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Auger electron spectroscopy studies of CO on Ni spectral line shapes and quantitative aspects
Surface Science 55 (1976) 741-746 0 North-Holland Publishing Company
AUGER ELECTRON SPECTROSCOPY STUDIES OF CO ON Ni SPECTRAL LINE SHAPES AND QUANTITATIVE
ASPECTS
Received 14 October 1975; manuscript received in final form 2 February 1976
The chemisorption of CO on Ni at 300 K is characterized by molecular-like adsorption associated with relatively weak bonding [l] . Studies of this adsorption system using electron beam techniques, such as low energy electron diffraction (LEED) and electron excited Auger electron spectroscopy (AES), are hampered due to electron beam induced decomposition of CO [l-3] and to date no Auger spectra of CO on Ni have been published. This letter describes work undertaken to study the Auger spectrum of CO on Ni (110) with a view to measuring (i) the C and 0 Auger line shapes, (ii) the relative C and 0 Auger currents and relative atomic concentrations, and (iii) the effects of electron beam interaction on them. The Auger spectra were obtained in their usual first derivative form Y_ E)(E) using a double pass cylindrical mirror analyzer (AE/E = O.OOS), the experimental details being given elsewhere [4]. The Ni (110) surface used here was cleaned by argon ion bombardment (500 eV) and exposed to CO without annealing to ensure that the surface was initially clean [4]. Although this (110) surface was not annealed molecular adsorption of CO would still be expected, as has been observed on other ion bombarded Ni surfaces [5]. Further, similar C and 0 Auger line shapes to those reported here have been observed by us from CO adsorbed on clean ion bombarded Ni foils that had been either annealed or left unannealed. To minimize electron beam induced effects a 1.5 MA, 1.5 keV electron beam was used for Auger excitation. Even under these conditions significant changes in the C and 0 Auger spectra could be observed after the beam was on for a few minutes. In order to confirm that the Auger data reported here are representative of adsorbed CO and were not affected by the electron beam before measurements could be made, the C Auger spectrum from freshly adsorbed CO was also excited using a 2 nA, 1.5 keV electron beam and measured using pulse counting techniques. The C Auger spectrum obtained in this manner (non-differentiated) was compared with the integral of the C Auger spectrum obtained in the usual way with the bigger beam current and no significant differences in Auger line shape were observed. Following CO exposure at 1 X lop4 Pa for 15 min the Auger spectrum shown in fig. 1 was obtained. The small background slope of the spectrum and the small size of the high energy Ni peaks relative to those at low energies are due to the relatively low electron beam energy used for Auger production. However, the most interesting aspects of this spectrum are the Auger line shapes of C and 0 as they are
742
M.P. Hooker, J. T Grant/AES studies of CO 011Ni
C
Ni
f
NI
I
100
300 ELECTRON
Fig. 1. First derivative Auger spectrum of CO adsorbed face. Data were taken Using a 1.5 PA, 1.5 keV electron used was 2 eV peak-to-peak.
500 ENERGY
700
900
(eV)
on a clean ion bombarded Ni(ll0) surbeam for excitation. The modulation
both quite different from those previously observed for CO on other metal surfaces such as Ta(lOO) [6] and Mo(ll0) [7]. The adsorption of CO on Ta and MO is much stronger than on Ni - CO actually decomposes on Ta at room temperature [6] and it is therefore felt that the C and 0 Auger line shapes observed here reflect the weak bonding of CO to Ni. In fact, the 0 Auger structures observed in o$her high resolution adsorption studies made to date involving strong bonding, such as CO and 0, on Mo(ll0) [7] or 0, on Ni(ll0) [4], are all quite similar to each other. The detail of the C and 0 Auger line shapes for CO on Ni(ll0) can be seen more clearly in fig. 2a. The feature on the high energy side of the main 0 Auger feature is enhanced in this spectrum compared with that in the other 0 spectra referred to above, and also only one strong Auger peak appears on the low energy side of the main peak whereas two peaks are present in the other 0 spectra. This peak on the low energy side also appears at an energy from the main peak different from either peak detected in the other 0 spectra and closely resembles structure in the Auger spectrum from gaseous CO [8]. Indeed, CO bonds to Ni through the C atom with the 0 atom furthest from the surface [ 1,9] so this similarity between the 0 Auger spectra from CO on Ni and CO gas is not too surprising. It is more difficult to compare the C Auger spectra because of their more complicated structures but differences would be expected because of the way CO bonds to Ni - bonding occurring through electron transfer from the Sa orbital of CO which is primarily centered on the C atom and by back-donation of electrons from the metal into the unoccupied antibonding 2n * orbital of CO [ 1,9]. It is felt, however, that the C Auger spectrum observed here might typify this type of CO-metal bonding and studies of other such systems are planned.
