Improvement of Magnetodielectric Coupling by Surface

Aug 28, 2014 - Ehab H. Abdelhamid , O. D. Jayakumar , Vasundhara Kotari , Balaji P. ... Biswajoy Bagchi , Nur Amin Hoque , Sukhen Das , Papiya Nandy...
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Improvement of Magnetodielectric Coupling by Surface Functionalization of Nickel Nanoparticles in Ni and Polyvinylidene Fluoride Nanohybrids Balaji P Mandal,*,† Katari Vasundhara,† Ehab Abdelhamid,§ Gavin Lawes,§ Hemant G. Salunke,‡ and Avesh K. Tyagi*,† †

Chemistry Division and ‡Technical Physics Division, Bhabha Atomic Research Centre, Mumbai − 400085, India § Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48202, United States S Supporting Information *

ABSTRACT: A facile surface hydroxylation treatment to modify the surface of nickel nanoparticles dispersed in ferroelectric polyvinylidene fluoride (PVDF) is reported. A remarkable increase in the amount of polar β phase of PVDF has been observed upon dispersion of nickel nanoparticles in the PVDF matrix. The leakage current reduces drastically in the composite with surface-modified nickel nanoparticles. This improvement has been observed with concurrent enhancement of electrical polarization in the surfacemodified nickel−PVDF composite. However, the magnetic moment of the modified composite is found to be slightly lower than that of the unmodified composite. Most importantly, the composite with surface-modified nickel exhibits magnetodielectric coupling significantly higher than that of the unmodified composite. The magnetodielectric coupling appears to be facilitated by surface hydroxyl groups on nickel. These results establish the potential of facile surface functionlization of nanoparticles toward the design of nanocomposites with higher magnetoelectric coupling.

1. INTRODUCTION Multiferroic materials simultaneously possess at least two “ferroic” orderings, among ferroelectricity, ferromagnetism, and ferroelasticity.1,2 The coexistence of ferroelectricity and ferromagnetism with eventual cross-coupling in a material is most desirable for next-generation device applications. These materials have been attracting ever increasing interest because of their wide applications in multistate data storages, spin filters, spintronics, etc.2,3 It is noteworthy that multiferroics with significant magnetoelectric coupling are very rare in singlephasic materials because of antagonistic symmetry requirement for each type of ferroic order. The magnetic ordering is associated with unpaired (partially filled) d or f electrons, whereas ferroelectricity requires an empty outer d orbital for stabilization of the nonpolar phase.4 Moreover, the magnetic and electric ordering of multiferroic materials must couple with each other in such a way that magnetization can be achieved by applying electric field and electrical polarization can be realized by applying magnetic field. In the past few years, several classes of compounds including BiFeO3, rare-earth manganates, BaMF4 (M= Co, Ni), etc. have been investigated. However, most of them have low Currie temperature and/or poor magnetoelectric coupling which prevents application in devices.5 These issues can be addressed by designing composite materials of different properties. Among composite materials, © XXXX American Chemical Society

different combinations in various geometries have been attempted during the past decade. In composite multiferroics, none of the constituent phases shows magnetoelectric (ME) effect, but the interaction between the two phases generates a third functionality, i.e., magnetoelectric (ME) effect.5,6 Venkataiah et al. established theoretically that magnetoelectric coupling occurs via interfacial effect in heterostructured multiferroics.7 For example, ferrite/piezoelectric ceramic composites, such as CoFe2O4/BaTiO3, CoFe2O4/lead zirconate-titanate (PZT), and Tb−Dy−Fe alloys (Terfenol-D)/ PZT, have been found to exhibit magnetoelectric coupling much higher than that of the single-phasic multiferroics.8 Narayanan et al. have shown that the Co@BaTiO3 core−shell nanotube structure gives significant magnetodielectric coupling.9 Traditionally, multilayer ceramic composites have been used for designing composite multiferroics; however, ceramics have several disadvantages, such as difficulty in processing, poor flexibility, heavy weight, inability to be designed in complicated shapes, etc. On the other hand, polymer-based multiferroic composites can circumvent some of these issues. Few studies on development of PVDF/metglas laminate composites and Received: July 2, 2014 Revised: August 21, 2014

