- Email: [email protected]
Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites
G Model
ARTICLE IN PRESS
JECS-10926; No. of Pages 9
Journal of the European Ceramic Society xxx (2016) xxx–xxx
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites T. Kavinkumar a , P. Senthilkumar b , S. Dhanuskodi b , S. Manivannan a,∗ a b
Carbon Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, 620 015, India School of Physics, Bharathidasan University, Tiruchirappalli, 620 024, India
a r t i c l e
i n f o
Article history: Received 25 July 2016 Received in revised form 11 November 2016 Accepted 11 November 2016 Available online xxx Keywords: Graphene oxide-barium titanate composite Electrical properties Micro capacitor Dielectric constant Ferroelectricity
a b s t r a c t Graphene oxide (GO)-barium titanate (BT) composite was prepared through sonication process at room temperature. The temperature dependent dielectric properties and transition of GO with different pellet thickness were studied. Electrical properties of GO-BT composites with different weight percentages of GO and BT (2:1, 1:1 and 1:2) from 30 to 200 ◦ C were investigated. The dielectric constant was calculated as 3701 and 1296 for GO(2)-BT(1) and GO(1)-BT(1) composites respectively which are higher than that of BT at 1 kHz. The improvement of ’ is attributed to the formation of microcapacitors by GO sheets segregated by BT particles. Curie temperature of BT was suppressed in the composites and effect of GO on the dielectric properties of the composite is predominant. The dielectric peaks of GO at 50 and 170 ◦ C were gradually shifted to high and low temperatures respectively with increasing BT content. Furthermore, polarization (P) vs electric field (E) was measured at room temperature specifying the decrease in ferroelectricity of composites from that of BT, indicating the highly conducting nature of these samples. The increasing content of GO in composite leads to decrease in remnant polarization because the introduction of GO would weaken the ferroelectricity of the composite. The present findings suggest that the new composite can be useful for fabrication of flexible electronic devices and high dielectric-based electronic and energy storage devices. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction High dielectric constant with low loss materials has attracted much attention due to their significant use in power capacitors in portable energy storage devices, electrical and electronic industries [1–4]. In recent years, there has been increasing interest in ferroelectric-based composites due to their enhanced performance, easy processing and flexibility [5–7]. Barium titanate (BT) is a perovskite mineral exhibiting ferroelectricity and has been used in capacitors due to its high dielectric constant [8,9]. The dielectric performance of BT has great variability with different dopants, morphology, preparation conditions, contribution of grain boundaries and internal material layers. Several methods have been suggested to enhance its dielectric properties. One such important approach is filling ceramic matrices with a small volume fraction of conductive materials such as metal particles and carbon materials to generate excellent energy storage capacity [10,11]. For instance, by combining ceramics materials with carbon materials like GO, graphene and
∗ Corresponding author. E-mail address: [email protected] (S. Manivannan).
carbon nanotubes possess special dielectric properties such as elevated dielectric constant and electrical conductivity, which leads to embedded capacitor applications [12,13]. GO based composites have attracted much attention because of its flexibility and high mechanical strength, which leads to many potential applications such as mechanical actuators and electronic devices. Thus, by incorporating GO the dielectric constant of BT can be significantly improved which may benefit to the fabrication of high energy density capacitors. Recently, Li et al. showed that the polyvinylidene fluoride (PVDF) composites with high dielectric constant and low loss tangent was obtained by loading relatively low content of graphene-encapsulated BT hybrid fillers. It is suggested that the dielectric constant of composites with 30 vol% of BT-reduced GO is 67.5 whereas the values for BT-GO/PVDF and BT/PVDF composites are 57.7 and 38.3 respectively at 1 kHz [12]. Shen et al. investigated the dielectric behaviour of three-phase graphene/BT/PVDF composites films. With the addition of GO, the dielectric constant of the composites mildly increases from 9 for 0 wt% of GO to 11 for 1.0 wt%. More substantial increase in dielectric constants was observed for the composite filled with reduced GO (rGO), i.e., from 9 for 0 wt% to 20 for 1.0 wt% [14]. Wang et al. reported the dielectric properties of functionalized rGO-BT/PVDF
http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model
ARTICLE IN PRESS
JECS-10926; No. of Pages 9
T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
2
Fig. 1. Schematic diagram for the preparation route of GO-BT composite structure.
nanocomposites with high dielectric constant (65) and a relatively low loss tangent (0.35) at 1 MHz [15]. According to the literature, it is still a great challenge to develop high performance capacitors with a high dielectric constant and low loss to meet electronic device requirements. A systematic investigation on dielectric and ferroelectric properties of composites are essential to understand their functional properties. However, to the best of our knowledge, no reports have been published on the investigation on electrical and ferroelectric behaviour of GO-BT composites. Here, for the first time we carried out the preparation and temperature dependent electrical properties of GO-BT composites with different weight percentages of GO and BT, prepared through sonication process. Moreover, the effect of thickness in the transition of the electrical properties of GO from 30 to 200 ◦ C in the frequency range of 100 Hz–1 MHz was studied. The existence of ferroelectricity in the GO, BT and GO-BT composites through P-E loop measurement at room temperature was also analyzed. 2. Experimental details 2.1. Preparation of GO-BT composites GO and BT were synthesized separately using modified Hummers and sol-gel methods respectively [16,17]. The desired amount of GO and BT were dispersed in the ethanol using sonication process, separately. Then, the BT suspension was poured into the GO mixer slowly. The obtained mixtures were stirred for 30 min and then transferred into a petri dish, dried at 60 ◦ C. The weight ratio of GO to BT was controlled to be 1:1, 1:2 and 2:1. The samples were named as GO(1)-BT(1), GO(1)-BT(2) and GO(2)-BT(1) for 1:1, 1:2 and 2:1 wt%, respectively. After drying, the powders were pressed
into pellets (8.0 mm in diameter and 1.0 mm in thickness) using a hydraulic press by applying uniform pressure of 20 MPa for 1 min. Subsequently, BT pellet was sintered for 3 h in a muffle furnace at 800 ◦ C. Rest of the samples is not sintered due to poor thermal stability. Because, after sintering GO and GO-BT composite transformed into rGO-BT due to reduction of GO at high temperature. The pellets were polished and silver paste was used to make ohmic contacts on both sides of the sample. The dielectric measurement was carried out using LCR meter connected to a furnace. The density of the samples were calculated as 2.786, 5.871, 3.144, 3.682 and 4.060 g/cm3 for GO, BT, GO(2)-BT(1), GO(1)-BT(1) and GO(1)-BT(2) composites respectively. The formation route to GO-BT composite is schematically illustrated in Fig. 1. 2.2. Characterization Powder X-ray diffraction (XRD) pattern was recorded (Rigaku Ultima III) at a scanning rate of 4◦ /min in the range of 5–80◦ with CuK␣1 radiation (1.5406 Å). Fourier-transform infrared (FTIR) spectra of all samples were collected using Perkin Elmer Frontier FT-IR spectrometer in the range 4000–400 cm−1 . Thermogravimetric analyses (TGA) were carried out in the temperature range 30–800 ◦ C at the heating rate of 10 ◦ C/min under N2 atmosphere (SIINT, EXSTAR 6200). Ultraviolet-visible-near infrared (UV–vis-NIR) spectra were carried out in the range of 200–800 nm using JASCO UV–vis-NIR (Model-V-670) spectrophotometer. The microstructure of sample was obtained using LabRAM HR Evolution Raman spectrometer with an excitation wavelength of 532 nm. The morphology of GO-BT composite was investigated using a field emission scanning electron microscope (FESEM) (Quanta 250 FEG, FEI) and existence of elements in composites were determined
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
Fig. 2. Powder XRD patterns of GO, BT and GO-BT composites.
