Langmuir 1995,11, 4505-4514
4505
Interaction of Silane Coupling Agents with the TiOs(110) Surface Lara Gamble, Linda S. Jung, and C. T. Campbell* Department of Chemistry, BG-10, University of Washington, Seattle, Washington 98195 Received January 19, 1995@ The interactions of prototypical silane coupling agents with a clean and hydroxyl-predosed TiOz(110) single-crystalsurfacehave been studied using surface spectroscopictechniques. The adsorption and reactions of vinyltriethoxysilane (VTES), diethyldiethoxysilane (DEDS), and (aminopropy1)triethoxysilane(AF’S) were studied by means of temperature programmed desorption (TPD)andX-rayphotoelectron spectroscopy (XPS). Even in the absence of surface water or hydroxyls, VTES dissociates rapidly to form Si(0Et)(CH=CH2), and two OEt,, where Et = CzH5 and “s” refers to surface species. The DEDS similarly dissociates to produce a mixture of s i E t ~ ,EtOSiEtz,,, ~, and one to two OEt,. Both of these silicon moieties are bound to the surface via Si-0-Ti bonds. Any EtO- ligands, whether bound to Ti cations or to the adsorbed silanes, decompose at -650 Kvia a-hydride elimination to create ethylene gas and a surface-boundhydrogen (OH,). This hydrogen further reacts with a second EtO- ligand to produce ethanol gas. Above 700 K, the ethyl ligands left on the adsorbed silane produced from DEDS decompose via p-hydride elimination to give ethylene gas. Similarly, the CH2=CH- ligands left on the adsorbed silane produced from VTES also decomposes through P-hydride elimination into acetylene gas and surface hydrogen, but they hydrogenate to give ethylene much more rapidly. When hydroxyls are predosed on the surface, they react with any Ti4+-boundEtO- ligands formed in the initial decomposition of VTES and DEDS t o produce ethanol gas at -350 K. Thus, the main function of the hydroxyls is to clear Ti4+sites of ethoxy groups, not to act as sites for the initial attachment of the silane to the surface. Neither TPD nor X P S data indicated that APS dissociated to a measurable extent on the TiOz(110)surface.
Introduction
chemistryofethyl- and diethylsilane on Si(11117x 77,8and trimethoxysilane on MgO(100) have also been investigated.g Here we examine the reaction of various functionalized ethoxysilanes with TiOz(110) using surface sensitive techniques including temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Specifically, we have studied reactions of gaseous vinyltriethoxysilane (VTES), diethyldiethoxysilane (DEDS), and (aminopropy1)triethoxysilane(APS)with a clean TiO2(110) single crystal surface. VTES has applications in the adhesion of vinyl polymers to oxide surfaces, and alkylsilanes like DEDS are used to render oxide surfaces hydrophobic. A surface that is functionalizedwith amine groups, as in APS,is useful for attaching a range of organic functional groups including amino acids and many biomolecules. Because the “classical” mechanism for silane-surface bond formation on oxides involves surface -OH groups, we also study here the coadsorption of VTES, DEDS, and APS with water on TiO~(110).The water is used to produce surface hydroxyls and to verify if they are needed to bond these silanes to TiOz. Thus our previous studyloof water on TiOz(110)is relevant background here. Water adsorbs partially molecularly and partially dissociatively on TiO2(110). It desorbs in TPD with a peak at 260 K, with a tail that extends to -350 K. The saturation ofthis peak, before the formation of a multilayer-like water TPD peak a t 110
Organofunctional silanes are used in a variety of fashions to modify surfaces. They are referred to as silane coupling agents (SCA)since they act as a bridge between organic and inorganic layers or phases. Functionalized silane coupling agents (SCA) generally take the form of R,S&-, where X is a hydrolyzable leaving group, which provides a site for attaching Si to the surface. Silane coupling agents readily react with water to form Si-OH bonds and HX. A silane-surface bond is formed by condensation of surface hydroxyls with these Si-OH bonds, or directly with a n Si-X b0nd.l Functional groups (R)can be chosen such that the silane coupling agents will act to (a) form a protective layer coating, (b) chemically modify electrodes or chromatographic columns, or (c)immobilize large functional groups such as organometallic complexes and biomolecules,2to give just a few examples. Strong silane-surface bonds are important for a robust coupling layer, whether it is used to bond a polymer to a n oxide surface for materials applications or to add a receptor to a sensor surface for analytical applications. Organosilane-surface interactions have been the subject of many previous studies. However, relatively little is understood about the interactions of functionalized silanes with a surface on the molecular scale. The majority of previous studies were performed on heterogeneous surfaces without well-defined geometry. On such illdefined surfaces, it is difficult to determine how the (3)Gamble, L.;Hugenschmidt, M. B.; Campbell, C. T.; Jurgens, T. structure and stability of the surface influence the A,; Rogers, J. W. J . Am. Chem. Soc. 1993,115, 12096-12105. (4) Danner, J. B.; Vohs, J. M. Appl. Surf. Sci. 1993,72,409-417. structure of the resulting silane adlayers and the kinetics ( 5 ) Crowell, J. E.;Tedder, L. L.; Cho, H.-C.; Cascarano, F. J . Vac. Sci. ofthe adsorption reactions. Exceptions have been studies Technol. A 1990,8,1864-1870. of tetraethoxysilane (TEOS) on single crystal T ~ O Z ( ~ ~ O ) , ~( 6 )Danner, J. B.; Rueter, M. A,;Vohs, J. M. Langmuir 1993,9,455MgO(100)and A1203(OOOl),4and Si(100) surface^.^,^ The 459. Abstract published inAdvance ACSAbstracts, October 15,1995. (1)Plueddemann, E. P. Silane Coupling Agents, 1st ed.; Plenum: New York, 1982. (2) Brennan, J. D.; Brown, S. T.; Manna, A. D.; Kallure, K. M. R.; Plunno, P. A.; Krull, U. J. Sens. and Actuators, B l993,11,109-119. @
(7) Coon, P. A.; Wise, M. L.; George, S. M. J . Chem. Phys. 1993,98, 7485-7495. (8)Coon, P. A.:Wise, M. L.; Walker, Z. H.; George, S. M. Surf. Sci. 1993,291,337-348. (9)Danner, J . B.; Vohs, J. M. Appl. Surf. Sci. 1992,62,255-262. (10)Hueenschmidt. M. B.: Gamble, L.; Campbell, C. T. Surf. Sci. 1994,302,-329-340.
