J. Phys. Chem. C 2010, 114, 14579–14582
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Acetone Adsorption on Oxidized and Reduced TiO2(110): A Scanning Tunneling Microscope Study Masa-aki Yasuo,† Akira Sasahara,†,‡,§ and Hiroshi Onishi*,† Department of Chemistry, Graduate School of Science, Kobe UniVersity, Rokko-dai, Nada, Kobe, 657-8501, Japan, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ReceiVed: June 25, 2010; ReVised Manuscript ReceiVed: July 29, 2010
Acetone adsorption on oxidized and reduced rutile TiO2(110) surfaces was examined in a vacuum with a scanning tunneling microscope. A vacuum-annealed TiO2 surface was oxidized with O2 and then exposed to acetone vapor at room temperature. The acetone-oxygen complex proposed in early studies appeared in constant-current topography. We assigned the complex to η2-2,2-propanediolates by comparing its topography with that of coadsorbed acetate. When the vacuum-annealed surface was directly exposed to acetone, mobile η1-acetone was observed on Ti-atom rows. 1. Introduction
2. Experimental Section
The photocatalytic decomposition of toxic organic compounds has received considerable attention,1,2 and a number of practical applications have been made.3,4 Organic compounds containing CdO groups are a major class of reactants that are decomposed to CO2 and H2O. Studies on well-defined, single-crystalline TiO2 surfaces are required to reveal the photochemical decomposition mechanisms. Experimental efforts were initially provided on carboxylates for this purpose. Carboxylates are chemisorbed on rutile5 and anatase6 surfaces prepared in an ultrahigh vacuum (UHV). Holes photoexcited in rutile (110) wafers7-9 and anatase (001) films10 attach to chemisorbed pivalates, (CH3)3CCOO. The attached pivalate decomposes to a CO2 and a t-butyl radial. On the other hand, ketones and aldehydes are less strongly adsorbed on the UHV-prepared TiO2 surfaces. Henderson11 recently found enhanced stability of adsorbed acetone on oxidized rutile (110) surfaces and proposed formation of an acetone-oxygen complex. The adsorbed complex was active for photochemical decomposition. Methyl radicals were released into the vacuum when irradiated by ultraviolet light.12,13 So far, photoinduced decomposition has been found with butanone14 and acetaldehyde15 adsorbed on oxidized rutile (110) surfaces. Ketone-oxygen and aldehyde-oxygen complexes are the key species in this series of photochemical decomposition. The composition and structure of those complexes are not yet known. Henderson raised η2-2,2-propanediolate as a possible species based on his high-resolution electron energy loss and isotope-labeled thermal desorption results.11 A density functional theory study16 supported the enhanced stability of the diolate. However, it is still difficult to obtain experimental evidence for the assignment because the number of acetone-oxygen complexes is limited to less than 0.1 monolayer (ML). In the present paper, we observed the acetone-oxygen complex on a rutile TiO2(110) surface by a UHV scanning tunneling microscope (STM). The observed topography of the complex supports the assignment to η2-2,2-propanediolate.
Experiments were done in a UHV STM (JSPM4500S, JEOL) equipped with an Ar ion gun (EX03, Thermo). A rutile TiO2(110) wafer (7 × 1 × 0.3 mm3, Shinko-sha) was clamped with a Si plate on a sample holder. The (1 × 1) surface was prepared by repeated Ar ion sputtering and vacuum annealing at 1100 K. The wafer temperature was monitored with an infrared pyrometer. Empty-state, constant-current topography was obtained at room temperature (RT) with positive sample bias voltages using electrochemically etched tungsten tips. Research-grade acetone (Wako) was degassed by trap-and-thaw cycles. Research-grade O2 gas was introduced in the microscope chamber without further purification. The sputter-annealed TiO2 surface was exposed to O2 or acetone at RT in a chamber separated by a gate valve from the microscope chamber. The purity of the dosed reactants was watched with a quadrupole mass analyzer (MKS, e-Vision+).
* To whom correspondence should be addressed. E-mail: oni@ kobe-u.ac.jp. † Kobe University. ‡ Japan Science and Technology Agency. § Present address: School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi, 923-1292, Japan.
