Multi Walls Carbon Nanotubes

...the realisation that edge plane pyrolytic graphite is an ideal electrode for many electrode reactions of analytical interest, as for example we have showed for the oxidation of NADH. The protein electrochemists have known this for two decades without quite fully realising why. Now that the difference between edge and basal plane zones of graphitic surfaces are much better appreciated there is and will increasingly be a significant trend for electroanalytical chemists to explore this material.

More generally the electroanalytical community en masse needs to realise that a little caution is required before jumping on bandwagons of the carbon nanotube type. It has been truly alarming seeing the sheer number of papers reporting naïve experiments in which nanotubes are scattered over an electrode which then assumes apparently magical electrocatalytic qualities in respect of some desired target molecule without the obvious control experiments ever being done. The work of Dr Craig Banks in my group has shown that in all the cases we have looked at, the catalysis either is no greater than seen at edge plane graphite or else results from metal oxide or metal nanoparticles trapped in the tubes as a result of the growth using CVD on substrates which are the source of the nanoparticles.

Nanoparticles of course are now highly interesting in their own right, for example as electrode surface modifiers and for contrasts in their chemical behaviour from the corresponding bulk material. Ultimately this will lead to designer interfaces chemically synthesised for specific analytical tasks. Hitherto modified electrodes have been considered mostly from a purely chemical point of view—the real attractions lie in simultaneously controlling the physical aspects, most notably diffusive transport.

Which research paper from the last two years are you most proud of, and why?

The last two years have been bumper years—the best ever for quantity. My group published 72 papers in 2004 and 77 in 2005. The best cited are a review article written with graduate students Marisa Buzzeo and Russell Evans concerned with electrochemistry in room temperature ionic liquids1 and a paper in Analytical Chemistry2 with Craig Banks (then a final year graduate student) and Ryan Moore (an undergraduate project student) showing that carbon nanotubes are in many cases no more electrocatalytic than the much vaunted multiwalled carbon nanotubes. Both of these already have more than 45 cites, as I write in mid-2006, so professionally speaking I guess I am most proud of these.

Scientifically however, I am most proud of a paper with Trevor Davies3 in which we show you can use simple cyclic voltammetry to measure the size of particles on the surface of an electrode. At first sight it seems that this is a simply geometrical blocking problem but since material can diffuse round the block it actually represents a very challenging theoretical problem, especially since the particles will be randomly distributed over the electrode surface and might be of any size. The match between experiment and theory was beautiful and really convinced me for the first time that we really understood the randomness problem and were tackling it correctly. The implications of this have had wide significance, not least in the rigorous characterisation of microelectrode arrays.4 Indeed this nicely illustrates a philosophy of the group; fully understand the physical chemistry of a problem and then exploit the insight for real world analytical measurements.

www.rsc.org; The "Analyst" profiles Richard Compton, Professor of Chemistry at the University of Oxford and the first and only recipient of both the RSC Medals in Electrochemistry and in Electroanalytical Chemistry.







Carbon Nanotubes in Electrochemistry: Applications to Nano-Electrochemical

A proportionally higher number of edge-plane-like defect sites occur on herringbone and bamboo MWCNTs (multi wall carbon nano tubes) because in these two cases the plane of the graphite sheets are at an angle to the axis of the tube requiring a high pro-portion of the graphite sheets to terminate at the surface of the tube. CNTs have two distinct possible reactive sites; these are edge plane like sites/defects and basal plane sites which occur on the side walls of the CNTs. The terms 'edge plane' and 'basal plane' are in comparison with the structure of highly ordered pyrolytic graphite. For the first time, we demonstrated edge plane like sites/defects are responsible for this reported electrocatalysis in most cases.

...Recently we also demonstrate that trapped metal impurities in carbon nanotubes
are responsible for certain electro-catalysis . The image shows the electrochemical
oxidation of hydrazine at various electrodes. The observed electro-catalysis signal observed at the CNTs is not due to edge-plane sites/defects on the CNTs but must derive from i r o n impurities.

Summary
- MWCNTs shows excellent and quantitative voltammetric responses corresponding to the
oxidation of ammonia
- The propylene carbonate molecule cannot penetrate between the concentric tubes and hence there is no intercalation within the carbon nanotubes
- MWCNTs provide new avenues in otherwise in intercalating media for sensing of electroactive species.


Comparison of palladium nanoparticle decorated glassy carbon electrode

The contrasting behaviour of the palladium decorated multi-walled carbon nanotubes where stripping does not remove the sub-nanoparticles, with that of the palladium decorated glassy carbon electrodes where there is complete stripping, suggests that the palladium exhibits an electrochemical metastability on the carbon nanotubes. However, that said, the palladium on the glassy carbon electrode appears not to have a metastability since it is fully anodically stripped off whilst being in the reported size region required for metastability to occur (< 20 nm). This in part suggests that rather than metastability the unique structural property of the carbon nanotubes supports are responsible of the observed stability of the palladium particles on the MWCNTs.

