CARBON ( 0 ) 0 –2 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon Purification of carbon nanotubes Peng-Xiang Hou, Chang Liu, Hui-Ming Cheng* Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China A R T I C L E I N F O A B S T R A C T Article history: It is predicted theoretically and understood experimentally that carbon nanotubes (CNTs) Received 27 June 2008 possess excellent physical and chemical properties and have wide-range potential applica- Accepted September 2008 tions However, only some of these properties and applications have been verified or real- Available online September 2008 ized To a great extent, this situation can be ascribed to the difficulties in getting highpurity CNTs Because as-prepared CNTs are usually accompanied by carbonaceous or metallic impurities, purification is an essential issue to be addressed Considerable progress in the purification of CNTs has been made and a number of purification methods including chemical oxidation, physical separation, and combinations of chemical and physical techniques have been developed for obtaining CNTs with desired purity Here we present an up-to-date overview on the purification of CNTs with focus on the principles, the advantages and limitations of different processes The effects of purification on the structure of CNTs are discussed, and finally the main challenges and developing trends on this subject are considered This review aims to provide guidance and to stimulate innovative thoughts on the purification of CNTs Ó 2008 Elsevier Ltd All rights reserved Contents Introduction 1.1 CNT synthesis techniques 1.2 Impurities coexisting with CNTs 1.3 Assessment of CNT purity 1.4 Purpose of this review Purification methods 2.1 Chemical oxidation 2.1.1 Gas phase oxidation 2.1.2 Liquid phase oxidation 2.1.3 Electrochemical oxidation 2.1.4 Brief summary 2.2 Physical-based purification 2.2.1 Filtration 2.2.2 Centrifugation 2.2.3 Solubilization of CNTs with functional groups 2.2.4 High temperature annealing * Corresponding author: Fax: +86 24 2390 3126 E-mail address: cheng@imr.ac.cn (H.-M Cheng) 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd All rights reserved doi:10.1016/j.carbon.2008.09.009 2004 2004 2004 2005 2006 2007 2007 2007 2009 2011 2011 2011 2012 2013 2013 2013 2004 CARBON ( 0 ) 0 –2 2.2.5 Other physical techniques 2.2.6 Combination of purification and separation 2.2.7 Brief summary 2.3 Multi-step purification 2.3.1 HIDE-assisted multi-step purification 2.3.2 Microfiltration in combination with oxidation 2.3.3 Sonication in combination with oxidation 2.3.4 High temperature annealing in combination with extraction 2.3.5 Brief summary 2.4 Applicability of typical purification techniques Challenges 3.1 Synthesis methods 3.2 Purification methods 3.3 Purity assessment Concluding remarks Acknowledgements References Introduction Elemental carbon in sp2 hybridization can form a variety of amazing structures, such as graphite (3D), graphene (2D), carbon nanotubes (CNTs, 1D) and fullerene (0D) CNTs defined by Iijima in 1991 [1] have a unique tubular structure with nanometer scale diameters and large length/diameter ratios CNTs may consist of one (single-walled CNTs, SWCNTs) or up to tens and hundreds (multi-walled CNTs, MWCNTs) seamless graphene cylinders concentrically stacked with an adjacent layer spacing of $0.34 nm Owing to the covalent sp2 bonds formed between individual carbon atoms, CNTs are stiffer and stronger potentially than any other known materials Thus, CNTs have ultra-high Young’s modulus and tensile strength, which makes them promising in serving as a reinforcement of composite materials with desired mechanical properties Because of the symmetry and unique electronic structure of graphene, the structure of a SWCNT determines its electrical properties For a SWCNT with a given (n, m) index [2], when (2n + m) = 3q (q is an integer), the nanotube is metallic, otherwise the nanotube is a semiconductor Not only these nanotubes show amazing mechanical and electronic properties, but also possess well-defined hollow interiors and biocompatibility with living systems As a result, CNTs are considered to be excellent candidates for many potential applications, including but not limited to: catalyst and catalyst supports [3,4], composite materials [5,6], sensors and actuators [7,8], field emitters [9,10], tips for scanning probe microscopy [11,12], conductive films [13,14], bio-nanomaterials [15], energy storage media [16,17] and nanoelectronic devices [18,19] 1.1 2014 2014 2015 2016 2016 2016 2017 2018 2018 2018 2020 2020 2021 2021 2021 2021 2021 benzene, ethanol, acetylene, propylene, methane, ethylene, CO, etc.) and growth of CNTs over the catalyst (usually transition metals such as Ni, Fe, Co, etc.) in a temperature range of 300–1200 °C Good alignment [22] as well as positional control on a nanometric scale [23] can be achieved by using CVD Control over diameter, shell number, and growth rate of CNTs are also realized with this method The chief drawback of CVD is the high defect density of the obtained CNTs owing to low synthesis temperatures, compared with arc discharge and laser ablation As a result, the tensile strength of the CNTs synthesized by CVD is only one-tenth of those made by arc discharge [24] Typical SWCNT content in as-prepared samples by CVD is $30–50 wt%, while the content of MWCNTs is in the range of $30–99 wt% depending on their diameters The by-products are usually aromatic carbon, amorphous carbon, polyhedral carbon, metal particles, etc Arc discharge uses two electrodes (at least one electrode is made of graphite) through which a direct current (DC) is passed in a gaseous atmosphere MWCNTs can be obtained by arc discharge without any metal catalyst, while mixed metal catalysts inserted into the anode are required when synthesizing SWCNTs by this method In laser ablation for producing CNTs, an intense laser beam is used to ablate/ vaporize a target consisting of a mixture of graphite and metal catalyst in a flow of inert gas This method favors the growth of SWCNTs with controlled diameter depending on reaction temperature [24] When using arc discharge and laser ablation for SWCNT synthesis, side products such as fullerenes, amorphous carbon, graphite particles, and graphitic polyhedrons with enclosed metal particles are also formed The record high-purity of the SWCNTs synthesized by arc discharge has been reported to be 80% by volume [25] CNT synthesis techniques 1.2 Nowadays, CNTs can be produced in large quantities by three dominant techniques: chemical vapor deposition (CVD, including high-pressure carbon monoxide (HiPco) process) [20], arc discharge [1], and laser ablation [21] CVD involves catalyst-assisted decomposition of hydrocarbons (commonly Impurities coexisting with CNTs As-synthesized CNTs prepared by the above methods inevitably contain carbonaceous impurities and metal catalyst particles, and the amount of the impurities commonly increases with the decrease of CNT diameter Carbonaceous impurities CARBON ( 20 ) 0 3–20 typically include amorphous carbon, fullerenes, and carbon nanoparticles (CNPs) (as shown in Fig 1) Because the carbon source in arc discharge and laser ablation comes from the vaporization of graphite rods, some un-vaporized graphitic particles that have fallen from the graphite rods often exist as impurity in the final product In addition, graphitic polyhedrons with enclosed metal particles also coexist with CNTs synthesized by arc discharge and laser ablation as well as high temperature (>1000 °C) CVD Fullerenes can be easily removed owing to their solubility in certain organic solvents Amorphous carbon is also relatively easy to eliminate because of its high density of defects, which allow it to be oxidized under gentle conditions The most knotty problem is how to remove polyhedral carbons and graphitic particles that have a similar oxidation rate to CNTs, especially SWCNTs Metal impurities are usually residues from the transition metal catalysts These metal particles are sometimes encapsulated by carbon layers (varying from disordered carbon layers to graphitic shells, as shown in Fig 1b and c) making them impervious and unable to dissolve in acids Another problem that needs to be overcome is that carbonaceous and metal impurities have very wide particle size distributions and different amounts of defects or curvature depending on synthesis conditions, which makes it rather difficult to develop a unified purification method to obtain reproducibly high-purity CNT materials To fulfill the vast potential applications and to investigate the fundamental physical and chemical properties of CNTs, highly efficient purification of the as-prepared CNTs is, therefore, very important 1.3 Assessment of CNT purity To evaluate the purity of CNTs, the efficiency of a purification method as well as changes in the structure of CNTs during purification, characterization methods with rapid, convenient and unambiguous features are urgently required Characterization of CNT samples falls into three groups: metal catalyst, carbonaceous impurity, and CNT structure variation (defects, functional groups, cap opening, cutting, etc.) Their characterization mainly depends on electron microscopy (EM, including scanning EM (SEM), and transmission EM (TEM)), thermogravimetric analysis (TGA), Raman spectroscopy and ultravioletvisible-near infrared (UV–vis-NIR) spectroscopy EM is a useful technique allowing for direct observations of impurities, local structures as well as CNT defects Owing to the small volume of sample analyzed and the absence of algorithms to convert images into numerical data, EM cannot give a quantitative evaluation of the purity of CNTs [28] 2005 TGA is effective in evaluating quantitatively the quality of CNTs, in particular, the content of metal impurity It is easy and straightforward to obtain the metal impurity content using TGA by simply burning CNT samples in air A higher oxidation temperature (>500 °C) is always associated with purer, less defective CNT samples The homogeneity of CNT samples can be evaluated by standard deviations of the oxidation temperature and metal content obtained in several separate TGA runs [29] The real difficulty is qualitative or quantitative assessment of carbonaceous impurity, which is influenced by the amount of defects, forms of carbon, and so on Raman spectroscopy is a fast, convenient and non-destructive analysis technique To some extent, it can quantify the relative fraction of impurities in the measured CNT sample using the area ratio of D/G bands under fixed laser power density In addition, the diameters and electronic structures of CNTs can be determined by using the resonance Raman scattering [30] However, the drawback of Raman spectroscopy is that it cannot provide direct information on the nature of metal impurities, and it is not as effective in studying CNT samples with a low content of amorphous carbon [31] UV–vis-NIR spectroscopy is a rapid and convenient technique to estimate the relative purity of