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Probing Interfaces and Surface Reactions of Zirconium Phosphate/Phosphonate Multilayers Using 31P NMR Spectrometry P. Kohli and G. J. Blanchard* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 Received May 28, 1999. In Final Form: September 16, 1999 We report our 31P magic angle spinning (MAS) NMR characterization of zirconium phosphate/phosphonate (ZP) multilayer assemblies grown on SiOx. The reaction of silica with excess POCl3 and treatment with collidine results in both physisorbed and chemisorbed HxPO4(x-3) being present at the SiOx surface. The 31P NMR spectrum of zirconium phosphate grown from silica shows no residual Cl-containing species, indicating essentially complete hydrolysis. The phosphate 31P resonances are broadened substantially upon complexation with Zr4+, and T1 data demonstrate the broadening to be inhomogeneous in nature. Data on maleimide-vinyl ether copolymer layers we reported recently [Langmuir 1999, 15, 1418] confirm that hydrolysis/deprotection by bromotrimethylsilane is efficient in converting phosphoesters to the corresponding phosphorus oxyacid in the synthesis of multilayers. Comparison of polymer multilayers with the relatively more ordered bisphosphonic acid-based multilayers indicates that essentially the same interlayer bonding chemistry is seen for both systems, with subtle differences arising as a consequence of the greater available free volume and more extensive disorder within the polymer.
Introduction The organization of mono- and multilayer molecular assemblies at solid surfaces provides a rational approach for fabricating interfaces with a well defined structure, composition, and thickness. These assemblies may ultimately find application in nonlinear optical devices,1,2 chemical sensing,3 surface passivation,4 photoreactivity,5-7 and separations.8,9 The ability to control interfacial processes has important implications for both fundamental and technological advances. Of particular significance is the ability to grow layered materials where there is substantial control over the layer thickness and uniformity. Layer-by-layer deposition of films provides spatial resolution over composition and molecular orientation relative to the substrate. Both of these properties can be critical to the macroscopic properties of the resulting system. We use metal bisphosphonate chemistry to produce multilayer interfacial materials because of its combined simplicity, versatility, and robustness.1,10-27 To grow welldefined layers having low defect densities, it is important * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 517-355-9715 x224. Fax: 517-353-1793. (1) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (2) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (3) Kepley, L. J.; Crooks, R. M.; Ricco, A. Anal. Chem. 1992, 64, 3191. (4) Swalen, J. P.; Allara, D. L.; Andrade, J. P.; Chandross, E. A.; Garoff, S.; Israelachvilli, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Tu, H. Langmuir 1987, 3, 932. (5) Calvert, J. M.; Georger, J. H., Jr.; Peckerer, M. C.; Perhsson, P. E.; Schnur, J. M.; Scheon, P.; E. Thin Films 1992, 114, 9188. (6) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Stenger, D. A.; Fare, T. L; Calvert, J. M. Science 1991, 252, 551. (7) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188. (8) Vrancken, K. C.; Van Der Voort, P.; Gillis-D’Hammers, I.; Vansant, E. F.; Grobet, P. J. Chem. Soc., Faraday Trans. 1992, 88, 3197. (9) Pfleiderer, B.; Albert, K.; Bayer, E. J. Chromatogr. 1990, 506, 343. (10) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618.
to understand surface reactions at various stages during the formation of multilayers. We report here on the characterization of layered interfaces formed by adsorption of selected compounds using zirconium phosphate/phosphonate chemistry. The substrate upon which the layers are grown is SiOx in the form of silica gel. We use 31P solid-state magic angle spinning (MAS) NMR spectrometry to characterize the surfaces after each reaction step. We chose 31P MAS NMR spectrometry because the chemical shift of 31P is very sensitive to its surrounding environment and is thus a good indicator of the species present during selected stages in layer-by-layer deposition reactions. The high sensitivity of the 31P NMR measurement is the result of the 100% natural abundance of 31P. The data we (11) Lee, H.; Kelley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1998, 92, 2597. (12) Yonemoto, E. H.; Saupe, G. B.; Schmehl, R. H.; Hubig, S. M.; Riley, R. L.; Iverson, B. L.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 4786. (13) Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855. (14) Cao, G.; Rabenberg, L. K.; Nunn, C. M.; Mallouk, T. E. Chem. Mater. 1991, 3, 149. (15) Rong, D.; Hong, H.-G.; Kim, Y.-I.; Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. Coord. Chem. Rev. 1990, 97, 237. (16) Thompson, M. E. Chem. Mater. 1994, 6, 1168. (17) Vermeulen, L.; Thompson, M. E. Nature 1992, 358, 656. (18) Vermeulen, L. A.; Snover, J. L.; Sapochak, L. S.; Thompson, M. E. J. Am. Chem. Soc. 1993, 115, 11767. (19) Snover, J. L.; Byrd, H.; Suponeva, E. P.; Vicenzi, E.; Thompson, M. E. Chem. Mater. 1996, 8, 1490. (20) Katz, H. E.; Scheller, G. J.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (21) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (22) Katz, H. E.; Bent, S. F.; Wilson, W. L.; Schilling, M. L.; Ungashe, S. B. J. Am. Chem. Soc. 1994, 116, 6631. (23) Ungashe, S. B.; Wilson, W. L.; Katz, H. E.; Scheller, G. R.; Putvinski, T. M. J. Am. Chem. Soc. 1992, 114, 8717. (24) Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Mater. 1991, 3, 699. (25) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (26) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567. (27) Katz, H. E. Chem. Mater. 1994, 6, 2227.
