What are Doped Carbon Nanotubes?
Doped carbon nanotubes are carbon nanotubes that have other elements incorporated into the structure to alter its electronic structure and physicochemical composition. The most widespread dopant elements are boron, nitrogen, sulphur, phosphorus and more. These dopant components can be incorporated into carbon nanotubes in various ways to give them new functionalities and application possibilities.
Types of Doped Carbon Nanotubes
Nitrogen-doped Carbon Nanotubes (N-CNTs): Nitrogen-doped carbon nanotubes can be synthesized by many techniques, including chemical vapor deposition (CVD) and high temperature carbonization of nitrogen-containing polymers. N-type conductivity and dipole scattering properties of nitrogen doping makes N-CNTs a popular material for catalysis and energy storage.
Boron-doped Carbon Nanotubes (B-CNTs): Boron-doped carbon nanotubes are made by CVD and other methods. They are of distinct electronic structures and physical attributes like good thermal conductivity and electrochemical properties. Boron-doped CNTs could also be used in electrochemical reactions, catalysis and hydrogen storage.
Sulfur-doped Carbon Nanotubes (S-CNTs): Sulfur-doped carbon nanotubes are produced by particular techniques, which have good electrochemical performance and stability. The specific capacity and cycling stability of carbon nanotubes, which could be applied to supercapacitors, is greatly enhanced by sulfur doping with alumina.
Phosphorus-doped Carbon Nanotubes (P-CNTs): Phosphorus-doped carbon nanotubes are often developed by CVD and are more efficient at both electronic and mechanical properties. P-CNTs have multiple uses in energy storage, sensors, and more.
Nitrogen/Sulfur Co-doped Carbon Nanotubes (N/S-CNTs): Nitrogen/sulfur co-doped carbon nanotubes are fabricated using hydrothermal process and are excellent in electrochemical and specific capacity. This co-doping strategy makes carbon nanotubes significantly better at storing electrochemical energy.
Boron/Nitrogen Co-doped Carbon Nanotubes (B/N-CNTs): Boron/nitrogen co-doped carbon nanotubes are made by particular processes and exhibit a very elaborate core-shell structure and great electronic properties. It is a method of co-doping with crucial uses in catalysis, electrochemistry and other fields.
Other Metal-doped Carbon Nanotubes: Examples are Copper, Nickel, and Cobalt-doped Carbon nanotubes, which are prepared through various synthesis procedures. These materials are very electrocatalytic and structurally stable, and are widely used in batteries, sensors, etc.
Preparation Methods of Doped Carbon Nanotubes
In-situ Doping: Dopant elements are added as carbon nanotubes are growing. By way of instance, boron or nitrogen sources could be included in carbon nanotube growth using graphite arc discharge, laser ablation and chemical vapor deposition (CVD) to dope them. Such a strategy gives good control of the dopant distribution and concentration, but with exact control of the growth conditions.
Post-treatment Doping: It is done by adding dopants into the carbon nanotubes which are already synthesized in pre-treatment. Most commonly used post-treatments are high-temperature pyrolysis, plasma treatment, ball milling and hydrothermal treatments. For instance, nitrogen doping is possible by high-temperature carbonisation of nitrogen-containing polymers or post-treatment with nitrogen.
Solution Doping: Carbon nanotubes are surface doped by a solution dopant. Dimethylformamide (DMF) solutions, for example, can be spin-coated on carbon nanotube surfaces and the doping can be varied by varying the concentration of the DMF solution. This approach is irreversible and doping effect can be controlled by altering the concentration of the solution.
Co-doping: It's a process in which various dopants are mixed together to enhance the performance of carbon nanotubes. Co-doping of boron and nitrogen, for instance, can be achieved using a new plasma-based hot-wire CVD process, which turns metallic carbon nanotubes into semiconducting ones. Further nitrogen/sulfur co-doping can be obtained through hydrothermal process which further enhances the electrochemical activity of the carbon nanotubes.
Special Doping Methods: For instance, Nitrogen and sulphur co-doping is possible using a polydopamine (PDA) platform. This process allows in-situ, continuous and high-concentration sulfur doping within the carbon system.
Any doping technique has its pros and cons. Choosing the best doping process should also depend on the desired use-cases and the desired properties of the carbon nanotubes.
Characterisation Methods of Doped Carbon Nanotubes
Doped carbon nanotubes (CNTs) exhibit special and enhanced properties that are desirable in many applications. But to have a proper grasp of how they perform, to ensure that they're performing to the specification, they must be characterized with care. Here are some of the most important methods for characterising doped carbon nanotubes:
Transmission Electron Microscopy (TEM) and High-Resolution Transmission Electron Microscopy (HRTEM): Used to detect the structure and shape of carbon nanotubes. For instance, TEM and HRTEM can detect the dimension, dispersion and lattice fringe of doped carbon nanotubes.
Atomic Force Microscopy (AFM): AFM measures height and roughness of carbon nanotubes for understanding their structure and distribution. AFM can, for instance, prove the height and spread of doped carbon nanotubes.
