Dear Maduru Narsimhulu,

The tauc plot is correct method. But you need the absorption coefficient (alpha). From your measurement you have diffuse reflectance data.

You need to convert reflectance data to the Kubelka-Munk function, which is proportional to alpha.

I have attached herewith the Research Article for the Determınatıon of Band Gap of a Nanopowdered Sample Usıng Kubelka Munk Theory.

Kindly go through the same.

Article Sımple Method For The Determınatıon of Band Gap of a Nanopow...

I hope I have answered your question.

With Best Wishes,

Samir G. Pandya

Dear Samir G. Pndya

Thank you very much