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Photochemical Reaction of Trimethyl Acetate on Pt/ TiO2(110) Hiroshi Uetsuka,†,‡ Chi Pang,†,§ Akira Sasahara,†,§ and Hiroshi Onishi*,†,§ Surface Chemistry Laboratory, Kanagawa Academy of Science and Technology, KSP, Sakado, Takatsu, Kawasaki, 213-0012 Japan Received June 22, 2005. In Final Form: September 14, 2005 Nanometer-sized Pt particles were prepared on an atomically flat surface of rutile TiO2. Trimethyl acetate (TMA) adsorbed on the Pt-modified surface was photochemically decomposed under ultraviolet light irradiation in a vacuum. Residing TMA anions were imaged by a scanning tunneling microscope to deduce the local rate of decomposition. Increasing the number density of Pt particles led to an enhancement of the initial reaction rate. The degree of this enhancement did not depend on the distance from the Pt particles.
1. Introduction Photochemical devices based on TiO2 have been developed for pollutant degradation,1 hydrophilic coatings,2 and solar cells.3 There is an intensive effort to modify materials with cocatalysts and dopants aiming at more efficient devices. Platinum is the most frequently examined cocatalyst for a wide range of applications. The charge transfer from Pt metal particles to a TiO2 substrate is thought to perturb the electrostatic potential in the substrate.4-6 Thermal reactions on the Pt surface may also play a role in multistep reactions initiated by the band gap excitation. However, at a site-specific level, little is known about how Pt modifiers enhance desired reactions. The scanning tunneling microscope (STM) can be used as a tool to observe photochemical reactions on a singlecrystal surface of TiO2. The degradation of trimethyl acetate (TMA) was imaged in a vacuum7 and in an O2 environment.8 The rate of TMA degradation was spatially and temporary modulated in O2 gas even on the modifierfree, originally uniform surface of rutile (110). In the present study, TMA degradation is examined on a TiO2(110) surface modified with Pt particles. TMA removal accompanied by OH generation, which happened on the Pt-free surface,7 was observed on TiO2 portions near and distant from a modifier particle. The initial rate of TMA * To whom correspondence should be addressed. E-mail: oni@ kobe-u.ac.jp. † Kanagawa Academy of Science and Technology. ‡ 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. (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (2) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigashi, M.; Watanabe, T. Nature 1997, 388, 431. (3) Hagfeldt, A.; Gra¨zel, M. Acc. Chem. Res. 2000, 33, 269. (4) Nosaka, Y.; Norimatsu, K.; Miyama, H. Chem. Phys. Lett. 1984, 106, 128. (5) Curran, J. S.; Domenech, J.; Jaffrezic-Renault, N.; Philippe, R. J. Phys. Chem. 1985, 89, 957. (6) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943. (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.
degradation was enhanced by increasing the number of Pt particles. The degree of this enhancement was uniform over the TiO2 surface, being independent of the distance from the Pt particles. These findings are interpreted for the electrostatic model including the electron transfer from TiO2 to the Pt modifier particles. 2. Experimental Section A rutile TiO2(110) surface of the (1 × 1) phase was prepared with argon ion sputtering and vacuum annealing in an ultrahigh vacuum chamber equipped with a scanning tunneling microscope (JEOL, JSPM-4500S). The sputter-annealed, light-blue wafer was cooled to room temperature (RT) without oxidation with O2 gas. A Pt droplet was heated on a tungsten filament in front of the sputter-annealed surface. The Pt-deposited surface was held at 920 K for 600 s to increase the size of the Pt particles and then exposed to trimethyl acetic acid vapor at RT. A trimethyl acetic acid molecule is dissociatively adsorbed on TiO2(110) to produce a TMA anion and a proton. The TMA-covered surface was irradiated in the vacuum with a 300 W Xe arc lamp. The light flux at the surface was estimated to be 100 W m-2 at wavelengths below 390 nm. Constant current topography of the surface was observed in the dark at RT.
