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Functionalization Of Carbon Nanotubes

Carbon nanotubes (CNTs) can be produced in several different ways, with each way having its own advantages and disadvantages. Some of the well-known methods of producing them involve high temperature techniques such as:

  • Laser Ablation
  • Arc Discharge
  • Chemical Vapour Deposition (CVD)

CNTs have certain unique mechanical, chemical and electronic properties that characterize them as the leading materials for a large variety of applications.

However, the outer walls of carbon nanotubes are normally conceived as chemically inert, something which is undesirable for the applications in many instances. One of the promising methods of overcoming such difficulty is by functionalization of carbon nanotubes.

Functionalization of carbon nanotubes

Functionalization of carbon nanotubes extends the CNTs properties, and as a result their application potential too. Through such functionalization, one can modulate the physicochemical properties of carbon nanotubes and increase the ease with which they can be processed, manipulated and dispersed, among many other things. Hence, understanding this process is very critical for fully exploiting the nanotubes' true potential. It basically involves the attachment of inorganic or organic moieties to their tubular structures.

Researchers have done a lot of hard work and developed some effective methods for functionalization of carbon nanotubes' surfaces. Some of these are:

  • Covalent modification
  • Non-covalent modification involving approaches like bio molecule binding, metal ion binding and polymer wrapping
  • Chemical and solid phase functionalization method
  • Hydro-mechanochemical functionalization method

Of these, it is the covalent and non-covalent methods of functionalization of carbon nanotubes that are discussed and delved into the most by researchers everywhere. Let's have a brief look at them below.

Non-covalent functionalization of carbon nanotubes

In this method, the carbon nanotubes are usually functionalized through harsh oxidative processes, for instance, by refluxing in a H2SO4 and HNO3 mixture to create defects on the CNTs' tube tips and sidewalls, which in turn can serve as the anchor groups for functionalization purpose and/or as sites for coordination chemistry.

This method of functionalization of carbon nanotubes is based on the carbon nanotubes sidewalls' extended n-system's ability to bind the guest molecules through n-n stacking interactions.

Non-covalent functionalization of carbon nanotubes has attracted a lot of attention in the recent times, as it doesn't induce any secondary structure and structural transformation in the nanotubes.

Covalent functionalization of carbon nanotubes

This type of functionalization of carbon nanotubes involves conversion of the carboxylic groups as well as the other oxygenated sites formed via oxidative purification. These carboxylic groups, which are mainly situated at the ends of carbon nanotubes, can be combined with various chemical groups.

First step: The nanotubes are put through an oxidative treatment for the growth of hydroxyl, carbonyl and carboxylic groups at their defect sites and ends, while simultaneously opening their caps. This may readily facilitate the direct bonding between the ends of the nanotubes with the matrix, through the carboxylic groups.

Second step: This involves reaction of the carboxylic groups with the other chemical functions, leading to either covalent or ionic bonds with these groups. Thereafter, the free amino functions on the carbon nanotube's surface would react with the polymer molecules, resulting in creation of equivalent bonds, and a better nanotube matrix bonding.

 
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