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CO2 Adsorption, Diffusion, and Electron-Induced Chemistry on Rutile TiO2(110): A Low-Temperature Scanning Tunneling Microscopy Study D. P. Acharya,†,§ N. Camillone, III,‡ and P. Sutter*,† † ‡
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States ABSTRACT: Low-temperature scanning tunneling microscopy (STM) has been used to study the adsorption of CO2 on rutile TiO2(110) from 80 to 180 K. For low CO2 doses, two molecular adsorption sites with different binding energies are identified, which are effectively isolated from one another by an apparent activation barrier to their interconversion. We identify the less tightly bound adsorption site as CO2 adsorbed atop 5-fold coordinated titanium surface atoms (Ti5f), without binding preferentially near oxygen vacancies. CO2 desorption from Ti5f occurs at ∼140 K. The more strongly bound site involves molecular CO2 binding at bridging oxygen vacancies (VO,br). We observe two distinct configurations of VO,br bound CO2 molecules. Despite its being bound to the vacancy, CO2 does not dissociate thermally but remains intact up to the desorption temperature of ∼175 K. At an elevated tunneling bias, the STM tip can selectively dissociate these CO2 molecules and thus trigger the healing of individual VO,br. At higher coverage, CO2 adsorption occurs predominantly at the more abundant Ti5f sites, with the distribution of CO2 molecules being determined by interactions both along the [001] and [110] directions.
1. INTRODUCTION The surface properties of TiO2 have long been a topic of active research due to their importance in a variety of processes, such as heterogeneous catalysis,1 photodecomposition of organic pollutants,2,3 surface self-cleaning,4 water splitting,5 and dye-sensitized solar energy conversion.6 Among the potential renewable energy conversion processes involving oxides such as TiO2, the photocatalytic reduction of CO2 is particularly interesting.7 Combined CO2 capture from stationary sources and renewable (solar) energy driven photocatalytic conversion of CO2 to fuels could form the basis for hydrocarbon-based fuel cycles with zero carbon emissions. However, such scenarios can only become viable at large scales if the yields and selectivities of photocatalytic reforming reactions are optimized and photocatalysts are identified that efficiently harness the solar spectrum. Ultraviolet (UV) photocatalytic CO2 reduction was first demonstrated by Somorjai on SrTiO3 exposed to gas phase (CO2, H2O)8 and by Honda on a range of different catalysts (TiO2, ZnO, SiC, CdS, etc.) in aqueous suspension.9 Products include methane (CH4) as well as formic acid (HCOOH), formaldehyde (CH2O), and methanol (CH3OH), suggesting different possible reaction pathways under UV exposure. Solutionphase experiments showed that the reaction yields could be increased by depositing transition metal catalyst particles on TiO2.1012 Work on gas-phase photocatalysis on TiO2 single crystals and nanoparticles confirmed the formation of the same hydrocarbon, oxygenate, and carboxylic acid product species during gas phase (CO2, H2O) exposure and UV irradiation.13,14 r 2011 American Chemical Society
The adsorption of CO2 is a key step in photocatalytic conversion reactions. A molecular-level understanding of the role of different sites and of coadsorbates, such as H2O, on both stoichiometric and defective TiO2 surfaces is essential for devising strategies for enhancing the reaction rates in large-scale processes and for optimizing the selectivity toward desired products. The adsorption of CO2 on rutile TiO2(110) has been studied using different experimental techniques, such as temperature programmed desorption (TPD),1518 Fouriertransform infrared spectroscopy (FTIR),11,19,20 high-resolution electron energy-loss spectroscopy (HREELS),16 surface conductivity and work function change,21 and X-ray photoelectron spectroscopy (XPS).22 The different spectroscopic methods consistently show that CO2 adsorbs molecularly. Using TPD, Henderson has concluded that CO2 molecules bind at lowcoordinated metal (Ti5f) and bridging oxygen vacancy (VO,br) sites, from which they desorb at 137 and 166 K, respectively.16 Recently, Thompson et al. have reported that CO2 desorption depends on the VO,br concentration,18 with the desorption temperature of VO,br bound CO2 molecules increasing with vacancy concentration. On the basis of FTIR, it has been proposed that adsorption involves a charge transfer from Ti3þ to CO2, leading to the formation of a negatively charged CO2 or CO3 species.11,19,20 Theoretical studies23,24 have identified the 1M-monodentate configuration (i.e., involving one bond to a Received: March 16, 2011 Revised: May 2, 2011 Published: May 31, 2011 12095
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The Journal of Physical Chemistry C single metal center in the surface) as the most stable configuration, albeit with significantly differing heats of adsorption (0.15 and 0.68 eV, respectively). In this configuration, the CO2 molecule is oriented perpendicular to the surface with one of its oxygen atoms directed toward the Ti (i.e., TiOC-O). The lack of an adsorption state at VO,br sites has been suggested both theoretically23 and experimentally.21 However, the weight of experimental evidence16,18,25 argues that the VO,br site is indeed the most stable adsorption site. In addition, recent B3LYP cluster calculations addressing molecular CO2 adsorption on bridging oxygen vacancies have identified bent geometries of the molecule with OCO bond angles of ∼130140° as the energetically favorable molecular adsorption states.26 Here, we report a molecular-scale imaging study of CO2 adsorption on rutile TiO2(110) using low-temperature scanning tunneling microscopy (STM). In agreement with previous conclusions from spectroscopy, we invariably observe molecular adsorption of CO2 at low temperatures in two different sites: low-coordinated Ti5f and VO,br. Our variable temperature STM imaging shows two clear desorption thresholds. CO2 desorbs from Ti5f sites at ∼140 K, whereas desorption from VO,br occurs at ∼175 K, in agreement with earlier TPD results.16,17 At 80 K, the lowest temperature used in our experiments, the CO2 molecules diffuse slowly along the Ti5f rows via single nearestneighbor Ti5f hops. This mechanism strongly contrasts with diffusion among VO,br sites, which involves an apparent longrange, rapid transfer from one VO,br to another VO,br several lattice constants distant from the first. The distinct diffusion behavior of CO2 for the two binding sites, taken together with the absence of evidence for transfer between the two sites, leads us to conclude that the two populations are essentially separate and that a substantial activation barrier to interconversion exists. Higher coverages of CO2 are primarily accommodated by adsorption at Ti5f sites, which are typically much more abundant than oxygen vacancy sites. STM images show configurations indicative of repulsive interactions between closely spaced CO2 molecules, both along the same Ti row and between neighboring rows. The CO2 hopping rate between Ti5f sites accelerates significantly as the coverage increases, consistent with a repulsive interaction between the molecules.18,24 Under noninvasive imaging conditions (V ∼1.0 V, I ∼0.1 nA), CO2 molecules adsorbed on both Ti5f and VO,br sites are stable and do not dissociate. In particular, we do not observe the thermal dissociation of CO2 and reaction with VO,br. However, at higher sample bias (V > 2.0 V) and tunneling currents, the nonthermal reaction of CO2 with oxygen vacancies can be activated by electrons tunneling from the STM tip. While this process heals the oxygen vacancy, the remaining CO is not observed due to its rapid diffusion or desorption from the reaction site. The identification of the STM contrast of individual CO2 molecules at distinct surface sites on TiO2(110) and their electron-mediated chemistry sets the stage for detailed studies of CO2/H2O coadsorption and of the reaction mechanisms underlying photocatalytic CO2 reduction at the molecular scale.
