I am working on various trimetallic catalysts of Pd and two other metals for ethanol oxidation.
All of them have resulted in one metallic phase upon analysis by X-ray diffraction. That means no individual metal peaks are separate from one another.
However, I, some times, see overlapping peaks with one main peak and another shoulder peak. I think this is not the same thing as the asymmetric peaks that is sometimes found in XRD even in one metal system. I am saying this because I also prepare each metal individually and compare the trimeallic peaks with single metal ones.
For example, in the attached graph is the (111) peak of trimetallic PdRhIr vs single Pd, Ir, and Rh. You can see the main peak in the trimetallic is close to that of single Pd while the shoulder is close to that of Ir. This drives another question about how to deconvolute those peaks? I have tried originlap using both Peak Analyser and Multipeak but both have not given a satisfying result.
The main question, however, having those overlapping peaks; how could I understand them and evaluate the alloy formation potential of the three metals?
Any idea or suggestion will be deeply appreciated.
Have a great day!
The data is showing very broad peaks; is it possible to improve your resolution by adjusting a few of your data collection parameters, e.g. longer collection times, smaller steps? Or are the broad peaks just a result of having very small domains (sub-micron) within the alloy itself?
It would be worthwhile checking the size of the domains / grains using the scanning electron microscope. If the domains are getting down to the nanometre range then you might be at the coherence limit for your x-ray diffraction analysis. If you have access to a transmission electron microscope you will be able to get diffraction data from much smaller domains although analysis of TEM diffraction comes with its' own traps.
Dear Ahmed,
me seems the crystallinity of your material is very low. The existance of peaks is connected with a periodicity of a lattice. Catalysts are nice examples for amorphes material. If you want to have a chance to use XRD I propose to anneal your material. This could create order in your structure.
With regard
R. Mitdank
Robert D Aughterson & Rüdiger Mitdank
Yes, I performed TEM (conventional no diffraction) and most of my trimetallic particles are less than 5 nm (see image attached). It, therefore, might be close enough to the coherence limit of XRD, but what is exactly the coherence limit? Is it similar to the resolution, which is 0.02 degree in the XRD machine I used?
Aside note the high broadness you see here might happen to you because I am enlarging only one peak (111) but the with the whole pattern (graph attached), I see more intensive peaks even though it might still be broad for this sample in particular compared to other samples.
Dear Ahmed,
I ask to to prove the Scherrer equation. This gives a relationship between grain size D and broadening delta:
D > = K*lambda/(delta*cos(teta)).
K is a shape factor near 1, lambda the wavelength and teta the Bragg angle.
The upper equation means the broadening is invers to the grain size.
With regard
R. Mitdank
Probably you are aware of this, but just to be emphatic: your 3-component diffraction pattern shows that you have a multiphase mixture, not an alloy. The observed pattern shows all the features of each of the individual components (although less clear because of overlap). Therefore, each of the component phases is still present.
The broadness in peaks is most likely a consequence of the small size of your grains (your TEM image certainly shows them to be small). I noticed that Rudiger Mitdank suggested annealing to improve crystallinity. A low temperature anneal might grow your grains a bit bigger making analysis via XRD more straight forward although my not knowing what you're trying to achieve the larger crystals will also likely change some of the material properties. The coherence length is related to the minimum size that your analysis technique is capable of getting any sensible information out of.
Your TEM image does seem to show at least a couple of distinct morphologies. If you have access to it, it might be worth your while doing TEM EDX analysis on individual grains. Even at a qualitative level this will tell you if you have an alloy or a multi-phase metal.
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