In situ Raman spectro-electrochemistry study of single-wall carbon nanotube mat

Abstract

A carbon nanotube-based electrochemical actuator has been investigated using in situ Raman spectro-electrochemistry. A sheet of single-wall carbon nanotubes (SWNTs) is dipped into an electrolyte and it expands or contracts if a voltage is applied between them and a counter electrode. It has been predicted that actuation is primarily due to changes in orbital occupation and band structure with a secondary contribution arising from double-layer electrostatic effects. However, the relative contribution and interaction between each of these mechanisms are unclear. We built a linear actuator from SWNT mat and studied its actuation in several alkali (Li, Na, K) and Ca halide and sulfate solutions in order to clarify the role of cation as mobile ions in the film. The variation of bonding with applied potential was monitored using in situ Raman spectroscopy. Raman can detect changes in CC bond length, because the radial breathing mode at ∼190 cm −1 varies inversely with the nanotube diameter and the G band at ∼1590 cm −1 varies with the axial bond length. In addition, the intensities of both the modes vary with the emptying or filling of the bonding and anti-bonding states. We discuss the variation of spectroscopic observables (intensity/frequency) of these modes providing valuable information on the charge transfer dynamics on the SWNTs mat surface and indirectly quantify some of the parameters that include in-plane compressive strain (∼−0.25%) and the charge transfer per carbon atom ( f c∼−0.005). The cyclic voltammetry and a.c. impedance spectroscopy are described briefly and help to demonstrate well-developed capacitive behavior of SWNT mat.