Barium Titanate Nanocomposites

Research Article www.acsami.org

Dielectric Enhancement in Graphene/Barium Titanate Nanocomposites Bingcheng Luo,† Xiaohui Wang,*,† Enke Tian,‡ Huiling Gong,† Qiancheng Zhao,† Zhengbo Shen,† Yan Xu,§ Xiaoyue Xiao,§ and Longtu Li*,† †

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China ‡ School of Science, China University of Geosciences, Beijing 100083, P. R. China § Nanjing SCF Nanotech Ltd., Nanjing 211800, P. R. China ABSTRACT: GN/BT nanocomposites were fabricated via colloidal processing methods, and ceramics were sintered through two-step sintering methods. The microstructure and morphology were characterized by X-ray diffraction, highresolution transmission electron microscopy, and field emission scanning electron microscopy. XRD analysis shows that all samples are perovskite phases, and the lattice parameters a and c almost decrease linearly with the increase of graphene nanosheets. The dielectric properties were tested by using precision impedance. The maximum dielectric constant at the Curie temperature for the nanocomposites with graphene addition of 3 wt % is about 16 000, almost 2 times more than that of pure BaTiO3 ceramics. The relaxation, band structure, density of states, and charge density distribution of GN/BT superlattices were calculated using first-principles calculations for the first time, and results showed the strong hybrid interactions between C 2p states and O 2p and Ti 3d orbitals. KEYWORDS: graphene, dielectric, first-principles, ferroelectric, BaTiO3

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the low thermal stability of graphene over 600 °C.19 Walker and colleagues reported that graphene was used to enhance the toughness of bulk silicon nitride ceramics, and results showed that the bulk ceramic’s fracture toughness was increased by up to 235% at 1.5% graphene platelets volume fraction.19 However, these researches are almost entirely involved with the structural ceramics related to the elastic and mechanical properties, and there are still no reports on the use of graphene additives in the dielectric, piezoelectric, ferroelectric, and other functional ceramics. As is well-known, ferroelectric ceramics have been widely used in capacitors, sensors, transducers, memory, and energy storage devices due to their unique dielectric, ferroelectric, and mechanical−electrical coupling properties. Barium titanate, one of the most important ferroelectric materials, has attracted extensive experimental and theoretical study since its discovery of ferroelectric behavior in 1945 and 1946. To the best of our knowledge, there is still no report about the graphene/barium titanate (GN/BT as abbreviation) composite bulk ceramics. As is wellknown, the sintering mechanics is crucial for nucleation and grain growth, thus affecting the properties of ceramics.

INTRODUCTION Graphene is the first two-dimensional atomic crystals with only one-atom-thick layers of sp2-bonded carbon packed in a hexagonal lattice and is a basic building block of well-known carbon materials such as graphite, diamond, carbon nanotubes, and fullerene.1 Since its discovery in 2004,2 graphene has shown extraordinary properties including a large theoretical specific surface area of about 2630 m2 g−1,3 outstanding electronic transport with a room-temperature electron mobility of 2.5 × 105 cm2 V−1 s−1,4 unique mechanical properties with a Young’s modulus of 1 TPa, an intrinsic strength of 130 GPa, a stiffness of 1060 GPa,5 an ultrahigh thermal conductivity of 5000 W m−1 K−1,6 and remarkable optical transparency with only 2.3% absorption of light intensity.2 For this reason, graphene has been used in sensors7 including biosensors,8 fieldeffect transistors,9 photovoltaic devices,10,11 memory devices,12 energy storage devices, and composites application. Graphenebased composites with polymer, metal, and metal oxides have been demonstrated to have performances thanks to the introduction of graphene in enhancing the electronic, mechanical, and electromechanical properties.13 Currently, the graphene-based composites are mainly composed of the polymer matrixes14,15 and inorganic composite bulks,16−18 but there are rare reports on adopting graphene as additives to improve the properties of bulk ceramics. This is attributed to © 2016 American Chemical Society

Received: November 20, 2015 Accepted: January 18, 2016 Published: January 18, 2016 3340

DOI: 10.1021/acsami.5b11231 ACS Appl. Mater. Interfaces 2016, 8, 3340−3348

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of fabrication of graphene barium titanate dielectric nanocomposites.

