J. Phys. Chem. 1988, 92, 2597-2601 reduction is observable with this sample, suggesting that the approach to ion-exchange equilibrium is slowed greatly by the presence of the porphyrin monolayer. This result is in accord with the cyclic voltammetric experiments and supports the assertion that the porphyrin is mediating all electron transfers to and from the encapsulated viologen ions. Conclusions. We have demonstrated that zeolite Y can be used to organize, via ion exchange and steric effects, electroactive molecules at an electrode surface. Cationic porphyrin molecules adsorb onto the zeolite in monolayer quantities, effectively sealing it against rapid exchange of viofogen molecules contained within. This microstructuring gives rise to electrochemical charge trapping
2597
reactions, like those observed with macrostructured polymer film devices. The application of these molecular assemblies to artificial photosynthesis is presently being explored.
Acknowledgment. We thank Jeffrey Cook for running the XPS and AES spectra and James Wallin and Dr. Ben Shoulders for the I3C N M R spectra. This research was supported by grants from Research Corp. and the Robert A. Welch Foundation. Registry No. ZnTMPyP4'.4CI-, 28850-44-4; CoTMPyPt.4C1-, 113451-63-1; BV2'.2Cl-, 1102-19-8; MV2+.2C1-, 1910-42-5; 3DQ2'.2Br-, 2895-98-9; MPVS+.Cf-, 113451-62-0; Sn02, 18282-10-5; Au, 7440-57-5; polystyrene, 9003-53-6.
Adsorption of Ordered Zirconium Phosphonate Multilayer Films on Silicon and Gold Surfaces Haiwon Lee, Larry J. Kepley, Hun-Gi Hong, Sohail Akhter, and Thomas E. Mallouk* Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 (Received: September 25, 1987)
Multilayer films of zirconium 1,lO-decanediylbis(phosph0nate) have been prepared on silicon and gold substrates and characterized by ellipsometry, XPS, and electrochemical measurements. The deposition technique requires first covalent attachment or adsorption of a phosphonic acid anchoring agent; HO(CH3)2Si(CH2)3P03HZ (I) and [-S(CH2)4P03H2]2(11) were used with Si and Au, respectively. The functionalized substrates are exposed alternately to aqueous ZrOC1, and 1 ,IO-decanediylbis(phosphonic acid) solutions to yield multilayer films. Ellipsometry shows an increase in film thickness, on Si, of 17 A/layer, which corresponds to the layer spacing in bulk Zr(0,PCloH20P03).Variable take-off angle X-ray photoelectron spectra from four-layer films have attenuated Si peaks but strong Zr and P peaks when the detector is 70° off the surface normal, implying that the films on Si are continuous. Electrochemical comparison of bare and functionalized Au shows facile electron transfer between Au or Au-I1 electrodes and 1 mM aqueous Fe(CN)?- but nearly complete blocking of electron transfer at Au-I1 electrodes immersed once each in ZrOClz and 1,lO-decanediylbis(phosphonicacid) solutions.
Introduction The chemical modifications of surfaces with rationally designed microstructures is interesting for both fundamental and practical reasons. On electrode surfaces, organized assemblies of molecules and/or polymers have been used to make devices with current rectifying behavior,'-3 to study the distance dependence of electron and energy transfer,"6 and to transport electrons or energy unidirectionally following visible light excitation.',* The limit of resolution in these surface microstructures is at the molecular level. In order to attain this kind of ordering, the classical LangmuirBlodgett (LB) synthetic procedure9 has most often been used. This powerful technique allows one to prepare surface multilayer structures in which each layer may contain a different chemical component and to control the distance between these components with angstrom resolution. Unfortunately, the technique suffers (1) (a) Abrufia, H. D.; Denisevich, P.; Umana, M.; Meyer, T.J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1. (b) Denisevich, R.; Willman, K. W.; Murray, R. W. J. Am. Chem. SOC.1981, 103, 4727. (c) Willman, K. W.; Murray, R. W. J. Electroanal. Chem. 1982, 133, 21 1. (d) Pickup, P. G.; Leidner, C. R.; Denisevich, P.; Murray, R. W. J . Electroanal. Chem. 1984, 164, 39. (e) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984,106, 1991. (f) Leidner, C. R.; Murray, R. W. J. Am. Chem. SOC.1985, 107, 551. (2) Smith, D. K.; Lane, G. A,; Wrighton, M. S. J. Am. Chem. SOC.1986, 108, 3522. (3) (a) Li, Z.; Mallouk, T. E. J. Phys. Chem. 1987, 91, 643. (b) Li, Z.; Wang, C. M.; Persaud, L.; Mallouk, T.E. J. Phys. Chem., in press. (4) Kuhn, H. Pure Appl. Chem. 1979, 51, 341. (5) (a) Moebius, D. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 848. (b) Moebius, D. Acc. Chem. Res. 1981, 14, 63. (6) Li, T. T.-T.; Weaver, M. J. J. Am. Chem. SOC.1984, 106, 6107. (7) Fromherz, P.; Arden, W. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 1045. ( 8 ) Arden, W.; Fromherz, P. J. Electrochem. SOC.1980, 127, 370. (9) (a) Blodgett, K. B. J. Am. Chem. SOC.1935.57, 1007. (b) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 51, 964.