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report here for both polymeric and alkanebisphosphonate multilayers indicate that essentially the same interlayer linking chemistry is operative for both types of system, but subtle differences in the efficiency of the layer formation arise from the disorder and permeability that is characteristic of the polymer multilayers. Experimental Section The alternating copolymer of N-phenylmaleimide (NPM) and (2-vinyloxy)ethylphosphonate (VEP), poly(NPM-VEP), was prepared by radical polymerization using azobisisobutyronitrile (AIBN) as an initiator.28,29,32 VEP was synthesized by the reaction of tri(isopropyl)phosphite and 2-chloroethylvinyl ether as reported previously.30,32 NPM was purchased from Aldrich Chemical Co. and used after recrystallization from hexanes. POCl3, 2,4,6trimethylpyridine (collidine), anhydrous acetonitrile, bromotrimethylsilane (BTMS), and AIBN were also purchased from Aldrich Chemical Co. and all were used as received. Silica gel (230-400 mesh) having a surface area of 450-550 m2/g was purchased from Spectrum Quality Products, Inc. We synthesized multilayers using the metal phosphonate (MP) chemistry pioneered by the Mallouk,10-15 Thompson,16-19 and Katz groups.20-27 Multilayers grown using MP chemistry are, in many cases, deposited on substrates having surface hydroxyl groups. The first step in layer formation on such surfaces is typically treatment of the substrate with ethoxydimethylaminopropylsilane to provide an amine-terminated surface, followed by reaction with POCl3 and a Lewis base such as collidine.24,31 Hydrolysis yields a phosphonate-terminated surface. We have reported the formation of robust multilayer assemblies containing polymers,32 and small bifunctional molecules33,34 using direct reaction of surface silanols with POCl3 as the substrate-priming step. Both ellipsometry and FTIR data reveal no significant differences between the layers grown with and without the silane priming chemistry. In both the cases the resulting layers were not susceptible to removal by polar solvents, nonpolar solvents, or aqueous electrolyte solutions.32,33 The direct chemisorption of phosphate on the substrate silanol groups was carried out by adding 0.4 M POCl3 in dry acetonitrile dropwise to a stirred suspension of silica gel in 0.4 M collidine in anhydrous acetonitrile under argon. The reacted silica gel was washed thoroughly with reagent grade acetonitrile and water. The phosphated surface was zirconated using 5 mM ZrOCl2 in 60% ethanol(aq) for 1-2 h at room temperature. The polymer layers were deposited on the zirconated substrate from a 10 mM solution of partially hydrolyzed poly(NPM-VEP) in acetonitrile at 45 °C for 12 h. Partial hydrolysis of the polymer prior to deposition was accomplished by deprotection of the phosphonate ester groups with 0.5 equiv of BTMS.32 Following initial layer formation, exhaustive phosphoester hydrolysis to activate the surface-bound polymer was accomplished using 4 equiv of BTMS in anhydrous CH2Cl2/CH3CN followed by washing several times with reagent grade CH2Cl2/CH3CN and water. Adsorption of the second polymer layer is accomplished with the same partial deprotection procedure described above.32 The 31P MAS NMR measurements were conducted on a Varian VXR 400 NMR spectrometer with 31P nuclei resonating at 161.9 MHz. All spectra are proton decoupled and nuclei are excited using a 90° pulse between 5 and 7.7 µs in duration. Samples were spun at speeds between 4 and 5.5 kHz. Spinning speeds for individual spectra are given in the figure captions. The relaxation delays for all spectra were g4T1. T1 values for phosphorylated and phosphorylated-zirconated silica were 0.21 and 0.77 s, respectively. The spinning sidebands were identified by acquisition of the spectra at several spinning speeds with molecular resonances exhibited a constant chemical shift. Comparisons (28) Olson, K. G.; Butler, G. B. Macromolecules 1984, 17, 2480. (29) Olson, K. G.; Butler, G. B. Macromolecules 1984, 17, 2486. (30) Rabinowitz, R. J. J. Org. Chem. 1961, 26, 5152. (31) Horne, J. C.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 12788. (32) Kohli, P.; Blanchard, G. J. Langmuir 1999, 15, 1418. (33) Bakiamoh, S. B.; Blanchard, G. J. Langmuir 1999, 15, 6379. (34) Flory, W. C.; Mehrens, S. M.; Blanchard, G. J. J. Am. Chem. Soc., in preparation.
