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Surface structural change of alumina induced by electron impact in CO2, CO, O2 and D2O
275
Journal of Nuclear Materials 151 (1988) 275-280 North-Holland, Amsterdam
SURFACE STRUCTURAL CHANGE IN CO,, CO, 0, AND D,O Siro NAGAI
OF ALUMINA
INDUCED
BY ELECTRON
IMPACT
*
Osaka Laboratory for Radiation Chemistry JAERI,
Mii-minami 25-1, Neyagawa, Osaka 572, Japan
Received 11 August 1987; accepted 14 October 1987
Change of surface structure of alumina by electron impact in CO,, CO, 0, and D,O has been compared with that in ultra-high vacuum (UHV) using Auger electron spectroscopy (AES). Electron impact in CO,, 0, or D,O suppresses the formation of Al metal on the surface which proceeds in UHV while the impact in CO has no effect on the reduction from alumina to Al metal and produces aluminium carbide on the surface. Experiments on the reaction of the Al metal produced by electron beams with CO, and CO showed that the observed effects due to CO, and CO were caused by reaction of the produced Al metal with 0 atoms from CO, and with C atoms from CO, respectively.
1. Introduction Extensive studies have been carried out on particle impact induced desorption of gases from solid surfaces which is one of the fundamental processes in plasma surface interactions in thermonuclear fusion reactors. According to our data compilation for electron stimulated desorption [l] and ion impact and photon stimulated desorption [2], however, most of these studies have been confined to metal and semiconductor surfaces, and only a few studies have been made on insulator surfaces. Since some insulators such as alumina (Al,O,) and MgAlO, are thought of as favored candidates for several critical components of fusion reactors [3], it is important to study the surface processes including desorption occurring during impact of plasma particles. In the previous report [4], we studied surface defects and secondary ion formation by electron and Ar+ ion impact on SiO,, Al,O, and MgO. The surface of SiO, and Al,O, was found to be richer in Si or Al under electron impact. Since such change of surface composition, referred to as surface reduction hereafter, of the insulators may play an important role in the adsorption and desorption phenomena of gases at the surface, we have studied the effects of energy and current of primary electrons, surface temperature, and impurity gases con* Present address: Department of Research, Takasaki Radiation Chemlistry Research Establishment, JAERI, Watanukimachi 1233, Takasaki, Gunma 370-12, Japan.
0022-3115/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
tained in UHV on the surface reduction. This paper reports the results of the effects of CO, and CO at room temperature, and those of the other effects will be reported elsewhere. Alumina was selected here because it was found to be more resistant than SiO, to charging up due to electron impact [4] so that AES measurement could be continued long enough to obtain necessary
data for analysis.
2. Experimental The AES/SIMS apparatus used is this study in the same as described previously [4] except that a differential ion gun (Physical Electronics Division, PerkinElmer) has since been added to the system for an ion source. Alumina samples used were single crystals of sapphire (0.5 mm thick, Shinko Co.). The surface of alumina was cleaned by bombarding it with 4 keV A? ions and 1.5 keV electrons simultaneously until the C KLL Auger signal due to contaminated carbon disappeared completely. The Auger electron spectrum was recorded during electron (1.5 keV, 20 PA) impact in UHV (I 2.7 x lo-’ Pa) or in an atmosphere of CO, (99.99% up, Seitetsu Kagaku), 0, (99.995% up, Seitetsu Kagaku), CO (99.95% up, Seitetsu Kagaku) or D,O (99.75%, Merck). The gas pressure was controlled in the range 1.3 x 10m6 to 6.7 x lop5 Pa. Temperature was ambient, - 300 K.
B.V.
216
S. Nagai / Surface structural change of alumina
3. Results 3. I. Reduction
of alumina surface in UHV
Electron impact on alumina in UHV produces Al metal on the surface, the concentration of which reaches a stationary value on prolonged impact, as reported previously [4]. Our subsequent study on the electron beam induced surface reduction of alumina revealed that the stationary concentration of Al metal depends on the pressure of residual gas, the extent of surface contamination due to carbon, the amount of water adsorbed on the surface, and temperature. The maximum concentration we observed under impact with 1.5 keV electrons at 2.7 x lo-’ Pa at room temperature was ca. 45% of the total Al on the surface. To obtain the depth profile of the Al metal, the alumina sample containing the Al metal produced by electron impact was bombarded with ArC and electrons simultaneously at a sputtering rate of 3.3 A/min while the Auger spectrum was monitored. The Al(M) signal due to Al metal decreased monotonically with the sputtering time and levelled off in 6 min at the level obtained when fresh alumina surface was bombarded with Ar+ and electrons under the identical conditions. This result indicates that the Al metal produced by electron impact is localized in a shallow region of the surface, not deeper than 20 A. The Al metal produced by electron impact was not stable in the absence of electron beams under the vacuum conditions employed here, ca. 2.7 X lo-’ Pa and residual gas consisting mainly of H,, CO, CO, and H,O, and decayed gradually with time. The decay can be ascribed to the reaction between Al metal on the surface and oxygen-containing gases, CO,, O,, and H,O since these gases were found to accelerate the decay of Al metal. 3.2. Electron impact in CO,,
0,
0
-0 Time (min)
Fig. 1. Variation of Auger signal intensity during electron impact on alumina in 1.3X10-’ Pa CO,. For notations, see text.
