Langmuir 1995,11, 3970-3974
3970
Characterization of Si02 Thin Film Obtained by the Sol-Gel Route from TEOS and Triton X45 P. Piaggio," A. Bottino, G. Capannelli, and E. Carosini Istituto di Chimica Industriale, Uniuersita di Genova, Corso Europa 30, 16132 Genoua, Italy '
A. Julbe
Laboratoire des Materiaux et Procedes Membranaires (CNRS UMR 9987), Ecole Nationale Superieure de Chimie, 8 Rue de l%cole Normale, 34053 Montpellier Cedex 1, France Received November 15, 1994. In Final Form: June 19, 1995@ Si02 thin films prepared by the sol-gel process from TEOS as alkoxide precursors and Triton X45 as additive have been investigated by vibrational spectroscopy,thermal analysis, and Nz adsorptioddesorption measurements. These studies indicate that microporous materials capable of retaining both high surface area and free silanol concentration at 600 "C were obtained. These findings are discussed in relation to the structure of the surface silanols, interaction between silanols, and the additive and the thermal degradation of the latter. On these bases a formation mechanism of the porous structure is proposed.
Introduction The sol-gel processing route allows the preparation of a wide variety of ceramic materials by controlling the hydrolysis and condensation reactions of the starting precurs0rs.l In the field ofmembrane technology the solgel process is often used to deposit a very thin microporous film onto the surface of a mesoporous ceramic A way of preparing stable, homogeneous, and defect-free films is provided by adding to the starting sol organic additives which allow control of the interfacial energy and/ or the drying rate during the p r o c e ~ s . ~The - ~ choice of suitable additives is strictly related to a good knowledge of the mechanism governing both sol and gel formation as well as the evolution of the gel during the successive firing treatment. Therefore a reliable characterization of the materials is required, and the use of sampling techniques which, as far as possible, do not involve contamination or structural changes, is particularly important. To this end we have investigated the possible application of vibrational techniques to follow the structural changes which occur during the preparation of the Si02 thin films. This paper deals with the study of unsupported films prepared from tetraethoxysilane (TEOS) as organic precursor and Triton X45 (TX). Indeed such nonionic surfactant belongs to the family of poly(ethy1ene glycol) phenyl ethers, which have been found to be very useful additives for preparing silica membranes with tailored microporous structures.8 It is shown that FT-Raman
* To whom correspondence should be addressed: phone, 39 10 353 8558;FAX, 39 10 353 8323;e-mail,
[email protected]. Abstract Dublished in Advance ACS Abstracts. SeDtember 1. 1995. @
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(1)Brinker, C. J.; Scherer, G. W. Sol-Gel Science: the physics and chemistry of sol-gelprocessing; Academic Press: San Diego, CA, 1990. (2) Leenars, A. F. M.; BurggraafA. J. J . Colloid Interface Sci. 1985, 105, 27. (3)Larbot, A.; Fabre, J. P.; Guizard, C.; Cot, L. J . Membrane Sci. 1988,39, 203. (4) Larbot,A.; Julbe, A,; Guizard, C.; Cot, L. J. Membrane Sci. 1989, 44, 289. (5)Uhlhorn, R. J. R.; Huis In't Veld, M. H. B. J.;Keizer, K.; Burggraaf, A. J. J. Mater. Sci. 1992,27, 527. ( 6 ) Julbe, A,; Guizard, C.; Larbot, A,; Cot, L.; Giroir-Fendler,A. J. Membrane Sci. 1993,77, 137. (7) Brinker, C. J.;Ward, T. L.; Sehgal, R.; Raman, N. K.; Hietala, S. L.; Smith, D. M.; Hua, D. W.; Headley, T. J. J. Membrane Sci. 1993, 77,165.
