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Construction of TiO2 Nanotubes/C/MnO2 Composite Films as a Binder-Free Electrode for a High-Performance Supercapacitor Zhirong Zhang, Zhongping Yao,* Yanqiu Meng, Dongqi Li, Qixing Xia, and Zhaohua Jiang School of Chemistry and Chemical Engineering, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150001, P. R. China

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF FLORIDA on 01/10/19. For personal use only.

S Supporting Information *

ABSTRACT: Although titanium dioxide (TiO2) exhibits excellent promise in electrode materials for supercapacitors, its poor conductivity and low areal specific capacitance hamper further development. In this work, we have designed a clever way to deposit manganese dioxide (MnO2) in order to improve its electrochemical performance via a facile and typical hydrothermal method. In a hydrothermal process, carbon (C), which deposited via new gas thermal penetration, acts as a reducing agent, while a potassium permanganate (KMnO4) solution acts as an oxidant. In this way, MnO2, which has a high theoretical capacity, is generated on TiO2 nanotube arrays (denoted as TNTs) successfully. Remarkably, a TNTs/C/MnO2 film prepared at a hydrothermal temperature of 90 °C and 0.3 g of KMnO4 revealed a superior electrochemical property with 55 mF/cm2 areal capacitance at a scan rate of 5 mV/s, 23 times more enhanced than that of a TNTs/C film. Also, the energy density of a TNTs/C/MnO2 film reached 46.8 Wh/cm2 when the power density was 0.12 mW/cm2, and the energy density still remained at 22.4 Wh/cm2 at a high power density of 0.8 mW/cm2. After 1000 cycle tests, 93.2% capacitance was still retained, indicating excellent reversibility and cycle stability of TNTs/C/MnO2 electrode. This work opens up a facile path for efficient growth of electrode materials with high performance for energy storage devices.

1. INTRODUCTION

Highly ordered TiO2 nanotube arrays have great potential as binder-free supercapacitor electrode materials because of their high surface area, wide potential window, and excellent stability, importantly open-ended nanotubular characteristics offering direct and continuous charge-transport pathways.8,9 However, their poor electrochemical activity and electrical conductivity have blocked their practical utilization as advanced electrode materials.10,11 To address this obstacle, many approaches, such as annealing in Ar, H2, and NH3 atmospheres, C modification,12 and electrochemical reduction, have been proposed.13−15 For instance, Zheng and coworkers16 proposed a facile method to dope C onto the anodic TNTs, for which the specific capacitance reached 2.5 mF/cm2 at a current density of 200 μA/cm2. Peighambardoust et al.17 proposed two effective methods (nitrogen doping and electrochemical reductive doping) to improve the areal specific capacitance of TNTs, and the reduced anodic TNTs delivered a high specific capacitance of 7 mF/cm2. More recently, we prepared TNTs/C films by gas thermal penetration with methanol as the drop agent.18,19 C was successfully deposited in a TNT film in the form of inorganic C and chemisorbed organic C, such as hydroxyl and carbonyl groups, which greatly

With the rapid development of portable electronic products and electric vehicles, high-performance energy storage devices with high security, long service life, and fast charging and discharging rates are urgently required in modern society.1,2 In this context, supercapacitors have attracted intensive attention because of their intriguing characteristics, including highefficiency, fast charging/discharging capability, excellent stability, and high power density.3,4 On the basis of chargestorage mechanisms, supercapacitors conventionally divided into nonfaradaic supercapacitors based on carbon materials and pseudocapacitance properties involved redox reactions, for example, manganese dioxide (MnO2), nickel oxide (NiO), nickel cobaltite (NiCo2O4), and titanium dioxide (TiO2).5−7 Traditionally, active materials with pseudocapacitors are usually attached to current collectors with binders. However, the addition of binders often hinders the conductivity of asprepared electrodes and reduces the electrolyte-accessible area. In addition, the robust adherence between the substrate and active materials is another crucial aspect in elaborating the pesudocapacitance property and electrochemical performance of supercapacitors. Therefore, great efforts have been devoted to designing and optimizing self-supported and binder-free electrodes for high-performance supercapacitors. © XXXX American Chemical Society

