Development Of A Reaxff Reactive Force Field For Titanium Dioxide/ Water Systems And Its Applications To Etching, Nanoparticles With Organic Solvents And Ion Adsorptions On Nanocrystalline Surfaces

Open Access
Author:
Kim, Sung-yup
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 18, 2012
Committee Members:
  • Adrianus C Van Duin, Dissertation Advisor
  • James David Kubicki, Committee Chair
  • Richard A Yetter, Committee Member
  • Donghai Wang, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
Keywords:
  • ReaxFF
  • Reactive force field
  • titanium dioxide
  • nanoparticle
Abstract:
A new ReaxFF reactive force field has been developed to describe reactions in the Ti-O-H system. The ReaxFF force field parameters have been fitted to a quantum mechanical (QM) training set containing structures and energies related to bond dissociation energies, angle and dihedral distortions, and reactions between water and titanium dioxide as well as experimental crystal structures, heats of formation and bulk modulus data. Model configurations for the training set were based on DFT calculations on molecular clusters and periodic systems (both bulk crystals and surfaces). ReaxFF reproduces accurately the QM training set for structures and energetics of small clusters. ReaxFF also describes the relative energetics for rutile, brookite and anatase. The results of ReaxFF match reasonably well with those of QM for water binding energies, surface energies and H2O dissociation energy barriers. To validate this ReaxFF description, we have compared its performance against DFT/MD simulations for 1 and 3 monolayers of water interacting with a rutile (110) surface. We found agreement within 10% error between the DFT/MD and ReaxFF water dissociation levels for both coverages. In addition to this, a new Ti-Cl force field parameter has been developed to describe reactions in Ti/O/H/Cl materials. The ReaxFF force field parameters are fitted against a quantum mechanical (QM) training set containing structures and energies related to bond dissociations, angle and dihedral distortions, and reactions between titanium and chlorine gases as well as heats of formation of TiClx crystals. These newly developed Ti-Cl force field parameters were combined with the recently developed Ti-O-H force field. ReaxFF accurately reproduces the QM training set for structures and energetics of small clusters and TiClx crystals. This force field was applied to etching simulations for titanium metal and titanium dioxide with chlorine and hydrogen chloride gases. In the etching simulations, titanium and titanium dioxide slab models with chlorine and hydrogen chloride gases were used in molecular dynamics simulations. The etching ratio between HCl and Cl2 are compared to experimental results and satisfactory results are obtained, indicating that this ReaxFF extension provides a useful tool for studying the atomistic-scale details of the etching process. In addition, simulations of TiO2 (both rutile and anatase) nanoparticles with water, methanol and formic acid were conducted by combining C parameter with the Ti/O/H parameters to investigate the characteristic behavior of reactivity to these organic solvents. The force field was validated by comparing water dissociative adsorption percentage and bond length between Na and O with DFT and experimental results. In the simulations, 1 nm rutile and anatase nanoparticles with water, methanol and formic acid were used, respectively. The numbers of attached hydroxyl with time and nanoparticles distortion levels are presented. We found that the rutile nanoparticle is more reactive than the anatase nanoparticle and that formic acid distorts nanoparticles more than water and methanol. By adding Ca parameters with the previously developed Ti/O/H/Cl force field, simulations of TiO2 nanocrystalline surfaces in aqueous solutions with Ca and Cl ions were conducted. Before that, the force field was validated by comparing bond length between Ca and O with experimental results. The energetic and structures of hydrated ions was investigated. The numbers of OH and Ca ions attached to the surfaces with respect to time were tracked. Also, the mechanism of ion adsorption caused by proton-induced charge transfer and Ca ions diffusion behavior was analyzed. In addition to these, the ion adsorption characteristic behaviors of 5 different specific faces for anatase and rutile were evaluated.