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## Words matter

Marc Monthioux

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Marc Monthioux. Words matter. 2024. hal-04806134

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Submitted on 26 Nov 2024

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## Featured Article: Words Matter

Dr Marc Monthioux, Centre National de la Recherche Scientifique - Institut National de Physique, Toulouse, France
[marc.monthioux@cemes.fr](mailto:marc.monthioux@cemes.fr)

There was a time where carbon was not going nano. Carbon science was handled by carbon scientists, and the synthesis of most of the carbon forms was carried out by chemists, *i.e.* people who know the role and interest of having a common nomenclature. It was possible to be aware of the past literature because it was not so abundant, and publishing time-constants were in the range of a year (or more) anyway, instead of few months as nowadays, hence racing for publishing was not even an option. At that time, everything was slow, but meanwhile, it was possible to know which carbon material an article was about merely by reading the title and the abstract. Times have changed, thank God (or whoever is in charge), but not all for the good.

The craze for carbon nanoforms which started with fullerenes (then nanotubes, then graphene, then... we will see) has brought many non-carbon-scientists to the field, in particular many physicists, who were attracted by the possibility to calculate the properties and model the structure of various carbon nanoforms, thanks to their limited number of atoms and intrinsic periodicities, which was not possible with non-nano carbon materials (and even when they were, such as carbon blacks). This had two bad consequences: the first is that a schism was created between the "traditional carbon" people, and the "nanocarbon" people, from which we still suffer today. The other is that articles started focussing on properties (for good reason, as they are many, and outstanding) but did not care too much about describing accurately the materials being studied, because what matters most for physicists is the right use and interpretation of physical laws, not polishing some descriptive vocabulary, a trivial concern left to entomologists and disciplines alike. The fact that "graphene people" are now back to more "traditional" carbons (e.g., for fuel cells and batteries) makes the situation even worse, as most of the carbon science developed within the 20th century is ignored. As a consequence, it is now necessary to read a whole paper and scrutinise the contained data to understand which carbon material an article is actually about, and even worse, to make sure that it actually is what it is said to be, just because of inappropriate use of vocabulary and terms.

This concern might look like it comes from a grumpy old chemist who has nothing else left to contribute but nit-picking about vocabulary issues that no-one really cares about. It does not. First, I am not grumpy, and I am not even a chemist. Second, words matter not only for literature, but also for science, a statement that my background (geology, a discipline fundamentally based on description) possibly made me be well aware of, and my time spent serving the CARBON journal as an editor for thirteen years even more. And I am not old, by the way.

Because carbon variability and complexity are unique within the material world, the related science has experienced tremendous development, with contributions from thousands of research laboratories worldwide since the 19th century. This has accelerated with the nanocarbon saga which started in the early nineties, thereby multiplying the risks for the scientific information to be blurred, to the detriment of overall knowledge and correct understanding. There are three instances of vocabulary issues specific to carbon science indeed:

1. The wrong use of terms: Terms such as graphite, graphitic, graphitised, graphitisable, amorphous... are misused in thousands of papers, thereby ignoring that carbon scientists (the old guys, those who knew that speaking a common language is essential for mutual understanding), on behalf of the International Union of Pure and Applied Chemistry (IUPAC), have struggled since 1975 to find a common agreement on the meaning of the terms above-mentioned, and many others. No need to rush to Google Scholar to find out, it is enough to consider the final recommendations gathering all the previous ones they published in 1995 in the PURE & APPLIED CHEMISTRY journal [1]. As this glossary reports on 144 terms, all carbon scientists, whatever their topic of interest, will find words that they use on a daily basis, and will be able to check whether they are using them correctly. How many times, in articles, do we read that a carbon material is 'graphitic' merely because transmission electron microscopy images show a bunch of straight fringes? How many times do we see a carbon

