ChemInform abstract impurities in graphenes and carbon nanotubes and their influence on the redox properties

It was discovered later on that the impurities present inherently in the carbon nanomaterials were in fact the component that was responsible for their observed ‘‘exceptional’’ redox pro

Chemical Science

ISSN 2041-6520

PERSPECTIVE

Martin Pumera et al.

Impurities in graphenes and carbon nanotubes and their infl uence on the redox properties

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Impurities in graphenes and carbon nanotubes and their influence on the redox

properties

Martin Pumera,* Adriano Ambrosi and Elaine Lay Khim Chng

Received 29th August 2012, Accepted 16th September 2012

DOI: 10.1039/c2sc21374e

Carbon nanomaterials, such as carbon nanotubes and graphene-related materials are currently being

heavily researched and widely proposed for numerous applications It is often underestimated that

these carbon nanomaterials are of complex nature, consisting of different components and often

containing impurities These impurities can dramatically influence, or even dominate various properties

of carbon nanotubes and graphenes Herein, we will show that impurities in such carbon nanomaterials

are capable of exhibiting a striking effect on their redox properties The impurities being discussed

include metallic, nanographitic and amorphous carbon-based impurities commonly found in carbon

nanotube samples; and metallic, nanographitic, and carbonaceous debris-based impurities in

graphenes We emphasize that the effects brought about by these impurities on the properties of the

carbon nanomaterials can, in many cases be rather significant As such, one needs to be cautious by

clearly accounting for these effects observed for the nanomaterials before assigning any properties to

the material itself

1 Introduction

Nanocarbon materials, such as carbon nanotubes and graphene,

have attracted enormous amount of interest and research

activities in the past two decades.1,2 These 1-dimensional (1D) and two-dimensional (2D) carbon nanomaterials exhibit a wide spectra of exceptional properties, such as ballistic electron conductivity, large thermal conductivity, outstanding mechan-ical, redox and optical properties, which are attractive for materials science, physics and chemistry.1–3 Their utility have been demonstrated in a myriad of applications, i.e as conducting

Martin Pumera

Prof Martin Pumera has been a faculty member of Nanyang Technological University (Sin-gapore) since 2010 He moved

to NTU from his tenured senior scientist position at the National Institute for Materials Science, Japan (2006-2009) In 2009, Prof Pumera received ERC-StG award He has broad inter-ests in nanomaterials and microsystems, specifically in electrochemistry and synthetic chemistry of carbon materials, in nanomotors, nano-toxicity studies, and lab-on-chip systems He is associate editor of STAM, member of Editorial

board of Electrophoresis, Electroanalysis, The Chemical Records

and eight other journals He published over 170 peer-reviewed

articles and has h-index 36

Adriano Ambrosi

Dr Adriano Ambrosi received his PhD degree from Dublin City University, Ireland in 2007

As postdoctoral researcher he worked at ICN (Spain), and then, in 2009, at NIMS (Japan), before joining in 2010 the research group of Prof

Martin Pumera at Nanyang Technological University (Sin-gapore) as Senior Research Fellow His research interests include the application of nano-materials to electrochemical biosensors, synthesis and funda-mental electrochemical studies

of graphene-related materials, and synthetic nanomotors

Division of Chemistry & Biological Chemistry, School of Physical and

Mathematical Sciences, Nanyang Technological University, Singapore,

637371, Singapore E-mail: pumera@ntu.edu.sg

Cite this: Chem Sci., 2012, 3, 3347

www.rsc.org/chemicalscience PERSPECTIVE

nanowires,4drug delivery vehicles,5electrochemical sensing and

energy storage devices6,7or biology probes,8just to name a few

The redox properties of these materials were studied in detail

with great excitement; however, it turned out subsequently that

the majority of the early reported studies mistakenly linked the

redox activity to the carbon nanostructures It was discovered

later on that the impurities present inherently in the carbon

nanomaterials were in fact the component that was responsible

for their observed ‘‘exceptional’’ redox properties.7,9In addition,

these impurities are one of the main reasons for the toxicity of the

carbon nanotubes.10,11This perspective article describes the story

of impurities in carbon nanotubes and graphenes and how they

are related to their redox properties

2 Carbon nanotubes

Carbon nanotubes (CNTs) are structures of sp2 hybridized

carbons resembling seamlessly closed structure of ‘‘rolled up’’