bulk SWCNTs based on the integrated intensity of S22 transitions compared with that of a reference SWCNT sample [28] It is convenient to determine the concentration of SWCNTs dispersed in solution once the extinction coefficient of SWCNTs is known [32] On the other hand, SWCNTs give rise to a series of predictable electronic band transitions between van Hove singularities in the density states of nanotubes (S11, S22, and M11), therefore this technique is also used to analyze SWCNT types, i.e., metallic or semiconducting [31,33,34], according to their electronic structure For small diameter SWCNTs individually dispersed in solution with the assistance of surfactants or DNA molecules, the (n, m) index assignment is also possible from UV–vis-NIR spectroscopy [33,34] The drawback of this method is the difficulty in repeatedly preparing the standardized SWCNT film or solution and controlling film thickness or solution concentration, making it difficult for quantification analysis Furthermore, it is not yet possible to provide an absolute value of the purity of SWCNTs because there is no 100% pure standard SWCNT sample or accurate extinction coefficient for SWCNTs Besides the above most commonly used techniques, X-ray photoelectron spectroscopy (XPS) is often used to characterize functional groups on the walls of CNTs, and energy dispersive spectroscopy (EDS) is also used to semi-quantitatively identify the metal content in CNT samples, especially for Fig – TEM images of (a) amorphous carbon and fullerene molecules on the surface of CNTs [26]; (b) metal nanoparticles covered by amorphous carbon layer, (c) metal nanoparticles covered by graphitic carbon multi-layer (reproduced with permission from [27], Copyright 2004 Amercian Chemical Society) 2006 CARBON ( 0 ) 0 –2 trace amounts The major purity and quality assessment techniques and their efficiency are summarized in Table It seems that no assessment technique mentioned above can give a precise and comprehensive quantification of CNTs (Table 1) Consequently, there is a need to develop an integrated method by which the type, amount, and morphology of CNT-containing materials can be accurately and precisely quantified [35] Alternatively, a combination of different assessment techniques may be a good choice to give a full understanding of CNTs but this takes more time Furthermore, a precise definition of purity should be established because ‘‘purity’’ can be different from different points of view, such as CNT content, structure integrity, and SWCNT content From this respect, we define the purity of CNTs as given in Table Meanwhile, the major purity assessment techniques and how to evaluate them are also briefly included 1.4 Purpose of this review As mentioned above, a series of problems involving the presence of impurities in CNTs, the non-uniformity in morphology and structure of both CNTs and impurities, as well as the absence of precise characterization methods limit the applications of CNTs Thus great attention has been paid to the issue of purification The developed purification schemes usually take advantage of differences in the aspect ratio and oxidation rate between CNTs and carbonaceous impurities In most cases, CNT purifications involve one or more of the following steps: gas phase oxidation, wet chemical oxidation/treatment, centrifugation, filtration, and chromatography, etc However, a reproducible and reliable purification protocol with high selectivity, especially for SWCNTs, is still a great challenge, because the purity of CNTs depends on not only purification itself, but also many other factors, including CNT type (SWCNTs or MWCNTs), morphology and structure (defects, whether or not they exist in bundles, diameter), impurity type and their morphology (particle size, defect, curvature, the number and crystallinity of carbon layers wrapping metal particles), purity assessment technique, and so on This article attempts to give a comprehensive survey and analysis of the purification of CNTs The challenges existing in the purification methods, synthesis techniques and purity assessments, which have to be overcome in order to enable the wide applications of CNTs, will be discussed The purity in this article generally is referred to as CNT content in the Table – Summary of commonly used techniques for detecting the impurities in CNT samples Technique C-Ia M-Ib F-Gc S-Dd C-Fe EM TGA Raman UV–vis-NIR XPS EDS a b c d e f g h Df J D J   g    J  D J    J Jh  D      J J   Advantages Limitations Direct observation Precise content of carbon and metals Diameter, quality and conductivity of SWCNTs Conductivity feature and content of SWCNTs Accurate assessment of F-G on CNTs Elemental contents, special for trace amounts A small amount of sample is analyzed CNTs analyzed are completely destroyed Invalid for MWCNTs and metal impurities Need 100% pure SWCNTs as standard Invalid to purity assessment Invalid to evaluate CNT content Carbonaceous impurity Metal impurity Functional groups Structure defects Conductivity feature Qualitatively valid Invalid Valid Table – Definition of purity for CNTs from different points of view and the corresponding assessment techniques Purity definition Assessment technique and methods CNT content The content of CNTs in sample containing CNTs, carbonaceous and metallic impurities Structure integrity Pure CNTs without large defects and faults, and no functional groups, amorphous carbon or fullerene adhered on the tube wall SWCNT content The content of SWCNTs in CNTs TGA Metal content can be calculated from the ash weight after complete oxidation, and carbonaceous impurity content can be calculated by corresponding peak area ratio from DTG curve CNTs without any other carbonaceous impurity are characterized by one DTG peak EM in combination with XPS EM can directly observe and qualitatively assess the amount of defects, amorphous carbons, fullerenes adhered on the wall of CNTs XPS can give a quantitative characterization of type and content of functional groups Raman spectroscopy 100% pure SWCNTs should be characterized by one G band with RBM and without D band CARBON ( 20 ) 0 3–20 as-prepared or purified samples, and the yield means the weight ratio of purified CNTs to that of the as-prepared CNT sample, unless specified otherwise Purification methods Purification methods of CNTs can be basically classified into three categories, namely chemical, physical, and a combination of both The chemical method purifies CNTs based on the idea of selective oxidation, wherein carbonaceous impurities are oxidized at a faster rate than CNTs, and the dissolution of metallic impurities by acids This method can effectively remove amorphous carbon and metal particles except for those encaged in polyhedral graphitic particles However, the chemical method always influences the structure of CNTs due to the oxidation involved The physical method separates CNTs from impurities based on the differences in their physical size, aspect ratio, gravity, and magnetic properties, etc In general, the physical method is used to remove graphitic sheets, carbon nanospheres (CNSs), aggregates or separate CNTs with different diameter/length ratios In principle, this method does not require oxidation, and therefore prevents CNTs from severe damage However, the physical method is always complicated, time-consuming and less effective The third kind of purification combines the merits of physical and chemical purification, and we denominate it as multi-step purification in this article This method can lead to high yield and high-quality CNT products Owing to the diversity of the as-prepared CNT samples, such as CNT type, CNT morphology and structure, as well as impurity type and morphology, it needs a skillful combination of different purification techniques to obtain CNTs with desired purity 2.1 Chemical oxidation The carbonaceous impurities co-existing with as-synthesized CNTs are mainly amorphous carbon and CNPs Compared with CNTs, these impurities usually have higher oxidation activity The high oxidative activity demonstrated by amorphous carbon is due to the presence of more dangling bonds and structural defects which tend to be easily oxidized; meanwhile the high reactivity of the CNPs can be attributed to their large curvature and pentagonal carbon rings [36,37] Therefore, chemical oxidation purification is based on the idea of selective oxidation etching, wherein carbonaceous impurities are oxidized at a faster rate than CNTs In general, chemical oxidation includes gas phase oxidation (using air, O2, Cl2, H2O, etc.), liquid phase oxidation (acid treatment and refluxing, etc.), and electrochemical oxidation The disadvantages of this method are that it often opens the end of CNTs, cuts CNTs, damages surface structure and introduces oxygenated functional groups (–OH, –C@O, and –COOH) on CNTs As a result, the purified CNTs in turn can serve as chemical reactors or a starting point for subsequent nanotube surface chemistry [38,39] 2.1.1 Gas phase oxidation In gas phase oxidative purification, CNTs are purified by oxidizing carbonaceous impurities at a temperature ranging from 225 °C to 760 °C under an oxidizing atmosphere The 2007 commonly used oxidants for gas phase oxidation include air [40–46], a mixture of Cl2, H2O, and HCl [47], a mixture of Ar, O2, and H2O [48–50], a mixture of O2, SF6 and C2H2F4 [51], H2S and O2 [52], and steam [53] High temperature oxidation in air is found to be an extremely simple and successful strategy for purifying arc discharge derived MWCNTs, which are metal free and have fewer defects on tube walls Ebbesen et al [40,41] first reported a gas phase purification to open and purify MWCNTs by oxidizing the as-prepared sample in air at 750 °C for 30 However, only a limited amount of pure MWCNTs (1–2 wt%) remained after the above purification, which can be ascribed mainly to two reasons One is uneven exposure of CNTs to air during oxidation, and the other is the limited oxidation selectivity between CNTs and carbonaceous impurities Therefore, two routes may be helpful to increase the purification yield using this simple air oxidation One is to ensure that the as-synthesized CNT samples are evenly exposed to air, and the other is to enhance the difference in oxidation resistance to air between CNTs and carbonaceous particles The above suggestions have been verified by some researchers As an example, Park and coworkers [42] increased the purification yield to $35 wt% by rotating the quartz tube in which the sample was placed, in order to evenly expose the CNTs and carbonaceous impurities to air at 760 °C for 40 To increase the difference in oxidation resistance to air between MWCNTs and carbon impurities, the difference in oxidation rates of graphite and intercalated graphite [43–45] was taken into account Graphite intercalation compounds are formed by the insertion of atomic or molecular layers of other chemical species between graphite layers This interaction causes an expansion of carbon interlayer spacing, which reduces