Neatly arranged mesoporous MnO2 nanotubes with oxygen vacancies for electrochemical energy storage

Man Shen , Yi Wang and Yu Xin Zhang*
State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. E-mail:

Received 5th August 2020 , Accepted 14th September 2020

First published on 15th September 2020

Intrinsically poor conductivity, sluggish ion transfer kinetics, and limited specific area are the three main obstacles that confine the electrochemical performance of manganese dioxide in supercapacitors. Herein, one-dimensional mesoporous MnO2 nanotubes were prepared using a polycarbonate film as a template and a large number of oxygen vacancies were introduced by calcination under a N2 atmosphere. The effects of calcination temperature on the crystal structure, micro-morphology and electrochemical performance of MnO2 nanotubes were studied. The presence of oxygen vacancies increases the redox capacity of ov-MnO2-300 nanotubes, and the unique one-dimensional mesoporous structure also provides an effective channel for ion transport. Therefore, the ov-MnO2-300 nanotube has an excellent specific capacitance of 459.0 F g−1 at a current density of 1 A g−1 and also has outstanding rate performance and cycle performance. An asymmetric supercapacitor assembled with ov-MnO2-300 nanotubes as the positive electrode and graphene@MoS2 as the negative electrode delivered an energy density of 40.2 W h kg−1 at a power density of 1024 W kg−1. The excellent capacitance performance is mostly attributed to the introduction of oxygen vacancies to increase the intrinsic conductivity of MnO2, and the unique one-dimensional mesoporous nanotube structure increases the active sites of redox reactions.


Thanks to its outstanding advantages, i.e. low cost, little environmental pollution, high reserves and superior theoretical specific capacitance (1370 F g−1), manganese dioxide (MnO2) is one of the most research-oriented transition metal oxides for supercapacitor electrodes. There are several studies which reviewed the latest developments in MnO2-based supercapacitors,1–4 and concluded that, for MnO2-based supercapacitors, a major issue that is critical to achieving excellent capacitance performance is the rapid transport of electrons and ions in materials or interfaces.3,5–7

Recently, some research studies have pointed out that the one-dimensional porous MnO2 nanotubes which were connected together exhibit outstanding capacitance properties and were relatively worthy of further discussion.8–10 Tubular structures usually have a large specific surface area and can provide an effective ion/electron transmission channel in the longitudinal axis direction; hence, the one-dimensional tubular structure can deliver higher specific capacitance.11–13 Besides, the one-dimensional mesoporous MnO2 nanotubes also shorten the diffusion distances of ions and electrons, thereby promoting the occurrence of Faraday redox reactions, and improving the electrochemical performance of MnO2.14–16 Although the electrochemical performance of MnO2 can be significantly improved by modification of MnO2 into nanotube-like MnO2 (i.e. the design of favorable structure or morphology), the performance is still confined to some degree, because the low intrinsic conductivity of MnO2 is another major obstacle. Therefore, apart from morphology design, some scholars have combined MnO2 nanotubes with other conductive materials, such as carbon nanotubes (CNTs)17and graphene,18,19 or by doping heteroatoms that can dramatically increase the intrinsic conductivity of MnO2, which is reported in our previous work.20 However, only a few research studies have reported that inducing defects in MnO2 nanotubes can further improve their electrochemical performance.

Herein, we focused on synthesizing single MnO2 nanotubes, and introduced oxygen vacancies into them through a simple calcination approach. Numerous studies have also demonstrated that the introduction of oxygen vacancies can effectively improve the conductivity of MnO2 nanomaterials.6,21 Based on this research, a neatly arranged δ-MnO2 nanotube array was prepared via a facile template method, and oxygen vacancies were successfully introduced by calcination under a N2 atmosphere to improve the conductivity of MnO2. The unique hollow mesoporous nanotube structure provides a large number of fast ion transmission channels, and effectively shortens the ion diffusion distance. At the same time, the introduction of oxygen vacancies reduces the lattice oxygen of MnO2, increases the surface adsorbed oxygen, and increases the redox activity of MnO2. Therefore, the specific capacitance of the MnO2 nanotubes with introduced oxygen vacancies increased to 459.0 F g−1 at a current density of 1 A g−1.

