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J. Phys. Chem. 1995, 99, 4639-4647
Thermal Stability of Hydroxyl Groups on a Well-Defined Silica Surface Ofer Sneh and Steven M. George* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 Received: August 4, 1994; In Final Form: December 1, 1994@
The thermal stability of hydroxyl groups was studied on a well-defined silica surface. The silica sample was a 5 pm thick Si02 layer grown on Si( 100)by a combination of thermal oxidation and chemical vapor deposition with Si& and 0 2 . The silica surface was cleaned and analyzed under ultrahigh vacuum conditions. H20 2SiOH) was achieved by a H20 plasma reaction in a small internal Hydroxylation (Si-0-Si Si-0-Si H20) of the silica surface high-pressure chamber. The extent of dehydroxylation (2SiOH was then investigated versus annealing temperature. The thermal stability of the hydroxyl groups was monitored by two different monolayer sensitive experimental methods. In the primary method, methanol (CH30H) was used to titrate the surface SiOH species by hydrogen bonding between the hydroxyl groups. Secondarily, laser-induced thermal desorption (LITD) was used to desorb directly H20 from hydroxyl groups on the surface. The CH30H temperature-programmed desorption (TPD) signal after saturation CH30H exposures represented the total SiOH surface coverage. In contrast, the LITD H20 signal appeared to originate only from neighboring (vicinal) SiOH groups. Both the CH30H TPD and H20 LITD experiments monitored the progressive decrease of the hydroxyl coverage versus thermal annealing from 100 to 900 "C. These thermal stability results are consistent with earlier measurements of hydroxyl species versus thermal annealing on high surface area silica powders. The H20 LITD measurements also indicated that the dehydroxylation proceeded quickly at each temperature and reached a fairly constant coverage in less than 1 min.
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1. Introduction
desorption kinetics have displayed a fast initial desorption rate that changes to a much slower rate at longer times. Complete Silica (SiOz) is one of the most common materials on earth. dehydroxylationis reportedly not obtained even at temperatures The surface properties of Si02 are important in many fields as high as 1100 0C.7,23Neighboring surface hydroxyls that are including catalysis,' soil chemistry,* and chromat~graphy.~.~ hydrogen-bonded together (vicinal) have been shown to be Surface processes on silica have been intensively studied, but desorbed easily as H20.697316 These vicinal hydroxyl groups many results are still a matter of debate. The high degree of desorb in the temperature range of 150 to 450-600 O C 6 s 7 The inconsistency between various silica surface studies has been isolated and geminal (HO-Si-OH) hydroxyl groups that are attributed to the different preparation method^.^,^ Many studies left behind represent 25 -30% of the initial fully hydroxylated have been performed on dried gels or powders obtained by silanol s ~ r f a c e .These ~ ~ ~ hydroxyl groups are much harder to fuming3 or sol-gel e x t r a ~ t i o nand ~ , ~ processed by a variety of desorb. Most of the isolated and geminal hydroxyl groups are chemical, mechanical, and thermal procedure^.^,^ These samples thermally removed in the range 600-900 "C. In addition, have varied substantially in porosity, intemal H2O content, defect vicinal and isolated surface hydroxyls display different reacdensity, surface curvature, structural integrity, gas flow cont i ~ i t i e sand ~ ~the isolated SiOH groups are usually more reactive ductivity, thermal conductivity, alkali ion content, and than vicinal groups. The clear distinction between isolated and Evidence that the structure and reactivity of silica depend on the thermal and chemical history of the sample has vicinal SiOH groups is well established from IR absorption added even more c ~ n f u s i o n . ~ - Consequently, '~-~~ measurements e ~ p e r i m e n t s ~ J because ~ . ~ ~ - ~different ~ absorption features can of surface processes on well-defined, preferably planar, silica discriminate between the types of hydroxyl group^.^,^^ surfaces will be useful to resolve the many conflicting observaThis work marks the beginning of our effort to study tions. adsorption, diffusion, reaction, and desorption on planar and Dehydroxylation (2SiOH Si-0-Si H20) is one of the well-defined silica surfaces. The experiments were conducted most studied reactions on silica surfaces. Dehydroxylation is on a silica surface that was cleaned, annealed, and analyzed in very important because the reactivity of silica surfaces changes ultrahigh vacuum (UHV) conditions. The silica substrate was dramatically between the dehydroxylated siloxane phase (Sia pure and stoichiometric Si02 film grown by a combination 0-Si) and the hydrated silanol (SiOH) ~ u r f a c e . ~The , ~ siloxane of thermal oxidation of a Si(100) wafer and chemical vapor surface is very inert, whereas the silanol surface has a hydroxyl deposition using S i b and 0 2 . This 5 p m thick silica film on functional group that has chemical reactivity. The thermal top of a 400 p m Si(100) silicon wafer was cleaned by a 0 2 or stability of hydroxyl groups on silica surfaces has been thoroughly investigated, both e ~ p e r i m e n t a l l y ~ ~ ~and . ' ~ ~ ' ~ H20 plasma discharge. The film was then annealed at 7501000 O C to obtain vitreous silica. Auger electron spectroscopy theoretically.6.18-22Although this entire set of information is (AES) was used to characterize the surface cleanliness, vitrevery inconsistent,6 the data that may be the least affected by and stoichiometry. The resulting surface yielded results sample preparation do display a fairly consistent p i ~ t u r e . ~ - ~ , ~ousness, ~ that were very reproducible and independent of the history of The thermal stability of the surface hydroxyl species on silica is strongly coverage and temperature d e ~ e n d e n t . ~ - The ~ * ~ ~ the experiments. In this paper, the stability of hydroxyl groups was studied versus thermal annealing on the well-defined silica surface. @Abstract published in Advance ACS Abstracts, March 1, 1995.
