The band gaps can be calculated via UV-Vis spectroscopy using Tauc Plots. Attached you can find one file for your assistance. Here we plot the graph between (ahv)1/2 on y-axis and hv on x-axis. a (alpha) is the absorbance calculated from UV and hv can be calculated by following way... hv = 1240/wavelength. Then you will extrapolate the vertical sagments of the plot to intersect on x-axis where y-axis is zero. That value is your band gap. It might be direct or indirect depending upon the formula.

How can I calculate the band gap from UV-Vis absorption spectra of thin films consisting of TiO2 and other metal complexes? - ResearchGate. Available from: https://www.researchgate.net/post/How_can_I_calculate_the_band_gap_from_UV-Vis_absorption_spectra_of_thin_films_consisting_of_TiO2_and_other_metal_complexes [accessed Jul 8, 2016].

here is the excel file which will plot the alpha hf square calculated from dffuse reflectance spectra for some _______semiconductor to calculate the band gap

Please find the attachment. 

Dear Dr. Tvarit Patel

Have a nice day and good times

Experimentally, You can get  the value of Eg by usually use the Tauc relation, which is given by this equation:                                

 αhν = A (hν - Eg )n

where α is absorption coefficient given by                

α = 1/t  ln [(1-R)2 / T]

where t  is the sample thickness, T and R are the transmission and  reflection, while  (hν) is the photon energy, where:

hν(eV) = 1240 / [incident wavelength (nm)]

If you plot a graph between (αhν)1/n  versus (hν), then you can get a straight line. This line intersects the X-axis at (αhν)1/n = 0 . The values of Eg have been estimated from this intercept.  The value of n is dependent on the electronic transition type. Where:

n=1/2 for direct allowed transition,

 n= 2 for indirect allowed transition, 

n=3/2  for direct forbidden transition and

n=3 indirect forbidden transition. You should try to select the suitable n according to your samples and their preparations.

The appended papers may help you; they are examples of direct and indirect allowed transitions.

Good luck

Your,

Ahmed Saeed Hassanien

https://www.researchgate.net/publication/283338024_Effect_of_Se_addition_on_optical_and_electrical_properties_of_chalcogenide_CdSSe_thin_films

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films

https://www.researchgate.net/publication/304150361_Study_of_optical_properties_of_thermally_evaporated_ZnSe_thin_films_annealed_at_different_pulsed_laser_powers

Article Influence of composition on optical and dispersion parameter...

Article Effect of Se addition on optical and electrical properties o...

Article Study of optical properties of thermally evaporated ZnSe thi...

 Dear Dr. Ahmed Saeed Hassanien

Thanks a lot for detail explanation of finding band gap. I have small query in that. here you have told that 't' represents the thickness of film, but I did the experiments of powder sample (which have mixture of different particles size) with dry UV-Vis method. what I suppose to do?  

Dear Tvait,

you can find energy gap without using thickness, this paper below  illustrated that  by using absorbance data only with the wavelength and by plot between 1/lamda on x-axis and (Absorbance/lamda)^2 on y-axis this for direct transition and by extrapolating the linear region and intersect with x-axis at 0 the find (1/lamda)=value and by reciprocal this value can you find lamda cutoff after that can use the relation to find energy gap Eg(eV)=1239.83/lamda cutoff (nm). With the best regards

Dear Dr.Tvait

Have a nice day

Yes, you can calculate the energy gap of your samples without knowing their thickness.

Dear Dr.Tvait

You don't tell us about if you could calculate the optical energy gap of your samples without knowing their thickness. 

I hope for you all the best and good luck

Here you can find good explanation and band gap calculator,

http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/

dear ahmed saeed pleased correct me in that with best of my knowledge the value of 'n' for tauc plot is as

1/2 for direct allowed transition  

3/2 for direct forbidden transition

2 for indirect allowed transition

3 for indirect forbidden transition

............?

Dear Muhammad Saleem

Al Salamu Alaikum

Hi Dr.

The value of n is dependent upon what does it was placed, either in the right or the lift side? If it is placed in R.H.S. like our case:

αhν = A (hν - Eg )n

And If you plot a graph between (αhν)1/n  versus (hν), then you can get a straight line. This line intersects the X-axis at (αhν)1/n = 0 . The values of Eg  can be estimated from this intercept.  The value of n is dependent on the electronic transition type. In this case if :

n=1/2 then the transition is direct allowed transition,

n= 2 then the transition is indirect allowed transition, 

n=3/2  then the transition is direct forbidden transition while if:

n=3 then the transition is indirect forbidden transition.

Thank you for drew my attention.

Good Luck Dear.

Hi, use Kubelka-Munk theory, more information in this ref.

S. K. Loyalka and C. A. Riggs, Appl. Spectrosc. 49 (1995) 1107-1110

 Sir, we downloaded the excel file. Please tell us what all need to be plotted. which is X axis and which is Y axis. Please explain. 

Dear Bilal Yaseen Taher

I made a solution of nanoparticle and attained absorbance,please guide me, how do i determine optical band gap ?

Dear s.a Hejazi 

I posted an answer on your page. Please go there.

pls see the attached file

Hi...

How can I calculate thickness?

The value of the band gap (Eg) can be obtained by using the Tauc relation, : αhν = A (hν - Eg )n where α is absorption coefficient given by α = 1/t ln [(1-R)2 / T] where t is the sample thickness, T and R are the transmission and reflection, while (hν) is the photon energy, where: hν(eV) = 1240 / [incident wavelength (nm)]. After getting these, if you plot a graph between (αhν)2 versus (hν) for direct allowed transition then you can get a straight line. extrapolate this line to the x axis where (αhν)2 = 0 . The value of the interception line on the x axis gives the value of Eg. Similarly, for indirect transition draw a (αhν)1/2 versus (hν) and deterime the value of Eg.

Dear Colleague,

You can check my article below

http://www.sciencedirect.com/science/article/pii/S0925838817316717

Regards

Actually, the band gap doesnot depend on the synthesied method. It depends on the several factors: such as crystallinity, ( defects, purity) crystal structure,crystallite size, grain size, particles size, thickess of the film and ysnthesied tempeture etc.

If you plot a graph between (αhν)1/n  versus (hν), then you can get a straight line. This line intersects the X-axis at (αhν)1/n = 0 . The values of Eg have been estimated from this intercept.  The value of n is dependent on the electronic transition type. Where:

n=1/2 for direct allowed transition,

 n= 2 for indirect allowed transition, 

n=3/2  for direct forbidden transition and

n=3 indirect forbidden transition. You should try to select the suitable n according to your samples and their preparation

Dear Prof. Ahmed Saeed Hassanien

I just would like to ask about n value in (ahv)^1/n. Does this n value is affected by whether our sample is thin film or powder ?

For example, is it equal to 1/2 for direct band gap regardless of whether or sample is powder or thin film ?

Thanks for your comment in advance

Dear Şevki Samet Kaplan

Greeting

the power factor n is independent upon the structure form of the sample. It depends only on the electronic transition.

Regards

Thanks for your fast and kind response dear proffesorAhmed Saeed Hassanien

You are welcome Dr. @Şevki Samet Kaplan

Don't hesitate to communicate and ask me.

Thanks Dr. A. S. Hassanien, for their informative answer.