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

A neural network parametrized coagulation rate model for <3 nm titanium dioxide nanoclusters

Coagulation is a key factor governing the size distribution of nanoclusters during the high temperature synthesis of metal oxide nanomaterials. Population balance models are strongly influenced by the coagulation rate coefficient utilized. Although simplified coagulation models are often invoked, the coagulation process, particularly for nanoscale particles, is complex, affected by the coagulating nanocluster sizes, the surrounding temperature, and potential interactions. Toward developing improved models of nanocluster and nanoparticle growth, we have developed a neural network (NN) model to describe titanium dioxide (TiO₂) nanocluster coagulation rate coefficients, trained with molecular dynamics (MD) trajectory calculations. Specifically, we first calculated TiO₂ nanocluster coagulation probabilities via MD trajectory calculations varying the nanocluster diameters from 0.6 to 3.0 nm, initial relative velocity from 20 to 700 m s^-1, and impact parameter from 0.0 to 8.0 nm. Calculations consider dipole-dipole interactions, dispersion interactions, and short-range repulsive interactions. We trained a NN model to predict whether a given set of nanocluster diameters, impact parameter, and initial velocity would lead to the outcome of coagulation. The accuracy between the predicted outcomes from the NN model and the MD trajectory calculation results is >95%. We subsequently utilized both the NN model and MD trajectory calculations to examine coagulation rate coefficients at 300 and 1000 K. The NN model predictions are largely within the range 0.65-1.54 of MD predictions, and importantly NN predictions capture the local minimum coagulation rate coefficients observed in MD trajectory calculations. The NN model can be directly implemented in population balances of TiO₂ formation.

Photonic Band Gap and Bactericide Performance of Amorphous Sol-Gel Titania: An Alternative to Crystalline TiO₂

In addition to its traditional application in white pigments, nanocrystalline titania (TiO₂) has optoelectronic and photocatalytic properties (strongly dependent on crystallinity, particle size, and surface structure) that grant this naturally occurring oxide new technological applications. Sol-gel is one of the most widely used methods to synthesize TiO₂ films and NPs, but the products obtained (mostly oxy-hydrated amorphous phases) require severe heat-treatments to promote crystallization, in which control over size and shape is difficult to achieve. In this work, we obtained new photocatalytic materials based on amorphous titania and measured their electronic band gap. Two case studies are reported that show the enormous potential of amorphous titania as bactericide or photocatalyst. In the first, amorphous sol-gel TiO₂ thin films doped with N (TiO2−xNx, x = 0.75) were designed to exhibit a photonic band gap in the visible region. The identification of Ti-O-N and N-Ti-O bindings was achieved by XPS. The photonic band gaps were found to be 3.18 eV for a-TiO₂ and 2.99 eV for N-doped a-TiO₂. In the second study, amorphous titania and amine-functionalized amorphous titania nanoparticles were synthetized using a novel base-catalysed sol-gel methodology. All the synthesized amorphous TiO₂ nanoparticles exhibit bactericide performance (E. coli, ASTME 2149-13).

The structural and electronic properties of reduced amorphous titania

Reduced amorphous titania has been modeled by removing oxygen atoms to clarify the properties of these materials.

Atomistic investigation into the mechanical behaviour of crystalline and amorphous TiO2 nanotubes

The mechanical properties of titanium dioxide (TiO₂) nanotubes are studied based on molecular dynamics simulations.

Amorphization and thermal stability of aluminum-based nanoparticles prepared from the rapid cooling of nanodroplets: effect of iron addition

Despite an intensive investigation on bimetallic nanoparticles, little attention has been paid to their amorphization in the past few decades. The study of amorphization on a nanoscale is of considerable significance for the preparation of amorphous nanoparticles and bulk metallic glass. Herein, we pursue the amorphization process of Al-based nanoparticles with classic molecular dynamics simulations and local structural analysis techniques. By a comparative study of the amorphization of pure Al and Fe-doped Al-based nanodroplets in the course of rapid cooling, we find that Fe addition plays a very important role in the vitrification of Al-based nanodroplets. Owing to the subsurface segregated Fe atoms with their nearest neighbors tending to form relatively stable icosahedral (ICO) clusters, the Fe-centred cluster network near the surface effectively suppresses the crystallization of droplets from surface nucleation and growth as the concentration of Fe attains a certain value. The glass formation ability of nanodroplets is suggested to be enhanced by the high intrinsic inner pressure as a result of small size and surface tension, combined with the dopant-inhibited surface nucleation. In addition, the effect of the size and the added concentration of nanoparticles on amorphization and the thermal stability of the amorphous nanoparticles are discussed. Our findings reveal the amorphization mechanism in Fe-doped Al-based nanoparticles and provide a theoretical guidance for the design of amorphous materials.