the oxidation resistance of the intercalated graphite Carbonaceous impurities have higher structural defect densities than CNTs, and are therefore more ready to act as reaction sites for intercalated atoms Thus the oxidation resistance difference between CNTs and carbonaceous impurities can be increased As an example, Chen et al [43] reported a combined purification process consisting of bromination and subsequent selective oxidation with oxygen at 530 °C for days Temperature programmed oxidation profiles of the CNT samples with and without bromine treatment are shown in Fig It is obvious that oxidation of the brominated sample occurs more readily than that without bromination TEM studies showed that CNTs with both ends open were enriched in the purified sample, and the yield obtained by the above process varied from 10 to 20 wt% with respect to the weight of the original carbon sample Furthermore, they found that the yield depended crucially on the flow rate of oxidant, the amount of initial sample, the manner of packing of the carbon, and the quality of the cathodic soot Although MWCNTs can be purified by a variety of gas phase oxidation [41–45], attempts to use similar procedures for SWCNTs result in nanotubes etching away For example, using the bromine and oxygen system, the yield was $3 wt% [47] for SWCNT purification, which implies that a large fraction of SWCNTs are consumed in the process This large difference between MWCNTs and SWCNTs results from two factors One is the larger amount of curvature experienced by the graphene sheet of SWCNTs, and the other is 2008 CARBON ( 0 ) 0 –2 Fig – Temperature programmed oxidation profiles of the cathodic soot before (CS) and after (BS) bromination (reprinted with permission from [43], Copyright 1996 Wiley– VCH Verlag GmbH & Co KGaA) metal impurities catalyzing the low-temperature oxidation of carbon There may therefore be two ways to increase the purification yield of SWCNTs using gas phase oxidation One is to select oxidants that can selectively oxidize carbonaceous impurities by a unique selective carbon surface chemistry while leaving SWCNTs intact The other is to remove metal particles before gas phase oxidation Some positive results have been obtained following the above suggestions Zimmerman et al [47] first reported suitable conditions allowing for the removal of amorphous or spherical carbon particles, with or without metal catalyst inside, while simultaneously protecting SWCNTs The purification incorporates a chlorine, water, and hydrogen chloride gas mixture to remove the impurities A SWCNT yield of $15 wt% and a purity of $90% indicate that the carbonaceous impurities are preferentially removed Based on their experimental observation, hydrogen chloride was required for selective removal of the unwanted carbon They proposed a mechanism for the purification Chlorine gas mixture interacted with the nanotube cap and formed a hydroxy-chloride-functionalized nanotube cap Hydrogen chloride in the gas phase purification mixture protected the caps that are more reactive, by preventing hydroxyl groups from deprotonating The disadvantage of this method is that only small quantities ($5 mg) of SWCNTs were purified each time Furthermore, the reagents and produced gases are toxic and explosive, which limits its practical use At the same time, some other oxidants that can selectively oxidize carbonaceous impurities were also reported For example, hydrogen sulfide was reported to play a role in enhancing the removal of carbon particles as well as controlling the oxidation rate of carbon A purity of $95% SWCNTs with a yield of 20–50 wt% depending on the purity of raw material was reported [52] In addition, steam at atm pressure [53], local microwave heating in air [46], air oxidation and acid washing followed by hydrogen treatment [54] were also reported to work well to improve the purification yield It was Chiang et al [48,49] who clearly elucidate the role of metals in oxidizing carbons and the need for their prior removal They found that metal particles catalyze the oxidation of carbons indiscriminately, destroying SWCNTs in the presence of oxygen and other oxidizing gases Encapsulated metal particles can be exposed using wet Ar/O2 (or wet air) oxidation at 225 °C for 18 h This exposure was attributed to the expan- sion of the particles because oxidation products have a much lower density (the densities of Fe and Fe2O3 are 7.86 and 5.18 g/cm3, respectively) Such significant expansion broke the carbon shells, and the particles were exposed as a result Based on the above results, they proposed a multi-stage procedure for purifying SWCNTs synthesized by the HiPco process Their method begins with cracking of the carbonaceous shells encapsulating metal particles using wet oxygen (20% O2 in argon passed through a water-filled bubbler) at 225 °C, followed by stirring in concentrated hydrochloric acid (HCl) to dissolve the iron particles After filtering and drying, the oxidation and acid extraction cycle was repeated once more at 325 °C, followed by an oxidative baking at 425 °C Finally, 99.9% pure SWCNTs (with respect to metal content) with a yield of $30 wt% were obtained The validity of this method was verified by another group [50] However, owing to the complicated purification steps, it is hard to purify SWCNTs in a large scale Xu et al [51] developed a controlled and scalable multistep method to remove metal catalyst and non-nanotube carbons from raw HiPco SWCNTs Their scalable multi-step purification included two processes: oxidation and deactivation of metal oxides In the oxidation, metal catalysts coated by nonnanotube carbon were oxidized into oxides by O2 and exposed by using a multi-step temperature increase program In the deactivation step, the exposed metal oxides were deactivated by conversion to metal fluorides through reacting with C2H2F4, SF6, or other fluorine-containing gases to avoid the catalytic effect of iron oxide on SWCNT oxidation The Fe content was remarkably decreased from $30 to $1 wt% and a yield of $25–48 wt% was achieved However, the shortcoming of this method is that it is limited to HiPco SWCNTs, in which the dominant impurity is metal catalyst Furthermore, the toxicity of the reagents used in this method and the resulting gases are undesirable features Gas phase oxidation is a simple method for removing carbonaceous impurities and opening the caps of CNTs without vigorously introducing sidewall defects, although it cannot directly get rid of metal catalyst and large graphite particles Thus it is a good choice to purify arc discharge derived MWCNTs, which contains no metal catalyst For purifying SWCNTs or MWCNTs (synthesized by other techniques), acid treatment to remove the metal catalyst is always necessary Another point worth noting is that CNTs (SWCNTs in particular) in agglomerates prevent oxidant gas from homogeneously contacting the whole sample In order to obtain high-purity CNTs, the amount of sample to be purified each time is quite limited (tens to a hundred milligrams) Therefore, methods that can cause the oxidant gas to homogeneously contact CNT samples are urgently required to obtain high-purity CNTs on a large scale Recently, Tan et al [55] mixed raw SWCNTs with zirconia beads to enhance air flow uniformity and increase the exposed surface of raw soot during thermal oxidation in air The final purified samples had a yield of $26 wt% and a metal impurity of $7% Although the purity is not very high, the technique suggests a way to purify SWCNTs on a large scale using gas phase oxidation This method can provide pure and opened CNTs without heavily damaging tube walls, which is a good choice for the application of open-ended CNTs as nano-size reaction tubes or chemical reactors [56,57] For achieving purified CNTs on a large scale, gas phase oxidation CARBON ( 20 ) 0 3–20 2009 need to be modified in the following ways: one is to look for a simple approach and non-toxic reagents to remove metal particles encapsulated by carbon layers; the other is to look for a way that can make oxidant gas homogeneously contact the as-prepared CNTs In addition, the gas phase oxidation can be combined with other techniques, such as filtration or centrifugation, to further enhance the purification efficiency 2.1.2 Liquid phase oxidation Although the merits of gas phase oxidation are obvious, it has a drawback that metal particles cannot be directly removed, and further acid treatment is needed In order to overcome this limitation, liquid phase purification that always simultaneously removes both amorphous carbon and metal catalyst was developed Oxidative ions and acid ions dissolved in solution can evenly attack the network of raw samples, and therefore selection of oxidant type and precise control of treatment condition can produce high-purity CNTs in a high yield The commonly used oxidants for liquid phase oxidation include HNO3 [58–60], H2O2 or a mixture of H2O2 and HCl [61–63], a mixture of H2SO4, HNO3, KMnO4 and NaOH [64–67], and KMnO4 [67–69] The shortcomings of this method are that it causes reaction products on the surface of CNTs, adds functional groups, and destroys CNT structures (including cutting and opening CNTs) Nitric acid is the most commonly used reagent for SWCNT purification for its mild oxidation ability, which can selectively remove amorphous carbon In addition, it is inexpensive and nontoxic, capable of removing metal catalysts and no secondary impurities are introduced Dujardin et al [58] reported a one-step method using concentric nitric acid to purify SWCNTs synthesized by laser ablation Briefly, as-synthesized SWCNTs were sonicated in concentrated nitric acid for a few minutes followed by refluxing under magnetic stirring at 120–130 °C for h The yield reached 30–50 wt% of the raw sample and the metal amount was decreased to $1 wt% One problem in the above purification is that the permeation rate during filtration was very low because SWCNTs packed together and the filter membrane was blocked This makes it difficult to purify CNTs on a large scale, and some small carbonaceous impurity particles cannot permeate the filter To solve this problem, Rinzler et al [59] adopted hollow-fiber cross-flow filtration (CFF) to filtrate SWCNTs that had been refluxed in 2.6 M HNO3 for 45 h Highly pure SWCNTs with a yield of 10–20 wt% were obtained with this readily scalable method, which opens up a way to purify SWCNTs on a large scale Even though the effectiveness of nitric acid treatment on the purification of SWCNTs is confirmed, the relationship between purification yield and purity with systematic and quantitative measurements was not reported before Hu et al.’s work [60] They established a systematic and quantitative relationship between yield and purity by using solution phase NIR spectroscopy In their experiments, g of the as-prepared SWCNT sample was refluxed in M nitric acid for 12, 24 and 48 h, in M nitric acid for and 12 h, and in concentrated nitric acid for and 12 h The weight percent of each