Synthesis of High‐Performance Polyvinylidene Fluoride Composites via Hydroxyl Anchoring Effect and Directional Freeze‐Drying Method

Polyvinylidene fluoride (PVDF) and its copolymers present extensive application prospects, especially in the field of wearable electronics. However, utilizing nanofillers for enhanced β‐phase and piezoelectric properties faces challenges like noncontinuous interfaces, poor compatibility between nanofillers and PVDF matrix, and the requirement of high‐voltage polarization, hindering extensive domain alignment on a large scale. Herein, a method is proposed to synthesize high‐performance PVDF composites by introducing hydroxylated barium titanate (H@BTO) nanoparticles and a directional freeze‐drying method to enhance β‐phase content and piezoelectric properties without polarization. Molecular dynamics simulations reveal robust binding interactions between Ba and F atoms along with OH surface terminations on H@BTO, facilitating hydrogen bonding within the PVDF matrix, resulting in dipole alignment and increased spontaneous polarization. The composite film achieves an 86.69% β phase content and a piezoelectric coefficient of ≈14.49 pm V−1 without electric polarization. The freeze‐drying PVDF‐H@BTO composite film paired with a PA6 membrane is used to fabricate triboelectric nanogenerator, demonstrating a current density of ≈107.5 mA m−2 and an output voltage of ≈832 V. Results demonstrate that the utilization of strong binding interactions between various atoms, the hydroxyl anchoring effect, and directional freeze‐drying method as a strategy holds promising prospects for synthesizing high‐performance piezoelectric composites.


Introduction
Piezoelectric polymers possess distinctive characteristics that differentiate them from their rigid counterparts.In contrast to traditional piezoelectric materials like ceramics, piezoelectric polymers offer numerous advantages, including flexibility, lightweight, ease of processing, and biocompatibility.Consequently, they hold significant promises for a wide range of applications, particularly in the field of wearable electronics.Polyvinylidene fluoride (PVDF) and its copolymers represent an exemplary blend of high piezoelectric properties and mechanical flexibility.They have found extensive utility in the fields of sensing, [1][2][3] transducing, [4] energy application, [5][6][7] water filtering, [8,9] and biomedical devices. [10,11]PVDF, as a highperformance ferroelectric polymer material, holds substantial value in scientific research and practical applications.It exhibits five different crystalline phases, namely α, β, γ, δ, and ε. [12] Among these phases, the β phase stands out as the most electroactive polar phase, demonstrating exceptional piezoelectric, pyroelectric, and ferroelectric properties. [13]n recent years, research on PVDF and its polymers has flourished, with extensive investigations into various approaches Polyvinylidene fluoride (PVDF) and its copolymers present extensive application prospects, especially in the field of wearable electronics.However, utilizing nanofillers for enhanced β-phase and piezoelectric properties faces challenges like noncontinuous interfaces, poor compatibility between nanofillers and PVDF matrix, and the requirement of high-voltage polarization, hindering extensive domain alignment on a large scale.Herein, a method is proposed to synthesize high-performance PVDF composites by introducing hydroxylated barium titanate (H@BTO) nanoparticles and a directional freeze-drying method to enhance β-phase content and piezoelectric properties without polarization.Molecular dynamics simulations reveal robust binding interactions between Ba and F atoms along with OH surface terminations on H@BTO, facilitating hydrogen bonding within the PVDF matrix, resulting in dipole alignment and increased spontaneous polarization.The composite film achieves an 86.69% β phase content and a piezoelectric coefficient of ≈14.49pm V À1 without electric polarization.The freeze-drying PVDF-H@BTO composite film paired with a PA6 membrane is used to fabricate triboelectric nanogenerator, demonstrating a current density of ≈107.5 mA m À2 and an output voltage of ≈832 V. Results demonstrate that the utilization of strong binding interactions between various atoms, the hydroxyl anchoring effect, and directional freeze-drying method as a strategy holds promising prospects for synthesizing high-performance piezoelectric composites.
aimed at increasing the β-phase content and crystalline phase.These endeavors encompass the incorporation of diverse nanomaterials, including carbon nanosheets, [14] PbTiO 3 nanosheets, [15] barium titanate (BaTiO 3 , BTO) nanoparticles, [16] multiwalled carbon nanotubes (MWCNTs). [17]This approach effectively enhances the output performance of PVDF-based triboelectric nanogenerators (TENGs).Furthermore, in comparison to other polymer materials, PVDF films could be synthesized by a number of well-established methods.These include phase transition annealing, [18] solvent casting, [19] copolymer development, [20] electro-spinning, [21] and the addition of nucleating fillers. [22]Moreover, some progress has been made in devising strategies for fabricating high-performance PVDF materials with enhanced β phase.For instance, Eom et al. [23] achieved oriented epitaxial growth of PVDF-TrFE films with the polarization axis perpendicular to the substrate by exploiting crystal matching between PVDF-TrFE and chitin.Zhou et al. [24] employed spincoating technique to deposit PVDF-TrFE films on a graphene/ high-index copper surface, followed by annealing to achieve epitaxial growth.Lopes et al. [25] utilized [C 2 mim][BF 4 ], a hydrophilic ionic liquid (IL), to directly induce crystallization of PVDF into the piezoelectric β phase during the melting process, resulting in a pore-free film that fully crystallizes in the β phase.Qi et al. [26] used PANI-MoS 2 to stabilize interfacial polarization in PVDF.By leveraging the interactions between Mo─S dipoles in MoS 2 , π electron clouds in PANI, and ─CH 2 dipoles in PVDF, they achieved a specific alignment of the ─CH 2 dipoles.This alignment created a stable, full-reverse planar zigzag configuration of the polarized β phase in PVDF.Additionally, methods such as strong electric field polarization, [27] mechanical stretching, [28] and electrophoretic growth [29] have been utilized to enhance piezoelectric properties of PVDF films, enabling the fabrication of high-performance PVDF film-based devices.
