Langmuir 1987,3, 1161-1167
1161
Electron-Stimulated Decomposition of Alkyl and Fluoroalkyl Ethers Adsorbed on A1203 L. Ng, J. G . Chen, P.Basu, and J. T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received May 5, 1987. I n Final Form: July 28, 1987 The interaction of model ether and fluoro ether molecules with atomically clean A1203has been studied by using electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), and thermal desorption spectrometry (TDS) methods. Thin (-5 A) films of A1203have been produced on an Al(111) surface for study with the adsorbed ether molecules. It has been found that (CF,H),O and (CH3)20are chemically unreactive with A1203and that thermal desorption occurs without molecular decomposition in both cases. (CF2H),0 is liberated from the adsorbed monolayer with an activation energy of -8 kcal/mol. EELS indicates that the ether molecules are adsorbed on A1203 without significant perturbation of the vibrational modes. Electron impact (300 eV) on both (CH3)20and (CF2H),0 layers leads to rapid electron-stimulated decomposition (ESD) with a cross section of the order of 7 X lo-'' cm2. Both F- and C-containing surface species are produced by ESD as observed by irreversible modification of the A1203 EELS spectra as well as by Auger spectroscopy. The implication of these findings to the degradation of fluoro ether lubricants on A1203is discussed. Introduction Molecules that act as weak donor ligands have been extensively studied in coordination and surface chemistry.' These ligands act as weak electron donors and coordinate to transition-metal atoms via a lone pair or K orbital. Examples of such molecules are water, alcohols, ethers, ketones, sulfoxides, and nitriles. Among these classes of molecules, ethers are generally characterized by their low reactivity with common oxidizing and reducing agents and their high solvation capability for most organic compounds. Ethers are excellent solvents for organic reactions including the preparation of Grignard reagents used extensively in organic chemistry. The Mg atom in a Grignard reagent is known to be stabilized by the lone-pair electrons on the 0 atom in the ether through coordination.2 The fluorinated analogues of the ether have also been studied and are found to be more reactive toward alkali metals but less reactive toward other reagentsq3 The adsorption of dimethyl ether, (CH3),0, on transition-metal surfaces has been reported in several papers.@ On clean polycrystalline P d surfaces, (CH3)ZO was found to chemisorb on the surface primarily via the lone-pair orbitals associated with the oxygen atoms.4 At 300 K, (CH3),0 undergoes a thermally activated decomposition reaction on P d resulting in the production of a layer of chemisorbed CO.4 For (CH3),0 adsorbed on clean Pt(ll1) and Cu(100) surfaces, the following behavior is found (1) the molecular ether weakly "chemisorbs" on both surfaces with low heats of adsorption; (2) the interaction with the surfaces occurs via the oxygen lone pair on the ether molecule with a stronger interaction for Pt(ll1) than for cu(100).59 The interaction of this class of molecules with other atomically clean metal surfaces such as A1 has not been (1)For a recent review, see: Albert, M. R.; Yates, J. T., Jr. In The Surface Scientist's Guide to Organometallic Chemistry; American Chemical Society: Washington, DC, 1987. (2) Solomons, T. W. G. In Organic Chemistry; Wiles New York, 1976. (3) Sheppard, W. A.; Sharts, C. M. In Organic Fluorine Chemistry; W. A. Benjamin: New York, 1969. (4)Liith, H.; Rubloff, G. W.; Grobman, W. D. Surf. Sci. 1977,63,325. ( 5 ) Holloway, P. H.; Madey, T.E.; Campbell, C. T.; Rye, R. R.; Houston, J. E. Surf. Sci. 1979,88,121. (6)Sexton, B. A. Surf. Sci. 1981,102,271. (7)Sexton, B.A.; Rendulic, K. D.; Hughes, A. E. Surf. Sci. 1983,124, 162. (8) Rendulic, K. D.; Sexton, B. A. J . Catal. 1982,78, 126. (9)Sexton, B. A.; Hughes, A. E. Surf. Sci. 1984,140,227.