Figure 2. BaTiO3 nanoparticles and graphene nanosheets. (a) TEM image of pure BaTiO3 nanoparticles. (b) TEM image of BaTiO3 nanoparticles with CTAB modification. (c, d) TEM images of graphene nanosheets with about seven layers. (e) High- and (f) low-resolution SEM images of graphene nanosheets. (g, h) SEM images of BaTiO3 nanoparticles. (i) The particle size distribution based on SEM images.

Conventional sintering, associated with some additions to control the sintering diffuse process, is usually employed to the densification and grain growth. Nevertheless, the relative density through conventional sintering is very low. Comparatively, spark plasma sintering and hot pressing sintering approaches are employed to produce fine ceramics with high densification. However, these methods are not economical depending on the application of these products. Chen and Wang20 proposed one two-step sintering method to fabricate the ceramics with relative density excess of 95% and applied the cost-effective preparation of various nanocrystalline materials. We did this in this work using a two-step sintering method to obtain GBT ceramics, with suppressing the final-stage grain

growth through exploiting the difference in kinetics between grain boundary diffusion and migration. Recently, first-principles calculations have been widely used in chemistry, physics, and materials, biological, and other fields. As far as we know, the first-principles investigation of GN/BT nanocomposites has not been reported, which might be attributed to the following reasons. The first is the lattice mismatch. The BaTiO3 has a tetragonal lattice with the symmetry P4mm, while graphene possesses a hexagonal lattice with the symmetry P6/mmm, producing the serious lattice mismatch. As a result, the common superlattice structure is unable to be modeled in a conventional method. The second reason is the size of these nanocomposites. The GN/BT nanocomposites in the present experiment are bulk ceramics, 3341


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Figure 3. (a) Thermogravimetric analysis (TGA) for graphene nanosheets. (b) Sintering mechanism for GN/BT composites. (c) XRD patterns of GN/BT composites with different graphene addition. (d) Lattice parameters derived from XRD patterns.

Table 1. Basic Physical Properties of GN/BT Composites with Two-Step Sintering and Linear Fitting Parameters of FrequencyDependent Dielectric Constants samples

graphene, wt %

density, g/cm3

ratio, %

γ

C′

residual sum of squares

Pearson’s r

adj. R square

GN-BT-0 GN-BT-1 GN-BT-2 GN-BT-3 GN-BT-4 GN-BT-5

0.0 0.3 0.5 0.7 1.0 3.0

5.940 5.900 5.926 5.918 5.854 5.842

98.7 98.0 98.4 98.3 97.2 97.0

0.7593 0.7284 0.9028 0.9089 0.9094 0.8709

81521 69444 154106 167672 175790 153788

0.046 0.044 0.024 0.003 0.101 0.056

0.9936 0.9935 0.9977 0.9997 0.9904 0.9943

0.9861 0.9857 0.9949 0.9993 0.9790 0.9875

the density and dielectric properties. Our previous works have reported the fabrication of high-density BaTiO3, Ni−Cu−Zn ferrite, Y2O3, and related nanograin ceramics combining the best powder synthesis and optimized two-step sintering.22−24 The key of two-step sintering is the apparent suppression of grain growth in the second step densification due to the “frozen” four-grain junctions which resist grain boundary migration even though grain boundary diffusion still proceeds.22 In our pioneering work in 2006, dense fine-grained BaTiO3 ceramics with 96% of relative density sintered at low temperature were fabricated using two-step sintering approaches.23 Through controlling the temperature and time in the two-step sintering process, the grain size and dielectric constant can be dominated, and our previous results showed that the maximum dielectric constant appears in ceramics with about 1 μm of grain size. Therefore, the two-step sintering approaches are applied to obtain a high density GN/BT composite according to the sintering mechanism shown in Figure 3. Through the thermogravimetric curve in Figure 3a, it can be seen that graphene is highly stable up to 600 °C in air. As a result, the composite disks are treated at 400 °C in air to remove the organics, followed by the two-step sintering displayed in Figure 3b. Table 1 lists the basic physical parameters after the two-step sintering. All samples have a