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2.4. Characterizations. The phase purity of the all the samples was characterized by XRD using a Philips PW1820-Xray diffractometer coupled with a PW 1729 generator operated at 30 kV and 20 mA. A graphite crystal monochromator was used for generating monochromatic Cu Kα radiation. Fourier transform infrared (FTIR) spectra of modified nickel, PVDF, 10%Ni-PVDF, and 10%Ni (modified)-PVDF were recorded using a Bomem MB102FTIR (model 610) equipped with a DTGS detector. Both sides of the thick films were painted with Electrolube’s conductive silver paint and dried at room temperature. Then the polarization and leakage current of the films were measured using aixACCT’s TF analyzer 2000. All the hysteresis polarization measurements were carried out at 100 Hz under the field 200 kV/cm at room temperature. The room-temperature magnetization measurements were carried out using Quantum Design physical property measurement system unit (PPMS). The zero-field-cooled (ZFC) measurements were performed by cooling the sample in zero field to 5 K, and then the magnetization is measured while warming in a field of 100 Oe. In field-cooled (FC) measurements, the sample is cooled in a field of 100 Oe to 5 K, and the moment is evaluated while warming to ∼300 K. A thin layer of Ag paint was applied to the top and bottom of the films separately to which Au wires were attached to make parallel plate capacitor. An Agilent 4284A LCR meter was used to measure the dielectric signal at a frequency of 30 kHz under a 100 mV excitation, while PPMS was used to provide the magnetic field. Background correction was made to extract the magnetodielectric data.

multilayered PVDF/terfenol-D 2-2 composites have been reported in the recent past. However, these works were focused on the importance of laminate configuration and highpermeability magnetostrictive layers in the composites. Some other works have also reported that the ferroelectric nature of the polymer phase has been increased in several ways, which subsequently enhances the ME effect.10 Nan’s group has found that the dielectric constant can be improved by adding Ni nanoparticles in PVDF matrix; however, that study focused on dielectric properties only.11 In multiferroic composites, when a magnetic field is applied, the magnetic phase undergoes a deformation because of magnetostriction and transfers the strain to the piezoelectric phase which in turns generates electrical polarization.10 Therefore, the coupling can be enhanced by tailoring the strain-transfer efficiency of the magnetic phase. No effort could be found in the literature which describes improvement of strain-transfer efficiency. In this work, a new method has been developed to enhance the magnetodielectric coupling of the composite consisting of ferromagnetic Ni and ferroelectric PVDF. The surface of the Ni nanoparticles has been functionalized by reaction with H2O2, and the nanocomposite was prepared with PVDF. The functionalized nickel surface gives better linkages between Ni and PVDF. A significant improvement in magnetodielectric coupling has been achieved in the case of surface-modified NiPVDF nanohybrid.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ni Nanoparticles. Initially, nickel nitrate hexa hydrate [Ni(NO3)2·6H2O] was dissolved in 30 mL of ethylene glycol, and then the required amount of hydrazine hydrate was added to this solution under stirring conditions. The solution was heated at 140 °C until the solution turned black. The obtained black Ni nanoparticles were separated using a strong magnet. The obtained Ni nanoparticles were washed 4−5 times with distilled water, once with methanol, and dried at room temperature overnight. 2.2. Surface Modification of Ni Nanoparticles. Ni nanoparticles (200 mg) were dispersed in 100 mL of H2O2 and refluxed at 100 °C for 4 h. The precipitate was then filtered from the H2O2 solution, washed several times with deionized water, and finally washed with methanol. The obtained particles were dried at room temperature overnight. 2.3. Preparation of Ni-PVDF Nanocomposite. Nanocomposite films with different concentrations of nickel nanoparticles in PVDF were prepared. A known amount of as-prepared nickel nanoparticles was dispersed in 10 mL of DMF sonicated for 15 min. PVDF (1 g) was added to the above solution, and the solution was sonicated again for 30 min until a well-dispersed gel type liquid was obtained. The solution was poured onto a glass slide and dried at 60 °C. The samples with following compositions were prepared: PVDF, PVDF-10 wt % Ni, PVDF-20 wt % Ni, and PVDF-40 wt % Ni. The samples are denoted as PNi1, PNi2, and PNi4 for the samples PVDF-10% Ni, PVDF-20% Ni, and PVDF-40% Ni, respectively. The surface-modified nickel nanoparticles were also dispersed in PVDF in the same manner. These samples are denoted as PNim1, PNim2, and PNim4 for the surfacemodified nickel−PVDF samples PVDF-10 wt % Ni (modified), PVDF-20 wt % Ni (modified), and PVDF-40 wt %Ni (modified), respectively.

