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Surface diffusion: the low activation energy path for nanotube growth
S Hofmann1, G Csányi, A C Ferrari
1Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom.
Physical Review Letters
|August 11, 2005
Summary
Plasma-enhanced chemical vapor deposition enables faster carbon nanofiber growth due to low activation energy for carbon diffusion on catalysts. This contrasts with slower thermal methods limited by gas decomposition.
Area of Science:
- Materials Science
- Chemical Engineering
- Nanotechnology
Background:
- Carbon nanofibers (CNFs) are crucial nanomaterials with diverse applications.
- Plasma-enhanced chemical vapor deposition (PECVD) offers advantages for nanomaterial synthesis.
- Understanding growth kinetics is vital for optimizing CNF production.
Purpose of the Study:
- To investigate the temperature dependence of CNF growth rates using PECVD.
- To determine the activation energy for CNF growth with Ni, Co, and Fe catalysts.
- To elucidate the rate-limiting steps in PECVD versus thermal growth.
Main Methods:
- Experimental determination of CNF growth rates via PECVD at varying temperatures.
- Computational analysis using ab initio plane wave density functional theory (DFT).
- Investigation of carbon diffusion and precursor molecule stability on catalyst surfaces.
Main Results:
- A common low activation energy (0.23-0.4 eV) was observed for PECVD growth.
- Carbon surface diffusion on Ni and Co (111) planes has a low activation energy (0.4 eV).
- Precursor dissociation barriers (C2H2: ~1.3 eV, CH4: ~0.9 eV) are higher than surface diffusion barriers.
Conclusions:
- Carbon surface diffusion is the rate-limiting step in PECVD growth of CNFs.
- PECVD growth is significantly faster than thermal growth due to lower activation energy barriers.
- Catalyst surface properties critically influence CNF growth kinetics.