J. Phys. Chem. C 2009, 113, 15533–15537
15533
Viable Route for Cobalt Oxide-Carbon Nanocomposites Fabio Lupo,† Radha Kamalakaran,‡ and Antonino Gulino*,† Dipartimento di Scienze Chimiche, UniVersita` di Catania and I.N.S.T.M. UdR di Catania, Viale Andrea Doria 6, 95125 Catania, Italy, and GE Global Research, Max-Planck-Institut für Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany ReceiVed: March 30, 2009; ReVised Manuscript ReceiVed: July 1, 2009
A one-step process to form carbon nanotubes coated with cobalt oxide nanoparticles, using spray pyrolysis of cobalt(II) acetylacetonate-xylene solutions, has been developed. Both microstructure and chemical composition of carbon nanotubes covered with cobalt oxide nanoparticles depend on the concentration of the Co(acac)2 solutions. High resolution transmission electron microscopy and high resolution electron energy loss spectroscopy studies reveal the microstructural and chemical nature of the cobalt oxide nanoparticles formed in situ on the surface of carbon nanotubes, during the spray pyrolysis process. Cobalt oxide nanoparticles, result to be embedded within graphite layers. X-ray photoelectronic data confirm the chemical composition and structure of the as-synthesized composite nanomaterials. Introduction
Experimental Details
Since their discovery,1 carbon nanotubes (CNT) have been the subject of extensive research due to their high-performances and unique electronic and mechanical properties.2 If metal and/ or metal oxides (guest) are embedded with/on CNT to form nanocomposites, the range of application is greatly enhanced.3,4 In many of these cases, surface modification treatments of CNTs involving multistep processes are required, in order to establish efficient tube-guest interactions.5 In this scenario, interesting results concerning the influence of cobalt oxides on growth and properties of CNT have been reported.3a Examples of them are applications as hydrogen, carbohydrates, thiols, CO sensing;7,8 improved Fischer-Tropsch synthesis;9 precursor-controlled formation of carbon/metal oxide nanocomposites;10 synthesis of alcohols from syngas;11 highly efficient materials for Libatteries;12 preparation of nanocomposites via self-assembly;13 as catalysts for NH3 decomposition;14 formation of CNT/Co3O4 nanocomposites for supercapacitors;15 fabrication of rectangular Co3O4 nanosheets;16 beaded cobalt oxide nanoparticles along carbon nanotubes for highly integrated electronic devices;17 and many other applications.18
The spray pyrolysis setup used in the syntheses has already been described elsewhere.20 In the present studies, the reactant solutions were prepared by dissolving and sonicating appropriate quantities of Co(acac)2 in 100 mL of xylene (Aldrich reagents) in order to obtain the 2, 10, and 50 wt % starting solutions/ dispersions. Quartz reactor tubes (one per each experiment) were heated to 1000 °C. Then, for every experiment, 10 mL of a given Co(acac)2 starting solution/dispersion was sprayed inside the reactor in 3 min time, using argon as carrier gas (at =1.5 atm). The total pressure inside the reactor was 1 atm. Afterward, the system was left to cool to room temperature in argon ambient and the pyrolysis products collected in glass vials. The as-synthesized samples were analyzed by scanning electron microscopy (SEM, JEOL JSM 6300F, equipped with a Noran instrument EDX detector) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 4000EX operated at 400 kV and Philips CM20 operated at 200 kV). A dedicated STEM VG-HB 501UX equipped with a Gatan UHVEnfina was used for high-resolution electron energy loss spectroscopy (HREELS). Samples for TEM analysis were ultrasonically (SonoPlus with tip MS73, Bandelin, power: 30%) dispersed in ethanol and dropped onto a holey carbon-coated copper grid. Sample preparation and measurements have been repeated 10 times per each starting solutions/dispersions. X-ray diffraction (XRD) data were recorded on a Bruker D-5005 diffractometer operating in a θ-2θ geometry (Cu KR radiation, 30 mA and 40 kV). X-ray photoelectron spectra (XPS) were measured at 45°, relative to the surface plane with a PHI 5600 Multi Technique System (base pressure of the main chamber 2 × 10-10 Torr).16 The spectrometer is equipped with a dual anode X-ray source; a spherical capacitor analyzer (SCA) with a mean diameter of 279.4 mm; an electrostatic lens system Omni Focus III. Samples were mounted on Mo stubs. Spectra were excited with Al KR radiation. The structure due to the KR2 satellite radiation has been subtracted from the spectra before the data processing. The XPS peak intensities were obtained after Shirley background removal.16 Procedures to account for steady state charging effect
