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Self-Assembled Monolayers Supported on TiO2: Comparison of C18H37SiX3 (X ) H, Cl, OCH3), C18H37Si(CH3)2Cl, and C18H37PO(OH)2 Roy Helmy and Alexander Y. Fadeev* Department of Chemistry and Biochemistry, Seton Hall University, South Orange, New Jersey 07079 Received July 16, 2002. In Final Form: September 9, 2002 The solution-phase reactions of octadecylsilanes with different headgroups (C18H37SiH3, C18H37Si(OCH3)3, C18H37SiCl3, and C18H37Si(CH3)2Cl) and of octadecylphosphonic acid (C18H37PO3H2) with titanium dioxide (anatase) were investigated. Chemical analysis and FTIR suggested that all the reactions, with the exception of that of C18H37Si(CH3)2Cl, yielded closely packed self-assembled monolayers (SAMs). SAMs were characterized with high grafting density (∼4.3-4.8 octadecyl groups/nm2) and with high degree of ordering of alkyl chains. Reaction of C18H37Si(CH3)2Cl yielded less ordered surfaces with grafting density ∼1.5 group/nm2. The kinetics plots were similar for all the reactions and illustrated two distinct regions, a rapid attachment followed by a slow growth of the grafting density. The uptake curves were adequately described by the first-order kinetics with two rate constants that differed from each other by 1-2 orders of magnitude. According to the rate constants, the following range of reactivity was established: C18H37SiCl3 . C18H37PO(OH)2 > C18H37Si(CH3)2Cl > C18H37Si(OCH3)3 > C18H37SiH3. Two distinct types of the SAM growth, uniform and islandlike ones, were proposed on the basis of the FTIR study of the SAMs at submonolayer coverage. Silanes capable of cross-linking (C18H37SiX3, X ) H, Cl, OCH3) gave SAMs with a high degree of ordering at relatively low surface coverage, suggesting nonuniform (islandlike) film growth. For SAMs of C18H37Si(CH3)2Cl and C18H37PO(OH)2, the order gradually improved with coverage and highly ordered SAMs were obtained only for high surface coverage, arguing for the uniform mechanism of the film growth. The thermal stability of the supported monolayers was characterized by TGA. All the SAMs showed good thermal and oxidative stability, and no mass loss was observed below ∼200 °C in air. The temperatures of the maximum mass loss rate were close for all SAMs (∼300 °C).
Introduction Selective surface modification techniques have been the focus of research and technology for a variety of applications, including sorption and separation media, wetting and adhesion, lubricants, pigments, sensors, and optical and electronic devices.1-3 Functional organosilanes of the general formula RnSiX4-n (n ) 0-3), where X is a readily hydrolyzable group (most often chloro or alkoxy), are widely used as effective surface modifying agents for different substrates. Covalent surface modification of titanium dioxide is of great interest in view of its importance in medical implants, catalysis and photocatalysis, polymer fillers, and so forth. Several surface modification techniques for titanium dioxide have been reported. Reactions of monofunctional silanes (R3SiX),4 tri- and tetrafunctional silane coupling agents,5,6 and various functionalized7-11 RSi(OEt)3 with (1) Leyden, D. E., Ed. Silanes, Surfaces, and Interfaces; Gordon and Breach: New York, 1986. (2) Pesek, J. J., Leigh, I. E., Eds. Chemically Modified Surfaces; Royal Society of Chemistry: Cambridge 1994. (3) Plueddemann, E. P. Silane coupling agents, 2nd ed.; Plenum: New York, 1991. Mittal, K. L., Ed. Silane and other coupling agents; Utrecht: VSP, 1992. (4) Amati, D.; Kova`ts, E. sz. Langmuir 1988, 4, 329. (5) Gamble, L.; Jung, L. S.; Campbell, C. T. Langmuir 1995, 11, 4505. (6) Gamble, L.; Henderson, M. A.; Campbell, C. T. J. Phys. Chem. B 1998, 102, 4536. (7) Moses, R. P.; Wier, L. M.; Lennox, J. C.; Finklea, H. O.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978, 50, 576. (8) Finklea, H. O.; Murray, R. W. J. Phys. Chem. 1979, 83, 353. (9) Tsubokawa, N.; Kogure, A. J. Polym. Sci. 1991, 29, 697. (10) Mahon, M.; Wulser, K. W.; Langell, M. A. Langmuir 1991, 7, 486.
