ARTICLE pubs.acs.org/JPCC
Interaction of CO with Oxygen Adatoms on TiO2(110) Junseok Lee,*,†,‡ Zhen Zhang,§ Xingyi Deng,†,‡ Dan C. Sorescu,† Christopher Matranga,† and John T. Yates, Jr.*,§ †
National Energy Technology Laboratory, Department of Energy, Pittsburgh, Pennsylvania 15236, United States URS, P.O. Box 618, South Park, Pennsylvania 15129, United States § Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States ‡
ABSTRACT: The interaction between CO and O adsorbed on TiO2(110) has been investigated using scanning tunneling microscopy (STM), electron stimulated desorption (ESD), temperature programmed desorption (TPD), and density functional theory (DFT). Coadsorption of CO and O produces CO-O and CO-O-CO surface complexes via weak attractive interaction as revealed by STM and DFT. The effect of the adsorbates interaction can also be observed in two ensembleaveraged techniques, ESD and TPD, strongly supporting the STM and DFT results. The CO molecules adsorbed near chemisorbed O cause a ∼70% decrease in the Oþ ion yield in ESD. The interaction between CO and O is considered to be electrostatic in nature due to the charge rearrangement upon chemisorption.
I. INTRODUCTION Understanding the fundamental steps of catalytic reactions on metal oxide surfaces provides useful insights in technological areas such as heterogeneous catalysis, gas sensors, and photocatalysis and in the photooxidation of organic pollutants.1,2 Rutile TiO2 is a wide band gap (3.0 eV) semiconductor, and its (110) surface has become a model system for the fundamental study of reactions of metal oxide surfaces.3 The study of molecular oxygen interaction with TiO2 surfaces has become a popular topic due to its role in photooxidation and water splitting reactions.3-5 Various aspects of the chemistry of oxygen on TiO2(110) have been studied by using temperature programmed desorption (TPD), electron stimulated desorption (ESD), and scanning tunneling microscopy (STM).6 For example, the physisorption of O2 was observed at low temperatures and high O2 pressures.7,8 More than one O2 molecule has been reported to chemisorb at an oxygen vacancy (Vo) site,9,10 and two separate O2 dissociation channels have been identified.11,12 To better understand the catalytic reactions involving oxygen on the TiO2 surface, studies of species coadsorbed with oxygen are necessary. Despite remarkable progress in the field, only one other STM investigation to understand the interaction between an oxygen adatom and CO adsorbed on TiO2(110) has been reported.13 In addition there has been little connection between the results obtained by atomic scale techniques and ensemble-averaged techniques. In this paper, we present a study of the interaction between molecular CO and atomic O adsorbed on a reduced TiO2(110) single crystal surface using STM, ESD, TPD, and density functional theory (DFT). STM and DFT results reveal that attractive r 2011 American Chemical Society
interaction exists between the two adsorbates as demonstrated by the formation of surface CO-O and CO-O-CO complexes. We also show results from ensemble-averaged techniques such as ESD and TPD to further support the atomic scale results.
II. STM STUDIES OF CO AND O INTERACTIONS ON TIO2(110)-(1 1) The STM experiments have been performed in a variable temperature ultrahigh vacuum (UHV) STM system (Omicron) at 40 K using liquid He. The typical base pressure was around 5 10-12 mbar. A reduced clean TiO2(110) surface has been obtained through Arþ sputtering and annealing in vacuum. A variable leak valve was attached close to the STM head for directional dosing of CO and O2 gases. STM images were obtained in a constant current mode at positive sample bias (1.2-1.5 V; 10-50 pA). ESD experiments were performed in a separate UHV chamber with a calibrated capillary array doser for gas dosing. A detailed description of the ESD setup can be found elsewhere.14 Spin-polarized DFT calculations reported in this study were done using the VASP package15 and the Perdew, Burke, and Ernzerhof (PBE)16 exchange-correlation functional. A plane-wave basis set with a cutoff energy of 400 eV has been employed while the sampling of the Brillouin zone was performed using a 2 2 1 MonkhorstPack17 grid of k points. The electron-ion interaction was described using the projector augmented wave (PAW) method as implemented by Kresse and Joubert.18 The rutile TiO2(110) Received: November 26, 2010 Revised: January 24, 2011 Published: February 23, 2011 4163
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Figure 1. (a) STM image (20 20 nm2, 1.5 V, 20 pA) of O/TiO2(110) surface after dosing O2 at 40 K and subsequent annealing to 200 K. The inset (3 3 nm2) is an image of a surface area with Ot and Vo features superimposed with a lattice grid. The grid is constructed so that vertical lines correspond to Ti rows and horizontal lines are aligned with Vo positions. It clearly shows two Ot atoms are located on top of Ti5c atoms. (b) STM image (7 7 nm2, 1.2 V, 10 pA) of CO/TiO2(110) surface after dosing CO at 40 K. The inset (3.9 3.1 nm2) shows the CO is also located on top of a Ti5c atom. (c) STM image of CO/O/TiO2(110) surface. A small amount of CO was dosed on O/TiO2(110) surface at 40 K.
