You need consider if these nano particles have agglomerated.

Surface area not only depends upon the nanosize of the particles, it also depends on crystalline density of the material, may be it will affect its high values.

agglomeration should not affect the BET values too much, because agglomerates just have point-like contacts, not area contacts if balls are contacting.

Gurumurthy Hedge, what material are you talking about?

From where do you know nanoparticle size is smaller than what you get from BET?

I would assume 2 possible reasons:

1) you measure the nanoparticle size in colloidal dispersion which may be stabilized by some "stuff"; then you dry and your "stabilizer stuff" builds some glue = cement around the particles and baked them together

2) you do not have such "glue" around your nanomaterial, but you have clean isolated ones (comparable to what I had in my research some time ago) - here, the surface did not like N2, N2 was not adsorbed so BET measurement using N2 was not the appropriate method.

Dear Bernhard,

thanks for the answer and sorry for late reply. Our materials are from biowastes based CARBON NANOSPHERES. CAN YOU SUGGEST SOME POINTS FOR THIS?

let me ask again: From where do you know nanoparticle size is smaller than what you get from BET?

Even if somebody said "these are carbon nanospheres", how does he know?

In my answer above I gave some hypothetical reasons for a difference between particle size and BET values - but now I ask: did you ever measure the particle size before using BET?

- SEM?

- TEM?

- Laser-Doppler or other methods to measure particle size in colloidal dispersion stage?

Yes, we measured SEM, TEM and in both measurements we found particles sizes are around 40-60 nm.