- Email: [email protected]
An ISS and AES examination of the interaction of oxygen with platinum-tin alloy
Surface Science 161 (1985) L583-L590 *North-Holland, Amsterdam
SURFACE
SCIENCE
L583
LETTERS
AN ISS AND AES EXAMINATION OF THE INTERACTION WITH A PLATINUM-TIN ALLOY Gar B. HOFLUND Department
and Douglas
OF OXYGEN
A. ASBURY
of Chemical Engineering, University of Florida, Gainesville, Florida 3261 I, USA
and Piotr KIRSZENSZTEJN Department
of Chemistry,
* and Herbert
A. LAITINEN
University of Florida, Gainesville, Florida 32611, USA
Received 28 March 1985; accepted for publication
13 June 1985
In this study ISS and AES were used to examine a platinum-tin alloy surface before and after exposure to oxygen. Annealing increases the tin-to-platinum concentration ratio at the surface while sputtering causes a decrease in this ratio. The ISS results suggest that the oxygen interacts with the platinum and not the tin.
Platinum-tin systems are very important as hydrocarbon reforming catalysts when supported on alumina. Compared to alumina-supported platinum, the bimetallic catalyst has improved stability and catalytic activity toward many of the reforming reactions. Much controversy exists in the literature concerning the state of the tin and the form of the platinum-tin interaction. Several studies suggest that a platinum-tin alloy is formed [l-3] while others suggest that the tin is not reduced to metal but exists as SnO, SnO, or a compound containing tin, aluminum and oxygen [4-71. In the case of platinum crystallites supported on a SnO, substrate, an Sn-0-Pt bond has been identified using ESCA [8,9]. The Sn-0-Pt species bonds the crystallite to the support and may be responsible for the alteration of the catalytic properties in the bimetallic platinum-tin catalysts. Platinum alloyed with palladium, iridium, ruthenium and tin have been studied previously with ESCA and AES. Bouwman, Toneman and Holscher [lo] used AES to show that the composition of the near surface region of a Pt/Sn alloy can be modified drastically by a number of treatments. Sputtering or exposure to hydrogen enriches the surface region in platinum while anneal* Permanent address: Department
of Chemistry, University
of A. Mickiewicz,
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Poznan, Poland.
L584
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
ing or exposure to oxygen enriches the surface region in tin. They also demonstrated that oxygen adsorbs differently on a PtsSn surface containing 40 at% Sn than on a PtSn surface containing 70 at% Sn (composition determined by AES). Specifically, they found that Pt3Sn more readily chemisorbs oxygen at lower temperatures than PtSn. This study was extended by Bouwman and Biloen [11,12] who compared ESCA and AES results from PtSn and PtsSn after the treatments mentioned above. Hilaire, Guerrero, L6gar6, Maire and Krill [13] have studied bimetallic platinum alloys containing palladium, ruthenium and iridium. They found that it is possible to oxidize the platinum in these alloys under conditions which do not oxidize pure platinum. Furthermore, the concentration of the second metal in the surface region must be above some critical value for the oxidation to occur. In this study ion scattering spectroscopy (ISS) was used to examine the interaction of oxygen with a platinum-tin alloy. Determination of the composition of the outermost one or two surface layers is possible with ISS [14], which also is highly sensitive to trace amounts of surface species [15]. Only a few other surface techniques are as surface sensitive as ISS. This makes ISS a particularly good technique for studying catalytic surfaces for which the composition of the outermost layer primarily determines the catalytic properties. In ISS a primary beam of nearly monoenergetic inert gas ions impacts the surface of the sample, and the scattered ions leaving the surface at some angle 0 are energy analyzed. It has been verified experimentally that the collisions between the inert gas ions and surface atoms can be treated as elastic, two-particle scattering events. Thus, the kinetic energy of a scattered ion E 1 of mass M 1 which hits a surface atom of mass M 2 is given by:
E°
(M1 + M2
/ M2 - sin20
(1)
where E 0 is the initial kinetic energy of the ion and 0 is defined as shown in fig. 1. Then scanning the kinetic energies of the scattered ions at a given 0 yields a mass spectrum (composition) of the outermost surface layer. The high surface sensitivity of ISS arises from the fact that an ion which penetrates beneath the surface has nearly a 100% probability of being neutralized. Two difficulties in using ISS are quantification and damage caused by sputtering effects. In quantifying ISS it is necessary to determine the scattering cross sections and ion neutralization probabilities. This has been reviewed previously [14-16] and will not be discussed further here. Damage due to sputtering effects can be minimized by using a low primary ion beam current density and keeping the exposure as small as possible while still obtaining a reasonable signal-to-noise ratio. The purpose of this investigation is severalfold. With respect to the
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L585
D
/
Fig. 1. Schematic diagram of the double-pass CMA used in these studies showing the geometry used in ISS. Components indicated are: A, ion sputter gun; B, electron gun; C, movable aperture; D, spiraltron amplifier; S, sample. The movable aperture allowed selection of the ion scattering angle. In these studies the spectra presented were taken using a scattering angle 8 of 147°.