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Q 1995 American Chemical Society
4640 J. Phys. Chem., Vol. 99, No. 13, 1995 INCIDENT CO2 LASER BEAM
Sneh and George
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Figure 1. Top: Top view of the upper level of the UHV apparatus. Bottom: Illustration of the internal high-pressure chamber (IHPC). The IHPC is located in the lower level of the UHV apparatus.
Laser-induced thermal desorption (LITD),28*29temperatureprogrammed desorption (TPD),30and Auger electron spectroscopy (AES) served as the main experimental techniques. The measured hydroxyl stability on the well-defined silica surface was then compared with earlier results in the literature on high surface area silica powders. The time scale of dehydroxylation at a given temperature was also examined and compared with similar measurements on high surface area silica powders. These comparisons should establish the possible differences in hydroxyl group stability on well-defined, planar silica surfaces and high surface area silica powders.
2. Experimental Section
A. Apparatus. Figure 1 illustrates the ultrahigh vacuum (UHV) apparatus used for the experiments. This apparatus had a base pressure of 2-5 x Torr. In addition, experiments at pressures as high as 1 atm were performed in-situ in a small internal high-pressure chamber.31 The sample surface was attached to a liquid nitrogen cryostat that provided sample cooling to below 100 K. The silicon substrate holding the silica thin film could be resistively heated to ~ 1 0 0 0"C. The temperature could be controlled to f 2 K in UHV and to f 5 K under high-pressure conditions. A low-energy electron diffraction (LEED) spectrometer (a 15-120) was used in the retarding field scheme32 for Auger electron spectroscopy (AES) of the surface composition. A UTI lOOC quadrupole mass spectrometer was utilized in the LITD experiments. An AMATEK Dycor quadrupole mass spectrometer was employed for low-background TPD studies. This mass spectrometer was enclosed in a glass shroud that was differentially pumped by a 25 L/s ion pump. Low-pressure dosing was performed with a glass capillary array d o ~ e r . ~ ~ A comprehensive description of the internal high-pressure chamber is given e l ~ e w h e r e .A ~ ~schematic of the internal high-
pressure chamber is shown at the bottom of Figure 1. Briefly, a combination of the sample holder with a knife-edge sealingflange around its perimeter and a moving cap is used to seal a -20 cm3 volume. This volume has a surface area of -145 cm2. Typically, a pressure of 1 atm in the high-pressure cell caused a pressure increase in the UHV apparatus of AP (12) x 10-lOTorr. The sample holder was attached to a liquid nitrogen cryostat through a helium gas thermal s w i t ~ h . ~The ~ ,thermal ~~ switch controls the thermal conductivity between 0.03 and 1.05 W/"C by varying the He pressure between 5 and 500 mTorr, re~pectively.~~ Sample temperatures could be achieved as low as 100 K. In addition, the sample holder temperature could be quickly raised to 300-400 K before dosing condensable gas reactants in the internal high-pressure chamber. This temperature rise is accomplished by a combination of evacuating the He in the thermal switch and resistive heating of the sample holder. A turnover time of -10 min was possible between highpressure experiments with H20 and UHV condition^.^^ This rapid transition was facilitated using a cryopanel attached to a He closed cycle refrigerator (CTI CT-350) with a pumping speed for H20 of ~ 5 0 0 0W s . B. Sample Preparation and Mounting. The sample silica surface was prepared by growing a 5 p m thin Si02 thin film on a Si(100) wafer. The Si02 film was grown by a combination of thermal oxidation with 0 2 to prepare a 1 p m Si02 film and low-temperature chemical vapor deposition (CVD) with Si& and 0 2 to obtain the 5 p m thick film (1 p m thermal oxide and 4 p m CVD oxide). N-type phosphorus-doped Si( 100) wafers were used with a typical resistivity of e = 0.02-0.03 8-cm. Phosphorus-doped wafers were employed because phosphorus segregates out of the Si02 film and into the silicon bulk and allows the preparation of a pure silicon oxide film.35 Square pieces of the Si( 100) wafer with dimensions of 0.75 in x 0.75 in served as the samples. The samples were cleaned inside the UHV apparatus using a high-frequency plasma discharge36 inside the internal highpressure chamber. Running a plasma in 0.3 Torr of 02, H2, or H20 for 30 s resulted in a silica surface displaying only oxygen and