Electrochemical synthesis of flower shaped morphology MOFs in an ionic liquid system and their electrocatalytic application to the hydrogen evolution reaction

Abstract

An important functional material, MOF-5, with unique flower shaped morphology, which is usually synthesized through hydrothermal or solvothermal methods at high temperature and pressure with high energy consumption, was successfully prepared by a mild in situ electrochemical synthesis method in a tunable ionic liquid (IL) system. In the reaction, H₂BDC (BDC = 1,4-benzene-dicarboxylate) was chosen as the organic ligand, and the ionic liquid was Bmim (Bmim = 1-butyl-3-methylimidazole) bromine which functioned as a templating agent. The π–π stacking interaction between the imidazole groups, and the ionic band between the Zn²⁺ and Cl⁻, cause the directional arrangement of the MOF-5 crystal. Results show that the reaction results in a more perfect MOF-5 crystalline phase in comparison to other methods. The product, MOF-5(IL), presents a distinctive flower shaped morphology with a diameter of about 10 microns, and possesses a homogeneous morphology, stable structure and high thermal stability (up to 380 °C in N₂ atmosphere). The electrochemical reaction in the ionic liquid Bmim bromine is a quasi-reversible redox reaction. The cyclic voltammetric curve of the MOF-5(IL) modified carbon paste electrode (CPE) illustrates that the flower shaped MOF-5(IL) has a better ability to catalyze the hydrogen evolution reaction than cubic MOF-5 prepared by other methods. The electrochemical method in the ionic liquid system can also be used to synthesize other MOF materials and nanomaterials by changing the metal ions, ligands and ionic liquid types.

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1. Introduction

In recent years, metal–organic framework (MOF) materials have gained great attention in the fields of gas storage,¹ catalysis,² sensors³ and separation⁴ for their tunable, highly porous structure and active sites of different strengths. In addition, the size of their channels and cavities falls within the range of that of many molecules of interest, and many MOFs present excellent ion exchange capabilities and exciting electronic properties. The self-assembled supramolecular porous network structure of MOFs is formed through the metal–ligand complexation between the organic ligands and metal ion building blocks.⁵ The main methods used for MOF synthesis are the diffusion method and the hydrothermal (solvothermal)⁶ method. In recent years sonochemical,⁷ microwave,⁸ and mechanosynthesis⁹ methods have also been introduced in the MOF synthesis field. The complex synthesis process, high energy consumption and long reaction times, with their demands on equipment, are needed to overcome the frequently high enthalpies of formation and/or to account for the slow kinetics of nucleation. These shortcomings ensure that these methods are not in accordance with the concept of “green chemistry”. Compared with traditional methods, the electrochemical method has several advantages such as mild reaction conditions, simplicity, and ease of control, cleaning and so on. Therefore, in the field of new nanomaterial synthesis, the mild, clean electrochemical synthesis method has numerous advantages over other methods. Metal ions can be produced in situ by anodic oxidation, and anions like nitrates (from metal salts) are avoided. Denayer,¹⁰ using the electrochemical method, synthesized thin HKUST-1 layers on copper mesh without metal salts as a metal source. Gascon¹¹ also prepared some archetypical Zn²⁺, Cu²⁺ and Al³⁺ MOFs by the electrochemical method. Kulandainathan¹² reported optimized conditions for the electrochemical synthesis of Cu₃(BTC)₂. The electrochemical synthesis of MOFs has attracted more and more attention, because the electrochemical method sharply decreases the reaction time and the requirements of the reaction devices, but the above reports prepared MOF materials under a N₂ atmosphere and using weak electrical conductivity systems. Electrosynthesis of MOFs in the air and at room temperature and pressure is still a challenge for scientists.