0743-746319512411-4505$09.00/0 0 1995 American Chemical Society
Gamble et al.
4506 Langmuir, Vol. 11, No. 11, 1995
K, is defined as one monolayer of water here, and it was estimated to have one water molecule per TiOz(110)unit cell. The “tail”of the molecular desorption peak, extending from -300 to 350 K, was attributed to the disproportionation of surface hydroxyl groups. Our studies of tetraethoxysilane (TEOS) on the TiOz(110)surface3have clarified how ethoxysilanes react with TiOz(110). TEOS is used as the “model”for understanding reactions of ethoxysilaneswith the TiOz(110)surface since TEOS has only ethoxy functionalities. A comparison of that previous TEOS data to the present data from functionalized ethoxysilanes will allow us to distinguish ethoxy group reactions from those involving other functional groups. The conclusions based on our TPD as well as XPS studies3of the adsorption and reactions of TEOS on clean and water (and DzO) predosed TiOz(110) may best be summarized by the following reaction mechanism.
TEOS adsorption a n d initial dissociation: 130 K:
Si(OEt),,,
180-440 K -250-350
-
Si(OEt),,,
K
(1)
Si(OEt),,,
-
Si(OEt),,,
Si(OEt),,,
(2)
+
2Si(OEt),,, 2Ti -OEt (3)
further reactions: without surface water
+ -
650 K
2Ti-OEt
650 K
Si(OEt),,,
650 K
SiO,
+ C2H4,, + 0, SiO, + EtOH, + C2H4,,
EtOH,
0,
(4)
(5)
SiO,,,
(perhaps concerted with part of ( 5 ) ) (6) net:
+ 2C,H4,, + SiO,,,
BEtOH,
Si(OEt),,,
(7)
with surface D20 130 K:
D,O,
~ 3 0 K: 0
D20,
-350 K
-
-
D20, (reversible) Ti-OD
Ti-OEt
650 K
Si(OEt),,,
650 K:
SiO,
+ 0,
+ O,,-D
-
+ O,,-D
-
(8) (reversible) (9)
EtOD, (occurs twice) (10)
SiO,
+ EtOH, + C,H,,,
(5)
SiO,,,
(perhaps concerted part of with ( 5 ) ) (6) Si(OEt),,,
+ D,O,
-
+
+
BEtOD, EtOH, C,H4,, + SiO,,, (11) The subscripts g and s here refer to gaseous and surface bound, respectively, while Ti-OD (or Ti-OEt) refers to an -OD (or -0Et) group bound to a Ti4+surface site, and Obr-D refers to a hydroxyl formed by a D atom sitting on a “bridging oxygen” atom (Obr) site. These “bridging oxygen” atoms are the only nondefect lattice oxygens on the clean TiOz(110) surface with a missing nearest neighbor (i.e., with a dangling bond). (We refer here to the model for the nearly perfect (1x 1)surface proposed by Kurtz et a1.I1) In this previous study of TEOS net:
a d ~ o r p t i o nwe , ~ did not distinguish between Ti-OD and Ob,-D, and we referred to both simply as OD,. Making this distinction became necessary in order to fully understand data concerning ethanol adsorption and reactions in ref 12, so we use this distinction here. In this case, the net reaction (11)simplifies to that shown above only when the additional possibility for the interconversion between Ti-OD and Ob,-D is included in the mechanism. This interconversion consumeslproduces an oxygen vacancy12 and is not shown above. To summarize the above reactions: TEOS molecularly adsorbs a t 130 K. At approximately 250-300 K TEOS dissociates to form two surface-bound ethoxy groups, leaving the silane with two silicon-surface oxygen bonds. The eliminated ethoxys are probably bound to the free Ti4+sitesS3J3This was proposed to be a n S N ~ reaction initiated by the bridging ~ x y g e n These .~ bridging oxygens are considered to be Lewis base sites14which are likely nucleophiles that initiate a nucleophilic substitution reaction with TEOS.3 The reaction occurs regardless of whether surface water or hydroxyls are present or not. When there are no surface hydroxyls present, the ethoxy groups, whether still on the silicon or bound to Ti4+,remain on the surface until 650 K a t which they undergo a p-hydride elimination reaction12to produce ethylene and ethanol gases in a 1:l ratio (reactions 4 and 5). Thus, when no surface hydroxyls are present, two ethylene molecules and two ethanol molecules are produced a t 650 K for every one TEOS that dissociates (net reaction 7). When water is dosed to the surface a t 130 K, it molecularly adsorbs and dissociates (reactions 8 and 9Ie1O These surface hydroxyls can combine with the surfacebound ethoxy groups which, as seen in reaction 10,removes them as ethanol a t -350 K. The two ethoxy that remain on the silicon do not react with surface hydroxyls and remain on the surface until they are removed a t 650 K by p-hydride elimination which is once again reaction 5. When “excess”water is dosed to the surface, the ratio of ethylene to ethanol becomes 3.0:1.0, as seen in the net equation (reaction llL3 A quantitative coverage of TEOS on this surface was estimated from the C(ls)/Ti(2p3/~) and Si(2p)/Ti(2p3/~) XPS ratios and from the attenuation of the Ti(2p312)peaka3The results for TEOS were consistent with a close-packed model for the adsorbed monolayer (ML), in which there are 0.018 TEOS molecules per pi2, or 0.35 per unit cell. (The TiOz(110)unit cell has an area of 19.21 pi2.) The product coverage after flashing to 220-440 K, a t which most of the molecularly adsorbed TEOS is desorbed, was estimated to be 0.12 Si(OEt)z,, species per TiOz(110)unit cell. TPD data indicate that twice as much TEOS dissociates if “excess water” is coadsorbed with it. Thus, the Si(OEt)z,, coverage was estimated to be 0.24 per unit cell after flashing to 220-440 K i n the presence of excess water (hydroxyls). (While the above coverages were correct, some of the TEOS coverages were incorrectly reported in ref 3. The interested reader should note that true coverages of 0.6, 0.8, and 0.9 ML were misstated throughout that paper as being 0.3, 0.4, and 0.6 ML, respectively.) Our studies of the reaction of TEOS on TiOz(110)have shown that there are two pathways for removal of Tibound ethoxy groups: They may decompose by p-hydride elimination a t 650 K or they may combine with surface hydroxyls and desorb as ethanol a t -350 K. The ethoxy (11)Kurtz, R. L.; Stockbauer, R.; Madey, T. E.; Roman, E.; Segovia, J. L. D. Surf. Sei. 1989,218, 178-200. (12) Gamble, L.;Jung, L.; Campbell, C. T. Surf. Sci, in press. (13)Kim, K.S.; Barteau, M. A. J.Mol. Catal. 1990,63, 103-117. (14)Tanabe, K.;Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases: Their Catalytic Properties; Elsevier: New York, 1989.