which are composed of tens of thousands of grains; it is difficult to study the whole bulks in principle. Thus, in this paper, we focus on the local interaction between graphene nanosheets and BaTiO3, and the structural and electronic properties are calculated through first-principles calculations. As for the whole properties of the nanocomposite bulks, they can be calculated by the future thermodynamic statistical model or molecular dynamical methods.

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RESULTS AND DISCUSSION The graphene barium titanate composite powders were fabricated by using colloidal processing methods, which have been reported as an effective way to improve the dispersion of graphene or carbon nanotubes. Figure 1 illustrates the fabrication of GN/BT composites. The highly dispersed GN/ BT nanocomposite slurries containing graphene nanosheets were prepared based on the pioneering work with graphene/ silicon nitride composites and single-walled carbon nanotube/ silicon nitride nanocomposites.19,21 The prepared graphene was composed of about seven-layer nanosheets as shown in Figure 2d, while the purchased BaTiO3 powders is in the uniform distribution of the size of 50 nm, as seen in Figure 2. After we fabricate the homogeneously dispersed nanocomposite particles, the crucial process is the sintering process, which affects 3342


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Figure 4. Frequency dependence of dielectric constant (a) and dielectric loss (b) of GN/BT composites. Temperature dependence of dielectric constant (c) and (d) Curie Weiss fitting curves of GN/BT nanocomposites. (e) Schematic images of the microstructure of GN/BT nanocomposites.

decreases in the whole range and fluctuates at the composition 0.3−1 wt % of graphene nanosheets addition. The decrease is consistent with Vegard’s law, which states that, in the absence of strong electronic effects, the variation of lattice parameters is linear with composition.26,27 It can be concluded that some carbon atoms may mix into the crystal lattice when much more graphene nanosheets are introduced. Figure 4 shows the frequency dependence of the dielectric constant and dielectric loss of GN/BT composites with different graphene addition measured in the frequency range of 40−10 000 Hz and at room temperature. At low frequencies, the GN/BT composites have a much higher dielectric constant for all samples. With the increase of frequency, the dielectric constant decreases slightly for all GN/BT composites. It can be clearly seen that the dielectric constant of GN/BT composites increases after the addition of graphene. As the content of graphene grows, the dielectric constant of composites increases. Meanwhile, the dielectric loss increases with the loading of graphene nanosheets. Figure 4c shows the temperature dependence of the dielectric constant of GN/BT composites with different graphene addition measured in the temperature range of 30−180 °C at 1000 Hz. The dielectric constant increases with the temperature and then reaches the maximum at the Curie temperature, which resulted from the phase transition from tetragonal to cubic phase. After the Curie temperature, the dielectric constant decreases quickly. From Figure 4c, it can be clearly seen that the maximum dielectric

high density with the relative ratio over 97%. The total density of samples decreases with the addition of graphene nanosheets with low-density increases. Figure 3 shows the X-ray diffraction (XRD, D8 Advance, BRUKER) patterns for GN/BT composites with various amounts of graphene nanosheets after two-step sintering. It can be seen that all composites could be fabricated in the desired perovskite phase with no second phase as shown in Figure 3a, identifying that no reaction between the matrix and graphene or decomposition of the matrix has occurred. The 2θ value obtained for pristine graphene was at 26.6°, which is in good accordance with previous results.25 The characteristic XRD patterns of the GN/BT nanocomposites exhibit six prominent peaks, which are indexed to the scattering from (001)(100), (101)(100), (111), (002)(200), (112)(211), and (202)(220) planes, respectively, which was in accordance with the powder diffraction peaks for tetragonal BaTiO3 (ICDD PDF #05-0626). The splitting peaks that appeared at 2θ = 45° further confirm the tetragonality of all GN/BT samples. It can be seen that the fundamental perovskite reflections at 2θ = 45° shift to the left with the increase of graphene addition and then shift to the right after the addition is over 0.7 wt %, revealing the difference of lattice parameters. Figure 3b illustrates that the changes of lattice parameters for GN/BT composites with increasing graphene nanosheets correspond with the phase. It is clear that the lattice parameters a and c almost decrease linearly with the increase of graphene nanosheets, while the c/a ratio 3343


