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Surface hydroxylation and silane grafting on fumed and thermal silica
Journal of Colloid and Interface Science 264 (2003) 354–361 www.elsevier.com/locate/jcis
Surface hydroxylation and silane grafting on fumed and thermal silica Vincent Dugas and Yves Chevalier ∗ Laboratoire des Matériaux Organiques à Propriétés Spécifiques, UMR 5041 CNRS, Université de Savoie, B.P. 24, 69390 Vernaison, France Received 4 March 2003; accepted 20 May 2003
Abstract The optimization of the surface functionalization of flat thermal silicon oxide by silanes was investigated. The difficulties are the low density of silanols at the surface of thermal silica, the lack of precise knowledge of the actual surface chemistry of thermal silica and of its hydroxylation, and the limited number of possible chemical analyses at flat surfaces of small area. This steered our study toward a comparative investigation of the hydroxylation and silane grafting of thermal silica and the well-known fumed silica. The silane grafting density for fumed silica that had undergone thermal treatments of dehydroxylation was related to the surface density of silanols. The surface density of silane on the flat thermal silica as measured by FTIR-ATR spectroscopy was 1.4 µmol/m2 , similar to that of fumed silica dehydroxylated at 1000 ◦ C. This moderate value was related to the low silanol density present on such silica surfaces. Several rehydroxylation treatments that proved their efficiency on dehydroxylated fumed silica did not lead to any noticeable improvement on thermal silicon dioxide. 2003 Elsevier Inc. All rights reserved. Keywords: Silica; Silane; Silanol; Chemical grafting; Infrared spectroscopy
1. Introduction The chemical grafting of silanes at the surface of thermal silica is one step of the surface functionalization of microelectronic devices used in different fields such as molecular electronics [1], chemical sensors (ISFETs) [2,3], and DNA chips [4], all of them emerging and gaining importance nowadays. The grafting of silanes onto silica requires the presence of silanol groups at the surface. There are many types of silica differing in their surface properties [5,6]. In particular, the density of silanol groups and the types of silanols (isolated, hydrogen bonded, geminal) vary very much. In the case of low silanol density, a hydrolysis of the surface that creates silanol groups from siloxane bridges is useful prior to silane grafting [7]. There is a large body of investigations into surface chemistry and silane coupling dealing with precipitated and pyrogenic silica [8], but thermal silica has received little attention, in spite of its technical importance in electronic devices. Thermal silica, which is made by means of thermal oxidation of silicon, has a low density of silanol groups at its surface. There are also a large variety of thermal silicon oxides that may differ in their surface chemistry [9]. Fumed * Corresponding author.
E-mail address: [email protected] (Y. Chevalier). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00552-6
silica and thermal silicon dioxide have often been confused. Fumed silica is prepared by combustion of silicon tetrachloride in a flame of hydrogen and oxygen; this aerosol gives a pulverulent powder of high specific area and having a very hydrophilic surface. Indeed, the oxidation into silica takes place in the presence of the water vapor produced in the flame. The accepted values for the silanol density of fumed silica are in the range 6–8 µmol/m2 [5,10]. The high silanol density values of fumed silica have often been assumed for thermal silica [11]. But the few literature reports that deal with the hydroxyl density of flat thermal silica reveal a much lower density of silanols at the surface [12–15]. For example, adsorption measurements on silica sputtered onto a germanium crystal by means of IR-ATR gave a silanol density of 2.3 µmol/m2 [15]. The purpose of this investigation is to optimize the grafting process on thermal silica with reference to the wellknown processes used on fumed silica. The advantage of fumed silica as a reference is that many chemical and spectroscopic analyses can be carried out because of the high specific area. In particular, conventional elemental analyses [16] of the grafted species and solid state 13 C and 29 Si NMR [17] can easily be performed. Such methods cannot be used at the surface of flat wafers having area a few cm2 . IR spectroscopy can be used both on fumed silica powder by transmission or diffuse reflectance (DRIFT) [18] and at
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the surface of thermal silica by attenuated total reflectance (ATR) [19]. Since fumed silica is highly hydroxylated at its surface, whereas the silanol density of thermal silica may be an order of magnitude lower, the grafting of silanes was investigated on thermally dehydroxylated fumed silica, which was expected to resemble thermal silica. The principle of the investigation method then consisted in comparing the silane grafting densities on thermal silica with those at the surface of fumed silica that had been dehydroxylated to variable extents. Since it is shown that the silane grafting density is related to the silanol density [20], the surface of thermal silica was considered as similar to that of the fumed silica that gave the same silane grafting density. One goal of this study is to find a type of silica of large specific area which could be used as a representative model of thermal silica. Last, attempts aimed at increasing the silane grafting density on thermal silica by applying methods efficient on fumed silica are reported. The method that is currently proposed is finally quite similar to the method of titration of the surface silanol of silica by means of reactions the Si–OH groups with various reagents. Various silanes and alcohols have been used for that purpose. This well-known method has been strongly criticized because some of the silanol groups may not be accessible, especially when the reagent is a bulky molecule. Because it suffers from this major drawback when the silanol density is high, alternative methods are preferred, either spectroscopic or by isotopic exchange with deuterium [21]. The chemical grafting method gives fairly accurate values of the Si–OH density when the densities to be measured are low [22,23]. The grafting density is controlled by steric interactions between grafted molecules for highly hydroxylated silica [24], so that the measured grafting densities are lower than the silanol densities of the underneath silica surface. Our purpose being to investigate thermal silica in comparison with fumed silica, a rather bulky silane was chosen because this functional molecule was interesting for further surface chemistry. This is not a limitation regarding the present study, because the silanol density of thermal silica is actually low. In cases where the silanol density is low, a hydrolysis process is applied to the surface of silica, which cleaves the siloxane bridges into supplementary silanol groups. The grafting process then involves two steps: the hydroxylation of the silica surface and the silane grafting reaction itself. Much attention was paid to the first step of hydrolysis, which was thought to be crucial for the thermal silica because of its low silanol density. The grafting reaction of a monofunctional silane was selected for the second step because it is simpler and more reproducible than those of multifunctional silanes. Here, the functionality of the silane is the number of reactive groups present on the silicon atom of the silane. Thus, there are several troubles with the use of trifunctional silanes such as trimethoxysilane or trichlorosilane because they may react with the surface hydroxyls by means of their three reactive
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groups, but this reaction is most often incomplete because it is not possible to attach the same silicon atom to the three oxygens present at fixed positions at the surface, keeping the bond lengths and angles close to reasonable values. Furthermore, the hydrolysis of the silane which takes place in the frequent case of incomplete drying of the silica surface leads to a concomitant polycondensation of the hydrolyzed silanes and condensation with the surface silanols; an illdefined polycondensate of organosilane grows from the surface and a thick and rough final silane layer results [25]. The amount of water at the surface of silica being very difficult to control, the grafting of multifunctional silanes is intrinsically of poor reproducibility. Such troubles do not occur with monofunctional silanes that bind to the surface by means of a single bond. Even in the case of hydrolysis of the silane next to the surface, the polycondensation of the hydrolyzed silanes amounts to a simple coupling reaction that leads to a nongrafted dimer (disiloxane). But monofunctional silanes have a lower reactivity than multifunctional ones [26]. This has to be compensated for by the choice of a highly reactive leaving group such as dimethylamino [27]. Thus, methyl [(dimethylamino)dimethylsilyl] undecanoate (Fig. 1) was selected as a model monofunctional silane for the present investigation. The presence of an ester group makes the IR analysis easier, using the C=O stretching absorption band at 1733 cm−1 . The grafting reaction takes place according to two different mechanisms: either direct condensation or hydrolysis and condensation as shown in Fig. 1.