Despite the potential of nanofillers to enhance β-phase formation and piezoelectric properties, numerous significant challenges remain.These include the agglomeration effect of the nanofillers, the formation of noncontinuous interfaces, poor compatibility between nanofillers and PVDF matrix, limited functional group availability, the inability to achieve extensive domain alignment on a large scale within the polymer matrix, [30] and the requirement of high-voltage polarization.These complex factors collectively pose a formidable obstacle to establishing a homogeneous and long-range molecular interaction with PVDF polymer chains.Consequently, the dipole polarization process is significantly impeded, hindering the attainment of an all-trans conformation in the synthesized PVDF composites.
Here, we demonstrate a facile and efficient strategy for tailoring the local dipole moment and β phase content of piezoelectric polymer composites by introducing hydroxylated barium titanate (H@BTO) nanoparticles and a directional freeze-drying process.Molecular dynamics (MD) simulation showed the robust binding interactions between Ba and F atoms and the OH surface terminations on the H@BTO surface.This facilitates strong hydrogen bonding with the PVDF matrix, resulting in dipole alignment and increased net spontaneous polarization.Based on the results of MD simulations, we employed a directional freeze-drying process to establish a strong temperature gradient to synthesize a rough topography and to enhance the hydroxyl anchoring effect of PVDF molecular chains on the H@BTO surface.This process effectively increased the β-phase content of the composite.An optimal concentration (2.5 wt%) of H@BTO nanoparticles significantly reinforced the interfacial connection between the inorganic nanofillers and the organic polymer matrix, thereby augmenting the β-phase content (F(β)).The F(β) value of the composite film reached 86.69%, a nearly 30% higher compared to those prepared by the conventional oven baking method.Moreover, the composite film exhibited substantial piezoelectric effects, with piezoelectric coefficients (d 33 ) reaching up to ≈14.49 pm V À1 without requiring electric polarization.We fabricated a TENG using a freeze-drying PVDF-H@BTO composite film and a polyamide-6 (PA6) membrane (20 Â 20 mm 2 ).The device exhibited a short-circuit current density ( J SC ) of ≈107.5 mA m À2 , an open-circuit voltage (V OC ) of ≈ 832 V, and the surface charge density (Q SC ) of ≈183.54 μC m À2 , almost two times higher than those of the control PVDF-BTO/PA6 TENG These demonstrated the effectiveness and efficiency of our strategy to synthesize high-performance PVDF composites.

MD Simulation of PVDF Chains with BTO Nanoparticles Interactions and Hydroxyl Anchoring Effect
In order to investigate the interactions between BTO nanoparticles and PVDF polymer chains, MD simulations [31][32][33] were performed with 80 PVDF "mer" chains and varying concentrations of BTO nanoparticles, as depicted in Figure 1a.The radial distribution function (RDF), a common tool for characterizing extended molecular structures, was employed to assess the probability of finding a pair of atoms at a distance 'r' relative to a full random distribution within a specified volume.The RDF outcomes for Ba-F interactions at varying concentrations of BTO nanoparticles additions (1.0, 2.5, 5.0, and 7.5 wt%) were analyzed, as shown in Figure 1b.Evident peaks within the 2.5-3.5 Å range indicate interactions between Ba atoms within BTO nanoparticles and F atoms within the PVDF chains.Notably, at a concentration of 2.5 wt% of BTO nanoparticles, the prominent BaÀF peak signifies the strongest binding interaction between Ba and F atoms. Figure 1c illustrates the RDF between Ba, Ti, and O atoms in the BTO nanoparticles and H and F atoms in PVDF chains, with the incorporation of 2.5 wt% of BTO nanoparticles.Remarkably, the intra-molecular RDF peak corresponding to Ba─F interactions surpasses the RDF peaks associated with Ba─H, Ti─H, Ti─F, O─F, and O─H interactions.This enhanced RDF peak can be attributed to the relatively stronger affinity between Ba and F atoms compared to other atomic pairs.The interatomic distance between Ba and F atoms within PVDF is determined to be 2.9 Å, indicating a robust interaction facilitated by the Ba-F binding between BTO nanoparticles and PVDF chains.
During the surface modification of BTO nanoparticles with hydrogen peroxide, a substantial number of hydroxyl (─OH) surface terminations are generated on the BTO surface.][35] Concurrently, in line with the principles of the freeze-drying process, the presence of temperature gradients simplifies the formation of hydrogen bonds during the fabrication of PVDF composite films. [36,37]As a result, the H@BTO surface establishes hydrogen bonding and electron dipole interactions with the C-H and C-F moieties of PVDF molecules, leading to robust intermolecular binding with the PVDF polymer chains, as illustrated in Figure 1d.The localized anchoring of PVDF chains on the H@BTO surface guides the in situ arrangement and orientation of CH 2 and CF 2 moieties, transitioning from the initial random coil conformation (left side of Figure 1d) to an extended all-trans conformation (right side of Figure 1d).This transition amplifies macroscopic out-of-plane polarization and enhances piezoelectricity.