0022-3654/88/2092-2597$01.50/0
SCHEME I
several drawbacks, the most important being the requirement for planar substrates, a sensitivity to environment contaminants, and the large number of mechanical manipulations required to produce complex structures. These problems arise from the fact that LB films are metastable structures and are prepared via transfer of preassembled monolayers to the substrate. In order to circumvent the shortcomings of the LB technique, recent efforts have been directed toward the preparation of self-assembling monolayers and m ~ l t i l a y e r s . ~ JThis ~ ' ~ approach is based on the spontaneous adsorption from solution of thermodynamically stable surface layers. Sagiv and co-workers12have shown that adsorption of a stable monolayer, followed by alternate chemical activation and adsorption steps, can yield organized multilayer structures without recourse to monolayer transfer (10) (a) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, I , 45. (b) Allara, D. L.; Nuzzo, R. G.Langmuir 1985, 1 , 52. (c) Nuzzo, R. G.; Fusco, F. A,; Allara, D. L. J . Am. Chem. SOC.1987,109, 2358. (d) Nuzzo, R. G.; Allara, D. L. J . A m . Chem. SOC.1983, 105, 4481. (d) Finklea, H. 0.;Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T.Lungmuir 1986, 2, 239. (e) Finklea, H. 0.; Avery, S.; Lynch, M.; Furtsch, T.Langmuir 1987, 3, 409. (11) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987, 109, 3559. (12) (a) Netzer,,L.; Sagiv, J. J . Am. Chem. SOC.1983, 105, 674. (b) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Sci. 1984, 100, 465. (c) Gun,J.; Iscovici, R.; Sagiv, J. J. Colloid Sci. 1984, 101, 201.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
--P
P P
i
PJ
\ 111
surface-active group
77777 Lletal
Phosphonite Film
E@
77777
Langmuir-Blodgett Film
Figure 1. Structural analogy between surface-adsorbedmetal phosphonate and Langmuir-Blodgett multilayer films.
techniques. Unfortunately, the a-chlorosilane-w-alkene system which they chose to illustrate this concept does not permit stacking of more than 2-3 layers before structural defects occur in the film.lZa We have foundi3that stable, well-ordered multilayer films based on metal phosphonates can be prepared via the sequential adsorption technique. Metal phosphonates are an ideal choice for this kind of synthesis because as bulk phases their morphol0 g y ~ ~ resembles 3'~ that of LB multilayers. The structural analogy is shown in Figure 1. Building up a metal phosphonate film on a surface requires first anchoring of molecules bearing the phosphonate functionality, either by adsorption or by covalent binding to the substrate. Multilayer films are then prepared by alternately adsorbing metal ions (Zr4+ in the case of this study) and a,w-bis(phosphonic acid)s from aqueous solution. The process of adsorbing these soluble components to form insoluble metal phosphonate multilayer films is shown in Scheme I. In this paper we report the adsorption of zirconium 1,I 0-decanediylbis(phosph0nate) (ZDBP) multilayers on silicon and gold surfaces. Apart from the two different phosphonate anchoring agents used (I for Si and I1 for Au surfaces), the technique and results are similar for the two substrate HO(CH3)2Si(CH2)3P03H2
[-S(CHZ),PO~HZ]Z
I I1 materials. Ellipsometric examination of these films reveals that they grow in a stepwise fashion, according to Scheme I, with the stacking axis of the layered phosphonate salt oriented perpendicular to the surface. XPS and electrochemical measurements reported here show that the films are insulating and essentially pinhole free. Experimental Section Materials. Single-crystal silicon wafers of 2-in. diameter with polished (100) faces were obtained from Aurel Corp. The wafers were rinsed for 5 min with trichloroethylene, 10 min with 2propanol, and then 20 min with deionized water immediately before use. Planar polycrystalline gold surfaces were prepared by evaporation of high-purity (99.999%) Au from a resistively heated molybdenum boat onto freshly cleaved muscovite (Spruce Pine Mica Co.) with the substrate held at 280 "C.I6 The (13) A preliminary account of our results with Si substrates has been communicated: Lee, H.; Kepley, L. J.; Hong, H.-G.;Mallouk, T.E. J . A m . Chem. SOC.1988, 110, 618. (14) (a) Alberti, G.; Constantino, U.; Alluli, S.; Tomassini, J. J . Inorg. Nucl. Chem. 1978, 40, 1113. (b) Alberti, G.; Constantino, U.; Giovagnotti, M. L. L. J. Chromarogr. 1979, 180, 45. (c) Casciola, M.; Constantino, U.; Fazzini, S.; Tosoratti, G. Solid State Ionics 1983, 8, 27. (15) (a) Dines, M. B.; DiGiacomo, P. Inorg. Chem. 1981, 20, 92. (b) Dines, M. B.; DiGiacomo, P.; Callahan, K. P.; Griffith, P. C.; Lane, R.; Cooksey, R. E. In Chemically Modified Surfaces in Catalysis and Electrocafalysis;Miller, J . , Ed.; American Chemical Society: Washington, DC, 1982: ACS Symp. Ser. No. 192, p 223. (c) Dines, M. B.; Griffith, P. C. Inorg. Chem. 1983, 22,567. (d) Dines, M. B.; Cooksey, R. E.; Griffith, P. C. Inorg. Chem. 1983. 22, 1003. (e) Dines, M. B.; Griffith, P. C . Polyhedron 1983, 2, 607.
