Genesis and Growth of Nanoparticles during Sol-Gel Synthesis
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Genesis and Growth of Nanoparticles during Sol-Gel Synthesis Nan Yao and King Lun Yeung* Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China Guoxing Xiong* and Shishan Sheng State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P. O. Box 110, Dalian 116023, P. R. China *Corresponding authors
ABSTRACT In situ observation of the genesis and growth of silica-alumina structures during the sol-gel process has been carried out. Although the gross structure of the gel network is kinetically stabilized, in situ AFM study reveals that at nanometer scale, the internal structure of the gel is dynamic as shown by the rich and complex morphological transformations. INTRODUCTION The structural evolution during the sol-gel process is an influential factor that determines the final properties of the material [1]. This phenomenon has been investigated at different length and time scales using various spectroscopic techniques. NMR spectroscopy is useful for identifying the different oligomeric species formed by the hydrolysis, condensation and polymerization reactions during the sol-gel process [2]. Infrared and Raman spectroscopy provide important information on the growth and structure of local inorganic frameworks and small-angle-scattering methods using X-rays (SAXS) and visible light give insights into the formation, growth and topology of colloidal sols and macromolecular gel networks [3,4,5]. It has been demonstrated that by controlling the morphology of the nanoparticles and their assembly, one could engineer the material, structural and chemical properties of the final product [6]. A better understanding of the dynamic phenomena at this length scale (i.e., 1-100 nm) is therefore vital for the development of new material with tailored properties (e.g., pore structure, optical, thermal, mechanical and catalytic). However, most of the information on the local structural environment is deduced from indirect measurements using photophysical and photochemical probe molecules that lack the ability to resolve the morphology of the individual subunits (i.e., colloids and nanoparticles). Electron microscopy could provide high-resolution snapshot pictures of the phenomenon, but due to the extreme nature of the sample preparation and the sensitivity of the sol-gel material this technique is difficult to use. In addition, it is unable to capture the dynamic processes that are occurring during the sol-gel synthesis. This paper demonstrates the use of atomic force microscopy (AFM) for in situ observation of the nanoscale morphological transformations that occur during the sol-gel process.
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EXPERIMENTAL DETAILS A silica-alumina sol was prepared by adding 4.7 ml of alumina sol (0.42 mol/L)[7] to 50ml water glass solution at pH of 10. The resulting precipitate was peptized using nitric acid (0.9mol/L) and then treated with ultrasound for 1 minute to obtain a stable sol. The pH value of the sol is
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