Functional Modification On Carbon Nanotubes(CNTs) Surface

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Functional Modification on Carbon Nanotubes(CNTs) Surface

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Functional Modification on Carbon Nanotubes(CNTs) Surface

March 18,2019.

Carbon nanotubes are one-dimensional nano tubular materials made of single-layer and multi-layer graphene platelet. They have good properties of high mechanical strength, good chemical stability, excellent electrical conductivity and electromagnetic shielding, so they are considered as ideal fillers for high performance composites. However, their surface lack active groups resulting in poor dispersibility and difficult processing, which limit their practical application. Therefore, the researchers improved their solubility and dispersibility through surface modification. At the same time, the multifunctional functional materials are prepared by chemically or physically connecting the desired functional groups to the surface of carbon nanotubes. At present, surface modification of the carbon nanotubes surface have become a hot research field.

Surface modification on carbon nanotubes

Surface functional modification of carbon nanotube is mainly divided into organic modification, mechanical modification and inorganic coating.

1. Organic modification
The organic modification and modification of carbon nanotubes mainly include covalent modification and non-covalent modification.

(1) Surface covalent modification of carbon nanotubes

Surface covalent modification of carbon nanotube is to introduce new covalent bonds on the wall of the tube with chemical reaction to optimize the performance of CNTs. The main reactions include oxidation reaction, free radical addition, electrochemical reaction and thermochemical reaction,etc. The oxidation reaction is to introduce relatively large polarity carboxyl or hydroxyl group by means of chemical method, thereby causing active group on the surface of CNTs, and then to introduce different functional groups with covalent crosslinking reaction.

A Shainghai research group used mixed acid (H2SO4:HNO3=1:3) and strong alkali (NaOH) to treat MWCNTs, and the absolute value of the zeta potential of the carbon nanotube solution became larger, indicating better dispersibility and stability, and MWCNTs have higher water solubility.

After the acid treatment, the infrared spectrum has an absorption peak at 1600 to 1700 cm-1, indicating the introduction of the C=O group. A weak absorption peak was observed at 1260 cm-1, which was mainly caused by the stretching vibration of the C-O group in the carboxyl group. At 3300 to 3500 cm-1, an absorption peak of a free hydroxyl group (-OH) was found.

(2) Non-covalent modification of CNTs

Non-covalent modification of carbon nanotubes means that no covalent chemical bonds are introduced on the surface, but by non-covalent bonds. This includes physical adsorption and surface coating. Non-covalent interactions include dispersive force, hydrogen bonding, dipole dipole force, π-π stacking, and hydrophobic interaction. The carbon atoms in CNTs are all SP2 hybridized to form highly delocalized π electrons, which can be modified with other π-electron-rich compounds by π-π stacking.

Non-covalent modification can not only introduce polymer on CNTs surface, but also surfactants to change surface activity. The surfactants comprise two parts respectively oleophilic end and hydrophilic end.

The advantage of non-covalent modification is that the obtained CNTs are structurally intact and retain their original properties.

2. Mechanical modification
Mechanical modification refers to using external force for surface modification of CNTs to optimize their performance. Mechanical modification methods include grinding, friction, vibration,etc. The above physical method can increase the internal energy of the carbon nanotubes, and react with some substances under certain external conditions, thereby achieving surface modification.

The advantages of mechanical modification: simple, fast and low cost.
Disadvantages: it is difficult to control during the grinding process, and the formation of lattice defects is easy to cause the length of the carbon nanotubes to be too short causing losting their original properties.

3. Inorganic coating
The inorganic coating methods of CNTs mainly include in-situ liquid phase synthesis and vapor phase deposition method.

(1) In situ liquid phase synthesis

In-situ liquid phase synthesis refers to the formation of new substances in situ on the surface of carbon nanotubes under liquid phase conditions. These substances are mainly non-ferrous metal oxides. By utilizing the inorganic coating on the surface of the carbon nanotubes, on the one hand, the metal oxide and the carbon nanotubes can excellent properties, and on the other hand, the energy of the surface of CNTs can be significantly reduced, thereby reducing the degree of aggregation. The crystallization method is classified into calcination crystallization and hydrothermal crystallization method.

The calcination crystallization method means that the metal oxide precursor is converted into a corresponding sol in an aqueous solution, adsorbed on the surface of CNTs, and calcined in an inert environment, and the calcination temperature should exceed the phase transition temperature of the metal oxide. The advantage of this method is that the process is relatively simple.

The hydrothermal crystallization method is a relatively common method in the research of inorganic coating. The advantage is that the size of the obtained materials are small and uniform. The disadvantage is that the production process is complicated.

(2) Vapor deposition method

Vapor deposition refers to the introduction of two or more gaseous raw materials into a reactor, causing the raw materials to collide with each other and undergo chemical reaction, thereby preparing a new material and depositing it on the surface of the substrate.
The advantages of vapor deposition are that the reaction speed is fast and thorough, and the deposition is uniform. It's often used to prepare core-shell materials. The researchers used gasified silicon oxide to react with carbon monoxide to form β-silicon carbide, and then uniformly deposited it on CNTs surface. The coating was uniform and complete.

Hongwu Nano offers customized surface modification on carbon nanotubes based on the rich experience and advantage technology to improve solubility and dispersibility and makes it get the best effect in practice. For further info or requirement, please don't hesitate to contact us now!

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