0743-7463/95/2411-3970$09.00/0
spectroscopy is valuable to study the sol phase while reliable information about gel and xerogels films can be obtained by in situ FTIR measurements at different equilibrium conditions of temperature and relative humidity. In particular the spectral changes observed during a few temperature cycles allowed us to clarify the structure of the silanols at the sample surface as well as their interaction with the organic additive. In addition thermal and porosimetric analyses have been carried out on the same materials and the results are discussed together with the spectroscopic findings. In such a way a fairly complete characterization of the porous films has been obtained.
Experimental Section Preparation of Silica Sols, Gels, and Xerogels. Freshly prepared bidistilled water, ethanol (EtOH, >99.8%, Aldrich), t-Oct-C~H4-(OCH~-CH2)xOH ( x = 5) (TritonX45,Fluka),and nitric acid (60%,Aldrich) were first mixed at room temperature and then heated at 60 "C before TEOS (98%,Aldrich) addition. The resulting solution (2 mol of HzO, 1 mol of EtOH, 0.02 mol of HN03, 1 mol of TEOS, 5.7% (vlv)TX) was allowed to react for 2 h at 60 "C to obtain a clear sol. In order to prepare gel film, a proper amount of sol was first poured in aluminum dishes at room temperature and then heated at 50 "C for 24 h. Xerogels were obtained by firingthe gel for 1h at the temperature indicated in the text. CharacterizationTechniques. FTIR spectra were obtained by using a Bruker IFS66 spectrometer while FT-Raman measurements were carried out using a Bruker RFSlOO instrument with a Nd-YAG laser (1064 nm). In both cases a resolution of 2 cm-l was used. In situ IR measurements were performed during temperature cycles, between 30 and 220 "C, by a Specac variable temperature cell. The relative humidity (RH) was controlled by fluxing the spectrometer with a dry air stream. Quartz cells or tubes were used for Raman samples. Surface area, pore volume, and pore size distribution were evaluatedby Nz adsorptioddesorptionmeasurementsperformed with a Micromeritics ASAP 2000 instrument. The samples, whose weight varied from 300 to 400 mg, were outgassed at 200 "C for 20 h with the high vacuum pump connected to the instrument. The duration of each measurement varied from 12 to 20 h depending on the weight of the sample as well as on its porosity. Pore volume and pore size distributionwere evaluated from the Density Functional Theoretic Models developed by Micromeritics, for slit-shaped pores. (8)Julbe, A,; Balzer, C.; Barthez,J. M.; Guizard, C.; Larbot, A.; Cot, L. J. Sol-Gel Sci. Technol., in press.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 10, 1995 3971
Si02 Thin Films
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Raman shift cm'l Figure 1. FT-Raman spectra of EtOH (a), TX (b), TEOS (4, sol (d), gel before (e) and after (0 firing at 450 "C.
Differential thermal (DTA) and thermogravimetric (TGA) analyses were carried out with a Mettler DSC 30 calorimeter and a Perkin-ElmerTG analyzer,respectively. The heating rate of 10 Wmin in nitrogen was used for both measurements.
