Re: [Ifeffit] Need an adviceMatt, the complication here is that the Co-O bonds will be present in the surface of Co particles because they are not just "metallic cobalt" but can be nano-size particles, and thus their surface contribution may be significant. If it is this type of a mixed phase, not the other type, when the particles are larger than, say, 5nm in size, then these bonds will count and it is not possible to deconvolute them from the Co-O bonds in the oxide. Therefore, the results of the Co-O and Co-Co bond coordination numbers cannot be easily interpreted in terms of the oxide/metal ratio if, again, the particles are smaller than 5nm and if the binding geometry of O on the Co surface is not known. For example, in fcc metal nanoparticles, ligands may bind above the bridge, the face, or the vertex. In each case, the coordination number of M-O will be different, and the oxide/metal ratio will thus change accordingly to the model used. I think, the best way to deal with it is to rely on TEM or, perhaps, XRD, to actually measure the particle size and then infer the Co-Co coordination numbers. The Co-Co coordination numbers measured from EXAFS can then be compared to the TEM-derived, and if the answers are different (namely, the experimental numbers are smaller than the TEM-derived), this discrepancy can be interpreted in terms of the "mixed phase" situation, and the mixing fraction of metal is exactly the ratio of these two numbers. What I've just described, is actually going to published, and I will send the reference when it is out. Regards, Anatoly ****************** Anatoly Frenkel, Ph.D. Associate Professor Physics Department Yeshiva University 245 Lexington Avenue New York, NY 10016 (YU) 212-340-7827 (BNL) 631-344-3013 (Fax) 212-340-7788 anatoly.frenkel@yu.edu http://www.yu.edu/faculty/afrenkel -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov]On Behalf Of Matt Newville Sent: Friday, May 06, 2005 4:06 PM To: XAFS Analysis using Ifeffit Cc: Gary L. Haller; Yanhui Yang; Yuan Chen; Lisa D. Pfefferle; Dragos Ciuparu Subject: Re: [Ifeffit] Need an advice Hi Steven, On Thu, 5 May 2005, Steven S. Lim wrote:
Dear all,
I have a question to ask all of you as follows.
We have been investigating the EXAFS of some partially reduced Co materials (by Co EXFAS) and would wish to obtain some qualitative information on the Co metal particles. If, in fact, the Co had some average coordination environment because we have a distribution of Co particle sizes with oxygen adsorption, analyzing the windowed Co-O and Co-Co peaks together as a linear combination and assessing the average oxygen and Co coordination from the Co edge absorption (what we do with Athena) would appear to give a useful answer. However, if what we have is closer to a physical mixture of small Co metal particles and Co oxide particles, it would seem that it would be more appropriate to analyze separately the windowed Co-O and Co-Co peaks in R-space, to somehow normalize the Co-Co to the fraction of metal in the sample and use this as a qualitative measure of metal particle size. Is there a way to do this with Athena or other method? Or do you know of a reference where this has been discussed? Of course, we can (and will) make experimental mixtures as references but wonder if there is a different way of doing the analysis or if there is some literature on this problem (which we have not been able to locate).
If I understand correctly, you are asking where there is way to distinguish a two possible cases: 1) a physical mixture of metallic cobalt + cobalt oxide (what I would call "mixed phases") ; and 2) a chemically intermediate phase, that was neither fully metallic nor oxidized. I don't know enough about the system you're working on to know the exact details. In general the EXAFS analysis (as with Artemis) allows one to distinguish these case, though it can sometimes be complicated. For the first case, you could create models of Co-Co and Co-O and fit the spectrum as a linear combination of those two phases without allowing the bond distances to change from the known values for metal and oxide phases. That gives one fit, with a result for metal / oxide fraction. For the second case, you could allow both Co-Co and Co-O contributions but allow the distances and coordination numbers to vary. If the distances move significantly from those of the isolated phases, that's good evidence that a simple physical mixture is not sufficient. But also, you can compare the goodness-of-fit parameters for the two cases to help decide which of the two cases better explain the data. Ideally, you could also include further "shells" in this analysis, especially for second neighbor Co-Co in CoO. This is more work, generally requires fairly good EXAFS data, but can really improve the confidence that you have a component that is the isolated oxide. Hope that's enough to get you started in the right direction. If not, let us know! --Matt _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit