Hydrogen in rutile TiO2

Tor S. Bjørheim ,  Svein Stølen ,  Truls E. Norby 

University of Oslo, Department of Chemistry, Oslo N-0315, Norway

Abstract
Rutile TiO2 is a wide-band gap metal oxide semiconductor with a variety of potential applications involving its electrical, optical and catalytic properties. The point defect chemistry governing its properties has been extensively studied since the 1960s, and has recently been reviewed by Nowotny et al.[1]. However, hydrogen defects, such as protons are often neglected. In this study, we utilize ab initio DFT calculations and molecular dynamics (MD) to elaborate on the behaviour of hydrogen defects in rutile TiO2, and predict the relative dominance of protons and oxygen vacancies as positive defects.

All calculations were carried out using DFT as implemented in VASP, using GGA-PBE  and the PAW method with a cut-off energy of 500 eV.

The calculations show that both OHOq and  HOq  form shallow donor levels (Fig. 1a). Of the three hydrogen defects, protonic defects have the lowest formation energy, even at low oxygen activities, and will thus be the dominating species. However, the formation energy of HO is only about 0.35 eV higher, and may thus form in small quantities. From the formation energy of OHO and vO••, we have determined the enthalpy of hydration of oxygen vacancies:

H2O(g) + OO× + vO••= 2OHO

to ∆HHydr = -1.57 eV. As such, rutile TiO2 can be expected to be dominated by protons at elevated temperatures, even under relatively dry conditions.

A MD simulation in the NVT ensemble at 2000K (Fig.1b and c) shows an anisotropic proton diffusion in rutile, with diffusion along the c-axis dominating over diffusion in the a-b-plane, in good agreement with experiments [2]

(a)

E-MRS_fig1b_2.PNG

(b)

(c)
Figure 1: (a) Defect formation energies as a function of Fermi level, (b) and (c) Proton positions extracted from a MD simulation of 50 ps at 2000K plotted on a static lattice, viewed along the c- and b-axis, respectively

1. M.K. Novotny, L.R. Shepard, T. Bak, J. Nowotny, J. Phys. Chem. C 112, (2008), 5275
2. O.W. Johnson, J.W. DeFord, S.H. Paek, Materials Science Res. 9, (1975), 253


 

Presentation: Oral at E-MRS Fall Meeting 2009, Symposium G, by Tor S. Bjørheim
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-25 11:53
Revised:   2009-11-25 10:33