ever, is much lower than that in a UHV, which suggests that the surface reduction of alumina is suppressed by CO,. In fact, no surface reduction of alumina was observed during electron impact under CO, pressures greater than 6.7 x low5 Pa, as can be seen from fig. 2 which shows the Auger signal intensity ratio, Al(M)/Al(Ox), in UHV and under three different CO, pressures. When electron beams were switched off, Al(M) decayed with time, the decay rate being greater in CO, than in UHV.
I
c
and D,O
Fig. 1 shows the change of intensity of Auger signals, 0 KLL, Al(Ox) due to a cross transition between Al and 0 atoms, and Al(M) due to Al metal, with time of electron impact on alumina in 1.3 X lo-’ Pa CO,. Both the OKLL and Al(Ox) signals decrease in intensity in the initial stage and a new signal of Al(M) grows in intensity with time. The intensities of all these three signals level off after 20 min. These changes in signal intensity agree qualitatively with those previously found by electron impact in UHV [4], showing the occurrence of surface reduction of alumina in a CO, atmosphere. The extent of decrease or increase in signal intensity, how-
Time
(mln)
Fig. 2. Variation of Auger signal intensity ratio of Al(M) to Al(Ox) during electron impact on alumina in UHV (0) and in CO, atmosphere at 6.7 X 10K6 Pa (A), 1.3 x 10e5 Pa (0) and 6.7~10~‘Pa(X).
217
S. Nagai / Surface structural change of alumina
In an attempt to learn the mechanism of the inhibition effect by CO, on the surface reduction, alumina was prebombarded with electrons in CO, at 6.7 X 10m6 Pa until the intensity of Al(M) reached a stationary value, and the resultant surface was brought in contact with 6.7 x lop5 Pa CO,, an environment where no electron impact induced surface reduction proceeded (fig. 2) either in the presence or absence of electron beams. As soon as the surface was exposed to 6.7 X 10m5 Pa CO, in the presence of electron beams, Al(M) disappeared completely and did not reappear under prolonged impact, and the intensities of Al(Ox) and Ox,, returned to the levels observed before electron impact in 6.7 X 10e6 Pa CO,. On the other hand, the exposure in the absence of electron beams resulted in a gradual decrease of Al(M) with a half-life of several min together with gradual increase of Al(Ox) and Ox,, intensities. Above results in the presence of electron beams indicate that Al metal produced by electron impact on alumina is rapidly oxidized to aluminium oxide in 6.7 X 10e5 Pa CO,. It can be expected, therefore, that Al metal, if produced, would not be observed during electron impact on alumina under CO, pressures as high as 6.7 X lop5 Pa, in agreement with the result in fig. 1. Both 0, and D,O were found to inhibit the surface
reduction of alumina during electron impact as well as CO,. The results obtained for 0, agree almost quantitatively with those for CO,: the variations of Al(M)/Al(Ox) with impact time in 6.7 X 10m6 and 1.3 x 10e5 Pa fit the corresponding curves obtained for CO, (fig. 2), and Al(M) did not appear at all in 6.7 X 10m5 Pa 0,. D,O showed a stronger inhibition effect on the surface reduction than CO, and O,, and no surface reduction took place in 1.3 X 10e5 Pa D,O. 3.3. Electron
impact in CO
The change of the Auger spectrum observed in the early stage of electron impact on alumina under CO pressures from 6.7 X 10e6 to 6.7 x 10K5 Pa is in accord with that in a UHV, indicating that CO apparently has no effect on the surface reduction of alumina. By prolonged impact on alumina in CO, however, drastic changes were observed in the Auger spectrum as shown in fig. 3. The C,,,(272) signal appeared, grew in intensity with time and finally leveled off. This C,,, signal is ascribed to carbon in aluminium carbide (Al,Cs) since both the peak energy and signal shape coincide with those reported for this compound [S]. Or&SOS) signal levelled off after an initial decrease in intensity. The spectral change in the low energy region is shown
Initial
Electron impact time(min) ; 41
272 eV 508 eV _
Electron Energy Fig.3. Changeof Auger spectrum of alumina during electron impact in 6.7 x lo-' Pa CO.