Summary
- The behaviour of the palladium decorated multi-walled carbon nanotubes where stripping does not remove the sub-nanoparticles shows that the palladium exhibits an electrochemical metastability on the carbon nanotubes.
- A generic approach for electrochemical detection of analytes has been presented in potential ranges that would normally be inhibited due to the stripping of the electrocatalytic metal.


http://compton.chem.ox.ac.uk/news/2006/poster%20Xiaobo.pdf
Refs:
- Banks, C. E.; Moore, R. R.; Davies, T . J.; Compton, R. G. chemical commun.
2004, 16, 1804
- Moore, R.R.; Banks, C.E. Compton, R.G. Anal Chemical 2004, 76, 2677
- Abrahamson, J; Wiles, P G; Rhoades, B L Carbon 1999


The influence of edge-plane defects and oxygen-containing surface groups on the voltammetry of acid-treated, annealed and “super-annealed” multiwalled carbon nanotubes


The role of edge-plane-like defects at the open ends of multiwalled carbon nanotubes (MWCNTs) and at hole defects in the tube walls is explored using cyclic voltammetry with two charged redox probes, namely potassium ferrocyanide and hexaamineruthenium(III) chloride in unbuffered aqueous solutions, and one neutral redox probe, norepinephrine, in pH 5.7 buffer. Further, the presence of oxygen-containing functional groups (such as phenol, quinonyl and carboxyl groups), which decorate the edge-plane defect sites on the voltammetric response of the MWCNTs, is also explored. To this end, three different pre-treatments were performed on the pristine MWCNTs made using the arc-discharge method (arc-MWCNTs). These were (a) arc-MWCNTs were subjected to acid oxidation to form acid-MWCNTs—open-ended MWCNTs also possessing numerous hole defects revealing a large number of edge-plane-like sites heavily decorated with surface functional groups; (b) acid-MWCNTs, which were subsequently vacuum-annealed at 900 °C to remove the functional groups but leaving the many undecorated edge-plane-like sites exposed (ann-MWCNTs); (c) ann-MWCNTs, which were subjected to a further vacuum “super-annealing” stage at 1,750 °C (sup-MWCNTs), which caused the hole defects to close and also closed the tube ends, thereby, restoring the original, pristine, almost edge-plane defect-free MWCNTs structure. The results of the voltammetric characterisation of the acid-, ann- and sup-MWCNTs provide further evidence that edge-plane-like sites are the electroactive sites on MWCNTs. The presence of oxygen-containing surface groups is found to inhibit the rate of electron transfer at these sites under the conditions used herein. Finally, the two charged, “standard” redox probes used were found to undergo strong interactions with the oxygen-containing surface groups present. Thus, we advise caution when using these redox probes to attempt to voltammetrically characterise MWCNTs, and by extension, graphitic carbon surfaces.


Andrew Holloway, Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford,
http://www.springerlink.com/content/f888g14jr714330k/



...There exist in the literature a variety of methods of fabricating arrays of metal nanoelectrodes on electrode surfaces, such as electrochemical or electroless deposition of metal nanoparticles onto a suitable electrode substrate [9,10]. These methods produce random arrays of nanoparticles on the electrode surface, such that the array rarely, if ever, is diffusionally independent on most practical experimental timescales. Lithographic techniques of fabricating ordered micrometer and nanometer sized metal arrays are progressing, such as the use of ion beam milling [11,12] and/or nanoimprinting [13], but these techniques are limited in that they can only produce nanoelectrodes with diameters of the order of 100 nm, whilst a significant fraction of the individual electrodes in the array are not in electrical contact with the substrate, forming “dead” electrodes [14]. Conducting forms of carbon, including graphite, glassy carbon, carbon nanotubes and boron doped diamond (BDD) have been shown to have desirable properties as electrode substrates for electroanalysis of a wide range of analytes.

Fabricating random arrays of boron doped diamond nano-disc electrodes: Towards achieving maximum Faradaic current with minimum capacitive charging; Lei Xiao, Ian Streeter, Gregory G. Wildgoose, Richard G. Compton; Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University

Science Direct, Jan 2008
Ref.:
[9] A.O. Simm, S. Ward-Jones, C.E. Banks, R.G. Compton, Novel methods
for the production of silver microelectrode-arrays: their characterisation
by atomic force microscopy and application to the electroreduction of
halothane, Anal. Sci. 21 (2005) 667–671.
[10] C.M.Welch, R.G. Compton, The use of nanoparticles in electroanalysis: a
review, Anal. Bioanal. Chem. 384 (2006) 601–619.
[11] Y.H. Lanyon, D.W.M. Arrigan, Recessed nanoband electrodes fabricated
by focused ion beam milling, Sens. Actuators B 121 (2007)
[12] Y.H. Lanyon, G. De Marzi, Y.E. Watson, A.J. Quinn, J.P. Gleeson, G.
Redmond, D.W.M. Arrigan, Fabrication of nanopore array electrodes by
focused ion beam milling, Anal. Chem. 79 (2007) 3048.
[13] M.E. Sandison, J.M. Cooper, Nanofabrication of electrode arrays by
electron-beam lithography and nanoimprint lithographies, Lab. Chip 8
(2006) 1020.
[14] O. Ordeig, C.E. Banks,T.J. Davies, J. del Campo, R. Mas, F.X. Munoz, R.G.
Compton, Regular arrays of microdisc electrodes: simulation quantifies the
fraction of ‘dead’ electrodes, Analyst 131 (2006) 440–445.