Langmuir, Vol. 11, No. 11, 1995 4507
Interaction of Silane Coupling Agents with Ti020 10) groups that remain on the silicon atom, however, can only be removed by the ,&hydride elimination reaction. The consequence of having different removal schemes for the two different types of ethoxy groups is that, when water is dosed to the surface, all, or almost all, of the Ti-bound ethoxys can be removed a t -350 K. This allows us to distinguish them from the undissociated ethoxy groups that are bound to the silicon.
1;
t
.-
Experimental Section The experiments are performed in a two-chamber ultrahigh mbar, vacuum (UHV)apparatus, with base pressures of described p r e v i ~ u s l y . ~Additionally, ~J~ the partial pressure of HzO is kept lower in the analysis chamber for the "minimalwater" experiments by a liquid Nz trap. The polished, oriented TiOz(110)single crystal is mounted on a tantalum support with a small amount of indium between the sample and the support to maintain thermal contact. The support is resistively heated, or cooled by liquid nitrogen, so that surface temperatures from -100 to 900 K can be obtained. Details can be found in previous paper~.~JO Temperatures up to 1000 K could be reached, but followingtreatments to above 900 K, indium can be detected on the surface using XPS. Thus 900 Kisgenerally used as the maximum temperature here. The TiO~(110) singlecrystal is prepared as previouslydescribed by Hugenschmidt et al.IO The surface is cleaned before each experiment by sputtering with 4 x low6mbar Ar+ at 0.5-0.7 keV, flashing the sample to 800 Kin 2 x mbar 0 2 , and then cooling slowlyin 1 x 10+ mbar 0 2 . This procedure gives a good ~ ( 1 x 1LEED ) pattern. This annealing procedure does not eliminate bulk vacancies created during the initial preparation of the sample, which were purposefully induced to give some conductivity, removing charging effects. However, XPS of the Ti(2p) peaks shows negligible intensity iri the Ti3+ region, indicating that the surface is stoichiometric TiOz. This surface is considered "nearly p e r f e ~ t " . ~ ~ J ~ J ~ X P S experiments are performed with an Al Ka radiation at a constant pass energy of 100 eV. The binding energies were calibrated to gold reference peaks previouslyreported.19 Carbon to titaniumXPS ratios are used to measure the amount ofcarbon on the surface after exposure to the silane coupling agents. The relative carbon coverageis determined by subtractingthe small constant background carbon signal (due to the sample holder) and taking the ratio of the resulting carbon signal to the clean titanium signal. Absolute carbon coverages are determined by comparison of this C(ls):Ti(Pp)XPS ratio to ratios for known coverages given by our TEOS experiment^.^ The silane couplingagents [Si(OCzH5)4,CHz=CHSi(OCzH5)3, NHzC3H&(OCzH5)3, and (CzH&Si(OCzH&l were purchased from Hulls America Inc. The SCAs were further purified by several freeze-pump-thaw cycles and dosed as a mixture with -85% helium as described p r e v i o ~ s l y . Exposure ~ . ~ ~ units (E.U.) are defined as mbar-sof this mixture, directed at the sample with a doser tube. Deionized distilled water and 99.9% (D) deuterated water were further purified with several freezepump-thaw cycles and dosed through a pinhole doser to the surface for the excesswater experiments. The purity of the water was checked with the mass spectrometer. All the TPD spectra are taken by resistively heating the support to produce a linear response of the thermocouple voltage with time. This produces a constant heating rate of 4.5 Ws above 250 K but somewhat faster rates below 250 K.I0J6 The water coverage was calculated from the time-integrated intensity of the mle = 18 TPD peak. This peak area was scaled to absolute coverage units by comparison to TPD peak areas for (15)Clendening, W.D.; Rodriguez, J. A.; Campbell, J. M.; Campbell, C. T. Surf. Sci. 1989,216,429. (16) Campbell, J. M.; Seimanides, S.;Campbell, C. T. J.Phys. Chem. 1989,93,815. (17)Pan, J. M.; Maschoff,B. L.; Diebold, U.; Madey, T. E. J.Vac.Sci. Technol. A 1992,10,2470-6. (18)Gdpel, W.;Rocker, G.; Feierabend, R. Phys. Rev. B 1983,28, 3497 I_-..
(19) Anthony, M. T.;Seah, M. P. Surf. Interface Anal. 1984,6,95. ( 2 0 ) Henn, F.; Bussel, M.; Campbell, C. T. J.Vac. Sci. Rel. Phenom. 1991,A9, 10.