silicon AES features. This resulting surface displayed charging effects and distorted Si AES features in the 63-78 eV range. Further annealing at 750-1000 "C for 2-10 h resulted in the elimination of these effects. The sample was then hydroxylated by a H20 plasma discharge in the internal high-pressure chamber at a H20 pressure of 0.3 Torr. The silica surface was subsequently dehydroxylated by annealing at 500-600 "C. This procedure was repeated several times. Repetitive hydroxylation and dehydroxylation is known to reduce surface defects by relaxing the surface into a stable cristobalite p h a ~ e . ~ .The ' ~ reproducibility of the dehydroxylation results provided strong evidence that the experiments performed on the silica surfaces prepared by repetitive hydroxylation and dehydroxylation had a low initial defect density.4326327 The sample temperature was determined by a ChromelAlumel thermocouple attached to the crystal with high thermalconductivity ceramic glue (AREMCO 5 16). Resistive heating was achieved by running current through the silicon sample. A 4000 8, Ta film was deposited on the back side of the sample to facilitate current The resistance of this sample was 1-2 Q. The surface mounting scheme was similar to the methods used for mounting silicon wafers for silicon chemistry studies described elsewhere.37 C. Hydroxylation. One of the most debated and inconsistent issues in the field of silica surface chemistry is the
J. Phys. Chem., Vol. 99, No. 13, 1995 4641
Stability of Hydroxyl Groups on a Silica Surface
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rehydroxylation (Si-0-Si H20 2SiOH) of siloxane surfaces. Many studies agree that rehydroxylation with either H20 vapor or solution is a very slow and unfavorable process. Rehydroxylation with H20 vapor is expected to be completed on a time scale ranging from a few hours to a few day^.^,^^^ To obtain a complete hydroxylation on a much faster time scale, we employed a plasma discharge at 0.3 Torr of H20 inside the intemal high-pressure chamber. The radicals *OHand 'H have previously been shown to react with dehydrated silica surface^.^^-^^ The Si-0-Si siloxane bridge sites are cleaved by attachment of .OH and .H radicals to form silanol (SiOH) groups. The rehydroxylation of these siloxane sites by .OH and .H radicals completely hydroxylated the silica surface within a few seconds. The reproducibility of the results following numerous plasma discharges indicated that the H20 plasma reaction did not modify the Si02 film. In contrast, running the plasma with only 0 2 at 0.3 Torr in the intemal high-pressure chamber resulted in a siloxane surface with no hydroxyl groups. D. TPD and LITD Techniques. The two main experimental techniques employed in this investigation were temperatureprogrammed desorption (TPD)30,42and laser-induced thermal desorption (LITD).z8,29,43,44In the TPD experiments, the desorption of adsorbates is monitored by a mass spectrometer while the surface temperature is progressively increased at a constant rate. The adsorbate surface coverage is proportional to the integrated intensity of the TPD signal. In our experiment, the substrate was positioned -5 mm in front of a glass shroud aperture that baffled the desorption flux into a differentially pumped mass spectrometer chamber. This scheme discriminated against desorption from the back of the sample or the sample h ~ l d e r . ~ ~The - ~ *surface heating rates varied from p = 0.1 to 4 us. In the LITD experiments, the beam of a pulsed C02 laser (Lumonics 930) was focused to a small spot of -450 pm on the silica surface. The original CO2 TEA laser was modified to become a TEM-00 laser by the installation of an 11 mm intracavity aperture.49 Two mirrors mounted on linear translators were used to raster the laser beam on the silica surface with an accuracy of f0.5pm.43,45,47The optimization of the laser power and pulse length to obtain the best performance and reduce the possible surface damage has been discussed e l ~ e w h e r e .During ~~ the LITD experiments, the C02 laser pulse at A = 10.6 pm produces a small heated area on the silica ~ u r f a c e . ~Heating ' rates higher than 1O'O K/s can be accomplished during laser heating.50 Heating to very high temperatures can be achieved with modest laser pulse energies. While the surface is hot, laser thermal desorption proceeds with a rate given by deldt = vOen exp(EdedRT),where n is the desorption order, Edes is the desorption activation barrier, and 8 is the surface coverage. The extent of desorption