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Langmuir 1994,10, 1472-1476
Chemical Vapor Surface Modification of Porous Glass with Fluoroalkyl-Functional Silanes. 1. Characterization of the Molecular Layer Hiroaki Tada' and Hirotsugu Nagayama Central Research Laboratory, Nippon Sheet Glass Co., Ltd., I , Kaidoshita, Konoike, Itami, Hyogo 664, J a p a n Received July 12, 1993, I n Final Form: February 28,1994" The hydrophilic surface of porous glass substrates became highly hydrophobic (static contact angle, &(water)= 119.4f 0.7O;&(n-hexadecane)= 80.7f 2-59by chemical vapor surface modification (CVSM) with (heptadecafluorodecy1)trichlorosilane (HFTS). Diffuse reflectance Fourier-transformed infrared (FT-IR)spectra of SiOzpowder used as a model adsorbent indicated that the HFTS molecules are anchored on the surface via condensation between their terminal functional groups and isolated surface Si-OH groups. The quite low reflectance of the pristine porous glass of 0.17% at the wavelength of 490 nm was only increased to 0.62% by means of the CVSM treatment, while the fluoroalkylsilane coating using a rubbing method raised it up to 4.55%. The slight red shift (-5 nm) of the wavelength having a minimum reflectance and data of FT-IR attenuated total reflection and X-ray photoelectron spectroscopies suggested that the HFTS molecules form a monolayer along the external and the internal surface, exposing mainly CFg groups outermost in the CVSM sample. The Cassie-Baxter theory on the composite surface consisting of open area and hydrophobic region covered with HFTS was reasonably used to account for the excess increase of the contact angle over that in the smooth surface.
Introduction Antireflective glass is quite useful not only for increasing efficiency of solar energy conversion devices such as solar collectors and solar cells but for decreasing the strain on the human eye in computer work.' In a single-overlayer design on glass, a refractive index of 1.23 for the coating material is needed in order to obtain zero reflectance. Even the index of MgF2, which is the lowest among the inorganic materials, is 1.38. The low index requirement can be satisfied only by using porous materials, because the introduction of porosity significantly reduces the index. However, surface porous glass (SPG) has an inherent problem that soil is easy to adhere to and difficult to be removed from the surface. T o resolve the problem, the surface free energy must essentially be decreased. In general, monolayers of foreign compounds may alter pronouncedly the surface properties, according to Langmuir's "principle of independent surface a ~ t i o n " . ~AlJ though the preparation and the characterization of monolayers on flat and smooth surfaces have been studied extensively,"O the investigation of porous surfaces is quite limited" and there is great room for pursuit. Oriented monolayers of amphiphilic compounds can be conveniently created from aqueous and nonaqueous solutions by the Langmuir-Blodgettl2 and self-assembly methods,13-17 Abstract published in Advance ACS Abstracts, April 1, 1994. (1) Prasad, A.;Balakrishnan,S.; Jain, S. K.; Jain, G. C. J.Electrochem. SOC.1982, 129, 596. (2) Langmuir, I. J. Am. Chem. SOC.1916, 38, 2221. (3) Langmuir, I. J. Am. Chem. SOC.1929, 6, 451. (4) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (5) Dote, J. L.; Mowery, R. L. J.Phys. Chem. 1988,92, 1571. (6) Chatzi, E.G.; Urban, M. W.; Ishida, H.; Koenig, J. L. Langmuir 1988, 4, 846. (7) Bardwell, J. A,; Dignam, M. J. J. Colloid Interface Sci. 1987,116, 1. (8) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (9) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (10) Kurth, D. G.; Bein, T. J.Phys. Chem. 1992,96,6707. (11) Ogawa, K.; Soga, M.; Takada, Y.; Nakayama, I. Jpn. J. Appl. Phys. 1993,32, L614. (12) For a general reference, see Ulman, A. An Introduction t o
Ultrathin Organic Films, From Langmuir-Blodgett t o Self-Assembly; Academic Press: Boston, MA, 1991. (13) Nuzzo, R. G.; A h a , D. L. J. Am. Chem. SOC.1983, 105, 4481.