surface has been studied using a 6 2 periodic supercell model (along the [001] and [110] directions) containing five (O-TiO) layers. During optimizations, the adsorbates and the surface atoms in the top three layers were allowed to relax while the bottom two layers of the slab were kept frozen. A typical STM image of oxygen adatoms (Ot) on a reduced clean TiO2(110) surface at 40 K is shown in Figure 1a. The image was acquired after dosing O2 on the TiO2(110) surface at 40 K followed by brief annealing to 200 K. The topographically high bridging oxygen rows appear dark and the topographically low Ti rows appear bright. The main features found on the surface are adsorbed atomic oxygen (Ot), bridging-oxygen vacancy site (Vo), and bridging hydroxyls (OHb). The Vo is imaged as a bright spot on dark bridging oxygen row and the OHb is imaged brighter and larger than the Vo feature as shown in Figure 1a. The Ot is imaged as a small, round feature located on the Ti row and the inset to Figure 1a shows the Ot is located on top of the 5-fold Ti (Ti5c) site as reported in previous studies.19,20 The apparent height of the Ot feature in Figure 1a measured from the Ti row along the [001] direction is 49 ( 7 pm. The apparent height varied with different tip conditions. Pairs of Ot atoms are also found on the surface confirming previous reports on the additional O2 dissociation channel on the Ti row.12 Figure 1b shows an STM image of CO molecules adsorbed on a clean TiO2(110) surface at 40 K. The CO molecule is also imaged as a bright protrusion on the Ti row. The CO feature is larger and higher than Ot with an apparent height of 60 ( 9 pm. The inset shows two CO molecules located on top of Ti5c atoms in accord with a previous study.21 Figure 1c shows an STM image after a small amount of CO adsorption on the O/TiO2(110) surface at 40 K. We could not find any significant interaction between CO and Ot due to the lack of thermal energy required for the diffusion of species at low temperature. After the TiO2(110) surface was exposed to O2, terminal hydroxyl (OHt) species are known to form on Ti5c as a result of reaction of the O2 with OHb at room temperature,22 even though it is unlikely at low temperature. Height analysis reveals the CO is lower than the OHt (105 ( 7 pm) feature allowing us to identify CO by its apparent height. Annealing the CO/O/TiO2(110) surface to 100-120 K (below the CO desorption temperature on TiO2(110)) induces thermal diffusion of the CO molecules. Figure 2a shows an STM image after annealing the surface to ∼100 K. Note that new pear-shaped features appear on Ti rows. Each lobe within the pear-shaped feature is located approximately on a Ti5c site, and the lobes are separated by one lattice spacing as shown in Figure 2b. We assign the pear-shaped feature to a CO-O surface complex. By judging from the apparent
Figure 2. (a) STM image (5 5 nm2, 1.2 V, 10 pA) of CO/O/ TiO2(110) surface after annealing to 110 K. The pear-shaped feature in the ellipse is a CO-O complex formed upon annealing. A line profile along the red line is shown in (d). (b) STM image (3.5 3.5 nm2, 1.4 V, 10 pA) of an area with CO-O, Ot, and three Vo features superimposed with a lattice grid. Two small black dots are the positions of CO and O in the CO-O complex. (c) STM image (8 8 nm2, 1.2 V, 10 pA) of CO/ O/TiO2(110) surface at higher coverage of CO. The peanut-shaped feature in the ellipse is a CO-O-CO complex. A line profile along the blue line is shown in (d). Two panels in the inset show a voltage pulsing experiment. A negative voltage pulse (-3.5 V, 30 ms) is applied to the upper CO molecule while scanning the area where the CO-O-CO complex is located (left panel). A CO-O complex is shown after a CO molecule is displaced from the CO-O-CO complex by the voltage pulse (right panel). (d) Line profiles along CO-O and CO-O-CO complexes along the red and blue line in (a) and (c), respectively. Arrows indicate the positions of CO and O.
