Letter pubs.acs.org/JPCL
Diffusion of CO2 on the Rutile TiO2(110) Surface Junseok Lee,*,†,‡ Dan C. Sorescu,† Xingyi Deng,†,‡ and Kenneth D. Jordan†,§ †
National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, Pennsylvania 15236, United States URS, P.O. Box 618, South Park, Pennsylvania 15129, United States § Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States ‡
S Supporting Information *
ABSTRACT: The diffusion of CO2 molecules on a reduced rutile TiO2(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO 2 molecule at an oxygen vacancy (VO) becomes increasingly streaky with increasing temperature, indicating thermally activated CO2 diffusion from the VO site. From temperaturedependent tunneling current measurements, the barrier for diffusion of CO 2 from the VO site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO2 along Ti rows of only 1.33 kcal/mol.
SECTION: Surfaces, Interfaces, Catalysis
C
hemical reactions on surfaces frequently occur at reactive sites, such as steps, kinks, and defects to which molecules diffuse and react.1−3 For this reason, understanding diffusion processes of adsorbed molecules is important in elucidating the mechanisms of catalytic reactions on surfaces. One of the most studied catalyst surfaces is rutile TiO2(110).4,5 The (110) and other surfaces of TiO2 have been the focus of intense research due to the use of TiO2 in photovoltaics as well as photocatalyst for the splitting of water and for other reactions.6−8 Recently, the photochemical reduction of CO2 on TiO2 surfaces to give useful organic products has attracted considerable attention. 9−11 There have been many studies of the chemistry of CO2 on various surfaces.12 The adsorption of CO2 on the rutile TiO2(110) surface has been studied using density functional theory (DFT) calculations13 and scanning tunneling microscopy (STM),14−16 with these studies confirming the CO2 adsorption sites proposed in previous temperature programmed desorption studies.17−19 Most significantly, CO2 is found to adsorb more strongly at the oxygen vacancy (VO) sites on TiO2(110) than on the defect-free surface.13,15 It has also been demonstrated that CO2 molecules adsorbed at VO sites can be dissociated by low-energy electrons from an STM tip.14−16 In a low temperature STM study,15 it was assumed that the diffusion of CO2 between VO sites occurs through a highly mobile state associated with the Ti row. In the present study, we determine using STM measurements the barrier for diffusion of CO2 from a VO site. Additional insight into the nature of diffusion of CO2 on the TiO2(110) surface is provided by DFT calculations. © 2011 American Chemical Society
Figure 1a shows an STM image of an area of the TiO2(110) surface at T = 130 K before dosing with CO2. Typical emptystate STM characteristics of TiO2 are observed including bright Ti rows, dark bridging oxygen (Obr) rows, and VO features on the Obr rows.20 CO2 was then introduced at a constant flux while scanning the same area. After the introduction of CO 2, a small number of sharp streaks and spikes started to appear intermittently at the VO sites. After reaching 0.5 V.E. (dosage in vacancy equivalent; see Experimental Methods in the Supporting Information), streaky features were observed at every VO site as shown in Figure 1b, in agreement with a recent STM study of Acharya and coworkers.15 This suggests that, under these experimental conditions, the CO2 molecules move between VO sites. A subsequent image of the same area (Figure 1c) at much higher dose (3.3 V.E.) shows an increased density of streaky features at VO sites. This coverage-dependent behavior at the VO sites demonstrates that the streaky features are induced by the adsorbed CO2 molecules. In previous studies carried out at T = 55 K and low coverage,13,14 CO2 molecules were found to adsorb at VO sites without showing streaky features. Thus the streaky features observed in Figure 1 are the result of thermally activated diffusion of CO2 molecules. Vibrational excitation or hindered translation of CO2 molecules at VO sites are excluded because clear VO STM features were observed after the CO2 molecules diffuse away from the VO sites.14 Note also that, even in Figure 1c, no CO2 features are observed in the areas other Received: October 3, 2011 Accepted: November 29, 2011 Published: November 29, 2011 3114
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Figure 2. (a) STM image (1.5 V, 5 pA, 10 × 10 nm2) of the TiO2(110) surface at T = 130 K and a CO2 dose of 0.85 V.E. Streaky features are visible at the VO sites. (b) STM image (1.5 V, 5 pA, 10 × 10 nm2) of the TiO2(110) surface at T = 130 K after dosing with O2. VO sites without streaky features are enclosed in squares. Retained streaky features are indicated by circles. Features due to oxygen adatoms (Oad) are also visible.
Figure 1. STM images (a−c, 1.5 V, 5 pA, 9 × 9 nm2) of the same area on the TiO2(110) surface at T = 130 K. (a) Before dosing with CO2. Three bright features on the right side of the image serve as reference objects. (b) At 0.5 V.E. of CO2, and (c) at 3.3 V.E. of CO2. V.E. represents the relative amount of CO2 with respect to the number of VO sites. (d) An STM image (1.5 V, 5 pA, 10 × 10 nm2) at 0.5 V.E and T = 70 K. Squares and circles represent, respectively, departing and arriving CO2 molecules at a VO site.