In this appealing panorama, in the present study we report on a new and simple approach to prepare multiwalled, cobalt oxide coated nanotubes in a one step process using a Co(II) precursor. These samples were prepared by spray pyrolysis of a solution of Co(acac)2 in xylene. The cobalt complex participates in the reaction by acting as a cobalt oxide precursor and, at the same time, providing the catalyst for CNT production. The as-synthesized product consists of CNTs with cobalt oxide nanoparticles attached to the external walls and embedded within a few graphene layers. Some acetylacetonates have already been used for CNT production;3a,19 thus, to the best of our knowledge, there is no report on the use of Co(acac)2 for syntheses of both pristine or modified CNTs. * To whom correspondence should be addressed. † Universita` di Catania and I.N.S.T.M. UdR di Catania. ‡ GE Global Research, John F. Welch Technology Center, Sy No 122, EPIP, Whitefield Road, Bangalore-560066, India.
10.1021/jp902857g CCC: $40.75 2009 American Chemical Society Published on Web 08/10/2009
15534
J. Phys. Chem. C, Vol. 113, No. 35, 2009
Lupo et al.
Figure 1. SEM images of MWNT from flakes (A) sample A, (B) sample B, and (C) sample C formed from spray pyrolysis of 2, 10, and 50 wt % Co(acac)2-xylene, respectively.
Figure 2. Representative HRTEM images from (A) sample A, (B) sample B, and (C) sample C; (D) TEM image of a thin edge of a representative flake from sample C.
Cobalt Oxide-Carbon Nanocomposites
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15535
Figure 3. HRTEM of a representative small portion of sample C CNT (shown in the inset) coated with cobalt oxide nanoparticles.
have been described elsewhere.16 Experimental uncertainties in binding energies lie within (0.45 eV. Result and Discussion In all cases, the obtained product occurred as a flaky black deposit on the inner walls of the quartz tube and was easily removed with a steel tool. SEM studies revealed the morphology of these flakes. Samples A (from the 2 wt % solution; yield ∼11 wt %) and B (from the 10 wt % solution; yield ∼15 wt %) show black flakes of similar dimension (∼0.2 × 0.2 cm) and morphology. These small flakes essentially consist of matted nanotubes as shown in Figure 1. In particular, sample A (Figure 1a) entirely consists of entangled tubes with a 40 ( 10 nm diameter and several µm in length. Sample B (Figure 1b) shows similar nanotubes as well as other rare particulates on their surface. Sample C, from the highest 50 wt % (yield ∼21 wt %) Co(acac)2 suspension precursor, reveals a different and irregular morphology because of the presence of bumps on the CNT surfaces. In fact, the as-synthesized CNTs show surfaces largely covered with nanoparticles. EDX studies confirmed the presence of carbon and cobalt in all samples. In addition, both samples B and C show the presence of oxygen. HRTEM investigations have been carried out to better understand the nanoscopic morphology of the tubes and particles seen on some tube surfaces. Figure 2 shows representative micrographs of samples A, B, and C. Sample A (Figure 2a) consists of rather clean MWCNT with a 40 ( 10 nm diameter partially filled with metallic Co, as inferred from HREELS analysis. Sample B, (Figure 2b) in addition to the MWCNT having a 40 ( 10 nm diameter shows sporadic surface coverage with nanosized (2-15 nm) particles. In contrast, sample C shows a different picture. The tube-like structures are CNTs almost totally coated with nanosized particles, as seen in Figure 2c.
