A Study of Contact Electrification Process on PVDF–Metal Interface: Effect of β Phase Composition

Recently, triboelectric nanogenerators (TENGs) are getting considerable attention as an energy harvesting tool that can convert random mechanical energy into electricity due to the wide material selection, low cost, and easy fabrication. TENGs work by contact electrification on the interface and electrostatic induction on the electrodes when two surfaces contact and separate. Herein, the study of the contact electrification process on the metal–polyvinylidene difluoride (PVDF) interface is conducted focusing on the effect of β phase content on the electrical properties of the PVDF films. It is found through the EFM and KPFM surface electrical studies that a higher β phase promotes stronger electrostatic interactions and enhances electron‐cloud overlap with the metal coated cantilever tip that leads to higher amount of charge transfer. Additionally, there is overall enhancement of the TENGs electric output performance for a higher β phase containing PVDF films and the maximum electric output of 8.1 V and 12.2 nA is obtained for the TENG made with 79% β phase PVDF film.


Introduction
Triboelectric nanogenerator (TENG) is a type of energy harvesting device that can convert mechanical energy into electricity by contact electrification (CE) at the interface when two materials contact, and subsequent electrostatic induction during DOI: 10.1002/admi.202300727separation. [1,2][5][6][7] Tremendous research has been conducted on improving their performance and optimizing structure of TENGs since its first development in 2012.As TENG's efficiency is primarily determined by the amount of surface charge generated during the CE process, a proper selection of triboelectric material pair based on their location in triboelectric series is crucial to maximize transferred charge density. [8,9,10][18] Wang et.al. used ionized charge injection in order to optimize the surface charge density of the triboelectric materials. [19][22][23] PVDF is a polymorphic polymer with stable , , and  crystalline phases.Among them,  phase PVDF is the thermodynamically most stable phase however, due to its non-polar nature it is not desirable material for nanogenerators.[26][27][28][29][30][31] However, further systematic quantitative study can be done on how  phase composition of PVDF influences charge transfer process during contact of two materials at the microscopic surface level as well as bulk macroscopic state.Herein, we prepared various  phase content PVDF films to study quantitatively the CE process on the PVDF-metal interface.The surface of the PVDF films were characterized by atomic force microscopy (AFM) and it was found that as the  phase ratio increased, the electrostatic force microscopy (EFM) amplitude was enhanced while Kelvin probe force microscopy (KPFM) surface potential became more negative after CE.The obtained results suggest that polar  phase increased electrostatic attraction between two materials and thereby assisted for improved electron transfer during the polymer and metal contact.PVDF-Cu TENGs were prepared, and the measured output performance agreed well with the AFM data confirming the importance of the polar  phase PVDF for enhancing charge transfer during the CE.Also, it was found that on a macroscopic level, physical parameters such as the thickness of dielectric layer affect induced charge density of TENGs, and it is reduced significantly with the decrease of thickness of PVDF.Additionally, due to piezoelectric activity of PVDF, electrical output of piezoelectric nanogenerators (PENGs) was tested and compared with TENGs output to investigate its contribution in contact triboelectric mode.It was found that piezoelectricity has a negligible contribution and induced charge mainly originates from triboelectricity.