0743-7463/94/2410-1472$04.50/0
respectively. In light of coverage, these methods do not seem to be necessarily appropriate for the coating of the porous materials. Recently, the formation of a highly oriented monolayer of 1,3,5,7-tetramethylcyclotetrasiloxane on flat Si02 substrates by a chemical vapor surface modification (CVSM) method has been reported.18 This paper describes the CVSM treatment of SPG substrates by (heptadecafluorodecy1)trichlorosilane (HFTS), with an emphasis of characterization of the surface layer formed. Moreover, the wetting properties are discussed on the basis of the Cassie-Baxter theory.
Experimental Section SPG (ARglass, Nippon Sheet Glass CO.)'~and smooth surface glass (SSG) substrates were soaked in an alkaline solution (pH = 13.7)for 30 s and subsequently rinsed by sonification in pure water (conductivity < 1 jtS cm-l) for 10min. After the substrates (38 mm X 40 mm) had been set in a vacuum chamber (2.83L) kept at 35 "C, it was evacuated until the pressure reached ca. 10 Torr and heated to 80 O C . (Heptadecafluorodecy1)trichlorosilane (CFs(CF&H&H2SiCl3, HFTS, >98%, Toshiba Silicone)of ca. 50 pL (0.15mmol) was dosed with an injector through the inlet sealed off by a rubber stopper and allowed to react with the substrates for 1 h under a closed system. Immediately after introduction of HFTS, its vaporization was observed through the glass cover of the chamber. Then the temperature was raised up to 99 O C , evacuatingfor additional1h to removethe unreacted HFTS. For comparison, a rubbing method was adopted. A 2-propanol solution of (heptadecafluorodecyl)trimethoxysilane (CFs(CF2)7CH&H2Si(OCHs)s,>98%,Toshiba Silicone) with a concentrationof 2.0 w t 5% was placed on the substrate and ita excess was removed by rubbing with clean paper towels. Static contact angles (8.) were measured by using a contact angle meter (ModelCA-D,Kyowa InterfaceScience Co.) at room temperature (20 1 "C). Water droplets with a diameter of approximately 2 mm were placed at six positions for one sample (14) Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC.1989,111,7164. (15) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5 , 1370. (16) Ulman,A.; Evans, S.D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. SOC.1991,113,1499. (17) Evans, S. D.; Sharma, R.; Ulman,A. Langmuir 1991, 7 , 156. (18) Tada, H.; Nakamura, K.; Nagayama, H. Submitted for publication in J. Phys. Chem. (19) Thomsem, S. M. RCA Reu. 1951,143.
0 1994 American Chemical Society
Langmuir, Vol. 10, No. 5, 1994 1473
Modification of Porous Glass and the average was adopted as the value of 0,. The contact anglesof n-hexadecane (>98%, TokyoKasei OrganicChemicals) were also determined in the same manner as with water. The tilted plane method was used to determine critical tilting angles (a), at which water droplets (10-100 mg) on the substrate begin to slide. Fourier-transformedinfrared attenuated totalreflection (FTIR-ATR) spectra of the substrates subjected to the CVSM treatment were obtainedusing a FT-IR spectrophotometer(JIR5500, Nishon Denshi Co.) equipped with a multiple ATR attachment. All spectra were measured in a samplecompartment purged for 5 min with dry air. The ATR attachmentwas designed so that the nonpolarized incident light can be reflected between KRS-5plate (50 mm X 20 mm X 3 mm in thickness) faces 17 times at the average incident angle of 45O. Both sides of the internal reflectionelementplate were used. In this configuration, the penetration depth calculated is approximately 1.57 pm at 1100 cm-1. Spectra were recorded at a resolution of 4 cm-l with 5 X 1 P coadded scansand computed with triangularapodization. DiffusereflectanceFT-IR measurementswere carried out in the same way as previously reported.20 Visible reflection spectra of the SPG substrates with and without the surface modification were measured on a Hitachi 330 spectrophotometer in the wavelength region between 400 and 800 nm. Incident angle was set at 5O from the normal to the surface and Al-evaporated (thickness 100 nm) glass was used as a reference. X-ray photoelectronspectroscopic(XPS)measurementswere performed with a ShimadzuElectronSpectrometer(ModelESCA 7000) using a Mg K a X-ray source (hv = 1253.6 eV). The X-ray sourcewas operatedat 30 mA and 8 kV. The residual gas pressure in the spectrometer chamber during data acquisition was less than le7Torr. Incident and detected angles were fixed at 90" and irradiation area was ca. 19.6mm2. The binding energy scales for the fluorocarbon samples were referenced by setting the hydrocarbon (CH,) peak maxima in the C1, spectra of 284.6 eV. The accuracy of the binding energy determined with respect to this standard value was within 10.3 eV.