3. Results 3.1. Pt Particles and Adatoms. Figure 1 presents the microscope topography of a TiO2 surface in the course of Pt modification. The Ti atom rows on the surface exhibit protruding topography (bright lines) in a constant-current STM image, whereas the O atom rows are presented as depressed topography (dark lines).9-11 A number of atomsized protrusions were resolved on O-atom rows in the image of the sputter-annealed surface shown in Figure 1a. They are assigned to O atom vacancies10 and H adatoms.12 It was difficult to identify each protrusion as an O vacancy or an H adatom, although different topographies of the two have been recognized in previous studies.13,14 TiOx islands on the sputter-annealed surface9 were observed as nanometer-sized bumps. (9) Onishi, H.; Fukui, K.; Iwasawa, Y. Bull. Chem. Soc. Jpn. 1995, 68, 2447. (10) Diebold, U.; Anderson, J. F.; Ng, K. O.; Vanderbilt, D. Phys. Rev. Lett. 1996, 77, 1322. (11) Berko´, A.; Magony, A.; Szo¨koˆ, J. Langmuir 2005, 21, 4562. (12) Suzuki, S.; Fukui, K.; Onishi, H.; Iwasawa, Y. Phys. Rev. Lett. 2000, 84, 2156. (13) Schaub, R.; Thostrup, P.; Lopez, N.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Besenbacher, F. Phys. Rev. Lett. 2001, 87, 266104. (14) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G., submitted.
10.1021/la051679t CCC: $30.25 © 2005 American Chemical Society Published on Web 10/29/2005
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Figure 1. Constant-current topography of a TiO2(110) surface (a) sputter-annealed, (b) subsequently exposed to Pt vapor for 10 min, and (c) annealed in the vacuum at 920 K for 600 s. (d) Topography of another TiO2 surface sputter-annealed, Pt-deposited for 2 min, and vacuum-annealed. Image size: 44 × 44 nm2. Sample bias voltages: (a) +1.3, (b) +2.0, (c) +0.7, and (d) +1.3 V. Tunnel current: 0.4 nA.
Figure 2. Constant-current topography of TMAs photodecomposed on the Pt/TiO2(110) surface of Figure 1d. The Pt-modified surface was exposed to 100 L of trimethylacetic acid at RT. A wide-scan image (a) and a magnified image (b) are presented. The TMA-covered surface was UV irradiated for (c) 1 and (d) 3 h. (e) Magnified image of the 3-h irradiated surface. Image sizes: (a) 40 × 40, (b) 5 × 5, (c) 40 × 40, (d) 40 × 40, and (e) 10 × 10 nm2. Sample bias voltages: (a) +1.3, (b) +1.6, (c) +1.6, (d) +1.6, and (e) +1.6 V. Tunnel current: 0.4 nA.
Platinum particles, 0.8-1.0 nm in height, appeared on the surface exposed to the Pt source for 10 min, as shown in the image of Figure 1b. In addition to the particles, atom-sized protrusions were observed on the Ti-atom rows but not on the O-atom rows and were assigned to Pt adatoms. This surface was vacuum annealed at 920 K for 600 s in the vacuum to increase the size of the Pt particles. Dulub et al.15 found TiOx thin film migrating onto Pt particles when a Pt-deposited surface was annealed at 973 K. The annealing temperature in the present study was below this temperature, and signs of the encapsulation were not observed. In the image of the annealed surface (Figure 1c), the width and height of Pt particles increased to 3-5 and 1.0-1.5 nm, respectively, leading to an averaged volume of 20 nm3 particle-1. One particle with this volume contains 1 × 103 Pt atoms by considering the atom density of Pt metal, 6.6 × 1028 atoms m-3. The number density of particles decreased to 8 × 1015 particles m-2 on the annealed surface. The total number of modifiers was 1 × 1019 Pt atoms m-2. This number corresponds to two Pt atoms per surface unit cell. The number of protrusions on O-atom rows (oxygen vacancies and hydrogen adatoms) decreased upon annealing. This is consistent with the finding on Au-deposited TiO2(110) that metal particles are nucleated on O vacancies.16 Another TiO2(110) surface exposed to the Pt source for 2 min and annealed at 920 K gave a smaller number density of 3 × 1015 particles m-2, as shown in Figure 1d. (15) Dulub, O.; Hebenstreit, W.; Diebold, U. Phys. Rev. Lett. 2000, 84, 3646. (16) Wahlstro¨m, E.; Lopez, N.; Schaub, R.; Thostrup, P.; Rønnau, A.; Africh, C.; Lægsgaard, E.; Nørskov, J. K.; Besenbacher, F. Phys. Rev. Lett. 2003, 90, 026101.