p l a t i n u m - t i n reforming catalysts, it is of interest to characterize the p l a t i n u m - t i n interaction in the alloy with and without the presence of oxygen. It is anticipated that information of this type will be useful in elucidating the p l a t i n u m - t i n interaction in the bimetallic reforming catalysts. It is also of interest to use a highly surface sensitive technique such as ISS to study alloy systems and to make comparisons with AES and ESCA results. This is particularly useful in assessing the performance of less surface sensitive techniques in studies of phenomena which are occurring at the outermost monolayer. Finally, the surface properties of alloys are an interesting and important area of study with regard to segregation, selective sputtering by ion b o m b a r d m e n t and oxidation. A Pt3Sn alloy sample was prepared using high purity, reagent grade materials. Platinum foil (Materials Research Corporation, VP Grade) of dimensions 7 x 10 × 0.13 m m was ultrasonically cleaned for 5 minutes in each of the following solvents: water, benzene, acetone, ~chloroform, acetone and methanol. The platinum foil was then used as a cathode in an electrochemical cell containing an electrolytic solution consisting of the stoichiometric amount of SnC14 required to produce Pt3Sn, 2.5 g of NHENH2H2SO4 and H20 for a total volume of 100 ml. Metallic tin was plated onto the platinum cathode at a potential of - 0 . 7 0 V versus SCE at a temperature of 55°C. The potential rose slowly during the early stages of deposition. After plating for about 1 hour, the electrolytic solution was neutralized to a p H of 7.00 using N H 4 O H . After washing with distilled water and drying at 60°C in vacuum, the sample was
L586
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
weighed to verify that the correct amount of tin had been deposited. Then the sample was placed in a quartz tube. It was heated at 450°C for 3 hours (slowly through the tin melting temperature) under flowing hydrogen and cooled slowly to 20°C. The sample was stored in an argon atmosphere. For the surface studies the sample was spot welded onto two 0.02" tungsten wires. It was supported at an angle of 45 ° with respect to the electrostatic analyzer as shown in fig. 1. Heating was accomplished radiantly using a tungsten filament mounted behind the sample. The temperature was measured using iron-constantan thermocouple wires spot welded to the back of the sample. AES and ISS were performed using a P H I 15-255GAR double-pass CMA containing an internal electron gun and a movable aperture for angular resolution of the emitted signal. AES was taken using standard procedures with a primary beam energy of 3 keV and a 0.5 Vpp oscillation. ISS was performed using a P H I Model 04-161 sputter gun mounted at an angle of 30 ° with respect to the sample normal. Scattered ions leaving the sample at an angle of 3 ° with respect to the sample normal were energy analyzed using a nonretarding mode. The angle selection was accomplished using the movable aperture as shown in fig. 1. The primary ion beam consisted of 1000 eV helium ions, and the beam current was about 500 nA over an area with an approximate diameter of 2 mm. This results in good signal-to-noise ratios, an adequate resolution of the platinum and tin peaks and minimal sputtering effects. After baking and pumping the system down to 10 -1° Torr, AES showed that the sample was slightly contaminated with carbon and oxygen. Both were removed easily with heating to 500°C and argon ion sputtering. Before ISS was performed, the general conclusions of Bouwman et al. [10-12] were checked qualitatively and substantiated. It was found that annealing the sample or exposing it to oxygen enriched the surface region in tin and that sputtering enriched the surface region in platinum. Fig. 2 shows ISS spectra after various treatments, and fig. 3 shows the corresponding AES spectra. Spectra from a clean surface are shown in figs. 