The X-ray diffraction patterns in Figure S1 reveal that Mn@NCNTs presented a higher peak intensity at 26.5° than pristine CNTs, which implied a higher graphitic degree and a better reducibility attributed to the removal of surface oxygen functionalities and incorporation of nitrogen dopants.In addition, typical diffraction peaks for manganese carbides were observed in a range of 30°–50°, which confirmed the successful decoration of Mn species into the carbon framework.Two characteristic peaks, D and G bands of carbon, can be observed in Raman spectra ( Figure S2 ), and the increased intensity ratio of the D band versus G band (I/I) unveiled the higher defective degree induced by nitrogen doping and manganese carbide incorporation. The effect of pyrolysis temperature was further investigated by Raman spectroscopy( Figure S3 ), suggesting that a higher graphitic degree could be achieved at elevated temperatures. However, the carbon skeleton of Mn@NCNTs started to collapse and then form irregular nanostructures at 900°C. In addition, Brunauer-Emmett-Teller (BET) analysis ( Figure S4 ) depicts that the specific surface area (SSA) of Mn@NCNTs dramatically increased with increased pyrolysis temperatures from 75.4 m(700°C) to 217.1 m(900°C).

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Figure 1 Structure and Morphology and the As-Made Mn@NCNTs Show full caption (A) SEM image of Mn@NCNTs-800. (B and C) TEM images of Mn@NCNTs-800. Yellow dashed box in (B) is magnified in (C). (D) TEM image of Mn@NCNTs-700 tip growing mechanism. (E) HRTEM image of Mn nanoparticle tip encapsulation. (F) TEM image of Mn@NCNTs-800 after acid treatment. (G) HAADF-STEM images of Mn@NCNTs-800 and the corresponding energy-dispersive X-ray spectroscopy elemental mappings of C, N, O, and Mn (from the third left to right).

Scheme 1 Schematic Illustration of the Procedure for the Preparation of Mn@NCNTs Preparation

The morphology and structure of Mn@NCNTs were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and their images are shown in Figures 1 and S5 Figures 1 A and 1B reveal the spring-like morphology of Mn@CNTs-800 with a smooth surface and dimensions of 3–5 μm in length and 20–40 nm in ring diameter. Interestingly, the TEM image in Figure 1 C shows that the CNTs presented a hollow tubing structure with an inner tunnel diameter of around 20 nm. The effect of temperature on the catalyst morphology is displayed in Figure S5 . As shown in the SEM image of Mn@CNTs-600 ( Figure S5 A), most of the melamine was converted to graphite and Mnwas reduced to zero-valent Mn and manganese carbide particles under a Nflow. However, no regularly tubing structures were found. When the temperature increased to 700°C, some curved CNTs were formed ( Figure S5 B), indicating the early-stage formation of helical tubes. By increasing the temperature to 800°C, Mn@CNTs reached the optimal temperature, under which most of the tubes presented in a helical shape ( Figure S5 C). Meanwhile, by further raising the temperature to 900°C, the CNTs were broken down into CNT rings ( Figure S5 D). From these SEM and TEM illustrations, the growing mechanism can be inferred. Firstly, Mnnanoparticles were formed at 600°C and started to serve as the catalyst for the formation of graphitic cages. As the temperature increased to 700°C–800°C, the graphite cages would serve as a building block for CNT formation and growth. Due to the fact that the high temperature may influence the metal species and carbon growth rate, the curved CNTs were bent and formed helical particles. After increasing the temperature to 900°C or above, the helical tubes were broken into ring-like CNTs due to the thermal cutting of CNTs and decomposition of Mnat over-high temperatures. As illustrated in Scheme S1 , Mn nanoparticles act as the catalysts at 800°C to convert the amorphous carbon into graphitic carbons. CNTs then start to grow and stretch from the tip of Mn nanoparticles, while the Mn/C clusters are gradually consumed and bonded in the stable form of CNTs and Mn Scheme 1 A–1C). After the harsh acid treatment, most of the superficial metals and oxides were removed, leaving the open-ended and hollow helical tubes ( Scheme 1 D and 1E). The unique helical shape of CNTs was formed due to the relatively high surface tension of Mn carbides compared with other transition metal carbides, such as iron (Fe) and lanthanum (La), which resulted in the distorted graphitic carbon networks from the irregular carbon deposition rate around the Mn core.Because of the high surface energy and closed structure, topological defects such as five-fold and seven-fold carbon rings can be formed, allowing the carbon tubes to grow in a tilted direction.The enlarged TEM image of the region marked with rectangles in Figure 1 E clearly illustrates the lattice fringes of 0.21 and 0.34 nm, corresponding to the d-spacing of (200) manganese carbide and (002) graphite basal planes, respectively.After the acid treatment, the superficial Mn species and tip Mn particles were removed, whereby a smooth and open-ended carbon nanotube was obtained as shown in Figure 1 F. High-angle annular dark-field imaging scanning TEM (HAADF-STEM) images ( Figure 1 G) display that N-dopants were uniformly distributed on the surface and the imbedded Mn species could be observed beneath the carbon layers. Furthermore, when the temperature was increased to 900°C, the CNTs started to break up into shorter tubes ( Figure S5 D), which is consistent with the aforementioned BET and Raman analyses.