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Langmuir 1995,11, 1720-1725
Characterization of Octadecylsilane and Stearic Acid Layers on Also3 Surfaces by Raman Spectroscopy Wade R. Thompson and Jeanne E. Pemberton" Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received July 18, 1994. In Final Form: February 7, 1995@ Results are presented which demonstrate the use of Raman spectroscopy for the characterization of octadecylsilane, octadecyldimethylsilane, and stearic acid layers on A 1 2 0 3 surfaces without the use of surface enhancement. Unlike previous attempts to study adsorbates at A 1 2 0 3 surfaces, an enhancing Ag adlayer was not used. These results represent the first report of the use of Raman spectroscopy for the characterization of alkylsilaneand stearic acid layers on A 1 2 0 3 surfaces. The spectral data are interpreted in terms of alkyl chain conformation within these layers. Raman data from the v(C-C) and v(C-H) spectral regions suggest that the alkyl chains comprising the octadecylsilane layer are more disordered than similar films formed on silica. The octadecyldimethylsilane monolayers are less ordered, suggesting an even greater concentration of gauche conformations in the alkyl chains of these films. The observed increase in disorder in films formed from octadecyldimethylsilane is attributed t o the influence of the bulky methyl groups on bonding density which prevent the alkyl chains from packing at their van der Waals radii. The Raman spectral data from self-assembled films formed from stearic acid suggest that these films are more ordered than the similar films from alkylchlorosilanes. The increase in order is believed to be due to the bonding orientation of the carbonyl group at the A 1 2 0 3 surface.
Introduction Al is an important industrial material used in chemical processing, including chromatographic and catalytic applications, the fabrication of building materials, and automotive and airplane parts. The range of applications for materials made from Al comes from its diverse physical properties. Products can be made to be highly reflective, electrically conductive, and chemically resistant and have good mechanical properties while still being relatively lightweight. Al easily passivates in the eresence of air forming a thin A 1 2 0 3 overlayer ca. 40-60 A thick which protects the underlying metal. The oxide overlayer completely covers and is firmly adherent to the underlying metal surface. The 4 2 0 3 layer formed either by air oxidation or by electrochemical anodization is amorphous and is referred to as a - A l 2 O 3 , but upon heating, this form of oxide changes to a harder crystalline form known as y - A 1 2 0 3 . 1 The A 1 2 0 3 layer contains both acid and base sites as shown in Figure L2 These characteristics are important to both catalytic and adsorption processes a t A 1 2 0 3 surface^.^ The Al ionic species, which act as Lewis acid sites, are not found directly in the surface layer but are exposed by oxygen and hydroxyl ion v a ~ a n c i e s . The ~ vacancies that expose the Al ionic species are not uniform, differing in size and nearest-neighbor configuration, which confers a range of acidic properties to the A 1 2 0 3 ~ u r f a c e . ~ The basic sites are associated with surface hydroxyls and adsorbed molecular water. The importance of materials made from Al for industrial applications brings relevance to the interfacial chemistry that occurs a t these surfaces. For instance, thin films
* Author to whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, April 1, 1995. (1)King, F. InAluminum and itsdlloys; West, E. G., Ed.; John Wiley and Sons: New York, 1987; Chapter 1. (2) Scott, R. P. W. In High Performance Liquid Chromatography; Brown, P. R., Hartwick, R. A,, Eds.; John Wiley and Sons: New York, 1989; Chapter 2. (3) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. In New Solid Acid And Bases: Their Catalytic Properties; Delmon, B., Yates, J. T., Eds.; Elsevier: New York, 1989; Vol. 51. (4)Peri, J . B. J . Phys. Chem. 1965,69,211. ( 5 ) Knozinger, H.; Ranassamy, P. Catal. Reu. Sci. Eng. 1978,17,31. @
Al Brtjnsted Acid - Base
Lewis Acid - Base
Figure 1. Schematic showing the A 1 2 0 3 surface with acid and base sites. formed by the adsorption of organic molecules at A 1 2 0 3 surfaces are important to many applications, including adhesion, lubrication, and corrosion inhibitior6 Organic coatings for Al surfaces are particularly important for improving adhesion of epoxies and paints to A l 2 O 3 surfaces, particularly in aqueous or high humidity environments. 7,8 Therefore, developing a detailed molecular description of the interfacial interactions that occur at A 1 2 0 3 surfaces is vital in designing surfaces with superior performance characteristics. The interactions of molecular adsorbates at A 1 2 0 3 surfaces such as carboxylic a c i d ~ , ~chromium -l~ phosphates,8 chlorosilanes,1° aromatic^,^^-^^ amine^,^ and thiols7J0have been characterized using X-ray photoelec-
.