2. Materials and methods 2.1. Reagents The synthesis-grade solvents isopentane and THF were purchased from SDS. Pentane was dried and stored over 3 Å molecular sieves and THF was dried by distillation over sodium metal before use. Methyl (chlorodimethylsilyl)undecanoate from Gelest and dimethylamine from Fluka were used as received. Deionized water (18 M cm) was prepared by percolation through ion-exchange resins. Methyl [(dimethylamino)dimethylsilyl]undecanoate was prepared by reaction of the methyl (chlorodimethylsilyl)undecanoate with dimethylamine according to Boksanyi et al. [28]. It was purified by distillation (125 ◦ C, 0.8 mbar) prior to use. IR (liquid compound absorbed into dry KBr pellets): 2928 and 2851 cm−1 (νC–H ); 1738 cm−1 (νC=O ester); 986 and 835 cm−1 (δ Si–N). 1 HNMR (in CDCl , δ in ppm from TMS): −0.07 (s, 6 H); 3 0.39 (m, 2 H); 1.3 (m, 10 H); 1.6 (quint, 2 H); 2.3 (t, 2 H); 3.6 (s, 3 H).
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Fig. 1. The two mechanisms of the grafting reaction of methyl [(dimethylamino)dimethylsilyl]undecanoate on silica: (a) direct condensation; (b) hydrolysis and condensation.
2.2. Substrates Fumed silica having specific area 200 m2 /g was purchased from Aldrich. Partial dehydroxylation of its surface was carried out by means of thermal treatments at high temperatures (from 150 up to 1000 ◦ C) in a tubular oven under dry argon. Thermal silica was prepared by oxidation of the surface of silicon samples. The samples were 10 × 5 × 1 mm parallellepipedic prisms of monocrystalline Si(111) designed as internal reflection elements for attenuated total reflectance IR spectroscopy (ATR-IR). They were purchased from SpectraTech. The Si samples were cleaned prior to their oxidation. A first cleansing in H2 SO4 /H2 O2 (10/1) at 90 ◦ C for 10 min was followed by rinsing with DI water, etching of the native silicon oxide by immersion in a HF solution for 10 min (buffer oxide etchant: 7/1 mixture of HF (49%) and NH4 F (40%)), and a final washing with anhydrous methanol. Placing the crystal in a tubular oven flushed with dry synthetic air at 800 ◦ C for 1 h formed thermal oxide. The thickness of the thermal oxide layer was then 60–70 Å [29]. 2.3. Grafting process The silica samples were dried under anhydrous gas flow at 140 ◦ C for 2 h. After cooling, the silica surface was covered with the silane by immersing the samples in a solution of silane in pentane followed by evaporation of the pentane under reduced pressure. A film of silane was also deposited on the flat surfaces in the same manner. The grafting reaction itself was then performed at 140 ◦ C for 16 h. Unreacted materials were removed by washing with THF in a Soxhlet extractor and the samples were subsequently dried. Grafting densities were measured on silica powders by means of elemental analyses of carbon at the Service Central d’Analyses of the CNRS (SCA-CNRS, B.P. 22, F69390
Vernaison) and by means of quantitative IR analysis. Quantitative analyses were carried out on the prisms of silicon covered by thermal silica by ATR-IR spectroscopy. 2.4. IR spectroscopy IR spectroscopy was performed on a Nexus FTIR (Nicolet) equipped with a DTGS detector. Fumed silica samples were analyzed by diffuse reflectance IR Fourier transform spectroscopy with SpectraTech DRIFT equipment. The fumed silica powder was mixed with dry potassium bromide (10% of silica in KBr) and spectra were recorded by collecting 16 scans at 4 cm−1 resolution. Flat silicon samples were analyzed by ATR on silicon prisms covered with thermal silica using a 4× beam condenser and microATR equipment from SpectraTech that allowed 10 reflections. Spectra were obtained by subtracting the spectrum of the uncoated oxidized ATR substrate from the spectrum of the coated substrate. For each spectrum, 1000 scans at 2 cm−1 resolution were recorded. In order to obtain quantitative information, a calibration was made with both powders and ATR crystals where known quantities of silane had been deposited. Linear calibration curves were obtained in all cases. For DRIFT, the comparisons of the different spectra were performed in Kubelka– Munk units using the Si–O vibration of silica at 1868 cm−1 as an internal calibration.
3. Results and discussion 3.1. Silanol density and grafting of the silane on fumed silica Since extensive studies dealing with the hydroxylation and chemical grafting of fumed silica have already been reported in the literature, the purpose of the present study was
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to collect data on fumed silica under the same experimental conditions as for the thermal silica and with the same silane, which has not been used in previous work. Thus, a relationship between the surface hydroxylation rate (density of silanols) and the density of chemical grafting was first established. 3.2. Silanol types and densities upon thermal treatments The surface of silica is dehydrated as the temperature of the thermal treatment increases. The dehydration comes from the elimination of adsorbed water and the thermal dehydration reaction, where two neighboring silanols condense into a siloxane bridge. These two phenomena are difficult to distinguish because water evolves in both cases. The first step of dehydration occurs under moderate temperature (<190 ◦ C) and consists in the elimination of water adsorbed on the silica surface. This preliminary treatment is required in order to obtain fully hydroxylated silica in a reproducible manner [7]. Water elimination at higher temperatures is also due to the hydroxyl pair condensation. Surface silanols are either free Si–OH groups or hydrogen-bonded with water or vicinal silanol groups. Because of their short distance (shorter than 3 Å), the later hydrogen-bonded silanols are identifiable by IR or NMR spectroscopy [10,30]. The SiO–H stretching vibration observed in IR spectroscopy is particularly useful. Thus, hydrogen-bonded silanols appeared as a broad band at 3600 cm−1 , whereas a sharp band at 3745 cm−1 corresponded to isolated silanols (Fig. 2). Dehydroxylation of fumed silica at increasing temperatures resulted in a decrease of the IR band of the hydrogen-bonded Si–OH at 3600 cm−1 . Conversely, the intensity of the IR bands of the isolated Si–OH at 3745 cm−1 first increased because dehydroxylation of hydrogen-bonded Si–OH created supplementary isolated Si–OH (Fig. 2). This later band finally decreased at the highest temperatures (above 700 ◦ C). The behavior of the surface silica hydroxyls as observed in the present study and according to the
Fig. 2. IR spectra of fumed silica in the SiO–H vibration domain after thermal treatment at 325 (solid), 700 (dashed), and 1000 ◦ C (dotted) for 70 h. The Si–O vibration at 1868 cm−1 marked with a star was used as an internal calibration of the intensities.
literature [10] is summarized in the scheme of Fig. 3 and in Fig. 4. Methyl [(dimethylamino)dimethylsilyl]undecanoate was grafted onto fumed silica powders that had been heat-treated for 70 h. The grafting density was measured by means of elemental analysis of carbon and quantitative DRIFT. Elemental analysis gives the total carbon content of the sample and is therefore very sensitive to contamination by residual reagents or solvents. Thus, excess silane was eliminated by repeated washings with THF in a Soxhlet extractor; the powder was filtered and subsequently dried under vacuum for two days in order to remove the THF to completion. This process allowed accurate determinations since a supplementary drying did not change the carbon content. Typical carbon analyses were between 5 and 10 wt%. DRIFT spectroscopy gives a quantitative analysis of the grafting density if the scattering conditions of different samples are repeatable. This could be successfully achieved by diluting the
Fig. 3. Schematic representation of the silanol types and densities upon thermal treatments.
