# Electrospinning-Based Carbon Nanofibers for Energy and Sensor Applications
## 3 Different Carbon Nanofibers Obtained by Electrospinning
### Carbon Nanofibers from Modified Single-Nozzle Electrospinning
#### 3.2.2 Metal-Oxide-Decorated Carbon Nanofibers  
Another method that has been used in the application of CNFs is to combine them with metal or metal oxide for further use. Although the metal or metal oxide can act as the main working material, the CNFs play the role of the current collector [75]. Due to the large surface area, CNFs can be an ideal template for metal or metal oxide decoration.  
One well-known method of decoration is to mix the metal or metal oxide precursor, which is usually in the form of a metal salt, into the polymer solution used for electrospinning. [PERSON] et al. used multichannel CNFs as a template for ruthenium (Ru) nanoparticles decoration (Figure 9a) [76]. The electrospun nanofibers were made using an electrospinningning system with a mixed solution of PAN and PS containing ruthenium chloride (RuCl\({}_{3}\)). Ru was chosen to detect dopamine because it belongs to the noble metals but is more cost-effective than gold or platinum [77; 78]. Oxygen plasma was applied for the generation of the oxygen functional groups that form chemical bonds with Ru prior to the carbonization process. RuCl\({}_{3}\) later decomposed during the PAN carbonization period and formed Ru metal particles. The Ru-containing CNFs were used as the main material for nonenzymatic field-effect transistor fabrication. As a result, the device exhibits high sensitivity that can detect the target at a low concentration of 1 fM. In addition, the device presents a long life span of up to 4 weeks.  
Figure 8: (a) Fabrication of multiscale porous CNFs from PAN and PS using steam activation [70]. (b) Fabrication of sponge\(-\)like CNFs using PTFE particles and PVA [72]. (c) Fabrication of porous CNFs using PAN and \(\beta\)-CD precursors [74].  
[PERSON] et al. fabricated CNFs covered with zinc oxide and tin oxide with the diameter of the fiber down to 100 nm (Figure 9b) [79]. During the electrospinning process, while the metal salt dispersed well in the fiber, the PAN was concentrated in the middle of the fiber as the core and the PVP formed the cover layer that acted as the shell material. Although the electrospun polymer nanofiber has a large diameter, the PAN core is only occupied in the center of the fiber. The PVP shell is decomposed completely during high-temperature calcination and leaves behind the metal-oxide particles, and the PAN, which is the central part of the fiber, is carbonized to leave the carbon fiber. The surface area of the small CNFs makes them highly sensitive to dimethyl methylphosphonate (DMMP). However, it was found that the sensitivity was the highest in the case of fibers obtained from a solution with a 1% metal salt. A higher concentration level of salt may result in a reduction in sensitivity due to the overlapping of the metal-oxide particles.  
Instead of a metal or a metal-oxide precursor, an initiator can be mixed and added to the polymer solution for electrospinning. [PERSON] et al. mixed 2-methylimidazole (2-MeIM) with the PAN solution to use for electrospinning (Figure 9c) [80]. The electrospun nanofibers were immersed in the solution containing Zn(NO\({}_{3}\))\({}_{2}\cdot\)6H\({}_{2}\)O/Co(NO\({}_{3}\))\({}_{2}\cdot\)6H\({}_{2}\)O and 2-MeIM, where the reaction between 2-MeIM and Zn(NO\({}_{3}\))\({}_{2}\)/Co(NO\({}_{3}\))\({}_{2}\) happened and formed the metal-organic framework (MOF). Later, the nanofibers came into contact with sodium molybate (NaMoO\({}_{4}\)) to dope molybdenum to the MOF material. After carbonization, the final collected material was Co\({}_{5}\)S\({}_{8}\)-MoS\({}_{2}\) nanocrystal embedded on the CNFs. The shape of the flakes helps to enhance the surface area as well as the electrocatalytic performance of the MOF catalyst.