Dissociative and molecular oxygen chemisorption channels on reduced rutile TiO 2 (110): An STM and TPD study Estephania Lira, Jonas Ø. Hansen, Peipei Huo, Ralf Bechstein, Patrick Galliker, Erik Lægsgaard, Bjørk Hammer, Stefan Wendt ⁎, Flemming Besenbacher Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark abstract article info Article history: Received 28 October 2009 Accepted 2 August 2010 Available online 10 August 2010 Keywords: TiO 2 O 2 dissociation O adatoms O 2 desorption Ti interstitials O vacancies Scanning tunneling microscopy (STM) Temperature-programmed desorption (TPD) High-resolution scanning tunneling microscopy (STM) and temperature-programmed desorption (TPD) were used to study the interaction of O 2 with reduced TiO 2 (110)–(1 × 1) crystals. STM is the technique of choice to unravel the relation between vacancy and non-vacancy assisted O 2 dissociation channels as a function of temperature. It is revealed that the vacancy-assisted, first O 2 dissociation channel is preferred at low temperature (~ 120 K), whereas the non-vacancy assisted, second O 2 dissociation channel operates at temperatures higher than 150 K–180 K. Based on the STM results on the two dissociative O 2 interaction channels and the TPD data, a new comprehensive model of the O 2 chemisorption on reduced TiO 2 (110) is proposed. The model explains the relations between the two dissociative and the molecular O 2 interaction channels. The experimental data are interpreted by considering the available charge in the near-surface region of reduced TiO 2 (110) crystals, the kinetics of the two O 2 dissociation channels as well as the kinetics of the diffusion and reaction of Ti interstitials. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide (TiO 2 ) is a reducible transition metal oxide that is widely used in a number of technological fields such as photocatalysis, heterogeneous catalysis, solar cells, hydrophilic films, gas sensors, waste remediation, and biocompatible materials [1–8]. Among these applications particularly in areas such as photocatalysis, photode- gradation of organic pollutants and the photo-generation of hydro- philic films, the interaction of O 2 with TiO 2 plays an important role [1,3,5,7,8]. For example, O 2 is a common oxidant and is also used in photocatalysis as a scavenger of the photo-excited electrons to prevent negative charge accumulation on the surface of the catalysts [1–3]. Furthermore, the interaction of O 2 with TiO 2 is interesting with a view to the possible photodynamic therapy of cancer, where the O 2 molecules play a vital role as oxidizing species [1]. To enhance the efficiencies of the applications listed above, it is essential to improve our understanding of how O 2 interacts with TiO 2 surfaces. The promising applications of TiO 2 -based materials have spurred tremen- dous research both in fundamental as well as in more applied fields. In early surface science studies under ultra-high vacuum (UHV) conditions, the interaction of molecular O 2 with TiO 2 single crystals was studied most frequently using photon-stimulated desorption (PSD) and temperature-programmed desorption (TPD) [8–22]. In all these works the rutile TiO 2 (110)–(1 × 1) surface (cf. Fig. 1), which is the most stable surface of rutile, and which is often considered the model system for transition-metal oxide surfaces [4,6,17,23–27], was studied. Very valuable information has been gained by using these desorption techniques on the O 2 interaction with rutile TiO 2 (110)–(1 × 1). For example, O 2 -PSD studies by the Yates group [8–10,16,17] have revealed that the O 2 -PSD occurs in UHV on O 2 -exposed TiO 2 (110) very much as it is observed on high-surface-area TiO 2 powder materials [28–30]. In addition, the thermal chemistry has been examined by Henderson and co-workers by combining detailed TPD studies with electron energy loss spectroscopy (EELS) measurements [12–14]. These studies disclosed the complex nature of the O 2 –TiO 2 (110) interaction and showed that dissociative and molecular channels co-exist on reduced TiO 2 (110) crystals. The desorption of O 2 at ~410 K was observed for TiO 2 (110)– (1 × 1) samples characterized by ~8%ML (monolayer) bridging oxygen (O br ) vacancies after O 2 adsorption at temperatures lower than ~180 K [12–14,16]. Interestingly, no sign of scrambling was found for the O 2 molecules that desorb in the TPD peak centered at ~410 K neither with the O br atoms on the surface nor with other O 2 molecules [12,15]. EELS measurements indicated that charge is transferred from the TiO 2 (110) surface to O 2 molecules adsorbed at 120 K and suggested the stabilization of a superoxo species (singly charged O 2 ) on the surface [12–14]. Recent low-temperature TPD studies by Dohnálek et al. [18] and Kimmel and Petrik [20] showed that physisorbed O 2 desorbs from the TiO 2 (110) surface at temperatures below 100 K. Utilizing the Surface Science 604 (2010) 1945–1960 ⁎ Corresponding author. Fax: +45 8612 0740. E-mail address: swendt@phys.au.dk (S. Wendt). 0039-6028/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.08.004 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc