The interaction of hydrogen with the {010} surfaces of Mg and Fe olivine as models for interstellar dust grains: a density functional theory study

Silicate-mediated interstellar water formation: A theoretical study

Water is one of the most abundant molecules in the form of solid ice phase in the different regions of the interstellar medium (ISM). This large abundance cannot be properly explained by using only traditional low temperature gas-phase reactions. Thus, surface chemical reactions are believed to be major synthetic channels for the formation of interstellar water ice. Among the different proposals, hydrogenation of atomic O (i.e., 2H + O → H2O) is a chemically "simple" and plausible reaction toward water formation occurring on the surfaces of interstellar grains. Here, novel theoretical results concerning the formation of water adopting this mechanism on the crystalline (010) Mg2SiO4 surface (a unequivocally identified interstellar silicate) are presented. The investigated reaction aims to simulate the formation of the first water ice layer covering the silicate core of dust grains. Adsorption of the atomic O as a first step of the reaction has been computed, results indicating that a peroxo (O 2 2 - ) group is formed. The following steps involve the adsorption, diffusion and reaction of two successive H atoms with the adsorbed O atom. Results indicate that H diffusion on the surface has barriers of 4-6 kcal mol-1, while actual formation of OH and H2O present energy barriers of 22-23 kcal mol-1. Kinetic study results show that tunneling is crucial for the occurrence of the reactions and that formation of OH and H2O are the bottlenecks of the overall process. Several astrophysical implications derived from the theoretical results are provided as concluding remarks.

Determination of the Optimal Sensing Temperature in Pt/Ta₂O₅/MoO₃ Schottky Contacted Nanobelt Straddling Heterojunction

Nanostructured Schottky barrier gas sensors have emerged as novel semiconductor devices with large surface areas and unique electronic characteristics. Although it is widely known that operating these gas sensors requires heating to an optimal temperature for the highest sensitivity, the fundamental mechanism that governs the temperature-dependent sensitivity has yet been well understood. In this work, we present new evidence to support that thermionic field emission (TFE) is the dominant transport mechanism for Schottky contacted nanostructured heterojunction gas sensors at their optimal sensing temperature. Through the fabrication and characterization of Pt/MoO₃ Schottky contacts, and Pt/Ta₂O₅/MoO₃ heterojunctions, we found a previously unreported connection between TFE transport and optimal gas sensing temperature. This connection enables the description of Schottky barrier gas sensing performance using transport theory, which is a major step towards systematic engineering of gas sensors with nanostructured high-k oxide layers.

Mineral Surface Rearrangement at High Temperatures: Implications for Extraterrestrial Mineral Grain Reactivity

Mineral surfaces play a critical role in the solar nebula as a catalytic surface for chemical reactions and potentially acted as a source of water during Earth's accretion by the adsorption of water molecules to the surface of interplanetary dust particles. However, nothing is known about how mineral surfaces respond to short-lived thermal fluctuations that are below the melting temperature of the mineral. Here we show that mineral surfaces react and rearrange within minutes to changes in their local environment despite being far below their melting temperature. Polished surfaces of the rock and planetary dust-forming silicate mineral olivine ((Mg,Fe)₂SiO₄) show significant surface reorganization textures upon rapid heating resulting in surface features up to 40 nm in height observed after annealing at 1200 °C. Thus, high-temperature fluctuations should provide new and highly reactive sites for chemical reactions on nebula mineral particles. Our results also may help to explain discrepancies between short and long diffusion profiles in experiments where diffusion length scales are of the order of 100 nm or less.

H2 Formation on Cosmic Grain Siliceous Surfaces Grafted with Fe+ : A Silsesquioxanes-Based Computational Model

Cosmic siliceous dust grains are involved in the synthesis of H₂ in the inter-stellar medium. In this work, the dust grain siliceous surface is represented by a hydrogen Fe-metalla-silsesquioxane model of general formula: [Fe(H₇ Si₇ O_12-n )(OH)_n ]⁺ (n=0,1,2) where Fe⁺ behaves like a single-site heterogeneous catalyst grafted on a siliceous surface synthesizing H₂ from H. A computational analysis is performed using two levels of theory (B3LYP-D3BJ and MP2-F12) to quantify the thermodynamic driving force of the reaction: [Fe-T7H₇ ]⁺ +4H→[Fe-T7H₇ (OH)₂ ]⁺ +H₂ . The general outcomes are: 1) H₂ synthesis is thermodynamically strongly favored; 2) Fe-H / Fe-H₂ barrier-less formation potential; 3) chemisorbed H-Fe leads to facile H₂ synthesis at 20≤T≤100 K; 4) relative spin energetics and thermodynamic quantities between the B3LYP-D3BJ and MP2-F12 levels of theory are in qualitative agreement. The metalla-silsesquioxane model shows how Fe⁺ fixed on a siliceous surface can potentially catalyze H₂ formation in space.

Does Fe(2+) in olivine-based interstellar grains play any role in the formation of H2? Atomistic insights from DFT periodic simulations

Using periodic DFT-D2 methods, atomistic simulations of interstellar H adsorption and H2 formation on a (010) Fe-containing olivine surface are presented. At variance with the (010) Mg2SiO4 surface and key to these processes are the large Fe/H interaction energies, suggesting that olivine surfaces are good reservoirs of H atoms for subsequent recombination to form H2.

