pubs.acs.org/Langmuir © 2010 American Chemical Society
Photoreaction of the Rutile TiO2(011) Single-Crystal Surface: Reaction with Acetic Acid E. L. Quah,† J. N. Wilson,†,§ and H. Idriss*,‡ †
‡
Department of Chemistry, The University of Auckland, Auckland, New Zealand, and Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, U.K., and Robert Gordon University, Aberdeen, U.K. § Present address: Watercare Laboratory Services, Manukau 2022 Auckland, New Zealand. Received October 28, 2009. Revised Manuscript Received December 30, 2009
The reaction of acetic acid with stoichiometric and reduced rutile TiO2(011) single-crystal surfaces has been studied under dark and UV illumination conditions. The surface coverage after the dissociative adsorption of acetic acid with respect to Ti was found to be 0.55. Monitoring XPS Ti, O, and C lines revealed that the surface population decreased incrementally with temperature up to 650 K. The decrease in the slope of both the -CH3- and -COO- XPS peaks was not monotonic and followed two slopes in agreement with TPD results. The first channel involves the removal of surface acetates to acetic acid by recombinative desorption, and the second mainly involves dehydration to ketene. UV-light illumination was conducted at 300 K in the absence and presence of molecular oxygen at different pressures: in the 10-6-10-9 Torr range. Acetate species were found to decrease with illumination time, and their decrease is seen to be dependent on the oxygen pressure. Plausible decomposition pathways are presented. Deliberately reducing the surface by electron bombardment prior to the adsorption of acetic acid did not affect the photoreaction rate within the experimental limits.
Introduction Titanium dioxide surfaces have received considerable attention for many years because of their participation in a wide range of chemical and biochemical processes and in particular since the discovery of water splitting to hydrogen by Fujishima and Honda in 1972.1 This has triggered many other investigations, including photomaterials in dye sensitizer solar cell devices,2 the production of hydrogen from organic compounds,3 gas sensors,4 biosensors,5 and photocatalysts for wastewater treatment6 as well as disinfection such as the removal of bacteria.7 The essential criteria of the photocatalytic process in the TiO2 semiconductor is the existence of a region of void energy between the valence band (mainly O 2p) and the conduction band (mainly Ti 3d). This void-energy region, also known as the band gap, has a typical energy of 3.0-3.2 eV for TiO2. The irradiation of photons, with energy equal to or greater than the band gap energy, hν g Eg, creates electron-hole pairs. The adsorbed organic layer will then react with the migrated electrons and holes (from the conduction band and valence band) as well as with surface hydroxyls. Molecular oxygen also plays a crucial role in most photocatalytic reactions because it captures electrons from the conduction band, initiating further electrontransfer reactions. This ultimately leads to the decomposition of the organic molecule, producing CO2 and H2O.8 *Corresponding author. E-mail:
[email protected]. (1) Akira, F.; Honda, K. Nature 1972, 238, 37. (2) Pan, K.; Zhang, Q.; Wang, W.; Liu, A.; Wang, D.; Li, J.; Bai, Y. Thin Solid Films 2007, 515, 4085. (3) Yang, Y. Z.; Chang, C.-H.; Idriss, H. Appl. Catal., B 2006, 67, 217. (4) Zakrzewska, K.; Radecka, M. Thin Solid Films 2007, 515, 8332. (5) Zhou, H.; Gan, X.; Wang, J.; Zhu, X. L.; Li, G. X. Anal. Chem. 2005, 77, 6102. (6) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid St. Chem. 2004, 32, 33. (7) Maness, P.-C.; Smolinski, S.; Jacoby, W. A. Appl. Environ. Microbiol. 1999, 65, 4094. (8) Waterhouse, G. W. N.; Idriss, H. Photoreaction of Ethanol and Acetic Acid over Model TiO2 Single Crystal Surfaces. In On Solar Hydrogen & Nanotechnology; Vayssieres, L., Ed.; John Wiley & Sons: New York, 2009.
