MoS₂-doped polyvinyl alcohol nanofiber films via electrospinning for high-performance triboelectric nanogenerators

Materials

PVA (Mowiol® 210, molecular weight 67,000), isopropyl alcohol (IPA), and molybdenum disulfide (MoS₂, ≥ 98%, M104967) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). The FEP film (thickness 100 um) was supplied by Dongguan Zhanyang Polymer Materials Co., Ltd. (Dongguan, China). Double-sided nickel (Ni, thickness 100 um) conductive tape and aluminum (Al, thickness 100 um) foil tape were purchased from Shenzhen Changdasheng Electronics Co., Ltd. (Shenzhen, China). All materials were used as received without further treatment.

Preparation of MoS₂-PVA films

The fabrication procedure is illustrated in Supplementary Figure 1. MoS₂ nanocrystals were dispersed in a 3:2 (v/v) IPA:deionized (DI) water mixture and ultrasonicated for 4 h in an ice bath, followed by centrifugation at 2,500 rpm for 10 min. The supernatant was vacuum-dried to obtain few-layer MoS₂ nanosheets, which showed lateral sizes of 20-30 nm and thicknesses < 4 nm by an [Supplementary Figure 2]. Separately, 9 wt.% PVA solution was prepared by dissolving PVA powder in DI water at 85 °C for 5 h. MoS₂ nanosheets (0, 15.2, 30.6, 46.4, and 62.5 mg) were added to 15 g of PVA solution to achieve 0-4 wt.% loadings, followed by 2 h stirring and 2 h ultrasonication to ensure uniform dispersion. Each mixture was stirred for 2 h and ultrasonicated for another 2 h to ensure uniform dispersion. The resulting solutions were transferred into 10 mL syringes for electrospinning.

Electrospinning was carried out for 1 h at a 0.7 mL/h feed rate and an applied voltage of 18-22 kV in 1 kV increments, under 23 ± 2 °C and 30% ± 3% RH (Relative Humidity), with a 10.5 cm needle-to-collector distance. The nanofiber mats were dried in a desiccator (20% RH) for 5 h to obtain 30 μm MoS₂-PVA ES films, labeled as x wt.% MoS₂-PVA ES films (x = 1, 2, 3, 4). MoS₂-PVA spin-coated films (20 µm) were prepared by depositing each solution onto glass substrates using a two-step spin process (500 rpm for 10 s and 1,000 rpm for 10 s), followed by overnight drying at 20% RH and peeling to obtain freestanding films. These were designated as x wt.% MoS₂-PVA spin-coated films and used as controls for comparison with MoS₂-PVA ES film-based samples and TENGs.

Fabrication of MoS₂-PVA/FEP TENG devices

MoS₂-PVA composite films and FEP membranes were used as the triboelectric positive and negative layers, respectively, to assemble vertical contact-separation-mode TENGs. ES MoS₂-PVA films were cut into 20 × 20 mm² pieces and laminated onto Ni conductive tape that had been attached to glass substrates, forming the electrode-tribolayer. The Ni tape was extended to 30 mm with an additional 10 mm overhang, wrapped in aluminum foil for external wiring. Spin-coated MoS₂-PVA films were assembled similarly to the control samples. For footwear energy harvesting, TENGs were housed between two acrylic panels (40 × 40 × 2 mm³), with glass substrates mounted on an acrylic base to define an effective contact area of 20 × 20 mm² [Supplementary Figure 3]. The assembly employed calibrated compression springs and adhesive bonding to ensure mechanical stability and durability under repetitive gait cycles.

Materials and TENG characterization

The surface morphology and elemental distribution of the MoS₂-PVA composite films were analyzed by field-emission scanning electron microscopy (FESEM, HITACHI S-4800, Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS). Samples were sputter-coated with a thin Au layer (KYKY SBC-12, Beijing, China) to improve conductivity. Raman spectra were obtained using a Horiba LabRAM Odyssey spectrometer (532 nm laser, Kyoto, Japan), and Fourier-transform infrared (FTIR) spectra were recorded with a Nicolet 5700 system (Thermo Fisher Scientific, Waltham, MA, USA). X-ray diffraction (XRD) patterns were acquired using a Shimadzu LabX XRD-6100 (Kyoto, Japan). Thermogravimetric analysis (TGA) was carried out using a TA Instruments TGA55 (New Castle, DE, USA), and differential scanning calorimetry (DSC) was performed on a TA Discovery DSC25 (New Castle, DE, USA). AFM characterization, piezoelectric charge constant (d₃₃) measurement via PFM, and KPFM surface potential mapping were carried out using a unified Bruker Multimode SPM system (Billerica, MA, USA).

Triboelectric performance was tested using a Popwil YPS-1 dynamic fatigue system (Hangzhou Popwil Physics Instrument Co., Ltd., China) to control contact force, frequency, and separation distance. Output voltage and current were measured with a Tektronix MDO 3022 oscilloscope (100 MΩ internal resistance, Beaverton, OR, USA) and a Keysight B2981A picoammeter (Santa Rosa, CA, USA), respectively. Charge density was calculated from the integration of the current. Capacitor voltages were also recorded using the Tektronix MDO 3022 oscilloscope.

