Bimetallic catalysts such as Fe-Co and Co-Mo have been used for efficient growth of bulk single-walled carbon nanotube (SWNTs) and vertically aligned SWNTs [1]. Recently, bimetallic catalysts such as Co-W [2] or Co-Cu [3] are employed for structure controlled growth of SWNTs. Here, roles of bimetallic catalysts are explored by newly proposed SiO₂ grid based in-plane TEM technique, which enables a direct TEM characterization of catalysts on SiO₂, CVD growth of SWNTs in normal furnace, and TEM characterization of catalysts and grown nanotubes after CVD.

The in-plane TEM results of the traditional Co-Mo bimetallic catalysts were consist with our previous report [1]; metallic Co nanoparticle embedded in Co-Mo oxide is responsible for the nucleation of SWNTs. By using Co-Cu catalysts, we can synthesize vertically aligned SWNTs with subnanometer diameters on quartz (and SiO₂/Si) substrates [3]. EDS-STEM and HAADF-STEM imaging of the Co-Cu bimetallic catalyst system showed that small Co catalysts were captured and anchored by adjacent Cu nanoparticles, which grew small diameter of SWNTs. High-melting point W₆Co₇ alloy produced from metal cluster precursors is reported to grow a single chirality (12,6) with over 90 % abundance [2]. Here, we show that a sputtered Co-W catalyst can selectively grow (12,6) SWNTs by CVD at lower reduction and growth temperatures [4]. Statistical Raman mapping analysis and optical absorption spectrum of the as-grown SWNTs reveal that the abundance of (12,6) is over 60 %. The morphology and structure of catalyst is investigated by the in-plane TEM before and after CVD growth. The catalyst alloy we produced was W₆Co₆C and the only metallic Co was left after the CVD of 5 min. By comparing the catalyst TEM images with variable CVD period, we can discuss the evolution of the alloy catalysts. The W rich initial catalyst is composed of bcc W bound with W₆Co₆C. Once ethanol is introduced, additional W will be gradually removed to the gas phase product of WO₃ and the ratio of W decreases. It is speculated that after the formation of pure W₆Co₆C, further loss of W will result the precipitation of Co; nucleation of Co cluster which is primary responsible for growth of an SWNT.

This work was supported by JSPS KAKENHI Grant Numbers JP25107002, JP15H05760, and IRENA Project by JST-EC DG RTD, Strategic International Collaborative Research Program, SICORP. Part of this work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). We also acknowledge supports from Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References:

[1] M. Hu, Y. Murakami, M. Ogura, S. Maruyama, T. Okubo, J. Catalysis, 225 (2004) 230.

[2] F. Yang et al., Nature 510 (2014) 522.

[3] K. Cui, A. Kumamoto, R. Xiang, H. An, B. Wang, T. Inoue, S. Chiashi, Y. Ikuhara, S. Maruyama, Nanoscale, 8 (2016) 1608.

[4] H. An, A. Kumamoto, H. Takezaki, S. Ohyama, Y. Qian, T. Inoue, Y. Ikuhara, S. Chiashi, R. Xiang, S. Maruyama, Nanoscale, 8 (2016) 14523.