by energy dispersive X-ray spectroscopy (EDX) (Oxford INCA 350 Energy). Dielectric measurement was carried out in the frequency range 102 to 106 Hz (HIOKI 3532-50). Ferroelectric studies were performed with P-E loop tracer (Radiant Technologies Inc., USA). 3. Results and discussion Powder XRD pattern of GO, BT, GO-BT composites are shown in Fig. 2. GO exhibits a sharp pattern at 9.40◦ corresponding to an interlayer spacing of 9.45 Å. This can be attributed to the expansion of the graphene sheets to accommodate the water molecules trapped between oxygen functional groups on graphene sheets. Diffraction patterns for prepared BT nanoparticles are observed at 22.29◦ , 31.53◦ , 38.92◦ , 45.31◦ , 50.87◦ , 56.34◦ , 65.93◦ , 70.48◦ , 75.03◦ and 79.44◦ and are representing the Bragg reflections from the (100), (110), (111), (200), (102), (211), (220), (212), (310) and (311) planes respectively. All the diffraction patterns reveal the tetragonal phase perovskite structure for BT particles (JCPDS card no. 79-2264) [18]. The GO-BT composite with different weight ratios of GO to BT showed XRD patterns similar to that of BT. As the GO weight ratio increases, the intensity of the GO pattern increase. After mixing of GO with BT in the weight ratio of 2:1, similar diffraction patterns of BT was observed. Additionally, a pattern at 10.41◦ is assigned to the GO, which confirms the successful formation of GO-BT composite. Meanwhile, the intensity of broad diffraction pattern of GO at 9.40◦ is decreased in GO(1)-BT(1) and disappeared in GO(1)-BT(2) composites, which might be due to the low amount of GO in composite, leading to relatively weak diffraction intensity of GO in GO-BT composites. In other words, the successful distribution of BT particles prevented the restacking of graphene sheets. Thus, the XRD pattern indicates that BT had been success-
3
fully incorporated with GO and presence of GO does not changes in preferential orientations. FT-IR was employed to detect the structural changes in GO, BT and GO-BT composites and are presented in Supplementary information (SI) 1. The absorption bands of GO at 1730, 1622, 1200 and 1045 cm−1 are ascribed to C O, sp2 hybridized C C, C O C and C O stretching vibrations, respectively. The absorption band around 3450 cm−1 corresponds to the O H stretching vibrations. The presence of these bands indicates the existence of various oxygen functional groups on the GO nanosheets. A strong absorption band appears at around 540 cm−1 is assigned to the Ti-O stretching vibration characteristic of BT. For GO(2)-BT(1) composite, similar absorption bands of GO were observed. In addition, a strong absorption band occurs at around 540 cm−1 and is assigned to the Ti-O stretching vibration which is characteristic of BT confirms the presence of BT in GO(2)-BT(1) composite. In the case of GO(1)-BT(1) composite, the intensity of all absorption peaks corresponding to oxygen functional groups (C O, C O, O H) of GO has a significant decrease which indicates domination of BT and is in good agreement with the results obtained from XRD analysis. The intensities of absorption bands of oxygen functional groups decreased for GO(1)-BT(2) composite. This demonstrates that the content of GO is relatively lower than other composites. These results explains that the presence of oxygen functional groups in GO allow the incorporation of BT and make the GO-BT nanocomposites easier. The diffuse reflectance spectra of GO, BT and GO-BT composites are shown in SI 2. The spectrum of GO appeared at 230 nm due to presence of oxygen functional groups. The strong band at around 350 nm is due to the absorption band of BT [19]. The spectrum of GO(2)-BT(1) composite exhibits two major peaks, which were caused by the existence of GO and BT in the composites. In addition, with the increasing of BT content, reflection intensity of composite increased in the range of 400 to 450 nm. No other significant peaks were observed in the composites. This result confirms the effective incorporation of BT particles on GO sheets. The significant structural changes from GO, BT and GO-BT composites are also reflected in the micro Raman spectra. The Raman spectra of all five samples in the wavenumber range 50 to 1000 cm−1 are presented in Fig. 3a. No remarkable peaks were observed in the GO nanosheets. BT exhibited four characteristic bands centered at 261, 307, 517, and 713 cm−1 originated from the tetragonal structure. A band at 261 cm−1 corresponding to the A1(TO2) mode and the band observed at 307 cm−1 arising from the Raman active B1 and E(LO + TO) lattice vibrations of the tetragonal unit cell. The asymmetric band around 517 cm−1 corresponds to E(TO) and A1(TO3) modes. The highest wavenumber band around 713 cm−1 related to E(LO) and A1(LO) phonon modes. The 307 and 261 cm−1 modes are ascribed to the O-Ti-O bending vibration [20,21]. No other peaks featured other than the tetragonal BT, revealed the absence of other impurity phases. Raman active modes attributed to tetragonal BT do not clearly appear for composite samples and only weak plasmon bands were observed. This confirms the incorporation of BT in GO nanosheets. The as prepared GO has a much stronger dispersion peak ranging from 1000 to 3000 cm−1 (Fig. 3b) and no remarkable peaks were observed in BT. For GO, the prominent D band observed at 1348 cm−1 . This is due to structural disorder and the presence of oxygen functional groups. The G band at 1598 cm−1 represents planar sp2 bonded carbon of GO [22]. The D band of the GO(2)-BT(1) composite shifted to 1341 cm−1 and then gradually shifted to higher wavenumber with increasing weight percentage of BT. The G band positions of composites were in the order of GO(2)-BT(1) (1591 cm−1 ) > GO(1)-BT(1) (1588 cm−1 ) > GO(1)-BT(2) (1587 cm−1 ). This might be caused by the interaction between C and Ti-O atoms during the sonication process. Furthermore, the intensity ratio of the D to G band (ID /IG ) for composites are calculated as
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model
ARTICLE IN PRESS
JECS-10926; No. of Pages 9
T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
4
Fig. 3. Micro Raman spectra of GO, BT and GO-BT composites in the wavenumber ranges from 50 to 1000 cm−1 (a) and from 1000 to 3000 cm−1 (b).
Table 1 EDX data analysis table of GO-BT composites. Samples
C (at.%)
O (at.%)
Ti (at.%)
Ba (at.%)
GO(2)-BT(1) GO(1)-BT(1) GO(1)-BT(2)
68.0 64.8 54.2
29.5 26.1 31.7
0.8 3.9 5.6
1.7 5.9 8.5
1.09, 1.10 and 1.15 respectively, which is slightly changed from GO (0.99) because of ultrasonication process. The morphology and microstructure of GO-BT composites were investigated using FE-SEM and are shown in Fig. 4. The FE-SEM demonstrates that the composite consists of many BT particles and GO nanosheets. It can be clearly seen that the highly segregated particles of BT were covered on GO sheets. When the weight ratio of BT to GO was increased, the distribution of BT particles are also increasing on the surface of the GO sheets and is no obvious aggregation was observed. This result confirms the successful deposition of BT on the surface of the GO sheets. The existence of carbon (C), barium (Ba), titanium (Ti), and oxygen (O) elements in GO-BT composites were confirmed by the EDX measurement and is listed in Table 1. The thermal behaviors of GO, BT and GO-BT composites were studied by TGA and are shown in SI 3. GO is thermally unstable and the weight loss is about 15% at around 110 ◦ C, which can be assigned to the removal of water molecules trapped between the GO sheets. Most of the oxygen functional groups are lost in the temperature range of 100–250 ◦ C (44%) and is ascribed to the pyrolysis of the labile oxygen functional groups in the forms of CO, CO2 and steam. When the temperature reached 550 ◦ C, the weight loss of GO is about 49% and almost no weight loss occurred above this temperature. No prominent changes were observed in BT and total weight loss was found to be below 2%. TGA curve of GO(2)-BT(1) composite exhibits major weight loss between 100 and 250 ◦ C due to thermal degradation of the unreacted oxygen functional groups of GO. The
addition of BT content in composite leads to increase in thermal stability and reduction in weight loss is caused by a high thermal stability of BT than GO. It can be seen that the weight loss decreases as the amount of BT is increased. It is evidenced that higher weight losses are due to the increased amount of GO in the composite. Temperature dependence of dielectric constant (’) for GO at different thickness is shown in Fig. 5. The dielectric behaviour of GO was investigated with varying temperature from 30 to 200 ◦ C at 1 kHz. GO samples with thickness 53 m and 1 mm were named as GO1 and GO2 respectively. At 30 ◦ C, the ’ of the GO2 (1.89 × 104 ) is increased by more than two orders of magnitude in comparison with GO1 (3.27 × 102 ). For GO1, ’ increases with increasing temperature and it attains a maximum value at 35 ◦ C and then it gradually decreases with further increase in temperature until 150 ◦ C. This is due to the thermal vibration of chemical bonds, as well as the change in the dipole moment and orientation polarization of the functional groups (-OH and −COOH) resulting in a low ’ at high temperatures. When the temperature is higher than 150 ◦ C, ’ of GO1 started to increase due to increase of reduced clusters in GO1 [23,24]. Thus, these electrical transitions are attained due to the loss of absorbed moisture in GO1. A similar trend is also observed for GO2 in the same temperature domain. However, GO2 shows the temperature dependent ’ and transition shifted towards higher temperature such as 50 and 165 ◦ C compared to GO1 (35 and 150 ◦ C). The ’ of GO2 is directly relative to the polarization (charge displacement) which further depends on the applied electric field and the responses of the different constituents (atoms, ions) of the solids. With increase in thickness of GO, more and more defects are produced and the movements of these defects result in increased ’ of GO2 than GO1. Fig. 6a and b shows the AC conductivities () and ’ of GO, BT and GO-BT composites respectively, over a wide range of frequency. In comparison to BT and GO-BT composites, the GO shows a strong frequency dependent dielectric response at 27 ◦ C. GO is considered as layered composite consisting of insulating sp3 matrix and con-
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
5
Fig. 4. FE-SEM images of a) GO b) BT (c) GO(2)-BT(1), (d) GO(1)-BT(1) and (e) GO(1)-BT(2) composites.
