Langmuir 2002, 18, 1695-1701
1695
Interaction of Ni/Al Interfaces with Oxygen A. Arranz and C. Palacio* Departamento de Fı´sica Aplicada, Facultad de Ciencias, C-XII, Universidad, Auto´ noma de Madrid, Cantoblanco, 28049-Madrid, Spain Received October 18, 2001 The interaction of oxygen with nickel deposited on polycrystalline aluminum surfaces has been investigated using Auger electron spectroscopy, X-ray photoelectron spectroscopy, and angle-resolved X-ray photoelectron spectroscopy. The growth of the nickel on the aluminum surfaces occurs in two stages: formation of NiAlx (x ≈ 0.45) islands 10 monolayers (ML) thick up to a coverage of θNiAlx ≈ 0.7, followed by the formation of metallic nickel islands 8 ML thick that grow over the intermetallic islands previously formed. For surfaces containing NiAlx islands alone, the chemical information obtained by the analytical techniques shows that oxygen exposure causes the formation of aluminum intermediate oxidation states Al+ and Al2+, in addition to Al3+, which are attributed to the formation of Al-O-Ni cross-linking bonds at the interface. The analysis of the Ni 2p peak shape shows that no nickel oxide is formed, the small changes observed in this band being attributed to Ni atoms in an aluminum-depleted layer at the interface. In contrast, nickel oxide is formed during the oxidation of surfaces containing nickel islands. In such a case, the oxide film is composed of a mixture of intermediate aluminum oxidation states that grow over a NiO oxide layer. At the interface between Al and Ni oxides, a NiAl2O4-like mixed oxide or Ni3+ defects are formed.
Introduction The Ni-Al alloy has attracted great interest during the past years due to its different technological applications. In particular, Ni-Al compounds are used as coatings for high-temperature applications because of their very good resistance to oxidation and corrosion, good thermal conductivity, and low density.1-12 For this reason, the oxidation of Ni-Al bulk alloys has been the subject of several works.6-12 The results of these studies show that the oxide layer that grows during the first stages of the oxidation of Ni-Al bulk compounds is formed of a complex mixture of Al2O3, NiO, and NiAl2O4 oxides, the structure and composition of this layer being dependent on the oxygen pressure and the oxidation temperature. The deposition of nickel and nickel oxide thin films on different substrates has been also the subject of several studies over recent years. This is due to the changes in the physical and chemical properties of these systems with respect to the properties of the single components.13-24 Moreover, * Corresponding author. Fax: ++34 91 3974949. E-mail:
[email protected]. (1) Intermetallic Compounds. Principles and Practice; Westbrook, H. H., Fleischer, R. L., Eds.; John Wiley & Sons: New York, 1995; Vol. 2, Chapter 1. (2) Shutthanandan, V.; Saleh, A. A.; Smith, R. J. J. Vac. Sci. Technol., A 1993, 11, 1780. (3) Ruckman, M. W.; Jiang, L.; Strongin, M. J. Vac. Sci. Technol., A 1990, 8, 134. (4) Arranz, A.; Palacio, C. Thin Solid Films 1998, 317, 55. (5) Bornstein, N. S. J. Phys. IV: Colloque C9, Supple´ ment au J. Phys. III 1993, 3, 367. (6) Young, E. W. A.; Rivie`re, J. C.; Welch, L. S. Appl. Surf. Sci. 1987, 28, 71. (7) Young, E. W. A.; Rivie`re, J. C.; Welch, L. S. Appl. Surf. Sci. 1988, 31, 370. (8) Venezia, A. M.; Loxton, C. M. Surf. Interface Anal. 1988, 11, 28. (9) Venezia, A. M.; Loxton, C. M. Surf. Sci. 1988, 194, 136. (10) Jaeger, R. M.; Huhlenbeck, H.; Freund, H. J.; Wuttig, M.; Hoffmann, W.; Franchy, R.; Ibach, H. Surf. Sci. 1991, 259, 235. (11) Bardi, U.; Atrei, A.; Rovida, G. Surf. Sci. 1992, 268, 87. (12) Haerig, M.; Hofmann, S. Appl. Surf. Sci. 1998, 125, 99. (13) Sotiropoulou, D.; Ladas, S. Surf. Sci. 1998, 408, 182. (14) Bourgeois, S.; Seigneur, P.; Perdereau, M. Surf. Sci. 1995, 328, 105. (15) Espino´s, J. P.; Ferna´ndez, A.; Gonza´lez-Elipe, A. R. Surf. Sci. 1993, 295, 402. (16) Marcus, P.; Hinnen, C. Surf. Sci. 1997, 392, 134.