278
S. Nagai / Surface structural change
Electron
of alumina
impact
time (min)
;
Fig. 5. Variation of Auger signal intensity during electron impact of alumina in 6.7 X 10K5 Pa CO.
58 eV -Electron
Energy
Fig. 4. Change of low energy Auger spectrum ( - 40-75 ev) of
alumina during electron impact in 6.7 X
lOUs
Pa CO.
in fig. 4 in an expanded scale. The decrease of Al(Ox) in intensity and the appearance of Al(M) and its increase in intensity with time observed in the early stage are the same as observed in UHV. After 40 mm, however, the intensity of Al(M) decreases with time and finally disappeared completely. At the same time, the Al(Ox) signal broadens in width and the peak energy shifts gradually from initial 55 eV to final 58 eV. The final peak energy of 58 eV coincides with that for a cross transition Al,C,C,, denoted here as Al(C), in Al,C, [5]. Therefore, the broad signal with peak energy of 58 eV in fig. 4f is thought to arise from unresolved superposition of Al(Ox) and Al(C). Fig. 5 shows the time dependence of the Auger signal intensity of Al(Ox), [Al(Ox) + Al(C)], Al(M), C KLL and OKLL. It can be seen that the intensity of C KLL increases with time until Al(M) disappears completely. After the disappearance of Al(M), the intensity Of cKLL remains constant as well as the intensities of 0 KLL and [Al(Ox) + Al(C)]. These results indicate that CO reacts with the Al metal on the alumina surface under electron impact to produce Al,C, and that the resultant alumina surface containing Al,C, no longer
undergoes surface reduction at all when electron impact was continued further. In order to find the role of electron impact in the formation of Al,C, from the reaction between the Al metal and CO, an experiment similar to that with CO, described above was carried out. Results are shown in fig. 6. At the first stage, alumina surface was continuously bombarded with electrons in UHV. When the intensity of Al(M) levelled off by 60 min impact, electron beams were switched off and the alumina was exposed to 6.7 x lop5 Pa CO for 60 min. Then the CO gas in the chamber was pumped out and electron beams were switched on again to record Auger spectra. It can be seen from fig. 6 that electron impact on alumina in UHV (first stage) induced a slight deposition of carbon on the surface together with the surface reduction as revealed from the intensity change in Al(Ox), Al(M) signal observed here is identical in and OK,,. The C,,, energy and shape with that observed by electron impact
2
‘O0r
e-EMON
Time (mgn)
Fig. 6. Variation of Auger signal intensity during electron impact in UHV after exposure of prebombarded alumina to 6.7 x 10V5 Pa CO for 60 min in the absence of electron beams (e-BM).
S. Nagai / Surface structural change
in 6.7 x 10m5 Pa CO (see fig. 3d) so that it can be ascribed to carbon in Al& produced from the reaction between the Al metal and CO contained in the residual gas. The intensities of Al(Ox), Al(M) and OKLL observed immediately after electron beams being switched on again are close to the corresponding initial ones at the first stage, indicating that the Al metal produced on alumina surface by electron impact in UHV is almost completely oxidized to alumina in a CO atmosphere in the absence of electron beams. Subsequent change of the intensities of Al(Ox), Al(M) and OKLL observed under prolonged impact is analogous to that during electron impact at the first stage except that the initial rate of the change is faster in the second series of electron impact. It is noted that the intensity of CKLL remained unchanged by exposure of the alumina surface containing Al metal to 6.7 X 10F5 Pa CO, indicating that Al,C, is not produced in the absence of electron beams. Therefore, it can be concluded that the formation of Al,C, is a result of electron beam induced reaction of the Al metal and CO.