The thin edges of the sample C flake also show nanotubes, covered with nanoparticles (Figure 2d). Figure 3 shows a HRTEM image of a representative part of sample C CNT (see inset). It is interesting to note that a few layers of graphitic carbon (13-26 Å) cover these surface particles whose sizes are in the 5-10 nm range. This observation confirms that cobalt oxide nanoparticles (vide infra) are intimately encapsulated within graphite, thus giving origin to a nanocomposite material and gives also a rationale for the dimension of these nanosized surface particles. In fact, nucleation and growth of the metal oxide (vide infra) nanoparticles on the CNT surfaces, compete with the graphite deposition. For this reason, the graphite deposition hampers the growth of these metal oxide nanoparticles that, in turn, remain nanosized and promote the formation of a nanocomposite material. It must be mentioned, that these results were highly repeatable on different samples C and on all the analyzed portions. Therefore, a rather homogeneous coverage of the nanotubes by the metal oxide nanoparticles emerged and confirmed the synthesis of a nanocomposite material. These results are in agreement with recent already reported studies on metal organic chemical vapor deposition of cobalt oxides using Co(II) β-diketonates as precursors.21 The chemical investigation of this sample C nanoparticles was performed by HREELS analysis. Bright and dark field HRTEM micrographs of the analyzed area are shown in Figure 4, panels a and b. HREELS spectra of the as-synthesized materials revealed the presence of ionization edges at ca. 285, 530, 780-797 eV corresponding to the C-K, O-K, and Co-Lexcitations (Figure 4c).22 The near edge fine structure of the carbon K-edge confirms that the material is graphitic.23 Elemental distribution profiles derived from the HREELS line scan analysis across a coated CNT is shown in Figure 4d. The cobalt profile matches the partial filling within the CNT core and the
15536
J. Phys. Chem. C, Vol. 113, No. 35, 2009
Lupo et al.
Figure 4. HREELS spectra from sample C: (A) bright and (B) dark field HRTEM micrographs of the area analyzed by HREELS; (C) ionization edges at ca. 285, 530, and 780-796 eV corresponding to the C-K, O-K, and Co-L-excitations; (D) elemental distribution profiles derived from the HREELS line scan analysis (marked with a line and two arrows in Figure 4B) across a CNT.
Figure 5. X-ray diffraction pattern, over a 30° < 2θ < 80° angular range, for sample C.
two surface particles (marked with arrows in Figure 4b). The oxygen profile matches only the two surface particles. This observation clearly indicates the presence of metallic cobalt in the CNT core and cobalt oxide particles on the CNT surface. The carbon intensity dominates the center of the profile (dashed line). Compositional quantification of elements from more than 20 EELS line scans, collected from the particles coating the tubes, was performed and a constrained power-law model was used to determine the background underneath the core-loss edges.24 Results were always repeatable and yielded a 1.0:1.0 ((5%) O:Co rate, thus corresponding to the CoO stoichiometry. Additional XRD measurements (Figure 5) of sample C provide evidence of cubic CoO (PDF-09-0402) and Co (PDF15-0806) phases. Only the CoO (200) reflection (θ ) 42.69) has been observed thus pointing to textured CoO. The molecular characterization of the sample C was also carried out with X-ray photoelectron spectroscopy. This technique is ideal since gives information on the bonding states and allows to estimate the surface elemental composition, making due allowance for the relevant atomic sensitivity factors.16 In
Figure 6. Al KR excited XPS of a representative sample C measured in the Co 2p energy region.
