*3.4. TEM Analysis*

To identify the structure of the CuCrO2 hollow nanotubes synthesized by annealing and followed by chemical etching, TEM was used to further confirm the hollow structures of the nanotubes. The nanotubes were formed by using Al2O3 fiber as a template and depositing copper chromium ions on the tube surfaces so that the inner core was Al2O3. As shown in TEM image in Figure 10, the inner template of Al2O3 was completely etched away by 2M H2SO4 solution. The inner diameter of the nanotubes was about 70 nm, which is consistent with the diameter of Al2O3 fiber. The tube wall which consists of CuCrO2 features a thickness of several tens of nanometer [39]. These results indicate that the chemical etching method was successful in making CuCrO2 hollow nanotubes. Based on previous report, CuCrO2 hollow nanotubes have more porous cavity than none-hollow CuCrO2 nanofibers due to annealing condition [10].

**Figure 10.** TEM images of the CuCrO2 hollow nanotubes.
