Fluorinated Titania Nanoparticle-Induced Piezoelectric Phase

Jun 7, 2019 - E. valuation of the piezoelectric performa. nce after electric. -. field poling. Output. voltage for the forward connection: PVDF films ...
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Article Cite This: Langmuir 2019, 35, 8816−8822

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Fluorinated Titania Nanoparticle-Induced Piezoelectric Phase Transition of Poly(vinylidene fluoride) Seung-Hyun Kim,†,‡ Jong-Wook Ha,‡ Sang Goo Lee,‡ Eun-Ho Sohn,‡ In Jun Park,‡ Hong Suk Kang,*,‡ and Gi-Ra Yi*,† †

School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea Interface Materials and Chemical Engineering Research Center, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: We prepared F-coated rutile titanium dioxide nanoparticles (rTiO2 NPs) via simple thermal annealing of titania NPs in poly(vinylidene fluoride) (PVDF) and demonstrated that the F-coated r-TiO2 NP-doped composite film could efficiently induce piezoelectric phase transition of nonelectroactive PVDF due to highly electronegative F bonds on the surface of these NPs. In the case of a 2.0 wt % composite film, 99.20% of the nonelectroactive PVDF was transformed into the electroactive phase. Additionally, utilizing the F-coated r-TiO2 NPs for a piezoelectric device led to an enhancement of the piezoelectric performance. With the 5.0 wt % composite film, the resulting piezoelectric device exhibited voltage generation of 355 mV, whereas a device with the innate r-TiO2 NPs exhibited voltage generation of only 137 mV. Furthermore, because of optical inactivity of F-coated r-TiO2 NPs, the piezoelectric films exhibited high stability under 64 h of photoirradiation at an intensity of 0.1 W/cm2. These results indicate that the F-coated r-TiO2 NP-doped composite films could be useful for various applications, including outdoor energy-harvesting, self-powered wearable devices, and portable sensors.

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used as filler materials.21−24 However, the required high concentration of a-TiO2 NPs reduces the flexibility and piezoelectric properties as the fraction of PVDF decreases.25,26 Additionally, because of the photoactive nature of a-TiO2 NPs, the piezoelectric PVDF matrix decomposes over time, degrading the device performance.22,27−29 To solve these problems, we have developed F-coated rutile TiO2 NPs (r-TiO2 NPs) as an alternative filler with no photocatalytic properties,30 but capable of effectively inducing the electroactive PVDF phase. We demonstrate that with only 2.0 wt % of F-coated r-TiO2 NPs, 99% of PVDF was transformed into electroactive-phase PVDF that can be operated under intense sunlight for a considerable amount of time. We also showed substantial improvement of the piezoelectric performance with PVDF doped with F-coated rTiO2 NPs. The voltage generated in a piezoelectric device with 5.0 wt % F-coated r-TiO2 NPs was 355 mV in comparison to 137 mV for a comparable device with uncoated r-TiO2 NPs.

INTRODUCTION Piezoelectric polymers have attracted considerable interest in the past decade due to their potential applications in selfpowered sensors; smart skins and wearables; and portable electronics.1−6 In particular, energy generation from irregular air flow/vibration, ultrasound waves, and persistent physical movement has been extensively explored using piezoelectric polymers. Moreover, the integration of such energy-generation systems with existing microelectronic technology is very simple.2,5,7 Recently, poly(vinylidene fluoride) (PVDF) has attracted considerable interest as a piezoelectric material for its outstanding pyroelectric, piezoelectric, and ferroelectric properties. 7−9 The PVDF polymer also has excellent mechanical properties, including high flexibility, durability, and low density, in addition to high thermal and chemical stability.7,10−16 Among five different crystalline polymorphs of PVDF, the βand γ-phases present strong piezoelectricity and good electroactive properties.7,17 In general, these electroactive PVDF polymorphs are obtained from naturally stable non-electroactive PVDF polymorphs (α-, δ-, and ε-phase) by stretching, heat treatments, electrical poling, or including fillers.13,16−18 Among these approaches, the filler addition is particularly interesting because a uniform electroactive-phase PVDF can be prepared with the significant enhancement of mechanical properties over a large area.19,20 Consequently, metastable anatase TiO2 nanoparticles (a-TiO2 NPs) have been widely © 2019 American Chemical Society

