Question

How to find the energy gap for Silica?

Answer 1

As silica is insulator its band gap is higher than 6 eV .

Answer 2

silicon dioxide glass is composed SiO4 tetrahedra connected at the oxygen atoms. The chemical ordering is very strong, with each silicon atoms connected to four oxygens, and each oxygen bonded to two silicons. The bonds are very strongly covalent, and the bulk material displays a band gap of approximately 9 eV. The chemistry between Si and O forces this order.

Answer 3

moreover you can calculate the band gap for any materials using photo luminacence - uv visible spectroscopy like techniques,