Anal. Chem. 1992, 64, 2480-2495
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Valence Band Spectra of Aluminum Oxides, Hydroxides, and Oxyhydroxides Interpreted by X a Calculations Sajan Thomas and Peter M. A. Sherwood' Department of Chemistry, Willard Hall, Kansas State University, Manhattan, Kansas 66506
The oxtdes, hydroxldes, and oxyhydroxldes of aluminum cannot be dlstingulshed by thelr core X-ray photoelectron (XPS) reglons, but have slgnlficantly dlfferent valence band reglons. The dltforencesobserved experlmentaliy can be explained by Xa calculations. By comblnlng the valence band XPS data to Identlfy surface composltlon, wlth bulk purity IdentHIed by X-ray powder dlffractlon, and the expected spectra from the calculations,we conclude that while the oxldesand hydroxldes are unchanged on the surface, the oxyhydroxldes are only seen as hydroxide (bayerlte or nordstrandlte)on thelr surfaces. The surface oxide on the "as received" metal is found to be in the form of glbbslte. The ablllty of valence band XPS to dlstlngulsh these compounds wlll aid In thelr IdentHlcatlon In corrosion and catalytlc sltuatlons.
INTRODUCTION Aluminum is an important structural material because, despite ita high reactivity toward water and oxygen, it is protected by a thin impervious stable oxide layer. Aluminum has a range of oxides, oxyhydroxides, and hydroxides, the principal ones being a- and 7-A1203,the oxyhydroxides a(diaspore) yA100H (boehmite), and the hydroxides (Al(OH)3)gibbsite, bayerite, and nordstrandite. All these compounds will be discussed in this paper. The aluminum-water system has been fully discussed in many places (e.g. ref 1). A more detailed examination of the corrosion behavior of aluminum would clearly need the type of oxide to be identified on the metal surface. There have been many studies of such oxide films using techniques such as X-ray diffraction to investigate the thick oxide films that grow under appropriate oxidation conditions (e.g. refs 1and 2). The oxidation of the metal has been examined in detail by many workers (e.g. see the review in ref 3), and many of these studies have used X-ray photoelectron spectroscopy (XPS or ESCA) and other surface science methods for the study (e.g. refs 4-7 oxidation by water and refs 8-26 oxidation by oxygen). (1)Alwitt, R.S.In Oxides and Oxide Films; Diggle, J. W., Ed.; Marcel Dekker Inc.: New York, 1974;Vol. 4,pp 169-254. (2) Wood, G. C. In Oxides and Oxide Films; Diggle, J. W., Ed.; Marcel Dekker Inc.: New York, 1973;Vol. 2, pp 167-279. (3)Batra, I. P.; Kleinman, L. J . Electron Spectrosc. Relat. Phenom. 1984,33, 175-241. (4)Netzer, F. P.; Madey, T. E. Surf. Sci. 1983,127,L102-LlO9. (5)Miller, J. E.; Harris, J. Phys. Reu. Lett. 1984,53,2493-96. (6)Makarychev, Yu. B.; Akimov, A. G. Pouerkhnost 1988,12,94-9. (7)Smith, P. B.; Bernasek, S. L. J . Electron Spectrosc. Relat. Phenom. 1989,49,149-58. (8)McConville, C. F.; Seymour, D. L.; Woodruff, D. P.; Bao, S. Surf. Sci. 1987,188,1-14. (9)Flodstrom, S.A.; Martinsson, C. W. B.; Bachrach, R. 2.; Hagstrom, S.B. M.; Bauer, R. S. Phys. Reu. Lett. 1978,40,907-910. (10)Pashutski, A.; Hoffman, A.; Folman, M. Surf. Sci. 1989,208,L91-
.
T.97
I".
(11)Barr, T. L.,J. Vac. Sci. Technol. 1977,14,660-5. (12)Fujikawa, T. J. Electron Spectrosc. Relat. Phenom. 1982,26,79. (13)Erskine, J. L.; Strong, R. L. Phys. Reu B. 1982,25,5547-50. (14)Baragiola, R.;Ferron, J.; Zampieri, G. Nucl. Instrum. Methods Phys. Res. Sect. E 1984,230,614-16.
