J. phys. Chem. 1982, 86, 3621-3626
3821
Infrared Absorption Spectroscopy of Chemically Modified Tltanlum Dioxldel Harry 0. Finklea' and R. VHhanago chsmkrby W r t m e n t , Vkghla PO&t&nlc
Instnute and State Unhfersity, Blecksbwg, VkginL 24061 (Received: Aprtl23, 1982)
IR spectroscopy has been used to examine a TiOz substrate reacted with methylsilanes (Mec,SiX,, n = 1-4, X = C1, OMe; and hexamethyldisilazane (HMDS)). Ti02powder, in the form of a pressed pellet, is dried and exposed to silane vapor while mounted in a vacuum IR cell. The spectra reveal bonded silanes with coverages on the order of one monolayer. Terminal surface 0-H groups react more facilely than bridging ones. Chemisorption and adsorption of the silanization byproduct is evident. Unreacted Si-X bonds are spectroscopically observable for MeSi(OMeI3and Si(OMe),. Hydrolysis of a modified substrate yields free Si-O-H only for the tetrafunctional silanes, SiC14and Si(OMe),.
Introduction Silanization has been a common method for binding molecules to electrodes.2 The synthesis is based on the reaction between a hydrolyzable Si-X bond and a surface hydroxyl group (eq 1). Because silanes can be attached
I M+
I
0-H
+
X-SI-
I
I
-
'
M+O-?i-
I
t HX
(1)
I
X = C1, OR,NHR to a variety of metal and semiconductor substrates, we are interested in probing the nature of the bound layer. Questions concerning the chemically modified electrode include the structure and the coverage of the silane layer, and any perturbations of the original electrode surface. Previously these questions have been addressed by X-ray photoelectron spectroscopy (XPS)."* XPS allows the detection of an attached molecular layer via a tag element and yields a rough estimate of the coverage. However, a more detailed picture of the chemically modified surface can be obtained by vibrational spectroscopy. The method for obtaining IR spectra of surface functionalities is well established; three books review the field up to 1975."" Provided that the substrate has a "window" in the infrared region, a pellet can be pressed from extremely fine powder and examined in the transmission mode. Chemical modification reactions can be performed in situ by exposing the pellet to reagent vapor. Surprisingly, few reporta exist of IR studies on silanized surfaces, and they are all on silica substrate^.'^-'^ (1) Presented in part at the 182nd National Meeting of the American Chemical Society in New York, Aug 1981, Abstract no. 119, and at the 160th National Meeting of the Electrochemical Society, 01% 1981, Abstract no. 564. (2) Murray, R. W. Acc. Chem. Res. 1980,13, 135-41 and references therein. (3) Moses, P. R.; Wier, L.; Murray, R. W. Anal. Chem. 1975, 47, 1882-6. (4) Mosee, P., R.; Murray, R. W. J.Am. Chem. SOC. 1976,98,7435-6. (5) Elliott, C. M.; Murray, R. W. Anal. Chem. 1976, 48, 1247-54. (6) Untereker, D. F.; Lennox, J. C.;Wier, L.M.; Moaea, P. R.; Murray, R. W. J. Ekctroanal. Chem. 1977,81,30+18. (7) Moses, P. R.; Wier, L. M.; L~MOX, J. C.; Finklea, H. 0.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978,50, 57685. (8) Umana, M.; Denieevich, P.; Rolison, D. R.; Nakahama, S.; Murray, R. W. Anal. Chem. 1981,53,117&5. (9) Kieelev, A. V.; Lygin, V. 1. 'Infrared Spectra of Surface Compounds"; Wiley: New York, 1975. (10) Hair, M. L.'Infrared Spectroscopy in Surface Chemistry";Marcel Dekker: New York, 1967. (11) Little, L. H. "Infrared Spectra of Adsorbed Species"; Academic Press: London, 1966. (12) Reference 9, pp 95, 113-9. (13) Hertl, W. J.Phys. Chem. 1968, 72, 1248-53 and 3993-7.
