Preparation of CuCrO2 Hollow Nanotubes from an Electrospun Al2O3 Template

A hollow nanostructure is attractive and important in different fields of applications, for instance, solar cells, sensors, supercapacitors, electronics, and biomedical, due to their unique structure, large available interior space, low bulk density, and stable physicochemical properties. Hence, the need to prepare hollow nanotubes is more important. In this present study, we have prepared CuCrO2 hollow nanotubes by simple approach. The CuCrO2 hollow nanotubes were prepared by applying electrospun Al2O3 fibers as a template for the first time. Copper chromium ions were dip-coated on the surface of electrospun-derived Al2O3 fibers and annealed at 600 °C in vacuum to form Al2O3-CuCrO2 core-shell nanofibers. The CuCrO2 hollow nanotubes were obtained by removing Al2O3 cores by sulfuric acid wet etching while preserving the rest of original structures. The structures of the CuCrO2-coated Al2O3 core-shell nanofibers and CuCrO2 hollow nanotubes were identified side-by-side by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The CuCrO2 hollow nanotubes may find applications in electrochemistry, catalysis, and biomedical application. This hollow nanotube preparation method could be extended to the preparation of other hollow nanotubes, fibers, and spheres.


Introduction
One-dimensional (1D) nanostructure materials such as nanotubes, nanobelts, and nanofibers have attracted wide interest in nanoscience and technology [1]. Regulating the size and shape of synthesized nanomaterials is of great technological interest nowadays. Particularly, hollow nanostructures have received considerable attention due to their high surface areas and structural uniqueness, thus they have been extensively applied in many fields, such as sensors, dye-sensitized solar cells, catalysts, supercapacitors, photoelectrochemical cells, electronics, and biomolecule devices. Hence, different approaches have been used in the development of hollow nanotubes and nanofibers for large-scale synthesis [2,3]. One of such structural approaches is electrospinning which has been widely applied to synthesize nanofibers from a variety of oxide materials [4].
Electrospinning is a fiber formation method that uses self-repulsion effect, which induces an electrostatic charge on a precursor material to stretch the liquid in an electric field into fiber structure.

Materials and Methods
All the high-purity chemicals used in this experiment were obtained from Sigma Chemical Co, Taiwan. The electrospun Al 2 O 3 microfibers precursor was prepared by the electrospinning method. Typically, the precursor solution was prepared by dissolving aluminum nitrate (Al(NO 3 ) 3 9H 2 O) into 14.4 mL of dimethylformamide (DMF) solvent to make a 0.04 M metal source solution. Then, 2.4 g polyvinylpyrrolidone (Mw = 1,300,000) was mixed into the aforementioned prepared metal source solution followed by constant stirring for 6 h. Finally, a viscous gel-like precursor solution of Al 2 O 3 was obtained. The Al 2 O 3 precursor solution was loaded into a horizontal programmable syringe pump. A schematic image of the fundamental electrospinning process is illustrated in Figure 1. An ordinary electrospinning set-up, a high-voltage source is combined with the metallic needle, which is connected to a syringe pump. This syringe pump was connected with Teflon tube (length = 125 mm, diameter = 4.2 mm) for conventional electrospinning setup. During the electrospinning process, the precursor solution was placed in a 10 mL syringe equipped with a stainless steel needle (ID = 0.5 mm). A voltage of 20 kV was applied to the stainless steel needle tip, and the collector was fixed at a distance of 16 cm from the needle tip with the flow controlled at 0.02 mL/h. The electrospun Al 2 O 3 precursor was distributed uniformly over the collector to form Al 2 O 3 precursor fibers (Step 1). After the electrospinning, the electrospun Al 2 O 3 precursor fibers were heated at a rate of 5 • C/min to the annealing temperature of 600 • C in a high-temperature furnace at air atmosphere and then held at that temperature for 2 h, after which Al 2 O 3 nanofibers were formed (Step 2) and the diameter of the Al 2 O 3 nanofibers is <100 nm. Nanomaterials 2019, 9, x FOR PEER REVIEW 3 of 10 pump. A schematic image of the fundamental electrospinning process is illustrated in Figure 1. An ordinary electrospinning set-up, a high-voltage source is combined with the metallic needle, which is connected to a syringe pump. This syringe pump was connected with Teflon tube (length = 125 mm, diameter = 4.2 mm) for conventional electrospinning setup. During the electrospinning process, the precursor solution was placed in a 10 mL syringe equipped with a stainless steel needle (ID = 0.5 mm). A voltage of 20 kV was applied to the stainless steel needle tip, and the collector was fixed at a distance of 16 cm from the needle tip with the flow controlled at 0.02 mL/h. The electrospun Al2O3 precursor was distributed uniformly over the collector to form Al2O3 precursor fibers (Step 1). After the electrospinning, the electrospun Al2O3 precursor fibers were heated at a rate of 5 °C/min to the annealing temperature of 600 °C in a high-temperature furnace at air atmosphere and then held at that temperature for 2 h, after which Al2O3 nanofibers were formed (Step 2) and the diameter of the Al2O3 nanofibers is <100 nm.

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).

