Surface hydroxylation and silane grafting on fumed and thermal silica

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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.

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 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], [13], [14], [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 well-known 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 13C and 29Si 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 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 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 ill-defined 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.

Section snippets

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 MΩ cm) was prepared by percolation through ion-exchange resins.

Methyl [(dimethylamino)dimethylsilyl]undecanoate was prepared by reaction of the methyl (chlorodimethylsilyl)undecanoate

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 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.

Silanol types and densities upon thermal treatments

The surface of

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

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