the Ni/Al2O3 and NiO/Al2O3 interfaces present interesting magnetic and catalytic applications.17-24 Therefore, the growth and characterization of ultrathin Ni-Al mixed oxide layers should play a key role in the understanding of the physical behavior of those applications. In contrast, no studies concerning the initial stages of the oxidation of the bimetallic system formed at the Ni/Al interface have been performed to our knowledge. Such a study could lead to the synthesis of Ni-Al oxide films with electronic and catalytic properties significantly different from those of the Ni and Al bulk oxides, that could be used not only in specific applications but also in model catalysis studies. The formation of the Ni/Al interface during the deposition of Ni on polycrystalline Al at room temperature has been already studied by Palacio et al.25 The purpose of the present work is to chemically characterize the oxide films formed at room temperature on different Ni/Al interfaces exposed to low oxygen pressure and to relate the morphology and composition of the different interfaces to the composition of the thin oxide film formed. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and angle-resolved X-ray photoelectron spectroscopy (ARXPS) were used to check in situ the formation of the oxide film as a function of oxygen exposure. Experimental Section High-purity polycrystalline aluminum substrates (nominal composition 99.994% Al, 0.004% Si, 0.001% Fe, and 0.001% Cu) manufactured by Toyo Aluminum Co., Japan, were used throughout this work. They were degreased by successively boiling in (17) Ranga Rao, G.; Rao, C. N. R. J. Phys. Chem. 1990, 94, 7986. (18) Zhong, Q.; Ohuchi, F. S. J. Vac. Sci. Technol., A 1990, 8, 2107. (19) Ohuchi, F. S.; Kohyama, M. J. Am. Ceram. Soc. 1991, 74, 1163. (20) Mukhopadhyay, S. M.; Chen, C. S. J. Vac. Sci. Technol., A 1992, 10, 3545. Sˇ arapatka, T. J. Chem. Phys. Lett. 1993, 212, 37. (21) Ealet, B.; Gillet, E.; Nehasil, V.; Moller, P. J. Surf. Sci. 1994, 318, 151. (22) Bolt, P. H.; ten Grotenhius, G.; Geus, J. W.; Habraken, F. H. P. M. Surf. Sci. 1995, 329, 227. (23) Kishi, K.; Fujiwara, K. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 51. (24) Kishi, K.; Hayakawa, Y.; Fujiwara, K. Surf. Sci. 1996, 356, 171. (25) Palacio, C.; Arranz, A. J. Phys. Chem. B 2000, 104, 9647.
10.1021/la015634d CCC: $22.00 © 2002 American Chemical Society Published on Web 01/22/2002
1696
Langmuir, Vol. 18, No. 5, 2002
Arranz and Palacio
carbon tetrachloride, acetone, and ethanol. Then the sample was introduced into the ultrahigh vacuum (UHV) chamber. The aluminum substrates were sputter-cleaned in situ with 3 keV Ar+ until no impurities were detected by AES. To minimize the surface roughness attained after sputter cleaning, the ion current density was kept below 2 µA/cm2. Such a low current density develops small and uniform roughness that does not influence, from a qualitative point of view, the spectroscopic results. Experimental details for nickel deposition and for AES and XPS characterization have been given in detail elsewhere25 and will only be summarized here. Nickel was deposited by sublimation of a directly heated filament of 99.98% purity onto the Al substrates at room temperature. The nickel deposition rate was ∼1.5 × 1015 atoms cm-2 min-1. Auger spectra were measured in the derivative mode using a cylindrical mirror analyzer (CMA) with a nominal resolution of 0.25%. A modulation voltage of 2 Vp-p was supplied to the CMA. A constant primary electron beam current density of 1 × 10-3 A/cm2 at 3 keV primary beam energy was used. XPS data were recorded using a hemispherical analyzer (SPECS EA-10 Plus). The pass energy was 15 eV giving a constant resolution of 0.9 eV. A twin anode (Mg and Al) X-ray source was operated at a constant power of 300 W, using Mg KR (1253.4 eV) radiation. The oxidation was carried out at room temperature. For the oxidation experiments, oxygen of 5N quality was introduced into the spectrometer chamber at a controlled partial pressure in the 10-8-10-5 Torr range. The partial pressure of oxygen and the purity of the gas were controlled by means of a quadrupole mass analyzer.
