Large-scale purification of single-wall carbon nanotubes: process, product, and characterization

We describe, in detail, a readily scalable purification process capable of handling single-wall carbon nanotube (SWNT) material in large batches. Characterization of the resulting material by SEM, TEM, XRD, Raman scattering, and TGA shows it to be highly pure. Resistivity measurements on freestanding mats of the purified tubes are also reported. We also report progress in scaling up SWNT production by the dual pulsed laser vaporization process. These successes enable the production of gram per day quantities of highly pure SWNT, which should greatly facilitate investigation of material properties intrinsic to the nanotubes. PACS: 81.15T; 72.80R; 61.48 In 1996, a dual pulsed laser vaporization (PLV) technique for the generation of single-wall carbon nanotubes (SWNT) was reported [1]. This produced 70–90 vol.% SWNT, organized in hexagonal close-packed bundles (ropes). As the result of prior theoretical predictions concerning their novel electronic properties [2] and anticipated extreme tensile strength, this finding led to a dramatically heightened interest and demand for this exciting new material. In an attempt to satisfy this demand the Rice group undertook to scale up SWNT production by the PLV process. This work revealed a high sensitivity of the material quality (fractional SWNT yield) to several parameters, some of which turn out to be incompatible, in an engineering sense, with large-scale production of the highest quality material. This compromise of raw-material quality for quantity required that a collateral battle be fought on the material purification front. We report on a process developed for purification of large batches of SWNT material resulting in high-purity SWNT, essentially independent of the material starting quality. The purified SWNT material is characterized by electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, and thermo-gravimetric analysis (TGA). Temperature-dependent resistivity measurements performed on freestanding mats of the purified tubes are also reported. Other techniques for purification of SWNT are found in the literature [3–5]. These all suffer from the problem that they are microscale techniques, of varying degrees of effectiveness, which have not proved useful for purifying large batches of moderate-quality material. The purification process reported here, in contrast, is a macroscale technique which may readily be scaled further to industrial levels of throughput when such volumes of SWNT material become available. We additionally report on some observations concerning the differences in SWNT material made under distinct growth conditions encountered during the production scale-up. In particular, it is found, in corroboration of an independent (and significantly more systematic) investigation [6] of the phenomena, that the SWNT diameter distribution shifts to larger diameters when the vaporization and SWNT growth takes place at a higher surrounding background temperature. 1 SWNT material production The limiting factor in the original 1′′-flow-tube system (described previously [1]) was plugging of the tube around the target by the web-like SWNT deposit. The materials used in the present investigations were made in two different PLV systems which were successive scale-ups from the original apparatus. A description of each and the production conditions pertaining to the resulting material follows.