How can I explain the fact that the size measured in SEM is different from the one I got by calculating crystallite size in XRD by Scherrer equation?

I have SEM images from ZnO nanoparticles whitch I've measured. Then I used the Scherrer equation to calculate the crystallite size but I can't get the result I expected, but a very smaller one.

14 April 2016 2,587 9 View