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Langmuir 1998, 14, 3202-3209
“Figure/Ground” Study of Colloidal Silica Nanoparticles and Corresponding Microporous Xerogels D. Laurence Meixner,* Andrew G. Gilicinski, and Paul N. Dyer Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195 Received October 10, 1997. In Final Form: March 11, 1998 A critical hurdle in the development of sol-gel techniques for the preparation of microporous inorganic materials is elucidating the effect of key synthesis and processing parameters on the structure of the resulting sols. Characterization of colloidal particles with diameters of less than approximately 10 nm is problematic using conventional techniques such as dynamic light scattering. Similarly, quantitative determination of pore size distribution in the resulting materials using gas adsorption isotherms is difficult in the microporous regime. The present work extends these traditional methods with the objective of obtaining information both on the particle size in an inorganic sol and on the pore size distribution in the resulting microporous xerogel. Silica sols synthesized at pH 3 and an H2O/TEOS ratio of r ) 83 were shown by atomic force microscopy to have a distribution of particle diameters centered around approximately 4-6 nm. Nitrogen and molecular probe adsorption studies on xerogels derived from the same sols indicated a mean micropore diameter of approximately 1 nm. These adsorption studies also suggested nearly close packing of the silica particles in the xerogel. The pore size determined via nitrogen and molecular probe adsorption experiments was consistent with nearly close packed particles of the dimension determined by the AFM experiments. The molecular probe adsorption studies, particularly the equilibrium adsorption of N(C4F9)3, indicated clearly that the smallest micropore diameters were achieved in xerogels derived from sols prepared at pH 3. Larger particle sizes and micropore diameters were determined for xerogels derived from sols prepared at pH 5. This observation was attributed primarily to particle agglomeration in the pH 5 sol, which was less stable to gelation than the sol prepared at pH 3. These complementary “figure/ ground” data on the silica particles in the sol and the interstitial microporosity in the corresponding xerogel provided a consistent picture of the microstructure formed in these silica materials.
Introduction Microporous inorganic materials have a wide variety of uses, including adsorption and membrane separation processes, catalysis, sensors, and numerous emerging specialty applications.1,2 Such materials may be crystalline, such as zeolites, or amorphous, such as silica gels.3 Sol-gel processing represents a low-temperature method for the development of microporous, typically amorphous, inorganic structures.4,5 Incomplete knowledge of the impact of synthesis and processing parameters both on the sols and on the resulting porous inorganic materials remains an obstacle to the development and commercialization of sol-gel preparation techniques. The generalized sol-gel process may be viewed as consisting primarily of the hydrolysis and condensation of a metal-containing precursor in solution. An excellent and thorough review of the principles of sol-gel science is given by Brinker and Scherer.6 Although the chemistry is relatively simple, variation of one or more experimental parameters can dramatically affect the sol properties, as well as the microstructure of the resulting xerogel. In the present case of silica sol-gel synthesis using tetraethox* To whom correspondence should be addressed. (1) Advances in Porous Materials; Komarneni, S., Smith, D. M., Beck, J. S., Eds.; Materials Research Society Symposium Proceedings Vol. 371; Materials Research Society Press: Pittsburgh, PA, 1995. (2) Ward, D. A.; Ko, E. I. Ind. Eng. Chem. Res. 1995, 34, 421. (3) Microporous and Macroporous Materials; Lobo, R. F., Beck, J. S., Suib, S. L., Corbin, D. R., Davis, M. E., Iton, L. E., Zones, S. I., Eds.; Materials Research Society Symposium Proceedings Vol. 431; Materials Research Society Press: Pittsburgh, PA, 1996. (4) Uhlmann, D. R.; Teowee, G.; Boulton, J. J. Sol-Gel Sci. Technol. 1997, 8, 1083. (5) Zarzycki, J. J. Sol-Gel Sci. Technol. 1997, 8, 17. (6) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990.
ysilane (TEOS) in aqueous solution, the hydrolysis reaction is base-catalyzed for pH > 7 and acid-catalyzed for pH < 7. However, the condensation reaction proceeds via a basecatalyzed mechanism above the isoelectric point of silica (pH ≈ 2) and is acid-catalyzed for pH values below the isoelectric point. Therefore, changes in the pH of the solution alter the relative rates of hydrolysis and condensation, yielding sols containing structures ranging from weakly branched silicate polymers to compact silica particles.7-9 The sols discussed in the present work were prepared at values of the H2O/TEOS molar ratio r much greater than stoichiometric values, which favored the formation of silica particles. The experimental parameters were chosen to yield sols with very small (
