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Preparation and Characterization of Macroporous Silicate Films Alexander N. Khramov, Jeff Munos, and Maryanne M. Collinson* Department of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, Kansas 66506-3701 Received January 22, 2001. In Final Form: June 18, 2001 A straightforward procedure for engineering porosity in thin silicate films has been demonstrated. Macroporous silicate films were fabricated by doping a silica sol prepared from the acid-catalyzed condensation of tetramethoxysilane with polystyrene latex spheres of diameter ranging from 0.3 to 1 µm. After film formation, the polystyrene spheres are etched out the film using chloroform so that the pores are completely exposed on both top and bottom surfaces of the film. The size and distribution of pores in the film and the extent at which the pores are accessible to species in solution were characterized using atomic force microscopy (AFM) and electrochemical methods. AFM reveals that the pores are uniform in size with dimensions defined by the diameter of the particles. The number density of pores in the silica surface was varied from 10 to 150 pores/100 µm2 by controlling the concentration of microspheres in the sol. Cyclic voltammetry with probe compounds (ruthenium hexaammine, potassium ferricyanide, ferrocene methanol, and ferrocenylmethyl trimethylammonium ion) shows that the extent of pore penetration is a strong function of the ionic charge of the redox probe and the size of the pores.
Introduction The sol-gel process has been frequently used to prepare stable host materials through the hydrolysis and condensation of alkoxysilanes.1 These materials have been widely used in the development of chemical sensors, in nonlinear and photochromic applications, and in solidstate electrochemical studies.2-4 From a scientific perspective, the sol-gel process is incredibly flexible in that various reagents can be permanently trapped into this stable matrix via their addition to the sol prior to gelation and because materials in various configurations (films, monoliths, and particles) can easily be prepared and utilized. In chemical sensing applications, thin films are most often utilized as the path length for diffusion is short, which enables quick response times and recovery rates. In contrast to silica monoliths, however, thin films are considerably less porous.1,5 Films prepared from acidcatalyzed silica sols, in particular are quite dense, leading to a reduction in the mobility and activity of entrapped reagents and the rate at which external molecules can diffuse into the films.6 The ability to engineer porosity into these materials would clearly improve their performance in analytical science. Among various strategies for the preparation of porous materials, template based sol-gel processing has rapidly gained popularity.7 In this approach, the silicate matrix is formed around a given template. After removal, cavities of the approximate shape and size of the template remain * To whom correspondence should be addressed. Tel.: (785) 5321468. Fax: (785) 532-6665. E-mail:
[email protected]. (1) Brinker, J.; Scherer, G. Sol-Gel Science; Academic Press: New York, 1989. (2) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Gun, J. Anal. Chem. 1995, 67, 22A-30A (3) Avnir, D. Acc. Chem. Res. 1995, 28, 328-334. (4) Collinson, M. M.; Howells, A. R. Anal. Chem. 2000, 72, 702A709A. (5) Frye, G. C.; Ricco, A. J.; Martin, S. J.; Brinker, C. J. Mater. Res. Soc. Symp. Proc. 1998, 121, 349-354. (6) Collinson, M. M.; Rausch, C. G.; Voight, A. Langmuir 1997, 13, 7245-7251. (7) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682-1701.
in the cross-linked host. A wide variety of templates have been used to form mesoporous and macroporous materials. These include small molecules such as D-glucose,8 dendrimers,9 organic functionalites,10 bridged organic ligands,11,12 surfactant assemblies,13-15 and latex spheres.16-19 Most of these investigations have focused on the preparation of three-dimensional periodic bulk materials and/or thick membranes.9-17 In this work, polystyrene microspheres have been used as the templating entities to fabricate thin macroporous silicate films with controllable porosity. Such polymer microspherical particles are readily available in a wide range of particle dimensions, which can be used to fabricate materials with a wide range of pore sizes. The ability to tailor the porosity of the silicate materials is particularly important in chemical sensing related applications because the response time and signal levels can be optimized. In a prior investigation, we have demonstrated the feasibility of utilizing polystyrene latex spheres as templates to form macroporous silicate films.20 In this report, we provide a detailed account of the morphology and (8) Wei, Y.; Xu, J.; Dong, H.; Dong, J. H.; Qiu, K.; Jansen-Varnum, S. A. Chem. Mater. 1999, 11, 2023-2029. (9) Hedrick, J. L.; Hawker, C. J.; Trollsas, M.; Remenar, J.; Yoon, D. Y.; Miller, R. D. Mater. Res. Soc. Symp. Proc. 1998, 519, 65-75. (10) Lu, Y.; Cao, G.; Kale, R. P.; Prabakar, S.; Lopez, G. P.; Brinker, C. J. Chem. Mater. 1999, 11, 1223-1229. (11) Hobson, S. T.; Shea, K. J. Chem. Mater. 1997, 9, 616-623. (12) Chevalier, P.; Corriu, R. J. P.; Delord, P.; Moreau, J. J. E.; Man, M. W. C. New J. Chem. 1998, 423-433. (13) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (14) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-43. (15) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Chem. Mater. 1998, 10, 3597-3602 (16) Holland, B. T.; Blanford, C. F.; Do, T.; Stein, A. Chem. Mater. 1999, 11, 795-805. (17) Gates, B.; Yin, Y.; Xia, Y. Chem. Mater. 1999, 11, 2827-2836. (18) Martin, J. E.; Anderson, M. T.; Odinek, J.; Newcomer, P. Langmuir 1997, 13, 4133-41. (19) (a) Ayral, A.; Guizard, Col, L. J. Materials Sci. Lett. 1994, 13, 1538-39. (b) Klotz, M.; Ayral, A.; Guizard, C.; Cot, L. Bull. Korean Chem. Soc. 1999, 20, 879-884.