(6) Boerio, F. J.;Chen, S. L. J. Colloid Interface Sci. 1980,73,176. (7) Walker, P. J . Adhesion Sci. Technol. 1991,5,279. (8) Ahern, A. M.; Schwartz, P. R.; Shaffer, L. A. Appl. Spectrosc. 1992,46,1412. (9) Cass, D. A,; Strauss, H. L.; Hansma, P. K. Science 1976,192, 1128. (10)Snyder, L. R.; Kirkland, J . J.Inlntroduction to Modern Liquid Chromatography, 2nd ed.; John Wiley and Sons: New York, 1979; p 362
(11)Brown, N. M. D.; Floyd, R. B.; Walmsley, D. G. J . Chem. SOC., Faraday Trans. 1979,75,261. (12) Golden, W. G.; Snyder, C. D.; Smith, B. J. Phys. Chem. 1982, 86,4675.
0743-'746319512411-1720$09.00/0 0 1995 American Chemical Society
Characterization of Layers on A1203 Surfaces tron spectroscopy(XPS),5inelastic tunneling spectroscopy (IETS),9J1r28 Raman spectroscopy,17J8,8,21~22*25~26-27 and an assortment of Fourier transform infrared (IR) spectroscopic techniques, including attenuated total reflectance (ATR)8,2Q and reflection-absorption infrared spectroscopy (RAIRS).12-15,30 The majority of vibrational spectroscopic studies have used IR spectroscopy to probe the molecular interactions of organic molecules at these surfaces. Raman spectroscopy has been used less frequently and more selectively, limited predominantly to the investigation of strong Raman scatters. Although Schatz31has theoretically predicted the surface enhancement values for Al to be comparable to Ag at ca. 520 nm, Al has not been observed to provide a significant amount of electromagnetic enhancement to adsorbates. Furthermore, previous surface Raman spectroscopic experiments at A1203 surfaces have been hindered by fluorescence from the Al2O3, particularly for high surface area samples.20,21 Although flat A1203 surfaces decrease the fluorescence background, they also reduce the number of Raman scattering centers. Meier and co-workers have used the layered NAl2O3 structure to study the Raman scattering ofp-nitrobenzoic acid adsorbed onto an air-oxidizedvapor deposited Al surface.22 Although the v,(NOz) mode of p-nitrobenzoic acid is a strong Raman band, the signalto-noise ratio obtained for the surface adsorbed species was only ca. 4. Due to the poor sensitivity of Raman spectroscopy,the majority of Raman experiments at A1203 surfaces use Ag island adlayers to provide electromagnetic enhancement to the surface adsorbed species.17J8,8,21,25-27 Although Ag islands do improve the sensitivity of the Raman spectroscopy experiments for the nonenhancing Al metal, this approach does not necessarily provide a true representation of the surface chemistry at the A1203 interface. The use of Raman spectroscopy for the investigation of octadecylsilane,octadecyldimethylsilane, and stearic acid at Al/Al2O3 surfaces is presented in this paper. The interactions of these organic systems a t A 1 2 0 3 surfaces have not been previously studied using Raman spectroscopy due to their weak Raman scattering. A thin A1203 layer formed on an Al surface is used to reduce the fluorescent background, and a thinned, back-illuminated charged coupled device detector is used to improve signal detection. The symmetric and asymmetric v(C-H) intensity ratio for the methylene groups is used as an indicator of the alkyl chain order. (13)Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986,132,93. (14) Sondag, A. H. M.; Tol, A. J. W.; Touwslager, F. J . Langmuir 1992,8,1127. (15)Allara, D. L.: Nuzzo. R. G. Langmuir 1985.1, 45. (16) Allara, D. L.; Nuzzo, R. G. Langmuir 1985,I , 52. (17) Moskovits, M.; Suh, J. S. J. A m . Chem. SOC.1985,107,6826. (18)Bello, J. M.; Stokes, D. L.;Vo-Dinh,T.AppZ. Spectrosc. 1989,43, 1325. (19) Tao, T. J . A m . Chem. SOC.1993,115,4350. (20) Pemberton, J. E.; Guy, A. L. In Materials Characterization, 9th ed.; Niemczyk, T., Ed.; American Society for Metals: Metals Park, OH, 1986: Vol. 10. D 126. (21) Egertoi, T. A,; Hardin, A. H.; Kozirovski, Y.; Sheppard, N. J. Catal. 1974,32,343. (22) Meier, M.; Carron, K. T.; Fluhr, W.; Wokaun, A.App1. Spectrosc. 1988.42.1066. ~ - - - - I
