Modeling hydroxylated nanosilica: Testing the performance of ReaxFF and FFSiOH force fields

Nucleation of zeolitic imidazolate frameworks: from molecules to nanoparticles

We have studied the clusters involved in the initial stages of nucleation of Zeolitic Imidazolate Frameworks, employing a wide range of computational techniques. In the pre-nucleating solution, the prevalent cluster is the ZnIm₄ cluster (formed by a zinc cation, Zn²⁺, and four imidazolate anions, Im^-), although clusters such as ZnIm₃, Zn₂Im₇, Zn₂Im₇, Zn₃Im₉, Zn₃Im₁₀, or Zn₄Im₁₂ have energies that are not much higher, so they would also be present in solution at appreciable quantities. All these species, except ZnIm₃, have a tetrahedrally coordinated Zn²⁺ cation. Small Zn_xIm_y clusters are less stable than the ZnIm₄ cluster. The first cluster that is found to be more stable than ZnIm₄ is the Zn₄₁Im₈₈ cluster, which is a disordered cluster with glassy structure. Bulk-like clusters do not begin to be more stable than glassy clusters until much larger sizes, since the larger cluster we have studied (Zn₁₄₄Im₂₈₈) is still less stable than the glassy Zn₄₁Im₈₈ cluster, suggesting that Ostwald's rule (the less stable polymorph crystallizes first) could be fulfilled, not for kinetic, but for thermodynamic reasons. Our results suggest that the first clusters formed in the nucleation process would be glassy clusters, which then undergo transformation to any of the various crystal structures possible, depending on the kinetic routes provided by the synthesis conditions. Our study helps elucidate the way in which the various species present in solution interact, leading to nucleation and crystal growth.

Structure and Properties of Nanosilicates with Olivine (Mg2SiO4) N and Pyroxene (MgSiO3) N Compositions

Magnesium-rich silicates are ubiquitous both terrestrially and astronomically, where they are often present as small particles. Nanosized Mg-rich silicate particles are likely to be particularly important for understanding the formation, processing, and properties of cosmic dust grains. Although astronomical observations and laboratory studies have revealed much about such silicate dust, our knowledge of this hugely important class of nanosolids largely rests on top-down comparisons with the properties of bulk silicates. Herein, we provide a foundational bottom-up study of the structure and properties of Mg-rich nanosilicates based on carefully procured atomistic models. Specifically, we employ state-of-the-art global optimization methods to search for the most stable structures of silicate nanoclusters with olivine (Mg₂SiO₄) _N and pyroxene (MgSiO₃) _N compositions with N = 1-10. To ensure the reliability of our searches, we develop a new interatomic potential that has been especially tuned for nanosilicates. Subsequently, we refine these searches and calculate a range of physicochemical properties of the most stable nanoclusters using accurate density functional theory based electronic structure calculations. We report a detailed analysis of structural and energy properties, charge distributions, and infrared vibrational spectra, where in all cases we compare our finding for nanosilicates with those of the corresponding bulk silicate crystals. For most properties considered, we find large differences with respect to the bulk limit, underlining the limitations of a top-down approach for describing these species. Overall, our work provides a new platform for an accurate and detailed understanding of nanoscale silicates.

Properties of hydrated TiO2 and SiO2 nanoclusters: dependence on size, temperature and water vapour pressure

Nanoscale titania (TiO₂) and silica (SiO₂) are massively produced technologically important nanomaterials used in a wide range of technological applications where nano-titania is the active component (e.g. water splitting, pollution remediation, self-cleaning coatings). Generally, these applications entail contact with water and a degree of hydration of these nano-oxides. Although the hydration of nano-silica has been fairly well studied, the corresponding level of microscopic understanding for nano-titania is severely lacking. Here, using accurate electronic structure calculations we perform a detailed and comprehensive study of the hydration of titania nanoclusters. Firstly, using global optimisation, we establish the most energetically stable structures of a set of (TiO₂)_M(H₂O)_N nanoclusters with sizes ranging through M = 4-16 and with N/M ratios of ≤ 1.0. Using this extensive dataset we investigate how the structures, energy gaps, and thermodynamic stabilities of these species depend on size, temperature and water vapour pressure. To provide a broader chemical context for our study we also provide this full set of data for the respective set of (SiO₂)_M(H₂O)_N nanoclusters which we use to compare and contrast their properties with those of nano-titania. Our broad systematic study thus provides a comparative and foundational reference study for a thorough understanding of how hydration affects the structure, energetics and properties of both nano-SiO₂ and nano-TiO₂.

Modelling the surface of amorphous dehydroxylated silica: the influence of the potential on the nature and density of defects

The nature and density of defects on the amorphous dehydroxylated silica surface are studied by molecular dynamics for information on the silanol groups of pretreated silica.