An electron-donating strategy to guide the construction of MOF photocatalysts toward co-catalyst-free highly efficient photocatalytic H2 evolution

Dapeng Dong a, Chaoxiong Yan a, Jindou Huang *a, Na Lu a, Pengyan Wu *b, Jian Wang b and Zhenyi Zhang *ac
aKey Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic Affairs Commission, Key Laboratory of Photosensitive Materials and Devices of Liaoning Province, School of Physics and Materials Engineering, Dalian Nationalities University, 18 Liaohe West Road, Dalian 116600, P. R. China. E-mail:;
bSchool of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou, 221116, P. R. China. E-mail:
cSchool of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450001, P. R. China

Received 9th June 2019 , Accepted 15th August 2019

First published on 16th August 2019

Herein, an interesting electron-donating strategy is proposed to guide the construction of novel CdS-based MOFs for co-catalyst-free photocatalytic H2 evolution in the presence of triethanolamine as the hole-scavenger. The optimally designed CdS-based MOF exhibits an exceptional H2-evolution rate of 26.1 mmol g−1 h−1 which is almost the highest rate among the reported co-catalyst-free MOF systems.

With dwindling fossil energy reserves and ever-increasing carbon emissions, the exploitation of clean and renewable energy sources has become an urgent issue in current society.1 Photocatalytic water splitting by using high-performance photocatalysts offers a great opportunity to convert abundant solar energy into the “green fuel” of hydrogen (H2).2 During the photocatalytic process, the half-reaction of photocatalytic proton reduction over the photocatalyst with suitable reduction potentials is a critical step for the production of H2.3 In order to facilitate this half-reaction, various kinds of sacrificial hole scavengers, such as methanol (MeOH), lactic acid (LA), triethanolamine (TEOA), etc., are usually introduced in the photocatalytic water solution.4 These scavengers can donate excess electrons to rapidly quench the photoinduced holes on the photo-oxidation active-sites of the adjacent photocatalyst, thereby prolonging the lifetimes of the photoinduced electrons on the photo-reduction active-sites for achieving photocatalytic H2 production.4 Notably, if an inappropriate hole scavenger is used in the photocatalytic system, there would be no H2 production even if a costly co-catalyst such as Pt is employed to separate the photoinduced charge-carriers and to lower the H2-overpotential of the photocatalyst.5 In this context, engineering spontaneous molecular interactions between hole scavengers and photocatalysts to improve the aforementioned electron-donating process may open a new way to realize the co-catalyst-free highly efficient photocatalytic H2 production. The major challenge for this approach is to selectively activate the photocatalysts for improving the hole scavenger-bonding ability on their photo-oxidation active-sites. However, it is almost impossible to achieve this goal in classical inorganic semiconductor photocatalysts, mainly owing to their inherent crystal structures and indistinguishable photo-redox active-sites.

As a class of burgeoning hybrid crystalline materials composed of organic molecule ligands and inorganic metal nodes, metal–organic frameworks (MOFs) possess abundant porous channels, tailorable modular structures, identifiable photo-redox active-sites, and short paths for photoinduced charge-carriers migrations.6 These unique merits make MOFs ideal candidates for photocatalyst design. However, until now, the reports about the construction of MOF photocatalysts for co-catalyst-free H2 production are still limited and the relevant design concept on these MOF photocatalysts has also been rarely mentioned. Herein, we propose an interesting electron-donating strategy to guide the rational design of MOF photocatalysts for executing co-catalyst-free H2 production in the presence of sacrificial hole scavengers. This strategy requires that the molecule ligand as the photo-oxidation active-site in MOF photocatalysts is capable of bonding to the sacrificial hole scavenger for improving the electron-donating process from the scavenger to the photocatalyst.

