Hi George,
Do you have a rough sense of the shape of your nanoparticles?
I've consistently found (and I think Anatoly Frenkel has as well) that these kinds of methods tend to underestimate the size of nanoparticles, and in fact give a "size" for bulk metals which suggests they're actually large-ish nanoparticles.
I've never been able to nail down why that is. The hypothesis that I used to favor is that EXAFS is emphasizing the small end of the size distribution, and so there's actually a low-end tail that is being measured. More recently I've started to wonder if it's related to the assumption that S02 is independent of R and k within the EXAFS region. It's been known since the 80's that this is not exactly true, and it seems possible to me that there's a slight systematic trend with R that gets confounded with the particle size effect. As I say, that's just a guess, but in any case the underestimate of particle size comes up quite often--I've discussed it in a number of papers.
Therefore, if you do have a sense of the shape of your nanoparticles (e.g. roughly spherical, or roughly raft-like), I'd suggest constructing a model that describes the morphology with a small number of free parameters. For a sphere-like shape, for instance, the only free parameter needed is the radius (and S02). Then calculate the effect on the coordination number as a function of those free parameters. Next, fit the copper foil using the model, and see what it finds for the free parameters-. Finally, apply to your nanoparticulate samples, setting S02 to the value you found for the foil using the model, and compare the results for the other free parameters, such as radius.
I suggest this procedure for several reasons:
*By using a relatively small number of free parameters, it reduces the ability of the fit to compensate for a host of other systematic errors by tuning each individual coordination number up and down.
*By comparing to the fit to the foil, and using the S02 for that fit, some kinds of systematic errors are more likely to apply equally to each fit, thus making the procedure useful for at least judging relative sizes.
*The procedure is relatively insensitive to the details of the shape of the nanoparticles; you don't have to get much further than "raft-like" or "sphere-like" to have it produce useful results.
Hopefully that helps!
--Scott Calvin
Sarah Lawrence College
On Oct 29, 2013, at 1:23 PM, Georges Siddiqi