A humidity-controlled environment was established using a dual-path N₂ system, with one stream bubbled through DI water. Mass flow controllers (MFCs, Sevenstar Electronics Co., Ltd., Beijing, China) regulated dry/humid flow rates to adjust RH at a total flow of 5 SLM, maintaining 28 ± 0.5 °C. RH was monitored in real time with a calibrated AS847 hygrometer (Smart Sensor, Dongguan, China), and the MD1101 humidity sensor (Zave, Shenzhen, China) was tested in a sealed chamber with uniform N₂ distribution.

Morphologies of MoS₂-PVA ES films

The microstructures of the MoS₂-PVA ES nanofiber films were analyzed by SEM, EDS, FTIR, XRD, Raman, TGA, and DSC to establish the morphology-property relationships governing triboelectric performance. Electrospinning and MoS₂ incorporation are expected to introduce structural features distinct from spin-coated films, including increased surface roughness, nanosheet-polymer interactions, and alignment-induced ordering. These detailed characterizations provide a solid foundation for correlating morphology and microstructure with surface potential modulation and piezoelectric coupling in the ES composites.

SEM images of ES nanofibers with 0-4 wt.% MoS₂ [Figure 1A-E] reveal continuous, bead-free networks. Higher magnification insets reveal distinct surface protrusions at 3-4 wt.% loading, suggesting MoS₂ aggregation at higher concentrations. AFM images [Supplementary Figure 4] further confirm the presence of granular features and increased surface roughness in these samples. Diameter distribution analysis (n = 35) in Supplementary Figure 5 shows a slight reduction in average diameter at 3-4 wt.% MoS₂, attributed to increased solution conductivity during electrospinning^[31,32]. In contrast, spin-coated MoS₂-PVA films [Supplementary Figure 6A] and pristine FEP film [Figure 1F] exhibit smooth, non-fibrous surfaces. The porous ES morphology provides a larger effective contact area, beneficial for enhanced TENG output.

Figure 1. SEM morphology and elemental analysis of MoS₂-PVA ES films. (A-E) SEM images of x wt.% MoS₂-PVA ES films (x = 0, 1, 2, 3, 4). (F) An SEM image of the FEP film. (G) SEM morphology of the EDS area. (H) EDS elemental mapping of C, O, Mo and S for the 2 wt.% MoS₂-PVA ES film. (I) Elemental content variation with different MoS₂ doping concentrations.

To evaluate the distribution of MoS₂ within the nanofiber mat, EDS elemental mapping was conducted on 0-4 wt.% MoS₂-PVA ES films. As an example, the 2 wt.% MoS₂ sample [Figure 1G and H] shows uniformity of C, O, Mo and S distributions across the mat, indicating effective dispersion of MoS₂ nanosheets during the electrospinning process. Quantitative EDS analysis [Figure 1I] further confirms the increase in Mo and S contents with doping level, rising from negligible levels at 1 wt.% (Mo: almost 0.1%, S: 0.2%) to detectable concentrations at 4 wt.% (Mo: 0.8%, S: 1.7%).

FTIR spectroscopy [Figure 2A] was used to examine the interfacial interactions between MoS₂ and the PVA matrix. Pristine PVA exhibits characteristic peaks at near 3,300 cm^-1 (O-H stretching), 2,924 and 2,853 cm^-1 (C-H stretching), 1,717 cm^-1 (C=O), 1,094 cm^-1 (C-O-C), and 829 cm^-1 (C-H bending)^[33-35], all of which are retained in the MoS₂-PVA films; no new peaks are observed, indicating no formation of new chemical bonds. A slight red shift of the O-H band to 3,304 cm^-1 with reduced intensity suggests weak hydrogen bonding or physical interactions between MoS₂ nanosheets and PVA chains^[35]. Meanwhile, Mo-S vibrational peaks at 1,103, 1,386.6, and 1,471 cm^-1 become increasingly pronounced with higher MoS₂ loading^[33], confirming the presence and progressive incorporation of MoS₂ within the polymer matrix.

Figure 2. Structural and thermal characterization of MoS₂-PVA ES films. (A) FTIR spectra. (B) XRD patterns. (C) Raman spectra. (D) TGA curves. (E) DSC endothermic thermograms. (F) DSC exothermic thermograms.

XRD analysis [Figure 2B] was used to examine the crystalline structure of the composites. Pristine MoS₂ exhibits distinct diffraction peaks at 2θ ≈ 14.3°, 32.9°, and 33.2°, corresponding to the (002), (100), and (101) planes^[33]. These MoS₂ peaks are retained in all MoS₂-PVA composites and increase in intensity with higher loading, indicating preserved MoS₂ crystallinity during electrospinning. Pristine PVA shows a broad semi-crystalline peak at 2θ ≈ 19.4°^[35], which remains essentially unchanged across all doped samples, suggesting minimal influence of MoS₂ on PVA crystallinity.