In this work, we present a successful paradigm to design and construct a novel CdS-based MOF, named DLNU-M-CdS(H2TD) (H2TD = 1,3,4-thiadiazole-2,5-dithiol), based on the above strategy for realizing co-catalyst-free highly efficient photocatalytic H2 production with TEOA as the sacrificial hole-scavenger. The choice of the H2TD ligand is based on the following considerations: (i) the high polarizability and the conjugate structure enable the electron-rich H2TD molecule to show good photoelectronic properties;7 (ii) the N atom distribution in H2TD provides more chances to connect with the hydroxyl-rich TEOA molecule through the intermolecular hydrogen bond. From the viewpoint of regular hard acid theory in coordination chemistry,8 the thiolate in H2TD provides a soft-base linkage, which should be easy to combine with a soft-acid linkage for constructing a stable coordination compound. Among soft-acid-type metal ions, Cd2+ ions are preferentially used to connect with the H2TD ligand, since the formed CdS-based MOFs would be capable of initiating the photocatalytic proton reduction by virtue of the suitable reduction potential of the Cd 5s orbital in the CdS crystal (Scheme 1).9 By combining first-principles calculations with time-resolved photoluminescence (TRPL) spectroscopy, we demonstrate that the electron-donating process from TEOA to H2TD occurs during the photocatalytic reaction, which induces fast directional electron transfer in the path of TEOA → H2TD → Cd → H+ to greatly hinder the recombination of photoinduced charge-carriers in the MOF. Upon UV-visible-light irradiation, the H2 production rate of the DLNU-M-Cd(H2TD) photocatalyst reaches 26.1 mmol g−1 h−1, which is almost the highest value among the data obtained from the reported co-catalyst-free MOF photocatalytic systems.

image file: c9ta06141j-s1.tif
Scheme 1 Schematic diagram of the design philosophy for constructing co-catalyst-free CdS-based MOFs based on the electron-donating strategy with TEOA as the hole scavenger.

The chemical interaction and the electron transfer process between the selected H2TD ligand and hole-scavenger TEOA were investigated based on the B3LYP/TZVP level at first. H2TD can be spontaneously connected with TEOA through intermolecular hydrogen bonds with two different modes: one is the double-hydrogen bond mode (C1), which forms between the two N atoms of the H2TD ligand and the two H atoms of TEOA; the other mode is the single-hydrogen bond between H and N atoms in the TEOA–H2TD complex (C2), as shown in Fig. 1. The energy calculations show that the binding energy of C1 (−28.2 kJ mol−1) is lower than that of C2 (−24.7 kJ mol−1), suggesting that the double-hydrogen bond is easier to form between H2TD and TEOA. For C1, the singlet excited state corresponding to the maximum absorption peak is dominantly formed by the transitions from HOMO−2 → LUMO (47%), HOMO−1 → LUMO+1 (24%) and HOMO−2 → LUMO+1 (19%). Note that HOMO−1 and HOMO−2 are mainly or partly localized on the TEOA moiety, whereas the LUMO and LUMO+1 mainly distribute in H2TD. These different distribution characteristics indicate that intermolecular electron transfer processes occur from TEOA to H2TD. Thus, the selection of H2TD as the ligand to construct MOF photocatalysts would achieve highly efficient electron donation from hole-scavenger TEOA to the MOF during photocatalytic water reduction, mainly owing to the spontaneous formation of the intermolecular double-hydrogen bond in the TEOA/H2TD complex.

image file: c9ta06141j-f1.tif
Fig. 1 The geometries of the TEOA–H2TD complex C1 formed through a double intermolecular hydrogen bond (I) and C2 formed through a single intermolecular hydrogen bond (II), and the shapes of frontier molecular orbitals related to their maximum absorption peaks.

The CdS-based MOF of DLNU-M-CdS(H2TD) was constructed by using CdI2 as the metal node precursor dissolved into the aqueous solution of the H2TD ligand with hydrothermal treatment at 140 °C for 3 days. The single-crystal X-ray diffraction analysis reveals that the obtained sample crystallizes in the monoclinic space group C2/c. Each fundamentally structural unit contains four unique Cd2+ cations and four TD2− ligands (Fig. 2A). The Cd1 cation is in a five-coordinated environment, and is coordinated by three sulfur atoms (S1A, S1B and S3) from three separate TD2− anions and two nitrogen atoms (N1B and N2C) from two separate TD2− anions to form the CdS3N2 polyhedral geometry. The H2TD ligand bridges two Cd cations through two terminal S atoms and two N atoms to form the [Cd2(TD)2] secondary building units, which are further interlinked by the terminal S atoms to form a 3D framework structure. Each S3 atom behaves as a μ1 metal linker joining one Cd atom, while each S1 atom behaves as a μ2 metal linker linking the two Cd atoms. As shown in Fig. 2B, plenty of N atoms are exposed on the ab plane of DLNU-M-CdS(H2TD), which provides abundant active sites to capture the hole-scavenger TEOA. Meanwhile, the metal Cd nodes on the surface of the MOF are simultaneously connected with two N atoms from the two different H2TD molecules (Fig. 2B and C). This fascinating structure–property relationship would shorten the migration distance of photo-excited charge-carriers between the Cd nodes, the H2TD ligands, and the hole-scavenger TEOA during the photocatalytic process.