- - I
(23) Heritage, J . P.; Allara, D. L. Chem. Phys. Lett. 1980,74,507. (24) Debe, M. K. AppZ. Surf. Sci. 1982,14,1. (25) Heaviside, J.; Hendra, P. J.; Paul, S. 0.;Freeman, J . J.; Friedman, R. M. Appl. Spectrosc. 1981,35,220. (26) Gao, Y.; Lopez-Rios, T. Surf. Sci. 1988,198,509. (27) Bello, J. M.; Narayanan, V. A,; Vo-Dinh, T. Spectrochim. Acta 1992,48A,563. (28) Higo, M.; Kamata, S. Anal. Chem. 1994,66, 818. (29) Sperline, R. P.; Song, Y.; Freiser, H. Langgmuir 1992,8,2183. (30) Fondeur, F.; Koenig, J . L. Appl. Spectrosc. 1993,47,1. (31) Zeman, E. J.; Schatz, G. C. J. Phys. Chem. 1987,91,364.
Langmuir, Vol. 11, No. 5, 1995 1721
Experimental Section Instrumentation. Raman spectral data were obtained using 100 mW of 514.5 nm radiation from a Coherent Innova 90-5 Arf laser on a Spex 1877 "Triplemate" spectrometer as described previously.32-34 The detector in these experiments was a Princeton Instruments charge coupled device (CCD)system based on a Tektronix TK-512T, 512 x 512, thinned, back-illuminated, antireflection-coated CCD chip cooled with liquid Nz to - 120 "C. The spectra acquired on 4 2 0 3 were particularly challenging to acquire due to the weak signal intensities. The reader is urged to pay careful attention to the integration times listed in the figure captions for these spectra as they provide an indication of the difficulty in acquisition of these spectra. Materials. Octadecyltrichlorosilane (>97%) was obtained from Huls America. Octadecyldimethylchlorosilane (95%) and hexane (99+%)were obtained from Aldrich. Toluene (reagent grade), obtained from EM Science, was distilled and stored over molecular sieves prior to use. Stearic acid (99+%)was obtained from Sigma. Ethanol (absolute) was purchased from Midwest Grain Products. Polycrystalline Ag disks (99.999%) were obtained from Johnson Matthey. Al(99.998%)was obtained from Johnson Matthey. Materials were used as received unless otherwise noted. Polishing supplies including alumina and pqlishing pads were purchased from Buehler. Molecular sieves (3 A, 4-8 mesh) were purchased from Aldrich. The molecular sieves were activated in a vacuum oven heated to 120 "C and evacuated to ca. 38 mmHg for 2 days. Procedures. Alkylsilane layers were formed on AlzOdAl surfaces as follows. Al surfaces were initially mechanically polished with LO-, 3.0-, and 0.5-pm alumina and sonicated for 5 min in HzO, rinsed with HzO, and resonicated in HzO for an additional 5 min. The surfaces were allowed to air-dry for 1h, which allowed formation of the thin oxide layer, prior to immersion into a 0.1 M alkylchlorosilane/lOO%distilled toluene solution for 12 h. Excess alkylsilane was removed by Soxhlet extraction in freshly distilled toluene and methanol for 3 h each. The preparation of stearic acid layers on Al and Ag surfaces was performed as follows. The Al and Ag surfaces used here were initially mechanically polished with LO-, 3.0-, and 0.5-pm alumina and sonicated in HzO for 5 min, rinsed with HzO, and resonicated in HzO for an additional 5 min. The surfaces were immersed into 5.0 mM stearic acid/100%ethanol or 0.1 M stearic acid/100%hexane solutions for 3 days. Prior to Raman spectral acquisition, the modified surfaces were removed from the solution and rinsed with copious amounts of the corresponding solvent, ethanol or hexane. The stearic acid films formed from both solutions were spectroscopically identical.
