- Email: [email protected]
STM investigation of the reaction of AgO added rows with CO2 on a Ag(110) surface
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surface science
ELSEVIER
Surface Science 344 (1995) L1207-L1212
Surface Science Letters
STM investigation of the reaction of A g - O added rows with CO2 on a A g ( l l 0 ) surface Yuji Okawa*, Ken-ichi Tanaka The Institutefor Solid State Physics, The University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan Received 17 July 1995; accepted for publication 18 September 1995
Abstract
The formation of carbonate species by a reaction of Ag-O added rows with CO2 was investigated by scanning tunneling microscopy (STM). STM observation during the reaction revealed that the Ag-O added rows were compressed from a (4 x 1) to a (2 x 1) phase, according to the growth of carbonate domains. Though the carbonate domains give a (1 × 2) diffraction pattern, STM images of this domain do not show a (1 × 2) structure, which indicates that the Ag substrate reconstructs to a (1 × 2) structure. Though the CO3 species thermally decompose at a lower temperature than Ag-O added rows, the Ag-O added rows can be selectively decomposed by ultraviolet irradiation.
Keywords: Carbon dioxide; Low index single crystal surfaces; Oxygen; Photochemistry; Scanning tunneling microscopy; Silver; Surface chemical reaction
The interaction of COz with pre-adsorbed oxygen on a Ag(110) surface was first reported by Bowker et al. [ 1], who demonstrated the formation of surface carbonate intermediates. Using labeled oxygen isotopes, they showed that the three oxygen atoms on the CO3 species were statistically equivalent, and the ratio of adsorbed CO2 to oxygen atoms was close to 1:1. Barteau and Madix [2] found that the formation of carbonate was strongly dependent on the initial oxygen coverage. They also found by low energy electron diffraction (LEED) experiments that the adsorption of CO2 upon a (n x 1) ordered oxygen adlayer at 300 K resulted in the formation of (1 x2) domains of CO3 which caused compression of the oxygen *Corresponding author. Fax: +81 3 3401 5169; E-mail: [email protected]. 0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 9 6 6 - 3
atoms to a (2 x 1) structure. Heating of the (1 × 2) sample to 520 K to decompose the carbonate results in restoration of the original (n x 1) oxygen LEED pattern. High resolution electron energy loss spectroscopy (HREELS) investigations [3,4] indicated that the carbonate species are present in a monodentate state. From a TPD study, Backx et al. [4] found that one CO2 molecule adsorbed per two oxygen atoms. However, XPS and TDS studies by Campbell and Paffett [5] supported the 1:1 ratio for the carbonate ions to oxygen atoms. They also found that the O(ls) XPS peak was slightly broadened, which suggested the existence of two unresolved peaks. X-ray absorption fine structure investigations [6,7] revealed that the carbonate species lay in a plane parallel to the plane of the surface. For steric reasons, Bader et al. [6] concluded that the observed ( l x 2 ) surface
Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
not derive from the carbonate overlayer, but a reconstruction of the substrate took place. Ricken et al. [ 8 ] studied the electronic structure of the carbonate species with angle-resolved photoemission, and showed that the point group symmetry was C~. When a A g ( l l 0 ) surface is exposed to oxygen, sequential (nx 1) L E E D patterns ( n = 7 to 2) appear. Scanning tunneling microscopy (STM) studies [-9] have revealed that this phenomenon can be explained by the "added row" mechanism. That is, it was shown that one-dimensional A g - O chains, which consisted of alternating silver and oxygen atoms, grew along the [001] direction. The A g - O chains on the A g ( l l 0 ) surface prefer to disperse in the [110] direction, so that a variety of (n x 1) structures appear as the density of A g - O chains increases. In this STM study, we report the microscopic details of the reaction of A g - O added rows with CO2 on a A g ( l l 0 ) surface at room temperature. We will show that the A g - O added