Ultrathin Single–Layered Membranes from Double–Walled Carbon Nanotubes

Abstract

Substantive work has been carried out on microscopic and one–dimensional carbon nanotubes (CNTs), owing to their outstanding mechanical and electrical properties and their attractive application potential. Macroscopic CNTs are also essential for diverse applications, such as composites, conductors, sensors, and catalyst templates. After the first preparation of 40 cm long single–walled CNT (SWCNT) strands, macroscopic CNT yarns have been fabricated by spinning well–aligned CNT arrays, directly spinning and pulling of CNTs in a reaction furnace, and chemical infiltration of CNTs. However, for applications with large two–dimensional lateral surface areas, such as field–emission displays, fuel cells, and supercapacitors, it is desirable to grow or transfer nanotubes directly onto an appropriate surface to form film–like structures with a controlled thickness. As a relatively new form of nanotubes, double–walled CNTs (DWCNTs) have attracted great research interest in recent years because of their unique coaxial structure and promising mechanical, electrical, optical, and thermal properties compared to SWCNTs and multiwalled CNTs (MWCNTs). Recent advances in the large–scale synthesis of DWCNTs have made them attractive for a variety of applications. DWCNT “buckypapers”, about 2.5 cm in diameter and more than tens of micrometers in thickness, were obtained by infiltrating purified DWCNTs. We have previously developed a two–step purification protocol to obtain pure and clean DWCNT films, which retain their film–like structures after H2O2 and HCl treatments. It is well known that the Langmuir–Blodgett (LB) technique can produce mono– and multilayered organic–molecule membranes, with controlled thicknesses ranging from about 1 nm up to tens of micrometers. LB films consisting of microscopic SWCNTs have indeed been fabricated from organic solutions. In this communication, we report a novel and simple approach, similar to the LB technique, to controllably fabricate two–dimensional DWCNT membranes of only a few tens of nanometers in thickness. The raw DWCNT samples, containing several to tens of thin layers, were prepared by a chemical vapor deposition (CVD) method, described in detail in our recent paper. The black and sticky films with a thickness of several micrometers are comprised mainly of DWCNT bundles, together with amorphous carbon and catalyst particles coated with several layers of graphenes. Ultrathin DWCNT membranes could be obtained via a post–purification treatment and the generation process is illustrated in Figure 1. The raw DWCNT films (DWCNT–1) were first immersed into a H2O2 solution (30 %) for 72 h (DWCNT–2), which was followed by treatment in HCl solution (37 %) (DWCNT–3). The treated samples were then rinsed in distilled water until a pH value of 7 was reached. Addition of a few drops of ethanol or acetone to the purified DWCNT in water led to the rapid flotation of a DWCNT film to the water surface, which subsequently extended to a large thin film. The resulting film can be collected easily from the water surface with any substrate, such as a silicon wafer, copper grid, foil, or hollow metal ring, for further characterization and use. The thickness of the DWCNT film was effectively reduced by the H2O2 and HCl purification processes as the amorphous carbon and metal catalyst particles were successfully removed. The resulting membranes are so thin that they appear to be fully transparent. This was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization, which showed a single–layered structure and, thus, these films differ significantly from the as–prepared DWCNT samples. In order to understand the fundamentals behind the property changes of the DWCNT surface during the purification C O M M U N IC A IO N S