M.P. Hooker, J. T. Grant/AES
I
Carbon
studies of CO on Ni
743
Oxygen
w+_wq#w
(c)
+, 50 ev
ELECTRON
ENERGY
-
Fig. 2. First derivative Auger spectra of C and 0 on Ni( 110): (a) following exposure of clean Ni(ll0) to 1 X lo4 Pa of CO for 15 min; (b) after exposure to the electron beam for 10 min; (c) after exposure to the electron beam for 40 min. The electron beam was 1.5 PA at 1.5 keV. The modulation used was 2 eV peak-to-peak.
The effects of the electron beam on the Auger line shapes of C and 0 are illustrated in fig. 2, (a) being the spectra of freshly adsorbed CO, (b) the result obtained after the electron beam was left on for 10 min and (c) after the beam was on for 40 min. Note that quite pronounced changes occur in both the C and 0 spectra. For C both the main peaks and the lower energy peaks change markedly, almost all the features being replaced by new ones after 40 min interaction. At this stage the C Auger features resemble those of nickel carbide [lo]. Corresponding changes in the 0 Auger spectrum are less marked but nevertheless significant. The intensity of the feature on the high energy side of the main 0 peak decreases relative to that of the main peak, and the feature approximately 2.5 eV below the main peak is eventually replaced by two new ones at the positions found for 02 on Ni although they are not lucid in the data shown in fig. 2c (see also below). The 0 Auger spectrum obtained (from previously adsorbed CO) after 40 min interaction time is therefore similar to that obtained from the adsorption of 0, on Ni(ll0) [8]. These AES results indicate that the electron beam decomposes molecularly adsorbed CO into surface carbide and oxide. Corresponding changes in the C and 0 Auger signal strengths and relative concentrations are discussed below. As CO adsorbs molecularly on Ni( 110) equal C and 0 atomic concentrations
744
M.P. Hooker, J.T. Grant/AES studies of CO on Ni
would be expected in the absence of electron beam effects. For comparison, the relative C and 0 concentrations can be calculated from the Auger data presented here, follow~g the procedure already outlined for studies of CO on ~o(lOO) ]I 11. First, the ionization cross section curves for carbon and oxygen on this Ni sample were both measured and assuming that the maximum cross sections vary as the reciprocal of the square of their threshold ionization energies, a relative carbon to oxygen ionization cross section of 4.1 obtains for an incident electron beam energy of 1 S keV. This means that for equal C and 0 concentrations on Ni( 110) the ratio of C and 0 Auger currents (corrected for analyzer energy-window effects [ 113) shouid equal 4.1. The relative C and 0 Auger currents cannot be measured directly from the derivative form data shown in fig. 2a because their Auger line shapes, are different from each other [ 121 nor can they be obtained by comparison with Auger spectra of CO on Mo(ll0) [l I] because the CO Auger spectra themselves are different. This problem in comparing Auger currents from different elements such as C and 0 can be overcome by integrating % g)(E) spectra twice over a fixed energy interval, as long as the background has been removed from the spectra [ 111. For the work reported here the CO Auger spectra were digitized and then a linear ramp (approximating the background shown in fig. 1) was subtracted from the C and 0 spectra before integration. The results obtained following one integration of the data are shown in fig. 3a. A second integration of C and 0 data over the same energy range as used earlier, 44 eV [l 11,gives the ratio of the measured C/O Auger currents
I
Corbon
ELECTRON
Fig. 3. First integrals
Oxygen
ENERGY
of the respective
-
data shown in fig. 2.
