Series of ZnSn(OH)6 Polyhedra: Enhanced CO2 Dissociation

Apr 24, 2017 - Faculty of Materials Science and Engineering, Key Laboratory of Advanced Materials of Yunnan Province, Kunming University of Science an...
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Series of ZnSn(OH)6 Polyhedra: Enhanced CO2 Dissociation Activation and Crystal Facet-Based Homojunction Boosting Solar Fuel Synthesis Lanqin Tang,†,‡,§ Zongyan Zhao,∥ Yong Zhou,*,⊥,#,†,‡ Bihu Lv,† Peng Li,∇ Jinhua Ye,∇,○ Xiaoyong Wang,† Min Xiao,† and Zhigang Zou†,‡ †

School of Physics, National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China # State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials of Sichuan Province, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, P. R. China ‡ Eco-Materials and Renewable Energy Research Center (ERERC), Nanjing University, Nanjing 210093, China § College of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, P. R. China ∥ Faculty of Materials Science and Engineering, Key Laboratory of Advanced Materials of Yunnan Province, Kunming University of Science and Technology, Kunming 650093, China ⊥ Key Laboratory of Modern Acoustics (MOE), Institute of Acoustics, Department of Physics, Nanjing University, Nanjing 210093, China ∇ Environmental of Remediation Materials Unit and International Center for Materials Nanoarchitectures (WPI-MANA), 1-1 Namiki, Tsukua, Ibaraki 305-004, Japan ○ TU-NIMS Joint Research Center, School of Materials Science and Engineering, and Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: A series of ZnSn(OH)6 polyhedra are successfully explored with well-controlled area ratio of the exposed {100} and {111} facets. Band alignment of the exposed facet-based homojunction of the elegant polyhedron facilitates spatial separation of photogenerated electrons and holes on {111} and {100} surfaces, respectively. Optimal area ratio of {100} to {111} is the prerequisite for pronounced CO2 photocatalytic performance of high-symmetry cuboctahedra into methane (CH4). The synergistic effect of the excess electron accumulation and simultaneously the enhanced CO2 absorption and low dissociation activation energy on {111} reduction sites promote the yield of CO2 photocatalytic conversion product.

1. INTRODUCTION

based on different band energies of the exposed crystal facets, which is a representative approach for improving charge separation. More positive CB energy level is favorable for the injection of electrons, and more negative VB energy level are beneficial to the accumulation of holes, which create spatially separated oxidation and reduction sites on different facets. Such facet-induced spontaneous charge separation has been observed in numerous metal oxide semiconductors, such as BiVO4,5 TiO2,6 SrTiO3,7 BiOCl,8 Ag3PO4,9 and CeO2.10 The activation of CO2 for photoreduction is one of the most challenging themes, because CO2 is a highly stable molecule requiring a high activation energy for CO bond cleavage.11

Photocatalytic reduction of CO2 with H2O into valuable hydrocarbon compounds, also known as “artificial photosynthesis”, can realize the conversion and storage of the inexhaustible and low-density solar power to chemical energy.1−3 H2O is oxidized by the photogenerated holes in valent band (VB) to generate hydrogen ions via the reaction of H2O → 1/2O2 + 2H+ + 2e− (E°redox = 0.82 V vs normal hydrogen electrode (NHE)); CO2 is reduced by the photogenerated electrons in conduction band (CB) to CH4 via the reaction of CO2 + 8e− + 8H+ → CH4 + 2H2O (E°redox = −0.24 V vs NHE). The photocatalytic efficiency was determined not only by intrinsic crystallographic structures of photocatalysts but also by exposed crystal facets.4 The surface homojunction is the junction layers made of the same semiconductor materials, © 2017 American Chemical Society