We have used IR absorption spectroscopy to systematically study the chemical modification of TiOz by silanes.18 The silanes chosen for this study are methylsilanes (Me,_,SiX,, n = 1-4, X = C1, OMe; and hexamethyldisilazane). These silanes are sufficiently volatile for gas-phase modifications. They incorporate three different leaving groups and the possibility of multiple bonding to the substrate. They are representative to the silanes used on chemically modified electrodes. TiOz is an interesting semiconductor most noted for its stability during the photoelectrolysis of water.lg IR studies have been performed on clean surfaces of both the anaand the rutile form.mal*s The surface chemistry of TiOz has been reviewed by Parfitt.26 The effects of silanization on the electrochemistry and photoelectrochemistry of TiOz have been reported.27
Experimental Section Ti02.Ti02 (P-25) was obtained from Degussa Corp. The manufacturer states that it is principally anatase, with a mean particle size of 0.03 pm and a silicon content less than 0.2% by weight. X-ray powder diffraction revealed both anataseand rutile phases with the anatase/rutile ratio being 8515 by weight.% The surface area by the BET method was 51.2 m2/g. The powder was heated to 500 "C in air for 4-6 h to remove carbonaceous impurities. This treatment lowered the surface area to 48.6 m2/g and did not change the anatase/rutile ratio. Typically 60 mg of powder was pressed in a 13-mm die at a pressure of 80-100 MPa to yield a translucent pellet. The pellet was then refired at 500 "C for several hours. The surface area decreased to (14) Hair, M. L.; Hertl, W. J.Phys. Chem. 1969, 73, 2372-8. (15) Hertl, W.; Hair, M. L. J. Phys. Chem. 1971, 75,2181-5. (16) Hsing, H. H.; Fettlemoyer, A. C. Prog. Colloid Polym. Sci. 1976, 61, 54-63. (17) Morrow, B. A.; Hardin, A. H. J. Phys. Chem. 1979,83,3135-41. (18) Finklea, H. 0.; Vithanage, R. ACS Symp. Ser. in press.. (19) Wrighton, M. S.; Ginley, D. 5.;Wolczanski, P. T.; Ellii, A. B.; Morse, D. L.; Linz, A. h o c . Natl. Acad. Sci. U.S.A. 1975, 72, 1518. (20) Yates, D. J. C. J. Phys. Chem. 1961,65,74653. (21) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1216-20. ~ -. ~ _ . (22) Munuera, G.; Rives-Ameau, V.; Saucedo, A. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 736-47. (23) Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971,67, 2669-78. 1971,67,2469-83. (24) Jackson, P.; Parfitt, G. D. Trans. Faraday SOC. (25) Griffiths. D. M.: Rochester. C. H. J.Chem. SOC..Faradav Trans. 1 1977, 73, 151G29. . (26) Parfitt, G. D. Prog. Surf. Membr. Sci. 1976,11, 181-226. (27) Finklea, H. 0.; Murray, R. W. J . Phya. Chem. 1979, 83, 353-9. (28) Primet, M.; Basset, J.; Matthieu, M. W.; Prettre, M. J. Phys. Chem. 1970, 74, 2868-74.
0 1982 American Chemical Society
8622
Finklea and Vithanage
Ths JocKnal of phvsiCel Chemistry, Vd. 88, No. 18, 1982 I
LO00
3000, cm-
I
2000
FI@uro1. I R spectra of (A) a clean TOp pellet after drying under vacwm for 2 h at 150 O C and (B) the same pellet after exposure to O& (1 h room temperatwe, 800 Pa) folkwed by drylng as in A. Bar = 0.1 absorbance unit.
31.5 m2/g, poeeibly because of particle-particle contact and sintering. A slight increase in rutile content (ca. 5 % ) was also observed. Pellet density was 2.4 f 0.1 g/cm3 (rutile 4.26, anatwe 3.84 g/cm3>. The pellet was stored in a humid atmosphere to achieve maximum hydration of the surface. Vacuum Line and IR Cell. All reactions were carried out on a "greaseless" vacuum line containing Teflon stopcocks and Viton O-ring joints. A mechanical and an oil-diffusion pump, both isolated by liquid-nitrogen traps, were capable of sustaining dynamic pressures below 0.13 Pa. The vacuum IR cell consisted of two collinear no. 15 O-ring joints, two CaF2 windows pressed against Teflon O-rings, and clamping platea with four bolts and wing nuts. The pellet was mounted in a Teflon holder with an aperture of 8 mm and inserted into the IR cell. Heating tape wrapped the body of the cell. A Teflon stopcock and third O-ring joint connected the cell to the vacuum line. The IR cell could be sealed, removed from the vacuum line, and inserted into the IR spectrometer. Infrared Spectrometer. All spectra were recorded on a Perkin-Elmer 283B spectrometer. The slit program was set for maximum energy; the half-bandwith was 11cm-' at 4000 cm-', 4 cm-' at 2000 cm-', and 2.5 cm-' at lo00 cm-'. The reference beam was attenuated with an empty vacuum IR cell. Syntheses. Silanes (Petrarch) were distilled and degassed by two freeze-pump-thaw cycles. Before any reaction, the pellet was dried a t 150 "C under vacuum for 2 h and an IR spectrum obtained. The pellet was then exposed to silane vapor in equilibrium with the thermostated liquid phase. Vapor pressures varied from 0.1 to 15 kPa, depending on the silane. After 1h excess vapor was pumped away. For each silane, the following experimental regimes were used, each one on a separate pellet. (1)The pellet was reacted at room temperature with the liquid silane at 0 "C. After a spectrum was obtained, the pellet was heated to 150 "C under vacuum for 2 h and a third spectrum obtained. The pellet was then exposed to humid air for 3 h, dried at 150 "C under vacuum for 2 h, and reexamined. (2) The pellet was reacted at 150 O C with the liquid silane at 0 OC. The remainder of this regimen was identical with the first one. (3) The pellet was reacted at 150 "C with the liquid silane at 15 "C. After a spectrum was obtained, the pellet was exposed to humid air for 3 h and dried, and the final spectrum obtained.