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This work aims to investigate the dark and photoreactions of a prototype organic molecule on the surface of a TiO2(011) single crystal. The organic molecule of choice is acetic acid because it is an ideal example of a carboxylic acid. It contains two carbon bonds, thus allowing for the interaction through the carboxylic function with the surface, leaving the other carbon (the CH3 group) further away from the interaction sites. Many studies have been conducted to investigate the interactions of acetic acid on rutile TiO2(110) single -crystal surfaces,9-14 the most studied rutile surface to date. All work has shown that acetic acid is dissociatively adsorbed with the two oxygen atoms adsorbed to two Ti cations along the [001] orientation, making a 2 1 reconstruction. Moreover, a nonnegligible number of photoreactions of small organic compounds were also studied on the (110) rutile surface. These include trimethyl acetic acid,15,16 ethanol,17 acetone,18-20 and acetaldehyde.21 Far less work has been devoted to studying carboxylic acids in general and acetic acid in particular on the TiO2(011) single crystal.22-24 This TiO2(011) singlecrystal surface has been the subject of intense computation and spectroscopic studies very recently. The stable surface is not the bulk-terminated surface but a reconstructed (2 1) structure. (9) Idriss, H.; Legare, P.; G. Maire, G. Surf. Sci. 2002, 515, 413. (10) Bates, S. P.; Kresse, G.; Gillan, M. J. Surf. Sci. 1998, 409, 336. (11) Wang, L. Q.; Ferris, K. F.; Shultz, A. N.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1997, 380, 352. (12) Fukui, K.i.; Iwasawa, Y. Surf. Sci. 2000, 464, L719. (13) Guo, Q.; Cocks, I.; Williams, E. M. J. Chem. Phys. 1997, 106, 2924. (14) Cocks, I. D.; Guo, Q.; Williams, E. M. Surf. Sci. 1997, 390, 119. (15) White, J. M.; Henderson, M. A. J. Phys. Chem. B 2005, 109, 12417. (16) White, J. M.; Szanyi, J.; Henderson, M. A. J. Phys. Chem. B 2004, 108, 3592. (17) Jayaweera, P. M.; Quah, E. L.; Idriss, H. J. Phys. Chem. 2007, 111, 1764. (18) Henderson, M. A. Langmuir 2005, 21, 3443. (19) Henderson, M. A. J. Phys. Chem. C 2008, 112, 11433. (20) Henderson, M. A. J. Phys. Chem. B 2005, 109, 12062. (21) Zehr, R. T.; Henderson, M. A. Surf. Sci. 2008, 602, 2238. (22) Idriss, H.; Barteau., M. A. Adv. Catal. 2000, 45, 261. (23) Wilson, J. N.; Idriss, H. J. Catal. 2003, 214, 46. (24) Wilson, J. N.; Idriss, H. J. Am. Chem. Soc. 2002, 124, 11284.
Published on Web 02/26/2010
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Initially, a surface model was proposed containing a titanyl group, (TidO).25-27 In this model, the reaction of few molecules has been studied, including acetic acid,28 by computational methods. A more recent model has been proposed on the basis of XP diffraction and STM and explains the 2 1 reconstruction without the need to invoke TidO groups29,30 (Figure 1). It has been suggested that the (011) surface has high photocatalytic activity when compared to other low-index surfaces. This suggestion was based on Ag ion reduction on different thin films of rutile TiO2.31,32 This activity is, however, not a general trend because other work has shown that the (110) surface is indeed more active for Pt ion reduction when compared to the (011) surface.33 None of these studies were conducted, however, on single crystals. We have previously investigated the reaction of acetic acid over this surface by steady state and monitored the gas-phase reaction.23,24 In this work, we focus on adsorbates and their population in the presence and absence of molecular oxygen under UV irradiation in order to understand their surface reaction and extract meaningful physical parameters such as the photoionization cross section. In addition, we have studied the same reaction over a deliberately reduced TiO2 surface to see the effect of reduced surface states on the photoreaction.
