You can estimate crystallite sizes via XRD using Scherrer formula. TEM and SEM give you the particle sizes. Each particle may contain two or more crystallite, So it is very normal Scherrer formula gives you lower amount than TEM or SEM. If you are going to report particle size TEM, SEM, DRS,... are more accurate. In my idea it can be good that you report both XRD and TEM results for your sample. I have reported in my paper with this link:

http://indjst.org/index.php/indjst/article/view/43611

I could see 2 possible explanations:

- your sample (particles) are not entirely crystallized (which is less probable, seen their small size), or each particle contains several very small crystallites: note that in x-ray diffraction you measure the size of the crtystallites (crystalline part), not the size of the particle (agregate). If you have agregates (100nm) of very small particles (10nm), you will measure this last one

- you need to correct from instrumental resolution (broadening). Thus the real FWHM becomes smaller, thus calculated size becomes bigger. There is a topics on the instrumental resolution and correction (using a reference powder), please have a look

https://www.researchgate.net/post/To_correct_the_instrumental_broadening_in_XRD_profiles_can_I_use_as_standard_a_different_material_from_the_material_which_I_am_analyzing

You can estimate crystallite sizes via XRD using Scherrer formula. TEM and SEM give you the particle sizes. Each particle may contain two or more crystallite, So it is very normal Scherrer formula gives you lower amount than TEM or SEM. If you are going to report particle size TEM, SEM, DRS,... are more accurate. In my idea it can be good that you report both XRD and TEM results for your sample. I have reported in my paper with this link:

http://indjst.org/index.php/indjst/article/view/43611

Thank You so much Cristian, Amir and Adrian for much needed responses.

1. As Cristian has already pointed out, you have made an all too common mistake in your Scherrer calculation. You can only compute a size of 10nm from the parameters you have described if you assume the instrumental broadening is zero, which is certainly not the case. If it is more like 0.5° or so for your diffractometer at this 2theta angle, then your crystallite size calculation will yield a MUCH larger number.

2. Even when you do the calculation correctly, a single peak Scherrer crystallite size calculation is based on a very simplistic model that almost certainly in not a good representation of your zeolite system, and so should not be taken with much confidence. Also the distinction between particle and crystallite (coherent domain) size is important.

The discussion above has covered the most salient points on the topic. But to add more, there are many more discussion done here in detail at RG on this and related topics. Please go thro. those discussions. You will learn more about it, I am sure.

Dear chandra;

Scherrer is a route for estimating the crystal size. however a particle is made up of several crystals; unless the particles become single crystal. Also I suggest you to take a look at this article:

http://ijmse.iust.ac.ir/browse.php?a_code=A-10-3-149&sid=1&slc_lang=en

Thank you all for your support....btw, MOFs are very porous materials. In fact ZIF-8 that i have reported here have been reported to possess porosity of over 1600 m2/g. Does this high porosity has to do anything about the size discrepancy that is observed between XRD and SEM?

Girish, I don't think so. Porosity should not affect the XRD or SEM observation. Amir and Cristian have rightly pointed out the basic reasons for this discrepancy. However, there is one more point which should be mentioned. The peak broadening is caused by basically three factors: Crystallite size, Lattice strain and Lattice defects. In Scherrer equation, only crystallite size effect is taken into consideration and the whole broadening is taken due to crystallite size effect solely. So, naturally, the value comes much less than what is should be (if other effects are taken into consideration). So, Scherrer equation is not the right choice for estimating crystallite size accurately from XRD Diffarctograms. Warren-Averbach Fourier Transform method is an alternative and better choice.

SEM reveals particle size which is generally an agglomerate of many crystallites. Due to porosity, sample preparation for SEM observation might be little tough, but it should not alter the observed particle size.

TEM Bright field image shows agglomerate of crystallites. But Dark field imaging is a good way of estimating crystallite size along a particular crystallographic direction. You can try these two methods and compare the results.

Thank you suchitra for your support

Please expand MOF & ZIF when convenient. Does the Scherrer equation have provisions for porosity in the "diffracting domain"? This would futher deteriorate the certitude of the results from the XRD analyses.

The effect of porosity (and meso-porosity) and its quantification using NDE XRD is of utmost interest to me. I'd like to evaluate a "Black Silicon" (http://en.wikipedia.org/wiki/Black_silicon) wafer sample with etchpits in the 2-20um (size, spacing & depth) range from NREL Colorado. These vertical etchpits were introduced to enhance the conversion efficiency of PV devices. Any suggestions?

Here is another relevant RG discussion that I suggest you poke your brain in to when convenient, to learn or teach:

https://www.researchgate.net/post/Why_is_there_a_difference_in_the_crystalline_size_of_the_nanoparticle_as_measured_from_TEM_image_and_XRD_analysis_of_the_same_sample#rg-injektor-generated-rg_modules_publictopics_actions_PostCommentItemProxy_5378bad2d3df3e10728b45a7-