## 3 Results and Discussion
### CNTs
The reason is that the active Fe nanoparticle, as the \"seed\" of the CNTs produced on the stainless-steel substrate, needs a suitable temperature range. These active nanoparticles act as catalysts during the growth of CNTs. The size and quantity of active Fe nanoparticles on the substrate increases with the increase of temperature. It makes the diameter and the precursor conversion efficiency of the CNTs increase. When the tube furnace temperature is 400 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, no CNTs grow on the substrate. The reason is that the surface activity of the stainless-steel substrate is too low to catalyze the growth of the CNTs when the temperature of the furnace is 400 \({}^{\circ}\)C. At 1000 \({}^{\circ}\)C, the surface of the stainless-steel substrate melts completely, which then cannot provide \"seeds\" for the growth of the CNTs. Figure 6 shows the HRTEM micrographs of CNT ethanol dispersions dropped on a carbon-coated TEM grid. The CNTs tangle with each other, and their layers are in the range of 10-20, respectively. The wall thickness and distance between layers of the CNTs at different temperatures were measured, as shown in Figure 6. The walls of the CNTs at all temperatures are tightly aligning with a high density of graphite sheets, with a distance between layers of 0.35-0.38 nm. The layer number and wall thickness of carbon nanotubes increase with the increase of temperature. A large number of amorphous carbon layers attached to the CNTs produced at low temperatures, as shown by red arrows in Figure 6. With the increase of the temperature, the content of amorphous carbon decreased. This phenomenon is consistent with our Raman analysis results. The CNTs prepared at 700 and 800 \({}^{\circ}\)C consist of hollow compartments, looking like bamboo, which are not apparent at 500 and 600 \({}^{\circ}\)C. The carbon walls of the hollow always bulge towards the root of the CNTs. According to the vapor-liquid-solid growth method, in the growth process of CNTs, active liquid Fe nanoparticles dissolve gaseous carbon particles and then precipitate carbon atoms to form CNTs. The carbon dissolution rate of active liquid Fe nanoparticles at the temperature of 700 and 800 \({}^{\circ}\)C is higher than that at 500 and 600 \({}^{\circ}\)C, so the growth rate of CNTs is faster. When the carbon dissolution rate of Fe nanoparticles is higher than that of precipitation, new carbon layers formed, resulting in the bamboo structure. Compared with the carbon walls of the CNTs, we found that the carbon layers of CNTs grown at high temperatures are smoother and cleaner than that grown at low temperatures. The reason is that high temperature makes the process of dissolving and precipitating carbon faster and makes it easier to grow continuous carbon layers. Figure 6j,k shows the magnified view of the joint between the wall and the hollow and the open end structure of the CNTs. XPS further characterized the CNTs prepared at 800 \({}^{\circ}\)C.  
XPS measurement was carried out to investigate the surface functional groups of the synthesized CNTs. As shown in Figure 7a,b, the CNTs contain C, N, and O, and the magnified view of **C1s** peak indicate that they are mainly composed of C-C/C=C (284.8 eV, 59.27%), and the surface defects of CNTs are -C-OH/C-N (285.7 eV, 5.33%), -C=O (287.1 eV, 26.73%), and -COOH (288.6 eV, 8.67%). The oxygenated functional groups on the surface of CNTs were mainly from the hydroxyl groups obtained from ethanol decomposition, which caused the high defect concentration of the CNTs. Figure 5: The SEM micrographs of the CNTs grown on the substrate with 200 sccm ethanol precursor at the temperature of 500, 600, 700, and 800 \({}^{\circ}\)C. (**a,d,g,i**): the low magnification images; (**b,e,h,k**): the high magnification images; (**c,f,i**): the diameter distribution of samples.