J. Phys. Chem. C 2010, 114, 3987–3990
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Direct Visualization of Water-Induced Relocation of Au Atoms from Oxygen Vacancies on a TiO2(110) Surface Xiao Tong,† Lauren Benz,‡ Steeve Chre´tien,† Horia Metiu,† Michael T. Bowers,† and Steven K. Buratto*,† Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106, and Department of Chemistry and Biochemistry, UniVersity of San Diego, San Diego, California 92110 ReceiVed: October 14, 2009; ReVised Manuscript ReceiVed: January 7, 2010
Variable-temperature scanning tunneling microscopy (STM) is used to show that Au1+ deposited onto a TiO2(110)-(1 × 1) surface under soft-landing conditions at 600 K results in a surface decorated with isolated gold atoms bound to oxygen vacancies. This result is in sharp contrast to the large, sintered islands which form from Au1+ deposited onto a hydroxylated TiO2(110)-(1 × 1) surface under soft-landing conditions at 300 K. The position of the isolated Au atoms prepared by deposition at 600 K changes from directly above the bridging oxygen rows to directly above 5c-Ti atoms when the substrate is allowed to cool from 600 to 300 K. The binding site of the Au atoms returns to directly over the bridging oxygen rows when the temperature is returned to 600 K, indicating that this process is reversible. We attribute the change in binding site to a competition between the Au atom and an adsorbed water molecule for an oxygen vacancy on the reduced TiO2 surface. Using density functional theory (DFT), we show that dissociative adsorption of water occurs at an oxygen vacancy occupied by an Au atom, displaces the Au atom, and forms a stable OH-Au-TiO 2 complex on the surface. Introduction Metal-oxide interfaces play a major role in many technological applications, including the development of heterogeneous catalysts, electronic devices, and sensors. The rutile TiO2(110) surface has become one of the key model systems for metal oxide surfaces. Au nanoclusters supported on TiO2(110) act as good catalysts for CO oxidation at low temperatures.1,2 Extensive experimental studies have shown that water plays an important role in CO oxidation,3-6 but a detailed understanding of this effect is not known. An initial step toward an understanding of the role of water in the activity of Au/TiO2 catalysts is determining how water affects the adsorption of Au on the TiO2(110)-(1 × 1) surface. The preparation of the rutile TiO2(110)-(1 × 1) surface in UHV conditions results in a partially reduced surface with approximately 10% oxygen vacancies formed by the removal of bridging oxygen atoms. The electrons occupying the O-atom vacancy reduce the formally (4+) titanium ions near the vacancy to a formal (3+) oxidation state. 7,8 It is well-known that water present in the background pressure will adsorb and react with an oxygen vacancy even under UHV conditions. The water molecule dissociates when it interacts with the oxygen vacancy, fills the vacancy with a hydroxyl group, and creates a second hydroxyl group as the hydrogen adatom binds to an adjacent bridging oxygen atom. 9-13 Further adsorption of water leads to a separation of these double hydroxyls and results in a surface decorated with single OH groups and no oxygen vacancies. 10,12,13 In typical UHV conditions of 2 × 10-10 Torr, water will * Corresponding author. Phone: (805) 893-3393. Fax: (805) 893-4120. E-mail:
[email protected]. † University of California, Santa Barbara. ‡ University of San Diego.
