Titanium Dioxide
Crystal Structure
Recently, TiO2 (B) has been added as the fourth naturally occurring TiO2 polymorph found at very low pressures (Banfield, Veblen et al. 1991). This implies that transition energetics is closely related to particle size and that anatase is the more stable form at the nanoscale (Barnard, Zapol et al. 2005).
Manufacture of nano-TiO 2
Yin, Li et al. 2002) of nano-TiO2, namely hydrolysis of titanium alkoxides and hydrolysis of titanium tetrachloride or titanium trichloride (Sun, Gao et al. 2003; Sun, Gao et al. 2003). The size and phase of the particles formed are greatly influenced by the solvent and the stabilizer used (Trung, Cho et al. 2003).
Overview of Dissertation
This proves that the orientation of nanoparticles is extremely important in the sintering process. The fact that the surface ions of nanoparticles have the same charge as the ions in the core.
SIMULATION METHODOLOGY
Background
In its early days, the molecular dynamics method gained popularity in materials science. The extent of the success of the molecular dynamics technique is dependent on the availability of computing power.
Newton’s equations of motion
- Forcefield for TiO 2
The relative mobilities of ions in the neck and core regions of the sintering nanoparticles are also reported. These vectors are the dipole vector ( .. r n), the vector connecting the two hydrogen atoms of the water molecule.
SINTERING OF TiO 2 NANOPARTICLES
Introduction
Six different mechanisms (Zeng, Zajac et al. 1998) are believed to contribute to the sintering of larger particles, namely (1) surface diffusion, (2) lattice diffusion from surface, (3) vapor transport, (4) grain boundary diffusion, (5) lattice diffusion from grain boundary and (6) lattice diffusion through dislocations. Zhu and Averback (Zhu and Averback 1995; Zhu and Averback 1996-a; Zhu and Averback 1996-b) performed molecular dynamics simulations of sintering of copper nanoparticles and nanocylinders, while Raut and colleagues (Raut, Bhagat et al. studies 1998 reported) of sintering aluminum nanoparticles.
Simulation Details
- Forcefield Selection
- Simulation Method
The coordination number of Ti ions along the nanoparticle radius is shown in Figure III.3. However, the temperature increase appears to be independent of the starting temperature of the simulation.
Introduction
There is also a hypothesis that the extremely high internal pressure of the smaller particles is responsible for the inapplicability of sintering models for these particles. Such a high internal pressure directly affects the diffusion of atoms or ions in the particle, which in turn affects the kinetics and mechanism of sintering. The sintering phenomenon was studied from an atomic point of view using molecular dynamics simulations in Chapter III.
In flame reactors where TiO2 nanoparticles are produced, the nanoparticles are found to be in an adiabatic environment for sintering as they are present in a low-pressure dilute gas phase. Rare collisions with gaseous molecules ensure that the total energy of the sintered particles remains constant and does not dissipate. Therefore, an accurate knowledge of the melting points of nanoparticles is required before modeling the sintering process.
Melting point of TiO 2 nanoparticles
- Simulation Details
- Simulated X-ray Diffraction
- Pair Correlation Function
- Diffusivity
- Lindemann Index
Loss of crystallinity is indicated by a decrease in peak height at ~3.05 Å. Since approximately 35% to 45% of the total ions reside in this layer for the nanoparticles considered here (particle diameters between 2.5 nm and 5 nm), the overall diffusivity of solid nanoparticles is expected to be slightly higher than that of solids in in bulk. Ionic diffusivity values are of the order of 10-11 m2/s at lower temperatures and increase to 10-9 m2/s at higher temperatures, indicating that melting has occurred.
Lindemann indices showing the temperature of anatase and rutile nanoparticles of different sizes are shown in Figure IV.11. The melting points of 5 nm anatase and rutile nanoparticles obtained by analyzing the X-ray diffraction patterns, pair correlation functions, ionic diffusivities and the Lindemann indices are shown in Figure IV.12. Tm is the bulk melting temperature in K, L is the latent heat of fusion in J/kg.
Model Development
- Surface Area Calculation
- Model Predictions
The assumption correctly predicts the overall trend, but breaks down when applied to time scales of the order of 100–103 picoseconds, which is the time scale relevant to the molecular dynamics simulations. The surface area must be calculated at each time step of the simulation to determine da/dt. If the mesh point under consideration is within the Van der Waals radii of any of the ions in the nanoparticles, then that point is counted in the volume calculation.
This may account for the slow decrease in area after about 25ps from the start of the simulation. It has also been shown that as the temperature increases, the surface of the particles becomes more agitated and liquid-like, which lowers the surface tension and pushes it towards the liquid surface tension value ("l=0.38J/m2 (Ikemiya, Umemoto) et al., 1993; Li and Ishigaki 2002).This gives credibility to the use of the Meyer method to determine what da/dt should be used in the model and therefore confirms the inapplicability of the Koch and Friedlander rate law to TiO2 nanoparticle sintering.
Conclusions
34;Effect of the brookite phase on the anatase-rutile transition in titanium oxide nanoparticles." Journal of the European Ceramic Society. 34;Synthesizing and comparing the photocatalytic properties of high surface area rutile and anatase titanium oxide nanoparticles." Journal of the Ceramic Society of America. 34;Atomistic simulation of crystal structures and bulk moduli of TiO2 polymorphs." Journal of Physics and Chemistry of Solids.
