*3.5. STEM Analysis*

Figure 11a shows a STEM image of CuCrO2 hollow nanotube formed by annealing and chemical etching. The average diameter of the CuCrO2 hollow nanotube was about 100 nm and that of the center hollow was approximately 20 nm. These results exhibit that the chemical etching method succeeded in producing hollow nanotube. The STEM-EDS signals of CuCrO2 nanotube showed the presence of (Figure 11b) Cu, (Figure 11c) Cr, and (Figure 11d) O. Besides, the STEM-EDS spectrum showed higher numbers of atoms present in the tube edge than inside the cavity, which clearly shows the successful formation of the CuCrO2 hollow nanotubes.

**Figure 11.** (**a**) STEM image of the CuCrO2 hollow nanotube, (**b**) Cu, (**c**) Cr, (**d**) O.