Oxidation begins by the chemisorption of the oxidizing species (such as water or oxygen) onto the metal surface3and often finally ends with the formation of a-A1203,but depending upon the conditions any of the AI-0-H species discussed above may be formed. In fact the ready hydration of aluminum oxide surfaces has been observed by many workers (e.g. refs 1and 27-32). The challenge then is to identify the surface species, especially on "practical" surfaces such as catalyst supports or corroded samples where the amorphous nature of the film may make analysis more difficult, and to be able to do so in the surface region for thin films (thick films can be readily analyzed by X-ray diffraction and vibrational spectroscopy). One approach that has been successfully applied is to use vibrational spectroscopic differences as revealed in the HREELS spectrum to distinguish between these oxides. Valence band XPS has not been widely used to distinguish subtle chemical changes on surfaces. It is a technique which we have found especially valuable in a wide range of systems.33-46 (15)Adem,E. H.; Seymour,D. L.; Pattinson, E. B. Surf. Sci. 1984,141, 1-12. (16)Gautier, M.; Duraud, J. P.; Vigourous, J. 0.; Le Gressus, C.; Shimizu, R.Scanning Electron Microsc. 1985,165-70. (17)Ocal, C.; Basurco, B.; Ferrer, S. Surf. Sci. 1985,157,233-43. (18)Crowell, J. E.; Chen, J. G.; Yates, J. T., Jr. Surf. Sci. 1986,165, 37-64. (19)Chen, J. G.;Crowell, J. E.; Yates, J. T., Jr. Phys. Reu. B 1986,33, 1436-9. (20) Testoni, A. L.; Stair, P. C. Surf. Sci. 1986,171, L491-L497. (21)Hoffman, A.; Maniv, T.; Folman, M. Surf. Sci. 1987,182,56-68. (22)Kim, S.T.; Choudhary, K. M.; Shah, S. N.; Lee, J. H.; Rothberg, G. M.; DenBoer, M. L.; Williams, G. P. J. Vac. Sci. Technol., A 1987,5, 623-6.
(23)Fritach, A.; Legare, P. Surf. Sci. 1987,186,247-55. (24)Astaldi, C.; Geng, P.; Jacobi, K. J. Electron Spectrosc. Relat. Phenom. 1987,44,175-82. (25)Lauderback, L. L.; Larson, S. A. Surf. Sci. 1990,233,276-82. (26)Lauderback, L.L.; Larson, S. A. Surf. Sci. 1990,234,135-42. (27)Hart, R. K. Trans. Faraday SOC.1957,53,1020-1027. (28)Hart, R. K.;Maurin, J. K. Corrosion 1965,21,222-234. (29)Alwitt, R. S.;Archibald, L. C. Corrosion Sci. 1973,13,687-88. (30)Chen, J. G.; Crowell, J. E.; Yates, J. E., Jr. J . Chem. Phys. 1986, 84,5906-9. (31)Wittberg, T. N.;Wolf, J. D.; Wang, P. S. J . Mater. Sci. 1988,23, 1745-7. (32)Thorne, N. A,; Thuery, P.; Frichet, A,; Gimenez, P.; Sarte, A. Surf. Interface Anal. 1990. 16. 236-40. (33) Thdmas, S.;Sherwood, P. M. A.; Singh, N.; Al-Sharif, A.; O'Shea, M. J. Phys. Reu. B 1989,39,6640-6651. (34)Welsh, I. D.; Sherwood, P. M. A. R o c . Electrochem. SOC.1989, 89-13 (Advances in Corrosion Protection by Organic Coatings; Scantlebury, D., Kendig, M., Eds.), 417-429. (35)Xie, Y.; Sherwood, P. M. A. Chem. Mater. 1989,1, 427-32. (36)Welsh, I. D.; Sherwood, P. M. A. Phys. Reo. B 1989,40,6386-92. (37)Sherwood,P. M. A. In Data Analysis inXP2?and AESinPractical Surface Analysis, Second Edition, Volume I-Auger and X-ray Photoelectron Spectroscopy, Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1990;Appendix 3,pp 555-86. (38)Xie, Y.;Sherwood, P. M. A. Appl. Spectrosc. 1990,44,797-803. (39)Xie, Y.; Sherwood, P. M. A. Chem. Mater. 1990,2,293-299. (40)Sherwood, P. M. A. Phys. Reu. B 1990,41,10151-54. (41)Xie, Y.; Sherwood, P. M. A. Appl. Spectrosc. 1990,44,1621-28. (42)Sherwood, P. M. A. J. Vac. Sci. Technol., A 1991,9,1493-7. (43)Xie, Y.; Sherwood, P. M. A. Chem. Mater. 1991,3,164-8. (44)Xie, Y.;Sherwood, P. M. A. Appl. Spectrosc. 1991,45,1158-65. (45)Welsh, I. D.; Sherwood, P. M. A. Chem. Mater. 1992,4,133-140. (46)Sherwood, P.M. A. J. Vac. Sci. Technol., A 1992,10,2783-87.
