The particle size calculated from TEM results is around 20 nm. But from UV-Vis spectra by using effective mass model the calculated particle size is 8 nm. The value differs high. What may be the reason for this? How to choose the correct value from this two analysis?
If a calibration of your TEM is correct and the pictures have a good focus, the real particle size is around 20 nm.
The optical methods for analysis of too small (less than 100 nm) particles have some artifacts and it is possible to have an error around 100% for diameter calculation. Electron microscopy and AFM are better for such tasks.
Actually, you can check the results by AFM, but it is high-probability that it will be agree with results of TEM.
Good luck!
You may also perform XRD measurements (is alos sensitive to the crystallit size). I guess that the onset of the bandgap is not correctly determined. Experimental examples for ZnO (TEM, XRD, UV-vis) may be found within the attached publication.
Article Synthesis and testing of ZnO nanoparticles for photo-initiat...
Data Supporting-information to Synthesis and testing of ZnO nanop...
You may add a screenshot of the UV-vis measurement to the question.
Uv vis absorption spectroscopy gives accurate values for particle size of quantum dots only.It is not applicable for particle size greater than 10nm.
The average particle size estimated from UV–VIS analysis should be in a good agreement with the TEM investigation This indicates the presence of strong quantum confinement regime in nanosize quantom dots.
In my experience particles measured by TEM are usually smaller than calculated from XRD or other methods... and your example looks just the other way around. I would suspect some gravity segragation of the particles during sample preparation (if I know how they were prepared - from size of the particles I would guess they were dispersed in some solution, and the sample was taken from "aged" sample from bottom part of the solution? )?
It is really hard to compare, unless your particle size distribution is very narrow. In TEM you are viewing only few 10s of particles, but in UV, you might have recorded for 1000s of particle, which makes significant different in the particle size values.
You can calculate the size from XRD. very good explanation is here on this link,
http://www.instanano.com/characterization/theoretical/xrd-size-calculation/
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