Experimental and Methodology The experiments are conducted in an ultrahigh vacuum chamber (UHV) equipped with XPS (Perkin-Elmer), a sputter gun, a dosing line, and other equipment needed to heat the sample. The (011)-oriented TiO2 (10 10 1 mm3) obtained from the MIT Corporation was glued (Ultra-Temp 516, a high-temperature ceramic adhesive with paste properties) as well as clamped onto Ta plates (0.1 mm width) and a ceramic holder with Ta wires (0.5 mm diameter) attached to it. The crystal surface is cleaned by successive sputtering and annealing cycles with a typical Ar pressure of 5 10-5 Torr, 2 to 3 kV beam voltage, and 25 mA emission current followed by several flashes to ∼700 K with and without oxygen. The procedure is repeated until no carbon peak was detected in the XPS scans. A narrow XPS Ti 2p3/2 line with a fwhm value of less than 1.4 eV and a nearly symmetric XPS O 1s line would indicate that the TiO2 crystal is indeed stoichiometric. XPS analyses are performed using an Al anode source at 14.5 kV, 20 mA current. Acetic acid is cleaned using a freeze-pump-thaw process several times to remove contamination. Acetic acid dosing is carried out with a backfilling method from a dosing line connected to the roughing pump with a base pressure of ∼10-3 Torr. When working on the experiments to investigate XPS C 1s and O 1s as a function of temperature, the surface is saturated with acetic acid ((1-5) 10-8 Torr) and then flashed to the indicated temperatures. After the desired temperature is reached, the crystal is allowed to cool before data acquisition. The XPS C 1s signal is collected at a pass energy (PE) of 50 eV whereas those of XPS Ti 2p and XPS O 1s, at a PE of 25 eV. (25) Beck, T. J.; Klust, A.; Batzill, M.; Diebold, U.; Di Valentin, C.; Tilocca, A.; Selloni, A. Surf. Sci. 2005, 591, L267. (26) Dulub, O.; Di Valentin, C.; Selloni, A.; Diebold, U. Surf. Sci. 2006, 600, 4407. (27) Di Valentin, C.; Tilocca, A.; Selloni, A.; Beck, T. J.; Klust, A.; Batzill, M.; Losovyj, Y.; Diebold, U. J. Am. Chem. Soc. 2005, 127, 9895. (28) McGill, P. R.; Idriss, H. Surf. Sci. 2008, 602, 3688. (29) Torrelles, X.; Cabailh, G.; Lindsay, R.; Bikondoa, O.; Roy, J.; Zegenhagen, J.; Teobaldi, G.; Hofer, W. A.; Thornton, G. Phys. Rev. Lett. 2008, 101, 185501. (30) Gong, X.-Q.; Khorshidi, N.; Stierle, A.; Vonk, V.; Ellinger, C.; Dosch, H.; Cheng, H.; Selloni, A.; He, Y.; Dulub, O.; Diebold, U. Surf. Sci. 2009, 603, 138. (31) Morris Hotsenpiller, P. A.; Bolt, J. D.; Farneth, W. E.; Lowekamp, J. B.; Rohrer, G. S. J. Phys. Chem. B 1998, 102, 3216. (32) Lowekamp, J. B.; Rohrer, G. S.; Morris Hotsenpiller, P. A.; Bolt, J. D.; Farneth, W. E. J. Phys. Chem. B 1998, 102, 7323. (33) Ohno, T.; Sarukawa, K.; Matsumura, M. New J. Chem. 2002, 26, 1167.
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Figure 1. Bulklike surface (a) and the 2 1 reconstructed surface (b) of the rutile TiO2(011) single crystal according to the models of refs 29 and 30. All Ti atoms on the surface are 5-fold coordinated whereas all O atoms are 2-fold coordinated compared to bulk values of 6 and 3, respectively. Black balls are O atoms, and gray balls are Ti atoms. TPD experiments are conducted in a stainless steel UHV chamber that has been described in detail previously.23,24 The pressure is in the 10-10 Torr range because of a 240 L/s turbomolecular pump, a Varian titanium sublimation pump, and a 220 L/s Varian ion pump. The system is equipped with a Spectra quadrupole mass spectrometer, with measurement capability of up to 300 m/z, enclosed in a Pyrex tube with a small aperture, a Perkin-Elmer ion gun for sample cleaning and sputtering, a threecoordinate sample manipulator, and a reactant dosing line (10-3 Torr) with leak valve and dosing needle positioned a few millimeters away from the crystal face. The TiO2(011) single crystal is cleaned in situ by repeated cycles of Arþ bombardment and annealing. A typical cycle consisted of sputtering with a beam voltage of 3 kV and an emission current of 25 mA for 30 min and argon pressure of (2-3) 10-5 Torr, followed by annealing at 750 K in a 1 10-7 Torr oxygen atmosphere for 30 min. Acetic acid was purified by heating gently prior to use in the dosing line. All dosing was performed at room temperature. A heating rate of 1 K/s was used in TPD experiments. UV steady-state reactions were conducted with a similar set of conditions to that of TPD with the exception