Characterization of Phase Composition of PVDF Films
PVDF films were spin coated on a pre cleaned glass substrate at 1000 and 2000 rpm speed.Spin coated films were annealed at 60 and 80 °C to promote formation of  phase PVDF and free-standing films were obtained.35] The freestanding PVDF films were analyzed from both sides as shown in Figure 1c due to the difference in surface morphology formed during the solvent evaporation and crystallization.The surface on the upside of peeled films is marked with a red arrow while the down side of films facing the substrate is marked with a blue arrow and corresponding samples were named with U and D for differentiation.Surface topography of films was characterized by AFM on the up (U) and down (D) sides.As shown in Figure S1 (Supporting Information) in supporting information, upside of the films have grainy morphology characteristic to the semicrystalline polymer film.The average roughness was reduced significantly from 116.5 ± 6.6 to 81 ± 2 nm when increasing spin speed while varying annealing temperature had a negligible effect.On the other hand, the downside of the films had a considerably smooth surface with smaller roughness which was around 11 ± 1.5 nm for all films which mainly originated from roughness of the glass substrate.Since the morphology and roughness difference can influence surface electrical properties and the CE process, up and down sides of films were considered separately throughout the study.
The attenuated total reflection mode fourier transform infrared (ATR-FTIR) spectroscopy was conducted to study phase composition of the polymer films and results are presented in  study the effect of spin coating speed on the  phase content, while keeping annealing temperature constant at 80 °C.[38][39][40][41] As the coating speed was increased to 2000 rpm the intensity of the peak at 763 cm − ¹ reduced relative to the absorbance peak at 839 cm − ¹ indicating occurrence of  to  phase transformation.The equations below were used for quantitative estimation of the  phase content of the PVDF films. [40] = − logT  (1) Here, A  is the absorbance at 763 cm − ¹, A  is the absorbance at 839 cm − ¹ and it was calculated using transmittance data at a corresponding wavenumber through Equations 1 and 2. K  = 6.1 × 10 4 cm 2 mol − ¹ and K  = 7.7 × 10 4 cm 2 mol − ¹ are absorption coefficients corresponding to  and  phase.In the case of films coated on pristine glass substrate the  phase content increased from 63% to 78% on the upside of the PVDF film and 68% to 82% on the down side of the PVDF films when coating speed was doubled.The enhancement is due to the increased polymer chain stretching at a larger centrifugal force that causes  to  phase transition upon doubling the spin coating speed. [42]oreover, there is a noticeable difference in the  phase composition of films depending on the substrate.For films coated on the plasma activated substrate,  phase content increased from 69% to 85% on the upside of film and 79% to 87% on the downside of film.[35] Furthermore, substrate treatment led to a more uniform distribution of the  phase throughout the film than in the PVDF films coated on the pristine glass that showed a larger variation as provided in Table S1 (Supporting Information).Additionally, the upside of the film has a slightly lower  phase content than the downside of the film which can be due to a faster solvent evaporation on surface layers than near substrate layers.Since the dimethylformamide (DMF) is polar solvent and it stabilizes polar  phase so its slower evaporation is preferred for promotion of  to  phase conversion. [42]o study the effect of annealing temperature on the  phase formation, temperatures of 60 and 80 °C were selected based on the previous works while keeping spin coating speed at 1000 rpm. [43,44]As demonstrated in Figure 2c,d   (Supporting Information), there is an overall increase in  phase content upon raising the annealing temperature since a higher temperature provides more activation energy to transform from thermodynamically stable  phase to a higher energy  phase state.However, the effect of temperature is negligible compared to the coating speed and the highest  phase of over 85% was obtained for samples coated on plasma activated glass substrate with 2000 rpm coating speed and 80 °C annealing temperature.

