Super-washing does not leave single walled carbon nanotubes iron-free

Abstract

We demonstrate with transmission electron microscopy and X-ray photoelectron spectroscopy that single-walled carbon nanotubes contain significant amounts of iron in the form of Fe₃O₄, which even after acid washing, is not removed.

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Carbon nanotubes are at the forefront of chemical research due to their reported unique structural and physical properties and receive extensive attention in a plethora of areas.^1–5 For example, both multi-walled and single-walled carbon nanotubes in electroanalysis and electro-catalysis are becoming to dominate due to the reported ‘catalytic’ properties.^6–10 Such ‘catalytic’ properties include increments in magnitude of voltammetric peak heights, enhanced sensitivities, lower detection limits and little or no surface fouling.¹¹

In many studies where carbon nanotubes are utilised as electrode modifiers we have advocated the alternative use of edge plane pyrolytic graphite electrodes (EPPG).¹² The EPPG electrode is an electrode constructed from highly ordered pyrolytic graphite where the graphite layers are perpendicular to the disc surface and are separated with an interlayer spacing of 3.35 Å.^13,14 Surface defects occur in the form of steps exposing the edges of the graphite layers. Conversely, a basal plane pyrolytic graphite (BPPG) electrode is fabricated such that the layers of graphite lie parallel to the surface.¹⁴

In a quest to understand the electrochemical activity of carbon nanotubes, direct comparison of the response of carbon nanotubes with EPPG and BPPG electrodes allowed us to demonstrate that the origin of the electrochemical response of carbon nanotubes is solely due to edge plane like-sites/defects which occur at the ends or along the tube axis.¹⁵ In particular, the direct comparison of the electrochemical response of carbon nanotubes with that of EPPG electrodes has also allowed an understanding of the role of surface passivation of nanotubes.¹⁶ In the case of the electrochemical oxidation of NADH, the surface passivation of NADH occurs solely at edge plane-like/sites defects. Moreover, due to the high density of edge plane-like sites/defects on the nanotubes the overall electrochemical response is undeterred.¹⁶

However, we have recently found that for the electrochemical oxidation of hydrazine¹⁷ and the reduction of hydrogen peroxide¹⁸ and halothane,¹⁹ the electrochemical response of multi-walled carbon nanotubes does not compare directly with that of an EPPG. That is, the electrochemical response does not originate from edge plane like-sites/defects. We have shown that metal impurities resulting from the fabrication process, which are likely trapped in graphite layers, are the dominate source of the observed electrochemical activity rather than edge plane like-sites/defects. These metallic impurities are predominantly iron oxide^17,18 but in certain cases, copper oxide¹⁹ depending on fabrication and post-production care. Without full characterisation of carbon nanotubes, a fundamental understanding of the origin of the electrochemical activity cannot be attained. In the area of electrochemistry, in particular, electroanalysis, researchers often misplace claims of electro-catalysis on carbon nanotubes.

For the first time, we demonstrate via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) that single-walled carbon nanotubes contain significant amounts of iron. XPS analysis indicates that this is in the form of Fe₃O₄, which even after acid washing, as advocated throughout the literature, does not remove the metal impurity.

In our work we use single-walled carbon nanotubes (SWCNTs) which are received from Carbon Nanotechnologies Inc. (USA). These are classed as “Purified HiPCo^® single-wall carbon nanotubes” which are of research grade and have been purified to remove large catalyst particles (<15 wt% ash content).²⁰ Researchers should note that the nanotubes come with a disclaimer of warranties and are sold “as is” and “that there may be variations in the characteristics of products, and CNI expressly disclaims any warranties related to any samples that CNI may from time to time provide to Purchaser.”²⁰

TEM was conducted on the as-received (unwashed) SWCNTs. Fig. 1 shows a TEM image of the SWCNTs where close inspection reveals tube like structures with metal particles, shown as dark dots on the tubes. Energy Dispersive X-ray Spectrography (EDAX) analysis was conducted which indicated that the particles are composed of iron. TEM images, shown in Fig. 2 reveals a single SWCNT indicating that the majority of the SWCNTs conjugate together as ropes of SWCNTs which makes them appear larger than they really are. Note also that this single SWCNT appears to be free from metal particles. Last, from the TEM images shown in Fig. 2 the metal particles can be estimated at ∼5 nm in diameter or less.