that acetic acid was continually leaked into the chamber from a dosing line. A 0.15-mm-wide stainless steel tube with an internal diameter of about 0.5 mm at 1 mm from the crystal face was used for the dosing of acetic acid and oxygen while the crystal was exposed to UV photons. Reaction products were monitored using the same quadrupole mass spectrometer, which is positioned about 1 mm from the crystal face. Analysis Method. A surface-coverage experiment is performed on the oxidized TiO2(011) single-surface crystal. The surface coverage is calculated by comparing the intensity of the Ti 2p and O 1s signals of the clean surface with signals obtained upon dosing surface saturation through the following equation: IB ¼ IB0 ½1 -φA þ φA expð -R=λ cos θÞ IB is the intensity after adsorption, IB0 is the intensity before adsorption, φA is the surface coverage, R is the diameter of acetic acid, λ is the attenuated length of photoelectrons, and θ is the X-ray angle normal to the crystal surface. Langmuir 2010, 26(9), 6411–6417
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Figure 2. (a) XPS of Ti 2p before (b) and after (O) acetic acid at saturation (1.3 L) on the TiO2(011) single crystal at 300 K. (b) XPS O 1s before and after the adsorption of acetic acid at saturation (1.3 L) on the TiO2(011) single crystal at 300 K; the height of the O 1s peak after adsorption is normalized to that before adsorption to highlight the contribution of the COO group on the high binding-energy side. The intensity of the peak is obtained using XPS peak-fitting software, and the size of the acetate molecule is obtained from the Spartan 06 code with DFT B3LYP 6-31þG(d,p). To undergo photoreaction, however, TiO2(011) is exposed to UV light with a wavelength of 360 nm through the window flanges of the UHV chamber. The lamp used was a 100 W Xe/Hg lamp that was focused onto the sample without an infrared filter. The surface-temperature increase of about 5 K was too small to affect the data because most of the IR radiation was absorbed by the Pyrex window. The flux of the lamp was measured using an HD 2302 photoradiometer (Delta Ohm, Bell Technology. Ltd.) after the Pyrex window similar to that used for the UHV chamber. The flux was found to be close to 1017 photons/cm2 s. The crystal is first dosed with 1 10 -8 Torr acetic acid for 120 s. The crystal was then illuminated with UV light at different time intervals with and without oxygen. This part of the UV illumination was repeated three times but with different oxygen pressures each time. The oxygen pressures were 1 10 -8, 1 10 -7, and 1 10 -6 Torr. Photoreaction with 1 10 -7 Torr O2 was repeated again, but on the reduced surface instead. The reduced surface was exposed under an electron beam gun (Perkin-Elmer) for 20 min at 3 kV and 20 mA emission current. The photoionization cross section of acetate was then calculated using the following equation: Ct ¼ C0 expð -ktÞ, k is the rate constant in s -1 Ct ¼ C0 expð -FQtÞ Ct is the acetate coverage after illumination at time t, C0 is the acetate coverage before illumination, F is the flux of UV photons, and and Q is the cross section. TPD Data Analysis. Desorption peak analysis is permitted by using ASTEK (analysis and simulation of the thermal equilibrium and kinetics of gases adsorbed on solid surfaces), written by Kreuzer and Payne (Helix Science Applications).34 From this data, the program extracts the isosteric heat of adsorption, the energy needed to move a particle from the adsorbate into the gas phase at a given temperature and coverage. By varying the initial coverage, one can extract the isosteric rate from Arrhenius parameters (coverage-dependent desorption energy and prefactor). dθ -Ed ðθ0 Þ x θo ¼ -veff ðθ0 Þ exp dt kB T (34) Kreuzer, H. J.; Payne, S. H. Thermal Desorption Kinetics. In Dynamics of Gas-Surface Interactions; Rettner, C. T., Ashfold, M. N. R., Eds.; Royal Society of Chemistry: Cambridge, U.K., 1991; pp 220-256.
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The coverage on the right-hand side of the above equation is set equal to the initial coverage for a given TPD trace, and the logarithm of the rate against 1/T from the initial temperature to that where a specified fraction has desorbed is plotted. In the equation, θ is the coverage, νeff is the prefactor, Ed is the desorption energy, x is the order of desorption, and kB is the Boltzmann constant.