Figure 1. (a) 31P MAS NMR spectrum of silica reacted with POCl3. Bands marked with asterisks are spinning sidebands (spinning speed 4.0 kHz). The bands at δ ) 0.6 and -11.8 ppm were fit using Gaussians and the area ratio is 1.5. (b) 31P MAS NMR spectrum of the same sample after washing with polar solvents. The integrated band area ratio has decreased to 0.91 (spinning speed 5.5 kHz). were made on samples prepared under the same conditions and data were acquired using the same instrumental parameters. All the 31P chemical shifts reported here are relative to 85% phosphoric acid (δ ) 0 ppm).
Results and Discussion The focus of this paper is on understanding the chemical speciation of phosphorus and its environment at selected points during the synthesis of zirconium phosphate/ phosphonate layered interfaces on silica gel. We are interested in establishing the details of multilayer growth from the perspective of the interlayer linking functionalities. We consider the surface reactions shown in Scheme 1 sequentially. The first reaction step, to produce the phosphatemodified surface and shown in Scheme 1 A, is the reaction of a silica surface with POCl3 followed by hydrolysis. We show the 31P MAS NMR spectrum of the reacted silica surface in Figure 1. The two sharp resonances at δ ) 0.60 and -11.8 ppm are associated with two distinct types of phosphorus (Table 1). Morrow et al.35 have studied the adsorption of PCl3 and POCl3 on silica and reported that the reaction of PCl3 with silica produces Si-O-P containing species such as SiOPCl2, SiOPdO(H)(OH), and (SiO)2PdO(H) (Table 1). Because we use an excess of POCl3 in the surface reaction, some of POCl3 can react with any water present either in solution or on the SiOx substrate particles to form phosphorus oxyacids. An unresolved peak at ∼-5 ppm could, in principle, be due to phosphite P-H species (Figure 1b). There is, however, no confirming IR evidence of P-H vibrational modes in our POCl3/collidine/ water treated sample.32 Phosphoric acid, the most likely hydrolysis product, is capable of hydrogen bonding with surface silanol groups36 and chemisorbed SiOPO3H2. The more intense peak at δ ) 0.60 ppm is indicative of a physisorbed phosphorus oxide moiety as we discuss below. Keeping all the parameters and conditions uniform, the ratio of integrated area of peak at δ ) 0.60 ppm to the (35) Morrow, B. A.; Lang, S. J.; Gay, I. D. Langmuir 1994, 10, 756. (36) Murashov, V. V.; Leszczynski, J. J. Phys. Chem. A 1999, 103, 1228.
31P
NMR Probe of Zr Phosphate/Phosphonate Multilayers
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Scheme 1. Surface Reaction Sequence Studied in This Papera
a A: Reaction of a silica surface with POCl . B: Zirconation of A. C: Deposition of a partially hydrolyzed layer of poly(NPM-VEP). 3 D: Hydrolysis and zirconation of the poly(NPM-VEP) layer. E: Deposition of a second polymer layer.