2. EXPERIMENTAL METHODS Our experiments were carried out in a commercial cryogenic STM (Createc) in the temperature range from 80 to 180 K. The STM was surrounded by a pair of concentric radiation shields cooled by liquid nitrogen, and the pressure inside the STM chamber was consistently below 1011 Torr. The base pressure
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Figure 1. STM of CO2 molecules adsorbed on TiO2(110). Emptystates STM image of CO2 on TiO2(110). Rectangles mark VO,br and OHbr. Dashed circles and ellipses: CO2 molecules on Ti5f and VO,br, respectively. Imaging conditions: V = 1.2 V, I = 0.21 nA. T = 80 K.
in the sample preparation chamber was 1010 Torr. Singlecrystal rutile TiO2(110) samples were cleaned by several cycles of Arþ sputtering (1 keV) and UHV annealing (900 K). STM imaging on the reduced samples showed typical VO,br concentrations of ∼8% of the available bridging oxygen sites.18,27 Electrochemically etched tungsten tips, cleaned by electron bombardment inside the UHV chamber, were used for STM imaging. CO2 (research purity, 99.998%) was directly dosed inside the cryogenic STM through a small dosing hole in the radiation shields, allowing us to image the same position on the surface before and after CO2 adsorption from the gas phase. All STM images were recorded in constant-current mode at positive sample bias (empty-state imaging).
3. RESULTS AND DISCUSSION A. Identification of Binding Sites and Geometries. To determine the appearance and adsorption sites of individual CO2 molecules on TiO2(110), we dosed small amounts (∼0.1 monolayers, ML, where 1 ML denotes 1 CO2 molecule per TiO2 surface unit mesh) of CO2 onto the freshly prepared surface at T = 80 K. The initial sticking coefficient of CO2 on TiO2(110) is ∼0.56,17 thus achieving a coverage of ∼0.1 ML entails exposure to ∼109 Torr of CO2 for ∼1 min. Figure 1 shows a typical STM image of the surface following CO2 exposure. Over most of the surface, the image reflects the usual appearance of clean TiO2(110)-1 1 in STM, i.e., alternating dark and bright rows along the [001] in-plane axis coinciding with rows of 2-fold-coordinated bridging oxygen atoms (Obr) and 5-fold-coordinated Ti surface atoms (Ti5f), respectively.28,29 Individual bridging oxygen vacancies (VO,br) are imaged as faint protrusions elongated along the [110] direction with an apparent height of ∼0.4 Å30,31 above the Obr atoms, whereas single bridging hydroxyls (OHbr) are imaged as somewhat higher protrusions (∼0.5 Å) and are easily identified via their particular desorption behavior by STM voltage pulses.32 Exposure to CO2 results in the appearance of new bright features across the surface, as seen in Figure 1. These features 12096
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Figure 2. Adsorption of CO2 molecules on TiO2(110): (a) empty-states STM image with overlaid height contours (contour spacing, 0.06 Å) of CO2 on Ti5f site. (b, c) Empty-states STM images with overlaid height contours (contour spacing 0.06 Å) of CO2 on VO,br sites in two different adsorption geometries: symmetric (b) and asymmetric (c). Imaging conditions: V = 1.2 V, I = 0.21 nA, T = 80 K.
