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. 2026 Feb 7;11(7):12254–12265. doi: 10.1021/acsomega.5c11588

SnSe/PVDF-HFP Composite Fibrous Membrane with Excellent Piezoelectric and Photocatalytic Dual Properties

Xu Li †, Meng-Nan Liu †, Tong Zhang †, Gang Zheng †, Ru Li ‡, Jun Zhang †, Qing-Ru Shao §, Wen-Peng Han †,*, Yun-Ze Long †,*
PMCID: PMC12947144  PMID: 41768690

Abstract

Currently, electrospun nanofiber membranes are widely used in sensor and photocatalysis research fields. In this study, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) flexible nanofiber membranes were successfully prepared by electrospinning technology. The narrow band gap semiconductor tin selenide (SnSe) was innovatively introduced as a functional filler, and a SnSe/PVDF-HFP composite fibrous membrane with excellent piezoelectric and photocatalytic dual properties was developed. By systematically optimizing the load process parameters of SnSe, including loading methods, hydrothermal duration for crystal size control, and ultrasonic time for loading amount regulation, the piezoelectric properties of the material were significantly improved. Experimental results demonstrate that the composite fibrous membrane exhibits outstanding dual functionality: On one hand, the flexible sensor based on its piezoelectric properties achieves an output voltage of 23.1 V and a sensitivity of 440 mV–1, enabling precise monitoring of human motion. On the other hand, the piezo-photocatalytic synergistic enhancement mechanism is proposed based on kinetic studies. Under the combined action of light irradiation and ultrasonic vibration, the composite fibrous membrane as a catalyst achieves 99.0% degradation rate of methylene blue within 120 min while maintaining excellent stability and recyclability, effectively addressing the recovery challenges of traditional powder catalysts. This work not only provides a novel material design strategy for integrating piezoelectric sensing and photocatalytic dual functions but also opens innovative pathways for developing wearable flexible electronic devices and easily recoverable catalysts.

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1. Introduction

In recent years, flexible piezoelectric sensors have garnered significant attention due to their potential applications in human motion monitoring and health status tracking. − To enhance the device performance, selecting materials with excellent piezoelectric properties has become crucial. Electrospinning technology, with its ability to construct three-dimensional porous fiber networks structures, imparts membrane materials with high specific surface area and excellent breathability, − emerging as one of the core processes for preparing high-performance nanofiber membranes. − PVDF-HFP shows significant advantages in this field due to its excellent chemical stability, mechanical flexibility, and piezoelectric properties. − Research indicates that the piezoelectric sensor based on PVDF-HFP achieves a sensitivity of 2.7 V N–1 (1.08 V kPa–1) and a maximum output voltage of 10.1 V, maintaining stable performance even after 14,000 cyclic impacts. These sensors have been successfully applied in human activity monitoring and gesture recognition. Furthermore, by incorporating materials such as BaTiO3, PVDF-HFP-based nanocomposites can further enhance the piezoelectric output to 7.7 V, demonstrating broad applications in flexible electronics. Meanwhile, these materials also exhibit promising applications in the piezocatalytic degradation of organic pollutants.

Although PVDF-HFP material itself has shown excellent piezoelectric properties, the researchers further optimized its performance through polarization treatment, thermal stretching processes, and material modifications. Among these, the load modification technique significantly enhances the piezoelectric response of composites by introducing inorganic nanomaterials such as ZnO, BaTiO3, and MXene, leveraging their intrinsic piezoelectric or dielectric properties to form synergistic effects with PVDF-HFP. For example, Li et al. prepared PVDF-HFP/ZnO composite nanofiber membrane-based sensors through electrospinning technology, which exhibited a high sensitivity of 1.9 V kPa–1 in the range of 0.02–0.5 N, a rapid response of 20 ms, and an outstanding durability of over 5000 cycles. Zhang et al. innovatively adopted a codoping strategy of MXene and ZnO, successfully developing PM/PZ-type triboelectric-piezoelectric sensors by constructing double-layer, interpenetrating, or core–shell structures. The design not only improved the piezoelectric output but also realized the sensitive detection of human body motion-induced bending deformation. However, current functional fillers are predominantly limited to wide band gap semiconductors or insulators, whose insufficient visible light utilization restricts their application in optoelectronic devices. Therefore, developing novel nanofillers that combine piezoelectric enhancement effects with excellent optoelectronic performance will become a critical breakthrough point for advancing the multifunctional development of PVDF-based composites.

SnSe, as a narrow band gap semiconductor material of the IV–VI group, demonstrates unique advantages in energy and sensing fields due to its high carrier mobility, wide spectral absorption range, and potential intrinsic piezoelectric properties. − ,− Research shows that SnSe crystals prepared by mechanical exfoliation can generate giant in-plane piezoelectric effects, and their integrated self-powered sensing unit with MoS2 has outperformed traditional devices. Notably, SnSe-100 powders synthesized by the hydrothermal method exhibits multilevel structures with microscale flower-like, nanorod, and nanosheet morphologies, achieving 98.2% degradation rate in the rhodamine B photocatalytic experiment. , Compared with traditional fillers, when SnSe is combined with polymer fibers, not only does it serve as an effective stress transfer unit and nucleating agent to significantly enhance piezoelectric performance but its narrow band gap also enables the composite material to respond to visible light, creating possibilities for photocatalytic applications. However, current SnSe photocatalysts remain primarily in the powder form, facing challenges such as difficult recovery and potential secondary pollution, which limit their practical applications.