Received: February 5, 2017 Published: April 24, 2017 5704

DOI: 10.1021/acs.inorgchem.7b00219 Inorg. Chem. 2017, 56, 5704−5709

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Inorganic Chemistry Surface chemistry of CO2 demonstrates that catalysts containing hydroxyl (−OH) groups are known to enhance the local CO2 concentration through noncovalent interactions with adsorbed reagent species, overcoming the slow kinetics of the reaction.12 CO2 adsorbed in the vicinity of −OH groups can also be easily protonated, converting the adsorbed CO2 species into bicarbonate species.13 Layered double hydroxides, as typical solid bases, promote the photoreduction of CO2 to CH4 with H2O through synergic effect of transit metal catalytic sites with basic OH.14 The presence of both the Lewis acid In(III) and the Lewis base OH surface sites of nonstoichiometric In2O3−x(OH)y affects the energetics and dynamics of CO2 adsorption and reaction as well as H2 dissociation for reverse water gas shift reaction, CO2 + H2 → CO + H2O, and is more active than stoichiometric In2O3 nanocrystals.15 Hydroxylation treatment of anatase TiO2 with aqueous NaOH solution promoted the chemisorption, activation, and photocatalytic CO 2 reduction.16 Rutile TiO2(100) surface or brookite TiO2 can produce more surface hydroxyls, leading to higher efficiency in the photocatalytic reduction of CO2 than their references.17 Zinc hydroxystannate [ZnSn(OH)6] is a kind of perovskite-structured metal hydroxide material with face-centered-cube closed packing.18 The metal atoms are octahedrally coordinated with oxygen atoms to form Sn(OH)6 and Zn(OH)6 polyhedra, which share their “O” corners to build the structural framework.19 ZnSn(OH)6, as an element earth-abundant compound, has attracted attention for various photocatalytic activities in environmental cleaning, such as long-term stability and high conversion ability in oxidation of benzene.20 In this Article, we systematically explore a series of ZnSn(OH)6 polyhedra with well-controlled area ratio (AR) (abbreviated as AR{100}/{111}) of exposed {100} and {111} facets. The microstructure experiences elaborative shape evolution from cubes (the corresponding sample is abbreviated as SC), to truncated cubes (ST‑C), high-symmetry cuboctahedra (SC‑O), truncated octahedra (ST‑O), and eventually to perfect octahedra (SO). Such morphology transit corresponds to a progressive shrinkage of {100} facets and an enlargement of {111} facets. The band alignment of the elegant polyhedron facilitates photogenerated electrons and holes to {111} and {100} surfaces, respectively, and results in a spatial charge separation for long survival. Optimal AR{100}/{111} is the prerequisite for pronounced CO2 photocatalytic performance of SC‑O into methane (CH4). The synergistic effect of the excess electron accumulation and simultaneously the enhanced CO2 absorption and low dissociation activation energy on {111} reduction sites promote the yield of CO2 photocatalytic conversion product.

Figure 1. SEM images and the corresponding schematic of various ZnSn(OH)6 polyhedra: (a1, a2, a3) SC, (b1, b2, b3) ST‑C, (c1, c2, c3) SC‑O, (d1, d2, d3) ST‑O, and (e1, e2, e3) SO.

Figure 2. TEM images and the corresponding SAED of (a, b) SC and (c, d) SO.

2. RESULTS AND DISCUSSION The SEM images of all the polyhedra with smooth surfaces are shown in Figure 1. SC and SO are bound with six square {100} and eight triangular {111} facets, respectively. Corner truncation of SC allows generation of 14-faceted ST‑C enclosed with six square {100} facets and eight triangular {111} with the AR{100}/{111} of ∼4. High-symmetry SC‑O is made of six square {100} and eight trapezoid {111} lattice planes with the AR{100}/{111} of ∼1. ST‑O is created by cutting the corners of the octahedron, featuring a small {100}-truncated surface and the AR{100}/{111} of ∼0.1. Well-aligned spots of the selected-area electron diffraction (SAED) patterns demonstrate the singlecrystal feature of the ZnSn(OH)6 polyhedra (Figure 2).

