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Spatially controlled oxidation by the tip of a scanning tunneling microscope operating inside a reactor
surface science ELSEVIER
Surface Science 331-333 (1995) 337-342
Spatially controlled oxidation by the tip of a scanning tunneling microscope operating inside a reactor U. SchriSder b,,, B.J. Mclntyre a, M. Salmeron a, G.A. Somorjai
a
a Material Science Division, Lawrence Berkeley Laboratory and Department of Chemistry, University of California, Berkeley, CA 94720, USA b Institut fiir Physikalische und Theoretische Chemie der Universitiit Bonn, Wegelerstrasse 12, 53115 Bonn, Germany
Received 10 August 1994; accepted for publication 25. November 1994
Abstract
The tip of a scanning tunneling microscope that operates inside an atmospheric pressure chemical reactor cell, has been used to locally oxidize carbonaceous fragments deposited on the surface of Pt(lll). The carbon fragments were produced by partial dehydrogenation of propylene. The reactant gas environment inside the cell consisted of pure 0 2 at 300 K. The Pt/Rh tip acted as a catalyst after activation by short voltage pulses. In this active state the clusters in the area scanned by the tip were reacted away with very high spatial resolution. Keywords: Alkanes; Alkenes; Catalysis; Low index single crystal surfaces; Platinum; Scanning tunneling microscopy; Surface chemical
reaction 1. Introduction
The interaction of oxygen with hydrocarbon clusters on a metal surface has been studied for a very long time due to its importance in a large number of catalytic reactions [1]. The processes by which these reactions occur still lack complete understanding. One reason for this is that at present, conventional tools and methods are limited to studying the local catalytic activity of surface sites and defects in an average way. With the development of a specialized version of the STM in our laboratory [2,3], it is now possible to study catalysis on surfaces on an atomic scale under controlled environments ranging from U H V to atmospheric pressures.
* Corresponding author. Fax: +49 228 732551.
The first experiments demonstrating the catalytic action of an STM tip were carded out in hydrogen containing environments [4]. In these experiments, carbonaceous fragments were first deposited on the surface of P t ( l l l ) by partial dehydrogenation of propylene. The reactor cell was then filled with pure hydrogen or a mixture of hydrogen(90%)/propylene(10%) up to a pressure of 1 atm. After activation of the tip by applying a voltage pulse (to remove contaminant material), the STM tip catalyzed with nanometer spatial resolution the rehydrogenation of the carbonaceous species. In these experiments it was observed that the activated tip had a definite 'lifetime' beyond which the tip-induced catalysis would no longer occur until another voltage ( ~ 3 V) pulse was applied. From these experiments, a model was proposed where the tip serves as a source of atomic hydrogen. The hydrogen is transferred to the
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00205-7
338
U. SchriSderet al. / Surface Science 331-333 (1995) 337-342
hydrocarbon fragments on the surface when the tip is brought into their proximity. One of the questions that arises from these results is whether or not the tip catalysis effect will only occur in hydrogen environments. We thus explored other possible reactions to determine the generality of this phenomenon. The oxidation of hydrocarbons on Pt is widely acknowledged to have lower activation barriers than hydrogenolysis of similar species [5]. The activation energy for hydrogenolysis is typically of the order of 50-80 kcal/mol whereas the activation energy for oxidation is approximately 15 kcal/mol. Similarly, both oxygen and hydrogen readily dissociatively chemisorb on Pt at room temperature [6]. From this perspective, it might be expected that if it is possible for STM tip-induced hydrogenolysis to occur, then STM tip-induced oxidation is also very likely and may even happen more readily. We present in this paper recent results that confirm this idea.