2a and 3a. The sample had been sputtered followed by annealing at 500°C for 2½ hours. During the annealing period, the pressure remained in the 10 10 Torr range. Scattering cross sections have not been determined for pure platinum and pure tin so it is not possible at this time to determine a surface composition. It can be seen that the platinum and tin peaks are fairly well resolved. Figs. 2b and 3b show spectra from the same surface after a 1000 L exposure to 02 at 200°C. Both ISS and AES show a peak due to adsorbed oxygen. In the ISS spectrum, it appears that the tin peak has grown with respect to the platinum peak. This is not the case because the scale factors of the ordinates of figs. 2a, 2b and 2c are arbitrary with respect to each other. This is discussed below. The energies of the oxygen, tin and platinum peaks are in good agreement with the values calculated from eq. (1) using the geometry shown in
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L587
i,,,,i,,,,l,,,,l,,,,i,,,,l,,,,i,,=,
X7 0 C
Sn
I
0
i
|]
IPt
b .,.':2
" I111[111 0.4
--1
L___
[1111[1111[1111[1111[IIII 0.6
0.8 EIE o
Fig. 2. ISS spectra of Pt3Sn taken using 1 keV helium ions. The spectrum shown in (a) was taken after annealing the sample at 500°C for 2½ h, the spectrum shown in (b) was taken after exposing the sample to 1000 L of oxygen at 200°C and the spectrum shown in (c) was taken after sputtering with the ISS beam for 1 h. The sensitivity of the technique is demonstrated by the oxygen signal shown in (c) increased by a factor of 70. The ordinates of (a), (b), and (c) are arbitrary, and they are not scaled relatively to each other.
fig. 1. Figs. 2c and 3c show the ISS and AES spectra taken after 1 hour of sputtering at room temperature with helium ions. AES shows complete removal of the oxygen and reduction of the tin with respect to the platinum, and ISS shows similar results. The high sensitivity of ISS is demonstrated by the spectrum in the 300 to 430 eV region which has been expanded by a factor of 70. A very minute amount of adsorbed oxygen is still present after the sputtering. Fig. 4 shows enlarged scans of the platinum and tin peaks of figs. 2a and 2b scaled appropriately with respect to each other. The important point is that there is no change in the tin peak upon oxygen adsorption, but the platinum
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L588
V
dN
I
I I I I I I I I I 100 KINETIC
I I I I I [ I I 300
1
till 500
INERGY [eV]
Fig. 3. AES spectra corresponding to the ISS spectra of figs. 2a, 2b and 2c respectively. The
ordinates of (a), (b), and (c) are arbitrary, and they are not scaled relatively to each other. peak is greatly reduced in size. The most probable explanation for this behavior is that the oxygen adsorbs to the surface platinum atoms and not to the tin atoms. This, in effect, covers the platinum atoms so they are not detected in ISS while the surface tin atoms are still detected. It is somewhat surprising that the oxygen bonds to the platinum and not the tin. This could be due to a drastic modification of the electronic properties of the tin by its interaction with the platinum. The fact that Hilare et al. [13] found that alloyed platinum is more easily oxidized than pure platinum supports this viewpoint. Unfortunately, the platinum-tin interaction is not understood well enough to be able to describe how the alloyed species interact with oxygen. These results suggest that the interaction of the alloy with oxygen is much different than expected from pure metallic properties. This interpretation is also consistent with the experimental results of Bouwman et al. [10] who found that oxygen more readily adsorbs on a Pt3Sn surface containing 40 at% Sn than on a PtSn
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L589
lllllllllllllllllllllllllllllllllll.
ne
Pt
IIIIIIIIIII111111111111111111111111 0~5
0.90
0.95
EIEo
Fig. 4. ISS Pt and Sn peaks from figs. 2a and 2b shown correctly scaled with respect to each other.
