Assessing the viability of silicate nanoclusters as carriers of the anomalous microwave emission: a quantum mechanical study

Nanosized silicate dust is likely to be abundant in many astronomical environments and it is a prime candidate for being the source of the anomalous microwave emission (AME). To assess the viability of silicate nanoclusters as AME carriers, their detailed properties need to be established. Using quantum chemical calculations, we compute the accurate chemical and electronic structures of three families of nanoclusters with astrophysically relevant compositions: Mg-rich olivine (Mg2SiO4)N, Mg-rich pyroxene (MgSiO3)N, and silicon monoxide (SiO)N, all in the ≤1 nm diameter size regime and for neutral and ± 1 charge states. From these fundamental data, we directly derive the shapes, ionization potentials, electron affinities, and dipole moments of all nanoclusters. The aspect ratio of the nanoclusters fluctuates significantly with N for small sizes, but especially for the olivine and pyroxene nanoclusters, it tends to stabilize towards ~1.3 for the largest sizes considered. These latter two nanocluster families tend to have mass distributions consistent with approximately prolate ellipsoidal shapes. Our calculations reveal that the dipole moment of all our nanoclusters can be substantially affected by changes in chemical structure (i.e. different isomers for a fixed N), ionisation, and substitution of Mg by Fe. Although all these factors are important, the dipole moments of our Mg-rich nanoclusters are always found to be large enough to account for the observed AME. However, (SiO)N nanoclusters are only likely to be potential AME contributors when they are both charged and their chemical structures are anisotropically segregated. We also model the emissivity per H of a representative (Mg2SiO4)3 nanocluster by directly calculating the quantum mechanical rotational energy levels and assuming a distribution of occupied levels in accordance with equilibrium Boltzmann statistics. We compare our bottom-up results with previously published classical models and show that a population of silicate nanoclusters containing only 1% of the total Si budget can reproduce the AME emissivity

Global optimisation of hydroxylated silica clusters: a cascade Monte Carlo Basin Hopping approach

We report on a global optimisation study of hydroxylated silica nanoclusters (SiO2)/w(H2O)(N) with sizes M = 6, 8, 10 12, and for each size with a variable number of dissociatively chemisorbed water molecules (N = 1, 2, 3...). Due to the high structural complexity of these systems and the associated ruggedness of the underlying potential energy landscape, we employ a 'cascade' global optimisation approach. Specifically, we use Monte Carlo Basin Hopping (MCBH) where for each step we employ two energy minimisations with: (i) a lightly parameterised but computationally efficient interatomic potential (IP) which does not distinguish between H-bonded conformational isomers, and then (ii) a more sophisticated IP which accounts for polarisation and H-bonding. Final energies from the MCBH search are then refined with optimisations using density functional theory. The reliability of our approach is first established via comparison with previously reported results for the (SiO2)(8).(H2O)(N) case, and then applied to the M = 6, 10 and 12 systems. For all systems studied our results follow the trend in hydroxylation energy versus N, whereby the energy gain with hydroxylation is found to level off at a point where the average tetrahedral distortion of the SiO4 centres is minimised. This optimal hydroxylation point is further found to follow an inverse power law with increasing cluster size (M) with an exponent close to -2/3, further confirming work in previous studies for other cluster sizes. (C) 2016 Elsevier B.V. All rights reserved

Does Processing or Formation of Water Ice Mantles Affect the Capacity of Nanosilicates to Be the Source of Anomalous Microwave Emission?

Anomalous microwave emission (AME) is detected in many astrophysical environments as a foreground feature typically peaking between 20-30 GHz and extending over a 10-60 GHz range. One of the leading candidates for the source of AME is small spinning dust grains. Such grains should be very small (approx. ≤1 nm diameter) in order for the rotational emission to fall within the observed frequency range. In addition, these nanosized grains should possess a significant dipole moment to account for the observed emissivities. These constraints have been shown to be compatible with spinning bare nanosilicate clusters, assuming that ∼1% of the total Si mass budget is held in these ultrasmall grains. Silicate dust can be hydroxylated by processing in the interstellar medium and is generally known to provide seeds for molecular water ice nucleation in denser regions. Herein, we use quantum chemical calculations to investigate how the dipole moment of Mg-rich pyroxenic (MgSiO3) nanoclusters is affected by both accretion of molecular water and dissociative hydration. Our work thus provides an indication of how the formation of water ice mantles is likely to affect the capacity of nanosilicates to generate AME

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

[eng] 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.[spa] 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 infraroja. 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

Computing Free Energies of Hydroxylated Silica Nanoclusters: Forcefield versus Density Functional Calculations

We assess the feasibility of efficiently calculating accurate thermodynamic properties of (SiO2)n·(H2O)m nanoclusters, using classical interatomic forcefields (FFs). Specifically, we use a recently parameterized FF for hydroxylated bulk silica systems (FFSiOH) to calculate zero-point energies and thermal contributions to vibrational internal energy and entropy, in order to estimate the free energy correction to the internal electronic energy of these nanoclusters. The performance of FFSiOH is then benchmarked against the results of corresponding calculations using density functional theory (DFT) calculations employing the B3LYP functional. Results are reported first for a set of (SiO2)n·(H2O)m clusters with n = 4, 8 and 16, each possessing three different degrees of hydroxylation (R = m/n): 0.0, 0.25 and 0.5. Secondly, we consider five distinct hydroxylated nanocluster isomers with the same (SiO2)16·(H2O)4 composition. Finally, the free energies for the progressive hydroxylation of three nanoclusters with R = 0–0.5 are also calculated. Our results demonstrate that, in all cases, the use of FFSiOH can provide estimates of thermodynamic properties with an accuracy close to that of DFT calculations, and at a fraction of the computational cost

Understanding the interplay between size, morphology and energy gap in photoactive TiO2 nanoparticles

Anatase TiO 2 nanoparticles (NPs) have the potential to photocatalyse water splitting using UV light, to thus provide hydrogen fuel in a clean and sustainable manner. Such NPs have optical gaps covering a small range of relatively high energy solar photons, giving rise to low photo-efficiencies. Although anatase NPs with 10-20 nm diameters thermodynamically prefer crystalline faceted morphologies, application of physico-chemical procedures can produce more rounded NPs with amorphous shells. Such engineered metastable core-shell NPs (so-called black TiO 2 NPs) have reduced band gaps due to shell-induced band edge broadening, resulting in higher photoactivities. For 2 nm, annealing yields NPs with anatase-cores and amorphous-shells. Like larger black TiO 2 core-shell NPs, we confirm that our core-shell NPs are metastable relative to faceted anatase NPs and have significantly smaller optical gaps than faceted NPs. Our calculated gaps are in excellent agreement with experimental data, strongly supporting the validity of our NP models. Energy gap narrowing in these core-shell NPs is found to be due to broadening of valence band states induced by the amorphous shell, analogous to the mechanism proposed for black TiO 2 NPs. Our stoichiometric NPs also show that this band narrowing effect does not require the disordered shells to be non-stoichiometric or for incorporation of other atom types. Instead, we find that this effect mainly arises from 4-coordinated Ti atoms in the amorphous shell. Our careful and systematic computational investigation, using NP models of unprecedented realism, thus provides direct confirmation that the enhanced photoactivity in small spherical TiO 2 NP observed in experiment is due to the formation of metastable core-shell NPs with 4-coordinated Ti centre