rows were compressed from a ( 4 x 1) to a ( 2 x 1) phase, according to the growth of carbonate domains. Although the carbonate domains give a (1 x 2 ) diffraction pattern, STM images of this domain do not show a (1 x 2 ) structure, which indicates that the Ag substrate reconstructs to a (1 x 2 ) structure. Finally, we will show that the A g - O added rows can be selectively decomposed by photoreaction using ultraviolet irradiation. The experiments were carried out in an ultrahigh vacuum apparatus equipped with a STM, fourgrid LEED-AES optics, an Ar ion gun, and a quadrupole mass analyzer. The STM used in this study was a commercial Rasterscope-3000 STM from Danish Micro Engineering. All STM images presented here were recorded in the constant current mode at room temperature using a tungsten tip. Typical tunneling currents were about 0.3 nA with sample bias voltages in the --0.2 to - 2 V range. The A g ( l l 0 ) crystal was cleaned by repeated cycles of Ar + sputtering, anneafing at 620 K for 15min, treating with 02 at 2 x 1 0 - 4 T o r r and 450 K for 30 s, and heating again to 670 K for l m i n . Exposure to oxygen at a pressure of 2 × 1 0 - 4 Torr on a clean A g ( l l 0 ) surface at 470 K
for 30 s leads to a (2 x 1) structure. A (3 x 1) and a (4 x 1)-O/Ag(ll0) surface was obtained by heating the (2 x 1) surface to 510 K for a few minutes. A high pressure mercury lamp (500W) was used for the photo-reactivity studies. The wavelength of the UV light was 310,-~ 510 nm. Fig. la shows a typical STM image of the (4 x 1) phase of A g - O added rows grown along the [-001] direction. When this surface was exposed to a CO2 background pressure at 1 x 10 .7 Torr, in situ STM measurements revealed that the Ag-O chains reacted with CO2 from the ends of the chains, and a new structure appeared. Fig. lb is a STM image obtained after exposure to CO2 for 20min. Corresponding to the growth of the domains of the new structure between the A g - O added rows, the unreacted A g - O added rows are compressed to a (3 x 1) phase. Fig. lc is a STM image obtained after exposure to COe for total 30 min. Now the domains of the new structure cover most of the surface. Concurrently, the remaining Ag-O added rows are compressed to a (2 x 1) phase. By further exposure to CO2, the A g - O added rows completely disappear and the new structure covers all of the surface. The L E E D pattern of this surface shows a (1 x 2 ) structure. Considering the previously rePorted results [ 1 - 8 ] , we conclude that the new structure observed in Figs. lb and lc is the carbonate-related structure. Compression of metal-O chains by coadsorption with other adsorbates also has been reported on N i - O with ammonia [ 10], benzene [11], and hydrogen [12] on a N i ( l l 0 ) surface. During the exposure to CO2, no detectable movement of step edges was observed, nor was island or trough formation observed on wide terraces. This indicates that the density of Ag atoms on the surface does not change drastically during the reaction. Fig. 2 shows STM images obtained after exposure to 105 L (2 x 1 0 - 4 Torr for 500 s) of COz on a ( 3 x l) A g - O / A g ( l l 0 ) surface. The L E E D pattern of this surface was a superimposition of a ( 2 x l ) and a ( l x 2 ) . In Fig. 2, we observe a coexistence of the (2 x 1) domains of unreacted A g - O added rows and the carbonate-related structure. The STM images were strongly dependent on the tip conditions. In Fig. 2a, two types of
Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
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Fig. 2. STM images obtained after exposure to 10s L of CO2 on a Ag(ll0) surface covered by a (3 x 1) phase of Ag-O added rows (86 x 86 ,~2). The coexistence of (2 x 1) domains of Ag-O rows and carbonate structure is observed. In (a), big and small protrusions are resolved in the carbonate domain (sample bias= ~ - 0 . 6 3 V). (b) The image obtained after the tip condition was changed (sample bias= ~ - 0 . 1 9 V ) . The dotted lines are drawn from protrusions in the (2 x 1) domain of Ag-O added rows. The protrusions in the carbonate domain are positioned on the lines.