the surface of oxidized titanium and titanium dioxide have been published. Chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane on the surface of titania and the subsequent modification of the methylsiloxane monolayers was described in refs 12-15. Recently, the preparation and properties of SAMs of organosilicon hydrides (RSiH3)16-18 and of organophosphonic acids (RPO(OH)2)19,20 on the surfaces of titania and other metal oxides were described. Although different synthetic approaches for the surface modification of titania are available, the comparison of the results is difficult because of different substrates, different reaction conditions, and different characterization techniques used. In the present work we report the comparative study of the reactions of octadecylsilanes with different headgroups and of octadecyl phosphonic acid for the surface modification of titanium dioxide (anatase). The work is focused on the structure, kinetics, and mechanism of the growth of the monolayers derived from different surface coupling agents on TiO2. (11) Xiao, S.-J.; Textor, M.; Spencer, N. D. Langmuir 1998, 14, 5507. (12) Tada, H.; Tanaka, M. Thin Solid Films 1996, 281-2, 404. (13) Tada, H.; Nakamura, K.; Nagayama, H. J. Phys. Chem. 1994, 98, 12452. (14) Tada, H. Langmuir 1995, 11, 3281. (15) Tada, H. Langmuir 1996, 12, 966. (16) Fadeev, A. Y.; McCarthy, T. J. J. Am. Chem. Soc. 1999, 121, 12184. (17) Shafi, K. V. P. M.; Ulman, A.; Yan, X.; Yang, N.-L.; Himmelhaus, M.; Grunze, M. Langmuir 2001, 17, 1726. (18) Fadeev, A. Y.; Helmy, R.; Marcinko, S. Langmuir 2002, 18, 7521. (19) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (20) Gawalt, E. S.; Avaltroni, M. J.; Koch, N.; Schwartz, J. Langmuir 2001, 17, 5736.
10.1021/la0262506 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002
SAMs Supported on TiO2
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Experimental Section
Chart 1. Octadecyl Surface Modifying Agents Studied
General Information. Solvents (HPLC quality) were purchased from Aldrich and Fisher. Octadecylsilane and octadecyltrimethoxysilane were purchased from Gelest (Tullytown, PA). Octadecyltrichlorosilane, octadecyldimethylchlorosilane, and octadecylphosphonic acid were purchased from Alfa Aesar (Ward Hill, MA). Titanium dioxide (anatase) with an average particle size of 2-30 µm and surface area (BET) of 10 ( 1 m2/g was purchased from Aldrich. Surface area measurements were performed with the Coulter 100CX using the low-temperature nitrogen adsorption. Surface area was calculated assuming the cross section for nitrogen a(N2) ) 13.5 Å2. This value has been demonstrated by Jelinek and Kova´ts21 to give more realistic surface areas for nonporous oxides than the traditional value of a(N2) ) 16.2 Å2. Use of a(N2) ) 16.2 Å2 will result in a ∼20% increase in the surface area and in a ∼20% reduction of the grafting densities calculated using eq 1 (see below). Infrared spectra were obtained with a Perkin-Elmer Spectrum One FTIR instrument with a DTGS detector. Spectra were collected in reflectance mode using a Harrick Seagull accessory (Harrick, Ossining, NY); spectra were taken at a 45° angle of incidence, with 100 scans and the resolution 4 cm-1. Thermogravimetric studies were performed with TA Instrument’s HiRes 2950 thermogravimetric analyzer. The temperature range was from room temperature up to 800 °C at a rate of 10°/min. Chemical analysis was performed by Schwarzkopf Microanalytical Lab (Woodside, NY) using a Perkin-Elmer 2400 CHN analyzer (ASTM method). Grafting density (F, group/nm2) of the monolayers was calculated using the formula22
C18H37SiH3 (1) C18H37SiCl3 (2) C18H37Si(OCH3)3 (3) C18H37Si(CH3)2Cl (4) C18H37P(O)(OH)2 (5)
F)
6 × 105(%C) 1 [1200nC - MW(%C)] S(BET)
(1)
MW is the molecular weight of the bonded group, nC is the number of carbon atoms in the grafted molecule, %C is the weight carbon percentage in the sample, and S(BET) is the N2 surface area of the bare oxide (m2/g). Surface Modification. Titanium dioxide (0.5 g) was placed in a GC vial and dried at 120 °C in an oven overnight. Ten milliliters of the solution of C18H37SiX3 (X ) H, Cl, OCH3), C18H37Si(CH3)2Cl, or C18H37PO(OH)2 in toluene was injected into the vial using a syringe. The solutions contained 25 µmol of modifier per each square meter of the titanium dioxide, which corresponded approximately to a 3-fold excess in respect to the complete monolayer coverage of the surface with alkyl groups (∼5 group/nm2). Upon addition of the solution, the reaction vessels were left at room temperature. After a given time (1-240 h), the reactions were quenched by filtering on a Bu¨chner funnel with a fritted disk. The samples were subsequently washed with reagent grade toluene, acetone, water-acetone (1:1), and acetone, then dried on the filter to a dry state, and then dried in an oven at 60 °C overnight.
