Elsevier

Carbon

Volume 46, Issue 6, May 2008, Pages 833-840
Carbon

Chemical oxidation of multiwalled carbon nanotubes

https://doi.org/10.1016/j.carbon.2008.02.012Get rights and content

Abstract

The effect of oxidation on the structural integrity of multiwalled carbon nanotubes through acidic (nitric acid and a mixture of sulfuric acid and hydrogen peroxide) and basic (ammonium hydroxide/hydrogen peroxide) agents has been studied. In order to purify the as-received material, a non-oxidative treatment (with hydrochloric acid) was also applied. Electron microscopy and thermogravimetric analysis clearly revealed that the nitric acid-treated material under reflux conditions suffered the highest degree of degradation, such as, nanotube shortening and additional defect generation in the graphitic network. Basic oxidative treatment led to the complete removal of amorphous carbon and metal oxide impurities but the structural integrity was found to be intact. X-ray photoelectron spectroscopy was employed to confirm the different functionalities produced for each oxidation agent, whereas titration measurements determined the relative concentration of carboxylic functions onto the graphitic surface. Moreover, a general relationship between the chemical treatment and the amount of non-graphitic carbon was established by means of Raman spectroscopy measurements. The possibility of controlling the required amount of functionality, carboxylic and hydroxyl, via these oxidation procedures is discussed.

Introduction

Carbon nanotubes (CNTs) have raised much interest during the recent years due to their inherent extraordinary electrical and mechanical properties [1]. For some of the potential applications of this carbon allotrope, highly purified material is necessary, whereas the chemical inertness of the graphitic network presents a major challenge when it comes to composite material fabrication. The oxidation of CNTs either by wet chemical methods [2], [3], [4], [5], [6], [7], [8], [9], photo-oxidation [10], [11], oxygen plasma [12], or gas phase treatment [13] has gained a lot of attention in an attempt to purify and also enhance the chemical reactivity of the graphitic network. Typically, through the above harsh treatments, the pristine CNTs can be effectively purified and oxygen-containing groups, mainly carboxyl and hydroxyl, have been found to decorate the graphitic surface. The presence of oxygen-containing groups facilitates the exfoliation of CNT bundles, and increases the solubility in polar media [2], [3]. This, in turn, affects the processing of CNTs and increases the possibility of further modification/functionalisation depending on application [14], [15]. Concerning the use of CNTs as reinforcements in composite materials, the incorporation of oxygen-containing functionalities onto the graphitic surface is a very crucial step for the enhancement of interfacial adhesion. As a result, the unique mechanical and electrical properties of CNTs can be transferred to the properties of CNT-based composites.

The effects of the commonly used acid and/or air oxidation at elevated temperature on CNT surface morphology have been well documented in previous studies. Independent studies by Hu et al. and Martinez et al. [5], [6] have found that treating single-wall CNTs with hot nitric acid leads to an efficient elimination of metal impurities and amorphous graphitic platelets. It is interesting to note that during that treatment, intercalation process of nitric acid molecules into the CNT bundle structure was found to take place [16], which is accompanied by bundle exfoliation and etching of the carbonaceous material. This process leads to the formation of additional amorphous carbon nanoparticles covering the remaining smaller bundles of CNT. In order to eliminate amorphous carbon, the nitric acid-treated material has to be annealed to 900 °C [6].

Concerning the oxidation reaction of sulphuric acid/hydrogen peroxide mixture (piranha) with CNTs, Ziegler et al. [7] have shown that the one-dimensional nanostructures can be cut in a controlled manner under specific conditions. At high temperatures, piranha was found to attack existing damage sites, generating vacancies in the graphene sidewall, and consume the oxidized vacancies to yield shorter nanotubes. Increased reaction time results in increasingly shorter tubes. However, significant sidewall damage also occurs as well as selective etching of the smaller diameter nanotubes. On the other hand, room-temperature piranha treatments showed the capability of attacking existing damage sites with minimal carbon loss, slow etch rates, and little sidewall damage.

The detailed observation and analysis of Zhang et al. [8] showed that the defects on CNTs, original or newly created, play a crucial role in the oxidation process. Following the defect-generating and defect-consuming steps, they explored the possible oxidation reactions and predicted the intermediate and final products. The presence of defects on CNT surface not only affects the structural stability of oxidized material but also determine its electronic properties. For instance, the wet oxidation experiment of Kovtyukhova et al. [17] showed that the disruption of the conjugated network of the tubes leads to an appreciable increase of the resistivity of about three-orders of magnitude.

On the other hand, there is very little information on the structural alteration of graphitic material under basic oxidative treatments. Recently, Kim et al. [18] have treated MWCNTs with an ammonium hydroxide/hydrogen peroxide mixture and showed that the resulting composite with epoxy as matrix had enhanced electrical conductivity due to the minor damages of the graphitic sidewalls.

In the present work, we perform a systematic study of the chemical oxidation of MWCNTs treated by various reagents that possess different degrees of oxidation power. In addition, a comparative study of acidic and basic oxidative treatments on the structural integrity of CVD grown MWCNTs, is made. Regulation of the amount of oxygen onto the CNT sidewalls is a prerequisite for building certain functional nanoscale structures and devices under special conditions. To modify the CNT surface, we treated the pristine material with concentrated HCl [19], hot nitric acid [20], piranha mixture [21], [22] and ammonium hydroxide/hydrogen peroxide mixture. Treated MWCNTs were compared with the pristine material with purity above 80 wt%. Oxidised MWCNTs were tested by X-Ray photoelectron spectroscopy in order to quantify and qualify the nature of oxygen present on the MWCNT surface. The characterisation of the oxidised MWCNTs was completed by thermogravimetrical analysis, Raman spectroscopy and scanning/transmission electron microscopy (SEM/TEM) and showed the prospect of building the desirable oxygen-containing functionality onto the MWCNT surface under controlled conditions.

Section snippets

Experimental section

MWCNTs synthesized by CCVD of purity around 80% and of diameter ranging between 10 and 20 nm were employed in this study. The specific batch was provided by Nanocyl (Belgium). Purification and chemical oxidation of MWCNTs was carried out with four different agents using chemicals supplied by Aldrich.

Treatment 1: In this step the as-received MWCNTs were treated with hydrochloric acid, which is an established method for the removal of impurities. One grams of MWNCT was placed in a 500 ml round

Results and discussion

As already mentioned [2], [3], [4], [5], [6], [7], [8], [9], acidic oxidative treatment to the CNT material may cause major alteration in its structural properties. Especially, pristine CNTs synthesized by the CVD method exhibit significant lack of conjugation and some precaution should be taken for the purification/surface modification of the material. The presence of dangling bonds at the as-produced material results in a graphitic network with a higher defect density [23]. This may lead to

Conclusions

Chemical oxidation of MWCNTs with reagents of different oxidation power was studied. The reference sample was hydrochloric acid-treated MWCNTs which contains very low amount of graphitic nanoparticles bearing oxygen functionalities. The oxidative reagents used were a non-acidic mixture of ammonium hydroxide/hydrogen peroxide, a piranha solution and refluxing nitric acid. All prepared samples form permanent suspensions in common polar media, which is necessary for better manipulation of the

Acknowledgements

We thank Dr V. Drakopoulos (ICE-HT, FORTH) and Mr D. Kastanis (Interdepartmental Polymer Programme, University of Patras) for conducting the SEM and TGA experiments, respectively. Financial support from the Marie Curie Transfer of Knowledge program CNTCOMP [Contract No.: MTKD-CT-2005-029876] is gratefully acknowledged.

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