In this study, SnSe powders were innovatively loaded on the electrospun PVDF-HFP fiber membrane, and a SnSe/PVDF-HFP multifunctional composite membrane was successfully constructed. By systematically adjusting the loading process parameters of SnSe, including loading methods, hydrothermal duration to control crystal size, and ultrasonic oscillation time to adjust the loading amount, the piezoelectric performance of the composite material was optimized. Experimental results demonstrate that the composite membrane not only exhibits excellent piezoelectric response characteristics and cyclic stability, enabling real-time monitoring of human joint movements and gait signals, but also shows significant advantages in photocatalytic applications. The three-dimensional network structure ensures mass transfer efficiency, while the flexible substrate supports damage-free recovery. Combined with in situ regeneration technology, this design effectively addresses the challenges of difficult recovery and secondary pollution associated with traditional powder catalysts. This study pioneers the integration of piezoelectric sensing and piezo-photocatalytic dual functions within a single material system, both providing novel sensitive materials for wearable electronics and establishing a theoretical framework with technical paradigms for developing recyclable piezo-photocatalysts. This breakthrough opens new avenues for the development of multifunctional composite materials.

2. Experimental Section

2.1. Materials and Reagents

Selenium (Se) powders and PVDF-HFP particles (P875306, M w ∼ 400,000) were purchased from Shanghai Macklin Biochemical Co., Ltd. Tin chloride dihydrate (SnCl2·2H2O) powders were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Sodium hydroxide (NaOH) powders, analytical grade reagents anhydrous ethanol (C2H5OH), N–N dimethylformamide (DMF), and acetone (CH3COCH3) were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was purchased from Qingdao Hongqiaoxin Environmental Engineering Co., Ltd.

2.2. Synthesis of SnSe Powders

SnSe powders were fabricated using hydrothermal synthesis techniques in which Se powders and SnCl2·2H2O were used as selenium and tin sources, respectively. First, 6 g of NaOH powder was precisely weighed, followed by dissolution in 80 mL of deionized water under stirring, yielding a clear NaOH aqueous solution. Next, 2.27 g of SnCl2·2H2O powder was added to the prepared NaOH solution and completely dissolved by magnetic stirring to obtain the mixed solution. Then, 0.4 g of Se powder and the mixed solution were added to a 100 mL polytetrafluoroethylene (PTFE) reactor liner to obtain the hydrothermal reaction solution, and the reactor liner was positioned inside a stainless reaction chamber to establish the experimental setup. After the seal was tightened, it was placed in a vacuum drying oven for hydrothermal synthesis. The temperature of the vacuum drying oven was set to 130 °C, and the insulation time was set to 30 h. The cooled reaction mixture was collected after natural cooling of the autoclave and then centrifuged to separate the solid phase. Purification was achieved through three successive washings with deionized water and ethanol, ultimately yielding a purified precipitate. Finally, the precipitate was dried in a vacuum oven at 60 °C for 2 h and then ground in a mortar for 0.5 h to obtain SnSe powders. In order to investigate the effect of different hydrothermal durations, SnSe powders were synthesized under the conditions of 10 and 20 h according to the above method.

2.3. Preparation of PVDF-HFP Fibrous Membranes

The PVDF-HFP fibrous membranes were fabricated via electrospinning. First, PVDF-HFP particles were dissolved in DMF and acetone in a mass ratio of 3:10:7 (equivalent to 15:50:35 wt %) and stirred magnetically at 25 °C for 24 h to obtain a uniformly mixed PVDF-HFP precursor solution. Subsequently, the PVDF-HFP polymer solution was dispensed from a 10 mL disposable syringe (21 gauge) through a plastic steel needle featuring a 0.41 mm internal diameter during the electrospinning procedure. The electrospun fibers were deposited onto a metal roller whose surface was covered with silicone oil-coated paper, functioning as the membrane collection platform. The electrospinning system operated with the following parameters: 15 kV applied voltage, 1 mL h–1 syringe pump flow rate, and 15 cm working distance between the spinneret and 200 rpm rotating collector. Environmental controls ensured stable processing at 25 ± 5 °C temperature and 30 ± 5% relative humidity over the 8 h duration. Finally, the electrospun fibers adhered to the silicone oil-coated substrate were subsequently vacuum-dried at 60 °C for 12 h to produce the PVDF-HFP fibrous membrane.

2.4. Preparation of SnSe/PVDF-HEP Composite Fibrous Membranes

To explore the influence of the SnSe loading mode on piezoelectric characteristics of the PVDF-HFP fibrous membrane, SnSe/PVDF-HFP composite fibrous membranes were prepared using three different methods: (a) electrospinning SnSe/PVDF-HFP solution, (b) in situ hydrothermal growth of SnSe on the surface of electrospun PVDF-HFP fibers, and (c) the dip-coating method.

  • •Preparation of the SnSe/PVDF-HFP composite fibrous membrane via electrospinning SnSe/PVDF-HFP solution: first, PVDF-HFP particles were dispersed in a DMF–acetone cosolvent system in a mass ratio of 3:10:7 (equivalent to 15:50:35 wt %), and continuous magnetic stirring at ambient temperature for 24 h yielded a uniform PVDF-HFP precursor solution for subsequent electrospinning. Then, SnSe powders were added to the PVDF-HFP solution according to a certain mass ratio. Through 12 h magnetic stirring at ambient temperature, three precursor solutions containing SnSe powers at varying mass fractions (1–3 wt % relative to PVDF-HFP) were successfully formulated for electrospinning. Subsequently, SnSe/PVDF-HFP precursor solutions with different concentrations of SnSe were filled into 10 mL (21-gauge) disposable syringes (using plastic steel needles with an inner diameter of 0.41 mm). A metal roller wrapped in silicone paper was employed as the collection device for the fiber membrane, and electrospinning was carried out under conditions of 15 kV, 1 mL h–1, 15 cm, 200 rpm, 25 ± 5 °C, and 30 ± 5%. Finally, the as-spun nanofibers accumulated on silicone oil-coated substrates over 8 h were subsequently vacuum-dried at 60 °C for 12 h to fabricate SnSe/PVDF-HFP composite membranes with different SnSe concentrations.

  • •Preparation of the SnSe/PVDF-HFP composite fibrous membrane through in situ hydrothermal growth: the electrospun PVDF-HFP fibrous membranes served as substrates for the hydrothermal deposition of SnSe nanostructures, allowing SnSe to grow in situ on the PVDF-HFP fibrous membrane. And SnSe size was regulated by controlling the hydrothermal duration. SnSe/PVDF-HFP composite fibrous membranes exhibiting progressively varied SnSe crystal sizes were successfully prepared by modulating hydrothermal duration with the range from 10 to 30 h. And then, the samples were placed in a ventilated area to air-dry naturally.