Results and Discussion Synthesis of SAMs. The solution-phase reactions of a series of octadecylsilanes and octadecylphosphonic acid with a titanium dioxide (anatase) surface have been investigated. The structures of the modifying agents used are given in Chart 1. The reagents in Chart 1 contain the same “tail” group (octadecyl) and vary by “head” groups. By varying the headgroup, we intended to investigate its role in the kinetics and mechanisms of self-assembly and on the structure of the SAMs supported on TiO2. Surface reactions were monitored by FTIR. IR spectra for all the reactions were consistent with the attachment of octadecylsiloxy (for silanes) and octadecylphosphonic groups (for phosphonic acid) to the surface. Assignment of the major peaks observed in the IR spectra of SAMs is (21) Jelinek, L.; Kova´ts, E. sz. Langmuir 1994, 10, 4225. (22) Unger, K. K. Porous silica, its properties and use as support in column liquid chromatography; J. Chromatogr. Library, v.16; Elsevier: Amsterdam, 1979; Chapter 2.
Table 1. Major IR Vibrations Observed for the C18 SAMs Supported on TiO2 peak frequency (cm-1) in the SAM prepared from vibration C-H
Si-O
νa(CH2) νs(CH2) δ(CH2) ν(Si-CH2) δ and ν(Si-CH3) ν(Si-O)
P-O
ν(PO3)
Si-C
1
2
3
4
5
2917 2849 1467 1176
2917 2848 1466 1180
2916 2849 1467 1178
2917 2848 1469
1128, 1040
1124, 1020
1132, 1028
2921 2852 1468 1182 1264, 820 1060
1155, 1075
Table 2. Grafting Densities and Selected IR Vibrations for Different C18 SAMs Supported on TiO2a C18 modifying agent
%C
grafting density (groups/nm2)
vA(CH2) (cm-1)
1 2 3 4 5
1.69 1.66 1.51 1.48 0.58
4.84 ( 0.1 4.75 ( 0.1 4.30 ( 0.1 4.26 ( 0.1 1.48 ( 0.2
2917 ( 1 2917 ( 1 2916 ( 1 2917 ( 1 2921 ( 1
a Data for the end points on the kinetics curves. Reactions are done at room temperature.
given in Table 1. We note that the samples were thoroughly washed prior to the analysis and no evidence of physisorbed (not grafted) molecules on the surface was found in the spectra. Grafting Density and Order in SAMs. Table 2 presents the grafting densities for the SAMs prepared, as determined from the carbon analysis. As one can see from Table 2, grafting densities for all SAMs, except for C18H37Si(CH3)2Cl, are high and close to ∼4.5-5 group/nm2, the value reported for the best SAMs of octadecyltrichlorosilanes on silica,23 SAMs of octadecylthiol on gold,24 and Langmuir-Blodgett monolayers of fatty acids and longchained alcohols.25 We note that SAMs of C18H37SiCl3 and C18H37SiH3 are somewhat more closely packed than SAMs of C18H37Si(OCH3)3. This agrees well with the results published previously for SAMs of alkylsilanes supported on silica.26 Grafting density for the SAM of octadecylphosphonic acid is in excellent agreement with the value reported for the closely packed monolayers of alkylphosphonic acids on titania and zirconia (∼4.2 group/nm2). Thus, the results of Table 2 suggest formation of wellpacked organic SAMs upon the reactions of C18H37SiX3 (X ) H, Cl, OCH3) and C18H37PO(OH)2 with titanium dioxide. Order in the SAMs can be assessed from the position of the CH2-stretching. As shown in previous studies,28,29 (23) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (24) Bain, C. D.; et al. J. Am. Chem. Soc. 1989, 111, 321. (25) Baker, H. R.; Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1952, 56, 405. Levine, O.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1069. (26) Bierbaum, K.; Kinzler, M.; Woll, C.; Grunze, M.; Hahner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (27) Randon, J.; Blanc, P.; Paterson, R. J. Membr. Sci. 1995, 98, 119. (28) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. Parikh, A. N.; Leidberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996.