3. Results and Discussion 3.1. Acetone Adsorption on Oxidized TiO2. Figure 1 shows a series of the constant-current topography of a TiO2 wafer. Flat terraces appeared on the vacuum-annealed surface, as seen in panel (a). The lateral size of terraces was 20 nm or larger. The stoichiometric, unreconstructed structure of TiO2(110) is established.17 The topmost layer contains two types of Ti atoms, 5-fold and 6-fold coordinated. The 5-fold coordinated Ti atoms are exposed to the ambient. Some oxygen atoms are protruding from the surface plane and bridge the 6-fold coordinated Ti atoms. Grooves with the 5-fold coordinated Ti atoms with the bottom, and ridges of the bridge oxygen atoms are parallel to the [001] direction. Rows of 5-fold coordinated Ti atoms are observed as bright lines in the constant-current topography.18,19 The height of the steps was 0.3 nm as expected from the lattice structure of rutile. Small protrusions randomly appeared between adjacent Ti rows. They are bridge-oxygen vacancies.20 The number density of the vacancies is sensitive to the history of TiO2 wafers.21 The density on our vacuum-annealed surface was 0.07 ML, where one ML is defined to be the density of the surface lattice units, 5.2 × 1018 m-2. It is known that an O2 molecule is dissociated on an oxygen vacancy. The vacancy is filled with one of the two oxygen
10.1021/jp105876u 2010 American Chemical Society Published on Web 08/11/2010
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Figure 1. Constant current topography of a TiO2(110) surface exposed to oxygen and acetone. The vacuum-annealed surface of image (a) was exposed to 100 L of O2 to make the surface of image (b). The oxygenexposed surface was further exposed to (c) 10 and (d) 100 L of acetone. Image size ) 20 × 20 nm2, sample bias voltage ) +1.3 V, and tunnel current ) 0.3 nA. The height distribution of the protrusions found on the surface of images (b) and (c) is shown in panel (e) with gray and dark columns, respectively. The coverage of acetone-induced protrusions is shown in panel (f) as a function of acetone exposure.
atoms. The other oxygen atom is transferred to one of the Ti atoms nearby.22 In constant-current topography, the transferred oxygen adatom is recognized as a protrusion on a Ti-atom row.23,24 The oxygen vacancy as the protrusion between Ti-atom rows disappears at the same time. In our study, the vacuum-annealed surface of panel (a) was exposed to 100 L of O2 (1 L ) 1.3 × 10-4 Pa s) at RT. The oxygen vacancies disappeared and oxygen adatoms appeared in the topography of panel (b). The number density of oxygen adatoms was 0.07 ML, being equal to the number of vacancies on the vacuum-annealed surface, as expected. When the surface of (b) was further exposed to O2 for 100 L, the number of the adatoms remained intact. This shows that oxygen vacancies on the vacuum-annealed surface were fully healed by an O2 exposure of 100 L. The oxygen-exposed surface of (b) was then exposed to acetone at RT. The constant-current topography of panels (c) and (d) was obtained on the surface exposed to 10 and 100 L of acetone, respectively. Protrusions larger than oxygen adatoms appeared on the surfaces. The height distribution of protrusions on the acetone-exposed surface of panel (c) is shown in panel (e) as dark columns, together with that of the oxygen-exposed surface of panel (b) as gray columns. The protrusions on the oxygen-exposed surface present a single peak at 0.1 nm stemming from the oxygen adatoms. Another peak appeared at 0.3 nm and the peak at 0.1 nm diminished, when the surface was exposed to acetone. The acetone-induced protrusions are hence assigned to some chemical species containing acetone and oxygen adatom(s). Oxygen-exposed TiO2 surfaces were similarly prepared and then exposed to acetone for 10, 100, and 1000 L. The number of the acetone-induced protrusions was counted in the constant-
Yasuo et al.
Figure 2. Acetone-oxygen complexes and acetates coadsorbed on a TiO2(110) surface. A constant-current topography is shown in image (a). Image size ) 7.5 × 7.5 nm2, sample bias voltage ) +1.3 V, and tunnel current ) 0.05 nA. The line scan along the A-B cross section is in panel (b). Possible structures of the complex are illustrated in (c), including η2-2,2-propanediolate.