integrated area of peak at δ ) -11.8 ppm is reduced from a value of 1.5 (Figure 1a) to 0.9 (Figure 1b) after successive washings with ethanol, acetone, and water. The interaction of H3PO4 and silica is strong enough to withstand this washing procedure to a significant extent. Recently Murashov et al.36 used ab initio calculations to determine that silanols can, in fact, hydrogen-bond strongly to phosphate functionalities. Their calculations suggested a modulation in electron density at the phosphate and silanol groups in such a way as to strengthen the P-O bond and weaken the PdO and C-O bonding in adsorbed organophosphate moieties. They calculated the hydrogen bonded silanol-phosphate complexes to be stabilized by ∼14 kcal/ mol per hydrogen bond.36 In our case, although the δ ) 0.6 ppm peak does not decrease greatly after washing with ethanol, acetone, and water, it is diminished substantially upon reaction with Zr4+. This finding is a clear indication that the strength of the Zr phosphate interaction
exceeds the ∼14 kcal/mol H-bonding energy, which is a fully expected result. Physisorbed H3PO4 can decrease the extent of chemisorption on silica because the physisorbed phosphoric acid can block surface silanol groups and preclude covalent bonding between surface hydroxyl groups and POCl3. We now focus on the species responsible for the resonance at δ ) -11.8 ppm. Morrow et al.35 and Mudrakovskii et al.37 obtained the 31P MAS NMR spectra of SiOx exposed to H3PO4 at various temperatures and attributed bands in the -8 to -12 ppm region to chemically bound SiOPO3H2 (Table 1). Morrow et al.35 also reported phosphorus oxychloride species bound to silica. Their experiments were conducted under vacuum so there was little or no water present to hydrolyze the P-Cl bonds. (37) Mudrakovskii, I. L.; Mastikhin, V. M.; Kotsarenko, N. S.; Shmachkova, V. P. Kinet. Catal. (Engl. Transl.) 1988, 29, 165.
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Table 1. Summary of 31P NMR Resonance Assignments Reported in the Literaturea chemical structure C18H37PO(OH)2 weak interactions with Cd2+ C18H37PO(OH)2 free acid C18H37PO(OH)2 (s) C18H37POO-(OH)Na+ C18H37PO32-(Na+)2 C18H37PO32-(Na+)2 weak interactions with Na+ C8H17PO32-Zn2+ C14H29PO32-Zn2+ C18H37PO32-Zn2+ R-Zr(HPO4)2‚H2O, HPO42γ-Zr(HPO4)2‚2H2O, H2PO4γ-Zr(HPO4)2‚2H2O, PO43P(OCH3)3 OPH(OCH3)2 OP(OCH3)2CH3 (SiO)2P(H)O (SiO)2P(CH3)O (CH3O)PCl2 (CH3O)2PCl (CH3)3SiOPO(H)(OH) ((CH3)3SiO)2PO(H) (CH3SiO)3PO SiOPO3H2 SiOPO(H)(OH) (SiO)2P(H)O a
chemical shift δ (ppm)
ref
22.5
44
28.5 31.5 26.5 23.8 28.1
44 50 48 48 48
33.6 and 32.6 33.8 and 32.6 33.8 and 32.7 -18.7 -9.4 -27.4 140 11 32 -5 9 with a shoulder at 20 181 169 -1 -13.6,35 -14.9,54 -14.355 -28.3 -8 to -12 -5 -16
48 48 48 41,42 41,42 41,42 51 51 51 52 52 53 53 35 35,54,55 53 37 35 35
Spectral shifts are measured relative to 85% H3PO4.
They observed an interaction between POCl3 and silica, and concluded that hydrogen bonding between surface silanol groups and POCl3 accounted for their data. In the work we report here, where we used excess POCl3 in the presence of a Lewis base, it is more likely that the silanol groups will react with POCl3 to form covalent SiOPOCl2 which can be hydrolyzed efficiently to yield SiOPO3H2. It is the phosphate chemically bonded to the silica substrate that is responsible for the sharp resonance at δ ) -11.8 ppm, in agreement with the literature.37 This resonance does not decrease in intensity after repeated washing in polar solvents followed by drying under vacuum, consistent with the δ ) -11.8 ppm band being associated with chemisorbed phosphate. We assume complete hydrolysis of the phosphorus oxychlorides either upon reaction with the SiOx substrate or immediately after the formation of the Si-O-P bond. After the initial reaction with POCl3, we do not attempt to maintain anhydrous conditions. We can test the assumption of essentially complete hydrolysis experimentally. The 31P chemical shifts characteristic of P-Cl bonds are in the δ ) 150-180 ppm region,35,38 and we find no resonances in this spectral window. After covalent bonding to the SiOx substrate, the remaining P-Cl functionalities are converted to P-OH as a result of their reaction with water. Another important issue relating to the surface bonding of the phosphorus oxides to SiOx surfaces is the number of bonding sites per phosphate. Multiply bound species such as (SiO)2PO2H and (SiO)3PO are characterized by 31P NMR resonances at δ ≈ -20 and -30 ppm, respectively, and we do not observe these bands in our data. We account for this finding on two grounds. The first is that our reaction conditions provide a stoichiometric excess of POCl3 relative to surface silanol groups, a condition that favors the formation of single attachment of the phosphorus oxychlorides to surface silanols. The second reason (38) Bogatyrev, V. M.; Brei, V. V.; Chuiko, A. A. Theor. Exp. Chem. (Engl. Transl.) 1988, 24, 603.