0743-7463/87/2403-ll61$01.50/0
studied to the best of our knowledge. Also, there has been no study of fluorinated ethers adsorbed on metal surfaces. We have studied in detail the interaction of dimethyl ether with Al(111), and the results will be presented elsewhere.1° In this paper, we report the interaction of dimethyl ether, (CH3)2O, and bis(difluoromethy1) ether, (CF,H),O, with an A1203 film produced on an Al(111) crystal. Our purpose is to gain some insight into the reactivity of this class of organic molecules with A1203. Experimental Section The details of the ultrahigh vacuum system used in the experiments described here have been given in a previous publication." Briefly, the three-level UHV chamber is equipped with facilities for EELS, AES,and TDS measurements. The chamber is pumped by a 150 L/s turbomolecular pump, a 220 L/s triode ion pump, and a liquid nitrogen cooled Ti sublimation pump. The mbar for EELS base pressure in the chamber was
Figure 6. Vibrational spectra of (CF,H),O adsorption on A1203 as a function of (CF,H),O exposure at 90 K.
L
e
c .-
e
-
Figure 3f is a typical vibrational spectrum of aluminum oxide and carbide based on previous studies.16J7 The Auger spectrum recorded for the surface layer of Figure 3f shows the presence of carbon on the surface. This observation thus indicates that, unlike the thermal behavior of (CH3)20/A1203, electron irradiation induces the
decomposition of (CHJ20 molecules and the formation of AI-C species on the surface. Adsorption of (CF,H),O on Al(ll1). Figure 4 shows the comparison of a representative EEL spectrum of (CF,H),O with one of (CH3)20of comparable exposure on Al(111). The (CF2H),0/A1(111) spectrum is dominated by a highly intense loss feature at 1170 cm-l which we assign to the C-F stretching mode, u(C-F).l* The small feature at 2330 cm-' is assigned to the double loss mode, 2u(C-F). There are also very weak features at 565 and 805 cm-'. The intensities of these features are less than 1% of that of the dominant v(C-F) peak. Adsorption of (CF,H),O on A1203. 1. Thermal Desorption of the (C-F2HJ20Adlayer. Thermal desorption spectra of (CF,H),O' a t -various exposures are presented in Figure 5. The adsorbed (CF2H),O/A&O3 layers were prepared by exposing the A1203 film to (CF,H),O molecules a t 90 K. The mass spectrometer signal for m u 51 (CF2H+)was monitored in each case and is the major mass spectrometer cracking product of (CF2H),0.19 A t the lowest exposure (Figure 5a), a single desorption feature, al,is observed a t -132 K. At higher exposures, additional desorption features appear, one a t Tpzi 121 K (a2)and another a t 117 K (a3).The latter is indicative of desorption from a multilayer of (CF2H),0. The activation energies of desorption calculated for the
(16) Chen, J. G.;Crowell, J. E.; Sates, J. T., Jr. Surf. Sci. 1986,172, 733. (17) Crowell, J. E.;Chen, J. G.; Sates, J. T., Jr. J. Chem. Phys. 1986, 85,3111.
(18) Herzberg, G. In Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand New Sork, 1945; p 315. (19) Mass spectrometer (70 eV) cracking products of (CF2H)20: CFZH', 66%; CHO*, 19%; CO', 9%; CFHO', 7%; (CF*H)20, el%.
2,
.-4l-n c
al
c
c
U
L
e,
c
E
E
+
0 e,
n
v, ln
ln
r" I
100
1
1
120
l
I
140
I
l
160
I
I
180
Temperature ( K )
Figure 5. Thermal desorption spectra of adsorbed (CF2H)20on A120,; dT/dt = 1 K 5-l.