Lee et al. evaporator operated with a base pressure of 1 X Torr, and the deposition rate was ca. 1 A/s. The thickness of the gold film was monitored with a quartz oscillator; films were typically 1700 A thick. After deposition, the chamber was back-filled with prepurified nitrogen. X-ray diffraction patterns showed that the Au films had predominantly (1 11) orientation. Scanning electron microscopy (SEM) gave no indication of visible surface roughness at lOOOOOX magnification. Care was taken to minimize the period of exposure of these films to the laboratory ambient prior to functionalization with 11. Au electrodes were made from high-purity gold wire (Johnson-Matthey) contacted via colloidal silver paint (Ted Pella, Inc.) to a copper wire and sealed either in glass or with insulating epoxy. Immediately before use, these electrodes were cleaned with hot H 2 0 2 / H Z S 0 4(1:lO) for 10 s and then cycled electrochemically between the hydrogen and oxygen evolution potentials in 0.5 M HzSO4 as described by Finklea et al.'" The electrodes were then dipped briefly in 0.1 M NaOH and rinsed thoroughly with deionized water. Zirconyl chloride octahydrate was obtained from Alfa Inorganics. All other chemicals were reagent grade, obtained from commercial sources, and were used as received. Deionized water purified to a resistivity of 18.3 M a c m with a Barnstead Nanopure system was used in all experiments. 1,10-Decanediylbis(phosphonic acid) (DBPA) was prepared by the Michaelis-Arbuzov reaction of 1,lO-dibromodecane (Aldrich) with triethyl phosphite. 1,lO-Dibromodecane (0.1 mol) and P(OC2H,), (0.25 mol) were refluxed at 150 "C for 6 h, with evolution of ethyl bromide; the solution was cooled to room temperature, and 150 mL of concentrated (37%) aqueous HC1 solution was added. Heating was resumed at 100 "C for 10 h; the aqueous layer was then separated and evaporated at room temperature. The white crystalline DBPA which precipitated was washed several times with HPLC grade CH3CN and dried in vacuo. Aqueous solutions of 3-(hydroxydimethylsilyl)propylphosphonic acid (I) were prepared by reacting 1,3-bis(3-chloropropy1)-1,1,3,3-tetramethyldisiloxane (Petrarch Systems), 0.05 mol, with 0.25 mol of P(OC2H,), at 150-160 OC for 5 h under flowing nitrogen. A 20-mL sample of 37% HCI was added and the mixture was refluxed for 5 h; addition of 50 mL of 6 N NaOH to the aqueous layer followed by heating at 100 "C for 4 h gave an alkaline solution of I. Aliquots of this solution were diluted with water and acidified with HC1 just prior to use. The disulfide 11 was prepared by oxidation of the corresponding thiol. 1,4-Dibromobutane (0.5 mol) and P(OC,H,), (0.1 mol) were reacted at 150 "C on an oil bath. Hydrolysis with 100 mL of concentrated (48%) HBr solution and evaporation of the aqueous layer gave (4bromobuty1)phosphonic acid. A 5-mmol sample of this acid was refluxed with 5 mmol of thiourea and 10 mL of ethanol for 3 h. Six milliliters of 3 N aqueous NaOH was added and refluxing continued for 3 h. The cooled solution was acidified in 0.5 M sulfuric acid and slowly added to an ice-cooled solution of 20 mL of 5 N NaOH. During the addition the temperature was maintained between I O and 15 "C. Excess 34% H 2 0 , solution was then added at such a rate that the temperature did not rise above 30 "C. This solution was stirred 30 min and extracted with diethyl ether. The ether extracts were dried over MgSO, and evaporated to give I1 as a white crystalline powder. Satisfactory carbon and hydrogen analyses for DBPA and I1 (Atlantic Microlabs, Atlanta, GA) were obtained. Instrumentation. Ellipsometric measurements were made with a Rudolf 437 ellipsometer equipped with a RR-2000 rotating analyzer detector and 6328-A (He-Ne laser) analyzing light. The angle 0 between the incident beam and surface normal was varied between 68" and 72". All data were recorded in air without controlling the humidity and represent averages of at least 50 measurements. Errors of A0.2" in \k and h0.5' in A for these data were estimated from scatter in the individual measurements. The data were processed with a Hewlett-Packard Model 9816 desktop computer equipped with a H P 7470A plotter. X-ray (16) Chopra, K. L.; Bobb, L. C.; Francombe, M. H. J . Appl. Phys. 1963, 34, 1699.
