(781b) In Situ Characterizing Catalyst Structure and Chemical State During Carbon Nanotube Growth

Carbon nanotubes (CNTs) are extensively used in various applications, such as biotechnology, nanotechnology, etc., depending upon their structure and morphology. In order to exploit their unique properties, controllable synthesis of monodispersed CNTs with the same structure and morphology and high yield is needed. A common route for CNT synthesis is catalytic chemical vapor deposition (C-CVD), which involves metal catalyst (i.e. Ni, Fe, Co) to decompose carbon precursors and to provide a nucleation site for growing CNTs. It is conceivable that understanding the structure and the chemistry of the catalyst nanoparticles during CNT nucleation and growth will lead to the conditions needed for their controlled synthesis. Environmental scanning transmission electron microscope (ESTEM) allows gas injections up to 2000 Pa, has been successfully employed to synthesize and monitor CNT nucleation and growth mechanism simultaneously.  It has been shown that in situ observation using ESTEM can also reveal the state of the catalyst during CNT growth at atomic scale[1,2].  However, the process of catalyst decomposing the carbon precursor and CNT nucleation remains unknown.  In addition, how the structure and the chemistry of catalysts during nucleation affect the CNT growth hasn’t been revealed.  Here, we used Ni and Fe NPs as catalysts on SiO_x film and acetylene as carbon precursor to nucleate CNT in the ESTEM.  Acetylene pressure and CNT growth temperature can be easily adjusted for different growth conditions. Video-rate high resolution images were recorded to capture the reduction of the oxidized catalysts and the CNT nucleation at atomic resolution in TEM mode. Moreover, in situ electron-energy loss spectroscopy (EELS) was also employed to evaluate the metal catalysts oxidation state and composition.  The changes in the structure and chemical state of the catalyst at nucleation stage under different growth conditions with respect to the CNT growth mechanism will be presented.

References

[1]        R. Sharma, et al., Nano Letters 2009, 9. 689

[2]        R. Robertson et al., J. Nano Sci. Nanotech. 2008, 8, 6105-6111