Experimental section

Synthesis of the ov-MnO2-X nanotubes

All the chemicals used were of analytical grade. All the solutions were prepared using deionized water. The MnO2/PC membrane was prepared according to the previous method.20,22 Different from the previous method mentioned, the polycarbonate membrane template was removed more quickly. The MnO2/PC membrane was placed in N,N-dimethylformamide (DMF) solution, stirred at 60 °C in a water bath for three hours, and washed with centrifugation. Then the slurry obtained after the previous centrifugation was added into a dichloromethane solution, washed in a 30 °C water bath for ten minutes, and then centrifuged and washed again. This process was repeated three times and finally it was washed with alcohol and dried in an oven at 60 °C for 12 h to obtain MnO2 nanotube powders. 40 mg (three parts) of dried MnO2 nanotube powder was weighed, placed in a crucible and spread evenly, and heated in a tube furnace under a nitrogen atmosphere at a heating rate to 5 °C min−1 The final temperature was 200 °C, 300 °C and 400 °C, and the temperature was maintained for 3 h. The three samples finally obtained were named ov-MnO2-200, ov-MnO2-300, and ov-MnO2-400, respectively.


The structures and morphologies of the as-prepared nanotubes were identified using focused ion beam scanning electron microscopy (FIB/SEM, ZEISS AURIGA) and transmission electron microscopy (TEM, Talos F200S). The crystallographic information and chemical compositions of the as-obtained nanostructures were characterized using powder X-ray diffraction (XRD, D/max 2500, Cu Kα). Nitrogen adsorption and desorption isotherms were measured using a micrometrics ASAP 2020 sorptometer. XPS spectra were recorded using a Physical Electronics ESCA 5600 spectrometer with a monochromatic Al Kα X-ray source (power: 200 W/14 kV) and a multichannel detector (Omni IV).

Electrochemical measurements

An electrochemical workstation and a three-electrode system were used to test the electrochemical properties of ov-MnO2-X nanotubes and the asymmetric devices in 1 M Na2SO4 electrolyte with MnO2 nanostructures as the working electrode, platinum plate as the counter electrode and the calomel electrode as the reference electrode. The positive electrode materials were studied by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). The CV curves at different scan rates of 10–100 mV s−1 were tested at a potential window of 0–0.8 V. The GCD curves were measured with current densities ranging from 1 to 10 A g−1 in the potential window of 0 to 0.8 V. The EIS spectra were recorded at frequencies ranging from 0.01 kHz and 100 kHz with a perturbation amplitude of 5 mV versus the open-circuit potential.

The asymmetric supercapacitor was measured by a two-electrode system using two slices of the electrode material of the same size, a piece of Whatman filter paper as the separator, and two pieces of nickel foil as the current collectors. In a two-electrode system, the ov-MnO2-300 nanotubes were used as the positive electrode and graphene@MoS2, which was prepared as graphene@MoS2, carbon black, and PVDF in NMP in a mass ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1, was used as the negative electrode.

In addition, the detailed calculation equations are given in the ESI.

Theoretical calculations

The Gaussian smearing width was set to 0.2 eV. The Brillouin zone was sampled with a 3 × 6 × 2 K point. All atoms were converged to 0.01 eV Å−1. A plane-wave basis fulfil cut-off energy of 500 eV was employed within the framework of the projector-augmented wave method. A 2 × 2 × 2 supercell of bulk MnO2 that includes 99 atoms was first relaxed and after optimization, a small number of oxygen atoms were removed from the lattice of MnO2.