  • •Preparation of the SnSe/PVDF-HFP composite fibrous membrane using the dip-coating method: first, 0.1 g of SnSe powder was added to 100 mL of anhydrous ethanol, and ultrasonic vibration was carried out for 0.5 h to evenly disperse the SnSe powders to obtain the SnSe dispersion solution. Subsequently, the electrospun PVDF-HFP fibrous membrane was immersed into SnSe dispersion solution, ultrasonic oscillation was performed for a certain period of time to make SnSe uniformly loaded on the PVDF-HFP fibrous membrane, and SnSe loading amount was regulated by controlling the ultrasonic oscillation time. SnSe/PVDF-HFP composite fibrous membranes with different SnSe loading amounts were obtained by setting the ultrasonic oscillation time as 1 min, 5 min, and 10 min, respectively. Finally, SnSe/PVDF-HFP composite fibrous membranes were dried in a vacuum oven at 50 °C for 2 h to evaporate anhydrous ethanol attached to the fibrous membranes.

2.5. Assembly of Flexible Piezoelectric Sensors

First, the SnSe/PVDF-HFP composite fibrous membranes were cut into a square of 3 cm × 3 cm. Then, conductive aluminum (Al) foil was attached to the upper and lower surfaces of the fibrous membrane as the electrode layer, and two copper wires were, respectively, connected to the electrode layer to export piezoelectric signals. Finally, polyimide (PI) tape was used for packaging to obtain the flexible piezoelectric sensor.

2.6. Piezo-Photocatalytic Degradation of Organic Dyes

In this paper, methylene blue (MB) was selected as the organic dye for the piezo-photocatalytic test. First, 2 mg of MB powder was weighed and dissolved in 100 mL of deionized water to prepare 20 mg L–1 MB solution. Then, 50 mL of MB solution was placed in a beaker for the piezo-photocatalytic test, and the amount of SnSe powders added as the catalyst was 50 mg. Considering that physical adsorption can cause organic dyes to fade, the MB solution containing SnSe powders should be placed in the dark environment for 30 min before the catalytic test to eliminate the interference of organic dyes on catalytic degradation. In order to investigate the influence of the photoelectric effect and piezoelectric properties of SnSe powders on catalytic degradation of organic dyes, the catalytic effects of SnSe powders on MB solution were tested under the three different catalytic modes of photocatalysis, piezocatalysis, and piezo-photocatalysis (light radiation, ultrasonic vibration, and simultaneous presence of light radiation and ultrasonic vibration), respectively. In the photocatalytic test, a xenon lamp served as the light source for simulating sunlight. The output power of the light radiation was 50 W, and the distance between the xenon lamp and the catalytic solution was 50 cm. In the piezocatalytic test, an ultrasonic cleaner was used as the ultrasonic source with a frequency of 99 kHz. In the piezo-photocatalytic test, a xenon lamp and an ultrasonic cleaner were used as the light source and ultrasonic source, respectively, and the test conditions were the same. During the testing process, the MB solution was extracted to be tested every 30 min. The extraction solution with SnSe powders as the catalyst required centrifugation, while the extraction solution with the fibrous membrane as the catalyst did not need to be treated. Then, quantification of organic dye degradation efficiency was achieved through UV–vis spectrophotometric analysis of the extracted solution’s concentration dynamics.

2.7. Characterizations

The microstructure and surface morphology of the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane were characterized by scanning electron microscopy (SEM, Phenom Pro G6). An X-ray diffractometer (XRD, Smart Lab) was used to determine the crystallographic structure of the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane. High-resolution TEM and selected area electron diffraction images of SnSe powder were observed using transmission electron microscopy (TEM, JEM2100F). The molecular architectures and compositions of the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane were analyzed via a Fourier transform infrared spectrometer (FTIR, Nicolet is50). The electrical signals of flexible piezoelectric sensors were tested by using a digital oscilloscope (GDS-2102). The photocurrent density of SnSe powder was obtained by an electrochemical test. An automated single fiber tester was used to test the mechanical toughness of the SnSe/PVDF-HFP fibrous membrane.

3. Results and Discussion

3.1. Crystal Structure and Microstructure of SnSe Powders

The crystal structure schematic diagram of SnSe powder prepared by hydrothermal synthesis is shown in Figure a,b. The structure of SnSe has noncentrosymmetry, whose surface charge will have an asymmetric distribution under external forces, resulting in the generation of electrical signals. Therefore, SnSe with Pnma symmetry has great potential in piezoelectric applications, especially in polarization along the armchair direction. Figure c demonstrates the SEM image of SnSe powders prepared by a hydrothermal synthesis. It can be seen that SnSe has an obvious layered structure and regular boundaries, and multiple layered structures of SnSe cluster together to form the cluster-like structure. To confirm that the high-purity SnSe powders were successfully prepared by hydrothermal synthesis, XRD analysis was performed to characterize its crystal phase. As shown in Figure d, it can be seen that the XRD diffraction peaks of SnSe can correspond one-to-one with those on the reference standard card, and the diffraction peaks of SnSe are enhanced at 2θ ≈ 31.08°. Subsequently, the high-resolution and diffraction patterns of SnSe were observed by using TEM, as shown in Figure e,f. It can be clearly seen that SnSe has an orthorhombic structure, and the small with bright spots in the diffraction pattern also indicate that SnSe has the single crystal structure. These further confirm that the high-purity single crystal SnSe has successfully been prepared through hydrothermal synthesis. As shown in Figure S1, SnSe exhibits a transient current response under periodic cycles illumination.

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(a,b) Crystal structure schematic diagrams of SnSe with Pnma symmetry; (c) SEM image of SnSe; (d) XRD pattern and standard card spectrum of SnSe; and (e,f) high-resolution TEM and electron diffraction images of SnSe.