Preparation of CuCrO 2 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. Al 2 O 3 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 Al 2 O 3 -Cu-Cr-O core (Step 3). The Cu-Cr-O ions deposited on Al 2 O 3 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 H 2 SO 4 to remove the Al 2 O 3 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 CuCrO 2 hollow nanotube ( Figure 2). Nanomaterials 2019, 9, x FOR PEER REVIEW 3 of 10 pump. A schematic image of the fundamental electrospinning process is illustrated in Figure 1. An ordinary electrospinning set-up, a high-voltage source is combined with the metallic needle, which is connected to a syringe pump. This syringe pump was connected with Teflon tube (length = 125 mm, diameter = 4.2 mm) for conventional electrospinning setup. During the electrospinning process, the precursor solution was placed in a 10 mL syringe equipped with a stainless steel needle (ID = 0.5 mm). A voltage of 20 kV was applied to the stainless steel needle tip, and the collector was fixed at a distance of 16 cm from the needle tip with the flow controlled at 0.02 mL/h. The electrospun Al2O3 precursor was distributed uniformly over the collector to form Al2O3 precursor fibers (Step 1). After the electrospinning, the electrospun Al2O3 precursor fibers were heated at a rate of 5 °C/min to the annealing temperature of 600 °C in a high-temperature furnace at air atmosphere and then held at that temperature for 2 h, after which Al2O3 nanofibers were formed (Step 2) and the diameter of the Al2O3 nanofibers is <100 nm.

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).

Characterization
The crystallized phase of Al 2 O 3 microfibers and CuCrO 2 hollow nanotubes was characterized with an X-ray diffractometer (XRD, D 2 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 µA and chamber was about 1.0 × 10 −6 to 3.0 × 10 −6 torr. The composition hollow nanotubes were confirmed by JOEL JEM2100F type scanning transmission electron microscope (STEM) attached with an energy dispersive spectrometer (EDS).

TGA Analysis
The TGA/DSC analysis of the Al 2 O 3 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 Al 2 O 3 fibers [40][41][42][43].

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 μA and chamber was about 1.0 × 10 −6 to 3.0 × 10 −6 torr. The composition hollow nanotubes were confirmed by JOEL JEM2100F type scanning transmission electron microscope (STEM) attached with an energy dispersive spectrometer (EDS).

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][41][42][43].  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  Figure 4 shows the XRD analysis of annealed Al 2 O 3 fibers prepared by electrospinning method. The Al 2 O 3 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 Al 2 O 3 fibers became crystallized when the annealing temperature was over 800 • C [44]. peaks of Al 2 O 3 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]. Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 10 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 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].  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 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 shows the XRD pattern of Al 2 O 3 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 H 2 SO 4 solution due to the strong acid and without the formation of impurities. That solution was employed because Al 2 O 3 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 CuCrO 2 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 Al 2 O 3 fibers with copper chromium ion solution deposited on the surfaces could be treated with 2M H 2 SO 4 solution and DI water to obtain CuCrO 2 hollow nanotube [39].

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 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.

SEM Analysis
The SEM micrographs of as-spun Al 2 O 3 precursor fibers have fine cylindrical with smooth surface morphology and shows in Scheme 1 [41]. Besides, the SEM image of Al 2 O 3 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 Al 2 O 3 fibers have continuous, one-dimensional structure and that the diameter of each Al 2 O 3 fiber is <100 nm. The morphology and dimension of Al 2 O 3 fibers are essentially similar in the case of annealing temperature of 600 • C and the counterpart in 800 • C. Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 10 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.

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 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 8 shows the morphology of Al 2 O 3 fibers immersed in copper chromium ion solution and then dried for 2 min at 80 • C on a hotplate. After that, the Al 2 O 3 -CuCrO 2 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 Al 2 O 3 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 Al 2 O 3 -CuCrO 2 nanofibers after immersion in 2M H 2 SO 4 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 CuCrO 2 nanotubes etched by 2M H 2 SO 4 . It was inferred that the Al 2 O 3 core was mostly removed by the H 2 SO 4 solution and remaining impurities were removed by DI water. um at 600 °C for 30 min (Figure 8a) and 60 min (Figure 8b), and at 700 °C for 30 min (Figure surfaces of the fibers are smooth, and there is no specific change compared with calcin rphous Al2O3 fibers. The copper chromium ion precursor solution, composed of mixed cop ate, chromium acetate, and ethanolamine, was dissolved in ethylene glycol monomethyl ethe   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.

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 nonehollow CuCrO2 nanofibers due to annealing condition [10].

TEM Analysis
To identify the structure of the CuCrO 2 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 Al 2 O 3 fiber as a template and depositing copper chromium ions on the tube surfaces so that the inner core was Al 2 O 3 . As shown in TEM image in Figure 10, the inner template of Al 2 O 3 was completely etched away by 2M H 2 SO 4 solution. The inner diameter of the nanotubes was about 70 nm, which is consistent with the diameter of Al 2 O 3 fiber. The tube wall which consists of CuCrO 2 features a thickness of several tens of nanometer [39]. These results indicate that the chemical etching method was successful in making CuCrO 2 hollow nanotubes. Based on previous report, CuCrO 2 hollow nanotubes have more porous cavity than none-hollow CuCrO 2 nanofibers due to annealing condition [10]. 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 nonehollow CuCrO2 nanofibers due to annealing condition [10].  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  Figure 11a shows a STEM image of CuCrO 2 hollow nanotube formed by annealing and chemical etching. The average diameter of the CuCrO 2 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 CuCrO 2 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 CuCrO 2 hollow nanotubes.

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

Conclusions
CuCrO 2 hollow nanotubes were successfully prepared by our proposed method using electrospun Al 2 O 3 fiber as core template. The amorphous Al 2 O 3 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 CuCrO 2 precursor and formed CuCrO 2 cladding layer by thermal annealing at 600 • C for 30 min. After removing amorphous Al 2 O 3 core fibers by using H 2 SO 4 , CuCrO 2 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.