J. Phys. Chem. B 2004, 108, 10621-10624
10621
Photoinduced Redox Reaction Coupled with Limited Electron Mobility at Metal Oxide Surface Hiroshi Uetsuka,†,‡ Hiroshi Onishi,*,†,§ Michael A. Henderson,⊥ and J. Michael White⊥,| Surface Chemistry Laboratory, Kanagawa Academy of Science and Technology, KSP, Sakado, Takatsu, Kawasaki, 213-0012 Japan, and Interfacial Chemistry and Engineering Group, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: April 12, 2004; In Final Form: June 11, 2004
Photoinduced oxidation of trimethyl acetate (TMA) was examined on an atomically flat surface of rutile TiO2. The macroscopic rate of reaction (the partial pressure of desorption products) was temporary modulated when a TMA-covered surface was UV-irradiated in oxygen gas. Scanning tunneling microscope imaging revealed spatially modulated distribution of TMA and OH on that surface. The limited surface mobility of electrons photoexcited and trapped at OH-associated Ti sites was proposed to cause the temporal and spatial modulations. The characteristic length of the spatial modulation, i.e., the lateral dimension of electron confinement, was a few lattice constants of the oxide.
(CH3)3CCOOH + Ti4+-O2--Ti4+ f
1. Introduction Photochemical reaction on semiconductor oxides1 has attracted attention from a broad community motivated by ambitions of degrading organic pollutants,2 hydrophilic coatings,3 disinfecting bacteria-contaminated surfaces,4 splitting water to produce H2 fuel,5 and developing efficient solar cells.6 Surfacemediated reduction and oxidation (redox) reactions follow the band-gap excitation of a semiconductor oxide to complete each of the practical processes. The redox reactions involve electrons and holes excited in the oxide bulk. The excited charges are delocalized in the three-dimensionally periodic lattice of atoms and transported from the bulk to the surface. Only little is known about the lateral transport across the surface. The present Letter reports limited mobility at an atomically flat surface of TiO2 when the laterally periodic lattice is significantly perturbed with adsorbed chemicals. The (110) plane of rutile has been extensively studied as a prototype of atomically flat surface of metal oxide.7-12 Trimethyl acetate (TMA or pivalate, (CH3)3CCOO) was chosen as an organic reactant that captures a photoexcited hole. When a TiO2(110) surface prepared by argon ion sputtering and vacuum annealing is exposed to trimethyl acetic acid vapor, a densely packed monolayer of TMA is formed at room temperature, where each TMA anion bridges two Ti4+ atoms. The acid proton is transferred to a bridge O2- atom.13 * To whom correspondence should be addressed. E-mail:
[email protected]. † Kanagawa Academy of Science and Technology, KSP. ‡ Current address: Technology Research and Development Department, General Technology Division, Central Japan Railway Company, Komaki 485-0801, Japan. § Current address: Department of Chemistry, Faculty of Science, Kobe University, Kobe 657-8501, Japan. ⊥ Interfacial Chemistry and Engineering Group, Pacific Northwest National Laboratory, Richland, WA 99352. | Visiting professor at Pacific Northwest National Laboratory. Permanent address: Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78712.
(CH3)3CCOO- + Ti4+-OH--Ti4+ (1) The carboxylate moiety is the main anchoring ligand for binding organic sensitizers to TiO2 surfaces.6 The TMA captures a hole and decomposes to a tert-butyl radical with a CO2 molecule.14
(CH3)3CCOO- + h+ f (CH3)3C + CO2
(2)
The tert-butyl radical rearranges by β-hydride elimination to release isobutene with a small amount of isobutane. This holederived channel resembles the photo-Kolbe reaction on rutile photoelectrodes.15 The accompanying electron is trapped as a Ti3+ cation bound to a bridge OH group.14
e- + Ti4+-OH--Ti4+ f Ti3+-OH--Ti4+
(3)
2. Experimental Section A rutile (110) surface was prepared with argon ion sputtering and vacuum annealing. The light blue wafer was exposed to trimethyl acetic acid vapor. The TMA-covered surface was irradiated with a 100 W Hg arc lamp. Photodesorbed species were analyzed with a quadrupole mass spectrometer at Richland. A similarly prepared surface was irradiated by a 300 W Xe arc lamp in a scanning tunneling microscope (JEOL, JSPM-4500S) at Kawasaki. Constant current topography was observed in the dark. The light flux at the TiO2 surface was estimated to be 80 (Hg lamp) and 10 (Xe lamp) mW cm-2 at wavelengths below 390 nm. 3. Results and Discussion A TMA-covered TiO2(110) surface was irradiated by UV light in the absence and presence of O2 atmosphere. Figure 1 presents the partial pressure of isobutene, i.e., the macroscopic rate of isobutene production. In a vacuum (curve A), UV irradiation with the Hg lamp caused immediate desorption of isobutene. The anaerobic production of isobutene excluded the
10.1021/jp0484027 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/01/2004
10622 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Letters
Figure 1. Photodesorption of isobutene on the TMA-covered TiO2(110) surface irradiated with a 100 W Hg arc lamp. (A) Light irradiation in a vacuum. Irradiation began at time ) 0. After 100 s, the surface was exposed to 400 L of O2 in the dark and then reexposed to UV light in the absence of O2. (B) Continuous light irradiation at 0-300 s in O2 atmospheres.