3.2. Photodecomposition of TMA. The surface of Figure 1d, modified with 3 × 1015 Pt particles m-2, was exposed to 100 L (1 L ) 1.3 × 10-4 Pa s) of trimethylacetic acid at RT. When a TiO2(110) surface is exposed to carboxylic acid vapor, a densely packed monolayer of carboxylate anions covers the surface. Each anion is chemisorbed on a pair of Ti4+ atoms17 with the acid proton transferred to a bridging O2- atom18
RCOOH (g) + Ti4+-O2--Ti4+ f RCOO- (a) + Ti4+-OH--Ti4+ (1) where (g) and (a) represent vapor and adsorbed species. Adsorbed TMA anions are recognized as (2 × 1)-ordered, regular-sized protrusions containing 2.6 × 1018 TMA anions m-2 as shown in Figure 2a. The one monolayer of TMA is defined at this coverage. A small-area image shown in Figure 2b reveals that the Pt particles are also covered with similar protrusions, presumably TMA. The TMA-covered surface was irradiated with UV light in the vacuum. The number of TMAs decreased with irradiation time, as traced in the images of Figure 2c and d. In magnified images of the irradiated surfaces, faint protrusions were recognized among the bright protrusions of TMA (not shown). These faint features were located on O-atom rows and assigned to bridging OH groups. TMA removal accompanied with OH generation is exactly what was observed on a Pt-free TiO2(110) surface irradiated in the same way.7 In the present study, no other features were recognized on the TiO2 surface even in the neighborhood of Pt particles. Accordingly, we propose that (17) Onishi, H.; Iwasawa, Y. Chem. Phys. Lett. 1994, 226, 111. (18) Sayago, D. I.; Polcik, M.; Lindsay, R.; Toomes, R. L.; Hoeft, J. T.; Kittel, M.; Woodruff, D. P. J. Phys. Chem. B 2004, 108, 14316.
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Figure 3. Constant-current topography of TMAs photodecomposed on the Pt/TiO2(110) surface of Figure 1c. The Pt-modified surface was exposed to 100 L of trimethylacetic acid and then UV irradiated for (a) 1 and (b) 3 h. A TMA-covered, Pt-free TiO2(110) surface was irradiated with the same UV flux for (c) 1 and (d) 3 h for comparison. Large bumps in c and d are TiOx islands,9 not Pt particles. Image size: 40 × 40 nm2. Sample bias voltage: +1.6 V. Tunnel current: 0.4 nA.
