TEM images showing particle size of 2-5nm while when I have calculated crystallite size from XRD using Scherrer equation 13-15 nm approximately.
Saqib Khan Yes, it is possible that crystallite size Is greater than particle size. The crystallite size obtained from XRD using the Scherrer equation represents the average size of the crystalline domains within the nanoparticles. In the case of biologically synthesized nanoparticles, the presence of organic coatings, capping agents, or other biomolecules on the particle surface can contribute to a larger overall particle size observed in TEM images compared to the crystallite size calculated from XRD. The additional layers or organic components surrounding the crystalline core can increase the apparent size of the nanoparticles in TEM images, leading to a situation where crystallite size is smaller than the observed particle size.
Alvena Shahid thank you for your response. can you please share any reference for this. I have synthesized metal oxide nanoparticles, and from Scherrer equation crystallite size is 13-15 nm while from TEM images particle size is much smaller 3-5 nm (while crystallite has to be smaller than particle).
It sounds like a possible error in the XRD analysis, perhaps the instrumental broadening has been overestimated (and subtracted from the data).
It could also be a sample preparation issue, i.e. only the smallest particles have been attached to the TEM grid.
No crystallite size cannot be greater than particle size. Scherrer methods have limitations.
Scherrer methods have limitations, so you should use the WH plot for the calculation of average crystallite size.
Where is TEM picture? Are you sure you prepared TEM specimen properly? It is possible you observed some contaminations instead of your particles.
No . The term "crystalline size" is occasionally referred to as sub-grain size. A particle is composed of numerous grains or, in some cases, a single crystal.
Your observation {size(XRD) > TEM (physical size)} is just reverse of existing literature/ understanding that physical size TEM >= XRD size.
(i) When crystallites are too small like 2-3 nm XRD peak widths are significantly large (~ 1-2 degree) and instrumental peak broadening contribution is quite small (~ 0.1 deg). In my opinion try to use Scherrer equation without removing instrumental contribution, if it matches with TEM result then instrumental contribution is expected to be overestimated.
If you still get XRD size 10-15 nm, then this may be due to following situations related to either sample heterogeneity (synthesized nanoparticles) or measurement technique or their combination:
(i) TEM probe only tiny amount of sample and probably not giving true average value as pointed out by Krister Svensson "It could also be a sample preparation issue, i.e. only the smallest particles have been attached to the TEM grid".
This possibility is more when sample consists of broad or bimodal/ multimodal size distribution.
(ii) Another point of difference is related to use of "Integral breadth (IB)" or "FWHM" of XRD peak in Scherrer formula and Scherrer constant as mentioned by Nita Dragoe.
If this is the case then this difference could be removed by accounting XRD peak "SHAPE" contribution in Scherrer constant using Scherrer constant relationship: K(FWHM) = phi*K(IB). phi = 0.94 & 0.64 for Gauss & Lorentz peak shapes, respectively. More details in following article:Article Significance of diffraction peak shapes in determining cryst...
In this work simulated data consist of constant FWHM with different peak shape and "FWHM" based Scherrer equation gives fixed crystallite size of about 21 nm. While use of IB and WPPM shows that size goes down to few nm (and expected from TEM) with changing peak shapes as shown in table 1 and Figure 4 of this work and corelate with this question.
(iii) Scherrer equation gives apparent crystallite size related with "VOLUME" weighted mean and in case of size distribution this mean is biased towards larger size (In case synthesized nanoparticles consists of a broad or bimodal size distribution).
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