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permselective properties of the templated silica films as a function of the size and the amount doped into the sol using a combination of microscopic, spectroscopic, and electrochemical methods. Experimental Section Reagents and Equipment. Tetramethoxysilane (TMOS, 99%), sodium dodecyl sulfate (SDS, 98%), and ferrocenemethanol (97%), and ferrocenylmethyl trimethylammonium iodide were purchased from Aldrich. Chloroform, hydrochloric acid, potassium nitrate, potassium phosphate, and potassium ferricyanide were purchased from Fisher Scientific. Ruthenium (III) hexaammine (99%) and ferrocene monocarboxylic acid (95%), and were purchased from Strem Chemicals. Aqueous suspensions of polystyrene uniform microspheres (PSMS) with diameter ranging between 0.1 and 1 µm were obtained either from Aldrich (2 wt/ v% of solids) or from Interfacial Dynamics (Portland, Oregon, USA) (8 wt %/v, surfactant-free, sulfated). Water was purified to Type I using a Labconco Water Pro PS four-cartridge system. Electrochemical measurements were performed using a BAS CV-50W voltammetric analyzer using a one-chamber, threeelectrode cell. The working electrode consisted of a glassy carbon electrode (A ) 0.2 cm2) prepared as previously described.6 The reference and auxiliary electrodes were a silver-silver chloride electrode (1 M KCl) and a platinum wire electrode, respectively. Film thickness was measured either with a Tencor Alpha Step 500 surface profiler or by means of atomic force microscopy (AFM) imaging. AFM measurements were performed in the contact mode with a Nanoscope IIIa multimode SPM microscope (Digital Instruments, Inc., Santa Barbara, CA) using a microfabricated Nanoprobe silicon nitride tip at a scan rate of 1-3 Hz. FT-IR spectra of the thin films were recorded in transmission mode with a Nicolet Nexus 670 spectrophotometer using a silicon wafer as a substrate. The spectra (average of 32 scans) were recorded from 4000 to 400 cm-1 at a resolution of 4 cm-1. Procedures. The silica sol was prepared by mixing 0.5 mL of TMOS with 0.43 mL of deionized water and 0.1 mL of 0.1 M hydrochloric acid followed by stirring for 5-10 min. The original aqueous suspension of polystyrene microspheres (PSMS, 0.1, 0.3, 0.5, 0.7, or 1 µm diameter) was sonicated for 5 min prior to use. The sample solutions of PSMS suspension were prepared by mixing the original suspension with water and optional 10 mM SDS followed by sonication for 2 min. The aged (3 days) silica sol was then added to a PSMS suspension in a ratio of 1:2 (v/v) unless otherwise stated. The final mixture was vigorously stirred and again sonicated for 1 min. The resultant suspension was cast on the surface of a glassy carbon electrode or on a silicon substrate at ca. 7000 rpm using an in-house built rotator. Prior to the spin-casting procedure, the electrodes were polished with 0.05-µm alumina particles on a napless polishing cloth, sonicated in water and methanol for 5-10 min, and then dried. The thin films were allowed to dry overnight under room conditions (4555% RH, ambient temperature). To remove the polystyrene latex spheres, the films were soaked in chloroform for 2 h. After airdrying, the films were placed in deionized water for 30 min before they were placed in the solution of the electroactive probe compound (see below).