Ether Decomposition of A1203
Langmuir, Vol. 3, No. 6, 1987 1165
three TPD features observed in the spectrum of Figure 5d are a1 = 8, a, = 7.5, and a3 = 7.2 kcal/mol, calculated by using an assumed first-order preexponential factor of 1013 s-1*20
I
2. EELS Study and Thermal Treatment of the (CF,H),O Adlayer. EELS was employed to study the adsorption of (CF,H),O on A1203. The spectra as a function of (CF,H),O exposures are shown in Figure 6. Figure 6a shows the clean A1203.All of the phonon modes of Al,03 are well resolved. The double loss and combination modes of A1203 are also visible. After the adsorption of a small amount of (CF,H),O at 90 K, the following vibrational modes related to the (CF,H),O overlayer were observed v(C-F) at -1135 cm-' and v(C-H) a t 3055 cm-l (Figure 6b,c). After increased exposure of the A1203layers to (CF,H),O, additional features are observed at -1345-1380 and -2270 cm-' (Figure 6d,e). The feature a t 1345-1380 cm-l is assigned to the 6(C-H) vibrational mode, analogous to the assignment given to (CH3),0. The 2270-cm-' feature is assigned to the double excitation, 2v(C-F), mode. The loss feature a t 1135 cm-I increased drastically in intensity and shifted to -1170 cm-' at an exposure of 3.6 X 1015molecules/cm2. A t this coverage the v(C-F) mode a t 1170 cm-I is the dominant feature in the spectrum. The v(C-H) vibrational mode a t -3055 cm-I also increased in intensity. A summary of the assigned vibrational modes for (CF2H),O/Al(1ll) and (CF2H)20/A1203is shown in Table 11. When the adlayer was heated, (CF,H),O desorbed molecularly in the temperature range 110-140 K, leaving behind a clean A1203surface as characterized by Figure 6f. Neither strong chemisorption nor thermally induced decomposition was observed. 3. Electron-Stimulated Decomposition of the (CF2H),0 Adlayer. To investigate the electron-stimulated reactivity of (CF2H),0 with A1203,the (CF2H),0 adlayer was systematically bombarded by 300-eY electrons as previously described. Analogous to the case of (CH3)2O, the controlled electron bombardment of the (CF2H)20/A1203 layer produced a dramatic surface reaction involving the A1203 layer. EELS studies of the electron-stimulated reaction as a function of electron fluence are shown in Figure 7. Figure 7a is the vibrational spectrum of clean A1203.The A1203 layer was exposed to 4.3 X 1015molecules/cm2 of (CF,H),O and an EEL spectrum taken (Figure 7b). This characteristic spectrum of (CF,H),O on Alz03shows the prominent v(C-F) vibrational mode at -1170 cm-l, the 6(C-H) mode at -1385 cm-l, and the v(C-H) mode at -3075 cm-l. Upon irradiation of this layer with an electron fluence between 2.5 X 1014and 4.8 X 1015e-/cm2, a gradual decrease in the intensities of the v(C-F), 6(C-H), and v(C-H) modes resulted. After bombardment by 2 x 10l6e-/cm2, vibrational modes related to molecularly adsorbed (CF,H)20 were no longer observed. Comparison of Figure 7f and 7a, however, shows that the nature of the surface resulting from the electron bombardment of the (CF2H)20/A1203 layer is vastly different from that of the clean A1203. This is indicated by the following: (1) in figure 7f, a very broad feature is centered a t -870 cm-', as compared to the well-resolved v(Al-0) features in Figure 7a; (2) the loss feature in Figure 7f is approximately 2.5 times more intense than the feature in Figure 7a. In an effort to identify the adsorbed species on the surface of Figure 7f, an Auger spectrum was recorded after
-
(20) Redhead, P. A. Vacuum 1962,12, 203.
Electron Stimulated Decomposition of (CF,H),O on AI,03/Al ( 1 1 1 ) '
!
'
,
!
,
I
,
(CF,H),O/AI,O,/AI(II~) 870
Tadr = 9 0 K Exposure :
I
x3\s
'.d\.
4 . 3 ~ 1 0molcculer.cm-' '~
,
x4I
V,c = 3- 0- 0- V
1710
-
\I105
200
2.5
0
I
I
1000 2000 3000 4000 Energy t o s s (cm-' 1
0
Figure 7. Vibrational spectra recorded following electron bombardment of an adsorbed layer of (CFzH)20on AZO, a function of electron fluence.
