## 3 Results and Discussion
### Morphologies and Structures
Figure 3d shows the SEM images of several isolated CNLs from the sample in Figure 3c with higher magnification, in which the folded two-dimensional flake structure of CNL is revealed. The relationship between the size of the catalyst particles and the thickness of the film is consistent with the findings of [PERSON] et al. [22]. Differently, the growth of carbon products is not limited in one mode. It is considered that the mixed structure of CNTs and CNLs may attribute to the mixed growth mode. Tip-growth is more likely to occur on the surface of catalyst particles with a radius below 10 nm, while root-growth tends to appear on particles bigger in size [22]. The size of the catalyst particles on the film of 100 nm were relatively small, so tip-growth was dominant. When the thickness of the Cu film increased to 500 nm, the increment in size results in the appearance of the root-growth mode, leading to the growth of CNL. Inevitably, on the Cu film of 500 nm, bamboo-shaped CNTs based on the tip-growth mode could grow on some small-sized particles synchronously, which is agreement with Figure 3c. To further prove our hypothesis, TEM images of CNL are shown in Figure 3e. The facts, that the top of CNL is narrower than the bottom and the presence of elongated catalyst at the end of CNL (indicated by an arrow and  
Figure 2: SEM images of different carbon nanostructures (CNS) after C\({}_{2}\)H\({}_{2}\)/H\({}_{2}\)-chemical vapor deposition (CVD): (**a**) 100 nm Cu film; (**b**) 500 nm Cu film; (**c**) 100 nm Cu-Cr alloy film; (**d**) 500 nm Cu-Cr alloy film. will be explained in the next paragraph), are strong evidence for the root-growth mechanism of this reed-leaf-like nanostructure. A possible root-growth mechanism for CNLs in our thermal system is shown in Figure 4. Firstly, carbon atoms of a certain concentration are supplied to the Cu catalyst due to the adsorption and catalytic decomposition of C\({}_{2}\)H\({}_{2}\). Then, the carbon atoms diffuse to lower the concentration (through bulk or surface diffusion). At last, precipitation and the formation of the graphite structure can occur. On bigger catalyst particles, the controlling step is the limited carbon diffusion rate. In a C\({}_{2}\)H\({}_{2}\)/NH\({}_{3}\) atmosphere, NH\({}_{3}\) inhibits the appearance of amorphous carbon at the initial stage of the synthesis, and thereby protects the Cu metal particles from being covered by amorphous carbon. The activity of carbon can be described as per the following equation [23]:  
\[\alpha_{C}\approx X_{C}\exp\left(\frac{G_{C}^{0}+\Omega_{CuC}-G_{C}^{0g}}{RT} \right)\exp\left(\frac{2X_{C}\Omega_{CuC}}{RT}\right)\exp\left(\frac{X_{N}W_{ NC}}{RT}\right) \tag{1}\]  
Figure 3: SEM (**a**) and TEM (**b**) images of carbon nanotubes (CNTs) grown on 100 nm Cu catalysts in NH\({}_{3}\). SEM (**c**), (**d**) and TEM (**e**) images of carbon nanostructures grown on 500 nm Cu catalysts in NH\({}_{3}\). EDX(**f**) spectra of carbon nanostructures grown on a 500 nm Cu catalyst in NH\({}_{3}\). \(X_{C}\) and \(X_{N}\) are the concentration of carbon and nitrogen atoms in the vapor-liquid-solid (VLS) growth mechanism of CNTs; R and T are the gas constant and temperature, respectively. \(G_{C}^{0}\) and \(G_{C}^{0\delta}\) are the Gibbs free energy of the carbon and graphite. \(\Omega_{CuC}\) is the interaction between the Cu and C, where \(W_{NC}\) is the interaction parameter between N and C at a Cu-N-C resolution. According to [23], \(\Omega_{CuC}\leq 0\) and \(W_{NC}>0\). After H\({}_{2}\) is replaced by NH\({}_{3}\), \(X_{N}\
eq 0\). Thus, from Equation (1), it is clear that the activity of carbon is enhanced with the increment of \(X_{C}\) and \(X_{N}\). The graphite layer is formed on the surface of copper particles one by one and the growth rates of the outer graphite layers are faster than the inner ones.