[Ifeffit] Need an advice

Anatoly Frenkel frenkel at bnl.gov
Fri May 6 15:20:16 CDT 2005

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.



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 at yu.edu

  -----Original Message-----
  From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at 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
  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!


  Ifeffit mailing list
  Ifeffit at millenia.cars.aps.anl.gov
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