Morphology-Controlled Synthesis of Ni-MOFs with Highly Enhanced

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Morphology-Controlled Synthesis of Ni-MOFs with Highly Enhanced Electrocatalytic Performance for Urea Oxidation Mengwei Yuan,† Rui Wang,† Zemin Sun, Liu Lin, Han Yang, Huifeng Li, Caiyun Nan, Genban Sun,* and Shulan Ma* Beijing Key Laboratory of Energy Conversion and Storage Materials and College of Chemistry, Beijing Normal University, Beijing 100875, China

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ABSTRACT: MOFs present potential application in electrocatalysis. The structure−activity of the Ni-MOFs with different morphologies, nanowires, neurons, and urchins is systemically investigated. The Ni-MOFs were controllably synthesized via the facile solvothermal method. Among them, the Ni-MOF nanowires are endowed with the highest electrocatalytic activity due to the unique structure, more exposed active sites, lower charge transfer resistance, and the fast and direct electron transfer in 1D structures. The typical morphology of the Ni-MOF nanowires is ca. 10 nm in diameter and several micrometers in length. When employed as an electrocatalyst in urea oxidation reaction, it exhibits a lower overpotential than and superior stability to the NiMOFs with other morphologies. Ni-MOF nanowires require a potential of ∼0.80 V (vs Ag/AgCl) to obtain 160 mA cm−2. In addition, after continuous electrocatalyzing for 3600 s at 0.40 V (vs Ag/AgCl), the current density retention of Ni-MOF nanowires could still reach more than 60% (>12 mA cm−2), which demonstrates Ni-MOF nanowires as promising electrocatalysts for urea oxidation.

1. INTRODUCTION The tunable inorganic metal ions and organic ligands on MOFs, to a great extent, endow them with distinguishing applications in various fields.1−5 The high specific surface area, tunable pore size, different capture capability, and tailorable electronic structures were demonstrated to be inherent advantages.6−9 However, the bulk morphology and low electronic conductivity became important facets to block their application in energy transition and storage, such as in Liion batteries, supercapacitors, and fuel cells.10−14 By introducing low dimensions, the local electronic environment of metal cations can be modified, and the electronic structure and electron transfer are improved in a certain direction, making it possible to design advanced electrocatalysts with high electrocatalytic activity.15 Currently, 2D MOFs have aroused the interests of researchers, and of course the brilliant results of the successful preparation and application of them has provided quite amazing accomplishments, such as ultrathin 2D NiCo-MOF nanosheets in the oxygen evolution reaction, ultrathin 2D Mn-MOF nanosheets as bifunctional electrocatalysts for Li−O2 batteries, and so on.11,16−19 One of the low-dimensional MOF structures, MOF nanowires with more metal active site exposure applied in electrocatalysis, were seldom reported. MOF nanowires not only inherited the characteristics of the bulk MOFs but also possessed a high percentage of exposed metal atoms on the surface, which was promising in surface-active application. More accessible active sites were exposed on the surface rather than enclosed in pores © XXXX American Chemical Society

or channels, facilitating more interactions between active sites and substrate molecules, which could enhance the performance of MOFs in catalysis, separation, and sensing.19−21 Fuel cells are one of the efficient electrochemical energy storage and conversion devices, directly converting the chemical energy to electricity, which has a low impact on the environment. Direct urea/urine fuel cells (DUFC) are one of the promising methods for the simultaneous wastewater treatment and electricity generation. DUFCs present a high open circuit voltage close to that of hydrogen FC (1.21 V) and have a high theoretical efficiency of 102.9% compared to 83% in the case of hydrogen FC.22 All in all, urea, owing to its nontoxicity, stability, and high energy density, is promising for application in fuel cells. However, the urea oxidation reaction (UOR) is kinetically sluggish due to the 6e− transfer process.23,24 Therefore, highly efficient electroncatalysts are required to realize the practical application of UOR in fuel cells.15,25−27 According to previous reports, Ni-based materials were promising electrocatalysts for UOR due to their unique electron structures,28−31 such as the Pd/Ni carbonized PVA nanofiber hybrid,32 NiMo/graphene nanocatalysts,33 CoNi/Cr trimetallic system,34 Fe-incorporated α-Ni(OH)2 hierarchical nanosheet arrays,28 Ni nanowires/N, S-doping CNFs,35 and Ni/C nanocatalysts.31 Among this research, the authors investigated heteratom doping, catalyst arrays, and the Received: April 17, 2019