height and the size, the larger lobe represents CO and the smaller lobe represents Ot. The CO-O complex forms via thermal diffusion of CO to Ot species that is immobile in this temperature range.23 Recent studies show Ot will remain on the surface until it reacts with interstitial Ti3þ producing TiOx islands at around 400 K.12,24 Figure 2c shows an STM image of CO/O/TiO2(110) surface at higher CO coverage. New peanut-shaped features appear on 4164
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Figure 3. (a) Temperature programmed desorption of CO from clean (black) and Ot-covered (red) TiO2(110) surface (O2 exposure: 9.6 1012 molecules/cm2). The exposure of CO was the same for both experiments (9.6 1012 molecules/cm2). Temperature ramping rate was at 2 K/s. The TPD curve maximum shifting from 133 to 139 K can be clearly observed (marked by two dotted lines). (b) Annealing temperature-dependent 18Oþ ESD yield change from 18O-covered TiO2(110) surface with (red circle) and without (blue square) CO. The ESD yields were measured at 81 K after annealing to a desired temperature. Three 18Oþ ESDIAD patterns at 80, 120, and 160 K are also shown above the yield curves.
Ti rows in addition to pear-shaped features already shown in Figure 2a. We assign the peanut-shaped feature to a CO-OCO surface complex. The low Ot feature between the two CO molecules is hardly visible in the STM image. To confirm the existence of Ot species in the CO-O-CO complex, a voltage pulse from the STM tip was employed. The results are shown in the inset to Figure 2c where a negative voltage pulse (-3.5 V, 30 ms) is applied to the upper CO molecule in the left panel. The panel on the right shows that the voltage pulse displaced a CO molecule away from its original position leaving a pear-shaped CO-O feature behind. The position of both CO-O and COO-CO features on the TiO2(110) surface are found to be uncoordinated to the position of Vo sites. As the binding energy and diffusion barrier of Ot on Ti5c sites are quite large, the positions of the two types of complexes are governed by the position of the Ot species. Figure 2d shows line profiles for the two features. The CO-O feature has a shoulder along the profile at the O position. In the CO-O-CO feature, the distance from central O to each CO position is about 3 Å, which is very close to the distance between adjacent Ti5c atoms. Annealing the CO/O/ TiO2(110) surface to above 200 K leads to almost complete desorption of CO leaving an Ot species (not shown). These STM studies agree very well with recently reported STM results by Wang.13
III. TPD AND ESD STUDIES OF CO AND O INTERACTION ON TIO2(110)-(1 1) To gain further insight, we employed temperature programmed desorption (TPD) and electron stimulated desorption
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(ESD). Figure 3a shows TPD spectra of CO from clean and O/ TiO2(110) surfaces at the same CO exposures (9.6 1012 molecules/cm2). The temperature peak maximum of CO TPD from the CO/TiO2(110) surface (black) is at 133 K but the peak maximum of TPD from the CO/O/TiO2(110) surface (red) shifts up to 139 K. The observed peak temperature increase indicates that the desorption activation energy of CO increases in the presence of coadsorbed Ot species which can be clearly explained by the formation of CO-O complexes due to the attractive interaction between CO and Ot as revealed in the STM results. Slight broadening of the CO TPD peak from the CO/O/ TiO2(110) surface further supports the presence of adsorbateadsorbate interactions. In the ESD experiment, we exploit the exceptionally high ESD cross section of Ot to yield Oþ and its sensitivity to the surrounding molecular environment to monitor the interaction between the CO and Ot.14 Figure 3b shows the temperaturedependent change in the Oþ ESD yield from the O/TiO2(110) and CO/O/TiO2(110) surface. To distinguish between lattice and adsorbed O, 18O2 molecules have been used to produce 18Ot species. Thus we could selectively monitor the 18Oþ ESD signal from 18Ot via mass spectrometry, i.e., the 18Oþ ESD ions are exclusively produced from 18Ot species.14 The 18Oþ ESD signal from 18O/TiO2(110) surface does not change significantly as temperature increases except for a small increase between 140 and 180 K as shown in the Figure 3b. After CO molecules (exposure: 9.6 1012 molecules/cm2) are dosed on the 18O/ TiO2(110) surface at 81 K, the 18Oþ ESD yield starts to decrease immediately after the first annealing to 100 K. Upon annealing to 120 K, the 18Oþ ESD yield drops dramatically to about 30% of its initial value indicating that the CO-O or CO-O-CO complex formation by CO diffusion