Results In an earlier work, the formation of the interface during the deposition of nickel on polycrystalline Al substrates was studied at room temperature using XPS and ARXPS.25 In that work, a two-stage mechanism for nickel growth was found: a first stage characterized by the formation of NiAlx (x ≈ 0.45) islands 10 monolayers (ML) thick up to a coverage of θNiAlx ) 0.7, followed by the formation of metallic nickel islands 8 ML thick that grow over the NiAlx islands formed previously. The simultaneous lateral growth of both kinds of islands was observed during the second stage. It is important to note that no metallic nickel was detected on the surface during the first stage. However, both NiAlx and Ni islands grow on the surface during the second stage. In present work, different Ni/Al interfaces corresponding to both the first and second stages of the Ni/Al interface formation have been oxidized at room temperature. For the oxidation experiments, the oxygen exposure was varied up to 5000 langmuir (1 langmuir ≡ 10-6 Torr s). To characterize the thin oxide film formed, the Ni LVV and O KLL Auger transitions as well as the Al 2p, Ni 3p, O 1s, and Ni 2p XPS bands have been measured for different nickel deposition times and subsequent oxygen exposure. Figure 1 shows the Ni LVV and O KLL Auger transitions of 5000 langmuir oxide films measured for different interfaces. The nickel deposition time as well as the coverage of NiAlx (θNiAlx) and Ni (θNi) are also indicated in Figure 1. The first stage, that is, θNi ) 0, corresponds to t e 9 min. The AES spectra for Ni only exhibit important changes (indicated by arrows) when Ni/Al interfaces corresponding to the second stage (θNi > 0) are exposed to oxygen. These changes can be attributed to the nickel oxidation at this stage. The O KLL peaks show a chemical shift as the nickel content increases. This shift is clearly observed for interfaces corresponding to the second stage of nickel growth. For a nickel coverage θNi ) 0.84, the O KLL spectrum is similar to that measured on a nickel substrate, labeled Ni in Figure 1, therefore indicating the nickel oxidation. The Al 2p, O 1s, and Ni 2p XPS bands were measured for different Ni/Al interfaces at oxygen exposures up to
Figure 1. Derivative Ni LVV and O KLL Auger spectra of the 5000 langmuir oxide films measured for different Ni/Al interfaces.
5000 langmuir. Measured spectra of aluminum, oxygen, and nickel, after background subtraction based on a modified Shirley method,26 are shown in Figure 2a-c, respectively. As shown in Figure 2a, Ni 3p and Al 2p bands overlap. Although oxygen exposures were carried out in the range of 0-5000 langmuir, for simplicity only results corresponding to an oxygen exposure of 5000 langmuir are given in Figure 2a-c. In Figure 2a, spectra labeled as Al and Ni are representative of clean Al and Ni substrates exposed to 5000 langmuir of oxygen. As the oxygen exposure increases (not shown), a broad shoulder appears on the high binding energy (BE) side of the Al 2p band. Also, an attenuation of the metallic Al 2p band is observed. Results corresponding to an exposure of 5000 langmuir show that the position of the Al 2p shoulder shifts to the lower BE as the nickel content increases. On the other hand, only a slight attenuation of the Ni 3p band is observed for t e 9 min as the oxygen exposure increases, whereas for t > 9 min a broad shoulder appears on the high BE side of this band. Figure 2b shows the measured spectra of the O 1s band for different nickel deposition times. The band exhibits a shift from ∼532 eV for the Al substrate to ∼529.5 eV for pure Ni, indicating the presence of at least four components. On the other hand, the Ni 2p band shows new peaks associated with NiO formation for nickel deposition times above 9 min (θNi > 0). Below 9 min, the band shifts ∼0.2 eV to higher BE and loses its asymmetry, but these changes cannot be related to oxide formation. To determine the bands associated with the different Al, Ni, and O species by peak deconvolution, synthetic spectra and a least-squares optimization were used. Five synthetic bands have been used to reproduce the Al 2p spectra in the whole range of nickel deposition times and oxygen exposures: an asymmetrical Gaussian-Lorentzian (GL) function, Al0, associated with metallic aluminum; a symmetrical GL function, AlNi, attributed to the intermetallic compound NiAlx; and three symmetrical GL (26) Proctor, A.; Sherwood, E. P. A. Anal. Chem. 1982, 54, 13.