Flguro 2. I R spectra of a pellet reacted with MefiCi, using reglme 1: (A) clean pellet; (B) after exposure to Me,SICi, vapor (5.45 kPa), followed by evacuation at room temperatwe; (C)after evacuating the pellet for 2 h at 150 "C. Eiar = 0.2 absorbance unit on the left side and 0.4 absorbance unit on the right side.
Results and Discussion nozBand Assignments. Figures 1A and 2A show the IR absorption spectrum of a dried TiOz pellet. Three distinct and sharp peaks appear at 3730,3650, and 3420 cm-';theae are assigned to surface 0-H groups. A shoulder and sometimes a peak are found at 3680 cm-'. The peaks sit on a broad absorption envelope attributable to hydrogen-bonded 0-H. A small amount of molecular water is indicated by an absorption band at 1620 cm-'. The Ti02 lattice absorbs strongly below 1200 cm-'. Hydrocarbon contamination is evident from the set of peaks at 3000-2800 cm-'. Exposure of the pellet to D20vapor effects a quantitative exchange of 0-H stretches to 0-D stretches (Figure 1B). The deuterated peak positions are 2750,2690, and 2530 cm-l with the shoulder at 2720 cm-'.The quantitative exchange proves that the 0-H groups are indeed on the surface and not interstitial.= More specific assignments of the various 0-H peaks are lese certain. Pure anatase21exhibits only two major peaks at 3715 and 3665 cm-', while pure rutile21*23-26 exhibits peaks at 3680,3655, and 3410 cm-'. Rutile spectra also have a peak at 3730 cm-' which has been attributed to Si-0-H from silica impurities.*% Since P-25 is a mixture of the two phases, we assign the 3410-cm-' peak and the 3680-cm-' shoulder to the rutile component, the 3730-cm-' peak to the anatase component, and the 3650-cm-' band to overlapping rutile and anatase peaks. The 3730-cm-' peak is more likely to be due to Ti-O-H rather than Si0-H since the silicon content is very low; also, silicon peaks are absent in the XPS spectrum. Griffitha and RochesteP argue that the 0-H peaks can be divided into two types, terminal and bridging. The assumption is made that the particle surfacea are composed predominantly of certain crystallographic planes.2s The preferred planes have high atomic density and maximally coordinated titaniums. For rutile the dominant plane is the (110); for anatase it is the (001). All surface 0-H groups occupy oxygen crystallographic sites which are a natural extension of the lattice. The (110) plane of rutile contains alternate rows of pentacoordinate and tetracoordinate titanium atoms.% When hydroxylated, the pentacoordiite titanium atoms gain single 0-H groups which project perpendicular to the surface,designated as terminal (29)
Harris,L. A,; Schumacher, R. J . Electrochem. SOC.1980, 127,
1186-8.
I R Spectroscopy of Chemically Modified TiO,
The Journal of Physical Chemistty, Vol. 86, No. 18, 1982 3023 I
I
p, ,
4000
W
3000
j
. cm-1
;
, 1800
J
W
, 1500
,
1
1200
Figure 3. I R spectra of pellets reacted with HMDS and Me3SiOMe using regime 1: (A) HMDS (vapor pressure, 130 Pa), bar = 0.2 absorbance unit; (B)MeSSiOMe(vapor pressure, 9.31 kPa), bar = 0.3 absorbance unit.
