Controlling Polymerization Initiator Concentration in Mesoporous

Dec 23, 2013 - Center of Smart Interfaces and Department of Material and Earth-Sciences Physics of Surfaces, Technische Universität Darmstadt,...
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Controlling Polymerization Initiator Concentration in Mesoporous Silica Thin Films Fabio Krohm,† Haiko Didzoleit,‡ Marcus Schulze,§ Christian Dietz,§ Robert W. Stark,§ Christian Hess,∥ Bernd Stühn,‡ and Annette Brunsen*,† †

Ernst-Berl Institute for Chemical Engineering and Macromolecular Science, Technische Universität Darmstadt, Alarich-Weiss-Str. 4, D-64287 Darmstadt, Germany ‡ Institute of Condensed Matter Physics, Technische Universität Darmstadt, Hochschulstraße 8, D-64289 Darmstadt, Germany § Center of Smart Interfaces and Department of Material and Earth-Sciences Physics of Surfaces, Technische Universität Darmstadt, Alarich-Weiss-Str. 10, 64287 Darmstadt, Germany ∥ Eduard Zintl Institute for Inorganic and Physical Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str. 8, D-64287 Darmstadt, Germany S Supporting Information *

ABSTRACT: We present a strategy toward controlled polymer density in mesopores by specifically adjusting the local amount of polymerization initiator at the pore wall. The polymerization initiator concentration as well as the polymer functionalization has a direct impact on mesoporous membrane properties such as ionic permselectivity. Mesoporous silica-based thin films were prepared with specifically adjusted amount of polymerization initiator (4(3-triethoxysilyl)propoxybenzophenone (BPSilane)) or initiator binding functions ((3-aminopropyl)triethoxysilane (APTES)), directly and homogeneously incorporated into the silica wall pursuing a sol−gel-based cocondensation approach. The amount of polymerization initiator was adjusted by varying its concentration in the sol−gel precursor solution. The surface chemistry, porosity, pore accessibility, and reactivity of the surface functional groups were investigated by using infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray reflectometry, ellipsometry, atomic force microscopy, and transmission electron microscopy. We could gradually modify the amount of reactive polymerization initiators in these mesoporous membranes. Mesopores were maintained for APTES containing films for all tested ratios up to 25 mol % and for BPSilane containing films up to 15 mol %. These films showed accessible and charge-dependent ionic permselectivity and an increasing degree of functionalization with increasing precursor ratio. This approach can directly result in control of polymer grafting density in mesoporous films and thus has a direct impact on applications such as the control of ionic transport through mesoporous silica membranes.

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Sol−gel prepared mesoporous silica films can be modified with organic functional moieties by co-condensation or postgrafting approaches. In particular, grafting-from polymerizations in confinement of mesoporous silica thin films demand a wellcontrolled initiator functionalization strategy to obtain a defined functional polymer density. On one hand, it is difficult to achieve a homogeneous distribution of functional groups along the entire film and to tune the amount of functional groups in a postgrafting approach.16−19 On the other hand, cocondensation influences the porous structure but allows a homogeneous and specific adjustment of the amount of functional groups with control in the z-direction by using a multilayer film preparation approach.20−24 Several functions

INTRODUCTION Controlling structure and function is a major challenge in the fabrication of functional nanodevices,1−3 and it is essential to gate ionic permselectivity through mesoporous membranes. The gating of biological ion channels is closely related to their functional and structural organization on the nanoscale, as for example investigated by modeling.4 The combination of mesoporous films as structural unit and surface grafted responsive polymers as functional unit is a versatile approach toward synthetic membranes with responsive surface properties, resulting in membranes with adjustable ionic permselectivity or catalytic properties.5−10 Mesoporous ceramic thin films are usually prepared via sol−gel chemistry and evaporationinduced self-assembly (EISA).11,12 The pore structure can be tuned in terms of framework nature, composition, crystalline structure, effective surface area, pore dimension, shape, accessibility, pore array symmetry, and interconnection.9,13−15 © 2013 American Chemical Society

Received: October 16, 2013 Revised: December 10, 2013 Published: December 23, 2013 369

