Electrospinning Equipment: Electrospun Cu-MOF Derived Nanofiber Membranes for Efficient Pollutant Removal

In the context of the global water crisis, the rational utilization and protection of water resources are of utmost importance, and wastewater treatment has become a crucial link. Although traditional wastewater treatment methods can handle sewage for discharge, they struggle to meet the demand for the reuse of purified water in multiple fields. Nanofiber membranes have attracted significant attention in the field of wastewater recovery. However, common preparation methods suffer from issues such as low efficiency and high impurity content. Electrospinning technology, as an emerging method that uses an electrospinning machine to produce nanofiber substrates, can produce nanofiber substrates with a large specific surface area and high porosity, showing great potential in wastewater treatment. Nevertheless, challenges remain in selecting suitable materials for this technology and water treatment applications. Materials such as metal-organic frameworks (MOFs) can enhance the performance of polymer nanofibers, but MOF-based materials also face numerous problems in practical applications, which urgently need to be addressed.  

A research team led by Professor Jian Luan from the College of Science and Technology, Shenyang University of Chemical Technology, in collaboration with multiple institutions, published their latest research findings titled "Fabrication of Cu-MOFs derived nanofiber membranes for efficient removal of environmental pollutants" in the journal Journal of Materials Chemistry C. The team synthesized two novel copper-based metal-organic frameworks (Cu-MOFs) through the hydrothermal method. Using these as precursors, they prepared a series of uniform nanofiber membrane materials via different high-temperature calcination, sol-gel, and electrospinning technology with the help of an electrospinning device. This nanofiber membrane provides new ideas for the removal of pollutants in wastewater treatment and is expected to become an ecologically sustainable and practical method for treating industrial wastewater.

The team synthesized two novel copper-based metal-organic frameworks (Cu-MOFs) using the hydrothermal method, designated as Cu-MOF-1 and Cu-MOF-2. Taking Cu-MOFs as precursors, they obtained Cu-1-X and Cu-2-X materials through calcination at different high temperatures. Then, by using the sol-gel method and electrospinning technology, Cu-1-1000 and Cu-2-1000 were mixed with polyacrylonitrile (PAN). After processes such as spinning, drying, and washing, Cu-1-1000@PAN and Cu-2-1000@PAN nanofiber membranes were fabricated. The morphology and element distribution are shown in Figure 1, indicating that the derived materials were successfully loaded onto the surface of PAN, and Cu-2-1000@PAN exhibited better uniformity.

Figure 1 (a) and (b) SEM images of Cu-1-1000@PAN. (c)-(e) EDX images of Cu-1-1000@PAN. (f) and (g) SEM images of Cu-2-1000@PAN. (h)-(j) EDX images of Cu-2-1000@PAN.

During the water purification process, the Cu-1-1000@PAN and Cu-2-1000@PAN catalysts demonstrated excellent photocatalytic degradation activity. Especially in the photocatalytic degradation experiment of gentian violet (GV), the degradation rate of Cu-1-1000@PAN reached 92.66%, while that of Cu-2-1000@PAN was as high as 96.92%. In contrast, the original PAN material showed almost no obvious degradation effect on dyes during a 120-minute dark reaction and a 240-minute light reaction, further highlighting the superior performance of Cu-1-1000@PAN and Cu-2-1000@PAN (see Figure 2). Moreover, after five cycles of use, the degradation rates of these two catalysts remained above 90%, indicating good stability and reusability. This suggests that Cu-1-1000@PAN and Cu-2-1000@PAN have great potential in practical wastewater treatment applications and can effectively remove organic pollutants in water.

Figure 2 (a) Diagrams of degradation efficiency and time for Cu-1@PAN. (b)-(d) Diagrams of degradation efficiency and time for Cu-1-X@PAN (X = 600, 800, 1000). (e) Diagrams of degradation efficiency and time for Cu-2@PAN. (f)-(h) Diagrams of degradation efficiency and time for Cu-2-X@PAN (X = 600, 800, 1000).

XPS analysis revealed the presence of a CuO peak in Cu-2-1000@PAN. This indicates that in Cu-2-1000@PAN, the recombination process of electrons and holes is relatively slow, and its band gap is relatively small. A decrease in the band gap means that the photocatalytic material has enhanced sensitivity to light and can more effectively utilize light energy to drive the flow of electrons and holes, thus improving the photocatalytic performance. Specifically, the band gap of Cu-2-1000@PAN is approximately 1.55 eV, while that of Cu-1-1000@PAN is approximately 1.78 eV. The narrower band gap enables Cu-2-1000@PAN to absorb a wider range of spectra, generating more electron-hole pairs to participate in the photocatalytic reaction. Therefore, the photocatalytic performance of Cu-2-1000@PAN is superior to that of Cu-1-1000@PAN (see Figure 3).