contribution of O2-derived adsorbates to the TMA reaction. This makes contrast to the photooxidation of CO 16, 2-propanol,17 and CH3Cl 18 on this TiO2 surface, with which reactants molecularly adsorbed oxygen is required. The anaerobic production of isobutene monotonically decayed as a function of irradiation time. The observed decay cannot be ascribed to the exhaustion of the reactant, because the photodesorption yield was enhanced by an exposure to O2 in the dark. We propose that the occupied electron traps having been produced in (3) efficiently recombine with holes,
Ti3+-OH--Ti4+ + h+ f Ti4+-OH--Ti4+
(4)
The hole-derived reaction (2) is poisoned as a result. The hydrogen atom released in the rearrangement of the tert-butyl radical is probably captured on the surface to produce the bridge OH with an excess electron
Ti4+-O2--Ti4+ + H f Ti3+-OH--Ti4+
(5)
and to similarly poison the reaction. The poisoned reaction was released when the trapped electrons were removed by the reaction with O2. It is known that the OH-associated Ti3+ was synthesized by the dissociation of water at an oxygen vacancy,19,20
(Ti3+- -Ti3+) + (Ti4+-O2--Ti4+) + H2O f 2(Ti3+-OH--Ti4+) (6)
and removed by the reaction with O221,22
2(Ti3+-OH--Ti4+) + 1/2O2 f 2(Ti4+-O2--Ti4+) + H2O (7) Light irradiation in O2 caused an additional, nonlinear feature on the isobutene desorption (curves in B); an additional rise followed the initial spike. As the oxygen pressure was increased, the rise shifted toward early irradiation time and gained intensity. The photodesorption enhanced in the oxygen atmospheres is consistent with the proposed mechanism of poisoning. Furthermore, the temporal modulation (decay and subsequent rise) of the macroscopic reaction rate suggested that reaction 2 was poisoned and released heterogeneously over the surface. Microscope imaging of the adsorbed chemicals (TMA and OH) revealed the heterogeneity of poisoning. The TiO2(110) surface annealed in the vacuum contained oxygen vacancies of 0.12 ML, where 1 ML is defined as 5.2 × 1014 sites cm-2. Neither molecularly nor atomically adsorbed oxygen was observed on the annealed surface. Image a of Figure 2 was obtained on a uniform, densely packed monolayer of TMA prepared on the TiO2 surface. Additional very bright features are of a different phase of titanium oxide that frequently appears on a sputter-annealed surface.23 The TMA-covered surface was maintained at 280 K and irradiated with the Xe lamp. Light irradiation in the vacuum resulted in random depletion of TMAs. This anaerobic reaction was poisoned in accordance with the decayed desorption in Figure 1A. A certain number of TMAs remained on the surface irradiated for 7 h (b). When the surface
Letters
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10623
Figure 2. STM topography of the light-irradiated TMA monolayer on TiO2(110). (a) A TMA monolayer prepared on a vacuum-annealed TiO2 surface. (b) The monolayer irradiated for 7 h in the vacuum. A separate monolayer irradiated in 1 × 10-7 Torr of O2 for (c) 10, (d) 15, (e) 20, and (f) 30 min. (g) Another monolayer irradiated in 1 × 10-6 Torr of O2 for 5 min. Imaging scans were done in the dark. Image size: 88 × 88 nm2. Sample bias voltage: +1.5 to +2.0 V. Tunnel current: 0.4 nA.
of a was irradiated in O2 of 1 × 10-7 Torr, TMAs randomly depleted in the initial 10 min (c). The uniform distribution of TMAs broke down upon further irradiation. Nanometer-scale domains of TMA-free surface appeared at 15 min (d) and then the reaction was accelerated. The TMA-free domains rapidly extended (e) and covered the terraces by 30-min irradiation in total (f). The delayed creation and rapid expansion of the domains reproduced the decay and rise of isobutene production. The scale of irradiation time was expanded in the observation of Figure 2 due to the small photon flux with the Xe lamp. A higher O2 pressure (1 × 10-6 Torr) led to domain formation at a shorter irradiation time of 5 min (g), being in accordance with the pressure-dependent shift of the rise. It is hence concluded that the spatially modulated rate of TMA reaction caused the temporally modulated rate of isobutene desorption. Zoomed-up images of e and g presented faint spots in TMAfree domains. They were assigned to a residual portion of the bridge OHs produced in (3) and (5). Similar STM features have been reported on TiO2(110) with water dissociation at oxygen vacancies 19,20 and with exposure to atomic H.24 Thermal desorption measurements after light irradiation supported the assignment to OH. The coverage of OHs was sensitive to the
oxygen pressure. Exposure to high-pressure oxygen reduced OH coverage as expected from reaction 7. Photoinduced adsorption of O2-derived species (atomic oxygen, O2-, O22-, or O3-) were not observed. Figure 3 illustrates how the rate of the TMA reaction (eq 2) is spatially modulated on the atomically flat, uniform surface of TiO2. Photoexcited holes randomly attach to TMAs in the initial stage of light irradiation, when the closely packed TMA monolayer blocks impinging O2. The number of trapped electrons produced in (3) and (5) increases with irradiation time. Holes diffusing to the surface efficiently recombine with the trapped electrons. The effective number of holes reduces at the surface and the TMA reaction is poisoned. When O2 molecules penetrate through a degraded portion of the monolayer, the occupied traps are locally removed by reaction 7. The poisoned reaction is released in that portion to make a domain of TMAfree surface. TMAs are populated outside of the domain while holes are available inside. Reaction 2 is therefore activated at the edge of the domain once formed. At the edge of the domains, local TMA coverage jumped from zero to the saturation as a function of lateral coordinate no more than a few lattice units. This abrupt modulation
10624 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Letters
Figure 3. Concentration of electrons and holes coupled with the lateral distribution of adsorbed chemicals.