745
M.P. Hooker, .I. T. Gran t/AES studies of CO on Ni
as 2.2 with an experimental uncertainty of 20% obtained by averaging over three adsorption runs. Applying the correction for the analyzer energy window [ 111, the ratio of C to 0 Auger currents from CO on Ni(ll0) as measured by double integration of the Auger spectra is then 2.2 X (5 10/270) = 4.2 with an uncertainty of about 20% which compares well with the value of 4.1 expected from cross section studies based on equal C and 0 atomic concentrations. It is important to note that the application of integration techniques to Auger spectra of CO on Ni and MO both gave C/O atomic ratios of unity whereas the ratio of C to 0 peak-to-peak heights in 72 g)(E) spectra of CO on Ni and MO differ by more than a factor of 2, even after normalizing the data for the same incident electron beam energy. The effects of the electron beam on the C and 0 Auger currents were also measured by double integration of the ?Z z)(E) data. The first integrals of the Auger data in fig. 2b and 2c are shown in figs. 3b and 3c respectively. Note that the two peaks on the low energy side of the main 0 peak obtained after exposure of adsorbed CO to the electron beam for 40 min are clearer in the integral spectrum, fig. 3c, than in the raw derivative form spectrum, fig. 2c, due to the improved signal/noise obtained on integration [ 131. By integrating the data of fig. 3 it is found that both the carbon and oxygen Auger currents decrease with increasing exposure to the electron beam, and that the oxygen Auger current decreases at a faster rate indicating disproportionation. The carbon and oxygen concentrations decreased by 10% and 35% respectively after 10 min beam exposure, and by 30% and 90% after 40 min exposure. In summary, the C and 0 Auger line shapes of CO adsorbed on a clean ion bombarded Ni(ll0) surface have been measured and found to be quite different from other CO adsorption systems studied to date, probably reflecting the weak molecular-like bonding of Co to Ni through the C atom. However, the Auger results show that equal numbers of C and 0 atoms are present on the surface following adsorption, and it is found that the relative surface concentrations of C and 0 are subsequently affected by electron beam interaction. Both C and 0 are removed by the electron beam, the 0 being removed at a faster rate. After prolenged exposure to the electron beam, studies of the C and 0 Auger line shapes show that the CO is decomposed into surface carbide and oxide. We wish to acknowledge
the technical assistance of J. Miller and G. Wolfe. M.P. HOOKER
and J.T. GRANT *
UniversalEnergy Systems, Inc., 319s Plainjield Road, Day ton, Ohio 45432, USA * Research was sponsored by the Air Force Materials Laboratory, United States Air Force, Contract No. F33615-74X-4017.
Air Force
Systems
Command,
M.P. Hooker, J. T. GrantIAES studies of d0 on Ni
146
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ 111 [ 121 [ 131
J.E. Demuth and T.N. Rhodin, Surface Sci. 45 (1974) 249. T.N. Taylor and P.J. Estrup, J. Vacuum Sci. Technol. 10 (1973) 26. H.H. Madden, J. Kiippers and G. Ertl, J. Chem. Phys. 58 (1973) 3401. M.P. Hooker, J.T. Grant and T.W. Haas, J. Vacuum Sci. Technol. 13 (1976) 296. D.E. Eastman, J.E. Demuth and J.M. Baker, J. Vacuum Sci. Technol. 11 (1974) 273. T.W. Haas, J.T. Grant and G.J. Dooley, in: Adsorption-Desorption Phenomena, Ed. F. Ricca (Academic Press, New York, 1972) p. 359. M.P. Hooker, J.T. Grant and T.W. Haas, J. Vacuum Sci. Technol. 12 (1975) 325. J.T. Grant and M.P. Hooker, Solid State Commun., in press. G. Doyen and G. Ertl, Surface Sci. 43 (1974) 197. J.P. Coad and J.C. Riviere, Surface Sci. 25 (1971) 609. J.T. Grant, M.P. Hooker and T.W. Haas, Surface Sci. 46 (1974) 672. l J.T. Grant, T.W. Haas and J.E. Houston, J. Vacuum Sci. Technol. 11 (1974) 227. J.T. Grant, T.W. Haas and J.E. Houston, Surface Sci. 42 (1974) 1.