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2.02−1.92 (m, 2H, CH2), 1.29−1.23 (t, 9H, CH2−CH3), 0.84−0.79 (m, 2H, Si−CH2). Synthesis of Mesoporous Benzophenone (BPSi) and APTES Containing Silica Films (NH2Si) by Co-condensation. Mesoporous silica films were synthesized via a sol gel method based on the oxide precursor tetraethoxysilane (TEOS) in combination with the organic function carrying precursor (3-aminopropyl)triethoxysilane (APTES) or the 4-(3-triethoxysilyl)propoxybenzophenone (BPSilane) in the presence of the template (Pluronic F127). The precursor solution was prepared using the following molar ratios: (1 − x)TEOS:xAPTES or xBPSilane:0.005 F127:24 EtOH:5.2 H2O:0.28 HCl, with x being between 0 and 25 mol %. This solution was stirred for 24 h and used to produce films by dip-coating on ITO, glass, or silicon wafer at 40−50% relative humidity at 298 K (2 mm s−1 withdrawing speed). Freshly deposited films were stored at 50% relative humidity in a chamber for 24 h. Then a stabilizing thermal treatment was carried out in two successive 24 h steps at 60 and 130 °C. After ramping the temerature up from 130 to 200 °C at 1 °C min−1, Finally, films were stabilized at 200 °C for 2 h. The template was extracted in acidic ethanol (0.01 M HCl in EtOH) for 3 days. Then films were rinsed with ethanol and stored under ambient conditions. Functionalization of Amino Groups with Trifluoraceticacid Anhydride (TFAA). Mesoporous aminosilica film were treated with a 0.21 M TFAA solution in toluene for 1 h at 60 °C under a nitrogen atmosphere. Free TFAA was removed by 3 h of Soxhlet extraction in toluene. Finally, the trifluoracetamide-functionalized NH2Si-films were washed with ethanol, 0.1 M hydrochloric acid in water, distilled water, and again with ethanol according to a literature protocol.44 Polymerization of [2-(Methacryloyloxy)ethyl]trimethylammonium Chloride (METAC) in Mesoporous Silica Thin Films. The BPSi-films were degassed by several consecutive vacuum−nitrogen cycles and immersed into a degassed aqueous solution of 10 wt % METAC under a nitrogen atmosphere. To initiate polymerization, the samples were irradiated in a photo-cross-linker (Vilber Lourmat Bio-Link BLX cross-linker) at 365 nm (P = 2 mW cm−2) for 12 min. After polymerization, the films were washed three times with deionized water and immersed for 1 h in deionized water to remove noncovalently attached monomer. Transmission Electron Microscopy (TEM). TEM measurements were made using a FEI CM20 TEM microscope with a maximum resolution of 2.3 Å, equipped with a LAB-6 cathode and a CCD camera (Olympus), using an acceleration voltage of 200 kV. Samples were scratched from glass substrates and dispersed in some drops of ethanol, and the suspension was placed onto an TEM grid. X-ray Reflectometry (XRR). X-ray reflectivity determines the intensity of the X-ray beam reflected from a planar surface. For an ideally flat surface of a bulk material one would receive an intensity that decays rapidly with increasing scattering angle according to Fresnel’s formula. Deviations from this law are due to a variation of electron density along the surface normal and are used to determine the thickness and internal structure of thin films on a substrate. The reflected intensity at a scattering angle 2θ or the related scattering vector qz = 4π/λ sin θ may be calculated exactly on the basis of the electron density profile. in a good approximation at larger q it is given as (Born approximation):

have been introduced into mesoporous silica or non-silica matrices by co-condensation such as thiols, amino groups, or aromatic moieties.20,23,25−36 To our knowledge, polymerization initiators were not co-condensated directly into a mesoporous network to achieve a homogeneous initiator distribution along the pore. A homogeneous initiator distribution is a key feature to consecutively control local polymer functionalization and local polymer functional density in mesopores,37 which we found to be essential to control ionic permselectivity and transport properties in mesoporous films.8,38,39 Most of the polymer functionalization strategies of mesoporous thin films lack this control on local functionalization and functional density. Furthermore, a direct co-condensation of polymerization initiators safes one postgrafting step in the further pore wall modification. So far, polymers have mostly been introduced into mesoporous films after initiator postgrafting, with the aim to consecutively investigate ionic permselectivity gating for example.5,6,40−42 Here, we present a mesoporous silica membrane with directly and homogeneously included type II photoinitiator 4(3-triethoxysilyl)propoxybenzophenon (BPSilane) by using cocondensation. We compare the concentration-dependent cocondensation of aminopropyltriethoxysilane (APTES) as an initiator binding function for initiator postgrafting with the direct co-condensation of the type II photopolymerization initiator 4-(3-triethoxysilyl)propoxybenzophenon (BPSilane) into a TEOS-based mesoporous thin film. By adjusting the APTES and BPSilane concentration in the film preparation solution, the amount of polymer binding units is predetermined, which inherently determines the maximum grafting density. The successful incorporation of functional units as well as their reactivity is investigated in detail. As proof of principle, a polymerization after direct co-condensation of a BPSilane initiator is shown, proving the reactivity of incorporated polymerization initiators. The hybrid meso-architectured interface and their functional features are studied with a combination of experimental techniques including ellipsometry, cyclic voltammetry, X-ray reflectometry, atomic force microscopy, and X-ray photoelectron spectroscopy.

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EXPERIMENTAL SECTION

Reagents. [2-(Methacryloyloxy)ethyl]trimethylammonium chloride (METAC) solution 80% in water and Pluronic F127 were purchased from Sigma-Aldrich. 3-(Triethoxysilyl)propylamine (APTES), 4-benzoylbenzoic acid, 4-hydroxybenzophenone, hexaammineruthenium(II) chloride, tetraethoxysilane (TEOS), triethoxysilane, and platinum (10% on carbon as catalyst) were purchased from Alfa Aesar. (Dicyclohexyl)carbodiimide was purchased from Interchim. Potassium chloride, potassium ferricyanide(III), and dimethylformamide were purchased from Merck Millipore. All chemicals were used as received unless otherwise noted. Synthesis of (4-(3-(Triethoxysilyl)propoxy)phenyl)(phenyl)methanone (BPSilane). BPsilane was synthesized according to a protocol published by Prucker et al.43 Briefly, 54 mL of triethoxysilane (47.79 g, 0.29 mol), 4-allyloxybenzophenone (5.18 g, 22 mmol), and Pt−C (70 mg, 10% Pt) on activated charcoal were refluxed in a Schlenk flask at 120 °C for 5 h. The catalyst was separated by filtration under a nitrogen atmosphere. Subsequently, the excess of triethoxysilane was removed by recondensation. The product was dried under high vacuum to yield a slightly brownish oil (86% yield). The synthesis was performed in the absence of water and oxygen with nitrogen as the inert gas. The product was characterized by 1H NMR. 1H NMR (300 MHz, CDCl3): δ 7.87−7.83 (tt, 2H, HAr), 7.80−7.76 (tt, 2H, HAr), 7.61−7.56 (tt, 1H, HAr), 7.52−7.48 (tt, 2H, HAr), 7.00−6.95 (tt, 2H, HAr), 4.08−4.04 (t, 2H, O−CH2), 3.91−3.84 (q, 6H,Si−O−CH2),