graphene sheet, creating a tube of diameter ranging typically

from 1 to100 nm with lengths typically of several mm.12The

ends of the CNTs can be opened or closed by polyhedral or

semi-fullerene-like caps The CNTs can be single-walled or

multi-walled, depending on the number of concentric graphenic

cylinders forming the carbon nanotube CNTs pristine walls and

open ends/defect sites are the different components affecting

their redox properties The reasons for this will be discussed in

the following paragraphs

2.1 Source of impurities in carbon nanotubes

At the beginning of carbon nanotubes research conducted by

chemists, the word ‘‘impurities’’ was rarely heard It is

under-standable, as chemists were used to working with pure

compounds and the eventual impurities do not have typically

major influence upon the properties of the compounds However,

neither carbon nanotubes, nor graphene can be classified as

‘‘chemicals’’ They are materials and thus are much more

complex than simple compounds There are different sources of

origin of the impurities of carbon nanotubes and graphenes

Carbon nanotubes are typically fabricated by CVD based

growth on template nanoparticles The mechanisms consist of

dissolution of carbon containing gas (i.e CH4) in metal

nano-particles, where the gas dissociates resulting in carbon nanotubes

growing from the metallic nanoparticles These nanoparticles can consist of practically any metal, i.e Au, Ag, but due to economic reasons, the most common metal is Ni, Fe, Co, Mo or a mixture

of those.13,14The facet orientation of the nanoparticle determines the shape of nanostructure, which can be classical carbon nanotubes, bamboo-like nanotubes or carbon nanofiber.15It is possible to control and favour a prevailing form of carbon nanostructure over another by setting proper conditions, but it must be highlighted that it is impossible to completely avoid the growth of other carbon nanostructures, i.e carbon nanoonions

or nanographite.16 In addition, small amounts of amorphous carbon are present.17 When synthesis is finalized, the carbon nanotubes can contain more that 30% wt of residual metallic catalyst nanoparticles.18The residual metallic nanoparticles are present not only at the tips of the CNTs, but they are also encapsulated within the nanotubes or covered with carbon onion shells.19 These carbon shells hinder their removal as shown below It should also be noted that the metallic particles are not only present in their metallic forms but also as carbides and oxides; where the reactivity and solubility of these binary compounds is dramatically different from pure metal and typi-cally hinders the removal of the residual catalyst nanoparticles

To summarize, after their synthesis, carbon nanotubes can contain the following impurities: (a) metal-based nanoparticles;

(b) nanographite; (c) amorphous carbon It is extremely difficult

to remove these metallic and carbon-based impurities from carbon nanotubes.18

Metallic impurities can be partially removed by a wide variety

of methods The removal is only partial as the majority of the methods proposed are only capable of reducing the metal content from the original30 to 50% wt in ‘‘as synthesized’’ CNTs to a range between 1–10% wt.18,20These methods include oxidative acid treatment, either at room or elevated temperature, in order

to dissolve the metallic impurities Due to the solubility of the metals in strong acidic solutions, it was assumed that such acid

‘‘washing’’ would be able to remove all metallic impurities.21

However, the kinetics of dissolution of such metallic particles in most of the cases is very slow, which is in part also due to the fact that they are protected by graphene sheets (Fig 1).20,22In addi-tion, it is important to mention that the acid treatment damages the structure of carbon nanotubes and creates large amounts of the amorphous debris.23 Alternative techniques, such as the hydrothermal treatment proved to be able to decrease the concentration of metallic impurities down to0.7% wt, but with very low yields.24 Another method, which allows almost the complete removal of the metallic impurities from CNTs consists

of a high temperature (>2000 C)25 treatment in vacuum At these temperatures, the metallic nanoparticles evaporate, leaving behind pure CNTs with only ppm levels of metal content

However, at these temperatures, structural changes to the CNTs are always produced and should be considered in relation to the specific application they are employed

Carbonaceous impurities, that is nanographitic and amorphous carbon, are even more difficult to remove than metallic impuri-ties since they have a chemical composition similar to the CNTs

Carbonaceous impurities can be eliminated exploiting two main characteristics: (i) they are generally smaller in size than CNT and therefore filtration or centrifugation methodologies could be effective (ii) The presence of dangling bonds and structural

