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In 1968, Stober and et al., reported an advanced method of synthesizing spherical and monodisperse silicon dioxide nanoparticles, from the aqueous solution of silicon alkoxide in the presence of aqueous ammonia as a catalyst, from 50 nm to 1 μm of different sizes of silicon dioxide nanoparticles with a narrow particle size distribution. The size of the particles depends on the type of silicon alkoxide and alcohol. The particles prepared in the methanol solution are the smallest, and the particle size increases as the chain length of the alcohol increases. When the long chain alcohol is used as the solvent, the particle size distribution is also widened. After that, a lot of research has been done in this area. In this study, two types of reactions are mainly concerned: (i) polycondensation reactions formed by hydrolysis of silicon hydroxyl groups and (ii) siloxane bridges:

Hydroxylation: Si- (OR) 4 + H2O → Si- (OH) 4 + 4R-OH, 

Condensation: 2Si- (OH) 4 → 2 (Si-O-Si) + 4H2O. 

The rate of condensation depends on the reaction conditions, which may result in the formation of a three-dimensional network of the structure, or the formation of a single monodisperse particles. For the preparation of larger particles, Bogush et al. have described a seed growth technique. The seed suspension in this technique was prepared by precipitation using Stober. When the reaction was complete, TEOS and water were added to the seed suspension at a molar ratio of 1:2. The disadvantage of this technique is that if the amount of TEOS exceeds a certain critical value, the second particle population will appear. With this technique, more monodisperse particles can be prepared and their mass fraction in the sol is increased, but in this way it is not possible to increase monodisperse particles in excess of 1 micron in size. The effect of the electrolyte on the size of the silicon dioxide nanoparticles was illustrated by Bogush and Zukoski, and in their study they reported that when the concentration of the electrolyte (NaCl) increased from 0 to 10-4M, the particles the size increases from 340 to 710 nm. 

Huang and colleagues have reported that the ultrasonic treatment in the reaction process can significantly increase the yield of carbodiimide-mediated amidation. In view of this, in this study, we have determined the effect of each reagent on the particle size, in addition to the effect of temperature on the ultrasonic treatment. As far as we know, this is the first report on the preparation of silicon dioxide particles using the sequential addition process in the sol-gel process.



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