surface containing 70 at% Sn. The ISS results presented here suggest that the appropriate explanation for their experimental results is that the Pt3Sn surface simply provides more Pt surface atoms or sites for oxygen adsorption than the PtSn surface. As discussed in previous studies, ESCA spectra of the platinum 4f peaks show large energy shifts in oxidizing platinum metal to PtO or PtO2 [8,9], and the tin 3d peaks shift by more than 2 eV in oxidizing metallic tin to SnO or SnO2 [9]. In this study ESCA was performed before and after the oxygen exposure. However, no differences in either the Pt 4f or Sn 3d peaks were observed. This suggests that oxides are not being formed by the conditions of this exposure. The nature of the adsorbed oxygen will be the subject of a future investigation using temperature programmed desorption (TPD) and electron stimulated desorption (ESD). It is also believed that prolonged high-temperature annealing followed by
L590
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
s p u t t e r i n g selectively removes tin from the sample. Evidence for this Comes f r o m the fact that m a n y s p u t t e r - a n n e a l - o x y g e n - e x p o s u r e cycles g r a d u a l l y resulted in a lower surface tin c o n t e n t as d e t e r m i n e d b y AES. A s m e n t i o n e d a b o v e the earlier A E S a n d E S C A studies of B o u w m a n et al. [10-12] find c o n s i d e r a b l y different b e h a v i o r for PtSn a n d Pt3Sn. A n effort is being m a d e to p r o d u c e the s a m e surface c o m p o s i t i o n with b o t h alloys over a b r o a d range. T h e use of ISS w o u l d then allow an investigation of the influence of b u l k p r o p e r t i e s on surface properties. Results from these studies will be r e p o r t e d later. The a u t h o r s a p p r e c i a t e financial s u p p o r t received f r o m N S F u n d e r grants CPE-8210776 a n d CHE-8309445 a n d the d o n o r s of the P e t r o l e u m R e s e a r c h F u n d a d m i n i s t e r e d b y the A C S u n d e r P R F # 15102-AC5.
Note added in proof T h e conclusions d r a w n in this s t u d y are b a s e d on a simple site-blocking m o d e l of the ISS data. R e c e n t s c a n n i n g A u g e r m i c r o s c o p y ( S A M ) a n d angleresolved E S C A e x p e r i m e n t s suggest that tin segregation to the surface also occurs d u r i n g the oxygen exposure a n d that a m o r e involved i n t e r p r e t a t i o n of the ISS d a t a is required. The question of whether oxygen b o n d s with the p l a t i n u m , tin or b o t h p l a t i n u m a n d tin will be discussed m o r e fully with r e g a r d to the recent S A M a n d A R E S C A e x p e r i m e n t s in the n e a r future.
References [1] H. Lieske and J. Volter, J. Catalysis 90 (1984) 96. [2] F.M. Dautzenberg, J.N. Helle, P. Biloen and W.M.H. Sachtler, J. Catalysis 63 (1980) 119. [3] R. Bacaud, P. Bussi~re and F. Figueras, J. Catalysis 69 (1981) 399. [4] A.C. Muller, P.A. Engeihard and J.E. Weisang, J. Catalysis 56 (1979) 65. [5] R. Burch, J. Catalysis 71 (1981) 348. [6] R. Burch and L.C. Garla, J. Catalysis 71 (1981) 360. [7] S.R. Adkins and B.H. Davis, J. Catalysis 89 (1984) 371. [8] D.F. Cox, G.B. Hoflund and H.A. Laitinen, Langmuir 1 (1985) 269. [9] G.B. Hoflund, D.A. Asbury and R.E. Gilbert, Thin Solid Films, in press. [10] R. Bouwman, L.H. Toneman and A.A. Holscher, Surface Sci. 35 (1973) 8. [11] R. Bouwman and P. Biloen, Anal. Chem. 46 (1974) 136. [12] R. Bouwman and P. Biloen, Surface Sci. 41 (1974) 348. [13] L. Hilaire, G,D. Guerrero, P. L6gar6, G. Maire and G. Krill, Surface Sci. 146 (1984) 569. [14] T.M. Buck, in: Methods of Surface Analysis: Methods and Phenomena, I, Ed. A.W. Czanderna (Elsevier, Amsterdam, (1975) p. 75. [15] D.J. Bell, T.M. Buck, D. MacNair and G.H. Wheatley, Surface Sci. 30 (1972) 69. [16] E.P.Th.M. Suurmeijer and A.L. Boers, Surface Sci. 43 (1973) 309.