Raman spectroscopy [Figure 2C] was used to probe the structural origin of the enhanced piezoelectric response. The 2 wt.% MoS₂-PVA ES film exhibits MoS₂ E_2g¹ and A_1g peaks at 380.0 and 404.7 cm^-1, with a peak separation of ~24.7 cm^-1 corresponding to approximately 5 MoS₂ layers^[36]. Since few-layer MoS₂ shows piezoelectricity only for odd layers (1, 3, 5) due to broken inversion symmetry^[36,37], the observed layer thickness indicates the likely presence of piezoelectric-active, odd-layered nanosheets within the film. The absence of major peak shifts further confirms that MoS₂ retains its layered crystalline structure and is physically embedded in the PVA matrix without strong chemical disruption. Minor hydrogen bonding or weak physical interactions between MoS₂ nanosheets and PVA chains remain, as shown later, though they are not easily detectable by FTIR or Raman at the low MoS₂ concentrations used.

TGA analysis [Figure 2D] was conducted to evaluate the thermal stability of the MoS₂-PVA ES film. All samples display three weight loss stages: adsorbed/bound water evaporation below 150 °C, PVA backbone degradation between ~220-380 °C, and carbonization above 400 °C. At 400 °C, the weight losses are 14.62% (0 wt.%), 23.73% (1 wt.%), 35.47% (2 wt.%), 28.93% (3 wt.%), and 33.93% (4 wt.%). At 800 °C, the corresponding residual masses are 2.23%, 5.22%, 4.00%, 8.67%, and 10.92%, respectively. The overall trend indicates that MoS₂ incorporation increases both the decomposition onset temperature and the residual char yield, indicating enhanced thermal stability. The 2 wt.% sample decomposes most slowly at 400 °C but accelerates between 500-700 °C, yielding the lowest residue (4.00%). This behavior suggests that optimized MoS₂ dispersion at moderate loading initially restricts chain scission but later promotes secondary degradation and oxidation once interfacial sites become destabilized.

DSC analysis [Figure 2E and F] reveals systematic shifts in the thermal transitions of the MoS₂-PVA ES films. In the endothermic thermograms, the melting temperatures increase from 176.96 °C (0 wt.%) to 183.62 °C, 185.39 °C, 186.89 °C, and 189.10 °C for 1-4 wt.% MoS₂, indicating restricted chain mobility and stabilized crystallites due to nanosheet-polymer interactions. In the exothermic thermograms, the cold-crystallization peaks shift from 133.57 °C (0 wt.%) to 145.91 °C, 149.44 °C, 151.50 °C, and 156.51 °C for 1-4 wt.% MoS₂, accompanied by decreasing peak intensities. These trends suggest that MoS₂ acts both as a heterogeneous nucleation site during fiber formation and as a confining agent that suppresses subsequent chain rearrangement, resulting in higher melting and crystallization temperatures and enhanced thermal robustness of the composite films.

Performance of MoS₂-PVA/FEP TENGs

To evaluate the triboelectric properties of the fabricated MoS₂-PVA ES films, we constructed vertical contact-separation mode TENGs. Figure 3A shows the TENG configuration, using the MoS₂-PVA ES film and FEP film as the positive and negative triboelectric layers, respectively. The working mechanism of the TENG device [Figure 3B] is based on contact electrification and electrostatic induction principle. During the contact stage, electrons transfer from surface of the MoS₂-PVA layer to that of the FEP layer, generating charges on the surfaces of the two friction layers with opposite polarity. Upon separation, an electric potential is built up between the two triboplates which drives electron flow through the external circuit, producing an electrical output. As the separation distance increases, the potential reaches its maximum. When contact resumes, a reverse electron flow occurs, resulting in an alternating current output.

Figure 3. Structure and output performance of MoS₂-PVA-based TENGs. (A) Device structure. (B) TENG working mechanism. (C-E) Output performance of 0-4 wt.% MoS₂-PVA ES films: (C) open-circuit voltage; (D) short-circuit current density; (E) summary of voltage, current density, and charge density. (F-H) Output performance of 0-4 wt.% MoS₂-PVA spin-coated films: (F) open-circuit voltage; (G) short-circuit current density; (H) summary of voltage, current density, and charge density. (I) COMSOL simulated potential distribution of spin-coated and ES MoS₂-PVA films (0 and 2 wt.%).