0-H. The tetracoordinate titaniums acquire 0-H groups which bridge each pair of Ti atoms. Griffiths and Rochester assign the 3655-cm-1 peak to the terminal 0-H group and the 3410-cm-' peak to the bridging 0-H groups. On anatase the most densely packed planes have only crystallographic sites for oxygen atoms bonded to single titaniums; there are no bridging sites. Surface 0-H groups are therefore expected to be terminal. The absence of a peak for anatase in the 3400-345o-cm-' region is consistent with this hypothesis. Evidence for Bonding of Silanes. After a pellet is reacted with dimethyldichlorosilane, new peaks appear in the IR spectrum (Figure 2B). A sharp pair of peaks at 2965 and 2907 cm-l are assigned to the symmetric and antisymmetric c-H stretches of the methyl groups. A methyl bending band is located at 1404 cm-'. The intense peak at 1260 cm-' is due to a Si-C stretching mode. For comparison, a solution of Me&C12 in CC14 produces IR peaks at 2978,2914,1407, and 1260 cm-'. Equivalent peaks are found for all methylsilanes. The spectrum indicates the presence of the dimethylsilyl moiety on the Ti02 surface. That the moiety is bonded, and not adsorbed, to the surface is illustrated by heating the pellet under vacuum (Figure 2C). The IR peaks assigned to dimethylsilyl moiety exhibit little, if any, change. These results are obtained for all of the silanes. Selective Reactivity of Surface 0-H Groups. From eq 1,it is evident that silanization should be accompanied by loas of 0-H peak intensity. This is indeed the observation (Figures 2 and 3). However, in every case, the terminal 0-H peaks (3750-3600 cm-') lose intensity to a far greater extent than the bridging 0-H peak (3410 cm-').% For the chlorosilanes the loss of terminal 0-H peak intensity is generally 100% under all reaction conditions, while the loss of bridging 0-H peak intensity varies between 20% and 60%. The methoxysilanes yield 90 f 10% loas for terminal 0-H groups and 15 f 15% loss for bridging 0-H groups. HMDS gives a 93 f 6% loss of terminal 0-H groups; the bridging 0-H peak overlaps a peak due to adsorbed ammonia (see below). The percent losses cannot be equated directly to the number of Ti-O-Si bonds formed because of side reactions which consume 0-H groups (see below). However, there is a distinct preference for silanes to bond to terminal 0-H groups rather than bridging 0-H groups. On the (110) rutile plane, the terminal 0-H groups project further above the plane than the bridging 0-H groups. We suggest that the selective reactivity is due to steric hindrance. The more accessible 0-H groups react first, and in doing so they screen the bridging 0-H groups from the silane.
I LOO0
I
I
I
I
cm-1
I
I
3000
Flgure 4. I R spectra of pellets exposed to silanization byproducts: (A) HCI (1 h, room temperature, 11.3 kPa), bar = 0.3 absorbance unit; (B) MeOH (1 h, room temperature, 4.79 kPa), bar = 0.2 absorbance unit (C) NH3(1 h, room temperature 660 Pa), bar = 0.1 absorbance unit.
Under most reaction conditions some 0-H peak intensity remains, the exception being SiC14 reacted at high temperature and pressure (regime 3). Consequently, silanized rutile contains unreacted 0-H groups, with one type predominating. AdsorptionlReaction of the Byproduct. During the reaction of a silane with a surface 0-H group, a byproduct molecule is released. The byproduct molecule can adsorb to the surface and react with surface 0-H groups. Chlorosilanes generate HC1, which is itself a potent reagent for sequestering 0-H groups on Ti02.23*24928 At moderate temperature and pressure all 0-H peaks are uniformly attenuated (Figure 4A), a large new peak appears at 3550 cm-', and the broad envelope of hydrogenbonded 0-H increases. These features are apparent on spectra of pellets reacted with chlorosilanes (Figure 2). In addition, a peak at 1600 cm-' develops (not shown). At higher temperatures and pressures the original 0-H peaks are completely removed. We interpret these results according to the reaction
