Article pubs.acs.org/JPCC
Millimeter-Tall Carpets of Vertically Aligned Crystalline Carbon Nanotubes Synthesized on Copper Substrates for Electrical Applications Eti Teblum,† Malachi Noked,† Judith Grinblat,† Anna Kremen,‡ Merav Muallem,† Yafit Fleger,† Yaakov R. Tischler,† Doron Aurbach,† and Gilbert D. Nessim*,† †
The Department of Chemistry and Institute of Nanotechnology and ‡The Department of Physics and Institute of Nanotechnology, Bar-Ilan University, Ramat Gan 52900, Israel S Supporting Information *
ABSTRACT: We synthesized millimeter-tall, dense carpets of crystalline CNTs on nonpolished copper substrates with a thin Al2O3 (below 10 nm) underlayer and Fe (1.2 nm) layer as a catalyst using chemical vapor deposition (CVD). Preheating of the hydrocarbon precursor gases and in-situ formation of controlled amounts of water vapor were critical process parameters. High-resolution microscopy showed that the CNTs were crystalline with lengths up to a millimeter. Electrical conduction between the CNTs and the copper substrate was demonstrated using multiple methods (probe station, electrodeposition, and hydrolysis of water). Through TEM characterizations of cross sections, we demonstrated that copper diffusion into the alumina layer during the thermal process was the key to explain the observed electrical conductivity. Additionally, the high electrical conductivity of a thermally processed sample compared to the insulating behavior of a pristine sample confirmed the mechanistic hypothesis. Adsorption isotherm measurements showed the mesoporous structure of the vertically aligned carbon nanotubes (VACNTs) with a surface area of 342 m2/g. Electrical conduction and high surface area of this nanostructure make it a promising platform to be functionalized for future battery electrodes.
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INTRODUCTION Dense carpets of vertically aligned carbon nanotubes (CNTs)1 grown on metal substrates are a very promising electrically conductive and mechanically stable framework that can be functionalized for applications as novel battery electrodes.2−4 For such applications, it is critical to have good electrical contact between the vertically aligned CNTs (VACNTs) and the metal current collector. Synthesizing CNTs directly on metallic layers simplifies the electrode assembly process by avoiding transfers that require binders and conductive additives.5 Therefore, the direct synthesis of CNTs on metal substrates using chemical vapor deposition (CVD) has attracted much attention in recent years.6−9 VACNTs synthesized on metallic layers can also be used for field emission,10 nanoscale electronic devices,11 lithium-ion batteries,12−14 and supercapacitors.15,16 A few teams have reported VACNT syntheses on various metal substrates such as stainless steel (SS),17−23 silver (Ag),24 gold (Au),25 and aluminum (Al).26 However, all the reported CNT carpets, which were directly grown on metallic layers, have heights in the range of tens of microns. Taller CNT carpets grown on metallic layers would be more suitable for battery applications. A two-step technique to fabricate tall carpets of CNTs on metallic layers consists of first synthesizing CNTs on a known insulating substrate/catalyst system and then transferring and © 2014 American Chemical Society
binding the CNTs to the metallic layer. For instance, Evanoff et al. first synthesized a millimeter-tall carpet of CNTs on Si that was then transferred to a copper substrate and showed electrical conductivity between CNTs and copper.27 However, direct synthesis of VACNTs on copper, which is the best current collector for negative electrodes in Li and Li ion batteries,28−31 is still challenging as copper is known to be a poor catalyst for CNT growth.16,32−37 Pure copper deactivates CNT growth since it does not catalyze decomposition of hydrocarbons; thus, the interaction of hydrocarbons with the copper surface does not lead to the formation of the specific carbon−carbon bonds.38 Furthermore, the solubility limit of carbon in solid copper (melting temperature 1085 °C) is extremely low (e.g., only 0.0001 wt % C in Cu at 1100 °C).39,40 Carbon cannot efficiently diffuse into nanoscale copper catalyst particles, and consequently these particles cannot act as nucleation sites for the formation of CNTs.41,42 An alternative approach is to use copper as a substrate and a layer of catalytic material (e.g., iron) as a catalyst. However, during high temperature CVD processing, iron will diffuse into copper43 at the intergrain voids.44 The iron diffusion coefficient in single-crystalline Received: February 13, 2014 Revised: July 10, 2014 Published: July 21, 2014 19345
dx.doi.org/10.1021/jp5015719 | J. Phys. Chem. C 2014, 118, 19345−19355
The Journal of Physical Chemistry C
Article
copper is above 10−12 cm2/s at 1000 K, at least 3 orders of magnitude higher than the diffusion coefficient in silicon dioxide (