Results and discussion

The crystalline structure and micro-morphology of MnO2 will change during high-temperature calcination in a special gas atmosphere. By controlling the calcined gas atmosphere, calcination temperature, and holding time, the crystal structure and micromorphology of MnO2 can be developed in a direction that is conducive to improving the electrochemical performance. In this paper, N2 was used as the calcination atmosphere. At the same holding time, MnO2 nanotubes were calcined at different temperatures. The changes in the crystal structure and micromorphology after calcination and their effects on the electrochemical performance of MnO2 nanotubes were studied. The XRD patterns of the as-prepared samples are shown in Fig. 1. Pure MnO2, ov-MnO2-200 and ov-MnO2-300 possess similar diffraction peaks at 2Θ = 37.36°and 65.47°, which are assigned to the (012) and (110) planes of δ-MnO2, respectively (JCPDS no. 86-0666).20 No other phases or impurities were observed, confirming that the structure of δ-MnO2 was retained after the calcination treatments. However, the diffraction peaks become weaker and wider with increasing calcination temperatures, indicating a decrease in crystallinity, which can provide more channels for ion permeation. In particular, when the calcination temperature is increased to 400 °C, MnO2 undergoes crystal transformation. ov-MnO2-400 possess diffraction peaks at 32.32°, 36.08°and 59.84°, which are assigned to the (103), (211) and (110) planes of Mn3O4 respectively (JCPDS no. 24-0734). Excessive temperature causes MnO2 to be completely reduced to Mn3O4.23,24 The valence state of the Mn element in Mn3O4 is also a mixed valence state. Thus, Mn2+ and Mn3+ exist simultaneously in the Mn3O4 crystal.
image file: d0dt02733b-f1.tif
Fig. 1 XRD patterns: MnO2, ov-MnO2-200, ov-MnO2-300, and ov-MnO2-400.

Since the electrochemical performance studies below show that ov-MnO2-300 has the best electrochemical performance, MnO2 nanotubes and ov-MnO2-300 nanotubes are selected for comparison in the subsequent characterization methods. XPS is used to obtain more detailed chemical composition information of MnO2 and ov-MnO2-300. For the Mn 2p spectra (Fig. 2a), the two major peaks at 642.3 and 653.9 eV (spin energy separation of 11.6 eV) are assigned to the Mn 2p3/2 and Mn 2p1/2 binding energies, respectively, which are in good agreement with characteristic MnO2.6,21 For ov-MnO2-300, the Mn 2p3/2 spectrum is further deconvoluted into two pairs of peaks at 642.3 and 645.05 eV, whereas the Mn 2p1/2 spectrum is deconvoluted into peaks at 653.50 and 656.25 eV; the peaks at 642.3, 653.50 eV and 645.05, 656.25 eV are assigned to Mn3+ and Mn4+, respectively. The two main peaks of MnO2 are also decomposed in the same way. Three fitted peaks at 530.1, 531.9 and 532.9 eV are observed in the O 1s spectra (Fig. 2b), which are assigned to the lattice oxygen (denoted as O2−), surface adsorbed oxygen (denoted as Ovoc) and water (denoted as H2O), respectively.21 According to the results of the Mn 2p and O 1s spectra (Fig. 2a and c), the Mn3+/Mn4+ ratio and O species content for the MnO2 and ov-MnO2-300 samples are calculated. As for pure MnO2, the Mn3+/Mn4+ ratio is 32.02%, which is increased to 55.9% for ov-MnO2-300. The results show that after introducing oxygen vacancies, part of Mn4+ in pure MnO2 is reduced to Mn3+. The higher Mn3+/Mn4+ ratio means enhanced redox capacity, which is beneficial for electrochemical performance. It can be clearly seen from Table 1 and Fig. 2b that a significant decrease in lattice oxygen content is observed after calcination under a N2 atmosphere, which assisted the delocalization of neighboring electrons nearby the low coordinated Mn atoms, affected the crystal growth mode and further changed the crystal morphology.25,26 In addition, after calcination, the lattice oxygen decreases, and the surface adsorbed oxygen increases, which increases the surface activity and redox capacity of MnO2, which is beneficial to improve the electrochemical performance. However, if the temperature is too high, too much oxygen is taken away and MnO2 is reduced to Mn3O4.

image file: d0dt02733b-f2.tif
Fig. 2 XPS patterns: (a) Mn 2p and (b) O 1s.
Table 1 Mn and O species content (at%) of the samples based on XPS results
Samples Mn4+ Mn3+ O2− Ovoc H2O
MnO2 75.74% 64.14% 66.2% 21.2% 12.6%
ov-MnO2-300 24.36% 35.86% 59.9% 33.5% 6.6%