Figure 2. (a) 29Si MAS NMR spectrum of the silica gel prior to reaction with POCl3 (spinning speed 3.5 kHz). (b) 29Si MAS NMR spectrum of the same sample following reaction with POCl3 (spinning speed 3.5 kHz). The absence of any features near δ ) -200 ppm indicates single point binding of the phosphorus oxychloride.
that we see predominantly single Si-O-P bonding is the unfavorable steric constraints associated with multiple bonding sites. We note that the 29Si NMR spectrum of these same samples (Figure 2) does not reveal any resonances in the δ ≈ -200 ppm region that would be consistent with the existence of 5- or 6-coordinate silicon.39 We next consider the phosphated and zirconated surface, as shown in Scheme 1 B. After reaction of the phosphated surface with ZrOCl2(aq), the silica-bound phosphates formed a complex with Zr4+ of the form of [SiOPO3ZrX2], where X can, in principle, be OH-, Cl-, or C2H5O- based on the species present in the reaction vessel. The counteranions X are necessary to maintain charge neutrality in zirconium phosphate/phosphonate complexes. Although the concentration of Cl- is 10 mM in the zirconation solution, much higher than either [OH-] or [C2H5O-], previous XPS studies of multilayer assemblies33 have not revealed detectable Cl in zirconium phosphate/carboxylate or zirconium phosphate/sulfonate multilayers, suggesting that OH- and/or C2H5O- are the dominant counterions in layered assembly growth. The pKa values for water and ethanol are 14.0 and 15.9, respectively,40 and assuming the pH of ZrOCl2 in a 3:2 ethanol:water solution is ∼4, the concentration of OH- is approximately twice that of C2H5O-. The probability of having OH- as a counterion for the [SiO(PO3)2-Zr4+] complex is higher than that of having C2H5O-, assuming the association constants for these anions with Zr4+ are similar. The zirconated surface exhibits several resonances distinct from those for the nonzirconated surface (Figure 3). The narrow resonance associated with chemisorbed SiOPO3H2 at δ ) -11.8 ppm broadens on complexation with Zr4+ and two new resonances are seen at δ ) -14.8 ppm and δ ) -19.1 ppm. The appearance of these new resonances is coincident with a substantial loss of intensity in the δ ) 0.6 ppm resonance, consistent with (39) Bernstein, T.; Fink, P.; Mastikhin, V. M.; Shubin, A. A. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1879. (40) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry, 2nd ed.; W. H. Freeman and Co.: New York, 1994; p 244.
31P
NMR Probe of Zr Phosphate/Phosphonate Multilayers
Figure 3. 31P MAS NMR spectrum of the phosphated and zirconated surface. There are distinct but poorly resolved resonances at δ ) 0.6, -11.8, -14.8, and -19.1 ppm indicating partial complexation of the surface as well as several different complexation conditions. The bands marked with asterisks are spinning sidebands (spinning speed 4.0 kHz).
either the removal or in situ complexation of physisorbed phosphorus oxyacids. Clayden,41 Nakayama,42 and Morgan and co-workers43 studied 31P isotropic chemical shift values of phosphate groups in R-Zr(HPO4)2 as a function of phosphate group deprotonation. They found that as the phosphate group is successively deprotonated, the 31P resonance shifts to lower frequencies. For example, the (H2PO4)-, (HPO4)2-, and (PO4)3- groups in R-Zr(HPO4)2 appeared around δ ) -10, -20, and -30 ppm, respectively (Table 1). Another factor that can also affect resonance position is the extent of hydrogen bonding between phosphate groups and other adsorbed molecules. MacLachlan et al.43 observed that the difference between anhydrous and hydrated R-Zr(HPO4)2 was ∼2-4 ppm, with the hydrated and presumably hydrogen-bonded forms being shifted downfield of the anhydrous form. Comparing the data in Figure 3 to data in the literature, we assign the resonance at δ ) -19.1 ppm to (SiOPO3)2- complexed with Zr4+,41-43 and the δ ) -14.8 ppm resonance to hydrated (SiOPO3‚ xH2O)2-Zr.43 Due to complex surface topology and roughness of silica after reaction with POCl3, there may be some residual, uncomplexed phosphate (SiOPO3H2) that exists following zirconation. The presence of residual SiOPO3H2 would account for the resonance at δ ) -11.8 ppm. It is important to note that there also remains a small amount of physisorbed H3PO4 (δ ) 0.6 ppm) following zirconation, and its existence is likely due to surface morphology effects as well. With the identities of the individual 31P resonances determined for phosphated and zirconated SiOx, we are in a position to consider the factors that contribute to the spectral width of the resonances in the -5 to -35 ppm spectral region. The width of NMR resonances following the formation of the zirconium phosphate complex provides insight into the structural freedom of the phosphate functionality in the ZP complex. Comparing the 31P NMR spectra of R-Zr(HPO4)2 obtained by Nakayama42 and MacLachalan43 with the spectrum of our ZP complex (Figure 3) shows the full width at half-maximum (fwhm) of the dominant resonance to be ∼25 times narrower than our spectrum. The fwhm of the 31P spectrum of R-Zr(HPO4)2 is ∼0.7-0.8 ppm42,43 while the fwhm of our spectrum is broadened from ∼3.0 (Figure 1b) to ∼18 ppm after (41) Clayden, N. J. J. Chem. Soc., Dalton Trans. 1987, 8, 1877. (42) Nakayama, H.; Eguchi, T.; Nakamura, N.; Yamaguchi, S.; Danjyo, M.; Tsuhako, M. J. Mater. Chem. 1997, 7, 1063. (43) MacLachlan, D. J.; Morgan, K. R. J. Phys. Chem. 1990, 94, 7656.