the TMA decomposition mechanism is not modified by the presence of Pt particles. The following series of redox reactions was proposed on the Pt-free surface. A TMA anion is attacked by a hole and decarboxylated to give a tert-butyl radical,
(CH3)3CCOO- + h+ f (CH3)3C + CO2
(2)
An accompanying electron is trapped at a bridging OH provided in the dissociative adsorption,
e- + Ti4+-OH--Ti4+ f Ti3+-OH--Ti4+
(3)
The tert-butyl radical rearranges to an isobutene releasing a hydrogen atom. The bridging OH group with a trapped electron produced in the latter reaction is identified as the faint protrusion on O-atom rows. The lateral distribution of residing TMA reflects the local rate of reaction 2 because chemisorbed TMA is immobile at RT.8 The TMA distribution remained random over the Pt-modified surface in the images of Figure 2c and d. This suggests that the reaction rate is constant throughout the surface. It was difficult in Figure 2e to recognize species adsorbed on UV-irradiated Pt particles. A future study observing desorption products is required to determine the fate of the TMA adsorbed on the particles. TMA decomposition was examined on the other Ptmodified surface of Figure 1c that possessed 8 × 1015 Pt particles m-2. TMA removal and OH generation were observed as shown in Figure 3a and b. The lateral distribution of TMA was again random, being independent of the distance from Pt particles. A TMA-covered, Pt-free surface was prepared and irradiated as a reference. Parts c and d of Figure 3 present the microscope images of the Pt-free surface irradiated with UV light of the same flux as used for the Pt-modified surfaces. TMA was removed at a lower rate compared to the removal rate for the two Pt-modified surfaces. Figure 4 shows the number of residing TMA anions as a function of the irradiation time. On the Pt-free surface (curve a), the TMA reacted with an initial rate of 0.4 ML h-1. The reaction stopped after 1 h of irradiation when 0.4 ML of TMA reacted to disappear. This initial rate corresponds to an apparent quantum yield of 2 × 10-6 TMA reacted per incident UV photon. When the surface was modified with 3 × 1015 Pt particles m-2 (curve b), the initial rate was enhanced by a factor of 2. On the surface modified with 8 × 1015 Pt particles m-2 (curve c), the initial rate was enhanced by a factor of 3, and 0.6 ML of TMA reacted until the reaction stopped.
Figure 4. Number density of TMA residing on TiO2(110)-(1 × 1) surfaces irradiated with UV light. A sputter-annealed surface was modified with (a) none, (b) 3 × 1015, and (c) 8 × 1015 Pt particles m-2.
4. Discussion Consider how Pt modifiers enhanced the initial rates of TMA reaction. A possible picture is based on the electron transfer from the TiO2 substrate to the Pt particles. Pt metal has a large work function of 5.6 eV,19 whereas TiO2 is an n-type semiconductor because of oxygen deficiency. The work function of a sputter-annealed TiO2(110) surface was reported to be 5.3 eV on the basis of the photoelectron emission threshold.20 Donors are ionized near the PtTiO2 interface to balance the Fermi levels. An electrostatic double layer is composed of negatively charged Pt particles and positively charged ionized donors. The depth profile of the electrostatic potential is illustrated on a laterally infinite interface (Figure 5). When a band gap excitation occurs in the double layer, the hole is driven toward the interface, and the electron is pulled into the TiO2 bulk. The recombination probability is reduced because the electrons and holes are driven in opposite directions.21 The surface concentration of holes is further enhanced by the directed transport to the surface. These two features (the reduced recombination probability and the directed transport of holes) are possible reasons for the enhanced reaction. (19) Michaelson, H. B. J. Appl. Phys. 1977, 48, 4729. (20) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 193, 33. (21) Anpo, M.; Chiba, K.; Tomonari, M.; Coluccia, A.; Che, M.; Fox, M. A. Bull. Chem. Soc. Jpn. 1991, 64, 543.
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Figure 5. Received band structure of an infinite interface of Pt and TiO2.
In microscope images of Figures 2 and 3, TMA anions randomly decomposed, being independent of the distance from Pt particles. This nonlocal enhancement results from a long decay length of the electrostatic field. In the model shown in Figure 5, the depth of the ionized donor layer (λ) is given by
∆φ )
eFλ2 20
(4)
and predicted to be 60 nm by assuming a work function difference (∆φ) of 0.3 eV and a relative dielectric constant () of 102, with e being the elementary charge and 0 being the dielectric constant of the vacuum. The donor density (F) was estimated to be 1024 m-3 on the basis of the electric resistivity of similarly prepared TiO2 wafers.22 The number of ionized donors Fλ ) 6 × 1016 m-2. When Pt particles of 8 × 1015 m-2 density receive electrons from the ionized donors, eight electrons are stored in a particle. However, a nanometer-sized Pt particle can receive a limited number of electrons. When one electron is confined in a metal sphere of 1 nm diameter, the electrostatic potential of the sphere placed in a vacuum is raised by 2.8 V. The ability of a Pt particle to ionize donors is limited by the raised potential, though the rise is reduced when the particle is close to highly polarizable TiO2. The illustration of Figure 5 is drawn for an infinite interface. On TiO2 surfaces modified with Pt particles, the potential is given by summing the double layers created by each particle. The surface hole concentration is therefore increased with the particle density. The lateral distance from one Pt particle to another was on the order of 10 nm, which is smaller than the ionized layer depth. Hence the surface hole concentration was constant as evidenced by the lateral distribution of residing TMA anions. Our TiO2 surface was prepared with ion sputtering and vacuum annealing. If most of donors (oxygen vacancies and interstitial Ti atoms) were near the surface, then the depletion layer would be much thinner than estimated. This was not the case however. The observed distribution of residing TMA indicated a uniform concentration of surface holes. In the model of Figure 5, the uniform hole (22) Li, M.; Hebenstreit, W.; Diebold, U.; Tyryshkin, A. M.; Bowman, M. K.; Dunham, G. G.; Henderson, M. A. J. Phys. Chem. B 2000, 104, 4944.