100
200
! ~ 300
~
, , ~ ~ ~ 400 500 Temperature [K]
,
,
600
l
~ , 700
,
I
~ 800
Figure 1. TPD spectra of molecular desorption of vinyltriethoxysilane (VTES). A representative mass (mle = 63) shows increasing amounts of molecular desorption, curves a-e, with increasing dose to clean TiOZ(110) at -140 K. VTES doses, given in exposure units (E.U.) were (a)2 E.U., (b) 4 E.U., (c) 4.7 E.U., (d) 6 E.U., and (e) 8 E.U. Curve e (8 E.U.) is approximately one monolayer. The insert shows 6 E.U. doses ofVTES with minimal (curve 0 and multilayer (curve g) water predoses. water coverageson TiOz(110)whose absolute values were known to within -35%.1°
Results TPD of Silane Coupling Agents (SCAs) on TiO(110). (a)Vinyltriethoxysilane (VTES). When vinyltriethoxysilane is dosed to the surface a t -150 K, a portion of it desorbs intact during TPD. Figure 1illustrates the molecular desorption of VTES as a function of increasing exposure at a n intermediate water coverage of about 0.35 HzO per Ti02 unit cell. The mass fragment followed here was mle = 63. Other masses (mle = 45,79,119,135,145) were followed and also found to give this same TPD peak line shape in a ratio of intensities characteristic of the VTES mass spectrometer cracking pattern. Submonolayer coverages ofVTES (a-e in Figure 1)partially desorb molecularly as a broad peak from 200 to 500 K, which is similar to results seen with TEOS (tetraethoxysilane) on TiOZ(110). Multilayer doses of VTES give a sharp peak at 180 K (not shown) due to desorption of the multilayer. The saturation of the molecular TPD peak, prior to multilayer TPD peak formation, is defined as one monolayer (ML) for all molecules studied here, the same definition used for TEOS on TiOn(110).3As seen in curve e, the monolayer peak exhibits a broad "tail" over the 300550 Kregion. The presence of the broad tail is dependent on the coverage of coadsorbed water on the surface. The insert in Figure 1 shows two equivalent doses of VTES. In curve f the amount of water on the surface was kept to a minimum, while in curve g a n excess of water, leading to multilayer HzO desorption, was dosed to the surface prior to the VTES dose. (Minimal water coverage corresponds to 0.07 f 0.02 HzO molecules per TiOn(110) unit cell throughout this paper.) The intensity ofthe 300500 K tail decreases with increasing water on the surface, as was also seen for TEOS on the same ~ u r f a c e .This ~ decrease in molecular desorption is coupled with an increase in the amount of decompositionproducts ofVTES seen a t higher temperatures, which is discussed below. In addition to molecular desorption, two decomposition products were easily seen whether the VTES was dosed at 150 K or at room temperature: ethylene and ethanol gas. These TPD products are shown with minimal water
,
,
/
,
~
Gamble et al.
4508 Langmuir, Vol. 11, No. 11, 1995
3
100
200
300
400
500
600
700
800
Temperature [K]
Figure 2. TPD spectra showingreaction products aRer 5 E.U. ofVTES are dosed to TiOz(110) at 150 K. Desorption of intact VTES is followed by mle = 63. The major contributor to m/e = 26 and 27 intensities is ethylene, with a minor contribution from ethanol. Ethanol is shown as mle = 31. Above -700 K acetylene is seen by the increase in mle = 26 relative to mle = 27. The mle = 18 TPD peak area, which corresponds to -0.07 HzO per T i 0 2 unit cell, is due to background adsorption. Note: This mass spectrometer has an unusually strong decrease in transmission with increasing mass, and therefore heavier ions show up much less intensely than in most instruments.
coverage in Figure 2 for a VTES dose of 5 exposure units (E.U.),the dose at which dissociation product peaks were saturated and molecular desorption was still minor. Some water contamination, shown by the small peak at mle = 18, could not be avoided, since a small amount of background water was always present in the chambers. The mle = 63 peak represents molecular desorption of VTES. The mle = 31 peak centered around 340 K was found to be ethanol by its cracking pattern (not shown). This will be called the “low-temperature”ethanol peak. In analogy with results from TEOS3and ethanol13on TiOz, it will be attributed to the reaction between Ti4+-bound ethoxy groups and surface OH, or reaction 10 above. A second peak which is centered a t 650 K consisted of coincident ethanol (mle = 31) and ethylene (mle = 26 and 27) peaks. By analogy with the TEOS data where the same peaks were seen,3this peakis attributed top-hydride elimination reactions in which two ethoxy groups produce ethylene and ethanol in equimolar ratios: reactions 4 and 5 above. It is thought that these reactions occur in two consecutive steps CH,CH,O, and CH3CH,0,
-
CH,=CH,,,
+ OH,
+ OH, - CH3CH,0Hg + 0,
(12a)
(12b)
where P-hydride elimination (reaction 12a) is the slower step.3 The subscript s here means “surface”,but does not distinguish here whether the groups are bound to the silicon, titanium, or oxygen atoms. This assignment is based on the similarity of these simultaneous ethylene and ethanol peaks to those also seen a t 650 K after dosing )~ and to those seen at 590 K TEOS to T i O ~ ( l l 0 surface after dosing ethanol to the (110)-faceted TiOz(100) surface.12J3 In those cases, the reaction was proven to be due to the ,&hydride elimination of one ethoxy group to form ethylene and a surface hydroxyl (reaction 12a) whose hydrogen combines with another ethoxy to form ethanol (reaction 12b).12 In the case of VTES and TEOS, this reaction occurred a t about the same temperature of 650
K whether the ethoxy was bonded to the Ti4+sites or still attached to the ~ i l i c o n .This ~ will be referred to as the “650 K” peak. A calibration experiment, in which the chamber was backfilled with similar pressures of ethylene and ethanol gases, showed that the mass spectrometer was -2.2 times as sensitive to ethylene a t mle = 26 as to ethanol a t mle = 31.3 When this correction is taken into account and any contribution of the ethanol cracking pattern is eliminated from the ethylene mass peaks, it was found in a series of experiments like those of Figure 2 that the average ethy1ene:ethanol yield ratio from the 650 K peak, integrated from 500 to 720 K, is (1.3 f 0.1): 1. The expected ratio for the p-hydrogen elimination reactions, (4)and (5)or (12)above, is l.O:l.O, assumingthat addition of an H, to a surface bound ethoxy is a rapid step. This inconsistency is believed to be due to a contribution to the ethylene yield from partial hydrogenation of the vinyl functionality, as discussed below. This produces extra ethylene and