The water remains adsorbed, thus accounting for the band at 1600 cm-'. The peak at 3550 cm-' shifts to 2625 cm-' upon exposure to D20. It is attenuated upon heating under vacuum or upon exposure to water vapor. It is therefore assigned to a terminal 0-H group which is perturbed by hydrogen bonding to adsorbed HC1 or a T i 4 group. The sharpness of the peak suggests a well-defined orientation of the hydrogen bond. The 3550-cm-' peak can be removed by exposing the pellet to water vapor for prolonged periods of time (i.e., days) and then drying. However, the original 0-H bands remain strongly attenuated. There are two possible causes. The Ti-C1 bonds may be very resistant to hydrolysis. Or, surface chloride catalyzes the thermal
3624
The Journal of Physical Chemistry, Vol. 86, No. 18, 1982
dehydroxylation of TiOp23924Heating a pellet previously exposed to HC1 to temperatures of 150 OC may promote the loss of surface 0-H according to eq 3; this reaction does 2(-Ti-OH)
-
-Ti-0-Ti-
+ H20
(3)
not occur at that temperature on clean surfaces. Rehydroxylation of dehydrated surfaces is very slow. From these results we conclude that silanization with chlorosilanes causes strong perturbations of the surface both by bonding of the silyl group and by chemisorption of the HC1 byproduct. Methanol also reacts with Ti02 (Figure 4B). The terminal 0-H peaks are attenuated, but the bridging 0-H peak actually increases slightly. In addition, a multitude of C-H stretching peaks appear. The principal peaks are at 2950,2925, and 2820 cm-', with smaller peaks at 2980, 2890, and 2845 cm-'. The methoxy moiety is indicated by a broad band at 1440-1460 cm-'. The intensity of the C-H peaks increases if the reaction is carried out at 150 "C; these peaks are thermally stable under vacuum. We conclude that terminal 0-H groups form Ti-O-CH3 moieties in a manner analogous to eq 2. Chemisorption of alcohols to bare titanium ions (forming a Lewis acidbase bond) has also been postulated.2e Exposure of the pellet to water vapor for 3 h greatly reduces the C-Hpeaks, but exposure for 2 days is required to regenerate the original 0-H peak spectrum. During all of these manipulations the bridging 0-H peak shows only minor changes in intensity. In contrast, reaction of a pellet with a methogysilane usually c a w a significant decrease (ca 20 f 15%) in the bridging 0-H intensity, but only if the methoxysilane is polyfunctional. Me3SiOMe bonds only to terminal 0-H groups, but the di-, tri-, and tetramethoxy derivatives do bond to the bridging 0-H groups. Ammonia, the byproduct of reaction with HMDS, generates an interesting four-peak pattern (3395,3345,3255, and 3145 cm-') in the N-H stretching region (Figure 4C). The peaks have been assigned to NH3 bonded to two different Lewis acid sites.3o There is a new peak at 1595 cm-l assigned to molecular NH3. The terminal 0-H peaks are again attenuated, while the bridging 0-H peak is obscured. However, the N-H peaks are not thermally stable, nor do they increase in intensity after reaction at elevated temperatures. The formation of a Ti-NH2 species is not indicated. As with methanol, days of exposure to water vapor are required to recover the original TiO, spectrum. The rather strong interactions of the byproducts with the surface is attributed to the absence of a liquid phase. The solvent may serve to remove adsorbed byproducts from the surface. Experiments with liquid-phase silanization under strictly anhydrous conditions have shown that the 3550-cm-' peak is observed for chlorosilanes while adsorbed NH3 is absent from HMDS-modified Ti02.31 Coverage. There are two possible spectroecopic methods for measuring the quantity of silane bonded to the Ti02 surface. The loss of 0-H peak intensity should correspond to the number of Ti-0-Si bonds formed. Given that and the number of surface 0-H groups present, the coverage of the silane can be estimated. Unfortunately, the loss of 0-H groups by other mechanisms (eq 2 and 3) prevents the use of this method. (30) Primet, M.;Pichat, P.; Matthieu, M.W. J. Phys. Chem. 1971, 75, 1221-6. (31) The reactions were carried out in sodium-dried toluene on the loose powder. Silane concentrations were 10% by volume. The powder wae filtered, dried, and premed into a pellet. Martha Gilliam, unpublixhed results.
Finklea and Vithanage
TABLE I:
Coverage for TiO, Reacted with
solu tiona
HMDS
~-
pelletb
-
-
rd h A 2960 140 2956 0.37 1.7 2900 25 2899 0.13 3.3 1406 10 1408 0.03 1.9 1254 205 1251 0.785 2.4 a 10% by volume in CCl,. Reacted according t o regime 3; pellet weight, 0.0669 g ; radius, 0.65 c m ; surface area, 31.5 m 2 / g . L / ( m o l c m ) , per SiMe, moiety. h
EC
pmol/m2.