In the field of MOF synthesis, N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), 1-methyl-2-pyrrolidone, water and ethanol are the conventional solvents widely used for dissolving both the inorganic and organic precursors. Ionic liquids, also known as molten salts,¹³ are organic substances composed of anions and cations which are liquid at room temperature. In contrast with conventional solvents, they are tunable solvents with essentially zero vapor pressure, a wide electrochemical window, nonflammability, high thermal stability, and a wide liquid range.¹⁴ These advantages mean that ionic liquids can be designed to meet the needs of specific applications by altering the physical and chemical properties. These characteristics of ionic liquids mean that they are very suitable for preparation of MOFs as an electrolyte material. As a medium for the synthesis of solid MOF materials, they will influence the final products. Just as Kwon¹⁵ reported, the cations of an ionic liquid can often function as templates and direct the framework structures, and the cations of the reagents can interact with the anions of the ionic liquid. Morris' recent paper¹⁶ clearly illustrated the formation of framework structures from the same reagents in different ionic liquids with different hydrophilicity/hydrophobicity.

MOF-5(Zn₄O(BDC)₃; BDC = 1,4-benzene-dicarboxylate) is a typical MOF material¹⁷ which is famous for its cubic geometry. Here, we describe a particular flower shaped MOF-5 prepared by in situ electrochemical synthesis in a tunable ionic liquid system. In the reaction, the ionic liquid Bmim (Bmim = 1-butyl-3-methylimidazole) bromine was chosen as a template for the synthesis of MOF-5. The reaction was carried out exposed to the air, meanwhile the use of the ionic liquid increased the electrical conductivity, and the anhydrous reaction system avoided the stripping of zinc metal deposited at the cathode. Scheme 1 shows a schematic diagram of the crystallization of MOF-5 arranged around the ionic liquid template. The π–π stacking interaction of the imidazole groups¹⁸ causes the parallel configuration of Br⁻, and the ionic band between Zn²⁺ and Br⁻ leads to the arrangement of Zn²⁺ and Br⁻. Both of the two oxygen atoms of the carboxylic ion in the BDC group connect to one zinc, and each zinc links three oxygens from three carboxylic acid ligand ions; the other four zincs share one oxygen (O²⁻), this then forms the [Zn₄O(COO)₆] structural unit (as shown in Scheme 2), with each zinc in a four ligand tetrahedral configuration. Meanwhile, each structural unit constitutes an octahedral configuration of secondary building units (SBU). These structural units are connected through organic ligands between two carboxylic acid ion groups. The coordination effect of BDC and Zn²⁺ results in the crystallization of MOF-5 according to the template configuration, and forms an infinite network. Meanwhile, to avoid the spalling of Zn produced by cathode deposition, an anhydrous ionic liquid electrolyte is optimized, which can prevent hydrogen and oxygen evolution occurring in the electrolytic process.

Scheme 1 A schematic diagram of the crystallization of MOF-5 arranged around the ionic liquid template.

Scheme 2 (a) The [Zn₄O(COO)₆] structural unit, and (b) a schematic diagram of the connection of two [Zn₄O(COO)₆] structural units.

2. Experimental

2.1. Electrochemical synthesis of MOF-5

A zinc (99.98% zinc) tablet (80 × 10 × 0.5 mm) was polished to remove the surface oxide layer using #600 sandpaper, and then washed with ethanol and distilled water. Terephthalic acid (H₂BDC 0.5 g, 3 mmol) was dissolved in 45 ml DMF, and then 0.65 g (2 mmol) zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O] was dissolved in the solution as the conductive medium. Because zinc ions are produced by anodic oxidation in the process of electrochemical synthesis, in the beginning stages of the reaction, the zinc ion concentration is very low. Zinc ions need an enrichment period before they can coordinate with organic ligands. The addition of a small amount of zinc nitrate can increase the initial concentration of zinc ions in the electrolyte and shorten the incubation period of the coordination reaction. 2.5 g Bmim bromine was added, and then the mixture was stirred in a magnetic agitator for 0.5 h. In the DC (direct current) power source, the zinc was used as the anode and the titanium sheet as the cathode, and the current density was set to 0.025 A cm⁻². As the reaction proceeded, a white flocculent substance was generated from the solution. Two hours later, the product was filtered and washed twice with DMF, and then washed twice with chloroform. The sample was dried in an oven at 80 °C. In the following discussion this sample is referred to as MOF-5(IL).