Fig. 5. The temperature dependence of the dielectric constant of GO1 and GO2 at 1 kHz.
ductive sp2 clusters. The conductivity showed less variation when the applied frequency was low due to more active grain boundaries. Variation at high frequency regions was due to less active grain boundaries. Intercalated water molecules and additional oxygen functional groups in GO played a major role in the electrical
conductivity. Conductivity of GO was controlled by the presence of insulating sp3 structure within the GO sheet. Compared to the pristine BT, the introduction of GO into BT increases the electrical conductivity of the GO(1)-BT(2) composite, which may be due to the introduction of polar functional groups. It is observed that the electrical conductivity of GO-BT composites increases with increasing content of GO. This reaches 3.68 × 10−3 Sm−1 for GO(2)-BT(1) composite. Because, the BT was grafted onto the surface of GO to form GO-BT composite and became more insulating than GO. ¨ Fig. 6(b) and (c) shows the frequency dependence of ’ and of the GO, BT and GO-BT composites at 27 ◦ C. All the samples have a frequency dependence of ’ and ¨in the low frequency region followed by a nearly frequency independent behaviour above 104 Hz. GO shows the highest value of ’ (3.3 × 105 ) at low frequency and decreases with increasing frequency and becomes constant in the high frequency range of 104 –106 Hz. It is obvious that the ’ and ¨ the samples were dependent on the applied frequency. The of possible cause of this decrease of ’ with increasing frequency may be attributed to the interfacial polarization. As the GO to BT weight ratio approached to 1:1 and 1:2, the ’ of the GO(1)-BT(1) and GO(1)-BT(2) composites decreased to 5630 and 1479 respectively. It can be clearly observed that the ’ of the composites can be enhanced by increasing weight ratio of GO nanosheets. When the GO to BT weight ratio approached about 2:1, a percolation transition from an insulator to semiconductor occurred, which is accompanied by a dramatically increased ’. The ¨increases with the increasing weight percentage of GO in the composite. For
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9 6
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
Fig. 6. The frequency dependence of (a) electrical conductivity (b) dielectric constant (c) dielectric loss and (d) imaginary part of electric modulus of GO, BT and GO-BT composites at 27 (C. Insert shows the expanded portion of dielectric constant and dielectric loss of BT and GO-BT composites at low frequency.
GO(2)-BT(1) composite, the ¨increased significantly due to the high thermal agitation/vibration of the dipoles on the GO basal plane [8]. Fig. 6d shows the frequency dependence of the imaginary part (M”) of the electric modulus of GO, BT and GO-BT composites at 27 ◦ C. The electric modulus formalism has been used to study the dipole relaxation process, interfacial polarization and long-range conduction process within the bulk of materials. Because, it minimizes the electrode interface capacitance contribution and other interfacial effects. The BT shows a relaxation peak at low frequency compared to GO and composites. This is due to the charge carriers in BT have a low mobility within the bulk materials. In such a case, there should be a considerable time interval required for charge carriers to reach an interface, in other word interfacial polarization takes long time to be established. Consequently, interfacial polarization appears at low frequencies for BT [25]. The variation of M” with applied frequency for GO-BT composites, the characteristic relaxation peak moves towards higher frequencies with increasing GO weight ratio, because the charge carriers accumulate on the surface of GO. This is the reason for the shift of the interfacial polarization relaxation peaks of composites to higher frequencies. For GO, the increase in peak frequency indicates a short relaxation time, which is consistent with an increase in the mobility of charge carriers.
The temperature dependences of the ’ of GO, BT and GOBT composites at three different frequencies are shown in Fig. 7. The dielectric behaviour of samples at different frequencies with increase in temperature was studied. The decrease of the ’ with frequency can be attributed to the contribution of multi components of polarizability, deformational and relaxation. GO exhibits temperature dependent behaviour over the entire frequency range. We observe that ’ of GO was increases with temperature (30–50 ◦ C) at 100 Hz. The ’ start to decrease with an increase in temperature from 55 to 165 ◦ C due to the change in the dipole moment, orientation polarization of the functional groups (-OH and −COOH) and the loss of absorbed moisture. Above 170 ◦ C, ’ of GO increased with increasing temperature, as shown in Fig. 7a. A similar trend is also observed for all the other frequencies in the same temperature domain. Increase of the charge carriers mobility with the increasing temperature gives rise to an increase of the interfacial polarization, which leads to an increase of the ’ [26,27]. For BT, the ’ increases with temperature and reaches maximum value (1446 at 150 ◦ C) corresponds to the ferroelectric to paraelectric phase transition of the BT phase (Fig. 7b). It reveals that BT exhibits Curie peak at 150 ◦ C measured at 1 kHz due to increase of grain size [17]. The ’ of the different weight ratio of GO-BT composite showed that the two major behaviors [Fig. 7c and SI 4)]. With GO adding into BT [GO(1)-
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
7
Fig. 7. Temperature dependence (a-c) dielectric constant of GO, BT and GO-BT composites at selected frequencies and (d) comparison of temperature dependence dielectric constant at 1 kHz for GO, BT and GO-BT composites. Insert shows the dielectric constant transition region of samples.
BT(2)], ’ increases significantly. It can be considered the addition of BT makes the destruction of the electric field resulting accumulation of charges at intergranular areas. Therefore, the formation of this macro dipoles dispersion impedes polarization and leads to the decrease of ’. Other word, the high values of ’ of GO play a leading role of the dielectric response in the composite, so the ’ increases with increasing amount of GO. The increase of ’ with increase of GO is ascribed to the large value of the ’ of the GO (20444) as compared to BT (1446) at 1 kHz. Curie temperature of BT was suppressed in the all composites and effect of GO on the dielectric properties of the composite is significant. The peak of GO was found at 50 ◦ C and this peak gradually moves to high temperature as well as the intensity of this peak decreases gradually when increasing the BT ratio in composite. In addition, valley peak at 170 ◦ C moves towards low temperature with increasing BT content ratio indicates that the domination of BT in composite (Fig. 7d). SI 5 depicts the AC conductivity of GO, BT and GO-BT composites as a function of frequency at different temperatures. GO, BT and composites exhibits frequency dependent conductivity over the entire frequency range and demonstrate an Arrhenius of behaviour [28]. The frequency dependence of lies in the relaxation phenomena arising due to mobile charge carriers. At 30 ◦ C, the of GO is 1.577 × 10−2 Sm−1 and with an increase in temperature it increases to 1.63 × 10−2 Sm−1 (55 ◦ C) due to enhanced ion mobility. The temperature between 60 and 165 ◦ C, ionic conductivity of GO is very low. This is due to an accumulation of charge carriers at the electrode leads to the formation of charge layers, which can shield
the sample from the electric field. Thus, the electrical conductivity decreases with temperature [23]. The of GO is increases rapidly at 170 ◦ C and above. A large increase of conductivity appears at 170 ◦ C due to deoxygenation. At 150 ◦ C, BT exhibits the maximum conductivity and then decreases with temperature due to phase transition of BT from tetragonal to cubic. The conductivity of GO-BT composite enhances with increasing content of GO as illustrated in SI 5c. This is due to the incorporation of GO in composites. The homogeneous dispersion takes place when the GO content is increased, which contributes to the enhancement in the electrical conductivity of the composite. Also found that the transition of GO at 170 ◦ C was shifted to lower temperature with increasing BT content. It is very similar to the variations of temperature dependence of ’ behaviour. SI 6 shows the temperature dependence of imaginary part of electric modulus (M”) of GO, BT and GO-BT composites. The M” was used to study dielectric relaxation mechanisms, because it minimizes the electrode interface contribution and other interfacial effects which associated with the contribution of small capacitance. For GO, the relaxation peak shifted to higher frequency when the temperature increasing from 30 to 60 ◦ C and then it gradually shifted to low frequency until 170 ◦ C. This is due to decrease of charge carrier density because the ion mobility increases with increasing temperature. The M” of the BT exhibits the loss peak in the low frequency range and peak shifted to high frequency when temperature is high. According to SI 6 (c-e), the variation of M” with frequency shows a typical relaxation peak that moves towards
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9 8
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
Fig. 8. Polarization (P) vs Electric filed (E) loops for GO, BT and GO-BT composites at room temperature.
lower frequencies by increasing BT. This implies that charge carriers in the composites become bounded. Therefore, their migration and accumulation process needs a relatively long time. In this case, the time interval required for charge carriers to reach an interface takes long time to be established, so interfacial polarization appears at low frequencies. The relaxation strength of GO-BT composites increases with decreasing BT content [25]. Fig. 8 shows variation in polarization with respect to the electric field (P-E responses) at 2 Hz for GO, BT and GO-BT composites. The P-E hysteresis loop illustrates the effect of adding GO content to the BT. BT shows a well-defined hysteresis loop with negligible leakage indicating better ferroelectric behaviour [17]. The GO and GO-BT composites displayed an unsaturated P-E curve with loss, which may be partially attributed to the presence of an interface between BT and GO. It has been realized from the figure that the ferroelectricity of composites decreases from that of BT, allowing for ferroelectric domain switch. Furthermore, the area of the loop also increased for composites than BT, which may be due to the
strong conductivity. The much increased leakage current indicates that the high remnant polarization in the GO might be attributed to the accumulation of charges on the surface of GO [29]. The increasing content of GO in composite leads to decrease in remnant polarization because the introduction of GO would weaken the ferroelectricity of the composite, which is in agreement with the dielectric results. This confirms that the presence of dielectric materials significantly affects the polarization response of the BT in the composites.
