Preparation and properties of bimetallic Co/Cu ZIF-67 for electrochemical application

Efforts to enhance the electrochemical properties of materials have become the focus of numerous studies because these properties are essential in various fields of application. Zeolitic imidazole framework-67 (ZIF-67) is a type of metal-organic framework (MOFs) expected to demonstrate excellent performance in electrochemical applications due to its numerous distinct properties. Consequently, various strategies and techniques have been developed to improve the electrochemical performance of ZIF-67. In this study, we employed bimetallic ZIF-67 constructed with cobalt (Co) and copper (Cu) metal ions within the imidazole frameworks. The use of bimetal is expected to increase conductivity and fine-tune the physicochemical properties of ZIF-67. Using coprecipitation methods, we synthesized both single-metal and bimetallic ZIF-67 and compared their characterizations. The addition of Cu metal ions does not alter the materials phase, ensuring compatibility with the single-metal ZIF-67 structure. However, the rhombic dodecahedron morphology of ZIF-67 shifts from a smooth to a concave and rough surface in Co/Cu ZIF-67. Furthermore, Co/Cu ZIF-67 exhibits higher peak current on their cyclic voltammetry (CV) curve by 46.15 µA. The results effectively illustrate the advantages of bimetal on ZIF-67 properties and performance. Finally, this study succesfully briefly demonstrate the potential development of Co/Cu-based ZIF-67 for various electrochemical applications.


Introduction
Electrochemical properties of materials are important characteristics because they are related to numerous applications, such as semiconductors, supercapacitors, thermoelectrics, and sensors.Good electrochemical performance depends on several material properties, such as electrical conductivity and surface area.Along with advances in nanotechnology, various materials have been developed that are suitable for modifying electrochemical devices, including noble metal nanoparticles [1,2], transition metal phosphates [3], and carbon materials [4,5].Noble metal nanoparticles exhibit superior catalytic performance, but their use is limited by high production costs.While the application of transition metal phosphates still has weaknesses due to the complicated and lengthy synthesis process [6,7].In addition, carbon material possesses a good surface area, yet its electrical conductivity still needs improvement.
Zeolitic imidazole frameworks (ZIF) are a type of metal-organic frameworks (MOFs) material.This material exhibits superior properties for electrochemical applications, including chemical stability, uniform porosity, a wide specific surface area, and easily modifiy configurations [8,9].One specific type of ZIF, namely ZIF-67, is composed of cobalt (Co) ions and imidazole-based ligands.ZIF-67 stands out as a ZIF variant with high metal content and a well-organized carbon framework [10].It has found widespread use in various electrochemical applications, as documented in several reviews [11,12].
Efforts to enhance the electrochemical properties of MOFs by incorporating second metal ions into the framework have been reported [13].This strategy involves the creation of secondary building units (SBUs) within the MOF structure, simultaneously generating synergistic effects.Numerous studies reporting on bimetallic MOFs have indicated that changes in metal ions and their proportions can potentially tune the physicochemical properties of the materials [14,15].The reported bimetallic MOFs have demonstrated property enhancements compared to their single metal counterparts.Several successfully synthesized and applied bimetallic MOFs include CoxFe-MOF for oxygen evolution [16], FeMn-MOF for water splitting [17], and FeCo/N-MOF for tetracycline degradation [18].Among the various types of metals, copper (Cu) stands out as a potential candidate for bimetallic incorporation into ZIF-67 due to its good catalytic activity [19,20].
In this study, bimetallic ZIF-67 was developed by introducing Cu 2+ ions into the Co 2+ sites within the methylimidazole framework.The bimetallic Co/Cu ZIF-67 was compared to the single metal ZIF-67 in various aspects, including phase and crystallinity, morphology, and electrochemical properties, which were obtained from XRD, SEM, and CV characterizations.This brief study aims to uncover the effects of adding Cu metal ions on the properties of ZIF-67 and explore the potential of Co/Cu ZIF-67 for various electrochemical applications

Materials
The ZIF-67 was prepared involving organic ligands in the form of 2-methylimidazole (2-MeIm) and cobalt(II) nitrate hexahydrate (Co(NO3)2•6H2O) as metal ion precursor.In addition, the bimetallic Co/Cu ZIF-67 adds copper(II) nitrate trihydrate (Cu(NO3)2•3H2O) as another metal ion precursor.All reagents were purchased from Sigma Aldrich, Singapore.The solvent used in this synthesis method was methanol, which was purchased from Merck, Germany.