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material said 'amorphous' merely because it is made of small graphenic entities randomly oriented, thereby
making a confusion between being amorphous and being isotropic? Existing definitions can even be ignored to
the point that going back to the original meaning can become impossible, and it becomes preferable to
amend the definition rather than enforcing the scientific community to change the habits, however bad they
are. This is what happened with "graphene", originally clearly defined as the single-atom thick hexagonal
array of carbon atoms [1], however afterwards used by many newcomers to the field to designate materials
made from two or more stacked graphenes. Facing such a situation, the editorial board of CARBON journal, in
2013, published a recommended nomenclature in order to tentatively bring some order and clarity to the field
[2]. In this article, (i) the IUPAC definitions of some terms are reminded (such as those related to graphite);
(ii) the definition of "graphene" is extended to a dual meaning, i.e., both the original one (the single layer, if
isolated) and a more generic one (such as water or bread) provided the term is combined with others
referring to the number of stacked layers (e.g., bi-layer graphene, few-layer graphene, multi-layer
graphene...); (iii) new terms are proposed, such as "graphenic" and "graphenised", obviously for describing
carbon materials made of graphene layers, yet not necessarily graphitic. Although this initiative was not
taken on behalf of IUPAC, the CARBON journal editorial board can be considered authoritative enough in
such a matter for its recommendations to be taken seriously, and followed. Alternatively, the International
Organization for Standardization (ISO), a profit-based non-governmental Company, has started publishing
definitions related to the field. Unfortunately, it has been performed without considering the standardisation
efforts made by the carbon community itself, and without consulting authoritative opinions except those of
individuals, instead of carbon-expert groups (such as journal editorial boards, national Carbon Societies...). As
a result, some of the ISO definitions do not exactly match that of 1995 or 2013 [1,2]. The lack of coordination
always generates messy situations, making worse what it was intended to clarify.

Vocabulary misuses do not only concern terms specifically related to carbon. I already mentioned
"amorphous"; another important one is "allotrope". Its initial definition was not so satisfactory when the term was
first introduced by J. Berzelius in 1841 (being based on discriminating single-element phases through their
physical properties [3]). However the development of diffraction methods in the early 20th century allowed
allotrope discrimination through atom positions, eliminating any possible ambiguity. Therefore, since then,
allotropy is defined as *"the phenomenon of the existence of an element in a crystalline solid state in at least two
distinct forms that differ from each other by the spatial arrangement of their atoms"*, as reminded by W. E.
Addison for instance, already long ago [4]. On the basis of this definition, terms such as "fullerene",
"nanotube", "graphene" do NOT designate allotropes, they are molecular forms which, if periodically
assembled, make the various related allotropic forms, which are commonly designated as fullerite,
nanotubulite, and graphite, each of them corresponding to a variety of crystallographic structures (for instance,
hexagonal graphite, rhombohedral graphite, and turbostratic carbon, for the latter). The fact that thousands of
papers say otherwise just because they repeat each other does not make them right. "Crystallite" is also
misused, sometimes, as some articles are found to discuss the "crystallite quality", which is an oxymoron
somehow, as a crystallite is perfect by definition, hence there is no point discussing its "quality".

2. The wrong choice of terms: Sometimes, vocabulary is lacking when it comes to describe a new carbon form or a new behaviour, at which point carbon scientists draw on wider vocabulary resources to put observations into words. A famous example is "glassy carbon" which was at first a commercial designation for a type of graphenic carbon whose mechanical behaviour was similar to that of glasses. Bad idea. Glasses are vitreous, i.e., amorphous, whereas "glassy carbons" are not. Since then, unfortunately, there are many instances where glassy carbons have been incorrectly used as standard materials to represent amorphous carbons. Other examples are many:

* When the craze about carbon nanotubes started, they were first described as "carbon needles comprising coaxial tubes of graphitic sheets" by S. Iijima [5] (which is about OK, except that they are not needles because needles exhibit a tapered form, and that "graphitic" is not appropriate), and then as "single-shell" and "multi-shell" nanotubes by the same two years later [6] (which was even better, except that it strictly works best with nanotubes with concentric texture). However, in the same issue of the NATURE journal as the latter, D. S. Bethune et al. introduced the expression "single-walled nanotube" [7]. For some obscure
reason, the latter won, and designating carbon nanotubes as "single-wall" and "multi-wall" is now of common
use. Not that it is so wrong, but geometry speaks, and a tube is a hollow cylinder, i.e., it is made of a wall
and an inner coaxial cavity. Therefore, a description difficulty may arise when it is needed to designate the
wall of a multi-wall nanotube as a whole. More accurate designations would have been "single-graphene
nanotube" and "multi-graphene nanotubes" then (SGNT and MGNT, instead of SWCNT and MWCNT),
allowing description of the wall of a carbon nanotube as being made of a single or several graphenes.

* I already pointed out the incorrect use of "graphene". It occurred in Geim and Novoselov's landmark paper in 2004 [8], from which all the mess came (from the mere lexical point of view, of course) because the expression "few-layer graphene" (FLG) was used in it to designate the graphenic objects obtained from the adhesive-tape peeling of HOPG (opening the door to kindred terms such as "bilayer graphene", "multi-layer graphene", etc.). With all due respect to my honourable colleagues, such an expression explicitly says that a graphene can be multi-layered, which is a nonsense (well, "was", since the definition is now adapted, see above). It would have been enough to merely inverse the terms "layer" and "graphene" as "few-graphene layer" (FGL) to designate the material accurately while staying respectful of the primary IUPAC definition.

* Using organic gels is a relatively recent way to obtain carbon materials with interesting pore distributions, specifically in the nanometre range. Such carbon materials are designated as "carbon aerogels" (or xerogels, depending on the synthesis method) by the scientists involved in the field, who understand each other with no ambiguity on this. But what about newcomers to the field, students, and above all, pure logic and scientific accuracy? Carbon aerogels are NOT gels. They are carbon materials (i.e., solid, 90-100 % pure carbon) obtained by carbonising organic precursors obtained by sol-gel routes. Why not name them "aerogel-based carbons", then? One word more, OK, but if this is the price to pay for being right and accurate, it is worth it. And using the acronym drawn from it (ABC) would always be possible.

* Talking about pores: BET methods discriminate between micropores, mesopores, macropores... Obviously, the terms refer to diameter ranges relative to each other, not relative to dimension units, as the scale was probably built at a time (the BET equation was proposed in 1938) where the methodology was barely able to measure pores in the nanometre range (or where the importance of such a porosity range in physico-chemical processes was ignored). But the term "micropore" is ambiguous as it suggests micrometre-sized diameters as well, and the need to extend the terminology as the measurement methods improved has led to the introduction of the term "ultra-micropore" to cover pore sizes smaller than 1 nm, which makes the term even more out of the common understanding. Would it not be a good time to revisit the designation of the porosity range measured by gas-adsorption methods, and move to a porosity scale reflecting their actual dimensions (e.g., sub-nanopores, ultra-nanopores, nanopores, sub-micropores, micropores...)?

There are more, but the case is made, I presume.

3. The lack of a standard description protocol: Carbon materials are multiscale materials, and the features to describe that the authors focus on depend on the property or behaviour they are willing to investigate. The result is that the description of carbon materials is often only partial. In addition, terms such as structure, texture, nanostructure, microstructure, microtexture, and so on, are used while covering different aspects from a paper to another. Therefore, ascertaining how far carbon materials used in different articles by different authors are similar or different is often not possible. It would mark a significant progress regarding the spreading of knowledge on carbon materials if each carbon material could be assigned an identity card based on a multiscale, multi-aspect description. For instance, the following is the four-descriptor protocol I have been using for more than twenty years to build such identity cards for the carbon materials we study [9]:

(a) **Morphology**: The term strictly designates the shape (outer or inner) of the material. Bulk, filamentous, porous, aligned, coiled, tubular, hollow, bamboo-like, flake, lamella, film, crumpled, folded, particle, polyhedral, facetted, etc. are morphological terms.