Results and Discussion Spectroscopy. The FT-Raman spectra of the sol, as well as of its neat components, are shown in Figure la-d. The comparison of these spectra reveals that the band at 650 cm-l, characteristic of TEOS (Figure IC),disappears in the sol spectrum (Figure Id) which, on the contrary, shows all the bands of the other two components (Figure la,b). This seems to indicate that the hydrolysis of the alkoxide is practically complete. Because of the very small Raman activity of SiOH and water vibrational modes, the spectrum of the sol is quite similar to that of a solution of TX in EtOH. With heating of the sol, evaporation of the EtOH occurs and the spectrum of the resulting gel (Figure l e ) is similar to that of TX (Figure lb). Further heating of this gel above 400 "C causes the elimination ofthe organic additive so that the spectrum (Figure If) does not present any significant band and only an appreciable enhancement of the background, often observed in high surface area inorganic material^,^ is shown. These results indicate that FT-Raman spectroscopy is valuable for studying the changes of the composition of starting solution during the sol formation but it is less useful for investigating the evolution of the silicic network structure of the gel. This can be properly done by transmission FTIR spectroscopy, given that very thin free standing samples can be easily obtained. In fact, the spectra of these samples, even if dominated by the very strong absorption of the Si-0-Si skeletal vibrations of the bulk below 1300 cm-l, offer a wealth of information for both the surface silanolic structure and the organic components in the higher frequency part.lOJ1 Parts a-c of Figure 2 show the spectral evolution of a film of gel (Figure 2a), successively heated up to 220 "C (Figure 2b) and hence recooled down (Figure 2c) in a dry atmosphere (RH < 8%). In the former two spectra an appreciable cut-off of the intensity of the strongest bands is observed but it disappears during the coolingtreatment (Figure 2c). These changes are surprising as the cut-off band is generally associated to the presence of holes in highly absorbing (9) Schrader, B.; Hoffmann, A.; Keller, S. Spectrochim. Acta 1991, 47A, 1135. (10)Legrand, A. P.; Hommel, H.; Tuel, A,; Vidal, A.; Balard, H.; Papirer, E.; Levitz, P.; Czernichowski, M.; Erre, R.; Van Damme, H.; Gallas, J.P.; Hemidy, J. F.; Clavalley, J.;Barres, 0.;Burneau, A.; Grillet, Y. Adu. Colloid Interface Sci. 1990,33, 91. (11)Lin-Vien, D.; Colthup, N. B.; Fateley W. G.; Grasselli, J. G. The Handbook ofrnfrared and Raman CharacteristicFrequencies oforganic Molecules; Academic Press: San Diego, CA, 1991.
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Figure 2. FTIR "in situ" spectra of a thin film of gel (a),heated at 220 "C (b) and hence cooled at 30 "C (c).FTIR spectrum of neat TX (d) (capillary film between KBr windows). samples. However the cut-off phenomenon can be also observed if a strong deviation from parallel flat surfaces is present as well as for a heterogeneous mixture ofhighly absorbing and transparent media. It is worth stressing that the spectra ofFigure 2 refer to "in situ"measurements on the same sample, i.e. without removing it from the radiation beam or changing the cell position, as well as that other gel samples behaved in a similar fashion. These findings seem to indicate that the thermal treatment leads to the suppression of the optical inhomogeneity present in the starting material and responsible for the band cutoff. This hypothesis is consistent with the observation of scattering effects and baseline changes in the spectra recorded during the heating process especially between 150 and 220 "C, i.e. in the temperature range where both relevant weight loss and exothermic effects, as it will be shown in the following, are observed. However further investigation is necessary to clarify this point and, in particular, light scattering techniques would be very useful. Further inspection of Figure 2 reveals that heating in a dry atmosphere causes the complete release of both EtOH and water and the spectrum of Figure 2c is assignable only to TX and silica matrix. The shape of the OH stretching band does not show any evidence of free SiOH absorption above 3700 cm-l and compared with that observed for the neat TX (Figure 2d) shows the maximum of absorption red shifted of about 60 cm-', indicative of a strenghtening of the H bonds. These findings agree quite well with the data reported for adsorption of alcohols on silica surfaces.12 However it is worth remembering that also silanols H bonded to ethers absorb in the 34003300 cm-l region.12 A deeper insight into the nature of the interactions between the OH groups can be achieved by considering the spectral changes induced by repeated heating and cooling of the sample between 30 and 220 "C in a dry atmosphere. A complete reversibility is observed during several cycles, showing that each spectrum corresponds to a n equilibrium condition and no mass transfer within the sample is involved. Typical spectra for selected temperature values are shown in Figure 3. It can be noticed that minor baseline changes, probably due to small variations of the thermal emissivity of the cell, are appreciable only below 1700 cm-l. The most relevant fact in Figure 3 is however the regular modification of the OH stretching bands; i.e. the low-frequency component, assigned to the H bonded OH groups of TX and silanols loses intensity and its maximum moves from 3340 to 3390 cm-l upon heating. All curves cross each other around 3515 cm-l, while the intensity of the component a t 3694 cm-l, which is characteristic ofweakly perturbed silanols, (12) Knozinger, H. In The Hydrogen bond; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976, Vol 111, p 1265.