indicates localized ability of the trapped electron to recombine with a hole. If delocalized electrons recombined with holes, the effective concentration of holes would be laterally uniform. This was not the case. The trapped electron more efficiently recombines with a hole than a delocalized electron in the conduction band does. The electron trap neutralized by the recombination in (4) is immediately reoccupied by a delocalized electron. In conclusion, the effective concentration of electrons and holes correlates with the lateral distribution of adsorbed chemicals. The rate of redox reactions is spatially modulated on an otherwise uniform surface of TiO2 as a result. The lateral dimension of the modulation is no more than a few lattice constants of the oxide. Charges excited in the bulk should be mobile for efficient transport to the surface. It is however oversimplified that the mobile and delocalized charges initiate the redox reactions at the surface. Acknowledgment. H.U. and H.O. acknowledge the support by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government, and also Core Research for Evolutional Science and Technology by Japan Science and Technology Agency. M.A.H. and J.M.W acknowledge support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. J.M.W. also acknowledges support from the Center for Materials Chemistry at the University of Texas at Austin and the Robert A. Welch Foundation. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by the Battelle Memorial Institute under Contract DE-AC06-76RLO1830. Part of the research reported here was performed in the William R. Wiley Environmental Molecular Science Laboratory, a Department of Energy user facility funded by the Office of Biological and Environmental Research.
References and Notes (1) Linsebigler, A. L.; Lu, G.; Yates Jr., J. T. Chem. ReV. 1995, 95, 735. (2) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1. (3) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigashi, M.; Watanabe, T. Nature (London) 1997, 388, 431. (4) Mills, A.; Le Hunte, S. J. Photochem. Photobiol. A 1997, 108, 1. (5) Kudo, A. Catal SurVeys Asia 2003, 7, 31. (6) Hagfeldt, A.; Gra¨zel, M. Acc. Chem. Res. 2000, 33, 269. (7) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (8) Sherrill, A. B.; Barteau, M. A. In Oxide Surfaces Woodruff, D. P., Ed.; Elsevier: Amsterdam, 2001; pp 409-442. (9) Egdell, R. G.; Jones, G. H. J. Mater. Chem. 1998, 8, 469. (10) Lai, X.; St. Clair, T. P.; Valden, M.; Goodman, D. W. Prog. Surf. Sci. 1998, 59, 25. (11) Bonnell, D. A. Prog. Surf. Sci. 1998, 57, 187. (12) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, England, 1994. (13) Onishi, H. In Chemistry of Nanomolecular Systems; Nakamura, T., Matsumoto, T., Tada, H., Sugiura, K.-I. Springer: Berlin, 2003; pp 7589. (14) Henderson, M. A.; White, J. M.; Uetsuka, H.; Onishi, H. J. Am. Chem. Soc. 2003, 125, 14974. (15) Kraeutler, B.; Bard, A. J. Am. Chem. Soc. 1978, 100, 2239. (16) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Chem. Phys. 1995, 102, 3005. (17) Brinkley, D.; Engel, T. J. Phys. Chem. 1998, 102, 7596. (18) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 7626. (19) Brookes, I. M.; Muryn, C. A.; Thornton, G. Phys. ReV. Lett. 2001, 87, 266103. (20) Schaub, R. et al., Phys. ReV. Lett. 2001, 87, 266104. (21) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. J. Phys. Chem. B 2003, 107, 534. (22) Trimethyl acetic acid is thus expected to be oxidized in O2 to produce isobutene, CO2 and water as (CH3)3CCOOH + 1/2O2 + hν f (CH3)2CdCH2 + CO2 + H2O. The water production was not checked in the present study. (23) Onishi, H.; Fukui, K.; Iwasawa, Y. Bull. Chem. Soc. Jpn. 1995, 68, 2447. (24) Suzuki, S.; Fukui, K.; Onishi, H.; Iwasawa, Y. Phys. ReV. Lett. 2000, 84, 2156.