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MATERIALS AND METHODS

Materials. N,N-Dimethylmethanamide (DMF, >97%, SigmaAldrich), PVDF (K-761, Elf Atochem of North America Inc.), and TiO2 particles (R960, Dupont Co.) were used as received.

Received: February 24, 2019 Revised: May 23, 2019 Published: June 7, 2019 8816

DOI: 10.1021/acs.langmuir.9b00546 Langmuir 2019, 35, 8816−8822

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Figure 1. Preparation and analysis of F-coated r-TiO2 NPs. (a) Schematic illustration of the preparation of F-coated r-TiO2 NPs. (b) XRD and (c) XPS wide-scan spectra for F-coated r-TiO2 NPs and untreated r-TiO2 NPs. Scanning electron microscopy (SEM) images of r-TiO2 (d) before and (e) after F coating. The insets indicate energy-dispersive X-ray spectroscopy (EDS) element-mapping images of F, Ti, and O atoms. Scale bars in the insets are 400 nm. Preparation of F-Coated r-TiO2 NPs. To prepare F-coated rTiO2 NPs, rutile TiO2 particles were annealed with PVDF. In a typical procedure, 1.0 g of rutile TiO2 powder and 4.0 g of the PVDF powder were annealed at 500 °C for 6 h in air. The resulting product was purified by successively using the following steps: ethanol treatment, cross-flow filtration, and drying under nitrogen for 10 h at 50 °C. This purification step was repeated three times in order to completely remove the residual PVDF. Fabrication of Composite Films. PVDF/F-coated r-TiO2 NP composite films were prepared by dispersing F-coated r-TiO2 NPs in DMF, followed by PVDF polymer addition. In a typical procedure, Fcoated rutile TiO2 powder (0.05 g) was dispersed ultrasonically in DMF (9.0 g). Then, the PVDF powder (0.95 g) was added and dissolved at room temperature using a magnetic stirrer. The resulting solution (2.0 mL) was coated onto overhead projector films, and dried for 5 min at 80 °C. Piezoelectric Tests of the Composite Films. To measure the piezoelectric properties of the TiO2/PVDF composite films, 2 × 2 cm2 samples with the thickness of 20 μm were prepared. The samples were positioned between two electrodes (Al foils), and flexible polyethylene terephthalate films were used to package the composite films. The poling process was performed in an electric field of strength 400 V for 40 min. Characterization. Colloidal particles and composite films were observed using a scanning electron microscope (Hitachi, S-4300) to measure their size and morphology. Additionally, chemical compositions were analyzed using an Fourier-transform infrared (FT-IR) spectrometer (FT/IR 4100, JASCO) and an X-ray photoelectron spectrometer (ESCALAB 250). The piezoelectric properties were analyzed using a linear power amplifier (PA-151, Labworks Inc.), an electrodynamic shaker (ET-139, Labworks Inc.), and a digital oscilloscope (TBS 2000, Tektronix). We performed all piezoelectric tests at an external force frequency of 3 Hz. Photostability was measured using a solar simulator (Newport 66902).

