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Notes Enhanced Bonding of Alkanephosphonic Acids to Oxidized Titanium Using Surface-Bound Alkoxyzirconium Complex Interfaces Ellen S. Gawalt, Gang Lu, Steven L. Bernasek,* and Jeffrey Schwartz* Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received July 12, 1999
Introduction We have reported the use of surface zirconium complexes, which are prepared by reaction of zirconium alkoxides with surface hydroxyl groups, to enhance bonding between carboxylic acids and metal oxides or oxidized metal surfaces.1-3 For example, we found that surface Zr η2-carboxylates1 are more stable with regard to thermal desorption1,2 or washing3 than are adducts formed spontaneously by simple reaction of the carboxylic acid and the oxide surface. Alkanephosphonic acids are another broad class of organic acids which have been used to protect, lubricate, and enhance adhesion properties of the surface of substrates as diverse as steel,4 aluminum,5 and paper.6 It may be that the phosphonic acids react with the metal surface native oxide layer7 by hydrogen bonding,8,9 or by (possibly reversible) proton transfer, as has been proposed for carboxylic acids.10-13 Enhancing the interaction between an alkanephosphonic acid and some metal oxide surfaces can be accomplished using linkers such as organosilanes14,15 or inorganic Zr species (after surface oxide phosphorylation16,17) or by using hightemperature processing conditions for the untreated (1) Aronoff, Y. G.; Chen, B.; Lu, G.; Seto, C.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1997, 119, 259. (2) Purvis, K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. Langmuir 1998, 14, 3720. (3) VanderKam, S. K.; Bocarsly, A. B.; Schwartz, J. Chem. Mater. 1998, 10, 685. (4) Hibi, T.; Morikawa, H.; Yamamoto, K.; Ikeda, J.; Tatsumi, K. Patent Jpn. Kokai Tokkyo Koho JP 03,258,897; Japan, 1992; Daido Chemical Industry Co. Ltd. Chem. Abstr. 116, 109915s. (5) Nitowski, G. A.; Wiserman, L. F.; Wefers, K. Patent US 5,277,788; 1994; Aluminum Co. of America. Chem. Abstr. 121, 20979w. (6) Tsutsui, K.; Yamaguchi, T.; Sato, K. Nippon Kagako Kaishi 1995, 68. (7) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (8) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (9) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (10) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (11) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (12) For example, see, Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (13) Sondag, A. H. M.; Raas, M. C. J. Chem. Phys. 1989, 91, 4926. (14) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (15) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (16) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, L.; Chidsey, C. E. D. Science 1991, 254, 1485. (17) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567.
oxide.18 We find that, consonant with their practical uses, alkanephosphonic acids react spontaneously with hydroxylated native oxide surfaces of Fe or Al to give adducts that are somewhat resistant to removal by either washing or tape-based peeling. Surprisingly, these acids do not react analogously with the hydroxylated native oxide surface of Ti, yet Zr alkoxides do react with it rapidly and under mild conditions to give Ti-surface Zr alkoxide complexes.19 We now report that this simple zirconium alkoxide-derived complex interface enables strong, covalent interaction between an alkanephosphonic acid and the native oxide coating of Ti and that such surface Zrphosphonate complexation provides stability against removal of the organic moiety from the Ti surface by washing or tape-based peeling. Experimental Section General. Titanium (0.25 mm, 99.6%), aluminum (0.25 mm, 99.0%), and iron (0.125 mm, 99.5%) foils (all obtained from Goodfellow, Inc.) were sanded and cleaned with methanol, cut into ca. 1 cm × 1 cm samples, and stored in an oven at 200 °C. The Ti foil was also used for ultrahigh vacuum (UHV) studies, in which background data were collected using a sample that had been rinsed with THF and evacuated at room temperature. The UHV chamber has been described in detail previously,20,21 and contains a Mattson Research Series FT-IR spectrometer, a quadrupole mass spectrometer (QMS), and an X-ray photoelectron spectrometer (XPS). Tetra(tert-butoxy)zirconium (Aldrich, 1) was distilled in vacuo and was stored under nitrogen. Aerosol spraying of solutions of phosphonic acids was performed in a simple glovebox under a N2 flow. “Peel” tests were performed by pressing a strip of red Scotch “471” tape onto a substrate and then rapidly peeling the tape off. Preparation of Surface [Ti]-[O]-Zr(OBut)3 (Ti-2). A Ti foil sample was exposed to vapor of 1 at 10-2 Torr in five “cycles” of alternating exposure for 15 min with external evacuation applied, followed by 30 min without such evacuation. The sample was then evacuated at 10-2 Torr for 1 h to remove any surface physisorbed 1. Diffuse reflectance Fourier transform (DRIFT) infrared analysis confirmed the formation of Ti-2 (νCH3(asym) ) 2977 cm-1). Surface [Al]-[O]-Zr(OBut)3 (Al-2). Aluminum foil samples were exposed to vapor of 1 for three “cycles” and analyzed by DRIFT, as described for Ti-2. Surface [Fe]-[O]-Zr(OBut)3 (Fe-2). Iron foil samples were exposed to vapor of 1 for five “cycles” and analyzed by DRIFT, as described for Ti-2. Surface [Ti]-[O]-Zr(OBut)2([HO]PO2C18H37) (Ti-4a). A solution of 0.8 mM octadecanephosphonic acid was prepared in dry tetrahydrofuran and was sprayed in a fine aerosol mist using N2 and a TLC sprayer (Supelco) onto both Ti-2 and a control sample foil. Samples were then evacuated at 0.1 Torr overnight to remove solvent and were then analyzed by DRIFT. Similar procedures were used for spraying methanephosphonic acids (0.5 mM) onto Ti-2 to give Ti-4b. (18) Hong, H.-G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, 3, 521. (19) Purvis, K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. Langmuir, in press. (20) Miller, J. B.; Bernasek, S. L.; Schwartz, J. Langmuir 1994, 10, 2629. (21) Miller, J. B.; Bernasek, S. L.; Schwartz, J. J. Am. Chem. Soc. 1995, 117, 4037.
10.1021/la990906m CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999
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Langmuir, Vol. 15, No. 26, 1999
Notes
Scheme 1. Preparation of Zirconium Alkanephosphonate (Aluminum) Surface Species
Surface [Al]-[O]-Zr(OBut)x([HO]PO2C18H37)2-x (Al-4a). This material was prepared as described for Ti-4a using Al-2 and was then analyzed by DRIFT. Surface [Fe]-[O]-Zr(OBut)x([HO]PO2C18H37)2-x (Fe-4). This material was prepared as described for Ti-4a using Fe-2 and was then analyzed by DRIFT. Analysis of ex Situ Prepared [Ti]-[O]-Zr(OBut)1([HO]PO2CH3)2 (Ti-4b) in UHV. A sample of Ti-4b was prepared and washed three times with THF in the glovebox as described above for Ti-4a. The sample was then mounted into the manipulator of the UHV apparatus under ambient conditions. No attempt was made to blanket the apparatus under N2 during this transfer, and the sample was exposed to the atmosphere for ≈5 min. TheUHV apparatus was then sealed and evacuated, without baking, for 4 days, until a background pressure of 1% and