Elaine Lay Khim Chng

Elaine Lay Khim Chng received her honours degree in Nanyang Technological University, Sin-gapore, in 2010 and stayed to carry on her PhD studies in the Pumera group Elaine has broad interests in electrochemistry and nanotoxicology studies, particu-larly pertaining to carbon nano-materials such as carbon nanotubes and graphene-related materials

defects render the carbonaceous impurities more chemically active and can therefore be more easily oxidized Oxidative processes with an oxidizing gas (Cl2, O2, air, H2S etc.) or with a chemical mixture in solution (HNO3, H2O2, KMnO4, etc.) are quite effective in specifically removing these types of impurities

Examples of purification methodologies are ultrasonication-assisted microfiltration,16 two-step procedure based on sonica-tion and ultracentrifugasonica-tion,26 single sonication process,17

microwave-assisted digestion27and gas-phase oxidations at high temperature.28Again, structural damages to the tubes such as the opening of the tips or the cutting and shortening, are common side-effects, as well as the introduction of chemical functionalities

2.2 Influence of metallic impurities on redox properties of CNTs

There is a huge influence of metallic impurities on the redox properties of CNTs, with several cases where their effect is predominant even at very low concentrations This phenomenon might not be surprising on hindsight as it is well-known that metals, contrary to carbon, catalyze many redox reactions

However, during the ‘‘gold rush’’ on carbon nanotubes,9 the possibility that secondary impurities may affect the extraordi-nary properties of the CNTs, was never taken into consideration

Only on a later date, it was realized that the metallic impurities may not only just dominate the electrochemical properties of the CNTs, but also that can play a very active role in toxicological events

The influence of the residual metallic impurities in CNTs on their redox properties was first noticed by Compton et al., who demonstrated that residual Fe3O4impurities were responsible for the ‘‘electrocatalytic’’ properties of CNTs towards the oxidation

of hydrazine.21 The same group demonstrated that iron oxide impurities within the CNTs exhibit a similar effect on the reduction of hydrogen peroxide (Fig 2).29 This was a highly significant discovery as hydrogen peroxide is used in a wide variety of biosensing schemes Consequently, they demonstrated that copper based impurities are electrocatalytic for the reduc-tion of halothane30 and for the oxidation of glucose.31 It was shown later that metallic impurities are not responsible for reduction of only hydrogen peroxide, but all peroxide moiety containing compounds.32Ni-based impurities within CNTs were also found to be responsible for low potential electrocatalytic oxidation of sulphides.33It is of interest to note that very often, the composition of the metallic impurities in CNTs is not limited

to only one metal, but is often multicomponent, either due to the fact that metallic nanoparticles used for their synthesis are of low purity, or because the mixture of two (or more) metals is used on purpose to tune the growth properties.14In such cases, even trace amount of one metal (i.e Fe in Co/Mo/Fe nanoparticles) can be responsible for their redox activity, as in the example on the reduction of H2O2 34, or all Co/Mo/Fe components can be responsible for oxidation of another compound, N2H4.35It was shown that bi-component Ni/Fe residual catalyst impurities within single-walled carbon nanotubes are responsible for the electrochemical oxidation of arginine, where both Fe and Ni participate; and histidine, where only Fe participates.36 This brings us to the influence of metallic impurities upon the redox

Fig 1 Slow kinetics of dissolution of metallic impurities from CNTs.

Evidence of kinetic limitations in acid dissolution during purification.

TEM images of commercial samples of CNTs showing a range of

observed morphologies following 3 M HCl treatment for 48 h The most

commonly observed morphologies are (a) empty shells and (b) shells with

intact Ni-containing nanoparticles This implies the presence of pore-free

carbon shells that are fully protective of embedded catalysts and the

presence of some defective shells that allow sufficient fluid access to

remove Ni Also seen, however, as minority features in (c) carbon shells

with partially etched Ni nanoparticles, implying that the acid dissolution

process was still underway at the end of the treatment time These shells

probably contain more subtle defects that allow only the slow

transport of etching reactants and products Reprinted with permission

from ref 19.