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Fig. 4. Density of total silanols (!), free silanols (E), and grafting density (") as a function of the temperature of thermal treatment.
silica powder into KBr powder. A 10% silica weight with respect to KBr gave the best results in terms of signal-tonoise ratio and reproducibility; the exact amount and the accuracy of sampling were not crucial since the combination vibration band of SiO2 at 1868 cm−1 was used for internal intensity calibration [31]. A calibration curve with samples of silica where known amounts of silane have been deposited allowed a quantitative analysis. Results from elemental analysis and DRIFT spectroscopy applied to both the silane bands at 2920 cm−1 (C–H) and 1733 cm−1 (ester) were similar. This gave confidence to the data since the IR ester band is not affected by the contamination by THF. Moreover, this method did not suffer from inaccuracies of the Kubelka–Munk theory coming from the oversimplification of the scattering [32], because the calibration was done with the same type of powder as for the grafted one. All these methods gave the amounts of grafted silane per gram of silica (mol/g), which were converted into grafting densities (mol/m2) using the specific area of the silica (200 m2 /g). The grafting method by impregnation and condensation gives a maximum grafting density because a thin film of bulk reagent is deposited in the impregnation step. The process is then similar to a reaction performed in bulk silane, which gave maximum efficiency [22]. Indeed, a supplementary grafting step increases the grafting density by some 10% only. The grafting reaction was made in the presence of a large excess of silane, so that the presence of a trace of water, if any, did not cause any trouble. The grafting density on untreated fumed silica (but nevertheless dried at 140 ◦ C) having the maximum density of silanols was 3.5 µmol/m2 . This value corresponds to a dense monolayer and is close to the maximum according to the bulkiness of the two methyl groups attached to the silicon atom [24]. When the silica surface was dehydroxylated by means of thermal treatments and directly used in the silane grafting reaction, the grafting density decreased because of the lower density of the underlying silanol groups (Fig. 4). This effect appeared for thermal treatments above
450 ◦ C, where the density of silanol groups was lower than 3.5 µmol/m2 . This result suggested that chemical grafting was a suitable way to determine silanols at densities below 3.5 µmol/m2 . The relationship between grafting density and silanol density taken from the literature [10] is shown in Fig. 4. The silane grafting density is lower than the silanol density below 450 ◦ C because of the lateral steric hindrance of the silane molecules, as explained before. Both densities are identical above 450 ◦ C, but the silane grafting density departs from the density of Si–OH at the highest temperatures above 700 ◦ C. This observation suggested that siloxane bridges have been opened during the vigorous grafting process, where pure silane was spread on the silica surface at 140 ◦ C for 16 h. The dimethylamine released by the grafting reaction might help in opening siloxane bridges. The last interesting point is that there was a significant grafting of silane (1.7 µmol/m2) after a thermal treatment at 1000 ◦ C where adsorbed water has been removed to completion. This indicates that the grafting path by means of direct condensation (see Fig. 1) is actually operative. Let us recall that a large number of reports on silane grafting describe the mechanism of hydrolysis and condensation as the sole reaction path. Even some authors claimed that the presence of water is absolutely required for silane grafting [33]. 3.3. Determination of the silanol density on thermal silica Silane chemical grafting was performed onto the surface of silicon ATR internal reflection elements where thermal oxide was grown by oxidation at 800 ◦ C. The oxide layer was 60–70 Å thick [29]. This fresh oxide layer was then used without further treatment for grafting the methyl [dimethyl(dimethylamino)silyl]undecanoate. The IR spectrum recorded in ATR (Fig. 5) showed the expected characteristic bands of the grafted silane (C–H stretching bands in the 2900 cm−1 region and C=O stretching band at 1733 cm−1 ). The silica bands did not appear because the spectrum of the same bare ATR crystal recorded before grafting was subtracted. The grafting density could be measured from the intensity of the ester band of the silane in ATR-IR with the help of the calibration by deposition. A calibration curve was performed by measuring the intensity of the IR bands for successive depositions of known silane solutions on the ATR crystal. The calibration curves for the bands at 2921 cm−1 and 1733 cm−1 were both linear, demonstrating the validity of such a measurement (Fig. 6). The absorbance was used directly; no normalization was necessary as in DRIFT spectroscopy on powders. The linear calibration curve for the C–H band at 2921 cm−1 did not pass through the origin, however, because there was always a residual C–H band coming from organic contamination on the sample, but also in different parts of the IR beam optical path. This might not impede a quantitative measurement, but the ester carbonyl band at 1733 cm−1 was nevertheless preferred, because the calibration line passed through the origin.
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Fig. 5. ATR-IR spectrum of methyl [(dimethylamino)silyl]undecanoate on the ATR internal reflection element oxidized at 800 ◦ C for 1 h.
The problems of organic contamination, which readily happen at this scale, were avoided by using the ester band for the analysis (Fig. 6). The density of silane grafted on thermal silica was then 1.4 µmol/m2 . This value is 2.5 times lower than that of the dense monolayer of 3.5 µmol/m2 measured on fumed silica and corresponds to that of the fumed silica dehydroxylated at 1000 ◦ C. This moderate value of the grafting density reflects the low density of silanol groups of thermal silica. The correlation between the silanol and silane grafting densities allowed assessing the silanol density, which could not be measured directly. On the basis of the silane grafting results, a good model for thermal silica would be fumed silica heated at 1000 ◦ C and not rehydroxylated. This is not that surprising, since thermal silica has been prepared by oxidation at high temperature (800 ◦ C) and it has been shown that the silanol densities of different silicas converge to a common value upon treatment at such high temperatures [23].
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Fig. 6. Intensities of the absorption bands at 2921 (!) and 1733 cm−1 (") as a function of the amount of deposited silane on the ATR crystal. The linear regression to the data at 1733 cm−1 gave y = 0.0051x (R 2 = 0.998).
3.4. Rehydroxylation studies Increasing the grafting density has thereafter been attempted by means of a rehydroxylation process. The simplest and efficient method for the rehydroxylation of dehydrated fumed silica is a hydrothermal treatment in boiling water [34]. As already observed [6,20,34,35], complete rehydroxylation was possible after dehydroxylation at moderate temperatures, but part of the dehydroxylation was irreversible at the highest temperatures. Thus, rehydroxylation for 4 h in boiling water restored the full IR intensity of the SiO–H bands when the dehydroxylation has been performed below 700 ◦ C. Dehydroxylations at higher temperatures, especially over long times, were partially reversible. Only part of the IR intensity could be recovered after rehydroxylation in boiling water (Fig. 7, left). The irreversible part of the dehydroxylation should not be ascribed to the irreversible removal of geminal silanols that takes place till 400 ◦ C [36]. The irreversible behavior is observed at higher
Fig. 7. Rehydroxylation of fumed silica. Left: DRIFT of fumed silica heated for 70 h at 325 (solid), 700 (dashed), and 1000 ◦ C (dotted) after rehydroxylation in boiling water for 4 h. Right: fumed silica heated for 70 h at 1000 ◦ C (dashed) and after rehydroxylation in boiling water for 4 h (solid); the inset shows the difference spectrum. The small bands at 2900–3000 cm−1 are due to organic contamination.
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Table 1 Determination of the grafting density of thermal silica by means of ATR-IR for various methods of hydroxylation Treatment Mean absorbance (10−3 ) Standard deviation Grafting density (µmol/m2 )
None
H2 O (boiling)
H2 SO4 /H2 O2
Etching with HF
7.1 0.9 1.4
6.5 0.5 1.3
7.2
4.4 0.7 0.9
Fig. 8. Grafting density after heat treatment (!) and after heat treatment and rehydroxylation in boiling water (").
