Letter pubs.acs.org/JPCL
Multiple Transmission-Reflection IR Spectroscopy Shows that Surface Hydroxyls Play Only a Minor Role in Alkylsilane Monolayer Formation on Silica Vikrant V. Naik, Maura Crobu, Nagaiyanallur V. Venkataraman, and Nicholas D. Spencer* Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093, Zurich, Switzerland S Supporting Information *
ABSTRACT: Multiple transmission-reflection (MTR)a recently developed infrared spectroscopy sampling method for surfaceshas been applied to the study of silane monolayer formation on silicon oxide. Thanks to the excellent signal:noise ratio of data obtained by MTR, spectra of silane monolayers on a silica substrate could be readily obtained. This system has been previously difficult to investigate by standard sampling methods. The data is particularly important for gaining insights into the nature of the silica-silane interaction. The results support a model in which the inherent strain caused by the mismatch of alkyl-chain van der Waals radius and Si−O−Si bond distance is relieved in silane monolayers by the formation of a structure resembling snow moguls or closely packed umbrellas.
SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
A
The silanes hydrolyze in solution to form silanols at a rate that depends on the leaving group, X.1,5,8 The order of reactivity or ease of hydrolysis is Cl > Br > I > OMe > OEt. In the classical mechanism, once the silanols are formed, they readily attach to the surface by their reactions to the surface OH by elimination of water. Once on the surface, these silanes polymerize internally to form Si−O−Si (siloxane) linkages. Although described sequentially, it has also been suggested that two or more of these steps could occur through a concerted mechanism.1 The classical reaction mechanism leaves a number of open questions. Primarily, it does not deal with the degree of polymerization of the Si−O−Si linkages. Current characterization techniques have been unable to quantify the siloxane linkages, and it is not known whether these exist as groups of a fixed number of silanes or the entire surface consists of a continuous cross-linked structure. The relative roles of surface water or hydroxide and residual solvent moisture in the hydrolysis of the silanes is also not fully understood. Another problem that the current model faces concerns the volume constraints of alkyl chains.5 For a self-assembled monolayer (SAM) formed by octadecyltrichlorosilane (OTS), a typical alltrans alkyl chain has a cross-sectional diameter of 4.9 Å.10,11 Considering a typical Si−O bond length of 1.8 Å and Si−O−Si angle of 105°, the maximum available space for the alkyl chain
lkylsilanes (silanes) have long been used for tailoring a wide variety of surfaces including silicates, aluminates, and titanates.1−3 They are currently used on an industrial scale for a wide variety of applications, such as coupling agents for glasses and polymers, as adhesion promoters, as cross-linking and dispersing agents, and for hydrophobization. Silanes readily react with surfaces through surface hydroxyl groups, or bound water to form a strongly bound coating that includes both covalent bonds and multiple van der Waals interactions.1,4,5 While a vast array of complex silanes is used commonly in industrial processes, most academic studies have modified surfaces by means of functionalized monoalkylsilanes with three leaving groups (R-SiX3). The R group is generally a simple short- or long-chain alkane (e.g., dodecyl or octadecyl) or an end-functionalized alkyl chain that still retains a Si−C bond.6−8 X in this case can be a halogen or an alkoxy group (e.g., 1aminopropyltriethoxysilane).9 Although the exact mechanism for the formation of the surface coating is still not completely understood, it has classically been portrayed that the surface reaction of silanes takes place as shown in Figure 1.1
Received: July 11, 2013 Accepted: July 30, 2013 Published: July 30, 2013
Figure 1. Classical view of the surface reaction of silanes. © 2013 American Chemical Society
2745
dx.doi.org/10.1021/jz401440d | J. Phys. Chem. Lett. 2013, 4, 2745−2751
The Journal of Physical Chemistry Letters
Letter
Figure 2. (a) Water and hexadecane contact angles and (b) ellipsometer thickness of an OTS monolayer on a double-sided silicon wafer as a function of immersion time in a 0.0001 M OTS solution in decalin.
has a diameter of 2.9 Å.6,12 Maoz et al. have suggested that there is a dynamic equilibrium and a continuous redistribution of the Si−O bonds within a two-dimensional network of oligomeric siloxane and silanol species.5 However, even that model cannot fully account for the fact that the voluminous alkyl chains need to be squeezed into the Si−O−Si distance. Many studies have been directed at this problem, but a conclusive picture of the Si−O−Si region still remains elusive.8,13−15 We have used Fourier-transform infrared (FTIR) spectroscopy to understand the structure of OTS SAMs on a silica surface, and to gain insights into the mechanism of SAM formation. FTIR is a proven tool for the investigation of alkylchain assemblies on surfaces.16 The use of infrared reflection and absorption spectroscopy (IRRAS) has provided invaluable information on the nature of SAMs on metal surfaces.16,17 However, because of the nonmetallic nature of silica surfaces, IRRAS is unsuitable for studying SAMs on Si. Simple transmission FTIR through a silicon wafer is possible, but lacks sensitivity. In the present work, we have used a recently developed technique, multiple transmission and reflection (MTR) IR spectroscopy, to study the OTS SAM with the aim of better understanding the nature of alkylsilane SAMs.18,19 A dilute (0.0001 M) solution of OTS was prepared in freshly distilled decahydronaphthalene (decalin) (cis−trans mixture) to coat 30 × 18 mm2 ⟨111⟩ double-sided polished silicon wafers (Si-Mat, Germany). Prior to this treatment, the wafers had been piranha cleaned for 20 min, dried and oxygen plasma cleaned for 2 min. The silicon wafers were then immersed in the OTS solution for different specified periods of time from 10 s to one day, to obtain the OTS coated surface. A detailed description of the process and instrumentation is provided as a part of the Supporting Information. The static water contact angle was measured as a function of immersion time and is shown in Figure 2a. It can be seen that after immersion times as low as 10 s, the surface is already rendered more hydrophobic, with a contact angle of 56°, compared to the clean-wafer value of