Fig. 1. (a) A STM image of a Ag(110) surface covered by ( 4 x l ) phase of Ag-O added rows (210x210A2). (b), (c) STM images taken after exposure to 1 x 1 0 - 7 Torr of COz for 20 and 30min, respectively. As the new structure grows between the Ag-O rows, the remaining Ag-O rows are compressed to ( 3 x l ) and ( 2 x l ) phases in (b) and (c), respectively.
2
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Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
protrusions are resolved in the carbonate structure. That is, big protrusions with a height of ,-~1.5 and small protrusions with a height of ,-~1.0 A are resolved. (The height of the Ag-O added rows is about 1.0 A in this figure). The protrusions form rows along the [110] direction. The distance between the rows is two [001] lattice spacings. On the other hand, the distance between the protrusions along the [110] row takes a random value from two to five [110] lattice spacings. These protrusions are very mobile at room temperature and they change their position from image to image. Fig. 2b is a STM image at the same place as in Fig. 2a, obtained after the tip condition was changed. Now the atomic resolution in the (2 x 1) domains of Ag-O added rows is obtained, though the resolution in the carbonate domain becomes worse. As indicated by dotted lines in the figure, the protrusions in the carbonate domain are positioned on the extended lines drawn from protrusions in the (2 x 1) domains of Ag-O chains along the [110] direction. It is very difficult to determine whether silver or oxygen atoms are imaged in the Ag-O added rows in Fig. 2b. However, in the case of Ni-O added rows on Ni(110) and Cu-O added rows on Cu(110), it has been accepted that the STM image corresponds to metal atoms in many cases [13]. Hence it is natural to assume that the Ag atoms are imaged by STM also in the Ag-O added rows in Fig. 2(b). According to this assumption, the positions of both big and small protrusions correspond to hollow sites of the original Ag substrate. Though sharp (1 x 2) spots were observed in the LEED pattern, the (1 x 2) periodicity has never been observed by STM. This indicates that the Ag substrate under the carbonate structure, which could not be observed by STM, is reconstructed to a (1 x 2) structure, giving the diffraction pattern as proposed by Bader et al. [6]. Very recently, Stensgaard et al. [ 14] also observed this structure by STM. In their higher resolution study, they resolved additional details of the (1 x 2) reconstruction. That is, they observed that [ll0]-directed triplets of atoms were added on the surface, and the carbonate ions were seen to reside on top of the triplets. This triplet structure, which is very mobile, primarily along the [1[0] direction, will
explain the observed ( l x 2 ) periodicity in the LEED pattern. The small and big protrusions observed in Fig. 2a can be assigned to the triplets and the carbonate ions on the triplets, respectively, though our STM images do not have enough resolution to resolve the triplet structure. Stensgaard et al. [ 14] indicated that the reconstruction of the surface should include oxygen atoms, which do not become part of the carbonate species. To investigate the chemical propertieS of the small protrusions (or "triplets"), we exposed them to CO or Hz gas at room temperature on a surface covered by the (1 x 2) carbonate structure. As a result, no detectable change was observed on either type of protrusion. Since Ag-O added rows are known to react with CO at room temperature [15], this indicates that the chemical property of the [ ll0]-directed "triplets" is quite different from the normal [001J-directed Ag-O added rows, even if the "triplet" structure contains oxygen atoms. Any diffraction patterns corresponding to the CO3 species themselves have never been reported in the past. However, when CO2 was exposed on a surface covered by (6 x 1) Ag-O rows, we found that very weak streaks run along the [001] direction from the a*/3 position in the LEED pattern, together with the weak (1 x 2) spots. These streaks may suggest that the average distance along the [ 110] direction between carbonate species is three lattice spacings. These streaks were not observed at higher coverages. Fig. 3 shows a STM image obtained after exposure to l x 1 0 -TTorr of CO2 for 10rain on a (3 x 1) Ag-O/Ag(ll0) surface. This figure indicates that the reaction of CO 2 with Ag-O added rows starts heterogeneously on the surface. That is, the formation of the carbonate structure starts preferentially at the upper side of the step edge. The UV irradiation study was performed on a surface on which coexisted the (2x l) Ag-O domain and the ( l x 2 ) carbonate structure domain, as shown in Fig 2, prepared by exposure to 10SL of COz on a (3x l) Ag-O/Ag(ll0) surface. Fig. 4 is one of the STM images obtained after UV light irradiated the surface for 12 min. All STM images obtained at different sample positions are essentially identical to Fig. 4, which shows that the Ag-O rows disappeared and only the
Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
Fig. 3. A STM image obtained after exposure to 1 x 10-7 Torr of CO2 for 10rain on a (3xl) Ag-O/Ag(ll0) surface (260 x 260 ,~z). The formation of the carbonate structure is observed only at the upper side of the step edge.
