Paper No. 203e Microstructured Porous Silica Obtained via Colloidal Crystal Templates O. D. Velev, T. A. Jede, R. F. Lobo and A. M. Lenhoff Department of Chemical Engineering, University of Delaware, Newark DE 19716 Prepared for Presentation at 1998 Annual AIChE Meeting: Session 203 - Nanostructured Materials Copyright Velev, Jede, Lobo and Lenhoff Date 09/1998 AIChE shall not be responsible for statements or opinions contained in papers or printed in its publications. This paper and updates can be downloaded from: http://www.che.udel.edu/~velev/
Abstract A novel scheme for using modified colloidal crystals as templates for silica polymerization is reported. By this method we were able to obtain highly structured silica materials of which the pore size, shape and ordering can be precisely controlled in a wide region that has been previously unattainable. 3D close-packed crystals of submicrometer latex microspheres are assembled on a membrane surface by filtration and serve as templates for the silica polymerization. To initiate polymerization, the surfaces of the assembled particles are functionalized by adsorption of cationic surfactant. The colloidal crystals are then infused with silica solution, which polymerizes in the cavities. Because of the in situ functionalization, the method is not sensitive to the surface properties of the particles used in the crystal assembly; this was demonstrated by using both positively and negatively charged latexes. In the final step, the latex particles are removed by heating, leaving behind porous silica of very low density. SEM observations demonstrate that the product has highly uniform pores ordered in hexagonal and square lattices, representing a negative replica of the original colloidal crystal. The porosity of the material estimated by TGA is near that of close-packed spherical cavities (74%). The size of the pores can be controlled by changing the size of the microspheres used, and we were able to obtain samples with pore diameters ranging from 180 to 1000 nm. The method appears inexpensive and controllable, and could be adapted for large-scale production. Its future development holds promise for the formation of advanced new materials with unique properties, which can find application in catalysis, separations, coatings, microelectronics and electro-optics. 2
1. Introduction Porous silicas of different types have great potential in applications like catalysis, separations, environmental technologies, sensors, membranes, etc. The most commonly studied mesoporous materials today are the M41S types of silica formed by using surfactant micelles and liquid crystals as templates (1-3). Although these materials are easily fabricated, the pore diameters are typically limited to less than 10 nm. The formation of inorganic materials with uniform pores of arbitrary size, shape and orientation remains a challenge, and therefore there is significant interest in the development of methods using other templates that can yield silica with different pore sizes and morphologies. In a separate line of research, colloidal crystals have been a subject of intense investigation, as their formation and properties reveal some of the basic interactions in the microscopic world (4). The ordered arrays of particles have remarkable optical properties and much effort has been directed to turning them into usable solid-state materials. The results of this effort have been quite modest so far, not least because of the poor mechanical properties of the materials obtained. In this paper, we present a method by which the structure of the colloidal crystals is replicated into the stable matrix of solid polymerized silica. This method produces highly structured silica of which the pore size, shape and ordering can be precisely controlled in a wide region that has been previously unattainable (5,6). 2. Experimental A schematic presentation of the method is shown in Figure 1. The diluted latex particles are assembled into a colloidal crystal by slow filtration through a smooth membrane with narrow pores. Monodisperse suspensions of amidine (positively charged) or sulfate (negatively charged) latexes from IDC (USA) were used. The latexes were diluted with deionized water or with HTAB solution to 0.1 vol% and then filtered for 2 hours through polycarbonate membranes with pore size of 0.1 µm. The use of a membrane support allows easy functionalization and mineralization of the crystal, as the corresponding liquids are simply washed through the ordered layers. 3
Figure 1. Schematic illustration of the stages of the synthesis method. To functionalize the surfaces of the microspheres to a state where they can serve as nuclei for silica polymerization, the latex layer was washed with HTAB solution for 20 min. The in situ treatment by cationic surfactant makes the method insensitive to the type of the latex used we have proven this by using both positively and negatively charged microspheres as templates. On the other hand, if this step is omitted, the silica polymerization is not successful and the material collapses on calcination. The mineralization of the cavities between the particles was carried out by infusion with 0.5 M aqueous solution of Si(OH) 4. The latex/silica composites were then dried in a vacuum oven for 30 mins at 50 o C and then heated in a programmable furnace for 4 h at 450 o C. The 4