0003-2700/92/0384-24SS$03.00/0 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, l Q Q 2 2489
Recently it has been shown to be of value in the study of catalytic s~pports,47-~9 and despite earlier suggestions that a- and 7-A1203 could not be distinguished,m recent papers47~~~51 show significant differences in the valence band of these two oxides. In this paper we discuss the use of valence band XPS as a means of distinguishing between the surface species and present a detailed analysis of the valence band spectra of the Al-O-H system for the first time. We have coupled the experimental data with spectra calculated by X a calculations. This approach is especially important because, as with any sample, one can never be sure that the surface will be representative of the bulk. Thus we have used X-ray powder diffraction to identify our prepared samples and the combination of valence band XPS and our predicted spectra to determine the nature of the surface species and to show that the change that we see between these compounds experimentally would be anticipated theoretically.
EXPERIMENTAL SECTION Our XPS measurements were made using a VSW HA100 spectrometer with a base pressure of 10-10 Torr using A1 Ka X-radiation (line width approximately 0.9 eV) with a power of 275 W. Spectra were recorded in FAT (fixed analyzer transmission mode) to achieve maximum instrument resolution (substantially better than the X-ray line width), and data were usually collected with at least 17 points/eV in order to be sure to identify any subtle features that might be lost at lower resolution and a larger step size. Calculated spectra were generated using a spectral generation program with a 50% mixed Gaussian-Lorentzian product funct i ~ n All . ~spectra ~ were repeated on different occasions with new samples at least once for each compound under study. Identical results were obtained for each repeated spectrum. a-A1203was obtained from Alfa Chemicals, yAl& was obtained from Johnson Matthey Chemical, and gibbsite (wA~(OH)~) was obtained from Fisher. Bayerite (P-Al(OH)3)was prepared from high purity aluminum53by cutting the metal into strips, and degreasing the metal strips with acetone. The metal pieces were covered with a layer of amalgam by a short immersion in a 0.1 M HgClz solution and were then thoroughly rinsed with quadruply distilled water. The metal pieces were left under quadruply distilledwater in a covered flask for several days, when white crystals of bayerite were slowly formed. The crystals were washed in quadruply distilled water and dried. Nordstrandite (Al(OHI3)was preparedM in a similar manner to Bayerite. In the final step the metal pieces were left under quadruply distilled water to which ethylene diamine was added, when crystalsof nordstrandite appeared slowlyduring a few days. Boehmite (yAl00H)was ~repared~~using the steps to prepare amalgamated aluminum metal strips as described for Bayerite. The amalgamated strips were then washed with quadruply distilled water and transferred to a flask. Enough quadruply distilled water was added to cover the aluminum pieces, and the water was brought to a boil, when a violent reaction resulted leading to the production of boehmite. The product was washed with quadruply distilled water and dried. All the compounds were checked for purity by using powder X-ray diffraction. These studies were carried out by using a (47)Barr, T. L.; Chen, L. M.; Mohsenian, M.; Lishka, M. A. J. Am. Chem. Soc. 1988,110,7962-75. (48)Barr, T. L. In Applications of Electron Spectroscopy to Heterogeneous Catalysis in Practical Surface Analysis, Second Edition, Volume 1 - Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah,M. P., Eds.; Wiley: New York, 1990;Chapter 8,pp 357-436. (49)Barr, T. L. Crit. Rev. Anal. Chem. 1991,22,229-325. (50)Balzarotti, A.; Bianconi, A. Phys. Stat. Sol. B 1976,76, 689-94. (51)Frederick, B. G.;Apai, G.; Rhodin, T. N. Surf. Sci. 1991,244, 67-80. (52)Sherwood, P. M. A. In Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy; Briggs, D., Seah,M. P., Eds.; Wiley: New York, 1983;Appendix 3. (53)Handbook of Preparative Inorganic Chemistry, 2nd ed.; Brauer, G., Ed.; Academic Press: New York, 1963;Vol. 1. (54)Hauschild, U.2.Anorg. Allg. Chem. 1963,324, 15-9.
Table I. Core Binding Energies (eV) 01s fwhmOls
gibbsite bayerite nordstrandite boehmite corundum y-Alz03
531.7 531.9 531.9 531.3 531.5 531.5
2.1
3.0 2.9 2.1
1.9 2.9
A12p
fwhmAl2p
73.5 73.8
2.1 2.2 2.1
73.9 73.6
73.4 74.2
2.2 2.1 2.2
Scintag XDS 2000 instrument. The X-ray radiation wavelength of Cu Kal is 1.540 59 A, and the power used was 1800 W. The data were collected using the 20 step scanning mode with a step size of 0.005'. The basis for spectral comparison was the on-line JCPDS data base.55 There was some residual hydrocarbon on the samples, but the level of contamination was such that it would contribute only to a negligible extent in the valence band region. The most intense hydrocarbon feature would be found around 18-20 eV due to carbon 2s features, but no such features were observed. All the calculations were performed on an IBM RISC/6000 system. The multiple scattered wave Xa calculationsMneeded significant computer time for the no-symmetry clusters.