In order to comprehend the interactions between the hydroxylated BTO surface and fluoropolymer, MD simulations were conducted employing the periodic lattice of BTO with ─OH surface terminations (H@BTO) and 60 "mer" chains of PVDF, as depicted in Figure 1e.To elucidate the functionality and mechanisms of hydrogen bonds between the ─CF 2 groups of PVDF and ─OH groups, the corresponding hydroxylated BTO surface without ─OH groups (untreated BTO surfaces) was constructed as well and the same 60 "mer" chains of PVDF were simulated simultaneously as shown in Figure 1f.Evidently, after the interaction, the PVDF chains anchored on the hydroxylated BTO surface spontaneously achieved an out-of-plane polarization of 1670.34D, significantly surpassing the out-of-plane polarization of 68.14 D induced on the BTO surface without -OH groups.This result signifies that molecular interactions activated through hydrogen bonding could greatly enhance self-assembly of highly oriented PVDF chains on the hydroxylated BTO surface.
Furthermore, the phase transition becomes distinctly evident through the fluctuations in dihedral angles that govern bond conformation within the PVDF chains.In particular, the optimal torsional bond arrangement features substituents positioned at 180°(trans or T) relative to each other, as opposed to those at AE60°(gauche or G). [13]In the case of α-phase of PVDF, the continuous arrangement of four main carbon atoms forms a trans-gauche-trans-gauche (TGTG) conformation with dihedral angles of 180°(T) and 60°(G).Conversely, in β-phase of PVDF, the dihedral angles for the main carbon atoms are all trans (TTTT), each measuring 180°.As a result, a higher proportion of local trans structures within the membrane corresponds to a higher proportion of the β crystalline form.Figure 1g,h depicts the distributions of dihedral angles for the backbone C─C─C─C of PVDF chains on the H@BTO and untreated BTO surfaces, respectively.Moreover, Figure 1g presents the dihedral angles distribution of PVDF chains, predominantly near the À60°(Gauche) and 180°(Trans) configurations.Meanwhile, Figure 1h illustrates the comprehensive distribution of PVDF chains' dihedral angles, spanning the full spectrum from À180°to 180°.It is noteworthy that on the hydroxylated BTO surface, PVDF chains exhibit a higher proportion of torsional bond conformations near 180°(right side of Figure 1g), while they have a lower proportion of torsional bond conformations near À60°( left side of Figure 1g).The result indicates that the hydrogen bond-induced anchoring effect leads to a transition from twisted conformations to trans conformations.This validates the role and functionality of hydrogen bonds in polymer chain alignment and the attainment of an all-trans conformation (i.e., polar β-phase), thereby promoting spontaneous polarization and piezoelectricity in fluoropolymer composite materials.
Hence, the heightened binding interaction between Ba and F atoms, in conjunction with the hydroxyl anchoring effect on the H@BTO surface, induces a unique zigzag conformation in the carbon backbone of the PVDF chain.This conformation aligns with the characteristic features associated with the β-crystalline phase.

Morphology and EDS Characteristics of Freeze-Drying PVDF-H@BTO Composites
The microstructures of composite films based on freeze-drying PVDF-H@BTO with different compositions are illustrated in Figure 2a-e.According to the principle of directional freezedrying, [38] a strong temperature gradient is established along the vertical direction by employing different coefficients of thermal expansion on the bottom and other sides.This temperature gradient facilitates the directional crystallization of both ice crystals and PVDF, leading to vertical growth of the PVDF structure.Simultaneously, according to the MD simulation, binding interactions between Ba and F atoms along with OH surface terminations on H@BTO promote the formation of β-PVDF.As a result, this process yields a composite film characterized by spontaneous polarization, which is pivotal for its functional properties.Furthermore, during this process, the frozen solvent undergoes a phase transition directly from the solid phase to the gas phase.Consequently, the surface exhibits a relatively rough topography as depicted in these figures.Scanning electron microscopy (SEM) results (Figure 2a-e) confirm highly rough surface structures and longitudinally layered cross sections with high porosity in PVDF-H@BTO composite films prepared with varying BTO compositions.Additionally, atomic force Figure 2. Morphology characterization and EDS results of freeze-drying PVDF-H@BTO composite films.SEM images of the film's surface with varying concentrations of H@BTO nanoparticles: a) 0 wt%, b) 2.5 wt%, c) 5 wt%, d) 10 wt%, e) 25 wt%.f ) SEM image of the film's cross-section with a BTO concentration of 2.5 wt%.EDS mapping spectra of different elements in the as-prepared composite film: g) Ba, h) Ti, i) the EDS analysis of all elements within the composite film.microscope (AFM) measurements confirm significant surface roughness (Figure S1, Supporting Information).In the comparative analysis of surface roughness, films prepared through the oven baking method exhibited a roughness value of approximately 532 nm.In contrast, the freeze-drying method resulted in significantly higher surface roughness, with values averaging around 1081 nm.This notable increase in roughness for the freeze-dried films underscores the distinct impact of the fabrication technique on the surface morphology of the films.These rough structures are advantageous for increasing the contact area if it is used to construct TENG, thereby enhancing its output performance.Furthermore, Figure 2f presents cross-sectional images of the film with a 2.5 wt% BTO concentration, showing a porous structure.Importantly, films with different BTO compositions have similar thicknesses due to the identical processing conditions.From the SEM measurements, the thickness of the freeze-drying PVDF-H@BTO composite films in this work is confirmed to be approximately 150 μm.Zoom-in images can be viewed in the Figure S2, Supporting Information.
Figure 2g,h displays the energy-dispersive spectrometer (EDS) mappings of Ba and Ti elements in the as-prepared composite film with a BTO concentration of 2.5 wt%.Both Ba and Ti elements are evidently distributed within the PVDF film, suggesting that H@BTO nanoparticles are uniformly dispersed within the PVDF film during the freeze-drying process.Furthermore, Figure 2i presents the EDS spectrum of all elements in the composite film.The presence of gold elements is observable in the EDS analysis and is due to the coating of a gold layer on the surface by sputtering process for SEM characterization.Additionally, the analysis detected Ba, Ti, C, and F elements in the composite film, with atomic percentages of 0.93, 1.41, 66.72, and 18.31%, respectively.