2. Experimental The experiments were carried out with a STM in a reactor cell, which has been described recently [2,3]. The Pt crystal was oriented in the (111) direction, however, a miscut angle of approximately 1.3 ° from this ideal direction resulted in a stepped surface having a terrace width of ~ 100 ~,. The tips were made of Pt(80%)-Rh(20%) wires etched in a NaCI/NaNO 3 solution. The sample was cleaned in a separate UHV system [7] (10-10 mbar base pressure) by Ar + ion bombardment and annealing in oxygen pressure. The cleanliness was checked by Auger electron spectroscopy (AES). Once clean and ordered, the crystal was protected from contamination by a monolayer of sulfur that formed a (v~× v~')R30 ° ordered structure. The sample was transferred to the reactor cell in a small transfer chamber at 10 - 6 mbar pressure. Once in the cell the protective S layer was removed by heating in a pure oxygen pressure of 1 atm. The oxygen was pumped down to 10 -5 mbar and a mixture of hydrogen (90%)/propylene (10%) was admitted to the chamber so that the hydrogen removed the oxygen from the surface and the adsorbed propylene forms ordered propylidyne structures under room temperature
conditions. Carbonaceous clusters were produced on the P t ( l l l ) surface by heating in vacuum (after pumping away the propylene/H 2 gas mixture) or after backfilling the cell with pure CO. To observe the tip induced catalysis, the cell was then filled up to one atmosphere of pure oxygen.
3. Results Images of the hydrocarbon clusters on the Pt surface show round islands having uniform diameters of ~ 100 A (see Fig. 1). These fragments consist o f CxHy fragments and occupy the entire width of the platinum terraces. Furthermore, a bimodal height distribution of the clusters were observed. The height values have been normalized by comparison with the height of the Pt step ( ~ 2.27 .A) in the same image (see Fig. 2). The 'low' clusters have a height of about 1 / 2 a Pt monoatomic step and the 'high' clusters have a height of approximately 1 Pt monoatomic step. The height of the lower clusters is in good agreement with the size of a carbon atom. It may be that the bimodal distribution is the result of one and two atom high particles on the surface. Experiments on decomposed ethylene [8] show a similar bimodal distribution.
~3t3 & Fig. 1. Scanning tunneling microscopy image of a Pt(111) surface covered with hydrocarbon species generated by exposure to propylene gas. The 1500× 1500 A image was taken in constant height mode at 0.1 V bias and 1 nA current.
U. Schr6der et al. / Surface Science 331-333 (1995) 337-342
After filling the reactor cell with pure 0 2, the hydrocarbon clusters were again imaged (Fig. 3a). A voltage pulse was applied to the tip and the area shown in Fig. 3a was imaged repeatedly. We observed a progressive removal of the hydrocarbon fragments as the image in Fig. 3b clearly illustrates. Throughout these experiments, this hydrocarbon removal was repeated many times, although the hydrocarbon clusters were never completely removed from a region, It was not possible to determine whether the hydrocarbons were removed more or less easily
I
3
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I
a~
I
in oxygen or in a hydrogen environment. It was observed, however, that once the tip was 'activated' in an oxygen atmosphere, it tended to remain active longer than when in an atmospheric pressure of propylene (10%)/hydrogen (90%). It is not yet clear, however, whether the tip lifetime in the propylene/ hydrogen environment is due to the presence of propylene in the background. In an attempt to better understand the proposed catalytic action of the STM tip, we have performed the following additional experiments [9]: (1) We
i
I
hydrocarboncluster 1/2 P~step height
2 ¸
.<
J
1
339
I
3 b)
I
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I
hydrocarboncluster 1 Pt st~/height
..~. 1
no
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-2
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,
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-3
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100
150
200
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Fig. 2. STM image showing 'low' and 'high' clusters. (a) Profile of a 'low' cluster which has a height of approximately 1 / 2 a monoatomic Pt step height (close to the carbon atom size). (b) Line cursor profile showing a cluster which has a height of approximately 1 Pt monoatomic step height or two carbon atoms. Throughout these experiments, a bimodal height distribution of the clusters was observed.
340
U. Schrfder et al. / Surface Science 331-333 (1995) 337-342 20
ascertained that the hydrocarbon clusters were not removed after bias pulsing in any case when the platinum STM tip was replaced with a gold tip. (2) The tip catalysis does not appear to depend upon electric field or polarity, as checked by changing the bias voltage. (3) A significant pressure dependence is
16
12 r-
8
4
0 . . . .
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. . . .
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,,,1
. . . .
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. . . .
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. . . .
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pressure [mbar] Fig. 4. Pressure dependence of the catalysis by a STM tip. The plot shows the number of scans needed to remove ~ 80% of the clusters from the surface as a function of 0 2 pressure.
