Dear Aniruddh Bahadur Yadav,

You must hold the solution at 70°C for one hour before aging. You can read the experimental part of my article where this is explained.

Article Influence of Pb and Co-doping on photocatalytic degradation ...

Dear A. B. Yadav,

Before mixing raise the temperature upto 60 °C and then add DEA slowly in the ZA solution.

I did that some time ago. It become thick gel like cream when heated the solution.

Is sol-gel preparation depends on the gravity and place of experiment.

Maybe you chose a high temperature that caused your solution to gel. As for gravity, I don't think it will have any effect.