titrate the oxygen vacancies and the surface will become completely hydroxylated within a few hours at room temperature. 13 Initial experiments using vapor deposition of Au atoms onto TiO2(110)-(1 × 1) surfaces at room temperature resulted in rapid sintering and agglomeration of the Au into three-dimensional islands of the order of 100 atoms as reported by STM. 14-16 Our group has also observed sintering for mass-selected Au atoms deposited on TiO2(110)-(1 × 1) at 300 K.17 In these experiments, the sintered islands had approximately 30 atoms as determined from the STM image. In contrast to these results, however, Anderson and co-workers used X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), and CO binding experiments to show, convincingly, that Au+ deposited from a mass-selected source resulted in isolated atoms on the surface bound at oxygen vacancies.18 This apparent discrepancy was reconciled by the recent experiments of the Besenbacher and Anderson groups. Besenbacher and co-workers showed that rigorous removal of water from the background pressure in UHV leads to a pristine, reduced TiO2(110)-(1 × 1) surface that remains hydroxyl-free for several hours.19 Vapor deposition of Au atoms onto this surface results in isolated Au atoms bound at the oxygen vacancies.19 The adsorption of Au atoms onto a hydroxylated TiO2(110)-(1 × 1) surface leads to efficient sintering due to the enhanced mobility of the Au on the hydroxylated surface at room temperature.19,20 Anderson and co-workers showed using XPS and ISS Au atoms are isolated and bound to vacancies when deposited on a pristine surface and that Au mobility and sintering is enhanced as the degree of surface hydration increases,20 consistent with the results of the Besenbacher group. Furthermore, Anderson and co-workers showed that the addition of water to the Au atom decorated surface led to more rapid sintering of the Au upon annealing. On the basis of this result, they concluded that the water displaced the Au atom from the vacancy.20
10.1021/jp9098705 2010 American Chemical Society Published on Web 02/17/2010
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In work described here, we use scanning tunneling microscopy (STM) to probe the interaction of isolated Au atoms supported on a reduced TiO2(110)-(1 × 1) surface, with water from the background pressure under UHV conditions. We show that the binding site of the Au atom depends on the degree of hydration of the surface. On a hydroxyl-free surface, the Au atoms bind to the oxygen vacancies as predicted by theory. If the surface is allowed to interact with water at room temperature, the water molecule will displace the Au atom from the vacancy replacing it with an OH group and form a stable OH-Au-TiO2 complex on the TiO2(110)-(1 × 1) surface. Experimental Section A beam of Au1+ was created using laser desorption from a gold rod illuminated with a pulsed YAG laser beam (532 nm, 500 mJ/pulse max power),21 producing a gold plasma. Positive ions were extracted following a pulse of argon carrier gas. The beam was focused, accelerated, and mass selected using a magnetic sector. An Au1+ beam, with a flux of ∼2 nA/cm,2 was focused onto a hydroxyl-free TiO2(110)-(1 × 1) sample in a UHV chamber with base pressure during deposition of less than 5 × 10-10 Torr (mainly due to the argon carrier gas). A positive sample bias lowered the incident kinetic energy of the ions to less than 2.0 eV, resulting in soft-landing on the surface. The total amount of metal deposited was calculated from the incident ion flux and compared to that calculated from the STM images. Typically, a coverage of 0.01 ML was used. The TiO2(110)-(1 × 1) surface was prepared by multiple cycles of Ar+ ion sputtering of the crystal (1 keV, 20 min), followed by flashing at ∼850 °C. To ensure that the surface remained hydroxyl-free, we maintained a temperature above 600 K during the deposition of Au1+. At this temperature, any surface hydroxyls recombined to form H2O and desorbed.22,23 After deposition, the Au-decorated surface was transferred to the STM chamber and imaged in a chamber with a base pressure of 1 × 10-10 Torr. Empty state STM images of the surface were acquired at either 300 or 600 K in constant current mode with a sample bias of +1.5 V and a tunneling current of 0.1-0.2 nA. High-resolution STM images of a hydroxyl-free TiO2(110)(1 × 1) substrate consisted of alternating gray and black rows, which are assigned to 5-fold coordinated titanium atom (5c-Ti) rows and bridging oxygen rows, respectively. The oxygen vacancies were observed as gray rectangles (same contrast as the 5c-Ti rows) within the bridging oxygen rows. Doublehydroxyls appeared as spots along the bridging oxygen rows with brighter contrast than the vacancies and elongated along the bridging oxygen row. Single hydroxyls along the bridging oxygen row appeared with a contrast slightly brighter than the 5c-Ti rows. An important point to note is that a pristine, reduced surface that becomes hydroxylated by dissociative adsorption of water has approximately 1.8 times the number of spots along the bridging oxygen rows as the hydroxyl-free surface. Results and Discussion The STM image of Figure 1a shows the result of Au+ softlanded onto a fully hydroxylated TiO2(110)-(1 × 1) surface at room temperature. The total coverage of Au was ∼0.010 ( 0.003 ML. At 300 K (Figure 1a), the Au atoms readily sinter and yield a surface decorated with large Au islands at a low surface density. This behavior has been observed previously in STM images of Au/TiO2(110) for mass-selected ion17 and vapor deposition of Au atoms.14,15,16,24 A broad height distribution was observed for the islands with an average value of 4.3 Å and a standard deviation of 1.7 Å. If we assume a hemispherical shape
