Ethanol Diffusion on Rutile TiO 2 (110) Mediated by H Adatoms Peipei Huo, Jonas Ø. Hansen, Umberto Martinez, Estephania Lira, Regine Streber, Yinying Wei, Erik Lægsgaard, Bjørk Hammer, Stefan Wendt,* and Flemming Besenbacher Interdisciplinary Nanoscience Center (iNANO), Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark * S Supporting Information ABSTRACT: We have studied the diffusion of ethanol on rutile TiO 2 (110)-(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Time-lapsed STM images recorded at ∼200 K revealed the diffusion of ethanol molecules both parallel and perpendicular to the rows of surface Ti atoms. The diffusion of ethanol molecules perpendicular to the rows of surface Ti atoms was found to be mediated by H adatoms in the rows of bridge-bonded O (O br ) atoms similarly to previous results obtained for water monomers. In contrast, the diffusion of H adatoms across the Ti rows, mediated by ethanol molecules, was observed only very rarely and exclusively on fully hydrogenated TiO 2 (110) surfaces. Possible reasons why the diffusion of H adatoms across the Ti rows mediated by ethanol molecules occurs less frequently than the cross-row diffusion of ethanol molecules mediated by H adatoms are discussed. SECTION: Surfaces, Interfaces, Catalysis T he understanding of diffusion processes on surfaces is interesting both from a fundamental point of view and with a view to applications in heterogeneous catalysis. For example, in heterogeneous catalysis, the overall reactivity is often determined by active sites such as kinks, steps, and point defects. 1-3 The adsorbates and intermediate species need to diffuse to the active sites where the actual reactions occur due to the special electronic environment at these sites. Alternatively, the adsorbates may be reactive at regular surface sites but can be trapped at particular sites on the surface where different reaction routes can be initiated 4 or where they are nonreactive. In either case, the overall reactivity is strongly influenced by the diffusivity of the adsorbates and inter- mediates, 3 and thus insights into the diffusion mechanism occurring on catalyst surfaces are of great interest. A very well-studied model catalyst surface is the rutile TiO 2 (110)-(1 × 1) surface. 5-11 Because of the potential applications of TiO 2 in photocatalysis, solar cells, super- hydrophilicity, and water splitting, much research has been focused on this material and the processes occurring on its surfaces. 12-14 Regarding the anisotropic rutile TiO 2 (110) surface, high-resolution STM studies have unraveled how O 2 molecules oxidize H adatoms (H ad , often also denoted as OH br species) 15,16 on the surface and Ti interstitials in the bulk. 11,17,18 In addition, STM studies have shown that hydrogen bonds between water molecules and H ad species make the diffusion across the rows possible. 8,19 Likewise, the interactions of alcohols with rutile TiO 2 (110) surfaces have been studied. 4,7,10,20-25 Previous studies showed that O br vacancies on rutile TiO 2 (110) are the most stable adsorption sites and that alcohols dissociate at O br vacancy sites via O-H scission. 4,10,20-22 Furthermore, room-temperature STM studies by Zhang et al. revealed a dramatic increase in H ad diffusion perpendicularly to the rows of surface Ti atoms, that is, in the [11̅0] direction, after high methanol 21 and 2-butanol 22 exposures, respectively. Among the alcohols, ethanol (CH 3 CH 2 OH or EtOH) is of particular interest because it holds promises both as a renewable energy carrier 26,27 and as feedstock for future green chemistry. 28,29 In addition, the decomposition of EtOH can be used to identify active sites on metal oxide surfaces. 4,10,25 Recently, we provided direct evidence that molecularly and dissociatively adsorbed EtOH species coexist on regular surface Ti sites on TiO 2 (110). 25 On the basis of time-lapsed STM images-so-called STM movies-and DFT calculations we were able to identify EtOH molecules and two types of EtO ethoxides. 25 In fact, in time-lapsed STM images, the hopping rates of diffusing species along with the observed surface reactions and the STM heights allow one to distinguish between various adsorbates, and in some cases the identity of the surface species can be unraveled with certainty. 1,8,10,15,25,30 Here we report on the diffusion of EtOH on rutile TiO 2 (110) studied by time-lapsed STM imaging and DFT calculations. The diffusion of EtOH molecules both parallel and perpendicular to the rows of O br and Ti atoms was observed. In contrast with the diffusion along the Ti troughs, the diffusion of EtOH perpendicular to the rows of surface Ti atoms is mediated by H ad species, similarly as previously observed for Received: December 8, 2011 Accepted: January 6, 2012 Published: January 7, 2012 Letter pubs.acs.org/JPCL © 2012 American Chemical Society 283 dx.doi.org/10.1021/jz201616z | J. Phys. Chem.Lett. 2012, 3, 283-288