Surface Characterization of PVDF Films
In atomic force microscopy (AFM) measurement, non-contact electrostatic force microscopy (EFM) mode was used to investigate the electrical properties of PVDF film surfaces.The sur-face of the PVDF films were scanned with the conductive Au/Cr coated cantilever tip while applying 1 V alternating current (AC) bias voltage at frequency of 17 kHz to the tip with sample bias voltage of + 1 V and the electrical response of samples were recorded.The inset images in Figure 3a,b display the EFM amplitude maps recorded during the measurement and it represents electrostatic force between the sample and the tip during the scanning process.As presented in Figure 3a,b the EFM amplitude on the up and down sides showed a steadily rising tendency with the increase of the  phase content of PVDF films.The EFM amplitude on the upside of the PVDF film enlarged up to 1.6 fold, rising from 10.3 to 16.9 mV when the  phase content increased from 62% to 85%.Likewise, the downside of the films, although having considerably smaller surface roughness than on the up side, exhibited similar surface electrical properties.The EFM amplitude was risen from 11.6 mV at 61% of  phase to 20.2 mV at 87%.So, based on the above findings, presence of a higher  phase in the PVDF seems to increase the electrostatic interactions with the metal coated conductive tip.It is known that electrostatic attraction is the driving force for charge transfer between electron acceptor and electron donor.Therefore, the presence of a higher content of  phase enhances electrostatic interactions between PVDF film and metal that can have an influence on the boosting of the charge transfer between two materials during CE.In one theoretical study, authors have calculated electrostatic potential distributions of the  and  phase PVDF molecular orbitals and determined that the lowest unoccupied molecular orbital (LUMO) level is lowered for the  phase PVDF. [45]Therefore,  phase PVDF would have a higher affinity to electrons upon contact with the metal and hence, experience stronger electrostatic attraction that will result in a higher amount of charge transfer which is in agreement with the obtained EFM result. [45]nlike EFM amplitude that represents electrostatic interactions, Kelvin probe force microscopy (KPFM) allows us to get quantitative information on the surface potential of the sample. [46]The CE process at the interface between PVDF and metal is complicated by the coexistence of several crystalline and amorphous states of the polymer.The measurement of surface potential shows the outcome of CE rather than the holistic picture; however, the interpretation of the data can give some clues on charge transfer behavior between two materials.Therefore, KPFM surface potential, also known as contact potential difference (CPD), of the PVDF films was mapped at three different areas before and after the CE process to study the charge transfer process at the polymer-metal interface.The insets of surface potential maps before and after CE in Figure 3c,d show that on average for all samples, the surface potential at the measured area became more negative after the CE suggesting that the electrons flow from the metal coated tip to the PVDF.On the other hand, the extent of the change for different samples varied and the difference in KPFM surface potentials before and after the CE (defined as ΔCPD) was calculated and plotted as a function of  phase content in the PVDF films.Figure c,d shows that overall on the up and the down side of PVDF films, the ΔCPD on the sample surface increases with a higher  phase content and the surface becomes more negatively charged as an outcome of electron transfer from the metal tip to the PVDF film.However, the exception was the sample with 63%  phase in which the direction of the electron flow was reversed and the PVDF surface became positively charged.This can be caused by the nonuniform distribution of  phase along with the presence of a high content of  phase whose electron donating (-CH 2 ) and withdrawing (-CF 2 ) regions overlap and thus, electron transfer can happen in both directions upon contact with the metal surface. [45]Meanwhile, the increase of the polar  phase content enhances electron withdrawing ability of PVDF.It can be seen from Figure 3c,d and Figure S2 (Supporting Information), that the largest change in the KPFM potential was observed for the down side of PVDF film containing 87% of  phase with ΔCPD = −94.5 mV where the surface potential became 178% more negative after charge the transfer.The above result is larger for around 6.2 and 2.1 times for 61%  phase PVDF with ΔCPD = −15.1 mV and for 77%  phase PVDF with ΔCPD = -44 mV correspondingly.The experimental result from KPFM potential ties well with the EFM amplitude data and it is clear that the presence of higher  phase improves charge transfer on a microscopic level.Figure 3e shows a conceptual band diagram of a charge transfer at the interface between metal and PVDF with different phase compositions.As the ratio of the polar  phase increases, it induces band bending and down shifting of a Fermi level in PVDF thus, enlarging the gap with a work function of a metal (ΦM) and hence, more electrons are promoted to flow from the metal to the PVDF.Additionally, unlike in the nonpolar  phase, in the  phase fluorine groups with high electronegativity are all located on one side of the polymer backbone and during contact with metal it will increase electron withdrawing ability of PVDF attracting more electrons.