occur in two distinct locations: (i) centered on the Ti5f rows and (ii) centered on the Obr rows. The features centered on the Ti5f rows appear as intense lobes (some of which are marked by circles in Figure 1) with circular projections. The features centered on the Obr rows are somewhat less intense, elliptically shaped protrusions (some of which are marked by ellipses in Figure 1) that are notably elongated along the [110] direction and are typically accompanied by a characteristic constriction of the bright contrast of the adjacent Ti5f rows. No new bright features atop Obr atoms were observed, and within our limited statistics no preference for adsorption on Ti5f near special sites (e.g., near VO,br) could be detected. Analysis of several images such as that shown in Figure 1 indicates that at low CO2 coverage and for the particular VO,br density considered here, the number of bright features at the two types of location differs significantly, with a much larger number occurring centered on the Obr rows than on the Ti5f rows. The positions of each of these features along the respective rows are in registry (as determined from the positions of the bridging oxygen vacancies) with the atomic positions of the Obr and Ti5f sites, indicating adsorption in atop sites. We find that the surface density of the bright features centered on the Obr rows closely follows the density of oxygen vacancies. Indeed, control experiments, discussed in greater detail below, show that each of these features is a CO2 molecule bound atop an oxygen vacancy defect; at sufficiently high exposures, each VO,br becomes decorated by an adsorbed CO2 molecule. These experiments also lead us to conclude that all of the new bright features observed following CO2 exposure represent the nondissociative, molecular adsorption of CO2 and, importantly, that CO2 molecules are bound at VO,br sites but do not spontaneously react with them. A more detailed analysis of our images reveals clear differences in STM contrast among the adsorbed CO2 molecules. From several scans recorded under identical imaging conditions as those of Figure 1, we have been able to identify three distinct motifs represented by the shaded contour plots of Figure 2. CO2 adsorbed on Ti5f invariably gives rise to the same STM contrast motif: a single protrusion centered atop a Ti5f site with an apparent height of ∼0.9 Å relative to that of the Ti5f rows (Figure 2a). For adsorption on VO,br, on the other hand, two distinct motifs are found. First, we observe a symmetric configuration (Figure 2b), wherein the CO2 molecule is imaged as a uniform elliptical lobe, elongated along the [110] direction and with an apparent height of ∼0.75 Å relative to the Ti5f height. Second, we find an asymmetric configuration, wherein the
molecule again appears elongated along [110] but exhibits a narrower apparent-height maximum that is offset toward one of the adjacent Ti5f rows (Figure 2c). At 80 K, the asymmetry is static: repeated images show no evidence of either change of the position of the narrow maximum nor of conversion of the asymmetric features to symmetric features (or vice versa). Within our limited statistics, we find a majority (∼65%) of the VO,br bound CO2 molecules in the symmetric configuration, with the remainder appearing asymmetric. Comparison of our findings with theoretical studies allows tentative assignments for the three observed binding configurations. First, density functional theory cluster calculations suggest an assignment of the two VO,br bound CO2 geometries. Computational work on small clusters designed to represent a VO,br site has shown that CO2 adsorbs in these sites with a bent molecular axis clearly preferred in comparison to linear adsorption geometries.26 In one of the bent geometries (with μ2η4 bonding, where the notation indicates that two Ti atoms are involved and there are four bonds between them and the CO2), the molecule is centered with the C atom above the middle of the vacancy and the O atoms extending symmetrically along the Obr row; both the C atom and the O atoms bond to the Ti atoms associated with the VO,br. Only one calculated structure is asymmetric with respect to mirror reflection across the Obr row: a carbonate-like species that involves the C atom bonding to an in-plane O atom adjacent to the VO,br site. The image contrast we observed is qualitatively consistent with these two bonding geometries. Thus we tentatively attribute the symmetric CO2/VO,br(I) feature (Figure 2b) to molecules bound in the μ2η4 geometry and the asymmetric CO2/VO,br(II) feature (Figure 2c) to a carbonate-like species.26 Regarding the features on the Ti5f rows, periodic HartreeFock calculations24 suggest that the bright features we observe are linear CO2 molecules bound with their axes perpendicular to the surface atop Ti5f sites through an oxygen lone pair, i.e., the 1M-monodentate configuration predicted by the calculations to be the preferred binding configuration for the defect-free (110) surface. More comprehensive computational studies on larger clusters or extended surfaces, in conjunction with DFT-based STM image simulations, are needed to make a conclusive assignment of the bonding geometries. To summarize, following CO2 adsorption on TiO2(110) at 80 K, STM images provide direct evidence for stable adsorption states at two sites: the Ti5f site and the VO,br site. On the basis of available theoretical work, the CO2 maintains a linear structure 12097
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when adsorbed at the Ti5f site, whereas adsorption at VO,br sites is expected to involve significant distortion from linearity. In addition, our images are consistent with the existence of two distinct adsorption states at VO,br sites, one of which (the asymmetric structure) may involve the formation of a CO3like species. B. Implications for the Adsorption Dynamics. The relative population of the Ti5f and the VO,br adsorption sites following exposure at 80 K argues strongly for the role of a mobile precursor state in the adsorption dynamics. The preference for adsorption at the VO,br minority sites at low coverage (∼0.1 ML) seen in Figure 1 is dramatic: we observe ∼80% of the CO2 to be adsorbed on VO,br despite the fact that they comprise only ∼6% of the available surface sites. We can begin to understand the implications of this observation for the adsorption dynamics by predicting the site occupation statistics in the absence of a mobile precursor state, i.e., assuming that each molecule either sticks at the site where it lands or scatters off the surface. In such a “hit-and-stick” scenario, we can write a simplified expression for the initial sticking coefficient: S0 ¼ SV PV þ STi PTi
ð1Þ
where PV (PTi) is the probability that an incident CO2 molecule lands on a VO,br (Ti5f) site, and SV (STi) denotes the initial sticking coefficient for adsorption at the VO,br (Ti5f) site. The ratio of the number of molecules adsorbed at VO,br sites to those adsorbed at Ti5f sites (in the zero coverage limit) is R ¼
SV PV STi PTi
ð2Þ
Solving eqs 1 and 2 for the two unknown quantities, SV and STi, gives Sv ¼
S0 R PV ðR þ 1Þ
ð3Þ
STi ¼
S0 PTi ðR þ 1Þ
ð4Þ