Table 1 lists the experimental parameters for the preparation of various ZnSn(OH)6 polyhedra. The amount of added NaOH mainly dominates the shape evolution of ZnSn(OH) 6 polyhedra. Maintaining the concentrations of Zn2+ and Sn4+ with a reaction time of 13 h in the presence of triethanolamine (TEA), a crystal growth modifier, a smaller quantity of NaOH (relative concentration: [Zn2+]/[Sn4+]/[OH−] = 1:1:6; see Experimental Section) exclusively produces uniform SC. Increasing the amount of NaOH ([Zn2+]/[Sn4+]/[OH−] = 1:1:9) allows SC to transit into ST‑O. Addition of more NaOH ([Zn2+]/[Sn4+]/[OH−] = 1:1:12) makes {100} facets completely disappear and generates SO. ZnSn(OH)6 as a face5705

DOI: 10.1021/acs.inorgchem.7b00219 Inorg. Chem. 2017, 56, 5704−5709

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Inorganic Chemistry

The time evolution of CH4 shows that SO displays the CH4 production amount for 5 h (3.5 ppm), 1.5 times as large as SC (2.4 ppm), demonstrating preferable capacity of {111} to CO2 photoreduction than {100} (Figure 3a). ST‑C enhances the CH4

Table 1. Experimental Parameters for the Preparation of Various ZnSn(OH)6 Polyhedraa

a Asterisks indicate that [Zn2+], [Sn4+], and [OH−] refer to the relative concentrations among them.

Figure 3. (a, b) CH4 generation amounts over ZnSn(OH)6 polyhedra as a function of irradiation time and (c) comparison of the photocatalytic CH4-production activity of the initial 5 h.

centered cubic crystal is of a general sequence of surface energies, γ {111} < γ {100} < γ {110} . 21 Raising the OH − concentration in the reaction system can obviously increase the reaction of Zn2+ and Sn4+ with OH− to form the microcrystal enclosed with {111} planes as the basal surfaces to minimize surface energy. Reaction time also determines morphology transition. A reaction time of 3 h in the case of [Zn2+]/[Sn4+]/[OH−] = 1:1:9 allows for generation of ST‑C. Elongation of the reaction time to 8 h increases the area proportion of {111} facets to produce elegant SC‑O. With the reaction time, {100} or {110} lattice planes grow faster and ultimately disappear during the crystal growth. The morphology evolution with synthetic parameters was summarily illustrated in Figure S1. The preliminary experiment demonstrates that TEA also plays a critical role in the formation of the ZnSn(OH)6 polymorph. In the absence of TEA, a mixture of irregular polyhedra with a smaller size of ∼1.0 μm as well as lots of nanoparticles were observed with [Zn2+]/[Sn4+]/[OH−] = 1:1:9 and a reaction time of 13 h (Figure S2), in sharp contrast to 10 μm sized ST‑O in the presence of TEA. This indicates that TEA can restrain the number of nuclei formation and favor the crystal-shaped growth. The XRD peaks of all the ZnSn(OH)6 polyhedra can be indexed as cubic perovskite-type ZnSn(OH)6 [JCPDS 20-1455; space group, Pn3̅m(224)] (Figure S3). No additional peaks were observed in the patterns. The sharpness and narrowness of the diffraction peaks indicate a high degree of crystallinity. The ZnSn(OH)6 polymorphs display strong absorption in the UV light region with the absorption edges at ∼330 nm and corresponding band gaps to ∼3.78 eV (Figure S4a). Alteration of AR{100}/{111} shows no obvious influence on the band gap. The Mott−Schottky measurement shows that the CB edge of SC‑O is estimated about −0.95 V (vs NHE), much more negative than E° (CO2/CH4) (−0.24 V vs NHE). The corresponding VB edge of SC‑O is consequently calculated about +2.83 V vs NHE, more positive than E° (H2O/H+) (+0.82 V vs NHE), indicating the capability of ZnSn(OH)6 for photocatalytic reduction of CO2 with H2O to produce CH4 (Figure S4b).

