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Preparation of Quaternary Ammonium Organosilane Functionalized Mesoporous Thin Films Eva M. Wong,† Michael A. Markowitz,*,† Syed B. Qadri,‡ Stephen L. Golledge,§ David G. Castner,§ and Bruce P. Gaber† Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, D.C. 20375, Surface Modification Branch, Code 6372, Materials Science and Technology Division, Naval Research Laboratory, Washington, D.C. 20375, and National ESCA & Surface Analysis Center for Biomedical Problems, Departments of Bioengineering and Chemical Engineering, Box 351750, University of Washington, Seattle, Washington 98195 Received October 9, 2001. In Final Form: December 7, 2001 The preparation of surfactant-templated periodic mesoporous silica thin films functionalized with N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride (TMAC) is described. The supramolecular self-assembly of the triblock copolymer, Pluronic 123, served as a template for the formation of these functionalized silane films. In the as-prepared and extracted conditions, the films exhibited the p6 mm type hexagonal mesostructure with a large unit cell (a ) 8.5-9.6 nm). Contraction of the films during heat treatment (250 °C for 2 h) reduced the pore spacing perpendicular to the substrate yielding Fm3m pore ordering, confirmed by X-ray diffraction and transmission electron microscopy. Ellipsometry measurements revealed the films to have thickness of ∼400 nm in the as-prepared condition and ∼300 nm in the heattreated condition, a total shrinkage of ∼25% perpendicular to the substrate. X-ray photoelectron spectroscopy measurements showed the incorporation of the TMAC to be 76-90% of the amount initially added following surfactant extraction.
Introduction The processing and fabrication of silica-based mesoporous thin films using surfactant aggregates as structuredirecting agents have garnered recent interest1-5 for applications such as membranes and sensors6 or surfaces for heterogeneous catalysis7 where powder samples cannot be readily used and a thin film geometry is essential. While nonfunctionalized silica materials are relatively inert, organosilanes can be added to impart functionality to the matrix. Judicious selection of the organic group can be made, such that the functionality of the material can be tailored to a specific application. For example, the preparation of transparent silica mesostructured thin films containing (aminopropyl)triethoxysilane with pHsensing capabilities has been reported.8,9 Surface-imprinted silicates with incorporated quaternary ammoni* To whom correspondence may be addressed. Tel: 202-4046072. Fax: 202-767-9594. E-mail:
[email protected]. † Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory. ‡ Surface Modification Branch, Code 6372, Materials Science and Technology Division, Naval Research Laboratory. § National ESCA & Surface Analysis Center for Biomedical Problems, Departments of Bioengineering and Chemical Engineering, University of Washington. (1) Yamada, T.; Asai, K.; Endo, A.; Zhou, H. S.; Honma, I. J. Mater. Sci. Lett. 2000, 19, 2167. (2) Ogawa, M.; Ishikawa, H.; Kikuchi, T. J. Mater. Chem. 1998, 8, 1783. (3) Hernandez, R.; Franville, A.-C.; Minoofar, P.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 2001, 123, 1248. (4) Grosso, D.; Balkenende, A. R.; Albouy, P. A.; Lavergne, M.; Mazerolles, L.; Babonneau, F. J. Mater. Chem. 2000, 10, 2085. (5) Pevzner, S.; Regev, O.; Yerushalmi-Rozen, R. Curr. Opin. Colloid Interface Sci. 2000, 4, 420. (6) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (7) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1997, 7, 1285. (8) Fan, H.; Lu, Y.; Stump, A.; Reed, S. T.; Baer, T.; Schunk, R.; Perez-Luna, V.; Lopez, G. P.; Brinker, C. J. Nature, 2000, 405, 56.