properties of biologically important compounds It was shown

that the redox behaviour of the regulatory peptideL-glutathione

is affected by the presence of nickel oxide impurities within

single-walled carbon nanotubes.37L-glutathione is an important

antioxidant present in every cell of the human body Hurt and

Kane et al demonstrated that toxicologically significant

amounts of iron can be mobilized from CNTs.38 This iron is

redox active and induces single-strand breaks in plasmid DNA in

the presence of ascorbate (Fig 3).38 In a similar manner, Ni

based, redox active, impurities were shown to be bioavailable at

toxicologically significant concentrations despite their apparent

encapsulation by carbon walls.39 One can ask the question: is

there any concentration of metallic impurities where they do not

dominate the redox properties of CNTs? We have investigated

this issue and found that while at a concentration of 100 ppm, the

Fe-based impurities still affect the redox properties of CNTs; at a

level of 10 ppm they have no effect and the tubes can be

considered as electrochemically pure.40Another important issue

to be addressed is related to the post-processing of CNTs A

widely used practice employed in most of the laboratories around

the world consists of the ultrasonication of CNTs dispersed in

different solvents This is to create homogeneous dispersions

separating the nanotubes that are aggregated in bundles as much

as possible We investigated the possible effects of such process to

the activity of metallic nanoparticles and found out that even

short (5 min) ultrasonication times strongly increases the

bioavailability of the metallic impurities.41

Metallic impurities in CNTs have to be considered not only as

active catalysts for the conversion of an external compound, but

also as species with an inherent electrochemical activity This property proved to be extremely useful because it allowed the direct electrochemical quantification of the bioavailable/mobi-lizable portion of Mo42and Ni43impurities within CNT samples

in relation to their total content (Fig 4).44,45 The presence of metal impurities may also be exploited advantageously to the development of electrochemical systems

If properly characterized and quantified, they can be purportedly used as electrocatalytic sites for determination of important analytes This direction was pioneered by Anik and Cevik.46

2.3 Influence of carbonaceous impurities on redox properties of CNTs

Nanographitic impurities have also shown to exhibit strong influence on the electrochemical properties of CNTs This is due

to the fact that for most of the compounds, the basal planes of

sp2hybridized carbons exhibit very slow heterogeneous electron transfer (HET) constant, while the edge plane of sp2 carbon exhibits fast HET.47–50 Pristine CNTs, whose surface mostly consist of basal-plane walls with only few edge-plane sites localized at their tips and at some structure defects, show overall

Fig 2 Catalytic reduction of H 2 O 2 on carbon nanotubes is due to

iron-based metallic impurities present in CNTs Reprinted with permission

from ref 29.

Fig 3 Iron-based impurities induce DNA damage via redox mechanism.

Reprinted with permission from ref 38.

Fig 4 Inherent electrochemistry of Mo and Ni-based impurities within CNTs (A) Cyclic voltammograms of CNT samples containing Mo-based impurities and which present oxidative current signals at around 0 V due

to the oxidation of molybdenum (phosphate buffer, pH 7.4) (B) Repetitive cyclic voltammograms in NaOH 0.1 M solution of SWCNTs containing Ni impurities Typical redox behaviour of Ni hydroxide in alkaline solution is presented Reprinted with permission from ref 44 and

45, respectively.

slow electron transfer kinetics.51Nanographite impurities on the

other hand expose a large number of highly electroactive edges

on their surfaces52and therefore, dominate the electrochemical

properties of CNTs even at modest amounts of 5% wt (Fig 5, left

panel).53This problem was first noted by Compton in his

pio-neering work on carbon nanoonion impurities in CNTs (Fig 5,

right panel).54 Nanographitic impurities exhibit effect upon a

large amount of compounds, starting with ferro/ferricyanide,53

hydroquinone,55azo compounds,56organic hydrazines,57signal

transducing amino acids,58enzyme cofactors,58carbamazepine,59

insulin, nitric oxide, and extracellular thiols.60They were also

found to be responsible for the previously excellent anti-fouling

properties of CNTs, in the case of phenolic compounds.61It was

claimed that CNTs can be used for the so called ‘‘low-potential’’

