Molecular Dynamics Simulations of Aggregation of Anatase (Titanium Dioxide) Nanocrystals

Open Access
Author:
Alimohammadi Zanjani, Mozhgan
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 17, 2011
Committee Members:
  • Kristen Ann Fichthorn, Dissertation Advisor
  • James David Kubicki, Committee Member
  • Darrell Velegol, Committee Member
  • Michael John Janik, Committee Member
  • Kristen Ann Fichthorn, Committee Chair
Keywords:
  • rutile
  • anatase
  • titanium dioxide
  • oriented attachment
  • aggregation
Abstract:
A quantitative understanding of the forces between colloidal nanoparticles would benefit numerous applications in sensing, nanoelectronics, composite materials, nanofluids, optics, and catalysis, which could exploit the unique properties associated with their small sizes. In applications involving crystallization, the aggregation of nanoparticles can be important. Recent experiments on specific materials, including anatase-TiO$_2$, have documented the phenomenon of oriented attachment, in which nanocrystals approach one another and merge along specific crystallographic directions to form twinned or single-crystalline structures. The ability to direct crystallization through oriented attachment is an exciting prospect that could allow for the creation of new nanostructures with well-defined sizes and shapes. To realize the full potential of oriented attachment for achieving the controlled synthesis of anatse nanocrystals, insight into its origins, which are not currently clear in all cases, would be beneficial. This dissertation employs molecular dynamics simulations to resolve the issues regarding anatse nanoparticle aggregation in a vacuum and in aqueous environments. By utilizing both symmetric and asymmetric nanocrystals, the majority of the observed synthesized anatase shapes have been covered. MD simulations have been carried out to determine the aggregation pathway of such nanocrystals, and by repeating the simulation with different initial configurations, the directional alignment of these nanocrystals has been discovered in vacuum environment. Also, by performing force, energy, dipole-dipole interactions, and other sorts of calculations, the origins of the preferred alignment of anatase nanocrystals in vacuum has been discovered. Oriented attachment was first discovered in experimental studies for the crystallization of anatase nanocrystals under hydrothermal aqueous conditions. For the aqueous anatase nanoparticles, the Bandura-Kubicki force field has been tested against results from first-principles calculations based on density-functional theory and refined to describe the binding of H$_2$O, H$^+$, and OH$^-$ to various crystal faces. Hence, the parameters for the available force fields have been modified to be used for anatase nanocrystals. Since MD is a very strong tool that allows solving larger systems with many water molecules, it has become possible to study the aggregation mechanism of anatase nanocrystals in an aqueous environment. Also, the difference in water adsorption on anatase surfaces under ambient and hydrothermal conditions has been studied, while hydration effects have been quantified.