Exposing Multiple Roles of H2O in High-Temperature Enhanced Carbon Nanotube Synthesis

The synthesis of single-wall carbon nanotubes (SWNTs), regardless of synthesis route, also generates significant amounts of undesired impurities such as catalyst particles, amorphous carbon, and graphitic particles. The need for pure samples is important and for certain applications or studies is paramount. A variety of postsynthesis purification procedures have been developed to remove impurities. Usually, a combination of purification steps are employed, most of which exploit oxidation. A disadvantage is that many lead to the partial etching of SWNTs and damage their structure. Less-aggressive oxidizing agents have also been explored. The use of steam can efficiently etch amorphous carbon and open the ends of single and multiwalled carbon nanotubes. CO2 is another weak oxidizer that can selectively remove amorphous carbon. Obviously, the direct synthesis of high purity SWNTs is far more attractive. A well-known example is the synthesis route developed by Hata et al. They demonstrated that small additions of water vapor (50-290 ppm) to the buffer gas during the chemical vapor deposition (CVD) of supported catalysts yields superdense and vertically aligned ultralong SWNTs. They claim this technique leads to SWNTs purities above 99%. This type of approach can eliminate the need for postsynthesis processing. However, SWNTs fabricated via CVD tend to possess higher structural disorder, large diameter distributions and usually include a fraction of multiwalled carbon nanotubes (MWNTs) as compared to floating catalyst techniques such as the highpressure carbon monoxide (HiPCO) process or laser evaporation. Remarkably, though, we have found no reports on the use of water vapor in laser ablation reactions for the formation of SWNTs. The potential advantages of high-purity SWNT samples with few defects and a narrow diameter distribution are obvious. In this work, we investigate the influence of water vapor on the formation of SWNTs in laser ablation. The SWNTs were synthesized using a furnace-based laser ablation process discussed elsewhere. In brief, the evaporation of the target was driven by a Nd:YAG laser (ca. 2 GW per pulse at 10 Hz). The ternary catalyst was composed from nickel, cobalt, and molybdenum (5:4:1 respectively). The oven temperature was maintained at 1200 °C in each test. All experiments were performed in a N2 atmosphere at a pressure of 101 kPa and a gas flow rate of 0.4 slpm. The water vapor was controlled via mass flow controllers from 3 to 2063 ppm (commercial dry N2 has a water vapor content of 3 ppm). N2 saturated with water vapor was achieved by bubbling the N2 through a stainless steel scrubber (22 °C). Typical ablation times were 120 s. The products were collected on a water-cooled copper finger behind the target. The average mass reduction of the target was 20 mg, which corresponds to an erosion rate rate of 10 mg/min. No differences in the erosion rate were observed when including H2O. The optical absorption spectroscopic (OAS) studies were conducted on a Bruker IFS 113V. A Bruker IFS100 spectrometer (1064 nm) was used for the Raman measurements. For transmission electron microscopy (TEM) investigations (FEI Tecnai F30), the samples were directly pressed on standard Cu TEM grids. Detailed TEM studies for all samples showed the samples to comprise SWNTs bundles, catalyst particles, and amorphous carbon. The amount of amorphous carbon was seen to decrease with increasing water content. In Figure 1, TEM images for 3 samples (3, 135, and 1227 ppm) show the * Corresponding author. E-mail: [email protected]. † IFW Dresden. ‡ Warsaw University. § Dresden University of Technology. | Vienna University. (1) Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H. J. Phys. Chem. B 2001, 106, 8297. (2) Magrez, A.; Seo, J. W.; Kuznetsov, V. L.; Forro, L. Angew. Chem., Int. Ed. 2007, 46, 441. (3) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362. (4) Rummeli, M. H.; Kramberger, C.; Loffler, M.; Jost, O.; Bystrzejewski, M.; Gruneis, A.; Gemming, T.; Pompe, W.; Buchner, B.; Pichler, T. J. Phys. Chem. B 2007, 111, 8234. (5) Rummeli, M. H.; Borowiak-Palen, E.; Gemming, T.; Pichler, T.; Knupfer, M.; Kalbac, M.; Dunsch, L.; Jost, O.; Silva, S. R. P.; Pompe, W.; Buchner, B. Nano Lett. 2005, 5, 1209. Figure 1. TEM micrographs of as-produced SWNTs from laser evaporation with different H2O content: (a, b) 3, (c, d) 135, and (e, f) 1227 ppm. 6586 Chem. Mater. 2008, 20, 6586–6588