Alternatively, the intensity of a peak assigned to the silane can be measured. The coverage in pmol/m2 can be estimated from eq 4. A is the measured absorbance, r is r =(103~~?)/(~4 (4) the pellet radius (cm), e is the molar extinction coefficient w is the pellet weight (g), and a is the surface (L/(mol cm)), area (ma/g). Equation 4 assumes that the pellet is a uniform cylinder whose volume can be calculated from its radius and thickness. It ale0 assumes that Beer's law holds for surface species and that the molar extinction coefficient does not change for the particular absorption band when the silane is bound to a surface. These latter assumptions can be questioned for several reasons. The vibrations of a parti& bond are likely to be perturbed when the silane bonds to the surface, thereby affecting the extinction coefficient. Secondly, the polarity of the surface environment is likely to be different from the solutions used to measure extinction coefficients. Finally, previous work has shown that the extinction coefficient can vary with coverage,"" invalidating Beer's law. With these caveats, we give a sample calculation. Table I lists the extinction coefficients of four peaks of HMDS. Each peak corresponds to a vibration in the trimethylsilyl moiety. All four peaks appear in the IR spectrum of a reacted pellet. It is apparent from the variation in calculation 'coverages that the relative intensities of the peaks are changed. The numbers, however, agree reasonably well with the expected value for a monolayer, Le., 4 pmol/m2 based on a close-pacM model. On silica surfaces modified with trimethylcldorosilane, coverages of 4-6 pmol/m2 have been r e p ~ r t e d . ~ , * ~ ~ We have calculated coverages of pellets reacted with all seven methylshes and for all three experimental regimes, using a value of 209 for the extinction coefficient of the Si-C peak. For a given silane, there is no observable diifference in coverage between regimes 2 and 3 (I' = 3.0 f 1 pmol/m2). Since these regimes differ in silane vepor preesure only, vapor pressure does not seem to be a critical factor in coverage. Regime 1, which is run at room temperature, yields a slightly lower coverage (I' = 2.3 f 1 pmol/m2). Comparisons between silanes are tentative because of the uncertain perturbations on the Si-C peak extinction coefficient. However, monomethoxysilane yjelda lower coverages (1.4 f 0.3 pmol/m2) than the rest of the silanes. The insensitivity of the silane coverage to vapor preasure and temperature suggests that the reactions are proceeding to completion for our experimental conditions. Because the Ti02 surface is virtually anhydrous, the silanes are expected to react until a monolayer has formed. The spectroscopic coverages agree with this prediction. Polyfunctional s h e a can form polymeric layers when excess water is present. The absence of polymeric layers dem(32) Gilpin, R. K.;Burke, M. F. Anul. Chem. 1973,45,1383-9. (33) Unger, K.Angew. Chem., Znt. Ed. Engl. 1972,Il, 267.
The Journal of Physlcal Chemistry, Vol. 86, No. 18, 1982 3825
IR Spectroscopy of Chemically Modified TIOP
iui T
I
1800
I
I
'50tom.,
I
I
1
I
1200
5. IR dmerence spectra af a pbbt reacted wlth SYOMe), d n g regime 1; the spectrum of the clean pellet has been subtracted from each spectrum: (A) after r e a m (vapor pressure, 130 Pa); (B) after evacuating the pellet for 2 h at 150 O C ; (C) after exposing the pellet to a humid atmosphere for 3 h, followed by evacuatkm for 2 h at 150 OC. Bar = 0.25 absorbance unit.
onstrates an advantageous characteristic of the vaporphase reaction method. Bonding. Polyfunctional silanes can form one, two, or possibly three bonds to the surface. In general, trifbctional silanes are believed to form an average of two bonds per silane.' If we compare the coverage of a silane monolayer (ca.4 pmol/m2) to the surface 0-H content of dry P-25 Ti02 (13.7F2lo,%and 7.5" pmol/m2), it is appakent that at least two bonds can be formed per silane. Thus, tri- and tetrafunctional silanes might have unreacted Si-X bonds after binding to the surface. In order to detect the presence of unreacted Si-X species, we attempted to observe new absorption bands which are specific to the Si-X moiety. S i 4 1 absorption bands (550-610 cm-') are syiectrwopically inaccessible on TiOz. HMDS has an intense absorption at 1181 cm-' assigned to the Si-N-Si moiety. On pellets reacted with HMDS a new peak appears at 1185-1190 cm-'; however, pellets reacted with ammonia also have a peak at 1180-1185 cm-'. The methoxysilanes have a characteristic peak at 1190-1195 cm-' assigned to the Si-0-C moiety. After Ti02 is reacted with Si(OMeI4, a peak is clearly evident at 1190-1200 cm-' (Figure 5A). Methanol exposure does not generate a peak at that position. The peak is thermally stable (Figure 5B) and is strongly attenuated when the pellet is exposed to water vapor (Figure 5C). Both facta are consistent with the assignment of the new peak to unreacbd S i 4 4 moieties. Pellets reacted with MeSi(OMe), exhibit a peak (11851190 cm-') with similar characteristics, but no such peak is distinguishable on pellets reacted with Me2Si(OMeI2. Unreacted Si-X bonds can hydrolyze to form Si-O-H when the pellet is exposed to water vapor. The silanol is easily observed as a new peak in the absorption spectrum (Figure 6, A and B). Pellets reacted with Sic&develop a peak at 3740 cm-' following hydrolysis, while Si(OMe)4 produces an equivalent peak at 3735 cm-'. Surprisingly, no other silane produces this peak upon hydrolysis. In particular,the silanol peak is absent on pellets reacted with (34)Boehm, H.P.Discus. Faraday SOC.1971, 264-75.