Electric Output Performance of Triboelectric and Piezoelectric Nanogenerators
Triboelectric nanogenerators (TENGs) were assembled to confirm the positive correlation between higher  phase of the PVDF films and improved surface electrical properties determined by AFM on a microscopic level.The vertical contact -separation mode TENGs were assembled with PVDF films as a tribonegative material and Cu metal as a tribopositive material, and the detailed schematics of the device is shown in Figure 4a.TENGs made with the two sides were considered separately due to the surface roughness variation on the up and the down side of PVDF films.
The TENGs were tested under a pushing tester while applying compressive load of around 8 N at 1 Hz frequency and the output performance is presented in Figure 4b,c and Figure S3 (Supporting Information).As the  phase (287 GPa) has a larger elastic modulus than  phase (148 GPa) PVDF, true contact area may reduce with the increase of  phase content in PVDF films. [47]owever, based on our estimations in the supporting information, the maximum change in contact area is only up to 16.7% which can be neglected as it seems electric output was enhanced with increment in  phase even though contact area is slightly reduced.Overall, there was an increase in the electric current and voltage output with the increase of  phase for the films coated at 1000 rpm.Moreover, it can be noticed that output performance of TENGs made with the downside of PVDF is relatively higher than that of with the up side.This can be due to a larger roughness on the upside that eventually lowered the true contact area on the interface as the force of 8 N was not enough to achieve maximum contact area.Mulvihill et.al. studied pushing force dependent electrical output of TENGs using triboelectrification layers with rough surface and found that at low pushing force the real contact area was smaller that led to lowered electric output. [48]s the applied force increased, the real contact area converged to maximum and hence, output performance was also improved. [48]n case of the TENGs with a more rough up side of the PVDF films, maximum output of about 3.0 V and 5.7 nA was observed for the sample with 69%  phase.The maximum output performance of 8.1 V and 12.2 nA was demonstrated in TENG made up of 79%  phase containing sample with the down smoother side contacting the tribopositive Cu surface.The results are consistent with EFM and KPFM data supporting the claim that the PVDF with a higher content of polar phase experiences a stronger electrostatic attraction that promotes increased electron flow between the atoms of the polymer and the metal.However, it can be seen from Figure 4b,e that as the coating speed increases up to 2000 rpm, the electric output is slightly reduced compared with that in films coated at 1000 rpm even though the  phase content surpasses 80%.The theoretical model of conductor to dielectric TENG is considered in order to explain the drop of the output for the samples coated at 2000 rpm. Figure S4a (Supporting Information) in supporting information shows the schematics of the model TENG system with the dielectric material and the electrodes.After the contact of the two materials, surface charges will be created as a result of contact electrification and during the separation induced charges will be created on the electrodes due to the potential difference.The amount of the induced charge can be estimated using the equation below. [49,50] Where ' is induced charge density,  is surface charge density due to contact electrification, x is the gap between the active lay-ers, d 1 and d 2 are thickness of the electrodes and dielectric layer,  n is the permittivity of vacuum,  1 and  2 are the dielectric constant of the metal and dielectric material.According to the Equation 4, the induced charge density and the dielectric thickness have an inverse relationship and a thinner layer has a higher induced charge and hence, surface charge due to triboelectrification should be also higher. [49,50]However, the dielectric constant which determines the charge storage properties of the material, also varies and must be taken into account to explain experimental values of lower induced charge density in Figure 4d,e for thinner films.