-
Results and Discussion Figure 1shows scanning electron micrographs (SEM) of the surface (A) and the cross section (B) of the SPG substrate. SEM A demonstrates the presence of many pores with exit sizes smaller than 20 nm on the surface. Also, in SEM B, a skeleton layer with a thickness of approximately 96 nm is observed. Figure 2 shows pore size distribution determined by the method of Cranston and Inkley.21 The pore size is distributed in a wide range between 1.8 and 10 nm around a main peak of ca. 3.5 nm. The difference of the pore size in Figures 1 and 2 means that the average exit size of the pores is larger than the average interior size. Figure 3 shows diffuse reflectanceFT-IR spectra of Si02 particles before (A) and after (B)the treatment with HFTS. In the difference spectrum of (C), a sharp negative peak a t 3745 cm-' is owing to the stretching vibration of isolated surface Si-OH groups (v(Si,-OH)). More intense positive peaks appear at 1253,1210, and 1169 cm-l, which can be assigned to the C-C stretching ((v(C-C)) vibration, the antisymmetric CF2 stretching (vaa(CF2)),and the symmetric CF2 stretching(v8(CF2))vibrations,respectively.22*23 Note that the strength of the broad peak (3500-3700 cm-') due to the Si8-OHgroups taking part in hydrogen bondings hardly changes. Also, no C1 was detected by the X-ray photoelectron spectroscopy (XPS) measurements of the sample after the treatment. It is likely that the HFTS (20)Tada, H.; Hyodo, M.; Kawahara, H. J. Phys. Chem. 1991, 95, 10185. (21) Cranston, R. W.; Inkley, F. A. Adu. Catal. 1957,9,143. (22) Peacock, C. J.; Hendra, P. J.; Willia, H. A.; Cudby, M. E. A. J. Chem. SOC.A 1970,2943. (23) Cho, H. G.; Strauss, H. L.; Snyder, R. G. J. Phys. Chem. 1992, M, 5290.
Figure 1. Scanningelectron micrographs of the surface (A) and the cross section (B) of the SPG substrate.
molecules preferentially react with the isolated Si,-OH groups via dehydrochlorination to be anchored chemically on the surface of SiO2. The residual C1 groups seem to be hydrolyzed by physisorbed H2O to yield Si-OH groups and result in the formation of Si-0-Si bonds between close neighbor moleculesvia d e h y d r a t i ~ n .However, ~~ the possibility of the same reaction via dehydration between HFTS molecules and Si,-OH groups cannot be ruled out thoroughly. (24) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647.
Tada and Nagayama
1474 Langmuir, Vol. 10, No. 5, 1994 I
c
I
0200
0 COO0
-0 0 2 0 0 AOOO
' ~ b 'a3 2 b 0
2ab0
'zaba 'zobo
lab0
12bo
sob
Wavenumberlcm-l
Figure 4. FT-IR-ATR difference spectra of the SSG (A) and the SPG (B) samples before and after surface modificationwith HFTS. Pore diameter/angstrom Figure 2. Pore size distribution in the SPG substratedetermined by the BET method. k
0 1000]
-0
IO00
10
o
'
aebo
32b0
lab0
2rbo
20b0
isba
12b0
sob
LOA
W.venumber/cm-l
Figure 3. Diffuse reflectance FT-IR spectra of Si02 particles before (A) and after (B) the treatment with HFTS; the average diameter of the particle is ca. 50 pm. Table 1. Static Contact Angles of Water and n-Hexadecane on the SPG and the SSG Substrates with and without the Surface Modification with HFTS contact angle/dego water n-hexadecane (y = 72.8 m N m-1) (y = 27.3 m N m-l) sample AR-glas -b AR-glass treated with HFTS 119.4f 0.7 80.7f 2.5 119.5c 91.W 122.86 48.6d 68.3f 2.3 107.6 0.6 flat glass treated with HFTS (1 The values were determined at 24.5 f 0.5 "C and 60.7 f 1.4% humidity. The surface was completely wet by the liquid. c,d Values calculated based on the Cassie-Baxter theory and Wenzel theory,
*
respectively.