[1 1 O]
[oo 1 Fig. 4. A STM image (90 x 90A2) obtained after irradiating the surface, on which coexisted the (2 x 1) Ag-O domain and the (1 x2) carbonate structure domain, with UV light. The Ag-O rows disappeared and only the carbonate structure remained. carbonate structure remained. The L E E D pattern of this surface also showed a disappearance of the ( 2 x l ) spot, and ( l x 2 ) spots and very weak streaks at the a*/3 position were observed. Because the CO3 species thermally decompose at a lower
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temperature than A g - O [1,2], the disappearance of A g - O is not a thermal effect of UV irradiation, but a photon-induced decomposition of the A g - O . The appearance of the streaks at the a*/3 position in the L E E D pattern suggests that the remaining carbonates spread over the surface. It should be pointed out that A g - O can be selectively decomposed by U-V irradiation, though the carbonate species thermally decompose at a lower temperature. This fact suggests the possibility of a "molecular mask" technique for surface modification. That is, we can "mask" a part of the A g - O rows by changing them to carbonate species, then decompose the remaining A g - O rows by UV irradiation, and finally we can restore the carbonate to Ag-O again by thermal decomposition. In summary, the formation of carbonate species by a reaction of A g - O added rows with CO2 on a Ag(110) surface was investigated by STM. The A g - O added rows were compressed from a (4 x 1) to a ( 2 x 1) phase, according to the growth of carbonate domains. The difference of the diffraction pattern and the observed periodicity in the STM images indicate that the Ag substrate under the carbonate structure reconstructs to a ( l x 2 ) structure. The A g - O added rows can be selectively decomposed by UV irradiation, though the carbonate species thermally decompose at a lower temperature than does A g - O . The authors acknowledge the Asahi Glass Foundation for the financial support of this work. This work was also supported by a Grant-in-Aid for Scientific Research (05403011) of the Ministry of Education, Science and Culture of Japan.
References [1] M. Bowker, M.A. Barteau and R.J. Madix, Surf. Sci. 92 (1980) 528. I-2] M.A. Barteau and R.J. Madix, J. Chem. Phys. 74 (1981) 4144. I-3] E.M. Stuve, R.J. Madix and B.A. Sexton, Chem. Phys. Lett. 89 (1982) 48. [-4] C. Backx, C.P.M. De Groot, P. Biloen and W.M.H. Sachtler, Surf. Sci. 128 (1983) 81. 1-5] C.T. Campbell and M.T. Paffett, Surf. Sci. 143 (1984) 517.
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[6] M. Bader, B. Hillert, A. Puschmann, J. Haase and A.M. Bradshaw, Europhys. Lett. 5 (1988) 443. [7] R.J. Madix, J.L. Solomon and J. St6hr, Surf. Sci. 197 (1988) L253. [8] D.E. Ricken, J.S. Somers, A.W. Robinson and A.M. Bradshaw, J. Chem. Phys. 94 (1991) 8592. [9] T. Hashizume, M. Taniguchi, K. Motai, H. Lu, K. Tanaka and T. Sakurai, Jpn. J. Appl. Phys. 30 (1991) L1529; M. Taniguchi, K. Tanaka, T. Hashizume and T. Sakurai, Chem. Phys. Lett. 192 (1992) 117; Surf. Sci. 262 (1992) L123; T. Hashizume, M. Taniguchi, K. Motai, H. Lu, K. Tanaka and T. Sakurai, Surf. Sci. 266 (1992) 282. [ 10] L. Ruan, I. Stensgaard, E. L~egsgaard and F. Besenbacher, Surf. Sci. 314 (1994) L873.