Given that S0 = 0.56 (as measured by Funk and Burghaus17), R = 4.5, given a VO,br density of 0.06 ML (as estimated from measurements such as those in Figure 1) and assuming that each VO,br site effectively screens adsorption at the two nearest Ti5f sites (PV = 0.12, PTi = 0.88), eq 3 gives the unphysical value of 3.8 for SV; i.e., even if the sticking coefficient at VO,br sites were unity, a simple hit-and-stick mechanism cannot explain the observed adsorption statistics given the measured sticking coefficient.17 Thus, our observations strongly suggest the existence of a precursor state. At least two models for the adsorption dynamics involving this precursor state are possible. First, we consider the possibility that each VO,br site effectively screens adsorption at the six nearest Ti5f sites. In this case, PV = 0.36, PTi = 0.64, and eqs 3 and 4 predict that SV = 1.3 and STi = 0.16. Given the somewhat limited statistics available from our images and the experimental uncertainty in the measurement of S0, we cannot reject this model out of hand, despite the fact that the value for SV exceeds unity. Instead, we can conclude that it is possible, within the experimental uncertainty, that there exists a precursor state associated with the VO,br sites and extending over ∼6 or more surface unit meshes that serve as a “gateway” to adsorption at the VO,br sites with a sticking coefficient near unity, while sticking at Ti5f sites outside the influence of the VO,br sites is ∼10 times less
probable. However, the decrease in sticking coefficient with increasing VO,br density observed by Funk and Burghaus17 argues strongly against such a model. Therefore, we propose that the strong preferential adsorption at VO,br sites is due to the existence of a mobile precursor state associated with regular lattice sites on the TiO2(110) surface, i.e, the molecules do not stick where they land but rather spend time in a relatively weakly bound state in which they rapidly diffuse across the surface prior to finally being trapped in one of the observed adsorption geometries on either VO,br or Ti5f. This mechanism is consistent with the Kisliuk precursor model33 and the observed coverage dependence of the sticking coefficient reported by Funk and Burghaus17 and Henderson.16 In contrast to the conclusion drawn by Funk and Burghaus that an extrinsic precursor state is operative, however, our direct imaging of the adsorption site distribution points to the importance of an intrinsic precursor state. Because Funk and Burghaus have shown that the Kisliuk parameter, K,33 is much greater than unity, our results suggest that the rate of adsorption is much larger than the rate of desorption out of the precursor state. Furthermore, the calculations of Markovits et al.24 provide grounds for speculation regarding the nature of this intrinsic precursor state. Specifically, they identify a 2M-bidentate structure (where the CO2 maintains its linear structure and lies parallel to a Ti row with each O atom bound to a Ti5f). Significantly, our time-lapse imaging of the diffusion at 80 K (discussed below) provides experimental evidence for a highly mobile bound-state associated with the Ti5f sites. C. Site-Dependent Diffusion and Desorption. To address the occupancy of the different CO2 adsorption sites, Ti5f and VO,br, we have imaged the adsorbates: (i) during annealing and thermal desorption of the CO2 molecules; (ii) after high temperature dosing; (iii) after cooling the annealed adsorbate populations; and (iv) in two distinct coverage regimes. As we will demonstrate, taken together our observations from these different types of experiments indicate that the CO2 adsorbs and desorbs molecularly intact; that thermal activation is effective at driving local rearrangement of the bonding geometry of VO,br bound CO2; and that despite the fact that thermal activation can efficiently drive hopping among Ti5f sites and VO,br sites separately, there is an apparent activation energy barrier to interconversion of VO,br and Ti5f bound CO2, thus exchange of CO2 between the two types of sites occurs rarely, if at all at low temperature. In the annealing and desorption experiments, an amount of CO2 somewhat more than sufficient to saturate all VO,br (although not all VO,br were populated because some CO2 initially adsorbed at Ti5f sites, see Figure 3a) was adsorbed on freshly prepared TiO2(110) at T = 80 K. The surface was then imaged by STM to characterize the CO2 population and diffusion behavior. The temperature was subsequently raised stepwise in ∼15 K increments to 180 K and the surface reimaged after stabilization at each step. Figures 3 and 4 show STM images representative of four characteristic temperature regimes: ∼80, ∼140, ∼150, and ∼175 K. In the following, we address each regime in turn, focusing in particular on the diffusion and desorption behavior. Figure 3, a time-lapse series of STM images, shows the starting point at 80 K where CO2 molecules are adsorbed both on the brighter Ti5f rows and at VO,br defect sites, as discussed in detail above with reference to a single image (Figure 1). The time-lapse measurements clearly show that the adsorbate distribution is not 12098
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Figure 3. Diffusion of CO2 molecules on TiO2(110). (ad) Time-lapse series of consecutive STM images, illustrating the different diffusion characteristics of CO2 molecules adsorbed on Ti5f and VO,br, marked by circles and ellipses, respectively. Rectangles denote empty VO,br sites without adsorbed CO2. Imaging conditions: V = 1.2 V, I = 0.2 nA, T = 80 K. (eh) Images cropped from a different area of the same time-lapse series to highlight the sudden appearance (indicated by an arrow) and disappearance of a mobile species on the Ti5f row.
Figure 4. STM of the evolution of CO2 coverage on TiO2(110) with increasing temperature, following adsorption at 80 K. (a) Partial coverage (∼0.1 ML) of CO2 on reduced TiO2(110) at 80 K. (bd) The same surface imaged at 140, 150, and 175 K. Imaging conditions: (ac) V = 1.2 V, I = 0.24 nA; (d) V = 1.27 V, I = 0.6 nA.
entirely static. At low temperature and low coverage, we observe diffusion of the CO2 molecules, albeit at a low frequency. We consistently observe four characteristics of the diffusion: (i) Ti5f bound CO2 diffuses along the Ti row by single lattice constant hops; (ii) VO,br bound CO2 apparently diffuses by long-range hops from one VO,br to another VO,br many lattice constants away without being detected at intermediate positions along the apparent diffusion path; (iii) transfer of a CO2 from a Ti5f site to a VO,br site (or vice versa) occurs so rarely, if at all, that we have