um-, dihydroimidizole-, or pyridine-terminated organosilanes have been demonstrated to improve the adsorption of organophosphorus compounds by up to 10 times.10 Porous mesostructures containing covalently coupled organic moieties may also be useful for optical chemical sensing based on fluorescence11 or interferometry12 where thin film geometries are required. In this report we show that quaternary amine-functionalized mesoporous silica thin films can be prepared with long-range, ordered porosity. Here we report on N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride functionalized thin films spin coated onto Si(100) surfaces with two-dimensional hexagonal pore structures obtained by ordering of a supramolecular assembly of the triblock copolymer, Pluronic P123 (PEO20PPO70PEO20). Experimental Section Anhydrous ethanol (reagent grade), tetraethyl orthosilicate (TEOS, 98%), and hydrochloric acid (reagent grade) were purchased from Aldrich Chemical Co. N-((trimethoxysilyl)propyl)N,N,N-trimethylammonium chloride (TMAC, 50% solution in methanol) was purchased from Gelest Co., and Pluronic P123 was purchased from BASF. All chemicals were used in the asreceived condition. Water used was deionized and distilled to 18 MΩ‚cm. Film solutions were prepared by refluxing 17.8 mL of TEOS, 13.9 mL of EtOH, 1 mL of H2O, and 0.4 mL of 0.01 M hydrochloric acid for 1 h. The mixture was cooled to room temperature and subsequently mixed with a P123/ethanol mixture (4.1 g of P123 in 78.9 mL of EtOH). This solution was allowed to stir for 1 h, then 3.2 mL of 0.10 M hydrochloric acid and 2.6 mL of H2O were added. TMAC was added in the molar ratio of 1:0.05 (TEOS/ (9) Wirnsberger, G.; Scott, B. J.; Stucky, G. D. Chem. Commun. 2001, 119. (10) Markowitz, M. A.; Deng, G.; Gaber, B. P.; Langmuir 2000, 16, 6148. (11) Lavin, P.; McDonagh, C. M.; MacCraith, B. D. J. Sol Gel Sci. Technol. 1998, 13, 641. (12) Grace K. M.; Shrouf K.; Honkanen S.; Ayras P.; Katila P.; Leppihalme M.; Johnston R. G.; Yang X.; Swanson B.; Peyghambarian, N. Electron. Lett. 1997, 33, 1651.
10.1021/la015623k CCC: $22.00 © 2002 American Chemical Society Published on Web 01/15/2002
Letters
Langmuir, Vol. 18, No. 4, 2002 973 Table 1. Film Thickness and Percent Shrinkage extracted (24 h in refluxing ethanol)
heat treated (250 °C/2 h in air)
t ( 3σ (nm)
t ( 3σ (nm)
t ( 3σ (nm)
410 ( 85 450 ( 51 440 ( 48 410 ( 56
380 ( 53 340 ( 74 390 ( 50 350 ( 49
as-prepared
control 10 min 30 min 90 min
∆t ∆d (%) (%) 7 24 11 15
18 9 15 8
230 ( 20 240 ( 21 240 ( 43 300 ( 50
∆t ∆d (%) (%) 44 47 45 27
31 21 30 20
kV. Fragments of the film were scraped from the substrate and suspended in ethanol. Drops of the ultrasonicated suspension were placed onto holey carbon TEM grids. Measurements of film thickness and refractive index were made on a Gaertner L115 Waferscan model ellipsometer with an incident angle of 70°. X-ray photoelectron spectroscopy (XPS) was performed using a Surface Science Instrument SSX-100 spectrometer equipped with a monochromatic Al KR X-ray source, hemispherical analyzer, and multichannel detector. Three spectra were taken for each sample, and the average and standard deviation were obtained from these data.
Results and Discussion
Figure 1. XRD patterns of MCM-41 type thin film functionalized with TMAC introduced 90 min before spin coating in the (a) as-prepared condition and (b) heat-treated condition (250 °C/2 h in air). Contraction of film during heat treatment yielded a change in pore ordering. TMAC) at intervals of 10, 30, and 90 min prior to spin coating. Films were made by filtering the solution through a 0.22 µm polyethylene filter followed by spin coating 0.4 mL of prepared solution at 2000 rpm for 30 s onto ∼6.25 cm2 sections of Si(100) wafers. All films were aged at room temperature for 2 days before extracting or heat treating. Extraction was performed by placing the films in a Soxhlet extractor with refluxing ethanol for 24 h (78.5 °C). Heat treatment was performed by heating the samples in air from room temperature to 100 °C at 0.5 °C/min, holding at 100 °C for 2 h, and then ramping from 100 to 250 °C at 0.5 °C/min and holding at 250 °C for 2 h. X-ray diffraction measurements were performed on a Rigaku Rotaflex Series model RU200B θ-2θ rotating anode diffractometer using Cu KR radiation. Transmission electron microscopy (TEM) was performed on a Hitachi H8100 TEM operating at 200
In the absence of the (110) reflections,13 it is proposed that the as-prepared samples had porosity ordered in 2-D hexagonal arrays with the long axis parallel to the substrate. For all samples in the as-prepared condition, a strong (100) reflection at 2θ ) 0.92-1.04° and weak (200) and (300) reflections were observed. As an example, the diffraction pattern of a sample with TMAC added 90 min prior to spin coating is shown in Figure 1a. Following extraction in ethanol for 24 h, the diffraction pattern did not change, but the d100 spacing decreased by 8-18%. Following heat treatment at 250 °C, the d100 spacing decreased by 20-30% in all samples; however XRD patterns of the TMAC-functionalized films revealed that the films were no longer hexagonally ordered. As shown in Figure 1b, indexing of the X-ray diffraction patterns for the heat-treated sample with TMAC added 90 min prior to spin coating shows the ordering to be 2-D cubic, with space group Fm3m and a calculated lattice parameter of 14.5 nm. The most likely cause of this transformation is uniaxial shrinkage of the film perpendicular to the solid silicon substrate, an effect that has also been documented by Babboneau et al. in silica films.4,14 The overall thicknesses of the films were determined using ellipsometry and are shown in Table 1. In the asprepared condition, the film thickness was found to be 400-450 nm. Following extraction and heat treatment, the percent shrinkage in film thickness was calculated based on the measured average thickness obtained from ellipsometry. Calculated values for the decrease in the d100 spacing obtained from XRD are shown for comparison. Shrinkage measured by ellipsometry was of the same order as decreases in d spacing obtained from XRD. Changes in total pore volume were not measured. Transmission electron micrographs of a heat-treated film prepared with TMAC added 90 min prior to spin coating are shown in Figure 2. Figure 2a shows a cross section of the film parallel to the long axis of the pores. The structure is highly ordered with a pore-to-pore spacing of 10 nm. The entire cross section is 308 nm, corresponding to a thickness of ∼300 nm measured from ellipsometry as shown in Table 1. Figure 2b shows a cross section of the (13) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1998, 10, 1230. (14) Grosso, D.; Balkenende, A. R.; Albouy, P.-A.; Ayral, A.; Amenitsch, H.; Babonneau, F. Chem. Mater. 2001, 13, 1848.