Results and Discussion Dark Reactions. Figure 2a represents XPS Ti 2p before and after the adsorption of acetate at saturation (1.3 L) on the TiO2(011) single crystal at room temperature. The two peaks at 458.9 and 464.3 eV coincide with the positions of Ti 2p3/2 and Ti 2p1/2 for Ti4þ cations, respectively. The surface coverage of acetate with respect to Ti atoms is calculated from the attenuation of the Ti 2p3/2 peak and is found to be 0.55. The approximated size of one acetate molecule is 5.10 A˚ in diameter, and the attenuation length of photoelectrons was taken to be 1.7 nm.35 This indicates that one molecule of acetate would be adsorbed for every two 5-fold Ti atoms on the surface in a bridging configuration. This is similar to the adsorption of carboxylic acids over the TiO2(110) surface.13,14 Figure 2b presents XPS O 1s before and after the adsorption of acetate on the single-crystal surface at room temperature. The initial O 1s peak signal found after adsorption was attenuated because of screening by both the carboxylate and methyl groups. In Figure 2b, the signal after adsorption is scaled to that of the clean-surface O 1s peak to highlight the difference. The lattice oxygen peak is at a binding energy close to 531.0 eV, and adsorbate oxygen is at a binding energy close to 532.8 eV. The 1.8 eV separation between them is consistent with other reported work.36,37 The peak at 532.8 eV is due to both surface carboxylates (COO) and surface hydroxyls resulting from the dissociative adsorption with a theoretical ratio of 2 (oxygen atoms of the carboxylate group) to 1 (oxygen from the hydroxyl group). Figure 3 presents the XPS of C 1s obtained upon exposing the surface to increasing amounts of acetic acid at 300 K. Two peaks are observed at 285.1 and 289.2 eV and are attributed to the CH3 and COO groups, respectively. The separation between these peaks (3.9 eV) and the fhwm of each carbon peak (1.7 eV for COO and 1.9 eV for CH3) are consistent with previous work reported for acetate on other surface crystals.36,38 The ratio of the carboxylate (35) Fuentes, G.; Elizalde, E.; Yubero, F.; Sanz, J. M. Surf. Interface Anal. 2002, 33, 230. (36) Kim, K. S.; Barteau, M. A. J. Catal. 1990, 125, 353. (37) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 193, 33. (38) Ashima, H.; Chun, W.-J.; Asakura, K. Surf. Sci. 2007, 601, 1822.
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surface depletion in Figure 5b represents two different reaction processes occurring: one at low temperature (up to about 500 K) and the other at high temperature (above 550 K). In the first domain, about 0.3 of the total coverage is removed, and in the second domain, most of the remaining surface coverage is removed; both domains are discussed next. Figure 6 presents TPD following acetic acid adsorption at 300 K. Two desorption domains are seen: one mostly due to acetic acid and water and the other one due to ketene (CH2dCO), CH4, CO2, and some acetic acid. Ketene formation has been seen previously from acetic acid on many oxide surfaces, including TiO222 and UO2,39,40 and represents the dehydration route as indicated by the following equations: CH3 COO-ðTiÞ2 þ O-H f CH3 COOH þ 2Tis þ Os
ð1Þ
CH3 COO-ðTiÞ2 þ O-H f CH2 dCO þ H2 O þ 2Tis þ Os ð2Þ Figure 3. Coverage dependence of acetic acid on the TiO2(011) single crystal monitored by XPS of C 1s. (Inset) Sum of C 1s peak areas as a function of exposure.
Figure 4. Effect of temperature on acetate population on the TiO2(011) single crystal monitored by XPS of C 1s.
carbon peak to the area of the methyl group carbon is 0.83. The inset in Figure 3 presents the total carbon signal as a function of exposure to acetic acid. It indicates that after an initial surface coverage nearing saturation a 10-fold increase in exposure (from 0.133 to 1.33 L) had increased the surface coverage by about 20%. This nonlinear behavior has been previously seen for many compounds, including carboxylic acids.39,40 Figure 4 presents the effect of temperature on the population of acetate after the surface has been saturated with acetate species and hydroxyls at 300 K. Note the small fraction of surface carbides at about 283.3 eV. Heating results in a decrease in the surface population resulting from thermal desorption and decomposition reactions. Figure 5a represents the ratio of peak areas of -CH3- to -COO- as a function of annealing temperature. The total peak areas of both -COO- and -CH3- groups are displayed in Figure 5b as a function of temperature. The decrease in the amount of surface acetate is not linear, thus more than one process for its removal is involved. The two slopes for (39) Chong, S. V.; Idriss, H. Surf. Sci. 2002, 504, 145. (40) Chong, S. V.; Idriss, H. J. Vac. Sci. Technol., A. 2000, 18, 1900.