Stability of A I - F Species Resulting from ESD of (CF, H 1, 0 on AI,O, I
I
AES F(KLL) TRANSITION
4 r"""
( d ) Heating to 700 K
V ( c ) Additional
1 . 210"e-/cm' ~
(b) After the electron induced decomposition of (CF,H),O ?.OXIO" e-/"
( a ) Clean Al,O, *cIIMyLyLu*.LNIyI
I
I
700 ELECTRON ENERGY (eV) Figure 8. Auger spectra of F(KLL) transition recorded (a) for clean A1203 (b) after exposure to 2.0 X 10l6e-/cm2, (c) after exposure to an additional 1.2 X 10" e-/cm2,and (d) after heating to 700 K.
600
the EELS experiment. In addition to the oxygen Auger feature, fairly strong Auger transitions for carbon and
1166 Langmuir, Vol. 3, No. 6, 1987 fluorine were observed (Figure 8b). Since the EEL measurement indicates that the C-F bond is being destroyed by electron impact and F remains on the A1203 surface, we conclude that it is most likely that an A1-F bond has been produced, on the basis of its extreme stability during heating and during further ESD, as will be shown. The stability of this A1-F bond was further studied by additional electron bombardment and by heating the crystal to 700 K. The F(KLL) Auger transition remained unchanged (Figure &,d), indicating no scission of the Al-F bond by either desorption procedure. The fact that the A1-F bond is stable toward both electron bombardment and heating indicates that once the A1-F bond is formed with the A1203 substrate, irreversible changes have occurred in the A1203film.
Ng et al. Electron Stimulated Decomposition of Two Ethers Adsorbed on A1203
(CF2H O ), /AI,O,
0
.
-1
t -31 0
I
'
4
'
'
8
'
'
12
'
'
16
20 x
io15
Discussion Comparison of the Thermal Decomposition and Electron-Stimulated Decomposition of (CH3)20and (CF,H),O. It is well-known that electron beam irradiation causes damage on surface layers and films by charging, heating, or electronic excitation of the adsorbed molecules.21*22Charge accumulation in nonconductors such as oxides and organic layers can cause enhanced diffusion of ions in the surface region that leads to changes in layer composition. Heating may cause segregation or chemical reaction. For chemisorbed and substrate molecules, electronic excitation may result in dissociation and/or desorption of the excited species. For weakly bound molecules, electronic excitation often leads to dissociation and adsorption of fragment species.21,22 A comparison of Figure 2f with Figure 3f and of Figure 6f with Figure 7f shows that the surface layers which remain after electron-stimulated decomposition of (CH3),0 and (CF2H),0 differ significantly from those which remain after thermal desorption of the adsorbed species. In Figures 2f and 6f, EELS spectra of the unperturbed Al,O, layers reappeared after thermal desorption of the adsorbed species. However, after ESD (Figures 3f and 7f), and the well-resolved Al,03 phonon modes were no longer observable. Previous studies in our laboratory'l on the electron bombardment (300 eV) of a pure A 1 2 0 3 thin f i i on Al(111) have shown no observable changes in the EEL spectrum after an electron fluence of 1.5 X 1019e-/cm2, which is over 700-fold higher than that used on the (CF2H),0/A1,03 layer and over 300 times higher than that used on the (CH3),O/Al2O3layer. The modification of the A1203film upon electron bombardment of both adsorbed ethers is therefore due to the adsorption of the dissociated ether fragments onto the surface. Due to the fact that the vibrational frequencies of v(Al-F) are at 570-835 cm-' 23 and of v(A1-C) are a t 400-800 cm-I,,* the broad peak observed in Figure 7f is likely due to overlapping v(Al-F), v(Al-C), and v(A1-0) vibrational modes, while that of Figure 3f is due to v(A1-C) and v(A1-0) vibrational modes. Although we cannot exclude the possibility that some ESD-induced polymerization may occur, the disappearance of v(C-H) intensity for both ethers would suggest that molecular fragmentation is a major surface process in ESD.
