Micro-/Nanotechnology

Will the real Graphene please stand up?

Welcome to one of our guest columns, where active researchers can share their views on topics relevant to materials science. We invited Professor Martin Pumera of Nanyang Technological University to share an opinion on a pressing issue in Graphene-related research. Here, he shares his views on the accurate identification and characterization of graphene in research reports. We’ve also spoken to Professor Pumera on this and other subjects in our regular “MaterialsViews Interviews” section; click here to read the results!

“Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice” – at least popular definition says so.[1]

Graphene[2] is more familiar than the Philosopher’s Stone of the Middle Ages. Consequently, the word “graphene” is frequently found in the titles of scientific articles. Rightfully so? One can wonder.

I do not want to suggest that graphene does not exist. Here, I only wish to underline that too many articles, which contain word “graphene” in the title, actually do not contain research on real graphene, but rather on its mystical projection, which is far from the real state of the material they actually work with. This might be intentional or the authors simply do not understand their work (or are they engaging in black magic?). In any case, misuse of the term harms the entire research field.

Strictly speaking, only a single-layer graphene sheet is graphene.[1-3] There are also multilayered structures, which are related to graphene such as double- and few-layer graphene (3-9 layers) and multilayer graphene, which are basically graphitic structures.[4] There are also graphene-related structures, which are restricted in two dimensions along the graphene plane. Such structures are often called (single-, double-, few- or multilayer) graphene nanoribbons.[4]

The problem with much of the published research on graphene and its applications is that it lacks any section on its characterization; there is only a mention that “graphene was prepared by modified/extended Brodie’s (or Hummers’ or Staudenmaier’s) method.” The problem in this method of reporting is that any of the bulk material production methods (until very recently) was able to produce 99% “graphene” nanoflakes containing several (3 or more) layers of graphene, effectively being highly oriented graphite (HOPG) nanoflakes and only ~1% single-layer sheets.[5] This is especially true for modified Brodie’s, Hummers’, and Staudenmaier’s methods. Interested readers can find additional information in references.[4,5] However, note that the material prepared using these methods is very often incorrectly called “graphene.”

In some cases, characterization of the materials is provided, and it actually contradicts the claim in the title of the article that the authors used graphene, i.e., TEM/ED analysis of “graphene” shows a distinctive diffraction pattern, typical for graphite. Yet, such work contains the word “graphene” in the title without the use of any adjectives. In other work, AFM shows that the thickness of the “graphene” is 3 nm. This statement actually refers to few-layer graphene and, once again, the authors contradict themselves.[6]

Not much time has passed since the carbon nanotube (CNT) disaster. CNTs were deemed electrocatalytical and hundreds or perhaps thousands of articles on their electrochemistry were published before it was revealed that the “excellent” and “inherent” catalytic properties of CNTs are due to the impurities they contain.[7,8] This revelation largely damaged the field before the eyes of other chemists.

To avoid similar graphene disasters, I wish to urge all scientists to use the proper terms when describing the material, to provide as much characterization as possible and, more importantly, to not misuse the word “graphene.” There are only a few things more important than scientific honesty (or honesty in general) and trust is a very fragile commodity.

[1] http://en.wikipedia.org/wiki/Graphene

[2] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 2004, 306, 666.

[3] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183 and references therein.

[4] a) D. R. Dreyer, S. Park, C. W. Bielawski, R. S. Ruoff, Chem. Soc. Rev. 2010, 39, 228; b) C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int. Ed. 2009, 48, 7752; c) Y. Zhu , S. Murali , W. Cai , X. Li , J. W. Suk , J. R. Potts, R. S. Ruoff, Adv. Mater. 2010.

[5] P. K. Ang, S. Wang, Q. Bao, J. T. L. Thong, K. P. Loh, ACS Nano 2009, 3, 3587.

[6] Here I decided not to cite the articles in question as there are too many similar cases and picking up only a few would not be fair to the authors.

[7] C. E. Banks, A. Crossley, C. Salter, S. J. Wilkins, R. G. Compton, Angew. Chem. Int. Ed. 2006, 45, 2533.

[8] M. Pumera, Chem. Eur. J. 2009, 15, 4970.

Martin Pumera

Chemistry and Chemical Biology

Nanyang Technological University

21 Nanyang Link, Singapore

Fax: (+65) 6791-1961

E-mail: pumera@ntu.edu.sg

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