4. Conclusion We have described the dielectric properties of GO with different thicknesses at 27 ◦ C. Thick sample of GO exhibits maximum ’ as 1.89 × 104 at 1 kHz and is due to presence of more defects. The ’ of the composite decreases with increase in frequency and increases with increase in the GO content. GO exhibits two transition at 50 and 170 ◦ C and is attributed to the presence of oxygen
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
functional groups. The ’ peak of GO at 50 ◦ C was gradually shifted to high temperature as well as the valley peak at 170 ◦ C moves towards low temperature with increasing BT content in composites. The increased amount of oxygen vacancies and free electron are the causal factor for the observed shift in the composites. The P-E loops imply that the effect of GO in ferroelectricity of BT in the composite. The increasing content of GO in composite leads to decrease in remnant polarization because the introduction of GO would weaken the ferroelectricity of the composite. The enhanced dielectric behaviour of composite finds potential application in dielectric and optoelectronic application. Acknowledgements The authors thank Dr. R. Justin Joseyphus, Dr. N. V. Giridharan and Dr. B. Karthikeyan for TGA, ferroelectric and micro Raman measurements respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2016.11.026. References [1] [2] [3] [4]
[5] [6]
[7]
[8]
[9]
[10]
J. Xu, C. Wong, Appl. Phys. Lett. 87 (2009) 082907. M. Arbatti, X. Shan, Z.Y. Cheng, Adv. Mater 19 (2007) 1369. C.C. Homes, T. Vogt, Nat. Mater. 12 (2013) 782. L. Zhang, P. Wu, Y. Li, Z.Y. Cheng, J.C. Brewer, Preparation process and dielectric properties of Ba0.5Sr0.5TiO3-P(VDF-CTFE) nanocomposites, Compos. B Eng. 56 (2014) 284. K. Ring, K. Kavanaph, Substrate effects on the ferroelectric properties of fine-grained BaTiO3 films, J. Appl. Phys. 94 (2003) 5982. N. Kumar, J. Pan, N. Aysha, U.V. Waghmare, A. Sundaresan, C.N.R. Rao, Effect of co-substitution of nitrogen and fluorine in BaTiO3 on ferroelectricity and other properties, J. Phys. Condens. Matter25 (2013) 345901. X. Wang, Y. Zhang, X. Song, Z. Yuan, T. Ma, Q. Zhang, C. Deng, T. Liang, Glass additive in barium titanate ceramics and its influence on electrical breakdown strength in relation with energy storage properties, J. Eur. Ceram. Soc. 32 (2012) 559–567. S.J. Pedro, P.M. Luis, J.M. Dia´ınez, P. Antonio, J.M. Criado, Mechanochemical preparation of BaTiO3-Ni nanocomposites with high dielectric constant, Compos. Struct. 92 (2010) 2236. S. Nayak, B. Sahoo, T.K. Chaki, D. Khastgir, Facile preparation of uniform barium titanate (BaTiO3) multipods with high permittivity: impedance and temperature dependent dielectric behavior, RSC Adv. 4 (2014) 1212. K.S. Kumar, S. Pittala, S. Sanyadanam, P. Paik, A new single/few-layered graphene oxide with a high dielectric constant of 106 : contribution of defects and functional groups, RSC Adv. 5 (2015) 14768.
9
[11] T. Zhou, J.W. Zha, R.Y. Cui, B.H. Fan, J.K. Yuan, Z.M. Dang, Improving dielectric properties of BaTiO3/ferroelectric polymer composites by employing surface hydroxylated BaTiO3 nanoparticles, ACS Appl. Mater. Interfaces 3 (2011) 2184. [12] Y. Li, Y. Shi, F. Cai, J. Xue, F. Chen, Q. Fu, Graphene sheets segregated by barium titanate for polyvinylidene fluoride composites with high dielectric constant and ultralow loss tangent, Compos. A 78 (2015) 318. [13] U. Yaqoob, A.S.M.I. Uddin, G.S. Chung, The effect of reduced graphene oxide on the dielectric and ferroelectric properties of PVDF-BaTiO3 nanocomposites, RSC Adv. (2016), http://dx.doi.org/10.1039/C6RA03155B. [14] Y. Shen, Y. Guan, Y. Hu, Y. Lei, Y. Song, Y. Lin, C.W. Nan, Dielectric behavior of graphene/BaTiO3/polyvinylidene fluoride nanocomposite under high electric field, Appl. Phys. Lett. 103 (2013) 072906. [15] D. Wang, T. Zhou, J.W. Zha, J. Zhao, C.Y. Shi, Z.M. Dang, Functionalized graphene-BaTiO3 /ferroelectric polymer nanodielectric composites with high permittivity, low dielectric loss, and low percolation threshold, J. Mater. Chem. A 1 (2013) 6162. [16] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [17] C.R. Xin, J. Zhang, Y. Liu, Q.L. Zhang, H. Yang, D. Cheng, Polymorphism and dielectric properties of Sc-doped BaTiO3 nanopowders synthesized by sol-gel method, Mater. Res. Bull. 48 (2013) 2220. [18] R.X. Wang, Q. Zhu, W.S. Wang, C.M. Fan, A.W. Xu, BaTiO3 -graphene nanocomposites: synthesis and visible light photocatalytic activity, New J. Chem. (2015), http://dx.doi.org/10.1039/c4nj02272f. [19] M.K. Mahata, T. Koppe, T. Mondal, C. Brüsewitz, K. Kumar, V.K. Rai, H. Hofsäss, U. Vetter, Phys. Chem. Chem. Phys. (2015), http://dx.doi.org/10.1039/ C5CP01874A. [20] D.Y. Lu, W. Cheng, X.Y. Sun, Q.L. Liu, D.X. Li, Z.Y. Li, Abnormal Raman spectra in Er-doped BaTiO3 ceramics, J. Raman Spectrosc. 45 (2014) 963. [21] J. Suchanicz, K. Swierczek, E. Nogas-Cwikiel, K. Konieczny, D. Sitko, PbMg1/3Nb2/3O3-doping effects on structural, thermal Raman, dielectric andferroelectric properties of BaTiO3ceramics, J. Eur. Ceram. Soc. 35 (2015) 1777–1783. [22] D. Yang, A. Velamakanni, G. Bozoklu, et al., Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy, Carbon 47 (2009) 145. [23] X. Huang, C. Zhi, P. Jiang, D. Golberg, Y. Bando, T. Tanaka, Temperature-dependent electrical property transition of graphene oxide paper, Nanotechnology 23 (2012) 455705. [24] N. Adhlakha, K.L. Yadav, R. Singh, Effect of BaTiO3 addition on structural, multiferroic and magneto-dielectric properties of 0.3CoFe2 O4 -0.7BiFeO3 ceramics, Smart Mater. Struct. 23 (2014) 105024. [25] Y. Li, X. Huang, Z. Hu, P. Jiang, S. Li, T. Tanaka, Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon carbide three-phase nanocomposites, ACS Appl. Mater. Interfaces 3 (2011) 4396. [26] B. Wang, Y.P. Pu, N. Xu, H.D. Wu, K. Chen, Dielectric properties of barium titanate-molybdenum composite, Ceram. Int. 38 (2012) S37. [27] S. Baryshnikov, E. Stukova, E. Koroleva, Dielectric properties of the ferroelectric composite (NaNO2 )0.9 /(BaTiO3 )0.1 , Compos. B Eng. 66 (2014) 190. [28] M.M. El-Nahass, H.A.M. Ali, Solid State Commun. 152 (2012) 1084. [29] O.D. Jayakumar, E.H. Abdelhamid, V. Kotari, B.P. Mandal, R. Rao, V.M.Naik Jagannath, R. Naik, A.K. Tyagi, Fabrication of flexible and self-standing inorganic-organic three phase magneto-dielectric PVDF based multiferroic nanocomposite films through a small loading of graphene oxide (GO) and Fe3O4 nanoparticles, Dalton Trans. (2015), http://dx.doi.org/10.1039/ c5dt01509j.