3. RESULTS AND DISCUSSION The transparent, free-standing, and flexible nature of PVDF in shown in photographs (Figure 1). After ferromagnetic nickel

Figure 1. Flexibility and free-standing nature of PVDF and PVDF-Ni films.

nanoparticles are loaded on PVDF, the color of the thick films turns to black without compromising its flexibility and freestanding nature, which are very important criteria for fabrication of new devices. The XRD patterns of pure PVDF and Ni/PVDF composites are shown in Figure 2. The peaks corresponding to any possible impurity phases were not observed in any of these XRD patterns. The XRD pattern of pristine PVDF is shown in Figure 2a. PVDF is known to crystallize in α, β, and γ forms at room temperature. The most intense peak at 20.3° is the summation of (110) and (200) peaks, and the minor peak at 21.5° is characteristic of γ phase of PVDF.12 The XRD peaks at 2θ = 17.7°, 18.4°, and 19.9° corresponding to α phase of PVDF can be observed, suggesting the PVDF used in this experiment is a B

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Figure 3. FTIR spectra of (a) hydroxylated Ni, (b) pristine PVDF, (c) PNi1, and (d) PNim1. Inset in panel a shows the presence of hydroxyl group on nickel nanoparticle.

Figure 2. Typical XRD patterns of (a) PVDF, (b) PNi1, and (c) PNim1 composite.

graphene oxide-loaded PVDF samples.19,20 The content of β phase further increases on using surface-modified nickel nanoparticles as the ratio I840/I760 becomes 8.7 for 10% modified Ni loaded PVDF (PNim1) sample. This observation establishes that the surface coating agent also has profund effect on phase formation of PVDF. Earlier, Martins et al. reported that different surfactants on CoFe2O4 nanoparticles significantly influence the formation of β phase content of PVDF.21,13 Highly nickel-loaded sample (20 and 40%) were nontransparent; therefore, FTIR could not be recorded on those samples. The characterization of ferroelectric materials from polarization versus electric field (P-E) loop is very important; however, very often it gives misleading inferences regarding the nature of the material. Scott has reported that in true ferroelectrics, the electrical polarization saturates at certain field and exhibits a concave nature in the polarization−electric field curves.22 In the present investigation, the samples were stretched a little bit before ferroelectric measurement to remove the center of symmetry of spherulitic structure.23,24 It is well-known that β-PVDF is the ferroelectric backbone which shows saturation in the polarization curve.21 In the nickel-loaded samples too, the saturation polarization and concave region could be observed as for true ferroelectrics. The saturation polarization (Ps) of the pristine PVDF film increases upon 10% nickel loading (PNi1), as shown in Figure 4. The increase in polarization in PNi1 could be due to formation of more polar β phase of PVDF in the presence of nickel nanoparticles. The increase in β phase was also observed in XRD and FTIR study discussed earlier. However, high electrical field (more than 250 kV/cm) could not be applied on this sample because of its lower breakdown volatge, owing to Ni loading. Other highly nickel-loaded samples, i.e., 20%Ni-PVDF (PNi2) and 40%Ni-PVDF (PNi4) (Figure S1 of Supporting Information) also suffer from similar problems. The breakdown voltages of these highly loaded NiPVDF samples are even lower. The ferroelectric or multiferroic materials with lower leakage current are desirable for possible applications in devices. The leakage current of a thick film of pure PVDF is found to be 0.01