Results and Discussion Alkylsilane Layers on AlzOs Surfaces. A1203 has become an important alternative to silica as a chromatographic stationary support, particularly in basic environments and other solvent conditions where silica is unstable.2J0 The attachment of alkylchlorosilanesto A1203 is believed to occur in a manner analogous to the attachment of alkylchlorosilanes to silica. The chlorosilanes react with surface hydroxyls on the A1203 surface forming Al-0-Si bonds, while releasing HC1 as a byproduct. The surface hydroxyl concentration for porous y-Al2O3and a-Al2O3 surfaces is estimated to be ca. between 2 and 10 hydroxylsI100 A2 depending upon preparation procedure^.^^^^^^^ In contrast, silica has a surface hydroxyl (32) Bryant, M. A.; Pemberton, J. E. J . A m . Chem. SOC.1991,113, 8284. _-_ .. (33) Pemberton, J. E.; Bryant, M. A,;Sobocinski, R. L.; Joa, S. L. J . Phys. Chem. 1992,96,3776. (34) Bryant, M. A.; Pemberton, J. E. J . A m . Chem. SOC.1991,113, 3629. (35)Unger, K. K. In Packing and Stationary Phases in Chromatographic Technisues: Unger, - K. K., Ed.; Marcel Dekker: New York, 1990; Chapter 6. (36) Knbzinger, H.; Ranassamy, P. Catal. Rev. Sci. Eng. 1978,17, 52.
Thompson and Pemberton
1722 Langmuir, Vol. 11, No. 5, 1995 I
800
1000
1200
10
Wavenumbers (cm-')
2800
2900
3000
Wavenumbers (cm-'1
Figure 2. Raman spectra in the v(C-C)region for (a) neat octadecyltrichlorosilane and (b) octadecylsilane attached to A l 2 0 3 : integration times (a) 10 min, (b) 60 min; excitation wavelength (a and b) 514.5 nm.
Figure 3. Raman spectra in the v(C-H) region for (a) neat octadecyltrichlorosilane and (b) octadecylsilane attached to A l 2 0 3 : integration times (a) 5 min, (b) 20 min; excitation wavelength (a and b) 514.5 nm.
Table 1. Raman Vibrational Assignments and Peak Frequencies for Octadecyltrichlorosilane (OTS) in the v(C-C) Region neat OTS surface OTS cm-l assignment
Table 2. Raman Vibrational Assignments and Peak Frequencies for Octadecyltrichlorosilane(OTS) and Octadecyldimethylchlorosilane(ODS) in the v(C-H) Region
790 848 871 890 1065 1080 1124 1176 1256 1303 1366
neat OTS, cm-l
sha sh 871 sh
2851 2890 2927
889 1062
sh
2852 2900
sh
0.9
1095 1127 1166 1294 1366
sh, shoulder. rk, rock. G, gauche. T,trans. e
ODs, cm-1
Y , stretch.
coverage of ca. 4.5 hydroxylsI100 A2.37 Therefore, coverages for the alkylsilanes attached to A1203 should be roughly comparable to those on silica. Raman Spectroscopy i n the v(C-C) Region. Figure 2 shows Raman spectra in the v(C-C) region for neat octadecyltrichlorosilane (spectrum a) and an octadecylsilane layer on an A1203 surface (spectrum b). The bands of interest in this region are the CHz rkG, CH3 rkT, CH3 r&, v(C-C)T,v(C-C)G,CHZtwiStG, and CH2 twishmodes. Vibrational assignments and peak frequencies are listed in Table 1. Assignments are based on those for other long alkyl chain systems.38 For surface-confined octadecylsilane (Figure 2b), the presence of the two (C-C)T bands (T = trans) a t ca. 1062 and 1127 cm-l, the in-plane methylene twist at 1294cm-l, and the methylene rocking mode at ca. 889 cm-l suggest that the alkyl chain portion of these films is relatively ordered. The narrow width of the in-plane methylene twist mode at 1294 cm-l is consistent with this picture. However, the shoulder on the v(C-C)T band a t ca. 1080 cm-l and the presence of the methylene rocking modes at ca. 790 and 848 cm-l indicate a small number of gauche (G) conformations in the octadecylsilane film.