Figure 1. STM images (500 Å × 350 Å) of the TiO2(110)-(1 × 1) surface, which was exposed to Au1 at 300 K (a) and at 600 K (b), respectively. Both images were recorded at 300 K. The bright strings denoted by the arrows in parts a and b are 1 × 2 defects common in STM of reduced TiO2(110)-(1 × 1) surfaces.7
and the lattice spacing of metallic gold, then the average island has approximately 30 atoms. This result is similar to that observed from neutral Au vapor deposition (assumed to be monomeric).24 The facile sintering of Au under these conditions of deposition indicates that the Au atoms are highly mobile on the hydroxylated TiO2(110)-(1 × 1) surface at room temperature. Heating the surface above 550 K removes the surface hydroxyls,10,23,26 leaving a pristine, reduced TiO2(110)-(1 × 1) surface with approximately 0.100 ( 0.006 ML oxygen vacancies (1 ML is the number of 5C-Ti atoms, 5.2 × 1014 cm-2). The STM image of Figure 1b shows the result of 0.010 ( 0.003 ML of Au+ soft-landed onto this pristine TiO2(110)-(1 × 1) surface at 600 K. The surface was allowed to cool to room temperature before the image of Figure 1b was acquired. In sharp contrast to the image of Figure 1a, the image of Figure 1b shows no large clusters but instead shows a high density of very small spots. Statistical analysis reveals that the density of spots is within a few percent of that expected for isolated Au atoms, thus supporting their assignment to single Au atoms. This result is somewhat surprising, as increasing the surface temperature generally enhances the mobility of atoms on the surface. The observation of small spots indicates that the Au atoms soft-landed onto the surface, while highly mobile, find a strong binding site before finding another Au atom. A temperature of 600 K is apparently not large enough, on the time scale of our experiment, to induce further diffusion from these strongly bound sites. It is possible to probe this binding site of the Au atoms by imaging at high resolution. The STM image of Figure 2a shows the positions of the Au atoms as bright spots when deposited on a pristine, reduced TiO2(110)-(1 × 1) surface at 600 K and imaged at 600 K. These spots all appear along bridging oxygen
Water-Induced Relocation of Au Atoms
Figure 2. STM images (70 Å × 40 Å) of a TiO2(110)-(1 × 1) surface exposed to Au1 at 600 K and imaged at 600 K (a), imaged at 300 K after cooling from 600 K (b), and imaged at 600 K after heating from 300 K (c), respectively. The white arrows in parts a, b, and c denote gold atoms. The black arrows in parts a and c denote oxygen vacancies, and the black arrow in part b denotes a hydroxyl group. The dashed oval in part b denotes the extent of the contrast of the OH-Au-TiO2 complex with the brightest spot over the 5c-Ti row and residual contrast over the bridging oxygen row (see text for details).
rows. The gray spots above the bridging oxygen rows are unfilled oxygen vacancies as described earlier (black arrow of Figure 2 a). 27 Note that we do not observe any hydroxyl groups when the STM image is acquired at 600 K, since hydroxyls are not present at this temperature. 22 Statistical analysis of multiple images indicates that the sum of the Au atoms and the remaining oxygen vacancies is on average 0.100 ( 0.007 ML (i.e., the vacancy density of the pristine, reduced surface). Both of these observations imply that the binding site of the Au atom is an oxygen vacancy on the TiO2(110)-(1 × 1) surface at 600 K. The STM image of Figure 2b was acquired after the Au atom decorated surface was cooled from 600 K to room temperature (300 K). The image of Figure 2b was from the same surface as the image of Figure 2a but not the same area. There are three distinct differences between images acquired at 300 K and images acquired at 600 K. First, the bright spots in Figure 2b are positioned over the 5c-Ti rows, as indicated by the white arrow. Second, each of the spots is larger, oval in shape, and asymmetric with the brightest region over the 5c-Ti rows and the dimmer region extending over the oxygen rows. Finally, the sum of the number of bright spots (Au atoms) and the number of the fainter gray spots positioned above the oxygen rows (indicated by the black arrow of Figure 2b) is on average ∼1.8 times greater than that of the Au atom decorated surface imaged at 600 K (Figure 2a). The increase in the number of spots is similar to what occurs when the clean surface becomes
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Figure 3. DFT structures (a) and (b) are low energy structures for coadsorption of an A atom and a water molecule on a reduced TiO2(110)-(1 × 1) surface. The balls in the structural models represent Ti (gray), Au (yellow), H (white), O(TiO2) (red), and O (OH) (blue), respectively. The structure in part b is 0.25 eV lower in energy than structure a. High resolution STM images (26 Å × 26 Å) of a single Au atom (bright spot) bound to TiO2(110)-(1 × 1) above the bridging oxygen row at 600 K (c) and 5C-Ti row at 300 K (d). Both of the surfaces in parts c and d were exposed to Au1 at a substrate temperature of 600 K.