current topography. The number of the protrusions was saturated at 0.07 ML, as shown in panel (f). The saturation coverage is coincident with the number density of the oxygen vacancies on the vacuum-annealed surface. The coincident numbers suggest that one acetone-induced protrusion was created in the cost of one oxygen adatom. The one-to-one relationship is consistent with the consumption of oxygen adatoms depicted in panel (e). 3.2. Coadsorption with Acetates. The structure of the acetone-oxygen protrusion was further examined on the basis of the constant-current topography. A vacuum-annealed TiO2 surface was exposed to O2 for 100 L and acetone for 100 L to make acetone-oxygen complexes, and then acetic acid for 1000 L at RT. The constant-current topography of the final surface is shown in panel (a) of Figure 2. Two adsorbed species, tall and short, are identified. The topographic heights of the tall and short species were 0.3 and 0.2 nm, as shown in the line scan of panel (b). The coverage of the tall species was 0.08 ML. The height and coverage of the tall species are identical to those of the acetone-oxygen complex observed in Figure 1. The tall species are thus ascribed to the acetone-oxygen complex. The short species was absent on the surfaces of Figure 1 and is attributed to the adsorbed acetate. It is established that acetic acid is dissociatively adsorbed on a vacuum-annealed TiO2(110) surface.5 A bridge-bonded acetate anion is on a pair of Ti atoms with its C2V axis perpendicular to the surface. The molecular plane of the acetate is parallel to the [001] direction, as illustrated in panel (c) and the TOC graphic. It is natural in preparing the surface of Figure 2 that acetic acid was dissociatively adsorbed on the TiO2 surface unoccupied by acetone-oxygen complexes. On the basis of these assignments, the constant-current topography of the acetone-oxygen complex is larger by 0.1 nm than that of the acetate. The constant-current topography of a bridge-bonded acetate is in the middle of two adjacent Ti atoms on a Ti-atom row.25 The acetates are arrayed on a (2 × 1)-ordered mesh over the TiO2(110) surface as a result. In Figure 2a, acetone-oxygen complexes are located along the (2 × 1) mesh. Each complex is hence in the middle of two adjacent Ti atoms, as the acetate is.
Acetone Adsorption on TiO2(110) Here, we consider the structure of the acetone-oxygen complex. A bridge-bonded η2-2,2-propanediolate is most probable among possible compounds illustrated in Figure 2c. An acetone-oxygen complex involves one oxygen adatom, as evidenced in section 3.1. The propanediolate is produced by addition of an oxygen adatom to an acetone molecule. The CdO of the acetone is polarized with the carbon atom to be positive. The positively charged carbon atom receives the oxygen adatom, a nucleophilic reactant. The produced diolate can be adsorbed with the two oxygen atoms on the top of two Ti atoms. This form of adsorption is consistent with the arrangement on the (2 × 1)-ordered mesh. Polyacetone, enolate, and mesityl oxide are not consistent with the (2 × 1)-ordered arrangement. The propanediolate is terminated with two CH3 groups, whereas the acetate ends with one CH3 group. The topographic difference of 0.1 nm is reasonable with one additional CH3 group. The constant-current topography of the acetate and pivalate was compared on the TiO2(110) surface.26 The pivalate with three CH3 groups presented a topography larger by 0.05 nm. If two acetones were involved in a complex, as is expected with mesityl oxide, the topography could be large and asymmetric. This was not the case in our STM results. A density functional study16 found that the bridge-bonded propandiolate is stable on the TiO2(110) surface. The predicted activation energy of acetone desorption is 1.3 eV, being consistent with the thermal desorption that peaked at 375 K.11 In the early study,11 the acetone-oxygen complex on the TiO2(110) surface presented a vibration peak at 1425 cm-1. This wavenumber is larger than expected for the O-C-O stretching modes of the bridge-bonded propanediolate. On the other hand, the force constants of the O-C-O stretching should be sensitive to the electron density on the diolate. The nucleophilic addition of the oxygen adatom suggests some negative charge on the diolate, though the amount of the charge is unknown. 3.3. Acetone Adsorption on Reduced TiO2. Acetone adsorption was examined on a reduced TiO2 surface free from oxygen adatoms for comparison. A vacuum-annealed TiO2 surface was exposed just to acetone for 100 L at RT. A small number of particles appeared on the constant-current topography. Four consecutively observed images are shown in Figure 3. Particles are identified on Ti-atom rows, some of which moved back and forth on Ti-atom rows or appeared and disappeared frame by frame. We ascribe the mobile particle to η1-acetone anchored on the top of a Ti atom. The thermal desorption of acetone peaked at 345 K on a reduced TiO2(110) surface and at 375 K on an oxidized surface.11 The limited amount of acetone that remained at RT on our reduced surface is consistent with the desorption spectra. A 5-fold coordinated Ti atom on the surface can be further coordinated by the lone pair electrons at the CdO end of an acetone molecule. The on-top acetone adsorption was predicted in the theoretical simulation.16 The vibration spectrum of adsorbed acetone was identical to that of multilayered, physisorbed acetone,11 suggesting a weak coordination bond of CO-Ti. Displacement of η1-acetone by coadsorbed water was suggested in a thermal desorption study.27 The mobility at RT found in the STM topography is consistent with the suggestion. Henderson11 found no evidence for either decomposition or preferential binding of acetone at oxygen vacancy sites on his reduced TiO2 surface. Ma´rquez et al.16 reported that adsorption on a vacancy site is stabilized by 0.2 eV from adsorption on the top of the 5-fold coordinated Ti site. In our time-lapse STM images, some adsorbates were mobile, whereas the others were
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Figure 3. Time-lapse topography of an acetone-exposed TiO2(110) surface. Images (a-d) were consecutively determined with an acquisition time of 90 s frame-1. Some mobile and immobile particles are marked with white arrowheads and white circles, respectively. Image size ) 18 × 18 nm2, sample bias voltage ) +1.3 V, and tunnel current ) 0.3 nA.