Synthesis of ZIF-67 and Bimetallic Co/Cu ZIF-67
In this research, ZIF-67 was prepared via a simple coprecipitation method.First, the ligand and metal ion precursor were prepared by dissolving 8 mmol of 2-MeIm and 2 mmol of Co(NO3)2•6H2O in 30 mL of methanol separately.After both solutions were stirred for around 30 minutes until homogeneous, the 2-MeIm solution was gradually transferred to the Co(NO3)2•6H2O solution, allowing the observation of a change in the solutions color.This mixture was then stirred for an additional 60 minutes before entering the precipitation stage.Material precipitation was conducted at room temperature by allowing the solution to stand for 24 hours.The precipitated product was subsequently separated and washed several times with methanol to clean the residue until a neutral pH was achieved.The final product was dried at 60 °C overnight.The Co/Cu ZIF-67 was obtained by adding Cu(NO3)2•3H2O to a metal ion solution using the same synthesis procedure.The ratio of Co:Cu used in this study was 7:3, resulting in a metal ion solution consisting of 1.4 mmol of Co(NO3)2•6H2O and 0.6 mmol of Cu(NO3)2•3H2O in 30 mL of methanol.

Characterization and Electrochemical Measurement
Material characterization was initially conducted using an X-ray diffractometer (XRD, Rigaku Miniflex 300/600 -Japan) to identify the diffraction patterns and phases of the material.Subsequently, the material's morphology was examined using a scanning electron microscope (SEM, Hitachi SU-3500 -Japan).Finally, the electrochemical performance was measured using a PalmSens4 potentiostat from the Netherlands.The electrochemical performance evaluated in this study encompassed cyclic voltammetry (CV) with a voltage range of -1.0 to 1.2, employing a scan rate of 10 -100 Additionally, the CV curves of ZIF-67 and Co/Cu ZIF-67 were compared at a scan rate of 20 mV/s to further analyze the impact of the bimetal modification that had been implemented.

Results and Discussion
The synthesis of ZIF-67 and Co/Cu ZIF-67 produced the purple powder with slightly differences, whereas the Co/Cu ZIF-67 resulting in darker purple due to the blue color from Cu precursor.These asprepared samples were characterized by XRD to determine its crystal structure.As shown in Figure 1, both samples were having the main peaks which appeared at 2θ of 7.3°, 10.4°, 12.7°, 14.7°, 16.41°, and 18.01° and could be attributed to the (011), ( 002), ( 112), ( 022), (013), and (222).These peaks have been recognized as distinctive features of ZIF-67, consistent with findings from various studies that have investigated the synthesis of ZIF-67 [21][22][23].The incorporation of Cu 2+ did not yield a significant change on the materials diffraction pattern, as evidenced by the similar XRD spectra of Co/Cu ZIF-67 with the single-metal ZIF-67.Nevertheless, a slight difference is observed in the XRD spectra of Co/Cu ZIF-67, characterized by a finer and narrower spectrum compared to ZIF-67, indicating an increase in crystallinity.