Received: November 1, 2018

A

DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

different temperatures and contents of KMnO4 for 60 min. The following chemical reaction describes the formation of MnO2.

improved the electrochemical activity and electrical conductivity of TNTs. The resistance of a TNTs/C film decreased by 4 orders of magnitude and the areal capacitance reached 12.10 mF/cm2 at a current density of 0.12 mA/cm2, 21.1 times more enhanced than that of a TNTs film. However, the specific capacitance of a TNTs/C film still needs further improvement. Commonly, as a typical transitionmetal oxide, MnO2 has been widely investigated as a supercapacitor electrode material because of its abundance, environment friendliness, wide operating potential window in wild electrolytes, and, especially, remarkable theoretical capacity (1380 F/g).20−22 In this work, we further utilized the strong reduction of carbon nanoparticles on TNTs/C films to react with KMnO4 during hydrothermal reaction for the in situ growth of MnO2 to construct a freestanding hybrid TNTs/ C/MnO2 film electrode. In such a composite, MnO2 with a pseudocapacitance property, as well as C, which promotes the conductivity, jointly contribute to the capacitance of TNTs without sacrificing the advantage of TNTs. Benefitting from the synergy effect of these three components, the as-prepared TNTs/C/MnO2 film exhibits excellent electrochemical properties. The highest areal specific capacitance is up to 55 mF/ cm2 at a scan rate of 5 mV/s, enhanced 23 times compared to the TNTs/C film. The energy density reached 46.8 Wh/cm2 at 0.12 mW/cm2 and still remained at 22.4 Wh/cm2 at 0.8 mW/ cm2. After 1000 cycles, the capacitance retention achieved 93.2%, which illustrates that the composite film has a superior cycling performance.

4MnO4 − + 3C + H 2O → 4MnO2 + CO32 − + 2HCO3−

(1)

Finally, the sample was washed with deionized water and dried at 70 °C in air. 2.2. Material Characterization. The morphologies and element composition of the films were characterized by scanning electron microscopy (SEM; JSM-6480A, Japan). The phase composition of the samples was examined by X-ray diffraction (XRD; D/max-rB, Ricoh, Japan) with Cu Kα radiation at 40.0 kV and 30.0 mA. The 2θ angle of the XRD pattern was obtained at a scanning rate of 10°/min from 20° to 75°. Raman spectra from 0 to 2000 cm−1 were recorded using a Renishaw InVia Raman microscope with an Ar+ laser (λ = 532 nm). The surface composition and valence state of the film were analyzed by X-ray photoelectron spectroscopy (XPS; Phi5700 ESCA system, U.S.A.), and single-color Al Kα was used as the radiation source (hν = 1486.6 eV; 10.0 kV). 2.3. Electrochemical Measurements. The electrochemical performances were done on a three-electrode system with a TNTs/ C/MnO2 film as the working electrode, a Pt plate as the counter electrode, and Ag/AgCl as the reference electrode, which were tested in a 0.5 mol/L sodium sulfate (Na2SO4) aqueous solution. We employed an electrochemical workstation (CHI660D, Shanghai Chenhua Co., China) to measure the properties of cyclic voltammetry (CV), galvanostatic charging/discharging (GCD) tests, and electrochemical impedance spectroscopy (EIS) tests. EIS measurements were performed between 100 kHz and 0.01 Hz with a 5 mV sinusoidal modulation at the open-circuit potential of each sample. The cycling stability of the samples was investigated by GCD tests performed for up to 1000 cycles at a current density of 0.25 mA/cm2. All of the electrochemical studies described above were carried out at a room temperature of ∼25 °C. The specific capacitance, energy densities at different current densities, and power densities of the electrode materials were calculated according to the following equations:

2. EXPERIMENTAL DETAILS 2.1. Preparation of TNTs/C/MnO2 Films. TNTs/C/MnO2 films were synthesized by a simple and convenient three-step process, as shown in Scheme 1. TNTs/C films were prepared by a two-step

Cs =

Scheme 1. Schematic Illustration of the Formation of TNTs/C/MnO2 Composites

∫ I dV (2)

svΔV

Cs = I Δt /sΔV

(3)

