I had prepared ZnO by gel combustion method where the absorption maximum is at 376 nm while the band gap calculated by Kubelka- Munk plot is 2.6eV (visible light-active in degradation of organics). The particle size calculated by TEM turned out to be 40- 50 nm. Can anyone help me with this anomaly?
Dear Ramasami
I think the absorption maximum at 476 nm. You can get your answer by searching " the silver nanoparticle".
The absorption maxima is the lambdamax (peak position). But the absorption edge, you will get after making a tangent at the position when its touching the wavelength axis. The absorption edge in your case must be 476.92 nm.
Various ways have been used to determine the band gap Eg from optical absorption measurements.1 If the absorption spectra is very sharpe at the absorption edge, one can consider the extrapolation of the tangent of that absorption edge or that of the 'Kubelka- Munk plot' and consider the X-axis cut point as band gap energy. But it should not be much less than the absorption maxima. If it is so then other methods as mentioned next can be used to calculate the band gap energy.1,2 A practical method is to equate Eg with the wavelength at which the absorption is 50% of that at the excitonic peak (or shoulder). If there is a lack of sharpe edge one can follow the derivative method(by differentiating the absorption curve) to extract the inflection point and consider that energy value as Eg. One should veriyfy or consider the other measurements like PL spectra, photoemission and inverse-photoemission spectroscopy.
1. 102, 29, 5566, Journal of Physical Chemistry B ://000075151300004
2. 615, 378, 2014, Journal of Alloys and Compounds http://www.sciencedirect.com/science/article/pii/S0925838814014431
A UV absorption of ZnO sample has been attached. One can calculate Eg quiet straight forward in this kind of curve.
Dear Pal, Das and Biswas,
Thank you for the answers. I got 3.23 eV band gap based on derivative method as my sample had no sharp absorption edge. Thank you once again for your contributions.
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