Figure 3 it is shown that, given the dimension of a representative sample C CNT, XPS is useful to probe the surface particles of the CNT and can hardly give information on the internal CNT content. Figure 6 shows the Al KR XPS of as deposited cobalt oxide films in the Co 2p binding energy (B.E.) region. The Co 2p features consist of the main 2p3/2 2p1/2, spin-orbit components at 779.7 and 796.0 eV, respectively.21 It has already been reported that the binding energy values of the Co 2p states do not allow a clear distinction between CoO [pure Co(II)] and Co3O4 [CoIICoIII2O4]. In contrast, this distinction can be afforded by the analysis of the shakeup satellites. In fact, Co3O4 shows low intensity shakeup satellites at 9 eV from the main spin-orbit components while, pure CoO shows intense shakeup satellites 6 eV above the primary spin-orbit features. In the present sample C, shakeup satellites centered at 785.7 and 802.0 eV, 6 eV from the main bands, characteristic of the CoO phase are evident (Figure 6). These satellites can be used as a fingerprint for the recognition of high-spin Co(II) species in
Cobalt Oxide-Carbon Nanocomposites CoO.21e,g The O 1s peak is centered at 530.1 eV.21 The C 1s spectrum shows a broad peak at 285.0 eV. Atomic concentration analysis of different portions of sample C always indicated a 15% cobalt oxide content on the CNT. Experimental uncertaity is confined within the 10% of this value. It is interesting to note that the XPS of sample A does not show any Co or O signals, thus demonstrating that oxygen is intimately related to the presence of surface cobalt. Moreover, this results confirms that the XPS probed depth is not enough to reveal metallic cobalt within the CNTs. XPS analysis was repeated 5 times at 10 day time intervals and always gave identical results. This examination confirms the absence of cobalt oxide aging (oxidation to higher oxides) since these nanoparticles are closely encapsulated within graphite layers that hamper oxygen permeability. Finally, we believe that similar functionalized CNTs can be produced by spray pyrolysis of other metal-acetylacetonates. Conclusion An efficient one-step process for the synthesis of bulk quantities of cobalt oxide-functionalized nanotubes has been developed using a simple and well-defined cobalt β-diketonate complex as precursor. The nature of the end product, i.e., formation of pristine or functionalized nanotubes, can be controlled simply by changing the precursor concentration. Concentrated cobalt complex solutions resulted in MWNTs homogeneously covered with CoO nanoparticles in turn, embedded within graphite layers. These coated nanotubes are potentially useful as catalysts for fuel cells, as sensing elements for recognizing chemicals, as fillers for ceramic or polymer based composites and their specific nanostructure deserves enough motivation for a forthcoming study. Acknowledgment. The authors thank NATO (SfP Project 981964) and MIUR, Roma for financial supports (PRIN 2005 and FIRB 2003). References and Notes (1) Ijima, S. Nature 1991, 354, 56. (2) (a) Gogotsi, Y. Carbon Nanomaterials, 1st ed.; Taylor & Francis: London, 2006. (b) Barsukov, I. V.; Johnson, C. S.; Doninger, J. E.; Barsukov, V. Z. New Carbon Based Materials for Electrochemical Energy Storage Systems: Batteries, Supercapacitors and Fuel Cells, 1st ed.; Springer: Dordrecht, The Netherlands, 2006. (c) Terrones, M. Ann. ReV. Mater. Res. 2003, 33, 419. (d) Zhan, G. D.; Kuntz, J.; Wan, J.; Mukherjee, A. K. Nat. Mater. 