Figure 1. XRD spectra obtained from the characterization of ZIF-67 and Co/Cu ZIF-67
The morphologies of ZIF-67 and Co/Cu ZIF-67 were characterized using SEM and produced the SEM images as shown in Figure 2. In this study, the synthesis succesfully resulting ZIF-67 with a typical rhombic dodecahedron morphology with a uniform average particle size of 519 nm (see Figures 2(a,b)).Dodecahedron is a commonly used morphology of ZIF-67, especially for electrochemical application [24,25].This morphology used because it was exposed more active sites on ZIF-67 surface which allows the enhancement of catalytic performance [26,27].However, variations in reactants and molar ratios of precursors could affect the morphology and phase [21,28].As shown in Figures 2 (c,d), Co/Cu ZIF-67 had bigger average particle size of 1.475 µm, showing less homogeneity, with a considerable number of smaller particles around ~900 nm being observed.In addition, the SEM images of Co/Cu ZIF-67 revealed particles with a concave surface, different with the smooth surface owned by ZIF-67 particles.
These observations can be attributed to the influence of Cu 2+ ions introduced into the metal-organic framework system between Co 2+ in the 2-MeIm framework.The different growth mechanism and coordination preferences to ligands of each metal ions leads to this morphological transformation.The bimetallic Co/Cu ZIF-67 has successfully synthesized ZIF with well-maintained rhombic dodecahedron morphology with bimetal which expected to lead the electrochemical ehancement.The electrochemical measurement was firstly performed by measure the CV curve at ten different scan rates in the range of 10 -100 mV/s.This stage aims to determine oxidation-reduction (redox) the peak response and materials stability to the changes in scan rate.Figures 3 (a) and (b) illustrate that both ZIF-67 and Co/Cu ZIF-67 yield an increase in current along with higher scan rate.On the other hand, the peaks position respect to the x-axis showing the slightly shifted to the right and left for oxidation and reduction CV peaks, respectively.At higher scan rate, more ions are reaching the electrode surface even though some of them are not contributing to the reaction.Consequently, does not enough time for electrolyte ions to be involved in the charge transfer reaction, resulting in different redox peak positions [29].Generally, changes in CV peaks that occur in ZIF-67 and Co/Cu ZIF-67 are still indicating both samples having a good stability.
The effect of bimetal on the electrochemical performance of ZIF-67 was confirmed by comparing the CV curves of the two samples at a scan rate of 20 mV/s, as shown in Figure 3 (c).It was observed that the peak current of Co/Cu ZIF-67 (46.15 µA) was higher than that of single-metal ZIF-67 (39.61 µA), indicating better electron transfer rate and electrocatalytic activity.Numerous studies have reported that the incorporation of second metal ions in MOFs can lead to excellent conductivity.Dolgopolova et al. (2017) reported a DFT study of 1,3,5-benzenetriccarboxylate-based MOF that incorporated Cu and Co metal ions, resulting in higher conductivity due to the decrease in band gap [30].Another explanation suggests that the exchange of metal ions in bimetallic MOFs provides more free charge carriers and increased interlayer distance, subsequently enhancing the conductivity [13,31].Furthermore, Lu et al. (2022) reported a significant increase in Brunauer-Emmett-Teller surface area and pore volume in Co/Cu-based MOFs, reaching 1834.71m²/g and 0.788 cm³/g, respectively [23].Therefore, the increase in peak current compared to single-metal ZIF-67 obtained in this study is plausible.In conclusion, the Co/Cu ZIF-67 produced in this study demonstrates great potential for electrochemical applications.

Conclusion
In this study, we successfully synthesized single metal and bimetallic ZIF-67 using coprecipitation methods.The effects of Cu metal ions on phase, morphology, and electrochemical properties have been discussed.It was confirmed that the phase of ZIF-67 did not change significantly from single metal to bimetal ZIF-67.However, the morphology transformed from smooth to concave rhombic dodecahedron with a larger particle size.Finally, the expected advantages were observed in the CV curve, where the bimetallic Co/Cu ZIF-67 produces higher current peaks, indicating better electrochemical properties.This enhancement is attributed to the conductivity improvement obtained from the incorporation of Cu metal ions.The results discussed in this study successfully demonstrate a bimetal-based strategy for enhancing the electrochemical properties of ZIF-67.In future studies, it will be necessary to conduct a more in-depth investigation of the influence of bimetal parameters on tuning the properties of ZIF-67, such as the molarity ratio and the metal ion precursor used.