I encountered with this many times. For example, in "Journal of Hazardous Materials 168 (2009) 253–261" the authors obtained N-doped TiO2 using different precursors. Actually, the preparation details do not matter. The object is a semi-insulator with the photoexcitation threshold of ~390 nm, which VIS- sensitivity has been enhanced by an impurity N(concentration matters, but there are 3 different levels of doping in the work and the constant absorbance appears even for the level of doping of 0.64 at.% N). The absorption curves(fig.5) were obtained by using the Kubelka- Munk theory from diffuse reflectance measurements. They contain areas where the absorbance equal constant(>>0) and the areas go right after the Photoexcitation threshold of the undoped TiO2 and to the 800 nm(the region after 800 nm is not shown). I have seen such graphs in other papers, but, I cannot explain it. Why the absorbance = constant there?
Article Visible light absorption ability and photocatalytic oxidatio...
Do you mean the region from ~450nm to ~800nm?
In this region, the non zero absorption is due to insufficient correction of baseline. The slight inclresase towards 800 nm is probably thin film interference
Thank you, Dr. Hugh J Byrne.
Yes, that region confused me.
So, the curves must meet at the same line with an ordinate of ~ zero?
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