(b) **Texture**: The term accounts for the long-range spatial relationship between crystallite ensembles (typically graphene stacks, for graphenic materials), when considering the material as a bulk. Isotropic, anisotropic, concentric, random, fibrous, radial, aligned, alternate, herringbone (fishbone), platelet, polycrystalline, etc. are textural terms.
(c) **Crystallinity**: The term (now preferred to "nanotexture", that I formerly used) accounts for the average crystallite dimensions (the well-known $L_a$ and $L_c$, for graphenic carbons) on the one hand, and the short-range spatial relationship between crystallites (e.g., misorientations) on the other hand.

(d) **Structure**: The term accounts for the crystallographic characteristics of the crystallites (if any). The names of the various allotropes (turbostratic, graphite, fullerite...), and the various crystallographic structures (face centred cubic, hexagonal, amorphous...) are structural terms.

The four descriptors above address strictly-independent features, whatever the size scale of the carbon object being described. The protocol was primarily designed for describing graphenic carbons (it is not suitable for carbon molecular forms, which are described otherwise) but it also works for all kind of carbon materials, e.g., diamond-related. For non-graphenic carbon materials, not all the descriptors might be necessary (for instance, there is no texture to describe for a carbon with amorphous structure). Of course, these four descriptors are not enough to describe a carbon material completely. Other descriptors may be added at will in order to fulfil the specific requirements of a specific study (for instance, the sp²C/sp³C ratio) but that is what an ID card does: it tells the basics, such as your name, date of birth, possibly height and eye colours, but it does not tell your shoe size, yet an important feature to know in specific circumstances.

Carbon materials are the most complicated materials to describe because of their versatility regarding various multi-scale aspects. Understanding them requires that information is exchanged between carbon scientists. Laziness, simplicity, ignorance, whatever the reason, we should pay attention not to blur the information in this communication process. This is actually a general concern with science, as accurately expressed in a quote ("*With saying things wrong, comes the risk of thinking them wrong*") by Etienne Klein, who is a French scientist, philosopher... and a physicist.

[1] E. Fitzer, K.-H. Köchling, H. P. Boehm, H. Marsh, *Recommended terminology for the description of carbon as a solid*. **Pure Appl. Chem.** 67 (1995) 473-506.

[2] A. Bianco, H.-M. Cheng, T. Enoki, Y. Gogotsi, R. Hurt H., N. Koratkar, T. Kyotani, M. Monthioux, C.-R. Park, J. M. D. Tascon, J. Zhang, *All in the graphene family – A recommended nomenclature for two-dimensional carbon materials*. **Carbon** 65 (2013) 1-6.

[3] J. Berzelius, **Rapport annuel sur les progrès des sciences physiques et chimiques**, Académie Royale de Stockholm (M. Plantamour, translator), Fortin-Masson, Paris (France), 1841.

[4] W. E. Addison, **The allotropy of the elements**. Oldbourne Press, London, 1964.

[5] S. Iijima, *Helical microtubules of graphitic carbon*. **Nature** 354 (1991) 56-58.

[6] S. Iijima, T. Ishihashi, *Single-shell nanotube of 1-nm diameter*. **Nature** 363 (1993) 603-605.

[7] D. S. Bethune, C. H. Klang, M. S. de Vries, G. Gorman, R. Savoy, J. Vasquez, R. Beyers, *Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls*. **Nature** 363 (1993) 605-607.

[8] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, *Electric field effect in atomically thin carbon films*. **Science** 306 (2004) 666-669.

[9] M. Monthioux, *Describing carbons*. **Carbon Trends** 14 (2024) 100325.