**4. Conclusions**

CuCrO2 hollow nanotubes were successfully prepared by our proposed method using electrospun Al2O3 fiber as core template. The amorphous Al2O3 fibers were prepared by annealing the as-spun alumina precursor fibers at 600 ◦C for 2 h. These continuous and one-dimensional fibers were then deposited with CuCrO2 precursor and formed CuCrO2 cladding layer by thermal annealing at 600 ◦C for 30 min. After removing amorphous Al2O3 core fibers by using H2SO4, CuCrO2 nanotubes with an inner diameter of 70 nm and tube wall thickness of 30 nm were obtained. This work demonstrated a simple solution-based approach for the synthesis of oxide nanotubes and could be further extended to synthesize oxide materials with various complicated hollow structures.

**Author Contributions:** Conceptualization, H.-J.W. and T.-W.C.; data curation, H.-J.W. and T.-W.C.; formal analysis, H.-J.W.; funding acquisition, Y.-J.F., T.-W.C., and S.-S.L.; investigation, T.-W.C.; methodology, H.-J.W.; project administration, T.-W.C.; resources, T.-W.C.; supervision, T.-W.C.; validation, Y.-J.F., S.S., T.-W.C., and S.-S.L.; visualization, H.-J.W. and S.S.; writing—original draft, H.-J.W., S.-S.W., and T.-W.C.; writing—review and editing, H.-J.W., S.S., T.-W.C., S.-S.L., and J.-H.P.

**Funding:** This research was funded by University System of Taipei Joint Research Program (USTP-NTUT-TMU-108-06) and Ministry of Science and Technology, Taiwan grant number (MOST106-2221-E-027-041).

**Acknowledgments:** This work was supported by the Ministry of Science and Technology of Taiwan (MOST106- 2221-E-027-041) and University System of Taipei Joint Research Program (USTP-NTUT-TMU-108-06). The authors appreciate the Precision Research and Analysis Center of National Taipei University of Technology (NTUT) for providing the measurement facilities.

**Conflicts of Interest:** The authors declare no conflict of interest.