To systematically investigate the effects of MoS₂ doping concentration and film structure on the output performance of TENGs, MoS₂-PVA films with varying doping levels were prepared by electrospinning and spin-coating, respectively. The tested conditions are contact force: 50 N; frequency: 2 Hz; separation distance: 3.5 mm. As illustrated in Figure 3C-E, the Voc, Jsc, and Qsc of the TENG with the MoS₂-PVA ES films show a clear increasing-decreasing trend with rising MoS₂ content. The pristine PVA ES film exhibits a Voc of 427.2 V, a Jsc of 34.55 mA^-2, and a Qsc of 72.69 μC·m^-2. Upon doping with 1 wt.% MoS₂, the output of the TENG increases to 659.2 V, 91.13 mA·m^-2, and 118.43 μC·m^-2, respectively. The highest performance of the TENG is achieved at 2 wt.% doping, with a Voc of 994 V, Jsc of 111.03 mA·m^-2, and Qsc of 136.29 μC·m^-2. However, further raising the MoS₂ content to 3 and 4 wt.% led to a performance decline, with Voc dropping to 569.6 and 462 V, Jsc to 74.1 and 56.98 mA·m^-2, and Qsc to 106.54 and 95.37 μC·m^-2, respectively. These results suggest that moderate MoS₂ doping, especially at 2 wt.%, significantly enhances TENG performance. When the MoS₂ concentration is higher (3 to 4 wt.%), agglomeration of the nanosheets occurs. This weakens their ability to modulate surface potential and disrupts the nanosheet alignment essential for piezoelectric enhancement, thereby diminishing TENG performance.

In comparison, the spin-coated films exhibited considerably lower performance across all doping levels [Figure 3F-H]. For instance, the spin-coated films with 0, 1, 2, 3, and 4 wt.% concentrations show Voc values of 259.2, 456.9, 587.2, 497.6, and 451.6 V, Jsc values of 29.08, 42.15, 55.18, 53.85, and 47.48 mA·m^-2, and Qsc values of 46.00, 57.48, 81.07, 64.75, and 51.39 μC·m^-2, respectively. These results confirm that MoS₂ incorporation improves the output of both film types, but the ES nanofiber structures provide a substantially greater enhancement than the dense spin-coated films.

As shown in Figure 3I, COMSOL simulations visualize the potential distribution with different film structures and MoS₂ doping levels. The model used a 3.5 mm separation distance and treated the ES films as rougher effective dielectrics. Dielectric inputs (specifically the dielectric constant (ε′) and dielectric loss (D) [Supplementary Figure 7]) were taken at 4 kHz: ε′ = 3.68, D = 0.28 for pristine PVA and ε’ = 4.50, D = 0.31 for the 2 wt.% film. These values were applied to both spin-coated and ES configurations. The four panels (pristine PVA spin-coated/ES and 2 wt.% MoS₂-PVA spin-coated/ES) show that MoS₂ incorporation and electrospinning-induced morphology both increase the potential difference between the triboelectric layers, consistent with the measured output enhancements. For the optimized 2 wt.% MoS₂-PVA/FEP TENG, load-matching and power density analyses [Supplementary Figure 8] show that the output peaks at a 58.5 MΩ external load, corresponding to an equivalent internal resistance of 58.5 MΩ and a maximum power density of 0.57 mW·cm^-2. Integration of the instantaneous power profile yields a single-cycle energy of 6.33 × 10^-6 J. Comparative data compiled in Supplementary Tables 1 and 2 indicate that the 2 × 2 cm² MoS₂-PVA ES device (Voc = 994 V, Jsc = 111.0 mA·m^-2 means Isc = 44.4 μA, power density = 0.57 mW·cm^-2) outperforms most previously reported PVA- and MoS₂-based TENGs.

In summary, the MoS₂-PVA/FEP TENGs exhibit significantly enhanced output performance due to the synergistic effects of MoS₂ doping and ES nanofiber structure. Compared with the spin-coated pristine PVA film, the ES pristine PVA film shows increases of 1.65 times in Voc, 1.19 times in Jsc, and 1.58 times in Qsc. At 2 wt.% MoS₂, the ES PVA film-based TENG delivers a Voc of 994.0 V, a Jsc of 111.0 mA·m^-2, and a Qsc of 136.3 μC·m^-2, corresponding to 1.69, 2.01, and 1.68 times improvements over the spin-coated 2 wt.% film, respectively. Compared with the spin-coated pristine PVA device, the ES 2 wt.% MoS₂-PVA film achieves 3.8 times higher Voc, 3.8 times higher Jsc, and 3.0 times higher Qsc. These results clearly demonstrate that both MoS₂ incorporation and the ES nanofiber morphology play crucial roles in boosting TENG output, offering an effective strategy for high-performance energy harvesting devices.

Influence of working conditions on TENG performance

To evaluate the operational robustness and practical adaptability, 2 wt.% MoS₂-PVA ES film-based TENG was tested under varying contact force, frequency, and spacer distance [Figure 4], with all measurements conducted at 23 ± 2 °C and 30% ± 3% RH under default conditions of 50 N, 2 Hz, and 3.5 mm unless otherwise specified.

Figure 4. Influence of working conditions on output performance of MoS₂-PVA-based TENG (2 wt.% MoS₂ doping). (A-C) Variation of (A) open-circuit voltage, (B) short-circuit current density, and (C) Combined output with contact force. (D-F) Corresponding outputs under different working frequencies. (G-I) Corresponding outputs under different spacer distances.

