Effects of heat treatment on the atomic structure and surface energy of rutile and anatase TiO2 nanoparticles under vacuum and water environments

Abstract

10Nanomaterials have become a widely used group of materials in many chemical engineering 11 applications owing to their ability to provide an enhanced level of functional properties 12 compared to their crystalline and bulk counterparts. Here we report fundamental level 13 advancements on how the anatase and rutile phase of TiO2 nanoparticles chemo-thermally 14 respond between room temperature and the melting temperature under both vacuum and water 15 environments. The current study is based on using molecular dynamics (MD) simulations. We 16 present results on the equilibrium crystal morphology of these phases, structural and surface 17 energy of TiO2 nanoparticles in the size range of 2-6 nm under different temperatures. 18Thermodynamic and structural properties, in the form of potential energy and Radial 19Distribution Functions (RDF's) respectively, are calculated for both forms of TiO2 20 nanoparticles. The temperature associated with the melting transition increased with an 21 increase in the particle size in both the phases. The potential energy change associated with 22 the melting transition for anatase was seen to be less than that for rutile nanoparticles. Also 23 the temperature at which the RDF's began to stretch and broaden was observed to be lower for

Structures and orientation-dependent interaction forces of titania nanowires using molecular dynamics simulations

“…Experimental unit cell parameters and space group for anatase (Horn et al, 1972;Naicker et al, 2005) The typical average length (L):diameter (D) ratio of nanowires used in previous experiments range between about 10:1 and 20:1 (Shirale et al, 2010;Moshofsky and Mokari, 2013;Gudiksen et al, 2001), however here, this ratio is limited to 6:1 due to computational restrictions on the total number of atoms. The information obtained from the current simulations is useful and indicative of the nano scale properties of TiO2 in different shapes (Okeke et al, 2016). The sizes of the particles and their corresponding number of Ti and O atoms are presented in Table 2.…”

“…Also, the radial distribution functions (RDF's) of the nanoparticles were linked to the variation of potential energy of the system with temperature. It was observed that changes in the gradient of a plot of potential energy versus temperature associated with the melting transition occurred at temperatures similar to those associated with the broadening of peaks in the RDFs (Okeke et al, 2016).…”

“…The direct Coulomb summation technique available in DL_POLY was used in calculating the electrostatic interactions. Simulations were carried out for a period of 1 ns, sufficiently long to achieve a steady state in the atomic structure of the particles (Okeke et al, 2016). A time step of 1 fs was used in the simulations.…”

“…The melting point of the TiO2 nano-rods can be estimated from the change in slope of the temperature dependence of the potential energy manifested in the total energy profiles. Melting points and phase transition temperatures have been estimated previously for anatase spherical nanoparticles from total energy profiles (Okeke et al, 2016), where the melting transition temperature was observed to range between 1500 and 2000 K for nanoparticles diameters in the range 3 to 4 nm, and 2000…”

“…It exists in three crystalline forms namely rutile, anatase and brookite, with rutile and anatase, being the most important forms. Using molecular dynamics simulations, studies on TiO2 nanospheres have been carried out previously, to understand their thermal characteristics such as melting transition temperature, as well as structural properties (Okeke et al, 2016). In this, the melting point of the nanospheres was evaluated from the variation in the potential energy of the system as a function of temperature.…”