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s stands for the surface, and -(Ti)2 indicates two surface Ti atoms. The desorption of CH4, CO, and CO2 at the same temperature as that of ketene is due to acetate decomposition. The desorption of acetic acid in two temperature domains is not a wellunderstood reaction, yet many other carboxylic acids on TiO2 and UO2 single-crystal surfaces have shown similar trends, including formic acid,41 glutaric acid,42 maleic anhydride,43 and acrylic acid44 on TiO2(011) faceted (001) single crystals and formic, acetic, and propionic acids39 on UO2(111). There are three interpretations of this behavior: (i) the first desorption is due to molecular adsorption, and the second one is due to recombinative desorption; (ii) both modes are due to the recombinative desorption of acetates, but the first desorption is due to the monodentate mode and the second one is due to the bidentate mode of adsorption, with the monodentate mode being less stable than the bidentate mode;28 (iii) both modes are due to recombinative modes of bidentate acetates, but the first one occurs on a perfect surface and the second one occurs on surfaces with oxygen defects. Many studies, including this one, indicate that at 300 K most of the carboxylic acids are dissociatively adsorbed; therefore, (i) can be neglected. The presence of monodentate acetates (carboxylates), although computationally stable, has not been seen by STM on TiO2 surfaces, and this suggest that (ii) is probably not the main contributor. The removal of water at the same time as carboxylic acid creates surface oxygen vacancies. These vacancies act as surface traps on which part of the carboxylates migrate,45 thus their desorption requires higher energy. This behavior makes (iii) a plausible explanation. We have computed the desorption energies of acetic acid during TPD using the ASTEK code,34 where several TPD experiments were conducted at different coverage and the data were fitted to the desorption equation to extract the activation energies for desorption and prefactors. Figure 7 presents the change in the desorption signal of acetic acid (m/z 60) at different coverage with temperature and the corresponding Arrhenius plots. From these, one can extract the desorption energy at a given coverage. The desorption energy near zero coverage is found to be close to 25 kcal/mol (or about 105 kJ/mol), and the associated prefactor (41) Idriss, H.; Lusvardi, V. S.; Barteau, M. A. Surf. Sci. 1996, 348, 39. (42) Wilson, J. N.; Idriss, H. Langmuir 2005, 21, 8263. (43) Wilson, J. N.; Titheridge, D. J.; Kieu, L.; Idriss, H. J. Vac. Sci. Technol., A 2000, 18, 1887. (44) Titheridge, D.; Barteau, M. A.; Idriss, H. Langmuir 2001, 17, 2120. (45) Hayden, B. E.; King, A.; Newton, M. A. J. Phys. Chem. B 1999, 103, 203.
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Figure 5. (a) XPS C 1s peak areas of [CH3]/[COO] of as function of temperature. (b) Sum of XPS C 1s peak areas of both -COO and -CH3 groups as a function of temperature correlated with the surface coverage computed from TPD.
Figure 6. TPD after acetic adsorption at 300 K over the 2 1 rutile TiO2(011) surface. The numbers in parentheses represent the product yield.
was equal to 1010 s-1. It is also to be noted that desorption energies decrease with increasing coverage, most likely because of the lateral repulsion of adsorbed acetates. UV Irradiation. The effect of UV illumination was studied next. XPS of C 1s before and after UV illumination of acetates in the absence of oxygen at different time intervals was first studied. No noticeable changes in the peak areas are seen, in line with other observations on TiO2(110)9,17 suggesting that surface O atoms are not directly involved in the photodecomposition. Figure 8 presents the XPS of C 1s of acetic acid in the presence oxygen for 30 and 60 min. There is a decrease in the C 1s carbon peaks after 30 and 60 min in the presence of 1 10-6 Torr of molecular oxygen. Figure 9 presents ln Ct/C0 as a function of time at different oxygen pressures, where Ct is the acetate peak area signal after time t of UV illumination, and C0 is the initial acetate peak area signal. A linear decrease is seen with time with a steeper slope at higher O2 pressures. The cross section of acetate removal, Q (cm-2), can be obtained by knowing the flux of the UV light and the rate of depletion. The cross sections obtained are 9 10-22, 8 10-22, and 5 10-22 cm-2 under oxygen pressure of 1 10-6, 1 10-7, and 1 10-8 Torr, respectively. As indicated in the Introduction, photoreaction involves the transfer of electrons (leaving positively charged holes behind) from the valence band to the conduction band. The negatively Langmuir 2010, 26(9), 6411–6417
Figure 7. Computation of the desorption energy from the experimental TPD of acetic acid over the TiO2(011) surface. The main plot presents the desorption energy at three different initial coverages. The equation presents the fitted desorption energies (y) with coverage (x). The two insets present the change in surface coverage with temperature and the corresponding Arrhenius plots. All data were computed using the ASTEK code as described in the Experimental and Methodology section.