hydroxylated via dissociative adsorption of water.13 This suggests that water interacts with the Au atom decorated surface in the same way as it interacts with a clean surface; water adsorption occurs to form the double-hydroxyl groups via dissociation of a residual water molecule. Each of the oxygen vacancies, including those occupied by a Au atom, become hydroxylated. Dissociative adsorption of a water molecule at a vacancy occupied with a Au atom appears to result in displacement of the Au atom according to the STM image. This situation is further elucidated by DFT theory.25 The two lowest energy structures are given in Figure 3a and b. In Figure 3b, the water molecule dissociates, displaces the Au atom from the vacancy, and replaces it with an OH group. The hydrogen atom binds to an adjacent bridging oxygen atom, yielding the familiar double-hydroxyl group on the surface. The displaced Au atom is bound to both the surface hydroxyl group and an adjacent 5-fold coordinated Ti atom, with the center of the Au atom directly above the 5c-Ti row. The desorption energy of the Au atom from this OH-Au-TiO2 complex is 1.65 eV, which is larger by 0.25 eV than the desorption energy of the Au atom from the oxygen vacancy in the structure of Figure 3a. We have included STM images in Figure 3 for comparison with the DFT structures. The image of Figure 3c (acquired at 600 K) shows an Au atom along a bridging oxygen row. Cooling the sample to 300 K allows water to interact with the surface, and the resulting image in Figure 3d shows the Au atom positioned over the 5c-Ti row consistent with the displaced Au atom in the structure of Figure 3b. In addition, there is residual contrast over the bridging oxygen row due to the double hydroxyl group, also consistent with the structure of Figure 3b. If we heat the surface to a temperature of 600 K, the bright spots return to their original position over the oxygen rows. It is important to note that we do not observe any sintering of the Au atoms at ∼600 K.
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Conclusions
References and Notes
The results described above also provide a deeper understanding of some of our early experiments. We have previously demonstrated that deposition of Ag1 or Au1 under hard-landing conditions (∼190 eV impact energy) onto a TiO2(110) surface at 300 K28,29 results in monomer-like spots that appear above the 5c-Ti rows. The lack of observation of surface damage and defects suggested that the high impact energy of clusters is mainly dissipated as thermal energy to the substrate, resulting in the recovery of the initial impact-induced surface damage around the landing point.28,29 The increased thermal energy would remove the surface hydroxyl groups near the impact point and the Au (or Ag) atom would bind to the vacancy left behind. Once the surface equilibrates, the water from the background then readsorbs and displaces the Au atom from the vacancy, creates the Au-OH-TiO2 complex described above, and yields an STM image with a spot above the 5c-Ti rows as observed.29 In summary, we have shown here that water molecules significantly affect the adsorption site, size, and distribution of Au atoms bound to rutile TiO2(110). Theory tells us that the Au atom bound to an oxygen vacancy on the TiO2(110)-(1 × 1) surface is unstable in the presence of water. The dissociating water molecule displaces the Au atom, which then reacts with the surface and results in the Au-OH-TiO 2 structure depicted in Figure 3b. We show using STM that the relocation of Au atoms from the oxygen vacancies to the 5c-Ti rows readily occurs as the surface becomes hydroxylated by the residual water in UHV. These images show for the first time that water molecules are not only capable of replacing an oxygen vacancy by a hydroxyl group but are also capable of forcing an Au atom out of the vacancy. Since water molecules exist in any real catalytic system, insight into the interaction between water and Au clusters on the surface will help in understanding the role of water in the catalytic activity of the Au clusters. In addition, our experiments provide a platform that can be used to study the interplay between oxygen vacancies, nanoclusters, and other small molecules such as ethylene, propylene, and methanol on oxide surfaces.
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Acknowledgment. This work was supported by the DURINT program of the Air Force Office of Scientific Research and the National Science Foundation (CHE-0749489). Use of computer time at the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.
JP9098705