Flgurr 8. I R spectra of pellets reacted with SICI, and MeSICI,: (A) SICI, (regime 2; vapor pressure, 11.O kPa), after evacuation for 2 h at 150 O C ; (B) the same pellet after exposure to humid air, followed by evacuation for 2 h at 150 OC; (C) MeSICI, (regime 2; vapor pressure, 7.2 kPa), fobwed by exposure to humid air for 2.5 days, and then evacuated for 2 h at room temperature. Bar = 0.3 absorbance unit.
the trichloro- and trimethoxymethylsilanes.Since there is evidence for unreaded Si-X moieties with these silanes, wb hypothesize that the newly formed silanols condense with surface 0-H groups or other nearby silanols. In the experimental regimes, exposure to water vapor is followed by a drying step (heating under vacuum), which could promote such a condensation. However, no Si-0-H peak is observed even if the drying step is omitted (Figure 6C). Tb explain this inconsistency, we must postulate facile condensation of the silanol at room temperature, or perhaps hydrogen bonding to the surface. The tetrafunctional silanes exhibit the peak characteristic of free Si-0-H because with three bonds between the silicon and the surface, the remaining S i - b H moiety must be directed away from the surface. Recently, Sindorf and Maciel have reported a ?Si NMR study of silica gel r e a M with chloromethylsilanes.35 The silanizations were carried out with the substrate at 200 "C and the silane vapor pressure at 1 atm; reaction times were 12 h. These conditions are similar to, but somewhat more forcing than,o m . Both the di- and trichlorosilanes yield 'T3i NMR spectra with rpultiple peaks assignable to the methylsilanes. The peaks change in intensity following exposure to air and moisture. From these spectra the authors conclude that both di- and trichlorosilanes contain unreacted Si-C1 species and Si-0-H species following silanization. The inconsistency between this conclusion and our infrared results is currently under investigation. Conclusions. These results illustrate well the advantages of surface vibrational spectroscopies as a probe of silanized surfaces. Structural ihformation can be obtained both on the bound molecular layer and on the original surface moieties. Of particular significance to the field of chemically modified electrodes are (1) the selective silanization of certain types of surface 0-H groups, (2) the absence of free Si-0-H groups for all silanes except the tetrafunctional ones, and (3) the strong and permanent perturbation of the oxide surface by HC1, the byproduct of chlorosilanes. However, certain disadvantages are evident. The pellet method is restricted to powdered substrates with submi(35) Sindorf, D.W.;Maciel, G.E.J. Am. Chem. SOC.1981,103,4263-5.
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J. Phys. Chem. 1982, 86, 3626-3634
cion particle size. The pellets cannot be used as electrodes because interparticle contact is insufficient for good electrical conduction. It would be very useful if vibrational spectroscopies could be routinely applied to normal electrode surfaces. In certain instances, vibrational spectra can be obtained on metal surfaces, but these are restricted to particular metals (surface-enhanced Raman (36)Jeanmairie, D. L.; Van D u p e , R. P. J.Electroanal. Chem. 1977, 84, 1.
(37)McQuillan, A. J.; Hendra, P. J.; Fleischmann, M. J.Electroanal. Chem. 1975,65,933.
or to highly polished flat metals (grazing angle spectroscopy38). Acknowledgment. This work was supported by NASA Langley Research Center under grant no. NAG-1-89. We thank Martha Gilliam for her work on liquid-phase silanization. (38)Bell, A. T., Hair, M. L., Eds. "Vibrational Spectroscopies for Adsorbed Species"; American Chemical Society: Washington, DC, 1980; ACS Symp. Ser. No. 137.
Gas-Phase Photolysis of 2,2-Dimethylbutane, 2,2,3-Trimethylbutane, 2,2,3-Trimethyl-2-silabutane, and 2,2,3,3-Tetramethyl-2-siiabutane at 147 nm Dsryl J. Doyle,' S. K. Tokach,t M. S. Gordon, and R. D. Koob Deperhent of chemistry, North Dekota State Unlverstty, Fargo, North Dakota 58105-5516 (Received: October 75, 1987; I n Final Form: April 19, 1982)
The 147-nm photolyses of 2,2-dimethylbutane, 2,2,3-trimethylbutane, 2,2,3-trimethyl-2-silabutane(isopropyltximethylsilane),and 2,2,3,3-tetramethyl-2-silabutane(tert-butyltrimethylsilane)are reported. In addition, the mercury-sensitized photolyses of i-C4Hio, trimethylsilane, and mixtures of i-C4H10 and trimethylsilane are reported which give disproportionation to combination (D/C) ratios of 2.1 f 0.2 and 0.28 f 0.05 for (CH3)3C + (CH3)3Cand (CH3)3Si+ (CH3I3Si,respectively, and D/C ratios of 1.86 f 0.15 and 0.55 f 0.08 for (CH3)3C + (CH3)3Sito form 2-methyl-2-silapropeneand i-C4H8,respectively. With the completion of this work, several trends and generalizations can be drawn concerning the importance of various processes in linear vs. branched alkanes and alkylsilanes. These conclusions are summarized in this report.