Fig. 3. The catalytic action of the STM Pt tip is illustrated in these two images (1000x 1000/~). (a) Surface prior to bias pulsing that activates the tip, clearly showing a high coverage of hydrocarbons. (b) Image of the same area after one scan with an activated tip (by application of a voltage pulse of ~ 3 V).
observed such that the tip catalysis slows down as the 02 pressure is reduced. To quantify this effect we measured the number of scans over a given area that were necessary to remove 80% of the hydrocarbon clusters. Fig. 4 displays these results. At 1000 mbar oxygen pressure the surface is almost clean with one single scan. With decreasing pressure a higher number of scans were needed to remove 80% of the islands. At 1 × 10 -2 mbar, more than 15 scans were required. By varying the tip-surface distance we could show that the tip catalysis is enhanced when the tip is closer to the surface, indicating a proximity effect as would be required for the transfer of O atoms to the surface. One can dismiss also the possibility of the tip mechanically removing or displacing the clusters since large areas ( > 5000 A on a side) were cleaned of clusters by an active tip without visible deposits of material being observed at the edges of the empty areas. Also, in UHV and CO backgrounds or using Au tips (or inactive Pt tips), no mechanical removal was observed at similar tunneling gaps.
U. Schri~deret al. / Surface Science 331-333 (1995) 337-342
4. Discussion Previous studies of the reaction of oxygen with pyrolytic graphite by modulated molecular beammass spectrometry have shown that molecular oxygen will react with graphite to form CO and to a lesser extent CO 2, even at room temperature [10]. We believe that in our experiments, the catalytic action by the active tip, which is presumably clean P t / R h on a small area at the end of the tip (cleaned by the bias voltage pulse), consists of atomizing O 2 from the gas phase and oxidizing C bonds of the clusters under it. Once reacted, the cluster is dissolved and remains invisible to the STM. This could be due to either desorption of the reaction products or to their rapid diffusion on the surface, like the propylidyne and ethylidyne fragments that remain invisible at room temperature to the STM. While the mechanism for oxygen transfer to the hydrocarbon fragments on the surface is unknown, it appears to be similar to the case of hydrogen, which requires the tip to be in close proximity to the clusters on the surface. Rough estimates of the tip height over the Pt surface would place the tip at approximately 5 ,~ over the Pt substrate. Because of the presence of the hydrocarbons, it is possible that the tip is closer than this (because of enhanced overlap with the more localized bonds of the hydrocarbon species), which means that the tip could be in van der Waals contact with the molecules on the surface. Therefore, it may be that when the oxygen atom is at the end of the tip it is in contact with the molecules on the surface, thus inducing oxidation. Since the residence time of the tip over the area occupied by one cluster is of the order of milliseconds at our scanning speeds, it is clear that the turnover frequency of the O transfer reaction from the tip is approximately 10 3 (s Pt site) -1 (assuming the atoms are being transferred from a single Pt atom on the STM tip). The pressure dependence of the catalysis showed that the effect slowed down with decreasing oxygen pressure. Lets assume the case where an oxygen atom adsorbs and dissociates on the Pt tip without significant oxygen diffusion on the tip (because the pulsing of the tip cleans only a small area and the remainder of the tip is contaminated, thus hindering O mobility). Then, according to kinetic gas theory
the number of particles ( d n / d t ) surface at 1 cm 2 s-1 is given by dn - -
dt
341 sticking on the
SP ~ S Z
~-
~
'
where S is the sticking probability, Z the rate of collision per area, P the pressure in mbar and m the mass of gas particle. Assuming a hydrocarbon monolayer capacity close to the Pt area density of 1015 Pt atomsfcm 2, a sticking probability of S -- 0.1 on a Pt foil [11] and a scan velocity of 0.2 ms per Pt site, the equation yields a critical pressure of 10 -2 mbar. At lower pressure the tip cannot provide one oxygen atom to every hydrocarbon molecule on the surface. This is in good agreement with the experimental measurements if we consider that the probability for one oxygen atom to stick on the tip is reduced by geometrical shadowing of the surface and other parts of the tip.