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
ARTICLE IN PRESS
JECS-10926; No. of Pages 9
Journal of the European Ceramic Society xxx (2016) xxx–xxx
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites T. Kavinkumar a , P. Senthilkumar b , S. Dhanuskodi b , S. Manivannan a,∗ a b
Carbon Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, 620 015, India School of Physics, Bharathidasan University, Tiruchirappalli, 620 024, India
a r t i c l e
i n f o
Article history: Received 25 July 2016 Received in revised form 11 November 2016 Accepted 11 November 2016 Available online xxx Keywords: Graphene oxide-barium titanate composite Electrical properties Micro capacitor Dielectric constant Ferroelectricity
a b s t r a c t Graphene oxide (GO)-barium titanate (BT) composite was prepared through sonication process at room temperature. The temperature dependent dielectric properties and transition of GO with different pellet thickness were studied. Electrical properties of GO-BT composites with different weight percentages of GO and BT (2:1, 1:1 and 1:2) from 30 to 200 ◦ C were investigated. The dielectric constant was calculated as 3701 and 1296 for GO(2)-BT(1) and GO(1)-BT(1) composites respectively which are higher than that of BT at 1 kHz. The improvement of ’ is attributed to the formation of microcapacitors by GO sheets segregated by BT particles. Curie temperature of BT was suppressed in the composites and effect of GO on the dielectric properties of the composite is predominant. The dielectric peaks of GO at 50 and 170 ◦ C were gradually shifted to high and low temperatures respectively with increasing BT content. Furthermore, polarization (P) vs electric field (E) was measured at room temperature specifying the decrease in ferroelectricity of composites from that of BT, indicating the highly conducting nature of these samples. The increasing content of GO in composite leads to decrease in remnant polarization because the introduction of GO would weaken the ferroelectricity of the composite. The present findings suggest that the new composite can be useful for fabrication of flexible electronic devices and high dielectric-based electronic and energy storage devices. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction High dielectric constant with low loss materials has attracted much attention due to their significant use in power capacitors in portable energy storage devices, electrical and electronic industries [1–4]. In recent years, there has been increasing interest in ferroelectric-based composites due to their enhanced performance, easy processing and flexibility [5–7]. Barium titanate (BT) is a perovskite mineral exhibiting ferroelectricity and has been used in capacitors due to its high dielectric constant [8,9]. The dielectric performance of BT has great variability with different dopants, morphology, preparation conditions, contribution of grain boundaries and internal material layers. Several methods have been suggested to enhance its dielectric properties. One such important approach is filling ceramic matrices with a small volume fraction of conductive materials such as metal particles and carbon materials to generate excellent energy storage capacity [10,11]. For instance, by combining ceramics materials with carbon materials like GO, graphene and
∗ Corresponding author. E-mail address: [email protected] (S. Manivannan).
carbon nanotubes possess special dielectric properties such as elevated dielectric constant and electrical conductivity, which leads to embedded capacitor applications [12,13]. GO based composites have attracted much attention because of its flexibility and high mechanical strength, which leads to many potential applications such as mechanical actuators and electronic devices. Thus, by incorporating GO the dielectric constant of BT can be significantly improved which may benefit to the fabrication of high energy density capacitors. Recently, Li et al. showed that the polyvinylidene fluoride (PVDF) composites with high dielectric constant and low loss tangent was obtained by loading relatively low content of graphene-encapsulated BT hybrid fillers. It is suggested that the dielectric constant of composites with 30 vol% of BT-reduced GO is 67.5 whereas the values for BT-GO/PVDF and BT/PVDF composites are 57.7 and 38.3 respectively at 1 kHz [12]. Shen et al. investigated the dielectric behaviour of three-phase graphene/BT/PVDF composites films. With the addition of GO, the dielectric constant of the composites mildly increases from 9 for 0 wt% of GO to 11 for 1.0 wt%. More substantial increase in dielectric constants was observed for the composite filled with reduced GO (rGO), i.e., from 9 for 0 wt% to 20 for 1.0 wt% [14]. Wang et al. reported the dielectric properties of functionalized rGO-BT/PVDF
http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model
ARTICLE IN PRESS
JECS-10926; No. of Pages 9
T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
2
Fig. 1. Schematic diagram for the preparation route of GO-BT composite structure.
nanocomposites with high dielectric constant (65) and a relatively low loss tangent (0.35) at 1 MHz [15]. According to the literature, it is still a great challenge to develop high performance capacitors with a high dielectric constant and low loss to meet electronic device requirements. A systematic investigation on dielectric and ferroelectric properties of composites are essential to understand their functional properties. However, to the best of our knowledge, no reports have been published on the investigation on electrical and ferroelectric behaviour of GO-BT composites. Here, for the first time we carried out the preparation and temperature dependent electrical properties of GO-BT composites with different weight percentages of GO and BT, prepared through sonication process. Moreover, the effect of thickness in the transition of the electrical properties of GO from 30 to 200 ◦ C in the frequency range of 100 Hz–1 MHz was studied. The existence of ferroelectricity in the GO, BT and GO-BT composites through P-E loop measurement at room temperature was also analyzed. 2. Experimental details 2.1. Preparation of GO-BT composites GO and BT were synthesized separately using modified Hummers and sol-gel methods respectively [16,17]. The desired amount of GO and BT were dispersed in the ethanol using sonication process, separately. Then, the BT suspension was poured into the GO mixer slowly. The obtained mixtures were stirred for 30 min and then transferred into a petri dish, dried at 60 ◦ C. The weight ratio of GO to BT was controlled to be 1:1, 1:2 and 2:1. The samples were named as GO(1)-BT(1), GO(1)-BT(2) and GO(2)-BT(1) for 1:1, 1:2 and 2:1 wt%, respectively. After drying, the powders were pressed
into pellets (8.0 mm in diameter and 1.0 mm in thickness) using a hydraulic press by applying uniform pressure of 20 MPa for 1 min. Subsequently, BT pellet was sintered for 3 h in a muffle furnace at 800 ◦ C. Rest of the samples is not sintered due to poor thermal stability. Because, after sintering GO and GO-BT composite transformed into rGO-BT due to reduction of GO at high temperature. The pellets were polished and silver paste was used to make ohmic contacts on both sides of the sample. The dielectric measurement was carried out using LCR meter connected to a furnace. The density of the samples were calculated as 2.786, 5.871, 3.144, 3.682 and 4.060 g/cm3 for GO, BT, GO(2)-BT(1), GO(1)-BT(1) and GO(1)-BT(2) composites respectively. The formation route to GO-BT composite is schematically illustrated in Fig. 1. 2.2. Characterization Powder X-ray diffraction (XRD) pattern was recorded (Rigaku Ultima III) at a scanning rate of 4◦ /min in the range of 5–80◦ with CuK␣1 radiation (1.5406 Å). Fourier-transform infrared (FTIR) spectra of all samples were collected using Perkin Elmer Frontier FT-IR spectrometer in the range 4000–400 cm−1 . Thermogravimetric analyses (TGA) were carried out in the temperature range 30–800 ◦ C at the heating rate of 10 ◦ C/min under N2 atmosphere (SIINT, EXSTAR 6200). Ultraviolet-visible-near infrared (UV–vis-NIR) spectra were carried out in the range of 200–800 nm using JASCO UV–vis-NIR (Model-V-670) spectrophotometer. The microstructure of sample was obtained using LabRAM HR Evolution Raman spectrometer with an excitation wavelength of 532 nm. The morphology of GO-BT composite was investigated using a field emission scanning electron microscope (FESEM) (Quanta 250 FEG, FEI) and existence of elements in composites were determined
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
Fig. 2. Powder XRD patterns of GO, BT and GO-BT composites.
