Structure, defects, and impurities at the rutile TiO 2 (0 1 1)-(2 · 1) surface: A scanning tunneling microscopy study Olga Dulub a , Cristiana Di Valentin b , Annabella Selloni c , Ulrike Diebold a, * a Department of Physics, Tulane University, New Orleans, Louisiana 70118, USA b Dipartimento di Scienza dei Materiali, Universita ` degli Studi di Milano-Bicocca, Milano, Italy c Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Received 23 January 2006; accepted for publication 28 June 2006 Available online 21 July 2006 Abstract The titanium dioxide rutile (0 1 1) (equivalent to (1 0 1)) surface reconstructs to a stable (2 · 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin, A. Selloni, Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, Ti@O) is supported by Scanning Tunneling Microscopy (STM) measurements. These Ti@O sites are imaged bright in empty-states STM. A few percent of these terminal oxygen atoms are missing at vacuum-annealed surfaces of bulk-reduced samples. These O vacancies are imaged as dark spots. Their number density depends on the reduction state of the bulk. Double vacancies are the most commonly observed defect configuration; single vacan- cies and vacancies involving several O atoms are present as well. Formation of oxygen vacancies can be suppressed by annealing a sput- tered surface first in vacuum and then in oxygen; annealing a sputtered surface in oxygen results in surface restructuring and a (3 · 1) phase. Anti-phase domain boundaries in the (2 · 1) structure are active adsorption sites. Segregation of calcium impurities from the bulk results in an ordered overlayer that exhibits domains with a centered (2 · 1) periodicity in STM. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Scanning tunneling microscopy; Surface defects; Segregation; Domain boundaries 1. Introduction Titanium dioxide is a versatile material that finds appli- cations in a wide range of technical fields. Its surface prop- erties play a role in many TiO 2 -based devices and TiO 2 has become one of the most investigated metal oxides in sur- face science [1]. Most surface studies of TiO 2 have focused on the thermodynamically most-stable rutile form, in par- ticular on the lowest-energy (1 1 0) surface. In this paper we discuss in detail the preparation and structural proper- ties of the rutile (0 1 1) surface. The rutile crystal structure is tetragonal, and the (1 1 0) plane is inequivalent to the (0 1 1) (or (1 0 1)) plane. Several surface chemistry investigations on {0 1 1}-faceted TiO 2 (0 0 1) surfaces exist, see for example Refs. [2,3]. Atomic-scale structural information is easier obtained from flat single crystals. Yet, with the exception of work from our group [4–6], very few previous experi- mental surface studies of flat TiO 2 (0 1 1) have been reported [7,8]. Three main reasons have motivated us to conduct the work presented in this paper: Firstly, the (0 1 1) face of rutile is a low-energy surface. First-principles DFT calculations of bulk-terminated rutile surfaces predict it to have the third-lowest surface energy, somewhat higher than (1 1 0) and (1 0 0) [9]. In a Wulff con- struction, a large part of the equilibrium-shape crystal terminates with the (0 1 1) orientation [9]. In nano- and microcrystalline powder materials the (0 1 1) surface planes are quite common, and it is essential to study this plane if one wants to obtain a complete surface characterization of rutile. 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.06.042 * Corresponding author. Tel.: +1 504 862 8279; fax: +1 504 862 8702. E-mail address: diebold@tulane.edu (U. Diebold). www.elsevier.com/locate/susc Surface Science 600 (2006) 4407–4417