I prepared different GO films using different graphite flakes (with different sizes) following the improved method Article Improved synthesis of graphene oxide. ACS Nano 4:4806-4814
When performing XPS, I got in some samples small N and F peaks at 402 and 689 eV respectively.
1/What can be the source of this nitrogen and fluorine?
2/Can they be ignored as their percentage is small (0.5 - 2%)?
3/If not, how can they affect the mechanical GO films?
Waiting forward to your help. Many thanks in advance.
Dear Zineb,
Fluorine is one of the contaminants that is usually detected in xps. At least using the XPS I use, I always (most of the time) see peaks assigned to F and the samples have no fluorine. In my case, the XPS I use has a reaction cell in the preparation chamber and I use gas mixtures for treatment. Nitrogen is less frequent and, in this case, it could come from some intermediate process that you have used for the preparation of the sample...
Best,
Fran
Zineb Benzait I look to F coming from PTFE and other similar F-containing polymers used for low chemical reactivity purposes and ubiquitous in chemical laboratories. N can come from NH3 or ammonium hydroxide used for pH control or nitric acid. Remember that XPS is an extremely surface sensitive technique reflecting at most the top 10 -15 atomic layers on the surface. In general, the bulk composition will be different to the surface through factors such as chemisorption induced segregation.
I agree with Alan F Rawle Sir,
Further, you can perform ion etching for 30-40 secs on your sample and again take the XPS spectra, if this belongs to surface contamination, then you will observe decreased/absent atomic %. If this belongs to your sample itself, then there may be some impurity introduced during the synthesis procedure.
Zineb Benzait Ar+ ion etching is always useful to explore further into the bulk of your sample. Expect to see differences.
Thank you very much for all your answers.
Dear Dr. Alan F Rawle many thanks. I remembered that I tried once Ar+ ion etching, it reduced F content but greatly reduced the oxygen quantity of graphene oxide as well
Zineb Benzait
In XPS, all surfaces that have been exposed to air or moisture will be enriched in 'O'. Furthermore, the oxygen in the air will preferentially pull species to the surface in what is called 'chemisorption induced segregation'. This means, for a mixture, that the bulk and surface compositions cannot ever be the same. It also means that most species will have elements present in their highest oxidation state. One personal example from my Ph.D thesis in the 1970's - I was making Au, PdAu and similar catalysts. Invariably Au would be found as Au3+ (not Au0) on the surface of the supported (mostly on SiO2) catalysts.
What about Si peak (1.23%)? Can it be also caused by surface contamination?
Thank you in advance
Zineb Benzait Since you are using NaNO3 in the oxidation step of graphite, make sure to carry out a work-up procedure to remove most of the N-containing compounds in your product, otherwise it will be detected.
Also, be careful how you are storing samples, make sure to use plastic container which are fluorine free, or else cover the sample with Al foil (just to be safe) - otherwise you'll easily contaminate the samples and eventually the chamber of your XPS.
Thank you dear Maria Margo.
But I didn't use NaNO3 at all. Just KMnO3 (as oxidant), H2SO4 and H3PO4 acids as in Tour's method.
Except that small N, F and Si peaks, the other peaks are fine, and the oxygen groups ratios are as expected. But I need also to comment on that N, F, Si peaks.
Unfortunately the XPS facility is in a university far away now, I can't repeat the analysis. And if the samples are already contaminated, repreparing them is needed, which is also currently impossible.
Anyway, thank you very much for your time.
Best,
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