image file: c9ta06141j-f2.tif
Fig. 2 (A) Structural unit of DLNU-M-CdS(H2TD); ball and stick representation of DLNU-M-CdS(H2TD) viewed along (B) the a axis and (C) the c axis; (D) experimental and simulated XRD patterns of DLNU-M-CdS(H2TD); (E) DOS and (F) UV-vis absorption spectrum of DLNU-M-CdS(H2TD).

The experimental XRD pattern of the DLNU-M-CdS(H2TD) MOF is shown in Fig. 2D, which matches very well with the simulated one that is obtained on the basis of single crystal structure analysis, indicating a good phase purity of the constructed MOF. Furthermore, the DLNU-M-CdS(H2TD) MOF possesses good thermal and chemical stabilities (Fig. S1 and S2). The electronic structure simulations of the DLNU-M-CdS(H2TD) MOF were completed using density functional theory with the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional, and the computational details are shown in the ESI. As illustrated in Fig. 2E, the predicted band gap of the MOF is about 3.32 eV, which coincides basically with its actual absorption edge position(∼388 nm for ∼3.2 eV) obtained from the UV-vis absorption spectrum (Fig. 2F). The level-off tail in the absorption spectra is believed to be due to the structural defects in the MOF. Furthermore, the projected density of states (PDOS) (Fig. 2E) shows that the valence band maximum (VBM) of the DLNU-M-CdS (H2TD) MOF is mainly composed of S 3p and N 2p orbitals while its conduction band minimum (CBM) is mainly constituted by C 2p and Cd 5s orbitals. Upon intrinsic photo-excitation of the MOF, the photoinduced electrons would transfer from the S 3p/N 2p to Cd 5s orbitals, simultaneously leaving the photoinduced holes in the S 3p/N 2p orbitals. Thus, the photo-oxidation and photo-reduction active-sites of the MOF are located on the S/N and Cd atoms, respectively. According to the crystal structure of the DLNU-M-CdS(H2TD) MOF and the interaction between the MOF and hole-scavenger TEOA, the photoinduced charge-carrier migration channels from the N 2p to the Cd 5s orbital in the MOF should play a key role in the improvement of photocatalytic activity for proton reduction.

The photocatalytic proton-reduction test of the DLNU-M-CdS(H2TD) MOF was conducted under UV-visible light irradiation with TEOA as the sacrificial hole-scavenger but without any additional co-catalyst or photo-sensitizer. Control experiments indicated that no H2 evolution occurred in the absence of either photocatalysts or light irradiation. As observed in Fig. 3A, the DLNU-M-CdS(H2TD) MOF exhibits a remarkable photocatalytic activity for H2 evolution. Furthermore, the H2-evolution amounts strongly depend on the content of the hole-scavenger TEOA, suggesting that the electron-donating process from the TEOA to the MOF serves as the leading factor for the photocatalytic proton reduction. In our case, the optimal photocatalytic activity of the DLNU-M-CdS(H2TD) MOF appeared in the photocatalytic system containing 15 vol% TEOA. When a higher content of TEOA was used for the photocatalytic reaction, the adsorption–desorption and diffusion behaviors of TEOA on the MOF surface may be limited due to the high viscosity of TEOA, thereby resulting in a decreased photocatalytic activity. What's more, the initial pH value for the TEOA water solution also influences the photocatalytic H2-evolution of the DLNU-M-CdS(H2TD) MOF (Fig. 3B). A low pH value of the photocatalytic solution would impede the formation of double-hydrogen bonds between TEOA and the ligand of the MOF due to the protonation of the N atom in the ligand. In this case, the electron-donating process from the TEOA to the MOF was greatly weakened, which therefore led to a fast recombination of charge-carriers and a low activity of the MOF for photocatalytic H2 evolution. Moreover, a high pH value of the photocatalytic solution is also not beneficial for the H2 evolution, due to the fact that a small amount of H+ ions is supplied to the photo-reduction active-sites (Cd atoms) of the MOF. As concluded from the above results, the optimal photocatalytic solution for the DLNU-M-CdS(H2TD) MOF is 15 vol% TEOA in water with the initial pH at 9. Upon UV-visible light irradiation, the MOF exhibits a good time-dependent photocatalytic H2 evolution rate of 26.1 mmol g−1 h−1 (Fig. 3C). Notably, this H2-evolution rate is almost the highest one among the values reported for other co-catalyst-free photocatalytic systems over the MOF photocatalysts (Table 1). Moreover, the apparent quantum efficiency (AQE) of the DLNU-M-CdS(H2TD) MOF reaches ∼1.38% at 365 nm by using only 1 mg of the photocatalyst. This AQE is comparable to the corresponding values obtained from the co-catalyst-assisted MOF photocatalysts.10 Interestingly, when we used the other sacrificial hole-scavengers with different bonding abilities to the H2TD ligand during the photocatalysis, the photocatalytic activity of the DLNU-M-CdS(H2TD) MOF for H2 evolution was changed obviously according to the electron-donating process from the scavenger to the MOF (Fig. S3 and Table S2). These results confirm the efficiency of the electron-donating strategy proposed in our work.