to Ot species is responsible for the ESD yield decrease. The 18Oþ ESD yield increases surpassing its original value after annealing to 140 K, reaching a maximum at 160 K indicating the desorption of CO, causing the 18Oþ yield to increase. On comparison with STM and TPD results, it is clear that the Oþ yield is strongly correlated to the existence of CO molecules around Ot species on the TiO2(110) surface. We postulate that the ESD yield decrease is due to the reneutralization of Oþ ions by the CO molecules in close proximity. Diebold and Madey reported a similar effect where charge transfer occurs from coadsorbed NH3 molecule to a desorbing Oþ ion originating from the lattice of TiO2(110), suppressing the detected Oþ ion yield.25 Three representative 18Oþ ESDIAD patterns are also shown in the Figure 3b. All three ESDIAD patterns are circular indicating that the direction of Oþ ion ejection is normal to the surface, i.e., the Ot-Ti5c bond direction is oriented at or close to the surface normal. There is slight broadening of the ESDIAD pattern upon annealing at 120 K and then cooling to 81 K, most likely due to the perturbed Oþ ion trajectories caused by the presence of CO in the vicinity.
IV. DFT STUDIES For the better understanding of the interaction between CO and Ot, DFT calculations were performed to support the experimental findings. A schematic model of the TiO2(110) surface is shown in Figure 4a where the positions of surface atoms are shown along with numbers representing the relative Ti5c positions from the Vo site. Figure 4b shows two sets of calculated adsorption energies of CO in the presence of Ot on TiO2(110) at different adsorption configurations. We fixed either CO or O at 4165
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Figure 4. (a) A top view of a schematic TiO2(110) surface adopted from the slab model used in our calculation. The numbers show the lattice distance of the position of Ti5c sites from Vo. (b) Calculated adsorption energies of CO coadsorbed with Ot at different relative positions. The adsorption energy of CO has been calculated as Eads = (ECO þ E(slabþO) - E(COþOþslab))/n, where ECO is the energy of the isolated CO, n represents the number of CO molecules in the supercell, E(slabþO) is the total energy of the slab with an adsorbed O species, and E(COþOþslab) is the total energy of the system. CO(1)-O(j) (green) represents the configuration where CO is fixed at site 1 and O positioned at sites j (j = 0, 2, 3, 4, 5). In CO(j)-O(1) (red), O is fixed at 1. Gray bar shows CO adsorption energy without Ot. Dotted line shows the adsorption energy of CO in CO-O-CO. (c) Charge density difference plot along the plane containing CO, O, and Ti5c row before and after CO adsorption on the O/TiO2(110) surface (gray, Ti5c; pink, Ot and O in CO; green, carbon). Red and blue areas denote increased and decreased electron density, respectively. The scale bar unit is in e-/Å3. The charge density difference is calculated as ΔF(r) = F(COþO)/TiO2(110)(r) - FO/TiO2(110)(r) - FCO(r), where F(COþO)/TiO2(110)(r) represents the total charge density of the coadsorbed system, FO/TiO2(110)(r) is the charge density of O adsorbed on TiO2 slab, and FCO(r) is the charge density of the isolated CO. The geometries for O, TiO2 slab, and CO are the same as those in the coadsorbed system.
position 1 and changed the adsorption position of the other adsorbate on the Ti5c row to obtain the adsorption energy of CO. The adsorption energy of CO adsorbed at position 1 on a clean TiO2(110) is indicated by a gray bar (7.6 kcal/mol), which is in good agreement with the previous experimental and theoretical values.21,26 The CO(1)-O(0) and CO(1)-O(2) configurations where CO and O are separated by one lattice spacing show the highest adsorption energies of CO (∼9.5 kcal/mol). When the distance between CO and O is larger than one lattice spacing, the adsorption energy of CO drops close to the value for clean TiO2(110). The same trend can be found for CO(j)-O(1) configurations. The adsorption energies of CO for CO(0)O(1) and CO(2)-O(1) are 8.6 and 9.2 kcal/mol, respectively. These values are slightly lower than the CO(1)-O(j) configuration but still higher than the adsorption energy of CO on clean TiO2(110). At larger distances for both configurations, the adsorption energy of CO becomes very similar to the value on clean TiO2(110), indicating that the interaction becomes negligible. The DFT calculation results strongly suggest that the increased adsorption energy is due to the attractive interaction between the CO and Ot when the two adsorbates are at the closest distance, i.e., when they form a CO-O complex.