steals the hydrogen needed to produce stoichiometric amounts of ethanol, thus raising the ethy1ene:ethanol ratio. A third reaction, acetylene production, is shown as a shoulder after the 650 K peak for mle = 26 in Figure 2. At -800-850 K the cold sample holder begins to outgas background gases which interfere with the mle = 26 signal. This outgasing makes it difficult to determine the temperature at which this acetylene desorption rate decreases back to baseline. However, several control experiments all proved that the intensity of these peaks in the 720800 K range is not due to simple outgasing of the sample holder. This shoulder a t mle = 26 is accompanied by mle = 28 and lower intensity from mle = 27 and 25. No higher masses were observed until the sample holder began to outgas above 800 K. The ratios of peaks in the mle = 25 and 28 range indicate that a mixture of acetylene and ethylene is desorbing from 700 to 800 K. The contribution from acetylene starts a t -700 K, when most of the ethoxy groups have already been consumed via reactions 4-5, or reaction 12. The ratio of mle 26:27 in this shoulder, integrated between 720 and 800 K is (2.4 i 0.2):l. A quantitative comparison of the ethylene and acetylene mass peaks with their mass spectrometer sensitivity factors measured in this same chamber shows that the two gases were produced in a (1.6 f 0.3):l ratio between 720 and 800 K, using ion gauge sensitivities compiled and published by NASA, June 1969.21The coincident ethylene and acetylene production a t 720-800 K could be a result of a /3-hydride elimination reaction of the vinyl group, which would produce acetylene and a surface hydrogen. Further rapid reaction of the surface hydrogen product with another vinyl group would produce ethylene simultaneously, in analogy with reactions 12 above. This mechanism would give an ethy1ene:acetylene ratio of 1:l. The somewhat higher value we observed is attributed to the fact that extra ethylene is still desorbing in this temperature range from the tail of the 650 K peak. This peak gives ethylene both from P-hydride elimination reactions of surface ethoxys, (4) and (5)or (12) above, and from the hydrogenation of vinyls by the OH, product of reaction 12a. TPD spectra such as Figure 2 show different product yields with varying water predose. These changes in product yields as a function of surface water (plushydroxyl) coverage are quantified in Figure 3. Here TPD peak areas for ethylene and ethanol are adjusted for sensitivity factors. The water coverage is quantified by the area under the water (mle = 18) TPD peaks. The ethylene product (21)Summers, R. L. Table for Ion Gauge Sensitivities for Various Gases; Lewis Research Center, NASA Cleveland, OH, 1969.
Interaction of Silane Coupling Agents with Ti02(110)
400k'
"
'
"
' "
1
"
v E
Langmuir, Vol. 11, No. 11, 1995 4509
--I
n
I
PL 1
200
i L - J
OO
500
1000
2000
1500
H,O Yield
2500
3000
[arb. units]
Figure 3. Relative desorption yield of VTES products as a function of water precoverage (corrected for cracking patterns and sensitivity factors): (a)ethylene yield (mle = 26) from 500720 K, (b) low-temperature ethanol yield (mle = 31) from 220 to 480 K (c) total ethanol and ethylene yield from 220 to 720 K. A relative HzO yield of -750 corresponds to one water
molecule per unit cell.lo
yield (curve a in Figure 3) is reported as the area under the 650 K peak, of mle = 27, taken from 500 to 720 K. This area does not include most of the high-temperature ethylene (plus acetylene) tail, but it includes a small amount of extra ethylene due to this higher temperature process. As previously mentioned, ethylene and ethanol are produced in the 650 K peak in a 1.3:l.O ratio. Thus the plot of ethanol produced a t 650 K (not shown) is very similar to that for ethylene in curve a, although at -75% of the yield. The 650 K TPD peak yields of ethylene and ethanol stayed relatively constant with increasing water predose. The amount of acetylene produced in TPD a t -730 Kincreased by -20% with increasing water predose. The low-temperature ethanol TPD peak yield (curve b) increased from -0.2, a t minimal water, to -2 times the amount of ethylene produced a t 650 K when a n excess of HzO or DzO was predosed to the surface. Extrapolating curve b for low-temperature ethanol production to zero water yield suggests that in the absence of water, lowtemperature ethanol production would be completely eliminated. Control experiments with predosed DZ0 produced CH3CHzOD in the low-temperature peak only, which supports a model where Ti-bound surface ethoxy groups are readily removed a t 340 K by reaction 10 above:
Ti-OEt
+ Ob,-D
-
EtOD,
(10)
The peak temperature of the deuterated ethanol decreased with increasing coverage, just as we saw for this step in experiments with TEOS.3 Ethanol production in the 650 K peak in the presence of excess water is attributed to reaction of ethoxy groups still bound to silicon sites, or reaction 5. This ethanol was not deuterated, consistent with this mechanism. The total amount of decomposition products (curve c) increased by -45% when a n excess ofwater was predosed to the surface. A decrease in the amount of molecular VTES desorption (insert, Figure 1)was also observed with increasing water predose to the surface. These results indicate more VTES dissociates during TPD a t higher water (or OH) coverages. This same effect was observed with TEOS3where it was attributed to the fact thatsurface hydroxyls remove ethoxys from Ti4+ sites via ethanol production during TPD, allowing more TEOS to dissociate into these vacated sites before they desorb. We propose the same model for VTES. Extrapolation of the ethylene (a) and total products (c) curves in Figure 3 shows that
100
200
300
400 500 Temperature [K]
600
700
800
Figure 4. TPD spectra showing reaction products of 5 E.U. of diethyldiethoxysilane (DEDS) with a multilayer of predosed water dosed at 140K to TiOz(110). Data have not been adjusted for ion gauge sensitivity to different molecules. (This mass spectrometer has a rapidly decreasing transmission with increasing mass and therefore lighter species show up more intensely without this correction.) (A 10 E.U. dose gives a saturation of the molecular peak which is defined as one monolayer.) Intact DEDS desorption is followed by mle = 63. Water desorption is followed by mle = 18, with its multilayer peak at 150 K. Ethanol desorption is shown by mle = 31. Ethylene desorption is followed by mle = 26. The small "bump" at -350 Kin the mle = 26 curve is due to the cracking pattern of ethanol.