3.2. Piezoelectric Properties of SnSe/PVDF-HFP Composite Fibrous Membranes

3.2.1. Influence of SnSe Loading Methods on Piezoelectric Properties of the SnSe/PVDF-HFP Composite Fibrous Membrane

This study successfully prepared SnSe/PVDF-HFP composite fibrous membranes using three different loading methods: electrospinning, in situ hydrothermal growth, and dip-coating. The surface morphologies and microstructures of the composite membranes are shown in Figure a–c. Obviously, the three loading methods successfully incorporated SnSe powders onto the PVDF-HFP fiber surfaces, resulting in SnSe/PVDF-HFP composite fibrous membranes. Compared with in situ hydrothermal growth and dip-coating methods, electrospinning the SnSe/PVDF-HFP solution only allowed a small amount of SnSe powders to be loaded onto the PVDF-HFP membranes. Compared to in situ hydrothermal growth, the SnSe/PVDF-HFP composite fibrous membranes obtained through the dip-coating method contain small-cluster SnSe instead of large flake-like structures. This phenomenon may be attributed to the presence of PVDF-HFP fibrous membranes during the in situ hydrothermal growth process, which inhibits the agglomeration of small SnSe particles. The open-circuit voltages of the composite fibrous membranes were tested under pressures ranging from 1 to 50 N (Figure d–f). The results demonstrated that the open-circuit voltage of the SnSe/PVDF-HFP composite fibrous membranes increased with applied pressure, indicating an enhancement in piezoelectric performance. This observation confirms that all three loading methods yielded SnSe/PVDF-HFP composite membranes with a high sensitivity to linear pressure responses.

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SEM images of SnSe/PVDF-HFP composite fibrous membranes prepared by electrospinning SnSe/PVDF-HFP solution (a), in situ hydrothermal growth (b), and dip-coating method (c); open-circuit voltages of SnSe/PVDF-HFP composite fibrous membranes prepared by electrospinning SnSe/PVDF-HFP solution (d), in situ hydrothermal growth (e), and dip-coating method (f) under different pressures; XRD (g) and FTIR (h) of the SnSe powder, PVDF-HFP fiber membrane, and SnSe/PVDF-HFP composite fiber film; and β phase content curves (i) of PVDF-HFP fiber membranes and SnSe/PVDF-HFP composite fiber membranes doped with different SnSe powders.

At the same time, in order to demonstrate the relationship between high β phase content and piezoelectric output, SnSe/PVDF-HFP composite fiber membranes with different doping amounts of SnSe powder were prepared, in which the doping amounts of SnSe powder were 0 wt %, 1, 2, and 3 wt %, respectively. As shown in Figure g, the strong and prominent diffraction peaks at 2θ ≈ 25.56°, 30.71°, 38.05°, and 49.94° are all from SnSe and the prominent diffraction peaks at 2θ ≈ 20.62°, 39.06°, and 56.4° are all from PVDF-HFP, and the results show that SnSe powder is successfully loaded onto the PVDF-HFP fiber membrane by electrospinning doping. As can be seen from Figure h, the characteristic peak of the α phase is observed at 736 cm–1 and the characteristic peak of the polar β phase is observed at 840 cm–1, which further illustrates that the SnSe/PVDF-HFP composite fiber membrane is composed of SnSe and PVDF-HFP, which also follows the results of the XRD spectrum. Subsequently, the content of the β phase in the PVDF-HFP fiber membrane and SnSe/PVDF-HFP composite fiber membrane with different SnSe powder doping amounts was calculated by the Lambert–Beer formula, and eq is shown below

F(β)=Aβ1.26Aα+Aβ 1

Among them, F(β) is the polar β phase content and A α and A β represent the absorption strength of the fiber membrane at 736 cm–1 and 840 cm–1, respectively. Figure i shows the β phase content curves of PVDF-HFP fiber membranes and SnSe/PVDF-HFP composite fiber membranes with different SnSe powder dopings. It can be seen from the figure that the β phase content of the SnSe/PVDF-HFP composite fiber membrane is significantly higher than that of the PVDF-HFP fiber membrane, and the β phase content increases with the increase of SnSe powder doping. The results showed that the doping of SnSe powder could help increase the β phase content of PVDF-HFP fiber membranes, thereby improving their piezoelectric properties.

Figure S2a shows the linear relationship comparison of output voltage of SnSe/PVDF-HFP composite fibrous membranes prepared by three loading methods: electrospinning SnSe/PVDF-HFP solution, in situ hydrothermal growth, and dip-coating under different loading pressures. As can be seen from the figure, compared with in situ hydrothermal growth and dip-coating methods, the SnSe/PVDF-HFP composite fibrous membranes prepared by electrospinning has a poor linear relationship with the load pressure. Compared with electrospinning and dip-coating methods, the SnSe/PVDF-HFP composite fibrous membranes prepared via in situ hydrothermal growth exhibit higher output voltage. However, its mechanical toughness is significantly inferior to that obtained with the other two methods due to the alkaline growth environment, which limits its practical applications, as shown in Figure S2b. Therefore, the dip-coating method proves to be more suitable for preparing SnSe/PVDF-HFP composite fibrous membranes with high piezoelectric properties.