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Figure 5. (a, b) Raman spectroscopy of graphene nanosheets and GN/BT nanocomposites. (c) XPS spectra of graphene nanosheets and GN/BT nanocomposites. (d) High-resolution signals of C 1s elements of GN/BT nanocomposites with 1 wt % graphene contents.

consisting of a thin layer of BaTiO3 ceramics and graphene nanosheets, as shown in Figure 4. After the incorporation of graphene nanosheets into the BaTiO3 ceramics, some microcapacitor structures are formed, resulting in the increase of dielectric constant relative to pure BaTiO3 ceramics. With the addition of graphene nanosheets, the accumulated conductive particles form much more microcapacitors, resulting in the very large permittivity at the 3 wt % addition of graphene nanosheets. After the addition of graphene nanosheets is over 5 wt %, the whole composites show conductor behavior, which might be caused by the connection of upper and lower electrodes via graphene nanosheets. The third reason is the contribution of Maxwell−Wagner−Sillars polarization for heterogeneous systems, which is associated with the enhancement of free charges between the insulator/conductor interfaces.29 Because of the Maxwell−Wagner−Sillars effects, some charges are blocked at the interfaces between the BaTiO3 and graphene nanosheets after the addition of graphene nanosheets, as shown in Figure 4, which enhances the dielectric permittivity of the nanocomposites. Raman spectroscopy is a versatile tool to study the electron− phonon interaction and has been used to investigate the properties of graphene such as electric and magnetic fields, strain, doping, disorder, and functional groups.30,31 Figure 5 shows the Raman spectroscopy of graphene nanosheets and GN/BT nanocomposites. The peaks of D, G, G*, and G′(or 2D) modes in the graphene powders can be clearly seen. The G mode that appears at 1581 cm−1 is originated from the in-plane stretching vibration of the C−C bond with E2g symmetry at the Γ-point. The G* mode at 2438 cm−1 and the G′ mode at 2719 cm−1 are arising from the double resonance scattering process involving two in-plane transverse optical phonons (G*) or the combination of a longitudinal phonon and a transverse phonon (G′).32,33 The D mode at about 1358 cm−1 is caused by the disorder band or defect band, which is very weak in pristine

constant at the Curie temperature increases sharply after the addition of graphene nanosheets. The maximum dielectric constant at the Curie temperature for pure BaTiO3 ceramics is about 8000, while that of the composites with a graphene addition of 3 wt % is about 16 000. To better understand the phase transition, the modified Curie−Weiss law was adopted to fit the temperature-dependent dielectric constant (T − Tm)γ 1 1 − = ε εm C′ ⎛1 1⎞ ln⎜ − ⎟ = γ ln(T − Tm) − ln C′ εm ⎠ ⎝ε

where ε is the dielectric constant, εm is the maximum dielectric constant at the phase transition temperature Tm, γ is the preexponential factor, and C′ is the constant. If γ is nearer to 2, it means the diffuse phase transition for materials, while the value nearer to 1 means the normal case. Figure 4d shows the plots of Curie−Weiss law of GN/BT composites with different graphene addition, and the fitted results are listed in Table 1. The Pearson’s r for all samples is higher than 99%, while the adjusted R square is higher than 98%, indicating reliability of the linear fitting. The slope of the linear fitting curves can be used to determine the γ value. The γ value of the BT ceramics and GN/BT composites is nearer to 1, implying the normal phase transition and showing obscurely evidence for the diffuse transition behavior in contrast to the case of BZT/CNTs composites.28 As for the origin of the enhancement in dielectric properties of GN/BT nanocomposites, it could be explained by the following three reasons. Above all, the incorporation of high conductivity particles increases the conductivity of the nanocomposites, and the homogeneous dispersion contributes to the enhancement in dielectric properties. The second is the formation of large amounts of microcapacitor networks 3344


