Based on the above consideration, three major parameters contribute
to the best result (7.45% mass fraction): (1) low temperature
stabilizes the solution; (2) low ratio of NH3/TEOS avoids the
coagulation; (3) low polarity of IPA benefits the formation of large
particles, and steric effect of IPA avoids the aggregation.
We can infer that much higher mass fraction of particles in the
range of 30–1000 nm may be achieved by optimizing reaction
parameters (temperature, reactants), selecting multi-alcohol (glycol
and glycerol) to increase the effect of steric hindrance, and
choosing appropriate high-polarity alcohol to reduce viscosity.