concentration suggests a depletion layer that is not thin enough. The lateral decay length of the potential will be experimentally determined when a particle-particle distance larger than examined in the current study is available. Preparing metal particles in a large separation was demonstrated with iridium on TiO2(110)-(1 × 2).23 The other feature to be interpreted in Figure 4 is the total number of decomposed TMAs. The TMA decomposition reaction 2 is initiated by the hole attachment. Our previous study7,8 showed that localized electrons are produced in reaction 3 and efficiently recombine with holes migrating from the bulk to the surface. The TMA reaction is thereby initially activated and then poisoned in a vacuum environment, where the localized electrons are accumulated with irradiation time. The relation rate decreased at 1 h or later on the three surfaces because of the poisoning. Curve b deviated from the expectation probably due to statistical errors arising from the finite number of TMA molecules counted in microscope images. On the Pt-free surface 0.4 ML of TMA reacted until the surface was poisoned, where the equivalent number of electrons was accumulated as Ti3+-OH--Ti4+. On the surface modified with 8 × 1015 Pt particles m-2, 0.6 ML of TMA was reacted. Assuming that the same number of localized electrons poisoned the two surfaces, electrons equivalent to 0.2 ML (5 × 1017 m-2) of TMA should have been stored somewhere on the Pt-modified surface. When the Pt particles receive the excess electrons, as often proposed in photocatalytic reaction mechanisms, a capacity of 60 electrons particle-1 is predicted. An averaged particle on this surface contains 1 × 103 Pt atoms based on the STM topography of Figure 1c. An electron capacity of 0.06 electron Pt-atom-1 is expected. These electron storage numbers (60 electrons Ptparticle-1 and 0.06 electron Pt-atom-1) sounds reasonable on one hand. On the other hand, it may be difficult to bring additional electrons into the negatively charged metal particles. We further point out that the model of Figure 5 is constructed on the contact potential difference of macroscopic Pt and TiO2 entities. The energy levels of the nanometer-sized particles can be perturbed. The amount and even direction of electron transfer across a particle-TiO2 interface remains unknown. A very small shift of the work function was indeed reported on a Ptdeposited TiO2(110) surface.24 Future studies with scanning Kelvin probe microscopy to determine the local work function of a TiO2 surface modified with cocatalysts are promising. The atomic-scale resolution of this scanning probe method has been demonstrated on Na-deposited TiO2(110).25 Acknowledgment. Discussions with M. A. Henderson and J. M. White are acknowledged. This work was supported by CREST of the Japan Agency of Science and Technology. LA051679T (23) Schierbaum, K. D.; Fischer, S.; Torquemada, M. C.; de Segovia, J. L.; Roma´n, E.; Martı´n-Gago, J. A. Surf. Sci. 1996, 345, 261. (24) Berko´, A.; Klive´nyi, G.; Solymosi, F. J. Catal. 1999, 182, 511. (25) Sasahara, A.; Uetsuka, H.; Onishi, H. Jpn. J. Appl. Phys. 2004, 43, 4647.