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complexation with Zr4+ (Figure 3). This result is not without precedent. The spectra of solid octadecylphosphonic acid (ODPA) compared to that of a 125-bilayer ODPA Langmuir-Blodgett (LB) film containing Cd2+ ions also shows the broadening of phosphonic acid 31P resonances in the LB films as a result of complexation with Cd2+.44 The breadth of the resonances following complexation with zirconium could be due to any of several factors. One likely contribution is the presence of overlapping resonances characterized by a distribution of chemical shifts, the result of different types of surface bonds and/or binding sites of phosphates and zirconium on the surface. Hydrogen bonding can alter the isotropic chemical shift of phosphate by 3-5 ppm.35,45 Rothwell et al.45 reported the 31 P spectrum of monocalcium phosphate, Ca(H2PO4)2‚H2O in which they found two 31P resonances separated by ∼4.5 ppm. They attributed these resonances to the hydrogen bonding between two inequivalent H2PO4- groups. In their model, one H2PO4- accepts two hydrogen-bonding protons, one from a water molecule and the other from the second H2PO4-, which can be considered a hydrogen-bond donor.45 If our data are understood in the context of inequivalent H-bonds, the exchange time for the different types of environment formed by the H-bonding phosphate groups must be fast at room temperature relative to the speed of the measurement. Our results are not inconsistent with H-bonding playing a role in the line broadening we observe upon complexation. In our case, there also exists the possibility of hydrogen bonding between the terminal hydroxide/ethoxide of the ZP complex with other neighboring hydroxide/ethoxide groups and/or water/ethanol molecules. The evidence of hydrogen bonding comes from FTIR data on a bilayer of poly(NPM-VEP) on silicon showing a broad resonance in the 3200-3700 cm-1 region.32 To help resolve the issue of line broadening upon zirconation, we have measured the 31P spin-lattice relaxation times (T1) for both phosphated (Figure 1b) and phosphated/zirconated silica (Figure 3). We found T1 ) 0.21 s for the phosphated substrate and T1 ) 0.77 s for the phosphated/zirconated surface. This finding suggests that the additional line width seen for the complexed phosphate must be due to inhomogeneous broadening, presumably dominated by an unresolved and rapidly exchanging distribution of environments. If the complexation step simply slowed the T1 relaxation time and did not add to the distribution of sites, we would expect a significantly narrower resonance for the complexed system. The change in T1 as a function of complexation suggests that the motional freedom of the surface-bound OPO3 groups is restricted upon zirconation. It is not immediately apparent that the restriction of motion is limited to a specific rotation, for example, and other effects could contribute to the change in T1 we observe. Restrictions on proton exchange within or between adjacent phosphate groups could give rise to the change in T1.45,46 Such motions have been observed in KD2PO4 in the paraelectric phase.46 Unfortunately, given the signal-to-noise ratio of our data, it is not possible to separate the various contributions to the additional 31P line width in the complexed system. (44) Fanucci, G, E.; Bowers, C. R.; Talham, D. R. J. Am. Chem. Soc. 1999, 121, 1088. (45) Rothwell, W. P.; Waugh, J. S.; Yesinowski, J. P. J. Am. Chem. Soc. 1980, 102, 2637. (46) Blinc, R.; Pirsˇ, J. J. Chem. Phys. 1971, 54, 1535.