3.2.2. Influence of SnSe Powder Size on the Piezoelectric Properties of the SnSe/PVDF-HFP Composite Fibrous Membrane

To investigate the influence of SnSe powder size on piezoelectric characteristics of SnSe/PVDF-HFP composite fibrous membrane, the size of SnSe powder was controlled by adjusting the hydrothermal duration, which is 10, 20, and 30 h, respectively. SnSe powders synthesized under different hydrothermal conditions were loaded onto PVDF-HFP fibrous membranes prepared under the same conditions using dip-coating, and SnSe/PVDF-HFP composite fibrous membranes with different SnSe powder sizes were obtained. Figure a–c shows SEM images of SnSe/PVDF-HFP composite fibrous membranes with hydrothermal durations of 10, 20, and 30 h, respectively. It is obvious that the size of SnSe powder increases significantly with the increase of hydrothermal duration, and the distribution of SnSe loaded onto the PVDF-HFP fibrous membrane through dip-coating also becomes sparse. This is due to the degree of hydrothermal synthesis intensifying with extended reaction time, causing SnSe powder to aggregate into clusters and consequently resulting in a more dispersed particle size distribution. To investigate the influence of load pressure on piezoelectric characteristics of SnSe/PVDF-HFP composite fibrous membranes containing different SnSe powder sizes, the open-circuit voltages of SnSe/PVDF-HFP composite fibrous membranes with hydrothermal durations of 10, 20, and 30 h were tested in the pressure ranges from 1 to 50 N, as shown in Figure d–f. The results show that with load pressure increases, the open-circuit voltages of SnSe/PVDF-HFP composite fibrous membranes with different SnSe powder sizes all increased, and their piezoelectric performances were improved accordingly, indicating that these three composite fibrous membranes have a good linear response to the load pressure.

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SEM images (a–c) and V oc response to pressure (d–f) of SnSe/PVDF-HFP composite fibrous membranes with hydrothermal durations of 10, 20, and 30 h, respectively.

Figure S3 shows the linear comparison of the output voltages of SnSe/PVDF-HFP composite fibrous membranes with different hydrothermal durations under different loading pressures. As shown in the figure, when the load pressure is 50 N, the output voltage of the SnSe/PVDF-HFP composite fibrous membrane with a hydrothermal duration of 20 h is the highest, followed by that of 30 h, and the output voltage of 10 h is the lowest. This is mainly due to the fact that when the hydrothermal time is extended from 20 to 30 h, the hydrothermal synthesis reaction becomes more complete, the quantity and purity of synthesized SnSe are higher, and the flake-like SnSe finds it easier to agglomerate into clusters. When the SnSe/PVDF-HFP composite fibrous membrane is subjected to pressure, the voltage output is low due to the inhomogeneous deformation of the clustered SnSe. When the hydrothermal duration is 10 h, the hydrothermal reaction is incomplete due to the short time, resulting in the inability to obtain high-purity SnSe, which affects its piezoelectric performance. The results show that compared with the hydrothermal duration of 10 and 30 h, the SnSe/PVDF-HFP composite fibrous membrane with a hydrothermal duration of 20 h has the highest sensitivity to the linear response of the load pressure.

3.2.3. Influence of SnSe Powder Loading Amount on the Piezoelectric Properties of the SnSe/PVDF-HFP Composite Fibrous Membrane

The loading amount of SnSe powder on the PVDF-HFP fibrous membrane can be controlled by adjusting the duration of dip-coating. To investigate the influence of SnSe powder loading amount on the piezoelectric properties of SnSe/PVDF-HFP composite fibrous membranes, SnSe powder with a hydrothermal duration of 20 h was first prepared. Then, it was loaded onto PVDF-HFP fibrous membranes prepared under identical conditions through dip-coating with durations of 1, 5, and 10 min, respectively. Finally, SnSe/PVDF-HFP composite fibrous membranes with different SnSe powder loading amounts were obtained. Figure a–c show SEM images of SnSe/PVDF-HFP composite membranes with dip-coating durations of 1, 5, and 10 min, respectively. The comparison revealed that a small amount of SnSe powders was loaded onto the PVDF-HFP membrane when the dip-coating duration was 1 min. When the duration increased to 5 min, the loading amount of SnSe powder significantly increased and was uniformly distributed on the membrane. However, with the ultrasonic duration reaching 10 min, excessive SnSe powders densely accumulated on the PVDF-HFP fibrous membrane. It can be seen that with the increase of dip-coating duration, the loading amount of SnSe powder on the PVDF-HFP fibrous membrane increases significantly. To investigate the influence of load pressure on piezoelectric characteristics of SnSe/PVDF-HFP composite fibrous membranes with different SnSe powder loading amounts, the open-circuit voltage of SnSe/PVDF-HFP composite fibrous membranes with dip-coating durations of 1, 5, and 10 min were tested, as shown in Figure d–f. As shown in the figure, the open-circuit voltage of SnSe/PVDF-HFP composite fibrous membranes with different SnSe powder loading amounts increases with increasing pressure, indicating that these composite fibrous membranes showed a good linear relationship with the load pressure.

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SEM images (a–c) and V oc response to pressure (d–f) of SnSe/PVDF-HFP composite fibrous membranes with dip-coating durations of 1, 5, and 10 min, respectively.

Figure S4 shows the linear comparison of output voltages of SnSe/PVDF-HFP composite fibrous membranes with different dip-coating durations under different load pressures. It can be clearly seen that the voltage output of the SnSe/PVDF-HFP composite fibrous membrane with a dip-coating duration of 5 min is significantly higher than those with 1 and 10 min durations. This is because as the dip-coating duration increases, the SnSe powder loading amount on the PVDF-HFP fibrous membrane progressively rises. When the dip-coating duration is 1 min, the amount of SnSe powder loaded on the PVDF-HFP fibrous membrane is small due to the short time and thus provided a weak piezoelectric output signal. This makes the piezoelectric performance of SnSe/PVDF-HFP composite fibrous membrane mainly come from PVDF-HFP fibrous membrane. When the dip-coating duration is extended to 5 min, the loading amount of SnSe powder increases proportionally with the extended dip-coating duration and can be uniformly distributed on the PVDF-HFP fibrous membrane. This synergistic interaction between the SnSe powder and PVDF-HFP fibrous membrane generated piezoelectric signals, significantly enhancing the piezoelectric performance of the SnSe/PVDF-HFP composite fibrous membrane. The system demonstrated a maximum output voltage of 23.1 V and achieved a sensitivity of 440 mV N1– in the range from 1 to 50 N. When the dip-coating duration is further increased to 10 min, SnSe powder accumulated densely on the PVDF-HFP fibrous membrane due to excessive loading caused by prolonged dip-coating, which affected the deformation of PVDF-HFP fibers under external force and led to a decrease in the piezoelectric properties of the SnSe/PVDF-HFP composite fibrous membrane.

