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Langmuir 1999, 15, 7092-7096
Reaction of Tetra(tert-Butoxy)Tin or -Zirconium with Hydroxylated Titanium in Ultrahigh Vacuum: Contrasting Reactivity with Hydroxylated Aluminum Substrate Kathleen L. Purvis, Gang Lu, Jeffrey Schwartz,* and Steven L. Bernasek* Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received April 13, 1999
Introduction Surface processing of “structural” metals with organometallics may provide a route for the preparation of new composites with desirable properties. Aluminum and titanium are two lightweight metals that form somewhat passivating oxide coatings; it was therefore of interest to compare organometallic chemical vapor deposition (OMCVD) reactions of two simple metal alkoxides with these species. We have previously reported1 that tetra(tertbutoxy)zirconium (1) deposits on the hydroxylated oxide layer of Al1-3 at 170 K, and on warming to 270 K, gives [Al]-[O]-Zr(OBut)3 (Al-2). If the Al surface was heavily hydroxylated, then [Al]-[O]2-Zr(OBut)2 (Al-3) formed on further heating to 350-400 K; however, if the surface was only lightly hydroxylated, then only total ligand decomposition occurred over 300 K. Analogous behavior was noted for the reaction of 1 with hydroxylated Al under “normal” laboratory conditions.4,5 In contrast, tetra(tertbutoxy)tin (4) deposits on and reacts with hydroxylated Al to give [Al]-[O]-Sn(OBut) (Al-5) as the only detectable tin organometallic, either in ultrahigh vacuum (UHV; 220-270 K), or at room temperature under “normal” conditions.6,7 No intermediate polyalkoxide complex precursors were found: because desorption of the multilayer of 4 occurs only at ca. 220 K in UHV, observation of any chemisorbed species of low thermal stability that might have been formed was precluded. We now report that, in contrast to observations on Al, reaction of 1 with the hydroxylated oxide on titanium metal gives only [Ti][O]-Zr(OBut)3 (Ti-2), whereas 4 gives rise to a complete set of thermally developed surface-bound tin alkoxide products, including [Ti]-[O]-Sn(OBut)3 (Ti-6) and [Ti][O]2-Sn(OBut)2 (Ti-7), as well as [Ti]-[O]3-Sn(OBut) (Ti5) (Scheme 1). Furthermore, whereas Al-5 is stable to only ca. 270 K, Ti-5 is only formed at ca. 400 K and is stable up to ca. 500 K. Experimental Section General. Tetra(tert-butoxy)zirconium (1) and tetra(tert-butoxy)tin (4) were distilled in vacuo and were stored under nitrogen (1) Lu, G.; Purvis, K. L.; Schwartz, J.; Bernasek, S. Langmuir 1997, 13, 5791-5793. (2) Miller, J. B.; Bernasek, S. L.; Schwartz, J. Langmuir 1994, 10, 2629-2635. (3) Miller, J. B.; Bernasek, S. L.; Schwartz, J. J. Am. Chem. Soc. 1995, 117, 4037-4041. (4) Aronoff, Y. G.; Chen, B. L., Lu, G.; Seto, C.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1997, 119, 259-262. (5) Miller, J. B.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1993, 115, 8239-8247. (6) Lu, G.; Schwartz, J.; Bernasek, S. L. Langmuir 1998, 14, 15321534. (7) Schwartz, J.; Bernasek, S. L.; Lu, G.; Keegan, J. P.; Purvis, K.; VanderKam, S. K. J. Mol. Catal. A: Chemical 1998, 133, 0000.
