NANO LETTERS
Controllability of Pore Size and Porosity on Self-Organized Porous Silica Particles
2002 Vol. 2, No. 4 389-392
Ferry Iskandar, Mikrajuddin, and Kikuo Okuyama* Department of Chemical Engineering, Graduate School of Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan Received November 8, 2001; Revised Manuscript Received January 28, 2002
ABSTRACT Silica powder containing organized pores was prepared by a spray drying method. Silica and polystyrene latex (PSL) nanoparticles colloids were mixed and atomized to form micrometer-sized droplets. Nitrogen carrier gas was used to carry the resulting droplets into a vertical reactor that contained two heating zones: 200 °C and 450 °C, which were used, respectively, to evaporate the dispersing medium (water) and to decompose the PSL particles to give a porous silica powder. The pores on the surface of the powders were found to be arranged into a hexagonal packing, indicating that a self-organization process occurred spontaneously during evaporation of the solvent. The pore size was controlled by changing the size of the PSL particles. By adding an additional zone (third zone) maintained at high temperatures, the produced powders could be in-situ annealed. A comparison of the average volume of the powder before and after annealing (at 1500 °C) indicated that the porosity of the powder was about 70%. Together with the TEM image, this result indicated that pores were present both on the surface of the powders, as well as, inside the powder particles.
Introduction Catalysts, chromatography, the controlled release of drugs, low dielectric constant fillers, sensors, pigments, microelectronics, and electrooptics represent examples of applications of porous materials.1-3 Among the methods for preparation of those materials, solid colloid templating seems to be the simplest one.1,4-6 The preparation consists of three main steps: the formation of colloidal template, infiltration of the voids between the colloidal beads with other materials (another colloid, or reactant gases in the case of a CVD reactor, or reactant ions in the case of an electrochemical cell), and, last, the removal of the colloidal beads chemically or thermally, leaving behind a porous material that is an inverse replica of the mesostructure of the template. This, however, requires numerous processing steps and is timeconsuming. The entire process requires several hours or longer for completion. Recently, we reported on an aerosolassisted spray drying method to produce powder particles which contain organized pores.7 This procedure required only several seconds to reach completion. The controllability of the pore size as well as the morphology of the materials is important for the development of size-selective filters and a selective optical cavity. The wavelength of the optical mode in a cavity depends on the cavity size and the band gap of the optical crystals depend * To whom correspondence should be addressed. E-mail: okuyama@ hiroshima-u.ac.jp. Phone: +81-824-247716. Fax: +81-824-247850. 10.1021/nl015662g CCC: $22.00 Published on Web 02/12/2002
© 2002 American Chemical Society
on the crystal periodicity. Here, we report on the controllability of pores size and the morphology of organized porous silica particles prepared by a spray drying method using polystyrene latex (PSL) and silica colloids as the source materials. The pores sizes can be controlled using colloidal particle of different sizes. This is very easy because a huge variety of PSL and silica colloids of various particles sizes are available in the market at present. The porosity of powders was investigated based on scanning electron micrograph (SEM) and by heat treatment. High temperatures treatment removed the pores in the powder to produce solid particles. The difference in powder particle volume before and after annealing determines the porosity of the powder.
Experiment Samples were prepared using procedures described in our previous paper.7 To summarize, colloidal mixtures of silica nanoparticles (sol type, Nissan Chemical Ind. Ltd.) and PSL (Japan Synthetic Rubber Corp.) nanoparticles were sprayed as droplets into a vertical reactor which contained two temperature zones (200 °C and 450 °C). The droplets were carried from the low-temperature zone to the high-temperature one. The solvent in the droplets was evaporated in the lower temperature zone to produce a powder composite consisting of silica and PSL nanoparticles. The PSL nanoparticles in the powder were evaporated in the high-
Figure 1. SEM images of silica powders prepared using 5 nm of silica particle size and 178 nm PSL particle size: (a) low magnification and (b) high magnification of surface particles. The close-packed hexagonal pores in the produced powders can be clearly observed, indicating that a self-organization of PSL particles had taken place during the drying process in reactor tube.
temperature zone to produce a porous silica powder. The size of the silica particles varied from 5 to 25 nm and the PSL from 42 to 178 nm in a given fraction. Water was added to form a dilute solution. The annealing process was performed in-situ by adding a third zone, the temperature of which varied from 1200 °C to 1700 °C. The prepared powder samples (un-annealed and annealed) were collected using a filter and were then observed by scanning electron microscopy (SEM, Hitachi S-5000, operated at 20 kV). The size distribution of the prepared powders were taken by measuring the Feret’s diameter of powders from SEM images. For transmission electron microscopy (TEM, Hitachi HF-2000, operated at 200 kV), a differential mobility analyzer (DMA) was connected directly to the outlet of reactor to select 300 nm particle size. The classified particles were collected at a Carbon grid placed in an electrical chamber set at 6 kVDC. The collection time was around 10 min.