4. Discussion The present experiments demonstrate that CO,, 0, and D,O inhibit the surface reduction of alumina which proceeds under electron impact in UHV whereas CO has no such inhibition effect and reacts with the Al metal to produce aluminium carbide (Al,C,) on the surface. Experiments on the reaction of the Al metal produced by electron impact on alumina surface with CO, show that oxidation of Al metal is accelerated by electron impact. This suggests that some electron beam effects are involved in the oxidation, such as electron beam induced adsorption of CO, onto the surface and dissociation of CO,. Although it is known that CO, is readily though weakly adsorbed on alumina [6], no significant carbon signals were observed by AES throughout the present experiment with CO,. This implies that the adsorption of CO, on alumina is weak enough to be desorbed from the surface by electron impact, preventing observation by AES. Accordingly, electron beam enhanced adsorption of CO, seems not to be a possible cause for electron beam acceleration of oxidation of the Al metal. It is possible, however, that some of the adsorbed CO, is dissociated to CO molecules and 0 atoms before desorption from the surface. Although both these two species may react with Al metal to produce the oxide, a reaction between CO and Al metal is not likely to have occurred since the formation of Al,C, that is predomi-
ofalumina
219
nantly produced from the reaction was not observed in COZ. On the other hand, reaction between 0 atoms and Al metal is consistent with the fact that no significant carbon accumulates on the alumina surface during the oxidation of Al metal in CO,. In addition, that this reaction is responsible for electron beam enhanced oxidation of Al has been shown by Falconer et al. [7] in their study on Al films and foils. Therefore, it can be concluded that inhibition of the surface reduction of alumina by CO, arises from the oxidation of the Al metal by ,O atoms produced from electron beam induced dissociation of CO,. The absence of reaction of CO simultaneously produced from the dissociation with Al metal implies that the reactivity is much lower than that of 0 atoms. The inhibition by 0, and D,O is thought to proceed analogously by 0 atoms produced by electron beam induced dissociation of these molecules. The results with CO show that surface reduction of alumina proceeds without any inhibition effect by CO and the Al metal produced reacts with CO to form Al,C, under electron impact while it undergoes oxidation in the absence of electron beams. The latter fact suggests that Al,C, would be produced from the reaction between the Al metal and C atoms produced by electron beam induced dissociation of CO. This reaction has already proposed in electron beam stimulated interaction of CO with a single crystal of Al, Al(111) [5]. On the other hand, the finding that the Al metal on alumina is oxidized in a CO atmosphere without formation of Al,C, in the absence of electron beams contrasts with the previous results on interaction of CO with both single crystals and polycrystals of Al at room temperature: dissociation of CO followed by formation of both aluminium carbide and oxide [8,9], molecular adsorption (51 and no adsorption [lo]. Our result, however, can be understood if we assume that CO does not adsorb on Al metal in the absence of electron beam in agreement with Bargeron et al. [lo], and the oxidation of the Al metal on alumina is induced not by CO but by CO, and H,O contained in a CO atmosphere. The fact that alumina surface containing Al,C, does not undergo surface reduction by electron impact suggests that not only Al,C, is stable in itself but also that it protects alumina from surface reduction. According to the criteria for the stability of ionically bonded surfaces in ionizing environments [ll], compounds with electronegativity difference greater than 1.7 suffer from decomposition by low energy ionizing radiation. The difference for Al,O, is 2.0 while that for Al,C, is 1.0. Thus, the stability of Al,C, found here under electron impact agrees well with the criteria. The protection
280
S. Nagai
/ Surface structural
effect by Al,C, on the surface reduction of alumina parallels with the fact that surface reduction is inhibited when the alumina surface is contaminated with carbon. The reason for this is not understood at present, but one possible reason may be that the presence of carbon species on an alumina surface favors adsorption of CO,, H,O or O2 which serves as a source of 0 atoms when bombarded with electrons.
5. Summary
The surface reduction which proceeds during electron impact in UHV is inhibited in CO,, 0, and D,O atmospheres. The inhibition effect by these gases is ascribed to oxidation of the Al metal once produced by electron impact by oxygen atoms produced from electron beam induced dissociation of these gases. On the other hand, CO has no such inhibition effect and reacts with the Al metal to produce aluminium carbide (Al,C,). The formation of Al,C, arises from the reaction between the Al metal and C atoms produced by dissociation of CO. The aluminium surface containing
change of alumina
Al,C3 is stable against electron impact in contrast to the original alumina surface. References T. Oshiyama, S. Nagai. K. Ozawa and F. Takeuchi, Japan Atomic Energy Research Institute Report, JAERI-M 84094 (1984). S. Nagai, K. Ozawa and F. Takeuchi. ]21 T. Oshiyama, JAERI-M 85-100 (1985). ]31 J.L. Scott, F.W. Clinard Jr. and F.W. Wiffen, J. Nucl. Mater. 133 & 134 (1985) 156. ]41 S. Nagai and Y. Shimizu, J. Nucl. Mater. 128 & 129 (1984) 605. and C.W.B. Martinsson, Appl. Surf. Sci. ]51 S.A. Flodstrbm 10 (1982) 115. WI D. Norfolk, Radiat. Res. Rev. 5 (1974) 373. ]71 J.L. Falconer, S.D. Bischke and G.J. Hanna, Surf. Sci. 20 (1984) 97. and Y. Katayama, Surf. Sci. ]81 Y. Shiraki, K.L.I. Kobayashi 77 (1978) 458; Y. Katayama, K.L.I. Kobayashi and Y. Shiraki, Surf. Sci. 86 (1979) 549. J. Darville and J.M. Gilles, J. Vat. Sci. ]91 K. Khonde, Technol. 20 (1982) 834; Surf. Sci. 126 (1983) 414. 1101 C.B. Bargeron and B.H. Nall, Surf. Sci. 119 (1982) L319. [ll] M.L. Knotek and P.J. Feibelman, Surf. Sci. 90 (1979) 78.