In summary, the SnSe powder can be successfully loaded onto a PVDF-HFP fibrous membrane by dip-coating. Moreover, the size and loading amount of SnSe powder can be controlled by adjusting the hydrothermal duration and dip-coating duration, respectively. When the hydrothermal duration is 20 h and the dip-coating duration is 5 min, the piezoelectric performance of the SnSe/PVDF-HFP composite fibrous membrane is the best. This is because the appropriate duration of hydrothermal and dip-coating can ensure uniform loading of SnSe powder with appropriate size onto the PVDF-HFP fibrous membrane. Consequently, the SnSe/PVDF-HFP composite fibrous membrane can undergo sufficient deformation under external forces, thereby exerting the synergistic effect of the SnSe powder and PVDF-HFP fibrous membrane and improving its piezoelectric performance.

To better represent the piezoelectric performance of the SnSe/PVDF-HFP composite fibrous membrane, it was compared with other PVDF-based piezoelectric sensors in terms of voltage output, as shown in Table . Obviously, compared with other PVDF-based piezoelectric sensors, the voltage output of the piezoelectric sensor based on the SnSe/PVDF-HFP composite fibrous membrane obtained by loading the SnSe powder onto the PVDF-HFP fibrous membrane using dip-coating is higher, indicating that its piezoelectric performance is better. There are two main reasons. First, the piezoelectric voltage output of the PVDF-HFP fibrous membrane prepared using direct electrospinning SnSe/PVDF-HFP solution is significantly higher than that of other fibrous membranes prepared by dip-coating because electrospinning can improve the content of the β phase in PVDF-HFP, thus enhancing its piezoelectric performance. This is attributed to the mechanical stretching forces during the spinning process and the rapid evaporation of the acetone solvent, which facilitates the nucleation of the electroactive β-phase. − Second, SnSe powder synthesized by hydrothermal syntheses exhibits good piezoelectric properties. Uniformly sized and appropriate amounts of SnSe powders are loaded onto the PVDF-HFP fibrous membrane through dip-coating. The layered structure of SnSe enhances piezoelectric output when subjected to forces parallel to its layers, thereby further enhancing the piezoelectric performance of the SnSe/PVDF-HFP composite fibrous membrane.

1. Output Voltages of SnSe/PVDF-HFP Composite Fibrous Membrane-Based Piezoelectric Sensors versus Other PVDF-Based Sensors.
Materials Voltage (V) ref
PZT/PVDF 2.51
Graphene/PVDF 7.90
Ag–BaTiO3/PVDF 14.00
PVDF 19.20
3D-CNTs/PVDF 11.00
MnO2/PVDF 3.20
ZnO/PVDF 0.06
SnSe/PVDF-HFP 23.10 this work
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3.3. Piezo-Photocatalytic Performance of the SnSe/PVDF-HFP Composite Fibrous Membrane

The SnSe/PVDF-HFP composite fibrous membrane with the optimal piezoelectric performance was prepared by loading SnSe powder synthesized by a hydrothermal method onto a PVDF-HFP fibrous membrane through dip-coating with a hydrothermal duration of 20 h and an ultrasonic oscillation duration of 5 min. In order to further explore the performance and application of the SnSe/PVDF-HFP composite fibrous membrane in piezo-photocatalysis, the micromorphology of samples was observed using SEM. Moreover, the catalytic degradation effect of the samples on organic dye MB was also tested. Figure a shows the SEM image of a SnSe/PVDF-HFP composite fibrous membrane prepared through ultrasonic oscillation. It can be clearly seen that SnSe powders are uniformly dispersed on the PVDF-HFP fibrous membrane. When the SnSe/PVDF-HFP composite fibrous membrane is exposed to light radiation, SnSe loaded on the surface of the PVDF-HFP fibrous membrane exhibits excellent photoelectric characteristics and good piezoelectric properties, resulting in electron–hole separation and participating in redox reactions, so that the organic dye can be catalyzed and degraded. When the SnSe/PVDF-HFP composite fibrous membrane is affected by ultrasonic vibration, SnSe and PVDF-HFP fibrous membrane can synergistically generate the spatial electric field due to their good piezoelectric properties, thereby improving the catalytic degradation efficiency of organic dyes.

5.

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SEM image of the SnSe/PVDF-HFP composite fibrous membrane (a) and its degradation curves (b–d), kinetic studies (e–g), catalytic degradation constant k (h), and cyclic stability of catalysis (i) of organic dye MB under conditions of light radiation, ultrasonic vibration, and simultaneous light radiation and ultrasonic vibration.

