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J. Phys. Chem. C 2009, 113, 13180–13191
Final State Distributions of O2 Photodesorbed from TiO2(110) David Sporleder,†,§ Daniel P. Wilson,† and Michael G. White*,†,‡ Department of Chemistry, Stony Brook UniVersity, Stony Brook, New York 11794, and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: February 5, 2009; ReVised Manuscript ReceiVed: May 26, 2009
The UV photodesorption of molecular oxygen from a reduced TiO2(110) single-crystal surface was investigated as a function of photon excitation energy, substrate temperature, and preannealing conditions. A pump-delayedprobe method using pulsed lasers for UV excitation (pump) and VUV ionization (probe) were used in conjunction with time-of-flight mass spectrometry to measure velocity distributions of the desorbing O2 molecules. The measured velocity distributions exhibit three distinct features, two of which are attributed to prompt desorption resulting in “fast” velocity distributions and one “slow” channel whose average kinetic energy tracks the surface temperature. The latter is assigned to trapping-desorption of photoexcited O2* which are trapped in the physisorption well prior to thermal desorption. The velocity distributions show no dependence on photon energy over the range studied (3.45-4.16 eV), consistent with a substrate-mediated, hole-capture desorption mechanism. The observed prompt desorption channels have mean translational energies of ∼0.14 and ∼0.50 eV and are attributed to the photodesorption of two distinct initial states of chemisorbed oxygen. The identities of the chemisorbed initial states associated with oxygen vacancy or interstitial defect sites are discussed in light of previous experimental and theoretical studies of oxygen on reduced TiO2(110) surfaces. I. Introduction The sustained scientific interest in titania (TiO2) is driven in part by its use in a wide range of technological applications, e.g., environmental photocatalysis, heterogeneous catalysis, and solar energy conversion, and in part by its use as a prototype for fundamental studies of interfacial chemistry on metal oxide surfaces.1-8 For both thermal and photoinduced reactions, adsorbed oxygen species (e.g., O-adatoms, O2/O2-, •OH) and oxygen atom vacancies (defects) play key roles in determining the catalytic activity of titania surfaces.2,6,9-11 For example, the photooxidation of organic species requires the presence of molecular oxygen, where it is thought to play multiple roles as a scavenger of photoexcited electrons (O2-), as well as a direct participant in the oxidation chemistry (O2, •OOH).1,2,4,8,12,13 The details of how molecular oxygen binds to titania surfaces and forms the various active species on defective surfaces or in the presence of UV radiation, however, remains unclear.10,14-23 Even for the more elementary process of photodesorption of O2 from a well-characterized single-crystal TiO2(110) surface, there is currently no consensus as to how the observed desorption behavior relates to initial binding sites of the chemisorbed oxygen.2,10,17,18,21 Developing a more complete understanding of the bonding and photodynamics of O2 on TiO2 surfaces will provide a basis for continued evolution of technological applications of titania and other metal oxide photocatalysts. Due to its thermodynamic stability and ease by which it can be reduced and oxidized, the TiO2(110) (rutile) single-crystal surface has been used extensively for fundamental investigations of O2 interactions and the role of O2 in photoinduced processes. The (1 × 1)-TiO2(110) surface is comprised of rows of 5-fold * To whom correspondence should be addressed. E-mail: mgwhite@ bnl.gov. Phone: (631) 344-4345. Fax: (631) 344-5815. † Stony Brook University. ‡ Brookhaven National Laboratory. § Present address: Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, MA 01731.
coordinated TiIV atoms, designated as TiIV(5f) in this work, that lie between alternating rows of bridging oxygen atoms (Obr).6,9 The Obr atoms extend beyond the surface plane and are bridge bonded to two TiIV atoms lying underneath in the surface plane. On a fully oxidized (defect free) surface, O2 is only weakly bound (physisorbed) at low temperatures (500 K), ion bombardment, or electron impact.6,27-32 Vacuum annealing is the most common approach and produces a reduced surface that contains defects associated with Obr(v) vacancies as well as TiIIIinterstitial atoms, denoted as TiIII(i). The latter diffuse to the near surface region at high temperatures and are responsible for the 3d defect state observed at ∼0.85 eV below the Fermi edge.6,22,32,33 Both Obr(v) vacancies and TiIII(i) interstitials provide excess negative charge at the TiO2(110) surface which enable molecular oxygen adsorption as O2- species. On a reduced surface, oxygen adsorbs as the superoxide (O2-) anion14,21,25 while other oxygen species such as the peroxide (O22-) anion have been postulated.21,34,35 Recent theoretical and experimental studies also suggest the possibility of a stable O42- species resulting from oxygen adsorption at very low temperatures.23,36 On the basis of thermal desorption studies, Henderson, et al. suggested that every Obr(v) vacancy could induce the adsorption of three oxygen molecules, one of which binds to the vacancy site and the other two at TiIV(5f) sites in the adjacent troughs.16 This observation is supported by theoretical studies21,35,37,38 that show that the excess charge associated with an Obr vacancy is delocalized and can induce oxygen adsorption at TiIV(5f) sites that are as much as 1 nm away.19 For surface temperatures >150 K, oxygen adsorption directly at an Obr vacancy results primarily in dissociation to form two O-adatoms, one of which fills the vacancy and the other which goes to a terrace site.16,39-41 Recent
10.1021/jp901065j CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
Final State Distributions of O2 Photodesorbed from TiO2 STM studies have also shown that O2- species adsorbed at the TiIII(i) near surface defects can undergo a thermally activated dissociation process with the resulting O-adatoms forming TixOy adlayer structures.22 Previous studies of O2 photodesorption from reduced TiO2(110) surfaces using cw excitation (UV lamps) and detection (electron impact mass spectrometry) have shown that O2 photodesorption exhibits nonexponential kinetics and strongly temperature dependent photoyields and rates.11,17,18,34,42,43 The observation of multiexponential photodesorption kinetics at low surface temperatures (
