Electrochemical properties of aligned nanotube arrays: basis of new electromechanical actuators

Carbon nanotubes are of major interest because they combine nanoscale dimensions, structural regularity, exceptional mechanical properties, and interesting electrical properties. We have been primarily concerned with the use of nanotube assemblies as novel electromechanical actuators. The use of nanotube sheets as electromechanical actuators (artificial muscles) has provided encouraging results. However, the mechanical performance observed to date for nanotube sheets are far below those inherent for the individual tubes, and this aspect has severely limited the stresses generated by actuation. The previously demonstrated nanotube actuators are based on tangled arrays of bundled single-wall nanotubes. We here show the first example of an electromechanical actuator based on multi-wall carbon nanotubes. In contrast with the disordered assembly of bundled nanotubes in the previous actuator sheets, the present actuator sheets are based on arrays of parallel non-bundled multi-wall nanotubes in which the tube direction is orthogonal to the sheet plane. The multi-walled nanotubes in these sheets have a defined length (in the range 5 to 40 mu m) and diameter (in the range 10 to 60 nm). While the previous single-wall nanotube actuators are based on chain-direction dimensional changes, the present actuators utilize the electrostatic repulsion between electrical double layers associated with parallel multi-wall nanotubes. The electrochemical properties of these aligned nanotube arrays was studied in a number of different electrolytes, since electrochemical charge injection is important for actuation and for other nanotube applications - such as for energy storage in a supercapacitor. In addition, we have demonstrated the electrochemical deposition of conducting polymer on the nanotube arrays, and have shown that the resulting polymer/nanotube composites provide a high pseudocapacitance.