image file: c9ta06141j-f3.tif
Fig. 3 (A) H2-evolution amount over DLNU-M-CdS(H2TD) in aqueous solution containing different amounts of TEOA under UV-visible-light irradiation for 4 h; (B) H2-evolution amount over DLNU-M-CdS(H2TD) in 15 vol% TEOA aqueous solution with different initial pH-values under UV-visible-light irradiation for 4 h; (C) plots of photocatalytic H2-evolution amounts over different CdS-based MOFs; (D) ball and stick representation of DLNU-M-CdS(H2TD)(H2O) viewed along the a axis; (E) DOS and (F) UV-vis absorption spectrum of DLNU-M-CdS(H2TD)(H2O).
Table 1 Photocatalytic activity of reported co-catalyst-free MOF-based photocatalysts for H2 evolution
MOFs Metal node Linker Photosensitizer Activity Ref.
[Ni2(PymS)4]n Ni Pyrimidine-2-thio Fluorescein 6 μmol h−1 11a
[CuII(RSH)(H2O)]n Cu Rhodamine-based carboxylate linker Eosin Y 7.88 mmol g−1 h−1 11b
Cu-DSPTP Cu H2DSPTP No 6.99 μmol h−1 11c
Cu–I-bpy Cu 4,4′-Bipyridine No 7.09 mmol g−1 h−1 11d
[Cu2I2(BPEA)](DMF)4 Cu BPEA No 75.89 mmol g−1 within 18 h 11e
Cu–Ni–POMs Cu–Ni PBPY Self-sensitized POMs 833 μmol g−1 h−1 11f
DLNU-M-CdS(H2TD) Cd H 2 TD No 26.1 mmol g −1 h −1 This work

To further generalize the electron-donating strategy in the design of co-catalyst-free MOF photocatalysts, another novel isomeric CdS-based MOF of DLNU-M-CdS(H2TD)(H2O) was constructed through a similar hydrothermal process by using CdCl2·5/2H2O as the metal node precursor (Fig. S4–S6). According to the single-crystal X-ray diffraction analysis, the DLNU-M-CdS(H2TD)(H2O) MOF belongs to the monoclinic space group P21/n (Fig. S7). As expected, this MOF also displays the co-catalyst-free photocatalytic activity for H2 evolution in the presence of the hole-scavenger TEOA upon UV-visible light irradiation (Fig. 3C, S8 and S9). However, after optimizing the photocatalytic solution parameters, the H2-evolution rate of DLNU-M-CdS(H2TD)(H2O) was only 6.9 mmol g−1 h−1, which is about 3.8 times lower than that of DLNU-M-CdS(H2TD). The different photocatalytic activity of these two CdS-based MOFs can be attributed to their discrepant crystal and electronic structures. As evidenced by the crystal structures (Fig. 3D and S10) and the inductively coupled plasma (ICP) results, the content of metal Cd nodes in DLNU-M-CdS(H2TD)(H2O) (∼12.3 wt%) is lower than that in the DLNU-M-CdS(H2TD) (∼22.9 wt%). Meanwhile, the Cd coordination node in DLNU-M-CdS(H2TD)(H2O) is only bonded with one N atom from the H2TD ligand. Similarly, DLNU-M-CdS(H2TD)(H2O) is also a UV-light-active semiconductor with the band gap larger than that of DLNU-M-CdS(H2TD), as concluded by both the theoretical calculation and experimental results (Fig. 3E and F). Importantly, the two isomeric MOFs possess the same orbital constitutions for their CB and VB, implying that the electron-donating strategy is also suitable for the DLNU-M-CdS(H2TD)(H2O) MOF toward the co-catalyst-free photocatalytic proton reduction. However, although electron donation from the hole-scavenger TEOA to the DLNU-M-CdS(H2TD)(H2O) MOF across the formed hydrogen bond O–H⋯N can effectively hinder the charge-carrier recombination, photoinduced electron transfer from the N to Cd atoms in the MOF is unsatisfactory due to only one electron-migration channel (Cd–N) in this MOF. Please note that there are two migration channels (N–Cd–N) in the DLNU-M-CdS(H2TD) MOF. Based on the above results and analyses, we deduce that the Cd nodes in the two MOFs are main photo-reduction active-sites for H2 evolution. Thus, the lower photocatalytic activity for H2 evolution over the DLNU-M-CdS(H2TD)(H2O) as compared to the DLNU-M-CdS(H2TD) can be ascribed to the following two reasons: (i) the low content of Cd nodes for providing photo-reduction active-sites; (ii) poor electron transfer from N to Cd atoms limits the generation of the available electrons for executing the proton reduction.