E = CV 2/2

(4) (5)

P = E /Δt

where Cs is the areal specific capacitance (mF/cm ), ∫ I dV is the area enclosed by the CV curve, s is the area of the sample (cm2), v is the scan rate (mV/s), I is the discharge current (mA), Δt is the discharge time (s), and ΔV is the voltage window (V). 2

3. RESULTS AND DISCUSSION 3.1. Morphological and Structural Characterization. Figures 1 and S1 depict the morphologies of as-permeating TNTs/C and TNTs/C/MnO2 films prepared under different temperatures. From Figure 1a, it is found that the aspermeating TNTs/C film displays that some C nanoparticles loaded onto the surface and between neatly arranged nanotubes. The pristine TNTs/C samples have an inner diameter of ∼75 nm and a wall thickness of ∼27 nm. Figure 1b shows that, after a hydrothermal reaction, the above and outer walls of the TNTs/C film are covered by MnO2 nanoparticles, resulting in the formation of TNTs/C/MnO2 films with thicker tube walls and a narrower gap between tubes, compared to the TNTs/C film. TNTs/C/MnO2 films have a smaller inner diameter of ∼65 nm and a thicker wall thickness of ∼52 nm; in addition, the tube wall becomes coarser because of the deposition and penetration of MnO2 nanoparticles. Also, we can find that not only are the C/MnO2 nanoparticles

process of anodic oxidation and a gas thermal penetration method on Ti foil, which was introduced in detail in our previous work.18 Briefly, the well-cleaned Ti foil was anodized under 20 V for 120 min in a two-electrode configuration with a Cu plate as the cathode, and the electrolyte contained 0.14 M NaF and 10 wt % H3PO4. Through this simple method, TNTs were obtained. Subsequently, the TNTs/C films were prepared with 500 mL of methanol via a gas thermal penetration method in a 550 °C drip furnace. Amounts of C were deposited on the TNTs film via this novel method. Then the TNTs/ C films were cut in half and placed in a Teflon-lined stainless steel autoclave containing 30 mL of deionized water and reacted at B

DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. SEM images of (a) TNTs/C and (b) TNTs/C/MnO2 films prepared at 90 °C via a hydrothermal method.

The decreased content of Ti illustrates that the TNTs/C films are surely covered by nanoparticles after the hydrothermal reaction, which is consistent with the results of SEM analysis in Figure 1. The appearance of Mn and the increased content of O after the hydrothermal treatment indicate that the deposited nanoparticles are MnO2 via a redox reaction involved in KMnO4 and C in terms of chemical reaction (1). Accordingly, the decrease of the C content in the composites is attributed to the consumption of C during the redox reaction, accompanying the deposition of MnO2. Analysis of the EDS results demonstrates that MnO2 is successfully deposited onto TNTs/C. It is worth noting that the relative content of Mn reaches the maximum value at 90 °C. At this temperature, the mass fraction of MnO2 calculated based on the EDS results reaches 10.56%. The higher content of MnO2 is expected to be beneficial to the electrochemical performance because of its excellent pseudocapacitance property. However, when the hydrothermal temperature is further increased, the KMnO4 in the solution may start to decompose because of the high pressure of the Teflon-lined stainless steel autoclave, which is, in turn, responsible for the a small decrease of the Mn content at 110 °C As shown in Figure 2a, the XRD patterns of the TNTs/C and TNTs/C/MnO2 films does not exhibit any significant differences. The mix phase of TiO2, which includes anatase and rutile, can be observed. The characteristic peaks are located at ∼25.5° and ∼48.1° for anatase TiO2, at ∼27.5° for rutile TiO2, and at ∼37.9°, ∼39.8°, and ∼53.1° for Ti substrate. However, no distinct peaks are related to C and MnO2, confirming that the phases of C and MnO2 in all samples are amorphous.23 Figure 2b presents the Raman spectra of composite films. For

attached to the TNTs well but also the advantages of the TiO2 nanotubes, including large surface area and easy accessibility to the electrolyte, are not sacrificed. As a benefit to such a structure, the advantages of each component can be exploited for better electrochemical properties. Figure S1 indicates that the hydrothermal temperature does not distinctly influence the morphologies of TNTs/C/MnO2 films. In order to analyze the composition of the samples in the composites, the samples are characterized by energy-dispersive spectrometry (EDS), XRD, and Raman spectral analysis. Table 1 contains the EDS results of TNTs/C and TNTs/C/MnO2 Table 1. EDS Results of the TNTs/C and TNTs/C/MnO2 Films atom % Ti TNTs/C 70 °C 90 °C 110 °C

