How to measure band gap of powder sample with using diffuse reflectance method
It's not too hard to determine band gap.  When the energy of a wavelength of light is higher than the energy of the band gap the light will be absorbed, any wavelengths that are lower in energy will reflect or transmit. Simply make the spectrum, there should be a cliff-looking cut-off in transmittance, determine the wavelength and corresponding energy and that is your band gap.