What is the formula for tauc plot? By using tauc plot. It is a direct band gap.
Band Gap Energy (E) = h*C/λ
h = Planks constant = 6.626 x 10-34 Joules sec
C = Speed of light = 3.0 x 108 meter/sec
λ = Cut off wavelength = 410.57 x 10-9 meters
H = 6.63E-34
C = 3.00E+08
λ = 4.11E-07
E = 4.84156E-19
eV = 3.025976
Where 1eV = 1.6 X 10-19 Joules (conversion factor)
Cut of wavelength can be calculated from UV-visible DRS spectra of the particular sample..
It depends on what kind of absorption spectra you have, ie transmission mode or ATR mode.
For transmission spectra you can use Tauc plot.
Since you mention powder sample, You might used ATR or reflection method, so you need to use Kubelka-Munk function to obtain band gap. The Kubelka-Munk function, F(R), allows the optical absorbance of a sample to be approximated from its reflectance: F(R) = (1-R)^2/2R. You can find many papers on Kubelka-Munk function.
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