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DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION The Ni-MOFs were prepared via a facile solvothermal method. With temperature increasing, the morphology of the Ni-MOFs exhibited quite a lot of change. As shown in Figure 1a and b,

synergistic effect of multicomponents. Recently, Zhu et al. reported an ultrathin Ni-MOF nanosheet in the UOR reaction,15 inferring the potential application of low dimensional MOFs in urea electrolysis. And in this work, we have introduced the 1D Ni-MOF nanowires employed as efficient elelctrocatalysts toward the UOR. And the high electrocataytic activity and the superior durability presented a large potential for its application in electrocatalysis. Our research focused on the preparation of Ni-MOFs in different dimensions and systemically investigated the structure−performance of them, providing a new insight into the design and development of MOFs in surface related applications.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ni-MOFs with Different Morphologies. NiMOFs were prepared via a facile solvothermal method. A total of 1.13 mmol of Ni(NO3)2·6H2O and 7.40 mmol of 2-methylimidazole were dissolved in 30 mL of CH3OH, respectively. A green solution was obtained by adding ligand solution into Ni(NO3)2/CH3OH methanol solution under magnetic stirring, and then the mixed solution was transferred into an autoclave with the temperature held at 60, 80, and 100 °C for 12 h. The corresponding products were named Ni-MOF60, Ni-MOF-80, and Ni-MOF-100, respectively. The bulk-Ni-MOF was prepared with the same materials and stirred at room temperature for 12 h. The sample was separated via centrifugation, and it was washed several times with CH3OH. Finally, the samples were dried in the oven. 2.2. Physicochemical Characterizations. A scanning electron microscopy (SEM) experiment was performed on SU-8010 with an accelerating voltage of 5 kV. The element mapping was obtained from the energy dispersive spectrometer connected with SEM. The microstructure was observed with a transmission electronmicroscope (TEM, an acceleration voltage of 200 kV, JEM-2010, JEOL). The N2 adsorption/desorption measurement was carried out using a Quadrasorb SI analyzer at 77 K. The pore size distribution was derived from the desorption branch using the HK method. X-ray diffraction (XRD) spectra were obtained using a Phillips X’pertProMPD instrument with Cu Kα radiation (40 kV, 40 mA). X-ray photoelectron spectra were recorded on a Thermo ESCALAB 250Xi spectrometer with an excitation source of Al Kα X-ray. Fourier transform infrared spectroscopy (FT-IR, Thermo Fisher Scientific Co.) was employed to detect the functional group of the as-prepared sample. ICP was used to analyze the amount of the Ni using an instrument (Jarrel-ASH, ICAP-9000). The contents of C, H, and N in Ni-MOF were analyzed using an instrument (Elementar Vario EL elemental analyzer). And, the chemical formula was calculated on the basis of the ICP and CHN results. The gaseous product was detected using a GC-2014C equipped with a thermal conductivity detector. Ar was used as a carrier gas at 40 mL min−1. 2.3. Electrochemical Characterizations. Electrochemical tests were conducted in a three-electrode system with an electrochemical workstation (CHI 660, Shanghai Chenhua). The fabrication of the working electrode was as follows: 5 mg of catalyst, 1 mg of KB, and 35 μLof Nafion (5%) were dispersed in 1 mL of 1:1 water/ethanol solvent and ultrasonicated for 30 min. A total of 10 μL of the dispersion and 0.05 mg (∼0.7 mg cm−2) of Ni-MOF were cast on the glassy carbon electrode (GCE, diameter 3 mm). The electrochemical tests were conducted using a Ag/AgCl reference electrode and a Pt wire counter electrode. All electrochemical tests were performed under alkaline conditions, containing 1 M KOH and 0.5 M urea. Linear sweep voltammetry (LSV) was conducted at the potentials between 0 and 1 V (versus Ag/AgCl) with 10 mV s−1. Cyclic voltammetry (CV) was conducted at the potentials between 0 and 0.05 V (versus Ag/AgCl) at a scan rate of 10, 20, 30, 40, and 50 mV s−1. The time-dependent current density curve was obtained at 0.4 V and over 3600 s. EIS measurement was conducted using the electrochemical workstation (IM6 German).

