I have Co/Al2O3 catalysts and carbon is deposited on the surface by decomposition of light hydrocarbon on the surface of catalysts, therefore the catalysts are containing cobalt carbides and some deposited carbon on the surface. By increasing the decomposition temperature, the content of carbon on the surface of the catalyst increased and the G and D bands were observed, while for the other samples (with lower gas decomposition temperature) there is not any G and D bands, while the 2D band with lower intensity and broader was observed in these samples. According to our XRD analysis, for the samples prepared at lower decomposition temeprature (No.1, 2, 3, 4) without G and D band and with only weak 2D band at 2670 cm-1, we have some cobalt carbides species, while for those with G, D and 2D bands (No. 5, 6) there was not any peak belong to cobalt carbides in XRD spectrum. I would like to know your opinion about the band at 2670 cm-1 in these Raman spectrums.
Thanks in advance.
For defect-free graphene or nanotubes, you can have a 2D mode without a D-mode because the D-mode is symmetry-forbidden without defects while the 2D-mode is symmetry-allowed both for defected and undefected material.
However, if a G mode is absent, I would be very careful to claim something is a 2D mode.
In the case of your spectra, it is hard to tell what kind of physics is going on for spectra 1-4. That feature is not really a sharp peak as you would expect from a regular Raman process.
I don't know if this could be possible, since even thought the 2D band is generally stronger than the G band in 1L graphene, it is also much weaker than the G band in bilayers, trilayers and so on. Another interesting point from your data is how thin your D band looks, which is not usually the case for amorphous carbon (check the book Raman spectroscopy in carbon related materials). Looking at a figure comparing the Raman spectra of different carbon based materials in the same book, your spectra reminded me in part of single-wall carbon nanohorns (Article Organic–Inorganic Ternary Nanohybrids of Single-Walled Carbo...
). In my opinion, one thing that could be happening, maybe evidenced from spectrum 5, is the formation of some SWNH. The broad contribution appearing around 2600 cm-1 could be something else, and I don't believe it would be related to cobalt carbide due to the high frequency. Maybe some of the gases are trapped? C-H vibrations happen close to this value at ~2900 cm-1, but maybe something else could be happening.
Check https://www.mdpi.com/2311-5629/5/4/69/pdf
https://hal.archives-ouvertes.fr/hal-02503604/document
Also look at this paper
https://www.mdpi.com/1996-1944/15/5/1636/pdf
Thanks a lot for all your answers. I have done the Raman analysis again with the longer exposure time, and this time the peaks at 2670 cm-1 were only observed for that samples 5, and 6, which now can confirm that it is 2D band, since for these two samples D (~1340 cm-1) and G1 (~1580 cm-1) bands were also observed. However there is not any other visible peaks belong to carbon for samples 1-4. There is a very small peak at 1560 cm-1 also in all samples, which I am not sure that if it is a noise only or could be belong to G2 band. So, it seems that however in our XRD results we had cobalt carbides in samples 1-4, but Raman cannot detect any peaks belong to carbide.
Please let me know if you have any idea about the new Raman results.
Thanks.
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