AUGER ELECTRON SPECTROSCOPY STUDIES OF CO ON Ni SPECTRAL LINE SHAPES AND QUANTITATIVE
ASPECTS
Received 14 October 1975; manuscript received in final form 2 February 1976
The chemisorption of CO on Ni at 300 K is characterized by molecular-like adsorption associated with relatively weak bonding [l] . Studies of this adsorption system using electron beam techniques, such as low energy electron diffraction (LEED) and electron excited Auger electron spectroscopy (AES), are hampered due to electron beam induced decomposition of CO [l-3] and to date no Auger spectra of CO on Ni have been published. This letter describes work undertaken to study the Auger spectrum of CO on Ni (110) with a view to measuring (i) the C and 0 Auger line shapes, (ii) the relative C and 0 Auger currents and relative atomic concentrations, and (iii) the effects of electron beam interaction on them. The Auger spectra were obtained in their usual first derivative form Y_ E)(E) using a double pass cylindrical mirror analyzer (AE/E = O.OOS), the experimental details being given elsewhere [4]. The Ni (110) surface used here was cleaned by argon ion bombardment (500 eV) and exposed to CO without annealing to ensure that the surface was initially clean [4]. Although this (110) surface was not annealed molecular adsorption of CO would still be expected, as has been observed on other ion bombarded Ni surfaces [5]. Further, similar C and 0 Auger line shapes to those reported here have been observed by us from CO adsorbed on clean ion bombarded Ni foils that had been either annealed or left unannealed. To minimize electron beam induced effects a 1.5 MA, 1.5 keV electron beam was used for Auger excitation. Even under these conditions significant changes in the C and 0 Auger spectra could be observed after the beam was on for a few minutes. In order to confirm that the Auger data reported here are representative of adsorbed CO and were not affected by the electron beam before measurements could be made, the C Auger spectrum from freshly adsorbed CO was also excited using a 2 nA, 1.5 keV electron beam and measured using pulse counting techniques. The C Auger spectrum obtained in this manner (non-differentiated) was compared with the integral of the C Auger spectrum obtained in the usual way with the bigger beam current and no significant differences in Auger line shape were observed. Following CO exposure at 1 X lop4 Pa for 15 min the Auger spectrum shown in fig. 1 was obtained. The small background slope of the spectrum and the small size of the high energy Ni peaks relative to those at low energies are due to the relatively low electron beam energy used for Auger production. However, the most interesting aspects of this spectrum are the Auger line shapes of C and 0 as they are
742
M.P. Hooker, J. T Grant/AES studies of CO 011Ni
C
Ni
f
NI
I
100
300 ELECTRON
Fig. 1. First derivative Auger spectrum of CO adsorbed face. Data were taken Using a 1.5 PA, 1.5 keV electron used was 2 eV peak-to-peak.
500 ENERGY
700
900
(eV)
on a clean ion bombarded Ni(ll0) surbeam for excitation. The modulation
both quite different from those previously observed for CO on other metal surfaces such as Ta(lOO) [6] and Mo(ll0) [7]. The adsorption of CO on Ta and MO is much stronger than on Ni - CO actually decomposes on Ta at room temperature [6] and it is therefore felt that the C and 0 Auger line shapes observed here reflect the weak bonding of CO to Ni. In fact, the 0 Auger structures observed in o$her high resolution adsorption studies made to date involving strong bonding, such as CO and 0, on Mo(ll0) [7] or 0, on Ni(ll0) [4], are all quite similar to each other. The detail of the C and 0 Auger line shapes for CO on Ni(ll0) can be seen more clearly in fig. 2a. The feature on the high energy side of the main 0 Auger feature is enhanced in this spectrum compared with that in the other 0 spectra referred to above, and also only one strong Auger peak appears on the low energy side of the main peak whereas two peaks are present in the other 0 spectra. This peak on the low energy side also appears at an energy from the main peak different from either peak detected in the other 0 spectra and closely resembles structure in the Auger spectrum from gaseous CO [8]. Indeed, CO bonds to Ni through the C atom with the 0 atom furthest from the surface [ 1,9] so this similarity between the 0 Auger spectra from CO on Ni and CO gas is not too surprising. It is more difficult to compare the C Auger spectra because of their more complicated structures but differences would be expected because of the way CO bonds to Ni - bonding occurring through electron transfer from the Sa orbital of CO which is primarily centered on the C atom and by back-donation of electrons from the metal into the unoccupied antibonding 2n * orbital of CO [ 1,9]. It is felt, however, that the C Auger spectrum observed here might typify this type of CO-metal bonding and studies of other such systems are planned.
M.P. Hooker, J. T. Grant/AES
I
Carbon
studies of CO on Ni
743
Oxygen
w+_wq#w
(c)
+, 50 ev
ELECTRON
ENERGY
-
Fig. 2. First derivative Auger spectra of C and 0 on Ni( 110): (a) following exposure of clean Ni(ll0) to 1 X lo4 Pa of CO for 15 min; (b) after exposure to the electron beam for 10 min; (c) after exposure to the electron beam for 40 min. The electron beam was 1.5 PA at 1.5 keV. The modulation used was 2 eV peak-to-peak.
The effects of the electron beam on the Auger line shapes of C and 0 are illustrated in fig. 2, (a) being the spectra of freshly adsorbed CO, (b) the result obtained after the electron beam was left on for 10 min and (c) after the beam was on for 40 min. Note that quite pronounced changes occur in both the C and 0 spectra. For C both the main peaks and the lower energy peaks change markedly, almost all the features being replaced by new ones after 40 min interaction. At this stage the C Auger features resemble those of nickel carbide [lo]. Corresponding changes in the 0 Auger spectrum are less marked but nevertheless significant. The intensity of the feature on the high energy side of the main 0 peak decreases relative to that of the main peak, and the feature approximately 2.5 eV below the main peak is eventually replaced by two new ones at the positions found for 02 on Ni although they are not lucid in the data shown in fig. 2c (see also below). The 0 Auger spectrum obtained (from previously adsorbed CO) after 40 min interaction time is therefore similar to that obtained from the adsorption of 0, on Ni(ll0) [8]. These AES results indicate that the electron beam decomposes molecularly adsorbed CO into surface carbide and oxide. Corresponding changes in the C and 0 Auger signal strengths and relative concentrations are discussed below. As CO adsorbs molecularly on Ni( 110) equal C and 0 atomic concentrations
744
M.P. Hooker, J.T. Grant/AES studies of CO on Ni
would be expected in the absence of electron beam effects. For comparison, the relative C and 0 concentrations can be calculated from the Auger data presented here, follow~g the procedure already outlined for studies of CO on ~o(lOO) ]I 11. First, the ionization cross section curves for carbon and oxygen on this Ni sample were both measured and assuming that the maximum cross sections vary as the reciprocal of the square of their threshold ionization energies, a relative carbon to oxygen ionization cross section of 4.1 obtains for an incident electron beam energy of 1 S keV. This means that for equal C and 0 concentrations on Ni( 110) the ratio of C and 0 Auger currents (corrected for analyzer energy-window effects [ 113) shouid equal 4.1. The relative C and 0 Auger currents cannot be measured directly from the derivative form data shown in fig. 2a because their Auger line shapes, are different from each other [ 121 nor can they be obtained by comparison with Auger spectra of CO on Mo(ll0) [l I] because the CO Auger spectra themselves are different. This problem in comparing Auger currents from different elements such as C and 0 can be overcome by integrating % g)(E) spectra twice over a fixed energy interval, as long as the background has been removed from the spectra [ 111. For the work reported here the CO Auger spectra were digitized and then a linear ramp (approximating the background shown in fig. 1) was subtracted from the C and 0 spectra before integration. The results obtained following one integration of the data are shown in fig. 3a. A second integration of C and 0 data over the same energy range as used earlier, 44 eV [l 11,gives the ratio of the measured C/O Auger currents
I
Corbon
ELECTRON
Fig. 3. First integrals
Oxygen
ENERGY
of the respective
-
data shown in fig. 2.