Table 1 lists contact angles of water and n-hexadecane on the SPG and the SSG substrates with and without the surface modification. Both the liquids completely wet the surface of the untreated SPG substrate. The HFTS treatment leads to drastic increases in the contact angle of each liquid. This is mainly due to a decrease in the surface free energy with the treatment.25t26 The C1, XPS spectra of the samples had peaks at binding energies of 291.6 and 294.0 eV, which are characteristic of CF2 and CF3 groups.27 The surface atomic ratios of C(F3) to C(F3 (25)Hare, E.F.; Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1964, 58,236. (26)Lindner, E.;Arias, E. Langmuir 1992,8, 1195.
for the SSG and the SPG substrates are determined to be 0.253 and 0.255, respectively. Both the values are 1.81 times greater than that of the HFTS molecule (0.14).This fact indicates that the HFTS molecules are oriented perpendicular to the surface to expose chiefly the terminal CF3 groups on the surface. Moreover, the contact angles are 11.8' (water) and 12.4' (n-hexadecane) greater, compared to those of the SSG substrate treated with HFTS. As discussed in detail later, these excess increases of the contact angle seem to be ascribable to the surface heterogeneity effect. Figure 4 shows FT-IR-ATR difference spectra of the SSG (A) and the SPG (B) before and after the surface modification. Five peaks are observed at 1236,1203,1145, 1134, and 1113 cm-' in both the spectra, which are very similar to the spectrum of n-C16F34 in a n-CleHas matrix As described above, all at 140 K reported by Cho et the peaks are assigned to the v(C-C)and v(CF2)vibrations. The fine structure of the band is more obvious than that of the particles in Figure 3 and the shift to the lower wavenumber of the corresponding peaks from 7 to 17 cm-1 is perceived for reasons which are unclear. Although strict quantification is impossible from the FT-IR-ATR measurements, comparison of spectra A and B indicates that the peak intensities in the SPG sample are drastically greater than those in the SSG sample. Therefore, in the SPG substrate, HFTS molecules are thought to be immobilized not only on the external surface but also in the interior of more pores. The XPS peak intensity ratios of C I , ( C F ~ ) / and S ~ ~C1,(CF2)/Si2p ~ in the smooth (r,) and rough (rJ surface samples were 0.225 (rs(CF3/Si)), 0.231 (r,(CFs/Si)), 0.888 (r,(CFdSi)), and 0.909 (r,(CF&i)), respectively. The ratios in the SPG sample are slightly larger than the corresponding values in the SSG, which seems to result from the "surface concentrated effect" by the surface asperities. In each spectrum, the negative peak at 3743 cm-l due to the disappearance of the isolated Si,OH groups is detected despite the fairly strong noise between 3200 and 4000 cm-'. The positive peak at 3743 cm-' is observed in the FT-IR-ATR spectrum of the untreated SPG as shown in the inset of Figure 4B. Consequently, in the SPG substrate, the HFTS molecules are considered to be chemically bonded on the external and internal surfaces via dehydrochlorination (or dehydration) between the HFTS molecule and the Si,-OH group. Figure 5 showsvisible reflection spectra of the untreated SSG (A) and SPG (B)substrates, and the SPG ones treated (27)Castner, D.G.; Lewis, K. B., Jr.; Fischer, D. A.; Ratner, B. D.; Grand, J. L. Langmuir 1993,9, 537.
Langmuir, Vol. 10, No. 5, 1994 1475
Modification of Porous Glass
composite consisting of air and the hydrophobic region covered with HFTS, the subsequent thermodynamic discussion can be given on the basis of the Cassie-Baxter the0ry.~O9~l The equilibrium contact angle of the porous sample (ee*) is related to that of the smooth surface sample (Oe):
400 450 500 550 600 650 700 750 800
Wavelength/nm
Air (open area fraction=&)
+Water/sample HFTS molecule
(E)
Figure 5. Visible reflection spectra of the pristine SSG (A) and SPG (B) substrates, and S P G treated with HFTS by means of the CVSM method (C) and the rubbing method (D). (E) Schematic representation of the S P G substrate treated by the
CVSM method.
