cobalt nanoparticles would not be suitable for biomedical applications. Surface modifications could be designed to preserve their favourable characteristics for future utilisation

Interaction of H2O with the Platinum Pt (001), (011), and (111) Surfaces: A Density Functional Theory Study with Long-Range Dispersion Corrections

Platinum is a noble metal that is widely used for the electrocatalytic production of hydrogen, but the surface reactivity of platinum toward water is not yet fully understood, even though the effect of water adsorption on the surface free energy of Pt is important in the interpretation of the morphology and catalytic properties of this metal. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the interaction of H2O with the Pt (001), (011), and (111) surfaces. During the adsorption of a single H2O molecule on various Pt surfaces, it was found that the lowest adsorption energy (Eads) was obtained for the dissociative adsorption of H2O on the (001) surface, followed by the (011) and (111) surfaces. When the surface coverage was increased up to a monolayer, we noted an increase in Eads/H2O with increasing coverage for the (001) surface, while for the (011) and (111) surfaces, Eads/H2O decreased. Considering experimental conditions, we observed that the highest coverage was obtained on the (011) surface, followed by the (111) and (001) surfaces. However, with an increase in temperature, the surface coverage decreased on all the surfaces. Total desorption occurred at temperatures higher than 400 K for the (011) and (111) surfaces, but above 850 K for the (001) surface. From the morphology analysis of the Pt nanoparticle, we noted that, when the temperature increased, only the electrocatalytically active (111) surface remained

Interaction of SO2 with the Platinum (001), (011), and (111) Surfaces: A DFT Study

Given the importance of SO2 as a pollutant species in the environment and its role in the hybrid sulphur (HyS) cycle for hydrogen production, we carried out a density functional theory study of its interaction with the Pt (001), (011), and (111) surfaces. First, we investigated the adsorption of a single SO2 molecule on the three Pt surfaces. On both the (001) and (111) surfaces, the SO2 had a S,O-bonded geometry, while on the (011) surface, it had a co-pyramidal and bridge geometry. The largest adsorption energy was obtained on the (001) surface (Eads = −2.47 eV), followed by the (011) surface (Eads = −2.39 and −2.28 eV for co-pyramidal and bridge geometries, respectively) and the (111) surface (Eads = −1.85 eV). When the surface coverage was increased up to a monolayer, we noted an increase of Eads/SO2 for all the surfaces, but the (001) surface remained the most favourable overall for SO2 adsorption. On the (111) surface, we found that when the surface coverage was θ > 0.78, two neighbouring SO2 molecules reacted to form SO and SO3. Considering the experimental conditions, we observed that the highest coverage in terms of the number of SO2 molecules per metal surface area was (111) > (001) > (011). As expected, when the temperature increased, the surface coverage decreased on all the surfaces, and gradual desorption of SO2 would occur above 500 K. Total desorption occurred at temperatures higher than 700 K for the (011) and (111) surfaces. It was seen that at 0 and 800 K, only the (001) and (111) surfaces were expressed in the morphology, but at 298 and 400 K, the (011) surface was present as well. Taking into account these data and those from a previous paper on water adsorption on Pt, it was evident that at temperatures between 400 and 450 K, where the HyS cycle operates, most of the water would desorb from the surface, thereby increasing the SO2 concentration, which in turn may lead to sulphur poisoning of the catalyst

CO2 interaction with violarite (FeNi2S4) surfaces: a dispersion-corrected DFT study

The unbridled emissions of gases derived from the use of fossil fuels have led to an excessive concentration of carbon dioxide (CO2) in the atmosphere with concomitant problems to the environment. It is therefore imperative that new cost-effective catalysts are developed to mitigate the resulting harmful effects through the activation and conversion of CO2 molecules. In this paper, we have used calculations based on the density functional theory (DFT), including two semi-empirical approaches for the long-range dispersion interactions (-D2 and -D3), to explore the interaction of CO2 with the surfaces of spinel-structured violarite (FeNi2S4). This ternary sulfide contains iron ions in the highest possible oxidation state, while the nickel atoms are in the mixed 2+/3+ valence state. We found that CO2 interaction with violarite is only moderate due to the repulsion between the oxygen lone pairs and the electronic clouds of the sulfur surface atoms. This suggests that the CO2 activation is not dictated by the presence of nickel, as compared to the pure iron-isomorph greigite (Fe3S4). These results differ from findings in (Ni,Fe) ferredoxin enzymes, where the Ni/Fe ratio influences the redox potential, which suggests that the periodic crystal structure of violarite may hinder its redox capability

Assisted surface redesign by perturbing its point cloud representation

[Abstract] This research study explores the use of point clouds for design geometrically complex surfaces based on genetic morphogenesis. To this end, a point-based genetic algorithm and the use of massive unstructured point clouds are proposed as a manipulation method of complex geometries. The intent of the algorithm is to improve the design experience, thus different solutions can be presented to designers. The main objective of this work is to provide examples to be adopted as user own or to help them in the creative process. This is not about providing them with a tool to ‘do’ the designer's creative work, but using it as a creative tool in which the user retains control of it. The powerfulness of this approach relies on the fact that the user can use any/diverse criteria (objective or subjective) to evaluate the individuals proposed as possible solutions. As part of this study, the convergence of the algorithm and the ability of diversity in the final populations of the search process will be demonstrated. Various examples of the use of the algorithm are displayed

Assisted surface redesign by perturbing its point cloud representation

[Abstract] This research study explores the use of point clouds for design geometrically complex surfaces based on genetic morphogenesis. To this end, a point-based genetic algorithm and the use of massive unstructured point clouds are proposed as a manipulation method of complex geometries. The intent of the algorithm is to improve the design experience, thus different solutions can be presented to designers. The main objective of this work is to provide examples to be adopted as user own or to help them in the creative process. This is not about providing them with a tool to ‘do’ the designer's creative work, but using it as a creative tool in which the user retains control of it. The powerfulness of this approach relies on the fact that the user can use any/diverse criteria (objective or subjective) to evaluate the individuals proposed as possible solutions. As part of this study, the convergence of the algorithm and the ability of diversity in the final populations of the search process will be demonstrated. Various examples of the use of the algorithm are displayed

Predicting vertical urban growth using genetic evolutionary algorithms in Tokyo’s minato ward

[Abstract] This article explores the use of evolutionary genetic algorithms to predict scenarios of urban vertical growth in large urban centers. Tokyo’s Minato Ward is used as a case study because it has been one of the fastest growing skylines over the last 20 years. This study uses a genetic algorithm that simulates the vertical urban growth of Minato Ward to make predictions from pre-established inputted parameters. The algorithm estimates not only the number of future high-rise buildings but also the specific areas in the ward that are more likely to accommodate new high-rise developments in the future. The evolutionary model results are compared with ongoing high-rise developments in order to evaluate the accuracy of the genetic algorithm in simulating future vertical urban growth. The results of this study show that the use of genetic evolutionary computation is a promising way to predict scenarios of vertical urban growth in terms of location as well as the number of future buildings