Oxygen vacancies versus fluorine at CeO2(111): a case of mistaken identity?
We propose a resolution to the puzzle presented by the surface defects observed with STM at the (111) surface facet of CeO 2 single crystals. In the seminal paper of Esch et al. [Science 309, 752 (2005)] they were identified with oxygen vacancies, but the observed behavior of these defects is inconsistent with the results of density functional theory (DFT) studies of oxygen vacancies in the literature. We resolve these inconsistencies via DFT calculations of the properties of both oxygen vacancies and fluorine impurities at CeO2(111), the latter having recently been shown to exist in high concentrations in single crystals from a widely used commercial source of such samples. We find that the simulated filled-state STM images of surface-layer oxygen vacancies and fluorine impurities are essentially identical, which would render problematic their experimental distinction by such images alone. However, we find that our theoretical results for the most stable location, mobility, and tendency to cluster, of fluorine impurities are consistent with experimental observations, in contrast to those for oxygen vacancies. Based on these results, we propose that the surface defects observed in STM experiments on CeO2 single crystals reported heretofore were not oxygen vacancies, but fluorine impurities. Since the similarity of the simulated STM images of the two defects is due primarily to the relative energies of the 2p states of oxygen and fluorine ions, this confusion might also occur for other oxides which have been either doped or contaminated with fluorine.
Department of Chemistry-Ångström, Uppsala University, Box 538, S-751 21 Uppsala, Sweden.
J
J
Kullgren
Department of Chemistry-Ångström, Uppsala University, Box 538, S-751 21 Uppsala, Sweden and Department of Physics & Astronomy, University College London, London WC1E 6BT, United Kingdom.
M J
MJ
Wolf
School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, United Kingdom.
C W M
CW
Castleton
Department of Chemistry-Ångström, Uppsala University, Box 538, S-751 21 Uppsala, Sweden.
P
P
Mitev
Computational Biophysics, Twente University, P.O. Box 217, AE Enschede 7500, The Netherlands.
W J
WJ
Briels
Department of Chemistry-Ångström, Uppsala University, Box 538, S-751 21 Uppsala, Sweden.
K
K
Hermansson
D002563:Cerium / Q000737:chemistry*; D003460:Crystallization; D005461:Fluorine / Q000737:chemistry*; D017348:Microscopy, Electron, Scanning Transmission; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D013499:Surface Properties
D005461:Fluorine; D002563:Cerium; C030583:ceric oxide; D010100:Oxygen