mechanical and optical properties of polymer composites for display or outdoor applications. For the piezoelectric phase transformation of PVDF, a-TiO2 NPs are widely preferred over r-TiO2 NPs due to a lack of strong polar or electronwithdrawing groups on the surface of r-TiO2 NPs, which are critical for inducing the electroactive phase in PVDF. To overcome this limitation, we have developed a simple method for introducing highly electronegative F groups on the surface of r-TiO2 NPs to induce the piezoelectric phase transformation of PVDF. Owing to the compatibility of F groups with the PVDF film, their dispersion stability can be significantly enhanced. Therefore, a small fraction of NPs (2.0 wt %) can induce complete piezoelectric phase transformation of PVDF. As shown in Figure 1a, the F-coated r-TiO2 NPs were prepared via thermal annealing with the PVDF powder (molecular weight: 440 000 g/mol) at 500 °C for 6 h in air. During the annealing process, PVDF decomposed to generate HF that reacts with the highly reactive r-TiO2 surface. We believe that electronegative Ti−F bonds were formed on the TiO2 surface as described in a previous report.31 The X-ray diffraction (XRD) spectra for the r-TiO2 NPs in Figure 1b confirm that the bulk crystalline phase remained the same after the F surface-functionalization process. The crystal phase of pure r-TiO2 is clearly indicated by the diffraction peaks at 27.2°, 36.1°, and 54.2°, which correspond to the (110), (101), and (211) planes, respectively. The three peaks near 41.3°, 56.3°, and 68.8° correspond to the (111), (220), and (301) planes, respectively, and the six weak peaks near 39.1°, 43.6°, 63.0°, 64.2°, 69.9°, and 76.7° correspond to the (200), (210), (002), (311), (112), and (202) planes, respectively.30 The F coating on the r-TiO2 NPs was confirmed with the Xray photoelectron spectroscopy (XPS), as shown in Figure 1c. Notably, the XPS wide-scan spectra for untreated r-TiO2 NPs exhibited Si 2p and Si 2s peaks, corresponding to 100 and 150 eV, respectively. We believe that commercial r-TiO2 NPs have SiO2 on their surface.32 During the thermal annealing process, SiO2 impurities were completely removed via chemical etching

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RESULTS AND DISCUSSION There are three major phases of titania (TiO2): anatase, brookite, and rutile. Although the anatase and brookite phases are metastable and photocatalytically active, the rutile phase is thermally stable and photocatalytically inactive.30 Therefore, it is desirable to utilize r-TiO2 NPs as fillers to enhance the 8817

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Figure 2. Electroactive phase transition of PVDF. (a) Electron-density map of Ti−F bonds. (b) Illustration of the phase transition from α-phase PVDF to β- and γ-phase PVDF. FT-IR spectra for PVDF films containing (c) uncoated r-TiO2 NPs and (d) F-coated r-TiO2 NPs. FT-IR data of a pure PVDF film is included for comparison. (e) Variations in the electroactive phase (β- and γ-phase) PVDF with respect to the concentrations of r-TiO2 NPs and F-coated r-TiO2 NPs.

between Ti and F is crucial for effective transformation of the non-electroactive α-phase PVDF into electroactive β- or γphase PVDF (Figure 2b).13,16,20 Because of the strong interaction between the negatively charged fillers and the dipolar moments of PVDF, the monomer units in PVDF were preferentially ordered in a trans-conformation for inducing the electroactive phase.33 Using FT-IR spectroscopy, we investigated the influence of the F-coated r-TiO2 NPs on the phase transition of PVDF, as shown in Figure 2c,d. PVDF/TiO2 NP composite films were cast and rapidly dried in a high-temperature air bath. Crosssectional SEM images of a PVDF composite film containing 5.0 wt % F-coated r-TiO2 NPs are presented in Figure S2. The TiO2 NPs were well dispersed in DMF and uniformly distributed in the final PVDF composite film. To evaluate the effect of the filler concentration, four different composite samples were prepared, with 0.5, 1.0, 2.0, and 5.0 wt % Fcoated r-TiO2 NPs or untreated r-TiO2 NPs. The FT-IR spectra for the PVDF/untreated r-TiO2 NP composite films (Figure 2c) exhibited high-intensity peaks corresponding to the non-electroactive α-phase, regardless of the amount used. This indicates that the untreated r-TiO2 NPs had no effect on the phase transition of PVDF. In contrast, the FT-IR spectra for the PVDF/F-coated r-TiO2 NP composite films (Figure 2d) exhibited a significant decrease in the α-phase peaks and a sharp increase in the electroactive β- and γ-phase peaks. Moreover, the peaks corresponding to the electroactive β- and γ-phases gradually increased with the filler concentration,