Characteristics of Freeze-Drying PVDF-H@BTO Composite Films
The X-ray diffractometer (XRD) and Raman spectroscopy were employed to characterize the freeze-drying PVDF-H@BTO composite films with different BTO concentrations as shown in Figure 3.In a typical XRD patterns of pure PVDF, characteristic diffraction peaks at 2θ = 17.7°((100)), 18.3°((020)), and 2θ = 19.9°represent the α-phase, while peaks at 2θ = 18.5°, 19.2°, and 20.0°are indicative of the γ-phase.The peak at 20.26°is associated with the (110) plane of the β-phase.During testing, some diffraction peaks may overlap.In Figure 3a, despite partial overlapping of diffraction peaks from different crystalline phases of PVDF, two distinct peaks at 20.1°are still observable in the spectra, confirming the presence and uniform distribution of the PVDF component in the film.With the addition of BTO nanoparticles, the XRD patterns also clearly show the presence of barium titanate (BTO).Diffraction peaks at 2θ = 22.2°, 31.5°, and 38.9°correspond to the characteristic diffraction peaks of BTO, representing the (100), (101), and (111) crystal phases, respectively.Notably, as the BTO concentration gradually increases, the peak intensity representing BTO steadily rises.However, when the BTO content exceeds 25 wt%, there is a rapid decrease in the peak intensity of PVDF.This indicates the strong aggregation effect of BTO nanoparticles under this composition.Furthermore, as shown in Figure 3b, Raman spectroscopy reveals that with increasing BTO nanoparticle content, the wavelength representing the PVDF vibration peak remains nearly constant, while the vibration peak representing BTO continuously increases, reaching its highest point at 25 wt%.This observation also reflects the distribution and aggregation effect of BTO nanoparticles within PVDF films under different compositions.Fourier transform infrared spectroscopy (FT-IR) was used to analyze band structures, phases, and substance content of materials.As mentioned earlier, PVDF has different crystalline phases of α, β, and γ phases.The vibrational bands for the α phase are located at 530, 615, 765, and 795 cm À1 , while the β phase exhibits vibrational bands only at 510 and 840 cm À1 .The corresponding spectral bands for the γ phase are found at 431, 776, 812, 833, and 1233 cm À1 .Studies have shown that the functional groups of BTO do not exhibit stretching vibrations beyond 600 cm À1 . [13]For inorganic salts like BTO, the specific characteristic functional group vibration frequencies of the anions are influenced by the cations and can be quite complex.The stretching vibration wave range typically falls below 500 cm À1 .Therefore, the addition of BTO does not have an additional impact on the infrared absorption peaks of PVDF beyond 600 cm À1 , and it does not affect the calculation of the β-phase content.As shown in Figure 3c, the infrared absorption peaks representing BTO are significantly enhanced in the composite films based on freeze-drying PVDF-H@BTO, particularly in the range before 500 cm À1 .The β-phase content can be calculated by the Gregorio's formula, [39] FðβÞ ¼ here, A α and A β represent the mass fractions of α and β phases, the absorption bands located at approximately 765 cm À1 and approximately 835 cm À1 are denoted as α and β, and the absorption coefficients are represented by K α and K β at specific wavenumbers.Figure 3d illustrates the dependency of the β phase fraction (F(β)) in PVDF on the concentration of BTO.This figure presents four distinct data series, corresponding to combinations of material treatments and processing techniques: hydroxylated BTO (H@BTO) subjected to freeze-drying, H@BTO processed by oven baking, untreated BTO with oven baking, and untreated BTO with freeze-drying.The trend lines clearly demonstrate that the freeze-drying process, applied to both H@BTO and untreated BTO, consistently produces a higher β phase fraction than oven baking across all BTO concentrations.Notably, the F(β) value for composites prepared by freeze-drying PVDF-H@BTO shows an initial increase followed by a subsequent decrease as the concentration of H@BTO nanoparticles rises, with an optimal β phase fraction of 86.10% achieved at 2.5 wt%.This pattern indicates a synergistic enhancement of the β phase formation due to the hydroxylated surface of BTO and the specific physical conditions afforded by freeze-drying.In contrast, the impact of oven baking process on β phase content is comparatively subdued, displaying a plateau effect at higher BTO concentrations, regardless of hydroxylation status.Significantly, H@BTO processed by oven baking does not exhibit marked differences from its untreated counterpart, suggesting that the influence of hydroxyl groups may be diminished under the thermal conditions of oven baking.In conclusion, the data substantiates that the method of freeze-drying, especially in conjunction with hydroxylated BTO, is more effective at inducing the β phase in PVDF, which is critical for enhancing its piezoelectric properties.Meanwhile, the peak at 1233 cm À1 is indicative of the γ phase. [40]It was noted that upon introducing BTO, there is a suppression of the 1233 cm À1 peak, concurrent with an enhancement of the peak at 835 cm À1 .Furthermore, while the addition of BTO led to a noticeable reduction in the γ phase, this attenuation showed minimal fluctuation across different compositional variations.Moreover, as indicated by the results of XRD and Raman spectroscopy, with further increase of BTO component, BTO aggregation effect occurs, resulting in a reduction in the crystallinity of the polymer and ultimately reducing the value of F(β). Figure 3e displays the displacement and phase curves for the freeze-drying PVDF-H@BTO composite film with a 2.5 wt% BTO concentration.The PFM phase diagram, represented by a dotted line in Figure 3e, consistently displays a 180°phase difference during voltage sweeping in both directions.Illustrated as a solid line in Figure 3e, the presence of a complete piezoelectric butterfly curve upon the reversal of bias voltage further substantiates the film's piezoelectric nature.The film's piezoelectric coupling coefficient (d 33 ) can be derived from the slope of the displacement-voltage curve, offering a quantitative evaluation of the film's piezoelectric properties.