detection of dopamine in the presence of ascorbic acid.62

However, it was shown later that when CNTs containing

nano-graphitic impurities are used for the electroanalysis of ascorbic

acid, they actually produce two peaks at different potentials –

one at a low potential and the second potential corresponding to

the oxidation of dopamine Therefore, nanographitic impurities

were actually responsible for this ‘‘low potential’’ redox activity

of ascorbic acid and in addition, these impurities actually lead

into the results which obscure the relevant analytical

informa-tion.63In a similar manner as nanographite impurities, another

carbonaceous impurity, amorphous carbon, which is a form of

carbon material containing both sp2and sp3hybridized C atoms

and not having large crystalline order, can also dominate the

electrochemical properties of CNTs at very low amounts.64

3 Graphene

Graphene is defined by IUPAC as ‘‘A single carbon layer of graphite structure, describing its nature by analogy to a poly-cyclic aromatic hydrocarbon of quasi-infinite size.’’65However,

in present days the word ‘‘graphene’’ is used more broadly;

graphene-related materials include, pristine graphene, graphene oxide and various graphenes with double, few and multilayer structures Mechanical exfoliation of graphite was the first method proposed that successfully isolated a single layer gra-phene.66 Despite the good quality of the graphene obtained resembling pristine graphene, this method is however only applicable for scientific investigations and is limited to a lab-scale use For large-scale production of graphene, two procedures are mostly being used: (i) using graphite as starting material, a chemical oxidative treatment generates graphite oxide which can

be subsequently exfoliated/reduced to graphene.67This exfolia-tion/reduction can be obtained in a single step by means of a thermal shock at high temperature which causes the expansion and the elimination of the oxygen-containing groups such as CO and CO2 68 with the simultaneous result of exfoliating and reducing the graphite oxide; or in two separated steps consisting

of a preliminary liquid-phase exfoliation of graphite oxide to graphene oxide by means of ultrasonication, followed by a chemical69 or electrochemical70 reduction to obtain reduced graphene (ii) Chemical vapour deposition (CVD) at high temperature of carbon atoms from a feedstock source over a catalyst surface (mostly Cu and Ni).71,72By tuning experimental conditions, single, few or multilayer graphene can be grown onto the metal catalyst even at large size.73Different methods have also been proposed and optimized to transfer the grown gra-phene onto arbitrary surfaces and which require in most of the cases the dissolution of the metal catalysts by chemical agents.74 These two synthetic routes generate graphene with very different physical, chemical, electronic and electrochemical properties which therefore should be considered accordingly to the appli-cation the graphene is intended Electrochemical appliappli-cations have benefited from the use of graphene produced by both methods but it is important to mention here that in all cases, the presence of impurities can play a significant role in the electro-chemical behaviour of the graphene material as elaborated in the following paragraph

3.1 Source of impurities in graphenes Graphene obtained by the top-down procedure which consists of the successive oxidation/exfoliation/reduction steps, requires graphite as starting material Graphite can be obtained from nature as mineral or it can be fabricated synthetically (from natural materials) by a thermal conversion Natural graphite is dug out of ground and transported for purification in a flotation plant where the graphite is separated from rock This purification process eliminates most of the weakly bonded superficial impu-rities, but it is unable to remove impurities which are ‘‘interca-lated’’ within the graphite grains or within graphene layers In addition, after this purification step, graphite is milled to obtain the desired grain sizes before the final packing Metallic mills represent another possible source of contamination due to the high pressures required to cut graphite particles Natural

Fig 5 Effect of presence of nanographite impurities in carbon nanotube

sample upon the electrochemical behavior of CNTs Left panel: effect

towards oxidation/reduction of 10 mM [Fe(CN) 6 ]3 /4 Shown is (A)

peak-to-peak separation (green bar for pure MWCNT-A; red bar for

MWCNT-A with nanographitic impurities (NG), and blue bar for

MWCNT-A with graphitic microparticles (GMP) Decreasing

peak-to-peak separation indicate faster HET; and (B) resulting k0obs for

MWCNT-A containing different amounts of nanographite and graphite impurities.