RESULTS AND DISCUSSION Core XPS Studies. Aluminum metal can be readily distinguished from ita oxides, hydroxides and oxyhydroxides by chemical shifts in the core A12p region. Thus the A12p binding energy of the metal is 71.8 eV while the oxides, hydroxides, and oxyhydroxides are around 74.5 eV. In addition the metal has a substantially narrower line width. These observations have been reported in numerous papers over the past 20 years. The difference can be readily used as a method for evaluating oxide thickness (e.g. refs 57 and 58). Unfortunately it is very difficult to unambiguously identify any differences in A12p binding energy between the oxides, hydroxides and oxyhydroxides themselves. Indeed calculations suggest59that such differences would be expected to be small. Certainly some subtleties arise, for example the separation between metal peak and oxide peak can be smaller than normal for very thin oxide films,23@and it has been suggested23that this might be strongly dependent upon the surface preparation step prior to metal oxidation. Experiments involving the chemisorption of oxygen on aluminum suggest that as oxidation starts to occur the binding energy of the oxidized material shifts towards the oxide value.8J7 Table I shows the core binding energy region in the 01s and A12p regions for all the compounds studied. There are certainly some differences in peak width, but little obvious difference in binding energy. For example the A12p peak for ?-A1203 is the widest at 2.9 eV (full width at half-maximum (fwhm)), and a-Al203 the narrowest at 1.9 eV, with similar variations in the 0 1 s region. The binding energies are all similar in the 0 1 s region. In the A12p region the binding energies are closest with the largest difference between aand 7-A1203. Some of these line width differences61 might be associated with differential sample charging,@ some with lifetime effects. However, our valence band spectra (see below) give no evidence of significant differential sample (55)JCPDS, International Center for Diffraction Data, 1601Park Lane, Swarthmore, PA 19081-2389. (56)Johnson, K. H. J. Chem. Phys. 1966,45,3085-95. (57)Fadley, C. S.J.Electron Spectrosc. Relat. Phenom. 1974,5,409435. (58)Strohmeier, B. R. Surf. Interface Anal. 1990,15, 51-58. (59)Wilson, G.R.; Sherwood, P. M. A., to be published. (60)Thomas, S.; Sherwood, P. M. A.; Lee, K.-M.; O'Shea, M. J. Chem. Mater. 1990,2, 7-12. (61)Kawai, J.; Ohta, M.; Nihei, Y. Spectrochim. Acta, B 1989,44B, 815-24. (62)Dickinson, T.;Povey, A. F.; Sherwood, P. M. A. J. Electron Spectrosc. Relat. Phenom. 1973,2,441-447.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992
Table 11. Details of A l u m i n u m Compounds Studied compound
mineral name
a-Al~O3 Y-Alz03
corundum
a-Al(oH)~
gibbsite
P-Al(OH)S Al(OH)3 a-Al00H Y- A100H
bayerite nordstrandite diaspore boehmite
structure
ref
rhombohedral Crz03 structure (a) defect spinel structure with cubic close packing and cell edge of 7.9 A where only 8 / of ~ the 24 available cation sites are occupied by Al a t any given time (b) defect hausmannite structure (a tetragonal distortion of the spinel arrangement) monoclinic; very irregular structure with an average A1-0 = 1.924 A, but with bond lengths as short as 1.81 A and as long as 2.09 A hexagonal; approximates to an octahedral unit with A1-0 = 1.939 A. hexagonal; similar to bayerite orthorhombic orthorhombic
67,68 69, 70
65 65 65 65 65 65
01
I \ \ 1.871 1.987
os
02 06
04
01 01
06
09
I'
1.858
i'
1.957
1.997
1.971
03
Flgurr 1. Clusters used for the X a calculations: (a) A120912- (the AI203 cluster used the same geometry with only All, A12, 01,03, and 0 4 atoms), (b) top view of the AI2O9l2-cluster, (c)Aloes- for a-AlnO3, (d) Aloes- for y-A1203 D4hsymmetry, (e) A10eH3B-for dlaspore, (f) A106H27-for boehmle, (9) A10aHE3-for glbbsite, (h) AI06He3- for bayeritelnordstrandie.
charging. Further it is interesting to note tht in the valence band region at a binding energy