Figure 1. SEM images for the as-prepared Ni-MOFs.

the Ni-MOF was a typical urchin-like type with a diameter of ca. 200 nm under a reaction temperature of 60 °C. Simultaneously, the Ni-MOF-60 was cross-linked and aggregated seriously. When the reaction temperature was increased to 80 °C, the Ni-MOF-80 transferred to a neuronlike morphology (Figure 1c and d). The neuron-like Ni-MOF was assembled with the urchins linked by the nanowires. A large amount of Ni-MOF-100 nanowires (Figure 1e and f) were obtained when the temperature reached 100 °C. The diameter of the typical nanowire was ca. 10 nm, and the length was more than several micrometers. As for the differences observed in Ni-MOFs, the dynamic viscosity was the main factor to limit the mass transfer via the temperature change. This result was also confirmed by the Ni-MOFs prepared under room temperature, which presented a similar structure of Ni-MOF-60 in Figure 1g and h due to the high viscosity of the 2-methylimidazole containing solution at a lower temperature. The TEM measurement was used to further analyze the inner structure of the Ni-NOF nanowires. Figure 2a presented the typical 1D structure of the Ni-MOF nanowires, and the accurate size of the nanowires was measured via the HRTEM image of the one nanowire shown in Figure 2b. The diameter of one nanowire was 12 nm. And in the nanowire, there were no evident diffraction fringes due to the low crystallinity. Figure 2c presented a STEM image of Ni-MOF nanowires, and the corresponding element mapping illustrated the uniform dispersion of the Ni, C, and N in Ni-MOF nanowires. The XPS spectrum was investigated to analyze the chemical states of the elements. Ni-MOF-60 and Ni-MOF were selected to study the chemical states of the main elements, including B

DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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the Ni, C, and N, and the XPS result is shown in Figure 3. The survey spectrum in Figure 3a and e demonstrated the clear peaks of Ni 2p, N 1s, and C 1s, indicating the existence of these three elecments. The ratio of n(N)/n(Ni) was close to 4, similar to that of ZIF-8 and ZIF-67. The peaks of Ni 2p in Figure 3b and f were very close. The peaks at 855.1 and 872.3 eV were assigned to the 2p 3/2 and 2p 1/2 of the Ni2+.36,37 The weak satellite peaks at ∼863.0 and 880.0 eV were ascribed to the typical Ni-MOFs according to the previous report.15 As for the peaks for C 1s and N 1s, Ni-MOF-60 and Ni-MOF-100 were also presented the same species. The peak at 285.6 eV in Figure 3c and g was attributed to the C−N species in Ni-MOF, which was consistent with the peak of N 1s at 398.8 eV (Figure 3d and h), indicating the pyrrolic N in as-prepared NiMOF.38−41 These similar results inferred the same structure of the as-prepared Ni-MOFs but not the morphology. The XRD

Figure 2. (a) TEM, (b) HRTEM, and (c) STEM images and the corresponding elemental mapping of the as-prepared Ni-MOF-100.

Figure 3. Survey spectra and defined spectra for (a−d) Ni-MOF-60 and (e,f) Ni-MOF-100. C

DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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pore size of ∼1.07 nm, which indicated the four Ni-MOFs assembled with the same structure, and the single pore structure inferred that these as-prepared coordination polymers were the MOFs according to the IUPAC recommended nomenclature. EIS was employed to investigate the electrochemical characteristics of the as-prepared electrocatalysts. The EIS of different electrodes shown in Figure 6 illustrated a similar

patterns were further employed to analyze the structure of the Ni-MOF with different morphology. The strong diffraction peak at ∼14° (Figure 4a−c) distinctly confirmed the crystalline

Figure 4. XRD patterns of the as-prepared (a) Ni-MOF-60, (b) NiMOF-80, and (c) Ni-MOF-100.