5. Summary In this work we have observed the oxidation of hydrocarbon adsorbates with nanometer spatial resolution by a catalytically active STM tip. It is possible that molecular oxygen dissociated on the P t / R h tip, which is cleaned (activated) by a short bias pulse of ~ 3 V. We propose that atomic oxygen at the end of the tip, in close proximity with the hydrocarbon clusters, causes their oxidization. Once reacted, the hydrocarbon molecule 'dissolves' (desorbing or diffusing), and becomes invisible to the STM. These results demonstrating the catalytic action of the STM tip open the way for very exciting experiments and applications from gaining insights to fundamental questions of catalysis and surface reactions by allowing the study of the local catalytic activity of surface sites and defects, to performing chemical lithography with atomic precision. Obviously the tip-induced catalysis effect should be further investigated. In particular, it would be very interesting to attempt to react away atomic or molecular adsorbates which could be resolved with the STM. The possibility of performing site specific reaction studies on catalytic systems which are known to be structure sensitive is extremely appealing. Other possible ex-
342
U. Schr6der et al. / Surface Science 331-333 (1995) 337-342
periments include replacing the substrate with a metal other than Pt to investigate the degree to which the tip is able to catalyze a reaction when the substrate is not a catalytically active one.
Acknowledgments
This work was supported by the German Academic Exchange Service (DAAD) and by the Director, Office of Energy Research, Office of Basic Energy Science, Material Science Division, of the US Department of Energy under contract No. DEAC03-76SF00098.
References [1] G.A. Somorjai, Chemistry in Two Dimensions: Surfaces (Cornell Univ. Press, Ithaca, 1981).
[2] B.J. Mclntyre, M. Salmeron and G.A. Somorjai, Rev. Sci. Instrum. 64 (1993) 687. [3] B.J. Mclntyre, M. Salmeron and G.A. Somorjai, J. Vac. Sci. Technol. A 11 (1993) 1964. [4] B.J. Mclntyre, M. Salmeron and G.A. Somorjai, Science 265 (1994) 1415. [5] J.M. Thomas and W.J. Thomas, Introduction to the Principles of Heterogeneous Catalysis (Academic Press, London, 1967). [6] K. Christmann, G. Ertl and T. Pignet, Surf. Sci. 54 (1976) 365; F.P. Netzer and G. Kneringer, Surf. Sci. 51 (1975) 526; S. Ferrer and J.P. Bonzel, Surf. Sci. 119 (1982) 234; B. Lang, R.W. Joyner and G.A. Somorjai, Surf. Sci. 30 (1972) 454. [7] D.M. Zeglinski, D.F. Ogletree, T.P. Beebe, Jr., R.Q. Hwang, G.A. Somorjai and M.B. Salmeron, Rev. Sci. Instrum. 61 (1990) 3769. [8] T.A. Land, T. Michely, R.J. Behm, J.C. Hemminger and G. Comsa, J. Chem. Phys. 97 (1992) 6774. [9] U. Schr6der, B.J. Mclntyre, M. Salmeron and G.A. Somorjai, to be published [10] D.R. Olander, R.H. Jones, J.A. Schwarz and W.J. Siekhaus, J. Chem. Phys. 57 (1972) 409. [11] H,P. Bonzel and R. Ku, Surf. Sci. 40 (1973) 85.
Surface Science 331-333 (1995) 337-342
Spatially controlled oxidation by the tip of a scanning tunneling microscope operating inside a reactor U. SchriSder b,,, B.J. Mclntyre a, M. Salmeron a, G.A. Somorjai
a
a Material Science Division, Lawrence Berkeley Laboratory and Department of Chemistry, University of California, Berkeley, CA 94720, USA b Institut fiir Physikalische und Theoretische Chemie der Universitiit Bonn, Wegelerstrasse 12, 53115 Bonn, Germany
Received 10 August 1994; accepted for publication 25. November 1994
Abstract
The tip of a scanning tunneling microscope that operates inside an atmospheric pressure chemical reactor cell, has been used to locally oxidize carbonaceous fragments deposited on the surface of Pt(lll). The carbon fragments were produced by partial dehydrogenation of propylene. The reactant gas environment inside the cell consisted of pure 0 2 at 300 K. The Pt/Rh tip acted as a catalyst after activation by short voltage pulses. In this active state the clusters in the area scanned by the tip were reacted away with very high spatial resolution. Keywords: Alkanes; Alkenes; Catalysis; Low index single crystal surfaces; Platinum; Scanning tunneling microscopy; Surface chemical
reaction 1. Introduction
The interaction of oxygen with hydrocarbon clusters on a metal surface has been studied for a very long time due to its importance in a large number of catalytic reactions [1]. The processes by which these reactions occur still lack complete understanding. One reason for this is that at present, conventional tools and methods are limited to studying the local catalytic activity of surface sites and defects in an average way. With the development of a specialized version of the STM in our laboratory [2,3], it is now possible to study catalysis on surfaces on an atomic scale under controlled environments ranging from U H V to atmospheric pressures.