2.2. Preparation of the MOF-5 modified CPE

As reported in the literature,¹⁹ the MOF-5 modified carbon paste electrode (CPE) was prepared as follows: 0.270 g spectroscopic grade carbon powder was mixed with 0.030 g MOF-5 in ethanol, and the ethanol was then evaporated off. The resulting powder was transferred to a mortar and mixed well with 0.1 ml liquid paraffin. The electrode body was filled with the modified paste and a copper wire was used for the electrode connection. The unmodified carbon paste electrode was prepared using the same procedure without the addition of MOF-5. The paste electrode surface was simply renewed by scraping off about 2–3 mm of the old surface and polishing the new surface using a piece of tracing paper.

2.3. Characterization and instruments

For SEM analysis, the samples were calcined for 4 hours in an oven at 250 °C to remove contaminating molecules from their holes. Scanning electron micrographs were taken using a JSM-7001F system operating at 10 kV. X-ray diffraction (XRD) data were collected using a D/max-2500 power diffractometer operating at 40 kV, 100 mA and at a rate of 8° min⁻¹, using Cu Kα radiation. Energy-dispersive spectrometry (EDS) was conducted using a German QUANTAX 200 system to determine the elemental composition. Infrared (IR) spectrum analysis was performed using a FTIR-8400S Fourier-transform infrared system. Thermogravimetric analysis (TG) was carried out using a SDTA851 thermal gravimetric analyzer under a N₂ atmosphere with a heating rate of 10 °C min⁻¹. Cyclic voltammetry (CV) was carried out using the Shanghai Chen Hua CHI660D electrochemical workstation to measure the cyclic voltammetry curves of the reaction system.

3. Results and discussion

3.1. SEM results

Fig. 1 shows a SEM image of a sample prepared by electrochemical synthesis in an ionic liquid. As can be seen from the figure, MOF-5(IL) synthesized by the electrochemical method in the ionic liquid Bmim chloride presents a distinct flower shaped morphology; the diameter is about 10 micrometers. This differs significantly from the regular cubic structure of MOF-5 synthesized by conventional methods. The different morphology illustrates that different synthesis methods have a great influence on the geometry of a substance. Meanwhile, the Bmim bromine ionic liquid solvent also plays an important role in the production of the distinct flower shaped morphology. The cations of an ionic liquid often function as templates and direct the structure of the framework,¹⁵ but an increased concentration of ionic liquid is not always better; a higher concentration of ionic liquid will lead to a higher viscosity of the solution, which will cause the conductivity of the solution to decrease. In the energized condition, distortion from the conventional cubic configuration of MOF-5 occurs, which may be due to the movement of ions leading to an oriented arrangement of Zn²⁺ and BDC²⁻ in the direction of the electric field. The electric current causing the configuration of the anion and cation is different from other synthesis methods which induce morphology differences in the synthetic products. The combination of the electric current and the Bmim bromine ionic liquid creates the unique flower shaped morphology.

Fig. 1 SEM images of MOF-5(IL).

3.2. XRD analysis

As can be seen from Fig. 2, the XRD pattern of MOF-5(IL) prepared by the electrochemical method is mostly the same as that of MOF-5 synthesized through the traditional method.²⁰ The correspondence of the four main peaks (6.8°, 9.7°, 13.7° and 15.4°) illustrates that MOF-5 can be successfully prepared in an ionic liquid system by the electrochemical method. The sharp peaks with strong intensity reveal the good crystallinity and size homogeneity of the product.¹⁷ In contrast with the spectrum of MOF-5, there is a distinct split at 9.7° in the MOF-5(IL) spectrum. The split may be due to the distortion of the cubic symmetry structure,²¹ which is consistent with the SEM images showing a calabash structure. The peaks at 31.5°, 34.6° and 36.1° show that there is a trace amount of ZnO doped in the samples.

Fig. 2 The XRD spectra of MOF-5(IL).

