*2.1. Preparation of CuCrO2 Hollow Nanotube*

Copper (II) acetate, chromium (III) acetate, and ethanolamine were dissolved in ethylene glycol monomethyl ether (30 mL) to obtain 0.2 M precursor. The prepared solution was stirred for 24 h to obtain a well-mixed solution without impurities. Al2O3 microfibers were dipped in Cu-Cr-O ion solution up to 3 sec to deposit Cu-Cr-O ions on the fiber surfaces and form an Al2O3-Cu-Cr-O core (Step 3). The Cu-Cr-O ions deposited on Al2O3 fibers were dried at 80 ◦C on a hotplate for 2 min. Then the coated fibers were annealed at 600 ◦C in vacuum (Step 4). After that, the prepared nanofibers were etched with 2 M H2SO4 to remove the Al2O3 and other minor impurities from the fibers (Step 5) [39]. The nanofibers were repeatedly rinsed with DI water and a centrifuge was used to separate the liquid and fibers. Finally, the collected nanofibers were dried in an oven at 80 ◦C to form CuCrO2 hollow nanotube (Figure 2).

**Figure 2.** Schematic illustration of CuCrO2 hollow nanotubes fabrication process.

#### *2.2. Characterization*

The crystallized phase of Al2O3 microfibers and CuCrO2 hollow nanotubes was characterized with an X-ray diffractometer (XRD, D2 Phaser, Bruker) with Cu Kα radiation (λ = 0.15418 nm) from 20◦ to 80◦, a working voltage of 30 kV, and current of 10 mA. The thermal decomposition behavior of the as-spun fibers was identified using a thermogravimetric analysis/differential scanning calorimeter (TGA/DSC, STA 449 F5, NETZSCH) at a heating rate of 10 ◦C/min. The surface morphology and structure of the nanofibers were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4700) SEM 15 kV, 10 cm SEI detector, and nanotubes were identified by transmission electron microscopy (TEM, JEM-2100F, JEOL) operated at a working voltage of 200 kV, working current was10 <sup>μ</sup>A and chamber was about 1.0 <sup>×</sup> 10−<sup>6</sup> to 3.0 <sup>×</sup> 10−<sup>6</sup> torr. The composition hollow nanotubes were confirmed by JOEL JEM2100F type scanning transmission electron microscope (STEM) attached with an energy dispersive spectrometer (EDS).

#### **3. Results**

#### *3.1. TGA Analysis*

The TGA/DSC analysis of the Al2O3 electrospun fibers studied at a heating rate of 10 ◦C/min in air is shown in Figure 3. Two discrete regions of electrospun fibers weight loss occurred at about 135 ◦C and 300 ◦C. The weight loss at around 135 ◦C could be attributed to DMF solvent. Exothermic peaks at 300 ◦C with a large weight loss of ~80% corresponded to the decomposition of nitrate, PVP polymer, and other minor organic constituents during the burning combustion. For temperature higher than 600 ◦C, there was almost no change in the TGA curve, which confirmed that the complete decomposition of organic materials and polymer during the formation of Al2O3 fibers [40–43].

**Figure 3.** Thermogravimetric-derivative thermal analysis of as-spun Al2O3 precursor microfibers recorded in air at a heating rate of 10 ◦C/min.

## *3.2. X-ray Di*ff*raction Investigation*

Figure 4 shows the XRD analysis of annealed Al2O3 fibers prepared by electrospinning method. The Al2O3 fibers were fabricated following the process mentioned in the last section with thermal annealing at elevated temperature for 2 h. We found no distinct diffraction peak for the as-spun fibers, but after the fibers were annealed at 600 ◦C, a clear amorphous phase was found. The XRD pattern indicated that the Al2O3 fibers became crystallized when the annealing temperature was over 800 ◦C [44].