3972 Langmuir, Vol. 11, No. 10, 1995 Y I ,
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Figure 3. FTIR spectra of a thin film of Figure 2c for equilibrium conditions at RH a 8%and 30 "C (a),90 "C (b), 160 "C (c), and 220 "C (d).
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Figure 4. FTIR spectra of a thin film of gel fired for l h at 400 8% "C (a) and after repeated temperature cycles at RH between 30 "C (b) and 220 "C (c).
rises. This frequency is lower than the 3720 cm-' value generally observed in the case of vicinal SiOH engaged in a H bond as proton acceptor alone12J3but fairly agrees with the band shiR found for the adsorption ofthe aliphatic hydrocarbons on silica.1°J2 On the contrary it is rather difficult to understand the spectral changes observed around 3600 cm-l. Two different types of donors (Si-OH and C-OH) and four acceptors (Si-OH, C-OH, C-0-C, n-electrons) are present and a complex pattern of H bonds12 is possible. Moreover spectral changes are also associated to the temperature dependence of the H bond distances.14J5 Therefore a detailed discussion of these effects will be presented elsewhere. The results of Figure 3 however as a whole allow the conclusion that almost a t room temperature a n appreciable amount of silanol is strongly associated to TX by H bonds. Treatments at higher temperatures cause marked chemical changes in the material, as shown by the FTIR spectra of xerogels obtained by firing the gels at 400 and 600 "C reported in Figures 4 and 5 , respectively. In both figures (a) refers to the initial sample while (b) and (c) indicate the equilibrium spectra recorded in dry atmosphere a t 30 and 220 "C, respectively, to evaluate the hygroscopicity of the xerogels. The comparison of the spectra in Figure 4 with that of Figure 2d demontrates that TX is degraded a t 400 "C. The aliphatic bands between 3000 and 2800 cm-l and below 1500 cm-l are (13) Burneau, A.; Barr&, 0.;Gallas J. P.;Lavalley, J. C. Langmuir 1990,6,1364. (14) Piaggio, P.;Tubino, R.; Dellepiane, G. J.Mol. Struct. 1983,96, 277. (15) Hadzi, D.;Bratos, S.In The Hydrogen bond Schuster, P.,Zundel, G., Sandorfy, C., Eds.;North-Holland: Amsterdam, 1976,VolII, p 567.
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Figure 6. FTIR spectra of a thin film of gel fired for 1h at 600 "C (a) and after repeated temperature cycles at RH a 8% between 30 "C (b) and 220 "C (c).