with HF, as confirmed by the XPS spectra as shown in Figure 1c, where several Ti-related peaks are observed, including Ti 2p and Ti LMM peaks at 450 and 1090 eV, respectively. Additionally, F-related peaks appeared, including F 1s and F KLL peaks at 685.7 and 830 eV, respectively. The other representative peaks for Ti (450 eV), F (685.7 eV), and O (530 eV) before and after the F-coating process are shown in Figure S1. SEM images and EDS data were used for further analysis of the F coating on the surface of the r-TiO2 NPs. The r-TiO2 NPs before the treatment (Figure 1d) had a rough surface; however, the surface became smooth afterward, probably because of the HF etching (Figure 1e). The EDS mapping results (insets of Figure 1e) revealed that F atoms were only present in the F-coated r-TiO2 NPs, confirming that the F coating on the r-TiO2 NPs was successful. According to the TEM EDX results, the ratios of elemental contents of F, Ti, and O were 2.91, 51.09, and 46.00% by weight, respectively. In addition, we observed that the F moiety uniformly covered the surface of TiO2 NPs. Because of inherent high electronegativity of F atoms, the Fcoated r-TiO2 NPs had significant inhomogeneity with regard to the electron density on the surface. As indicated by the electron-density distribution in Figure 2a, which was obtained using the Avogadro and Chem3D simulation tools, a large number of electrons were located around F atoms. The electronegativity difference between Ti and F was 2.5, which is significantly higher than that (1.9) between Ti and OH in the case of a-TiO2 NPs. A large electronegativity difference 8818

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Figure 3. Evaluation of the piezoelectric performance. Output voltage for the forward connection: (a) pure PVDF film, (b) PVDF film containing 5.0 wt % r-TiO2 NPs, and (c−f) PVDF films containing F-coated r-TiO2 NPs with concentrations of (c) 0.5, (d) 1.0, (e) 2.0, and (f) 5.0 wt %. Output voltage for the reverse connection: (g) pure PVDF film, (h) PVDF film containing 5.0 wt % r-TiO2 NPs, and (i−l) PVDF films containing F-coated r-TiO2 NPs with concentrations of (i) 0.5, (j) 1.0, (k) 2.0, and (l) 5.0 wt %.

phase of PVDF increased only slightly as the amount of r-TiO2 NPs increased to 1.0 wt %. As the amount of r-TiO2 NPs increased from 1.0 to 5.0 wt %, the fraction of the electroactive phase remained almost identical, reaching a maximum value of 30%. In contrast, for the composite film with F-coated r-TiO2 NPs, the fraction of the electroactive phase of PVDF sharply increased as the amount of F-coated r-TiO2 NPs increased. Interestingly, with 2.0 wt % F-coated r-TiO2 NPs, the α-phase completely disappeared, and the maximum electroactive phase fraction was 98.20%. These results confirm that the proposed F-coated r-TiO2 NPs are highly effective for inducing the electroactive phase of PVDF, even at low concentrations. To evaluate the piezoelectric performance of the TiO2/ PVDF composite films, 2 × 2 cm2 samples with the film thickness of 20 μm were prepared and connected to electrodes, as shown in Figure 3. The piezoelectric output was obtained by applying an external force of 2.5 N/cm2 and an external force frequency of 3 Hz. When the composite films were subjected to a compressive stress, a piezoelectric field was established across their thickness, driving the electrons to flow in alternating directions in the external circuit, which resulted in an alternating output. The piezoelectric outputs for (i) a pure PVDF film, (ii) a PVDF/untreated r-TiO2 NP composite

implying that the electroactive-phase amount depended on the filler amount. The fraction of the electroactive phase of PVDF in the film can be quantitatively calculated using the following equation.34,35 Fraction of electroactive phase in PVDF film Xe = Xα + Xe Ae = Ke Aα + Ae K