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Figure 6. First-principles calculation of GN/BT superlattices. Superlattice structures before (a) and after (b) full relaxation. (c) Charge density distribution of GN/BT superlattices along the [001] axis. Band structures of graphene superlattices (d), BaTiO3 superlattices (e), and GN/BT superlattices (f). Projected density of states of graphene superlattices (g), BaTiO3 superlattices (h), and GN/BT superlattices (i) along the highsymmetry direction. The Fermi level is located at zero energy, indicated by the dotted line.

peaks at 285.2 eV are partially ascribed to the residual Ocontaining group of graphene and the O atom of BaTiO3, while the peaks at 288.2 eV might be arising from the interactions among C, Ti, and O atoms, which can be further verified by the density of states from first-principles calculations. In order to better interpret the inner mechanism of the interaction between graphene and BaTiO3, the first-principles calculations based on density functional theory are presented to investigate the interface properties of graphene and BaTiO3. The superlattice structure of GN/BT nanocomposites before and after full relaxation is shown in Figure 6a,b. It can be seen that the lattice distorts obviously after the relaxation, implying the intense interaction between BaTiO3 and graphene nanosheets. The charge density distribution of GN/BT nanocomposites shows the clear interactions of C−O and C−Ti atoms, as seen in Figure 6c. To understand the electronic properties of the nanocomposites and the details of the interaction between the BaTiO3 layers and graphene superlattices, the projected density of states (PDOS) and band structure were calculated by projecting the electron wave functions onto spherical harmonics centered on each type of atom, as shown in Figure 6.34 The Fermi level was referenced at zero energy. Before investigating the electronic properties of the nanocomposites GN/BT superlattices, the electronic properties of pure graphene and BaTiO3 superlattices were studied. Figure 6d shows the band structures of graphene superlattices. It can be seen that the conduction and the valence

graphene, as shown in Figure 5. For the GN/BT nanocomposites, the G mode, G* mode, and 2D mode of graphene can be observed at 1500−3500 cm−1, identifying the existence of graphene. The difference of Raman shifting from the pristine graphene might be caused by the local interaction between C atoms and Ti and O atoms during the sintering process. The Raman mode of BaTiO3 is mainly located at 100−800 cm−1, as shown in Figure 5. It can be observed that four characteristic bands located at 255, 308, 518, and 720 cm−1 and the optical modes for polarized tetragonal form are Raman active. The broad peaks centered at 255 cm−1 exhibit A1(TO) mode, and the peaks at 518 cm−1 correspond to E(TO) and A1(TO3) modes, while the peaks at 720 cm−1 are arising from E(LO) and A1(LO) modes. The Raman peaks that appeared at 307 cm−1 exhibit B1 and E(LO + TO) modes, verifying the tetragonal phase of BaTiO3 in great accordance with the XRD analysis, as shown in Figure 3. X-ray photoelectron spectroscopy (XPS) is employed to further confirm the graphene in GN/BT nanocomposites, as shown in Figure 5. It can be clearly seen that only C 1s peaks were found in pristine graphene nanosheet powders and C 1s peaks appear in all the samples of GN/BT nanocomposites, identifying the existence of graphene. The Ba 3d peaks were resolved into two components Ba 3d5/2 and Ba 3d3/2. From the high-resolution signals of C 1s peaks of GN/ BT nanocomposites with 1 wt % graphene contents, it can be seen that the C 1s spectra are mainly composed of the strong peak at 284.7 eV corresponding to sp2-hydridized carbon. The 3345