yet to observe a single such event; and (iv) there is clear evidence for highly mobile species that transiently occupy Ti5f sites. The images in Figure 3 were recorded sequentially, without a significant pause between frames, at a rate of 1 frame per ∼3 min. The pair of images in frames a and e in Figure 3 (and, similarly, frames b and f, c and g, and d and h) were cropped from a single larger image to highlight the observed diffusion events. In almost every pair of images in the sequence it is possible to identify pairs of sites consisting of one occupied and one unoccupied VO,br site that appear to exchange a CO2 molecule during the time required to acquire an image. Instances of such events are explicitly marked in Figure 3 by dashed circles and rectangles, indicating pairs of occupied and unoccupied VO,br sites, respectively, between which a CO2 molecule appears to have been exchanged. The low frequency of these events, in concert with observations, discussed below, which show higher mobility at higher temperature, indicates that the diffusion is an activated process. A most intriguing feature of these transport events is the long distance separating the involved sites and the lack of detected intermediate stopping points. These observations indicate that the molecule is transferred into a highly mobile state when hopping out of a VO,br site. The adsorption energy in this state is sufficient to keep the molecule from desorbing; however, the barrier to hopping between adjacent sites in this state must be substantially lower than the barrier to hopping out of a VO,br site. Furthermore, the barrier to hopping into a VO,br site must be similarly low.34 Comparison of frames c and d in Figure 3 reveals a second type of motion: single lattice constant hops (marked by dashed circles) by CO2 bound (as imaged in Figure 2a) at Ti5f sites. This hopping is rare at 80 K, and the diffusion of molecules bound in this state is correspondingly very slow. Thus, the hops appear to be activated, with a barrier similar to or greater than that for hopping out of VO,br sites. In this context it is also important to note that we do not observe the sudden appearance of Ti5f bound CO2, indicating that the CO2 does not hop out of VO,br sites into the Ti5f bound state. This fact, together with the slow diffusion behavior strongly suggests that the Ti5f bound 12099
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The Journal of Physical Chemistry C adsorption state, as imaged in Figure 2a and tentatively assigned as the 1M-monodentate configuration, cannot serve as the intermediate for transport between VO,br sites. Finally, as highlighted in frames f, g, and h in Figure 3, we detect a highly mobile species that briefly pauses over sites on the Ti5f rows but is much less frequently observed and much lower in apparent height than the localized Ti5f bound CO2 (highlighted in Figure 2a). The arrow in frame g of Figure 3 indicates one of these species. They are imaged as relatively dim protrusions with distinctive streaks, which appear at previously unoccupied positions (see frame f in Figure 3) and disappear by the next frame (i.e., within ∼3 min). The streaks indicate motion of the species under the tip on the time scale over which the species is imaged. At 80 K we detect these species with a frequency similar to that for exchange between VO,br sites and Ti5f site hopping. We have not yet acquired sufficient images to perform a valid statistical analysis of the various hopping rates at low temperature, and we do not have evidence directly linking this transient species to the other observed adsorbate states. However, the transient nature of this state is consistent with the expected characteristics of an intermediate involved in CO2 exchange between VO,br sites, and such a state may also be the precursor to adsorption from the gas phase. Because we do not observe the sudden appearance of the localized Ti5f bound CO2, it is apparent that conversion from the transient state to the localized state on the Ti5f rows is infrequent or absent at low temperature, suggesting an activation barrier between the mobile state and the localized state. Because we do observe the sudden appearance of VO,br bound CO2, we hypothesize that there is no such barrier for adsorption into VO,br sites from the mobile state. Thus the CO2 appears to be transported freely along the Ti5f rows in a mobile state from which it is much more easily trapped at VO,br sites than at Ti5f sites. Future work at lower temperatures should elucidate the nature of this mobile species and the mechanisms for adsorption and diffusion. The development with annealing from the adsorption temperature (80 K) to higher temperatures is shown in Figure 4. With increasing temperature, the CO2 molecules adsorbed on VO,br undergo a subtle change. Starting at 100 K, the VO,br bound molecules are no longer imaged as well-defined lobes with an elliptical footprint, but their appearance becomes streaky, indicating an increased motion of the molecule at the binding site. In addition, the streaking is reversible, between 80 and ∼150 K the degree of streaking increases with increasing temperature, but it completely disappears upon cooling to 80 K. Hence, the molecular motion of CO2 at the VO,br site, seen in STM as streaks, is likely thermally activated and not induced by the STM tip. As the sample temperature reaches ∼140 K, a major change is detected. STM images such as those in Figure 4b show CO2 molecules now only adsorbed on VO,br sites. Multiple images recorded from large sample areas show no evidence of the localized Ti5f bound CO2. STM also shows no indication of rapidly diffusing Ti5f bound CO2 molecules, which, if present, would be expected to manifest themselves as streaks along the Ti5f rows. Hence, we conclude that the CO2 removed from the Ti5f sites has desorbed. This finding is consistent with previous TPD experiments, which have shown that CO2 desorbs in two distinct waves: one with a peak at 137 K and a second with a peak at 166 K.16,17 Our observations at 140 K are consistent with the desorption of Ti5f bound CO2 at the lower temperature and the retention of CO2 in the VO,br sites. In addition, we note that all of
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Figure 5. STM images of TiO2(110) before and after exposure to CO2: (a) freshly prepared TiO2(110) surface (T = 145 K), (b) STM image of the same sample area after exposure to CO2 (∼0.06 ML) at 145 K. Imaging conditions: V = 1.12 V, I = 0.3 nA.
the VO,br sites are decorated, suggesting that some of the Ti5f bound CO2 molecules have migrated to VO,br sites. Because this process was not observed at 80 K, we conclude that there is an activation barrier to the conversion similar in magnitude to the barrier to desorption from Ti5f sites. We suggest, therefore, that Ti5f to VO,br conversion competes with desorption at temperatures near ∼140 K. Figure 4c shows an image of the same sample surface recorded at 150 K. At this temperature, the STM contrast of individual CO2 molecules becomes strongly streaky and smeared out but remains localized at oxygen vacancies. It becomes difficult to discern single molecules and to identify their exact positions. While we cannot exclude that hopping, i.e., place exchanges between pairs of molecules adsorbed on different VO,br, takes place, our data clearly show that each VO,br remains decorated by CO2 at 150 K, i.e., no net loss of CO2 occurs from these binding sites. This situation changes as the temperature is increased above ∼170 K. Figure 4d, a STM image of the same sample recorded at 175 K, shows the complete desorption of all molecules from the surface. The sample appears clean and cannot be distinguished from a freshly prepared one. In particular, the streaking in the vicinity of the VO,br sites is absent and their apparent height indicates that they are unoccupied. The vacancy concentration is nearly identical to that on the freshly prepared sample, but a small density of bridging hydroxyl species (e.g., marked by squares in Figure 4d) indicates that some vacancies 12100
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Figure 6. TiO2(110) with higher CO2 coverage: (a) larger STM image of the surface immediately after adsorption of ∼0.3 ML CO2; (b, c) subsequent STM images showing the diffusion of some of the CO2 molecules, indicated by arrows. Imaging conditions: V = 1.1 V, I = 0.39 nA, T = 80 K.