depends on the desorption order, the desorption kinetics, and the heating time.50 Longer time durations are always favorable when the desorption order is higher than 1; e.g., n = 2 for the dehydroxylation reaction 2SiOH Si-0-Si H20. However, the time scale of the laser heating is limited by the thermal diffusivity. On metallic and semiconductor surfaces, heat can diffuse -1 p m in 1 Therefore, the best LITD results are achieved with laser pulse durations of about 100 ns to keep the laser heating localized during the laser heating. The current experiment utilized C02 laser pulse widths of %lo0 ns. These pulse widths were obtained with a gas mixture of 7% C02 and 7% N2 in He. The TEM-00 C02 laser output energies were %30-50 rd.The laser beam was focused on the sample using a ZnSe lens with a focal length of 38 cm. A
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Figure 2. Spatial autocorrelation curve used to evaluate the size of the LITD desorption area. From the H20 LITD signals versus beam displacement, the elliptical area with a 1.54 aspect ratio has a diameter of 480 pm for the minor axis.
series of five or ten successive pulses was applied to the desorption area. This series of pulses averaged over laser energy fluctuations and better defined the edges of the desorption area.50 The desorption area dimensions were obtained using the spatial autocorrelation technique.29 Figure 2 displays a spatial autocorrelation curve obtained by measuring the H20 LITD signal from surface silanol species (2SiOH Si-0-Si H20) versus beam displacement. The C02 laser beam was incident on the silica surface at 49" relative to the surface normal. The desorption area was elliptical with an aspect ratio of 1.54 and a 450-480 p m minor axis obtained from the spatial autocorrelation measurements. The desorption event creates a temporal increase of the HzO partial pressure above the surface. This pressure transient was monitored by the UTI mass spectrometer that was located about 50 mm from the silica surface. The mass spectrometer has a line-of-sight to the silica surface as shown in Figure 1. Flight times between the surface and the ionizer of the mass spectrometer were -120 ps for H20. The time widths of the H2O LITD signals were -100 ps. E. Measurement of Hydroxyl Coverage. Probing the hydroxyl surface coverage on a planar silica surface is not straightforward. Techniques such as IR absorption spectroscopy may require a multireflection geometry to enhance the surface sensitivity and detect monolayer and submonolayer coverages.52 A multiple total internal reflection scheme is currently incompatible with our setup. High-resolution electron energy loss spectroscopy (HREELS) techniques could also be implemented but would require a charge-compensating beam to prevent sample ~ h a r g i n g . Our ~ ~ .chamber ~~ is not designed for HREELS experiments. Chemical reactions with the surface hydroxyls, such as SiOH ClSiR3 SiOSiR3 HC1, could also be used to determine the hydroxyl groups ~ o v e r a g e . ~ ,However, *~ this approach creates the additional problem of detecting a new surface species. The surface reactions also may not titrate all the hydroxyl surface species because of steric hindrance.24 This method can also lead to surface modification^.^^'^^^^ Another possible technique to measure the hydroxyl group coverage is titration of the SiOH species by probe molecule^.^^^^^
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Sneh and George
4642 J. Phys. Chem., Vol. 99, No. 13, 1995
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Temperature (K) Figure 3. Methanol TPD spectra from a siloxane surface following CH30H exposures at 120 K. The TPD signals are calibrated using the
integrated TPD peak area of a saturated monolayer of CHsOH hydrogen-bonded on a fully hydroxylated silanol surface. Various probe molecules may be adsorbed if the molecules have a strong hydrogen-bonding interaction with the surface hydroxyl groups. Alcohols have a hydroxyl group that can hydrogenbond with the SiOH surface species. In this study, methanol was used as the probe molecule to titrate the hydroxyl groups. The second technique that was employed to probe the hydroxyl group coverage was the LITD of H20 from surface hydroxyl groups. HzO can be desorbed from surface SiOH species by the dehydroxylation reaction 2SiOH -... Si-0-Si H20. The H20 LITD signal will represent the hydroxyl coverage if the dehydroxylation kinetics are rapid enough that all the hydroxyl groups are desorbed in the laser heated area.