mixture of nonpolar α phase and polar β phase. The ratio of intensity of peaks at 20.3 and 18.4 ratio (I20.3/I18.4) provides an estimate of α and β phase content in the mixture. In pure PVDF, the ratio is found to be 0.7, which increases to 3.4 in PNi1. This result suggests that β content of PVDF increases with nickel loading. The content of β phase further increases wjem using surface-modified nickel nanoparticles as the ratio (I20.3/I18.4) becomes 5.9 for 10% modified Ni loaded PVDF (PNim1) sample. Similar observations have been found in FTIR studies also. Fourier transform infrared spectroscopy is one of the best techniques for distinguishing the α, β, and γ phases of PVDF. However, some bands are simultaneously associated with β and γ phases, whereas some are asscoiated to amorphous phase of PVDF.13 FTIR study has been performed on surface-modified nickel, pristine PVDF, nickel, and modified nickel-loaded PVDF samples (Figure 3). The band in the range of 3200−3600 cm−1 (Figure 3a) corresponds to the hydroxyl groups that are attached to the nickel nanoparticle.14 The absorption at ∼570 cm−1 also corresponds to the Ni−OH bond.15 These observations confirm the formation of hydroxyl groups on the surface of nickel nanoparticles. The FTIR spectrum of PVDF exhibited the presence of α and β forms of PVDF (Figure 3b). The infrared bands have been assigned following the previous work on PVDF.16 The characteristic bands observed at 487, 612, 760, 796, 853, and 975 cm−1 correspond to α phase of PVDF, whereas the bands at 840, 511, 878, and 472 cm−1 indicate the presence of β phase of PVDF.17,18 The relative amount of β and α phase of PVDF in different samples have been estimated by calculating the relative ratio of the intensity of the characteristic peaks of α (760 cm−1) and β phase (840 cm−1). The ratio I840/I760 for virgin PVDF and 10% Ni loaded PVDF (PNi1) are found to be 1.9 and 3.9, respectively, suggesting the amount of polar β phase increases with nickel loading. Perhaps the nickel nanoparticles are acting as nucleating agents for formation of β phase in PVDF/Ni composites. The improvement of β content has been found earlier in silver, carbon nanotube, and C

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Figure 5. Comparison of leakage current of (a) PVDF, (b) PVDF-10% Ni, and (c) PVDF-10% Ni (surface functionalized).

Figure 4. Electrical field-dependent polarization of (a) pure PVDF, (b) Ni-PVDF, and (c) hydroxylated Ni-PVDF composite.

lower amount of nickel; therefore, it cannot form a conductive network. The reduction of the leakage current and improvement in polarization in hybrid films containing functionalized Ni is attributed to the improved interface between ferromagnetic and ferroelectric phases. The schematic in Figure 6 illustrates the

μA/cm2 at 100 kV/cm and shows a sudden increase upon Ni loading (Figure S2 of Supporting Information). The leakage current for PNi1 increases to 4.332 μA/cm2, whereas the PNi2 sample exhibits abrupt increase in leakage current above the field 50 kV/cm. The sudden rise in leakage current for PNi4 starts at an even lower field, i.e, 20 kV/cm onward. With an increase in nickel amount in the composites, the proximity of the conducting nickel nanoparticles increases, which leads to a decrease in resistivity; therefore, the ferroelectric samples become more and more leaky. Among the samples, the polarization of 10%Ni-PVDF composite exhibits the highest polarization; however, its moderately high leakage current is a point of concern for practical application. To achieve higher polarization without compromising the leakage current, the surface of the nickel nanoparticles was modified and 10% modified nickel nanoparticles were dispersed in the PVDF matrix. Other samples showed relatively less polarization and more leaky behavior; therefore, the data corresponding to only 10% nickel-PVDF samples have been discussed here. A remarkable improvement in polarization (0.5 μC/cm2) has been observed in the surface-modified nickel (10%)-PVDF composite (PNim1), as shown in Figure 4. A very high field (550 kV/cm) could be applied to the sample. The leakage current of the surface-modified nickel-PVDF was measured and is presented in Figure 5. Strikingly, the leakage current of the surface-modified sample decreases markedly compared to that of the unmodified sample. Another important observation is that the breakdown voltage considerably increases in the sample with surface-modified nickel and PVDF. Similar kind of observation has also been found in earlier literature on BaTiO3−PVDF.25,26 The contribution of leakage current toward the polarizations of the samples (PNi1 and PNim1) is expected to be very low because the resistivity of both the samples is very high (109− 1011Ω cm). To further reduce the effect of leakage current, if at all there, the polarization measurement has been done at relatively high frequency (100 Hz). It is worth mentioning that the effect of leakage current in electrical polarization is low in the percolative composites having conductive filler concentration below percolation threshold. Dang et al. reported that in a Ni-PVDF system the percolation threshold has been found to be ∼17 vol % (equivalent to 50.6 wt %) of nickel.11 The sample used in this experiment, i.e., 10% wt. Ni-PVDF, contains a