( ) α

(1)

Here, Xe and Xα are the mole fractions of the electroactive phase and α-phase, respectively; Ke =0.150 and Kα =0.365 μm−1 are the permeability coefficients of the electroactive phase and α-phase, respectively; and Ae =840 cm−1 and Aα =764 cm−1 are the absorption intensities of the electroactive phase and α-phase, respectively, as shown in the FT-IR spectra (Figure 2c,d). The fractions of the electroactive phase in the PVDF composite film according to eq 1 are plotted in Figure 2e. These results clearly indicate that for the composite film with uncoated r-TiO2 NPs, the fraction of the electroactive 8819

DOI: 10.1021/acs.langmuir.9b00546 Langmuir 2019, 35, 8816−8822

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Figure 4. Photodegradation of the PVDF piezoelectric films due to F-coated r-TiO2 NPs and a-TiO2 NPs. (a) Degree of decomposition of the PVDF piezoelectric films due to light irradiation for different periods and corresponding SEM images. The yellow dotted lines in the SEM image for a light-irradiation period of 4 h indicate the PVDF residue. The scale bars in the insets represent 1 μm. (b) FT-IR measurements for the PVDF/ F-coated r-TiO2 NP composite film for different light-irradiation periods.

film (5.0 wt %), and (iii) PVDF composite films with 0.5, 1.0, 2.0, and 5.0 wt % F-coated r-TiO2 NPs are shown in Figure 3 for both forward and reverse connections. The pure PVDF film and the PVDF composite film with 5.0 wt % untreated r-TiO2 NPs exhibited voltage outputs of 135 and 127 mV, respectively. A slight performance decrease was observed for the PVDF composite film with untreated r-TiO2 NPs compared with the pure PVDF film, which is ascribed to the decrease in the amount of PVDF per unit area. This clearly indicated that the amount of untreated r-TiO2 NPs in the PVDF matrix had no effect on enhancing the piezoelectricity. Notably, for the PVDF composite films with F-coated r-TiO2 NPs, the piezoelectric output increased as the amount of Fcoated r-TiO2 NPs increased (157 mV for 0.5 wt %, 256 mV for 1.0 wt %, 311 mV for 2.0 wt %, and 355 mV for 5.0 wt %), as shown in Figure 3a−f. It is clear from Figure 3 that the signals appeared successively and persistently. To confirm that the detected electrical signals were indeed due to the piezoelectric effect of the composite films, a polarity-switching test (reverse connection) was conducted, and the results are shown in Figure 3g−l. The output voltage was reversed because of the negative polarization of the composite film. The asymmetric profiles of output voltages in forward and reverse connection are ascribed to roughness difference between top and bottom surfaces of the film (Figure S3). Furthermore, we measured the piezoelectric performance for films with 5.0 wt % untreated r-TiO2 NPs and 5.0 wt % Fcoated r-TiO2 NPs, as shown in Figure S4. The output currents for these films were 0.5 and 1 μA, respectively, indicating that the F-coated r-TiO2 NPs enhanced not only the output voltage but also the output current. The enhancement of the device performance was due to the increase of the electroactive phase of PVDF, as shown in Figure 2. The piezoelectric performance can increase considerably in the presence of a poling E-field. Figure S5 shows that the PVDF films containing 5.0 and 1.0 wt