As depicted in Figure 3f, d 33 values of the composite films, fabricated using two distinct methods and varying component concentrations, were assessed.Without electric poling, the piezoelectric coefficient of freeze-drying composite films increases from 5.35 pm V À1 at a 0 wt% BTO concentration to 14.49 pm V À1 at a 2.5 wt% BTO concentration.Nevertheless, as the BTO component concentration continues to rise, the d 33 value exhibits a subsequent decline, reaching 3.17 pm V À1 at a 25 wt% composition.This can be attributed to the agglomeration effect of BTO particles.In stark contrast, composite films prepared using the oven baking method (Figure S3, Supporting Information) display significantly lower piezoelectric coefficients compared to those synthesized by the freeze-drying method, with the highest recorded d 33 value being a mere 2.62 pm V À1 at a 5 wt% BTO concentration.The results indicate that the freeze-drying method strengthens the hydroxyl anchoring effect on the H@BTO surface, consequently inducing spontaneous polarization and achieving a relatively high d 33 value.It should be emphasized that these samples were not electrically poled, and the piezoelectric properties are induced mostly by the hydroxyl induced spontaneous polarization and freeze-drying process.

XPS Characterization of PVDF-H@BTO Films and BTO Powders
Figure 4 affords a detailed XPS assessment of PVDF films fabricated via oven baking and freeze-drying methods, alongside a comparative analysis of the surface chemistry between hydroxylated barium titanate (H@BTO) and untreated BTO powders.In Figure 4a, the spectra of the films are displayed, revealing pronounced peaks for fluorine (F1s), carbon (C1s), and oxygen (O1s).Notably, the intensity of peaks associated with O, Ba, and Ti is markedly subdued.This can be attributed that most of the nanoparticles are located inside PVDF matrix at a depth that is greater than the depth of the XPS analysis.The highresolution C1s core-level XPS spectra of the PVDF films prepared by oven baking, as shown in Figure 4b, reveal distinct peaks at 285.4 eV and 289.8 eV, which correspond to the CH 2 and CF 2 carbon species, respectively.A notable shift in these peaks to higher binding energies is observed in the freeze-drying films, with CH 2 and CF 2 peaks manifesting at 285.8 and 290.5 eV, respectively.This shift in the freeze-drying spectrum indicates a significant alteration in the electronic environment of the carbon dipoles in PVDF.It is postulated that this is due to the electrostatic interaction between the PVDF molecular chains and H@BTO nanoparticles.Furthermore, it is possible to understand whether polarization and reorientation occurs during the interfacial interaction of H@BTO with PVDF by analyzing the peak separation(Δ) of two characteristic regions, for CF 2 and CH 2 dipoles, from figure 4b, it is clear that the Δ of the oven baking method (Δ 1 ) is relatively smaller than the Δ of the freeze-drying method (Δ 2 ).It can be seen that the preparation of PVDF films by freeze-drying method leads to a decrease in the content on the surface CF 2 groups, which can be attributed to the hydroxyl anchoring effect.This is in good agreement with the FTIR data, where it was shown that the freeze-drying method leads to a huge increase in the fraction of the β-phase of PVDF. Figure 4c compares the F1s spectra of the films, where the freeze-drying sample exhibits a notably more broadening peak.Thus, it can be assumed that it is due to the interfacial interaction of positively charged H@BTO nanoparticles with negatively charged CF 2 dipoles. [40]Moreover, XPS peaks of F1s for freeze-drying PVDF-H@BTO shift to higher binding energies, due to the chemical shift induced by the surface interfacial interactions. [41]he spectra of the BTO powders, depicted in Figure 4d, reveal clear distinctions in peak intensities and binding energy positions, particularly within the O1s region.The two peaks at 528.6 and 531.6 eV ae assigned to the lattice oxygen (bulk BTO) and hydroxyl group. [42,43]A more detailed examination of the O1s region for BTO powders in Figure 4e showcases a singular peak for untreated BTO, typically associated with lattice oxygen (bulk BTO).In contrast, the H@BTO spectrum, as observed in Figure 4f, displays an additional peak at higher binding energies, attributed to the surface-bound hydroxyl groups.This feature provides evidence for the successful hydroxylation of BTO surfaces.

Device Structure and Performance of the Freeze-Drying PVDF-H@BTO/PA6 TENGs
In the realm of materials science and nanotechnology, it is widely acknowledged that the integration of piezoelectric nanoparticles into polymer matrices provides notable advantages, particularly in enhancing surface charge density.This effect substantiates the use of such materials as exceptional candidates for constructing high-performance TENGs. [44]Given the elevated F(β) and hydroxyl-induced spontaneous polarization, it is reasonable to expect that TENGs fabricated from freeze-drying PVDF-H@BTO composite films should exhibit superior output performance.
The 3D schematic illustration of the freeze-drying PVDF-H@BTO/PA6 TENG and a photo of the test setup are shown in Figure 5a,b, respectively.In this work, a porous PA6 membrane was chosen as the positive tribo-material paired with the freeze-drying PVDF-H@BTO membranes to fabricate TENGs.The porous PA6 membrane, prepared by the phaseinversion method, has been proven to be one of the best positive tribo-materials for high-performance TENGs. [45]A series of experiments were then conducted to investigate the influence of BTO concentration, contact force, and working frequency on the performance of TENGs.
