Nanomaterials have been treated as effective dopants for enhancing mechanical and ion-conductive properties of polymer membranes. Among various nanomaterials, metal–organic frameworks are attracting enormous attention from researchers because of their intriguing structural and functional properties. Here we report a gentle and simple synthesis method of ZIF-8 nanomaterials, which are applied as dopants for polyvinyl alcohol composite membranes. This nanomaterials display uniform size distribution and high purity through various structural investigations. The as-prepared polymer composite membranes present enhanced mechanical and ion-conductive properties compared to pristine samples. This work provides a novel ideal on the design of nanomaterial dopants for high-performance polymer membranes.
Nanomaterials are emerging as mainstream materials in the modern world by the virtue of their special structural and functional properties, which are usually essential to specific application scenarios (1,2,3,4). Metal–organic frameworks (MOFs) are these kinds of functional nanomaterials composed of organic ligands and transitional metal centers (5,6,7). Zeolitic imidazolate framework-8 (ZIF-8) is a typical category of MOFs with high chemical stability and good functionality, which has been widely studied and applied in catalysis and energy fields (8,9,10). Even if MOF nanomaterials have been successfully utilized in many scientific fields including energy storage and conversion, their synthesis conditions are complicated and the costs are very high when compared with conventional materials (11,12,13). Thus, it is critical to develop simple synthesis method of MOF nanomaterials under gentle conditions for reducing the production costs and then obtaining widespread applications in modern industry.
In this work, a simple but effective synthesis method of ZIF-8 nanomaterials under gentle conditions has been reported, and the as-synthesized nanomaterials are verified to be an effective dopant with a capability of enhancing mechanical and ion-conductive properties of polyvinyl alcohol (PVA) membranes. It is observed that the as-prepared ZIF-8 nanomaterials present uniform size distribution according to the morphology characterizations. And the nanomaterials are investigated with high chemical purity by the structural spectra, which is important for subsequent doping processes. The ZIF-8/PVA composite membrane gives tensile modulus of 13.03 MPa and break elongation of 15.64%, whose performances are higher than those of pristine PVA sample. In addition, the composite membrane displays higher ion conductivity of 0.26 × 10−4 S cm−1 than that of pristine one of 0.14 × 10−4 S cm−1. The enhanced ion conductivity of composite membrane improves its electrochemical properties, which are greatly critical to the energy storage applications. It is hoped that this work will shed light on gentle and simple synthesis of MOFs nanomaterials for polymer composite doping applications.
2 Experimental section
Zn(NO3)·6H2O and 2-methylimidazole were bought from Millipore Sigma company. Methanol and dimethyl formamide (DMF) were obtained from Sinopharm Chemical Reagent. PVA was got from Aladdin Reagent.
2.2 Synthesis of ZIF-8 nanomaterials
First, 100 mg Zn(NO3)2·6H2O was mixed with 50 mL methanol under stirring for 2 h to form Solution 1. Second, 150 mg 2-methylimidazole was mixed with 30 mL methanol to make Solution 2. Third, Solution 2 was dropped into Solution 1 with stirring condition. Fourth, the mixture was put on desk without touching for 24 h after stirring for 1 h. Fifth, the ZIF-8 resultant was obtained after rinsing with methanol for three times and drying in vacuum condition at 70°C for 12 h.
2.3 Preparation of polymer membranes
First, 100 mg PVA materials and 3 mg ZIF-8 nanomaterials were dispersed with 10 mL DMF under ultrasonic conditions. Second, the uniform dispersion was poured on Teflon mold under 50°C. Third, the PVA/ZIF-8 membrane was peeled off from the mold after drying for 24 h in vacuum condition. The pristine PVA membrane was prepared following the same process without addition of ZIF-8 nanomaterials.
SEM and TEM results are made on SIGMA VP and FEI TALOS F200X. X-ray powder diffraction pattern is obtained from Antron-Paar TTK 450. Fourier-transform infrared spectroscopy (FTIR) spectra is made on Nicolet 6700. Strain–stress curves are conducted with AGS-X equipment. Ionic conductivity is tested by Bio-logic Potentiostat VMP3 equipment.
3 Results and discussion
3.1 Synthesis method of ZIF-8 nanomaterials
Schematic for the synthesis process of ZIF-8 nanomaterials is presented in Figure 1, including nucleation and crystallization processes. Typically, Zn(NO3)2 was mixed into methanol under ultrasonic condition for achieving uniform solution. Subsequently, the nucleation and crystallization of ZIF-8 nanomaterials were realized by adding 2-methylimidazole solution dropwise into the above solution. The reaction was made in aqueous solutions under gentle conditions at room temperature owing to the reduction of nucleation energy and thus promoting coordination organic ligands. Growth mechanism is mainly based on the formation of covalent bonds between metal ions and organic ligands. Water is not an appropriate solvent for ZIF-8 synthesis because of the instability under water solutions. Conventional synthesis of MOF nanomaterials is realized by hydrothermal methods, which always needs complicated manipulations and high temperature above 100°C with high production cost (14,15,16). The high cost and complicated synthesis processes of MOF nanomaterials will definitely impede their wide applications in catalysis and energy fields.