where Q is the total cross section (cm2)for decomposition of the species, No is the initial surface concentration, I / A is the electron current density (A/cm2),and E is the electron charge. We also estimate that the ratio of the intensity of the v(C-H) or v(C-F) feature to the intensity of the corresponding v(A1-0) feature is proportional to N(t)/No. A plot of In (IC-H/IA1-O) or In (IC-F/IA1-O) versus electron fluence26will, with these assumptions, allow an estimate to be made of the total cross section for electron-stimulated destruction of the overlayer. Such plots are presented in Figure 9. From Figure 9, the cross section for electron-stimulated decomposition (300 eV) for (CH3)20/A1,03is found to be cm2 and that for (CF,H)20 is 8.8 X cm2. 6.8 X The values for electron-stimulated decomposition cross sections are of the same order of magnitude as typical cross
(21)Klauber, C.; Alvey, M. D.; Yates, J. T., Jr. Surf. Sci. 1985,154, 139. (22)Bozack, M.J.; Choyke, W. J.; Muehlhoff, L.; Yates, J. T., Jr. Surf. Sci. 1986, 176,541. (23)(a) Peacock, R.D.; Sharp, D. W. A. J . Chem. SOC.1969,2762.(b) Weidlein, 3.; Krieg, V. J . Organomet. Chem. 1968,11, 9. (24)Chen, J. G.;Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Am. Chem. Soc. 1987,109,1726.
(25)(a) Pacansky, J.; England, C. D.; Waltman, R. Appl. Spectrosc. 1986,40,8. (b) Hussla, I.; Philpott, M. R. J. Electron Spectrosc. Relat. Phenom. 1986,39,255. (26)Plotting In (intensity ratio) versus electron fluence gives a cross section of the same order of magnitude as a plot of In (intensity) versus electron fluence. However, in the former case the error in the experimental data due to differences in the electron reflectivity of the surface is greatly reduced.
Electrons/cm'
Figure 9. Plot of In (IcH/IN4) and In ( I G F / I ~versus ~ ) electron fluence. The straight line show least-square fits of the experimental points. The slopes of the lines give the correspondingcross sections for electron-stimulated decomposition of (CH3)*0and (CF,H)20 on A1,0,.
In addition, infrared spectra of polyfluoro ethers show characteristic strong vibrational bands centered at 1235 and 1305 cm-' indicative of the CF, twisting and the CF, rocking modes.25 We have seen no evidence of these vibrational modes in Figure 7c-f. In the case of the fluorinated ether, the electron beam stimulated the breaking of the C-F bond, with F forming an A1-F bond. Estimated Cross Sections for the Electron-Stimulated Decomposition of (CH3),0 and (CF2H),0 on Al,03. An estimate of the cross sections for electronstimulated decomposition of (CH3),0 and (CF,H)20 was made by measuring the disappearance of the v(C-H) and v(C-F) features relative to the corresponding v(A1-0) feature. By analogy with the electron-stimulated desorption processes in an adsorbed monolayer, we assume that the concentration of undamaged species in the surface region at time t , N ( t ) ,obeys the first-order relation
N ( t ) = No exp(-QIt/Ac)
(1)
Langmuir 1987,3, 1167-1169 sections for dissociative ionization of adsorbed or gaseous molecules (V, = 100 to >lo00 eV).27-29 They are also in excellent agreement with cross sections of electron beam damage in ESD found by other researcher^.^?^^ Implications for Fluoro Ether Lubricant Degradation. Fluorinated ethers are widely used as lubricants in high-technology applications. In this study, (CF2H),0 is used as a model for higher molecular weight ether lubricants. Since it is likely that the chemical interaction between the 0 moiety in an ether or a fluoro ether and a surface of interest controls the overall surface chemistry, model studies of this type can be useful in defining chemical interaction routes which are of importance in more complex fluoro ether lubricants. In addition, the use of clean, in situ prepared A1203 surfaces, free of OH contaminants or other contaminants, permits the observation of fundamental