Basically, grain size is always bigger than particle size because a grain can be made up of several agglomerated particles. Each particle can be a single perfect crystal, than it corresponds to the crystallite size; but if the particle has some low angle defect (stacking faults, twin defects...) the crystallite size, which is the region with perfect crystalline order, will be smaller. Crystallite size is what you measure by XRD, which is usually smaller than what you see by electronic microscopy or AFM.

Basically, grain size is always bigger than particle size because a grain can be made up of several agglomerated particles. Each particle can be a single perfect crystal, than it corresponds to the crystallite size; but if the particle has some low angle defect (stacking faults, twin defects...) the crystallite size, which is the region with perfect crystalline order, will be smaller. Crystallite size is what you measure by XRD, which is usually smaller than what you see by electronic microscopy or AFM.

Thanks for your quick reply

ok when we do TEM, we see number of particles are agglomerated. So according you, the bourndaries form by agglomeration of particles are called grain boundary. Am i right

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Usually, the term "grain boundary" is used for the surface that separates the crystals forming an agglomerate. In that case the "grain" is the single crystal. There'a a little confusion in these terms. In your case the grain boundaries are the surfaces that separate your particles in the agglomerate, if they are single crystals.

I am not surprised about your confusion but this is the reason why one should not depend on only one technique to characterize one's materials. What you see in the TEM is more likely the correct size of the crystallites with the added advantage that you can see the boundaries if your samples are thin enough and the boundaries are properly oriented to give you enough contrast. On the other hand, AFM imaging is affected by the size of the probe you use and it can often give you the impression that the nanoparticles are bigger than they really are. Therefore, if your nanoparticles are smaller than about 10nm you are better off doing Scherer analysis using XRD (measure of the broadening of the peaks) and TEM imaging. On the other hand, AFM is a great tool to image larger sizes and for measuring the heights of nanoparticles. See one of my recent papers where we use several of these different techniques (Thin film solids doi:10.1016/j.tsf.2011.11.052).

The confusion is arised from the terminology used by various people. Actually the grain are the crystallites arrange in a particular orientation of crystal plane. The grains of different orientation make a polycrystalline crystal and so we have grain boundaries in a crystal. We get different peaks in xrd pattern due to these grain where atom are arranged in some direction which represent the crystal plance. So by using Scherrer formula we get grain size or the crystallite size. However, what we see in TEM depending upon the magnification, is mostly whole particle. This particle may or may not have different grains or crystallites in it. If the particle is composed of different grain, one can see the grain boundaries also. On the other hand, some people used the word "grain" for agglomerated particles also. AFM images the whole particle only. One can not study the microstructural properties of the material with AFM. For further details you can go through any standard book on crystallography, X-ray diffraction and one of my recent paper (Nanotwinning in CdS quantum dots, Physica B: Condensed Matter, DOI:10.1016/j.physb.2012.04.029)

In general, grain size is always bigger than particle size because a grain can be made up of several agglomerated particles.  Each particle can be a single perfect crystal, than it corresponds to the crystallite size; but if the particle has some low angle defect, the crystallite size, which is the region with perfect crystalline order, will be smaller.  The grains are the crystallites OR particles arrange in a particular orientation of crystal plane. More over grain size>particle size>crystallite size.

Dear colleague Shekhar,

Crystallite Size is Different than Particle Size. A particle may be made up of several different crystallites or just one crystallite so in this case (particle size = crystallite size)

Crystallite size often matches grain size, but there are exceptions

Crystallites are coherent diffraction domains in X-ray diffraction.

Particles are chunks/pieces (usually very small, below 1 mm) of solid matter, ensembles of atoms. Particles can be as small as two atoms (the nitrogen particle for example, N2)

Grains are volumes, inside crystalline materials, with a specific orientation.

Particles can be polycrystalline, single crystal or amorphous. A 100 nanometer particle of gold, for instance, can be made of:

a single gold crystal,

many grains with a grain size