until transfered to the UHV chamber. The UHV chamber has been described in detail previously2,3 and contains a Mattson Research Series FT-IR spectrometer, a quadrupole mass spectrometer (QMS), and an X-ray photoelectron spectrometer (XPS). The polycrystalline Ti metal sample was cleaned using Ar+ ion sputtering and annealing cycles and was analyzed in situ by XPS. Preparation of Surface [Ti]-[O]-Zr(OBut)3 (Ti-2) in UHV. Surface oxidation of the cleaned Ti sample was accomplished through heating to 600-650 K followed by exposure to oxygen for 3 min. Multiple cycles of water-dosing with subsequent heating were performed to ensure maximum hydroxylation, as determined by XPS and FT-reflectance absorbance IR spectroscopy (FT-RAIRS). Vapor phase deposition of 1 onto the hydroxylated titanium surface occurred at 170 K. XPS analysis of the multilayer showed C/Zr ) 16 (for intact 1, expected C/Zr ) 16), and FT-RAIRS showed strong bands at 2978 and 1188 cm-1. A thermal desorption spectroscopic (TDS) profile of the mutilayer (monitoring m/z ) 15, 57, and 73) showed evidence for only one surface complex, (Ti-2) determined (at 308 K) by XPS (C/Zr ) 11.6; C[1s] binding energies for C-O ) 286.9 eV; for C-C ) 285.2 eV; ratio 1:3) and FT-RAIRS (2974 and 1200 cm-1). Preparation of Surface [Ti]-[O]-Sn(OBut)3 (Ti-6), [Ti][O]2-Sn(OBut)2 (Ti-7), and [Ti]-[O]3-Sn(OBut) (Ti-5) in UHV. Vapor phase deposition of 4 onto the hydroxylated titanium surface occurred at 170 K. XPS analysis of the multilayer showed C/Sn ) 16 (for intact 4, expected C/Sn ) 16), and FT-RAIRS showed strong bands at 2973 and 1197 cm-1. A thermal desorption spectroscopic (TDS) profile of the mutilayer (monitoring m/z ) 15 and 57) exhibited four different (though not completely resolved) desorption regions, corresponding to the title compounds, with stoichiometries of major species and ligand types determined by XPS and FT-RAIRS.
Results and Discussion A multilayer of 1 was adsorbed onto the hydroxylated titanium metal surface at 170 K. Desorption of intact 1 up to 270 K was monitored by TDS, XPS (Figure 1), and FT-RAIRS (2974, 1361, 1203 cm-1; Figure 2), which showed the first product of chemisorption to be [Ti][O]-Zr(OBut)3 (2). TDS-monitored careful heating of 2 above 280 K showed essentially complete decomposition of all alkoxide ligation, and evidence for only trace formation of other surface Zr alkoxides was recorded (Figure 3). In similar fashion, a multilayer of 4 was adsorbed onto the hydroxylated titanium metal surface at 170 K (FTRAIRS characteristic of the tert-butoxy ligand: 2978, 2870, 1242, 1180 cm-1). Desorption of intact 4 up to ca. 210 K was monitored by XPS, which showed that the first product of chemisorption was likely [Ti]-[O]-Sn(OBut)3 (Ti-6; XPS measured ratios: C/Sn ) 11.8 ( 1.1; expected, 12; some intact 4 is still present). TDS-monitored heating (Figure 4) of Ti-6 to 280 K showed loss of a second tertbutoxy ligand and formation of [Ti]-[O]2-Sn(OBut)2 (Ti7; XPS measured ratio: C/Sn ) 7.8 ( 0.5; expected, 9; Figure 5a and 5b). This stoichiometry differs from that obtained by UHV reaction of 4 with the hydroxylated Al(110) surface at 270 K (Al-5, [Al]-[O]3-Sn[OBut]).6 Only on further thermal evolution of Ti-7 up to 405 K was the Ti analogue of Al-5 produced (Ti-5, XPS measured ratio: C/Sn ) 3.8 ( 0.7; expected, 4; Figure 6a and 6b). In contrast to observations on Al, only tin alkoxide species were observed by C(1s) and Sn(3d5/2) XPS for Ti-5 and Ti-7;
10.1021/la990449+ CCC: $15.00 © 1999 American Chemical Society Published on Web 07/28/1999
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Figure 1. (a) Zr(3d) and (b) C(1s) XP spectra of [Ti]-[O]-Zr(OBut)3 (2). Scheme 1. Synthesis and Thermal Evolution Reactions of Ti-6
there was no evidence for concomitant formation of tin oxides, tin metal, or graphitic carbon. Indeed, only on heating at temperatures greater than 500 K did decomposition to SnO2 occur, and no graphitic carbon was produced (as monitored by XPS, Figure 7). It has been shown that the surface-OH group content of hydroxylated Al determines the thermal profile for the evolution of surface zirconium alkoxide complex stoichiometry vs decomposition.1 The Al surface reacts more
readily with water to give a hydroxylated surface1,2 than does Ti.8 Indeed, although titanium metal is easily oxidized, water can either adsorb molecularly or can dissociate on the surface, depending on the presence of Lewis acid surface sites.9-11 A mechanism for chemical (8) Lu, G.; Bernasek, S. L.; Schwartz, J., unpublished results. (9) Fahmi, A.; Minot, C. Surf. Sci. 1994, 304, 343. (10) Suda, Y.; Morimoto, T. Langmuir 1987, 3, 786. (11) Lo, W. J.; Chung, Y. W.; Somorjai, G. A. Surf. Sci. 1978, 71, 199.