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3. Results Methanol was dosed onto the silica surface using the capillary array d ~ s e r .Figure ~ ~ 3 displays a set of TPD experiments for methanol adsorbed on the siloxane (Si-0-Si) surface. This surface was obtained by perfonning several plasma cleaning procedures with 0 2 , followed by annealing at 900 "C for 5 h. As expected, the CH30H desorption spectra display only a zeroorder desorption feature57characteristic of a methanol multilayer on the siloxane surface. No TPD features were observed after exposures at temperatures higher than 150 K. The silica surface was then subjected to a plasma discharge at 0.3 Torr of H20 followed by annealing at 150 "C to remove the physisorbed water. After a CH30H dose at 130 K, Figure 4a shows that the CH30H TPD spectra display a new feature at a temperature higher than the multilayer CH30H TPD peak. This CH30H TPD peak is attributed to methanol hydrogenbonded to surface hydroxyl groups.25 CH30H exposures at T e 150 K displayed only the high-temperature CH30H TPD peak shown in Figure 4b. The intensity of this peak was independent of CH30H dosing time for exposures larger than -100 L at 150 K. Exposures of 500 L were used to ensure that a saturation coverage of methanol was obtained on the hydroxyl groups of the SiOH surface species.
150
200 Temperature (K)
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Figure 4. Top: Methanol TPD spectrum from a fully hydroxylated silanol surface following CHsOH exposures at 130 K. Both the hydrogen-bonded feature and the methanol multilayer feature are
present. Bottom: Methanol TPD spectrum from the fully hydroxylated silanol surface after annealing for a few minutes at 150 K to desorb the methanol multilayer. The thermal stability of silanol groups was studied by a series of annealing experiments in the temperature range from 100 to 870 "C. This experiment assumed that the hydrogen-bonded methanol coverage corresponds to the hydroxyl coverage. After annealing at a particular temperature, the SiOH coverage was determined by exposing the surface to a saturation methanol exposure at 150 K and then recording the CH30H TPD spectrum. Figure 5 displays a set of methanol TPD spectra obtained after different annealing temperatures. Note the reduction and shift of the CH30H TPD peak to lower temperatures after annealing at higher temperatures. This behavior indicates that there are no steric problems but rather attractive interactions between the adsorbed methanol molecules at higher hydroxyl coverages and higher corresponding CH30H coverages. A closer examination indicates that the CH30H TPD spectra taken after annealing in the temperature range from 550 to 870 "C display a similar peak temperature but a progressively smaller intensity at higher annealing temperatures. In addition, the TPD peak height is fairly constant after annealing in the temperature range from 100 to 550 "C. The peak area is reduced as the CH30H TPD peak becomes narrower and shifts to lower temperatures after annealing at progressively higher temperatures. The integrated area of the CH3OH TPD peak represents the methanol coverage titrating the hydroxyl groups. The CH3OH TPD peak areas versus thermal annealing are summarized in Figure 6 . No difference was observed for annealing experiments with 1 and 10 min annealing times. The CH30H TPD experiments may be affected by CH30H desorption from the sample holder or the cryostat. LITD experiments verified that the high-temperature methanol TPD feature originates from the silica surface. In these experiments,
J. Phys. Chem., Vol. 99, No. 13, 1995 4643
Stability of Hydroxyl Groups on a Silica Surface
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Temperature (K) Figure 5. Methanol TPD spectra obtained after saturation CH30H exposures at 150 K. The initial surface was a fully hydroxylated silanol surface. The CH30H TPD spectra change as a function of surface annealing temperature prior to CHsOH adsorption at 150 K.
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OH surface coverage. This behavior indicates that the CH30H TPD spectrum originates from the silica surface. One potential difficulty with methanol titration is the possible esterification reaction:,13 i.e. SiOH CH30H .+ SiO-CH3 H20. AES was used to check for carbon that may be deposited by the esterification r e a c t i ~ n .No ~ carbon was measured above the AES detection limit of %0.4%. This observation indicates that the esterification reaction is negligible at the temperatures and pressures used in these methanol titration experiments. The low AES detection limit is achieved using a 300 V biased screen and a frequency filter.32 The H20 LITD signal was also used as a direct probe of the surface hydroxyl coverage, e.g. SiOH SiOH SiOSi H20. In these experiments, the H20 LITD signal was measured at 150 "C after annealing the surface to the desired temperature. The H20 LITD results are displayed in Figure 8. The different symbols show the results of several separate LITD experiments. These H20 LITD measurements of the hydroxyl coverage are somewhat different than the methanol titration results. Most intriguing is the fact that no H20 LITD signals were measured after annealing to temperatures of 600 "C and higher. The time dependence of the H20 LITD signal was also examined as a function of annealing time. Figure 9 shows the results of dehydroxylation at a variety of annealing temperatures between 150 and 450 "C. Very little change in the H20 LITD signal is observed at 150 "C. At T > 150 "C, the H20 LITD signal falls rapidly and then levels off at a signal intensity that is progressively lower for higher annealing temperatures.