Fig. 1 TEM image of the as-received single-walled carbon nanotubes.

Fig. 2 TEM images of the single-walled carbon nanotubes.

Next, XPS was conducted on the unwashed SWCNTs. Analysis of the XPS data, shown in Fig. 3 from the unwashed SWCNTs indicated that the sample was 93.6 atomic% carbon, 4.9 atomic% oxygen, 0.7 atomic% iron and 0.8% atomic chloride. Typically the nanotubes are fabricated via chemical vapour deposition using a metal catalyst which can be either nickel, cobalt or more commonly, iron. After the nanotubes are fabricated they undergo an acid purification which leaches away the large metal catalysts.²⁰ The presence of chloride as identified via XPS is likely from the initial acid washing procedure, likely to be concentrated HCl, as performed by the supplier. Returning to the XPS, analysis indicates that the iron present on the surface of the SWCNTs is likely Fe₃O₄.²¹ It is important to note that XPS only identifies elements on the surface (<10 nm depth) of the bundle of multi-walled carbon nanotubes (MWCNTs) so that any metal impurities in the centre of a MWCNT bundle may not be detected by XPS because of the surface sensitivity (<10 nm depth) of the technique.

Fig. 3 XPS spectra of unwashed (as-received), washed and super-washed SWCNTs.

Next we explored acid washing the SWCNTs which is recommended prior to the use of carbon nanotubes.^22–25 First a sample of the SWCNTs was placed into a covered beaker containing 2 M nitric acid and stirred for 20 h. This was then filtered and washed with copious amounts of water after which it was allowed to dry overnight at room temperature; this procedure is that advocated by Wang et al. the pioneering group in this area.^22–25 These nanotubes will be referred to hitherto simply as ‘washed CNTs’. Another sample of the unwashed CNTs was taken and subjected to a ‘super-wash’. The only difference between this and the ‘washed CNTs’ is the length of time which they are stirred in 2 M nitric acid, this being 36 h for ‘super-washed’. XPS was conducted on the washed and super-washed samples, the spectra of which are shown in Fig. 3. Analysis of the washed and super-washed nanotubes with XPS indicated identical responses as that of the as-received carbon nanotubes; clearly there is no effect of acid washing the nanotubes.

It is interesting to note that XPS data analysis of as-received MWCNTs¹⁷ indicated that the sample was 98.9 atomic% carbon, 1.0 atomic% oxygen, 0.1 atomic% iron and a trace (<0.1 atomic%) copper and sulfur. Comparison of XPS data from the MWCNTs with the SWCNTs indicates a greater amount of iron is present on the SWCNTs.

While it may be noted that larger catalyst particles, ∼100 nanometres in diameter are removed during post-production,²⁰ from the above data it is clear that for SWCNTs, iron impurities, remaining from the growth process of the nanotubes, cannot be simply removed by acid washing. From TEM data we are at present, unable to determine whether the iron is within or on the outside of the SWCNTs. Given the likely growth process of the nanotubes via metal catalysts, and the fact that large particles are removed during post-fabrication²⁰ we surmise that the iron is likely trapped within several layers of graphite and cannot be removed by the acid washing.

In summary we advocate that researchers assign caution in using SWCNTs in electrochemistry, in particular electroanalysis, and do not take it for granted that their electrochemical or ‘electro-catalytic’ response originates solely from edge plane sites/defects^15,16 but rather metallic impurities remaining from the fabrication process of the SWCNTs may also play an important role.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section. See DOI: 10.1039/b615824b

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