Tag Archives: good practice

Calibrating XPS spectra. Trivial thing?

This is a schoolbook task. Everybody dealing with XPS spectra knows that if charge neutralizer is used to compensate for charging effect for not fully conductive samples, that causes spectra to shift to lower BE. In order to correct for that spectra must be calibrated/charge corrected/shifted – whatever the word you’re using to describe this procedure.

Adventitious carbon contamination is a vice but its virtue is that all samples (almost) have it so we can reliably use it for calibrating spectra for lots and lots of materials. Its position is assumed to be 284.8 eV. Put maximum of C 1s spectra to that position and use the same shift for all other spectra from that position for that sample and voila!

But what if carbon is a major part of the material you’re designing or studying? Is it mainly graphitic (284.4 eV), aromatic (284.7 eV), aliphatic (285 eV)? Are there lots of surface oxides causing big secondary shift of C at 285.5 eV? How can you reliably use C as internal standard when you may know nothing about the carbon itself? Putting maximum of a carbon peak at 284.8 eV (assumed to be representative of internal hydrocarbon), how sure are we that we are not calibrating all of the spectra by graphitic or secondary carbon? The difference of 0.4-0.6 eV may not seem significant but we can not claim to being able to resolve peaks as close as 0.2-0.3 eV and accurately identify them at such accuracy of calibration, can we?

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If we put Au or Ag reference material (which are available as paints, pens or powders) onto each sample individually, we can see the effect. Figure shows that for bipyridine the difference between using C and Au for calibration is small, being only 0.2 eV.

However, for another sample shown, CoTMPP,  the difference in calibration is 1eV. So if we would’ve used C (and we did) we would incorrectly identified types of Co and N and O present within the sample.

The purpose of surface analysis of most functional advanced materials is to be able to correlate surface chemistry to whatever parameter of performance or of interest. In this particular example state of N is of critical importance in understanding what is the structure of active site in the electrocatalysts. Pyrrolic N and pyridinic N are among the suspected species responsible for oxygen reduction. And, coincidentally, the difference in BE between N in pyridine and pyrrole environment is … Yes, you guessed it right – 1 eV – exactly as we have found the difference between calibrating by Au and by C. So, if we would’ve used C, we would’ve found that majority of N is in pyrrolic state and if we would’ve used Au, we would conclude that it is pyridinic that is the main type of N.

Now wonder that out of 35+ manuscripts reviewed, N speciation in pyropolymers or fuel coals as determined by XPS shows huge spread of reported values for all major types of N.

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Is it because all of them used carbon as internal standard for calibrating their spectra?

I think you know the answer…

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