Zirconium Phosphonate Multilayer Films photoelectron spectroscopy (XPS) was done with a Kratos Series 800 system equipped with a hemispherical analyzer, using Mg K a (1253.6 eV) radiation in fixed analyzer transmission mode. Resolution of the analyzer is about 0.7 eV based on the width of the Ag 3d calibration line. Data were taken at low resolution (80-eV pass energy). Silicon wafers modified with ZDBP layers were mounted with carbon paste on a tantalum foil. The latter was spot-welded to a stage grounded to the UHV chamber, in which a base pressure of 1 X Torr was maintained. Binding energies were calibrated with Ag 3d (binding energy 368.2 eV) as a reference. No ev-idence of sample charging, judging from the position (285-eV binding energy) of the carbon line, was seen in any of the spectra. The take-off angle (the angle between the surface normal and the detector) could be varied continuously between 0' and 90' by rotating the sample in the chamber. Electrochemical measurements were carried out in one-compartment cells with gold working electrodes, platinum counter electrodes, and a saturated calomel (SCE) reference, with an EG&G/PAR Model 175/363 programmer/potentiostat and a Kipp & Zonen BD90 x-y recorder.
Results and Discussion Surface Multilayer Synthesis. Incubation of silicon samples with I, or gold samples with 11, gives a surface containing reactive phosphonic acid groups at approximately monolayer coverage. In the case of Si, a 2 mM solution of I was prepared by diluting and acidifying the alkaline stock solution to pH 3 with HC1. Clean Si wafers were then heated to 50 "C for 3 days in this solution. In acidic aqueous solution, covalent binding" of monofunctional silanols to the native oxide layer of Si competes with dimerization to form the corresponding (unreactive) siloxane. Ellipsometric and XPS data presented below suggest that a reasonably compact monolayer of pendant phosphonic acid groups is formed under these conditions. For gold surfaces it is knownlocslsthat bifunctional molecules such as I1 bind via the disulfide group to form compact monolayers. The orientation of the gold substrate does not appear to be of critical importance, since insulating monolayers can be formed on randomly oriented, polycrystalline gold electrodes;' the electrochemistry of ZDBP layers on gold, discussed below, indicates that pinhole-free layers of I1 are similarly adsorbed. Typically, I1 was adsorbed from a 1 mM solution by incubation for 16 h at room temperature. The phosphonic acid modified surfaces were immersed alternately, at room temperature, in 5.0 mM zirconyl chloride and 1.25 mM DBPA aqueous solutions, according to Scheme I, to give the desired number of ZDBP layers. To avoid precipitation of Zr(03PCloH20P03) as a bulk solid, the substrate was rinsed for 30 min in flowing deionized water between all adsorption steps. Measurements of film growth using an ellipsometer flow cellI3 show that adsorption of either the zirconium or phosphonic acid component of the film is largely complete aftzr 5-10 min. In order to ensure complete surface coverage, incubation times of 4-6 h were used in this study. Ellipsometry. Reflection ellipsometry measures the change in polarization of a polarized, specular beam upon reflection from a optically flat substrate. The change in the state of polarization is routinely characterized by the change in the phase difference (A) and amplitude ratio (tan 'k) of the component plane waves aligned parallel and normal to the plane of i n ~ i d e n c e . ' ~The (17) (a) Haller, I. J . A m . Chem. SOC.1978, 100, 8050. (b) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984, Vol 13, p 191. (18) Nuzzo, R. G.; Zegarski, B. G.; Dubois, L. H. J . A m . Chem. SOC. 1987, 109, 733. (19) The ellipsometrically measured parameters A and P are related to the film thickness d and indexes of refraction nnlm,nair,and nsubJtra,c through a complex function p tan (9) M i A ) = ~(X,8,dinfilm,nai,,nr,b,,,3 where X and 8 are the wavelength and angle of incidence of the analyzing light. The explicit form of p is given by: Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977; pp 332-340.
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