Journal of Nuclear Materials 151 (1988) 275-280 North-Holland, Amsterdam
SURFACE STRUCTURAL CHANGE IN CO,, CO, 0, AND D,O Siro NAGAI
OF ALUMINA
INDUCED
BY ELECTRON
IMPACT
*
Osaka Laboratory for Radiation Chemistry JAERI,
Mii-minami 25-1, Neyagawa, Osaka 572, Japan
Received 11 August 1987; accepted 14 October 1987
Change of surface structure of alumina by electron impact in CO,, CO, 0, and D,O has been compared with that in ultra-high vacuum (UHV) using Auger electron spectroscopy (AES). Electron impact in CO,, 0, or D,O suppresses the formation of Al metal on the surface which proceeds in UHV while the impact in CO has no effect on the reduction from alumina to Al metal and produces aluminium carbide on the surface. Experiments on the reaction of the Al metal produced by electron beams with CO, and CO showed that the observed effects due to CO, and CO were caused by reaction of the produced Al metal with 0 atoms from CO, and with C atoms from CO, respectively.
1. Introduction Extensive studies have been carried out on particle impact induced desorption of gases from solid surfaces which is one of the fundamental processes in plasma surface interactions in thermonuclear fusion reactors. According to our data compilation for electron stimulated desorption [l] and ion impact and photon stimulated desorption [2], however, most of these studies have been confined to metal and semiconductor surfaces, and only a few studies have been made on insulator surfaces. Since some insulators such as alumina (Al,O,) and MgAlO, are thought of as favored candidates for several critical components of fusion reactors [3], it is important to study the surface processes including desorption occurring during impact of plasma particles. In the previous report [4], we studied surface defects and secondary ion formation by electron and Ar+ ion impact on SiO,, Al,O, and MgO. The surface of SiO, and Al,O, was found to be richer in Si or Al under electron impact. Since such change of surface composition, referred to as surface reduction hereafter, of the insulators may play an important role in the adsorption and desorption phenomena of gases at the surface, we have studied the effects of energy and current of primary electrons, surface temperature, and impurity gases con* Present address: Department of Research, Takasaki Radiation Chemlistry Research Establishment, JAERI, Watanukimachi 1233, Takasaki, Gunma 370-12, Japan.
0022-3115/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
tained in UHV on the surface reduction. This paper reports the results of the effects of CO, and CO at room temperature, and those of the other effects will be reported elsewhere. Alumina was selected here because it was found to be more resistant than SiO, to charging up due to electron impact [4] so that AES measurement could be continued long enough to obtain necessary
data for analysis.
2. Experimental The AES/SIMS apparatus used is this study in the same as described previously [4] except that a differential ion gun (Physical Electronics Division, PerkinElmer) has since been added to the system for an ion source. Alumina samples used were single crystals of sapphire (0.5 mm thick, Shinko Co.). The surface of alumina was cleaned by bombarding it with 4 keV A? ions and 1.5 keV electrons simultaneously until the C KLL Auger signal due to contaminated carbon disappeared completely. The Auger electron spectrum was recorded during electron (1.5 keV, 20 PA) impact in UHV (I 2.7 x lo-’ Pa) or in an atmosphere of CO, (99.99% up, Seitetsu Kagaku), 0, (99.995% up, Seitetsu Kagaku), CO (99.95% up, Seitetsu Kagaku) or D,O (99.75%, Merck). The gas pressure was controlled in the range 1.3 x 10m6 to 6.7 x lop5 Pa. Temperature was ambient, - 300 K.
B.V.
216
S. Nagai / Surface structural change of alumina
3. Results 3. I. Reduction
of alumina surface in UHV
Electron impact on alumina in UHV produces Al metal on the surface, the concentration of which reaches a stationary value on prolonged impact, as reported previously [4]. Our subsequent study on the electron beam induced surface reduction of alumina revealed that the stationary concentration of Al metal depends on the pressure of residual gas, the extent of surface contamination due to carbon, the amount of water adsorbed on the surface, and temperature. The maximum concentration we observed under impact with 1.5 keV electrons at 2.7 x lo-’ Pa at room temperature was ca. 45% of the total Al on the surface. To obtain the depth profile of the Al metal, the alumina sample containing the Al metal produced by electron impact was bombarded with ArC and electrons simultaneously at a sputtering rate of 3.3 A/min while the Auger spectrum was monitored. The Al(M) signal due to Al metal decreased monotonically with the sputtering time and levelled off in 6 min at the level obtained when fresh alumina surface was bombarded with Ar+ and electrons under the identical conditions. This result indicates that the Al metal produced by electron impact is localized in a shallow region of the surface, not deeper than 20 A. The Al metal produced by electron impact was not stable in the absence of electron beams under the vacuum conditions employed here, ca. 2.7 X lo-’ Pa and residual gas consisting mainly of H,, CO, CO, and H,O, and decayed gradually with time. The decay can be ascribed to the reaction between Al metal on the surface and oxygen-containing gases, CO,, O,, and H,O since these gases were found to accelerate the decay of Al metal. 3.2. Electron impact in CO,,
0,
0
-0 Time (min)
Fig. 1. Variation of Auger signal intensity during electron impact on alumina in 1.3X10-’ Pa CO,. For notations, see text.