component calculated from TGA and NIR spectra is plotted in Fig It is clear that the purity and the yield of SWCNTs with nitric acid treatment depend on the concentration of the nitric acid and the time of reflux The treatments of Fig – Mass balance of the normalized weight percentage of all components including SWCNTs, metal, carbonaceous impurities, and weight loss of the SWCNT samples (reprinted with permission from [60], Copyright 2003 Amercian Chemical Society) M HNO3 for 12 h and M HNO3 for h were the most efficient Nitric acid treatment destroys SWCNTs, leading to the production of carbonaceous impurities Nevertheless, with the ability to dissolve the metal catalyst, intercalate SWCNT bundles, attack amorphous carbon, and break large carbon particles, the nitric acid treatment can be a viable first step for SWCNT purification The key to achieving high-purity SWCNTs is a subsequent process for removing the carbonaceous impurities that remain in the sample after nitric acid treatment In this case, a preferred step is hollow-fiber CFF [59] Hydrogen peroxide (H2O2) is also a mild, inexpensive and green oxidant, which can attack the carbon surface The disadvantage of H2O2 is also obvious It cannot remove metal particles Therefore, it is usually used together with HCl HCl is a widespread chemical that can be easily converted into a harmless salt Therefore, purifying CNTs using H2O2 followed by HCl treatment to remove metal particles has also been intensely investigated Macro-scale purification, including a first refluxing treatment in H2O2 solution and then rinsing with HCl, was reported by Zhao et al [61,62] Their experimental results showed that the size of Fe particles has a great influence on the oxidation of amorphous carbon However, this was still a question about the effect of Fe before Wang’s work [63] Wang et al [63] tried to explain the above question They combined two known reactions (oxidation of amorphous carbon with H2O2 and removal of metal particles with HCl) into a single pot, which simplified the process Surprisingly, the product yield and purity were improved Typically, carboncoated iron impurities were simply dissolved by reacting with an aqueous mixture of H2O2 and HCl at 40–70 °C for 4–8 h With this treatment, the purification yield was significantly increased to $50 wt% and the purity was up to 96 wt% According to Wang, the effect of this process on the purification can be summarized as following First, Fe particles act as a catalyst by Fenton chemistry [70], producing hydroxyl radicals (ÅOH), a more powerful oxidant than H2O2 Second, HCl dissolves the iron nanoparticles upon their exposure The exposed iron releases ferrous ions as a result of dissolution of 2010 CARBON ( 0 ) 0 –2 the Fe particles in the acid solution The ferrous ions quickly diffuse into the acid solution, thereby eliminating iron and iron hydroxide precipitation and their unwanted catalytic effect (Fig 4) Therefore, by confining the catalytic effect to the vicinity of the carbon-coated iron nanoparticles, both a high selectivity in removing iron particles and carbonaceous shells and a low consumption of SWCNTs are accomplished At almost the same time, microwave-assisted inorganic acid treatment for the effective removal of metal particles was reported [71–76] The principle of this method is that inorganic acids such as HNO3, HCl, and H2SO4 can rapidly absorb microwave energy and dissolve metals efficiently without damaging the tube wall structure in a short time As discussed above, HNO3, H2O2, as well as microwave-assisted inorganic acid treatments can effectively remove metal particles, but they are not so effective in removing carbona- Fig – Scheme of localized catalytic reaction of H2O2 with carbon-coated iron nanoparticles (not drawn to scale, reprinted with permission from [63], Copyright 2007 Amercian Chemical Society) ceous particles owing to the relative mildness in their oxidation In order to get rid of carbonaceous impurities, liquid oxidants with stronger oxidation activity were also investigated These oxidants are predominantly mixture of acids and KMnO4 Liu et al [64] use a mixture of concentrated H2SO4/HNO3 (3:1 by volume) to cut highly tangled long ropes of SWCNTs into short, open-ended pipes, and thus produced many carboxylic acid groups at the open ends Wiltshire et al [65] reported that liquid phase oxidation could be a continuous diameter-selective process, eliminating SWCNTs with smaller diameter by oxidizing the sidewalls Li and coworkers [66] investigated the purification effectiveness of concentrated H2SO4/HNO3 (3:1) treatment and compared this with M HNO3 treatment Typical TEM images of purified SWCNTs after different treatment conditions are shown in Fig 5, from which it can be concluded that concentrated H2SO4/HNO3 (3:1) is more effective than nitric acid in removing impurities Furthermore, it was reported that the best purification condition could reach 98% purity of SWCNTs with a yield of 40 wt% within h, without decreasing the number of small diameter nanotubes for a h reflux process using a concentrated H2SO4/HNO3 mixture (3:1) Colomer et al [68] reported an effective method for removing amorphous carbon by refluxing as-prepared MWCNTs in acidified KMnO4 at low-temperature (80 °C) According to them, amorphous carbon was completely removed at the cost of more than 60% carbon loss TEM observation of the purified CNTs indicated that all amorphous carbon aggregates were removed and the CNT caps were opened Hernadi et al [69] verified the above conclusion They obtained MWCNTs with oxygen functional groups which were free from amorphous carbon by KMnO4 oxidation Zhang et al [67] investigated the effect of KMnO4 in alkali solution on the purification of SWCNTs KMnO4 in alkali solution is a much more moderate Fig – TEM images of purified SWCNTs: (a) sonication in M HNO3 for h, (b) refluxing in M HNO3 at 120 °C for h, (c) refluxing in concentrated H2SO4/HNO3 mixture (3:1) at 120 °C for h, (d) refluxing in concentrated H2SO4/HNO3 mixture at 120 °C for h (reprinted with permission from [66], Copyright 2004 Institute of Physics Publishing) CARBON ( 20 ) 0 3–20 oxidant than in acidic solution The solution cannot effectively open the tube, while it is strong enough to attack the nanotube walls and generate abundant functional groups The problem of this process is that additional steps are needed to remove the MnO2 generated during the oxidation It is desirable to remove carbonaceous impurities by converting them into soluble or volatile products, and from this point of view, KMnO4 seems to be a less suitable oxidation agent Liquid phase oxidation is a continuous process that can eliminate impurities on a large scale, and it is hoped that it can be widely used for industrial application in the future This method often leads to surface modification that preferentially takes place on CNT sidewalls, which increases the chemical activity and the solubility of CNTs in most organic and aqueous solvents This surface modification effect shows great potential for improving their physical and chemical properties for specific applications, e.g., in making mechanically reinforced composites, in use as scanning probe microscopy tips with tailored chemical sensitivity, and in producing nanotube derivatives with altered electronic structures and properties [77–80] Furthermore, CNTs can be cut into short fragments decorated with oxygen functional groups under suitable treatment conditions, which greatly increases their dispersibility and facilitates their practical applications For example, the application of CNTs in the field of emerging biotechnology is based on the premise that short CNTs are dispersible in water [81,82] The main problem of this liquid oxidation strategy is the damage to CNTs, the inability to remove large graphite particles, and the loss of a large amount of SWCNTs with small diameter It is very difficult to obtain purified SWCNTs with high-purity and high yield without damage by simply using liquid phase oxidation 2.1.3 Electrochemical oxidation As with liquid phase oxidation and gas phase oxidation, carbon materials with fewer defects usually show a lower corrosion rate under electrochemical oxidation Therefore, it is reasonable to deduce that CNTs with fewer defects should show higher electrochemical oxidation resistance than carbon impurities with more defects Fang et al [27] investigated the electrochemical cyclic voltammetric (CV) oxidation behavior of an arc discharge derived SWCNT sample in KOH solution Amorphous carbon in the as-grown SWCNT sample was effectively removed by the CV oxidation, as confirmed by analyzing the sp3/sp2 carbon ratio from C1s XPS spectra and TEM observations The removal of amorphous carbon led to the exposure of metal nanoparticles, hence facilitating the elimination of the metal impurities by subsequent HCl washing The redox peaks from the electrochemical redox reactions of Fe and Ni impurities can be considered as an indication of the extent of removal of the amorphous carbon, and the optimum electrochemical oxidation time for the purification of the as-grown SWCNT sample can be determined in real time during the CV oxidation treatment The above electrochemical oxidation was performed in KOH solution, which needs further acid treatment to remove metal particles This makes the purification complex If the solution is acidic, the post-treatment should be omitted, which makes the purification easier Ye et al [83] verified this 2011 They recently reported an ultra-fast and complete opening and purification of MWCNTs through electrochemical oxidation in acid solution The vertically aligned MWCNT (with herringbone structure) arrays investigated were grown on a carbon microfiber network through DC plasma-enhanced CVD Electrochemical oxidation for tip opening and purification of MWCNT arrays was performed in an aqueous solution of 57% H2SO4 at room temperature SEM and TEM images before and after purification (Fig 6) indicated that the CNT tips were opened, and entrapped metals were removed during the electrochemical oxidation The results of inductively coupled plasma-mass spectrometry indicate that 98.8% of the Ni was removed after the electrochemical oxidation in acid The authors also investigated a series of electrolyte solutions for electrochemical opening of CNT tips at room temperature They concluded that if electrochemical oxidation was performed in neutral or basic aqueous solutions, no significant tip opening was observed If aqueous solutions of common strong or medium strength acids (5% H2SO4, 5% HNO3, or 25% HNO3 + 25% H2SO4, 5% H3PO4 and 5% CH3COOH) were used, not only can the amorphous carbon be readily etched but also the metal catalyst can be dissolved Superior to the gas phase oxidation and wet oxidation, the optimum time and degree of electrochemical oxidation for CNT purification can be easily determined This method can get rid of impurities to some extent, particularly for selectively opening and purifying vertically aligned