by energy dispersive X-ray spectroscopy (EDX) (Oxford INCA 350 Energy). Dielectric measurement was carried out in the frequency range 102 to 106 Hz (HIOKI 3532-50). Ferroelectric studies were performed with P-E loop tracer (Radiant Technologies Inc., USA). 3. Results and discussion Powder XRD pattern of GO, BT, GO-BT composites are shown in Fig. 2. GO exhibits a sharp pattern at 9.40◦ corresponding to an interlayer spacing of 9.45 Å. This can be attributed to the expansion of the graphene sheets to accommodate the water molecules trapped between oxygen functional groups on graphene sheets. Diffraction patterns for prepared BT nanoparticles are observed at 22.29◦ , 31.53◦ , 38.92◦ , 45.31◦ , 50.87◦ , 56.34◦ , 65.93◦ , 70.48◦ , 75.03◦ and 79.44◦ and are representing the Bragg reflections from the (100), (110), (111), (200), (102), (211), (220), (212), (310) and (311) planes respectively. All the diffraction patterns reveal the tetragonal phase perovskite structure for BT particles (JCPDS card no. 79-2264) [18]. The GO-BT composite with different weight ratios of GO to BT showed XRD patterns similar to that of BT. As the GO weight ratio increases, the intensity of the GO pattern increase. After mixing of GO with BT in the weight ratio of 2:1, similar diffraction patterns of BT was observed. Additionally, a pattern at 10.41◦ is assigned to the GO, which confirms the successful formation of GO-BT composite. Meanwhile, the intensity of broad diffraction pattern of GO at 9.40◦ is decreased in GO(1)-BT(1) and disappeared in GO(1)-BT(2) composites, which might be due to the low amount of GO in composite, leading to relatively weak diffraction intensity of GO in GO-BT composites. In other words, the successful distribution of BT particles prevented the restacking of graphene sheets. Thus, the XRD pattern indicates that BT had been success-
3
fully incorporated with GO and presence of GO does not changes in preferential orientations. FT-IR was employed to detect the structural changes in GO, BT and GO-BT composites and are presented in Supplementary information (SI) 1. The absorption bands of GO at 1730, 1622, 1200 and 1045 cm−1 are ascribed to C O, sp2 hybridized C C, C O C and C O stretching vibrations, respectively. The absorption band around 3450 cm−1 corresponds to the O H stretching vibrations. The presence of these bands indicates the existence of various oxygen functional groups on the GO nanosheets. A strong absorption band appears at around 540 cm−1 is assigned to the Ti-O stretching vibration characteristic of BT. For GO(2)-BT(1) composite, similar absorption bands of GO were observed. In addition, a strong absorption band occurs at around 540 cm−1 and is assigned to the Ti-O stretching vibration which is characteristic of BT confirms the presence of BT in GO(2)-BT(1) composite. In the case of GO(1)-BT(1) composite, the intensity of all absorption peaks corresponding to oxygen functional groups (C O, C O, O H) of GO has a significant decrease which indicates domination of BT and is in good agreement with the results obtained from XRD analysis. The intensities of absorption bands of oxygen functional groups decreased for GO(1)-BT(2) composite. This demonstrates that the content of GO is relatively lower than other composites. These results explains that the presence of oxygen functional groups in GO allow the incorporation of BT and make the GO-BT nanocomposites easier. The diffuse reflectance spectra of GO, BT and GO-BT composites are shown in SI 2. The spectrum of GO appeared at 230 nm due to presence of oxygen functional groups. The strong band at around 350 nm is due to the absorption band of BT [19]. The spectrum of GO(2)-BT(1) composite exhibits two major peaks, which were caused by the existence of GO and BT in the composites. In addition, with the increasing of BT content, reflection intensity of composite increased in the range of 400 to 450 nm. No other significant peaks were observed in the composites. This result confirms the effective incorporation of BT particles on GO sheets. The significant structural changes from GO, BT and GO-BT composites are also reflected in the micro Raman spectra. The Raman spectra of all five samples in the wavenumber range 50 to 1000 cm−1 are presented in Fig. 3a. No remarkable peaks were observed in the GO nanosheets. BT exhibited four characteristic bands centered at 261, 307, 517, and 713 cm−1 originated from the tetragonal structure. A band at 261 cm−1 corresponding to the A1(TO2) mode and the band observed at 307 cm−1 arising from the Raman active B1 and E(LO + TO) lattice vibrations of the tetragonal unit cell. The asymmetric band around 517 cm−1 corresponds to E(TO) and A1(TO3) modes. The highest wavenumber band around 713 cm−1 related to E(LO) and A1(LO) phonon modes. The 307 and 261 cm−1 modes are ascribed to the O-Ti-O bending vibration [20,21]. No other peaks featured other than the tetragonal BT, revealed the absence of other impurity phases. Raman active modes attributed to tetragonal BT do not clearly appear for composite samples and only weak plasmon bands were observed. This confirms the incorporation of BT in GO nanosheets. The as prepared GO has a much stronger dispersion peak ranging from 1000 to 3000 cm−1 (Fig. 3b) and no remarkable peaks were observed in BT. For GO, the prominent D band observed at 1348 cm−1 . This is due to structural disorder and the presence of oxygen functional groups. The G band at 1598 cm−1 represents planar sp2 bonded carbon of GO [22]. The D band of the GO(2)-BT(1) composite shifted to 1341 cm−1 and then gradually shifted to higher wavenumber with increasing weight percentage of BT. The G band positions of composites were in the order of GO(2)-BT(1) (1591 cm−1 ) > GO(1)-BT(1) (1588 cm−1 ) > GO(1)-BT(2) (1587 cm−1 ). This might be caused by the interaction between C and Ti-O atoms during the sonication process. Furthermore, the intensity ratio of the D to G band (ID /IG ) for composites are calculated as
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model
ARTICLE IN PRESS
JECS-10926; No. of Pages 9
T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
4
Fig. 3. Micro Raman spectra of GO, BT and GO-BT composites in the wavenumber ranges from 50 to 1000 cm−1 (a) and from 1000 to 3000 cm−1 (b).
Table 1 EDX data analysis table of GO-BT composites. Samples
C (at.%)
O (at.%)
Ti (at.%)
Ba (at.%)
GO(2)-BT(1) GO(1)-BT(1) GO(1)-BT(2)
68.0 64.8 54.2
29.5 26.1 31.7
0.8 3.9 5.6
1.7 5.9 8.5
1.09, 1.10 and 1.15 respectively, which is slightly changed from GO (0.99) because of ultrasonication process. The morphology and microstructure of GO-BT composites were investigated using FE-SEM and are shown in Fig. 4. The FE-SEM demonstrates that the composite consists of many BT particles and GO nanosheets. It can be clearly seen that the highly segregated particles of BT were covered on GO sheets. When the weight ratio of BT to GO was increased, the distribution of BT particles are also increasing on the surface of the GO sheets and is no obvious aggregation was observed. This result confirms the successful deposition of BT on the surface of the GO sheets. The existence of carbon (C), barium (Ba), titanium (Ti), and oxygen (O) elements in GO-BT composites were confirmed by the EDX measurement and is listed in Table 1. The thermal behaviors of GO, BT and GO-BT composites were studied by TGA and are shown in SI 3. GO is thermally unstable and the weight loss is about 15% at around 110 ◦ C, which can be assigned to the removal of water molecules trapped between the GO sheets. Most of the oxygen functional groups are lost in the temperature range of 100–250 ◦ C (44%) and is ascribed to the pyrolysis of the labile oxygen functional groups in the forms of CO, CO2 and steam. When the temperature reached 550 ◦ C, the weight loss of GO is about 49% and almost no weight loss occurred above this temperature. No prominent changes were observed in BT and total weight loss was found to be below 2%. TGA curve of GO(2)-BT(1) composite exhibits major weight loss between 100 and 250 ◦ C due to thermal degradation of the unreacted oxygen functional groups of GO. The
addition of BT content in composite leads to increase in thermal stability and reduction in weight loss is caused by a high thermal stability of BT than GO. It can be seen that the weight loss decreases as the amount of BT is increased. It is evidenced that higher weight losses are due to the increased amount of GO in the composite. Temperature dependence of dielectric constant (’) for GO at different thickness is shown in Fig. 5. The dielectric behaviour of GO was investigated with varying temperature from 30 to 200 ◦ C at 1 kHz. GO samples with thickness 53 m and 1 mm were named as GO1 and GO2 respectively. At 30 ◦ C, the ’ of the GO2 (1.89 × 104 ) is increased by more than two orders of magnitude in comparison with GO1 (3.27 × 102 ). For GO1, ’ increases with increasing temperature and it attains a maximum value at 35 ◦ C and then it gradually decreases with further increase in temperature until 150 ◦ C. This is due to the thermal vibration of chemical bonds, as well as the change in the dipole moment and orientation polarization of the functional groups (-OH and −COOH) resulting in a low ’ at high temperatures. When the temperature is higher than 150 ◦ C, ’ of GO1 started to increase due to increase of reduced clusters in GO1 [23,24]. Thus, these electrical transitions are attained due to the loss of absorbed moisture in GO1. A similar trend is also observed for GO2 in the same temperature domain. However, GO2 shows the temperature dependent ’ and transition shifted towards higher temperature such as 50 and 165 ◦ C compared to GO1 (35 and 150 ◦ C). The ’ of GO2 is directly relative to the polarization (charge displacement) which further depends on the applied electric field and the responses of the different constituents (atoms, ions) of the solids. With increase in thickness of GO, more and more defects are produced and the movements of these defects result in increased ’ of GO2 than GO1. Fig. 6a and b shows the AC conductivities () and ’ of GO, BT and GO-BT composites respectively, over a wide range of frequency. In comparison to BT and GO-BT composites, the GO shows a strong frequency dependent dielectric response at 27 ◦ C. GO is considered as layered composite consisting of insulating sp3 matrix and con-
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
5
Fig. 4. FE-SEM images of a) GO b) BT (c) GO(2)-BT(1), (d) GO(1)-BT(1) and (e) GO(1)-BT(2) composites.
