Scanning tunneling microscopy of oxygen adsorption on the Ag(110) surface

Scanning tunneling microscopy of oxygen adsorption on the Ag(110) surface

Surface Science 266 (1992) 282-284 North-Holland surface science Scanning tunneling microscopy of oxygen adsorption on the Ag(ll0) surface T o m i h...

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Surface Science 266 (1992) 282-284 North-Holland

surface science

Scanning tunneling microscopy of oxygen adsorption on the Ag(ll0) surface T o m i h i r o H a s h i z u m e a, M. T a n i g u c h i b K. M o t a i a, H u a L u a, K. T a n a k a b a n d T. S a k u r a i a "Institute for Materials Research (IMR), Tohoku University, Sendai 980, Japan b The institute for Solid State Physics, The Unieersity of Tokyo, Minato-ku~ Tokyo 106, Japan Received 5 August 1991; accepted for publication 9 August 1991

Our high-performance UHV-mode FI-STM/STS (field ion-scanning tunneling microscopy/spectroscopy) was used for the analysis of oxygen adsorption on the Ag(110) clean surface. When the surface was exposed to oxygen, one-dimensional linear chains with various separation widths were observed, corresponding to the (n × 1) LEED patterns. It was concluded that these linear chains consist of A g - O - A g components, similar to the case of the Cu(110)-O system.

1. Introduction A large number of STM investigations on semiconductor surfaces have been reported and great progress has been made in semiconductor physics through the use of the STM [1,2]. Nevertheless, there have been only a handful of STM studies on metal surfaces reported. The metal surfaces that have been studied successfully so far by STM have been limited to Au(110/111/100) [3-7], Pt(ll0) [8,9], Ni(ll0) [10], and C u ( l l 0 / 111/100) [11-20]. The most probable leason is that the typical corrugation of a metal surface is too small ( < 0.1 A) for sufficient resolution in the case of most STM's in operation. It is well-'~own that the resolution of the STM is almost exclusively determined by the atomic configurations of a scanning tip, which is rather difficult to characterize as well as control on an atomic scale [21-23]. The field ion-scanning tunneling microscope (FI-ST~'..) was designed and con:tructed with a major aim of shaping a tip into an ideal single a t o m tip in nrdm" t,~ ~,-h;,~.,e a . . . . , , - , ~ . , , . ~ . u reproduceable atomic resolution in the S'IM [24,25]. We report on our successful application of the FI-STM to a metal surface: the A g ( l l 0 ) 1 × 1 surface and its exposure to molecular oxygen. SiNer is known to be a unique "real-world" in-

dustrial catalyst for the epoxidation of ethylene [26,27]. Therefore, extensive investigations have been carried out in order to understand interactions of atomic and molecular oxygen on the silver surface [27]. Those studies showed that the sticking coefficient of oxygen is rather small initially, of the order of 10 -3 , and decreases even further with increasing coverage. This finding is difficult to comprehend when one recalls that oxygen permeates silver quite readily. Indeed, it is quite wellknown that silver is used as an ox-ygcn diffuser in the U H V research because of its high permeability. There has been another intriguing observation with respect to the LEED study of this surface. The LEED pattern changes with coverage from 7 × 1 to 6 × 1 , 5 × 1 , 4 × 1 , 3 x l and 2 x 1 finally [28].

2. Experimental procedure The Ag(110) specimen mas prepared by cutting a m d of single crystal and the clean surface was produced using the standard procedure. After being introduced into the FI-STM UHV (2 × 11~-'- ..... - t~ Torr) ,.n~muer, the specimen was Atsputtered (500 V bias potential, 3-4 #.A ion current, 5 × 10 -5 Torr Ar pressure) for !0 min

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T. Hashizume et aL / STM of oxygen adsorption on Ag(110)

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and annealed at 400°C for 10 min in the presence of 1 x 10 - 7 Torr 0 2 to remove any carbon residue. After several cycles of this cleaning process, AES spcctra showed little presence of impurities except ox~sen. The oxygen layer could be removed by annealing at 480°C in the U H V condition ( < 2 x 10 -1° Torr) for 10 min and the clean Ag(ll0) surface was obtained with its characteristic 1 x 1 L E E D pattern. A P t ( l l l ) tip was used for the STM study of the A g ( l l 0 ) - O system, after we found that the W tip upon oxidation ,~aused adverse effects on STM images.

3. Experimental results and discussion

I Fig. 1 is a typical STM micrograph of the clean Ag(ll0)l x 1 surface. The rows rm~ning alon~ the (110) direction are separated by 4.0 ( + 0.3) A, in agreement with crystallographic data of the Ag crystal. Individual Ag atoms are clearly resolved along the rows with a minimum spacing of 2.8 A. The measured corrugation is approximately 0.2 ~, 4.09A

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t 10A i Fig. !. A typical STM image of the clean Ag(ll0) l x l surface.

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Fig. 2. A STM image of the Ag(ll0) surface exposed to 500 I oxygen, showing both weak and bright linear chains along the (001) direction.

along the Ag rows in this image, which is only 5% of that (t..4 A) of the Si(lll)7 × 7 surface. As reported by Kuk et al. with respect to the case of Cu [11], the monoatomic steps of the clean Ag surface are not stable and flow considerably. This Ag surface was exposed to oxTgen in a pressure range of l0 -9 Torr and 10 -~ T o r t at room temperature, and the surface structural change was continuously monitored by the STM. As soon as the A g i l l 0 ) 1 × 1 surface was exposed to oxygen, linear chains of atom~ were obseiwed to form in the (001) direction, which is perpendicular to the Ag atom rows on this surface. As the oxygen exposure was increased, the spacings of the linear chains became narrower although there was some variation noted in the spacing. An example of the STM images is shown in fig. 2a. Both bright and weak linear lines run along the (001) direction. There are considerable amounts of Ag surface vacancy patches. The brighter chains were found to be rather unstable. These iines were quite mobile and some segments shifted sideways even during the STM tip scanning. We speculate that the weaker lines in fig. 2 consist of A g - O - A g - O - A g - - - A g - O - A g on top of the original surface Ag layer in the (001)

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T. Ha.~hizume et al. / STM of oxygen adsorption tm Ag(l lO)

direction and the brighter lines consists of another type of atomic oxygens sitting over the weaker lines. This model is new to the Ag(110)-O system and is capable to account all the experimental observations as well as its intriguing catalyst action. The assignment on the weaker lines is essentially identical to the one, now being established for the C u ( l l 0 ) - O system (the a d d e d row model) [12]. The rather small sticking coefficient of O and its further decrease with cove.rage can be explained as follows: Although O can move almost freely on the surface and permeate into the bulk of silver, each oxygen must find at least one mobile Ag atom before becoming stabilized. These free Ag atoms must come from the step or must be created by vacancy formation. Once t~e added rows are formed, the supply of free Ag atoms are further limited because Ag can move only within the region surrounded by the added rows. Detailed study is currently underway. The brighter lines can only form after the weaker atomic chemisorption lines were established on the surf:ee. The coverage of the molecular oxygen is a strong function of the O 2 gas pressure. When the O 2 gas supply was terminated, these brighter lines became blurred and disappeared, exposing the underlying A g - O - A g weaker lines. The second type of atomic oxygen is loosely bound to the surface, and thus, can be dispensed readily for the epoxidation of ethylene. tn conclusion, we have successfully applied our FI-STM for the "real-world" catalyst silver surface and its interaction with oxygen. Atomic-resolution STM images of the Ag(ll0) 1 × 1 surface were obtained for the first time and the "added row model" was proposed for the A g ( l l 0 ) - O system.

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