O2 at the same dose as that of the original CO2 exposure, the number of streaky features at VO sites is significantly reduced, and oxygen adatoms (Oad) are observed on the Ti rows (Figure 2b).21,22 On the oxygen treated surface, many of the VO sites (indicated in squares in Figure 2b) give STM images characteristic of empty VO sites on a clean TiO2(110) surface as shown in Figure 1a. At some of the VO sites (circles), there are streaky features, but with significantly less density compared to those in Figure 2a. It is clear from these measurements that far fewer VO sites are accessible to CO2 molecules after exposure of the surface to O2. This is due to the “blockage” of diffusion of CO2 along Ti rows by Oad and, possibly also, intact molecular O2 species, which would be invisible to STM.23 To scrutinize the dynamics of thermally activated diffusion of CO2 on TiO2(110), we measured the distribution of residence times of CO2 molecules at VO sites. Specifically, the STM tip was positioned at the center of a streaky feature at a V O site for a given time, and fluctuations in the tunneling current were measured as a function of time. A typical tunneling current trace at T = 99.5 K is shown in Figure 3a from which it is seen that the current is bistable, oscillating between values near 1 pA and near 5 pA. When inward diffusion of a CO2 molecule (to a VO site) occurs, the tunneling current is in the “high” state (∼5 pA), whereas it changes to the “low” state (∼1 pA) when there is a diffusion of a CO2 molecule out of a VO site. Statistical analysis of the tunneling current traces for many VO sites provides the mean lifetime of CO2 molecules at VO sites. When the temperature is increased to 104.7 K, the residence time in the “high” current state becomes shorter (Figure 3b). The distribution of the residence times for T = 104.7 K is reported in Figure 3c. We followed a method used in previous works to extract the residence time (see Supporting Information). 14,24 A fit to a single exponential decay yields a mean residence time (τ) of 15.1 ms for CO2 at the VO site. Thus the rate of outward diffusion (1/τ) of CO2 from the VO site at T = 104.7 K is 66.2 s−1. Possible tip effects are minimized by selectively analyzing the current traces from the less-perturbing situation (see Figure S2). The apparent barrier for outward diffusion can be deduced from the Arrhenius plot using the rates of diffusion obtained at different temperatures as shown in Figure 3c. At 0.5 and 1.5 V.E. dosage, the apparent barriers for outward diffusion are determined to be 3.31 ± 0.23 kcal/mol and 4.18 ± 0.32 kcal/ mol, respectively. It is likely that at a higher coverage, CO 2 molecules bound to the Ti rows in the vicinity of VO sites are
than the VO sites, despite the fact that the number of CO 2 molecules adsorbed on the surface is about 3 times higher than the number of VO sites (3.3 V.E.). There are two possible interpretations of this observation: (1) At higher coverage, more than one CO2 molecule could adsorb at a VO site and (2) CO2 molecules on the Ti rows are not observed in the STM images because of their fast diffusion along the Ti rows. The first possibility is ruled out on the basis of our DFT calculations that show that it is highly unfavorable to adsorb two CO2 molecules at a VO site. To support the second possibility, two pathways for diffusion of CO2 along the Ti rows were investigated computationally (see Figure S1, Supporting Information). One involves simple molecular translation, and the other involves a cartwheel rotation. The latter is predicted to have the lower barrier (1.33 kcal/mol). This low barrier is consistent with the second possibility mentioned above. When a diffusing CO2 molecule encounters a VO site, it localizes there, making observation via STM possible, as shown in Figure 1b,c. However, at T = 130 K, the residence time of CO2 at the VO site is relatively short, as manifested by the streaky or spiky features in the STM images. At low temperatures (e.g., T = 70 K), CO2 diffusion is considerably slowed down as shown in Figure 1d where partial CO2 features (rather than streaky features) are observed at the V O sites. Because the scanning direction was upward ([001]), the features that have only the upper portion of a full CO2 feature (circles) represent CO2 molecules newly arrived at VO sites during the scanning (inward diffusion). When only the lower portion of a full CO2 feature is visible (square), the adsorbed CO2 molecule is considered to have moved away from the V O site (outward diffusion). At temperatures below 50 K, the partial features are rarely observed, indicating that the diffusion of CO2 is a thermally activated process. This behavior is reversible over the entire temperature range examined in our experiments up to T = 130 K. Much more frequent diffusion of CO2 in/out of VO sites at T = 130 K makes more streaky features visible as shown in Figure 1b,c. Adsorption of another adsorbate can have significant impact on the diffusion of CO2 on TiO2(110). Figure 2a shows an STM image of TiO2(110) showing streaky CO2 features (0.85 V.E.) at the VO sites at T = 130 K. After exposing the surface to 3115
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Figure 4. (a) Minimum energy pathway for diffusion of a CO2 molecule from a Ti row to a VO site (images #0−10) followed by the precession of the molecule around a vertical axis leading to a change in the tilt angle (images #10−20) and diffusion out of the VO site to the Ti row (images #20−30). The barrier for outward diffusion corresponds to the difference in energy between the structures depicted in images #4 and #10 (or equivalently between #26 and #20). Likewise, the inward diffusion barrier corresponds to the difference in energy between images #0 and #4 (or #30 and #26). (b) Four representative configurations (top and side views) along the energy profile are shown. Only the surface layer of the TiO 2 slab is shown for clarity.
