Hi,

The XPS analysis may be used to analyze the phases present in mixed (bimetallic and trimetallic) metallic nanoparticles. The XPS analysis can tell about the oxidation states & elemental bonding and hence can confirm the formation of alloy.

Thanks

Ahmed Elsheikh

Your question ‘Is there a solid and rigorous analysis for intermetallic overlapping XRD peaks versus alloy formation?’ is correctly answered ‘No’. Please define ‘alloy’ for me…

It will depend on the bi- or trimetallic system and what phases they can form over the composition range plus how close the separate metal XRD peaks may be, and the crystallite size of the metals concerned. We always have to consider the heterogeneity of the system (especially on the nanoscale) – we can easily have ‘patches’ of alloyed and non-alloyed components plus the separate components themselves. I worked on PdAg and PdAu systems which form a set of totally miscible solid-state systems. This is not true for other systems such as Cu-Ni. In the case of fully miscible systems then one expects Vegard’s rule to be followed (i.e. an approximation to linearity over the composition range). Non-alloyed systems could be gaged here by either separate peaks (and here making an artificial mixture of the separate components is really useful) or peak distortion. You need to look at the phase diagrams and if these indicate different phases over the compositional range then life is more complicated.

XPS is extremely useful but a few cautionary notes. The surface composition, not the bulk, is assessed and in the case of bi- and tri-metallic systems in air or in the presence of oxygen (e.g. an oxide support in catalysis) then the bulk and surface compositions CANNOT be the same due to chemisorption induced segregation. In my case, surfaces were enriched in Pd as oxygen in the air would preferentially pull this to the surface. Significant Ar+ ion etching was needed to get down to the bulk composition. Further, depending on the system then it’s unlikely that one could see < 1 molar % of one constituent in the presence of the other. Also, it can be seen in XPS (or Auger) that the surface is fully oxidized – we saw Au3+ and not Au0 on the surface (top 5 – 10 atomic layers). So, oxidation states may be contradictory or difficult to interpret. How would one expect the oxidation states to differ in an alloy as opposed to a non-alloy?

I have some issues with the term ‘alloy’ as this implies a chemical rather than physical combination. Sure, there is interaction between the components and that can be strong or weak.

So, you’ll need to be like a detective and look at all the evidence. Consider the phase diagram of the systems first. Consider the Gibbs Free Energy of the components and work out what component(s) the surface will be enriched in. After all, it’s the surface that interacts with the immediate environment. Plus, there is unexpected information too that should be borne in mind – the PdAu and PdAg systems show a maximum in many chemical reactions (e.g. hydrogenation) at the Pd60OtherMetal40 point. Look at the XRD peak positions and peak shapes (bearing in mind that oxides can complicate the matter – Ag is always found on the surface as the Ag+ species) for both bi- and tri-systems plus artificial mixtures of the individual components.

Dear Ahmed

The direct answer to your question is no.

In the meantime, one should mention that Bragg's law ;n (,lamda)=2d Sin(theta) , can be used to identify separare elements in metallic alloys either by XR- structure analysis or by XR-spectroscopy.

In the XR-structure analysis, by knowing the wavelength (lamda)of the the XR used and by detecting the diffraction angle (theta) during the XR scanning over the sample, one can determine the spacing(d) between the atomic layers,i.e., integer numer (n) of layers, of

a specific element. Since each element has unique d value,therefore the element can be determined.

In contrast,in RX-spectroscopy by knowing/expecting that examined metallic alloys would have certain elements, i.e., d values become known/expected,and by detecting the incident angle (theta) which corresponds to consequent-peak spectra (lamda) as a characteristic signature for each element present in the metallic alloy, one can identify the elements present in the examined metallic alloy.

So there is no direct solid/ regorous analysis for intermetallic overlapping XRD peaks versus alloy formation.

Thank you dear Ahmed Elsheikh for this interesting discussion. I would like to say yes when the bi-metallic or tri-metallic crystalline structure gives sharp and well defined Bragg peaks with no more peak overlapping, while the diffuse diffraction with peaks overlapping are indicative of a product structure that either has amorphous crystal structure or weakly ordered crystal structure or is crystalline but with range of infinitely small that around few nm in size or less. In this case it is difficult to understand the peak overlapping and alloy formation potential. So the structures integrity of alloying and intermetallic product is the index to the characterization ability of the XRD technique and so for solid and rigorous analysis.

Best regards