Interaction of Ni/Al Interfaces with Oxygen
Langmuir, Vol. 18, No. 5, 2002 1697 Table 1. Parameters of the Synthetic Bands Used for Deconvolution of XPS Spectra of Figure 3 band Al0 AlNi Al+ Al2+ Al3+ Al 2p Al 2p O1 O2 O3 ONi Ni0 NiAl Niox
Figure 2. XPS spectra of the 5000 langmuir oxide films measured for different Ni/Al interfaces: (a) Al 2p-Ni 3p band; (b) O 1s band; (c) Ni 2p band.
functions, Al+, Al2+ and Al3+, associated with 1+, 2+, and 3+ Al oxidation states, respectively. Al+ and Al2+ species are also present for exposures below 10 langmuir and therefore can be also related to chemisorbed oxygen in this range of oxygen exposures. Since the X-ray source is not monochromatic, the KR3 and KR4 satellites of the Al 2p band are also observed and two symmetrical GL functions have been used to reproduce them. For the Ni 3p band, three synthetic components, Ni0, NiAl, and Niox, attributed to metallic nickel, nickel aluminide, and nickel oxide, respectively, have been used. Each component is the sum of three symmetrical GL functions. Two of them correspond to the Ni 3p3/2,1/2 doublet, and the third one corresponds to the characteristic satellite of nickel. The shape of the Niox synthetic band has been obtained from a 5000 langmuir oxygen exposed clean nickel substrate (see spectrum labeled Ni in Figure 2a). The Niox 3p component is only necessary to reproduce the Ni 3p spectra for deposition times above 9 min (θNi > 0). The O 1s band shows four components in the same range of nickel deposition times and oxygen exposures. The synthetic spectra related to these components are three
2p 2p 2p 2p 2p KR3 satellite KR4 satellite 1s 1s 1s 1s 3p3/2 3p1/2 satellite 3p3/2 3p1/2 satellite 3p3/2 3p1/2 satellite
E0 (eV)
fwhm (eV)
72.7 72.3 73.9 74.6 75.4 64.3 62.5 531.05 ( 0.05 531.6 532.1 529.55 ( 0.05 65.7 67.4 69.8 66.7 ( 0.05 68.4 ( 0.05 69.4 ( 0.10 67.3 ( 0.05 69.4 ( 0.05 71.4 ( 0.10
1.0 1.0 1.8 1.8 1.8 1.1 1.1 1.9 1.9 1.9 1.6 2.3 2.3 3.5 2.3 2.3 2.7 2.3 2.3 3.0
symmetrical GL bands, O1, O2, and O3, with O1 and O2 being attributed to the formation of intermediate Al oxidation states and oxygen chemisorption, and O3 to Al2O3; and an asymmetrical GL band, ONi, associated with nickel oxide, NiO. The reference band ONi was obtained from the measured O 1s spectra of the 5000 langmuir oxygen exposed clean nickel substrate. Table 1 summarizes the parameters, binding energy (E0) and full width at half-maximum (fwhm), defining the synthetic bands used for the deconvolution of the Al 2p, Ni 3p, and O 1s spectra. Further details on the deconvolution procedure have been given elsewhere.25,27 To determine the bands associated with the Ni 2p spectra by peak deconvolution, synthetic reference spectra were used. In a first step, synthetic peaks for the Ni 2p bands of the clean Ni/Al interfaces, Nimet, were obtained by a least-squares procedure using the measured Ni 2p spectra of the clean Ni/Al interfaces as a reference. This allows one to use peak heights as the only adjustable parameter. As the oxygen exposure increases, new features appear, on the high BE side of the Ni 2p spectra, as a consequence of the formation of new species related to the oxidation. In a second step, the number of additional bands was obtained by subtraction of the Ni spectrum of the clean interface from the measured spectra of the oxygenexposed surfaces. The Ni spectrum of the clean interface was normalized by a factor which minimizes the difference in the BE interval between the Ni 2p3/2 maximum of the clean interface spectrum and 848 eV. Figure 3 shows the subtracted spectra as a function of oxygen exposure, for nickel deposition times of 6, 15, and 22 min. For deposition times corresponding to the first stage (t e 9 min), the subtracted spectra of the oxygen-exposed interfaces up to 5000 langmuir are shifted ∼0.4 eV to higher binding energies with respect to the metallic reference, and no other features are observed, that is, the spectra are essentially metallic. This behavior would be consistent with the presence of Ni atoms in an aluminum-depleted layer at the interface, in agreement with analogous results on the Fe/Al system.27 The band associated with this state will be denoted as Ni0+, and its analytical shape was obtained from the subtracted spectra. As observed in Figure 3, for deposition times corresponding to the second stage of growth (t > 9 min), the subtracted spectra show the formation of additional (27) Palacio, C.; Arranz, A. J. Phys. Chem. B 2001, 105, 10805.