37.73 24.87 24.32 23.99

Mn

O

C

2.42 3.01 2.81

52.32 66.23 65.73 66.12

9.95 7.92 7.34 7.08

films prepared at 70, 90, and 110 °C. After the hydrothermal treatment, Mn, besides Ti, O, and C, begins to appear in the composite. It is worth noting that, with an increase of the hydrothermal temperature, the contents of Ti and C gradually decrease, while the content of Mn undergoes a trend of first increasing and then decreasing. Moreover, the content of O is increased after the hydrothermal treatment, and then the relative content remains around 66%.

Figure 2. (a) XRD patterns of TNTs/C and TNTs/C/MnO2 films prepared at 70, 90, and 110 °C. (b) Raman spectra of TNTs/C and TNTs/C/ MnO2 films prepared at 90 °C. C

DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) XPS full and (b) Mn 2p high-resolution spectra of TNTs/C/MnO2 films prepared at 70, 90, and 110 °C. (c) C 1s and (d) O 1s highresolution spectra of the TNTs/C/MnO2 film prepared at 90 °C.

Table 2. Component Ratios of C 1s and O 1s in TNTs/C/MnO2 Films Based on XPS Analysis species containing C (%)

species containing O (%)

temperature (°C)

C−C

CO

C−O

Ti−O

Mn−O

CO

O−H

70 90 110

78.24 72.21 80.78

6.33 7.85 5.02

15.43 19.94 14.2

25.31 25.07 26.67

25 28 27

10.75 10.32 10.80

38.95 36.56 35.53

the TNTs/C film, the mix phase of TiO2 and G and D bands of C can be clearly observed. In contrast, the band of MnO2 appears after the hydrothermal treatment, further demonstrating that MnO2 is deposited onto the pristine TNTs/C film through redox reaction. Figure 3 gives XPS analysis of the samples. According to the XPS full spectra (Figure 3a), the bands of Ti, Mn, O, and C can be detected in the TNTs/C/MnO2 films, which is consistent with the EDS results. The high-resolution Mn 2p spectra are shown in Figure 3b; the two distinct peaks observed at 643.2 and 654.7 eV correspond to Mn 2p3/2 and Mn 2p1/2, respectively. The spin-energy separation between the two peaks (≈11.5 eV) is obviously consistent with the characteristic binding energy of MnO2,24 suggesting that the valence of Mn is Mn4+ in composites. Parts c and d of Figure 3 are the C 1s and O 1s high-resolution spectra of the TNTs/C/ MnO2 film prepared at 90 °C. The two spectra are wide and asymmetric, indicating that different kinds of chemical states exist. The C 1s spectrum can be deconvoluted into inorganic C including the C−C (graphite C), chemisorbed C−O (CH2OH), and CO components, which are centered at binding energies of 284.6, 285.6, and 288 eV, respectively.25,26 With the graphite C deposited on the film and chemisorbed organic C, including CH2OH and CO, originating from the dropping agent and its pyrolysis products during gas thermal penetration treatment, the conductivity of TNTs would be improved. The O 1s spectrum can be divided into four peaks