To confirm the above assumption, a series of photoinduced carrier-dynamics tests were performed on the two MOFs. Fig. 4A shows the measurement results of the incident-photon-to-current-conversion efficiency (IPCE) and the transient photocurrent responses (TPR). It can be seen that the IPCE curves of the two samples match well with their respective intrinsic absorption curves, indicating that the defect-induced visible-light absorptions of these samples do not contribute to the separation of charge-carriers for executing the H2 evolution. Furthermore, the maximum IPCE of DLNU-M-CdS(H2TD) is about 6.7% at 300 nm. This value is ∼3.0 times larger than the corresponding IPCE of DLNU-M-CdS(H2TD)(H2O) (∼2.2%). Meanwhile, the transient photocurrent of the DLNU-M-CdS(H2TD) (∼1.12 mA) is ∼3.5 higher than that of DLNU-M-CdS(H2TD)(H2O) (∼0.32 mA) during several on–off cycles of irradiation by UV-visible light. Thus, we believe that the Cd nodes in the MOF act as the main supports for releasing the photoinduced electrons. And, the corresponding bonding mode between the Cd nodes and their adjacent N atoms affects the separation process of the photoinduced charge-carriers in the MOF. Please note that the above IPCE and TPR values of the two MOFs should be much smaller than their real values, because the insulating organic binder is introduced to fix the MOFs onto the electrodes during the tests.

image file: c9ta06141j-f4.tif
Fig. 4 (A) IPCE and TPR curves (inset) of two isomeric CdS-based MOFs; (B) Mott–Schottky plots of two isomeric CdS-based MOFs; (C) steady-state PL spectra: (b) DLNU-M-CdS(H2TD) and (d) DLNU-M-CdS(H2TD)(H2O) in water as compared to those of (a) DLNU-M-CdS(H2TD) and (c) DLNU-M-CdS(H2TD)(H2O) in TEOA aqueous solution; (D) time-resolved transient PL decay of DLNU-M-CdS(H2TD) in (a) water and (b) TEOA aqueous solution; (E) time-resolved transient PL decay of DLNU-M-CdS(H2TD)(H2O) in (a) water and (b) TEOA aqueous solution (the inset data were calculated using eqn (S1) and (S2)); (F) schematic diagram showing the directional electron transfer process from TEOA to DLNU-M-CdS(H2TD), and to protons during the photocatalytic H2 evolution.