1698
Langmuir, Vol. 18, No. 5, 2002
Arranz and Palacio Table 2. Parameters of the Synthetic Bands Nix+, Niint, and NiO Used for Deconvolution of Ni 2p Spectra band
Figure 3. XPS Ni 2p subtracted spectra as a function of oxygen exposure, for nickel deposition times of 6, 15, and 22 min. The subtraction procedure is explained in the text. NiO and Niint reference spectra used in the deconvolution procedure of the Ni 2p band are also given in the bottom of the right panel.
chemical states. The Ni 2p3/2 binding energy of NiO has been indicated by arrows at 854 eV.7,28-31 Moreover, for lower oxygen exposures (∼4-10 langmuir) an additional broad feature, denoted as Nix+, can be observed in the subtracted spectra at ∼853 eV. Since Nix+ and Ni0+ features are close and Nix+ is much broader than Ni0+, no attempt to use the Ni0+ band in the peak-fitting procedure, for deposition times corresponding to the second stage of growth, has been done. Therefore, for t > 9 min, the band denoted as Nix+ should involve information associated with the formation of any NiOx (x < 1) suboxide, with oxygen chemisorption during the first stages of the oxidation,28 and with Ni atoms in the aluminum-depleted layer at the interface. The evolution of the subtracted spectra for the 22 min Ni/Al interface, as a function of the oxygen exposure, is similar to that observed for a pure Ni substrate. The Nix+ synthetic band has been deduced from the subtracted spectra of the 22 min Ni/Al interface and from the oxygen-exposed pure Ni substrate, whereas the analytical shape of NiO was obtained from a 5000 langmuir oxygen exposed Ni substrate after subtracting the metallic component. The parameters defining the synthetic bands are given in Table 2. The calculated NiO spectrum is shown as a reference in Figure 3. This spectrum is in good agreement with those reported in the literature for stoichiometric NiO.31-33 The Ni 2p3/2 component of NiO is characterized by peaks a, b, and c, at ∼854, 856, and 861 eV, respectively. Peaks a and c are attributed to the c-13d9L-1 and c-13d8L final states, respectively, where c-1 and L-1 denote a hole in the Ni 2p and O 2p bands, respectively.32,33 According to the nonlocal screening model,34-36 peak b should be attributed to a local configuration of mainly c-13d9L character at the core hole site, (28) Kim, K. S.; Winograd, N. Surf. Sci. 1974, 43, 625. (29) McIntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208. (30) Norton, P. R.; Tapping, R. L.; Goodale, J. W. Surf. Sci. 1977, 65, 13. (31) Kuhlenbeck, H.; Odo¨rfer, G.; Jaeger, R.; Illing, G.; Menges, M.; Mull, T.; Freund, H. J.; Po¨hlchen, M.; Staemmler, V.; Witzel, S.; Scharfschwerdt, C.; Wennemann, K.; Liedtke, T.; Neumann, M. Phys. Rev. B 1991, 43, 1969. (32) Oku, M.; Tokuda, H.; Hirokawa, K. J. Electron Spectrosc. Relat. Phenom. 1991, 53, 201. (33) van Elp, J.; Eskes, H.; Kuiper, P.; Sawatzky, G. A. Phys. Rev. B 1992, 45, 1612. (34) van Veenendaal, M. A.; Sawatzky, G. A. Phys. Rev. Lett. 1993, 70, 2459. (35) Alders, D.; Voogt, F. C.; Hibma, T.; Sawatzky, G. A. Phys. Rev. B 1996, 54, 7716.
