Defect formation in graphene nanosheets by acid treatment: an x-ray absorption spectroscopy and density functional theory study

Defect formation in graphene nanosheets by acid treatment: an x-ray absorption spectroscopy and density functional theory study.

Coleman, V.A., Knut, R., Karis, O., Grennberg, H., Jansson, U., Quinlan, R., Holloway, B.C., Sanyal, B., Eriksson, O.

Journal of Physics D: Applied Physics, 2008, 41(6), 062001

And now, for something completely different. You might remember when I griped about a theoretical paper called "graphene nanoribbons with chemically modified edges"; apparently, those authors weren't alone. Many physicists (or P-Chemists) were excited enough about broken graphene to do a lot of calculations on the subject, since this may give metallic as opposed to semi-metallic properties (or something). Our current paper details an attempt to actually put holes in graphene sheets and check out a variety of the properties.

Let me begin by saying that the authors use a form of graphene I haven't encountered before, which they call carbon nanosheets (CNS). They use "radio frequency plasma enhanced chemical vapor deposition" to get a structure best described by the picture from the text:


The authors claim (or imply) that this structure will be an adequate analog for actual graphene sheets, and proceed to treat it with HCl for a few hours. They then did the same treatment on another sample using distilled water instead of HCl and took some SEM images to make sure they still had carbon nanosheets. The authors moved on to analyze the sheets with X-ray absorption spectroscopy (XAS, pdf summary from the University of Calgary here) which, in analogy to XPS, gives information on what kind of bonds are present in the sample (pi* C-C, sigma* C-O, pi* C=O, etc.). The authors find that, compared to a control, their acid-treated samples have a significantly higher peak that they ascribe to C-O bonds and a lower peak in the area attributed to C-C pi* and sigma* bonds. They theorize that the acid broke C-C bonds, which were replaced by hydroxyl groups, and back this up by mentioning the higher wettability (read: lower hydrophobicity) of the film and by XPS measurements showing a much higher proportion of C-O bonds (we've seen this before).

After establishing that acid does indeed add oxygen (and probably hydroxyls) to their films, the authors start doing some calculations of broken graphene layers (essentially calculating the situation where some C-C bonds are absent, which they say gives the same results as having pendant hydrogens where the bonds broke). I'll spare myself the details, but the energy states they find are consistent with their XAS measurements, and the calculations predict a metallic state in the graphene. They conclude that although they did not attempt to measure any metallic properties in the graphene, their calculations are probably valid since they correctly predicted their XAS results.

Maybe I'm missing something, but treating graphene/graphite with acid and finding that it oxidized isn't exactly new. The XPS and XAS data presented look very close to the relevant data for graphene oxide, and although it's surprising that graphene can be oxidized without a strong oxidant like HNO3, there's no reason to think oxidized defects introduced in this manner will be different than oxidized defects induced by more conventional acids. And although I'm out of my league here, I don't understand how they would get away with modeling the defects as simply breaking the C-C bonds (or using hydrogens), instead of having electron-donating hydroxyl groups present.

All in all, Coleman et al. did a good job communicating what they did, and I'm always glad when people try to test theory with wet lab research. These defects will have a huge role in modulating the electronic properties of graphene, and more work needs to be done on the subject. It just seems like the end result (and a lot of hard work, I'm sure) went into a study that told us what we already knew.



Coleman, V.A., Knut, R., Karis, O., Grennberg, H., Jansson, U., Quinlan, R., Holloway, B.C., Sanyal, B., Eriksson, O. (2008). Defect formation in graphene nanosheets by acid treatment: an x-ray absorption spectroscopy and density functional theory study. Journal of Physics D: Applied Physics, 41(6), 062001. DOI: 10.1088/0022-3727/41/6/062001