Size-dependent pressure-induced amorphization in nanoscale TiO2

The glass transition and thermodynamics of liquid and amorphous TiO(2) nanoparticles

The glass transition and thermodynamics of spherical liquid TiO(2) nanoparticles, with different sizes ranging from 2 to 5 nm, have been studied in a model under non-periodic boundary conditions. We use the pairwise interatomic potentials proposed by Matsui and Akaogi. Models have been obtained by cooling from the melt via molecular dynamics (MD) simulation. The structural properties of liquid nanoparticles at 3500 K have been analyzed in detail through the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Moreover, we also show the radial density profile in nanoparticles. Calculations show that size effects on the structure of a model are significant and that liquid TiO(2) nanoparticles have a distorted pentahedral network structure with the mean coordination numbers Z(Ti-O)≈5.0 and Z(O-Ti)≈2.5, while amorphous TiO(2) nanoparticles have an octahedral network structure. The temperature dependence of the surface structure and surface energy of the nanoparticles has been obtained and is presented. In addition, the size dependence of the glass transition temperature and the temperature dependence of the diffusion constant of atomic species have been found and are discussed.

The glass transition and diffusion in simulated liquid TiO2

The glass transition and diffusion in liquid TiO₂ have been studied in a model containing 3000 atoms via molecular dynamics (MD) simulation. The density dependence of the glass transition temperature, T_g, of liquid TiO₂ has been found and is discussed. Diffusion of atomic species in 3.80 g cm^-3 TiO₂ models has been investigated over a wide temperature range from 2100 to 7000 K. We found that the temperature dependence of the diffusion constant of atomic species follows an Arrhenius law at relatively low temperatures above the melting point, and at higher temperatures it deviates from an Arrhenius law. Differences between the structures of amorphous TiO₂ models at three different densities in the range from 3.80 to 4.20 g cm^-3 have been found and are discussed. In addition, a transition from a low-density liquid (ldl) form to a high-density liquid (hdl) form was found and is discussed.

Nanomaterials under high-pressure

The use of high-pressure for the study and elaboration of homogeneous nanostructures is critically reviewed. Size effects, the interaction between nanostructures and guest species or the interaction of the nanosystem with the pressure transmitting medium are emphasized. Phase diagrams and the possibilities opened by the combination of pressure and temperature for the elaboration of new nanomaterials is underlined through the examination of three different systems: nanocrystals, nano-cage materials which include fullerites and group-14 clathrates, and single wall nanotubes. This tutorial review is addressed to scientist seeking an introduction or a panoramic view of the study of nanomaterials under high-pressure.