Role of Sub-stoichiometric Defects in the Formation of Nano-particles

Kenneth T. Park (CNMS User), Baylor University; Minghu Pan, Vincent Meunier (CNMS Staff);
and E. Ward Plummer (ORNL & University of Tennessee)

Achievement
The key step in the formation of face-sharing TiO_x (x < 2) sub-oxides on surfaces is the surface oxidation of Ti interstitials. Although Ti is regarded as reactive, little is known concerning the reactivity of Ti interstitials toward oxygen on the surface. In this work, a combination of scanning tunneling microscopy and density functional theory allows the accurate determination of the structure and stoichiometry of Ti interstitial-based defects and their reactivity toward molecular oxygen (attached Figure and Park et al. Phys. Rev. B, 2007). Ab initio molecular dynamics calculations show that the row of Ti interstitials, as a highly under-coordinated cationic site, readily dissociates molecular oxygen. The dissociated oxygen surrounds a Ti interstitial to form an oxygen plane of a partial octahedron. On the other hand, the partially oxidized Ti₂O₂ strand exhibits much subdued reactivity. Although the line defect neither dissociates nor adsorbs molecular oxygen, it serves as a nucleation site for an oxidized Ti interstitial such as a TiO₂ molecule and for further growth of the line defect.

Significance
The crucial role that Ti interstitials play in surface reconstructions and formation of sub-stoichiometric defects has been recently elucidated by our CNMS team (Park et al. Phys. Rev. Lett. 96, 226105, 2006). As for the first case study, this work establishes the relationship between distinct local structures and stoichiometry of these Ti interstitial-based, surface defects and their chemical reactivity, exemplified in the reaction with molecular oxygen. The results from this work should be of broad interest as the interaction of oxygen with TiO₂ is essential in many photochemical and catalytic processes including the oxidation of CO by gold nanoparticles supported on titania as well as other reducible oxide supports.

Publications
K.T. Park, M. Pan, V. Meunier, and E.W. Plummer, “Re-oxidation of TiO₂(110) via Ti interstitials and Line Defects,” Phys. Rev. B 75, 245415 (2007).

K.T. Park, M. Pan, V. Meunier, and E.W. Plummer, “Surface Reconstructions of TiO₂(110) Driven by Sub-Oxides,” Phys. Rev. Lett. 96, 226105 (2006).

The theoretical research used resources of the National Center for Computational Sciences at ORNL. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. KTP is supported by the Baylor University Research Committee.