charged electron at the surface reacts with gas-phase molecular oxygen to produce O2- 3 (O2 minus radical) that in turn reacts with acetates on the surface to form other intermediates with the final reaction products being CH4/C2H6 and CO2.23,24,46-48 The two main reactions are ð3Þ CH3 COOHðaÞ f CO2 þ CH4 ðgÞ 2CH3 COOHðaÞ þ Os f 2CO2 ðgÞ þ CH3 CH3 ðgÞ þ H2 OðgÞ þ VO
ð4Þ 1 VO þ O2 ðaÞ f Os 2
ð5Þ
s stands for the surface, (a) indicates adsorbed, and VO represents surface oxygen defects with two associated electrons. Steady-state reactions were conducted under UV irradiation to monitor the reaction products. Figure 10 presents ethane (46) Nosaka, Y.; Koenuma, K.; Ushida, K.; A Kira, A. Langmuir 1996, 12, 736. (47) Liao, L.-F.; Lien, C.-F.; Lin, J.-L. Phys. Chem. Chem. Phys. 2001, 3, 3831. (48) Muggli, D. S.; J. L. Falconer, J. L. J. Catal. 1999, 187, 230.
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Figure 10. Steady state under UV excitation of acetic acid on the (011) surface of rutile TiO2. Ethane desorption is monitored by m/z 30, and O2 is monitored by m/z 32. Other products were also monitored, including CO2, methane, and water. Ketene was absent. The UV label indicates the time domain in which the UV light was turned on. The acetic acid signal is offset by 3 10-9 Torr for a better display of the data.
Figure 8. XPS of C 1s of acetic acid adsorbed on TiO2(011) at 300 K before and after illumination with UV light in the presence of 1 10-6 Torr O2 for 30 and 60 min. (Inset) XPS of C 1s indicating the decrease in the acetate concentration after UV illumination.
assumes that the full coverage of acetates occurs in a bidentate mode, then all Ti atoms on terraces are bound to the O of the carboxylic function of acetates. It is thus reasonable to assume that molecular O2 is adsorbed on surface steps and that the reaction occurs in a domain-type way, as has been seen for trimethyl acetic acid over TiO2(110) by STM.49 Surface oxygen defects may play many roles for chemical reactions, yet specifically in this study they may act in at least two ways: (i) They would trap molecular oxygen where one molecule would heal a surface point defect and the other would recombine with another one to desorb (reaction 5). Reaction 5 may not be the only pathway. Reports of on-top oxygen atoms (on top in this case means that one O atom is above one surface Ti atom that is “presumed” to be fully oxidized) have been given by a few authors on the basis of STM observations over reduced TiO2(110) surfaces:50-53 VO þ O2 f Os þ OðaÞ
Figure 9. ln Ct/C0 extracted from XPS C 1s total peak area ratios as a function of UV illumination under different oxygen pressures. Ct represents the peak areas of C 1s at time t, and C0 represents the initial peak areas.
desorption (the most dominant product) under an acetic acid partial pressure of ca. 5 10-9 Torr and as a function of increasing O2 pressure. The O2 gas and acetic acid vapor were introduced via two separate dosing lines. Acetic acid is introduced via a dosing needle at the face of the crystal, and O2 was introduced by the backfilling method. The partial pressures of reactants and products are measured with the online quadrupole mass spectrometer. Initially, in the presence of only background O2 pressure, ethane is formed but then decreases with time because of the removal of surface oxygen per eq 4. Adding O2 to the chamber restored ethane formation. The small increase in the acetic acid signal under UV irradiation is due to some surface desorption. The restoration of ethane formation due to O2 is thus an indication of the contribution of surface oxygen atoms during the photoreaction. Sites for Molecular Oxygen and Surface Defects. It is unclear where the adsorption of molecular oxygen occurs. If one 6416 DOI: 10.1021/la9040985
ð6Þ
The exact electronic nature of this species is not yet fully understood for the following reasons. Because one O2 molecule, once split, would contain two oxygen atoms, each with six electrons in the second shell (2s2, 2p4), and because one of them would take two electrons (from VO) to become O2-, indicated above as Os, the other one is short of two electrons and cannot make bonds on its own. It is therefore to be stressed that because O(a) is short of two electrons and Ti4þ cations cannot donate them, subsurface defects have been proposed to play an important role in stabilizing such a species. (49) Henderson, M. A.; White, J. M.; Uetsuka, H.; Onishi, H. J. Catal. 2006, 238, 153. (50) Zhang, Z.; Du, Y.; Petrik, N. G.; Kimmel, G. A.; Lyubinetsky, I.; Dohnalek, Z. J. Phys. Chem. C 2009, 113, 1908. (51) Du, Y.; Deskins, N. A.; Zhang, Z.; Dohnalek, Z.; Dupuis, M.; Lyubinetsky, I. Phys. Rev. Lett. 2009, 102, 096102. (52) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Nat. Mater. 2006, 5, 189. (53) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlstrom, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Laegsgaard, F.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Surf. Sci. 2005, 598, 226.