in the case of minimal water, the total amount of decomposition products is reduced, but not eliminated. The high temperature yields of ethylene and ethanol were roughly the same in minimal water and excess water. Thus, VTES does not require water or hydroxyls to react with the TiOZ(110) surface but merely to open sites for dissociation. (b)Diethyldiethoxysilane(DEDS).The TPD results of diethyldiethoxysilane on TiOz(110)were very similar to those of VTES. Low coverages of DEDS deposited on a cold (-140 K) surface partially desorb molecularly as a broad peak from -220 to 500 K (seen as mle = 63 in Figure 4). As seen with VTES and TEOS3 the molecular desorption peak had a broad tail extending over the 300500 K region which diminished with increasing water predose. At higher coverages of DEDS, a sharp multilayer peak grows in at 180 K (not shown). (Masses 26,31,59, 171, and 173 were also followed to ensure that the mle = 63 intensity was truly representative of only the intact molecule.) The ethanol and ethylene TPD peaks seen at higher temperatures were also similar to the decomposition peaks found for VTES and TEOS.3 A typical TPD spectrum of DEDS dosed a t 140 K with multilayer water precoverage is shown in Figure 4. The broad peak that desorbs from -260 to 500 K (shown as mle = 31) follows the ethanol cracking pattern. The intensity of this peak was found to increase with increasing water precoverage, until monolayer amounts of water were exceeded. The peak maximum shifted to lower temperatures with increasing water precoverages, reaching -350 K with multilayer water precoverages. Again, this is very similar to the behavior of the ethanol peak resulting from the reaction between surface OH and Ti-bound surface ethoxys deposited by the decompositionofVTES and TEOS3on TiOz(110),or reaction 10 above. The higher temperature peaks, centered around 650 K, fit the cracking patterns of a combination of ethylene (shown as mle = 26) and ethanol (shown as mle = 31)and are again attributed to ap-hydride
4510 Langmuir, Vol. 11, No. 11, 1995
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D
t-
OO
u p -
500
,u I--
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1500
i
2000
H,O Yield [arb. units]
Figure 5. Relative desorption of DEDS products as a function of water precoverage (corrected for cracking patterns and sensitivity factors): (a)ethylene yield (mle = 26) from 500-720 K (b)low-temperatureethanol yield (mle = 31) from 220 to 480 K (c) total ethanol and ethylene yield from 220 to 720 K. A relative HzO yield of -750 corresponds to one water molecule per unit cell.1o elimination reaction involvingthe ethoxy groups, reactions 4 plus 5 or 12 above. The intensities of mle = 26 and 31were integrated from 520 to 720 K, subtracting a horizontal background determined by the constant intensities below 220 K. The ratio of these integrated intensities was 3.9 f 0.1. From this, the product ratio of ethylene to ethanol in this 650 K peak was determined to be (1.7 f 0.l):l.O after the contribution from ethanol was subtracted from the mle = 26 intensity and correction was made for mass spectrometer sensitivity factors. This increase in the ethylene to ethanol yield ratio from the typical 1.O:l.O ratio of /?-hydride elimination of ethoxy groups (see above) suggests that an additional reaction is producing extra ethylene in this region. We propose this to be aB-hydride elimination reaction of the ethyl ligands on the silane. This reaction gives rise to a third desorption peak whose maximum is above 650 K and thus is observed as an unresolved shoulder of the 650 K ethylene peak. Again, outgasing of the sample holder above 800-850 K makes it difficult to determine the temperature a t which this additional ethylene desorption peak decreases back to baseline. Unlike VTES, the cracking pattern in the 720800 K range in TPD of DEDS matches the ethylene cracking pattern, and no acetylene is evident. The mass signals for other possible desorption products, including ethane and H2, were monitored but no characteristic TPD peaks were detected. Figure 5 shows the intensities of the various decomposition peaks of DEDS as a function of water predose coverage for a fixed (5 E.U.) dose of DEDS a t 120 K, a dose at which the dissociation products peaks were saturated. The signals are scaled to absolute yields using the proper sensivity factors. The yield of the 650 K ethylene peak integrated from 500 to 720 K (curve a)decreases by -20% with increasing water predose. The amount of ethanol produced at 650 K (not shown) has a water dependence similar to ethylene, but with only -60% of the yield. The “high temperature” ethylene shoulder (integrated from 720 to 800 K), however, appeared to increase by -1020% with increasing surface water coverage (not shown). The low-temperature ethanol yield (curve b) increased from 0.5 to -2 times the 650 K peak’s ethylene yield. The total amount of decompositionproducts below 720 K (curve c) was seen to increase with increasing water precoverage and was accompanied by the decrease in molecular desorption discussed previously. These results indicate
that an increase in surface water coverage increases the amount of DEDS dissociating on the surface, as seen with VTES and TEOS3 As with VTES and TEOS, the increase in low-temperature ethanol production with increasing surface water coverage is attributed to removal of the dissociated ethoxy groups by the surface hydroxyl groups, or reaction 10, which frees up sites for further dissociation of adsorbed DEDS during TPD before it desorbs. Ethanol desorption was still seen in the 650 Kpeakwhen an excess of water was predosed to the surface. Another type ofexperiment was performed where excess water and then 5 E.U. of DEDS were dosed to the surface. The sample was then flashed to 500 K to remove the lowtemperature ethanol TPD peak. After the sample was cooled, a multilayer of water was again dosed to the surface. The resulting TPD spectra (not shown) demonstrated h r t h e r ethanol desorption in the low-temperature peak (-350 K) and a reduction of the high-temperature (650 K) ethanol production. These results are quite different from results of a similar experiment performed with TEOS, where the second dose of water did not affect the 650 K ethanol peak because two ethoxys per TEOS were removed as low-temperature ethanol and the remaining two ethoxys were not reactive below 500 K.3 Similar results were expected for DEDS. Since DEDS has only two ethoxy groups, both of them should have desorbed as ethanol a t low temperatures and no ethanol should have been produced a t 650 K even without the second water dose. Our results suggest that not all of the ethoxys are eliminated from the surface by 500 K in the first flash. Note that -350 K is the temperature a t which all of the adsorbed hydroxyls and water are removed by desorption. Thus, a second dose of water can produce additional hydroxyls to react with the Ti-bound ethoxys that were produced between 350 and 500 K during the first flash. However, even the second dose of water did not remove all of the ethoxys, which was evident by residual ethanol desorption at 650 K. This suggests that some of the DEDS molecules remain bound to the surface through only one Ti-0-Si bond even a t 500 K, leaving some ethoxy groups still bound to Si. Such Si-bound ethoxys are unreactive with water below 500 K but produce ethanol (and ethylene) at 650 K via reaction 5.3 DEDS does not dissociate as readily on the TiOz(110) surface as did VTES and TEOS. TPD peak areas show that DEDS dissociates to produce only -V2--V3 as much dissociated silane as TEOS and VTES. (c) (Aminopropyl)triethoxysilane (APS).(Aminopropyl)triethoxysilane(APS)was vapor deposited on the TiO~(110) surface a t -130 K and molecularly desorbed in a broad peak from 220 to 500 K, as shown in Figure 6 for minimal water coverage. The monolayer peak shifts to lower temperatures as a function of increasing exposure which is attributed to repulsive interactions between the APS molecules. The monolayer peak saturates a t -340 K. At higher coverages, a sharp multilayer peak appears a t -200 K. No TPD peaks were observed which might suggest dissociation of the APS. (A small ethanol peak at -400 Kwas proven to be due to background impurities in the doser line.) Unlike VTES or TEOS,3the molecular peak of APS appears unaffected by predosing water. The lack of any other desorption products and the independence ofthe APS molecular peak on water predose indicate that the APS molecule is not dissociating to a significant extent ( 1.06ethoxys are removed per bound silane in the low-temperature ethanol peak in TPD of DEDS. This is well below the value of 2.0 seen from TEOS and VTES. (24) Rueter, M. A.; Vohs, J. M. Surf. Sci. 1992,262, 42-50. (25)Johnson, 0.W.; Paek, S.-H.; DeFord, J. W. J.Appl.Phys. 1976, 46, 1026-1033.