noticeably reduced while the aromatic ones (3100-3000, 1605,and 1507cm-l) are less affected. Anew rather broad band is observed around 1720 cm-l where the stretching of the carbonylic groups is expected. Moreover the presence of a sharp band a t 3744 cm-l clearly shows that many isolated silanols have been generated.13J6-18 The broader absorptions around 3670 and 3500 cm-l can be assigned to OH groups engaged in H bonds as proton acceptors or donors, re~pective1y.l~ The contribution of both silanols and water to this band can be easily discriminated by comparison between spectra a and b of Figure 4. As can be seen, the bands of adsorbed water (3670,3400 and 1630 cm-l) are not present in Figure 4b, which shows only a smaller band a t 3575 cm-' and a broadening on the low-frequency side of the 3745 cm-' band. These features are indicative of the presence also of several vicinal H bonded SiOH groups. The spectrum of Figure 4c shows that heating causes a decrease of the intensity of the low-frequencyside of the band and, hence, of the silanols engaged in H bonds. Moreover the shift from 1720to 1735cm-l ofthe C=O band ofthe degradation products is very likely indicative of the loosening of H bonding with silanols. All these findings are quite in agreement with the spectra of Figure 5 , which show that the firing a t 600 "C removes completely the organic materials but does not cause any relevant condensation of the free silanols, as the relative intensity of the OH stretch (3744 cm-') with respect to the S i 0 combination modes (2000- 1800 cm-l) is slightly affected. Even if less apparent than in spectra a and b of Figure 4, the water absorption bands in Figure 5a disappear after the temperature cycles (Figure 5b). Moreover the lower frequency side of the OH stretching band in Figure 5b (or Figure 5c) is strongly reduced with respect to what was observed in Figure 4b (or Figure 4c). This clearly shows that the firing at 600 "C causes a relevant condensation ofthe neighboring H bonded SiOH groups, still present after the 400 "C treatment. Thermal Analysis. The results of thermogravimetric and differential thermal analyses of the gel are reported in the Figures 6 and 7a, respectively. The former figure shows that the weight loss is faster around 50, 200, and 460 "C. The first loss is unambiguously related to the release of water and EtOH while that observed a t the highest temperature is due to degradation of TX. According to this behavior, the DTA curve of Figure 7a shows endotherms around 100 "C and between 4 6 and 500 "C. (16) Gallas. J. P: Lavallev. J. C.: Burneau A,: Barres. 0.Lawmuir 1991,7, 1235. (17) Morrow, B. A,; McFarlan, A. J. Langmuzr 1991,7,1695. (18) Morrow, B. A.; McFarlan, A. J. J.Phys. Chem. 1992,96,1395. I
Si02 Thin Films
Langmuir, Vol. 11, No. 10, 1995 3973
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More troublesome is the interpretation of the change of the loss rate observed in Figure 6 at the intermediate temperature as well as the rather complicatedcurve profile in the same temperature range (Figure 7a) due to the superimposition of exothermic and endothermic effects. These features are observed neither for a sample kept for 10 min a t 220 "C (Figure 7b) nor for a gel obtained without TX (Figure 7c). Indeed, in the first case (Figure 7b) as the solvent has been completely removed no endotherm appears in the low-temperature range and only the decomposition peak of TX(Figure 7d) is observed. On the contrary the sample without TX shows only a simple endotherm due to the solvent loss (Figure 7c). All these findings, together with the results of the spectroscopic analysis of the samples obtained after the thermal treatments, can be reasonably explained as follows. Above 100 "C H bonding of solvent molecules is loosen and the increased concentration of TX, due to the solvent evaporation, favors the formation of new types of H bonds between TX and solvent molecules with consequent possible formation of adducts. However these latter are not very stable so that further heating causes the complete release of the solvent. Finally it must be noticed that some broad features observed above 220 "C in the DTA curves (spectra a and b) of Figure 7 could be reasonably associated to the silanol condensation process induced by heating. This interpretation agrees also with the reduction of the intensity ratio between the SiO-H stretching and S i 0 combination bands, observed when the firing temperature is increased, as shown by comparison of Figures 3-5. In order to get a deeper insight into these facts, further investigation is necessary, especially as far as the effects related to changes in the scan rate are concerned.