Nanotubes

....Researchers are also getting excited about the mechanical properties of nanotubes. Single-walled tubes are very strong, and are stiffer than the conventional carbon fibres currently used in composite materials. They can also be bent double and yet spring back into shape without suffering any apparent damage.

Such properties have led to some surprising applications. For example, the resolution of scanning probe microscopes — which provide images of a surface by moving a fine tip across it — depends on the sharpness of the probe's tip. And the longer the tip, the more deeply the microscope can probe into surface cavities. In 1996, Smalley and colleagues showed that mounting a nanotube on the microscope's tip improves both resolution and penetration2 — and the tube's strength means that it will simply bend, instead of snapping, if it is pushed a little too far into the surface. Charles Lieber's group at Harvard University has since extended the concept by adding chemical groups to the end of the tips, so that the microscope can recognize the chemical composition of the surface being studied14. Further work indicates that a wide range of different substances, including biological molecules, can be attached to the tubes.

Together with his colleague Philip Kim, who is now at Columbia University in New York, Lieber has also used nanotubes as extremely fine tweezers3. They attached two nanotubes to a glass rod patterned with two electrodes. Giving these electrodes different charges creates an electrostatic attraction between the nanotubes, which makes them bend towards each other. This allowed Lieber and Kim to use the tubes to pick up and move tiny polymer beads. A team led by Yoshikazu Nakayama at Osaka Prefecture University in Japan has recently created similar tweezers on the tip of a scanning probe microscope17, raising the possibility that the microscope could be used both to observe and to manipulate samples, such as individual cells.

Nature, Roll up for the revolution, Nature 414, 142-144 (8 November 2001)
www.nature.com/nature/journal/v414/n6860/full/414142a0.html



Resonance Raman Spectroscopy to Study and Characterize Defects on Carbon Nanotubes and other Nano-Graphite Systems

The use of resonance Raman spectroscopy (RRS) to study and characterize single wall carbon nanotubes (SWNTs) is discussed, focusing on preliminary efforts for the development of the RRS to characterize defects in SWNTs. The disorder-induced D-band, disorder-induced peaks just above the first-order allowed graphite G-band, as well as the intermediate frequency modes (IFMs) appearing between the RBM and the D/G spectral region are addressed. RRS on nanographite ribbons and on a step-like defect in highly ordered pyrolytic graphite (HOPG) sheds light into the problem of characterizing specific defects in nano-related carbons.

Materials Research Society

http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2717&DID=114975&action=detail



High-resolution electron microscopy of multi-wall carbon nanotubes in the subcutaneous tissue of rats.

The atomic structure of multi-wall carbon nanotubes (MWCNTs) implanted in the subcutaneous tissue of rats was examined by means of high-resolution transmission electron microscopy (HRTEM). Clusters of the MWCNTs implanted in the subcutaneous tissue were well recognized by the TEM observations. It was indicated that some nanotubes were taken in phagocytes after the 1-year implantation. The deterioration of crystalline structure of the nanotubes in phagocytes was shown by the HRTEM observation. It was suggested that the deterioration of the nanotubes was due to the peeling of the outer graphene layers in the phagocytes.

Journal Electron Microscopy, 2008
http://www.ncbi.nlm.nih.gov/pubmed/18799809?dopt=Abstract





Reversible Metal−Insulator Transitions in Metallic Single-Walled Carbon Nanotubes

We report on reversible metal to insulator transitions in metallic single-walled carbon nanotube devices induced by repeated electron irradiation of a nanotube segment. The transition from a low-resistive, metallic state to a high-resistive, insulating state by 3 orders of magnitude was monitored by electron transport measurements. Application of a large voltage bias leads to a transition back to the original metallic state. Both states are stable in time, and transitions are fully reversible and reproducible. The data is evidence for a local perturbation of the nanotube electronic system by removable trapped charges in the underneath substrate and excludes structural damage of the nanotube. The result has implications for using electron-beam lithography in nanotube device fabrication.

Christoph W. Marquardt et al,
http://pubs.acs.org/cgi-bin/abstract.cgi/nalefd/2008/8/i09/abs/nl801288d.html
Nano Lett., 8 (9), 2767–2772, 2008. 10.1021/nl801288d
Web Release Date: August 14, 2008