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Figure 7. Comparison between the methanol TPD spectrum from the hydrogen-bonded CH30H on the fully hydroxylated silanol surface (solid line) and the corresponding methanol LITD signals (solid circles). The LITD signal monitors the methanol coverage (e),whereas the TPD signal represents the methanol desorption rate (dO/dt). The differential of the methanol LITD signals (open circles) is a measure of -dO/dt and is consistent with the methanol TPD spectrum.
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Temperature ('C) Figure 6. Methanol TPD peak areas after saturation CH3OH exposures at 150 K. The initial surface was a fully hydroxylated silanol surface. The CH30H TPD peak areas decrease after annealing th~ssurface to higher temperatures prior to CH30H adsorption at 150 K.
the CH30H TPD spectrum (deldt) was compared with the CH3OH LITD signals obtained during a linear temperature ramp at the same heating rate. The CH30H LITD signal probes only the CH30H surface coverage (8).Figure 7 shows that the CH3OH TPD spectrum correlates with the disappearance of the CH3-
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4. Discussion
A. Methanol Hydrogen-Bonding on Hydroxyl Groups. Methanol physisorption on a siloxane surface occurs at 5 150 K. This temperature corresponds to the methanol condensation t e m p e r a t ~ r e .Figure ~ ~ 4b shows that higher adsorption energies are obtained on a silanol surface. This behavior is attributed to hydrogen-bonding of CH30H to hydroxyl groups of surface
Sneh and George
4644 J. Phys. Chem., Vol. 99, No. 13, I995
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Figure 8. H2O LITD signals from a fully hydroxylated silanol surface as a function of annealing temperature. Data is displayed from four
different experiments.
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Annealing Temperature
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initial silanol surface for four different annealing temperatures. SiOH species.25 At higher temperatures of 2200 "C and highpressures, methanol can react with the hydroxyl groups according to SiOH HOCH3 SiOCH3 H20 and esterify the ~urface.~ On~ a, ~completely ~ hydroxylated surface, this reaction is known to proceed to completion. This behavior indicates that there are no steric effects between the methyl group^.^ BET experiments have obtained 4.8-5.6 physisorbed methanol molecules per nm2 on various oxide surfaces.60-62 A coverage of ~ 4 . 7 - 4 . 8 molecules per nm2 was observed for methanol, ethanol, and n-butanol on silica and femc oxide surfaces.60161 The correspondence between this coverage and the hydroxyl coverage of 4.6 per nm2 on a fully hydroxylated silica surface indicates that the short-chain aliphatic alchohols are hydrogen-bonded to the surface hydroxyl groups with the CH3(CH2), group pointing away from the ~ u r f a c e . ~Conse~,~~ quently, methanol is expected to titrate the surface hydroxyl groups on the planar, well-defined silica surface with a 1:1 ratio. This hypothesis can be tested by examining CH30H TPD spectra versus hydroxyl coverage. A repulsive interaction at high methanol coverages would indicate steric hindrance and a CH30WSiOH titration ratio less than 1:l. The experimental
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Figure 10. Comparison between the thermal stabilities of hydroxyl groups on the Well-defined, planar silica surface and previous results The data for obtained from high surface area silica hydroxyl thermal stabilities on high surface area silica powders are taken from refs 64-68 (open squares), refs 64 and 65 (open circles), and ref 65 (open triangles).