Introduction The gas-phase photolyses of 2,2-dimethylbutane, 2,2,3(isopropyltrimethylbutane, 2,2,3-trimethyl-2-silabutane trimethylsilane), and 2,2,3,3-tetramethyl-2-silabutane (tert-butyltrimethylsilane) complete two sequences of compounds, one alkane and one alkylsilane, in which three primary hydrogens of neopentane and tetramethylsilane are sequentially replaced with methyl groups (neopentane,' 2,2-dimethylbutane, 2,2,3-trimethylbutane, and 2,2,3,3tetramethylbutane2 for the alkane series and tetra2,2-dimethyl-2-silabutane (ethyltrimethylmethyl~ilane,~ ~ i l a n e ) 2,2,3-trimethyl-2-silabutane, ,~ and 2,2,3,3-tetramethyl-2-silab~tane~*~ for the alkylsilane series). 2,2-Dimethylbutane (22DMB) is the hydrocarbon analogue of recently studied 2,2-dimethyl-2-silabutanelabutane.q While highly branched, 22DMB still contains two geminal secondary hydrogens as does the extensively studied propane molecules and thus provides an opportunity to compare and contrast the influences governing the photodissociation processes in linear vs. branched hydrocarbons in the same molecule. With the exception of the photolysis of isobutane117there are few examples of photolysis of compounds which have tertiary hydrogens. Isopropyltrimethylsilane and 2,2,3trimethylbutane are examples of alkylsilane and alkane which contain tertiary hydrogen. The results from the photolyses of these two compounds, with emphasis placed on the role of tertiary hydrogens, along with the results from tert-butyltrimethylsilane are compared to previously 'Current address: 3 M Center, St. Paul, MN 55144. OO22-3654/82/2O66-362680 7.2510
reported alkanes and alkylsilanes.
Experimental Section Isopropyltrimethylsilane (IPTMS),* tert-butyltrimethylsilane (t-BTMS)? prepared by standard methods, and 2,2,3-trimethylbutane (223TMB), purchased from (1)R. E.Rebbert, S. G. Lias, and P. Ausloos, J.Photochem., 4,121 (1975). (2)(a) P. Boudjouk and R. D. Koob, J. Am. Chem. Soc., 97,6595 (1975);(b) S. K. Tokach and R. D. Koob J. Phys. Chem., 84,6(1980). (3)(a) S.K. Tokach and R. D. Koob, J.Phys. Chem., 83,774(1979); (b) L. Gammie, C. Sandorfy, and 0. P. Strausz, ibid.,83,3075(1979); (c) E. Bastian, P. Potzinger, A. Ritter, H. P. Schuchman, C. von Sonntag, and G. Weddle, Ber Bumenges. Phys. Chem., 84,56 (1980). (4)D. J. Doyle and R. D. Koob, J. Phys. Chem., 85, 2278 (1981). (5)S. K.Tokach, Ph.D. Thesis, North Dakota State University, Fargo, ND, 1979. (6) (a) H. Okabe and J. R.McNesby, J.Chem. Phys., 37,1340 (1962); (b) P. Ausloos, S. G. Lies, and I. B. Sandoval, Discuss. Faraday Soc., 36, 66 (1963);(c) P. Aualocs, R. E. Rebbert, and S. G. Lias, J. Photochem., 2,267(1973/74);(d) J. H. Vorachek and R. D. Koob, Can. J. Chem., 51, 344 (1973);(e) K. Obi, A. Akimoto, Y.Ogata, and I. Tamaka, J. Chem. Phys., 55, 3822 (1971); (f) J. H. Vorachek and R. D. Koob, J. Phys. Chem., 74,4455(1970); (9) J. R. McNesby, Actions Chem. Biol.Radiat., 9,59 (1966);(h) A. H.Laufer and J. R. McNesby, J.Phys. Chem., 70, 4094 (1966);(i) P. Auslocs and S. G. Lias, J. Chem. Phys., 44,521(1966); (j) J. H.Vorachek, Ph.D. Thesis, North Dakota State University Fargo, ND, 1971. (7)H. Okabe and D. A. Becker, J. Am. Chem. Soc., 84,4004 (1962). (8)lsopropyltrimethyleie was synthesized by converting ieopropyl bromide to the correaponding Grignard compound which was then added to trimethylchclorosilane. (9)(a) tert-Butyltrimethylsilane was prepared by the reaction of methylmagneaium bromide on tert-butyldimethylchlorosilane(for use in ref 5 ) . (b) tert-Butyltrimethylsilane was prepared by reacting tert-butyldimethylchlosilane with methyllithium (for use in ref 12).
0 1982 American Chemical Society