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Langmuir, Vol. 16, No. 2, 2000 Table 2. Summary of
a
Kohli and Blanchard 31P
NMR Resonance Assignments Reported in This Worka
chemical structure
chemical shift δ (ppm)
(1) treatment of silica with POCl3/collidine/water (2) (1) + ZrOCl2‚8H2O (3) (2) + 1 layer of poly(NPM-VEP) (4) (3) + BTMS hydrolysis + ZrOCl2‚8H2O (5) (4) + 2nd layer of poly(NPM-VEP) CH2dCHOCH2CH2P(O)(OCH(CH3)2) poly(NPM-VEP), solution phase H2O3P(CH2)11PO3H2 (UBPA)
0.60, -11.8 0.60, -11.8, -14.5, -19.1 28 (broad), 0.60, -11.8, -14.8, -19.1 0.60, -11.8, -14.8, -19.1, ∼-23.8 21.2, -11.8, -14.8, -18.0, -25.4 ∼27-28 (singlet, sharp) ∼24-28 (broad) ∼28.7 (sharp)
Spectral shifts are measured relative to 85% H3PO4.
Figure 4. 31P MAS NMR spectrum of the surface with a single layer of partially hydrolyzed poly(NPM-VEP). The band marked with an asterisk is a spinning sideband (spinning speed 4.0 kHz).
The next step in layer growth is the addition of the first layer of poly(NPM-VEP), as shown in Scheme 1 C. Figure 4 shows the 31P MAS NMR spectrum after adsorption of the first layer of partially hydrolyzed poly(NPM-VEP). The dominant feature associated with the addition of the polymer is the appearance of a new, broad resonance at δ ≈ 28 ppm. This feature is attributed to a phosphonate ester and/or uncomplexed phosphonic acid pendant groups on the poly(NPM-VEP) polymer. The solution phase 31P NMR spectrum of poly(NPM-VEP) contains only one broad band in the δ ) 24-28 ppm region. The width of this peak in the solution phase spectrum suggests that, in addition to slow motion of the polymeric chains, there are many conformations of the phosphonate ester and/or phosphonic acid and that inter- and intramolecular hydrogen bonding plays a significant role in the dynamics of the polymer. With the addition of the polymer layer, the relative intensity of the 31P resonance at δ ) 0.6 ppm diminishes further, consistent with its assignment as physisorbed phosphoric acid on silica. The bands in the δ ) -10 to -20 ppm region also become less well-resolved, suggesting an increase in the distribution of available environments upon complexation. At this point it is useful to compare the data for the polymer layers with the corresponding information on ZP layers formed using alkanebisphosphonates. We compare the 31P NMR spectrum of a relatively well-ordered monolayer of 1,11-undecylbisphosphonic acid (UBPA) (Figures 5) with a poly(NPM-VEP) monolayer. The upfield region of monolayer UBPA spectrum shows features similar to those of poly(NPM-VEP) monolayer (Figure 6a). For the UBPA layer, the band at δ ) -11.8 ppm is due to SiOPO3H2, and/or mono deprotonated ZP complex, i.e., Si(OPO3H-)Zr(OH-)3. The δ ) -18.4 ppm and δ ) -14.5 ppm bands arise from SiOPO32- and hydrogen-bonded (SiOPO3)2- in the ZP complex respectively (Figure 6b). A broad, relatively weak band in the δ ) 10-30 ppm region
Figure 5. (a) 31P MAS NMR spectrum of a monolayer of UBPA on primed SiOx (spinning speed 4.0 kHz). (b) The same sample that has been zirconated (spinning speed 4.0 kHz).
Figure 6. (a) 31P NMR MAS spectrum of a single layer of poly(NPM-VEP) after hydrolysis and zirconation (spinning speed 4.0 kHz). (b) Spectrum of the same sample with a second layer of poly(NPM-VEP) added (spinning speed 4.0 kHz).
