Abstract
This review focuses and summarizes recent studies on the functionalization of carbon nanotubes oriented perpendicularly to their substrate, so-called vertically aligned carbon nanotubes (VA-CNTs). The intrinsic properties of individual nanotubes make the VA-CNTs ideal candidates for integration in a wide range of devices, and many potential applications have been envisaged. These applications can benefit from the unidirectional alignment of the nanotubes, the large surface area, the high carbon purity, the outstanding electrical conductivity, and the uniformly long length. However, practical uses of VA-CNTs are limited by their surface characteristics, which must be often modified in order to meet the specificity of each particular application. The proposed approaches are based on the chemical modifications of the surface by functionalization (grafting of functional chemical groups, decoration with metal particles or wrapping of polymers) to bring new properties or to improve the interactions between the VA-CNTs and their environment while maintaining the alignment of CNTs.
Highlights
Carbon nanotubes (CNTs) have stirred the curiosity of the scientific community for two decades
Considerable progress has been made in the synthesis of vertically aligned CNTs since the first report, in 1996, by Li et al [23] showing the synthesis of aligned CNTs from acetylene chemical vapor deposition (CVD) catalyzed by iron nanoparticles embedded in mesoporous silica
Hata et al (2004) reported the growth of superdense, vertically aligned single-wall carbon nanotubes with heights up to several millimeters achieved by increasing the activity and lifetime of the Fe-based catalysts during the ethylene CVD by adding small amounts of water vapor, which acts as an oxidizer [26]
Summary
Carbon nanotubes (CNTs) have stirred the curiosity of the scientific community for two decades now. The first important research outcomes in this area were the synthesis on a large scale, the reproducibility, and the control of the diameter and number of walls of the CNTs using different synthesis techniques such as arc discharge, laser ablation or chemical vapor deposition After these issues had been addressed, the focus in carbon nanotube research shifted towards obtaining control in the engineering of organized architectures with determined orientations, such as vertically aligned carbon nanotubes (VA-CNTs). Important characteristic of a post-growth treatment is the removal of the amorphous carbon layers that can be often observed on the as-grown CNTs [21] In this context, physical functionalization, such as plasma treatment where the functionalization features depend on the plasma parameters, has been reported. A part of this review is dedicated to a frequent consequence of the functionalization (voluntarily searched or not): the bundling of CNTs
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