surface ODs, cm-'
2847 2881 2924sh
2851 2895 2928
1.3
1.3
assignments v,(CH~) vJCH~) vs(CH2,FR) I~,(CHZ)II,JCH~)
Raman Spectroscopy in the v(C-H) Region. Figure 3 shows Raman spectra in the v(C-H) region for neat octadecyltrichlorosilane and an octadecylsilane layer covalently attached to A1203. The important vibrational modes that are prominent are the vs(CH2),va(CH3),and vs(CH2,FR) modes. Vibrational assignments and peak frequencies are listed in Table 2. The intensity ratio of the methylene asymmetric and symmetric v(C-H) bands [Z,,(CH2)/Zv,(CH2)1provides an indication of the degree of alkyl chain order and a means of monitoring conformational changes within the alkyl chain. Previous studies have shown that the relative intensities of the va(CH2) and vs(CH2) bands change significantly from a disordered state, such as a liquid, to a more ordered state, such as a crystalline solid. In liquid alkanes, for example, the intensity ratio of these methylene bands is ca. 0.6-0.7, whereas in the crystalline solid, this ratio increases to ca. 2.0.39340 The methylene ratios reported here were calculated using the peak height intensities in analog-to-digital units (ADUs) from background-subtracted spectra. For neat octadecyltrichlorosilane, Figure 3a, this ratio has a value of ca. 0.9. It increases to ca. 1.3 for an octadecylsilane layer attached to A12O3, Figure 3b. The large increase in this ratio suggests that the octadecylsilane layer on A1203 is partially ordered. Similar changes in this region are also observed for octadecylsilane layers attached to silica, which exhibit a corresponding ratio of ca. 1.5.41Interestingly, when bulk octadecyltrichlorosilane is hydrolyzed, ~
(37) Dorsey, J. G.; Dill, K. A. Chem. Reu. 1989,89, 331. (38) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. In The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991.
1.0
surface OTS, cm-'
~~~~
~~~
(39) Wallach, D. F. H.; Varma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979, 559, 153. (40) Gaber, B.; Peticolas, W. L. Biochim. Biophys. Acta 1977, 465, 260. (41) Thompson, W. R.; Pemberton, J. E.Ana1. Chem. 1994,66,3362.
Characterization of Layers on A1203 Surfaces
27’00
2800
2900
3000
Wavenumbers (cm-’) Figure 4. Raman spectra in the v(C-H) region for (a) octadecyldimethylchlorosilaneand (b)octadecyldimethylsilane attached to & 0 3 : integration times (a) 5 min, (b) 120 min; excitation wavelength (a and b) 514.5 nm. the methylene intensity ratio is ca. 1.4.41 Comparison of the ratios for octadecylsilane on Al2O3, octadecylsilane on silica, and hydrolyzed octadecyltrichlorosilane suggests that there is slightly more disorder in the extended alkyl chains ofthe octadecylsilanelayers on A1203 surfaces based on the smaller ratio. Surface hydroxyl concentration would be expected to have a significant effect on the bonding density and, therefore, degree of order of the alkyl chains. Previous work has suggested that the surface hydroxyl concentration a t an a-A12O3 surface is comparable to that on s i l i ~ a . ~Additional , ~ ~ - ~ ~ research estimating the surface density ofthe alkyl chains and the concentration of surface hydroxyl density on these thin a-A1203layers would be helpful. Octadecyldimethylchlorosilane has a single reactive chlorine, and therefore, each molecule can only react with a single surface hydroxyl. A single reactive group does not permit cross-linking with neighboring molecules or further polymerization. Moreover, the two pendant methyl groups provide steric hinderance between molecules. As a result, silanization reactions using octadecyldimethylchlorosilane result in submonolayer coverages at best. Layers ofthis molecule onA1203were investigated as an interesting comparison to those formed from the corresponding trichlorosilane species. Vibrational assignments and peak frequencies for this molecule in the v(C-H) region are listed Table 2. Figure 4 shows the Raman spectra for neat octadecyldimethylchlorosilane (spectrum a ) and an octadecyldimethylsilane layer on A 1 2 0 3 (spectrum b). The spectrum for the octadecyldimethylsilane