Figure 6. Schematic of hydroxylated Ni-PVDF composite.

interaction between the surface-modified nickel nanoparticles and PVDF. The −OH groups on nickel nanoparticles can support strong dipole interaction between the fluorine atoms of PVDF, which results in the better dispersion of the nanoparticles in the polymer matrix. In addition, the charge trapping by surface hydroxyl groups minimizes possible charge conduction pathways in the film, resulting in considerable reduction in the leakage currents. A similar observation was recorded in dielectric study of hydroxylated BaTiO3−polymer composites.25,26 The magnetic field-dependent magnetization data for hydroxylated nickel nanoparticles at 5 and 300 K are shown in the inset of Figure 7 indicating the ferromagnetic nature of the samples. The coercivity of hydroxylated nickel nanoparticles was found to be 275 and 160 Oe, whereas saturation magnetization was 20 and 18.8 emu/g at 5 and 300 K, respectively. Nickel is a well-studied ferromagnetic material with saturation magnetization of 58 emu/g for bulk samples.27,28 However, the sample used in this experiment showed a lower magnetic moment. The plausible reason could be that the particles were in the nano regime and some hydroxyl groups were attached to these nanoparticles. It has been reported earlier that with decrease in particle size the saturation magnetization decreases.29,30 Additionally, it has D

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Figure 7. Magentization of (a) Ni-PVDF (PNi1) and (b) functionlized Ni-PVDF (PNim1) composite. Inset shows the magnetization of functionalized nickel nanoparticle at 5 K (red) and 300 K (black).

been reported that a decrease in magnetization of metal nanoparticles takes place when the nanoparticles are functionalized with different ligands, which supports our findings.31,32 The magnetic field-dependent magnetizations of PNim1 and PNi1, i.e., 10% modified Ni and nonmodified Ni dispersed in PVDF matrix, are shown in Figure 7. The saturation magnetic moments were found to be 2.1 and 2.0 emu/g for PNi1 and PNim1, respectively. The decrease in moment of the composite from pure magnetic nanoparticles is obviously due to the presence of diamagnetic PVDF phase, which reduces the fraction of magnetic constituent. On the other hand, the small decrease in magnetic moment of the modified composite (PNim1) in comparison with that of the unmodified composite is attributed to the extra hydroxyl groups present on the nickel nanoparticles. Temperature-dependent magnetization study has been done on these composites. The ZFC curves for functionalized nickel show that the magnetic succeptibility decreases with decreasing temperature, whereas the FC curve shows nearly constant magnetic succeptibility. The sample PNim1 composite also shows a similar trend (Figures S3 and S4 of Supporting Information). These curves also indicate that there is no secondary phase in the system. The magnetization studies on pristine PVDF reveal it to be diamagnetic in nature, as expected (figure not shown). The coexistence of ferromagnetic and ferroelectric order in these multiferroic composites is not enough for the proposed applications as transducer, multistate data storage devices, or magnetically tunable electronics devices. The composite materials need to show appreciable coupling between these physical properties. The coupling between magnetic and electric polarizations can be measured by observing the change in dielectric constant under magnetic field. The change in dielectric constant with applied magnetic field is characterized by the magnetodielectric constant defined as MD= (εH − ε0)/ ε0 × 100%, where εH and ε0 are the dielectric constants with and without the applied magnetic field, respectively. The PVDF-modified Ni10% (PNim1) sample showed the highest polarization along with the lowest leakage current among the samples; therefore, it has been chosen for magnetocapcitive measurement. In order to compare, the composite PNi1 was also tested under similar conditions (Figure 8). Both the

Figure 8. Magnetodielectric (MD = (εH − ε0)/ε0 × 100%) coupling of the (a) Ni-PVDF and (b) hydroxylated Ni-PVDF samples. The curves have been fitted in parabolic functions. The errors in fitting are 1.6% and 1.8% in panels a and b, respectively.