Right panel: HRTEM images of: (C) arc-MWCNTs showing their

closed-ended nature and lack of significant amounts of amorphous

carbon; (D) swiss-roll-like and closed shell giant nanoonions impurities in

CNT samples Reprinted with permission from ref 53 and 54,

respectively.

graphite has a typical purity of 80–98% with iron and nickel as

the common impurities generally found Synthetic graphite is

typically prepared from less ordered carbon materials, such as

coal tar and petroleum cokes These materials are heated to

2500C to convert amorphous materials to crystalline graphite

It should be noted that the crystallinity of synthetic graphite is

typically lower that the one of natural graphite Depending on

the starting material, synthetic graphite exhibits purity of 98.0–

99.9% wt These impurities are typically intercalated between the

graphene layers of graphite During the exfoliation/reduction

procedure for the fabrication of graphene, they persist and

remain linked to the graphene material altering its

electro-chemical properties as discussed in the following section

When considering carbonaceous impurities in graphenes, one

can consider few or multilayer graphene (multilayer graphene

equals to extra thin graphite or to graphite) which are products

of non-ideal exfoliation of graphite as being the impurities of

graphene Oxygen containing groups on incompletely reduced

graphene oxides during production of graphene (which are

produced by most of the reduction methods)75,76can be

consid-ered as another ‘‘point’’ impurity Amorphous carbon created

during the fabrication of graphene (i.e during digestion with

strong oxidants) should also be considered as an impurity of

graphene

3.2 Influence of metallic impurities on redox properties of

graphenes

Until very recently, metallic impurities in graphenes were not

considered as a problem as they were assumed to be absent.77

Therefore there have been many reports describing excellent

electrochemical properties of graphene electrodes towards the

redox reactions of many compounds, i.e sulphides78or

hydra-zines.79We investigated these properties in depth and discovered

that the ‘‘excellent’’ redox properties of the graphene materials

in some cases were due to the metallic impurities instead of the

carbon material As discussed above, graphene is typically

prepared from graphite undergoing oxidation in strong mineral

acids in the presence of strong oxidants, such as KClO3

(Stau-denmaier or Hofmann method)80,81 or KMnO4 (Hummers

method)82yielding graphite oxide and consequently exfoliated/

reduced Exposing graphite oxide to thermal shock at1000C

is one of the routes of preparing reduced graphene

Interest-ingly, it was shown that even after a prolonged treatment using

strong acids and powerful oxidizing agents and the consequent

high temperature exfoliation/reduction, the resulting graphene

material still contained residual metallic impurities at levels of

hundreds of ppm for Fe and Ni and tens of ppm for Co, Cu and

Mo These impurities were found to be responsible for the

electrocatalytic reduction of cumene hydroperoxide, and

oxidation ofL-glutathione and sulphides.83We have also tested

different graphite starting materials (natural and synthetic) and

other routes of preparation of graphene, via Staudenmaier or

Hummers oxidation, followed by the ultrasonication and

liquid-phase reduction with hydrazine, to evaluate the metal content of

the resulting graphene materials (Fig 6) In general the

impu-rities can originate from starting graphite (natural or synthetic)

or be introduced by chemicals used for its treatment (i.e acids

or hydrazine) It was found that metallic impurities are present

in chemically reduced graphenes prepared by this route and they originate from the starting graphite material (Fig 7) They exhibit a strong influence on the oxidation of hydrazine and the reduction of cumene hydroperoxide (Fig 8).84 An attempt to purify the graphene material from metallic impurities or decrease their effects on the electrochemical reactions was carried out by (a) soaking and refluxing in aqua regia (mixture

of a concentrated hydrochloric and nitric acid); (ii) sonication in

a mixture of hydrogen peroxide (H2O2) and HCl; and (iii) thermal treatment in a chlorine atmosphere While methods (i) and (ii) did not result in any improvement of the purity of the graphene, treatment (iii) in Cl2 atmosphere at 1000 C for 30 min (after treating graphene in H2atmosphere at 1000 C to reduce metal oxides to metals and facilitate their subsequent oxidation and removal as metal halogens using the halogen gas) resulted in a partial decrease of the metal content and more importantly, a significantly reduced influence of the impurities upon the graphene electrochemical properties This was likely due to the fact that the more accessible metallic impurities were removed by the Cl2treatment while those sheathed by graphene layers remained electrochemically inaccessible.84We emphasize that detailed characterization of graphene materials evaluating the presence of impurities should always be carried out prior any electrochemical investigation to avoid misleading attribu-tion of properties to the graphene employed It is highly prob-able that those works showing extraordinary catalytic properties of graphene towards the oxidation of sulphides and

Fig 6 Origin of impurities in graphenes Schematic for the preparation

of chemically reduced graphene Synthetic or natural graphite are preliminarily oxidized to graphite oxide (GO) using the modified Hummers or Staudenmaier methods Chemically reduced graphene (CRG) is obtained by the chemical reduction of GO using hydrazine.