structure of the as-prepared MOFs, which was the same as in a recent report.42 It can be observed that all three MOFs illustrated a similar body frame. Both the XPS and XRD measurements did not distinguish the obvious difference between Ni-MOF-60 and Ni-MOF-100 nanowires due to the just surface defect and coordinative unsaturation. The ICP, CHN, and IR (Figure S1) measurements were investigated to analyze the composition of the Ni-MOF nanowires (Table S1). According to the elemental analysis via ICP and CHN, as well as the XPS results of the quantitative analysis, it was clearly observed that the ratio of n(N)/n(Ni) was ∼4, indicating that each of the Ni atoms was coordinated with four N atoms in asprepared Ni-MOFs. In addition, the pore size of the asprepared Ni-MOFs was 1.07 nm, which was close to that of ZIF-67.43 Thus, we suspected that Ni-MOF is endowed with a similar structure to ZIF-67, in which the metal ions were coordinated with four 2-methylimidazole molecules. The specific surface area and the inner pore structure of the as-prepared Ni-MOFs were derived from the N2 adsorption/ desorption isotherm illustrated in Figure 5. The Ni-MOF nanowires demonstrated a high specific area of 295 m2 g−1, which was higher than those of Ni-MOF-60 (246 m2 g−1) and Ni-MOF-80 (257 m2 g−1). This result suggested that the reduction of the dimensional of the MOFs could increase the specific surface area, similar to that of nanostructure materials compared with the bulk. The inner structure was also studied. All of Ni-MOFs exhibited the micropore structure in a close

Figure 6. EIS spectra for as-prepared Ni-MOFs.

spectrum. The curves presented a semicircle at high frequency and a linear section at low frequency. The charge transfer resistance was related to the semicircle section.11,44 The semicircle section of the EIS was calculated based on the equivalent circuits, Rs(CPE1R1)(CPE2R2). There were three types of components in equivalent circuits: the equivalent series resistance (Rct), the constant phase element (CPE), and the charge-transfer resistance (Rct).33 The Rs contained contact resistances, intrinsic resistances of electrocatalysts, and electrolytes. The Rct could be attributed to the kinetic resistance of electron transfer in the interfaces between electrolytes and electrode. Table S2 listed the fitting values of EIS parameters (Rs, Rct, CPE, X2). It could be observed that the Rct value (1.147 ohm cm2) of the Ni-MOF-100 was smaller than those of Ni-MOF-80 (1.815 ohm cm2), Ni-MOF60 (3.683 ohm cm2), and Ni-MOF-RT (100.4 ohm cm2), which suggested that it had the lowest charge transfer resistance among the three MOFs, which was important to improving the sluggish kinetics of the electrocatalysis.11 A comparsion of the three MOFs illustrated that the order of conductivity was Ni-MOF-100 > Ni-MOF-80 > Ni-MOF-60, confirming that the reduction of the dimension of the MOF

Figure 5. N2 adsorption−desorption isotherms and pore size distribution for as-prepared (a) Ni-MOF-60, (b) Ni-MOF-80, and (c) Ni-MOF-100. D

DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) LSV curves for Ni-MOF-100 nanowires in different electrolytes. (b) LSV and (c) Tafel curves of Ni-MOFs for urea oxidation. (d) The LSV curves of the Ni-MOF nanowire for UOR at different rotation rates and the (e) fitting results of I−1 vs ω−1/2. (f) i−t curves of the NiMOFs at 0.4 V.