Raman Spectroscopy: Raman spectroscopy can be a very useful technique for characterizing the carbon nanotubes to look for defects, lattice vibration modes, etc. Changes in the G-band and D-band can be used, for instance, to identify the doping effect of carbon nanotubes from Raman spectroscopy.
X-ray Diffraction (XRD): The crystal structure and lattice properties of carbon nanotubes are determined by XRD. So, for instance, the crystal plane diffraction peaks of doped carbon nanotubes can be determined using XRD, which also tells us about the crystal structure.
Fourier Transform Infrared Spectroscopy (FTIR): FTIR analysis of chemical functional groups in the carbon nanotube surface. For instance, FTIR could register and alter oxygen-groups in doped carbon nanotubes.
X-ray Photoelectron Spectroscopy (XPS): XPS studies the chemical composition and elemental makeup of the surface of doped carbon nanotubes. For instance, XPS can determine what chemical bonds the dopant elements share.
Scanning Electron Microscopy (SEM): The macroscopic morphology and surface properties of carbon nanotubes are measured using SEM. SEM can show, for instance, the diameter, length and surface shape of doped carbon nanotubes.
Electron Energy Loss Spectroscopy (EELS): EELS is used to study the electronic composition and defect states of carbon nanotubes. EELS can observe electron transfer and defect formation, for instance, in doped carbon nanotubes.
It is critical to define doped carbon nanotubes in order to understand their special properties and prepare them for future application. SEM, TEM, Raman spectroscopy, XPS and others combine to give us the complete picture of the structural, chemical and electrical structure of doped CNTs. With these characterization techniques, suppliers and scientists can take advantage of the potential of doped CNTs in electronics, energy storage, composites and beyond.
Applications of Doped Carbon Nanotubes
The applications for doped carbon nanotubes (CNTs) are many, in part because doping techniques can significantly alter the electronic composition and behavior of carbon nanotubes. Doping is normally achieved by adding nanoparticles of heteroatoms (boron, nitrogen, sulphur, phosphorus etc.) In the carbon nanotubes which brings new features and properties.
Electrochemical Performance: Doped carbon nanotubes perform very well in the electrochemical sphere. N-doped carbon nanotubes, for example, are very electrocatalytic in ORR, and thus a candidate for fuel cells and metal-air batteries. Moreover, nitrogen/sulfur co-doped carbon nanotubes are also extremely electrochemically strong, which can be applied to supercapacitors.
Energy Storage: Carbon nanotubes dotted with an anti-microbial layer work brilliantly in lithium-ion batteries and supercapacitors. Nitrogen-doped carbon nanotubes, for instance, can boost the storage capacity of lithium-ion batteries; boron-doped carbon nanotubes improve the field emission of multi-walled carbon nanotubes, important for high power density electronics.
Catalytic Performance: Doped carbon nanotubes act as catalyst carriers and are good at all the catalytic reactions. Nitrogen-doped carbon nanotubes, for example, are very active in electrocatalytic oxygen reduction reactions with enormous potential to be used in environmental and energy-conversion technologies. Boron-doped carbon nanotubes also effectively adsorb gases such as formaldehyde, and have a high catalytic activity.
Composites: Carbon nanotubes are common in composites. Boron-doped carbon nanotubes, for instance, can be used to create high-performance composites like boron-doped carbon nanotube/cement composites and boron-doped carbon nanotube/magnesium composites. Such composites have high potential applications in building, aircraft, etc.
Sensors: The application of enriched carbon nanotubes extends into the area of sensors too. Boron-doped carbon nanotubes, for example, could be used to develop electrochemical biosensors that are sensitive and reproducible, which can be used for drug discovery and early disease detection.
Environmental Protection: Doped carbon nanotubes can also be useful in conservation. For instance, tin-doped carbon nanotubes (due to their chemical and catalytic properties) are particularly promising for environmental remediation.
Doped carbon nanotubes are very promising across multiple fields but they also still need to deal with the issue of dopant implantation, doping depth and doping concentration. Next generation studies will have to further hone doping approaches to achieve better-performing doped carbon nanotubes and make them available in more applications.
References
- Terrones, M. et al. "Doped carbon nanotubes: synthesis, characterization and applications." Carbon nanotubes: advanced topics in the synthesis, structure, properties and applications. Berlin, Heidelberg: Springer Berlin Heidelberg, 2007. 531-566.
- Lee, W. J., et al. "Nitrogen-doped carbon nanotubes and graphene composite structures for energy and catalytic applications." Chemical Communications 50.52 (2014): 6818-6830.
- Palm, I., et al. "Nitrogen and sulphur co-doped carbon-based composites as electrocatalysts for the anion-exchange membrane fuel cell cathode." International Journal of Hydrogen Energy 55 (2024): 805-814.
- Wang, X., et al. "Co-and N-doped carbon nanotubes with hierarchical pores derived from metal–organic nanotubes for oxygen reduction reaction." Journal of Energy Chemistry 53 (2021): 49-55.
