High Levels of Electrochemical Doping of Carbon Nanotubes:  Evidence for a Transition from Double-Layer Charging to Intercalation and Functionalization

Abstract

We studied the transition from the electrochemical double-layer charging regime to intercalative doping of bundled single-walled carbon nanotubes (SWNT) in KCl and HCl aqueous solution. For this purpose we used high doping levels by applying constant potentials above 1000 mV approaching and slightly exceeding the oxidation potential for Cl -ions. At each potential in situ Raman measurements of the radial breathing mode (RBM), the high-energy tangential mode (HEM), and the disorder-induced (D) mode were performed. Furthermore, the conductivity and reflectivity of a set of SWNT samples were measured as a function of doping and subsequently the samples were examined by X-ray photoelectron spectroscopy (XPS). From a comparative analysis of the results we conclude that above 1000 mV a significant penetration of chlorine species into the interstitial channels of the SWNT bundles and possible covalent functionalization take place.

Spectroelectrochemistry of Carbon Nanotubes

“…[19] The nanotube sample is usually a thin solid film on a working electrode support, such as Pt, Au, Hg, indium-tin oxide (ITO) and so forth, but a self-standing buckypaper can also be used. [20][21][22][23][24][25][26][27] Experiments with SWNT solutions employ either surfactant-dispersed SWNT in aqueous or aprotic solvents [28] or "true solutions" of polyelectrolyte/SWNT salts (with Na + or K + counterions) in dimethylsulfoxide (DMSO). [9] The electrochemical charging is also carried out in a geometry mimicking the field-effect transistor (FET).…”

“…Furthermore, supporting electrolytes like chlorides may trigger reactions with the SWCNT at anodic potentials. [27] The aprotic solvents were acetonitrile, [17,20,34, tetrahydrofuran, [25] DMSO, [9] ethylene carbonate/dimethylcarbonate [25] or ethylene carbonate/diethyl carbonate, [73] but the broadest electrochemical window is available in ionic liquids. [18] Polymer electrolytes, such as LiClO 4 + PEO (polyethylene oxide), [29,74,75] polyethyleneimine [76] or PC/PMMA (propylene carbonate/polymethylmethacrylate) [32,77,78] are useful for single nanotube studies.…”

“…Exceeding this window triggers additional faradayic reactions, including intercalation and covalent functionalization of SWNT. [27] 4. Vis/NIR Spectroelectrochemistry ).…”

“…Stronger charging causes even chemical modification of SWNT which can also be followed by this method. [27] Wang et al [14] suggested that the attenuation of Raman intensity (resonating with DE 22 S ) can be used for determination of the exciton binding energy. This conclusion provoked some debate [16] and lead, eventually, to the different interpretation that the electrochemical energy gap is a superposition of the exciton binding energy with the DE 11 S .…”

Identification and separation of metallic and semiconducting carbon nanotubes

“…The method and the results up to 2007 are summarized in the review of Kavan and Dunsch (Kavan and Dunsch 2007). An intriguing possibility has been put forward recently: at high enough applied potential, enough chem- ically active ingredients are produced in the electrochemical cell to form covalent bonds and thereby lower the conductivity (Rafailov et al 2008). The transition was monitored by reflectivity and XPS measurements.…”