CNT arrays The desired vertical orientation can be maintained and facilitates the use of CNT arrays as fuel cell electrodes, sensor platforms, nanoreactors, field emitter components, and other applications However, little polyhedral carbon, graphite particles, and metal particles enwrapped by carbon layers with fewer defects can be removed by the CV oxidation Moreover, the purity of the obtained sample greatly depends on the starting materials, and the amount of sample purified for each batch is too small to make the method practical 2.1.4 Brief summary Chemical-based purification can effectively remove amorphous carbon, polyhedral carbon, and metal impurities at the expense of losing a considerable amount of CNTs or destroying CNT structures Gas phase purification is characterized by opening the caps of CNTs without greatly increasing sidewall defects or functional groups Liquid phase oxidation introduces functional groups and defects preferentially on CNT side walls, and may cut CNTs into shorter ones with different lengths The electrochemical oxidation is suitable for purifying CNT arrays without destroying their alignment These features allow chemical purification adopted by researchers to fulfill different requirements The most serious problem of this technique is that the structure of CNTs may be destroyed by the reactants, and hence limits the applications of CNTs in some fields, for example, electronic devices 2.2 Physical-based purification A big problem in chemical purification is that it always destroys the structure of CNTs or changes their natural surface properties To elucidate the inherent physical and chemical properties of CNTs, purifications that not involve oxidative 2012 CARBON ( 0 ) 0 –2 Fig – SEM (a, b) and TEM (c, d) images of MWCNT arrays: (a, c) as-grown, (b, d) purified (reprinted with permission from [83], Copyright 2006 Amercian Chemical Society) treatment are highly desirable The morphology and physical properties of CNTs, such as aspect ratio, physical size, solubility, gravity, and magnetism are different from impurities These differences enable one to separate CNTs from impurities by adopting some physical techniques Therefore, physical-based methods including filtration, chromatography, centrifugation, electrophoresis, and high temperature (1400– 2800 °C) annealing, have been extensively investigated The most striking feature of these methods is a non-destructive and non-oxidizing treatment Another feature is that the purifications are mostly performed in solution, which requires the as-prepared samples to have a good dispersibility in the solutions To meet this requirement, surfactants and/ or sonication are often used 2.2.1 Filtration Separation by filtration is based on the differences in physical size, aspect ratio, and solubility of SWCNTs, CNSs, metal particles and polyaromatic carbons or fullerenes Small size particles or soluble objects in solution can be filtered out, and SWCNTs with large aspect ratio will remain Polyaromatic carbons or fullerenes are soluble in some organic solvents, such as CS2, toluene, etc The impurities can be easily removed by immersing the as-prepared sample in these organic solutions followed by filtering The impurity particles smaller than that of the filter holes flow out with the solution during filtration, while large impurity particles and small ones adhering to the CNT walls remain One problem of this technique is that CNTs or large particles deposited on the filter often block the filter, making the filtering prohibitively slow and inefficient Therefore, a stable suspension of CNTs and a technique preventing them from deposition and aggregation are very important during filtration Thus, surfactants are widely used to make a stable CNT suspension, and ultrasonication is usually adopted to prevent the filter from being blocked Bonard et al [84] first applied filtration assisted with sonication to purify MWCNTs The as-prepared MWCNTs were dispersed in water with sodium dodecyl sulfate (SDS), and a stabilized colloidal suspension was formed The suspension was filtered using a filtration apparatus with a funnel large enough to allow the sonication of the colloidal suspension to extract larger particles In order to enhance the separation yield, successive filtrations were carried out until the desired purity is reached Shelimov et al [85] used the above procedure to purify SWCNTs and obtained SWCNT material with a purity of more than 90% (estimated by EM) and a yield of 30–70 wt% Bandow et al [86] developed a purification process (shown in Fig 7) consisting of filtration and microfiltration under an overpressure ($2 atm) of N2 to separate CNSs, metal nanoparticles, polyaromatic carbons and fullerenes from SWCNTs The microfiltration was repeated three times to minimize the amount of residual CNSs and metal particles Using this technique, $84, 10, and wt% of purified SWCNTs, CNSs, and CS2 extracts were separated from the as-prepared lasersynthesized SWCNTs A major advantage of filtration is that it is driven by pure physicochemical interactions of carbon products with amphiphilic molecules and the filter membrane, leaving the nanotubes undamaged However, this procedure relies on the quality of raw samples and is time-consuming In addition, CARBON ( 20 ) 0 3–20 As-prepared carbonaceous sample in CS2 (SWCNT, CNS, C60, C70, polyaromatic carbons) Solids caught on filter Extract Sonication in aqueous solutions (0.1% surfactant) Evaporate CS2 Microfiltration under over pressure (~ atm) Fullerenes (C60, C70), polyaromatic carbons Liquid Evaporate and collect solids CNSs Solids caught on filter Purified SWCNTs Fig – A diagram illustrating the technique used for separating coexisting CNSs, metal nanoparticles, and polyaromatic carbons or fullerenes from the laser-synthesized SWCNTs (modified with permission from [86], Copyright 1997 Amercian Chemical Society) amorphous and spherical carbon particles stuck on the tube walls cannot be effectively removed 2.2.2 bon and CNPs is shown in Fig The drawback of this process is that CNTs need to be first treated with nitric acid, which introduces functional groups on their surface 2.2.3 Filtration Centrifugation Centrifugation is based on the effect of gravity on particles (including macromolecules) in suspension because two particles of different masses settle in a tube at different rates in response to gravity On the other hand, centrifugation can also separate amorphous carbon and CNPs based on the different stabilities in dispersions consisting of amorphous carbon, SWCNTs, and CNPs in aqueous media The different stabilities resulted from the different (negative) surface charges introduced by acid treatment [87,88] Low-speed centrifugation (2000g) is effective in removing amorphous carbon and leaving SWCNTs and CNPs in the sediment High-speed centrifugation (20000g) works well in settling CNPs, while leaving SWCNTs suspended in aqueous media The effectiveness of centrifugation in separating SWCNTs from amorphous car- 2013 Solubilization of CNTs with functional groups The principle of this purification step is to solubilize CNTs by introducing functional groups onto their surface These soluble nanotubes allow for the application of other techniques such as filtration or chromatography as a means of tube purification To regain reasonable quantities of un-functionalized but purified nanotubes, the functional groups should be removed by thermal treatment or other techniques Coleman et al [89,90] described a one-step, high yield, nondestructive purification for MWCNTs containing soot using a conjugated organic polymer host (poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene (PmPV)) in toluene PmPV is shown to be capable of suspending nanotubes indefinitely whilst the accompanying graphitic particles settle out Finally the host polymer was removed by Buchner filtration, giving CNTs with a purity of 91% (estimated from electron paramagnetic resonance) In this case, a yield of 17 wt% pristine nanotubes was reclaimed from the soot Yudasaka et al [91] mixed as-grown SWCNTs with a 2% monochlorobenzene (MCB) solution of polymethylmethacrylate (PMMA) with an ultrasonic cleaner The mixture was homogenized through an ultrasonic-homogenizer and filtered The MCB was removed by evaporation at 150 °C, and PMMA was removed by burning it off in 200-Torr oxygen gas at 350 °C At the same time, azomethine ylides and solution phase ozonolysis (À78 °C) were also reported to solubilize CNTs via 1,3 dipolar cycloaddition [92–94] Recently, Jeynes et al [95] and Sanchez-Pomales et al [96] reported a method for purifying CNTs using RNA and DNA Briefly, arc discharge derived CNTs were sonicated in deionized water at °C (in an ice-water bath) for 30 with 0.5 mg/mL total cellular RNA The solution was then centrifuged to pellet the insoluble particles RNA-wrapped CNTs were treated with enzyme ribonuclease to remove the RNA and thereby precipitate the CNTs Jeynes et al [95] also suggested that RNA/DNA was more efficient in solubilizing CNTs than SDS as there is a large surface area of phosphate backbone which interacts with water, while similarly there are many bases to bind the CNTs The advantage of this process is that it can always preserve the surface electronic structure of CNTs This property Fig – TEM images of (a) SWCNT-COOH material showing embedded catalyst particles, (b) purified SWCNT-COOH fraction, and (c) carbon particle fraction (reprinted with permission from [88], Copyright 2006 Amercian Chemical Society) 2014 CARBON ( 0 ) 0 –2 is of fundamental importance for the use of nanotubes as biosensors [97] On the other hand, the capability of dispersing CNTs in solution is a very important step for using CNTs as vectors to deliver therapeutics (drug, nucleic acid) [97] However, the effectiveness of this technique is not high for CNT samples containing a large amount of impurities or bundled SWCNTs 2.2.4 High temperature annealing For some applications of CNTs, such as their use as bio-materials, complete removal of metal particles is of particular importance However, it is very difficult to achieve this by acid washing because most of the metal particles are enwrapped by carbon layers The physical properties of carbon and metals are different at high temperature (>1400 °C) under inert atmosphere or high vacuum It is well known that, graphite is stable even at 3000 °C, while metal evaporates at temperatures higher than their evaporation point Therefore, it is expected that high temperature annealing can effectively remove metal particles Lambert et al [98] first attempted to remove metal catalyst particles from SWCNTs by heating the material above the evaporation temperature of the metal The results showed that this might be a good way to eliminate the catalyst particles The effectiveness of removing metal particles in MWCNTs using high temperature annealing was also verified by a few reports [99–101] Their results suggest that high-purity (99.9%) CNTs with respect to metal particles can be