As shown in Figure 4A-C, increasing the contact force from 10 to 70 N enhances the output, with Voc rising from 840.0-1,071.1 V, Jsc from 82.2-118.1 mA·m^-2, and Qsc from 113.3-137.2 μC·m^-2. The improvement originates from increased effective contact area and deformation-induced capacitance, confirming the good mechanical compliance of the ES fibrous structure. The improvement originates from the enlarged effective contact area at higher impact forces, which increases surface charge generation, and from deformation-induced enhancement of interfacial capacitance that promotes more efficient charge storage and transfer.

Under fixed contact force and spacer distance, raising the frequency from 1-5 Hz [Figure 4D-F] increases Voc from 666.0-1,072.5 V and Jsc from 54.1-113.4 mA·m^-2, while Qsc remains nearly constant, indicating faster charge generation per unit time but unchanged charge transfer per cycle. This stability reflects the robust charge-generation mechanism of MoS₂-PVA ES films.

The effect of spacer distance on device performance is depicted in Figure 4G-I. As the distance increases from 1-3.5 mm, Voc increases from 354.0-994.0 V, Jsc from 36.6-111.0 mA·m^-2, and Qsc from 80.9-136.3 μC·m^-2. The increased separation enhances electrostatic potential, strengthening the driving field for charge flow. The moderate growth of Qsc suggests that distance mainly influences output intensity rather than total transferred charge.

Durability testing further confirms device stability. After 6,000 continuous cycles at 50 N, 3.5 mm, and 3 Hz, Voc remains stable at about 1,000 V [Supplementary Figure 9A], and no cracks are observed on the glass substrate [Supplementary Figure 9B]. SEM characterizations before and after cycling [Supplementary Figure 6B-F] reveal no noticeable changes in fiber morphology, with the nanofiber network remaining intact and free from collapse or fracture. These results collectively demonstrate that the MoS₂-PVA ES TENG maintains excellent mechanical integrity and triboelectric stability under prolonged operation.

Mechanisms for performance enhancement of MoS₂-PVA TENGs

To elucidate the origin of the output enhancement in ES MoS₂-PVA film-based TENGs compared with those with spin-coated MoS₂-PVA films, we systematically investigated the underlying mechanisms from both experimental and theoretical perspectives. Our results reveal that the performance arises from the synergistic contribution of three factors: the enlarged effective contact area associated with the fibrous porous structure, the elevated surface potential induced by MoS₂ incorporation and electronic structure modulation, and the enhanced piezoelectric polarization originating from nanosheet alignment and molecular-level interactions.

Structural contribution: enlarged effective contact area by electrospinning

The superior triboelectric performance of TENG with ES PVA films compared with spin-coated ones can be primarily ascribed to their distinct structural characteristics. This difference underscores the decisive role of film morphology in governing triboelectric output. Spin-coated films are dense and possess a smooth surface. In contrast, ES films exhibit a three-dimensional fibrous network with a substantially rougher surface, as further evidenced by AFM analysis (Arithmetical mean surface roughness (Ra) = 0.55 nm, Root-mean-square surface roughness (Rb) = 0.68 nm for spin-coated films vs. Ra = 255 nm, Rb = 318 nm for ES films, Figure 5A). The convex fiber surfaces are more susceptible to deformation upon contact, thereby increasing the active contact area. This structural advantage is consistently observed at different doping levels: for both pristine PVA and 2 wt.% MoS₂-PVA composites, TENGs with ES films outperform spin-coated counterparts by a significant margin, highlighting the universality of the electrospinning strategy in optimizing triboelectric interfaces. Such enhancement via structural modulation is well-aligned with previous reports^[38,39], where porous or fibrous architectures were shown to effectively increase interfacial efficiency in TENGs.

Figure 5. Surface morphology and surface potential analysis of pristine and MoS₂-doped PVA films. (A) AFM images and corresponding roughness profiles of 2 wt.% MoS₂-PVA spin-coated film (top) and 2 wt.% MoS₂-PVA ES film (bottom). (B) CPD values extracted from KPFM for spin-coated and ES films. (C) Direct comparison of CPD values between pristine PVA, 2 wt.% MoS₂-PVA films, and FEP. (D) KPFM results of pristine spin-coated PVA film: (i) potential energy profile, (ii) surface morphology, and (iii) schematic illustration of electron cloud distribution in the triboelectric pair before and after contact with FEP. (E) Corresponding results for pristine PVA ES film. (F) Corresponding results of 2 wt.% MoS₂-PVA spin-coated film. (G) Corresponding results of 2 wt.% MoS₂-PVA ES film.