Micro-morphology is also one of the important factors affecting electrochemical performance. Fig. 3 shows the SEM image of all samples at the same magnification. MnO2, ov-MnO2-200 and ov-MnO2-300 are nanotubes composed of nanosheets. At the same magnification, the nanosheets of pure MnO2 and ov-MnO2-200 are too small to see obvious nanosheet structures. However, for ov-MnO2-300, the nanotube structure composed of nanosheets is very clear. With the increase in calcination temperature, the MnO2 nanosheets grow gradually, and the contact area with the electrolyte increases. However, when the temperature was increased to 400 °C, the crystal structure changed, and the nanosheets constituting the nanotubes were also transformed into nanospheres. Nanotubes composed of nanospheres are not very dense in structure; therefore, they have a large contact area with the electrolyte and can obtain a relatively good specific capacitance. The clear porous nanotube structure of pure MnO2 and ov-MnO2-300 can be seen in the transmission electron microscope picture (Fig. 4). From the low-power transmission electron microscope, the morphology of these two samples is not much different and both are composed of small Nanosheets made of nanotubes. Because the crystallinity of the two samples is not very high, the crystal plane that can be seen in the TEM picture is the (001) crystal plane of δ-MnO2, and the corresponding crystal plane spacing is 0.70 nm. There are some minor differences in the HRTEM images of the two samples. The lattice fringes of pure MnO2 are relatively continuous, while the lattice fringes of ov-MnO2-300 are shorter and discontinuous due to the presence of oxygen vacancies. The TEM results corresponded well with the XRD and XPS results, proving the existence of oxygen vacancies.

image file: d0dt02733b-f3.tif
Fig. 3 SEM images: (a) MnO2, (b) ov-MnO2-200, (c) ov-MnO2-300, and (d) ov-MnO2-400.

image file: d0dt02733b-f4.tif
Fig. 4 TEM images: (a)–(c) MnO2 and (d)–(f) ov-MnO2-300.

In order to investigate the supercapacitor performance of the calcined MnO2 nanotubes, electrochemical performance tests were performed on all samples in a 1 M Na2SO4 electrolyte. Fig. S1 shows the cyclic voltammetry curves of all samples at different scan rates and the charge and discharge curves at different current densities. Except for the CV curve of ov-MnO2-400, which has a very obvious redox peak, the GCD curve has a clear charge and discharge platform. The other three samples have no obvious redox peaks in this figure, but the CV curve is a regular rectangular structure, and the GCD curve is also a symmetrical triangle structure. These are the typical characteristics of a pseudo-capacitor supercapacitor. The CV curves of pure MnO2 and ov-MnO2-X at a constant scan rate of 10 mV s−1 are shown in Fig. 5a. At this scan rate, both ov-MnO2-300 and ov-MnO2-400 have obvious redox peaks, indicating that redox peaks appear after the introduction of oxygen vacancies, which indicates that the pseudocapacitor performance of MnO2 is enhanced. In contrast, the integrated area of the CV curve of the ov-MnO2-300 electrode is higher than that of other electrodes. As shown in Fig. 5b, the discharge time of ov-MnO2-300 is longer than that of the other electrodes, which corresponds to the CV result. When the current density is 1 A g−1, the specific capacitance of pure MnO2 is 279.4 F g−1, while 307.2, 459.0, and 396.3 F g−1 correspond to ov-MnO2-200, ov-MnO2-300, and ov-MnO2-400 electrodes, respectively. This result indicates that the specific capacitance of δ-MnO2 can be significantly improved by introducing a suitable oxygen vacancy concentration, which is attributed to the higher surface adsorbed oxygen concentration, resulting in increased surface activity and enhanced redox capacity. In addition, as shown in Fig. 5c, an electrochemical impedance spectroscopy (EIS) analysis was performed to further study the electrochemical behavior of all electrodes. In the high-frequency region, the intercept of the curve and the semi-circular arc on the real axis correspond to the internal resistance (Rs) and charge transfer resistance (Rct), respectively. It can be clearly seen in Fig. 5d that the ov-MnO2-300 electrode's Rs and Rct (Rs = 2.2 Ω, Rct = 2.8 Ω) are smaller than those of the other electrodes (Table 2), which is consistent with their enhanced performance. In the low frequency region, the slope of the straight line represents the Warburg resistance (Rw), which corresponds to the electrolyte diffusion resistance inside the electrode. By comparison, the ov-MnO2-200 electrode has the highest slope and a more vertical shape, which represents a lower Rw, which enables faster charge transfer kinetics. These results are mainly due to the introduction of oxygen vacancies, which is necessary for fast charge/discharge kinetics. When the current density is increased to 10 A g−1, the capacitance retention rates of pure MnO2, ov-MnO2-200, ov-MnO2-300, and ov-MnO2-400 are 50.8%, 60.2%, 79.3%, and 63.1%, indicating that reducing the lattice oxygen concentration can improve the rate performance of MnO2 (Fig. 5e). Fig. 5f shows the cycle performance of all samples. Among all samples, the cycle performance of ov-MnO2-300 nanotubes is the best. After 3000 cycles, 94.8% of the initial capacitance of ov-MnO2-300 nanotubes is still maintained.