The influence of BTO concentration was first investigated with the other conditions held constant: a working frequency of 4 Hz and an impact force of 100 N.Moreover, the thickness of both freeze-drying PVDF-H@BTO membrane and PA6 membrane was fixed at about 100 μm, while the contact area of both membranes was fixed at 20 mm Â 20 mm.The TENGs were thoroughly evaluated by measuring the V OC and calculating the J SC using precision instruments such as an oscilloscope and a picoammeter.The detailed output results are graphically presented in Figure 5c-e.From the graphical representation, it is evident that under these experimental conditions, the TENG's performance exhibits a characteristic trend of initially increasing and subsequently decreasing with the increase in BTO concentration.Notably, the V OC achieves its peak value of 832 V at a BTO component concentration of 2.5 wt%, coinciding with a maximum J SC of 107.5 mA m À2 and a maximum Q SC of 183.54 μC m À2 .In comparison to the normal PVDF-BTO/PA6 TENG, composed of a conventional thermally baked PVDF film and a PA6 film (Figure S4, Supporting Information), the performance of the PVDF-H@BTO/PA6 TENGs shows about 120% enhancement, e.g., J SC increases by 127.4% and V OC increases by 118.9%.However, as the BTO component concentration continues to increase, the TENG's output performance undergoes a diminishing phase.For instance, at a BTO component concentration of 25 wt%, the V OC decreases to 426 V, the J SC decreases to 50.1 mA m À2 and the Q SC decreases to 109.68s μC m À2 , signifying a pronounced attenuation in the output performance.It is worth noting that, due to the soft and elastic nature of the films, the negative peak of TENGs outputs enhances significantly, thereby greatly increasing the open-circuit output voltage.Thus, the concentration of BTO for all the devices mentioned later was fixed at 2.5 wt%, unless specified.The enhancement of freeze-drying PVDF-H@BTO/PA6 TENGs can be attributed to the synergistic effect of the piezoelectricity of BTO nanoparticles, the increase in the β-phase content within the PVDF film, and hydroxyl-induced spontaneous polarization.Additionally, the performance decrease of freeze-drying PVDF-H@BTO/ PA6 TENG when the concentration of BTO exceeds 25 wt% can be explained by the agglomeration of BTO, which is mentioned in the analysis of XRD patterns and FTIR spectra before.
Figure 5f displays the open-circuit voltage of freeze-drying PVDF-H@BTO/PA6 TENG under contact forces ranging from 10 to 150 N. As the contact force increases from 10 to 100 N, the V OC increases gradually from 488 to 864 V, then reaches 1060 V at a contact force of 150 N. The conclusions that can be drawn from the earlier results are as follows: As the contact forces gradually increases, the output of the TENG also increases gradually.However, when the contact force reaches a certain level, the tribo-electrical output tends to saturate.The main reasons for this behavior are as follows: Due to the elastic properties of the freeze-drying PVDF-H@BTO and PA6 films, the surfaces of the materials are not perfectly smooth.They still exhibit a rough morphology.When the contact forces are low, the surfaces of the two tribo-materials do not make full and effective contact, resulting in a small effective contact area.However, as the contact force increases, the effective contact area gradually increases, allowing for the generation of more charges and higher triboelectrical output performance.However, when the contact forces continue to increase and reach a certain threshold, the effective contact area reaches its peak.Therefore, under the influence of high contact force, the tribo-electrical output of the TENG tends to saturate.
The open-circuit voltage of the freeze-drying PVDF-H@BTO/ PA6 TENG as a function of working frequency was investigated with the results shown in Figure 5g.When the working frequency changes from 1 to 7 Hz, the V OC increases gradually from 638 to 880 and finally reaches 896 V at a working frequency of 10 Hz.Therefore, as the working frequency increases, the V OC of the TENG gradually increases.According to previous research, at the short-circuit condition, the transferred charges can be expressed as [46] Q And the output voltage of the TENG can be approximated as where Q represents the transferred charges between the electrodes, S is the surface area, d 1 , d 2 , ε 1 , and ε 2 are the thicknesses and relative dielectric constants of the two dielectric layers, x(t) is the separation distance, and σ is the charge density of the dielectric layers.
The reason behind this phenomenon lies in the fact that the higher working frequency accelerates the contact-separation speed of the TENG's two electrodes.When the TENG operates under a fixed contact force and separation distance, the charge density on the tribo-material surface is almost the same.The increase in motion velocity simply shortens the motion cycle, allowing the same amount of induced charge to be transferred in a shorter time period.This rapid transfer of induced charges results in an increase in voltage output in nonideal open-circuit test condition.
In order to substantiate the applicability of freeze-drying PVDF-H@BTO/PA6 TENG, the output of the device was connected to an external circuit through a full-bridge rectifier, as shown in Figure 5h.The device was tested under the fixed condition: a working frequency of 4 Hz and the impact force of about 100 N. Consequently, upon rectification of the output through a rectifier circuit, the TENG device effectively powered and illuminated over 120 blue LEDs connected in series (Figure 5i and Video S1, Supporting Information).It is evident that the freeze-drying PVDF-H@BTO/PA6 TENG exhibits relatively superior tribo-electrical output performance and reliability.This presents a promising and sustainable alternative power source with substantial potential for applications in emerging fields such as new energy sources, Internet of Things (IoT), and smart sensing.
