Faceted anhydrous anatase TiO2 nanoparticles (NPs) are found to exhibit higher than bulk electronic energy gaps and be more energetically stable than their spherical counterparts for diameters >∼2 nm. Annealing the latter gives rise to core–shell NPs with significantly lower energy gaps and improved potential for photocatalysis.

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 Mgrich 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 (Mg 2 SiO 4 ) N and pyroxene (MgSiO 3 ) 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.

Infrared (IR) spectroscopy is the main technique used to identify and characterize silicate dust grains in astronomical environments. From IR spectra, the fraction of crystalline dust particles can be estimated and used to help understand the processing of dust occurring in different astronomical environments such as the interstellar medium (ISM) and circumstellar shells. Narrow spectral lines are assigned to crystalline grains, while broad signals are usually assumed to originate from amorphous material. Herein, we accurately calculate the IR spectra and energetic stabilities of several amorphous and crystalline silicate nanograins with an astronomically common Mg-rich olivinic (Mg 2 SiO 4 ) stoichiometry and with sizes ranging from hundreds to thousands of atoms. First, unlike at larger length scales, crystalline forsterite-like grains at the nanoscale are found to be energetically metastable with respect to amorphous grains. However, from our careful analysis, we further show that the IR spectra of such nanosilicate grains cannot be unambiguously used to identify their structural nature. In particular, our work indicates that amorphous and crystalline silicate nanograins both exhibit broad IR spectra typical of noncrystalline grains, raising potential issues for estimates of the fraction of crystalline silicate dust in the ISM.

The stabilities and properties of globally optimised (TiO2)M(H2O)N and (SiO2)M(H2O)N clusters with M = 4–16 and a range of N/M ratios are studied with respect temperature and water vapour pressure. Our systematic study provides a comparative reference for understanding hydration of nano-silica and nano-titania.