ever, is much lower than that in a UHV, which suggests that the surface reduction of alumina is suppressed by CO,. In fact, no surface reduction of alumina was observed during electron impact under CO, pressures greater than 6.7 x low5 Pa, as can be seen from fig. 2 which shows the Auger signal intensity ratio, Al(M)/Al(Ox), in UHV and under three different CO, pressures. When electron beams were switched off, Al(M) decayed with time, the decay rate being greater in CO, than in UHV.
I
c
and D,O
Fig. 1 shows the change of intensity of Auger signals, 0 KLL, Al(Ox) due to a cross transition between Al and 0 atoms, and Al(M) due to Al metal, with time of electron impact on alumina in 1.3 X lo-’ Pa CO,. Both the OKLL and Al(Ox) signals decrease in intensity in the initial stage and a new signal of Al(M) grows in intensity with time. The intensities of all these three signals level off after 20 min. These changes in signal intensity agree qualitatively with those previously found by electron impact in UHV [4], showing the occurrence of surface reduction of alumina in a CO, atmosphere. The extent of decrease or increase in signal intensity, how-
Time
(mln)
Fig. 2. Variation of Auger signal intensity ratio of Al(M) to Al(Ox) during electron impact on alumina in UHV (0) and in CO, atmosphere at 6.7 X 10K6 Pa (A), 1.3 x 10e5 Pa (0) and 6.7~10~‘Pa(X).
217
S. Nagai / Surface structural change of alumina
In an attempt to learn the mechanism of the inhibition effect by CO, on the surface reduction, alumina was prebombarded with electrons in CO, at 6.7 X 10m6 Pa until the intensity of Al(M) reached a stationary value, and the resultant surface was brought in contact with 6.7 x lop5 Pa CO,, an environment where no electron impact induced surface reduction proceeded (fig. 2) either in the presence or absence of electron beams. As soon as the surface was exposed to 6.7 X 10m5 Pa CO, in the presence of electron beams, Al(M) disappeared completely and did not reappear under prolonged impact, and the intensities of Al(Ox) and Ox,, returned to the levels observed before electron impact in 6.7 X 10e6 Pa CO,. On the other hand, the exposure in the absence of electron beams resulted in a gradual decrease of Al(M) with a half-life of several min together with gradual increase of Al(Ox) and Ox,, intensities. Above results in the presence of electron beams indicate that Al metal produced by electron impact on alumina is rapidly oxidized to aluminium oxide in 6.7 X 10e5 Pa CO,. It can be expected, therefore, that Al metal, if produced, would not be observed during electron impact on alumina under CO, pressures as high as 6.7 X lop5 Pa, in agreement with the result in fig. 1. Both 0, and D,O were found to inhibit the surface
reduction of alumina during electron impact as well as CO,. The results obtained for 0, agree almost quantitatively with those for CO,: the variations of Al(M)/Al(Ox) with impact time in 6.7 X 10m6 and 1.3 x 10e5 Pa fit the corresponding curves obtained for CO, (fig. 2), and Al(M) did not appear at all in 6.7 X 10m5 Pa 0,. D,O showed a stronger inhibition effect on the surface reduction than CO, and O,, and no surface reduction took place in 1.3 X 10e5 Pa D,O. 3.3. Electron
impact in CO
The change of the Auger spectrum observed in the early stage of electron impact on alumina under CO pressures from 6.7 X 10e6 to 6.7 x 10K5 Pa is in accord with that in a UHV, indicating that CO apparently has no effect on the surface reduction of alumina. By prolonged impact on alumina in CO, however, drastic changes were observed in the Auger spectrum as shown in fig. 3. The C,,,(272) signal appeared, grew in intensity with time and finally leveled off. This C,,, signal is ascribed to carbon in aluminium carbide (Al,Cs) since both the peak energy and signal shape coincide with those reported for this compound [S]. Or&SOS) signal levelled off after an initial decrease in intensity. The spectral change in the low energy region is shown
Initial
Electron impact time(min) ; 41
272 eV 508 eV _
Electron Energy Fig.3. Changeof Auger spectrum of alumina during electron impact in 6.7 x lo-' Pa CO.