temperatures. The siloxane bridges are of better stability because less strained bonds are formed at high temperatures. Facilitated proton migration at the surface may contribute to the formation of siloxane bonds of optimum geometry. Since H-bonded Si–OH were formed, the density of free Si–OH decreased upon rehydroxylation (see the difference spectrum in Fig. 7 right). This phenomenon is the reverse of the behavior observed in the dehydroxylation process. The rehydroxylation treatment allows a better grafting of the silane for silica samples that have been extensively dehydroxylated. But the grafting ability was not completely recovered by a rehydroxylation treatment (Fig. 8). Thus, grafting densities close to 4 µmol/m2 were reached after a rehydroxylation treatment in boiling water, slightly higher than for the raw fumed silica. But the grafting density decreased significantly for silica heated above 700 ◦ C. The grafting density of fumed silica heated at 1000 ◦ C was 1.7 µmol/m2 and was increased to 3.4 µmol/m2 upon rehydroxylation in boiling water. Rehydroxylation was indeed beneficial but some useful reactive sites could not be recovered because of the partial rehydroxylation. There is again a good correlation between the grafting density and the density of silanols. In all cases, the recovery of the grafting density is good. The limiting value reached after rehydroxylation is close to that of the original untreated fumed silica. Among the rehydroxylation treatments studied, that in boiling water was the most efficient. In particular, strongly oxidizing and acidic solutions such as sulfochromic acid or ‘Piranha’ solution (H2 O2 /H2 SO4 mixture), which were often used to clean the surfaces of wafers, were of poor efficiency for the rehydroxylation. The intensities of the SiO–H
1.4
bands were lower than for the rehydroxylation with boiling water. On the other hand, basic solutions cannot be used because they are too much aggressive for the silica surface. The same methods of rehydroxylation have been investigated on thermal silicon dioxide, where the grafting of silane gave low densities (Table 1). The hydroxylation in either boiling water or ‘Piranha’ solution was inefficient in increasing the grafting density. A chemical etching in HF solution, which is commonly used in microelectronic technology, was also checked because this process cleaves the siloxane bridges. It was expected that a slight etching followed by an abundant rinsing with water could create new silanols groups at the surface. But the actual effect was disastrous regarding the silane grafting (Table 1). Because of the correlation between the silanol and silane grafting densities, the lack of variation of the grafting density indicates an inefficient hydrolysis of the surface siloxane bonds. The hydroxylation of thermal silica appears very difficult as compared to that of fumed silica dehydroxylated at 1000 ◦ C, which is a major difference between them. On the basis of the silane grafting results, a good model for thermal silica would be fumed silica heated at 1000 ◦ C and not rehydroxylated. But an important difference remains, since such fumed silica can be partially rehydroxylated, while thermal silica cannot. 4. Conclusions The comparative investigation of thermal and fumed silica as regards their surface silanol densities and silane grafting allowed to get new information on the surface chemistry of thermal silica. As was already well documented in the literature, fumed silica can be dehydroxylated by thermal treatments and rehydroxylated by hydrolysis of siloxane bonds in hot water. Chemical grafting of monofunctional organosilane permitted the establishment of a relationship between the density of silanols groups at the silica surface and the grafting density. This relationship is valid at low silanol densities (below ca. 3.5 µmol/m2). On this basis, the ATR-IR analysis of the derivatization of thermal silicon dioxide made it possible to estimate the surface hydroxyl density for the studied thermal silica. Quantitative IR spectroscopy is well suited for monitoring the grafting density, making use of absorption bands that are not affected by unavoidable organic contaminations. The density of silanol groups at the surface of thermal silica is low and the related silane grafting is also low
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(1.4 µmol/m2). Thermal oxide is stable against the hydrolysis processes studied and therefore differs from dehydroxylated fumed silica in this respect. It appeared very difficult to increase the density of the silane grafting on such a substrate by means of a surface treatment. Last, there are alternatives in order to increase the silane coverage of wafers in spite of the low density of reactive sites. Thus, self-assembled monolayers form with silanes having long n-alkyl chains that pack on the surface into a dense 2-dimensional layer because of lateral interactions between paraffinic chains [37]. This process yields a dense monolayer of adsorbed silane in the crystalline or liquid-condensed state prior to the condensation reaction with the surface sites. There are many reports of dense coverage of thermal silica with long-chained silanes (e.g., octadecyltrichlorosilane). This process may, however, appear difficult when a functional group attached at the end of the alkyl chain disturbs the formation of the self-assembled layer. References [1] M.C. Petty, M.R. Bryce, D. Bloor, An Introduction to Molecular Electronics, Oxford Univ. Press, New York, 1995. [2] I. Willner, E. Katz, Angew. Chem. Int. Ed. 39 (2000) 1180–1218. [3] P. Cléchet, N. Jaffrezic-Renault, C. Martelet, in: S. Yamauchi (Ed.), in: Chemical Sensor Technology, Vol. 4, Kodansha, Tokyo, 1992, pp. 205–225. [4] S.L. Beaucage, Curr. Med. Chem. 8 (2001) 1213–1244. [5] R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. [6] A.P. Legrand, The Surface Properties of Silicas, Wiley, Chichester, 1998. [7] J. Gobet, E.Sz. Kovátz, Adsorpt. Sci. Technol. 1 (1984) 77–92. [8] E.P. Plueddemann, Silane Coupling Agents, Plenum, New York, 1982. [9] W.A. Pliskin, J. Vac. Sci. Technol. 14 (1977) 1064–1081. [10] L.T. Zhuravlev, Colloids Surf. A 173 (2000) 1–38. [11] (a) L. Bousse, N.F. de Rooij, P. Bergveld, IEEE Trans. Electron. Devices ED-30 (1983) 1263–1270; (b) L. Bousse, N.F. de Rooij, P. Bergveld, Surface Sci. 135 (1983) 479–496; (c) A. van der Berg, P. Bergveld, D.N. Reinhoudt, E.J.R. Sudhölter, Sensors Actuators 8 (1985) 129–148; (d) C.D. Fung, P.W. Cheung, W.H. Ko, IEEE Trans. Electron. Devices ED-33 (1986) 8–18; (e) R.E.G. van Hal, J.C.T. Eijkel, P. Bergveld, Adv. Colloid Interface Sci. 69 (1996) 31–62. [12] K.H. Beckmann, N.J. Harrick, J. Electrochem. Soc. Solid State Sci. 118 (1971) 614–619. [13] D.L. Angst, G.W. Simmons, Langmuir 7 (1991) 2236–2242. [14] M.-J. Azzopardi, Réaction des organosilanes à l’interface silice/solution. Étude in situ par spectroscopie infrarouge, Ph.D. thesis, Université Paris VI, 1994.
