Computational Modelling of TiO2 and Mg-silicate nanoclusters and nanoparticles - Crystallinity and Astrophysical Implications

Abstract

The research presented in this thesis contributes to the understanding of both titania and silicate nanosystems by providing new information on energetic stability and properties of nanometer sized particles using computational modelling. Particular emphasis is placed on the importance of two nanosized regimes: i) tens of atoms, and ii) several hundred up to thousands of atoms. We differentiate these two size regimes by naming nanoclusters the structures containing between tens up to a hundred of atoms, and using the term nanoparticles (NPs) for the structures containing hundreds to thousands of atoms. Titania (TiO2) is the most studied photocatalyst, and thus research is mostly focused on understanding the electronic properties of different morphologies of TiO2 NPs. In detail, for TiO2 the present thesis benchmarks the ability of several interatomic potentials (IPs) to reduce the computational cost of Density Functional Theory (DFT) calculations, as well as a detailed analysis of the energetic stability of three different morphologies of NPs together with an analysis of their band-gap. We show that the Anatase crystal structure becomes the most stable for particle sizes of ~2-3 nm in diameter, while for smaller sizes amorphous particles are the most stable. Within the Anatase structure, we see that Wulff construction is the most stable for large sizes (above 2 nm), but amorphous shell-crystalline core nanoparticles are within the same energy range below a radius of 2 nm. We also find that spherical particles have a band-gap consistent with the so-called black TiO2. On the other hand, research on silicates is mainly focused on calculating the properties of nanoclusters and NPs, with the objective of obtaining a better understanding of the relevance of such species in interstellar space. In detail, we propose global minima (GM) candidates for numerous nanoclusters based on extensive global optimization (GO) searches and compare their spectroscopic and chemical properties with literature values, where the later values are mostly derived from extrapolation using macroscale laboratory samples. The GO searches were done with a reparameterization of the FFSiOH where we included the Mg element. We also evaluate whether silicate nanoclusters can be the origin of the anomalous microwave emission (AME), a foreground emission in the microwave (MW) region of the spectra from an unknown source and find that indeed nano silicates have the appropriate dipole moments in order to be a strong source of the AME. We indicate that the amount of nano silicates in the interstellar medium is constrained by the AME emission. Finally, the IR spectra of large NPs of around 4 nm in diameter is compared on the basis of their crystallinity. We find that for such sizes, the IR spectra of the crystalline particle corresponds to a broad band similar to the amorphous material, which we ascribe to the large fraction of surface atoms. We conclude that the IR spectra is not sufficient to characterize the crystallinity of astronomical silicates with sizes of several nanometers in diameter. We also show that amorphous silicate nano particles with sizes of ~1 nm in diameter are more stable than their crystalline counterparts. We extrapolate the tendency and propose that the crystalline nanoparticles become more stable than amorphous particles at particle sizes of ~12 nm in diameter.

En la presente tesis se estudian la estructura y propiedades de nano partículas de TiO2 y de silicatos de magnesio. Mediante cálculos de teoría del funcional de la densidad y cálculos de potenciales interatómicos se muestra, para TiO2, cómo la estructura de la nano partícula tiene un factor clave para entender la diferencia de energía entre la banda de conducción y la banda de valencia. Además, Comparamos la estabilidad energética de nano partículas de Anatasa con diferente geometría respecto a nano partículas y nano clúster amorfos, mostrando como la geometría de Wulff es energéticamente la más estable a partir de 2nm, pero para tamaños inferiores los clúster amorfos son más estables. Para silicatos de magnesio, se describe la estructura, espectro infra-rojo y microondas de clúster de MgSiO3 y Mg2SiO4, así como la similitud del espectro infra-rojo en materiales cristalinos y amorfos de tamaños ~4 nm. Los resultados se comparan con los modelos usados para entender las propiedades del medio interestelar. Se muestra que para nano partículas de hasta algunos nanómetros de radio no es posible diferenciar el material cristalino del amorfo en base a espectroscopia infrarroja. Finalmente, se calcula la intensidad de emisión en microondas (10-60 GHz) de nano clústers de silicatos de ~100 átomos y se demuestra a nivel teórico la capacidad de los silicatos como fuente viable de la emisión anómala de microondas.