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

View Article Online / Journal Homepage / Table of Contents for this issue Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM Chemical Science ChemSpider can help you! www.rsc.org/chemicalscience Volume | Number 12 | December 2012 | Pages 3333–3544 I need to know the structure of this compound We know that chemical naming is hard and that trivial names hide complex structures We want to make it easy for you to find this information wherever you are: z In the lab z At home z At a conference A simple and intuitive text search Once you’ve found a structure, save it in a format that can be opened in any chemical drawing program; use it again and again View the image in 3D And remember, ChemSpider gives you access to a database containing 28 million chemical structures and all of this information: FREE, for Anyone, Anytime, Anywhere ISSN 2041-6520 www.chemspider.com PERSPECTIVE Martin Pumera et al Impurities in graphenes and carbon nanotubes and their influence on the redox properties Registered Charity Number 207890 C Chemical Science Dynamic Article Links < Cite this: Chem Sci., 2012, 3, 3347 PERSPECTIVE Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM www.rsc.org/chemicalscience 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 Introduction Nanocarbon materials, such as carbon nanotubes and graphene, have attracted enormous amount of interest and research Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore E-mail: pumera@ntu.edu.sg Prof Martin Pumera has been a faculty member of Nanyang Technological University (Singapore) 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 ERCStG award He has broad interests in nanomaterials and microsystems, specifically in electrochemistry and synthetic Martin Pumera chemistry of carbon nanomaterials, in nanomotors, nanotoxicity 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 This journal is ª The Royal Society of Chemistry 2012 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 mechanical, 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 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 (Singapore) as Senior Research Fellow His research interests include the application of nanomaterials to electrochemical biosensors, synthesis and fundamental electrochemical studies of graphene-related materials, and synthetic nanomotors Chem Sci., 2012, 3, 3347–3355 | 3347 Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM nanowires,4 drug delivery vehicles,5 electrochemical sensing and energy storage devices6,7 or biology probes,8 just 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.7,9 In addition, these impurities are one of the main reasons for the toxicity of the carbon nanotubes.10,11 This perspective article describes the story of impurities in carbon nanotubes and graphenes and how they are related to their redox properties 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 to $100 nm with lengths typically of several mm.12 The ends of the CNTs can be opened or closed by polyhedral or semifullerene-like caps The CNTs can be single-walled or multiwalled, 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 understandable, as chemists were used to working with pure compounds and the eventual impurities 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 nanoparticles, where the gas dissociates resulting in carbon nanotubes Elaine Lay Khim Chng received her honours degree in Nanyang Technological University, Singapore, in 2010 and stayed to carry on her PhD studies in the Pumera group Elaine has broad interests in electrochemistry and nanotoxicology studies, particularly pertaining to carbon nanomaterials such as carbon nanotubes and graphene-related materials Elaine Lay Khim Chng 3348 | Chem Sci., 2012, 3, 3347–3355 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.13,14 The facet orientation of the nanoparticle determines the shape of nanostructure, which can be classical carbon nanotubes, bamboo-like nanotubes or carbon nanofiber.15 It 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.18 The 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 typically 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 original $30 to 50% wt in ‘‘as synthesized’’ CNTs to a range between 1–10% wt.18,20 These 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).20,22 In addition, 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 to $0.7% 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 impurities 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 This journal is ª The Royal Society of Chemistry 2012 Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM 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 ultrasonicationassisted microfiltration,16 two-step procedure based on sonication and ultracentrifugation,26 single sonication process,17 microwave-assisted digestion27 and gas-phase oxidations at high temperature.28 Again, 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 Fig 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 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 This journal is ª The Royal Society of Chemistry 2012 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 extraordinary 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 Fe3O4 impurities 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 reduction 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.32 Ni-based impurities within CNTs were also found to be responsible for low potential electrocatalytic oxidation of sulphides.33 It 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.14 In 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 H2O234, or all Co/Mo/Fe components can be responsible for oxidation of another compound, N2H4.35 It 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 Chem Sci., 2012, 3, 3347–3355 | 3349 Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM 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/mobilizable portion of Mo42 and Ni43 impurities 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 Fig Catalytic reduction of H2O2 on carbon nanotubes is due to ironbased metallic impurities present in CNTs Reprinted with permission from ref 29 properties of biologically important compounds It was shown that the redox behaviour of the regulatory peptide L-glutathione is affected by the presence of nickel oxide impurities within single-walled carbon nanotubes.37 L-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 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.40 Another 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 Fig Iron-based impurities induce DNA damage via redox mechanism Reprinted with permission from ref 38 3350 | Chem Sci., 2012, 3, 3347–3355 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 sp2 hybridized 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 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 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 This journal is ª The Royal Society of Chemistry 2012 Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM slow electron transfer kinetics.51 Nanographite impurities on the other hand expose a large number of highly electroactive edges on their surfaces52 and therefore, dominate