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Figure 1. Kinetics plots of the beginning stages of the reactions of C18H37SiCl3 ([), C18H37PO(OH)2 (2), C18H37Si(OCH3)3 (O), and C18H37SiH3 (]) with TiO2 at 24 °C.
the frequency of the CH2-stretching is characteristic of the order in SAMs of long-chained alkyls. For completely disordered structures, the frequency of the CH2-stretching is close to that of a liquid alkane (νa ∼ 2924 cm-1). For well-ordered SAMs, the frequency is shifted to lower wavenumbers and is close to that of a crystalline alkane30 (νa ∼ 2915-2918 cm-1). The data of Table 2 show that, for the SAMs of C18H37SiX3 and C18H37PO(OH)2, νa(CH2) is ∼2916-2917 cm-1. This indicates SAMs with a high degree of order, which is comparable to that observed for the best quality C18 SAMs reported in the literature.27-29 Surfaces prepared from C18H37Si(CH3)2Cl were less ordered and showed substantially lower grafting density as compared to that of the monolayers of C18H37SiX3 (Table 2). This is consistent with the well-known fact established for the monolayers supported on silica. Because of two methyl groups at the silicon atom, alkyldimethylchloroslanes cannot form closely packed structures on the surface (max. grafting density ∼ 2.4-3 group/nm2)51,52 and are more loosely packed and less ordered than SAMs of alkyltrichlorosilanes.31-33 Kinetics of Surface Modification: Effect of the Headgroup. Kinetics studies were performed for the solution-phase reactions of the compounds from Chart 1 with titanium dioxide at room temperature. This portion of the study investigates the effect of the headgroup on the rates of SAMs formation. Figure 1 presents an overlay of the kinetic plots for the reactions of different C18 modifiers with titania. Surface coverage in the SAMs (grafting density) was determined from the chemical analysis data (%C). As one can see from Figure 1, the reaction process is characterized by two distinct phases. The initial process is a relatively rapid growth in grafting density followed by a rather slow period, during which surface coverage approaches its final value. Depending on the headgroup of the modifier, the time needed to level off and to achieve the maximal grafting density may vary from ∼24 h (C18H37SiCl3) up to ∼240 h (C18H37SiH3). The reactions were considered complete when grafting density showed no further increase with the reaction time. The uptake plots in Figure 1 cannot be adequately described by any single rate constant process. We found that a good fit can be obtained with a linear (29) Kojio, K.; Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1998, 14, 973. Britt, D. W.; Hlady, V. Langmuir 1999, 15, 1770. (30) Snyder, R. G.; Straus, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (31) Horr, T. J.; Ralston, J.; Smart, R. St. C. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 97, 183. (32) Rye, R. R.; Nelson, G. C.; Dugger, M. T. Langmuir 1997, 13, 2965. (33) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759.
Figure 2. Best curve fit obtained with two exponential functions (solid line; eq 2 in the text). Individual functions are shown by dotted and dashed lines. The data points are for the reaction of C18H37Si(OCH3)3 with TiO2. Table 3. Kinetic Parameters for the Formation of C18 SAMs on TiO2 best fit parameters of eq.2 octadecyl modifier
k1 (L/mol‚h)
k2 (L/mol‚h)
1 2 3 4 5
1562 345 172 85 19.5
20.8 2.2 1.1 0.47 0.37
combination of two exponential functions:
θ ) 1 - Re-k1Ct - βe-k2Ct θ0
(2)
R+β)1 θ is the grafting density at time t, θ0 is the maximum grafting density, and C is the concentration of the modifier in solution. Equation 2 is a modification of the Langmuir adsorption kinetics, which is widely used to describe kinetics of monolayer growth.34 The parameters k1 and k2 of eq 2 were obtained through a curve fitting procedure (MathCad 8.0) and are presented in Table 3. The best-fit curves for selected reactions are shown in Figure 2 by solid lines. The data in Table 3 show that, for all of the reactions studied, the parameters k1 and k2 differ from one another by 1-2 orders of magnitude. We note that the results obtained are somewhat typical for the self-assembly on the surface. Kinetic behavior similar to that shown in Figure 1 has been reported previously for SAMs of alkylthiols on gold24 and alkyl alcohols on chlorinated silica.50 This kinetic behavior can be rationalized as follows. Initial growth is due to the reaction of a modifier with the surface when all reaction sites are available (k1). This process is effectively described by the “fast exponent” alone (Figure 2). As the surface coverage reaches ∼70% of its maximum, further growth is described entirely by the “slow exponent” (Figure 2). The slowing of the process may be caused by several factors, for example, slow diffusion of new coming molecules to the surface through the layer of molecules that are already grafted. Physisorbed molecules of solvent and of a modifier may also block the reactive surface sites. (34) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151.