I(qz) ∝ RF

∫ ddρz eiq z dz

2

z

Here RF denotes the Fresnel reflectivity which may be approximated as RF ∝ 1/qz4. The experiment therefore allows one to determine the internal structure of thin films. The reflectometer is based on a D8 Advance diffractometer (Bruker AXS, Germany), which is designed to measure reflectivities in θ−θ geometry. X-ray source and detector were mounted on goniometer arms which can be moved independently with a precision of 0.001°. The X-ray beam was generated by a conventional X-ray tube with a Cu anode to yield Cu Kα radiation with a wavelength of λ = 1.54 Å. The generated X-ray beam had a line focus and was monochromized by a Goebel mirror (W/Si multilayer mirror). Through a narrow slit of 0.1 mm the beam passed an absorber 370

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Figure 1. Schematic representation of the functionalization strategy of mesoporous silica films by a direct co-condensation of polymerization initiators 4-(3-triethoxysilyl)propoxybenzophenone (BPSilane) or initiator binding functions such as aminopropyltriethoxysilane (APTES) into the mesoporous silica network. This strategy allows a precise control of the initiator density on the pore wall surface. roughness of mesoporous silica-based thin films (Cypher AFM, Asylum Research, Santa Barbara, CA). To track frequency shifts, an external phase locked loop device HF2PLL (Zurich Instruments AG, Zurich, Switzerland) was connected to the AFM. We used Tap525 Cantilevers (Bruker AFM probes, Camarillo, CA) with resonance frequencies f 0 = 500 Hz and nominal flexural spring constants k = 200 N/m. The exact spring constants were determined by the method of Sader et al.46 All measurements were performed in constant excitation mode47,48 with free oscillation amplitudes of about A0 = 12 nm. To keep the tip−sample force low, the frequency set point was adjusted to Δf = −35 Hz with respect to the undisturbed resonance frequency, and the scan velocity was limited to 461 nm/s during imaging. XPS. X-ray photoelectron spectra were recorded using a Surface Science Laboratories SSX-100 X-ray photoelectron spectrometer equipped with a monochromatic Al Kα X-ray source (100 W). The X-ray spot size was 250−1000 μm. The binding energy scale of the system was calibrated using Au 4f7/2 = 84.0 eV and Cu 2p3/2 = 932.67 eV from foil samples. Charging of the powder samples was accounted for by setting the peak of the C 1s signal to 285.0 eV. A Shirley background was subtracted from all spectra. Peak fitting was performed with Casa XPS using 70/30 Gauss−Lorentz product functions. Atomic ratios were determined from the integral intensities of the signals, which were corrected by empirically derived sensitivity factors. Cyclic Voltammetry. Quantitative variations in permselectivity were studied by following the changes of voltammetric peak currents associated with cationic (Ru(NH3)62+/3+) and anionic (Fe(CN)64−/3−) redox species, diffusing across the mesoporous film49 were recorded with an Autolab PGSTAT302N (Metrohm). As recently summarized by Walcarius,50 changes in the voltammetric response of mesoporous electrodes reflect the changes in probe concentration or diffusion in response to external stimuli or the architecture of the pore. To this end, mesoporous films modified with redox-responsive polymer were prepared on bare indium tin oxide (ITO) electrodes. The measurements were performed with a 2 mM solution of the probe molecule in a 100 mM solution of KCl as supporting electrolyte. The pH was adjusted with NaOH or HCl. An Ag/AgCl electrode (BASi RE-6) was used as reference electrode, and various scan rates between 10 and

(calibrated Cu attenuator) which was used for high intensity near the critical angle in order to remain within the linear response regime of the detector. A second 0.1 mm slit was placed after the absorber to cut out the Kβ-line (which is also reflected by the monochromator). Intensity was detected by a Våntec-1 line detector (Bruker AXS, Germany) providing the possibility to measure the specular reflected intensities and the diffuse reflected intensities simultaneously in an angular range of Δθf = 2° for a given incident angle θi. Within one measurement the detector collected intensity over all qx (line focus). For each incident angle a single intensity I(θf) contained the specular and off-specular condition. The intensity of the reflected beam was determined as the integral of the specular peak corrected for background measured as the diffuse intensity. The specular reflectivities were analyzed using the Motofit Reflectometry package, rev. 409 for Igor Pro. Infrared (IR) Spectroscopy. IR measurements were performed on a Spectrum One (PerkinElmer) instrument in attenuated total reflection (ATR) mode. Mesoporous film was scratched from the substrate to record IR spectra in a range from 4000 to 600 cm−1. The measured spectra were corrected for the background and normalized to the Si−O−Si band at ∼1080 cm−1. NH2Si samples were immersed into 100 mM KCl adjusted to pH ∼ 3 by using HCl for 20 min at ambient conditions with subsequent rinsing with ethanol before being measured. Ellipsometry. The dry thickness of the surface-attached films on silicon wafers was measured using a Nanofilm EP3 imaging ellipsometer. One zone angle of incidence (AOI) variation measurements were captured between AOIs of 40° and 80° with a 658 nm laser. The apparent film thickness was calculated from the measured angles Ψ and Δ using the EP4 analysis software supplied with the instrument. The fitting parameters for the silicon oxide layer thickness (dSiOx = 2.5 nm measured separately prior to polymer film immobilization) and the refractive index of the polymer layer (npolymer = 1.5) were held constant. To determine the porosity out of refractive indices, the Brüggemann effective medium approximation was used. Details can be found in the Supporting Information (Table S3). AFM. We conducted measurements with a frequency-modulation atomic force microscope45 (FM-AFM) to characterize the surface 371