Results and Discussion Preparation and Characterization. The method used to fabricate the templated silicate films involves the preparation of the sol, physical entrapment of the template, and gelation of the sol. Polystyrene microspheres were chosen as the templating entities because they can be obtained in a wide variety of sizes and with different functional groups. This flexibility enables one to control the size of the cavities induced in the matrix and hence the porosity of the film. In this work, sulfate stabilized polystyrene microspheres with diameters ranging from 0.1 to1.0 µm were doped into a silica sol prepared from (20) Khramov, A. N.; Collinson, M. M. Chem. Commun. 2001, 8, 767768.
Figure 1. (A) 2.5 µm × 2.5 µm AFM image of a silicate film prepared from a sol containing 0.5 µm polystyrene microspheres, 1.3 wt/v % after chloroform treatment. (B) Cross sectional view of a single hole.
tetramethoxysilane at a concentration that was varied from 0.7 to 5%. No visible phase separation in silica solmicrosphere suspension due to sedimentation of aggregated particles was observed in any of the preparations. Stable films of a thickness of ca. 100 nm can easily be cast with good batch to batch reproducibility. In AFM images of the templated film, the polystyrene particles embedded within the silicate film appear as the “hills” randomly distributed through the topography of the film.20 Several different methods can be used to remove the polystyrene templates from the silicate framework while preserving the integrity of the cavity thus formed. Calcination has been shown to be very effective in removing organic material from the cross-linked host.7,14-16 This procedure, however, will not work for the samples prepared in this study as it will also result in the destruction of the electrode surface as well as any reagent or organic functionality trapped within. A milder chemical treatment should be used such as dissolving the spheres in an appropriate solvent.16,17 In this work, chloroform was used as the solvent of choice as polystyrene is very soluble in this solvent and the silicate films are stable. Figure 1 shows an AFM image of a macroporous silica film prepared with 0.5 µm diameter polystyrene particles. As can be seen, craters visibly open at the top are randomly distributed through the silicate film. The shape of the crater mimics the shape of the bottom half of the polystyrene particle. The inner diameter of the cavity is approximately the same size as the diameter of the latex spheres used to prepare the film. Upon formation of the film, the silica likely pulls away from the polystyrene microsphere exposing the top of the particle and building up around the sides. A schematic illustration of this is shown in Figure 2. Part of the electrode underneath the polystyrene microsphere becomes exposed after removal of the particle. This area is referred to as the “contact zone” in Figure 2 and gives rise to electrochemical response
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Figure 2. A schematic representation of the templating of silica films with polystyrene microspheres. Table 1. Properties of Polystyrene Templated Silicate Films Prepared from a Silica Sol Containing 0.5 µm Polystyrene Microspheres at Different Concentrations polystyrene content, %
approximate particle numbera per 100 µm2
iL ((SD)b, µA
0.7 1.3 2.5 5.0
7 40 47 54
2.2 ( 0.4 7.8 ( 0.2 8.2 ( 0.5 12.3 ( 2.6
Figure 3. FTIR spectra of a silicate film before (solid line) and after (dashed line) chloroform treatment. Film was prepared from a silica sol containing 0.5 µm polystyrene microspheres (1.3 wt/v %).
a Obtained by counting the number of holes in a 10 µm × 10 µm AFM image. b Statistical data represents mean (SD (N ) 6).
Table 2. Characteristics of Silica Films Templated with Polystyrene Latex Spheres (1.3 w/v%) of Different Sizes PSMS diameter, µm
approximate particle numbera per 100 µm2
iL ((SD)b, µA
0.1 0.3 0.5 0.7 1.0
617 ( 29 140 ( 13 40 ( 6 20 ( 3 10 ( 2
N/A 1.7 ( 0.4 8.3 ( 0.9 3.5 ( 0.6 4.0 ( 0.2
a Obtained by counting the number of holes in a 10 µm × 10 µm AFM image. b Statistical data represents mean (SD (N ) 6).