Piezoelectric contribution: nanosheet alignment and molecular-level interactions

Piezoelectric polarization plays a crucial role in enhancing TENG performance because the induced internal electric field supports surface charge separation and retention during the contact-separation cycles. In a hybrid piezoelectric-triboelectric system, the piezoelectric effect increases the effective surface charge density and thus improves overall output, mostly exceeding the sum of individual mechanisms^[42,43]. MoS₂ is a well-known 2D material that exhibits piezoelectricity when reduced to monolayer or few-layer form with broken inversion symmetry^[37]. In this work, few-layer MoS₂ nanosheets were incorporated into the PVA mat, as confirmed by Raman analysis, ensuring the retention of piezoelectric activity. Given the layered nature of the nanosheets incorporated into the PVA matrix, the resulting composite films are expected to exhibit improved piezoelectric behavior. To probe this effect, PFM was employed to characterize the piezoelectric response of the ES nanofiber films, with piezoelectric constant d₃₃ extracted from the displacement-voltage (D-V) curves. The displacement and phase response of the 2 wt.% MoS₂-PVA ES film [Figure 6A and B] exhibit characteristic features of piezoelectric materials, including a butterfly-shaped displacement loop and a phase shift of approximately 180°, confirming a reversible polarization switching behavior. The d₃₃ was extracted from the slope of the D-V curve using

(2) $$ \begin{equation} \begin{aligned} \text{d}_{33}=\frac{\Delta { Displacement }}{\Delta { Voltage }} \end{aligned} \end{equation} $$

where ΔDisplacement and ΔVoltage are the variations in displacement and applied bias voltage extracted from the D–V curve, respectively.

Figure 6. Piezoelectric response and nanosheet alignment in MoS₂-PVA films. (A and B) PFM displacement and phase curves of 2 wt.% MoS₂-PVA ES films. (C) d₃₃ comparison between spin-coated and ES films. (D) COMSOL simulation of nanosheet alignment during electrospinning: (i) schematic (The electrospinning needle and collector: Created in BioRender. Chen, C. (2025) https://BioRender.com/2asz6pv), (ii) nanosheet distribution inside the needle, (iii) alignment at the needle tip. (E) COMSOL simulation of nanosheet orientation and predicted piezoelectric response along the thickness of ES films. (F) COMSOL simulation of nanosheet-induced piezoelectric response along the thickness of spin-coated films. (G) MD simulations of MoS₂-PVA interactions: (i) initial structural model, (ii) without electric field, (iii) under a high z-axis electric field. (H) Radial distribution function (RDF) analysis of atomic interactions between PVA chains and MoS₂ nanosheets.

As shown in Figure 6C, the pristine PVA films show no piezoelectric response (0.27 pm/V for ES film and 0.24 pm/V for spin-coated film can be regarded as the experimental scattering), and the incorporation of MoS₂ nanosheets markedly enhances the piezoelectric response. Upon nanosheet loading, the ES films show a sharp increase in piezoelectric coefficients, reaching 6.79, 9.63, 9.00, and 9.45 pm/V for 1, 2, 3, and 4 wt.% samples, respectively. Although the d₃₃ values generally increase with MoS₂ loading, the maximum TENG output does not coincide with the highest piezoelectric coefficient. Specifically, while the 4 wt.% MoS₂-PVA ES film reaches a relatively high d₃₃ of 9.45 pm/V, its triboelectric output remains lower than that of the 2 wt.% MoS₂ TENG. This discrepancy suggests that excessive nanosheet incorporation introduces aggregation and local field distortion, which, although enhancing nanoscale piezoelectric response, does not translate into effective macroscopic polarization along the film thickness. Consequently, the piezoelectric enhancement saturates, and its contribution to charge transfer efficiency diminishes. These findings indicate that the piezoelectric and triboelectric mechanisms act in a synergistic but composition-sensitive manner, with 2 wt.% MoS₂ representing the optimal balance. By contrast, the spin-coated films display much weaker responses, with d₃₃ values of only 1.30, 2.41, 1.21, and 1.23 pm/V at the same MoS₂ loadings. Notably, the 2 wt.% MoS₂-PVA ES film achieves the highest d₃₃ (9.63 pm/V), nearly four times greater than its spin-coated counterpart (2.41 pm/V). This significant difference in d₃₃ values between spin-coated and ES films suggests that factors beyond nanosheet content alone influence the effective piezoelectric response. Specifically, the orientation of MoS₂ nanosheets within the polymer mat plays a critical role, as the alignment along the film thickness direction can markedly enhance thickness-direction polarization^[44].

Figure 6D illustrates the formation process of nanofibers during electrospinning and the corresponding orientation and alignment changes of the incorporated MoS₂ nanosheets as the solution is ejected from the needle [Supplementary Figure 12]. It can be observed that the nanosheets tend to align preferentially along the longitudinal axis of the nanofiber during electrospinning, driven by the strong stretching and electric field. Although the fibers are generally arranged parallel to the film surface, the random stacking of fibrous layers introduces orientation components with non-negligible out-of-plane (thickness-direction) contributions. In contrast, in spin-coated films, centrifugal spreading causes most nanosheets to orient randomly within the substrate plane, leading to negligible thickness-direction polarization. It is noted that for each MoS₂ nanosheet, a vertical potential gradient is established under mechanical deformation when oriented along the thickness direction, contributing effectively to the overall piezoelectric response of the film. The presence of partial out-of-plane components in ES films enables the establishment of vertical potential gradients under mechanical deformation, thereby contributing more effectively to the overall piezoelectric response. Consequently, the effective piezoelectric component in ES films is significantly larger than that in spin-coated films.