production amount to 5.3 ppm, indicative of favorable effects of the coexistence of {111} and {100} facets on photocatalytic activity. SC‑O of enlarged proportion of {111} reaches the maximum value of 9.3 ppm. Further augment of percentage of {111} facets for ST‑O contrarily decreases CH4 production rate of 3.7 ppm. Obviously, optimal AR{100}/{111} is the prerequisite for high CO2 photoconversion efficiency of SC‑O. Loading Pt and MnOx as electron and hole sinkers allows >3× enhancement of CO2 photoconversion efficiency of SC‑O (Figure 3b). The quantum yield of CO2 photoreduction for SC‑O was determined as 0.06% at 300 nm using monochromatic light (Figure S5). The isotopically labeled 13CO2 experiment demonstrates that the carbon source was derived from input CO2 (Figure S6). Although the CO2 yield on the present ZnSn(OH)6 polyhedra is still low due to wide bandgap of the ZnSn(OH)6 with UV light response, the doping process will narrow the bandgap to further improve photoconversion efficiency. The particle size of the synthesized ZnSn(OH)6 is in the micrometer scale; the photocatalytic activities may be further improved if the particule size is reduced to nano or submicro size. No appearance of CH4 production was detected when a CO2 reduction experiment was performed in the dark or in the absence of the photocatalysts, proving that the CO2 reduction reaction is driven by light irradiation of the photocatalysts. To explore the reasons for the ZnSn(OH)6 shape-dependent CO2 photocatalytic activity, density functional theory (DFT) calculations were carried out with ultrasoft pseudopotential plane wave method, which is implemented in the Cambridge Serial Total Energy Package (CASTEP) code.22 The transition state (TS) on the reaction path from the initial state [IS, i.e. molecular adsorption of CO2 on {111} surface and {100} surface] to the final state [FS, i.e., dissociation adsorption of CO2 on {111} surface and {100} surface, in other words, one of the C−O bonds is broken; Figure S7 shows the computational adopted models] was researched with the method of complete linear synchronous transit/quadratic synchronous transit (complete LST/QST). The adsorption energies at IS are 5706

DOI: 10.1021/acs.inorgchem.7b00219 Inorg. Chem. 2017, 56, 5704−5709

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Inorganic Chemistry both small, 0.152 eV for {111} surface and 0.048 eV for {100} surface, while the adsorption energies at FS are relatively large, 1.526 eV for {111} surface and 0.905 eV for {100} surface (Figure 4). These calculated results indicate that the

Figure 6. In situ FT-IR spectra of SC and SO for CO2−H2O adsorption at room temperature. The peak assignment was listed in the right panel.

Figure 4. Reaction path of CO2 decomposition from IS to FS, through TS and IP, on {111} surface and {100} surface of ZnSn(OH)6.

carbonate (m-CO32−), which are related to the change of the O−C−O bond angle during the chemisorption of CO2.23−26 HCO3− was formed from CO2 interaction with OH groups, which is the possible intermediate for CO2 photoreduction to CO and C1 fuels (e.g., CH4) once the dissociative H atom is available.27,28 b-CO32− originates from CO2 coordination with an unsaturated O2 and OH produced by surface H2O. It is suggested that CO32− may poison the catalyst by occupying active sites.29,30 Thus, it is favorable that the catalyst is covered with more HCO3− and less CO32−, and it additionally demonstrates that photocatalytic CO2 reduction preferably occurs on SO with weak CO32− peaks. Controlling the exposed facet of a particle can affect the statistical likelihood of correct charge carrier location, i.e., holes at oxidation sites and electrons at reduction sites.9 Probably because of the same cubic system, ZnSn(OH)6 particles can also be regulated with {100} and {111} facets exposed, which is quite similar to our previous work about CeO2 with coexposed {100} facets of hexahedra and {111} facets of octahedra.10 To determine whether the band alignment between the {100} and {111} facets can support the formation of the surface heterojunction, the band edges of these two facets of ZnSn(OH)6 are calculated. It is noted that the energy bands (especially the top of VB and the bottom of CB) of {111} facet are relatively down-shifting in comparison with those of {100} facet (Figure 5a). Thermodynamically, the photoinduced electrons tend to transfer from {100} to {111} facets in CB, while holes transfer in the opposite direction in VB, which facilitates the separation of electron−hole pairs (Figure 5b). Consequently, ZnSn(OH)6 with coexposed {100} and {111} facets shows enhanced photocatalytic activity. The photoluminescence decay profiles show that the average decay lifetimes of SC and So were detected at 1.24 and 2.99 ns, respectively (Figure 7). SC‑O possesses the longest average decay lifetimes of 3.27 ns, owing to spatial charge separation.