To explore the catalytic performance of the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane, the catalytic degradation of organic dye MB was tested under conditions of light radiation, ultrasonic vibration, and simultaneous presence of light radiation and ultrasonic vibration, respectively. Figure b shows catalytic degradation curves of MB under light radiation conditions for the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane. When catalytic duration is 120 min, the catalytic degradation efficiencies of the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane are 88.7%, 1.3%, and 81.5%, respectively. Obviously, the catalytic degradation effect of SnSe powders on MB is better than that of the PVDF-HFP fibrous membrane and SnSe/PVDF-HFP composite fibrous membrane under the condition of light radiation. The main reason may be that SnSe has an excellent photoelectric effect, which causes the separation of electrons and holes under the condition of light radiation, thereby participating in the redox reaction and effectively catalyzing the degradation of MB solution. The PVDF-HFP fibrous membrane mainly causes the fading of MB solution through physical adsorption, and due to the high porosity and small fiber diameter of the fibrous membrane, the adsorption quickly saturates, resulting in low catalytic degradation efficiency of MB. For thev SnSe/PVDF-HFP composite fibrous membrane, the catalytic degradation efficiency of MB solution can be improved due to the surface loading of SnSe with photocatalytic properties. However, the PVDF-HFP membrane exhibits limited physical adsorption capacity. Additionally, the internal loading of SnSe impairs light radiation absorption, resulting in diminished photoelectric performance and consequently reduced catalytic degradation efficiency for MB solutions. Figure c shows the catalytic degradation curves of MB under ultrasonic vibration conditions for SnSe powder, the PVDF-HFP fibrous membrane, and the SnSe/PVDF-HFP composite fibrous membrane. When the catalytic duration is 120 min, the catalytic degradation efficiencies of the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane are 62.5%, 2.4%, and 85.6%, respectively. It can be seen that under ultrasonic vibration conditions, the catalytic degradation effect of the SnSe/PVDF-HFP composite fibrous membrane on MB is better than that of SnSe powder and the PVDF-HFP fibrous membrane. This is because SnSe powders have good piezoelectric properties, which can generate electrical signals under vibration conditions, thus promoting the catalytic degradation efficiency of MB. However, the catalytic degradation effect on MB of the PVDF-HFP fibrous membrane is not ideal due to the weak piezoelectric properties. In contrast, the SnSe/PVDF-HFP composite fibrous membrane is a PVDF-HFP fibrous membrane loaded with SnSe powders on the surface through dip-coating. And under the ultrasonic vibration condition, the internal electric field generated by the PVDF-HFP fibrous membrane under ultrasonic vibration promotes the charge separation of SnSe, thereby improving the catalytic degradation efficiency of MB. Figure d shows the catalytic degradation curves of MB under the simultaneous conditions of light radiation and ultrasonic vibration for SnSe powder, the PVDF-HFP fibrous membrane, and the SnSe/PVDF-HFP composite fibrous membrane. When catalytic duration is 120 min, the catalytic degradation efficiencies of the SnSe powder, PVDF-HFP fibrous membrane, and SnSe/PVDF-HFP composite fibrous membrane are 98.7%, 3.7%, and 99.0%, respectively. It can be seen that compared with the conditions of light radiation and ultrasonic vibration, the catalytic degradation efficiencies of SnSe powder and the SnSe/PVDF-HFP composite fibrous membrane are significantly improved under the simultaneous presence of light radiation and ultrasonic vibration. Especially, the degradation efficiency of the SnSe/PVDF-HFP composite fibrous membrane is as high as 99.0% when catalytic time is 120 min. This is mainly due to the synergistic effect of optoelectronics and piezoelectricity. The internal electric field generated by SnSe/PVDF-HFP composite fibrous membrane under ultrasonic vibration conditions promotes the separation of photogenerated carriers and charges generated by vibration, allowing more electrons to participate in redox reaction and significantly improving its catalytic degradation efficiency of MB.

The dynamics fitting of the catalytic degradation process under the three conditions of light radiation, ultrasonic vibration, and simultaneous presence of light radiation and ultrasonic vibration was carried out. Figure e–g shows the dynamics diagrams of the conditions of light radiation, ultrasonic vibration, and simultaneous presence of light radiation and ultrasonic vibration, respectively. The relationship between ln (c/c 0) and time was plotted using the Langmuir–Hinshelwood kinetic equation, as shown in Figure h. Equation is as follows

ln(c/c0)=−kt 2

where k represents the catalytic rate constant, c 0 represents the initial concentration of the reaction, and c represents the concentration at the reaction time t. It can be clearly seen that the k value of the SnSe/PVDF-HFP composite fibrous membrane is the highest and is 0.039 min–1 under the condition of simultaneous presence of light radiation and ultrasonic vibration. Ultrasonic vibration not only provides mechanical force to deform PVDF-HFP fibrous membranes and SnSe powders with piezoelectric properties to generate piezoelectric charges to participate in the redox reactions but also increases the contact frequency between MB and catalysts, thereby improving their catalytic degradation efficiency. To investigate the stability of the SnSe/PVDF-HFP composite fibrous membrane as a catalyst, the catalytic experiments were repeated five times on the same sample. As shown in Figure i, there was no significant difference in the degradation efficiency of the sample after five repeated catalytic degradations, indicating that the SnSe/PVDF-HFP composite fibrous membrane as the catalyst exhibits good cyclic stability. Meanwhile, compared with the catalyst of SnSe powder, the PVDF-HFP composite fibrous membrane with good recycling performance can be reused after a simple cleaning treatment, avoiding secondary pollution caused by insufficient recycling, which makes it extremely convenient for recycling and reuse. To better demonstrate the catalytic performance of the SnSe/PVDF-HFP composite fibrous membrane, the degradation efficiency and time of several different catalysts are listed in Table . Comparative analysis demonstrated that the catalytic performance of the SnSe/PVDF-HFP fibrous membrane is comparable to that of other catalysts in the current research fields. Moreover, the performance of catalysts based on the PVDF composite is significantly better than that of other catalysts, mainly due to the internal electric field generated by PVDF’s piezoelectric effect, which promotes the charge separation in the loaded substances and further improves the catalytic degradation efficiency.

2. Comparison of Catalytic Performance of Different Catalysts.

Catalysts Organic dyes Degradation rate % Catalytic time (min) ref
Bi2VO5.5 MB 84.0 240
α-Fe2O3/PVDF MB 99.5 60
BiFeO3/PVDF MB 95.0 60
CN/PVDF MB 89.0 60
FTO/BaTiO3/AgNPs MB 94.0 180
SnSe/PVDF-HFP MB 99.0 120 this work
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3.4. Applications of the SnSe/PVDF-HFP Composite Fibrous Membrane

3.4.1. Flexible Piezoelectric Sensor Based on the SnSe/PVDF-HFP Composite Fibrous Membrane for Monitoring Human Activities

The packaged flexible piezoelectric sensor based on the SnSe/PVDF-HFP composite fibrous membrane can be fixed at the finger joints of humans for monitoring bending activities. As shown in Figure a, when the finger joint is bent at 30°, 90°, and 120°, the maximum voltage output by the sensor is 2.9, 7.6, and 9.7 V, respectively. Moreover, the output voltage increases with the increase in the bending angle. In addition to the finger joints, the sensor can also monitor the activity status of the wrist and elbow joints, as shown in Figure b,c. When the wrist and elbow joints are active, the sensor outputs different voltage signals accordingly. Furthermore, the sensor can also be fixed to the sole of the shoe to monitor the activity of walking and running. As shown in Figure d–e, compared with the walking, the voltage signal output is higher and the frequency is faster in the running, so that the daily training of track and field athletes can be monitored. To ensure the stability and durability of the sensor, it underwent 2000 cycles of impact testing, as shown in Figure f. The results indicate that the sensor has good stability and durability and can adapt to frequent human activities.