samples showed a linear drift in dielectric constant. The dielectric constants were adjusted by subtracting the linear background. The obtained curve could be fitted well in parabolic equation as (εH − ε0)/ε0 = γH2, originating from the lowest-order coupling terms polarization (P) and magnetization (H) in the free-energy term, as described in the literature.33 The value of γ in unmodified samples was found to be 6.1 × 10−12, whereas that for the modified sample was 7.9 × 10−12. The higher value of γ in PNi1m suggests that the surfacemodified Ni-PVDF composite has shown magnetodielectric coupling higher than that of the unmodified analogue. Magnetodielectric effect arises from terms proportional to P2M2 in a symmetry-allowed Ginzburg−Landau free energy, where P and M are electrical polarization and magnetization, respectively. In these samples, the magnetodielectric showed quadratic dependence on magnetization, which is observed in many magnetoelectric multiferroic materials.34,35 As is mentioned above, the magnetodielectric coupling arises in the composites as a result of the product of the magnetostrictive effect of the magnetic phase and the piezoelectric effect of the ferroelectric phase.36 The coupling of electric and magnetic phenomena takes place through elastic interaction between these two phases. Hence, the magnetoelectric effect in these composite systems is extrinsic in nature, which depends on the composite microstructure and coupling interaction across the magnetic and ferroelectric interfaces.37 In these multiferroic hybrids, the applied magnetic field probably modifies the magnetic alignment of nickel nanoparticles, generating a stress on adjoining ferroelectric PVDF phase via magnetostriction, and eventually leading to some surface charges by the piezoelectric effect.38 The hydroxyl groups on nickel nanoparticles cause better bonding with the ferroelectric E