obtained by a high temperature (P1800 °C) annealing treatment Further research indicated that high temperature annealing (>1400 °C) can change the structure of CNTs, such as removing structural defects [102], enlarging diameter [103], transforming SWCNTs to MWCNTs [104] or MWCNTs to double-walled CNTs (DWCNTs) [105] at appropriate temperatures In brief, high temperature annealing of CNTs is one of the most efficient methods for the removal of metal particles at the tips or in the hollow core of CNTs [98–105] and also for structural evolvement from disordered to straight, crystalline layers [106] High temperature annealing not only increases the mechanical strength and thermal stability of CNTs but also affects their electronic transport property The drawback of this method is that carbonaceous impurities still exist and become more difficult to remove after graphitization Therefore, this method can be used to remove residual metal particles of purified CNTs obtained by other techniques for achieving metal free CNTs It can also be used to remove the metal particles in as-prepared CNT samples that contain a small amount of carbonaceous impurities or in the case where the existence of carbon impurities is not of much concern 2.2.5 Other physical techniques Some other physical methods were also explored to remove metal particles, including a magnetophoretic technique [107], supercritical fluid carbon dioxide (Sc-CO2) extraction [108], and a mechanically ejecting technique [109] These techniques are reported to be effective in removing metal particles entrapped by carbon layers without changing the inherent properties of the CNTs Kang and Park [107] demonstrated magnetophoretic purification of SWCNTs (produced by HiPco process) from superparamagnetic iron-catalyst impurities in a microfluidic device The flow of a fluid through a microfluidic channel is completely laminar and no turbulence occurs due to the small dimension of the microchannels By employing microfluidic and a magnetic field-induced saw-tooth nickel microstructure, a highly enhanced magnetic force in adjoining microchannels was exploited The iron impurities of SWCNTs were attracted towards areas of higher magnetic-flux density in the microchannels where magnetic field was asymmetrically generated perpendicular to the streamline SWCNTs with a purity of $98–99% with respect to metal content were obtained However, some SWCNTs containing iron particles were also removed, which decreased the yield of this method Sc-CO2 has both gas-like and liquid-like properties; thus it can penetrate into small pores like a gas and dissolve organic substances like a liquid Based on the above mechanism, Wang and coworkers [108] developed a two-step purification using Sc-CO2 as a solvent to clean metal impurities from asgrown SWCNTs produced by the HiPco process The first step of this method is a pretreatment procedure using bulk electrolysis with ethylenediaminetetraacetic acid The second step is an in situ chelation/(Sc-CO2) extraction to remove metal particles Over 98% of the iron impurity (measured by EDS) in the as-grown SWCNTs were removed using this two-step extraction Thien-Nga et al [109] developed a mechanical purification to remove ferromagnetic particles from their graphitic shells The basic principle of the method is like a snooker game, where the energy of elastic impact between encapsulated catalysts and small hard inorganic particles is used to eject metal kernels and trap them by a strong magnet Typically, SWCNTs were first dispersed in various solvents Insoluble nanoparticles (zirconium oxide, diamond, ammonium chloride, or calcium carbonate) in the given medium were then added to the suspension This slurry was sonicated typically for 24 h This process enables the production of laboratory quantities of SWCNTs containing no magnetic impurities 2.2.6 Combination of purification and separation Following the purification of CNTs to remove foreign materials such as catalyst, amorphous carbon, and carbon-coated nanoparticles, the sorting of SWCNTs according to their length becomes particularly important in light of their potential applications Various techniques have been employed to purify CNTs and simultaneously sort them by length [110– 113] Chromatography is useful for the length fractionation of shortened CNTs less than 300 nm in length [110,111] For longer CNTs, techniques such as capillary electrophoresis (CE) [112] and field-flow fractionation (FFF) appear to be more applicable [113] These techniques are simple and nondestructive for the purification and length-dependent separation of CNTs These techniques require CNTs to be purified are individually dispersed Therefore, complex procedures are needed to obtain this kind of highly dispersible CNT solution Chromatography is a separation method that relies on differences in partitioning behavior between a flowing mobile phase and a stationary phase to separate the components CARBON ( 20 ) 0 3–20 in a mixture Materials that are smaller than the pore size can enter the pores and therefore have a longer path and longer transit time, while larger materials that cannot enter the pores are eluted first Different molecules, therefore, have different total transit times through the column depending on their size and shape Chromatography was first used to separate CNTs from carbonaceous impurities [114–117] After this, chromatography is usually used to separate CNTs by length For example, purification and length separation of oxidatively shortened SWCNTs were achieved by this technique in an alkalescent water solution [110] and SWCNTs with different lengths were separated Huang and coworkers [111] used chromatography to purify DNA-wrapped CNTs and sort them into fractions of uniform length As observed by atomic force microscopy (AFM), the length variation was typically within 10% or less for each of the measured fractions (Fig 9) Electrophoresis is caused by electrostatic forces, which are generated by applying an alternating current (AC) or DC electric field between an electrode and a charged body Electrophoresis can be achieved regardless of the electric field’s uniformity A charged particle is pulled along the field lines toward the electrode carrying an opposite charge to that of the particle In the same field, a neutral body is merely polarized The result may produce a torque, but not a net translational force, without which the body as a whole will not move towards either electrode Electrophoresis can be appreciable even when the free charge per unit weight of the particle is quite small Therefore, it is possible to purify CNTs in an electric field by using the motion difference between CNTs and carbon impurities [118] This motion depends not only on the intrinsic electric properties but also on the diameter and length of CNTs Therefore, this technique can also be used to separate CNTs with length, diameter, and conductivity by refining experimental conditions Yamamoto et al [119] first reported a purification and orientation method based on AC electrophoresis in isopropyl alcohol To increase separation rate and effectiveness, Doorn et al [112] adopted CE to purify and separate CNTs by size The CE was performed in narrow tubes (in the order of lm) and resulted in rapid separation based on charge- and size-dependent mobility of solution phase species under the influence of an applied electric field And AFM observations on fractions demonstrated a lengthbased separation mechanism that leads to elution of short tubes first, followed progressively by longer tubes Further work [120] indicated that CE has the potential to separate 2015 CNTs, not only by differences in length [112] but also by differences in size or other geometric factors, such as diameter or cross section At the same time, AC dielectrophoresis was reported to be capable of separating metallic SWCNTs from semiconducting SWCNTs in SDS suspension [121] This method takes advantage of the difference in the relative dielectric constants of two species with respect to the solvent, resulting in an opposite movement of metallic and semiconducting tubes along the electric field gradient Metallic tubes are attracted toward a microelectrode array, leaving semiconducting tubes in the solvent An enrichment of metallic tubes up to 80% was achieved by a comparative Raman spectroscopy study on the dielectrophoretically deposited tubes and a reference sample In addition, FFF was also developed to separate CNTs by length [113,122] FFF is a chromatography-like separation and sizing technique based on elution through a thin empty channel The main difference between FFF and chromatography is that FFF separation is (ideally) induced only by physical interactions with an external field rather than physicochemical interactions with a stationary phase Compared with chromatography and CE, FFF can separate CNTs by length over a larger range and in larger quantities These techniques can separate CNTs according to their size or electronic properties, which represents an important improvement in size and conductivity selectivity This will promote the development and application of CNTs in the analytical, nanotechnology and nanoelectronics fields [123,124] Therefore, they are not merely purification methods A common feature of these techniques is that they require high dispersibility of isolated CNTs in solution However, the CNT surface is hydrophobic, and the existing state of CNTs, especially for SWCNTs, is interconnected or in a thick bundle In order to obtain high-purity CNTs, pre-treatment is required to obtain isolated CNTs having high dispersibility in solution 2.2.7 Brief summary Physical-based purification can maintain the intrinsic structure of CNTs, which is desirable for elucidating their properties Furthermore, some techniques such as chromatography, electrophoresis, and FFF can separate CNTs according to their differences in length or conductivity in addition to their purification function, which is a key step for using CNTs in devices such as nano- and micro-electronics However, Fig – AFM images of three representative chromatography fractions of SWCNTs deposited onto alkyl silane-coated SiO2 substrates (reprinted with permission from [111], Copyright 2005 Amercian Chemical Society) 2016 CARBON ( 0 ) 0 –2 there are still some problems in these techniques that need to be solved One is that these methods are not very effective in removing impurities Another is that they require CNT samples be highly dispersible Therefore, the as-prepared sample is always first dispersed in solution by adding surfactants or treated by a chemical process to cut and/or add functional groups before purification The third problem is the limited amount of sample that can be purified each time Based on the above facts, physical methods are more suitable for use as an assistant step combined with chemical purification, except for the case where a small amount of CNTs with a particular structure or property are required 2.3 Multi-step purification As discussed