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Output voltage of the finger joint (a), wrist joint (b) and elbow joint (c) with different degrees of bending; output voltage during walking (d) and running (e); and output voltage for 2000 cycles (f).

3.4.2. SnSe/PVDF-HFP Composite Fibrous Membrane for Piezo-Photocatalytic Degradation of MB

The research results show that the SnSe/PVDF-HFP composite fibrous membrane can serve as catalysts for the degradation of organic dyes and has a wide application prospect in wastewater treatment. Under the simultaneous presence of light radiation and ultrasonic vibration, the composite fibrous membrane can achieve a degradation rate of 99% for 20 mg L–1 MB catalyzed for 120 min, with a catalytic rate constant of 0.39 min–1. It significantly exceeds the simple addition of the individual photocatalytic (81.5%) and piezocatalytic (85.6%) effects. This “nonlinear enhancement” suggests that the piezoelectric field of the PVDF-HFP substrate effectively modulates the band structure of SnSe, likely facilitating the separation of photogenerated electron–hole pairs. Compared with powder catalysts, fibrous membranes can achieve 100% recovery through physical cleaning, solving the problem of secondary pollution. Meanwhile, the performance retention rate of the fibrous membrane after five cycles is more than 95.0%, indicating that it has good cyclic stability. Figure depicts an idealized conceptual model of industrial organic wastewater treatment. The model envisions a vertically integrated processing architecture designed to simulate the synergy between piezoelectric catalysis and photocatalytic techniques. In this theoretical framework, organic wastewater is pooled through the top module and undergoes efficient physicochemical degradation as it passes through the reaction chamber, eventually being output as a reusable clean water source at the bottom. This design demonstrates a potential path to a zero-discharge cycle in industrial water in the future.

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Schematic diagram for treating industrial wastewater.

While this study demonstrates the excellent physical recoverability of the SnSe/PVDF-HFP membraneeffectively solving the issue of suspended solids associated with powder catalystswe acknowledge a limitation regarding the chemical stability assessment. Specifically, although the high-performance retention (>95.0%) over five cycles implies structural integrity, we did not perform trace element analysis to quantify potential Sn or Se ion leaching. Given that “green water treatment” necessitates zero eco-toxicity, future investigations must rigorously verify that ion leaching levels remain within safety thresholds, especially under varying wastewater pH and temperature conditions, to rule out chemical secondary pollution.

4. Conclusions

In summary, the SnSe/PVDF-HFP composite fibrous membrane with dual functions of piezoelectric sensing and piezo-photocatalysis is innovatively prepared through hydrothermal synthesis, electrospinning, and dip-coating. By optimizing the dip-coating load parameters (hydrothermal synthesis for 20 h and ultrasonic dispersion for 5 min), uniform dispersion of SnSe powers in the PVDF-HFP fiber network was achieved, and the increase of the β phase content significantly enhanced the piezoelectric response of the material. The flexible piezoelectric sensor based on the composite membrane exhibits a high output voltage of 23.1 V and a sensitivity of 440 mV N–1 under the pressure of 50 N, with excellent cyclic stability, successfully achieving the real-time monitoring of human joint motion and gait signals. In the field of catalysis, the synergistic enhancement mechanism of piezoelectric sensing and photocatalysis in the SnSe/PVDF-HFP composite fibrous membrane is demonstrated: the piezoelectric polarization electric field generated by the PVDF-HFP matrix under ultrasonic vibration can significantly promote the separation efficiency of SnSe photogenerated carriers, which is significantly improved compared with single photocatalysis. Under the combined conditions of light radiation and ultrasonic vibration, the degradation rate of organic dye MB catalyzed by the composite fibrous membrane reached 99.0% (k = 0.039 min–1) after 120 min, and the performance retention rate was more than 95.0% after 5 cycles. Compared to powder catalysts, fibrous membranes can achieve 100% recovery through physical cleaning, fundamentally solving the problem of secondary pollution. This work addresses the key issues of rigidity limitations, low photoelectric conversion efficiency, and difficult recovery of 2D SnSe in flexible electronic and catalytic applications. It not only provides a universal strategy for the flexibility of 2D piezoelectric materials but also opens up a promising approach for self-powered catalytic systems, which has important application value in the fields of wearable electronics and green water treatment.

Supplementary Material

ao5c11588_si_001.pdf (357.9KB, pdf)

Acknowledgments

This work was supported by the National Natural Science Foundation of China (52273077) and the State Key Laboratory of Bio-Fibers & Eco-Textiles, Qingdao University (ZDKT202108).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c11588.

  • Photocurrent density curve of SnSe; physical properties (mechanical toughness) and linear comparison of output voltage of SnSe/PVDF-HFP composite fiber membranes (different loading pressures, different loading methods, different hydrothermal durations, different dip-coating durations) (PDF)

∥.

X.L. and M.N.L. contributed equally to this work. Xu Li: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writingoriginal draft. Meng-Nan Liu: Conceptualization, Methodology, Formal analysis, Writingoriginal draft. Tong Zhang: Resources, Data curation, Visualization, Validation. Gang Zheng: Data curation, Visualization, Validation. Ru Li: Visualization, Supervision. Jun Zhang: Validation, Supervision. Wen-Peng Han: Writingreview and editing, Project administration. Yun-Ze Long: Writingreview and editing, Funding acquisition, Supervision.

The authors declare no competing financial interest.

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Supplementary Materials

ao5c11588_si_001.pdf (357.9KB, pdf)

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