image file: d0dt02733b-f5.tif
Fig. 5 The electrochemical performance of the MnO2 nanotube electrode and ov-MnO2-X nanotube electrodes. (a) CV curves measured at a scan rate of 10 mV s−1, (b) galvanostatic charge/discharge curves at a current density of 1 A g−1, (c) electrochemical impedance spectrum at open circuit potential at frequencies ranging from 0.01 Hz to 100 kHz, (d) the partial enlargement of (c), (e) specific capacitance under different current densities, and (f) the cycling performance at the current density of 10 A g−1.
Table 2 Resistance data fitted by EIS
Samples Rs Rct
MnO2 2.5 2.98
ov-MnO2-300 2.2 2.8

Combining the above electrochemical performance analysis, we can conclude that after the introduction of oxygen vacancies, the surface of MnO2 nanotubes adsorbs oxygen and increases surface activity.27–29 The conductivity of MnO2 is also improved, which increases the rate of redox reactions, and thus increases the specific capacitance performance of MnO2. The electronic structure change after introducing oxygen vacancies is proved by DFT theoretical calculation. Fig. 6 shows the calculated TDOS value of MnO2 and MnO2 after introducing oxygen vacancies. From the figure, we can see that the original δ-MnO2 has a band gap of 1.42 eV, but after introducing oxygen vacancies, the band gap of MnO2 becomes very narrow. It is fully proved that the conductivity of MnO2 will increase after the introduction of oxygen vacancies, and the conductivity of ions and electrons will also increase accordingly.

image file: d0dt02733b-f6.tif
Fig. 6 Calculated TDOS of MnO2 and ov-MnO2.

Therefore, the ov-MnO2's pseudocapacitive performance is superior to pure MnO2.

To improve the energy density of asymmetric supercapacitors, a suitable anode material is essential. The specific capacitance of our commonly used activated carbon and reduced graphene oxide is too low to match the performance of MnO2 nanotubes with improved conductivity. Then, it is important to choose a negative electrode material with good performance. MoS2 has high specific capacitance because of its good conductivity.30–32 In this paper, the prepared graphene@MoS2 nanomaterial is used as the negative electrode material of asymmetric supercapacitor. The graphene@MoS2 nanomaterial was tested for the same electrochemical performance as the cathode material. As shown in Fig. S2, the potential window of the graphene@MoS2 nanomaterial is −0.9 V to −0.2 V, and the shape of the CV curve is still well maintained at large scan rates, showing good capacitance performance. Fig. S2b shows the charge and discharge curves at different current densities. The graphene@MoS2 nanomaterial has a specific capacitance of 310 F g−1 at a current density of 1 A g−1, but when the current density increases to 10 A g−1, the specific capacitance becomes 190.7 F g−1 and the specific capacitance value decreases greatly, indicating that the material has poor rate performance. The reason for the poor rate performance may be the poor conductivity of the material and the poor stability of the structure. Similarly, as shown in Fig. S2c, after 3000 cycles of the material, the specific capacitance is 75.5% of the initial specific capacitance. Such a cycling performance is very general. The impedance diagrams before and after the cycling (Fig. S2d) also confirm that the initial resistance of the sample does not change much after cycling, but the increase in transmission resistance is relatively large. The electrochemical performance diagrams of asymmetric supercapacitors composed of graphene@MoS2 nanomaterials and ov-MnO2-300 nanotubes are shown in Fig. 7 and Fig. S3. The absolute value of the potential window of the positive and negative materials is added to 1.7 V, but after the two kinds of materials are matched, the potential window of the supercapacitor is generally greater than 1.7 V due to the generation of overpotential. As shown in Fig. 7a, when the potential window is enlarged to 2.2 V, the CV curve begins to polarize and so the potential window of the ov-MnO2-300//graphene@MoS2 supercapacitor should be 0–2.0 V. The CV curves of different scanning speeds (Fig. 7b) show that even if the scanning rate is increased to 100 mV s−1, the CV curve is not deformed, which illustrates the good charge and discharge performance of the capacitor. From the charge–discharge curve (Fig. 7c), we can calculate that at 1 A g−1, the specific capacitance of ov-MnO2-300//graphene@MoS2 ASCs is 70.3 F g−1, but it is affected by the negative electrode material. When the current density is increased to 8 A g−1, only 32 F g−1 remains for the specific capacitance (Fig. S3a). Fig. S3b and c show the cyclic performance chart of ov-MnO2-300//graphene@MoS2 ASCs and the impedance change chart before and after the cycle. After 3000 cycles, the specific capacitance remained at 60.2% of the initial capacitance, which did not reach the requirements of the application and the change in impedance also confirms this. The negative electrode material has a higher specific capacitance at low current density, but its rate performance is poor; in general, cycling performance reduces the electrochemical performance of a supercapacitor. Therefore, the comprehensive performance of the material should be fully considered when selecting the negative electrode material. Fig. 7d shows the energy–power density diagram of ov-MnO2-300//graphene@MoS2 ASCs. When the power density is 1024 W kg−1, the energy density reaches 40.2 W h kg−1. However, due to the excellent electrochemical performance of the positive electrode material, the performance of the supercapacitor still exceeds the performance of many currently reported MnO2 or supercapacitors, but a reasonable negative electrode material is still worth further investigation.