layer on A1203 is considerably weaker than that measured for the octadecylsilane layer on Al2O3. Assuming comparable Raman scattering cross sections, the weaker intensity for the octadecyldimethylsilane layer suggests that fewer molecules are covalently bound in the sampled area than in the octadecylsilane layer. The spectrum of neat octadecyldimethylsilane exhibits a methylene intensity ratio of ca. 1.0, while that of octadecyldimethylsilane attached to A1203 shows a methylene intensity ratio of ca. 1.3. The smaller methylene ratio for the octadecyldimethylchlorosilane film relative to that for octadecylsilane implies that films made from a monochloro precursor are less ordered than the films formed from precursors with
Langmuir, Vol. 11, No. 5, 1995 1723 multiple reactive chlorines. The smaller bonding density of octadecyldimethylsilane reduces the intermolecular forces that hold the alkyl chains in an ordered configuration, permitting the alkyl chains more flexibility and, hence, a greater number gauche conformations. Alkanoic Acid Layers on AlzOs and Ag Surfaces. A molecule structurally related to octadecylsilaneis stearic acid. The carbon backbone of stearic acid [CH3(CH2)16COOHI has the same number of carbon atoms as octadecyltrichlorosilane and octadecyldimethylchlorosilane. Variations in interaction at A1203 surfaces are expected, however, based on differences in the headgroup. Investigating the structural differences of these two types of molecules might provide insight into the role of the headgroup in dictating alkyl chain order in these films. Previous studies of alkanoic acid monolayers have been riddled with a number of inconsistent observations and conclusions. Golden and co-workers characterized arachidate (CH3(CH)1&OOH) monolayers (prepared by selfassembly and Langmuir-Blodgett deposition) at A1203 surfaces using reflection-absorption IR spectroscopy (RAIRS).12 They observed that films formed by Langmuir-Blodgett deposition were highly ordered with the alkyl chains extending away from the A1203 surface. The films formed by self-assembly were not as ordered, and the alkyl chains did not extend away from the AI203 surface. Brown and co-workers studied shorter n-alkanoic acids formed on AZO3surfaces using inelastic tunneling spectroscopy (IETS).ll They observed that the shorter n-alkanoic acids were predominantly in an all-trans configuration suggesting an ordered film. Hansma and co-workers came to the opposite conclusion on the basis of their IETS measurements observing both G and T conformations for similar systems formed by self-assembly on More recently Allara and Nuzzo investigated n-alkanoic acids adsorbed onto A 1 2 0 3 surfaces from solution using ellipsometry, XPS,and RAIRS.15 They observed that the oxide layer is highly adsorbent to contaminants from the ambient environment, which can significantly affect the adsorption kinetics of the alkanoic acids. Monolayer formation proved to be a very slow process with tremendous inconsistencies for similar samples in the same solution. These results suggest that surface defects andor monolayer defects may play an important role in the adsorption process. Their observations also suggest that some of the previous discrepancies in the literature may possibly be attributable to incomplete film formation. Raman Spectroscopy in the v(C-C) Region. Figure 5 shows the Raman spectra in the v(C-0 region for crystalline stearic acid (spectrum a) and stearic acid adsorbed to a smooth Ag surface (spectrum b). Data for stearic acid adsorbed to A 1 2 0 3 in the v(C-C) region could not be observed due to the very weak signals. The bands of interest in this region are the CH3 rkG, CH3 rkT, v(CC)T, v(C-C)o, CH2 twisb, and the CH2 twist, modes. Vibrational assignments and peak frequencies are listed Table 3.35 For stearic acid adsorbed to smooth Ag, the 14c-c)~ bands at ca. 1063 and 1126 cm-’ and the CH2 twistr at ca 1296 cm-l are clearly present but are very weak in intensity. Although CH2 rkG modes are present at ca. 841 and 687 cm-l, the vibration associated with the v ( C - 0 ~ conformation, which occurs between ca. 1070 and 1080 cm-l does not appear in Figure 5b. The intensities of the v((2-C)~ bands and the absence of the Y(C-C)G band suggest that the carbon backbone of stearic acid is largely in an all-trans conformation. This conclusionis consistent with the previous surface Raman spectroscopic work of Moskovits on short-chain alkanoic acids adsorbed onto