To showcase the application capabilities of the freeze-drying PVDF-H@BTO/PA6 TENG, a TENG device based on a spring structure and an acrylic base was constructed and integrated into an insole for harvesting energy from walking, as depicted in Figure 6a,b.This device has an effective contact area of 30 Â 30 mm 2 .In real-world testing conditions, the device achieved a peak open-circuit voltage (V rpoc ) of approximately 467 V, as shown in Figure 6c. Figure 6d illustrates the application of the TENG, where mechanical energy generated by human walking was stored in a capacitor through a TENG and a fullbridge rectifier.The electricity is then supplied to sensors or other devices controlled by a mechanical switch, simultaneously enabling wireless transmission of sensor data via a Bluetooth module.Figure 6e demonstrates the application of the TENG in powering a small calculator.Furthermore, Figure 6f and Video S2, Supporting Information, illustrate the device powering a temperature and humidity sensor, transmitting the current temperature and humidity data to a host cell phone via the wireless Bluetooth module.Thus, the freeze-drying PVDF-H@BTO/PA6 TENG exhibits reliable electrical output capability, effectively converting mechanical energy into electric energy required by electronic devices in real time.As demonstrated earlier, the freeze-drying PVDF-H@BTO/PA6 TENG has good reliability and a wide range of application prospects.In future, it can be applied in motion sensing, flexible electronic devices, and biocompatible devices, etc.
The working principle of freeze-drying PVDF-H@BTO/PA6 TENGs is illustrated in Figure 7.When the PA6 layer and PVDF-H@BTO layer make close contact under an external force (as illustrated in Figure 7a), electron transfer occurs due to the difference in electronegativity of the two frictional materials.The PVDF-H@BTO film gains electrons, becoming negatively charged, while the PA6 film becomes positively charged.Surface charges of opposite polarity are generated on the films.As the frictional films separate from each other (Figure 7b), the surface charges remain due to insulating properties, leading to a potential difference between them.Electrostatic induction induces opposite charges on the electrodes, resulting in a voltage/current pulse when connected to an external load.
With increasing separation distance, the shielding effect weakens until negligible, and the TENG reaches equilibrium (Figure 7c).Upon reapplication of external force, the shielding effect strengthens, causing electron flow in the reverse direction through the load, creating a reverse voltage/current pulse (Figure 7d).When the films make complete contact again, returning to the shielding state, the voltage or current returns to zero, completing one working cycle.

Conclusions
Here, we present a straightforward and effective approach to tailor the local dipole moment and enhance the β phase content of piezoelectric polymer composites.This is achieved by introducing hydroxylated-BTO nanoparticles using a freeze-drying process.MD simulations help us understand the strong binding interaction between Ba and F atoms, along with the presence of OH surface terminations on the H@BTO surface.These factors promote hydrogen bonding with the PVDF matrix, leading to dipole alignment and increased spontaneous polarization.An optimal concentration of H@BTO nanoparticles (2.5 wt%) significantly strengthens the connection between the inorganic nanofillers and the organic polymer matrix, thereby boosting the β-phase content.This results in a remarkable increase in the F(β) value of the composite film to 86.69%, significantly higher compared to samples prepared using conventional baking method.A TENG using freeze-drying PVDF-H@BTO composite  films and PA6 membranes was fabricated, which exhibited impressive performance metrics, including a current density of ≈107.5 mA m À2 , an output voltage of ≈832 V, and a maximum charge density of ≈183.54 μC m À2 .These values are almost twice as high as those achieved by the control PVDF-BTO/PA6 nanogenerator.All the results demonstrated the effectiveness and efficiency in synthesis of high-performance PVDF composites by our newly proposed strategy.

Experimental Section
Materials: PVDF resin (Molecule weight ≈625 000) was supplied by Beijing Epsilon Technology Co., Ltd.BaTiO 3 powder (99.9%, <100 nm) and hydrogen peroxide (30  Fabrication of Freeze-Drying PVDF-H@BTO Composite Films: In order to make the surface of BTO particles hydroxylated, the BTO particles were grinded and mixed with 30% hydrogen peroxide (H 2 O 2 ), then dispersed evenly by ultrasonic cleaner (SKE-KJE PS-60A).After that, the mixed solution was heated at 80 °C for 2 h and stirred.Centrifugal separation was carried out by microcentrifuge (Thermo Sorvall Legend Micro 17).Then the supernatant was poured away and the white solid particles were taken.After cleaning with isopropyl alcohol and deionized water, the BTO particles treated with hydrogen peroxide (H@BTO) were finally obtained after drying in a vacuum drying oven.After that, PVDF powder was mixed with H@BTO nanoparticles, and the mass ratio was 0.025:1, 0.05:1, 0.075:1, 0.1:1, 0.25:1, respectively.To get the precursor fluid, H@BTO nanoparticles of different components and DMSO solution were mixed evenly by magnetic stirring using multiple magnetic stirrer (JOANLAB MMS6-Pro).Then the mixed liquid was poured into the directional freezer mold, which was made of stainless steel at the bottom and sealed with acrylic plates for the sides and top.After the precursor liquid was frozen in the À80 °C refrigerator for 12 h, the frozen materials were freeze-drying by a Benchtop Freeze Dry System (LABCONCO FreeZone 6L) for 36 h to obtain 0, 2.5, 5.0, 7.5, 10, and 25 wt% PVDF-BTO composite films based on freeze-drying.
Fabrication of TENG: The freeze-drying PVDF-H@BTO films were cut into 20 mm Â 20 mm (the active areas of the TENGs), then the vertical contact-separation TENGs was constructed.The substrate of the TENG was two glass plates (20 mm Â 20 mm Â 1 mm), and the negative layers were freeze-drying PVDF-H@BTO films with different components.Meanwhile, the positive layers were PA6 films.Al sheets and Ni types were adhered onto the substrates to act as electrodes.The electrodes had a dimension of 20 mm Â 30 mm, leaving 10 mm extension for electrical tests.At last, the vertical contact-separation TENG is obtained by connecting the positive and negative electrodes with the external circuit through the wire.