* Corresponding author. Fax: +49 228 732551.
The first experiments demonstrating the catalytic action of an STM tip were carded out in hydrogen containing environments [4]. In these experiments, carbonaceous fragments were first deposited on the surface of P t ( l l l ) by partial dehydrogenation of propylene. The reactor cell was then filled with pure hydrogen or a mixture of hydrogen(90%)/propylene(10%) up to a pressure of 1 atm. After activation of the tip by applying a voltage pulse (to remove contaminant material), the STM tip catalyzed with nanometer spatial resolution the rehydrogenation of the carbonaceous species. In these experiments it was observed that the activated tip had a definite 'lifetime' beyond which the tip-induced catalysis would no longer occur until another voltage ( ~ 3 V) pulse was applied. From these experiments, a model was proposed where the tip serves as a source of atomic hydrogen. The hydrogen is transferred to the
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00205-7
338
U. SchriSderet al. / Surface Science 331-333 (1995) 337-342
hydrocarbon fragments on the surface when the tip is brought into their proximity. One of the questions that arises from these results is whether or not the tip catalysis effect will only occur in hydrogen environments. We thus explored other possible reactions to determine the generality of this phenomenon. The oxidation of hydrocarbons on Pt is widely acknowledged to have lower activation barriers than hydrogenolysis of similar species [5]. The activation energy for hydrogenolysis is typically of the order of 50-80 kcal/mol whereas the activation energy for oxidation is approximately 15 kcal/mol. Similarly, both oxygen and hydrogen readily dissociatively chemisorb on Pt at room temperature [6]. From this perspective, it might be expected that if it is possible for STM tip-induced hydrogenolysis to occur, then STM tip-induced oxidation is also very likely and may even happen more readily. We present in this paper recent results that confirm this idea.
2. Experimental The experiments were carried out with a STM in a reactor cell, which has been described recently [2,3]. The Pt crystal was oriented in the (111) direction, however, a miscut angle of approximately 1.3 ° from this ideal direction resulted in a stepped surface having a terrace width of ~ 100 ~,. The tips were made of Pt(80%)-Rh(20%) wires etched in a NaCI/NaNO 3 solution. The sample was cleaned in a separate UHV system [7] (10-10 mbar base pressure) by Ar + ion bombardment and annealing in oxygen pressure. The cleanliness was checked by Auger electron spectroscopy (AES). Once clean and ordered, the crystal was protected from contamination by a monolayer of sulfur that formed a (v~× v~')R30 ° ordered structure. The sample was transferred to the reactor cell in a small transfer chamber at 10 - 6 mbar pressure. Once in the cell the protective S layer was removed by heating in a pure oxygen pressure of 1 atm. The oxygen was pumped down to 10 -5 mbar and a mixture of hydrogen (90%)/propylene (10%) was admitted to the chamber so that the hydrogen removed the oxygen from the surface and the adsorbed propylene forms ordered propylidyne structures under room temperature
conditions. Carbonaceous clusters were produced on the P t ( l l l ) surface by heating in vacuum (after pumping away the propylene/H 2 gas mixture) or after backfilling the cell with pure CO. To observe the tip induced catalysis, the cell was then filled up to one atmosphere of pure oxygen.
3. Results Images of the hydrocarbon clusters on the Pt surface show round islands having uniform diameters of ~ 100 A (see Fig. 1). These fragments consist o f CxHy fragments and occupy the entire width of the platinum terraces. Furthermore, a bimodal height distribution of the clusters were observed. The height values have been normalized by comparison with the height of the Pt step ( ~ 2.27 .A) in the same image (see Fig. 2). The 'low' clusters have a height of about 1 / 2 a Pt monoatomic step and the 'high' clusters have a height of approximately 1 Pt monoatomic step. The height of the lower clusters is in good agreement with the size of a carbon atom. It may be that the bimodal distribution is the result of one and two atom high particles on the surface. Experiments on decomposed ethylene [8] show a similar bimodal distribution.