Thompson and Pemberton
1724 Langmuir, Vol. 11, No. 5, 1995
800
I
1ioo
1000
Table 3. Raman Vibrational Assignments and Peak Frequencies for Stearic Acid in the v(C-C) Region stearic acid. cm-I Ag Surface, cm-l assignment 848 867 892 909
841
sh 887 921 999
1007 1040 1063 1072 sh 1103 1128
1032 1063 1126 1156
1173 1185 1295
1266 1296
Ag c01loids.l~ Interestingly, Moskovits noted in this previous report that spectra from alkanoic acids with chain lengths longer than dodecanoic acid could not be observed. This statement is consistent with the very weak intensities observed here for stearic acid layers. Insight into the possible chemistry responsible for the weak intensities measured in this system comes from recent work of Porter in which the occurrence of surface corrosion of reactive metals such as Ag and Cu was noted during the selfassembly of alkanoic acid monolayers from solutions of these species in ethanol at short times or hexadecane at longer times.42 If surface corrosion of the Ag surfaces used here were occurring, films of relatively poor quality such as the spectrum shown in Figure 5b might be expected. Such behavior would also explain the presence of unexpected peaks such as the one at ca. 999 cm-l. The only other Raman spectral data in this spectral region in the literature for alkanoic acids at Ag surfaces were reported by Moskovits and Suh for colloidal Ag suspensions.17 The spectra obtained by these workers do not resemble the Raman spectra obtained for selfassembled films on smooth Ag surfaces. Their Raman spectra exhibit significantly larger v(C-C)T and CH2 twistsT bands suggesting that stearic acid films formed (42) Smith, E.L.; Porter, M.
D.J.Phys. Chem. 1993,97,8032.
I
Wavenumbers (cm-’)
Wavenumbers (cm-l) Figure 5. Raman spectra in the v(C-C) region for (a) solid stearic acid and (b) stearic acid adsorbed to smooth Ag: integration times (a) 5 min, (b) 40 min; excitation wavelength (a and b) 514.5 nm.
I
00 2800 2900 3000 3100
Figure 6. Raman spectra in the v(C-H) region for (a) solid stearic acid, (b) stearic acid adsorbed to A l 2 0 3 , and ( c ) stearic acid adsorbed to smooth Ag: integration times (a) 2 min, (b) 120 min, (c) 6 min; excitation wavelength (a-c) 514.5 nm. Table 4. Raman Vibrational Assignments and Peak Frequencies for Stearic Acid in the v(C-H) Region stearic acid, AUAlz03 surface, Ag surface, cm-I cm-1 cm-l assignments 2846 2880 2924 2.0
2853 2884 2911/2934 1.4
2851 2882 2924 1.3
vS(CH2) va(CH2) vs(CH2,FR) Iva(CH2)IIv,(CH3)
on colloidal Ag are highly ordered with the alkyl chains extending away from the Ag surface. If, in fact, the hydrocarbon chains do extend away from the Ag surface, as suggested by Allara15J6 and Porter,42 then an appreciable v(C-C) Raman signal, comparable to that observed from other CIS molecules such as octadecanethi01,~~a~ would be expected. Clearly, the Raman spectroscopy of these alkanoic acid monolayer systems requires further attention before all of the nuances associated with their behavior will be elucidated. Raman Spectroscopy in the v(C-H) Region. Spectra a-c in Figure 6 show Raman spectra in the v(C-H) region for neat stearic acid, a layer of stearic acid on Al2O3, and a layer of stearic acid on smooth Ag, respectively. The vibrations of interest in this region are the vs(CH2),va(CH3), and v,( CH2,FR)modes. Vibrational assignments and peak frequencies are listed Table 4.35 The spectrum from crystalline stearic acid shown in Figure 6a exhibits a large methylene intensity ratio of ca. 2.0, indicative of a highly ordered system. The spectral response from the alkyl chains for the layer of stearic acid adsorbed on AlzO3, shown in Figure 6b, has a smaller methylene intensity ratio of ca. 