may have reacted with residual H2O in the chamber.32,35 These features are clearly identified as OHbr and are readily distinguished by their appearance and stability from the possible byproducts of CO2 dissociation: oxygen adatoms, by their apparent height,36 and CO, by their stability at 175 K, a temperature above the desorption threshold of CO.37 Thus we can confidently conclude that the initial VO,br concentration prior to CO2 dosing, the density of lobes centered on the Obr rows after CO2 dosing and heating to ∼140 K, and the recovered vacancy concentration after CO2 desorption are all equal within experimental uncertainty. We consider this clear evidence of intact molecular CO2 adsorption and desorption above VO,br sites. Importantly, despite the high reactivity of oxygen vacancies in a variety of surface reactions,4,28,36 CO2 does not dissociate at the vacancy site at any temperature up to its eventual desorption at ∼170 K. To investigate the nature of the streaky protrusions seen at higher temperatures, CO2 was adsorbed on a freshly prepared sample at elevated temperature. The surface was exposed to a very small dose of CO2 such that the CO2 surface density was less than that of the VO,br sites. Figure 5a shows a STM image of the clean TiO2(110) surface at 145 K. The scanned area comprises a number of oxygen vacancy defects, as well as two larger protrusions used as registration landmarks. An STM image of the same sample area after exposure at 145 K (Figure 5b) shows a small coverage (∼0.06 ML) of streaky protrusions, comparable to those shown in Figure 4b, which appear only at the positions previously occupied by oxygen vacancies. At the low CO2 dose employed here, some of the vacancies have remained unchanged indicating that they remain vacant. The fact that some vacancies maintain their original appearance clearly shows that the streaking is inherent to the modification of the VO,br by adsorption of
CO2 and not an artifact due to a possible change to the STM tip. Also, no lobes or streaks are observed on the Ti rows, consistent with our observation of desorption from these sites at ∼140 K. Cooling this sample to 80 K suppressed the streaking and again showed the two distinct types of VO,br bound species with identical contrast as observed after CO2 exposure at 80 K (Figure 1). Given our observation of two distinct adsorption geometries of CO2 on oxygen vacancies, the streaks observed at elevated sample temperatures suggest that, given sufficient thermal energy, the CO2 molecules can change between different configurations on a time scale that is short compared to the observation interval in STM. The two distinct desorption temperatures of CO2 bound to Ti5f and VO,br sites allow us to further assess the diffusion of CO2 molecules on the TiO2(110) surface. We used experiments in which CO2 was dosed to low coverage at 80 K, followed by annealing to 140 K, to prepare adsorbate populations in which CO2 was bound exclusively on oxygen vacancies. Cooling such samples to lower temperatures brings them back to a temperature at which CO2 bound to Ti5f is stable. Hence, if CO2 molecules diffuse away from VO,br and populate the localized Ti5f state, STM imaging at lower temperatures should detect such species. CO2 covered surfaces annealed to 140 K, cooled to 80 K, and imaged by STM consistently showed only CO2 adsorbed on VO,br. The absence of any localized Ti5f bound molecules implies that either (i) CO2 molecules adsorbed on oxygen vacancies are immobile, i.e., do not diffuse away from their binding site at temperatures below 140 K, or (ii) these adsorbates can diffuse but the diffusion is not mediated by transient occupation of the localized Ti5f bound state. Since both transfer of CO2 between VO,br sites and hopping between Ti5f bound states are observed in our experiments, we conclude that 12101
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Figure 7. Histogram of CO2CO2 separations along [001]. CO2 coverage ∼0.3 ML; VO,br concentration ∼8%; T = 80 K.
the two CO2 adsorbate populations, those occupying the stable Ti5f bound state detailed in Figure 2a and those bound in VO,br sites, are separated by a substantial thermal activation barrier, of magnitude similar to that for thermal desorption from the less tightly bound state. In addition, these annealing experiments in conjunction with the time-lapse imaging at low temperature strongly suggest that their mass transport involves different diffusion mechanisms, specifically that mass transport among the vacancy sites is mediated by a highly mobile Ti5f row state while that among localized Ti5f atop sites occurs by simple nearest-neighbor site hops. Hence, our experiments show that the origin of the pronounced “two-step desorption” of CO2 from TiO2(110)16,17 lies in the fact that adsorption in two different surface sites produces two distinct, isolated adsorbate populations with very little exchange between them. With the appearance of CO2 adsorbed in the two types of adsorption sites identified, Ti5f and VO,br, we can use timedependent imaging at higher CO2 coverage to explore the nature of the interadsorbate interactions. Since the oxygen vacancies bind the CO2 tightly and comprise only a small fraction of the surface, they are readily saturated at rather low exposures, as observed in Figure 1. The adsorption of higher doses of CO2 should then progressively take place at the much more abundant Ti5f sites. This is confirmed by STM images following the adsorption of ∼0.3 ML CO2 at 80 K, shown in Figure 6. The surface is now highly populated with bright protrusions, whose contrast is very similar to that of Ti5f bound CO2 observed at lower coverages (Figures 1 and 2a). At this higher coverage, the hopping of individual Ti5f bound CO2 molecules is readily observed. Individual diffusion jumps take place both along the Ti row (i.e., the [001] direction) as well as between neighboring rows (i.e., along [110]). Jumps between Ti rows are less frequent, indicating a lower energy barrier for hops along [001] than along [110]. Figure 6 b,c illustrates this hopping in zoomed-in images at higher resolution. We can use a statistical analysis of our data to determine the distribution of CO2 molecules along Ti rows as well as between neighboring rows. Figure 7 shows a histogram of molecule molecule separations, obtained at 80 K for CO2 coverage of ∼0.3 ML, primarily on Ti5f sites (VO,br coverage ∼8% of the Obr sites). For this particular temperature and coverage, the distribution shows a strong dip at small separations, i.e., pairs of CO2 molecules avoid occupying nearest neighbor (spacing a) and next-nearest neighbor (2a) sites along the Ti row, indicating a repulsion of closely spaced molecules. Hence, at fractional surface coverage CO2 does not coalesce into dense 2-D islands
Figure 8. Electron stimulated healing of oxygen vacancies by reaction with adsorbed CO2: (a) STM image of the TiO2(110) with adsorbed CO2, bound to oxygen vacancies; (b) same sample area, imaged after scanning the outlined region at V = 2.0 V, I = 1.0 nA. Imaging conditions: V = 1.2 V, I = 0.25 nA, T = 140 K.