centered at binding energies of 530, 530.4, 531.2, and 532.5 eV. Two peaks at 530 and 530.4 eV are assigned to Ti−O (TiO2) and Mn−O (MnO2) of the film, which further confirms the successful deposition of MnO2,27,28 while the peaks at 531.2 and 532.5 eV are ascribed to CO and O−H, respectively.18 The groups of −OH and CO are reportedly used to enhance the electrochemical properties of the TNTs because of their pseudocapacitance effects.27,29 Table 2 gives the relative ratios of the main components of C 1s and O 1s in TNTs/C/MnO2 films, based on XPS analysis from Figures S2 and S3. For C 1s, with increasing hydrothermal temperature, the trend of graphite C is in contrast to the total amount of organic C. It can be inferred that it is graphite C that participates in the reaction with a KMnO4 solution. On the basis of O 1s, the MnO2 content is at a maximum when the hydrothermal temperature is 90 °C, which is in agreement with the results of EDS analysis. 3.2. Electrochemical Performances. In order to investigate the electrochemical performances of the TNTs/C and TNTs/C/MnO2 films, EIS, CV, and GCD tests were conducted using a three-electrode configuration with Ag/AgCl as the reference electrode, Pt as the counter electrode, and 0.5 M Na2SO4 as the electrolyte. EIS analysis is a powerful and informative technique to evaluate the intrinsic electrical conductivity of materials, and Nyquist plots of the TNTs/C and TNTs/C/MnO2 films are presented in Figures 4 and S4. At high frequency, the EIS curves exhibit a semicircular arc, D

DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX

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rectangular characteristics, indicating a low resistance. However, upon comparison with Figure 5a,b, the CV curves of the TNTs/C/MnO2 films deviate from the rectangle, which is consistent with the results of EIS, namely, implying a higher resistance because of the presence of MnO2. Conversely, it can be clearly seen that, compared to the TNTs/C film, the TNTs/C/MnO2 films have much larger areas (Figure 5c); as it turns out, the capacitance of the TNTs/C film is almost negligible, and it mainly works as a skeleton to load MnO2. The increase of the capacitance can be attributed to the pseudocapacitance of MnO2, which involves the transition between Mn3+ and Mn4+. The main redox reaction as follows:30,31 MnO2 + Na + + e− ↔ MnOONa Figure 4. EIS analysis of the TNTs/C and TNTs/C/MnO2 films prepared at 90 °C.

(6)

Figure 5d shows the areal specific capacitance reaching a maximum value at 90 °C. Combined with the characterization of the composites, especially the EDS and XPS results, it can be inferred that the trend of the areal specific capacitance is ascribed to the amount of MnO2 deposition. When the hydrothermal temperature is 90 °C, the largest capacitance is up to 42.2 mF/cm2 at a scan rate of 5 mV/s, 17.8 times that of the TNTs/C film. The GCD tests of the TNTs/C and TNTs/C/MnO2 films prepared at 70, 90, and 110 °C at different current densities are shown in Figures 6 and S6. Comparing parts a and b of Figure 6, we can see that both GCD curves of the TNTs/C film at different current densities exhibit better trigonometric symmetry and small IR drop, indicating the good electronic conductivity of the pristine TNTs/C film. From Figure 6c, TNTs/C/MnO2 films prepared at various temperatures have longer discharge times than the TNTs/C film at a discharge current density of 0.5 mA/cm2. With an increase of the hydrothermal temperature, the discharge time increased first and then decreased, reaching 31.2 s at 90 °C. Also, it can be

which is related to the charge-transfer resistance (Rct). It reveals that the semicircle becomes significantly larger after hydrothermal reaction, which indicates that the resistances of the TNTs/C/MnO2 films increased by an order of magnitude compared to that of the TNTs/C film. Accordingly, it can be inferred that MnO2 with poor conductivity accounts for an increase of the resistance. As shown in Figure 4, in the lowfrequency range, the Nyquist plot of all composites shows a nearly vertical line, indicating a characteristic of the capacitive behavior that arises from the diffusive resistance (Zw) in the electrode materials. The slope of the TNTs/C/MnO 2 composite decreases compared with that of the TNTs/C material, which is attributed to the deposition of poorly conductive MnO2. The CV curves of the TNTs/C and TNTs/C/MnO2 films at different scan rates are shown in Figure 5 and S5. The CV curves of the TNTs/C film at different scan rates have good

Figure 5. CV curves of the (a) TNTs/C and (b) TNTs/C/MnO2 films prepared at 90 °C. (c) TNTs/C and TNTs/C/MnO2 films prepared at 70, 90, and 110 °C at a scan rate of 50 mV/s. (d) Areal capacitances as a function of the scan rates. E

DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. GCD curves of the (a) TNTs/C and (b) TNTs/C/MnO2 films prepared at 90 °C. (c) TNTs/C and TNTs/C/MnO2 films prepared at 70, 90, and 110 °C at a current density of 0.5 mA/cm2. (d) Areal capacitances as a function of the current density.

