(86a) Study of Water Adsorption on Nanoparticle Surface and Phase Transformation during Nanoparticle Sintering Via Molecular Dynamics Simulations of Tio2 Nanoparticles

Vishal N. Koparde and Peter T. Cummings
Vanderbilt University,
Department of Chemical Engineering, Nashville, TN 37235

Nanoparticles have been the area of active research in the recent years due to their novel and unique properties, which distinguish them from the bulk¹. Given the difficulties associated with experimental analysis at the nanoscale, these systems are good candidates for study using molecular modeling methods.

Titania nanoparticles are used as white pigments in paints and paper, as photocatalysts in wastewater treatment², in solar cells and also as a gas sensor. Titania has three naturally occurring polymorphs, namely, anatase, rutile and brookite³. As anatase and rutile are predominantly used in the prior mentioned applications, we concentrate our studies on these phases.

The DLPOLY⁴ molecular dynamics package has been used to perform all the simulations. The Matsui-Akaogi⁵ forcefield has been chosen for TiO₂ as it is computationally less demanding and gives results comparable to more complex counterparts. Water is modeled in the simulations using the SPC/E⁶ potential. For ion-water interactions, the ab initio-derived parameters⁷ are used.

Constant pressure, constant temperature simulations of anatase and rutile nanoparticles in water were performed. As the particles are manufactured under hydrothermal conditions⁸, we have chosen P = 50 kbar and T = 523K for our simulations. The simulations are repeated for 2.5, 3, 3.5 and 4 nm. Particles in water are found to be more crystalline than those in vacuum, corroborating results previously obtained for other nanoparticles⁹. Examination of water density profiles around the nanoparticles suggests that water coverage increases with particle size for both phases but that for rutile is always more than anatase at all sizes. Ordering of water molecules is restricted to a distance of about 3.2Ã? from the surface beyond which behavior similar to bulk is observed. Water residence times on the nanoparticle surfaces range from 50 to 100 ps.

Constant volume, constant energy simulations of various combinations of 3nm anatase, rutile and amorphous nanoparticles in close proximity with each other have provided interesting results. Initial temperatures of 973 and 1473K, similar to those found in the flame reactors, are used in these simulations. For rutile+anatase and rutile+amorphous simulations, phase transformation occurred leading to a dumb-bell shaped rutile particle. No phase transformations where observed in anatase+amorphous and rutile+anatase+amorphous simulations.

We acknowledge the National Science Foundation for the financial support.

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