the electrochemical properties of CNTs even at modest amounts of 5% wt (Fig 5, left panel).53 This problem was first noted by Compton in his pioneering 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,55 azo compounds,56 organic hydrazines,57 signal transducing amino acids,58 enzyme cofactors,58 carbamazepine,59 insulin, nitric oxide, and extracellular thiols.60 They were also found to be responsible for the previously excellent anti-fouling properties of CNTs, in the case of phenolic compounds.61 It 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 nanographitic 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 information.63 In a similar manner as nanographite impurities, another carbonaceous impurity, amorphous carbon, which is a form of carbon material containing both sp2 and sp3 hybridized C atoms and not having large crystalline order, can also dominate the electrochemical properties of CNTs at very low amounts.64 Graphene Graphene is defined by IUPAC as ‘‘A single carbon layer of graphite structure, describing its nature by analogy to a polycyclic aromatic hydrocarbon of quasi-infinite size.’’65 However, 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 graphene.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.67 This exfoliation/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 CO268 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).71,72 By tuning experimental conditions, single, few or multilayer graphene can be grown onto the metal catalyst even at large size.73 Different methods have also been proposed and optimized to transfer the grown graphene 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 application the graphene is intended Electrochemical applications 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 electrochemical behaviour of the graphene material as elaborated in the following paragraph 3.1 Fig 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]3À/4À 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-topeak separation indicate faster HET; and (B) resulting k0obs for MWCNTA 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 This journal is ª The Royal Society of Chemistry 2012 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 impurities, but it is unable to remove impurities which are ‘‘intercalated’’ 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 Chem Sci., 2012, 3, 3347–3355 | 3351 Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM 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 2500  C 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 electrochemical 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)75,76 can be considered 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 sulphides78 or hydrazines.79 We 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 (Staudenmaier or Hofmann method)80,81 or KMnO4 (Hummers method)82 yielding graphite oxide and consequently exfoliated/ reduced Exposing graphite oxide to thermal shock at $1000  C is one of the routes of preparing reduced graphene Interestingly, 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 of L-glutathione and sulphides.83 We 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 liquidphase reduction with hydrazine, to evaluate the metal content of the resulting graphene materials (Fig 6) In general the impurities 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 3352 | Chem Sci., 2012, 3, 3347–3355 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 (after treating graphene in H2 atmosphere 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 Cl2 treatment while those sheathed by graphene layers remained electrochemically inaccessible.84 We emphasize that detailed characterization of graphene materials evaluating the presence of impurities should always be carried out prior any electrochemical investigation to avoid misleading attribution of properties to the graphene employed It is highly probable that those works showing extraordinary catalytic properties of graphene towards the oxidation of sulphides and Fig 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 This journal is ª The Royal Society of Chemistry 2012 Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM Fig Imaging the metallic impurities in graphenes Electron microscopy images of (A) natural graphite, (B) chemically reduced graphene (NCRG) 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 mm (A, C and D) and 100 nm (B) 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/inhomogeneous 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.89 Another 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)90–92 and therefore interfere or hide the electrochemical signals generated by the analyte in question This is especially so as the reduction of these moieties is significantly intense at modest potentials ($ À0.7 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.93 It was also demonstrated that thermally reduced graphene shows properties that are very similar to amorphous carbon.94 This journal is ª The Royal Society of Chemistry 2012 Fig Metallic impurities in chemically reduced graphenes dominate their redox properties Cyclic voltammograms recorded in the presence of 10 mM cumene hydroperoxide (CHP) (a) Fe3O4 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 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 important 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 impurities 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 Chem Sci., 2012, 3, 3347–3355 | 3353 Published on 08 October 2012 Downloaded by RMIT University Library on 3/20/2021 2:00:12 PM Fig Inherent electrochemical activity of graphene oxide due to oxygen containing groups Cyclic voltammetric profiles obtained from electrochemical reduction of graphene oxide and chemically reduced (CR) graphene oxide in a 50 mM phosphate buffer solution, pH 7.4 The bare glassy carbon (GC) electrode (dashed black line) is also shown for comparison Reprinted with permission from ref 92 challenging task considering the fact that even ppm levels of impurities can alter the electrochemical properties of the materials, 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 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 Notes and references C N R Rao, A K Sood, K S Subrahmanyam and A Govindaraj, Angew Chem., Int Ed., 2009, 48, 7752 R H Baughman, A A Zakhidov and W A de Heer, Science, 2002, 5582, 787 A K Geim and K S Novoselov, Nat Mater., 2007, 6, 183 F Schwierz, Nat Nanotechnol., 2010, 5, 487 Z Liu, J T Robinson, X Sun and H Dai, J Am Chem Soc., 2008, 130, 10876 K Scida, P W Stege, G Haby, G A Messina and C D Garcıa, Anal Chim Acta, 2011, 691, M Pumera, Chem Soc Rev., 2010, 39, 4146 X 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2012, 3, 3347–3355 | 3355 ... were in fact the component that was responsible for their observed ‘‘exceptional’’ redox properties. 7,9 In addition, these impurities are one of the main reasons for the toxicity of the carbon nanotubes. 10,11... describes the story of impurities in carbon nanotubes and graphenes and how they are related to their redox properties Carbon nanotubes Carbon nanotubes (CNTs) are structures of sp2 hybridized carbons... 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