above, gas phase oxidation is effective in removing amorphous carbon and polyhedral carbon at the cost of losing some CNTs but fails to remove a significant fraction of graphite particles and metal impurities Liquid phase oxidation with strong oxidants is effective in removing carbonaceous impurities and metal particles simultaneously, whereas purified CNTs are always cut, opened and damaged Physical purifications are effective in partly removing isolated carbonaceous or metallic impurities, while amorphous and spherical particles stuck to sidewalls or metal particles encapsulated in CNTs remain In order to achieve desirable CNT purity with high yield, combinations of chemical and physical purifications are being intensely investigated According to different needs, various kinds of multi-step purification methods are reported For example, it is difficult to remove carbonaceous impurities adhering to the sidewalls of CNTs for both chemical and physical purifications To solve this problem, hydrothermally initiated dynamic extraction (HIDE) [44,125–127] or sonication [128–133] was adopted in many chemical purification procedures Graphite particles existing in CNTs synthesized by arc discharge or laser ablation are hard to remove by chemical oxidation, so filtration is adopted in some purification [134,135] It is clear that metal particles catalyze the low-temperature oxidation of carbons indiscriminately, destroying SWCNTs in the presence of oxygen and other oxidizing gases To overcome this problem, purification combining gas phase oxidation and acid treatment were widely investigated [136–145] In fact, several techniques such as oxidation, sonication, HIDE, or filtration are simultaneously adopted in one purification procedure to obtain high-purity CNTs with high yield 2.3.1 particles and this allows them to be dissolved by hydrochloric acid in the final step of the treatment [125] Tohji et al [125,126] first reported a multi-step method to purify SWCNTs by combining HIDE with other processes as illustrated in Fig 10 SWCNTs with a purity of 95 wt% and a yield of about wt% were obtained by this process Graphite fragments from the graphite rod cannot be removed by oxidation To solve this problem, the as-prepared MWCNTs were treated by a multi-step process (shown in Fig 11) combining wet grinding, HIDE, oxidation and other techniques [127] TEM observations indicated that this process was effective in removing graphite and carbonaceous particles and opening CNT caps However, the yield was only $2 wt% due to the high oxidation temperature (700 °C) To increase the purification yield, we developed a multistep method combining sonication, HIDE, bromination, gas phase oxidation and acid treatment [44] It was found that bromination can increase the purification yield from $25 to $50 wt% The effect of bromination on the purification of CNTs was also verified by Fan et al [148] The problem with this technique is that it is not suitable for purifying SWCNTs On the other hand, the onset burning temperature of MWCNTs was decreased after purification, suggesting that defects or functional groups were introduced 2.3.2 Microfiltration in combination with oxidation Bandow et al [134] purified SWCNTs synthesized by laser ablation by combining microfiltration [86] with oxidation in air In a typical procedure, as-prepared soot containing SWCNTs was first purified using microfiltration to remove large CNSs The obtained SWCNTs were then oxidized in air at 450 °C for 20 to remove CNSs adhering to the SWCNT walls, followed by soaking in concentrated HCl (36%) for 1–2 days at room temperature to remove metal particles The purity of the SWCNTs after purification was greater than 90% To remove metal particles before oxidation, Kim and Luzzi [135] developed magnetic filtration carried out in a magnetic field They investigated the efficiency of using magnetic filtration alone, or combining it with chemical-based or annealingassisted oxidation treatments Using magnetic filtration alone, the catalyst content was reduced from 11.7 to 3.7 wt%, much better than obtained in oxidation or chemical As-produced SWCNTs HIDE-assisted multi-step purification HIDE provides comminution on a microscopic scale as a result of collision between soot particles and water molecules during thermal treatment [146] Thus during HIDE, water molecules break the network between SWCNTs, amorphous carbons and metal particles, and also attack the graphitic layers encapsulating metal particles As a result, almost all graphitic nanoparticles and CNSs are washed out from the soot The graphitic sheets of the CNSs, for the most part, have defects and dislocations, in contrast to SWCNTs [147] It is believed that the reaction of H2O with carbon breaks the graphitic layers that wrap the metal particles Consequently, incorporating HIDE in the purification procedure exposes the metal HIDE treatment for 12 h Purified SWCNT Removing exposed metal M HCl treatment Filtration and dry Removing Soxhlet extraction fullerenes in CS2 Removing amorphous carbon Oxidation at 470ºC for 20 Fig 10 – A diagram showing the purification of SWCNTs with a multi-step process incorporating HIDE treatment CARBON 2017 ( 20 ) 0 3–20 Cathode deposit (850 mg) Sediment (320 mg) Remove small graphite Grinding in water Ultrasonic irradiation Centrifugation Ultrasonic irradiation Add surfactant Dispersed material, 68 mg 37 μm filtration HIDE treatment Oxidation, 700 ºC for 15 Filtration (390 mg) Purified MWCNTs, 16 mg Remove large graphite Cake (460 mg) Fig 11 – A diagram showing the purification of MWCNTs with a multi-step process incorporating HIDE and wet grinding treatment (modified with permission from [127], Copyright 2001 Amercian Chemical Society) treated samples By combining chemical and magnetic purification, the metal catalyst content was reduced to 0.3 wt% These results allowed the authors to conclude that magnetic filtration is effective in removing metal catalysts, producing CNTs with high-quality and yield It is well known that metal particles can catalyze carbon oxidation in the presence of oxidants Therefore, effective removal of metal particles is desirable for the following purification of CNTs using chemical oxidation Thus magnetic filtration combined with chemical purification opens a new way to obtain purified CNTs with high yield 2.3.3 Sonication in combination with oxidation Sonication is identified as one of the effective processes to get rid of the amorphous impurities adhering to the walls of CNTs using suitable solvents [149] During sonication, the solvent molecules are able to interact with CNTs and hence lead to solubilization, which can improve purification effectiveness when some other steps are followed We [128] developed an effective multi-step purification approach to purify SWCNTs by combining sonication with oxidation in air The biggest problem in purifying SWCNTs synthesized by arc discharge is how to remove graphite particles produced from the graphite rod Ultrasonication in ethanol was adopted to first remove the graphite particles Because the SWCNTs used were rope-like, we decanted the alcohol solution containing graphite particles and other organic impurities after sonication for about This procedure was repeated five times The graphite-free material was oxidized and then soaked in HCl to remove amorphous carbon and metal particles TEM observations (Fig 12) and Raman spectra verified the effectiveness of the above purification procedure With this procedure, a 41 wt% yield of SWCNTs with a purity of about 96% was achieved However, the above procedure is only applicable to SWCNTs synthesized by hydrogen arc discharge, in which SWCNTs exist as ropes with fewer defects Fig 12 – TEM images of (a) the as-prepared SWCNTs, and (b) the purified SWCNTs [128] We [129] also developed a multi-step method to purify SWCNTs synthesized by CVD, which includes acid washing, ultrasonication and freezing treatments in liquid nitrogen After purification, SWCNTs with a purity of 95% (estimated from EDS and SEM) and a yield of 40 wt% were obtained and the procedure did not destroy the SWCNT bundles Montoro and Rosolen [130] reported a four-step method to purify SWCNTs synthesized by arc discharge as shown in Fig 13 This new procedure is efficient and appropriate for obtaining highly pure SWCNTs with minimum damage to the CNT walls and minimum modification in the CNT length Wang et al [132] developed a three-step method to purify and cut SWCNTs synthesized by CVD This method included refluxing in 2.6 M HNO3 to remove metal particles, ultrasonication in acid solution (H2SO4/HNO3, H2SO4/H2O2) to cut and polish the SWCNTs, and heat treatment in an NH3 atmosphere to remove carbon impurities and heal structural defects Recently, they [133] further improved their method mainly by replacing the above acid solution with an (NH4)2S2O8/H2SO4 solution In addition, ultrasonication time 2018 CARBON ( 0 ) 0 –2 As-prepared SWCNTs Fullerenes and soluble impurities Sohxlet extraction with toluene Amorphous carbons Sonication in H2O2 solution Metallic particles Graphite and protected metallic impurities Extracted material Sonication in HNO3 /HF/ SDS Separation in SDS solution High purity SWCNTs Fig 13 – A flowchart illustrating the multi-step purification of SWCNTs The removed species are listed to the left of the chart (modified after [130]) was extended to 4–30 h to control the length of the SWCNTs The final SWCNT product had a metal content lower than wt%, a length distribution between and lm, and very few defects 2.3.4 High temperature annealing in combination with extraction As discussed above, high temperature annealing is an effective method to remove metal particles, while it makes the removal of carbonaceous impurities much more difficult in the following chemical treatments To solve this problem, Zhang et al [150] proposed a method involving high temperature annealing followed by a dispersant extraction treatment using a polymer Briefly, the MWCNT samples with micrometer lengths and diameters of 20–50 nm were annealed at 2600 °C for 60 to remove metal catalyst The graphitized MWCNT product was sonicated in a solution of dispersing agent 710 (basic urethane copolymer) to remove CNPs Dispersing agents for carbon black have a two-component structure: one is specific anchoring groups which can be strongly adsorbed on the particle surface; the other is polymeric chains which dangle into the solvent and repel other polymers Considering that CNPs and MWCNTs present different dimensionalities, the interaction potential between two parallel, infinitely long, and perfect MWCNTs as well as two CNPs was calculated and compared on the basis of the continuum Lennard-Jones model (Fig 14) It was found that the attractive potential between two CNPs was relatively low and of short range Therefore, the polymer chain length and surface coverage for the dispersion of CNPs was lower than that needed for the dispersion of MWCNTs On the basis of this theoretical Fig 14 – Potential energy (per atom) of