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[15] L. Gueneau, Sites de surface, greffage et photodégradation d’organosilanes sur SiO2 et TiO2 , Ph.D. thesis, Université Paris-Sud, 1998. [16] Z. Elbhiri, Y. Chevalier, J.-M. Chovelon, N. Jaffrezic-Renault, Talanta 52 (2000) 495–507. [17] G.E. Maciel, D.W. Sindorf, V.J. Bartuska, J. Chromatogr. 205 (1981) 438–443. [18] J.P. Blitz, R.S.S. Murthy, D.E. Leyden, J. Colloid Interface Sci. 121 (1988) 63–69. [19] (a) D.R. Scheuing (Ed.), Fourier Transform Infrared Spectroscopy in Colloid and Interface Science, ACS Symp. Ser., Vol. 447, American Chemical Society, Washington, DC, 1991; (b) M.-J. Azzopardi, H. Arribart, J. Adhesion 146 (1994) 103–115. [20] A.P. Legrand, H. Hommel, A. Tuel, A. Vidal, H. Balard, E. Papirer, P. Levitz, M. Czernichowski, R. Erre, H. Van Damme, J.P. Gallas, J.F. Hemidy, J.C. Lavalley, O. Barres, A. Burneau, Y. Grillet, Adv. Colloid Interface Sci. 33 (1990) 91–330. [21] (a) L.T. Zhuravlev, A.V. Kiselev, Russ. J. Phys. Chem. 39 (1965) 236– 238; (b) B.A. Morrow, A.J. McFarlan, Langmuir 7 (1991) 1695–1701. [22] D.W. Sindorf, G.E. Maciel, J. Phys. Chem. 87 (1983) 5516–5521. [23] J.P. Gallas, J.C. Lavalley, A. Burneau, O. Barres, Langmuir 7 (1991) 1235–1240. [24] L. Boksányi, O. Liardon, E.Sz. Kovátz, Adv. Colloid Interface Sci. 6 (1976) 95–137. [25] (a) C.P. Tripp, M.L. Hair, Langmuir 11 (1995) 149–155; (b) M.W. Daniels, L.F. Francis, J. Colloid Interface Sci. 205 (1998) 191–200; (c) W. Yoshida, R.P. Castro, J.-D. Jou, Y. Cohen, Langmuir 17 (2001) 5882–5888. [26] B. Evans, T.E. White, J. Catalysis 11 (1968) 336–341. [27] (a) K. Szabó, N.L. Ha, P. Schneider, P. Zeltner, E.Sz. Kovátz, Helv. Chim. Acta 67 (1984) 2128–2142; (b) K.D. Lork, K.K. Unger, J.N. Kinkel, J. Chromatogr. 352 (1986) 199–211. [28] L. Boksányi, O. Liardon, E.Sz. Kovátz, Helv. Chim. Acta 59 (1976) 717–727. [29] C.R. Helms, B.E. Deal, The Physics and Chemistry of SiO2 and the Si–SiO2 Interface, Plenum, 1993. [30] D.R. Kinney, I.S. Chuang, G.E. Maciel, J. Am. Chem. Soc. 115 (1993) 6786–6794. [31] D. Gorski, E. Klemm, P. Fink, H.-H. Hörhold, J. Colloid Interface Sci. 126 (1988) 445–449. [32] H.G. Hecht, Anal. Chem. 48 (1976) 1775–1779. [33] (a) C.P. Tripp, M.L. Hair, Langmuir 8 (1992) 1961–1967; (b) M.L. Hair, C.P. Tripp, Colloids Surf. A 105 (1995) 95–103. [34] A.A. Agzamkhozhaev, L.T. Zhuravlev, A.V. Kiselev, Ya. Shengeliya, Colloid J. USSR 36 (1974) 1036–1039. [35] D.W. Sindorf, G.E. Maciel, J. Am. Chem. Soc. 105 (1983) 1487–1493. [36] J.A. Hockey, B.A. Pethica, Trans. Faraday Soc. 57 (1961) 2247–2262. [37] (a) J. Sagiv, J. Am. Chem Soc. 102 (1980) 92–98; (b) A. Ulman, Chem. Rev. 96 (1996) 1533–1554; (c) C. Carraro, O.W. Yauw, M.M. Sung, R. Maboudian, J. Phys. Chem. 102 (1998) 4441–4445; (d) S. Navarre, F. Choplin, J. Bousbaa, B. Bennetau, L. Nony, J.-P. Aimé, Langmuir 17 (2001) 4844–4850.
Surface hydroxylation and silane grafting on fumed and thermal silica Vincent Dugas and Yves Chevalier ∗ Laboratoire des Matériaux Organiques à Propriétés Spécifiques, UMR 5041 CNRS, Université de Savoie, B.P. 24, 69390 Vernaison, France Received 4 March 2003; accepted 20 May 2003
Abstract The optimization of the surface functionalization of flat thermal silicon oxide by silanes was investigated. The difficulties are the low density of silanols at the surface of thermal silica, the lack of precise knowledge of the actual surface chemistry of thermal silica and of its hydroxylation, and the limited number of possible chemical analyses at flat surfaces of small area. This steered our study toward a comparative investigation of the hydroxylation and silane grafting of thermal silica and the well-known fumed silica. The silane grafting density for fumed silica that had undergone thermal treatments of dehydroxylation was related to the surface density of silanols. The surface density of silane on the flat thermal silica as measured by FTIR-ATR spectroscopy was 1.4 µmol/m2 , similar to that of fumed silica dehydroxylated at 1000 ◦ C. This moderate value was related to the low silanol density present on such silica surfaces. Several rehydroxylation treatments that proved their efficiency on dehydroxylated fumed silica did not lead to any noticeable improvement on thermal silicon dioxide. 2003 Elsevier Inc. All rights reserved. Keywords: Silica; Silane; Silanol; Chemical grafting; Infrared spectroscopy
1. Introduction The chemical grafting of silanes at the surface of thermal silica is one step of the surface functionalization of microelectronic devices used in different fields such as molecular electronics [1], chemical sensors (ISFETs) [2,3], and DNA chips [4], all of them emerging and gaining importance nowadays. The grafting of silanes onto silica requires the presence of silanol groups at the surface. There are many types of silica differing in their surface properties [5,6]. In particular, the density of silanol groups and the types of silanols (isolated, hydrogen bonded, geminal) vary very much. In the case of low silanol density, a hydrolysis of the surface that creates silanol groups from siloxane bridges is useful prior to silane grafting [7]. There is a large body of investigations into surface chemistry and silane coupling dealing with precipitated and pyrogenic silica [8], but thermal silica has received little attention, in spite of its technical importance in electronic devices. Thermal silica, which is made by means of thermal oxidation of silicon, has a low density of silanol groups at its surface. There are also a large variety of thermal silicon oxides that may differ in their surface chemistry [9]. Fumed * Corresponding author.
E-mail address: [email protected] (Y. Chevalier). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00552-6
silica and thermal silicon dioxide have often been confused. Fumed silica is prepared by combustion of silicon tetrachloride in a flame of hydrogen and oxygen; this aerosol gives a pulverulent powder of high specific area and having a very hydrophilic surface. Indeed, the oxidation into silica takes place in the presence of the water vapor produced in the flame. The accepted values for the silanol density of fumed silica are in the range 6–8 µmol/m2 [5,10]. The high silanol density values of fumed silica have often been assumed for thermal silica [11]. But the few literature reports that deal with the hydroxyl density of flat thermal silica reveal a much lower density of silanols at the surface [12–15]. For example, adsorption measurements on silica sputtered onto a germanium crystal by means of IR-ATR gave a silanol density of 2.3 µmol/m2 [15]. The purpose of this investigation is to optimize the grafting process on thermal silica with reference to the wellknown processes used on fumed silica. The advantage of fumed silica as a reference is that many chemical and spectroscopic analyses can be carried out because of the high specific area. In particular, conventional elemental analyses [16] of the grafted species and solid state 13 C and 29 Si NMR [17] can easily be performed. Such methods cannot be used at the surface of flat wafers having area a few cm2 . IR spectroscopy can be used both on fumed silica powder by transmission or diffuse reflectance (DRIFT) [18] and at
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the surface of thermal silica by attenuated total reflectance (ATR) [19]. Since fumed silica is highly hydroxylated at its surface, whereas the silanol density of thermal silica may be an order of magnitude lower, the grafting of silanes was investigated on thermally dehydroxylated fumed silica, which was expected to resemble thermal silica. The principle of the investigation method then consisted in comparing the silane grafting densities on thermal silica with those at the surface of fumed silica that had been dehydroxylated to variable extents. Since it is shown that the silane grafting density is related to the silanol density [20], the surface of thermal silica was considered as similar to that of the fumed silica that gave the same silane grafting density. One goal of this study is to find a type of silica of large specific area which could be used as a representative model of thermal silica. Last, attempts aimed at increasing the silane grafting density on thermal silica by applying methods efficient on fumed silica are reported. The method that is currently proposed is finally quite similar to the method of titration of the surface silanol of silica by means of reactions the Si–OH groups with various reagents. Various silanes and alcohols have been used for that purpose. This well-known method has been strongly criticized because some of the silanol groups may not be accessible, especially when the reagent is a bulky molecule. Because it suffers from this major drawback when the silanol density is high, alternative methods are preferred, either spectroscopic or by isotopic exchange with deuterium [21]. The chemical grafting method gives fairly accurate values of the Si–OH density when the densities to be measured are low [22,23]. The grafting density is controlled by steric interactions between grafted molecules for highly hydroxylated silica [24], so that the measured grafting densities are lower than the silanol densities of the underneath silica surface. Our purpose being to investigate thermal silica in comparison with fumed silica, a rather bulky silane was chosen because this functional molecule was interesting for further surface chemistry. This is not a limitation regarding the present study, because the silanol density of thermal silica is actually low. In cases where the silanol density is low, a hydrolysis process is applied to the surface of silica, which cleaves the siloxane bridges into supplementary silanol groups. The grafting process then involves two steps: the hydroxylation of the silica surface and the silane grafting reaction itself. Much attention was paid to the first step of hydrolysis, which was thought to be crucial for the thermal silica because of its low silanol density. The grafting reaction of a monofunctional silane was selected for the second step because it is simpler and more reproducible than those of multifunctional silanes. Here, the functionality of the silane is the number of reactive groups present on the silicon atom of the silane. Thus, there are several troubles with the use of trifunctional silanes such as trimethoxysilane or trichlorosilane because they may react with the surface hydroxyls by means of their three reactive
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groups, but this reaction is most often incomplete because it is not possible to attach the same silicon atom to the three oxygens present at fixed positions at the surface, keeping the bond lengths and angles close to reasonable values. Furthermore, the hydrolysis of the silane which takes place in the frequent case of incomplete drying of the silica surface leads to a concomitant polycondensation of the hydrolyzed silanes and condensation with the surface silanols; an illdefined polycondensate of organosilane grows from the surface and a thick and rough final silane layer results [25]. The amount of water at the surface of silica being very difficult to control, the grafting of multifunctional silanes is intrinsically of poor reproducibility. Such troubles do not occur with monofunctional silanes that bind to the surface by means of a single bond. Even in the case of hydrolysis of the silane next to the surface, the polycondensation of the hydrolyzed silanes amounts to a simple coupling reaction that leads to a nongrafted dimer (disiloxane). But monofunctional silanes have a lower reactivity than multifunctional ones [26]. This has to be compensated for by the choice of a highly reactive leaving group such as dimethylamino [27]. Thus, methyl [(dimethylamino)dimethylsilyl] undecanoate (Fig. 1) was selected as a model monofunctional silane for the present investigation. The presence of an ester group makes the IR analysis easier, using the C=O stretching absorption band at 1733 cm−1 . The grafting reaction takes place according to two different mechanisms: either direct condensation or hydrolysis and condensation as shown in Fig. 1.