E0 (eV)
fwhm (eV)
2p3/2 satellite 2p3/2 2p1/2 satellite 2p1/2
Nix+ 853.0 860.0 870.3 876.3
1.6 3.4 2.6 6.0
2p3/2 satellite 2p3/2 satellite 2p3/2 asymmetry 2p1/2 satellite 2p1/2
Niint 856.0 ( 0.1 862.1 ( 0.1 867.0 ( 0.1 874.0 ( 0.1 881.0 ( 0.1
2.6 ( 0.1 4.9 ( 0.1 4.0 ( 0.1 4.2 ( 0.1 5.2 ( 0.1
2p3/2, peak a 2p3/2, peak b satellite 2p3/2, peak c satellite 2p3/2 asymmetry 2p1/2 satellite 2p1/2
NiO 854.0 856.2 861.1 865.9 872.8 879.8
2.2 2.2 4.8 3.8 4.4 4.8
being the Ni 2p core hole screened by an electron of a neighboring NiO6 unit (′), with local configuration c′3d′8L′-1. In contrast to the 22 min Ni/Al interface, for the Ni/Al interfaces grown in the range 9 min < t < 22 min, the intensity ratio between peaks a and b of the subtracted spectra is different from that of the NiO reference. An intense shoulder is observed at ∼856 eV, suggesting the formation of a new species related to nickel oxidation. This species will be denoted as Niint, since as we will discuss later it is located at the aluminum oxide-nickel oxide interface. The analytical shape of the Niint component has been obtained by a trial and error procedure from subtracted spectra of 12 and 15 min Ni/Al interfaces and is shown as a reference in Figure 3 (see also Table 2). According to the literature, this additional component should be related either to the formation of a NiAl2O4-like mixed oxide,6,22,29 where the isolation of the NiO6 units would promote the local screening mechanism,36 or to the formation of Ni3+ defects in the NiO layer.28,30,37-40 In both cases, the Ni 2p3/2 band is characterized by a unique peak shifted ∼1.5-2 eV to the high BE side of the NiO. Kishi et al.23,24 have observed a similar Ni 2p spectrum to that of the Niint reference shown in Figure 3, during the first stages of nickel deposition onto vanadium oxide. They attributed this peak to nickel atoms at the interface that are bonded to oxygen atoms of the substrate. To summarize, Nix+, Niint, and NiO synthetic bands are the sum of four, five, and six symmetrical GL functions, respectively, which allows reproduction of the Ni 2p3/2,1/2 doublet and their characteristic satellites at higher BE. The parameters defining these bands are given in Table 2. Examples of the deconvolution carried out for Al 2p-Ni 3p, O 1s, and Ni 2p spectra are given in Figure 2a-c. Figure 4 shows (a) the evolution of the Al, O, and Ni XPS peak areas as a function of the oxygen exposure, for a nickel deposition time of 6 min, that corresponds to the first stage of the Ni/Al interface formation. In addition, in (b) the evolution of the same XPS signals as a function of the oxygen exposure for a nickel deposition time of 15 min, corresponding to the second stage of nickel deposition, is also given. The intensities I (peak areas) are normalized (36) Altieri, S.; Tjeng, L. H.; Tanaka, A.; Sawatzky, G. A. Phys. Rev. B 2000, 61, 13403. (37) Roberts, M. W.; Smart, R. St. C. Surf. Sci. 1981, 108, 271. (38) Tomellini, M. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3501. (39) Gonza´lez-Elipe, A. R.; Holgado, J. P.; Alvarez, R.; Munuera, G. J. Phys. Chem. 1992, 96, 3080. (40) Carley, A. F.; Jackson, S. D.; O’Shea, J. N.; Roberts, M. W. Surf. Sci. 1999, 440, L868.
Interaction of Ni/Al Interfaces with Oxygen
Langmuir, Vol. 18, No. 5, 2002 1699
Figure 4. Normalized intensities (normalization is explained in the text) as a function of oxygen exposure of Al 2p, O 1s, and Ni 2p signals for (a) a Ni/Al interface corresponding to the first stage of nickel deposition on aluminum and (b) a Ni/Al interface corresponding to the second stage of deposition.
Figure 5. Normalized intensities as a function of the nickel deposition time of Al 2p, O 1s, and Ni 2p signals measured for the 5000 langmuir oxide films.
to the corresponding sensitivity factors, SAl ) 0.11, SO ) 0.63, and SNi ) 5.4.41 On the other hand, Figure 5 shows the evolution of Al, O, and Ni XPS signals as a function of the nickel deposition time, for an oxygen exposure of 5000 langmuir. Figures 4 and 5 show that the composition of the thin oxide film formed depends on both the nickel deposition time and the oxygen exposure. (41) Handbook of X-ray Photoelectron Spectroscopy; Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; Perkin-Elmer Corp.: Eden Prairie, MN, 1979.
Figure 6. Variations of different XPS peak area ratios as a function of takeoff angle for a 5000 langmuir oxide film grown on (a) an interface corresponding to the first stage of deposition (t ) 3 min) and (b) an interface corresponding to the second stage (t ) 15 min).