2003, 2, 38. (e) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (f) Thostenson, E. T.; Ren, Z.; Chou, T. W. Comp. Sci. Technol. 2001, 61, 1899. (3) (a) Zhi, L.; Hu, Y. S.; El Hamaoui, B.; Wang, X.; Lieberwirth, I.; Kolb, U.; Maier, J.; Mu¨llen, K. AdV. Mater. 2008, 20, 1727. (b) Wildgoose, G. G.; Banks, C. E.; Compton, R. G. Small 2006, 2, 182. (c) Jiang, K.; Eitan, A.; Schadler, L. S.; Pulickel, M.; Ajayan, M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Nano Lett. 2003, 3, 275. (d) Liu, Z.; Lin, X.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18, 4054. (e) Guerret-Plecourt, C.; Le Bouar, Y.; Lolseau, A.; Pascard, H. Nature 1994, 372, 761. (4) (a) Lu, A.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (b) Cui, G.; Zhi, L.; Thomas, A.; Kolb, U.; Lieberwirth, I.; Mu¨llen, K. Angew. Chem., Int. Ed. 2007, 46, 3464. (c) Cui, G.; Zhi, L.; Thomas, A.; Lieberwirth, I.; Kolb, U.; Mu¨llen, K. Chem. Phys. Chem. 2007, 8, 1013. (d) Stura, E.; Nicolini, C. Anal. Chim. Acta 2006, 568, 57. (e) Nishijo, J.; Okabe, C.; Oishi, O.; Nishi, N. Carbon 2006, 44, 2943. (f) Lukaszewicz, J. P. Sens. Lett. 2006, 4, 53. (g) Suh, W. H.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 12007. (h) Arico, A. S.; Bruce, P. G.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (i) Zhang, L.; Cheng, B.; Samulski, E. T. Chem. Phys. Lett. 2004, 398, 505. (j) King, W. D.; Corn, J. D.; Murphy, O. J.; Boxall, D. L.; Kenik, E. A.; Kwiatkowski, K. C.; Stock, S. R.; Lukehart, C. M. J. Phys. Chem. B 2003, 107, 5467. (k) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652. (l) Modi, A.; Koratkar, N.; Lass, E.; Wei, B.; Ajayan, P. M. Nature 2003, 424, 171. (m) Rabinovich, L.; Lev, O. Electroanal. 2001, 13, 265. (n) Joo, S. H.;
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15537 Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (o) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (p) Hayashi, T.; Hirono, S.; Tomita, M.; Umemura, S. Nature 1996, 381, 772. (5) (a) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 597. (b) Kim, K.; Lee, S. H.; Yi, W.; Kim, J.; Choi, J. W.; Park, Y.; Jin, J. I. AdV. Mater. 2003, 15, 1618. (c) Shi, J.; Qin, Y.; Li, X.; Guo, Z. X.; Zhu, D. Carbon 2004, 42, 455. (d) Whitsitt, E. A.; Barron, A. R. Nano Lett. 2003, 3, 775. (e) Hernadi, K.; Ljubovic´, E.; Seo, J. W.; Forro´, L. Acta Materialia 2003, 51, 1447. (f) Coleman, J. N.; Blau, W. J.; Dalton, A. B.; Munoz, E.; Collins, S.; Kim, B. G.; Razal, J.; Selvidge, M.; Vieiro, G.; Guillermo, G.; Baughman, R. H. Appl. Phys. Lett. 2003, 82, 1682. (g) Viswanathan, G.; Chakrapani, N.; Yang, H. C.; Wei, B. Q.; Chung, H. S.; Cho, K. W.; Ryu, C. Y.; Ajayan, P. M. J. Am. Chem. Soc. 2003, 125, 9258. (h) Liu, Z.; Lin, X.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18, 4054. (i) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 195. (j) Seeger, T.; Redlich, P.; Grobert, N.; Terrones, M.; Walton, D. R. M.; Kroto, H. W.; Ru¨hle, M. Chem. Phys. Lett. 2001, 339, 41. (k) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (l) Tsang, S. C.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159. (m) Tsang, S. C.