Mechanism of Electron-Induced Hydrogen Desorption from Hydroxylated Rutile TiO 2 (110) D. P. Acharya, † C. V. Ciobanu, ‡ N. Camillone III, § and P. Sutter* ,† Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973, United States, DiVision of Engineering, Colorado School of Mines, Golden, Colorado 80401, United States, and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973, United States ReceiVed: August 2, 2010; ReVised Manuscript ReceiVed: October 7, 2010 The mechanism of hydrogen desorption from rutile TiO 2 (110)-(1 × 1) was studied by injecting electrons with controlled energy and flux into single surface hydroxyls (OH) in cryogenic scanning tunneling microscopy (STM). Desorption proceeds without a clear threshold already at much lower energies than reported previously. 1 Our analysis identifies a transfer of H atoms from the TiO 2 surface to the STM tip, triggered by vibrational heating due to inelastic electron tunneling, as the desorption mechanism. The reversible H-atom transfer between sample and tip can be used as a tool to discriminate OH from other surface species on TiO 2 and to control the density and configuration of OH by selective removal and redeposition of H atoms on the oxide surface. Introduction Photocatalysts are widely used in water and air treatment, 2 organic waste remediation, 2,3 and have the potential to become key materials for the renewable conversion of solar energy to fuels, for example, via water splitting 4 or greenhouse gas reforming. 5 The (110) surface of rutile titanium dioxide is often seen as the prototype model system for studying surface chemistry and photocatalysis on metal oxides. Extensive experimental and theoretical work has been focused on under- standing the properties of atomic-scale defects giving rise to reactivity, such as metal interstitials (Ti i 6 ) and oxygen vacancies (V O,br 7,8 ). The simplest adsorbed species, a single hydrogen atom whose chemisorption on the bridging oxygen (O br ) rows on TiO 2 (110) generates a bridge-bonded hydroxyl (OH br ), has received attention because it is expected to participate as an intermediate in a variety of photocatalytic reactions, including water splitting, 9 the decomposition of organic molecules, 2,3,5,10 and the hydrogenation of CO 2 . 10 Whereas the primary mechanism of OH br formation on reduced TiO 2 surfaces prepared in vacuum - via the reaction of H 2 O with V O,br - is well-documented, 9,11 the desorption of H from OH br remains less well understood. 1,12 Various scanning tunneling microscopy (STM) studies have shown that H desorption from hydroxylated TiO 2 can be induced by the STM tip, either during imaging at elevated bias or by the application of voltage pulses, 1,13 similar to STM tip-stimulated H-desorption from Si(100) 14 and Ge(111). 15 This finding not only provides a means for identifying OH br among other surface species with similar STM contrast (e.g., V O,br ), but it also opens up the possibility of using the atomically precise injection of charge carriers at well-defined energy and current into single OH br species to gain a fundamental understanding of the H desorption mechanism. Here, we report the analysis of the controlled, STM tip- induced desorption of individual H atoms from OH br on TiO 2 (110) at cryogenic temperatures (77 K). The extreme stability of the tunneling gap in low-temperature STM has been exploited primarily for the manipulation of atoms and molecules on metal surfaces, but the potential of atomically precise manipulation for studies of elementary processes on oxides has remained largely unexplored. The present study is a step in this direction. Injecting charge carriers with variable energy and dose into individual OH br , we find that voltage pulses remove single H atoms from O br , transferring them to the STM tip. The yield of this single-H removal from TiO 2 (110) depends strongly on both the electron energy and tunneling current, but the STM- induced desorption proceeds without a clear onset voltage already at a sample bias as low as 1.3 V. The dependence on the carrier injection rate (i.e., tunneling current) shows that the H-removal involves both one- and two-electron processes, without detectable (H, D) isotope effect. By considering several possible mechanisms for the electron-stimulated desorption, we conclude that vibrational heating by inelastic electron tunneling is the mechanism responsible for H desorption. Besides understanding the nonthermal, electron-stimulated desorption of an adsorbate ubiquitous on the TiO 2 (110) model photocata- lyst, establishing the conditions for STM tip-induced H desorp- tion has practical implications for experiments on TiO 2 surfaces. Our findings suggest that conditions exist in which the H atoms in OH br species are selectively removed without affecting other adsorbates or surface defects. Hence, STM-induced desorption can be used, for example, as a means to identify OH br - discriminating it from other adsorbates with similar STM contrast - and to control the local OH br population in STM studies by selectively desorbing H from extended surface areas. Methods Our experiments were performed in a low-temperature STM system (Createc), liquid nitrogen cooled to operate in cryogenic ultrahigh vacuum (UHV, T ) 77 K, P < 10 -11 Torr). Two different types of rutile TiO 2 (110) single crystals (Princeton Scientific; Commercial Crystal Laboratories) were used, fol- lowing preparation in UHV by several cycles of Ar + sputtering and annealing to 910 K. Electrochemically etched W tips, electron bombardment annealed in UHV, were used for imaging and hydrogen atom manipulation. * Corresponding author. † Center for Functional Nanomaterials, Brookhaven National Laboratory. ‡ Division of Engineering, Colorado School of Mines. § Chemistry Department, Brookhaven National Laboratory. J. Phys. Chem. C 2010, 114, 21510–21515 21510 10.1021/jp107262b  2010 American Chemical Society Published on Web 11/19/2010