## 3 Results and Discussion  
### Morphologies and Structures  
Figure 1 shows the AFM images of different catalyst films after annealing at 650 \({}^{\circ}\)C in a vacuum for 60 min. The morphologies of the Cu films in Figure 1a,b are different from those of the Cu-Cr films in Figure 1c,d. After a high temperature treatment, the surface of the film was granular. Meanwhile, the size and surface roughness of the catalyst film increased with the thickness of the film. For the film with larger thickness, a longer deposition time during the magnetron sputtering process results in the increment in substrate temperature and nucleation rate, which leads to a larger particle size on the surface of the catalyst film [13]. Further, the melting point of the alloy is generally lower than that of the single metal, so the particle size and surface roughness of the Cu-Cr catalyst film is bigger than that of the Cu catalyst film, as shown in Figure 1c,d, respectively. Figure 2 shows SEM images of different CNSs after C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\)-CVD, where the inserts are the SEM images of the corresponding catalyst films annealed in a H\({}_{2}\) atmosphere. As shown in Figure 2a,b, CNTs with a diameter of nearly 40 nm dispersed on the Cu catalyst. The density of the CNTs increased with the thickness of the catalyst film. As for the Cu-Cr catalyst film of 100 nm, there were many graphite coated particles mixed in the CNT products, as shown in Figure 2c. Comparing Figure 2c with Figure 2a,b, the density of the CNT products on the Cu-Cr catalyst decreased significantly. When the thickness of the Cu-Cr film increased to 500 nm, in addition to some sub-micron CNFs, the density of the CNTs increased slightly. This difference in morphology can mainly be attributed to the fact that the size of surface particles in the Cu catalyst film are smaller than in the Cu-Cr film, as shown in the inserts, resulting in the faster growth rate of CNTs during the CVD process. It is known that the bonding of metal with carbon atoms will increase with the number of unfilled d-orbitals. Cu (3d\({}^{10}\)4s) has no d-vacancy, which is expected to show minimal affinity with the CNTs. Even so, the low solubility of carbon in Cu is enough for the carbon atoms to form CNTs of small diameters, according to the vapor-liquid-solid (VLS) mechanism [14]. However, catalyst particles of larger sizes are not conducive to the growth of CNTs, so the density of CNTs decreases on Cu-Cr catalysts, as shown in Figure 2c,d. In addition, some lattice distortions are produced when Cr atoms are introduced into Cu crystals and act as interstitial sites after co-sputtering and annealing, which increases the adsorption and deposition rate of carbon atoms on the surface of catalyst particles according to the defect growth mechanism [15]. Half-filled d-orbitals of Cr enhance the affinity with CNTs. As a result, the concentration of carbon atoms dissolved in bigger Cu-Cr particles is so high that sub-micron CNFs are formed. Figure 1: AFM images of different catalysts annealing at 650 \({}^{\circ}\)C for 60 min. (**a**) 100 nm Cu film; (**b**) 500 nm Cu film; (**c**) 100 nm Cu-Cr alloy film; (**d**) 500 nm Cu-Cr alloy film. AFM: Atomic Force Microscope. Previous works [15; 16; 17; 18; 19; 20; 21] have reported that ammonia gas has an etching effect and is favorable for the growth of CNTs. In this study, when H\({}_{2}\) was replaced by NH\({}_{3}\), the morphologies (shown in Figure 3) of carbon products changed greatly. The diameter of CNTs on the Cu film of 100 nm increased greatly, and they become shorter and more distorted. Figure 3b is the TEM image of CNTs in Figure 3a, where CNTs with a bamboo-like structure (indicated by circle) can be found (a similar structure is commonly observed in works using C\({}_{2}\)H\({}_{2}\)/NH\({}_{3}\)-CVD [2]), indicating a tip-growth mechanism. In Figure 3c, there are many CNLs mixed with CNTs.