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Figure 2. Representative transmission electron microscope (TEM) images of aminopropyltriethoxysilane (APTES) functionalized mesoporous silica thin films (NH2Si) with increasing molar ratio of co-condensated APTES from 10 to 25 mol %: (a) 10 mol % NH2Si; (b) 15 mol % NH2Si; (c) 20 mol % NH2Si; (d) 25 mol % NH2Si. In the bottom row, mesoporous silica thin films with increasing molar ratio of co-condensated 4-(3triethoxysilyl)propoxybenzophenone (BPSilane) from 10 to 25 mol % (BPSi) are presented: (e) 10 mol % BPSi; (f) 15 mol % BPSi; (g) 20 mol % BPSi; (h) 25 mol % BPSi. Corresponding porosity values as extracted from ellipsometry are shown beneath. The scale bar corresponds to a length of 200 nm (a−g) and to 50 nm (h).

Figure 3. Specular reflectivity curves for two different series of silica networks with functional groups: NH2Si (a) and BPSi (b). The curves have been shifted for clarity. Kiessig fringes and one or two Bragg peaks are clearly visible within the profiles. (c) Intensity of the Bragg reflections for both series of samples. A clear decrease of scattering contrast with increasing concentration of BPSilane is observed, whereas the contrast for NH2Si is unchanged with the increasing concentration of APTES. (d) Bragg peak position and correspondingly the sublayer thickness do not significantly change for the BPSi films (blue circles). The NH2Si films show an increase of the Bragg peak position with increasing APTES concentration (red circles). 1000 mV s−1 were measured. The measured electrode area was 0.21 cm2.

pore arrays can be observed for NH2Si films at all APTES concentrations in the range of 0−25 mol % as well as for BPSi films with a ratio of 10 mol % BPSilane. TEM results show slightly increasing pore size for NH2Si films with increasing APTES ratio from about 5.9 nm (±0.8 nm) to 8.9 nm (±0.8 nm). Simultaneously, the interpore distance depends on the direction looking at the pore and decreases with increasing ATPES ratio from 1.6 nm (±0.7 nm) to 2.3 nm (±1.1 nm) for the smallest distance and from 3.9 nm (±0.5 nm) up to 3.0 nm (±0.9 nm) for the larger distance (Table S2). For 20 mol % NH2Si thin films Calvo et al. observed comparable porous structure in TEM as was observed in our measurements (Figure 2). They reported a cubic-derived mesostructure for these film based on SAXS-2D measurements.44 In general, long aging times at high relative humidity (about 55%) during the EISA film preparation process lead to cubic phases whereas short

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RESULTS AND DISCUSSION The synthetic concept, based on increasing concentration of 4(3-triethoxysilyl)propoxybenzophenone (BPSilane, Figure 1) and aminopropyltriethoxysilane (APTES, Figure 1) between 0 and 25 mol % into a mesoporous tetraethyl orthosilicate (TEOS) thin film, is schematically depicted in Figure 1. Structural Characterization of Initiator-Modified, Cocondensated Mesoporous Films. All mesoporous films were characterized in terms of structure by TEM, XRR, AFM, and ellipsometry. TEM images for NH2Si and BPSi films are presented in Figure 2 as well as in the Supporting Information (Figure S1) for the corresponding 0% pure silica films. TEM pictures were obtained from scratched films. Highly regular 372

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with the data from ellipsometry. We note that in ellipsometry a small spot of 0.1−1.1 mm diameter is measured, whereas the XRR experiment relates to an illuminated area in the order of cm2. The observed difference may therefore be due to the heterogeneity of film thickness in the samples. In general, film thickness variation of up to 10−30 nm perpendicular to the coating direction can be observed for as-prepared films. At low q in Figures 3a and 3b two steps in the detected reflectivity mark the angles of total reflection of the film and the substrate. They correspond to the known electron density of the substrate (SiO2, 670 e/nm3) and the mesoporous layer. In the case of APTES we find 330 and 400 e/nm3 for the benzophenone containing silica layer. The most prominent feature of the reflectivity profiles is the existence of one or two peaks with an intensity depending strongly on the content of the added amino or benzophenone groups. These peaks result from interference due to an internal multilayer structure of the films. Assuming sublayers with an electron density difference Δρ, the ratio between the experimental reflectivity R and the Fresnel reflectivity of the substrate may be shown to exhibit Bragg peaks at positions qz,n = 2π/dl. Here dl denotes the thickness of the sublayer. The intensity of the resulting Bragg peak is proportional to the electron density difference Δρ2.52 In Figure S6 we therefore show R/RF for both series of films. Indeed, both series show a peak, which shifts slightly to higher values of qz with increasing amount of APTES and BPSilane, respectively (Figure 3d). This demonstrates a decreasing thickness of the sublayers, which might be due to hindrance of micelle formation or organization because of the interaction with the unpolar benzophenone groups. The values obtained for the sublayer thickness are given in Table 1. The entire film consists of 19−30 sublayers. The variation of intensity is clearly different for the NH2Si and BPSi films (Figure 3c). Whereas the intensity is nearly constant for the NH2Si films, we find a strong decrease for BPSi. Unfortunately, the second-order Bragg reflection cannot be determined from the reflectivity data.