Fig. 5. The temperature dependence of the dielectric constant of GO1 and GO2 at 1 kHz.
ductive sp2 clusters. The conductivity showed less variation when the applied frequency was low due to more active grain boundaries. Variation at high frequency regions was due to less active grain boundaries. Intercalated water molecules and additional oxygen functional groups in GO played a major role in the electrical
conductivity. Conductivity of GO was controlled by the presence of insulating sp3 structure within the GO sheet. Compared to the pristine BT, the introduction of GO into BT increases the electrical conductivity of the GO(1)-BT(2) composite, which may be due to the introduction of polar functional groups. It is observed that the electrical conductivity of GO-BT composites increases with increasing content of GO. This reaches 3.68 × 10−3 Sm−1 for GO(2)-BT(1) composite. Because, the BT was grafted onto the surface of GO to form GO-BT composite and became more insulating than GO. ¨ Fig. 6(b) and (c) shows the frequency dependence of ’ and of the GO, BT and GO-BT composites at 27 ◦ C. All the samples have a frequency dependence of ’ and ¨in the low frequency region followed by a nearly frequency independent behaviour above 104 Hz. GO shows the highest value of ’ (3.3 × 105 ) at low frequency and decreases with increasing frequency and becomes constant in the high frequency range of 104 –106 Hz. It is obvious that the ’ and ¨ the samples were dependent on the applied frequency. The of possible cause of this decrease of ’ with increasing frequency may be attributed to the interfacial polarization. As the GO to BT weight ratio approached to 1:1 and 1:2, the ’ of the GO(1)-BT(1) and GO(1)-BT(2) composites decreased to 5630 and 1479 respectively. It can be clearly observed that the ’ of the composites can be enhanced by increasing weight ratio of GO nanosheets. When the GO to BT weight ratio approached about 2:1, a percolation transition from an insulator to semiconductor occurred, which is accompanied by a dramatically increased ’. The ¨increases with the increasing weight percentage of GO in the composite. For
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9 6
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
Fig. 6. The frequency dependence of (a) electrical conductivity (b) dielectric constant (c) dielectric loss and (d) imaginary part of electric modulus of GO, BT and GO-BT composites at 27 (C. Insert shows the expanded portion of dielectric constant and dielectric loss of BT and GO-BT composites at low frequency.
GO(2)-BT(1) composite, the ¨increased significantly due to the high thermal agitation/vibration of the dipoles on the GO basal plane [8]. Fig. 6d shows the frequency dependence of the imaginary part (M”) of the electric modulus of GO, BT and GO-BT composites at 27 ◦ C. The electric modulus formalism has been used to study the dipole relaxation process, interfacial polarization and long-range conduction process within the bulk of materials. Because, it minimizes the electrode interface capacitance contribution and other interfacial effects. The BT shows a relaxation peak at low frequency compared to GO and composites. This is due to the charge carriers in BT have a low mobility within the bulk materials. In such a case, there should be a considerable time interval required for charge carriers to reach an interface, in other word interfacial polarization takes long time to be established. Consequently, interfacial polarization appears at low frequencies for BT [25]. The variation of M” with applied frequency for GO-BT composites, the characteristic relaxation peak moves towards higher frequencies with increasing GO weight ratio, because the charge carriers accumulate on the surface of GO. This is the reason for the shift of the interfacial polarization relaxation peaks of composites to higher frequencies. For GO, the increase in peak frequency indicates a short relaxation time, which is consistent with an increase in the mobility of charge carriers.
The temperature dependences of the ’ of GO, BT and GOBT composites at three different frequencies are shown in Fig. 7. The dielectric behaviour of samples at different frequencies with increase in temperature was studied. The decrease of the ’ with frequency can be attributed to the contribution of multi components of polarizability, deformational and relaxation. GO exhibits temperature dependent behaviour over the entire frequency range. We observe that ’ of GO was increases with temperature (30–50 ◦ C) at 100 Hz. The ’ start to decrease with an increase in temperature from 55 to 165 ◦ C due to the change in the dipole moment, orientation polarization of the functional groups (-OH and −COOH) and the loss of absorbed moisture. Above 170 ◦ C, ’ of GO increased with increasing temperature, as shown in Fig. 7a. A similar trend is also observed for all the other frequencies in the same temperature domain. Increase of the charge carriers mobility with the increasing temperature gives rise to an increase of the interfacial polarization, which leads to an increase of the ’ [26,27]. For BT, the ’ increases with temperature and reaches maximum value (1446 at 150 ◦ C) corresponds to the ferroelectric to paraelectric phase transition of the BT phase (Fig. 7b). It reveals that BT exhibits Curie peak at 150 ◦ C measured at 1 kHz due to increase of grain size [17]. The ’ of the different weight ratio of GO-BT composite showed that the two major behaviors [Fig. 7c and SI 4)]. With GO adding into BT [GO(1)-
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
7
Fig. 7. Temperature dependence (a-c) dielectric constant of GO, BT and GO-BT composites at selected frequencies and (d) comparison of temperature dependence dielectric constant at 1 kHz for GO, BT and GO-BT composites. Insert shows the dielectric constant transition region of samples.
BT(2)], ’ increases significantly. It can be considered the addition of BT makes the destruction of the electric field resulting accumulation of charges at intergranular areas. Therefore, the formation of this macro dipoles dispersion impedes polarization and leads to the decrease of ’. Other word, the high values of ’ of GO play a leading role of the dielectric response in the composite, so the ’ increases with increasing amount of GO. The increase of ’ with increase of GO is ascribed to the large value of the ’ of the GO (20444) as compared to BT (1446) at 1 kHz. Curie temperature of BT was suppressed in the all composites and effect of GO on the dielectric properties of the composite is significant. The peak of GO was found at 50 ◦ C and this peak gradually moves to high temperature as well as the intensity of this peak decreases gradually when increasing the BT ratio in composite. In addition, valley peak at 170 ◦ C moves towards low temperature with increasing BT content ratio indicates that the domination of BT in composite (Fig. 7d). SI 5 depicts the AC conductivity of GO, BT and GO-BT composites as a function of frequency at different temperatures. GO, BT and composites exhibits frequency dependent conductivity over the entire frequency range and demonstrate an Arrhenius of behaviour [28]. The frequency dependence of lies in the relaxation phenomena arising due to mobile charge carriers. At 30 ◦ C, the of GO is 1.577 × 10−2 Sm−1 and with an increase in temperature it increases to 1.63 × 10−2 Sm−1 (55 ◦ C) due to enhanced ion mobility. The temperature between 60 and 165 ◦ C, ionic conductivity of GO is very low. This is due to an accumulation of charge carriers at the electrode leads to the formation of charge layers, which can shield
the sample from the electric field. Thus, the electrical conductivity decreases with temperature [23]. The of GO is increases rapidly at 170 ◦ C and above. A large increase of conductivity appears at 170 ◦ C due to deoxygenation. At 150 ◦ C, BT exhibits the maximum conductivity and then decreases with temperature due to phase transition of BT from tetragonal to cubic. The conductivity of GO-BT composite enhances with increasing content of GO as illustrated in SI 5c. This is due to the incorporation of GO in composites. The homogeneous dispersion takes place when the GO content is increased, which contributes to the enhancement in the electrical conductivity of the composite. Also found that the transition of GO at 170 ◦ C was shifted to lower temperature with increasing BT content. It is very similar to the variations of temperature dependence of ’ behaviour. SI 6 shows the temperature dependence of imaginary part of electric modulus (M”) of GO, BT and GO-BT composites. The M” was used to study dielectric relaxation mechanisms, because it minimizes the electrode interface contribution and other interfacial effects which associated with the contribution of small capacitance. For GO, the relaxation peak shifted to higher frequency when the temperature increasing from 30 to 60 ◦ C and then it gradually shifted to low frequency until 170 ◦ C. This is due to decrease of charge carrier density because the ion mobility increases with increasing temperature. The M” of the BT exhibits the loss peak in the low frequency range and peak shifted to high frequency when temperature is high. According to SI 6 (c-e), the variation of M” with frequency shows a typical relaxation peak that moves towards
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9 8
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
Fig. 8. Polarization (P) vs Electric filed (E) loops for GO, BT and GO-BT composites at room temperature.
lower frequencies by increasing BT. This implies that charge carriers in the composites become bounded. Therefore, their migration and accumulation process needs a relatively long time. In this case, the time interval required for charge carriers to reach an interface takes long time to be established, so interfacial polarization appears at low frequencies. The relaxation strength of GO-BT composites increases with decreasing BT content [25]. Fig. 8 shows variation in polarization with respect to the electric field (P-E responses) at 2 Hz for GO, BT and GO-BT composites. The P-E hysteresis loop illustrates the effect of adding GO content to the BT. BT shows a well-defined hysteresis loop with negligible leakage indicating better ferroelectric behaviour [17]. The GO and GO-BT composites displayed an unsaturated P-E curve with loss, which may be partially attributed to the presence of an interface between BT and GO. It has been realized from the figure that the ferroelectricity of composites decreases from that of BT, allowing for ferroelectric domain switch. Furthermore, the area of the loop also increased for composites than BT, which may be due to the
strong conductivity. The much increased leakage current indicates that the high remnant polarization in the GO might be attributed to the accumulation of charges on the surface of GO [29]. The increasing content of GO in composite leads to decrease in remnant polarization because the introduction of GO would weaken the ferroelectricity of the composite, which is in agreement with the dielectric results. This confirms that the presence of dielectric materials significantly affects the polarization response of the BT in the composites.