Metallic impurities (Ni, Fe) present in graphite, still remain after the chemical treatments Reprinted with permission from ref 84.

hydrazine mistakenly attributed such effect to the carbon

material instead of the metallic impurities present in the sample

Similarly to the example of CNTs,46 the presence of metallic

impurities can be exploited to improve the electrocatalytic

performances of sensing devices and in this sense we proposed

an enhanced detection of cumene hydroperoxide by adopting

impure graphene sheets as electrocatalytic surface.85

3.3 Influence of carbonaceous impurities on redox properties of

graphene

Few- and multi-layer graphene (multilayer graphene is graphite)

can be considered as an impurity in the graphene samples These

graphite and nanographite impurities are residues either from the

uncompleted exfoliation of graphite or

improper/inhomoge-neous growth of CVD graphene However, it has been shown

that few and multilayer graphene exhibit almost interchangeable

redox properties towards many molecules, such as dopamine,

ascorbic acid, uric acid and nitroaromatic explosives86–88

although some differences were observed for the oxidation of

DNA bases.89Another redox active behaviour which cannot be

observed from pure graphite or graphene materials originates

from the presence of oxygen containing groups on the graphene

sheets Some of these oxygen moieties are inherently electroactive

(peroxy, epoxy and aldehyde moieties), can be oxidized or

reduced (Fig 9)90–92and therefore interfere or hide the

electro-chemical signals generated by the analyte in question This is

especially so as the reduction of these moieties is significantly

intense at modest potentials ( 0.7 V vs Ag/AgCl) limiting the

working potential range.91 It was shown that carbonaceous

debris that resided in graphene oxide/reduced graphene oxide can

profoundly affect their electrochemical behaviours.93It was also

demonstrated that thermally reduced graphene shows properties

that are very similar to amorphous carbon.94

4 Summary and outlook

Carbon nanotubes and graphenes are complex materials which contain small amounts of undesired components, originating from the impurities of the raw materials or introduced during the fabrication processes These components/impurities, often significantly affect the properties of both carbon materials Here

we discussed scientific contributions highlighting the influences

of different types of impurities on the redox properties of carbon nanotubes and graphenes It should be pointed out that the presence of such impurities can also affect several other impor-tant properties of both CNT and graphene materials In fact altered electronic, mechanical and physical properties of these fascinating materials have also been discovered and attributed to the presence of impurities which therefore no longer can be ignored during scientific investigations With regards to the electrochemical properties, the extremely active metallic impu-rities as potential reaction catalysts can play a significant role in the redox properties of both graphenes and carbon nanotubes as shown in some cases where their effect is completely dominant

Finding an effective purification method represents an extremely

Fig 7 Imaging the metallic impurities in graphenes Electron

micros-copy images of (A) natural graphite, (B) chemically reduced graphene

(N-CRG) produced from natural graphite, (C) synthetic graphite and (D)

chemically reduced graphene (S-CRG) produced from synthetic graphite.

The dark spot in (A) indicated by the arrow represents an Fe-based

metallic impurity Scale bars represent 1 mm (A, C and D) and 100 nm

(B) Reprinted with permission from ref 84.

Fig 8 Metallic impurities in chemically reduced graphenes dominate their redox properties Cyclic voltammograms recorded in the presence of

10 mM cumene hydroperoxide (CHP) (a) Fe 3 O 4 NPs-modified GC electrode and EPPG electrode; (b) electrode modified with chemically reduced graphene obtained from natural (N-CRG) and synthetic (S-CRG) graphite Reprinted with permission from ref 84.

challenging task considering the fact that even ppm levels of

impurities can alter the electrochemical properties of the

mate-rials, and lowering those levels is almost impossible without

substantial damages to the structure materials Therefore, future

efforts should focus not only on improving the purification

techniques commonly available but also on finding procedures

able to make the impurities electrochemically inactive so that

they do not alter the electrochemical behaviour of CNTs and

graphenes even though they are still present We stress again the

importance of a comprehensive characterization of the carbon

materials, evaluating not only their physical and chemical

features, but also their real composition opening to the possible

presence of extraneous components that may have a significant

role in the investigated properties

Acknowledgements

M.P thanks to MINDEF-NTU #JPP-11-02-06 grant for

support

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