aging times at a humidity of about 40% rather resulted in a hexagonal phase as reported for silica films prepared with CTAB as template.16,51 The pore diameter for BPSi seems to be slightly smaller comparing NH2Si and BPSi films with a ratio of 10 mol % of organic precursor. The pore diameter for these 10 mol % BPSi films as observed in TEM was 5.9 nm (±0.6 nm) with a regular interpore distance of 2.3 nm (±0.8 nm). For molar ratios of BPSi > 15 mol % we could not observe a clear porous structure in the TEM images, pointing toward the problem of co-condensating hydrophobic functional groups into a hydrophilic wall material in high ratios.20 Further structural characterization was carried out by X-ray reflectometry. The XRR results on NH2Si films as well as for BPSi films are displayed in Figures 3a and 3b, respectively. The reflectivity shows Kiessig oscillations which allow the determination of the total film thickness ranging from 150 to 250 nm. The results are compiled in Table 1 and compared Table 1. XRR Results Extracted from the Measured Spectra Depicted in Figure 3 and Compared to Ellipsometry Results total thickness (ellipsometry)a (nm)

total thickness XRR (nm)

sublayer thickness XRR (nm)

no. of layers XRR

sample

168 183 214 214 150 175 175 184

187.8 192.8 251.3 206.4 153.4 179.4 181.9 188.2

9.7 9.1 8.9 8.6 6.4 7.0 6.6 6.3

19 21 28 24 24 26 28 31

NH2Si 10% NH2Si 15% NH2Si 20% NH2Si 25% BPSi 10% BPSi 15% BPSi 20% BPSi 25%

a As discussed in the text variation in film thickness of 10−30 nm perpendicular to the coating position can be observed. To generate a homogeneous film thickness, large substrates are advantageous.

Figure 4. FM-AFM topographical images (300 × 300 nm2) of mesoporous silica-based thin films. (a) Mesoporous silica based thin film without functionalization. Roughness (rms): Rq = 0.24 nm. (b) BPSilane (10 mol %) functionalized mesoporous silica film. Rq = 0.30 nm. (c) APTES functionalized mesoporous silica film (20 mol). Rq = 0.22 nm. (c) APTES functionalized mesoporous silica film (20 mol). Rq = 220 pm. The white square indicates the position of Figure 5a. The bottom row shows the corresponding profiles of the horizontal cross sections as indicated with the red arrows. Imaging parameters: f 0 = 497 kHz, Δf = −35 Hz, and k = 85 N/m. 373

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Figure 5. Topographical image of an APTES (20 mol %) functionalized mesoporous silica film measured with a FM-AFM. Image size: (100 × 100) nm2. (a) Enlarged image of the region of interest (white frame) in Figure 4c. Red circles indicate single pores in the silica-based film. (b) 3D visualization of the same area. The arrows point to the pores highlighted in (a).

Figure 6. (a) ATR-IR spectra of pure silica, 20 mol % NH2Si, and 20 mol % BPSi. Spectra are normalized to the Si−O−Si vibration at 1047 cm−1, and relevant frequencies are indicated. The spectra are shifted in absorbance to facilitate visualization. (b) Details of the 1400−1800 cm−1 zone within the Si−O−Si normalized spectra of BPSi with increasing molar ratio of BPSilane (gray: 0 mol %; black: 10 mol %; red: 15 mol %; green: 20 mol %; blue: 25 mol %), showing the bending O−H and νCO signal. (c) Details of the 1400−1800 cm−1 region within the Si−O−Si normalized spectra of NH2Si with increasing molar ratio of APTES at pH 3 (gray: 0 mol %; black: 10 mol %; red: 15 mol %; green: 20 mol %; blue: 25 mol %).

refractive index within a small humidity range. Based on previous data, the change in refractive between 0% and 50% humidity for calcinated mesoporous silica was below 10%.54 The porosity values obtained by this procedure are compared in Table S3 and in Figure S2. Introducing 10 mol % of NH2Si or BPSi reduces the porosity compared to pure, calcinated mesoporous silica. For NH2Si modified silica, porosity decreases from ∼40% for 0% APTES to ∼19% for an APTES ratio of 15−25 mol % under these measurement conditions. For NH2Si thin films containing a ratio of 20 mol % of NH2Si, a pore volume of 25−32%, an average major pore size of ∼8 nm, and an average neck radius of ∼3 nm are reported based on measurements at 0% humidity. According to our TEM and XRR data, this pore size corresponds to the pore height (multilayer in XRR). The higher porosity might be explained by our measurement conditions as discussed above.54,55 for BPSi the porosity was determined to be 21% for a fraction of 10 mol % BPSilane, which is comparable to NH2Si films with a similar fraction of APTES. For increasing ratios of BPSilane the porosity decreases to 9% for 15 mol % BPSilane and to 0% for ratios of 20 mol % BPSilane and higher. These results are

Therefore, it is not possible to obtain information on the structure of the sublayer. Assuming both sublayers to consist of more spherical pores or connecting tunnels, the difference between both becomes smaller in the case of increasing BPSilane concentration. This is consistent with TEM results which showed no ordered porous structure for BPSilane concentrations of 20 mol % or higher. Additionally, these films are not accessible by ionic probe molecules as discussed below. The total pore volume of the mesoporous films is estimated from the ellipsometric data by using the refractive index and applying the effective medium theory.53 Assuming a twocomponent system composed of silica walls with a refractive index of 1.458 and pores (air) with a refractive index of 1, the ratio of pores compared to silica walls can be extracted from the measured refractive index. One has to be aware that ellipsometry measurements were performed under ambient conditions and not at controlled 0% humidity. Thus, absolute values have to be considered with care and real porosity might be slightly larger. For calcinated mesoporous silica films, using the same template polymer, water adsorption mainly takes place at a humidity >50% and leads to a steep increase in 374