(26)Kim, K.S.;Barteau, M. A,; Farneth, W. E. Langmuir 1988,4, 533.
Langmuir, Vol. 11, No. 11, 1995 4513
Interaction of Silane Coupling Agents with Ti02(110) Furthermore, when the low-temperature ethoxys are removed by flashing the adsorbed DEDS to 500 K and water is once again added to the cooled surface, more adsorbed ethoxys desorb as ethanol at 350 Kin the second TPD. This result for DEDS further indicates that two ethoxys per silane are not yet eliminated from the surface by 500 K as “low temperature” ethanol in one TPD. As we have seen in our experiments of HzO on TiOz(110), surface hydroxyls desorb as HzOby -350 K.l0 Therefore, the adsorbed ethoxys that are produced from silane dissociation after 350 K would no longer have water available to remove them. The second water dose produced more OH,, which removed the ethoxys that were eliminated in the first flash between 350 and 500 K. Even a second water dose, however, did not increase the ethanol yield up to two ethoxy groups per DEDS molecule. This indicates that some of the dissociated DEDS molecules have only one Si-surface bond and still have one intact Si-OEt bond, unlike TEOS andVTES which both appear to have two Si-0-Ti bonds and two eliminated ethoxys per molecule. We believe, therefore, that the dissociated DEDS molecules are bound to the surface at 450 K by a mixture of one or two Si-0-Ti bonds. The DEDS that are only bound by one Si-0-Ti bond have one ethoxy group on the silicon that is removed a t 650 K by /?-hydride elimination. This is reminiscent of a study of the reaction mechanism of gaseous dimethyldimethoxysilane with silica by HertlZ7who reported that about 60% dimethyldimethoxysilane bonded monofunctionally while 40% bonded difunctionally with a silica surface a t -500 K.27 Analysis of DEDS TPD peak areas revealed that, in a n excess of water, ’53% of the ethoxy groups are removed in the first flash as low-temperature ethanol via combination with hydroxyls. However, since not all of the ethoxy groups have been eliminated from the DEDS molecules before the hydroxyls desorb as water, a more accurate measure of the percent dissociated ethoxys requires quantifying the total low-temperature ethanol removed after two doses ofwater. Once all ofthe eliminated ethoxys removed as ethanol in the two flashes are taken into account, our experiments indicate that ’30% of the dissociated DEDS bonds to the surface with two Si-0-Ti linkages, and < 70% monofunctionally, which is in good agreement with Hertl’s estimate of 40% and 60%, respectively, on Si02.27 Interestingly, the vinyl groups of Si(OEt)(CH=CHz),, the product of VTES, hydrogenate readily during TPD above 500 K to give ethylene, while the ethyl groups of SiEtz,, and EtOSiEtz,,, the products of DEDS, do not hydrogenate to ethane. This is similar to the well-known trend that vinyls in metal complexes reductively eliminate more readily than alkyls.28 A comparison of the XPS data for VTES and DEDS to that for TEOS showed, with high water coverage, that VTES deposited 88%as much surface carbon and DEDS 63% of that deposited by TEOS after a similar flash to -480 K. The flash to 480 Kremoved the low-temperature ethanol, which in the presence ofexcesswater was believed to consist of two ofthe initial ethoxy ligands per silane for both TEOS and VTES. Since both these silane coupling agents had two carbons per ligand, and four ligands per silane, it is reasonable to assume that the C:Ti ratios can be used to compare their overall reactivity by comparing the amount of remaining carbon after this 480 K flash. Thus VTES gives -88% as much covalently bonded silane as TEOS, which produces 0.24 covalently bonded silanes per unit celL3 Thus, VTES produces 0.21 bonded silanes
per unit cell, or 1.1 x 1014 silanes per cm2. DEDS gives about 63% as much carbon in the covalently bonded silane as TEOS. However, as discussed previously, DEDS bonds via a mixture of one and two Si-0-Ti bonds and thus only -1.3 ethoxys are removed a t the lower temperature. A calculation, assuming 70% of the DEDS bonds monofunctionally (see above), shows that DEDS deposits about 50% as much covalently bonded silane as TEOS to the TiOz(110) surface (or 0.12 DEDS-derived silanes per unit cell). This is in agreement TPD product yields which indicated that DEDS has 33-50% as much dissociated product as TEOS and VTES. The TPD experiments with VTES showed that its dissociation products saturated at a dose of -5 E.U., where there was still little molecular desorption, and that 8 E.U. saturated the monolayer’s molecular desorption peak. Thus, m 5 / 8 of the monolayer dissociates. XPS showed that the amount which dissociates corresponds to 1.04 x 1014 silanes per cm2, so the monolayer coverage corresponds to -1.7 x 1014molecules per cm2, A similar calculation with DEDS, where 5 E.U. saturates the dissociation products, but 10 E.U. are needed to saturate the monolayer, shows that the monolayer corresponds to -1.3 x 1014molecules per cm2. These monolayer coverages are within 10-30% of the value of 1.8 x 1014molecules per cm2for a monolayer of TEOS.3 For TEOS, this coverage approximates a close packing of mole~ules,~ which suggests a similar model for all these silanes. Our results for both VTES and DEDS on TiOz(110) indicate that as the surface water is reduced, there is still appreciable product desorption (Figures 3 and 5). Our previous studies of TEOS on TiOZ(110) also show that silane coupling chemistry can occur on the TiOz(110) surface even in the absence of surface water or -OH groups. While the total amount of product desorption is less with minimal surface water coverage than