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hazardous and noxious substances are used and no waste liquid is produced, which is safe and environmentally friendly. Before fabricating the nanocomposites, surface functionalization is carried out via chemical coating methods with a cationic surfactant, cetyltrimethylammonium bromide (CTAB, ≥99%, Sigma-Aldrich), to improve the dispersion. CTAB is an amine-based cationic quaternary surfactant and has been reported in the application in the functionalization of graphene.16,35,36 GN/BT nanocomposites were fabricated by colloidal processing methods. Appropriate amounts of graphene and BaTiO3 powders were added to separate containers of DMAC with CTAB predissolved.19 Then, the graphene and BaTiO3 solutions were sonicated for 30 min, respectively. The dispersed solutions were then combined and sonicated for an additional 20 min. After sonication, the slurries were ball-milled in DMAC using zirconia media for 24 h. After drying, the powders were pressed into disks 10 mm in diameter and about 1 mm in thickness under a pressure of 5 MPa. Then, all disks were sintered using two-step sintering methods as shown in Figure 3b. The detailed schematic of fabrication of GN/BT nanocomposites is shown in Figure 1. Characterization. X-ray diffraction (XRD) patterns of all samples were collected at room temperature on a Bruker D8 Advance A25 Xray diffractometer using Cu Kα radiation of wavelength λ = 0.154056 nm. X-ray scans were performed over a wide range of 2θ (10° ≤ 2θ ≤ 90°). Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA449F3 apparatus at a heating rate of 10 °C/min in air. The surface characteristics of the samples were investigated using Raman spectroscopy (LabRAM HR Evolution, Horiba scientific) and ESCALAB 250Xi XPS (Thermo Scientific). Morphology analysis was performed with a field emission scanning electron microscope (FESEM, Merlin, ZEISS, Germany). Highresolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM-2010 microscope operating at 200 kV. To characterize the dielectric properties, silver paste was coated on both two sides of samples and the disks were sintered at 550 °C for 30 min. The frequency dependence of dielectric constant and loss tangent of the nanocomposites were measured by employing a precision impedance analyzer (Agilent HP4194A, Agilent Technologies, Santa Clara, California, USA) at room temperature at a frequency from 40 Hz to 10 MHz. The temperature dependence of dielectric properties were measured by employing a HP4192A precision impedance (Agilent Technologies, Santa Clara, California, USA) at 1000 Hz from room temperature to 180 °C. First-Principles Calculations. The symmetry of the tetragonal BaTiO3 crystal lattice is described by the space group P4mm with the lattice parameters a = b = 3.995 Å and c = 4.034 Å for one formula unit per unit cell, while that for graphene is P6/mmm, with the nearest C− C bond length l = 1.42 Å. Superlattice structures of GN/BT nanocomposites were modeled with 58 atoms based on 2 × 2 × 2 BaTiO3 and 3 × 3 × 1 graphene, as shown in Figure 6a. All calculations presented in this study were performed within firstprinciples calculations based on the density functional theory (DFT) framework.37,38 The pseudopotentials used for GN/BT superlattice models were constructed by the electron configurations as Ba 5s25p66s2 states, Ti 3s2sp63d24s2 states, C 2s22p2, and O 2s22p4 states. The generalized gradient approximation (GGA) with the PBE exchange-correlation functional with Hubbard correction and hybrid functional HSE06 implemented in the Vienna Ab initio Simulation Program VASP were used for the initial BaTiO3 crystals and graphene superlattices.37,39−41 As for the case of GN/BT superlattice models, the GGA-PBE exchange-correlation functional with Hubbard correction was employed.42−44 The cutoff energy was chosen to be 600 eV after carefully tested to be fully converged for the total-energy calculation. Monkhorst−Pack mesh grids with a 8 × 8 × 8 special kpoint mesh for the GN/BT superlattice were carried out for the special points sampling integration over the Brillouin zone.45 The careful tests showed that the cutoff energy and the k-points mesh used in this work are enough for this system. The energy tolerance was 1 × 10−8 eV/ atom, while the force tolerance was set to 0.001 eV/Å.