image file: d0dt02733b-f7.tif
Fig. 7 (a) CV curves of ov-MnO2-300//graphene@MoS2 measured at different potential windows at a scan rate of 100 mV s−1. (b) CV curves measured at different scan rates between 0 and 2.0 V. (c) Charge–discharge curves at different current densities. (d) Ragone plots of ov-MnO2-300//graphene@MoS2.


The mesoporous MnO2 nanotubes prepared by the template method were calcined under an N2 atmosphere to generate oxygen vacancies. Different calcination temperatures have a great influence on the crystal structure and electrochemical performance of MnO2 nanotubes. When the temperature rises to 400 °C, the δ-MnO2 nanotubes were completely reduced to Mn3O4 nanotubes with a spinel structure. The basic unit of the nanotubes also changed from nanosheets to nanospheres. The electrochemical properties of ov-MnO2-X nanotubes as electrode materials for supercapacitors in 1 M Na2SO4 electrolyte were investigated. Due to the introduction of oxygen vacancies, ov-MnO2-300 has the best electrochemical performance. It can reach 459.0 F g−1 at a current density of 1 A g−1 and retain 94.8% of the initial capacitance after 3000 cycles. The introduction of oxygen vacancies increases the surface activity and redox capacity of MnO2, thereby improving the capacitor performance. However, although Mn3O4 has good redox activity, its crystal structure is not conducive to the insertion/extraction of ions. Furthermore, the ov-MnO2-300//graphene@MoS2 asymmetric supercapacitor reversibly charges/discharges in 1 M Na2SO4 electrolyte at 2.0 V operating voltage, providing 40.2 W h kg−1 energy density at a power density of 1024 W kg−1. This article essentially increases the conductivity of MnO2 and increases the specific capacitance contribution of MnO2. The study of pure MnO2 as a supercapacitor has far-reaching significance.

Conflicts of interest

There are no conflicts to declare.


The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 51908092), the Graduate Scientific Research and Innovation Foundation of Chongqing, China (Grant No. CYS20002), the Joint Funds of the National Natural Science Foundation of China-Guangdong (Grant No. U1801254), the Chongqing Special Postdoctoral Science Foundation (XmT2018043), the Natural Science Foundation Project of Chongqing for Post-doctor (cstc2019jcyjbsh0079), the Technological projects of Chongqing Municipal Education Commission (KJZDK201800801), Project No. 2020CDJXZ001 supported by the Fundamental Research Funds for the Central Universities, the Innovative Research Team of Chongqing (CXTDG201602014) and the Innovative Technology of New materials and metallurgy (2019CDXYCL0031). The authors also thank the Electron Microscopy Center of Chongqing University for materials characterization studies.

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Electronic supplementary information (ESI) available. See DOI: 10.1039/D0DT02733B
These authors contributed equally.

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