The CO adsorption energy in the CO-O-CO complex is 9.1 kcal/mol, which is 1.5 kcal/mol higher than that found for a single CO adsorbed on clean TiO2(110). To deduce electron rearrangements upon CO adsorption on O/TiO2(110), we show a charge density difference plot along the plane containing CO, Ot, and a Ti5c site in Figure 4c. In addition to the accumulation of charge just below C atom, we note that there is a noticeable change in the region between CO and O where charge accumulation occurs around oxygen and depletion around CO. PDOS calculations (not shown) on CO and O show there is no overlap between CO and O electronic states. Thus the attractive interaction between CO and O is considered to be mainly electrostatic in nature.
V. SUMMARY OF RESULTS This work has employed both microscopic and ensembleaveraged probes of the interaction between adsorbed CO and O species on the TiO2(110)-(1 1) surface. A number of results have been obtained as listed below: 1. Adsorbed Ot species, produced dissociatively by O2 adsorption at temperatures near 200 K, are found to 4166
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2. 3. 4.
5.
attractively adsorb either one CO molecule or two CO molecules, depending upon surface conditions. STM observes the migration of CO species to Ot species at temperatures in the range 100-120 K. Annealing the mixed species layer to temperatures above 200 K removes CO completely with the CO desorption process beginning near 120 K. It is found that CO-O and CO-O-CO complexes cause the Oþ ESD yield to diminish by ∼70%, and this is postulated to be due to Oþ reneutralization through local interactions of neighbor CO molecules with the departing Oþ ions. The Oþ ions desorb normally from the surface, either from the pure Ot layer or from the CO þ Ot layer. Theoretical results show that attractive CO-O interactions occur over one to two lattice spacings separating Ot from CO. The attractive energy for various configurations of CO-O(-CO) complexes is in the range of 1.5-1.9 kcal/mol and is probably electrostatic, due to charge rearrangements in the adsorbed species.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] and
[email protected].
’ ACKNOWLEDGMENT The technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES Contract DE-FE0004000. Reference in this work to any specific commercial product is to facilitate understanding and does not necessarily imply endorsement by the US Department of Energy. The work at the University of Virginia was supported by the Department of Energy, Office of Basic Energy Sciences under DOE Contract Number DE-FG02-09ER16080. ’ REFERENCES (1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxide; Cambridge University Press: Cambridge, 1994. (2) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (3) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (4) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (5) Thompson, T. L.; Yates, J. T., Jr. Chem. Rev. 2006, 106, 4428. (6) Dohnalek, Z.; Lyubinetsky, I.; Rousseau, R. Prog. Surf. Sci. 2010, 85, 161. (7) Dohnalek, Z.; Kim, J.; Bondarchuk, O.; White, J. M.; Kay, B. D. J. Phys. Chem. B 2006, 110, 6229. (8) Green, I. X.; Yates, J. T., Jr. Phys. Chem. C 2010, 114, 11924. (9) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F. J. Phys. Chem. B 1999, 103, 5328. (10) Kimmel, G. A.; Petrik, N. G. Phys. Rev. Lett. 2008, 100, 196102. (11) Epling, W. S.; Peden, C. H. F.; Henderson, M. A.; Diebold, U. Surf. Sci. 1998, 412-413, 333. (12) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.; Hansen, J. Ø.; Matthiesen, J.; Blekinge-Rasmussen, A.; Lægsgaard, E.; Hammer, B.; Besenbacher, F. Science 2008, 320, 1755. (13) Wang, Z.; Zhao, Y.; Cui, X.; Tan, S.; Zhao, A.; Wang, B.; Yang, J.; Hou, J. G. J. Phys. Chem. C 2010, 114, 18222. (14) Lee, J.; Zhang, Z.; Yates, J. T., Jr. Phys Rev. B 2009, 79, 081408. (15) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (17) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. 4167
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