% F-coated r-TiO2 NPs showed output voltages of 2.37 and 0.8 V, respectively. The F-coated r-TiO2 NPs allowed efficient phase transition of PVDF and demonstrated high photostability even under intense sunlight. Such high photostability has rarely been achieved for PVDF films with a-TiO2 NPs and is ascribed to inactive photocatalytic property of the r-TiO2 NPs. This photostable characteristic of the PVDF composite films with rTiO2 NPs allows for their potential use in outdoor energyharvesting devices, such as outdoor wearable devices and selfpowered automatic devices, as well as power plants. To demonstrate the photostability of the F-coated r-TiO2 NPs, we performed a photodegradation experiment using a lamp that illuminates a broad spectrum of waves ranging from 200 to 1500 nm with an intensity of 0.1 W/cm2. After the light exposure, the composite films were examined using SEM, and the degree of decomposition was estimated by measuring the ratio of the decomposed PVDF area to the total area before light irradiation using the open-source image analysis program ImageJ. Figure 4a shows the photodecomposition of the piezoelectric film with respect to the irradiation time and the corresponding SEM images. For the PVDF composite film with a-TiO2 NPs, the SEM images revealed that the morphology of PVDF was severely damaged even for a short period, even though PVDF is generally rated as a good photoresistant polymer. The crack propagation on the surface at the early stages of light exposure is attributed to the absorption of light energy on the surface, which led to the generation of a considerable number of free radicals from a-TiO2, causing surface degradation. In the case of the PVDF composite film with a-TiO2 NPs, 99% of PVDF decomposed after only 64 h; thus, only a-TiO2 NPs remained (inset SEM image in Figure 4a). As previously mentioned, the PVDF composite film with F-coated r-TiO2 NPs exhibited good photostability (red line in Figure 4a). It retained its 8820

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original morphology even after 64 h. Inset SEM images in Figure 4a showed nearly identical images to the pure PVDF film. To evaluate the effect of the light irradiation on the phase transition of PVDF, FT-IR measurements were performed on the PVDF composite film with F-coated r-TiO2 NPs with different light-irradiation periods (Figure 4b). The PVDF crystallinity did not change with increase of the lightirradiation time; the profiles of PVDF in the composite films with F-coated r-TiO2 NPs were unchanged after light irradiation. Calculations using eq 1 indicated that all lightirradiated samples had an electroactive-phase ratio of >99.9%, confirming that our PVDF composite films with F-coated rTiO2 NPs were highly stable against photodegradation.

CONCLUSIONS We have developed an F-coated r-TiO2 NP filler that can effectively induce the electroactive phase of the PVDF composite films with high photostability. The key idea was the utilization of optically inactive r-TiO2 NPs rather than aTiO2 NPs, which are commonly used as PVDF phasetransition fillers. Because bare r-TiO2 NPs lack polar surface groups critical for inducing the electroactive phase in PVDF, we modified the surface of r-TiO2 NPs with the highly electronegative fluorine. With the PVDF composite film doped with 2.0 wt % of F-coated r-TiO2 NPs, 99.20% of the nonelectroactive-phase PVDF was transformed into electroactivephase PVDF, significantly enhancing the piezoelectric performance. In particular, the voltage generation for a piezoelectric device increased from 137 mV for the PVDF film with 5.0 wt % of bare r-TiO2 NPs to 355 mV for the PVDF film with 5.0 wt % of F-coated r-TiO2 NPs. Furthermore, the PVDF film with F-coated r-TiO2 NPs retained its original performance even after a long light exposure, confirming that the proposed PVDF/F-coated r-TiO2 NP composite film is highly robust to photodegradation. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00546. Magnified XPS spectra of the TiO2 particles, SEM images of PVDF/F-coated r-TiO2 NPs composite film, output current of composite films, and output voltage of composite film after electric-field poling (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.K.). *E-mail: [email protected] (G.-R.Y.). ORCID

Hong Suk Kang: 0000-0002-6581-6639 Gi-Ra Yi: 0000-0003-1353-8988 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the KRICT basic research fund and NRF grant no. 2019R1C1C1004967. G.-R.Y. acknowledges support from NRF (Korea) under award no. 2017M3A7B8065528. 8821

DOI: 10.1021/acs.langmuir.9b00546 Langmuir 2019, 35, 8816−8822

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DOI: 10.1021/acs.langmuir.9b00546 Langmuir 2019, 35, 8816−8822