dissociation adsorption is the stable state for CO2 on ZnSn(OH)6 surfaces, and the interaction between CO2 (and its dissociated production) and {111} surface is relatively stronger. On the reaction path, the active energy (the difference between IS and TS) of {111} surface is very small, 0.038 eV. However, an intermediate product (IP) was found on the {100} surface, so there are two active energies on the reaction path, 2.441 and 1.254 eV. Therefore, the dissociation reaction of CO2 is more easily carried out on the {111} reductive surface than the {100} oxidative one. Figure 5a shows the partial density of states (DOS) of {111} surface and {100} surface of ZnSn(OH)6. The rich surface states could be found in the forbidden band, owing to the dangling bonds on these two surfaces. Especially, the surface states on {111} surface are more obvious, which can explain why the interaction of CO2 or its dissociated product with {111} surface is stronger. CO2 adsorption performance on {100} and {111} facets was experimentally investigated. While the surface area of SC (1.49 m2/g) is ∼1.5 times as large as that of SO of 1.02 m2/g, nevertheless, the quantity of CO2 adsorbed on SO is measured as larger than SC (Figure S8). It apparently indicates stronger interaction and more activation sites of CO2 molecules with {111} facets than {100} facets. The DFT calculation reveals that the density of surface hydroxyls of {111} surface (6.5 nm−2) is larger than that of {100} surface (2.8 nm−2). More surface hydroxyls on {111} may lead to stronger interaction of CO2. The obvious distinction of in situ Fourier transform infrared (FT-IR) absorption peaks of CO2 in the presence of water vapor on SC and SO further reveals different CO2 absorption behaviors on two facets (Figure 6). SO exclusively enclosed with {100} leads to the formation of bidentate carbonate (b-CO32−) and bicarbonate (HCO3−). SC solely bound with {111} produces very strong HCO3− and b-CO32−, and some weak carboxylate (CO2−) and monodentate

Figure 5. (a) Partial DOS of {111} surface and {100} surface of ZnSn(OH)6. (b) Illustration of charge transfer cross {111} and {100} surface homojunction. 5707

DOI: 10.1021/acs.inorgchem.7b00219 Inorg. Chem. 2017, 56, 5704−5709

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Inorganic Chemistry

min, and then the background spectrum in the presence of the sample was collected. Finally, in situ FT-IR spectra were recorded as a function of time to investigate the dynamics of reactants (CO2 and H2O) adsorption. The adsorption of reactants on the catalyst surface was studied by introducing a CO2−H2O mixture to the IR cell for 40 min, and the in situ FT-IR spectra were recorded. Photocatalytic Reactions. The gas-phase photoreduction reaction was carried out in a closed circulation system equipped with a vacuum line. The ZnSn(OH)6 photocatalyst (0.1 g) was dispersed uniformly on a quartz reaction cell with an area of 4 cm2. Ultrapure water (0.4 mL) was injected into the system as reducer. The system was vacuumed, and then 710 Torr of CO2 was introduced. A 300 W Xe lamp with a light intensity of ∼800 mw/cm2 was employed as the UV−vis source. The apparent quantum efficiency was calculated as in the following equation, in which N(CH4) and N(Photons) signify the molecular number of generated CH4 per unit time and the number of incident photons per unit time, respectively.

Figure 7. Photoluminescence decay spectra of SC, SC‑O, and SO.

EQ = N (CH4) × 8/N (Photons) × 100%

The MnOx oxidation and Pt reduction cocatalysts were individually photodeposited on SC‑O. The SEM images clearly show that MnOx was formed selectively on the square {100} facet (Figure S9) and Pt was formed on the hexagonal {111} facet (Figure S10), experimentally proving the accumulation of the photogenerated holes on {100} facets for H2O oxidation and electrons on {111} facets for CO2 reduction.