4. Conclusion We have described the dielectric properties of GO with different thicknesses at 27 ◦ C. Thick sample of GO exhibits maximum ’ as 1.89 × 104 at 1 kHz and is due to presence of more defects. The ’ of the composite decreases with increase in frequency and increases with increase in the GO content. GO exhibits two transition at 50 and 170 ◦ C and is attributed to the presence of oxygen
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026
G Model JECS-10926; No. of Pages 9
ARTICLE IN PRESS T. Kavinkumar et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
functional groups. The ’ peak of GO at 50 ◦ C was gradually shifted to high temperature as well as the valley peak at 170 ◦ C moves towards low temperature with increasing BT content in composites. The increased amount of oxygen vacancies and free electron are the causal factor for the observed shift in the composites. The P-E loops imply that the effect of GO in ferroelectricity of BT in the composite. The increasing content of GO in composite leads to decrease in remnant polarization because the introduction of GO would weaken the ferroelectricity of the composite. The enhanced dielectric behaviour of composite finds potential application in dielectric and optoelectronic application. Acknowledgements The authors thank Dr. R. Justin Joseyphus, Dr. N. V. Giridharan and Dr. B. Karthikeyan for TGA, ferroelectric and micro Raman measurements respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2016.11.026. References [1] [2] [3] [4]
[5] [6]
[7]
[8]
[9]
[10]
J. Xu, C. Wong, Appl. Phys. Lett. 87 (2009) 082907. M. Arbatti, X. Shan, Z.Y. Cheng, Adv. Mater 19 (2007) 1369. C.C. Homes, T. Vogt, Nat. Mater. 12 (2013) 782. L. Zhang, P. Wu, Y. Li, Z.Y. Cheng, J.C. Brewer, Preparation process and dielectric properties of Ba0.5Sr0.5TiO3-P(VDF-CTFE) nanocomposites, Compos. B Eng. 56 (2014) 284. K. Ring, K. Kavanaph, Substrate effects on the ferroelectric properties of fine-grained BaTiO3 films, J. Appl. Phys. 94 (2003) 5982. N. Kumar, J. Pan, N. Aysha, U.V. Waghmare, A. Sundaresan, C.N.R. Rao, Effect of co-substitution of nitrogen and fluorine in BaTiO3 on ferroelectricity and other properties, J. Phys. Condens. Matter25 (2013) 345901. X. Wang, Y. Zhang, X. Song, Z. Yuan, T. Ma, Q. Zhang, C. Deng, T. Liang, Glass additive in barium titanate ceramics and its influence on electrical breakdown strength in relation with energy storage properties, J. Eur. Ceram. Soc. 32 (2012) 559–567. S.J. Pedro, P.M. Luis, J.M. Dia´ınez, P. Antonio, J.M. Criado, Mechanochemical preparation of BaTiO3-Ni nanocomposites with high dielectric constant, Compos. Struct. 92 (2010) 2236. S. Nayak, B. Sahoo, T.K. Chaki, D. Khastgir, Facile preparation of uniform barium titanate (BaTiO3) multipods with high permittivity: impedance and temperature dependent dielectric behavior, RSC Adv. 4 (2014) 1212. K.S. Kumar, S. Pittala, S. Sanyadanam, P. Paik, A new single/few-layered graphene oxide with a high dielectric constant of 106 : contribution of defects and functional groups, RSC Adv. 5 (2015) 14768.
9
[11] T. Zhou, J.W. Zha, R.Y. Cui, B.H. Fan, J.K. Yuan, Z.M. Dang, Improving dielectric properties of BaTiO3/ferroelectric polymer composites by employing surface hydroxylated BaTiO3 nanoparticles, ACS Appl. Mater. Interfaces 3 (2011) 2184. [12] Y. Li, Y. Shi, F. Cai, J. Xue, F. Chen, Q. Fu, Graphene sheets segregated by barium titanate for polyvinylidene fluoride composites with high dielectric constant and ultralow loss tangent, Compos. A 78 (2015) 318. [13] U. Yaqoob, A.S.M.I. Uddin, G.S. Chung, The effect of reduced graphene oxide on the dielectric and ferroelectric properties of PVDF-BaTiO3 nanocomposites, RSC Adv. (2016), http://dx.doi.org/10.1039/C6RA03155B. [14] Y. Shen, Y. Guan, Y. Hu, Y. Lei, Y. Song, Y. Lin, C.W. Nan, Dielectric behavior of graphene/BaTiO3/polyvinylidene fluoride nanocomposite under high electric field, Appl. Phys. Lett. 103 (2013) 072906. [15] D. Wang, T. Zhou, J.W. Zha, J. Zhao, C.Y. Shi, Z.M. Dang, Functionalized graphene-BaTiO3 /ferroelectric polymer nanodielectric composites with high permittivity, low dielectric loss, and low percolation threshold, J. Mater. Chem. A 1 (2013) 6162. [16] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [17] C.R. Xin, J. Zhang, Y. Liu, Q.L. Zhang, H. Yang, D. Cheng, Polymorphism and dielectric properties of Sc-doped BaTiO3 nanopowders synthesized by sol-gel method, Mater. Res. Bull. 48 (2013) 2220. [18] R.X. Wang, Q. Zhu, W.S. Wang, C.M. Fan, A.W. Xu, BaTiO3 -graphene nanocomposites: synthesis and visible light photocatalytic activity, New J. Chem. (2015), http://dx.doi.org/10.1039/c4nj02272f. [19] M.K. Mahata, T. Koppe, T. Mondal, C. Brüsewitz, K. Kumar, V.K. Rai, H. Hofsäss, U. Vetter, Phys. Chem. Chem. Phys. (2015), http://dx.doi.org/10.1039/ C5CP01874A. [20] D.Y. Lu, W. Cheng, X.Y. Sun, Q.L. Liu, D.X. Li, Z.Y. Li, Abnormal Raman spectra in Er-doped BaTiO3 ceramics, J. Raman Spectrosc. 45 (2014) 963. [21] J. Suchanicz, K. Swierczek, E. Nogas-Cwikiel, K. Konieczny, D. Sitko, PbMg1/3Nb2/3O3-doping effects on structural, thermal Raman, dielectric andferroelectric properties of BaTiO3ceramics, J. Eur. Ceram. Soc. 35 (2015) 1777–1783. [22] D. Yang, A. Velamakanni, G. Bozoklu, et al., Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy, Carbon 47 (2009) 145. [23] X. Huang, C. Zhi, P. Jiang, D. Golberg, Y. Bando, T. Tanaka, Temperature-dependent electrical property transition of graphene oxide paper, Nanotechnology 23 (2012) 455705. [24] N. Adhlakha, K.L. Yadav, R. Singh, Effect of BaTiO3 addition on structural, multiferroic and magneto-dielectric properties of 0.3CoFe2 O4 -0.7BiFeO3 ceramics, Smart Mater. Struct. 23 (2014) 105024. [25] Y. Li, X. Huang, Z. Hu, P. Jiang, S. Li, T. Tanaka, Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon carbide three-phase nanocomposites, ACS Appl. Mater. Interfaces 3 (2011) 4396. [26] B. Wang, Y.P. Pu, N. Xu, H.D. Wu, K. Chen, Dielectric properties of barium titanate-molybdenum composite, Ceram. Int. 38 (2012) S37. [27] S. Baryshnikov, E. Stukova, E. Koroleva, Dielectric properties of the ferroelectric composite (NaNO2 )0.9 /(BaTiO3 )0.1 , Compos. B Eng. 66 (2014) 190. [28] M.M. El-Nahass, H.A.M. Ali, Solid State Commun. 152 (2012) 1084. [29] O.D. Jayakumar, E.H. Abdelhamid, V. Kotari, B.P. Mandal, R. Rao, V.M.Naik Jagannath, R. Naik, A.K. Tyagi, Fabrication of flexible and self-standing inorganic-organic three phase magneto-dielectric PVDF based multiferroic nanocomposite films through a small loading of graphene oxide (GO) and Fe3O4 nanoparticles, Dalton Trans. (2015), http://dx.doi.org/10.1039/ c5dt01509j.
Please cite this article in press as: T. Kavinkumar, et al., Dielectric transition and ferroelectric properties of graphene oxide-barium titanate nanocomposites, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.11.026