278
S. Nagai / Surface structural change
Electron
of alumina
impact
time (min)
;
Fig. 5. Variation of Auger signal intensity during electron impact of alumina in 6.7 X 10K5 Pa CO.
58 eV -Electron
Energy
Fig. 4. Change of low energy Auger spectrum ( - 40-75 ev) of
alumina during electron impact in 6.7 X
lOUs
Pa CO.
in fig. 4 in an expanded scale. The decrease of Al(Ox) in intensity and the appearance of Al(M) and its increase in intensity with time observed in the early stage are the same as observed in UHV. After 40 mm, however, the intensity of Al(M) decreases with time and finally disappeared completely. At the same time, the Al(Ox) signal broadens in width and the peak energy shifts gradually from initial 55 eV to final 58 eV. The final peak energy of 58 eV coincides with that for a cross transition Al,C,C,, denoted here as Al(C), in Al,C, [5]. Therefore, the broad signal with peak energy of 58 eV in fig. 4f is thought to arise from unresolved superposition of Al(Ox) and Al(C). Fig. 5 shows the time dependence of the Auger signal intensity of Al(Ox), [Al(Ox) + Al(C)], Al(M), C KLL and OKLL. It can be seen that the intensity of C KLL increases with time until Al(M) disappears completely. After the disappearance of Al(M), the intensity Of cKLL remains constant as well as the intensities of 0 KLL and [Al(Ox) + Al(C)]. These results indicate that CO reacts with the Al metal on the alumina surface under electron impact to produce Al,C, and that the resultant alumina surface containing Al,C, no longer
undergoes surface reduction at all when electron impact was continued further. In order to find the role of electron impact in the formation of Al,C, from the reaction between the Al metal and CO, an experiment similar to that with CO, described above was carried out. Results are shown in fig. 6. At the first stage, alumina surface was continuously bombarded with electrons in UHV. When the intensity of Al(M) levelled off by 60 min impact, electron beams were switched off and the alumina was exposed to 6.7 x lop5 Pa CO for 60 min. Then the CO gas in the chamber was pumped out and electron beams were switched on again to record Auger spectra. It can be seen from fig. 6 that electron impact on alumina in UHV (first stage) induced a slight deposition of carbon on the surface together with the surface reduction as revealed from the intensity change in Al(Ox), Al(M) signal observed here is identical in and OK,,. The C,,, energy and shape with that observed by electron impact
2
‘O0r
e-EMON
Time (mgn)
Fig. 6. Variation of Auger signal intensity during electron impact in UHV after exposure of prebombarded alumina to 6.7 x 10V5 Pa CO for 60 min in the absence of electron beams (e-BM).
S. Nagai / Surface structural change
in 6.7 x 10m5 Pa CO (see fig. 3d) so that it can be ascribed to carbon in Al& produced from the reaction between the Al metal and CO contained in the residual gas. The intensities of Al(Ox), Al(M) and OKLL observed immediately after electron beams being switched on again are close to the corresponding initial ones at the first stage, indicating that the Al metal produced on alumina surface by electron impact in UHV is almost completely oxidized to alumina in a CO atmosphere in the absence of electron beams. Subsequent change of the intensities of Al(Ox), Al(M) and OKLL observed under prolonged impact is analogous to that during electron impact at the first stage except that the initial rate of the change is faster in the second series of electron impact. It is noted that the intensity of CKLL remained unchanged by exposure of the alumina surface containing Al metal to 6.7 X 10F5 Pa CO, indicating that Al,C, is not produced in the absence of electron beams. Therefore, it can be concluded that the formation of Al,C, is a result of electron beam induced reaction of the Al metal and CO.