Characterization. X-ray powder diffraction (XRD) patterns were obtained on a powder X-ray diffractometer equipped with graphite monochromatism Cu Kα radiation (λ = 1.54178 Å, D8 Advanced, Bruker, Germany). The morphologies of the samples were recorded by scanning electron microscopy (SEM, S-3400N II, Hitachi Co., Japan) with an acceleration voltage of 20 kV. The diffuse reflection spectra were recorded on a UV−visible spectrophotometer (UV-2550, Shimadzu, Japan) and then converted into absorption spectra via Kubelka−Munk transformation. Mott−Schottky analysis was performed in a three-electrode electrochemical cell, using a platinum wire and an Ag/AgCl electrode as the counter electrode and reference electrode, respectively. Samples prepared on indium tin oxide (ITO) glasses served as the working electrodes, and 1.0 M NaOH aqueous solution (pH = 13.6) was used as the electrolyte. The products of photoreduction CO2 were measured by using a gas chromatograph (GC-2014, Shimadzu Co., Japan) equipped with a flame ionization detector (FID) according to the standard curves. The isotope analysis of 13C was analyzed by using a gas chromatograph mass spectrum (JEOL-GCQMS, JMS-K9 and 6890N Network GC system, Agilent Technologies). The transient time-resolved photoluminescence decay measurements were recorded on a fluorescence spectrometer (FLS920, Edinburgh Instruments Ltd., U.K.).

3. CONCLUSIONS A series of elegant ZnSn(OH)6 with tunable AR of the coexposed {111} and {100} facets were synthesized for photocatalytic reduction of CO2 into methane. Photogenerated electrons and holes were efficiently separated and migrate onto {111} and {100} facets for long duration, respectively, owing to band alignment of different facet potential. Appropriate AR{100}/{111} is important for good performance for CO2 photocatalytic conversion into CH4. The present work may provide a new clue to design facet-featured photocatalysts, considering not only separation of charge carriers but also spontaneous surface activation of absorbed species.

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4. EXPERIMENTAL SECTION Preparation of ZnSn(OH)6 Polyhedra. ZnSn(OH)6 polyhedra were successfully synthesized by adding triethanolamine (C6H15NO3, TEA) during the facile one-step hydrothermal process. Zinc acetate dihydrate [(CH3 COO) 2Zn·2H2 O, ZnA] and stannic chloride pentahydrate (SnCl4·5H2O, SnC) were used as raw materials. It is found that TEA acts as a crystal growth modifier for the formation of regular faceted shape; otherwise, a mixture of irregular polyhedra as well as nanoparticles is formed, which is quite different from that reported by Gao and co-workers.18 Furthermore, the shape evolution of ZnSn(OH)6 polyhedra is associated with the amount of added NaOH. In a typical process, 4 mL of 0.5 M ZnA solution and equimolar amounts of SnC were mixed under stirring, and then 5 mL of TEA was added dropwise into the mixture. After that, 4 mL of 3.0 M NaOH solution was added into the mixture and stirred for 10 min. Finally, the mixture was hydrothermally treated at 100 °C for 13 h. Accordingly, ZnSn(OH)6 cubes (Sc) were obtained. ZnSn(OH)6 octahedra (So) were prepared only by changing the volume of added NaOH solution to 8 mL. Three other kinds of ZnSn(OH)6 cuboctahedra (ST‑C, SC‑O, and ST‑O) were synthesized with 6 mL of 3.0 M NaOH solution and reaction times for 3, 8, and 13 h, respectively. In Situ DRIFTS for CO2−H2O Adsorption. In situ FT-IR measurement was carried out in the reconstructive FT-IR-6300 spectrometer (Shimadzu Co., Japan), equipped with a liquid N2 cooled MCT detector, and a three-window DRIFTS cell with two KBr windows allowing IR transmission and a third (quartz) window allowing transmission of irradiation. Prior to the illumination experiments, the as-synthesized samples were purged by N2 for 60

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00219. XRD spectra, UV−vis spectra, Mott−Schottky plots, gas chromatogram and mass spectra, SEM, DFT calculation data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yong Zhou: 0000-0002-9480-2586 Jinhua Ye: 0000-0002-8105-8903 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by 973 Programs (Nos. 2014CB239302 and 2013CB632404), NSF of China (Nos. 21473091, 21473082, 51272101, 51202005, and 21603183), 5708

DOI: 10.1021/acs.inorgchem.7b00219 Inorg. Chem. 2017, 56, 5704−5709

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Inorganic Chemistry

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NSF of Jiangsu Province (Nos. BK2012015 and BK20130425), and Jiangsu Postdoctoral Science Foundation (1601062B).

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DOI: 10.1021/acs.inorgchem.7b00219 Inorg. Chem. 2017, 56, 5704−5709