bands either are separated by a gap or overlapped each other, which intersects in two inequivalent points known as Dirac points in the first Brillouin zone. The band structure of BaTiO3 superlattices shows the indirect band gap characteristic at Γ and R points. The band gap of 3.2 eV was calculated by using the GGA-PBE exchange functional with Hubbard corrections, and hybrid functional HSE06 calculations obtained the same results, which is in great agreement with the experimental values. Figure 6f displays the band structure of GN/BT superlattices, and new bands appear near the Fermi level compared with the BaTiO3 superlattices, implying the effects of graphene on the electronic properties of GN/BT superlattices. These new bands are originated from the C 2p states of graphene, as shown in Figure 6i. The strong hybrid interactions between O 2p orbitals and Ti 3d orbitals can be also seen. This hybridization shortens the distance between O and Ti atoms, causing the distortion of the [TiO6] octahedron, and then further produces the imbalance of charges for the whole crystal. After being applied with the external electric field, these substances can demonstrate a reversible spontaneous electric polarization. As a result, the strong hybrid interactions between O 2p orbitals and Ti 3d orbitals are the origin of ferroelectricity of GN/BT superlattices. The major effects of graphene on the BaTiO3 are reflected in the hybridization between C 2p states and O 2p and Ti 3d orbitals, as is clearly shown at both valence band and conduction band, which increases the conductance of the GN/ BT superlattices and demonstrated the large permittivity.

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CONCLUSIONS In this work, GN/BT nanocomposites were fabricated via colloidal processing methods and sintered through two-step sintering methods. The dielectric properties and the firstprinciples calculations of GN/BT nanocomposites were carried out for the first time. All samples have a high density with the relative ratio over 97%. XRD analysis shows that all samples are perovskite phases and the lattice parameters a and c almost decrease linearly with the increase of graphene nanosheets. Raman spectra and XPS analysis were employed to identify the existence of graphene in GN/BT nanocomposites. The maximum dielectric constant at the Curie temperature for the nanocomposites with graphene addition of 3 wt % is about 16 000, almost 2 times more than that of pure BaTiO3 ceramics. The origin of the enhancement in dielectric properties has been explained. The relaxation, band structure, density of states, and charge density distribution of GN/BT superlattices were calculated by employing the pseudopotential plane-wave approach based on the density functional theory framework. The strong hybrid interactions between O 2p orbitals and Ti 3d orbitals determinates the ferroelectric properties of BaTiO3 ceramics, while the hybridization between C 2p states and O 2p and Ti 3d orbitals contributes to the large permittivity.

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MATERIALS AND METHODS

Preparation of GN/BT Nanocomposites. Commercial BaTiO3 nanoparticles with an average size of about 50 nm were used as raw materials. Analytical reagent N,N-dimethylacetamide (DMAC, ≥99%) was used as the solvent and purchased from Sinopharm Chemical Reagent Co., Ltd. Graphene nanosheets were prepared by the mechanical exfoliation method. The organic functional group or ions were imported into the graphene layers to fabricate the graphite intercalation compounds via intercalation chemistry technology. Then, combining the effects of the temperature difference of 300 °C and the differential pressure of 50 MPa, the functional groups were exploded, to implement the high efficient exfoliation. It is worth noting that no 3346


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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 10 62784579. E-mail: [email protected] (X.W.). *E-mail: [email protected] (L.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Ministry of Sciences and Technology of China through the National Basic Research Program of China (973 Program 2015CB654604), the National Natural Science Foundation of China for Creative Research Groups (Grant No. 51221291), the National Natural Science Foundation of China (Grant No. 51272123), the Fundamental Research Funds for the Central Universities (Grant No. 2652013105), and also supported by Nanjing SCF Nanotech Ltd. and CBMI Construction Co., Ltd.

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Barium Titanate Nanocomposites

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