[53][54][55][56][57] One of the major contributions to the dielectric constant is the thickness dependent grain size.[57] As presented in AFM topography images in supplementary materials, when the spin coating speed is raised, the grain size and average roughness reduced significantly for thinner samples.Moreover, the dielectric permittivity of various thickness PVDF films in Figure S5 (Supporting Information) measured by broadband dielectric spectroscopy (BDS) from 10 − ¹ to 10 7 Hz, drastically decreased, possibly due to the reduction of grain size for thinner films.Therefore, as the thickness of the PVDF films reduced, although the KPFM surface potential became more negative indicating more surface charges transferred, the induced charge derived from integrating TENG's output current became smaller for thinner samples due to a lowered dielectric constant that reduced charge storing ability of the triboelectrification layer.In this study, the films coated at 2000 rpm had a twice lower thickness and smaller grain size than 1000 rpm, which can explain the superiority of the output performance of TENG based on 1000 rpm coated, 79%  phase PVDF film.However, within 2000 rpm coated samples the electric output is increasing again with the increase of the  phase suggesting its benefits for enhancing the charge transfer.
Since PVDF is also a ferroelectric material, piezoelectric charge contribution to the output of TENGs cannot be ignored. [58,59]The piezoelectric nanogenerators (PENGs) were also assembled using PVDF films with varying  phase content and the piezoelectric current output was compared with the outputs of nanogenerators operating in the triboelectric mode.In Figure S6a (Supporting Information), it can be seen that the magnitude of the piezoelectric signal is about 17 times lower than that of the triboelectric mode signal which were 0.09 and 1.53 nA cm −2 respectively.The induced charge of the PENG and TENG are 0.01 and 0.102 nC cm −2 correspondingly and hence, it can be inferred from the result that in TENGs charge mainly originates from the triboelectrification processes.As the polar  phase content goes up from 69% to 79% while the thickness remains almost constant, number of dipole-dipole interactions increases leading to a larger dipolar polarization in PVDF films and as a result, the piezoelectric output current increases by about 80% showing 0.16 nA cm −2 while induced charge almost doubled and reached 0.019 nC cm −2 .Interestingly, in the triboelectric mode, output current and induced charge further improved to the same extent as in piezoelectric mode and were 2.77 and 0.22 nC cm −2 respectively, suggesting positive effect of the larger polarization to the enhancement of triboelectrification.However, the absolute amount of current and charge in the TENG output was significantly greater than the increment of the PENG output, suggesting that major contribution arises from contact electrification effect due to a stronger electrostatic interaction with the metal atoms that promote charge transfer for a higher  phase containing PVDF samples.
The power outputs of the TENGs with varying  phase content of PVDF was measured using load resistance from 1 to 65 MΩ and results are shown in Figure 5a.The peak power density increased from 0.0119, 0.0164, and 0.0175 nW cm −2 as the  phase content of the PVDF were 61%, 77%, and 79% respectively.As presented in Figure 5b, the CPD for the above TENGs, measured before and after CE with KPFM was −15, −44, and -47 mV correspondingly, suggesting that charge transfer enhances with the increase of  phase content and therefore, power density is also improved.