1.4, suggesting an environment not as ordered as crystalline stearic acid. However, comparison of the ratio from the stearic acid layer on AZO3with the corresponding ratios for alkylchlorosilanes adsorbed to A 1 2 0 3 indicates that the alkyl chains of stearic acid are more ordered than those of the alkylsilane layers. Agreater degree of order for the stearic acid layer relative to the alkylsilane layers is consistent with the greater packing density of the adsorbed stearic acid layer as compared to the covalently bound alkylsilane layers. The spectrum of stearic acid adsorbed to a smooth Ag surface shown in Figure 6c suggests that this layer is slightly less ordered than the stearic acid layer on A 1 2 0 3
Characterization of Layers on A1203 Surfaces
Langmuir, Vol. 11, No. 5, 1995 1725
Octadecylsilanelayers on A1203 are observed to be more disordered than similar films formed on silica, suggesting a greater concentration of G conformations in the alkyl chains. The methylene intensity ratio for octadecyldimethylsilane layers on A1203 surfaces suggests that the alkyl chains are even less ordered than films formed from octadecyltrichlorosilane due to a lower bonding density. However, none of the alkylsilane systems studied possess “liquid-like”degrees of disorder. The substantial difference in integration times used to acquire spectra of octadecylsilane and octadecyldimethylsilane a t A1203 surfaces suggests that alkyl chain bonding densities for 0’20 dQ octadecylsilane are larger than those for octadecyldiAg A1203 methylsilane and may be an indication of multilayer formation. Figure 7. Schematic showing the adsorption of stearic acid to a smooth Ag surface and A 1 2 0 3 . Stearic acid adsorbs to A1203 surfaces and appears to produce slightly more ordered films than the alkylsilanes based on the slightly lower methylene intensity ratio of on either A1203 or silica. The data suggest that the bulky 1.3. These different degrees of order are believed to be methyl groups of octadecyldimethylchlorosilane and the due to the different interactions of the headgroup at the cross linking of octadecyltrichlorosilane prevent the alkyl surface. Observations made by Allara and N u z z o , ~ ~ J ~ chains from forming as ordered a film. On Al2O3, stearic Soundag and co-workers,14and Smith and Portef12indicate acid only adheres to the surface through a single oxygen that stearic acid adsorbs to Ag and A1203 in slightly atom. When stearic acid adsorbs to Ag surfaces, it adsorbs different orientations. Both studies suggest that alkanoic through both oxygens as a carboxylate, which may reduce acids adsorb to AZO3surfaces a t Lewis acid sites through the van der Waals interactions of the alkyl chains and a single oxygen atom tilted ca. 10”from the surface normal. create a slightly less ordered film. This picture would be However, alkanoic acids adsorb to Ag surfaces as the consistent with the slightly lower value of the methylene alkanoate through both oxygen atoms via a delocalized intdnsity ratio. Until all effects associated with the double bond, with the alkyl chain extending away from possible corrosion of the Ag surface can be eliminated, the surface along the surface normal. These orientations however, no definitive conclusions about film order can are shown schematically in Figure 7. be drawn from these data. Studies on alkanoic acid layers deposited from the vapor phase after the manner of Smith Conclusions and Porter42are underway and will be reported at a later The results presented here demonstrate the utility of date. surface Raman spectroscopy for the investigation of organic thin films at A1203 surfaces. Previous attempts Acknowledgment. The authors gratefully acknowlto study organic adsorption at A1203 surfaces have been edge the support of this work by the National Science limited to the use of strong Raman scatters or deposition Foundation (CHE-9121469 and CHE-9023687). of surface enhancing metal island films for making the surface SERS active. LA9405657 Stearic Acid [CH3(CH2)1&OOH1
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