Figure 7. Areal capacitances of TNTs/C/MnO2 films prepared with 0.1, 0.2, 0.3, and 0.4 g from (a) CV curves and (b) GCD curves. (c) Ragone plot and (d) cycle test of TNTs/C/MnO2 films prepared with 0.3 g of KMnO4 and at 90 °C.

After determining the reaction temperature of 90 °C, we further changed the concentration of the KMnO4 solution range from 0.1 to 0.4 g. The calculated capacitances of the TNTs/C/MnO2 films based on the CV (Figure S7) and GCD (Figure S8) tests are shown in Figure 7. Remarkably, in Figure 7a, when the content of KMnO4 is 0.3 g, the capacitance is up to 55 mF/cm2 at a scan rate of 5 mV/s, 23 times that of the

clearly seen that the areal specific capacitance of the TNTs/C film is quite less than that of TNTs/C/MnO2 films (Figure 6d). The highest capacitance is obtained at a discharge current density of 0.25 mA/cm2 with a value of 19.2 mF/cm2 when the hydrothermal temperature is 90 °C, 20.2 times that of the TNTs/C film. F

DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry TNTs/C film. Also, on the basis of the GCD test, the same variation trend is obtained with an increase of the content of KMnO4 (Figure 7b). The highest areal capacitance reached 26 mF/cm2 at a discharge current density of 0.25 mA/cm2, 27 times that of the TNTs/C film. Ragone plots (power density vs energy density) of TNTs/C/MnO2 films prepared with 0.3 g of KMnO4 and at 90 °C are displayed in Figure 7c. It shows that the energy density of the composites reaches 46.8 Wh/ cm2 at 0.12 mW/cm2 and even remains at 22.4 Wh/cm2 at 0.8 mW/cm2. Furthermore, the TNTs/C/MnO2 films exhibit superior cycle stability, with the capacitance retention stable up to 93.2% after 1000 cycles (Figure 7d). It is tested by GCD at a current density of 0.5 mA/cm2 and with the first test as the reference (100%), which illustrates the outstanding cycle stability of the TNTs/C/MnO2 film. As a binder-free electrode, the superior electrochemical performances of TNTs/C/MnO2 films are mainly due to the synergistic effects of three components, including TiO2 nanotubes (providing a direct, continuous pathway for the electron and high accessibility for the electrolyte ions), C nanoparticles (ensuring comparable conductivity), and, more importantly, MnO2 nanoparticles (possessing high pseudocapacitance property).

Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (Grant 2015DX07).

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4. CONCLUSION In summary, a hybrid nanostructure comprising TNTs, C, and MnO2 has been successfully synthesized via anodic oxidation, gas thermal penetration, and a hydrothermal method. During the hydrothermal process, MnO2 were generated through C nanoparticles deposited onto the TNTs reacting with KMnO4. Benefitting from the contribution of TNTs, C, and MnO2, when the hydrothermal temperature comes to 90 °C, the capacitance is up to 55 mF/cm2 at a scan rate of 5 mV/s, enhanced by 23 times that of the TNTs/C film. The energy density reached 46.8 Wh/cm2 at 0.12 mW/cm2 and still remained at 22.4 Wh/cm2 at 0.75 mW/cm2. The TNTs/C/ MnO2 film exhibited superior reversibility and cycling stability, which remained at 93.2% after 1000 cycle tests. Therefore, this work provides a good avenue for creating an ideal species via chemical reaction, while simplifying the complex synthetic process through a facile method.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03094. Detailed information on SEM, XPS, EIS, EDS, CV, and GCD (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.Y.). Tel: (86) 15663835116. ORCID

Zhongping Yao: 0000-0002-1044-1948 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 51571076) and State Key G

DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03094 Inorg. Chem. XXXX, XXX, XXX−XXX