for excess water, there is still a significant amount of SCA dissociation. With minimal water, 34% of a monolayer of TEOS (or 0.12 per unit cell) was found to dis~ociate,~ which greatly exceeds the defect concentration thought to be below 3%.1° The bridging oxygensof the Ti02(110)surface are basic sites which can act as nucleophiles to initiate an Sp~2mechanism that eliminates alkoxy groups and binds the silicon to the surface via Si-0-Ti bonds.3 The eliminated ethoxy groups are believed to coordinate to the surface a t nearby Ti4+~ i t e s . ~Other J ~ recent reports indicate nucleophilic attack via a surface oxygen in dissociative adsorption of trimethoxysilane on Lewis base sites of MgO(100)9 and in alkoxysilane chemistry on strained Si-0-Si defect sites of dehydroxylated silica surface.23 Neither the TPD nor XPS data indicated that APS dissociated to a measurable extent on the TiOZ(110) surface. Studies of solution phase deposition of AI’S showed dissociative adsorption through Si-0 b o n d ~ . ~ ~ - ~ O However evidence was also reported for amine group interaction with s u r f a ~ e . ~It~is- highly ~ ~ likely the APS molecule initially bonds molecularly to TiOz through its amine group’s lone pair interacting with Ti4+sites, rather than through surface interactions with the methyl groups of its ethoxys, as must be the case for TEOS. This could naturally make it more difficult to abstract ethoxy ligands from the adsorbed APS molecule, since it would lead to a rather different adsorption geometry which might sterically inhibit attack a t the Si-0 bonds. Also, this (29) Chiang, C.-H.; Ishida, H.; Koenig, J. L. J.Colloid Interface Sei. 1979, 74, 396-423.
(27) Hertl, W. J . Phys. Chem. 1968,72, 3993-3997. (28) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G .
Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987.
(30) Culler, S. R.; Ishida, H.; Koenig, J. L. J . Colloid Interface Sci.
1985, 106, 334.
(31)Homer, M.; Boerio, F.; Clearfield, H. J.Adhes. Sci. Technol. 1992, 6, 1-22.
4514 Langmuir, Vol. 11, No. 11, 1995
Gamble et al.
Table 1. Summary of Ethoxysilane Adsorption Studies on TiOZ(110)
monolayer coverage (per cm2)
maximum bound silane coverage (per cm2)
av no. of Ti- 0-Si bonds per bound silane 2.0
2.0 1.3
relative yields of high-temperature gaseous products 500-720 K, CzH4:EtOH= 1 500-720 K, CzH4:EtOH= 1.3 i 0.1 720-800 K, CzH4:CzHz = 1.6 f 0.3 520-720 K, CZH4:EtOH = 1.7 f 0.1 720-800 K, CzH4
amine interaction might self-block Ti4+sites needed for accommodating the eliminated ethoxy ligands. Thus, desorption ofAPS wins in its competition with dissociation as it is heated, so that no measurable amount of Ti-0-Si bond formation is observedwith this moleculeduring TPD. However, during TPD the molecule only spends one lifetime on the surface. As the surface is warmed, the molecule must either dissociate or desorb, but it only has one chance to do either. This is quite different than what occurs during silane dosing at room temperature and high pressure or in solution, where the molecule can make many, many collisions with the surface and adsorb and desorb many times before finally dissociating. In the latter case, the molecule has a much better chance ofdissociating if the rate constant for desorption is much bigger than that for dissociation, as it seems to be in the case of APS. Thus, there is nothing inconsistent with our present observations that APS does not dissociatively adsorb to a measurable extent during TPD and the possibility that APS might also dissociatively adsorb in higher pressure or liquid-phase dosing a t room temperature.
linkages per bound silane are also tabulated here. The ethoxy groups on each silane react with the same mechanisms, or reactions 3- 13 above, a t almost the same temperatures as those on TEOS. Exceptions to this are that the ethoxys on DEDS were less reactive (possibly due to steric constraints related to the fact that there are only two per molecule) and those on APS were not measurably reactive (attributed to the adsorption geometry of this silane via the amino group). Thus VTES dissociates rapidly to form Si(OEt)(CH=CH2)s,bound to the surface with two Ti-0-Si links, while DEDS produces a mixture of SiEtz,, and EtOSiEtZ+bound by two and one Ti-0-Si links, respectively. The vinyl groups on VTES have additional reaction steps, (14) and (E),involving their hydrogenation and P-hydride elimination. These give rise to additional ethylene and acetylene in TPD at 600-800 and 700-800 K, respectively. Here, the relative yield of ethylene is larger than that of acetylene, because hydrogenation turns on a t a lower temperature. The ethyl group on DEDS also undergoes P-hydride elimination, step 16, a t 600 to ’800 K, giving additional ethylene in TPD.
Conclusions Table 1summarizes many of our results for these three ethoxysilanes, and compares them to previous results for TEOS3 The monolayer coverage refers to the maximum coverage before the multilayer TPD peak appears. The maximum “bound”silane coverage refers to the maximum coverage of covalently bound silane, Le., silanes linked to the surface through Ti-0-Si bonds, achieved in TPD to -480 K in excess water. The number of such Ti-0-Si
Acknowledgment. The authors gratefully acknowledge the industrial sponsors of CPAC (the Center for Process Analytical Chemistry) for their funding and support of this research. The authors also acknowledge the partial support for this work by the Department of Energy, Office of Basic Energy Sciences, Chemical Science Division. Helpful discussions with Karen Goldberg and James Mayer are also gratefully acknowledged. LA950039V