but tends to be uniformly distributed across the surface with gaps between adjacent molecules. The observation of intermolecular repulsion is consistent with the shift of the TPD peak to lower temperature with increasing coverage observed by Thompson et al.,18 and the decrease in binding energy with coverage predicted in HartreeFock calculations by Markovits et al.24 D. Confirmation of Molecular Adsorption at VO,br. Finally, we have used the ability of STM to dose electrons with welldefined energy (given by the tunneling bias, V) and rate (determined by the current, I) with atomic spatial resolution into specific surface sites to shed light on the striking lack of thermally activated reactions between VO,br and CO2. While neither thermal excitation nor STM imaging under standard conditions (V = 1.2 V; I < 0.3 nA) triggers a chemical reaction between VO,br and CO2, Figure 8 demonstrates that scanning (or local pulses) at elevated bias and increased tunneling current can be used to drive CO2 dissociation at the vacancy site and the concomitant “healing” of the vacancy. Figure 8a shows an emptystate STM image recorded at V = 1.2 V; I = 0.25 nA after exposure to CO2 at 140 K, which leads to adsorption only on VO,br. As described above, streaky bright protrusions appear on each oxygen vacancy due to CO2 adsorption. A dashed square marks an area which was subsequently scanned at higher bias (2.0 V) and current (1.0 nA), thus injecting electrons with up to ∼1.6 eV excess energy above the conduction band minimum of TiO2.32 A 12102
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Figure 9. Electron stimulated processes on CO2/TiO2(110): (a, b) STM image of the surface following CO2 exposure at T = 80 K. Left, raw data; right, same image with positions of different species marked. (c, d) Same surface area after scanning at 2.0 V; 0.85 nA. Left, raw data; right, same image with markers as in part b. Imaging conditions: 1.2 V; 0.25 nA.
subsequent image of the same area, again obtained under standard imaging conditions (V = 1.2 V; I = 0.25 nA) shows that all CO2 adsorbates inside the rectangle have disappeared from the surface (Figure 8b). Indeed, the surface within the field scanned at elevated bias and current is not only completely free of CO2 adsorbates, but it also does not contain any oxygen vacancies. At the same time, the areas not exposed to the high bias/current scan have remained unchanged, i.e., the number of CO2 molecules neither increased nor decreased in these areas. The observed changes, from a population of VO,br, each of which was decorated by a CO2 molecule, to a defect- and adsorbate-free TiO2 surface, allow us to conclude that the electron injection at higher flux and energy from the STM tip drives the reaction of VO,br bound CO2 molecules with the underlying oxygen vacancies. This reaction heals the vacancy, thus leaving a stoichiometric, defect-free surface behind: CO2 ðV O, br Þ þ V O, br f Obr þ COðgÞ At 140 K, the remaining reaction product, CO, is highly mobile and weakly bound on TiO2 and either rapidly diffuses out of the field of view or desorbs from the surface.3739 We have also been able to heal vacancies one at a time by driving the reaction with bias pulses on single CO2 molecules on VO,br. In the lowcoverage limit, the STM induced reaction of CO2 with oxygen vacancies can thus be used to engineer the local distribution of VO,br bound CO2, providing a useful tool for future STM experiments on chemical reactions and photocatalytic conversion of CO2. Furthermore, our observations suggest the possibility that nonthermal energy input, e.g., from absorption of UV radiation, may stimulate the reaction of adsorbed CO2 with VO,br. However, our data also show that rather high photon energies (3.0 eV bandgap þ1.6 eV excess energy) may be needed to activate this process. To further characterize the STM-tip activated reaction between CO2 and VO,br, we repeated the same experiment at 80 K
(Figure 9). Figure 9a shows the raw STM image of TiO2(110) after exposure to CO2 at 80 K, imaged at V = 1.2 V and I = 0.24 nA. Figure 9b shows the same image with different surface species identified and marked by overlaid crosses in different colors: CO2 adsorbed on Ti5f (17 molecules, black crosses); CO2 adsorbed on VO,br (38 molecules, blue); and unfilled VO,br (15 vacancies; white). The same area was then scanned at 2.0 V and 0.85 nA, followed by imaging at 1.2 V bias and 0.25 nA current. The raw image following high-energy electron injection is shown in Figure 9c. Figure 9d shows this image with overlaid markers in positions as in Figure 9b, illustrating changes induced by electron injection. Inspection shows that 5 (of 17) Ti5f bound CO2 molecules are missing and 4 have diffused a distance of one or more lattice parameters; 35 (of 38) VO,br bound CO2 have filled their underlying vacancy; and 13 (of 15) unfilled VO,br have remained unchanged. Two previously empty VO,br were also healed (rectangles). As in Figure 8, these images show that injection of electrons from the STM tip at higher bias can dissociate the adsorbed CO2 molecules and heal the underlying vacancy. The fact that some of the initially unoccupied VO,br sites are also healed indicates that CO2 molecules bound to Ti5f are displaced to nearby VO,br and suggests, therefore, that nonthermal excitation, here by tunneling electrons, can activate the conversion of CO2 molecules from the Ti5f bound state to the VO,br state. This conversion was notably not observed at low temperature and lower biases in the time-lapse imaging detailed above. The possibility of driving the conversion with higher energy electrons is consistent with our conclusion that a significant activation barrier to this process exists. In addition, the disappearance of some Ti5f bound CO2 suggests that simple molecular desorption from the Ti5f sites can be driven by nonthermal excitation and that desorption is competitive with conversion from the Ti5f bound state to the VO,br state, similar to the case for the corresponding thermal processes as determined from images recorded during thermal annealing. Further detailed experiments with excitation at the single molecule level 12103
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The Journal of Physical Chemistry C (i.e., by local bias pulses instead of high-bias scanning) are needed to clarify the competition between nonthermally activated CO2 diffusion and reaction with VO,br, as well as the efficiency of these different events as a function of electron energy and flux.