Ni-MOF-60 and Ni-MOF-80. The lower Tafel slope (Figure 7c) for the Ni-MOF-100 (90 mV dec−1), lower than those of Ni-MOF-80 and Ni-MOF-60, inferred that the Ni-MOF nanowire could efficiently facilitate the UOR, and it was also concluded that the nanowire was endowed with higher electrocatalytic activity compared to the that of nanospheres and neuron-like Ni-MOFs.46−48 The LSV curves of the Ni nanowires at different rotation rates were illustrated in Figure 7d. The area of the GCE electrode used in this reaction was 0.25 cm−2 with a mass loading of 0.16 mg. It could be obviously observed that with the rotation rate increasing, the current density increased as well, which was due to the diffusion-controlled effect. According to the Koutecky−Levich equations I−1 = IK−1 + ID−1, ID = (0.2nFAD02/3 Cv−1/6)ω1/2 (n, F, D0, C, v, and ω are the transferred electron number, Faraday constant, diffusion coefficient, concentration, kinetic viscosity, and rotation speed), and I−1 = Bω−1/2 + IK−1, the I−1 was linear with the ω−1/2.49 Figure 7e presented the fitting results of I−1 vs ω−1/2. The intercepts recorded from the plots at different potentials are always greater than zero, indicating that the electrocatalytic oxidation of urea in alkaline media is a mixed control mechanism, where the currents are contributed by both diffusion and electrode kinetics in this system.50 In addition, stability was another pivotal factor for practical applications.51 An i−t measurement for three Ni-MOFs was performed. As shown in Figure 7f, Ni-MOF-100 nanowire exhibited strong durability over 3600 s. After i−t measurement,

could efficiently promote the conductivity and provide good potential for improving the electrocatalytic activity. A series of electrochemical measurements were investigated to evaluate the electrocatalytic activity of as-prepared NiMOFs for UOR. According to the previous report,45 in order to exhibit the high performance of the electrocatalysis to a great extent, the 1.0 M KOH was widely employed as the electrolyte in UOR measurements. Figure 7a presents the LSV curves of the electrocatalyst Ni-MOF-100 in different electrolytes. In 1.0 M KOH, the OER presented an oxidation peak at ∼0.36 V (vs Ag/AgCl electrode), which belonged to the formation of NiOOH.15,23 After adding the N2H4CO, NiMOF-100 exhibited a greatly enhanced current density, indicating the oxidation of the urea, which was similar to that of Ni-MOF-80, Ni-MOF-60, and Ni-MOF-RT (Figures S2−S4) with an impressive promotion of current density with urea addition. The on-set potential of UOR was quite close to the potential of the NiOOH that appeared, which inferred that the NiOOH species were the active sites for UOR. The LSV curves of Ni-MOF-60, Ni-MOF-80, and Ni-MOF-100 in 1.0 M KOH containing 0.50 M urea were illustrated in Figure 7b. When the potential (@40 mA cm−2) became more positive, the current density of Ni-MOF-100 exhibited an increase to a great extent. However, the other two MOFs (Ni-MOF-60 and Ni-MOF-80) only showed a limited increase. In particular, the Ni-MOF-100 nanowire required a low potential of ∼0.80 V to reach 160 mA cm−2, which was lower than other two MOFs, E

DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the current density retention of Ni-MOF-100 can still reach more than 12 mA cm−2, which was 3 and almost 6 times higher than those of Ni-MOF-80 and Ni-MOF-60. It also demonstrated a superior performance compared with the NiMOF-RT, which just remained at 3 mA cm−2 after 3000 s (Figure S3). When compared with the previous reports in Table S3, it was clear that the Ni-based electrocatalysts presented efficient UOR performance. And the Ni-MOF nanowire demonstrated a comparable and even superior electrocatalytic performance compared to the previous reports under similar conditions.23,31,32,35,52−55 All these results mentioned above clearly demonstrated Ni-MOF-100 as a high performance electrocatalyst for UOR due to its unique structure. In order to study the reaction mechanism of the UOR in this system, the product and electrocatalysts before and after longtime reaction were detected. Gas chromatography was employed to detect the gaseous product of the electrolysis. The gaseous product after electrolysis was collected from a type-H electrolytic tank. The cathode was prepared via pasting the slurry on carbon paper (1 cm × 1.5 cm). After a long-time continuous reaction at 0.5 V, the gas was injected in GC to detect the composition. Compared with the highly pure N2, the gas collected from the electrolytic tank presented the same retention time in Figure 8. It was clearly confirmed that the N2

Figure 9. SEM images of Ni-MOF nanowires (a, b) before and (c, d) after electrolysis. (e) Ni 2p spectrum after electrolysis.