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Figure 7. (a) Maximum current density (j) as extracted from cyclic voltammograms (compare Figures S7−S9) for BPSi mesoporous thin films with redox probe molecule Ru(NH3)63+ at pH ≤ 3 and pH ≥ 8, as well as for the redox probe molecule Fe(CN)63− at pH ≤ 3 and pH ≥ 8. (b) Maximum current density (j) as extracted from cyclic voltammograms for NH2Si mesoporous thin films with redox probe molecule Ru(NH3)63+ at pH ≤ 3 and pH ≥ 8 as well as for the redox probe molecule Fe(CN)63− at pH ≥ 8.

scratched mesoporous films. The ATR-IR spectra show the characteristic bands of the inorganic frameworks and the organic functions incorporated into the mesoporous samples. In Figure 6 the entire normalized spectra of NH2Si 20% sample (Figure 6a) in comparison to BPSi 20% (Figure 6a) and unmodified mesoporous silica (Figure 6a) are presented. Inorganic framework Si−O−Si bands corresponding to transverse optical Si−O−Si modes can be observed for both, mesoporous films in the 800 cm−1 (νs) and in the 950−1300 cm−1 zone (νas), accompanied by a longitudinal optical mode, which appears as a shoulder at 1250 cm−1 for all samples showing the presence of the silica matrix.55,57 Stretching vCH bands (2900−3000 cm−1), corresponding to methylene residues belonging to the aminopropyl or benzophenonpropyl groups, are present in hybrid samples after template extraction, indicating the presence of the organic functional groups within the silica matrix. Additionally, several bands between 1300 and 1500 cm−1 correspond to C−H bending. Amino and ammonium functions are visible in the co-condensated NH2Si samples as broad bands in the 3000−3500 cm−1 region (νNH), which superimpose to the large O−H stretching bands of adsorbed water and surface hydroxide groups with extended Hbonding. Si−O−H bands are observed at 950 cm−1. Figure 6b depicts the bending zone between 1400 and 1800 cm−1 for BPSi films with increasing BPSilane molar ratio. Here, a continuous increase of the aryl ketone signal at 1650 cm−1 with increasing BPSi:TEOS ratio in the film precursor solution is observed, indicating a proportional increase of BPSi within the film with increasing ratio in the precursor solution. This of course does not automatically reflect a linear increase of BP groups available for later polymerization on the pore wall surface but only the proportional increase within the pore wall. Figure 6c shows details of the bending zone located in the 1400−1800 cm−1 for NH2Si films with increasing molar ratio of APTES within the precursor solution. The bending O−H bands, corresponding to adsorbed water, are centered at 1660 cm−1. While spectra of pure silica samples present only δO−H bands hybrid samples also display absorption bands corresponding to nitrogen containing species, δN−H bending consistent with reported results on hybrid mesoporous thin

consistent with TEM measurements as well as with XRR results based on interpretation of the Bragg peak (Figure 3). For surface imaging a frequency-modulated atomic force microscope45 (FM-AFM) was operated in constant excitation mode,47,48 a technique which is very sensitive to topographical variations. Imaging (Figure 4) corroborated that consolidated and extracted mesoporous thin silica films without functionalization (Figure 4a), BPSi films (Figure 4b), and with NH2Si films (Figure 4c) were very smooth. The measured root-meansquare (rms) roughness values were smaller than Rq = 0.3 nm. The cross sections in the bottom row illustrate representative surface profiles. From the cross-sectional profiles the minimum distance between two adjacent pores was estimated to d = 14, 10, and 9 nm for APTES, BPSilane functionalized films, and silica films without functionalization, respectively. Subtle differences in the morphology of the samples with different functionalization were apparent. The mesoporous film without functionalization have the finest morphology followed by BPSi (10 mol %), whereas the NH2Si sample (20 mol %) reveals the coarsest morphology with respect to the pore distribution. These results corroborate the trend found with the TEM (compare Table S2) yet do not match the exact values of the interpore distances. A regular pattern as detected by TEM imaging could not be observed. Resolving the exact size of the pores from AFM images is crucial due the geometric “convolution” of the tip shape with the sample morphology. The finite geometric size of the AFM tip leads to dilation and erosion of surface features in the image. In other words, elevated features appear broader whereas holes appear smaller or cannot be resolved at all.56 Because pores that are smaller than the tip apex can hardly be resolved, the interpore distance obtained with the AFM overestimates the distances as compared to the TEM measurements. Figure 5a shows a magnification of the region of interest indicated in Figure 4c. Single pores can be identified (red circles). In the threedimensional plot of the same area in Figure 5b the pores are visualized. Functional Characterization of Two-Component Cocondensated Mesoporous Films. Functional characterization was carried out by ATR-IR spectroscopy of the 375

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Figure 8. (a) XPS results showing the increasing ratio of amino groups (black squares) with increasing ratio of APTES in the sol (expected values: black empty squares). Additionally, the increasing amino ratio is reflected in an increase of detected Fluor per silica after TFAA functionalization. Thereby the measured values (blue circles) reflect a ratio of ∼22% of the introduced amino groups reacted with one TFAA molecules and subsequently carrying three Fluor atoms. (b) Normalized IR spectra before (black) and after (red) polymerization of (2-methacryloyloxy)ethyltrimethylammounium chloride (METAC) in a BPSi film with a BPSilane ratio of 10 mol % by exposure at 365 nm and subsequent extraction of the unbound monomer. The CO stretching band at 1730 cm−1 emerges upon exposure and indicates successful polymerization.

