[Ifeffit] particle size and coordination number in Runanoparticles on carbon

[Dreher Marian]

Dear Ifeffit Community,
 
I have a question regarding the signal damping I mentioned in my last
post (I searched the archive but didn't find an answer. If this topic
has been discussed before, please just send me the link).
 
As I mentioned before, I'm trying to determine the particle size of a Ru
catalyst. I have 3 different reference spectra from Ru that I can use to
determine the s0^2 for bulk Ru. From that I can calculate the Ru-Ru CN
in my catalyst.
 
Now, two of my references give a very similar EXAFS signal in terms of
amplitude. When fitting the first Ru-Ru shell I get a s0^2 of 0.6 to
0.8, depending on k-weight. One of the reference spectra was obtained
from a pellet containing Ru powder ("Ru pellet Sigma Aldrich"), the
other from a thick Ru filter ("Ru filter foil").
 
I also have a reference spectrum from an unknown Ru sample. The
amplitude of the EXAFS is much higher compared to the other two spectra.
However, the edge step is considerably smaller. Here, a Ru-Ru first
shell fit gives me a s0^2 of 0.9 to 1.1, depending on k-weight. Even
before fitting, the experimental data is very close to the theoretical
spectrum.  
 
Obviously, the CN I calculate for my Ru catalyst depends on these
reference values for s0^2.
 
I can see how poor sample preparation leads to signal damping and thus
to a low amplitude in the EXAFS, but I can't imagine how it could cause
an amplitude which is too high. Should I therefore take the unknown
reference since it has a higher amplitude? 
 
I attached an Athena project file with all 3 spectra.
 
Many thanks,
 
Marian.
 
 
________________________________

From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Dreher
Marian
Sent: Mittwoch, 21. September 2011 21:07
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] particle size and coordination number in
Runanoparticles on carbon



Thanks everyone for your help, now everything is working fine. My Ru
reference was simply way too thick which caused a lot of dampening in
the signal. 

 

From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Wayne W
Lukens Jr
Sent: Dienstag, 20. September 2011 20:26
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] particle size and coordination number in Ru
nanoparticles on carbon

 

Hi Marian,

There is a very nice paper by Scott Calvin that describes this in some
detail:

J. Appl. Phys. 94:778-83 (2003)

Sincerely,

Wayne Lukens



On Tue, Sep 20, 2011 at 11:04 AM, Scott Calvin <
scalvin at sarahlawrence.edu> wrote:

Hi Marian,

My big tip is to look at the second nearest-neighbor CN as well, and
even further out if you can get it. It's sometimes hard to pin down the
near-neighbor coordination number because of issues like the one you
describe. But the ratio of higher CNs to near-neighbor CN is quite
diagnostic of small nanoparticles. (Actually, for reasons too lengthy to
get in to at the moment, this method does tend to produce results a bit
biased to the small side, so it's best to try the identical fitting
model on a bulk standard for comparison.)

--Scott Calvin
Sarah Lawrence College


On Sep 20, 2011, at 1:50 PM, Dreher Marian wrote:

Dear Ifeffit Community,


I am currently trying to extract the particle size of Ru supported on
carbon from EXAFS data. I also recorded spectra of a Ru foil as a bulk
reference sample.
So, my approach is to fit the first coordination shell of my bulk
reference, setting CN to 12. From that I get a value for s02. I'm then
using this value in the first shell fit for the Ru nanoparticles in
order to get the coordination number.

However, the fit suggests that CN is still pretty much 12. What is
changing drastically, though, is sigma^2 which usually doubles in value
compared to the bulk sample. That makes sense, I guess, since there
might be higher disorder in the nanoparticles.

Obviously, there is a chance that my nanoparticles are rather big, in
which case CN would be close to 12. However, STEM and HRTEM pictures
suggest that the Ru particles are between 1 and 2 nm in diameter.

I'm just starting to get into EXAFS analysis and pretty clueless right
now about how to approach this.

Happy about any input,

many thanks,

Marian.

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