745
M.P. Hooker, .I. T. Gran t/AES studies of CO on Ni
as 2.2 with an experimental uncertainty of 20% obtained by averaging over three adsorption runs. Applying the correction for the analyzer energy window [ 111, the ratio of C to 0 Auger currents from CO on Ni(ll0) as measured by double integration of the Auger spectra is then 2.2 X (5 10/270) = 4.2 with an uncertainty of about 20% which compares well with the value of 4.1 expected from cross section studies based on equal C and 0 atomic concentrations. It is important to note that the application of integration techniques to Auger spectra of CO on Ni and MO both gave C/O atomic ratios of unity whereas the ratio of C to 0 peak-to-peak heights in 72 g)(E) spectra of CO on Ni and MO differ by more than a factor of 2, even after normalizing the data for the same incident electron beam energy. The effects of the electron beam on the C and 0 Auger currents were also measured by double integration of the ?Z z)(E) data. The first integrals of the Auger data in fig. 2b and 2c are shown in figs. 3b and 3c respectively. Note that the two peaks on the low energy side of the main 0 peak obtained after exposure of adsorbed CO to the electron beam for 40 min are clearer in the integral spectrum, fig. 3c, than in the raw derivative form spectrum, fig. 2c, due to the improved signal/noise obtained on integration [ 131. By integrating the data of fig. 3 it is found that both the carbon and oxygen Auger currents decrease with increasing exposure to the electron beam, and that the oxygen Auger current decreases at a faster rate indicating disproportionation. The carbon and oxygen concentrations decreased by 10% and 35% respectively after 10 min beam exposure, and by 30% and 90% after 40 min exposure. In summary, the C and 0 Auger line shapes of CO adsorbed on a clean ion bombarded Ni(ll0) surface have been measured and found to be quite different from other CO adsorption systems studied to date, probably reflecting the weak molecular-like bonding of Co to Ni through the C atom. However, the Auger results show that equal numbers of C and 0 atoms are present on the surface following adsorption, and it is found that the relative surface concentrations of C and 0 are subsequently affected by electron beam interaction. Both C and 0 are removed by the electron beam, the 0 being removed at a faster rate. After prolenged exposure to the electron beam, studies of the C and 0 Auger line shapes show that the CO is decomposed into surface carbide and oxide. We wish to acknowledge
the technical assistance of J. Miller and G. Wolfe. M.P. HOOKER
and J.T. GRANT *
UniversalEnergy Systems, Inc., 319s Plainjield Road, Day ton, Ohio 45432, USA * Research was sponsored by the Air Force Materials Laboratory, United States Air Force, Contract No. F33615-74X-4017.
Air Force
Systems
Command,
M.P. Hooker, J. T. GrantIAES studies of d0 on Ni
146
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ 111 [ 121 [ 131
J.E. Demuth and T.N. Rhodin, Surface Sci. 45 (1974) 249. T.N. Taylor and P.J. Estrup, J. Vacuum Sci. Technol. 10 (1973) 26. H.H. Madden, J. Kiippers and G. Ertl, J. Chem. Phys. 58 (1973) 3401. M.P. Hooker, J.T. Grant and T.W. Haas, J. Vacuum Sci. Technol. 13 (1976) 296. D.E. Eastman, J.E. Demuth and J.M. Baker, J. Vacuum Sci. Technol. 11 (1974) 273. T.W. Haas, J.T. Grant and G.J. Dooley, in: Adsorption-Desorption Phenomena, Ed. F. Ricca (Academic Press, New York, 1972) p. 359. M.P. Hooker, J.T. Grant and T.W. Haas, J. Vacuum Sci. Technol. 12 (1975) 325. J.T. Grant and M.P. Hooker, Solid State Commun., in press. G. Doyen and G. Ertl, Surface Sci. 43 (1974) 197. J.P. Coad and J.C. Riviere, Surface Sci. 25 (1971) 609. J.T. Grant, M.P. Hooker and T.W. Haas, Surface Sci. 46 (1974) 672. l J.T. Grant, T.W. Haas and J.E. Houston, J. Vacuum Sci. Technol. 11 (1974) 227. J.T. Grant, T.W. Haas and J.E. Houston, Surface Sci. 42 (1974) 1.