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(6) van Suchtelen, J. Product Properties: A New Application of Composite Materials. J. Philips Res. Rep. 1972, 27, 28−37. (7) Venkataiah, G.; Shirahata, Y.; Itoh, M.; Taniyama, T. StrainInduced Manipulation of Magnetic Coercivity of Fe Film in Fe/ BaTiO3 Heterostructures by Electric Field. Appl. Phys. Lett. 2011, 99, 102506. (8) Kim, D. H.; Aimon, N. M.; Sun, X.; Ross, C. A. Compositionally Modulated Magnetic Epitaxial Spinel/Perovskite Nanocomposite Thin Films. Adv. Funct. Mater. 2014, 24, 2334−2342. (9) Narayanan, T. N.; Mandal, B. P.; Tyagi, A. K.; Kumarasiri, A.; Zhan, X.; Hahm, M. G.; Anantharaman, M. R.; Lawes, G.; Ajayan, P. M. Hybrid Multiferroic Nanostructure With Magnetic−Dielectric Coupling. Nano Lett. 2012, 12, 3025−3030. (10) Jin, J.; Lu, S.; Chanthad, C.; Zhang, Q.; Haque, M. A.; Wang, Q. Multiferroic Polymer Composites with Greatly Enhanced Magnetoelectric Effect under a Low Magnetic Bias. Adv. Mater. (Weinheim, Ger.) 2011, 23, 3853−58. (11) Dang, Z. M.; Lin, Y. H.; Nan, C. W. Novel Ferroelectric Polymer Composites with High Dielectric Constants. Adv. Mater. (Weinheim, Ger.) 2003, 15, 1625−1629. (12) Ansari, S.; Giannelis, E. P. Functionalized Graphene Sheet Poly(Vinylidene Fluoride) Conductive Nanocomposites. J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 888−897. (13) Martins, P.; Lopes, A. C.; Lanceros-Mendez, S. Electroactive Phases of Poly(vinylidene fluoride): Determination, Processing and Applications. Prog. Polym. Sci. 2014, 39, 683−706. (14) Hiroki, A.; LaVerne, J. A. Decomposition of Hydrogen Peroxide at Water−Ceramic Oxide Interfaces. J. Phys. Chem. B 2005, 109, 3364−3370. (15) Hengbin, Z.; Hansan, L.; Xuejing, C.; Shujia, L.; Chiachung, S. Preparation and Properties of the Aluminum-Substituted α-Ni(OH)2. Mater. Chem. Phys. 2003, 79, 37−42. (16) Lanceros-Mendez, S.; Mano, J. F.; Costa, A. M.; Schmidt, V. H. FTIR and DSC Studies of Mechanically Deformed β-PVDF Films. J. Macromol. Sci., Part B: Phys. 2001, 40, 517−527. (17) Betz, N.; Le Moel, A.; Balanzat, E.; Ramillon, J. M.; Lamotte, J.; Gallas, J. P.; Jaskierowicz, G. A FTIR Study of PVDF Irradiated by means of Swift Heavy Ions. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 1493−1502. (18) Ahn, Y.; Young, L. J.; Soon Man, H.; Jaerock, L.; Jongwook, H.; Jin, H. C.; Seo, Y. Enhanced Piezoelectric Properties of Electrospun Poly(Vinylidene fluoride)/Multiwalled Carbon Nanotube Composites due to High β-Phase Formation in Poly(Vinylidene fluoride). J. Phys. Chem. C 2013, 117, 11791−11799. (19) Mandal, D.; Henkel, K.; Schmeisser, D. Comment on “Preparation and Characterization of Silver-Poly(Vinylidene Fluoride) Nanocomposites: Formation of Piezoelectric Polymorph of Poly(Vinylidene Fluoride). J. Phys. Chem. B 2011, 115, 10567−10569. (20) Yu, S.; Zheng, W.; Yu, W.; Zhang, Y.; Jiang, Q.; Zhao, Z. Formation Mechanism of β-Phase in PVDF/CNT Composite Prepared by the Sonication Method. Macromolecules 2009, 42, 8870−8874. (21) Martins, P.; Caparros, C.; Goncalves, R.; Martins, M.; Benelmekki, P. M.; Botelho, G.; Lanceros-Mendez, S. Role of Nanoparticle Surface Charge on the Nucleation of the Electroactive β-Poly(vinylidene fluoride) Nanocomposites for Sensor and Actuator Applications. J. Phys. Chem. C 2012, 116, 15790−15794. (22) Scott, J. F. Ferroelectrics Go Bananas. J. Phys.: Condens. Matter 2008, 20, 021001. (23) Kawai, H. The Piezoelectricity of Poly (vinylidene fluoride). Jpn. J. Appl. Phys. 1969, 7, 975−976. (24) Martins, P.; Costa, C. M.; Ferreira, J. C. C.; Lanceros-Mendez, S. Correlation Between Crystallization Kinetics and Electroactive Polymer Phase Nucleation in Ferrite/Poly(Vinylidene Fluoride Magnetoelectric Nanocomposites. J. Phys. Chem. B 2012, 116, 794− 801. (25) Almadhoun, M. N.; Bhansali, U. S.; Alshareef, H. N. Nanocomposites of Ferroelectric Polymers with Surface-Hydroxylated

PVDF phase; therefore, the strain arising because of magnetostriction of nickel nanoparticles could easily be transferred to the ferroelectric PVDF phase. Therefore, the magnetodielectric coupling of the functionalized Ni-PVDF composite is found to be higher in magnitude.

4. CONCLUSIONS Flexible and free-standing films of PVDF loaded with Ni can be fabricated successfully. The amount of polar β phase of PVDF increases upon loading of nickel nanoparticles in PVDF. Fielddependent electrical polarization measurements confirm the ferroelectric behavior of the composite films with low leakage current. The surface functionalization on nickel could substantially improve the polarization and also reduce the leakage current. Magnetodielectric coupling in these nanocomposite films is also studied by magnetocapacitance measurements which show that surface functionalized nickelPVDF composite exhibit magnetodielectric coupling higher than that of the unmodified one. This new facile method for improving the magnetodielectric coupling in composite multiferroic samples can easily be applied to other systems too, thereby opening new possibilities in the emerging field of multiferroic composites.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Electrical field-dependent polarization of Ni-PVDF composites (Figure S1); comparison of leakage current of different composites (Figure S2); variation of magnetic susceptibility with temperature of modified nickel nanoparticle (Figure S3); and variation of magnetic susceptibility with temperature of 10% modified Ni-PVDF (PNim1) (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Department of Atomic Energy’s Science Research Council (DAE-SRC) is acknowledged for supporting this work via Sanction 2010/21/9-BRNS/2025 dated 7/12/2010.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp5065787 | J. Phys. Chem. C XXXX, XXX, XXX−XXX