described later. The number of particles (holes) per area can be directly obtained from the AFM images by counting the number of holes in a 10 µm × 10 µm image. As can be seen in Table 1, increasing the amount of polystyrene particles in the silica sol increases the amount of craters in the film. Likewise, increasing the diameter of the particles increases the width of the craters. Nearly identical results were observed for films prepared with polystyrene particles of diameter 0.3-1.0 µm. The average number of cavities in each film as determined from AFM are listed in Table 2. For films prepared with 0.1 µm latex spheres, no change in the topography of the silicate film before and after chloroform treatment was observed. In this case, the thickness of the silicate film was larger than the particle diameter thus causing the polystyrene particles to become deeply embedded in the silicate matrix and not readily extractable with chloroform. In addition to AFM characterization of the physical structure of the film surface, FTIR spectroscopy was also used to examine the composition of the films before and after chloroform treatment. The FTIR spectra of both chloroform treated and untreated composite films cast on silicon wafers are shown in Figure 3. Bands attributed to the silicate matrix are observed at 1070, 940, 800, and 440 cm-1 along with the broad absorption band near 3300 cm-1.1 These features are essentially identical for the silicate film both before and after chloroform treatment. The bands attributed to polystyrene at 3027, 2925, and 697 cm-1, however, disappear after the film is treated with chloroform for 2 h indicating that the polystyrene
Figure 4. 2.5 µm × 2.5 µm AFM image of a silicate film after copper electrodeposition. Film was prepared from a sol containing 0.5 µm polystyrene microspheres, 1.3 wt/v %.
microspheres have been removed from the film. In the case of the films prepared with the 0.1 µm spheres, no change in the FTIR spectra was observed before and after chloroform treatment. Electrochemical Characterization. All the craters appear to be open at the top and bottom as evidenced from cross sectional images such as that shown in Figure 1. To experimentally verify the underlying electrode surface is indeed exposed after template removal, electrodeposition experiments were undertaken. In these experiments, the film coated electrode was placed in a solution of 5 mM copper sulfate in 0.1 M sulfuric acid. The copper was then electrochemically reduced by applying -0.2 V to the electrode for 30 s and film imaged using AFM. A 2.5 µm × 2.5 µm image is shown in Figure 4. As can be seen, an additional island of material (e.g., copper metal) appears in the center of each crater. Upon examination of a 10 µm × 10 µm area, it can be ascertained that more than 90% of the craters contain copper. This result indicates that the majority of holes in the film are indeed open and are available for electrochemical reactions. To obtain more qualitative and quantitative information about the presence of template induced cavities in the
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Figure 6. Cyclic voltammograms of 1 mM Ruthenium hexaammine in 0.1 M KNO3 at a (A) polystyrene templated film and (B) control film after (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, (f) 100, and (g) 120 min in solution. Scan Rate: 100 mV/s.
Figure 5. Cyclic voltammograms of 1 mM (A) ferrocene methanol, (B) FcTMA+, (C) ruthenium hexaammine, (D) ferrocene monocarboxylic acid, and (E) potassium ferricyanide in 0.1 M KNO3 at a polystyrene templated film. Solid lines: after template removal; Dashed lines: before template removal. Films were prepared from a silica sol containing 0.5 µm polystyrene microspheres, 1.3 wt/v %. Scan rate: 100 mV/s.
silicate film, electrochemical probe techniques were used.21,22 In these experiments, the templated silicate film cast on an electrode surface is placed in a solution containing an electroactive “probe” molecule (i.e., potassium ferricyanide (FeCN63-), ruthenium hexaammine (Ru(NH3)63+), ferrocene methanol (FcCH2OH), ferrocenyltrimethylammonium ion (FcTMA+), and ferrocene monocarboxylic acid) and cyclic voltammetry obtained at different scan rates. If the probe molecule is able to reach the underlying electrode surface via diffusion through a defect site or a template induced cavity, Faradaic current will be observed. The shape of the voltammetric curve and the magnitude of the current can provide valuable information about the distribution and size of the cavities formed in the films after template removal.21,22 Figure 5 shows cyclic voltammograms (CVs) of 1 mM solutions of Fe(CN)63-, FcCOO-, FcTMA+, FcCH2OH, and Ru(NH)63+ at the silicate film both before and after template removal. The CVs were acquired a minute or so after the electrode was placed in solution. Prior to treatment in chloroform, no Faradic current can be seen with any of the electroactive probes. This is consistent (21) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329-1340. (22) Che, G.; Cabrera, C. R. J. Electroanal. Chem. 1996, 417, 155161.
with our prior work that has shown that films fabricated with a silica sol prepared via the acid catalyzed hydrolysis of tetramethoxysilane are compact and essentially defect free.6 After the polystyrene spheres are removed from the dense matrix, however, sigmoidal shaped CVs are obtained for FcTMA+, FcCH2OH, and Ru(NH3)63+ due to electron transfer at the electrode-solution interface produced by the template (see Figure 2). For Fe(CN)63- and FcCOO-, the voltammetric curves were nearly identical to the background voltammograms except at low sweep rates (