Figure 2. SEM images of the surfaces morphology of silica powders using (a) 79 nm, (b) 136 nm, and (c) 178 nm PSL particles.
Results and Discussion Figure 1 shows SEM photographs of organized porous silica powders prepared using 10 mL of 5 nm Silica (0.05wt %) and 1 mL of 178 nm PSL (3.2 × 1012 particles/ml). The resulting powders are almost spherical. Ordered pores with a close-packed hexagonal arrangement are clearly observed on their surfaces indicating that a self-organization of the PSL particles occurred spontaneously during the evaporation process in the first zone. The area of organization (arrangement) increased with an increase in powder particle size, which was obtained from a larger sized droplet. The prepared silica powder size can be adjusted by changing the precursor concentration as well as the droplet size. The ultrasonic nebulizer used in our experimental functions at 1.7 MHz, and is able to generate around 5 micrometer droplet sizes with a geometric standard deviation of around 1.5. The standard deviation of the porous particles was around 1.4. The mechanism of porous formation has been discussed in our previous paper.7 Figure 2 shows SEM photographs of a powder surface prepared using different PSL sizes. The ratio of silica and PLS colloid used was that which gave the optimum organization result (based on SEM picture). We used a fixed 390
Figure 3. Combination of PSL sizes and silica nanoparticles sizes that exhibited successful arrangement (open circle) and unsuccessful arrangement (solid three angle).
silica colloid, i.e., 10 mL of silica sol (5 nm, 0.05wt %) mixed with different PSL colloids. To obtain the sample in Figure 2(a) we used 1 mL of PSL colloid with particles sizes of 79 nm (3.6 × 1013 particles/ml). To obtain the sample in Figure 2(b) we used 1 mL of PSL colloid with particles sizes of 136 nm (7.2 × 1012 particles/ml). To obtain the sample in Figure 2(c) we used 1.5 mL PSL colloid with particles sizes of 178 nm (3.2 × 1012 particles/ml). The pores sizes on the powder surfaces were similar. As expected, the pore sizes increased with PSL particle size. We also measured the sizes of about five hundreds pores on the powder surfaces and found that the average pores sizes were identical to the average size of the PSL particles with a standard deviation of 1.05. This suggests that no shrinkage of the PSL particles occurred during the heating period in the reactor. We have tried several combinations of PSL and silica nanoparticles sizes. Some combinations resulted a selforganized pores and another combinations resulted no organization. Figure 3 shows combinations which produced successful (open circle) and unsuccessful (solid three angle) Nano Lett., Vol. 2, No. 4, 2002
Figure 4. SEM images of the surfaces morphology of silica powders using: (a) 5 nm, (b) 15 nm, and (c) 25 nm silica particle sizes and 178 nm PSL particles.
organizations. Specifically, Figure 4 shows the evolution of particle morphology by altering the ratio of PSL and silica nanoparticles sizes by using a fixed value of PSL size (178 nm) and changed the size of silica nanoparticles. In this sample, we found that the maximum ratio of silica nanoparticle and PSL particle sizes to produce organization arrangement was 25/178. This observation can be explained qualitatively as following. Consider three contacting PSL particles developing hexagonal structure. The maximum diameter, d, of a silica particle filling the space between the three contacting PSL particles is d ) (2x3/3-1)D with D denotes the diameter of a PSL particle. Therefore, the maximum ratio of silica nanoparticle and PSL sizes to produce organization arrangement is d/D ) 0.155. This value is represented by a straight line in Figure 3. Most of our experiment results are consistent with this calculation. The successful combinations lay below the line and the unsuccessful combinations lay above the line. A small deviation at small particles sizes (42 nm PSL and 5 nm silica) was observed in our experimental conditions. It was possibly due to large Brownian motion of small particles at high reactor temperature. A TEM image of a DMA classified particle prepared using 79 nm PSL and 5 nm silica nanoparticles is shown in Figure 5. This image clearly shows the presence of 79 nm sized pores inside the particle. However, based only on this image, it is rather dificult to calculate the porosity of the particles since the information regarding the inner pores arrangment is still lacking. To determine the porosity of particles more easily, we annealed the produced particles at high temperatures. Some silica nanoparticles collapse from the pore wall and give rise to smaller pore sizes inside the particles. Annealing the powder at 1500 °C completely removed the pores and gave solid silica particles. Figure 6 shows the evolution of powder morphology with annealing temperatures. In this case, a powder prepared by mixing ratio of 10 mL of silica (5 nm, 0.05wt %) and 1 mL Nano Lett., Vol. 2, No. 4, 2002
Figure 5. TEM picture of a DMA classified particles (set at 300 nm) prepared using 79 nm PSL particle and 5 nm silica nanoparticles.