[11] I. Stensgaard, L. Ruan, E. L~egsgaard and F. Besenbacher, Surf. Sci. 337 (1995) 190. [12] P.T. Sprunger, Y. Okawa, E. Besenbacher, I. Stensgaard and K. Tanaka, Surf. Sci. 344 (1995) 98. [13] L. Ruan, F. Besenbacher, I. Stensgaard and E. L~egsgaard, Phys. Rev. Lett. 70 (1993) 4079; F.M. Leibsle, Surf. Sci. 311 (1994) 45. [14] I. Stensgaard, F. Besenbacher and E. L~egsgaard, Eighth International Conference on Scanning Tunneling Microscopy/Spectroscopy and Related Techniques, Snowmass Village, USA (1995); I. Stensgaard, E. L~egsgaard and F. Besenbacher, to be published. [15] H. Albers, W.J.J. van der Wal, O.L.J. Gijzeman and G.A. Bootsma, Surf. Sci. 77 (1978) 1.
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surface science
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Surface Science 344 (1995) L1207-L1212
Surface Science Letters
STM investigation of the reaction of A g - O added rows with CO2 on a A g ( l l 0 ) surface Yuji Okawa*, Ken-ichi Tanaka The Institutefor Solid State Physics, The University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan Received 17 July 1995; accepted for publication 18 September 1995
Abstract
The formation of carbonate species by a reaction of Ag-O added rows with CO2 was investigated by scanning tunneling microscopy (STM). STM observation during the reaction revealed that the Ag-O added rows were compressed from a (4 x 1) to a (2 x 1) phase, according to the growth of carbonate domains. Though the carbonate domains give a (1 × 2) diffraction pattern, STM images of this domain do not show a (1 × 2) structure, which indicates that the Ag substrate reconstructs to a (1 × 2) structure. Though the CO3 species thermally decompose at a lower temperature than Ag-O added rows, the Ag-O added rows can be selectively decomposed by ultraviolet irradiation.
Keywords: Carbon dioxide; Low index single crystal surfaces; Oxygen; Photochemistry; Scanning tunneling microscopy; Silver; Surface chemical reaction
The interaction of COz with pre-adsorbed oxygen on a Ag(110) surface was first reported by Bowker et al. [ 1], who demonstrated the formation of surface carbonate intermediates. Using labeled oxygen isotopes, they showed that the three oxygen atoms on the CO3 species were statistically equivalent, and the ratio of adsorbed CO2 to oxygen atoms was close to 1:1. Barteau and Madix [2] found that the formation of carbonate was strongly dependent on the initial oxygen coverage. They also found by low energy electron diffraction (LEED) experiments that the adsorption of CO2 upon a (n x 1) ordered oxygen adlayer at 300 K resulted in the formation of (1 x2) domains of CO3 which caused compression of the oxygen *Corresponding author. Fax: +81 3 3401 5169; E-mail: [email protected]. 0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 9 6 6 - 3
atoms to a (2 x 1) structure. Heating of the (1 × 2) sample to 520 K to decompose the carbonate results in restoration of the original (n x 1) oxygen LEED pattern. High resolution electron energy loss spectroscopy (HREELS) investigations [3,4] indicated that the carbonate species are present in a monodentate state. From a TPD study, Backx et al. [4] found that one CO2 molecule adsorbed per two oxygen atoms. However, XPS and TDS studies by Campbell and Paffett [5] supported the 1:1 ratio for the carbonate ions to oxygen atoms. They also found that the O(ls) XPS peak was slightly broadened, which suggested the existence of two unresolved peaks. X-ray absorption fine structure investigations [6,7] revealed that the carbonate species lay in a plane parallel to the plane of the surface. For steric reasons, Bader et al. [6] concluded that the observed ( l x 2 ) surface
Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
not derive from the carbonate overlayer, but a reconstruction of the substrate took place. Ricken et al. [ 8 ] studied the electronic structure of the carbonate species with angle-resolved photoemission, and showed that the point group symmetry was C~. When a A g ( l l 0 ) surface is exposed to oxygen, sequential (nx 1) L E E D patterns ( n = 7 to 2) appear. Scanning tunneling microscopy (STM) studies [-9] have revealed that this phenomenon can be explained by the "added row" mechanism. That is, it was shown that one-dimensional A g - O chains, which consisted of alternating silver and oxygen atoms, grew along the [001] direction. The A g - O chains on the A g ( l l 0 ) surface prefer to