films.55 Comparing the N−H signal (∼1590 cm−1) in dependence of APTES concentration in the precursor solution, an increase in the δN−H bending with added APTES concentration in the precursor solution is observed. Due to the pH-dependent protonation/deprotonation equilibrium of surface amino groups, the pH was kept constant for all IR measurements by prior incubation into a pH-controlled destilled water. Accessibility of Two-Component Co-condensated Mesoporous Films. The ionic permselectivity of the cocondensated mesoporous thin films was probed electrochemically using Fe(CN)63− and Ru(NH3)63+ as anionic and cationic redox probes. These redox probe molecules are detected by cyclic voltammetry at an ITO substrate electrode after diffusing across the mesoporous film deposited on a conductive ITO substrate.49,58 Porosity, hydrophilicity, nanoporous structure, and proton concentration act as chemical and structural parameters responsible for allowing or hindering the diffusion of ionic redox probes from the thin film solution interface to the thin film electrode interface.49,58 By variation of solution pH, different electrostatic conditions inside the pore are defined. It is well established that the emergence of permselectivity effects or ionic gating requires the presence of surface charges on mesoporous membranes.6,38,39,54,58−60 At pH values at which the mesoporous framework displays no surface charges, permselectivity is mainly determined by accessible porosity, pore size, and wetting of the pore walls. At pH values at which the mesoporous walls present a surface charge electrostatic interaction and thus Debye length and pore size have to be taken into account. In the case of BPSi (Figure 7a), only the silanol groups of the silica framework induce charges at the pore wall. Thus, for solution pH ≤ 3, a neutral pore is expected, and permselectivity mainly depends on porosity and wetting. For basic solution pH values (pH ≥ 8), the pore wall surface is negatively charged due to the deprotonated silanol groups, which electrostatically hinders diffusion of Fe(CN)63− probe molecules and leads to a preconcentration of the countercharged Ru(NH3)63+. Figure 7a depicts the measured maximum current density as extracted from cyclovoltammetric measurements for BPSi films deposited on an ITO electrode at pH ≤ 3 and at pH ≥ 8 for both probe molecules Fe(CN)63− and Ru(NH3)63+, respectively. The original cyclic voltammorgrams are summarized in the Supporting Information (Figures S7−S9).

A decreasing maximum current density (jp) for Ru(NH3)63+ at an acidic pH (Figure 7a, black) with increasing ratio of BPSi reflects the decreasing porosity as measured by TEM, XRR, and ellipsometry (Figures 2 and 3). Permselectivity ranges from open pores and an maximum current density comparable to unmodified mesoporous silica54 down to zero reflection and no accessible porosity for a BPSi molar ratio of 25 mol %. The Ip decreases from 180 μAcm−2 for 10 mol % BPSi to 70 μAcm−2 in case of 15 mol % BPSi while the porosity decreases from 21% to 9%. For a BPSi ratio of 25 mol % no porosity was detected, and simultaneously no diffusion of redox probe can be observed independent of pH or redox-probe charge. Increasing the solution pH to ≥8, the deprotonated pore wall silanol groups result in negatively charged mesopore walls. Attractive electrostatic interaction with the positively charged Ru(NH3)63+ probe molecules result in a larger and broader signal (Figure S7). The most remarkable increase in current density, compared to pH ≤ 3, is observed for the 20 mol % BPSi film with almost no detectable porosity. This indicates a preconcentration of the countercharged probe molecule and the change in surface charge of the pore walls and shows the effect of an attractive force in particular in the case of low porosity or very small accessible pores or defects. Additionally, the peak separation slightly increases, indicating a slower electrode kinetic compared to a fast electrode reaction, as well indicating a diffusion barrier effect due to low film accessibility and electrostatic interaction. The jp is still determined by the decreasing porosity with increasing molar BPSilane ratio. Looking at the diffusion of a negatively charged redox probe Fe(CN)63−, the behavior at pH ≤ 3 again reflects the porosity as observed for Ru(NH3)63+ but with no significant difference between 10 and 15 mol % BPSi films. Increasing the pH to pH ≥ 8, pores are electrostatically closed for Fe(CN)63− due to the negatively charged silanol groups. For NH2Si mesoporous films (Figure 7b) the situation is slightly different because the amino groups on the surface are protonated at acidic pH. Thus, for an acidic pH (pH ≤ 3) an electrostatic repulsion of a positively charged redox probe is expected, as soon as the number of protonated amino groups reaches a critical concentration. Additionally, one has to consider the pH-dependent interaction between surface silanol groups and protonated amino groups.55,61 At pH ≤ 3 a decreased Ru(NH3)63+ signal with increasing APTES content is observed. This indicates an increasing number of amino functions on the surface that can be protonated resulting in 376