Figure 6E and F illustrates the COMSOL simulation results for the ES and spin-coated films with 2 wt.% MoS₂ doping. They reveal a pronounced difference in the distribution and magnitude of the nanosheet-induced piezoelectric components along the film thickness. Under the COMSOL modeling conditions [Supplementary Figure 13], the maximum piezoelectric field strength in the ES film is approximately four times higher than that of the spin-coated film, consistent with the experimental observations. These findings suggest that the enhanced piezoelectric response in MoS₂-PVA ES films is largely due to mechanical stretching and electric-field-driven alignment of nanosheets during electrospinning, which promotes vertical orientation of nanosheet components along the film thickness. In contrast, spin-coated films, dominated by centrifugal forces, produce randomly distributed nanosheets, resulting in a limited contribution to thickness-direction polarization.

To gain molecular-level insight into the role of MoS₂ nanosheets in enhancing the piezoelectric and triboelectric response, MD simulations were performed [Figure 6G, with full simulation details provided in Supplementary Note 1]. In the initial structural model [Figure 6G-i], PVA chains and MoS₂ nanosheets are randomly dispersed, with a total nanosheet dipole moment of -2.54 Debye. As the system evolves in the absence of an external electric field [Figure 6G-ii], the PVA chains gradually aggregate around the nanosheet surface due to intermolecular interactions, leading to an increase in the nanosheet dipole moment to 9.43 Debye. When a strong electric field (0.05 V/Å along the z-axis) was applied to mimic the electrospinning process [Figure 6G-iii], the nanosheets exhibited a dramatic dipole enhancement, reaching 75.86 Debye, nearly an order of magnitude higher than the electric field-free state. This pronounced dipole amplification under external bias confirms the critical role of electric-field-driven alignment in promoting nanosheet polarization, consistent with the experimental and COMSOL results.

To further probe the nature of MoS₂-PVA interactions, radial distribution function (RDF) analysis was conducted [Figure 6H]. The RDF profiles reveal the correlation between Mo and O atoms, suggesting the formation of localized interactions at the nanosheet-polymer interface. Such interfacial coupling not only facilitates charge transfer but also stabilizes the nanosheet orientation under an applied field, thereby enhancing the effective piezoelectric response of the composite film. These atomic-level insights provide a molecular explanation for the experimentally observed synergy between nanosheet incorporation and electrospinning-induced alignment.

In summary, the MD simulations revealed that PVA chains adsorb and wrap around MoS₂ nanosheets, while the nanosheets undergo pronounced dipole moment amplification under an external electric field (electrospinning). Mo-O interfacial interactions further facilitate efficient polarization transfer. These molecular-level insights corroborate the COMSOL simulations and experimental results, establishing a coherent multiscale mechanism in which nanosheet alignment and interfacial coupling govern the macroscopic piezoelectric-triboelectric enhancement in MoS₂-PVA film-based TENGs. Quantitative analysis [Supplementary Note 2] indicates that 40% of the total output improvement arises from structural triboelectric enhancement and 60% from MoS₂-induced piezoelectricity, confirming that the overall performance gain originates from the synergistic rather than simply additive interaction of structural, electronic, and piezoelectric effects.

Applications of MoS₂-PVA ES film-based TENG

To validate the applicability of the TENG, a compact 20 × 20 mm² device was fabricated using a 2 wt.% MoS₂-PVA ES film paired with a FEP layer (same type of TENGs used hereafter unless specified). The device [Supplementary Figure 3] demonstrated excellent energy harvesting capabilities under ambient mechanical stimuli. Integrated with a full-wave rectification circuit [Figure 7A], the TENG delivered a stable rectified output voltage [Figure 7B]. Using this rectified output, 180 green light-emitting diodes (LEDs) connected in series were directly powered [Figure 7C]. In addition, the rectified electricity could be stored in commercial capacitors (1-100 μF). For example, a 47 μF capacitor was charged to 2 V within 141 s [Figure 7D], and the charged capacitor was able to drive a commercial calculator for approximately 10 seconds [Figure 7E].

Figure 7. Applications based on MoS₂-PVA/FEP TENG. (A) Configuration of the TENG-based energy harvesting circuit used to power LEDs or electronic devices. (B) Output voltage of the TENG after rectification. (C) 180 green LEDs powered by the rectified TENG output. (D) Charging curves of various capacitors using the TENG output. (E) A calculator powered for 1-2 s by a charged capacitor. (F) Photograph of the energy harvesting device integrated into a shoe. (G) Energy harvesting performance during jumping, running, and walking. (H) Energy harvesting during hand clapping. (I) Energy harvesting during arm bending.