SAMs Supported on TiO2
Langmuir, Vol. 18, No. 23, 2002 8927 Scheme 1. Two Types of SAM Growth on TiO2
Figure 3. Plots of the CH2-stretching vs surface coverage for different SAMs.
The effect of the headgroup of a modifier on the reaction kinetics can be illustrated using the data of Table 3. It is noted that the rate constants differ significantly (by several orders of magnitude) depending on the headgroup. In accord with the constants k1, the following range of reactivity was established: C18H37SiCl3 . C18H37PO(OH)2 > C18H37Si(CH3)2Cl > C18H37Si(OCH3)3 > C18H37SiH3. For the silanes this range is consistent with the activity of the Si-X group in the substitution reactions, for example, susceptibility of silanes to hydrolysis with water.35,36 Speculations on the mechanisms of the selfassembly are given further in the text. SAMs Growth and Ordering: Uniform versus Islandlike Growth. Study of the ordering in the SAMs at submonolayer coverage provides insight into the mechanism of the SAM growth. By plotting the frequency of the CH2-stretching (parameter of the order of the monolayer) versus surface coverage, one can assess the information about the mechanism of the film growth.37 The results presented in Figure 3 show two types of dependences observed. For the reactions of C18H37SiX3 (X ) H, Cl, OCH3), SAMs with relatively high degree of ordering (va(CH2) ∼ 2920 cm-1) are obtained for the (35) Tiertykh, V. A.; Belyakova, L. A. Chemical Reactions with Participation of Silica Surface; Naukova Dumka: Kyev, 1991. (36) Siloxane Polymers. Clarson, S., Semylen, J. A., Eds.; PTR Prentice Hall: Upper Saddle River, New Jersey, 1993. (37) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054.
intermediate surface coverage at 40-50% of the maximum (Figure 3). This is arguing for the formation of highly ordered aggregates of grafted molecules at the early stages of the reaction. As surface coverage approaches its maximum value, SAMs undergo an “ordering transition”, which appears as a sharp decrease of the CH2-stretching down to 2916-2917 cm-1. For the reactions of C18H37PO(OH)2 and C18H37Si(CH3)2Cl, monolayers are completely disordered up until ∼50% surface coverage with the CH2-stretching being similar to that of alkyls in a liquid state. The early stages of the reactions can be pictured as isolated grafted molecules randomly distributed on the substrate. As the surface coverage increases, the order in the monolayers gradually increases (va(CH2) decreases), approaching the final highly ordered state (Figure 3). The difference in the film growth for C18H37SiX3 (X ) H, Cl, OCH3) and C18H37Si(CH3)2Cl and C18H37PO(OH)2 can be rationalized in view of the functionality of the reacting molecules. It is noted that only C18H37SiX3 are capable of cross-linking (after hydrolysis with adsorbed water), which is believed to cause an aggregation at the early stages of the SAM formation (Scheme 1). The islandlike mechanism of the film growth has been reported for the SAMs of C18H37SiCl3 on silica by IR,37 NEXAFS,26 and AFM studies.26,38-41 It has been proposed in the literature42-48 that the reactions of RSiCl3 and RSi(OAlk)3 with hydrated silica proceed through the hydrolysis of silane with surface water followed by the assembly of trisilanols into SAMs. Formation of a SAM is believed to be driven primarily by “lateral” interactions between (38) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (39) Huang, J. Y.; Song, K. J.; Lagoutchev, A.; Yang, P. K.; Chuang, T. J. Langmuir 1997, 13, 58. (40) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102 (23), 4441. (41) Goldmann, M.; Davidovits, J. V.; Silberzan, P. Thin Solid Films 1998, 329, 166. (42) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (43) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (44) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (45) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (46) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (47) Parikh, A. N.; Leidberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996. (48) Britt, D. W.; Hlady, V. J. Colloid Interface Sci. 1996, 178, 775.