observable would be lower adsorption energies at higher methanol and hydroxyl coverages. Consequently, the CH30H TPD peak would shift to lower temperatures at high hydroxyl coverages. Figure 5 shows that just the opposite behavior is observed for the methanol TPD spectra. The TPD peak shifts to higher temperatures at higher CH30H coverages corresponding to larger hydroxyl coverages. This behavior is consistent with attractive interactions between methanol molecules and a 1:1 titration ratio between methanol and the hydroxyl groups of SiOH surface species. Figure 10 compares the CH30H TPD titration results for the hydroxyl coverage versus thermal annealing with various sets of previous results.6.64-68 These earlier measurements of hydroxyl group thermal stability were obtained on silica powders using infrared, NMR, and chemical titration investigations.64-68 The agreement is extremely good between the CH30H titration results and these literature data. This correspondence is very encouraging because the earlier results on powders could have been seriously affected by many uncontrolled variables such as surface defects, surface strain, and surface contamination. The agreement between the CH30H titration results obtained on the Si02 thin film on Si(100) and the earlier data also indicates that the well-defined, planar silica surface is representative of a silica surface. Silica powders have a highly porous nature and conductance-limited pathways to their internal surface area. This porosity precludes serious time-dependent adsorption, diffusion, desorption, and reaction kinetics measurements. In contrast, our well-defined silica surface can be used as a model for planar silica surfaces for various surface kinetic studies. B. HzO LITD from Silanol Surface. The correspondence between the H20 LITD signals in Figure 8 and the previous literature data is not nearly as good. The H20 LITD signals are negligible after annealing temperatures of 600 "C and higher. In contrast, approximately 25-30% of the hydroxyl coverage still remains on the silica surface after annealing to 600 "C. This behavior suggests that the H20 LITD signal may be very dependent on the hydroxyl coverage. There may be some hydroxyl groups that cannot be desorbed as H20 by LITD. This behavior may be expected because the
J. Phys. Chem., Vol. 99, No. 13, 1995 4645
Stability of Hydroxyl Groups on a Silica Surface
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Temperature ("C) Figure 11. Proposed mechanism for hydrogen radicals reacting with a silica surface partially dehydroxylated at 600 "C.
Figure 12. H20 LITD signal as a function of annealing temperature for a fully hydroxylated silanol surface and a fully hydroxylated silanol surface first partially dehydroxylated by annealing at 600 "C and then reacted with the Hz plasma.
isolated and geminal hydroxyl groups become much further apart at lower hydroxyl coverage. For a hydroxyl coverage signal following the H2 plasma processing on a siloxane surface 525% of the coverage on a fully hydroxylated silanol surface, was 535% of the H20 LITD signal from a fully hydroxylated the average distance between hydroxyl groups is 2 10 A. The surface. diffusion and recombinative desorption of these isolated hyThe H20 LITD signal appears to probe only the vicinal droxyl groups by surface rearrangement' is expected to be hydroxyl groups. This assumption predicts that the H20 LITD difficult on the 100 ns time scale of the laser heating.50 signal can be superimposed on the literature data for the thermal In addition, the temperature range where LITD is able to stability of hydroxyl groups by adding a constant hydroxyl desorb H2O correlates well with the predicted range of H20 coverage. Because the H20 LITD signal is negligible at 600 desorption from the neighboring vicinal hydroxyl g r o ~ p s . ~ ~ ~ ~ ~ ~ ~ "C, the H20 LITD results can be offset by the hydroxyl coverage These vicinal hydroxyl groups are approximately 4.7 apart obtained after annealing at 600 "C. These baseline-shifted H20 and do not require surface rearrangement for dehydr~xylation.~ LITD signals shown in Figure 10 display a very good agreement Consequently, the H20 LITD signals are consistent with H20 with all the other data. derived only from the vicinal hydroxyl groups. Vicinal and isolatedgeminal hydroxyl species desorbing as To check for the existence of isolated and geminal silanol H20 in the ranges of 150-600 and 2600 "C, respectively, are species that cannot be laser desorbed, the silica surface was consistent with the shapes of the CH30H TPD spectra displayed reacted with a H2 plasma after annealing at 600 "C. Hydrogen in Figure 5. Methanol molecules adsorbing on isolated hydroxyl radicals are expected to cleave the siloxane bridge bonds and groups left after thermal annealing at 2600 "C are separated form an hydroxyl group and a silicon monohydride speby 210 8, and are too far apart to interact attractively. c i e ~ , ~Le.