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159. (6) Guo, K.; Jayatissa, A. Mater. Sci. Eng., C: Mater. Biol. Appl. 2008, 28, 1556. (7) Buratti, S.; Brunetti, B.; Mannino, S. Talanta 2008, 76, 454. (8) Wu, R. J.; Wu, J. G.; Yu, M. R.; Tsai, T. K.; Yeh, C. T. Sens. Actuators, B 2008, B131, 306. (9) Tavasoli, A.; Abbaslou, R. M. M.; Trepanier, M.; Dalai, A. K. Appl. Catal., A 2008, 345, 134. (10) Zhi, L.; Hu, Y. S.; El Hamaoui, B.; Wang, X.; Lieberwirth, I.; Kolb, U.; Maier, J.; Mu¨llen, K. AdV. Mater. 2008, 20, 1727. (11) Wu, X. M.; Guo, Y. Y.; Zhou, J. M.; Lin, G. D.; Dong, X.; Zhang, H. B. Appl. Catal., A 2008, 340, 87. (12) Du, N.; Zhang, H.; Chen, B.; Wu, J.; Ma, X.; Liu, Z.; Zhang, Y.; Yang, D.; Huang, X.; Tu, J. AdV. Mater. 2007, 19, 4505. (13) Li, J.; Tang, S.; Lu, L.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 9401. (14) Zhang, J.; Comotti, M.; Schueth, F.; Schloegl, R.; Su, D. S. Chem. Commun. 2007, 19, 1916. (15) Shan, Y.; Gao, L. Mater. Chem. Phys. 2007, 103, 206. (16) Ni, X.; Song, J.; Li, D.; Zhang, Y.; Zheng, H. Chem. Lett. 2007, 36, 146. (17) Fu, L.; Liu, Z.; Liu, Y.; Han, B.; Hu, P.; Cao, L.; Zhu, D. AdV. Mater. 2005, 17, 217. (18) (a) Qi, J. L.; Wang, X.; Zheng, W. T.; Liu, J. W.; Tian, H. W.; Li, Z. P.; Liu, C. J. Phys. D: Appl. Phys. 2008, 41, 205306. (b) Fu, L.; Liv, X.; Liu, Y.; Liu, Z.; Cao, L.; Wei, D.; Wang, Y.; Yu, G.; Hu, W.; Han, B. J. Phys. Chem. C 2007, 111, 8098. (c) Chai, S. P.; Zein, S. H. S.; Mohamed, A. R. Chem. Phys. Lett. 2006, 426, 345. (d) He, K. X.; Wu, Q. F.; Zhang, X. G.; Wang, X. L. J. Electrochem. Soc. 2006, 153, A1568. (e) Shan, Y.; Gao, L. Chem. Lett. 2004, 33, 1560. (f) Flahaut, E.; Peigney, A.; Bacsa, W. S.; Bacsa, R. R.; Laurent, C. J. Mater. Chem. 2004, 14, 646. (g) Ji, L.; Lin, J.; Zeng, H. C. Chem. Mater. 2000, 12, 3466. (19) (a) Moisala, A.; Nasibulin, A. G.; Kauppinen, E. I. J. Phys.: Cond. Mat. 2003, 15, 3011. (b) Terrones, H.; Hayashi, T.; Mun˜oz-Navia, M.; Terrones, M.; Kim, Y. A.; Grobert, N.; Kamalakaran, R.; Dorantes-Da´vila, J.; Escudero, R.; Dresselhaus, M. S.; Endo, M. Chem. Phys. Lett. 2001, 343, 241. (c) Nasibulin, A. G.; Moisala, A.; Brown, D. P.; Kauppinen, E. I. Carbon 2003, 41, 2711. (20) Kamalakaran, R.; Lupo, F.; Grobert, N.; Lozano-Castello, D.; Phillip, N. Y. J.; Ru¨hle, M. Carbon 2003, 41, 2737. (21) (a) Gulino, A.; Fragala`, I. Inorg. Chim. Acta 2005, 358, 4466. (b) Gulino, A.; Dapporto, P.; Rossi, P.; Anastasi, G.; Fragala`, I. J. Mater. Chem. 2004, 14, 2549. (c) Gulino, A.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem. Mater. 2003, 15, 3748. (d) Gulino, A.; Fiorito, G.; Fragala`, I. J. Mater. Chem. 2003, 13, 861. (e) Barreca, D.; Massignan, C.; Daolio, S.; Fabrizio, M.; Piccirillo, C.; Armelao, L.; Tondello, E. Chem. Mater. 2001, 13, 588. (f) Shen, Z. X.; Allen, J. W.; Lindberg, P. A. P.; Dessau, D. S.; Wells, O. B.; Borg, A.; Ellis, W.; Kang, J. S.; Oh, S. J.; Lindau, I.; Spicer, W. E. Phys. ReV. B 1990, 42, 1817. (g) McIntyre, N. S.; Johnston, D. D.; Coatsworth, L. L.; Davidson, R. D.; Brown, J. R. Surf. Interface Anal. 1990, 15, 265. (22) (a) Briggs, D. In Practical Surfaces Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley-VCH: Weinheim, Germany, 1995; Vol. 1, p 244. (b) Repoux, M. Surf. Interface Anal. 1992, 18, 567. (23) Redlich, P.; Carroll, D. L.; Ajayan, P. M. Curr. Opin. Solid State Matter. Sci. 1999, 4, 325. (24) (a) Kothleitner, G.; Hofer, F. Ultramicroscopy 2003, 96, 491. (b) Mitterbauer, C.; Kothleitner, G.; Grogger, W.; Zandbergen, H.; Freitag, B.; Tiemeijer, P.; Hofer, F. Ultramicroscopy 2003, 96, 469.
JP902857G