~3t3 & Fig. 1. Scanning tunneling microscopy image of a Pt(111) surface covered with hydrocarbon species generated by exposure to propylene gas. The 1500× 1500 A image was taken in constant height mode at 0.1 V bias and 1 nA current.
U. Schr6der et al. / Surface Science 331-333 (1995) 337-342
After filling the reactor cell with pure 0 2, the hydrocarbon clusters were again imaged (Fig. 3a). A voltage pulse was applied to the tip and the area shown in Fig. 3a was imaged repeatedly. We observed a progressive removal of the hydrocarbon fragments as the image in Fig. 3b clearly illustrates. Throughout these experiments, this hydrocarbon removal was repeated many times, although the hydrocarbon clusters were never completely removed from a region, It was not possible to determine whether the hydrocarbons were removed more or less easily
I
3
•
I
I
a~
I
in oxygen or in a hydrogen environment. It was observed, however, that once the tip was 'activated' in an oxygen atmosphere, it tended to remain active longer than when in an atmospheric pressure of propylene (10%)/hydrogen (90%). It is not yet clear, however, whether the tip lifetime in the propylene/ hydrogen environment is due to the presence of propylene in the background. In an attempt to better understand the proposed catalytic action of the STM tip, we have performed the following additional experiments [9]: (1) We
i
I
hydrocarboncluster 1/2 P~step height
2 ¸
.<
J
1
339
I
3 b)
I
I
I
I
hydrocarboncluster 1 Pt st~/height
..~. 1
no
\
.0')
no
\
-1
-2
-3
-2 I
0
~
I
50
,
I
1O0
i
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I
150
200
distance
[A]
,
I
250
-3
I
I
I
I
I
0
50
100
150
200
i
I
250
distance[,~]
Fig. 2. STM image showing 'low' and 'high' clusters. (a) Profile of a 'low' cluster which has a height of approximately 1 / 2 a monoatomic Pt step height (close to the carbon atom size). (b) Line cursor profile showing a cluster which has a height of approximately 1 Pt monoatomic step height or two carbon atoms. Throughout these experiments, a bimodal height distribution of the clusters was observed.
340
U. Schrfder et al. / Surface Science 331-333 (1995) 337-342 20
ascertained that the hydrocarbon clusters were not removed after bias pulsing in any case when the platinum STM tip was replaced with a gold tip. (2) The tip catalysis does not appear to depend upon electric field or polarity, as checked by changing the bias voltage. (3) A significant pressure dependence is
16
12 r-
8
4
0 . . . .
104
I
. . . .
10-3
t
,
,,,1
. . . .
10-2 10-1
I
. . . .
100
I
. . . .
10 ~
I
. . . .
102
I
. . . .
103
104
pressure [mbar] Fig. 4. Pressure dependence of the catalysis by a STM tip. The plot shows the number of scans needed to remove ~ 80% of the clusters from the surface as a function of 0 2 pressure.
Fig. 3. The catalytic action of the STM Pt tip is illustrated in these two images (1000x 1000/~). (a) Surface prior to bias pulsing that activates the tip, clearly showing a high coverage of hydrocarbons. (b) Image of the same area after one scan with an activated tip (by application of a voltage pulse of ~ 3 V).
observed such that the tip catalysis slows down as the 02 pressure is reduced. To quantify this effect we measured the number of scans over a given area that were necessary to remove 80% of the hydrocarbon clusters. Fig. 4 displays these results. At 1000 mbar oxygen pressure the surface is almost clean with one single scan. With decreasing pressure a higher number of scans were needed to remove 80% of the islands. At 1 × 10 -2 mbar, more than 15 scans were required. By varying the tip-surface distance we could show that the tip catalysis is enhanced when the tip is closer to the surface, indicating a proximity effect as would be required for the transfer of O atoms to the surface. One can dismiss also the possibility of the tip mechanically removing or displacing the clusters since large areas ( > 5000 A on a side) were cleaned of clusters by an active tip without visible deposits of material being observed at the edges of the empty areas. Also, in UHV and CO backgrounds or using Au tips (or inactive Pt tips), no mechanical removal was observed at similar tunneling gaps.