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Letters Table 2. Measured N/Si Ratio Obtained from XPS and Calculated Percent Incorporation of TMAC (Shown in Parentheses)
as-prepared
extracted (24 h in refluxing ethanol)
heat treated (250 °C/2 h in air)
0 0 0.006a 0.031 ( 0.001 0.043 ( 0.004 0.013 ( 0.003 (65 ( 2%) (90 ( 9%) (28 ( 7%) 30 min 0.028 ( 0.001 0.037 ( 0.008 0.017 ( 0.004 (59 ( 1%) (77 ( 16%) (36 ( 8%) 90 min 0.030 ( 0.001 0.036 ( 0.001 0.014 ( 0.004 (64 ( 1%) (76 ( 2%) (30 ( 8%) a N incorporation may be the result of contamination during heat treatment.15 control 10 min
were found in the heat-treated sample, most likely resulting from contamination during heat treatment or sample handling.15 Incorporation of TMAC was on the order of 60-90%. Calculated TMAC incorporation was higher in the extracted samples, which may result from removal of unbound SiO2 during the extraction process (in ethanol) thereby increasing the N/Si ratios. Chloride initially bound to the nitrogen in the as-prepared condition was removed following extraction or heat treatment suggesting that an exchange reaction occurred with OHduring these processes. In conclusion, we have found that TMAC can be incorporated into a mesoporous silicate film while maintaining an ordered structure. The time at which TMAC is added to the TEOS solution does not affect the properties of the film. This report describes the first steps toward incorporating quaternary ammonium organosilanes to TEOS. Further studies will be required to determine whether these films can be used as adsorbents or sensing components due to the added functionality imparted by the amine terminal group. Figure 2. TEM micrographs showing a heat-treated (250 °C/2 h in air) thin film functionalized with quaternary ammonium organosilane added 90 min prior to spinning: (a) cross section parallel to the long axis of the pores showing a pore-to-pore spacing of 10 nm; (b) cross section perpendicular to long axis of the pores showing pore arrangement. Outline of unit cell is marked for reference.
film illustrating the regular spacing of the pore network. An outline of the unit cell is overlaid with a measured lattice parameter of ∼15 nm, which corresponds well with the information gathered from XRD, as shown in Figure 1. XPS of the films was performed to measure the relative C, O, N, Cl, and Si content. The ratio of nitrogen to silicon was used to calculate the percentage of TMAC that was incorporated into the films by comparing to the amount added to the solution. Both the measured N/Si ratio and the calculated percentage incorporation of TMAC are shown in Table 2. As expected, nitrogen was not detected in the control samples, although small amounts of nitrogen
Acknowledgment. This work was performed while E. M. Wong held a National Research Council Research/ Naval Research Laboratory Research Associateship Award. This project was funded by the Office of Naval Research through a Naval Research Laboratory Accelerated Research Initiative. The National ESCA and Surface Analysis Center for Biomedical Problems at the University of Washington is an NIH-supported research center, RR01296. Supporting Information Available: XRD patterns for all film samples, measured O/C ratio obtained from XPS, C-H and C-O-C ratios from high-resolution XPS peak fits, concentration of all measured species by XPS. This information is made available free of charge via the Internet at http://pubs.acs.org. LA015623K (15) Although the exact source of this contamination is not known, we note that the nitrogen content is small in relation to the silicon content and is not evident in the other control samples and therefore is not systemic.