reaction routes under conditions likely to be present in tribological situations where friction and mechanical damage can lead to exposure of fresh surfaces to the lubricant. Our study has shown that neither (CF2H),0nor (CHJ2O adsorbates are strongly chemically interactive with A1203 surfaces. Adsorbate binding energies for (CF,H),O on A1203are of the order of 8 kcal/mol, and thermal desorption of both (CH3)2O and (CF2H),0 occurs from the adsorbed layer without molecular decomposition. Thus, comparing the two ether molecules, the partial fluorination of the alkvl moietv does not lead to enhanced reactivitv with A1,O;. (27) Madey, T. E.; Yates, J. T., Jr. Surf. Sci. 1977, 63, 203. (28) Madey, T. E.; Yates, J. T., Jr. J. Vac. Sci. Technol. 1971,8,525. (29) Menzel, D. In Topics in Applied Physics; Gomer, R., Ed.; Springer-Verlag: Berlin, 1975; Vol. 4, p 101. (30) Pantano, C. G.; Madey, T. E. Appl. Surf.Sci. 1981, 7, 115. (31) Takeda, S.;Tarao, R. Bull. Chem. SOC.Jpn. 1966,38, 1567.
1167
On the other hand, both (CF,H),O(ads) and (CH3),0(ads) on A1203exhibit very high electron-stimulated decomposition (ESD) cross sections (-7 X cm2),reminiscent of the cross sections exhibited for the ESD of gas-phase molecules. The A1203 substrate is very effective in reacting with the chemical fragments produced by ESD of both the alkyl and the fluorinated ethers, as seen by the modification of the EEL spectrum of the phonon modes of the M203as well as by AES. Auger spectroscopyclearly shows that electron impact effectively anchors F moieties to AZO3when (CF2H)20is lightly bombarded with 300-eV electrons. The irreversible bonding of F, presumably as A1-F, and the production of strongly bound C on A1203 suggest that active atomic or molecular fragments are efficiently produced by ESD and enter into surface reactions with A1203. It is also possible that surface polymerization and oxidation processes could be initiated by ESD in technological systems involving fluoro ether lubricants due to the interaction of active molecular fragments with each other and with ambient Oz(g). Thus, when fluorinated ethers are used for lubrication, one can expect that static electrical effects involving local charging and discharging will be very effective in producing irreversible surface chemical processes involving molecular fragments produced by the electron-stimulated decomposition processes.
Acknowledgment. This work was supported by a research grant from IBM Research Laboratories, San Jose, CA. P. Basu acknowledges a fully supported educational leave from Alcoa. Registry No. Al,O,, 1344-28-1;(CH3),0, 115-10-6;(CF,H),O, 1691-17-4.
Letters Observation of Preferential Orientation of Mineral Surfaces at the Air/Liquid Interface by X-ray Diffraction Oliver Lindqvist and Kjell Stridh* Department of Inorganic Chemistry, Chalmers University of Technology and University of Goteborg, S-41296 Goteborg, Sweden Received April 6, 1987. In Final Form: July 9,1987 A structural study of surface interaction aspects in flotation of minerals has been commenced. For the investigation of orientation effects on mineral particles a method, flotation 9-8 diffraction, has been developed for in situ flotation in a large-angle X-ray solution (LAXS) diffractometer. The method is used to evaluate what cleavage planes in the mineral are the most active and thus are orientated to the air phase in the flotation process. Calculated diffraction intensities, information on preferred cleavage planes obtained from literature, and the powder diffraction pattern of dry samples are compared with the diffraction pattern obtained from the flotated material.
(1) h j a , J. Surface Chemistry of Froth Flotation, Plenum: New York, 1982.
(2) King, R. P. Principles of Flotation; South African Institute of Mining and Metallurgy: Johannesburg, 1982.
0743-746318712403-1167$01.50/0 0 1987 American Chemical Society