4. Discussion The present experiments demonstrate that CO,, 0, and D,O inhibit the surface reduction of alumina which proceeds under electron impact in UHV whereas CO has no such inhibition effect and reacts with the Al metal to produce aluminium carbide (Al,C,) on the surface. Experiments on the reaction of the Al metal produced by electron impact on alumina surface with CO, show that oxidation of Al metal is accelerated by electron impact. This suggests that some electron beam effects are involved in the oxidation, such as electron beam induced adsorption of CO, onto the surface and dissociation of CO,. Although it is known that CO, is readily though weakly adsorbed on alumina [6], no significant carbon signals were observed by AES throughout the present experiment with CO,. This implies that the adsorption of CO, on alumina is weak enough to be desorbed from the surface by electron impact, preventing observation by AES. Accordingly, electron beam enhanced adsorption of CO, seems not to be a possible cause for electron beam acceleration of oxidation of the Al metal. It is possible, however, that some of the adsorbed CO, is dissociated to CO molecules and 0 atoms before desorption from the surface. Although both these two species may react with Al metal to produce the oxide, a reaction between CO and Al metal is not likely to have occurred since the formation of Al,C, that is predomi-
ofalumina
219
nantly produced from the reaction was not observed in COZ. On the other hand, reaction between 0 atoms and Al metal is consistent with the fact that no significant carbon accumulates on the alumina surface during the oxidation of Al metal in CO,. In addition, that this reaction is responsible for electron beam enhanced oxidation of Al has been shown by Falconer et al. [7] in their study on Al films and foils. Therefore, it can be concluded that inhibition of the surface reduction of alumina by CO, arises from the oxidation of the Al metal by ,O atoms produced from electron beam induced dissociation of CO,. The absence of reaction of CO simultaneously produced from the dissociation with Al metal implies that the reactivity is much lower than that of 0 atoms. The inhibition by 0, and D,O is thought to proceed analogously by 0 atoms produced by electron beam induced dissociation of these molecules. The results with CO show that surface reduction of alumina proceeds without any inhibition effect by CO and the Al metal produced reacts with CO to form Al,C, under electron impact while it undergoes oxidation in the absence of electron beams. The latter fact suggests that Al,C, would be produced from the reaction between the Al metal and C atoms produced by electron beam induced dissociation of CO. This reaction has already proposed in electron beam stimulated interaction of CO with a single crystal of Al, Al(111) [5]. On the other hand, the finding that the Al metal on alumina is oxidized in a CO atmosphere without formation of Al,C, in the absence of electron beams contrasts with the previous results on interaction of CO with both single crystals and polycrystals of Al at room temperature: dissociation of CO followed by formation of both aluminium carbide and oxide [8,9], molecular adsorption (51 and no adsorption [lo]. Our result, however, can be understood if we assume that CO does not adsorb on Al metal in the absence of electron beam in agreement with Bargeron et al. [lo], and the oxidation of the Al metal on alumina is induced not by CO but by CO, and H,O contained in a CO atmosphere. The fact that alumina surface containing Al,C, does not undergo surface reduction by electron impact suggests that not only Al,C, is stable in itself but also that it protects alumina from surface reduction. According to the criteria for the stability of ionically bonded surfaces in ionizing environments [ll], compounds with electronegativity difference greater than 1.7 suffer from decomposition by low energy ionizing radiation. The difference for Al,O, is 2.0 while that for Al,C, is 1.0. Thus, the stability of Al,C, found here under electron impact agrees well with the criteria. The protection
280
S. Nagai
/ Surface structural
effect by Al,C, on the surface reduction of alumina parallels with the fact that surface reduction is inhibited when the alumina surface is contaminated with carbon. The reason for this is not understood at present, but one possible reason may be that the presence of carbon species on an alumina surface favors adsorption of CO,, H,O or O2 which serves as a source of 0 atoms when bombarded with electrons.
5. Summary
The surface reduction which proceeds during electron impact in UHV is inhibited in CO,, 0, and D,O atmospheres. The inhibition effect by these gases is ascribed to oxidation of the Al metal once produced by electron impact by oxygen atoms produced from electron beam induced dissociation of these gases. On the other hand, CO has no such inhibition effect and reacts with the Al metal to produce aluminium carbide (Al,C,). The formation of Al,C, arises from the reaction between the Al metal and C atoms produced by dissociation of CO. The aluminium surface containing
change of alumina
Al,C3 is stable against electron impact in contrast to the original alumina surface. References T. Oshiyama, S. Nagai. K. Ozawa and F. Takeuchi, Japan Atomic Energy Research Institute Report, JAERI-M 84094 (1984). S. Nagai, K. Ozawa and F. Takeuchi. ]21 T. Oshiyama, JAERI-M 85-100 (1985). ]31 J.L. Scott, F.W. Clinard Jr. and F.W. Wiffen, J. Nucl. Mater. 133 & 134 (1985) 156. ]41 S. Nagai and Y. Shimizu, J. Nucl. Mater. 128 & 129 (1984) 605. and C.W.B. Martinsson, Appl. Surf. Sci. ]51 S.A. Flodstrbm 10 (1982) 115. WI D. Norfolk, Radiat. Res. Rev. 5 (1974) 373. ]71 J.L. Falconer, S.D. Bischke and G.J. Hanna, Surf. Sci. 20 (1984) 97. and Y. Katayama, Surf. Sci. ]81 Y. Shiraki, K.L.I. Kobayashi 77 (1978) 458; Y. Katayama, K.L.I. Kobayashi and Y. Shiraki, Surf. Sci. 86 (1979) 549. J. Darville and J.M. Gilles, J. Vat. Sci. ]91 K. Khonde, Technol. 20 (1982) 834; Surf. Sci. 126 (1983) 414. 1101 C.B. Bargeron and B.H. Nall, Surf. Sci. 119 (1982) L319. [ll] M.L. Knotek and P.J. Feibelman, Surf. Sci. 90 (1979) 78.