[Ifeffit] Estimation of Pt and Au particle size

Scott Calvin SCalvin at slc.edu
Tue Apr 19 08:28:37 CDT 2005


Hi David,

Sure thing. I'll send you a Pt file as a separate email; I'll also 
send you a copy of the paper we're currently preparing for 
publication. The final versions we used we're big Ifeffit scripts 
rather than Artemis files, but I'm sure I can scare up a 
representative Artemis file from earlier in the process.

Just a brief comparison of the two methods I've seen out there:

1) Fit the coordination number of each shell. Anatoly Frenkel has 
several publication related to this.

2) Assume the crystallites are "roughly" spherical and constrain the 
coordination numbers in a fit to reflect this. The only size-related 
fitting parameter is then R, the radius of the crystallite. I have 
written a few papers related to this method.

Method 1 has the advantage that it can in principle handle any 
morphology; Anatoly has used it with supported catalysts (they may 
even have been Pt, but I don't recall offhand) and distinguished 
between shapes like rafts, hemispheres, etc..

Method 2 has the advantage that is uses only 1 size-related 
parameter. For a complicated fit or one with limited data quality, 
that's nice. On the other hand, if the morphology is far from 
spherical (e.g. needles), then it doesn't work very well. This method 
can even be used with an empirical standard (as opposed to a 
theoretical standard like FEFF) to give a (rough) model-independent 
estimate of size.

So I tend to think method 1 is better when you have a very uniform 
sample, as can happen with supported catalysts, while method 2 is 
often appropriate when you have a "messier" sample with roughly 
spherical particles, as is often the case when wet chemical methods 
are used.

It should also be noted thatusing both techniques there have been 
many reports that for polydispersed samples EXAFS yields a mean 
smaller than that found by other methods (e.g. Scherrer analysis). 
Part of that can be explained by the different weightings inherent in 
the different techniques (EXAFS reads a 10 nm and 10 micron 
crystallite as essentially identical, but "notices" a 1 nm 
crystallite, while XRD would find the 10 micron signal dominant), but 
I am not yet convinced that there isn't something else at work as 
well. My group is currently planning a follow-up study of Pt to 
resolve this question.

--Scott Calvin
Sarah Lawrence College

>Ifeffit Community:
>
>Over the past year or so I have seen several discussions of 
>estimation of metal cluster size (collections of several to hundreds 
>of metal atoms) using coordination numbers derived from EXAFS 
>fitting.  I was wondering if anyone on the list would be willing to 
>share an example of their Artemis project file where this analysis 
>was done successfully for Pt and/or Au particle sizes.  As a first 
>pass I am just interested in roughly estimating an average particle 
>size with future considerations for estimations of particle size 
>distributions if data quality and theory allows.
>
>Thanks in advance to anyone willing to share their project files. 
>Given the number of experts in the field reading this list it just 
>seemed to make sense to build upon the back giants.



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