First-principle modelling of forsterite surface properties: Accuracy of methods and basis sets

The seven main crystal surfaces of forsterite (Mg2 SiO4 ) were modeled using various Gaussian-type basis sets, and several formulations for the exchange-correlation functional within the density functional theory (DFT). The recently developed pob-TZVP basis set provides the best results for all properties that are strongly dependent on the accuracy of the wavefunction. Convergence on the structure and on the basis set superposition error-corrected surface energy can be reached also with poorer basis sets. The effect of adopting different DFT functionals was assessed. All functionals give the same stability order for the various surfaces. Surfaces do not exhibit any major structural differences when optimized with different functionals, except for higher energy orientations where major rearrangements occur around the Mg sites at the surface or subsurface. When dispersions are not accounted for, all functionals provide similar surface energies. The inclusion of empirical dispersions raises the energy of all surfaces by a nearly systematic value proportional to the scaling factor s of the dispersion formulation. An estimation for the surface energy is provided through adopting C6 coefficients that are more suitable than the standard ones to describe O-O interactions in minerals. A 2 × 2 supercell of the most stable surface (010) was optimized. No surface reconstruction was observed. The resulting structure and surface energy show no difference with respect to those obtained when using the primitive cell. This result validates the (010) surface model here adopted, that will serve as a reference for future studies on adsorption and reactivity of water and carbon dioxide at this interface.

Interstellar H adsorption and H₂ formation on the crystalline (010) forsterite surface: a B3LYP-D2* periodic study

The physisorption/chemisorption of atomic hydrogen on a slab model of the Mg2SiO4 forsterite (010) surface mimicking the interstellar dust particle surface has been modeled using a quantum mechanical approach based on periodic B3LYP-D2* density functional calculations (DFT) combined with flexible polarized Gaussian type basis sets, which allows a balanced description of the hydrogen/surface interactions for both minima and activated complexes. Physisorption of hydrogen is barrierless, very weak and occurs either close to surface oxygen atoms or on Mg surface ions. The contribution of dispersion interactions accounts for almost half of the adsorption energy. Both the hydrogen adsorption energy and barrier to hydrogen jump between equivalent surface sites are overestimated compared to experimental results meant to simulate the interstellar conditions in the laboratory. The hydrogen atom exclusively chemisorbs at the oxygen site of the forsterite (010) surface, forming a SiOH surface group and its spin density being entirely transferred to the neighboring Mg ion. Barrier for chemisorption allows rapid attachment of H at the surface at 100 K, but prevents the same process from occurring at 10 K. From this H-chemisorbed state, the second hydrogen chemisorption mainly occurs on the neighboring Mg ion, thus forming a Mg-H surface group, giving rise to a surface species stabilized by favorable electrostatic interactions between the OHH-Mg pair. The formation of molecular hydrogen at the (010) forsterite surface adopting a Langmuir-Hinshelwood mechanism takes place either starting from two physisorbed H atoms with an almost negligible kinetic barrier through a spin-spin coupling driven reaction or from two chemisorbed H atoms with a barrier surmountable even at T higher than 10 K. We also suggest that a nanosized model of the interstellar dust built from a replica of the forsterite unit cell is able to adsorb half the energy released by the H2 formation by increasing its temperature by about 50 K which could then radiate in about 0.02 s.

Trends in the adsorption and reactivity of hydrogen on magnesium silicate nanoclusters

We investigate the potential role of ultrasmall silicate grains in interstellar hydrogen-based chemistry by modelling H adsorption and H₂ formation/dissociation on nanosilicates.

B3LYP periodic study of the physicochemical properties of the nonpolar (010) Mg-pure and fe-containing olivine surfaces

B3LYP periodic simulations have been carried out to study some physicochemical properties of the bulk structures and the corresponding nonpolar (010) surfaces of Mg-pure and Fe-containing olivine systems; i.e., Mg2SiO4 (Fo) and Mg1.5Fe0.5SiO4 (Fo75). A detailed structural analysis of the (010) Fo and Fo75 surface models shows the presence of coordinatively unsaturated metal cations (Mg(2+) and Fe(2+), respectively) with shorter metal-O distances compared to the bulk ones. Energetic analysis devoted to the Fe(2+) electronic spin configuration and to the ion position in the surfaces reveals that Fe(2+) in its quintet state and placed at the outermost positions of the slab constitutes the most stable Fe-containing surface, which is related to the higher stability of high spin states when Fe(2+) is coordinatively unsaturated. Comparison of the simulated IR and the corresponding reflectance spectra indicates that Fe(2+) substitution induces an overall bathochromic shift of the spectra due to the larger mass of Fe compared to Mg cation. In contrast, the IR spectra of the surfaces are shifted to upper values and exhibit more bands compared to the corresponding bulk systems due to the shorter metal-O distances given in the coordinatively unsaturated metals and to symmetry reduction which brings nonequivalent motions between the outermost and the internal modes, respectively.