Atomic layer deposition-derived ultra-low-density composite bulk materials with deterministic density and composition

A universal approach for on-demand development of monolithic metal oxide composite bulk materials with air-like densities (<5 mg/cm(3)) is reported. The materials are fabricated by atomic layer deposition of titania (TiO2) or zinc oxide (ZnO) using the nanoscale architecture of 1 mg/cm(3) SiO2 aerogels formed by self-organization as a blueprint. This approach provides deterministic control over density and composition without affecting the nanoscale architecture of the composite material that is otherwise very difficult to achieve. We found that these materials provide laser-to-X-ray conversion efficiencies of up to 5.3%, which is the highest conversion efficiency yet obtained from any foam-based target, thus opening the door to a new generation of highly efficient laser-induced nanosecond scale multi-keV X-ray sources.

A theoretical study of the optical properties of nanostructured TiO2

Optical properties of TiO(2) nanoclusters (with more than 30 TiO(2) units) were calculated within a fully atomistic quantum dynamic framework. We use a time dependent tight-binding model to describe the electronic structure of TiO(2) nanoclusters in order to compute their optical properties. We present calculated absorption spectra for a series of nanospheres of different radii and crystal structures. Our results show that bare TiO(2) nanoclusters have the same adsorption edge for direct electronic transition independently of the crystal structure and the nanocluster size. We report values of the adsorption edge of around 3.0 eV for all structures analyzed. In the present work we demonstrate that, for small clusters, both the direct transition absorption edge and the blue shifting phenomena are masked by thermal disorder.

Anodic growth of large-diameter multipodal TiO2 nanotubes

We report on the formation of a new class of nanostructures, namely, multipodal hollow titania nanotubes possessing two or more legs, achieved during the electrochemical anodization of titanium in diethylene glycol (DEG)-based electrolytes. The unique multipodal porous structure is expected to extend and enhance the applications of TiO(2) nanotube arrays. Multipodal nanotubes form by a process we term "nanotube combination", which only occurs in viscous electrolytes at high anodization potentials in the presence of a low concentration of fluoride-bearing species. The mechanism of formation of multipodal nanotubes is considered, and the tube length at which nanotube combination occurs is predicted theoretically using a simplified analytical model. The results suggest that capillary forces strong enough to bend the TiO(2) nanotubes by tens of degrees are generated during the imbibition of electrolyte into and out of the intertubular spaces between adjacent tapered nanotubes.

Modelling nano-clusters and nucleation

We review the growing role of computational techniques in modelling the structures and properties of nano-particulate oxides and sulphides. We describe the main methods employed, including those based on both electronic structure and interatomic potential approaches. Particular attention is paid to the techniques used in searching for global minima in the energy landscape defined by the nano-particle cluster. We summarise applications to the widely studied ZnO and ZnS systems, to silica nanochemistry and to group IV oxides including TiO(2). We also consider the special case of silica cluster chemistry in solution and its importance in understanding the hydrothermal synthesis of microporous materials. The work summarised, together with related experimental studies, demonstrates a rich and varied nano-cluster chemistry for these materials.

Molecular dynamics simulation of liquid and amorphous Fe nanoparticles

Liquid and amorphous Fe nanoparticles have been studied in spherical models with sizes of 3, 4 and 5 nm. We use the molecular dynamics (MD) method with the Pak-Doyam interatomic pair potential. Structural properties of Fe nanoparticles have been studied via radial distribution function (RDF), interatomic distances, coordination number distribution and radial density profile. Local icosahedral order was detected via using Honeycutt-Andersen (HA) analysis compared with that observed in the bulk counterparts. Surface and core structures of Fe nanoparticles have been studied in detail. Potential and surface energies of liquid and amorphous Fe nanoparticles have been calculated and discussed in addition to the size dependence of a glass transition temperature.

Static and thermodynamic properties of liquid and amorphous Fe(2)O(3) nanoparticles

Molecular dynamics (MD) simulation of the structure and thermodynamic properties of liquid and amorphous Fe(2)O(3) nanoparticles, with different sizes ranging from 2 to 5 nm, has been carried out based on the Born-Mayer pair potentials under non-periodic boundary conditions. The partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances of liquid nanoparticles at 3500 K were calculated in detail. In addition, we show the radial density profile and stoichiometry in nanoparticles. The temperature dependence of the surface structure and surface energy of nanoparticles has been obtained and presented. Moreover, the size dependence of the glass transition temperature has been found and discussed.

Surface structure and structural point defects of liquid and amorphous aluminosilicate nanoparticles

The surface structure of liquid and amorphous aluminosilicate nanoparticles of composition Al(2)O(3)·2SiO(2) has been investigated in a model of different sizes ranging from 2.0 to 5.0 nm with the Born-Mayer type pair potential under non-periodic boundary conditions. Models have been obtained by cooling from the melts at a constant density of 2.6 g cm(-3) via molecular dynamics (MD) simulation. The surface structure has been investigated via the coordination number, bond-angle distributions and structural point defects. Calculations show that surface effects on surface static and thermodynamic properties of models are significant according to the change in the number of Al atoms in the surface layers. Evolution of the local environment of oxygen in the surface shell of nanoparticles upon cooling from the melt toward the glassy state was also found and discussed. In addition, the nanosize dependence of the glass transition temperature was presented.