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Figure 4. TGA of bare TiO2 (1) and TGA and differential TGA graphs for TiO2 reacted with C18H37SiCl3 (2 and 2′), C18H37Si(OCH3)3 (3 and 3′), and C18H37SiH3 (4 and 4′).
molecules (Si-O-Si bonds, hydrogen bonding, and van der Waals interactions between alkyl chains) rather than by “vertical” bonding with the surface groups. On the basis of the kinetics results presented above, it is suggested that a similar mechanism of self-assembly is realized for the reactions of RSiX3 with TiO2. (We note that for RSiH3 there is an alternative reaction path that includes surface catalyzed oxidation of RSiH3 to RSi(OH)3 with oxygen. This mechanism is, presumably, dominating over hydrolysis for the reactions with dehydrated surfaces. Since the reactions reported here were performed with hydrous titanium dioxide, we believe hydrolysis of RSiH3 is a principal reaction preceding formation of SAM. The reactions of RSiH3 with MO2 surfaces are discussed in more detail in ref 18.) In the cases of C18H37Si(CH3)2Cl and C18H37PO(OH)2, each molecule in the monolayer is bound (covalently) to the surface, forming covalently attached monolayers (CAMs). The strong bonding in these systems distinguishes them from SAMs prepared from RSiX3, which are strongly bonded with neighbors and to a small extent with the surface.47,49 For CAMs the film growth is random and no aggregation is observed as the (49) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (50) Major, R. C.; Zhu, X.-Y. Langmuir 2001, 17, 5576. (51) Boksanyi, L.; Liardon, O.; Kova´ts, E. sz. Adv. Colloid Interface Sci. 1976, 6, 95. (52) Fadeev, A. Y.; Lisichkin, G. V. Structure and Molecular Organization of Bonded Layers of Chemically Modified Silicas. In Adsorption on New and Modified Inorganic Sorbents. Dabrowsky, A., Tyertykh, V. A., Eds.; Studies in Surface Science and Catalysis Series, vol. 99; Elsevier: Amsterdam, 1995; pp 191-246.
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adsorbate-adsorbate (van der Waals) interactions are less important than the adsorbate-substrate (covalent bonding) interactions. Thermal and Oxidative Stability of SAMs. Figure 4 presents an overlay of the TGA and DTGA curves of different C18 SAMs supported on titanium dioxide. The most evident feature in Figure 4 is the weight loss in the temperature region from ∼200 to ∼500 °C, which is not present for the bare titania. Obviously, this weight loss should be assigned to the degradation of the surface grafted organic species. It is noted that the Tmax values of the DTGA curves for different SAMs are very close to each other, arguing that the decomposition of the monolayers is independent of the nature of the anchoring group. In air, supported SAMs show no weight loss up to ∼200 °C, and in nitrogen, no weight loss is observed until ∼300 °C. In more detail, the thermal degradation of the TiO2 supported SAMs of organosilanes is discussed in our previous work.18 As it was suggested,18 the degradation of the SAMs proceeds by oxidative destruction of the organic groups and yields silica-like surfaces supported on TiO2. Conclusion We have investigated the reactions of a series of octadecylsilanes and of octadecylphosphonic acid with the surface of titanium dioxide. Trifunctional silanes (C18H37SiX3) and octadecylphosphonic acid (C18H37PO(OH)2) gave SAMs of similar quality with close packing and a high degree of ordering of the octadecyl chains. Reaction of C18H37Si(CH3)2Cl yielded less ordered surfaces with grafting density ∼ 1.5 group/nm2. The rates of the monolayers’ formation showed strong dependence of the nature of the headgroup of a coupling agent. On the basis of the kinetics measurements, the following range of reactivity was established: C18H37SiCl3 . C18H37PO(OH)2 > C18H37Si(CH3)2Cl > C18H37Si(OCH3)3 > C18H37SiH3. FTIR study of SAMs at the submonolayer coverage suggested two different mechanisms of the monolayers growth. An islandlike mechanism of the film growth was suggested for silanes capable of cross-linking, for example, C18H37SiX3 (X ) H, Cl, OCH3). A uniform mechanism of the film growth was suggested for SAMs of C18H37Si(CH3)2Cl and C18H37PO(OH)2. Acknowledgment. The use of the TA instrument supported by a grant from the New Jersey Commission of High Education is acknowledged. The authors also acknowledge Merck, Inc. for support. LA0262506
