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