disperse in the [110] direction, so that a variety of (n x 1) structures appear as the density of A g - O chains increases. In this STM study, we report the microscopic details of the reaction of A g - O added rows with CO2 on a A g ( l l 0 ) surface at room temperature. We will show that the A g - O added rows were compressed from a ( 4 x 1) to a ( 2 x 1) phase, according to the growth of carbonate domains. Although the carbonate domains give a (1 x 2 ) diffraction pattern, STM images of this domain do not show a (1 x 2 ) structure, which indicates that the Ag substrate reconstructs to a (1 x 2 ) structure. Finally, we will show that the A g - O added rows can be selectively decomposed by photoreaction using ultraviolet irradiation. The experiments were carried out in an ultrahigh vacuum apparatus equipped with a STM, fourgrid LEED-AES optics, an Ar ion gun, and a quadrupole mass analyzer. The STM used in this study was a commercial Rasterscope-3000 STM from Danish Micro Engineering. All STM images presented here were recorded in the constant current mode at room temperature using a tungsten tip. Typical tunneling currents were about 0.3 nA with sample bias voltages in the --0.2 to - 2 V range. The A g ( l l 0 ) crystal was cleaned by repeated cycles of Ar + sputtering, anneafing at 620 K for 15min, treating with 02 at 2 x 1 0 - 4 T o r r and 450 K for 30 s, and heating again to 670 K for l m i n . Exposure to oxygen at a pressure of 2 × 1 0 - 4 Torr on a clean A g ( l l 0 ) surface at 470 K
for 30 s leads to a (2 x 1) structure. A (3 x 1) and a (4 x 1)-O/Ag(ll0) surface was obtained by heating the (2 x 1) surface to 510 K for a few minutes. A high pressure mercury lamp (500W) was used for the photo-reactivity studies. The wavelength of the UV light was 310,-~ 510 nm. Fig. la shows a typical STM image of the (4 x 1) phase of A g - O added rows grown along the [-001] direction. When this surface was exposed to a CO2 background pressure at 1 x 10 .7 Torr, in situ STM measurements revealed that the Ag-O chains reacted with CO2 from the ends of the chains, and a new structure appeared. Fig. lb is a STM image obtained after exposure to CO2 for 20min. Corresponding to the growth of the domains of the new structure between the A g - O added rows, the unreacted A g - O added rows are compressed to a (3 x 1) phase. Fig. lc is a STM image obtained after exposure to COe for total 30 min. Now the domains of the new structure cover most of the surface. Concurrently, the remaining Ag-O added rows are compressed to a (2 x 1) phase. By further exposure to CO2, the A g - O added rows completely disappear and the new structure covers all of the surface. The L E E D pattern of this surface shows a (1 x 2 ) structure. Considering the previously rePorted results [ 1 - 8 ] , we conclude that the new structure observed in Figs. lb and lc is the carbonate-related structure. Compression of metal-O chains by coadsorption with other adsorbates also has been reported on N i - O with ammonia [ 10], benzene [11], and hydrogen [12] on a N i ( l l 0 ) surface. During the exposure to CO2, no detectable movement of step edges was observed, nor was island or trough formation observed on wide terraces. This indicates that the density of Ag atoms on the surface does not change drastically during the reaction. Fig. 2 shows STM images obtained after exposure to 105 L (2 x 1 0 - 4 Torr for 500 s) of COz on a ( 3 x l) A g - O / A g ( l l 0 ) surface. The L E E D pattern of this surface was a superimposition of a ( 2 x l ) and a ( l x 2 ) . In Fig. 2, we observe a coexistence of the (2 x 1) domains of unreacted A g - O added rows and the carbonate-related structure. The STM images were strongly dependent on the tip conditions. In Fig. 2a, two types of
Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
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Fig. 2. STM images obtained after exposure to 10s L of CO2 on a Ag(ll0) surface covered by a (3 x 1) phase of Ag-O added rows (86 x 86 ,~2). The coexistence of (2 x 1) domains of Ag-O rows and carbonate structure is observed. In (a), big and small protrusions are resolved in the carbonate domain (sample bias= ~ - 0 . 6 3 V). (b) The image obtained after the tip condition was changed (sample bias= ~ - 0 . 1 9 V ) . The dotted lines are drawn from protrusions in the (2 x 1) domain of Ag-O added rows. The protrusions in the carbonate domain are positioned on the lines.
Fig. 1. (a) A STM image of a Ag(110) surface covered by ( 4 x l ) phase of Ag-O added rows (210x210A2). (b), (c) STM images taken after exposure to 1 x 1 0 - 7 Torr of COz for 20 and 30min, respectively. As the new structure grows between the Ag-O rows, the remaining Ag-O rows are compressed to ( 3 x l ) and ( 2 x l ) phases in (b) and (c), respectively.
2
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Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
protrusions are resolved in the carbonate structure. That is, big protrusions with a height of ,-~1.5 and small protrusions with a height of ,-~1.0 A are resolved. (The height of the Ag-O added rows is about 1.0 A in this figure). The protrusions form rows along the [110] direction. The distance between the rows is two [001] lattice spacings. On the other hand, the distance between the protrusions along the [110] row takes a random value from two to five [110] lattice spacings. These protrusions are very mobile at room temperature and they change their position from image to image. Fig. 2b is a STM image at the same place as in Fig. 2a, obtained after the tip condition was changed. Now the atomic resolution in the (2 x 1) domains of Ag-O added rows is obtained, though the resolution in the carbonate domain becomes worse. As indicated by dotted lines in the figure, the protrusions in the carbonate domain are positioned on the extended lines drawn from protrusions in the (2 x 1) domains of Ag-O chains along the [110] direction. It is very difficult to determine whether silver or oxygen atoms are imaged in the Ag-O added rows in Fig. 2b. However, in the case of Ni-O added rows on Ni(110) and Cu-O added rows on Cu(110), it has been accepted that the STM image corresponds to metal atoms in many cases [13]. Hence it is natural to assume that the Ag atoms are imaged by STM also in the Ag-O added rows in Fig. 2(b). According to this assumption, the positions of both big and small protrusions correspond to hollow sites of the original Ag substrate. Though sharp (1 x 2) spots were observed in the LEED pattern, the (1 x 2) periodicity has never been observed by STM. This indicates that the Ag substrate under the carbonate structure, which could not be observed by STM, is reconstructed to a (1 x 2) structure, giving the diffraction pattern as proposed by Bader et al. [6]. Very recently, Stensgaard et al. [ 14] also observed this structure by STM. In their higher resolution study, they resolved additional details of the (1 x 2) reconstruction. That is, they observed that [ll0]-directed triplets of atoms were added on the surface, and the carbonate ions were seen to reside on top of the triplets. This triplet structure, which is very mobile, primarily along the [1[0] direction, will
explain the observed ( l x 2 ) periodicity in the LEED pattern. The small and big protrusions observed in Fig. 2a can be assigned to the triplets and the carbonate ions on the triplets, respectively, though our STM images do not have enough resolution to resolve the triplet structure. Stensgaard et al. [ 14] indicated that the reconstruction of the surface should include oxygen atoms, which do not become part of the carbonate species. To investigate the chemical propertieS of the small protrusions (or "triplets"), we exposed them to CO or Hz gas at room temperature on a surface covered by the (1 x 2) carbonate structure. As a result, no detectable change was observed on either type of protrusion. Since Ag-O added rows are known to react with CO at room temperature [15], this indicates that the chemical property of the [ ll0]-directed "triplets" is quite different from the normal [001J-directed Ag-O added rows, even if the "triplet" structure contains oxygen atoms. Any diffraction patterns corresponding