2. Materials and methods 2.1. Reagents The synthesis-grade solvents isopentane and THF were purchased from SDS. Pentane was dried and stored over 3 Å molecular sieves and THF was dried by distillation over sodium metal before use. Methyl (chlorodimethylsilyl)undecanoate from Gelest and dimethylamine from Fluka were used as received. Deionized water (18 M cm) was prepared by percolation through ion-exchange resins. Methyl [(dimethylamino)dimethylsilyl]undecanoate was prepared by reaction of the methyl (chlorodimethylsilyl)undecanoate with dimethylamine according to Boksanyi et al. [28]. It was purified by distillation (125 ◦ C, 0.8 mbar) prior to use. IR (liquid compound absorbed into dry KBr pellets): 2928 and 2851 cm−1 (νC–H ); 1738 cm−1 (νC=O ester); 986 and 835 cm−1 (δ Si–N). 1 HNMR (in CDCl , δ in ppm from TMS): −0.07 (s, 6 H); 3 0.39 (m, 2 H); 1.3 (m, 10 H); 1.6 (quint, 2 H); 2.3 (t, 2 H); 3.6 (s, 3 H).
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Fig. 1. The two mechanisms of the grafting reaction of methyl [(dimethylamino)dimethylsilyl]undecanoate on silica: (a) direct condensation; (b) hydrolysis and condensation.
2.2. Substrates Fumed silica having specific area 200 m2 /g was purchased from Aldrich. Partial dehydroxylation of its surface was carried out by means of thermal treatments at high temperatures (from 150 up to 1000 ◦ C) in a tubular oven under dry argon. Thermal silica was prepared by oxidation of the surface of silicon samples. The samples were 10 × 5 × 1 mm parallellepipedic prisms of monocrystalline Si(111) designed as internal reflection elements for attenuated total reflectance IR spectroscopy (ATR-IR). They were purchased from SpectraTech. The Si samples were cleaned prior to their oxidation. A first cleansing in H2 SO4 /H2 O2 (10/1) at 90 ◦ C for 10 min was followed by rinsing with DI water, etching of the native silicon oxide by immersion in a HF solution for 10 min (buffer oxide etchant: 7/1 mixture of HF (49%) and NH4 F (40%)), and a final washing with anhydrous methanol. Placing the crystal in a tubular oven flushed with dry synthetic air at 800 ◦ C for 1 h formed thermal oxide. The thickness of the thermal oxide layer was then 60–70 Å [29]. 2.3. Grafting process The silica samples were dried under anhydrous gas flow at 140 ◦ C for 2 h. After cooling, the silica surface was covered with the silane by immersing the samples in a solution of silane in pentane followed by evaporation of the pentane under reduced pressure. A film of silane was also deposited on the flat surfaces in the same manner. The grafting reaction itself was then performed at 140 ◦ C for 16 h. Unreacted materials were removed by washing with THF in a Soxhlet extractor and the samples were subsequently dried. Grafting densities were measured on silica powders by means of elemental analyses of carbon at the Service Central d’Analyses of the CNRS (SCA-CNRS, B.P. 22, F69390
Vernaison) and by means of quantitative IR analysis. Quantitative analyses were carried out on the prisms of silicon covered by thermal silica by ATR-IR spectroscopy. 2.4. IR spectroscopy IR spectroscopy was performed on a Nexus FTIR (Nicolet) equipped with a DTGS detector. Fumed silica samples were analyzed by diffuse reflectance IR Fourier transform spectroscopy with SpectraTech DRIFT equipment. The fumed silica powder was mixed with dry potassium bromide (10% of silica in KBr) and spectra were recorded by collecting 16 scans at 4 cm−1 resolution. Flat silicon samples were analyzed by ATR on silicon prisms covered with thermal silica using a 4× beam condenser and microATR equipment from SpectraTech that allowed 10 reflections. Spectra were obtained by subtracting the spectrum of the uncoated oxidized ATR substrate from the spectrum of the coated substrate. For each spectrum, 1000 scans at 2 cm−1 resolution were recorded. In order to obtain quantitative information, a calibration was made with both powders and ATR crystals where known quantities of silane had been deposited. Linear calibration curves were obtained in all cases. For DRIFT, the comparisons of the different spectra were performed in Kubelka– Munk units using the Si–O vibration of silica at 1868 cm−1 as an internal calibration.
3. Results and discussion 3.1. Silanol density and grafting of the silane on fumed silica Since extensive studies dealing with the hydroxylation and chemical grafting of fumed silica have already been reported in the literature, the purpose of the present study was
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to collect data on fumed silica under the same experimental conditions as for the thermal silica and with the same silane, which has not been used in previous work. Thus, a relationship between the surface hydroxylation rate (density of silanols) and the density of chemical grafting was first established. 3.2. Silanol types and densities upon thermal treatments The surface of silica is dehydrated as the temperature of the thermal treatment increases. The dehydration comes from the elimination of adsorbed water and the thermal dehydration reaction, where two neighboring silanols condense into a siloxane bridge. These two phenomena are difficult to distinguish because water evolves in both cases. The first step of dehydration occurs under moderate temperature (<190 ◦ C) and consists in the elimination of water adsorbed on the silica surface. This preliminary treatment is required in order to obtain fully hydroxylated silica in a reproducible manner [7]. Water elimination at higher temperatures is also due to the hydroxyl pair condensation. Surface silanols are either free Si–OH groups or hydrogen-bonded with water or vicinal silanol groups. Because of their short distance (shorter than 3 Å), the later hydrogen-bonded silanols are identifiable by IR or NMR spectroscopy [10,30]. The SiO–H stretching vibration observed in IR spectroscopy is particularly useful. Thus, hydrogen-bonded silanols appeared as a broad band at 3600 cm−1 , whereas a sharp band at 3745 cm−1 corresponded to isolated silanols (Fig. 2). Dehydroxylation of fumed silica at increasing temperatures resulted in a decrease of the IR band of the hydrogen-bonded Si–OH at 3600 cm−1 . Conversely, the intensity of the IR bands of the isolated Si–OH at 3745 cm−1 first increased because dehydroxylation of hydrogen-bonded Si–OH created supplementary isolated Si–OH (Fig. 2). This later band finally decreased at the highest temperatures (above 700 ◦ C). The behavior of the surface silica hydroxyls as observed in the present study and according to the
Fig. 2. IR spectra of fumed silica in the SiO–H vibration domain after thermal treatment at 325 (solid), 700 (dashed), and 1000 ◦ C (dotted) for 70 h. The Si–O vibration at 1868 cm−1 marked with a star was used as an internal calibration of the intensities.