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Article
(ii) The other effects that the creation of surface O defects would have is the reduction of some Ti4þ to Ti3þ cations (or even lower oxidation states), and it has been proposed that these Ti3þ cations would act as centers for fast electron-hole recombination and therefore decrease the photoreaction cross section54 but in other work they have also been correlated to hole trapping and therefore the increasing lifetime of excited electrons, which in turn increases the photocatalytic activity.55 To see the effects of surface defects, we have studied the adsorption and reactions of acetic acid over an electron-bombarded surface. We have opted for electron bombardment instead of Ar ion bombardment because it has been seen that the surface would still maintain some order after electron bombardment56 whereas Ar ion bombardment would in general result in a disordered surface. Figure 11 presents the XPS of C 1s after acetic acid adsorption on the electron-bombarded (reduced) surface (as indicated in the Experimental and Methodology section) as well as after UV illumination in the presence of molecular oxygen. There is no qualitative difference between this reduced surface (Ti 2p inset of Figure 11) covered with acetates and the fully oxidized surface. More importantly, there is no quantitative difference in its photocatalytic reactivity when compared to that of the stoichiometric surface. The extraction of the photoionization cross section from the C 1s decay indicates a very similar number to that observed on the stoichiomteric surface (about 7 10-22 cm-2). Although this observation may be rationalized as XPS is nonlocal and the rate seen is that from the total surface of the single crystal, curve fitting of the shoulder attributed to Ti3þ with a peak at 457.6 eV indicated about an 11% contribution of the overall Ti 2p3/2 signal. Considering that the attenuation depth of the Ti p3/2 photoelectron is close to 1.7 nm, the corresponding number of reduced surface states would be far higher. These results, however, indicate that the reduction of a fraction of the surface of TiO2 did not noticeably affect the photoreaction rate.
Conclusions This work presents dark and photoreaction conditions of acetates on stoichiometric and reduced TiO2(011) single-crystal (54) Murakami, S. Y.; Kominami, H.; Kera, Y.; Ikeda, S.; Noguchi, H.; Uosaki, K.; Ohtani, B. Res. Chem. Intermed. 2007, 33, 285. (55) Ke, S.-C.; T.-C. Wang, T.-C.; Wong, M.-S.; Gopal, N. O. J. Phys. Chem. B 2006, 110, 11628. (56) Dulub, O.; Batzill, M.; Solovev, S.; Loginova, E.; Alchagirov, A.; Madey, T. E.; Diebold, U. Science 2007, 317, 1052.
Langmuir 2010, 26(9), 6411–6417
Figure 11. XPS of C 1s before (a) and after (b, c) irradiation with
UV light in the presence of 10-7 Torr molecular oxygen. (RightHand Inset) Ti 2p XPS of oxidized (i) and the TiO2(011) single crystal bombarded with electrons for 20 min (ii). (Left-Hand Inset) ln of the ratio of the total carbon atoms’ signal after irradiation (Ct) over the initial surface carbon signal (C0) as a function of irradiation time.
surfaces. The surface coverage of dissociatively adsorbed acetate on the stoichiometric surface with respect to Ti was found to be equal to 0.55. One fraction of this (about 0.3) is removed by recombinative desorption below 500 K, and the remainder desorbs at higher temperature together with ketene (formed by dehydration) and the decomposition products (CO2, CO, and methane). Upon UV illumination of the stoichiometric surface, the population of acetates decreases with time depending on the gas-phase molecular oxygen pressure, with increasing reaction cross section at higher O2 pressures. In the 10-8 to 10-6 Torr range, the cross section was found to change by about half (from 9 10-22 cm-2 at 10-6 Torr to 5 10-22 cm-2 at 10-8 Torr). These relatively low cross-section numbers indicate that the rutile (011) surface has no particular photocatalytic activity when compared to other rutile TiO2 surface, in line with the new proposed structural model.29,30,57 The reduced surface prepared by prior electron bombardment, creating Ti3þ cations and associated oxygen vacancies, was shown to be equally active for the photoreaction within experimental limits. (57) Chamberlin, S. E.; Hirschmugl, C. J.; Poon, H. C.; Saldin, D. K. Surf. Sci. 2009, 603, 3367.
DOI: 10.1021/la9040985
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