Figure 3 (a) Band gap of Cu-2-1000@PAN. (b) Fluorescence emission spectra of Cu-2@PAN and Cu-2-X@PAN. (c) Impedance of Cu-2@PAN and Cu-2-X@PAN. (d) Cyclic stability of Cu-2-1000@PAN. (e) Other material comparison diagram.

In the radical trapping experiments, during the photocatalytic degradation process of Cu-1-1000@PAN and Cu-2-1000@PAN, ·OH (hydroxyl radicals), h⁺ (holes), and ·O₂⁻⁻ (superoxide radicals) were the main active species. In the experiments, tert-butanol (TBA), ammonium oxalate (AO), and p-benzoquinone (BQ) were used as the scavengers for ·OH, h⁺, and ·O₂⁻⁻, respectively. As shown in Figure 4, after adding these scavengers, the degradation efficiency of GV by Cu-1-1000@PAN and Cu-2-1000@PAN decreased significantly. This indicates that these active species play a crucial role in the photocatalytic reaction and can effectively degrade dye molecules such as gentian violet (GV). By precisely controlling the experimental conditions and using specific scavengers, researchers were able to accurately identify the main active species in the photocatalytic degradation process, thus gaining a deeper understanding of the photocatalytic reaction mechanism.

Figure 4 UV-vis spectra of the GV solution on Cu-1-1000@PAN in the presence of TBA (a), AO (b), and BQ (c). (d) The rates at which GV photodegrades on the Cu-1-1000@PAN when different scavengers are present. UV-vis spectra of the GV solution on Cu-2-1000@PAN in the presence of TBA (e), AO (f), and BQ (g). (h) The rates at which GV photodegrades on the Cu-2-1000@PAN when different scavengers are present.

In addition, by analyzing the energy band structure and optical properties of the catalyst, the research team proposed a possible photocatalytic mechanism (see Figure 5). Under light irradiation, the catalyst absorbs photon energy, and electrons in the valence band are excited to the conduction band, forming electron-hole pairs. These electrons and holes are separated and migrate to the surface of the catalyst under the action of an electric field, where they undergo redox reactions with the dye molecules adsorbed on the surface, thereby achieving the degradation of the dyes. The smaller band gap of Cu-2-1000@PAN enables it to more effectively absorb light energy and generate more electron-hole pairs. Meanwhile, the presence of copper ions helps to capture electrons, suppressing the recombination of electron-hole pairs and further enhancing the photocatalytic activity. Moreover, photoluminescence (PL) spectral analysis shows that Cu-1-1000@PAN and Cu-2-1000@PAN have low PL intensities, indicating a low electron-hole pair recombination rate, which is also an important factor contributing to their excellent photocatalytic performance.

Figure 5 Cu-1-1000@PAN and Cu-2-1000@PAN photocatalytic mechanism diagram of nanofiber materials.

In summary, two novel copper-based MOFs were synthesized through a simple hydrothermal synthesis method and used as precursors. Then, copper-based derived materials were further synthesized by calcination at three different temperatures, followed by electrospinning with an electrospinning device to generate membrane materials for the photocatalytic degradation of azo dyes. The results show that the nanofiber materials Cu-1-1000@PAN and Cu-2-1000@PAN have better photocatalytic performance, especially in the degradation of gentian violet (GV), with degradation rates reaching 92.66% and 96.92%, respectively. In addition, XPS and band gap analysis revealed that the presence of a CuO peak in Cu-2-1000@PAN indicates slow electron-hole recombination and a small band gap, making the photocatalytic performance of Cu-2-1000@PAN superior to that of Cu-1-1000@PAN. Radical trapping experiments showed that ·OH, h⁺, and ·O₂⁻⁻ are the main active species involved in the photocatalytic degradation of Cu-1-1000@PAN and Cu-2-1000@PAN. A possible photocatalytic mechanism was proposed by analyzing the energy band structure and optical properties of the catalyst. In the development of new photocatalysts, it is essential to further investigate the influence of the catalyst structure on the photocatalytic degradation performance, including controlling the band gap of the catalyst and managing the hole/electron recombination through the integration of interfaces and manufacturing defects. Therefore, it is beneficial to optimize the precursor MOFs by selecting different ligands and changing the coordination modes between atoms to achieve a wider range of catalyst structures. The preparation method of electrospinning is not only simple to operate but also suitable for large-scale production, with good photocatalytic efficacy, providing an environmentally friendly and practical alternative for industrial wastewater treatment.

Article source：https://doi.org/10.1039/D5TC00233A