Figure 9. DFT derived pore size distribution for gel fired at 300 "C (+), 400 "C (*), 500 "C (01, and 600 "C (A). Table 1. Surface Area and DFT Derived Pore Volume for Xeroeels of Figure 9 ~~
firing temp ("C)
Langmuir surface area (mz/d
DFT pore volume (cm3/d
300 400 500 600
731 1166 868 877
0.2218 0.3685 0.2596 0.2596
Porosimetry. Adsorption isotherms and DFT derived pore size distribution curves of samples fired a t different temperatures are reported in Figures 8 and 9, respectively. In the former figure type I isotherms, which are characteristisc of microporous materials,lg are observed. The same figure also reveals that for any given relative pressure the volume of Nz adsorbed is maximum for the sample fired at 400 "C. From Figure 9 it appears that by varying the temperature the profile of the pore size distribution curve does not sensibly change. The values of surface area and pore volume of different samples are listed in Table 1. The lowest values observed for samples treated a t 300 "C are very likely due to the presence of the TX into the pores. With the gel heated to 400 "C, TX degrades to small molecular weight products, which in part are released, allowing the emptying of the pores. This explanation seems to be not completely consistent with the the infrared spectra of Figure 4 which show that degraded organic products are stillpresent after firing at 400 "C. However it is worthwhile to observe that (19)Gregg, S. J.;Sing,K. S. W.Adsorption, surfacearea andporosity; Academic Press: San Diego, CA, 1982.
3974 Langmuir, Vol. 11, No. 10, 1995 the experimental conditions in the two cases are quite different. Namely the very high vacuum conditions required for the adsorptioddesorption technique can make easier the emptying of the pores, while some higher thermal energy is needed to obtain the same result a t atmospheric pressure, like in the case ofthe FTIR analysis. The firing a t 500 and 600 "C causes a decrease of the surface area and porosity, in agreement with the spectral changes observed for the structure of the silica matrix, already shown by discussing the FTIR spectra of Figures 4 and 5 .
Conclusions The characterization of thin unsupported films of silica prepared by the sol-gel method from TEOS and TritonX45 has been performed by vibrational spectroscopy, thermal analysis, and Nz adsorption. By integrating the calorimetric data with the infrared spectra, it is possible to clarify the mechanisms of formation of the porous film as well as the function of the additive. In particular it has been demonstrated that in situ spectroscopic measurements a t different temperatures and in controlled humidity conditions are necessary to determine the type of surface silanols and to understand the nature of the interactions between the additive and the silica matrix. In fact the equilibrium conditions, for a given humidity value, can be easily obtained by a few temperature cycles and directly checked by FTIR techniques. Porosimetric measurements show that a substantial increase of porosity can be obtained by removing the additive through firing a t least a t 400 "C. Further firing a t 500 and 600 "C only sligtly affects the porous structure of these xerogels. FTIR spectroscopyand thermal analysis show that between 100 and 200 "C a relevant rearrangement of the H bonds between TX, solvent molecules, and silanols occurs. It is our opininion that water, after the
Piaggio et al.
breaking of its H bonds, can bind the 0 atoms of TX molecules, whose motion is slow due to the high mass. However these new H-bonds cannot resist a t a temperature exceeding 200 "C and then the release of the water occurs. It follows that the rate of the drying process is significantly reduced in comparison with the case of the sol without TX. An appreciable limitation to the condensation reactions and, hence, to the growing of silica particles, is still operative at higher temperature as the interparticle cavities are filled by aggregates of TX. Indeed the condensation of silanols occurs preferably along the walls of the cavity, avoiding the formation of a dense material. Above 300 "C TX begins to degrade and the residual small molecules can be completely removed by vacuum around 400 "C or at higher temperature at normal pressure. In air, TX degradation mainly occurs via oxidative mechanism of both t-octyl group and the polyether chain. As final result a highly porous ceramic film is obtained and an appreciable number of isolated silanols is still present on the surface of the pores. As these SiOH groups must be a t a distance greater than 0.44 nm,13 their condensation is practically hindered unless a strong distortion of the S i 0 network occurs. The firing up to 600 "C is not enough to do it significantly but causes the almost complete condensation of residual neighboring silanols. This latter process forms mainly strained Si-0-Si structures and the vicinal silanols can be restored by simply rehydrating the material. Finally the obtained porosimetric data together with the complete absence of residual organic material confirm promising applications of these xerogels in the membrane field.
Acknowledgment. The work was supported by the Commission of the European Communities under the Contract Brite Euram n. BREU. CT91-0406. LA940914X