is due to uncomplexed or weakly complexed phosphonic acids (Table 1). We also find a band at δ ) 22.9 ppm in the UBPA monolayer (Figure 5b). The UBPA mono- and multilayers are relatively ordered compared to the polymer layers, but IR data on the aliphatic portions of the layers indicate the extent of organization is less than that of alkanethiol/ gold monolayers.47 The presence of free phosphonic acids is most likely due to the terminal groups at the top of the
31P
NMR Probe of Zr Phosphate/Phosphonate Multilayers
interface that remain to be zirconated. Recently, Fanucci et al.44 reported a 31P NMR spectrum of LangmuirBlodgett films of bisphosphonic acid acids and they found that the weak interactions between free phosphonic acids and nearby Cd2+ ions shifts the isotropic 31P resonances slightly upfield. Similarly, mono- and disodium salts of octadecylphosphonates are shifted upfield from octadecylphosphonic acid.48 We consider that from the point of view of the phosphonate groups, essentially the same chemistry is operative, save for the protection/deprotection steps used in the polymer (vide infra), for both types of layered material. We consider next the protection/deprotection chemistry of the polymers that allows us to grow multiple, uniform polymer layers. This step is depicted as the transition between C and D in Scheme 1. To hydrolyze the remaining phosphonate ester present in the monolayer of poly(NPMVEP), we react the layer with 4 equiv of bromotrimethylsilane (BTMS) in anhydrous CH3CN for 12 h. We use a stoichiometric excess of BTMS at this point in the layer growth because we want to achieve complete deprotection. To within our detection limit, the broad resonance at ∼28 ppm associated with the phosphonate ester is removed by BTMS/hydrolysis and subsequent zirconation, indicating essentially complete deprotection. A new shoulder at δ ) -23.8 ppm appears as a result of the reaction (Figure 6a). We tentatively assign this band to complexation of the new phosphonic acid functionalities in the polymer that are able to interact with any available (i.e., incompletely complexed) zirconium. Further study is needed to understand the origin of this band more fully. The Page group49 reported recently that the hydrolysis of phosphoesters in ZP multilayers formed on silica with BTMS is (47) Hong, H.-G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, 3, 521. (48) Gao, W.; Dickinson, L.; Morin, F. G.; Reven, L. Chem. Mater. 1997, 9, 3113. (49) Neff, G. A.; Page, C. J.; Meintjes, E.; Tsuda, T.; Pilgrim, W. C.; Roberts, N.; Warren, W. W., Jr. Langmuir 1996, 12, 238. (50) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (51) Gay, I. D.; McFarlan, A. J.; Morrow, B. A. J. Phys. Chem. 1991, 95, 1360. (52) Lang, S. J.; Gay, I. D.; Morrow, B. A. Langmuir 1995, 11, 2534. (53) Mark, V.; Dungan, C. H.; Crutchfield, M. M.; Van Wazer, J. R. Top. Phosphorus Chem. 1967, 5, 227. (54) Livantsov, M. V.; Prishchenko, A. A.; Lutsenko, I. F. J. Gen. Chem. USSR (Engl. Trans.) 1985, 55, 1976. (55) Brazier, J. F.; Houalla, D.; Wolf, R. Bull. Soc. Chim. Fr. 1970, 1089.
Langmuir, Vol. 16, No. 2, 2000 701
less efficient than our data indicate. They used relatively more ordered materials, however, that are less porous than our polymer multilayers. The availability of the phosphoester functionalities in their layers is very likely lower than it is in our polymers. The spectrum of the bilayer shown in Scheme 1 E is shown in Figure 6b. There are few new features present in this spectrum. The peak at δ ≈ -20 ppm is characteristic of the complex formed between zirconium and phosphonic acids on polymer chains, corresponding to the analogous peak in Figure 4. A new peak at δ ) 21.2 ppm is likely associated with free phosphoesters in the second layer. The position of the band suggests that weak interactions of these phosphoesters with zirconium ions may be present. The addition of additional polymer layers is expected to produce results that are consistent with the addition of the second layer. Conclusion We have reported the step-by-step layer growth of poly(NPM-VEP) and UBPA using 31P NMR spectrometry and summarize our results in Table 2. We resolve the several forms of phosphorus oxyacids that are present during the reaction steps shown in Scheme 1. Our data indicate that for the initial reaction of POCl3 with SiOx, physisorption of H3PO4 (from hydrolyzed POCl3) competes efficiently with chemisorption processes. Subsequent washing with polar solvents does not remove the physisorbed species completely, and we understand this effect in terms of the relatively strong hydrogen bonds formed between silanols and phosphorus oxyacids.36 Reaction of the surface with Zr4+ is required to remove the nonchemically bound phosphate. For layer growth, despite some differences in the details, there is broad similarity between the 31P NMR data for the poly(NPM-VEP) and UBPA layers, indicating that essentially the same chemistry is operative in both types of layered material. The differences between the 31P NMR response of these two materials arise primarily for steric reasons. Acknowledgment. We are grateful to the National Science Foundation for the support of this work through Grant CHE 95-08763. We are grateful to Kermit Johnson of the Max T. Rogers NMR facility for his assistance in performing these experiments. LA990668V