3.2 Morphology of ZIF-8 nanomaterials
Morphology is a key factor for evaluating the quality and purity of ZIF-8 nanomaterials, and morphology analysis is carried out with the aid of SEM and TEM techniques. The SEM images of ZIF-8 nanomaterials synthesized in this work are presented in Figure 2a and b. It is observed that the as-prepared ZIF-8 nanomaterials display homogeneous and hierarchical structure with regular polyhedron shapes. The materials also show no obvious aggregation. The size distribution is analyzed and presented in the inset. It can be found that the particle size is distributed in the range from 80 to 140 nm and mainly concentrated around 100 nm. The TEM images of ZIF-8 nanomaterials are shown in Figure 2c and d. The particle size of polyhedrons distributes uniformly with nearly the same size and is about 100 nm. The inorganic nanoparticles with uniform size and high modulus are potential dopants for polymer membranes with the purpose of improving their mechanical properties (17,18). And the hierarchical porous structures are also believed to be promising to enhance ionic concerning the properties of polymer membranes, since the polymer membranes may be utilized as solid electrolytes in electrochemical and electrocatalytic applications.
3.3 Structural characterization of ZIF-8 nanomaterials
X-ray diffraction (XRD) characterization is made to examine the chemical composition of the nanomaterials, and the corresponding XRD spectra is displayed in Figure 3a. As a result, the XRD pattern of prepared ZIF-8 nanomaterials is in accordance with the reported simulated spectra, whose typical peaks show the good crystallinity degree of ZIF-8 nanomaterials (19,20). The inset of Figure 3a presents the optical image of the nanomaterials as synthesized, and the nanomaterials are in fine powder state displaying white color. The FTIR spectra of ZIF-8 nanomaterials is shown in Figure 3b for evaluation of the chemical groups in molecular structures. The obvious band at the wavenumber of 421 cm−1 belongs to the formation of Zn–N bonds during synthesis process (21). Peak positions at wavenumbers of 3325 and 1637 cm−1 belong to the stretching and bending states of hydroxyl, and the peak value at 2222 cm−1 is attributed to the stretching vibration mode of N–H in imidazole rings (22,23).
3.4 Mechanical and ion-conductive properties of ZIF-8-doped polymer membranes
Stress–strain analysis of the polymer membranes was made to evaluate their mechanical properties. As shown in Figure 4a, the ZIF-8/PVA membrane gives higher tensile modulus and break elongation compared to pristine PVA sample. The tensile modulus of ZIF-8/PVA membrane is 13.03 MPa with a break ratio of 15.64%, and those of PVA membrane are 9.65 MPa and 13.58%, respectively. The enhanced mechanical properties are mainly due to the plastication effect of ZIF-8 polyhedrons in PVA membranes. As illustrated in Figure 4b, ionic conductivity of ZIF-8/PVA membrane is achieved at 0.26 × 10−4 S cm−1, which is nearly two times higher than that of pristine PVA sample (0.14 × 10−4 S cm−1). Based on the cyclic voltammetry (CV) curves in Figure 4c, the rectangular types indicate capacitive ion storage mechanism and the ZIF-8/PVA membrane gives higher current density. The nearly triangular shapes in charge–discharge curves in Figure 4d verify good coulombic efficiency, and the composite membrane delivers better ion storage performance than pristine PVA membrane. The results absolutely indicate that ZIF-8 nanomaterials are ideal dopants for polymer membranes, which will significantly improve mechanical and ion-storage properties. This hybrid membrane with good mechanical property and ionic conductivity has great potentials as separators for flexible electrochemical devices, including supercapacitors, batteries and fuel cells. In addition, the cost-effective preparation method will enable mass production for industrial applications.
In conclusion, a simple and gentle synthesis method of ZIF-8 nanomaterials has been reported and then the nanomaterials are supplied as effective dopants for polymer membranes. The high-quality powder of ZIF-8 nanomaterials is obtained under gentle reactive conditions and also verified as narrow size distribution and regular polyhedron shapes through morphology and structural characterizations. Since the plasticization effect of nanomaterials on polymer membranes, the composite membrane in this work displays higher mechanical and ion-conductive properties compared with the pristine samples. The mechanical and electrochemical measurements obviously verify the enhanced performances, possessing a great potential for ion kinetic applications in the future. This study presents a new design method of nanodopants for polymer membranes.
Research funding: This work was supported by the Earth Engineering Center, and Center for Advanced Materials for Energy and Environment at Columbia University.
Author contributions: Chao Lu: writing – original draft, methodology, formal analysis; Hang Xiao: formal analysis, visualization; Xi Chen: writing – review and editing, project administration, resources.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: All data generated or analyzed during this study are included in this published article.
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