to the CO3 species themselves have never been reported in the past. However, when CO2 was exposed on a surface covered by (6 x 1) Ag-O rows, we found that very weak streaks run along the [001] direction from the a*/3 position in the LEED pattern, together with the weak (1 x 2) spots. These streaks may suggest that the average distance along the [ 110] direction between carbonate species is three lattice spacings. These streaks were not observed at higher coverages. Fig. 3 shows a STM image obtained after exposure to l x 1 0 -TTorr of CO2 for 10rain on a (3 x 1) Ag-O/Ag(ll0) surface. This figure indicates that the reaction of CO 2 with Ag-O added rows starts heterogeneously on the surface. That is, the formation of the carbonate structure starts preferentially at the upper side of the step edge. The UV irradiation study was performed on a surface on which coexisted the (2x l) Ag-O domain and the ( l x 2 ) carbonate structure domain, as shown in Fig 2, prepared by exposure to 10SL of COz on a (3x l) Ag-O/Ag(ll0) surface. Fig. 4 is one of the STM images obtained after UV light irradiated the surface for 12 min. All STM images obtained at different sample positions are essentially identical to Fig. 4, which shows that the Ag-O rows disappeared and only the
Y. Okawa, K. Tanaka~Surface Science 344 (1995) L1207-L1212
Fig. 3. A STM image obtained after exposure to 1 x 10-7 Torr of CO2 for 10rain on a (3xl) Ag-O/Ag(ll0) surface (260 x 260 ,~z). The formation of the carbonate structure is observed only at the upper side of the step edge.
[1 1 O]
[oo 1 Fig. 4. A STM image (90 x 90A2) obtained after irradiating the surface, on which coexisted the (2 x 1) Ag-O domain and the (1 x2) carbonate structure domain, with UV light. The Ag-O rows disappeared and only the carbonate structure remained. carbonate structure remained. The L E E D pattern of this surface also showed a disappearance of the ( 2 x l ) spot, and ( l x 2 ) spots and very weak streaks at the a*/3 position were observed. Because the CO3 species thermally decompose at a lower
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temperature than A g - O [1,2], the disappearance of A g - O is not a thermal effect of UV irradiation, but a photon-induced decomposition of the A g - O . The appearance of the streaks at the a*/3 position in the L E E D pattern suggests that the remaining carbonates spread over the surface. It should be pointed out that A g - O can be selectively decomposed by U-V irradiation, though the carbonate species thermally decompose at a lower temperature. This fact suggests the possibility of a "molecular mask" technique for surface modification. That is, we can "mask" a part of the A g - O rows by changing them to carbonate species, then decompose the remaining A g - O rows by UV irradiation, and finally we can restore the carbonate to Ag-O again by thermal decomposition. In summary, the formation of carbonate species by a reaction of A g - O added rows with CO2 on a Ag(110) surface was investigated by STM. The A g - O added rows were compressed from a (4 x 1) to a ( 2 x 1) phase, according to the growth of carbonate domains. The difference of the diffraction pattern and the observed periodicity in the STM images indicate that the Ag substrate under the carbonate structure reconstructs to a ( l x 2 ) structure. The A g - O added rows can be selectively decomposed by UV irradiation, though the carbonate species thermally decompose at a lower temperature than does A g - O . The authors acknowledge the Asahi Glass Foundation for the financial support of this work. This work was also supported by a Grant-in-Aid for Scientific Research (05403011) of the Ministry of Education, Science and Culture of Japan.
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