with HFTS by means of the CVSM method (C) and the rubbing method (D). The reflectance of the SSG substrate monotonously decreases from 8.75 % at 400 nm to 7.89% at 800 nm. In the SPG substrate, it remarkably decreases in the whole visible range, having a minimum of 0.17 % at 490 nm that is 8.29% smaller than that of the SSG substrate at the same wavelength. The CVSM treatment increases the reflectance; however, the increment is only 0.45% at 490 nm. On the other hand, the reflectance at 490 nm of the sample treated by the rubbing method is 4.42% , being approximately 6.1 times greater than that for the CVSM sample. This finding is ascribable to an increase in the refractive index of the skeleton layer, caused by the filling up of the pores with the fluoroalkylsilane molecules; which is also supported by the fact that the wavelength with minimum reflectance (Amin) is red-shifted only 5 nm in the CVSM sample, while in the rubbing sample the amount of shift reaches as much as 80 nm. Note that Xmin is proportional to the refractive index of the skeleton layer.28 These results also lead to the conclusion that the HFTS monolayer is formed along the external and internal surfaces in the CVSM sample. Figure 5E represents an ideal scheme. Realistically, since some micropores with diameters smaller than twice that of the HFTS molecular length (- 1.7 nm)29are present, there would be pores not covered with HFTS molecules completely. If the interface between water and the SPG substrate subjected to the CVSM treatment is assumed to be a (28) Yoldas, B. E. Appl. Opt. 1980,19, 1425. (29) The value is estimated based on the CPK model, assuming a helical structure of the HFTS molecules; for the structure of the fluorocarbon, see: Bunn, C. W.; Howells, E. R. Nature 1954,174, 549.
where the subscript e denotes equilibrium and c is open area fraction. It is necessary to note that the experimental values (e,) are not equilibrium ones but quasi-equilibrium Although the equilibrium value is very ones (eqe, difficult to determine experimentally, the quasi-equilibrium value can certainly take any values between the advancing (8,) and receding (6,) contact angles, and the equilibrium value also lies between them. According to previous papers, in the smooth surface, the hysteresis of the contact angle (Ad) for the fluorocarbon is fairly large (16-20' for poly(tetrafluoroethy1ene),33,34 compared to that for the hydrocarbon (e.g., a selfassembled monolayer of octadecyltriethoxysilane on mica (A6 = 3') and silicon (A0 = 6°)).35 However, Johnson and Dettre theoretically indicated that 8, is very close to Ba (-0, = 6qe) in the highly hydrophobic surfaces with large Furthermore, in the present system, the A0 value of the HFTS-treated SPG sample is smaller than that of the smooth surface (vide infra). The similar trend that A8 reduces with increasing roughness of poly(tetrafluor0ethylene) is reported by Morra et al.33 and Dettre et al.34 Presuming that 8, e,, and Oe* Oqe* are valid from these above, one may rewrite eq 1
-
-
where f is the proportionality constant dependent on the shape of the roughness and the position of the water/ sample interface. From the thickness of the skeleton layer (t 96 nm) and 490 nm of Amin, the refractive index of the skeleton layer (n,) can be estimated to be 1.28 (n, = A,in/4t). With this and the refactive index of glass ( n= 1.52),the porosity of the skeleton layer is found to be 51.3% (p = 100 X {l - (nc2- l)/(n2- 1))).28Substitution of the values of p , e,, (4,= 107"),and a typical f value of 0.5537for the sphere packing system yields 120' as Oqe*, which is in good agreement with the experimental value of 119'. In the sphere (radius = R ) monolayer system with the highest packing density, the value of 0.55 means that water penetrates into the pores at the distance of 0.68R from the plane formed by the top of the spheres (see Figure 5E). The cylindrical pore array structure can also be inspected as another model system, in which E is equal to p , Le., f = 1. In this case, the eqe*value is calculated to be 133.0°, being 13.6" greater than the experimental one. As the system whose f value is known is limited, the other models could not be examined; however, it is found that the value of e,,* is strongly dependent on the shape of the porous
-
(30) Cassie, A. B. D.; Baxter, S. Trans. Faraday SOC.1944, 40, 546. (31) Baxter, S.; Cassie, A. B. D. J . Text. Inst. 1945, 36, T67. (32) Although a slow decrease in the value of 8, with time was observed, the decrement is confirmed to be quite small (