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electrostatic repulsion of Ru(NH3)63+. This observation of increasing surface amino group concentration with increasing molar APTES ratio in the precursor solution is consistent with XPS results (Figure 8). Because porosity and wetting are identical for these films according to ellipsometry results (Figure 2) and contact angle measurements (Figure S3), the variation in jp can be purely attributed to the electrostatic interaction. Within our experimental conditions of 100 mM monovalent supporting electrolyte (KCl) and 2 mM probe molecule the Debye length is approximately 1 nm. This value is smaller than the pore diameter but in the range of the neck diameter (connection between two pores). In summary, an APTES ratio of 20 mol % is sufficient to achieve complete electrostatic closure of the pores at acidic pH in the presence of 100 mM KCl. Increasing the pH to pH ≥ 8, the amino groups are deprotonated (neutral), whereas the surface silanol groups are deprotonated. Consequently, the positively charged redox probe Ru(NH3)63+ is preconcentrated within the pores, and the observed signal increases and broadens compared to pH ≤ 3 for all samples. The detected maximum current decreases slightly with increasing APTES ratio, pointing toward the reduced number of silanol groups and small structural changes. These results prove the preconcentration and the electrostatic interaction being the determining component in transport control for these NH2Si films. All NH2Si films show accessible pores for Ru(NH3)63+ at pH ≥ 8 with higher jp compared to Ru(NH3)63+ at pH ≤ 3. For the same reason the diffusion of the negatively charged redox probe Fe(CN)63− is strongly hindered at pH ≥ 8, showing that the presence of deprotonated silanol groups is sufficient to electrostatically block ionic transport for all APTES ratios up to 25 mol %. Reactivity of Amino Groups on the Pore Wall Surface. To reveal the reactivity of aminogroups in the NH2Si films, an elemental analysis was carried out by XPS on the outer mesoporous film interface. NH2Si films were modified with trifluoracetic acid anhydride (TFAA) to introduce easily detectable Fluor atoms as described in the literature.44 Figure 8a shows the F 1s and the N 1s signal in relation to the Si 2p signal for APTES molar ratios of 10−25 mol % before and after TFAA modification. Measured values corresponding to the TFAA modified NH2Si films and the unmodified NH2Si films are summarized in Tables S1 and S1b (Supporting Information). This signal was normalized to the Si 2p signal and depicted in Figure 8a as blue filled circles. The measured quantity of Fluor atoms fits to a theoretically expected increase of Fluor with increasing amount of APTES in the precursor solution (Figure 8a, blue empty circles). Based on the reaction stoichiometry, one amino group, reacting with TFAA, carries three Fluor atoms. Consequently, XPS data indicate TFAA modification for approximately 22% of the detected amino groups. This is comparable to reports in the literature for similar films44 and means that ∼22% of the introduced amino groups at the pore wall surface are reactive. This ratio is constant for varying ratio of APTES concentration in the film between 0 and 25 mol %. Consequently, by variation of the APTES ratio in the precursor sol, the binding points for polymerization initiators can be adjusted. Thus, the grafting density in porous systems can be controlled by following this strategy. Figure 8 as well shows the N 1s elemental analysis of the NH2Si films (Figure 8a, black squares). The expected ratio based on the introduced amount of APTES into the precursor sol is shown as empty squares.

The differences are due to experimental errors. In general, the measured relative quantity of amino groups fits the expected values based on stoichiometric calculation, showing no discrimination in film formation between the two precursor silane components. After TFAA functionalization the spectra in the N 1s region show peaks with two overlaying components at 400 and at 401.5 eV: a component due to amino groups and −NHCOCF3 groups (400 eV) and one due to ammonium (NH3+ at 401.5 eV).62 Reactivity of Benzophenone Groups on the Pore Wall Surface. To prove the reactivity of co-condensated benzophenone groups, photoinitiated polymerization of (2methacryloyloxy)ethyltrimethylammounium chloride (METAC) was performed on a 10 mol % BPSi mesoporous silica film by irradiation at 365 nm. IR analysis (Figure 8b) proves the presence of PMETAC after 12 min (1.4 J cm−2) of exposure and extensive subsequent extraction in water to remove unreacted monomer and free polymer based on the CO stretching at 1730 cm−1. Additionally, the presence of the Si−O−Si signal at 1053 cm−1 proves the presence of the silica and thus the stability of the mesoporous film. IR spectra were normalized to this Si−O−Si vibration. These XPS and IR experiments clearly indicate an increasing number of amino groups available for reactions on the pore wall surface, and even for the lowest ratio of benzophenone functions a clear proof of reactive functions on the surface available for polymerization is observed. This qualifies these films to adjust e.g. grafting density of polymers in a controlled manner for future experiments and opens a new important way of functional density control.

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CONCLUSION

In summary, we are able to gradually adjust the amount of reactive polymerization initiators (benzophenone) or initiator binding function (amino groups) in mesoporous silica films by co-condensation sol−gel chemistry, which is the basis for controlled variation of polymer grafting density in confined mesopores. Thereby, increasing concentration of organic function in the mesoporous film affects structure, ionic transport, and reactive functions available at the pore wall surface. For APTES ratios up to 25 mol % porosity and ionic permselectivity were not significantly affected by increasing APTES concentration, whereas BPSilane ratios should be kept below 15 mol %. A ratio of 22% of the total amount of directly co-condensated polymerization initiator binding functions (10−25 mol % APTES) were reactive on the pore wall surface, and the pH-dependent flux of ions could be gradually tuned by adjusting structural or electrostatic parameters. The polymerization initiators (benzophenone) were also reactive at the pore wall surface, and in the presence of monomer they initiate a polymerization at the mesopore wall surface. The well-defined concentration and the reactivity of the initiator after incorporation into the mesoporous silica film are the basis not only for tuning functional density by co-condensation but also for the precise adjustment of the grafting density in mesoporous silica thin films. Thus, we expect a direct impact on surface functionalization of porous materials with polymers and on the modulation of membrane properties such as ionic accessibility. 377

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S9 and Tables S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Landesoffensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz (LOEWE Soft Control) for financial support of this work as well as the Robert Bosch Stiftung and the Fonds der chemischen Industrie. We especially thank Ulrike Kunz and Prof. Kleebe from the Material Science Department of the Technical University Darmstadt for their support with TEM measurements. We thank Karl Kopp for his support with the XPS experiments.

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dx.doi.org/10.1021/la404004f | Langmuir 2014, 30, 369−379