As shown in Fig. 4B, the Mott–Schottky plots reveal that the flat-band potentials of the two MOFs are suitable for initiating the photocatalytic proton reduction. And, the photocatalytic reducibility of DLNU-M-CdS(H2TD) is stronger than that of DLNU-M-CdS(H2TD)(H2O) (−0.34 V vs. NHE) due to the smaller flat-band potentials (−0.72 V vs. NHE) of the former. According to these results, it can be concluded that the photoinduced holes in the VB of the MOFs can be quickly captured by the hole-scavenger TEOA due to the existence of deep hole-potential-wells between the VB of the MOFs and the TEOA (Fig. S11). That is to say, the electrons in the TEOA can be easily donated to the VB of the CdS-based MOFs for strongly hindering the photoinduced charge-carrier recombination of the CdS-based MOF (Fig. S12). When moving the two MOFs from the water solution to the TEOA aqueous solution, their PL intensities obviously decreased (Fig. 4C). Correspondingly, the emission lifetimes caused from either the interband charge-recombination (the shorter lifetime τ1) or the charge-recombination in the defect states (the longer lifetime τ2) of the two MOFs were simultaneously shortened, as evidenced by the TRPL spectroscopy measurements (Fig. 4D and E). The quenched PL and the decreased emission lifetimes suggest an effective transfer of the photoinduced electrons from the CB of the MOFs to the free protons due to the presence of TEOA in the photocatalytic solution.12 This transfer process is dependent on the electron-donating ability of the TEOA to the MOFs' VB, because highly efficient electron donation can strongly suppress the charge-carrier recombination in the MOFs. According to the eqn (S3), the rate constant of photoinduced electron transfer from the DLNU-M-CdS(H2TD) to the protons is estimated to be 4.5 × 107 s−1, which exceeds the constant (1.4 × 107 s−1) of the DLNU-M-CdS(H2TD)(H2O) system by ∼3.2-fold. These observations further confirm that the N–Cd–N bonds in the DLNU-M-CdS(H2TD) MOF can greatly enhance directional electron transfer from the surface-adsorbed TEOA to main photo-oxidation sites (N atoms), and further to photo-reduction sites (Cd atoms) of the MOF for realizing highly efficient photocatalytic water reduction (Fig. 4F). However, the photocatalytic stability of the DLNU-M-CdS(H2TD) MOF is not good enough due to the photocorrosion process during the photocatalytic H2 evolution (Fig. S13). As we know, the photo-stability problem is a great challenge for most of the MOF photocatalysts,6a–d,13 which needs to be investigated in detail in the CdS-based MOF system in our future work.

In summary, an electron-donating strategy has been proposed, for the first time, to guide the design and construction of MOF photocatalysts toward co-catalyst-free highly efficient photocatalytic H2 evolution. Based on this strategy, two novel isomeric CdS-based MOFs have been successfully constructed through a simple hydrothermal method by using H2TD as the ligand. The H2TD ligand in the MOFs can spontaneously form intermolecular double-hydrogen bonds with the hole-scavenger TEOA during the photocatalytic process. This chemical bond opens a high-speed transfer channel to improve the electron -donation from the TEOA to the VB (photo-oxidation sites) of MOFs, thereby greatly prolonging the lifetimes of the photoinduced electrons in the CB (photo-reduction sites) of MOFs. These long-lived photoinduced electrons are capable of reducing the protons for releasing H2 without additional co-catalysts. Among the two MOFs, the DLNU-M-CdS(H2TD) possesses the optimal chemical structure for realizing fast directional electron transfer in the path of TEOA → H2TD → Cd → H+. Thus, upon UV-visible-light irradiation, the photocatalytic H2 evolution rate of the DLNU-M-CdS(H2TD) could reach 26.1 mmol g−1 h−1 in the aqueous solution containing 15 vol% TEOA at pH 9. To the best of our knowledge, this H2-evolution rate is higher than almost all the rates reported for other co-catalyst-free MOF photocatalytic systems. Our work offers a new insight into the future design and development of high-performance low-cost MOF photocatalysts for fulfilling the co-catalyst-free solar-to-fuel conversion.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the National Natural Science Foundation of China (Grant No. 51772041), the Natural Science Foundation of Liaoning Province (Grant No. 20180550100 and 2017054190) and the Program for Dalian Excellent Talents (Grant No. 2017RQ148 and 2016RQ069), the Program for Liaoning Excellent Talents in University (LNET) (Grant No. LR2017004), Dalian Science Foundation for Distinguished Young Scholars (2018RJ05), the LiaoNing Revitalization Talents Program (XLYC1807176) and the Natural Science Foundation of Jiangsu Province for Outstanding Youth (No. BK20180105). Prof. Zhenyi Zhang acknowledges the support from the Liaoning BaiQianWan Talents Program.

Notes and references

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Electronic supplementary information (ESI) available: Experimental section; Fig. S1–S13. CCDC 1912087 and 1912088. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9ta06141j

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