Influence of peptization and peptizing agents on the crystalline phases and phase transitions." Journal of the American Ceramic Society. 34; Molecular dynamics calculation of nanoparticle sintering in the gas phase: a comparison with phenomenological models." Journal of Aerosol Science. 34; Understanding the behavior of polymorphic phase transformation during the growth of nanocrystalline aggregates: insights from TiO2." Journal of Physical Chemistry B.
WATER ADSORPTION ON TiO 2 NANOPARTICLE SURFACE
Introduction
In the interfacial region, water properties and nanoparticle structure both deviate from the characteristic bulk phases (Stumm 1992). TiO2 nanoparticles are insoluble in water and found various photocatalytic and photovoltaic applications in aqueous form as slurries or colloids (Ohtani, Okugawa et al. 1987; Augugliaro, Loddo et al. 1995). Molecular dynamics has proven to be an essential technique for studying the fundamentals of adsorption at the atomic level.
Most of these studies carried out using molecular dynamics were aimed at studying water structure at planar surfaces (Cummings, Predota et al. Very few simulation attempts to investigate water (or other liquid) adsorption at the surface of nanoparticles ( Qin and Fichthorn 2003; Ju) 2005) are found in the literature. Since rutile and anatase are more important polymorphs of TiO2, due to their large number of applications, brookite is not considered in the study.
Simulation Details
- Forcefield Selection
- Simulation Method
Molecular dynamics simulations at constant temperature and constant pressure were performed using DL_POLY version 2.13. DL_POLY uses Verlet's leapfrog algorithm in combination with the multiple time step method to integrate Newton's laws of motion over time. The size of the box is chosen such that the interaction between the particle and its own image in the adjacent box is negligible.
The solvent involved in the wet process is generally hydrothermal water (Cheng, Ma et al. For simulations at room (ambient) temperature and pressure, values of 300 K and 1 bar are used. To generate the initial configuration file, nanoparticles equilibrated with vacuums obtained at the end of the vacuum simulations described in Chapter II are enclosed in a box and then the box is filled with SPC/E water molecules using a tool provided with DL_POLY.
Results and Discussion
- X-ray Diffraction Patterns
- Coordination Number Distributions
- Density profiles
- Water Coverage
- Water Residence Time
- Water Orientation Distributions
- Bivariate Plots
The width of the density peak is due to the fact that the surface of the nanoparticle is very rough. To distinguish between the two orientations in the first hydration shell, we defined different regions on the water diffusion profile from the surface. To investigate the strength of water adsorption at the metal oxide nanoparticle surface, we calculated the time correlation function for the water molecules within the first hydration layer from the nanoparticle surface.
To determine the orientational preference of water molecules, the probability density functions of the cosines of the angles and ". This indicates that the water molecules in region 2 have one of their r OH vectors aligned to the surface, parallel but opposite in direction of the surface normal. The uniform spatial distribution of water molecules in bulk water would lead to uniform angle distribution.
Conclusions
34; Performance of Heterogeneous Photocatalytic Systems: Influence of Operational Variables on Photoactivity of Aqueous Suspension of TiO2." Journal of Catalysis. 34; Temperatures and Kinetics of Anatase to Rutile Transformation in Doped TiO2 Heated in Microwave Field." Journal of Thermal Analysis and Calorimetry. 34;Grain growth enhanced by anatase-to-rutile transformation in gel-derived nanocrystalline titanium dioxide powders.” Journal of Alloys and Compounds.
34;Phase transformations of precipitated TiO2 nanoparticles." Materials Science & Engineering A-Structural Materials Properties Microstructure and Processing. 34;MSEED - Program for rapid analytical determination of accessible surfaces and their derivatives." Journal of Computational Chemistry. Structure of surfaces and interfacial water from molecular dynamics using ab initio potentials." Journal of Physical Chemistry B.
PHASE TRANSFORMATIONS DURING TiO 2 NANOPARTICLE
Introduction
- Insight from Prior Simulations
Rutile is the only stable phase in the bulk form, and most brookite and anatase are metastable and irreversibly converted to rutile upon heating. Many efforts have been made to understand and control the transition of anatase to rutile, as these phases have extremely different physical properties in the nanometer range. Similar reductions in the activation energy required for anatase to rutile transition have been reported for various dopants by Borkar and Dharwadkar (Borkar and Dharwadkar 2004).
The transformation of anatase to rutile is reported to be enhanced with a decrease in pH (Suresh, Biju et al. 1998), which is believed to be due to the contributions of an intermediate brookite phase (Hu, Tsai et al. 2003-a; Hu, Tsai et al. 2003-b). It is believed that the main reason for the transformation of anatase particles into rutile upon heating is the increase in particle size during enhanced sintering at higher temperatures. Brookite could therefore transform directly into rutile, while anatase could transform either directly into rutile or into brookite and then into rutile.
Simulation Details
- Forcefield Selection
- Methodology
Results and Discussion
34;Molecular dynamics simulations of solid-to-liquid phase transition in small water aggregates." Computational Materials Science. 34;A simple model for the evolution of the characteristics of aggregate particles undergoing coagulation and sintering." Aerosol science and technology. 34; Fast Molecular Surface Approximation via the Use of Boolean Logic and Lookup Tables." Journal of Computational Chemistry.
34;Effect of coalescence energy release on the temporal shape evolution of nanoparticles." Physical review B 63 (20): art. 34; Photocatalytic activity of titania powders suspended in aqueous silver nitrate solution: correlation with pH-dependent surface structures." Journal of Physical Chemistry. 34;Effect of initial powder size of alpha-Si3N3 on microstructural evolution and phase transformation during sintering of Si3N4 ceramics.