The double-layer capacitance (Cdl) was positively proportional to the electrochemical active surface area, which was investigated via cyclic voltammogram collected at a potential range with nonfaradaic process (Figure 10a−c).57−59 It can be observed in Figure 10d that the Cdl of Ni-MOF-100 was 334.1 μF cm−2, which was 1.62 and 2.38 times higher than those of Ni-MOF-80 (206.0 μF cm−2) and Ni-MOF-60 (140.2 μF cm−2), revealing that the high density of the active site for the Ni-MOF-100 nanowire made an evident contribution to the exceptional UOR performance.60 On the basis of these results, we can suspect a possible mechanism for the urea electrooxidation. Under alkaline conditions, the Ni2+ in Ni-MOF nanowires was first oxidized into Ni3+ (NiOOH). In the urea electro-oxidation process, the urea molecule was directly adsorbed over the active species of NiOOH via the Ni−O and O−C bridging coordination bond, and the Ni3+ was further chemically reduced to form Ni 2+ via urea. 61 Simultaneously, the chemical bonds inside the urea were chemically broken to form its products (N2, CO2, and H2O). There were more exposed Ni atoms in the electrode/ electrolyte interface, as well as the 1D nanowire with high conductivity, to provide more active sites and electrocatalytic activity for UOR. This result strongly inferred the relationship of structure−activity for the low dimensions with more exposed active sites, lower charge transfer resistance, and fast and direct electron transfer to enhance the electrocatalytic performance, even though there are other surface-active applications.

Figure 8. Spectra of the gas chromatography for different gas.

was the cathodic product, which indicated that the 6e− transfer process occurred in this system. To investigate the composition and morphology of the catalyst after 200 cycles in the range of 0−0.6 V, SEM and XPS measurements were conducted. The SEM images before and after the cycles are shown in Figure 9a−d. Typical nanowires could be observed in Figure 9a and b. After a long time cycle, the nanowire morphology (Figure 9c and d) still remained due to the superior electrocatalytic stability. The XPS produced the Ni 2p spectrum in Figure 9e. The strong peak at 855.2 and a weak one at 856.7 eV were attributed to the 2p3/2 of the Ni2+ and Ni3+ in Ni-MOF and NiOOH, respectively, which indicated that the composition of the Ni-MOF has not much changed after the long-time electrolysis.56 Simultaneously, the existence of NiOOH indicated that the formed NiOOH was the active site for UOR. These results suggested that the as-prepared NiMOFs were endowed with superior stability in the electrocatalysis reaction. The electrocatalytic activity was dependent on the inherent activity and the number of active sites of the electrocatalyst.

4. CONCLUSIONS In summary, a novel electrocatalyst of 1D Ni-MOF for UOR was controllably synthesized. The electrochemical property of the as-prepared Ni-MOFs was systemically investigated. And the strong structure−activity relationship was established to reflect the performance and the dimensions of the MOFs in an F

DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. (a−c) CV curves for different Ni-MOFs and (d) the Cdl curves.

electrocatalytic field. The typical morphology of the Ni-MOF nanowire was ca. 10 nm in diameter and several micrometers in length. When employed as an electrocatalyst in UOR, it exhibited a lower overpotential than and superior stability to the Ni-MOFs with nanospheres or neuron-like morphology. The Ni-MOF nanowire demonstrated remarkable electrocatalytic activity and superior durability for UOR, which may be a promising electrocatalyst for practical applications. More importantly, this work may provide a new perspective in design and preparation of other low dimensional MOF-based catalysts.

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Author Contributions †

These authors contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundations of China (21771024, 21871028, U1832152 and 21606021) and Natural Science Foundation of Beijing Municipality (2182029).

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01124. Elemental contents and IR spectrum of the as-prepared Ni-MOF-100; fitting values of EIS parameters of NiMOFs; comparison of the Ni-based electrocatalysts for UOR; and partial performance of Ni-MOF-60, Ni-MOF80, and Ni-MOF-RT (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Huifeng Li: 0000-0002-0257-3967 Genban Sun: 0000-0001-9005-8123 Shulan Ma: 0000-0002-8326-3134 G

DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01124 Inorg. Chem. XXXX, XXX, XXX−XXX