literature [10] is summarized in the scheme of Fig. 3 and in Fig. 4. Methyl [(dimethylamino)dimethylsilyl]undecanoate was grafted onto fumed silica powders that had been heat-treated for 70 h. The grafting density was measured by means of elemental analysis of carbon and quantitative DRIFT. Elemental analysis gives the total carbon content of the sample and is therefore very sensitive to contamination by residual reagents or solvents. Thus, excess silane was eliminated by repeated washings with THF in a Soxhlet extractor; the powder was filtered and subsequently dried under vacuum for two days in order to remove the THF to completion. This process allowed accurate determinations since a supplementary drying did not change the carbon content. Typical carbon analyses were between 5 and 10 wt%. DRIFT spectroscopy gives a quantitative analysis of the grafting density if the scattering conditions of different samples are repeatable. This could be successfully achieved by diluting the
Fig. 3. Schematic representation of the silanol types and densities upon thermal treatments.
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Fig. 4. Density of total silanols (!), free silanols (E), and grafting density (") as a function of the temperature of thermal treatment.
silica powder into KBr powder. A 10% silica weight with respect to KBr gave the best results in terms of signal-tonoise ratio and reproducibility; the exact amount and the accuracy of sampling were not crucial since the combination vibration band of SiO2 at 1868 cm−1 was used for internal intensity calibration [31]. A calibration curve with samples of silica where known amounts of silane have been deposited allowed a quantitative analysis. Results from elemental analysis and DRIFT spectroscopy applied to both the silane bands at 2920 cm−1 (C–H) and 1733 cm−1 (ester) were similar. This gave confidence to the data since the IR ester band is not affected by the contamination by THF. Moreover, this method did not suffer from inaccuracies of the Kubelka–Munk theory coming from the oversimplification of the scattering [32], because the calibration was done with the same type of powder as for the grafted one. All these methods gave the amounts of grafted silane per gram of silica (mol/g), which were converted into grafting densities (mol/m2) using the specific area of the silica (200 m2 /g). The grafting method by impregnation and condensation gives a maximum grafting density because a thin film of bulk reagent is deposited in the impregnation step. The process is then similar to a reaction performed in bulk silane, which gave maximum efficiency [22]. Indeed, a supplementary grafting step increases the grafting density by some 10% only. The grafting reaction was made in the presence of a large excess of silane, so that the presence of a trace of water, if any, did not cause any trouble. The grafting density on untreated fumed silica (but nevertheless dried at 140 ◦ C) having the maximum density of silanols was 3.5 µmol/m2 . This value corresponds to a dense monolayer and is close to the maximum according to the bulkiness of the two methyl groups attached to the silicon atom [24]. When the silica surface was dehydroxylated by means of thermal treatments and directly used in the silane grafting reaction, the grafting density decreased because of the lower density of the underlying silanol groups (Fig. 4). This effect appeared for thermal treatments above
450 ◦ C, where the density of silanol groups was lower than 3.5 µmol/m2 . This result suggested that chemical grafting was a suitable way to determine silanols at densities below 3.5 µmol/m2 . The relationship between grafting density and silanol density taken from the literature [10] is shown in Fig. 4. The silane grafting density is lower than the silanol density below 450 ◦ C because of the lateral steric hindrance of the silane molecules, as explained before. Both densities are identical above 450 ◦ C, but the silane grafting density departs from the density of Si–OH at the highest temperatures above 700 ◦ C. This observation suggested that siloxane bridges have been opened during the vigorous grafting process, where pure silane was spread on the silica surface at 140 ◦ C for 16 h. The dimethylamine released by the grafting reaction might help in opening siloxane bridges. The last interesting point is that there was a significant grafting of silane (1.7 µmol/m2) after a thermal treatment at 1000 ◦ C where adsorbed water has been removed to completion. This indicates that the grafting path by means of direct condensation (see Fig. 1) is actually operative. Let us recall that a large number of reports on silane grafting describe the mechanism of hydrolysis and condensation as the sole reaction path. Even some authors claimed that the presence of water is absolutely required for silane grafting [33]. 3.3. Determination of the silanol density on thermal silica Silane chemical grafting was performed onto the surface of silicon ATR internal reflection elements where thermal oxide was grown by oxidation at 800 ◦ C. The oxide layer was 60–70 Å thick [29]. This fresh oxide layer was then used without further treatment for grafting the methyl [dimethyl(dimethylamino)silyl]undecanoate. The IR spectrum recorded in ATR (Fig. 5) showed the expected characteristic bands of the grafted silane (C–H stretching bands in the 2900 cm−1 region and C=O stretching band at 1733 cm−1 ). The silica bands did not appear because the spectrum of the same bare ATR crystal recorded before grafting was subtracted. The grafting density could be measured from the intensity of the ester band of the silane in ATR-IR with the help of the calibration by deposition. A calibration curve was performed by measuring the intensity of the IR bands for successive depositions of known silane solutions on the ATR crystal. The calibration curves for the bands at 2921 cm−1 and 1733 cm−1 were both linear, demonstrating the validity of such a measurement (Fig. 6). The absorbance was used directly; no normalization was necessary as in DRIFT spectroscopy on powders. The linear calibration curve for the C–H band at 2921 cm−1 did not pass through the origin, however, because there was always a residual C–H band coming from organic contamination on the sample, but also in different parts of the IR beam optical path. This might not impede a quantitative measurement, but the ester carbonyl band at 1733 cm−1 was nevertheless preferred, because the calibration line passed through the origin.
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Fig. 5. ATR-IR spectrum of methyl [(dimethylamino)silyl]undecanoate on the ATR internal reflection element oxidized at 800 ◦ C for 1 h.
The problems of organic contamination, which readily happen at this scale, were avoided by using the ester band for the analysis (Fig. 6). The density of silane grafted on thermal silica was then 1.4 µmol/m2 . This value is 2.5 times lower than that of the dense monolayer of 3.5 µmol/m2 measured on fumed silica and corresponds to that of the fumed silica dehydroxylated at 1000 ◦ C. This moderate value of the grafting density reflects the low density of silanol groups of thermal silica. The correlation between the silanol and silane grafting densities allowed assessing the silanol density, which could not be measured directly. On the basis of the silane grafting results, a good model for thermal silica would be fumed silica heated at 1000 ◦ C and not rehydroxylated. This is not that surprising, since thermal silica has been prepared by oxidation at high temperature (800 ◦ C) and it has been shown that the silanol densities of different silicas converge to a common value upon treatment at such high temperatures [23].
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Fig. 6. Intensities of the absorption bands at 2921 (!) and 1733 cm−1 (") as a function of the amount of deposited silane on the ATR crystal. The linear regression to the data at 1733 cm−1 gave y = 0.0051x (R 2 = 0.998).
3.4. Rehydroxylation studies Increasing the grafting density has thereafter been attempted by means of a rehydroxylation process. The simplest and efficient method for the rehydroxylation of dehydrated fumed silica is a hydrothermal treatment in boiling water [34]. As already observed [6,20,34,35], complete rehydroxylation was possible after dehydroxylation at moderate temperatures, but part of the dehydroxylation was irreversible at the highest temperatures. Thus, rehydroxylation for 4 h in boiling water restored the full IR intensity of the SiO–H bands when the dehydroxylation has been performed below 700 ◦ C. Dehydroxylations at higher temperatures, especially over long times, were partially reversible. Only part of the IR intensity could be recovered after rehydroxylation in boiling water (Fig. 7, left). The irreversible part of the dehydroxylation should not be ascribed to the irreversible removal of geminal silanols that takes place till 400 ◦ C [36]. The irreversible behavior is observed at higher
Fig. 7. Rehydroxylation of fumed silica. Left: DRIFT of fumed silica heated for 70 h at 325 (solid), 700 (dashed), and 1000 ◦ C (dotted) after rehydroxylation in boiling water for 4 h. Right: fumed silica heated for 70 h at 1000 ◦ C (dashed) and after rehydroxylation in boiling water for 4 h (solid); the inset shows the difference spectrum. The small bands at 2900–3000 cm−1 are due to organic contamination.