Conclusion
Herein, the systematic study of contact electrification on metalpolymer interface was conducted using PVDF and metals as triboelectrification surfaces.The PVDF films were prepared with different  phase contents in order to see its effect on the electrical output.It was found through the EFM and KPFM measurements that a higher  phase promotes stronger electrostatic interactions with the metal coated cantilever tip which led to a higher amount of electron flow determined from KPFM surface potential studies.Although contact electrification happens at the interface of two materials and is mainly governed by surface electrical properties, it was found that bulk parameters such as thickness also affect the electric output of TENGs.There was overall enhancement of the PVDF-Cu TENGs electric output performance for the 1000 rpm coated films in agreement with the AFM electrical studies.However, in the case of TENGs with 2000 rpm coated PVDF samples with a higher  phase, although the EFM amplitude and change in the KPFM surface potential were the largest, the electric output was reduced due to the dielectric layer thickness effect.The maximum electric output of 8.1 V, 12.2 nA, and 0.0175 nW cm −2 was obtained for the TENG made with 79%  phase PVDF film.Additionally, the electric output contribution of the dipolar polarization charge and surface triboelectrification charge due to CE was compared and based on the results obtained, triboelectrification charge can be considered as the dominant source of charges.

Experimental Section
Polyvinylidene Difluoride Film Preparation and Teng Assembly: Polyvinylidene difluoride (PVDF) pellets (Mw ≈180 000 by GPC from Sigma-Aldrich) were dried in a convection oven at 80 °C for 2 h prior to use.Anhydrous N, N─Dimethylformamide (DMF) (99.8% from Sigma-Aldrich) was filtered through the Al 2 O 3 (basic, from Sigma-Aldrich) powder column for removing water impurities from the solvent.The 15 wt.%PVDF solution was prepared by dissolving 7.5 g of PVDF pellets in 42.5 g of the anhydrous DMF.The solution was magnetically stirred at 100 °C and 100 rpm for 2 h until complete dissolution and left to degas for 24 h.For a substrate, microscopic glass slide with the size of 7.5 × 2.5 cm 2 was cleaned in three steps by sonication in acetone, ethanol, and distilled water consecutively and blow dried with N 2 gas.Moreover, some of the substrates were activated with oxygen plasma treatment (COVANCE, Femto Science) at 100 W power and 100 sccm, 99.999% pure O 2 gas flow for 5 min to create hydroxyl (-OH) groups on the glass surface.The polymer solution was spin coated on both pristine and surface activated glass at 1000 and 2000 rpm for 30 s. followed by drying on a 35 °C hot plate for 5 min.Afterward, the samples were annealed at 60 and 80 °C for 24 h and peeled off in DI water to obtain freestanding PVDF films with different  phase content.Triboelectric nanogenerators were made using copper (Cu) foil as an electrode and a tribopositive material and PVDF films attached on conductive Cu tape as a tribonegative material.Vertical contact separation mode TENGs, with size of 1.0 × 1.5 cm 2 and a separation distance of 10 mm, were assembled by attaching the triboelectrification layers on a PDMS support and connecting Cu wires for electrical measurements.The piezoelectric nanogenerators (PENGs) were assembled by sandwiching PVDF films between Cu tapes with Cu/PVDF/Cu structure and 1.5 × 1.5 cm 2 dimension.
Characterizations: Attenuated total reflection mode fourier transform infrared spectroscopy (ATR-FTIR, Spectrum Two FT-IR, Perkin Elmer) was performed to study phase composition and determine  phase content of PVDF films.Field emission scanning electron microscopy (FE-SEM, S-4700, Hitachi) was used for surface morphology characterizations.Atomic force microscopy (AFM, Park NX10, Park Systems) in electrostatic force microscopy (EFM) mode was used for surface topography and surface electrical property characterizations.EFM amplitude of samples was measured in non-contact EFM mode.Kelvin probe force microscopy (KPFM) potential measurements were done to study the CE process between PVDF and metal coated cantilever tip.First, KPFM potential was measured in non-contact EFM mode and recorded.Afterward, dynamic contact mode EFM was used to scan the PVDF surface and initiate CE when the conductive cantilever tip touches the surface of the sample and charge transfer occurs.Finally, KPFM potential was measured again in non-contact mode EFM.TENGs were triggered with a pushing tester driven by linear motor (LS mecapion) and electrical output voltage and current were measured with an oscilloscope (Keysight, DSOX2012A) equipped with low noise current preamplifier (SR570, Stanford Research Systems).High voltage probe with 100 MΩ resistance was used for voltage measurements while current measured with 10 MΩ probe.Power output was characterized from voltage and current measurements with a load resistance from 1 kΩ to 65 MΩ.

Figure 1 .
Figure 1.a) Molecular structure of  and  phase PVDF.b) Surface activation of glass substrate by RF oxygen plasma treatment.c) Preparation procedure of high  phase containing PVDF films (i) spin coating of PVDF solution, (ii)  phase formation after annealing, and (iii) obtaining free standing film in DI water.

Figure 2 .
PVDF solutions were coated at 1000 and 2000 rpm to

Figure 2 .
Figure 2. The ATR-FTIR spectrum of a) upside and b) downside of the PVDF films spin coated at 1000 rpm and 2000 rpm and annealed at 80 °C.The ATR-FTIR spectrum of c) upside and d) down side of the PVDF films spin coated at 1000 rpm and annealed at 60 and 80 °C.(* for PVDF films coated on surface plasma activated glass).
and TableS1

Figure 3 .
Figure 3.The  phase content dependent EFM amplitude of the PVDF measured on the a) upside and b) downside of the films.The  phase content dependent change in the KPFM potential of the PVDF measured on the c) upside and d) downside of the films.e) Energy band diagram of a charge transfer between a metal tip and PVDF with different  phase ratio.

Figure 4 .
Figure 4. a) Schematic representation of the vertical contact-separation mode PVDF-Cu TENG. phase content dependent output current of the PVDF-Cu TENGs with the CE on b) upside and c) downside of the films. phase content dependent induced charge density of the PVDF-Cu TENGs with the CE on d) upside and e) downside of the films.

Figure 5 .
Figure 5. a) Load resistance dependent power output for the 77%  phase PVDF-Cu TENG.b)  phase content dependent CPD measured with KPFM and power density of the TENGs.