Figure 5 shows the XRD pattern of Al2O3 fibers with copper chromium ions deposited on the surfaces after annealing in vacuum at 600 ◦C for 30 min and 60 min, and at 700 ◦C for 30 min. The fibers were composed of an Al2O3 core and the copper chromium ion solution. The XRD studies show the peaks of Al2O3 for the fibers annealed at 600 ◦C for 60 min. It is presumed that the prolonged annealing time caused the crystallization of alumina [39,44].

**Figure 4.** XRD patterns of electrospun Al2O3 precursor fibers annealed for 2 h in the air at various temperatures (600 ◦C–800 ◦C).

**Figure 5.** XRD patterns of Al2O3 microfibers with copper chromium oxide deposited on the surfaces after annealing at 600 ◦C and 700 ◦C in vacuum.

Figure 6 shows the XRD pattern of Al2O3 fibers with copper chromium ion solution deposited on the surfaces after annealing at 600 ◦C for 30 min in vacuum followed by leaching with 2M H2SO4 solution due to the strong acid and without the formation of impurities. That solution was employed because Al2O3 is an amphoteric oxide and reacts with both acid and alkaline solutions. From comparing Figure 6 with Figure 5, it is clear that the main phase of CuCrO2 can be clearly seen in the XRD pattern after the acid immersion. For comparison, NaOH solution was also used to remove alumina cores. As can be seen from the figures, after immersion of the fibers in NaOH solution, only the CuO phase remain while the chromium oxide disappeared. Therefore, we concluded that Al2O3 fibers with copper chromium ion solution deposited on the surfaces could be treated with 2M H2SO4 solution and DI water to obtain CuCrO2 hollow nanotube [39].

**Figure 6.** XRD patterns of Al2O3 microfibers with copper chromium oxide deposited on the surfaces after annealing at 600 ◦C in vacuum followed by leaching with 2M H2SO4 and NaOH solution.

#### *3.3. SEM Analysis*

The SEM micrographs of as-spun Al2O3 precursor fibers have fine cylindrical with smooth surface morphology and shows in Scheme 1 [41]. Besides, the SEM image of Al2O3 electrospun fibers annealed for 2 h in air at 600 ◦C and 800 ◦C are presented in Figure 7. The morphology of the fibers reveals that the Al2O3 fibers have continuous, one-dimensional structure and that the diameter of each Al2O3 fiber is <100 nm. The morphology and dimension of Al2O3 fibers are essentially similar in the case of annealing temperature of 600 ◦C and the counterpart in 800 ◦C.

**Figure 7.** SEM images of electrospun Al2O3 microfibers annealed for 2 h at (**a**) 600 ◦C and (**b**) 800 ◦C.

Figure 8 shows the morphology of Al2O3 fibers immersed in copper chromium ion solution and then dried for 2 min at 80 ◦C on a hotplate. After that, the Al2O3-CuCrO2 fibers were annealed in vacuum at 600 ◦C for 30 min (Figure 8a) and 60 min (Figure 8b), and at 700 ◦C for 30 min (Figure 8c). The surfaces of the fibers are smooth, and there is no specific change compared with calcined amorphous Al2O3 fibers. The copper chromium ion precursor solution, composed of mixed copper acetate, chromium acetate, and ethanolamine, was dissolved in ethylene glycol monomethyl ether.

Figure 9 shows a SEM image of Al2O3-CuCrO2 nanofibers after immersion in 2M H2SO4 and oven-drying at 80 ◦C for 1 day. As can be seen from the SEM morphology, there is a hollow-like structure at the tip of the CuCrO2 nanotubes etched by 2M H2SO4. It was inferred that the Al2O3 core was mostly removed by the H2SO4 solution and remaining impurities were removed by DI water.

**Figure 8.** SEM images of Al2O3-CuCrO2 nanofibers annealed at 600 ◦C for (**a**) 30 min, (**b**) 60 min, and at 700 ◦C for (**c**) 30 min.

**Figure 9.** SEM images of CuCrO2 hollow nanotubes after removal of Al2O3 core and impurities by 2M H2SO4 for 2 days and DI water.