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Table 1 Determination of the grafting density of thermal silica by means of ATR-IR for various methods of hydroxylation Treatment Mean absorbance (10−3 ) Standard deviation Grafting density (µmol/m2 )
None
H2 O (boiling)
H2 SO4 /H2 O2
Etching with HF
7.1 0.9 1.4
6.5 0.5 1.3
7.2
4.4 0.7 0.9
Fig. 8. Grafting density after heat treatment (!) and after heat treatment and rehydroxylation in boiling water (").
temperatures. The siloxane bridges are of better stability because less strained bonds are formed at high temperatures. Facilitated proton migration at the surface may contribute to the formation of siloxane bonds of optimum geometry. Since H-bonded Si–OH were formed, the density of free Si–OH decreased upon rehydroxylation (see the difference spectrum in Fig. 7 right). This phenomenon is the reverse of the behavior observed in the dehydroxylation process. The rehydroxylation treatment allows a better grafting of the silane for silica samples that have been extensively dehydroxylated. But the grafting ability was not completely recovered by a rehydroxylation treatment (Fig. 8). Thus, grafting densities close to 4 µmol/m2 were reached after a rehydroxylation treatment in boiling water, slightly higher than for the raw fumed silica. But the grafting density decreased significantly for silica heated above 700 ◦ C. The grafting density of fumed silica heated at 1000 ◦ C was 1.7 µmol/m2 and was increased to 3.4 µmol/m2 upon rehydroxylation in boiling water. Rehydroxylation was indeed beneficial but some useful reactive sites could not be recovered because of the partial rehydroxylation. There is again a good correlation between the grafting density and the density of silanols. In all cases, the recovery of the grafting density is good. The limiting value reached after rehydroxylation is close to that of the original untreated fumed silica. Among the rehydroxylation treatments studied, that in boiling water was the most efficient. In particular, strongly oxidizing and acidic solutions such as sulfochromic acid or ‘Piranha’ solution (H2 O2 /H2 SO4 mixture), which were often used to clean the surfaces of wafers, were of poor efficiency for the rehydroxylation. The intensities of the SiO–H
1.4
bands were lower than for the rehydroxylation with boiling water. On the other hand, basic solutions cannot be used because they are too much aggressive for the silica surface. The same methods of rehydroxylation have been investigated on thermal silicon dioxide, where the grafting of silane gave low densities (Table 1). The hydroxylation in either boiling water or ‘Piranha’ solution was inefficient in increasing the grafting density. A chemical etching in HF solution, which is commonly used in microelectronic technology, was also checked because this process cleaves the siloxane bridges. It was expected that a slight etching followed by an abundant rinsing with water could create new silanols groups at the surface. But the actual effect was disastrous regarding the silane grafting (Table 1). Because of the correlation between the silanol and silane grafting densities, the lack of variation of the grafting density indicates an inefficient hydrolysis of the surface siloxane bonds. The hydroxylation of thermal silica appears very difficult as compared to that of fumed silica dehydroxylated at 1000 ◦ C, which is a major difference between them. On the basis of the silane grafting results, a good model for thermal silica would be fumed silica heated at 1000 ◦ C and not rehydroxylated. But an important difference remains, since such fumed silica can be partially rehydroxylated, while thermal silica cannot. 4. Conclusions The comparative investigation of thermal and fumed silica as regards their surface silanol densities and silane grafting allowed to get new information on the surface chemistry of thermal silica. As was already well documented in the literature, fumed silica can be dehydroxylated by thermal treatments and rehydroxylated by hydrolysis of siloxane bonds in hot water. Chemical grafting of monofunctional organosilane permitted the establishment of a relationship between the density of silanols groups at the silica surface and the grafting density. This relationship is valid at low silanol densities (below ca. 3.5 µmol/m2). On this basis, the ATR-IR analysis of the derivatization of thermal silicon dioxide made it possible to estimate the surface hydroxyl density for the studied thermal silica. Quantitative IR spectroscopy is well suited for monitoring the grafting density, making use of absorption bands that are not affected by unavoidable organic contaminations. The density of silanol groups at the surface of thermal silica is low and the related silane grafting is also low
V. Dugas, Y. Chevalier / Journal of Colloid and Interface Science 264 (2003) 354–361
(1.4 µmol/m2). Thermal oxide is stable against the hydrolysis processes studied and therefore differs from dehydroxylated fumed silica in this respect. It appeared very difficult to increase the density of the silane grafting on such a substrate by means of a surface treatment. Last, there are alternatives in order to increase the silane coverage of wafers in spite of the low density of reactive sites. Thus, self-assembled monolayers form with silanes having long n-alkyl chains that pack on the surface into a dense 2-dimensional layer because of lateral interactions between paraffinic chains [37]. This process yields a dense monolayer of adsorbed silane in the crystalline or liquid-condensed state prior to the condensation reaction with the surface sites. There are many reports of dense coverage of thermal silica with long-chained silanes (e.g., octadecyltrichlorosilane). This process may, however, appear difficult when a functional group attached at the end of the alkyl chain disturbs the formation of the self-assembled layer. References [1] M.C. Petty, M.R. Bryce, D. Bloor, An Introduction to Molecular Electronics, Oxford Univ. Press, New York, 1995. [2] I. Willner, E. Katz, Angew. Chem. Int. Ed. 39 (2000) 1180–1218. [3] P. Cléchet, N. Jaffrezic-Renault, C. Martelet, in: S. Yamauchi (Ed.), in: Chemical Sensor Technology, Vol. 4, Kodansha, Tokyo, 1992, pp. 205–225. [4] S.L. Beaucage, Curr. Med. Chem. 8 (2001) 1213–1244. [5] R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. [6] A.P. Legrand, The Surface Properties of Silicas, Wiley, Chichester, 1998. [7] J. Gobet, E.Sz. Kovátz, Adsorpt. Sci. Technol. 1 (1984) 77–92. [8] E.P. Plueddemann, Silane Coupling Agents, Plenum, New York, 1982. [9] W.A. Pliskin, J. Vac. Sci. Technol. 14 (1977) 1064–1081. [10] L.T. Zhuravlev, Colloids Surf. A 173 (2000) 1–38. [11] (a) L. Bousse, N.F. de Rooij, P. Bergveld, IEEE Trans. Electron. Devices ED-30 (1983) 1263–1270; (b) L. Bousse, N.F. de Rooij, P. Bergveld, Surface Sci. 135 (1983) 479–496; (c) A. van der Berg, P. Bergveld, D.N. Reinhoudt, E.J.R. Sudhölter, Sensors Actuators 8 (1985) 129–148; (d) C.D. Fung, P.W. Cheung, W.H. Ko, IEEE Trans. Electron. Devices ED-33 (1986) 8–18; (e) R.E.G. van Hal, J.C.T. Eijkel, P. Bergveld, Adv. Colloid Interface Sci. 69 (1996) 31–62. [12] K.H. Beckmann, N.J. Harrick, J. Electrochem. Soc. Solid State Sci. 118 (1971) 614–619. [13] D.L. Angst, G.W. Simmons, Langmuir 7 (1991) 2236–2242. [14] M.-J. Azzopardi, Réaction des organosilanes à l’interface silice/solution. Étude in situ par spectroscopie infrarouge, Ph.D. thesis, Université Paris VI, 1994.
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