U. Schri~deret al. / Surface Science 331-333 (1995) 337-342
4. Discussion Previous studies of the reaction of oxygen with pyrolytic graphite by modulated molecular beammass spectrometry have shown that molecular oxygen will react with graphite to form CO and to a lesser extent CO 2, even at room temperature [10]. We believe that in our experiments, the catalytic action by the active tip, which is presumably clean P t / R h on a small area at the end of the tip (cleaned by the bias voltage pulse), consists of atomizing O 2 from the gas phase and oxidizing C bonds of the clusters under it. Once reacted, the cluster is dissolved and remains invisible to the STM. This could be due to either desorption of the reaction products or to their rapid diffusion on the surface, like the propylidyne and ethylidyne fragments that remain invisible at room temperature to the STM. While the mechanism for oxygen transfer to the hydrocarbon fragments on the surface is unknown, it appears to be similar to the case of hydrogen, which requires the tip to be in close proximity to the clusters on the surface. Rough estimates of the tip height over the Pt surface would place the tip at approximately 5 ,~ over the Pt substrate. Because of the presence of the hydrocarbons, it is possible that the tip is closer than this (because of enhanced overlap with the more localized bonds of the hydrocarbon species), which means that the tip could be in van der Waals contact with the molecules on the surface. Therefore, it may be that when the oxygen atom is at the end of the tip it is in contact with the molecules on the surface, thus inducing oxidation. Since the residence time of the tip over the area occupied by one cluster is of the order of milliseconds at our scanning speeds, it is clear that the turnover frequency of the O transfer reaction from the tip is approximately 10 3 (s Pt site) -1 (assuming the atoms are being transferred from a single Pt atom on the STM tip). The pressure dependence of the catalysis showed that the effect slowed down with decreasing oxygen pressure. Lets assume the case where an oxygen atom adsorbs and dissociates on the Pt tip without significant oxygen diffusion on the tip (because the pulsing of the tip cleans only a small area and the remainder of the tip is contaminated, thus hindering O mobility). Then, according to kinetic gas theory
the number of particles ( d n / d t ) surface at 1 cm 2 s-1 is given by dn - -
dt
341 sticking on the
SP ~ S Z
~-
~
'
where S is the sticking probability, Z the rate of collision per area, P the pressure in mbar and m the mass of gas particle. Assuming a hydrocarbon monolayer capacity close to the Pt area density of 1015 Pt atomsfcm 2, a sticking probability of S -- 0.1 on a Pt foil [11] and a scan velocity of 0.2 ms per Pt site, the equation yields a critical pressure of 10 -2 mbar. At lower pressure the tip cannot provide one oxygen atom to every hydrocarbon molecule on the surface. This is in good agreement with the experimental measurements if we consider that the probability for one oxygen atom to stick on the tip is reduced by geometrical shadowing of the surface and other parts of the tip.
5. Summary In this work we have observed the oxidation of hydrocarbon adsorbates with nanometer spatial resolution by a catalytically active STM tip. It is possible that molecular oxygen dissociated on the P t / R h tip, which is cleaned (activated) by a short bias pulse of ~ 3 V. We propose that atomic oxygen at the end of the tip, in close proximity with the hydrocarbon clusters, causes their oxidization. Once reacted, the hydrocarbon molecule 'dissolves' (desorbing or diffusing), and becomes invisible to the STM. These results demonstrating the catalytic action of the STM tip open the way for very exciting experiments and applications from gaining insights to fundamental questions of catalysis and surface reactions by allowing the study of the local catalytic activity of surface sites and defects, to performing chemical lithography with atomic precision. Obviously the tip-induced catalysis effect should be further investigated. In particular, it would be very interesting to attempt to react away atomic or molecular adsorbates which could be resolved with the STM. The possibility of performing site specific reaction studies on catalytic systems which are known to be structure sensitive is extremely appealing. Other possible ex-
342
U. Schr6der et al. / Surface Science 331-333 (1995) 337-342
periments include replacing the substrate with a metal other than Pt to investigate the degree to which the tip is able to catalyze a reaction when the substrate is not a catalytically active one.
Acknowledgments
This work was supported by the German Academic Exchange Service (DAAD) and by the Director, Office of Energy Research, Office of Basic Energy Science, Material Science Division, of the US Department of Energy under contract No. DEAC03-76SF00098.
References [1] G.A. Somorjai, Chemistry in Two Dimensions: Surfaces (Cornell Univ. Press, Ithaca, 1981).
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