To obtain further information on the concentration depth profiles of the different Al and Ni species, ARXPS measurements were carried out for all Ni/Al interfaces exposed to 5000 langmuir. Figure 6 shows different peak area ratios between Al and Ni species, as a function of the takeoff angle for (a) t ) 3 min (first stage of Ni deposition) and (b) t ) 15 min (second stage). In this figure, Alox corresponds to the Al+ + Al2+ + Al3+ contributions. As discussed below, a comparative analysis of the evolution
1700
Langmuir, Vol. 18, No. 5, 2002
of these ratios will give qualitative information on the in-depth composition of the oxide film formed. Discussion For the Ni/Al interfaces corresponding to the first stage, none of the species associated with nickel oxidation, ONi, Nix+, Niint, and NiO, are observed during oxygen exposure. In Figure 4a, Al2+ and O1 species are detected at low oxygen exposures indicating oxygen chemisorption up to ∼2-4 langmuir. The O1 signal shows a maximum at ∼6 langmuir, decreasing afterward to completely disappear for high oxygen exposures, whereas Al2+ reaches saturation at ∼20 langmuir. Al+, O2, and Ni0+ species exhibit a sigmoidal shape reaching saturation for oxygen exposures above 20 langmuir. The signals related to the stoichiometric Al2O3 (Al3+ and O3) appear at ∼10-20 langmuir and continuously increase. Sigmoidal shapes of the kinetic curves have been also observed during the first stages of oxidation for pure Al,42 Ni,43 and the Fe/Al system.27 This behavior is commonly explained by using an oxidation model that involves chemisorption of oxygen, oxide island nucleation and growth until coalescence, and thickening of the oxide film. An intermediate Al oxidation state has been also observed during the oxidation of NiAl and Ni3Al bulk compounds,9-11 at low oxygen pressures and temperatures of ∼500 °C. This state was attributed either to the presence of defects in the oxide film10 or to Al atoms at the oxide/ alloy interface bonded to the Ni of the alloy substrate.10,11 Also, it has been attributed to a precursor oxide species on the outer surface of the oxide, partially formed by Ni atoms that have not been able to diffuse inside the substrate.9 In the present work, two intermediate oxidation states, Al+ and Al2+, in addition to Al3+, are observed during the oxidation of Ni/Al interfaces, in good agreement with the results of Faraci et al.44,45 for Al deposition in the presence of atomic oxygen on SiO2 and graphite substrates and with results on the Fe/Al system.27 Furthermore, the O 1s band shows three features, O1, O2, and O3, following evolutions similar to those of the Al+, Al2+, and Al3+ bands, respectively. As observed in Figure 4a, not only Al3+ (Al2O3) but also Al intermediate oxidation states, Al+ and Al2+, are formed with increasing oxygen exposures. Moreover, Figure 5 shows that the signals Al3+ and O3 decrease with increasing nickel deposition time. This behavior can be related to the observed decrease of the available surface of the Al substrate, which is covered by NiAlx islands, with increasing nickel deposition time. This evolution is also accompanied by the formation of Al intermediate oxidation states, characterized by the Al+, Al2+, O1, and O2 species (see Figure 5). During the first stage, all the deposited Ni would react to form the intermetallic compound, NiAlx. Since no “free” metallic nickel is available on the surface, no chemical species associated with the nickel oxidation, (ONi, Nix+, Niint, and NiO) are expected to be produced during oxidation, in good agreement with observed experimental results. Only Ni0+ was identified, which should be attributed to Ni atoms in an Al-depleted layer at the interface, in good agreement with the results found for the Fe/Al system.27 The shift to lower BE in the nonmetallic part of the Al 2p band (see Figure 2a) may arise from a decrease in the (42) Arranz, A.; Palacio, C. Surf. Sci. 1996, 355, 203. (43) Holloway, P. H. J. Vac. Sci. Technol. 1982, 18, 653. (44) Faraci, G.; La Rosa, S.; Pennisi, A. R.; Hwu, Y.; Margaritondo, G. Phys. Rev. B 1993, 47, 4052. (45) Faraci, G.; La Rosa, S.; Pennisi, A. R.; Hwu, Y.; Margaritondo, G. J. Appl. Phys. 1995, 78, 4091.