The TENG device based on a spring structure and an acrylic base was constructed and integrated into an insole for harvesting energy from walking.This device has an effective contact area of 30 Â 30 mm 2 .Clean acrylic plates (30 mm Â 30 mm Â 1 mm) were employed as the supporting substrates.Then the freeze-drying PVDF-H@BTO films and PA6 films were cut into 30 mm Â 30 mm and attached onto the acrylic substrates with conductive nickel tapes that were used as the electrodes.Then, the acrylic plates were secured using springs and glue to assemble the device.
Material Characterization and Device Measurements: The obtained freeze-drying PVDF-H@BTO films with different concentration were characterized by various techniques.The morphology of surface and cross section was measured by field-emission scanning electron microscopy (FE-SEM) (Hitachi SU5000) after coating Au nanoparticles using ion sputtering instrument (KYKY SBC-12).The XRD patterns of these F-D films were acquired on multifunctional XRD (Shimadzu LabX XRD-6100).
The piezoelectric charge constant d 33 was measured using a Quasi-static d 33 measurement instrument (ZJ-3AN, IACAS).To acquire the Raman spectrum of freeze-drying PVDF-H@BTO films mentioned earlier, the Raman spectrometer (Horiba LabRAM Odyssey) was used.Meanwhile, the element distribution of BTO in PVDF freeze-drying films was obtained by the EDS (Bruker XFlash6130).The FT-IR spectra of freeze-drying PVDF-H@BTO were acquired on Nicolet 5700 (Thermo Electron Scientific Instruments Corp).The surface roughness was tested by AFM (Bruker ICON).The XPS spectrum was tested by X-ray photoelectron spectrometer (ESCALAB).
To evaluate the performance of TENG with different material combinations, we used a dynamic fatigue testing system (Popwil Model YPS-1) to control the periodic contact-separation motion between the two tribomaterials or different stresses applied.Besides, the environment humidity was maintained below 40%.Output voltage and short-circuit current of TENG were measured by an oscilloscope (Tektronix MDO3032) with an internal load resistance of 100 MΩ and a picoammeter (Keysight B2981A), respectively.The transferred charge density was calculated by integrating the current curves with time for one contact/separation cycle.

Figure 1 .
Figure 1.MD simulations of Interactions of PVDF chains with BTO nanoparticles and hydroxyl anchoring effect.a) MD simulation model using PVDF 'mer' chains and BTO nanoparticles.b) RDF analysis of Ba, Ti, and O atoms in BTO nanoparticles with H and F atoms in PVDF chains with a BTO concentration of 2.5 wt% c) RDF analysis of Ba-F interactions across different BTO concentrations (1.0, 2.5, 5.0, and 7.5%).d) Schematic of PVDF polymer chain in situ stretching and alignment via -OH surface terminations on H@BTO surface for enhanced spontaneous polarization (P s ).Final MD snapshots of PVDF polarization on e) H@BTO and f ) untreated BTO surfaces.g) Dihedral angles distribution of PVDF chains near the À60°(Gauche, left) and 180°(Trans, right).h) Dihedral angles distribution of PVDF chains ranging from À180°to 180°.

Figure 3 .
Figure 3. Characteristics of freeze-drying PVDF-H@BTO composite films.a) XRD patterns, b) Raman spectroscopy spectra, c) FT-IR spectra, d) displacement (solid line) and phase (dotted line) curves of the piezoelectric response of the freeze-drying PVDF-BTO composite film with a BTO concentration of 2.5 wt% under flip voltage e) F(β) value, f ) Absolute value of d 33 in the composite films with varying BTO concentrations by freeze-drying and oven baking processes respectively.

Figure 4 .
Figure 4. Comparative XPS analysis of PVDF-H@BTO films and BTO powders.a) XPS spectra of oven baking and freeze-drying films.High-resolution b) C 1s and c) F 1s XPS valence band spectra of oven baking and freeze-drying films.d) XPS spectra of H@BTO and untreated BTO powder.High resolution e) C 1s and f ) F 1s XPS valence band spectra of H@BTO and untreated BTO powder.

Figure 5 .
Figure 5. Device structure and performance of the freeze-drying PVDF-H@BTO TENGs.a) 3D schematic illustration of the freeze-drying PVDF-H@BTO TENGs, b) a photo of the test setup, c) voltage output, d) current output, e) peak-to-peak voltage (Vpp), current density and charge density of TENG made of freeze-drying PVDF-BTO composite films with different BTO concentrations (0, 2.5, 5, 10, and 25 wt%).f ) Voltage output of TENG made of freezedrying PVDF-BTO composite films with a BTO concentration of 2.5 wt%. at different force.g) Voltage output of TENG made of freeze-drying PVDF-BTO composite films with a BTO concentration of 2.5 wt%. at different frequencies.h) Configuration of the rectified circuit with external circuit.i) 120 blue LEDs in series were lit up by the rectified output of the TENG.

Figure 6 .
Figure 6.Applications based on the freeze-drying PVDF-H@BTO TENG.a,b) Photos of the designed freeze-drying PVDF-H@BTO TENG device integrated in the insole.c) Rectified output voltage from the TENG.d) The configuration of TENG energy harvesting circuit with connections to sensors and devices.e) Application of TENG in powering a calculator.f ) Application of TENG in powering a wireless temperature and humidity monitoring system.