J. Phys. Chem. C 2009, 113, 4401–4405
4401
Multicomponent Metallic Impurities and Their Influence upon the Electrochemistry of Carbon Nanotubes Martin Pumera*,† and Hideo Iwai‡ International Center for Materials Nanoarchitectonics and Biomaterials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Materials Analysis Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ReceiVed: January 4, 2009; ReVised Manuscript ReceiVed: January 12, 2009
Multicomponent metallic residual catalyst impurities, which are present within carbon nanotubes even after their purification with mineral acid, are responsible for the observed “electrocatalytic” oxidation of hydrazine on carbon nanotubes. We demonstrate that in the case where multicomponent Co/Mo/Fe impurities are present within double-walled carbon nanotubes (DWCNTs), all three impurity components govern the electrochemical response of DWCNTs toward the oxidation of hydrazine. Introduction Carbon nanotubes (CNT) are seen as a new class of materials that would have a profound impact on a wide range of applications. 1-10 However, despite a tremendous amount of effort and money invested in the field in the past two decades, only a few practical applications have materialized. The main problem associated with CNTs is their heterogeneity.11,12 A typical CNT sample contains a variety of carbon-based (i.e., nontubular graphitic carbon, amorphous carbon)11 and inorganic compound-based (i.e., residual metal catalyst impurities, silica support)12 impurities in surprisingly large amounts. Even after tedious purification procedures,13 carbon nanotube materials still contain significant amounts of metallic nanoparticle (NP) impurities in a typical span of 0.5-10% w/w14,15 because these impurities are sheathed by several graphene sheets.15,16 The presence of residual catalyst impurities is a very serious issue in general because of the proved toxicity of typical catalyst nanoparticles such as cobalt, nickel, iron, and molybdenum.17-21 Carbon nanotubes can bring a variety of advantages to electrochemical applications such as energy storage devices and (bio)sensors.1-5 These advantages include increased voltammetric currents,22-25 increased heterogeneous electron transfer rates,26 insignificant surface fouling of CNT-based electrodes,27,28 and an apparent “electrocatalytic” effect toward the reduction or oxidation of important compounds such as hydrogen peroxide or hydrazine.29,30 It has been shown that iron impurities are responsible for the observed electrocatalytic effect of CNTs toward the reduction of hydrogen peroxide31,32 and oxidation of hydrazine.32-34 In addition, it was discovered that copper impurities are responsible for the observed electrocatalytic behavior of CNTs toward the reduction of halothane.35 More recently, we have demonstrated that the issue of metallic impurities in CNTs is more complex. Residual catalyst impurities can consist of several components, either in an atomic ratio of about 1:1 (e.g., for Fe/Ni)15 or in an atomic ratio of about 99:1 (Fe/Mo or Co/Mo/Fe),15,36 where the minor metallic component is actually an impurity of the main catalyst nano* Corresponding author. Fax: (+)81-29-860-4714. E-mail: pumera.
[email protected]. † International Center for Materials Nanoarchitectonics and Biomaterials Center. ‡ Materials Analysis Station.
particle. We have demonstrated that the iron present in these multicomponent metallic impurities can participate in “electrocatalytic” reduction of hydrogen peroxide on the surface of CNT-based electrodes.15,36 This is true in both cases, when the amount of Fe in a nanoparticle is either 50% or 98% (Fe/Ni or Fe/Mo NPs, respectively)15 or when the amount of Fe in a residual nanoparticle is about 1% (Co/Mo/Fe NPs).36 We have also demonstrated that, while traces of iron within these multicomponent metallic impurities have a profound effect upon the reduction of hydrogen peroxide, the other metallic components that are present in overwhelmingly large amounts (Co, Mo) have no significant influence upon the reduction of hydrogen peroxide.36 The purpose of this Article is to study the influence of multicomponent metallic impurities in carbon nanotubes upon “electrocatalytic” oxidation of hydrazine. We investigate whether all or only some components of these multicomponent NP impurities are responsible for the observed “electrocatalytic” effect on the oxidation of hydrazine by using an example of double-walled carbon nanotubes (DWCNTs)37,38 containing multicomponent Co/Mo/Fe impurities. We will demonstrate that, in contrast to the reduction of hydrogen peroxide, where only traces of iron within Co/Mo nanoparticle impurities were responsible for the “electrocatalysis”,36 in the case of hydrazine, all components of such multicomponent metallic impurities participate in the electrochemical oxidation of hydrazine. Experimental Section Materials. Double-walled carbon nanotubes (o.d. × i.d. × length: 5 nm × 1.3-2.0 nm × 5-15 µm; max content of noncarbon impurities 10% w/w, residual metallic impurities diameter