SURFACE
SCIENCE
L583
LETTERS
AN ISS AND AES EXAMINATION OF THE INTERACTION WITH A PLATINUM-TIN ALLOY Gar B. HOFLUND Department
and Douglas
OF OXYGEN
A. ASBURY
of Chemical Engineering, University of Florida, Gainesville, Florida 3261 I, USA
and Piotr KIRSZENSZTEJN Department
of Chemistry,
* and Herbert
A. LAITINEN
University of Florida, Gainesville, Florida 32611, USA
Received 28 March 1985; accepted for publication
13 June 1985
In this study ISS and AES were used to examine a platinum-tin alloy surface before and after exposure to oxygen. Annealing increases the tin-to-platinum concentration ratio at the surface while sputtering causes a decrease in this ratio. The ISS results suggest that the oxygen interacts with the platinum and not the tin.
Platinum-tin systems are very important as hydrocarbon reforming catalysts when supported on alumina. Compared to alumina-supported platinum, the bimetallic catalyst has improved stability and catalytic activity toward many of the reforming reactions. Much controversy exists in the literature concerning the state of the tin and the form of the platinum-tin interaction. Several studies suggest that a platinum-tin alloy is formed [l-3] while others suggest that the tin is not reduced to metal but exists as SnO, SnO, or a compound containing tin, aluminum and oxygen [4-71. In the case of platinum crystallites supported on a SnO, substrate, an Sn-0-Pt bond has been identified using ESCA [8,9]. The Sn-0-Pt species bonds the crystallite to the support and may be responsible for the alteration of the catalytic properties in the bimetallic platinum-tin catalysts. Platinum alloyed with palladium, iridium, ruthenium and tin have been studied previously with ESCA and AES. Bouwman, Toneman and Holscher [lo] used AES to show that the composition of the near surface region of a Pt/Sn alloy can be modified drastically by a number of treatments. Sputtering or exposure to hydrogen enriches the surface region in platinum while anneal* Permanent address: Department
of Chemistry, University
of A. Mickiewicz,
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Poznan, Poland.
L584
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
ing or exposure to oxygen enriches the surface region in tin. They also demonstrated that oxygen adsorbs differently on a PtsSn surface containing 40 at% Sn than on a PtSn surface containing 70 at% Sn (composition determined by AES). Specifically, they found that Pt3Sn more readily chemisorbs oxygen at lower temperatures than PtSn. This study was extended by Bouwman and Biloen [11,12] who compared ESCA and AES results from PtSn and PtsSn after the treatments mentioned above. Hilaire, Guerrero, L6gar6, Maire and Krill [13] have studied bimetallic platinum alloys containing palladium, ruthenium and iridium. They found that it is possible to oxidize the platinum in these alloys under conditions which do not oxidize pure platinum. Furthermore, the concentration of the second metal in the surface region must be above some critical value for the oxidation to occur. In this study ion scattering spectroscopy (ISS) was used to examine the interaction of oxygen with a platinum-tin alloy. Determination of the composition of the outermost one or two surface layers is possible with ISS [14], which also is highly sensitive to trace amounts of surface species [15]. Only a few other surface techniques are as surface sensitive as ISS. This makes ISS a particularly good technique for studying catalytic surfaces for which the composition of the outermost layer primarily determines the catalytic properties. In ISS a primary beam of nearly monoenergetic inert gas ions impacts the surface of the sample, and the scattered ions leaving the surface at some angle 0 are energy analyzed. It has been verified experimentally that the collisions between the inert gas ions and surface atoms can be treated as elastic, two-particle scattering events. Thus, the kinetic energy of a scattered ion E 1 of mass M 1 which hits a surface atom of mass M 2 is given by:
E°
(M1 + M2
/ M2 - sin20
(1)
where E 0 is the initial kinetic energy of the ion and 0 is defined as shown in fig. 1. Then scanning the kinetic energies of the scattered ions at a given 0 yields a mass spectrum (composition) of the outermost surface layer. The high surface sensitivity of ISS arises from the fact that an ion which penetrates beneath the surface has nearly a 100% probability of being neutralized. Two difficulties in using ISS are quantification and damage caused by sputtering effects. In quantifying ISS it is necessary to determine the scattering cross sections and ion neutralization probabilities. This has been reviewed previously [14-16] and will not be discussed further here. Damage due to sputtering effects can be minimized by using a low primary ion beam current density and keeping the exposure as small as possible while still obtaining a reasonable signal-to-noise ratio. The purpose of this investigation is severalfold. With respect to the
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L585
D
/
Fig. 1. Schematic diagram of the double-pass CMA used in these studies showing the geometry used in ISS. Components indicated are: A, ion sputter gun; B, electron gun; C, movable aperture; D, spiraltron amplifier; S, sample. The movable aperture allowed selection of the ion scattering angle. In these studies the spectra presented were taken using a scattering angle 8 of 147°.