Arranz and Palacio
positive charge on Al atoms in the Al+ and Al2+ intermediate oxidation states as a consequence of the formation of Al-O-Ni cross-linking bonds at the interface. This interpretation is also supported by the presence of O1 and O2 bands at intermediate BE due to a decrease in the negative charge of the oxygen. The parallel evolution in the shift of the nonmetallic part of the Al 2p band and that of the O 1s band with increasing nickel deposition time (Figure 2a,b) suggests a decrease of the charge transfer from Al to O atoms at the interface as a consequence of the formation of cross-linking bonds with Ni atoms. Such a type of cross-linking bond has been also observed during the oxidation of the Fe/Al system27 and during the first stages of deposition of thin TiO2 films on SiO2 and Al2O3 substrates.46,47 Complementary information on the composition of the oxide film formed can be obtained from the ARXPS results in Figure 6a. The data can be interpreted by a simple and idealized model which considers the superposition of three layers: aluminum oxide with Al3+ states on top of Al+ and Al2+ states followed by a layer enriched in nickel on top of the substrate. The angular measurements of Figure 6a allow one to establish that the Al+ and Al2+ intermediate oxidation states are located at the interface between an Al2O3 layer and the substrate, just above the nickelenriched layer (Ni0+). This result is in good agreement with the localization proposed by Jaeger et al.10 and Bardi et al.11 for the intermediate Al oxidation state observed during the oxidation of NiAl and Ni3Al bulk compounds and rules out the presence of the precursor oxide species proposed by Venezia and Loxton at the outer surface.9 From Figures 2 and 4b, it is observed that the reaction of the Ni/Al interfaces, formed during the second stage of growth, with low-pressure oxygen is characterized by the formation of ONi, Nix+, Niint, and NiO species. Also, new features appear in the Ni LVV Auger transition (Figure 1) indicating the oxidation of nickel. As observed in Figure 4b, which corresponds to the oxidation of an interface of the second stage of growth, Al+, Al2+, and O1 species are already detected at very low oxygen exposures. The signal intensities of Al+ and O1 increase up to saturation at ∼100 langmuir, whereas Al2+ reaches a maximum at 2 langmuir, slightly decreasing above this exposure. For exposures above 1 langmuir, Nix+, Niint, and NiO species are detected. The Niint and NiO species have similar evolution, increasing to reach saturation at ∼100 langmuir. However, the signal intensity of Nix+ reaches a maximum at ∼20 langmuir, decreasing above this exposure. It is concluded that at exposures above 100 langmuir all the nickel oxide species and those of aluminum oxide may probably coexist. For nickel deposition times g22 min, when θNi is comparable to θNiAlx, the evolution of the signal intensities (not shown) of Nix+ and NiO as a function of the oxygen exposure is similar to that observed during the oxidation of a high-purity nickel substrate, therefore indicating that the oxidation is dominated by the oxidation of the nickel islands. Moreover, the Nix+ and Niint signals detected at 5000 langmuir decrease as the nickel deposition time increases (Figure 5), and only NiO is observed for high nickel deposition times. XPS line shape analyses of Al 2p, O 1s, and Ni 2p bands of Figure 5 indicate that the features O1, O2, O3, and ONi follow evolutions similar to those of the Al+, Al2+, Al3+, and NiO bands, respectively, with increasing nickel (46) Lassaletta, G.; Ferna´ndez, A.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. J. Phys. Chem. 1995, 99, 1484. (47) Sa´nchez-Agudo, M.; Soriano, L.; Quiro´s, C.; Avila, J.; Sanz, J. M. Surf. Sci. 2001, 482-485, 470.
Interaction of Ni/Al Interfaces with Oxygen
deposition time. The Al+, Al2+, O1, O2, and Nix+ species are observed, at high exposures, during the second stage of growth. This confirms that the Nix+ synthetic band, used for Ni 2p spectra deconvolution during the second stage, involves information associated not only with chemisorption and suboxide formation but also with the Ni atoms in the Al-depleted layer at the interface. Additional insight into the film composition can be obtained from the ARXPS results in Figure 6b. These results show that the in-depth distribution of species is consistent with the sequence aluminum oxides (Al3+ + Al2+ + Al+), NiO, Nix+, and substrate when going from the outer surface to the substrate, being Niint species located at the aluminum oxide-NiO interface. It must be pointed out that this model is idealized. In reality, concentration gradients are to be expected in the layers. In fact, at high nickel deposition times, the aluminum oxide layer is mainly formed of intermediate aluminum oxidation states with Ni atoms incorporated in the network. Conclusions The oxidation of nickel deposited on polycrystalline aluminum surfaces has been studied at room temperature and low oxygen pressures, using AES, XPS, and ARXPS.
Langmuir, Vol. 18, No. 5, 2002 1701
The growth of the nickel on the aluminum surfaces occurs in two stages. The reaction of the oxygen with the interfaces formed during the first stage leads to the formation of aluminum intermediate oxidation states Al+ and Al2+, in addition to Al3+. These intermediate oxidation states are attributed to the formation of Al-O-Ni crosslinking bonds at the interface. The analysis of the Ni 2p band shows small changes that can be attributed to Ni atoms in an aluminum-depleted layer at the interface. However, no nickel oxide is detected. The oxidation of the interfaces formed during the second stage of nickel growth can be described by a model postulating an outer film of aluminum oxide, mainly composed of Al+ and Al2+ states, on top of a NiO layer. A NiAl2O4-like oxide or Ni3+ defects seem to be formed at the interface between Al and Ni oxides. Acknowledgment. The authors thank D. Dı´az for technical assistance. This work is a part of a research project supported by the Spanish Comisio´n Interministerial de Ciencia y Tecnologı´a (Project MAT99-0830CO3-02). LA015634D