3.3. IR analysis

The IR spectra of MOF-5 and MOF-5(IL) are shown in Fig. 3. The peak shapes and positions in the two spectra are similar, further illustrating that MOF-5 was successfully synthesized by the electrochemical method in an ionic liquid system. The peaks observed at 1501 cm⁻¹ and 1588 cm⁻¹ are the asymmetric stretching vibrations of the carboxylic acid groups in BDC, and the peak at 1388 cm⁻¹ is the symmetrical stretching vibration of that group.²² The peak at 1640 cm⁻¹ is attributed to the hydroxyl group absorption band of water in the KBr window. Peaks in the range of 1284–730 cm⁻¹ can be attributed to the in-plane vibration of the BDC group. The two peaks in the range of 750–800 cm⁻¹ are =CH aromatic plane bends, which show that the phenyl ring is 1,4-substituted. Absorption peak shapes and peak intensities show slight differences in the spectra of the two substances at the same wavenumber, which is mainly due to the different reaction conditions (temperature, solvent, reaction time, current density and so on) inducing different hydrogen bonds, coupling between the molecules, and purities of the products.

Fig. 3 The infrared spectra of MOF-5 and MOF-5(IL).

3.4. EDS results

From Fig. 4 we can see that the elemental composition of MOF-5(IL) synthesized in the ionic liquid Bmim bromine by the electrochemical method is 78.88% C, 7.77% Zn, 6.77% O, 6.25% N and 0.33% Br. The formula of MOF-5 is Zn₄O₁₃C₂₄H₁₂,¹⁶ and so the content of C atoms in the sample is 33.60% points higher than the standard 45.28%. This observation illustrates that some small solvent molecules are adsorbed in the holes of MOF-5(IL); this can be further confirmed by the EDS pattern of MOF-5(IL) which contains a certain amount of N atoms and Br atoms. From the ratio of O atoms and Zn atoms, there are small amounts of ZnO impurities in the sample, and this is consistent with the XRD spectrum.

Fig. 4 The EDS spectrum of MOF-5(IL).

3.5. TG analysis

Fig. 5 shows the TG curve of MOF-5(IL). There are two obvious weight loss processes. The first one is at 150 °C–295 °C with a mass loss of about 17%, and this can be attributed to the loss of small molecules adsorbed in the holes of MOF-5(IL). The sharp decline of about 35.8% mass loss between 380 °C and 540 °C is mainly due to the elevated temperature; the skeleton of MOF-5(IL) begins to break at 380 °C, and the sample begins to decompose. Above 540 °C the weight of sample is basically unchanged, and the final product is ZnO.

Fig. 5 The TG curve of MOF-5(IL).

3.6. The CV curve in the 1-butyl-3-methylimidazole bromine ionic liquid system

Fig. 6 shows the cyclic voltammetry curve, over the potential range −3 V to +1 V and with a scan rate of 50 mV s⁻¹, for a three-electrode system in which the reference electrode is the saturated calomel electrode in the ionic liquid electrolyte. Two oxidation peaks can be observed at potentials of −2.25 V and +0.75 V, indicating that the Zn electrode is oxidized to Zn²⁺; Zn²⁺ will be released into the reaction system, counteracting the Zn²⁺ loss caused by the coordination reaction with the organic ligand, and maintaining the Zn²⁺ balance in the reaction system.²³ The two distinct reduction peaks at about −2.0 V and −0.25 V can be attributed to the cathodic reduction process of Zn²⁺ to Zn. The two pairs of redox peaks labelled 1–1′ and 2–2′ can be ascribed to two consecutive two-electron processes involving the Zn ions.²⁴ The ratio of oxidation to reduction from the peak currents approximates to 1, so we predict that the reaction system undergoes a quasi-reversible reaction.

Fig. 6 The CV curve in the Bmim bromine ionic liquid system.