p l a t i n u m - t i n reforming catalysts, it is of interest to characterize the p l a t i n u m - t i n interaction in the alloy with and without the presence of oxygen. It is anticipated that information of this type will be useful in elucidating the p l a t i n u m - t i n interaction in the bimetallic reforming catalysts. It is also of interest to use a highly surface sensitive technique such as ISS to study alloy systems and to make comparisons with AES and ESCA results. This is particularly useful in assessing the performance of less surface sensitive techniques in studies of phenomena which are occurring at the outermost monolayer. Finally, the surface properties of alloys are an interesting and important area of study with regard to segregation, selective sputtering by ion b o m b a r d m e n t and oxidation. A Pt3Sn alloy sample was prepared using high purity, reagent grade materials. Platinum foil (Materials Research Corporation, VP Grade) of dimensions 7 x 10 × 0.13 m m was ultrasonically cleaned for 5 minutes in each of the following solvents: water, benzene, acetone, ~chloroform, acetone and methanol. The platinum foil was then used as a cathode in an electrochemical cell containing an electrolytic solution consisting of the stoichiometric amount of SnC14 required to produce Pt3Sn, 2.5 g of NHENH2H2SO4 and H20 for a total volume of 100 ml. Metallic tin was plated onto the platinum cathode at a potential of - 0 . 7 0 V versus SCE at a temperature of 55°C. The potential rose slowly during the early stages of deposition. After plating for about 1 hour, the electrolytic solution was neutralized to a p H of 7.00 using N H 4 O H . After washing with distilled water and drying at 60°C in vacuum, the sample was
L586
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
weighed to verify that the correct amount of tin had been deposited. Then the sample was placed in a quartz tube. It was heated at 450°C for 3 hours (slowly through the tin melting temperature) under flowing hydrogen and cooled slowly to 20°C. The sample was stored in an argon atmosphere. For the surface studies the sample was spot welded onto two 0.02" tungsten wires. It was supported at an angle of 45 ° with respect to the electrostatic analyzer as shown in fig. 1. Heating was accomplished radiantly using a tungsten filament mounted behind the sample. The temperature was measured using iron-constantan thermocouple wires spot welded to the back of the sample. AES and ISS were performed using a P H I 15-255GAR double-pass CMA containing an internal electron gun and a movable aperture for angular resolution of the emitted signal. AES was taken using standard procedures with a primary beam energy of 3 keV and a 0.5 Vpp oscillation. ISS was performed using a P H I Model 04-161 sputter gun mounted at an angle of 30 ° with respect to the sample normal. Scattered ions leaving the sample at an angle of 3 ° with respect to the sample normal were energy analyzed using a nonretarding mode. The angle selection was accomplished using the movable aperture as shown in fig. 1. The primary ion beam consisted of 1000 eV helium ions, and the beam current was about 500 nA over an area with an approximate diameter of 2 mm. This results in good signal-to-noise ratios, an adequate resolution of the platinum and tin peaks and minimal sputtering effects. After baking and pumping the system down to 10 -1° Torr, AES showed that the sample was slightly contaminated with carbon and oxygen. Both were removed easily with heating to 500°C and argon ion sputtering. Before ISS was performed, the general conclusions of Bouwman et al. [10-12] were checked qualitatively and substantiated. It was found that annealing the sample or exposing it to oxygen enriched the surface region in tin and that sputtering enriched the surface region in platinum. Fig. 2 shows ISS spectra after various treatments, and fig. 3 shows the corresponding AES spectra. Spectra from a clean surface are shown in figs. 