not. The immobile adsorbate may possibly be acetone on vacancy sites. 4. Conclusions Acetone adsorption on the oxidized and reduced TiO2(110) surfaces was examined using STM. On the oxidized surface, an oxygen adatom was attached to an acetone molecule to produce a η2-2,2-propanediolate adsorbed on two adjacent Ti atoms. On the reduced surface, the η1-acetone was observed as mobile particles on Ti-atom rows. Acknowledgment. This study was supported by a Grant-inAid for Scientific Research (KAKENHI) on Priority Areas [477] “Molecular Science for Supra Functional Systems” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References and Notes (1) Thompson, T. L.; Yates, J. T., Jr. Chem. ReV. 2006, 106, 4428. (2) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (3) Hashimoto, K.; Irie, H.; Fujishima, A. Jpn. J. Appl. Phys. 2005, 44, 8269. (4) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (5) Onishi, H. Carboxylates adsorbed on TiO2(110). In Chemistry of Nanomolecular Systems -Towards the Realization of Molecular DeVices; Nakamaura, T., Matsumoto, T., Tada, H., Sugiura, K., Eds.; Springer: Berlin, 2003; p 75. (6) Tanner, R. E.; Sasahara, A.; Liang, Y.; Altman, E. I.; Onishi, H. J. Phys. Chem. B 2002, 106, 8211. (7) Henderson, M. A.; White, J. M.; Uetsuka, H.; Onishi, H. J. Am. Chem. Soc. 2003, 125, 14974. (8) Uetsuka, H.; Onishi, H.; Henderson, M. A.; White, J. M. J. Phys. Chem. B 2004, 108, 10621. (9) Henderson, M. A.; White, J. M.; Uetsuka, H.; Onishi, H. J. Catal. 2006, 238, 153. (10) Ohsawa, T.; Lyubinetsky, I.; Du, Y.; Henderson, M. A.; Shutthanandan, V.; Chambers, S. A. Phys. ReV. B 2009, 79, 085401. (11) Henderson, M. A. J. Phys. Chem. B 2004, 108, 18932. (12) Henderson, M. A. J. Phys. Chem. B 2005, 109, 12062. (13) Henderson, M. A. J. Catal. 2008, 156, 287. (14) Henderson, M. A. Surf. Sci. 2008, 602, 3188. (15) Zehr, R. T.; Henderson, M. A. Surf. Sci. 2008, 602, 2238.
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(16) Ma´rquez, A. M.; Plata, J. J.; Sanz, J. F. J. Phys. Chem. C 2009, 113, 19973. (17) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (18) Onishi, H.; Fukui, K.; Iwasawa, Y. Bull. Chem. Soc. Jpn. 1995, 68, 2447. (19) Onishi, H.; Iwasawa, Y. Surf. Sci. 1994, 313, L783. (20) Diebold, U.; Anderson, J. F.; Ng, K.-O.; Vanderbilt, D. Phys. ReV. Lett. 1996, 77, 1322. (21) Li, M.; Hebenstreit, W.; Diebold, U.; Tyrshkin, A. M.; Bowman, M. K.; Dunham, G. G.; Henderson, M. A. J. Phys. Chem. B 2000, 104, 4944. (22) Epling, W. S.; Peden C., H. F.; Henderson, M. A.; Diebold, U. Surf. Sci. 1998, 412/413, 333.
Yasuo et al. (23) Diebold, U.; Lehman, J.; Mahmoud, T.; Kuhn, M.; Leonardelli, G.; Hebenstreit, W.; Schmid, M.; Varga, P. Surf. Sci. 1998, 411, 137. (24) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlstro¨m, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Surf. Sci. 2005, 596, 228. (25) Onishi, H.; Iwasawa, Y. Chem. Phys. Lett. 1994, 226, 111. (26) Sasahara, A.; Uetsuka, H.; Ishibashi, T.; Onishi, H. J. Phys. Chem. B 2003, 107, 13925. (27) Henderson, M. A. Langmuir 2005, 21, 3443.
JP105876U