4. CONCLUSIONS The adsorption of CO2 on rutile TiO2(110)-1 1 was studied by low-temperature scanning tunneling microscopy. Our measurements show unambiguously that CO2 adsorbs molecularly at two different surface sites: atop 5-fold coordinated Ti and at bridging oxygen vacancies. While molecules adsorbed on Ti5f are imaged in a single conformation, two different adsorption geometries, one symmetric and one asymmetric with respect to the bridging oxygen row, are identified for vacancy-bound CO2 molecules. We also see evidence for a highly mobile state above the Ti5f rows. CO2 desorbs from the Ti5f atop sites at ∼140 K and from VO,br at ∼175 K. The surface diffusion of CO2 molecules bound to Ti5f involves nearestneighbor hops and with increasing coverage appears to be facilitated by repulsive interadsorbate interactions. The diffusion of CO2 away from VO,br is hindered by a large activation energy, but once removed from a vacancy, the adsorbate is rapidly transferred to another VO,br site. This transfer cannot be mediated by the Ti5f atop adsorption state, in which CO2 diffusion is slow, but may be mediated by the alternative, highly mobile state associated with the Ti5f rows. We infer that there is an activation barrier that effectively isolates the Ti5f and VO,br adsorbed CO2 populations. This conclusion is consistent with the separate, distinct desorption features at different temperatures for the two species. The STM images provide direct real-space evidence supporting the understanding derived from less direct methods. Site population analysis also provides new insight into the adsorption dynamics. Injection of tunneling electrons at elevated bias voltage (i.e., electron energy) can lead to a number of nonthermal excitations. Most importantly, electron injection can lead to the healing of VO,br by reaction with CO2, a process that is not thermally activated up to the desorption temperature. We also observe that thermally activated processes including desorption of CO2 from Ti5f sites and diffusion of CO2 away from the adsorption sites are readily activated by STM electron injection, and we infer that electron injection can also initiate the conversion between Ti5f and VO,br adsorbed CO2. Our results have important implications on understanding basic aspects of the photocatalytic reforming of CO2 on TiO2(110). In particular, the issue of molecular CO2 adsorption on oxygen vacancies has been clarified. While CO2 adsorbs at VO,br sites in two different conformations at low temperatures (80 K), STM suggests that the two (and possibly others) are sampled dynamically at elevated temperatures. Hence, the search for the optimal adsorption geometry needs to focus not only on local energy minima26 but also on the intervening kinetic barriers, which appear to be comparable to the thermal energy at ∼140 K. Future work will use concurrent UV exposure and low-temperature STM imaging to identify processes excited by light absorption and to extend the present studies on CO2 to the coadsorption of CO2 and H2O,16 in an effort to use molecular-scale imaging to understand the mechanisms of photocatalytic CO2 reforming. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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Present Addresses §
Fundamental and Computational Sciences Directorate, Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
’ ACKNOWLEDGMENT Work was performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC02-98CH1-886. Experiments were carried out at the Center for Functional Nanomaterials, a Nanoscale Science Research Center supported by the Office of Basic Energy Sciences, U.S. Department of Energy and supported by the Office of Basic Energy Sciences, Chemical Imaging Initiative, FWP CO-023. ’ REFERENCES (1) Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Perez, M. Science 2007, 318, 1757. (2) Thompson, T. L.; Yates, J. T., Jr. Chem. Rev. 2006, 106, 4428. (3) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515. (4) Fujishima, A.; Hashimoto, K.; Watanabe, H. TiO2 Photocatalysis: Fundamentals and Applications; BKC, Inc.: Tokyo, Japan, 1997. (5) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (6) Gratzel, M. Nature 2001, 414, 338. (7) Halmann, M. M. Chemical Fixation of Carbon Dioxide: Methods for Recycling CO2 into Useful Products; CRC Press: Boca Raton, FL, 1993. (8) Hemminger, J. C.; Carr, R.; Somorjai, G. A. Chem. Phys. Lett. 1978, 57, 100. (9) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637. (10) Ishitani, O.; Inoue, C.; Suzuki, Y.; Ibusuki, T. J. Photochem. Photobiol., A 1993, 72, 269. (11) Rasko, J.; Solymosi, F. J. Phys. Chem. 1994, 98, 7147. (12) Adachi, K.; Ohta, K.; Mizuno, T. Solar Energy 1994, 187. (13) Yamashita, H.; Kamada, N.; He, H.; Tanaka, K.-i.; Ehara, S.; Anpo, M. Chem. Lett. 1994, 23, 855. (14) Tan, S. S.; Zou, L.; Hu, E. Catal. Today 2006, 115, 269. (15) Henderson, M. A. Surf. Sci. 1996, 355, 151. (16) Henderson, M. A. Surf. Sci. 1998, 400, 203. (17) Funk, S.; Burghaus, S. Phys. Chem. Chem. Phys. 2006, 8, 4805. (18) Thompson, T. L.; Diwald, O.; Yates, J. T., Jr. J. Phys. Chem. B 2003, 107, 11700. (19) Rasko, J. Catal. Lett. 1998, 56, 11. (20) Ramis, G.; Busca, G.; Lorenzelli, V. Mater. Chem. Phys. 1991, 29, 425. (21) G€opel, W.; Rocker, G.; Feierabend, R. Phys. Rev. B 1983, 28, 3427. (22) Tanaka, K.; Mirahara, K.; Toyoshima, I. J. Phys. Chem. 1984, 88, 3504. (23) Wu, X.; Selloni, A.; Nayak, S. K. J. Chem. Phys. 2004, 120, 4512. (24) Markovits, A.; Fahmi, A.; Minot, C. A. J. Mol. Struct.: THEOCHEM 1996, 371, 219. (25) Liao, L.-F.; Lien, C.-F.; Shieh, D.-L.; Chen, M.-T.; Lin, J.-L. J. Phys. Chem. B 2002, 106, 11240. (26) Indrakanti, V. P.; Kubicki, J. D.; Schobert, H. H. Fuel Process. Technol. 2011, 92, 805. (27) Pan, J.-M.; Maschhoff, B. L.; Diebold, U.; Madey, T. E. J. Vac. Sci. Technol. A 1992, 10, 2470. (28) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (29) Diebold, U.; Anderson, J. F.; Ng, K.-O.; Vanderbilt, D. Phys. Rev. Lett. 1996, 77, 1322. (30) Diebold, U.; Lehman, J.; Mahmoud, T.; Kuhn, M.; Leonardelli, G.; Hebenstreit, W.; Schmid, M.; Verga, P. Surf. Sci. 1998, 411, 137. (31) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlstr€om, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; 12104
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