Metal-Catalyst-Free CVD Synthesis of Single Wall Carbon Nanotubes
|There are several advantages to metal-catalyst-free Carbon nanotubes, from use in a broader range of applications to easier purification, mitigated toxicity concerns, and lower prices of pure CNT materials. Synthesis of single wall carbon nanotubes (SWNT) on non-metallic catalysts, as recently carried out by Liu B. et al. and Huang S. et al., demonstrates that one could generalize standard synthesis processes to capture the fundamental physics underlying the CNT nucleation and growth. |
A review by Hyung Gyu Park, Ph.D, Assistant Professor, D-MAVT, ETH Zurich, Zurich, Switzerland
- Liu B. et al. 2009. Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes, J. Am. Chem. Soc. 131 (6): 2082-2083. DOI: 10.1021/ja8093907
- Huang S. et al. 2009. Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes on Substrates, J. Am. Chem. Soc. 131 (6): 2094-2095. DOI: 10.1021/ja809635s
Since the unique physical properties of Carbon nanotubes (CNT) were recognized, critical research has centered on tailoring the properties of nanotubes at the synthesis stage. The unique properties of CNT are believed to arise from their nanoscale morphology; therefore the primary synthesis issues include controlling nanotube geometry such as diameter, length, chirality, etc. A host of research during the last decade has pointed out that reactant gas composition, thermodynamic conditions of the reactor chamber, additives and catalysts are some of the most important control factors in the CVD synthesis of CNT. Of these, catalysts remain the primary but less thoroughly-explored factor. Until a few years ago, it was believed that CVD synthesis of single wall carbon nanotubes (SWNT) should incorporate transition metal nanoparticles, such as Fe, Ni, and Co, as catalysts. However, recent investigations on SWNT CVD are expanding this range of catalyst materials toward non-transition metals, noble metals, and most promisingly toward non-metallic materials.
CNT growth on non-metallic nanostructures can facilitate application of the CNT materials to semiconductor technology, pharmaceutical technology, membrane technology and basic sciences. Metal-free SWNT will remove a detrimental incompatibility issue in semiconductor electronics fabrication (CMOS) process that has not allowed adaptation of CNTs, as-synthesized or transplanted, onto an electronics substrate because of the contamination issues associated with metal catalyst particles. Metal-catalyst-free CNTs would enable much easier purification, mitigate a toxicity concerns, and lower the price of pure CNT materials, which may further extend the scope of these material applications. Membrane- or CNT-Nanofluidics-based research and industry could also benefit from the metal-free CNT in order to bypass one or more manufacturing steps, leading toward cost-effectiveness.
SWNTs free of metal catalysts push the envelope of CNT synthesis and their potential applications. A narrow size distribution of oxide nanoparticles can engender a narrow diameter distribution of as-grown SWNTs: 50% distributed within the ± 0.1-nm window; 84% within ± 0.3-nm. Unlike metal nanoparticle catalysts, oxide nanoparticles are not likely to diffuse on the surface or agglomerate on one another to enlarge the catalyst particle size. This effect implies that a precise control of CNT diameter could be possible at the catalyst preparation stage. As additional advantages, the metal-free characteristic can increase the compatibility with other micro/nanomanufacturing techniques; avoid generation of malicious defects during the purification process; and enable cost-effective simplification of any device fabrication by ruling out selective metal etching steps. However, challenges associated with the metal-free technique include catalyst density control, uniform distribution of CNT lengths, effect of additives such as water vapor during the CVD, and the role of hydrogen. Further systematic investigations will be able to address these challenges. Precise tuning of both SWNT diameter and chirality is on the horizon.
Images reproduced with permission from Liu B. et al. 2009. Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes, J. Am. Chem. Soc. 131 (6): 2082-2083 and Huang S. et al. 2009. Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes on Substrates, J. Am. Chem. Soc. 131 (6): 2094-2095. Copyright 2009 American Chemical Society.
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