3.7. The CV curves of the CPE and the MOF-5 modified CPE in 1 M H₂SO₄ solution

Fig. 7 shows the cyclic voltammetry curves of the CPE, the MOF-5 modified CPE and the MOF-5(IL) modified CPE in 1 M H₂SO₄ solution. The working electrodes are the CPE, the MOF-5 modified CPE and the MOF-5(IL) modified CPE, respectively. The auxiliary electrode is a platinum rod and the reference is an Ag/AgCl electrode. The scanning range is from −1.0 V to +1.0 V, and the scanning rate is 10 mV s⁻¹. The hydrogen zone in the low voltage range from −1.0 V to −0.5 V occurs due to the desorption of the hydrogen atoms. The oxidation peaks Ha, Hb and Hc correspond to the oxidation desorption reaction of the hydrogen atoms. No corresponding cathodic reduction of the adsorbed hydrogen atoms produces a reduction peak, indicating that the process is irreversible. The peak currents of Hb and Hc are obviously higher than that of the Ha peak current, which means that MOF-5 and MOF-5(IL) have the ability to improve the oxidation desorption reaction of the hydrogen atoms, and MOF-5(IL) produces the greatest improvement. Meanwhile, at around −0.05 V, the CV curve of the MOF-5 modified CPE shows a distinct reduction peak while the CV curve of the pure CPE shows no obvious redox peaks, indicating that MOF-5 can catalyze the hydrogen evolution reaction.²⁵ There is no oxidation peak corresponding to peak 1, showing that the hydrogen evolution reaction is a irreversible process. Compared with the CV curve of the MOF-5 modified CPE, the CV curve of the MOF-5(IL) modified CPE has two pairs of obvious redox peaks, which means that MOF-5(IL) has a higher catalytic activity for the hydrogen evolution reaction than MOF-5 prepared by the traditional method. The intensity ratio of peaks 2–2′ and 3–3′ is nearly 1. The results show that electrochemical synthesis of MOF-5(IL) in an ionic liquid system improves the catalytic ability for the hydrogen evolution reaction compared to MOF-5 prepared by another method. There may be two reasons for this phenomenon. Firstly, the π–π stacking interactions of the ionic liquid cause a more regular channel configuration and form a flower lamella structure which can increase the contact area for the catalytic hydrogen evolution reaction;²⁶ secondly, using the electrochemical synthesis process, there are more electron holes²⁷ in the surface of the flower shaped MOF-5(IL) than the conventional cubic MOF-5, and these electron holes can cause the catalytic hydrogen evolution reaction to proceed more easily.

Fig. 7 The CV curves of the CPE and the MOF-5 modified CPE in 1 M H₂SO₄ solution.

4. Conclusions

The metal–organic framework material MOF-5 was successfully synthesized by an electrochemical method in the Bmim bromine ionic liquid system, which provides zinc nitrate as a partial metal source and terephthalic acid as an organic ligand. The π–π stacking interaction of the imidazole groups causes the parallel configuration of Br⁻, and the ionic band between Zn²⁺ and Br⁻ leads to the arrangement of Zn²⁺ and Br⁻. Then the coordination effect of BDC and Zn²⁺ results in the crystallization of MOF-5 according to the template configuration, and forms an infinite network. The sample structure, morphology, thermal stability and the electrochemical properties of the reaction system were analysis by XRD, SEM, EDS, IR, TG and cyclic voltammetry measurements.

The SEM results showed that MOF-5(IL) synthesized in the Bmim bromine system by the electrochemical method presents a distinctive flower shaped morphology; the diameter size is about 10 microns. The combination of the electric current and the Bmim bromine ionic liquid created the unique flower shaped morphology. The XRD and IR results illustrated that the crystalline phase and the functional groups of the synthesized MOF-5(IL) are consistent with reported MOF-5 data, and revealed the good crystallinity and size homogeneity of the product. The EDS results have shown that the elemental composition of MOF-5(IL) synthesized in the ionic liquid is 78.88% C, 7.77% Zn, 6.77% O, 6.25% N and 0.33% Br. The thermal stability temperature of MOF-5(IL) is 380 °C. The cyclic voltammogram of the reaction system has shown that a quasi-reversible redox reaction occurs in the Bmim bromine ionic liquid, and there are two consecutive two electron processes involving the Zn centers. The CV curve of the MOF-5(IL) modified CPE illustrated that MOF-5(IL) can significantly improve the oxidation desorption reaction of the hydrogen atoms, and that MOF-5(IL) has a better ability to catalyze the hydrogen evolution reaction than MOF-5. This kind of MOF has promising applications in gas storage, protein encapsulation, catalysis and separation. The electrochemical method in the ionic liquid system can be used to synthesize other MOF materials and nanomaterials by changing the metal ions, ligands and ionic liquid types.

Acknowledgements

This work is jointly funded by the National Natural Science Foundation of China and Shenhua Group Corp. (grant nos U1261103). The authors thank the anonymous reviewers for their helpful suggestions for the improvement of the quality of our present paper.

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