~ 2H , ~ ~ Si-0-Si , ~ ~ SiH SOH. Assuming Therefore, the CH30H TPD peaks in Figure 5 show no complete dehydroxylation at 600 "C, the predicted HzO LITD temperature shift versus coverage and the desorption kinetics signal after the H2 plasma processing should be approximately should represent the CH30WSiOH pair interaction. On silica half the H20 LITD signal obtained from a fully hydroxylated surfaces containing vicinal hydroxyl groups after annealing at surface. However, if there are isolated hydroxyl groups on the lower temperatures 5600 "C, the methanol molecules are close surface that cannot be desorbed by LITD, the creation of enough to interact attractively. The CH30H desorption peaks additional surface hydroxyl groups by the H2 plasma will in Figure 5 shift to higher temperatures, indicating attractive, produce more vicinal hydroxyl species. The resulting H20 coverage-dependent desorption kinetics at higher CH30H and LITD signal should then be greater than half the H20 LITD SiOH coverages. signal from a fully hydroxylated surface. This experiment is Assuming that the H20 LITD signal probes the vicinal represented schematically by the cartoon in Figure 11. hydroxyl groups, the time scale of desorption from vicinal A fully hydroxylated surface was annealed to 600 "C and hydroxyl groups was obtained at various annealing temperatures. then reacted with a HZplasma. This surface produced a H20 A series of annealing experiments were performed as a function LITD signal that was about 65% of the H20 LITD signal from of annealing time. The vicinal hydroxyl coverage was obtained a fully hydroxylated surface. In addition, a series of thermal from the H20 LITD signal. These experiments should be valid annealing experiments were performed following the H2 plasma in the temperature range 150-600 "C, because no substantial processing. The results are displayed in Figure 12 together with desorption of isolated or geminal silanols is apparent at these the results for a fully hydroxylated surface. These results are temperatures.16 As displayed in Figure 9, the hydroxyl coverage consistent with the initial fully hydroxylated silica surface having monitored by the HzO LITD signals displays an initial rapid a substantial coverage of hydroxyl groups after annealing at decrease and then levels off at a signal intensity that decreases 600 "C. Apparently, most of the previously undetected hydroxyl progressively for higher temperatures. groups after annealing at 600 "C can be laser desorbed as H20 This dehydroxylation behavior is consistent with results from following the HZplasma processing. In contrast, the H20 LITD high surface area silica powders.6 However, the time scale
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reported for high surface area silica surfaces6is at least 2 orders of magnitude longer than the -1 min time scales observed in Figure 12. This deviation may be related to the high porosity and conductance-limited transport of H20 from the high surface area silica p o ~ d e r s .These ~ differences illustrate some of the problems that may be encountered when using porous samples for surface kinetics experiments.
5. Conclusions High surface area silica has been utilized previously to study surface chemistry on silica surfaces. These studies average over many surface inhomogeneities including various surface defects and contaminants, surface curvature and strain, and varying porosities and surface areas. In this study, a well-defined, planar silica surface was prepared to overcome these possible inhomogeneities. This silica sample was a 5 p m thick Si02 film grown on Si(100) by a combination of thermal oxidation and chemical vapor deposition with SiH4 and 0 2 . The thermal stability of hydroxyl groups was investigated on this well-defined, planar silica surface using temperatureprogrammed desorption (TPD) and laser-induced thermal desorption (LITD) techniques. Methanol was used to titrate the surface SiOH species by hydrogen-bonding between the hydroxyl groups. In contrast to the methanol multilayer peak at 155-160 K, a broad, hydrogen-bonded CH30H TPD peak was observed at 170-190 K from the hydroxylated silanol surface. The area under this CH30H TPD peak was used to measure the surface hydroxyl coverage versus thermal annealing. The hydroxyl coverage was observed to decrease progressively versus annealing temperature from 100 to 900" C. These results were consistent with previous studies on high surface area silica powders. The H20 LITD signals were also employed to measure the hydroxyl species by direct H20 desorption from SiOH surface species, i.e. 2SiOH Si-0-Si H20. The H20 LITD signals were negligible after thermal annealing at 2600 "C. These results suggested that the H20 LITD signals were only produced from vicinal hydroxyl groups. Experiments utilizing HZplasma processing after thermal annealing at 600 "C were consistent with the presence of isolated hydroxyl groups on the silica surface that could not be desorbed by the CO2 laser. These isolated hydroxyl groups could be converted to vicinal hydroxyl groups and laser desorbed following the H2 plasma treatment. The initial measurements of the kinetics of dehydroxylation on the well-defined, planar silica surface displayed a large difference in time scales compared with earlier measurements on high surface area silica powders. These differences may reflect the conductance-limited transport out of high porosity silica powders and illustrate the limitations of using high surface area silica for studies of adsorption, desorption, and reaction kinetics on silica surfaces. The agreement between the thermal stabilities of the hydroxyl groups on the well-defined silica surface and the high surface area powders also indicates that the planar Si02 film grown on Si(100) is an effective model substrate for kinetic studies on silica surfaces.
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Acknowledgment. This research was supported by the Office of Naval Research under Contract No. N00014-92-J1365 and by the National Science Foundation Grant No. CHE9215247. The authors thank Alex Grabbe from Sandia National Laboratories for stimulating discussions. O.S. is grateful to the Israeli Academy of Sciences for a Wolfson postdoctoral fellowship. S.M.G. acknowledges the National Science Foundation for a Presidential Young Investigator Award (19881994).
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