2a and 3a. The sample had been sputtered followed by annealing at 500°C for 2½ hours. During the annealing period, the pressure remained in the 10 10 Torr range. Scattering cross sections have not been determined for pure platinum and pure tin so it is not possible at this time to determine a surface composition. It can be seen that the platinum and tin peaks are fairly well resolved. Figs. 2b and 3b show spectra from the same surface after a 1000 L exposure to 02 at 200°C. Both ISS and AES show a peak due to adsorbed oxygen. In the ISS spectrum, it appears that the tin peak has grown with respect to the platinum peak. This is not the case because the scale factors of the ordinates of figs. 2a, 2b and 2c are arbitrary with respect to each other. This is discussed below. The energies of the oxygen, tin and platinum peaks are in good agreement with the values calculated from eq. (1) using the geometry shown in
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L587
i,,,,i,,,,l,,,,l,,,,i,,,,l,,,,i,,=,
X7 0 C
Sn
I
0
i
|]
IPt
b .,.':2
" I111[111 0.4
--1
L___
[1111[1111[1111[1111[IIII 0.6
0.8 EIE o
Fig. 2. ISS spectra of Pt3Sn taken using 1 keV helium ions. The spectrum shown in (a) was taken after annealing the sample at 500°C for 2½ h, the spectrum shown in (b) was taken after exposing the sample to 1000 L of oxygen at 200°C and the spectrum shown in (c) was taken after sputtering with the ISS beam for 1 h. The sensitivity of the technique is demonstrated by the oxygen signal shown in (c) increased by a factor of 70. The ordinates of (a), (b), and (c) are arbitrary, and they are not scaled relatively to each other.
fig. 1. Figs. 2c and 3c show the ISS and AES spectra taken after 1 hour of sputtering at room temperature with helium ions. AES shows complete removal of the oxygen and reduction of the tin with respect to the platinum, and ISS shows similar results. The high sensitivity of ISS is demonstrated by the spectrum in the 300 to 430 eV region which has been expanded by a factor of 70. A very minute amount of adsorbed oxygen is still present after the sputtering. Fig. 4 shows enlarged scans of the platinum and tin peaks of figs. 2a and 2b scaled appropriately with respect to each other. The important point is that there is no change in the tin peak upon oxygen adsorption, but the platinum
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L588
V
dN
I
I I I I I I I I I 100 KINETIC
I I I I I [ I I 300
1
till 500
INERGY [eV]
Fig. 3. AES spectra corresponding to the ISS spectra of figs. 2a, 2b and 2c respectively. The
ordinates of (a), (b), and (c) are arbitrary, and they are not scaled relatively to each other. peak is greatly reduced in size. The most probable explanation for this behavior is that the oxygen adsorbs to the surface platinum atoms and not to the tin atoms. This, in effect, covers the platinum atoms so they are not detected in ISS while the surface tin atoms are still detected. It is somewhat surprising that the oxygen bonds to the platinum and not the tin. This could be due to a drastic modification of the electronic properties of the tin by its interaction with the platinum. The fact that Hilare et al. [13] found that alloyed platinum is more easily oxidized than pure platinum supports this viewpoint. Unfortunately, the platinum-tin interaction is not understood well enough to be able to describe how the alloyed species interact with oxygen. These results suggest that the interaction of the alloy with oxygen is much different than expected from pure metallic properties. This interpretation is also consistent with the experimental results of Bouwman et al. [10] who found that oxygen more readily adsorbs on a Pt3Sn surface containing 40 at% Sn than on a PtSn
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
L589
lllllllllllllllllllllllllllllllllll.
ne
Pt
IIIIIIIIIII111111111111111111111111 0~5
0.90
0.95
EIEo
Fig. 4. ISS Pt and Sn peaks from figs. 2a and 2b shown correctly scaled with respect to each other.