MWCNTs and CNPs (reproduced with permission from [150], Copyright 2006 Amercian Chemical Society) prediction, CNPs and MWCNTs were effectively separated from each other by choosing an appropriate polymer and suitable concentration (for instance, 0.04–0.1 wt% for the dispersing agent 710) Thus an effective and nondestructive method for the purification of MWCNTs with high yield ($90 wt%) was developed Furthermore, the high temperature annealing step in this purification can improve the structures of CNTs, which will be of great importance for the exploitation of their excellent mechanical and electrical properties in various applications 2.3.5 Brief summary Based on the above description of multi-step purification, it can be concluded that the processes can be effective in removing both carbonaceous and metal impurities These methods have been rapidly developed and widely used in recent years Most importantly, one can selectively open tips, cut CNTs, add functional groups on sidewalls, separate CNTs according to length or conductivity, or maintain an undamaged CNT structure by skillfully combining different techniques The key point of this general method is how to combine different methods according to one’s requirement and the quality of the raw CNTs Although considerable progress has been made, some merits of physico-chemical techniques have not been fully used and combined For example, some physical methods capable of removing metal particles (magnetophoretic, Sc-CO2 extraction, etc.) are rarely reported to be combined with gas oxidation This combination may greatly improve gas phase purification yield owing to the early elimination of metal particles, which can catalyze the CNT oxidation [48,49] 2.4 Applicability of typical purification techniques A high yield of high-purity CNTs can hardly be obtained using a physical or chemical method alone because of specific limitations, while high-purity and high yield CNTs can be obtained using multi-step purification by using the advantages of the both processes Furthermore, it is possible to control the morphology and structure of CNTs by selecting suitable methods To enable one to fully understand the characteristics of each method and make a wise combination based on Table – Applicability of some typical purification technologies Technologies Characteristics Yield (wt%) Applicability Carbonaceous impurity Cama/Cnpb Chemical method Gas phase Liquid phase Electrochemical Filtration Centrifugation Solubilization with functional groups Multi-step method a b c d e f g h i j k l m n HIDE, wet grinding, filtration, oxidation, sonication, centrifugation Filtration/magnetic filtration, oxidation, annealing Sonication in H2O2, HNO3 /HF/ SDS, filtration High temperature annealing, extraction Adding functional groups, cutting or opening CNTs Suitable for aligned CNTs Non-destructive Non-destructive, small amount Improve crystallinity High selectivity to metal Separate CNTs by length or conductivity High-purity, low yield High-purity respect to metal High-purity, little damage Metal free, improving crystallinity Csole Mexpf Mpolg Mcnth $2–35 $15 $30 25–48 $30–50 10–75 30–75 10–60 $80 Ji J J J J J J J* J* j      J*   J*k J* J* J* J* J* J* J*           Jml Jm Jm J J J J J Jm J*mm J*m J*m J J* J* J J*  J*m J*m J*m J* J* J* J* J* J/J* [41–45] [47] [48,49] [51] [58,59] [61–63] [64–69] [71–76] [27,83] $30–84 $10–40 $17–50 J* J* J* J* J* J*    J J J J* J*  J* J* J*    [84–86] [87,88] [89–92] $70–90 $10–/n /   J   J       J J J* J J J* J J  [98–102] [107–109] [110–113] $2% J J J* J J J* J* [127] $9–20% $25 $90% J J J*  J* J* J J* J J J J J J J J J* J J* J* J [135] [130] [150] ( 20 ) 0 3–20 High temperature annealing Other techniques to remove metal particles Chromatography, electrophoresis, FFF Opening CNT caps, low yield, less damage to tube wall Cadd CARBON Physical method Air Cl2, H2O, HCl H2O, Ar, O2 O2, C2H2F4, SF6 HNO3 H2O2, HCl Mixture of acid or KMnO4 Microwave in inorganic acid Alkali or acid solution Cgc Ref Metal impurity Amorphous carbon CNP Graphite particles Carbon impurity adhering to CNT walls Soluble carbon in some organic solutions (CS2, toluene) Exposed metal Metal wrapped by polyhedral carbon Metal encapsulated in CNTs Effective Not effective Partly removed Further HCl treatment Further HCl treatment and partly removed No report 2019 2020 CARBON ( 0 ) 0 –2 one’s need and the quality of the as-prepared CNTs, it is essential to give a clear image of the basic characteristics of different techniques Based on up-to-date knowledge, we summarize the applicability of representative methods in Table In general, chemical oxidation is effective in eliminating amorphous carbon, adding functional structures, and cutting and/or opening CNTs; inorganic acid treatment can effectively remove exposed metal particles Physical treatment is capable of removing metal particles, sorting CNTs by length or conductivity, and, to some extent, separating isolated carbonaceous particles by size It is well known that the content, morphology and structure of impurities in as-prepared CNT samples strongly depend on their synthesis methods and conditions Therefore, there is no normalized method for routinely creating highquality CNTs Here we summarize a flow chart showing representative procedures based on the function and characteristics of the purifications (Fig 15) It should be a reference for obtaining purified CNTs according to different requirements Challenges Obviously, notable progress has been made on the purification of CNTs in recent years, and small amounts of high-purity CNTs can be readily obtained in spite of the disadvantages of high cost and long time involved However, there are still many technical barriers to be overcome when cost, reproducibility, environment compatibility, and scaling-up are taken into account Considering that the purification of CNTs is a complex issue involving the quality of the raw CNT sample, purification and purity assessment, we point out the following three main challenges: synthesis methods, purification and purity assessment 3.1 Synthesis methods Carbonaceous and metallic impurities are produced as byproducts during the synthesis of CNTs If pure CNTs without any impurity were directly synthesized, the purification would be obviated A decade ago, Kyotani et al [151] synthesized MWCNTs by a template CVD using an anodic aluminum oxide film as template The resulting MWCNTs are free of carbonaceous and metal impurities Furthermore, the length, diameter and wall thickness of the CNTs can be uniformly controlled by easily controlling the structure of the template Recently, Li et al [152] reported that pure SWCNTs with a purity of $96% were produced by CVD using ethanol as carbon source The only impurity was iron nanoparticles that were exposed on the surface and could be easily removed by acid Batch-scale preparation of ultralong SWCNT arrays with high- As-prepared CNT sample CS2 extraction to remove fullerene or aromatic carbon Purity evaluation Low purity High purity Physical separation method Removing metal impurity Wrapped by carbon layers Exposed metal Improve crystallinity Maintain CNT structure High temperature annealing Other physical techniques HCl treatment Removing carbonaceous impurity Removing carbonaceous impurity Polymer extraction Adding functional groups or cut CNTs Liquid phase oxidation Maintain structure Physical method Opening caps Gas phase oxidation Separation Conductivity Length < 300 nm Length > 300 nm Dielectrophoresis Chromatography Electrophoresis or FFF Fig 15 – A flow chart showing the representative procedures based on the function and characteristics of the purifications CARBON ( 20 ) 0 3–20 quality was achieved by ultralow flow rate CVD [153] At the same time, cloning growth from an existing nanotube segment was also reported [154,155], which ties synthesis and selectivity in chirality into one single step Despite the progress achieved in directly synthesizing high-purity CNTs, the following challenging issues in synthesis still remain: (a) Large-scale synthesis of high-purity CNTs with controllability in diameter, length, wall thickness, chirality and conductivity (b) Synthesis of isolated CNTs with desired structures (c) Growth of CNTs at preset positions for device applications 3.2 Purification methods As pointed out above, most of the as-prepared CNTs contain a certain amount of impurity To obtain high-quality CNTs, purification is inevitable The demand for obtaining uniform, high-purity CNTs with low cost and high efficiency on a large scale requires the following major challenges to be overcome (a) How to efficiently combine different purification techniques according to the requirements of various applications and the characteristics of the raw CNT samples (b) How to solve the problems brought about by the scaling-up of CNT purification, such as the uniformity of the CNTs and the homogeneous contact between impurities and oxidants (c) How to develop novel techniques that can effectively achieve purification, structure tailoring, and structure/ property sorting of CNTs 3.3 the purification strategy must vary according to the features of the raw CNTs, and the specific requirement and target applications of the purified CNTs Although considerable progress has been achieved, it is still difficult to find a simple method that can purify CNTs effectively Multi-step methods that combine the advantages of different purifications provide a solution for getting CNTs with higher purity With regard to the different types of CNTs, the purification of MWCNTs is relatively easy and effective, whereas the purification of SWCNTs and DWCNTs always results in low yield and defect formation Because of this, more efforts aimed at selectively sorting SWCNTs by length, diameter, transport properties, and even chirality, are required Therefore, challenges still remain in the purification of CNTs High-purity, high yield, low cost, and environmental friendliness are essential Establishing a CNT purity assessment standard is necessary for evaluating and improving the validity of purification Without question, the exploitation of efficient CNT purification techniques that can produce high-quality and homogeneous CNTs will greatly facilitate fundamental research and practical applications of CNTs Acknowledgements We acknowledge financial support from MOST (2006 CB932703), NSFC (90606008, 50672103), and Chinese Academy of Sciences The authors appreciate very much Prof Peter Thrower’s advice and comments We thank Dr Ren WC and Liu BL for useful discussion on Raman and UV–vis-NIR characterization methods R E F E R E N C E S Purity assessment An important issue on the purification of CNTs is purity evaluation Although several techniques have been used for evaluating the purity of CNTs, a standard and well-recognized purity assessment protocol has not been established This leads to uncertainty in the purity of the products obtained as well as the efficiency of 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