Figure 3. (a) and (b) Tunneling current traces at T = 99.5 and 104.7 K, respectively. These were obtained by placing the STM tip over a streaky feature while measuring the tunneling current change at a voltage of 1.5 V. Initial set current was 5 pA. The duration of the “high” current state represents the residence time of CO2 at the VO site. (c) Binned distribution of residence times of CO2 molecules in the high current state (sampling bin width = 5 ms) at T = 104.7 K. A fit to a single exponential decay function, A exp(−τ/t), yields a decay constant τ = 15.1 ms. (d) Arrhenius plot of the outward diffusion rate (1/τ) versus inverse temperature at 0.5 V.E. and 1.5 V.E. Linear fits to the data are indicated by red solid lines, giving apparent diffusion barriers of 3.31 ± 0.23 and 4.18 ± 0.32 kcal/mol at 0.5 and 1.5 V.E., respectively.
rapid switching between the two orientations. The barrier for outward diffusion of CO2 from the VO site (from #10 to #4) is calculated to be 3.80 kcal/mol, and that for the inward diffusion of CO2 to the VO site (from #0 to #4) is calculated to be 2.06 kcal/mol. The calculated barrier for diffusion of CO2 from a VO site is intermediate between the experimental values of 3.31 and 4.18 kcal/mol obtained at 0.5 and 1.5 V.E, respectively. DFT calculations (see Figure S3) show that adsorption of a CO 2 molecule on a Ti atom adjacent to a VO site increases the barrier for diffusion of a CO2 molecule from the VO site. In summary, we have examined using STM and DFT calculations diffusion of CO 2 molecules on the rutile TiO2(110)-(1×1) surface. The STM images reveal temperature-dependent streaky features associated with the diffusion of CO2 molecules at VO sites. The density of streaky features at VO sites increases as a function of CO2 dose. From the experimental measurements, the barriers for diffusion at low (0.5 V.E.) and high (1.5 V.E.) CO2 doses were deduced to be 3.31 and 4.18 kcal/mol, respectively. Repulsive interactions are considered to be responsible for the higher diffusion barrier at the higher coverage. Subsequent exposure of the surface to O 2 causes a reduction in the number of streaky features at V O sites due to blocking of CO2 diffusion by oxygen adatoms on Ti rows. The DFT calculations show that the outward diffusion of CO2 from a VO site takes place via “cartwheel”-like motion with a barrier of 3.80 kcal/mol. The calculated barrier for diffusion of a CO2 molecule on a Ti row is much smaller (1.33 kcal/
responsible for the increase in the height of the barrier for escape of CO2 from the VO site. The minimum energy pathway for diffusion of a CO 2 molecule between a VO site and an adjacent Ti row shown in Figure 4a was determined using the DFT calculations and the CI-NEB method.25,26 This pathway involves a cartwheel-like motion of CO2. Images #10 or #20 refer to minima at the VO site13 and images #0 and #30 place the CO2 molecule on the Ti row on either side of the VO site. Key configurations (images #0, #4, #10, and #15) along the diffusion pathway are shown in Figure 4b. Image #4 corresponds to the transition state for diffusion between a Ti row (image #0) and a VO site (image #10). In this structure, the molecular axis of the CO2 molecule is nearly horizontal to the surface. The oxygen atoms pointing toward the surface in images #0 and #10 belong to opposite ends of the molecule. Image #15 in Figure 4b depicts the transition state for flipping between the two symmetric tilted configurations at a VO site. The barrier for this process is calculated to be only 0.58 kcal/mol. This barrier is so small that at the temperatures of the STM measurements there should be 3116
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mol), consistent with rapid motion of CO2 molecules between VO sites except at very low temperatures.
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ASSOCIATED CONTENT S Supporting Information * Experimental methods, theoretical methods, and supporting figures are included. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected].
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ACKNOWLEDGMENTS The research was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES contract DE-FE0004000. We acknowledge a grant of computer time at Pittsburgh Supercomputer Center.
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