Figure 6. Morphology of porous particles before and after the annealing process. (a) as-prepared, after annealed at (b) 1200 °C, and (c) 1500 °C.
of PSL (100 nm, 1.8 × 1013 particles/ml) was used. The morphology of the powder become smoother and smaller at 1200 °C, due the sintering of silica, which occurs at this temperature. When the annealing temperature reached 1500 °C, the pores collapsed and, finnaly, solid spherical silica particles were obtained. Figure 7 shows the size distribution of silica particles before and after the annealing process at 1500 °C. Before the annealing process, the peak of size distribution of powders was 340 nm, and after the annealing process, the peak shifted to 220 nm. We also annealed the particles at 1700 °C and the size distribution was not changed compared with samples prepared at 1500 °C. This indicates that the silica particle is completely sintered at 1500 °C under the 391
layer pores is approximately u′ ) 4π/3[(R - D/2)3 - (R 3D/2)2]f]. The total pores volume (the outer and the inner layers) then becomes u + u′ ) 4π/3[R3 - (R - 3D/2)3]f, or the porosity is φ′ ) [1 - (1-3D/2R)3]f ) 0.9, which is larger than experimental data. Thus, we concluded the inner pores were either arranged in nonhexagonal packing or disorder. This conclusion is supported by Figure 1(a) that the arrangement of PSL particles in small size droplets was harder than in large size droplets. The arrangement of inner layer pores took place in smaller volume after the outer layer has been formed.
Conclusion
Figure 7. Size distribution of silica particles before and after the annealing process. (a) Un-annealed and (b) annealed at 1500 °C.
experimental conditions used herein. Using the peak of size dirtibution, the volume of the powder before and after the annealing process was V ) 4πR3/3 and V′ ) 4πR′3/3, respectively, with R denotes the radius of unannealed particles and R′ is the radius of annealed particles. The porosity of the powder becomes φ ) 1 - V′/V ) 1 - R′3/ R3. From Figure 7(a) we have R ) 360/2 nm and R′ ) 240/2 nm, so that φ ) 0.7. The appearance of surface pores are close to half hemisphere. The further question is weather the inner pores arranged in hexagonal packing or others. We used a simple calculation determine the true arrangement of inner pores. The total volume occupied by surface pores is approximately u ) 4π/3[R3 - (R - D/2)3]f with D is the diameter of PSL particle and f is the packing fraction. For a hexagonal arrangement, f ) π/2x3. If pores appeared only on the particle surface, the porosity of powder should be φ ) u/V ) [1 - (1 - D/2R)2]f. Using R ) 180 nm (from Figure 7(a)) and D ) 100 nm, one obtains φ ) 0.56. Because φ < φ, one concluded that the pores are not only present on the particle surface, but also inside the powder. Because R < 2.5 D, no more than two pores layers can appear in each particles powder. If the inner pores are arranged in hexagonal packing too, the volume of the inner
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The porous silica particles with controllable porous sizes were produced by a spray drying method. The pores were observed to be arranged into a hexagonal packing, indicating that a self-organization process occurred spontaneously during the solvent evaporation. The pore size changes with PSL particle size, and the morphology also changes as result of the annealing process, so that the pore size of the particles can be easily controlled. The size combination between the silica and PSL particles are also parameters for selforganizing pores formation. Acknowledgment. A scholarship provided by the Hashiya Scholarship Foundation and a grant provided by Hosokawa Micron of the Promotion of Engineering Foundation for F. I. and a scholarship provided by the Japanese Ministry of Education, Science, Sports, and Culture for Mikrajuddin are gratefully acknowledged. This work was also supported, in part, by the New Energy and Industrial Technology Development Organization (NEDO)'s “Nanotechnology Materials Program - Nanotechnology Particle Project” based on funds provided by the Ministry of Economy, Trade, and Industry, Japan (METI). References (1) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lemhoff, A. M. Nature 1997, 389, 447. (2) Huo, Q.; Feng, J.; Schuth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14. (3) Ozin, G. AdV. Mater. 1992, 4, 612. (4) Velev, O. D.; Kaler, E. W. AdV. Mater. 2000, 12, 531. (5) Park, S. H.; Qin, D.; Xia, Y. AdV. Mater. 1998, 10, 1028; Gates, B.; Qin, D.; Xia, Y. AdV. Mater. 1999, 11, 466. (6) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C. X.; Khayrullin, I.; Dantas, S. O.; Marti, I.; Ralchenko, V. G. Science 1998, 282, 897. (7) Iskandar, F.; Mikrajuddin; Okuyama, K. Nano Lett. 2001, 1, 231.
NL015662G
Nano Lett., Vol. 2, No. 4, 2002