surface containing 70 at% Sn. The ISS results presented here suggest that the appropriate explanation for their experimental results is that the Pt3Sn surface simply provides more Pt surface atoms or sites for oxygen adsorption than the PtSn surface. As discussed in previous studies, ESCA spectra of the platinum 4f peaks show large energy shifts in oxidizing platinum metal to PtO or PtO2 [8,9], and the tin 3d peaks shift by more than 2 eV in oxidizing metallic tin to SnO or SnO2 [9]. In this study ESCA was performed before and after the oxygen exposure. However, no differences in either the Pt 4f or Sn 3d peaks were observed. This suggests that oxides are not being formed by the conditions of this exposure. The nature of the adsorbed oxygen will be the subject of a future investigation using temperature programmed desorption (TPD) and electron stimulated desorption (ESD). It is also believed that prolonged high-temperature annealing followed by
L590
G.B. Hoflund et al. / Interaction of oxygen with platinum-tin alloy
s p u t t e r i n g selectively removes tin from the sample. Evidence for this Comes f r o m the fact that m a n y s p u t t e r - a n n e a l - o x y g e n - e x p o s u r e cycles g r a d u a l l y resulted in a lower surface tin c o n t e n t as d e t e r m i n e d b y AES. A s m e n t i o n e d a b o v e the earlier A E S a n d E S C A studies of B o u w m a n et al. [10-12] find c o n s i d e r a b l y different b e h a v i o r for PtSn a n d Pt3Sn. A n effort is being m a d e to p r o d u c e the s a m e surface c o m p o s i t i o n with b o t h alloys over a b r o a d range. T h e use of ISS w o u l d then allow an investigation of the influence of b u l k p r o p e r t i e s on surface properties. Results from these studies will be r e p o r t e d later. The a u t h o r s a p p r e c i a t e financial s u p p o r t received f r o m N S F u n d e r grants CPE-8210776 a n d CHE-8309445 a n d the d o n o r s of the P e t r o l e u m R e s e a r c h F u n d a d m i n i s t e r e d b y the A C S u n d e r P R F # 15102-AC5.
Note added in proof T h e conclusions d r a w n in this s t u d y are b a s e d on a simple site-blocking m o d e l of the ISS data. R e c e n t s c a n n i n g A u g e r m i c r o s c o p y ( S A M ) a n d angleresolved E S C A e x p e r i m e n t s suggest that tin segregation to the surface also occurs d u r i n g the oxygen exposure a n d that a m o r e involved i n t e r p r e t a t i o n of the ISS d a t a is required. The question of whether oxygen b o n d s with the p l a t i n u m , tin or b o t h p l a t i n u m a n d tin will be discussed m o r e fully with r e g a r d to the recent S A M a n d A R E S C A e x p e r i m e n t s in the n e a r future.
References [1] H. Lieske and J. Volter, J. Catalysis 90 (1984) 96. [2] F.M. Dautzenberg, J.N. Helle, P. Biloen and W.M.H. Sachtler, J. Catalysis 63 (1980) 119. [3] R. Bacaud, P. Bussi~re and F. Figueras, J. Catalysis 69 (1981) 399. [4] A.C. Muller, P.A. Engeihard and J.E. Weisang, J. Catalysis 56 (1979) 65. [5] R. Burch, J. Catalysis 71 (1981) 348. [6] R. Burch and L.C. Garla, J. Catalysis 71 (1981) 360. [7] S.R. Adkins and B.H. Davis, J. Catalysis 89 (1984) 371. [8] D.F. Cox, G.B. Hoflund and H.A. Laitinen, Langmuir 1 (1985) 269. [9] G.B. Hoflund, D.A. Asbury and R.E. Gilbert, Thin Solid Films, in press. [10] R. Bouwman, L.H. Toneman and A.A. Holscher, Surface Sci. 35 (1973) 8. [11] R. Bouwman and P. Biloen, Anal. Chem. 46 (1974) 136. [12] R. Bouwman and P. Biloen, Surface Sci. 41 (1974) 348. [13] L. Hilaire, G,D. Guerrero, P. L6gar6, G. Maire and G. Krill, Surface Sci. 146 (1984) 569. [14] T.M. Buck, in: Methods of Surface Analysis: Methods and Phenomena, I, Ed. A.W. Czanderna (Elsevier, Amsterdam, (1975) p. 75. [15] D.J. Bell, T.M. Buck, D. MacNair and G.H. Wheatley, Surface Sci. 30 (1972) 69. [16] E.P.Th.M. Suurmeijer and A.L. Boers, Surface Sci. 43 (1973) 309.