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Table 1. Thermal Evolution of Surface Sn(OBut)n Species on Hydroxylated Ti XPS temp. range (K) 170-210 210-280 280-405 405-500 >500
predominant complex obsd
species
binding energy (eV)
multilayer Sn(OBut)4 (4)
Sn 3d5/2 C 1s
488.7 285.3, 286.9
C-O : C-C 1:3
[Ti]-[O]-Sn(OBut)3 (Ti-6)
Sn 3d5/2 C 1s
488.4 285.3, 287.0
1:2.9
[Ti]-[O]2-Sn(OBut)2 (Ti-7)
Sn 3d5/2 C 1s
487.4 284.6, 286.2
1:3
[Ti]-[O]3-Sn(OBut)1 (Ti-5)
Sn 3d5/2 C 1s
486.6 284.0, 285.4
1:3.1
SnO2
Sn 3d5/2
484.3
C : Sn 16 11.8 7.8 3.8 -
Figure 2. FT-RAIRS spectrum of [Ti]-[O]-Zr(OBut)3 (2).
Figure 3. TDS analysis of the reaction of 1 with hydroxylated Ti metal.
vapor deposition (CVD) of 4 onto hydroxylated Al has been proposed, based on analogy with 1, which involves
reversible coordination of the metal alkoxide to a surface oxygen, followed by rate-determining proton transfer from
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Langmuir, Vol. 15, No. 20, 1999 7095
Figure 4. TDS analysis of the reaction of 4 with hydroxylated Ti metal.
Figure 5. (a) C(1s) XPS analysis of [Ti]-[O]2-Sn(OBut)2 (Ti-7). (b) Sn(3d) XPS analysis of Ti-7.
an -OH group to the alkoxide ligand.5,12,13 Organotin compounds likely undergo chemisorption analogously. Differences in deposition stoichiometry of 1 versus 43,5,6 observed on Al at less than 200 K have been proposed to (12) Miller, J. B.; Schwartz, J. Inorg. Chem. 1990, 29, 4579-4581.
be derived from relative Lewis acidities of (less electronegative) Zr versus (more electronegative) Sn and their binding affinities with surface -OH groups. This proposal (13) Miller, J. B.; Schwartz, J. Acta Chem. Scand. 1993, 47, 292295.
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Figure 6. (a) C(1s) XPS analysis of [Ti]-[O]3-Sn(OBut)1 (Ti-5). (b) Sn(3d) XPS analysis of Ti-5.
surface alkoxide complex, a competition exists between a thermally driven chemisorption reaction with an additional surface-OH group and thermolytic decomposition of the ligand. For example, on Al, where the surface concentration of -OH may be relatively high,1 [Al]-[O]Zr(OBut)3 can give [Al]-[O]2-Zr(OBut)2 (Al-2) competitively with ligand decomposition to ZrO2. But on Ti, where the surface concentration of -OH is relatively low,8-11 Zr coordination of an -OH group may be less probable, and decomposition to ZrO2 kinetically overwhelms additional chemisorption steps. However, because of its greater affinity for -OH binding, 14,15 the putative, initial product of chemisorption on the more hydroxylated Al surface, [Al]-[O]-Sn(OBut)3 (Al-6), apparently can rapidly convert to Al-5; even on the less hydroxylated Ti surface, further protolytic reaction of Ti-6 to give Ti-7 and Ti-5 can compete kinetically with ligand decomposition, which gives the oxide. Acknowledgment. The authors thank the National Science Foundation for support of this research. Figure 7. Sn(3d) XPS analysis of the thermolysis product of Ti-5 at 600 K.
is consistent with crystallographic data14,15 for the metal alkoxide adducts with alcohols that show stronger alcohol -OH coordination for Sn versus Zr. In addition, for every (14) Reuter, H.; Kremser, M. Z. Anorg. Allg. Chem. 1991, 598/599, 259-268.
Supporting Information Available: FT-RAIRS of Ti-6 and C(1s) and Sn(3d5/2) XP spectra of Ti-6 and the multilayer of 4 on Ti. This material is available free of charge via the Internet at http://pubs.acs.org. LA990449+ (15) Vaartstra, B. A.; Huffman, J. C.; Gradeff, P. S.; Hubert-Pfalzgraf, L. G.; Daran, J.-C.; Parraud, S.; Yunlu, K.; Caulton, K. G. Inorg. Chem. 1990, 29, 3126-3131.