organic components of the material were burned out during the calcination, leaving behind silica flakes of very low density. The results of the observation and characterization presented below demonstrate that the material comprises highly uniform and ordered pores. The samples were studied by optical microscopy in transmitted illumination, SEM, and TGA analysis. 3. Results and Discussion The final products of the calcination were off-white silica flakes of very low density. Investigation of the flakes by optical microscopy showed them to be highly porous, though the discrete structure of the material was below the resolution of the microscope. The discrete morphology of the material could, however, be observed by SEM along cracks and abrasions on the surfaces of the flakes. Large 3D ordered arrays of spherical cavities that obviously represented a negative replica of the original colloidal crystal embedded in the silica were observed (Figure 2). A unique feature of the materials obtained that can eventually make them usable in high technology applications is the long-range ordering of the uniformly sized pores. In the case of 200-500 nm particles, we have observed ordered domains of almost perfect hexagonal or square lattices including more than 10 3 pores of identical size in the first visible layer. Figure 2. SEM of representative sample of microstructured silica prepared with a 560 nm amidine latex. 5
One of the biggest advantages of the proposed method is the ability to control easily the dimensions of the pores obtained by varying the size of the latex beads in the templates. By using different suspensions of latex microspheres we have been able to obtain organized materials with pore sizes ranging from ca. 150 nm to 1 µm. Examples of the materials prepared with the smallest and the biggest particles used are shown in Figure 3. Figure 3. Examples of how the size of the pores can be tuned by changing the original latex suspension: (a) Sample obtained from 190 nm amidine latex. (b) Sample obtained from 980 nm sulfate latex. Both scale bars correspond to 1 µm. 6
To obtain additional information on the porosity of the material and the thermal aspects of its formation, thermogravimetric analysis (TGA) experiments on the calcination of the latex/silica composites were carried out. Typical results comparing the thermal decomposition of the latexsilica composite and of pure latex crystals are shown in Figure 3. The data were processed by the formula ε = w s wl ρl ρ + w s where ε is the porosity, w l and w s are the corresponding weights of latex and silica in the sample, ρ l and ρ s are their densities (assumed to be 1.05 g/cm 3 for the polystyrene latex and 2.20 g/cm 3 for the fused silica). Substituting the above TGA values, an estimated porosity of 78 vol% is obtained. This value is close to that expected for a material with close-packed ordered spherical pores ( 74%). l ρ l 140 120 100 Weight, % 120 100 80 60 40 20 500 400 300 200 100 Temp., deg. C Weight, % 80 60 Latex-Silica Comp. Latex only 0 0 0 100 200 300 400 500 Time, min 40 20 0 50 100 150 200 250 300 350 400 450 Temperature, deg. C Figure 4. TGA plot obtained during calcination of materials formed from 300 nm latex. The latex-silica composite sample is normalized to 100% after evaporation of the residual water. Thus we believe that we have proven experimentally the applicability of our method to the assembly of silica samples with pores of defined size and excellent ordering. A reasonable and pragmatic approach for the next steps should concentrate on adapting the method to the formation of samples that are larger in volume and better ordered. Interesting possibilities could 7
be found by reverse templating or introducing catalyst metal particles inside the ordered cavities via the latex template. 4. Conclusions We have developed a novel method by which we obtained silica samples with an ordered lattice of pores ranging from 190 to 1000 nm in size (5,6). The novel features of the method that make it promising from fundamental and practical viewpoints are: Two important research fields colloidal crystallization and microporous silica were linked. This is the first study that makes use of 3D colloidal crystals as a basis for the production of durable solid-state materials. In contrast to MCM-41 phases, the pore size of the materials is precisely tunable in a wide range. The concept of modifying the surface properties of the latex particles by adsorption of surfactant makes the method applicable to colloidal crystals prepared from a wide variety of particles. The symmetry and the shape of pores can, in principle, be regulated. This is a step-by-step controllable process. It uses cheap and clean synthetic materials and appears to have promise for a quick transition to practical production of advanced materials. 5. Referencess. 1. Kresge, C.T., et al., Nature, 359, 710 (1992). 2. Beck, J.S. et al., J. Am. Chem. Soc., 114, 10834 (1992). 3. Monnier, A. et al., Science, 261, 1299 (1993). 4. Arora, A.K. and Rajagopalan, R. in Ordering and Phase Transitions in Charged Colloids ; Arora, A.K., Tata, B.V.R. Eds.; VCH Publ.: New York, 1996. 5. Velev, O.D., Jede, T.A., Lobo, R.F. and Lenhoff, A.M., Nature, 389, 447 (1997). 6. Velev, O.D., Jede, T.A., Lobo, R.F. and Lenhoff, A.M., Chem. Mater., 10, 3597 (1998). 8