For biomechanical energy harvesting, the TENG was embedded in a shoe insole and tested under different movement modes [Figure 7F]. It generated output voltages of 647.2 V during walking, 804.4 V during jumping, and reached up to 880.9 V during running [Figure 7G]. These results highlight the TENG's robust response to various mechanical inputs and its potential for wearable energy harvesting. Furthermore, the TENG was evaluated as a self-powered motion sensor. Distinct voltage outputs were recorded in response to different human actions: 119.69 and 338.18 V for slight and forceful clapping [Figure 7H], and 22.59 and 41.83 V for slight and vigorous arm swings [Figure 7I].

After demonstrating the TENG’s robust response across diverse motion modes, we further confirm its sensitivity and versatility in distinguishing motion intensity and type, suggesting strong potential for gesture recognition, fitness monitoring, and interactive human-machine interfaces. In addition, the device exhibits stable performance under humidity variation, sweat exposure (with polydimethylsiloxane (PDMS) encapsulation), and long-term mechanical deformation, together with good cytocompatibility, as detailed in Supplementary Note 3 and 4 (containing Supplementary Figures 14-18).

To extend the functionality of the MoS₂-PVA ES film-based TENG beyond direct energy harvesting and gesture sensing, we designed a self-powered wireless sensing system in which the TENG only serves as the energy source, while capacitive sensors were used as sensors for motion and humidity monitoring. The system operates on the principle of resonant frequency modulation of an LC (inductor-capacitor) circuit^[45]. When an external stimulus, such as mechanical deformation or humidity variation, alters the capacitance of the sensor, the resonant frequency of the resistor-inductor-capacitor (RLC) oscillating signal shifts accordingly. These frequency changes are wirelessly transmitted via inductive coupling and are detected at the receiver end, where Fast Fourier Transform (FFT) is employed to extract variations in frequency or amplitude. In this configuration, physical stimuli are converted into electrical signals without the need for any external power supply.

The system overview is shown in Figure 8A, and the circuit diagram is presented in Figure 8B. The system consists of a transmitter and a receiver. The transmitter consists of a capacitive sensor, an electronic switch, an inductive coil, and a TENG. For the RLC oscillation circuit, the resonant frequency, f, is governed by:

where C represents the capacitance of the sensor and L is the inductance of the transmission coil. The receiver comprises another inductive coil and a series resistor, and the output is monitored on an oscilloscope.

Figure 8. Self-powered wireless sensing system. (A) System overview. (The arm bending illustration was created with BioRender; Chen, C. (2025) https://BioRender.com/z63tmpw). (B) Circuit diagram. (C) Signals of the transmitter side at different arm bending angles. (D) Corresponding transmitted signals after FFT analysis. (E) Corresponding signals of the receiver side. (F) Corresponding received signals after FFT analysis. (G) Capacitance variation of the capacitive sensor with humidity. (The inset is the box with the humidity sensor.) (H) One transmitted signal with encoded humidity information. (I) Received signals containing humidity information after FFT.

For wireless biomechanical sensing, a MoS₂-PVA capacitive sensor was fabricated. The sensor consists of interdigitated electrodes (IDEs) formed on a polyethylene terephthalate substrate (100 um thickness). The 2 wt.% MoS₂-PVA solution was coated on the IDEs, forming a capacitive sensor [Supplementary Figure 19]. The capacitance of the sensor would change under pressures with high sensitivity [Supplementary Figure 20] due to the pressure-induced variation in the effective dielectric constant and thickness of the 2 wt.% MoS₂-PVA film. As shown in Figure 8A, the sensor was mounted on a human arm for motion monitoring. The transmitter-side output signals for various bending angles are presented in Figure 8C, with the corresponding FFT spectra shown in Figure 8D. The receiver-side time-domain signals and their respective FFT spectra are displayed in Figure 8E and F. Increasing the bending angle increases the capacitance, resulting in a downshift of the resonant frequency. FFT analysis reveals distinct frequency shifts in both the transmitter and receiver spectra. Notably, the receiver spectrum exhibits two resonance peaks: the lower-frequency peak corresponds to the transmitter resonance, while the higher-frequency peak originates from the receiver coil itself.

The same wireless sensing circuit was adapted for humidity detection by replacing the MoS₂-PVA@IDEs sensor with a commercial humidity-sensitive capacitor (MD1101). As ambient humidity increased, the capacitance of this sensor decreased monotonically from 188 pF at 70% RH to 171 pF at 10% RH, thereby raising the resonant frequency of the RLC circuit [Figure 8G]. The transmitter signals [Figure 8H] and corresponding FFT spectra [Figure 8I] show distinct frequency shifts, confirming the reliable and uniform response of the system to humidity variation. These results demonstrate that the MoS₂-PVA TENG can function not only as a power supply but also as an enabling element for flexible wireless sensing platforms, offering promising potential for wearable healthcare monitoring, environmental sensing, and low-power Internet of Things (IoT) applications.