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Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric constant nanocomposites
Nano Energy (2015) 17, 302–307
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric constant nanocomposites Haixiong Tanga, Zhi Zhoua, Christopher C. Bowlanda, Henry A. Sodanob,c,n a
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United States b Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI 48109, United States c Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, United States Received 25 May 2015; received in revised form 29 August 2015; accepted 3 September 2015 Available online 11 September 2015
KEYWORDS
Abstract
Calcium copper titanate; Nanowires; Nanocomposites; Dielectric constant; Silicon dioxide
The discovery of a giant dielectric constant of 4105 in calcium copper titanate (CCTO) has generated significant interest for application in electronic devices. Over the past decade, many methods have been developed to synthesize CCTO particles in an effort to capitalize upon the outstanding dielectric properties. Polymer film capacitors are one application where CCTO could greatly increase the typically low dielectric constant of the polymer. Recent work has shown that the aspect ratio of the dispersed filler can greatly affect the dielectric constant of the polymer with high aspect ratio nanowires (NWs) producing greater dielectric constant and energy density than spherical particles. While CCTO NWs hold great potential to improve the properties of nanocomposite capacitors, no method currently exists for their preparation and their integration is challenged by the barrier layer capacitance which leads to high dielectric loss. Here, a novel two-step hydrothermal reaction is developed for the large-scale synthesis of CCTO NWs and an insulating barrier layer is applied to greatly reduce the loss while preserving the high dielectric constant. & 2015 Published by Elsevier Ltd.
n Corresponding author at: Department of Aerospace Engineering and Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, United States. E-mail address: [email protected] (H.A. Sodano).
http://dx.doi.org/10.1016/j.nanoen.2015.09.002 2211-2855/& 2015 Published by Elsevier Ltd.
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric
Introduction High dielectric constant (high-κ) materials are widely used in semiconductor manufacturing processes to replace traditional dielectric layer materials (silicon dioxide, silicon nitride) [1,2]. The relatively low κ of SiO2 (3.9) limits its use in transistors as gate lengths scale down to tens of nanometers [1–4]. This has motivated the development of high-κ dielectrics to allow further miniaturization of microelectronic components [3,4]. Ferroelectric materials with a perovskite-type structure, such as BaTiO3 [5,6], PbxZr1 xTiO3 (PZT),[7,8] and BaxSr1 xTiO3,[9,10] are widely applied in microelectronics because of their high dielectric constants. Recently, the discovery of calcium copper titanate, CaCu3Ti4O12 (CCTO), has generated intense research interests because of its giant dielectric permittivity (104–105) [11,12]. This high dielectric constant makes it an ideal candidate for application in microelectronic components, such as advanced transistors and energy storage capacitors [1,2,9,10]. Additionally, the dielectric properties of CCTO are independent of temperature over a wide range, 100– 600 K, as a result of its stable cubic structure [11,13]. Traditional high dielectric ferroelectric materials, such as PZT and BaTiO3, are limited to low temperature applications due to the instability of their dielectric constant as a function of temperature especially above their Curie temperatures, where the dielectric permittivity changes sharply [12]. Therefore, CCTO can provide a high performance dielectric material that is suitable for high temperature applications, which provides significant improvements over existing high-κ materials and traditional dielectric layers. Recently, nanowires (NWs) have been widely studied due to their unique properties and their potential for fabrication into high density nano-scale devices including capacitors [6,7,9,14,15], sensors [16,17] and electronics [18,19]. For example, NWs with high dielectric permittivity have been widely utilized in the fabrication of nanocomposites for capacitor and energy storage applications [6,7,9]. Recent modeling and experimental efforts by Andrews et al. and Tang et al., respectively, have shown that the aspect ratio of filler impacts the effective properties of the nanocomposite, such as dielectric constant and energy density [7,20]. Tang et al. showed that nanocomposites with high aspect ratio PZT and BaTiO3 NWs had higher dielectric constants and energy densities as compared to low aspect ratio nanoparticles [6,7]. The authors later showed that the orientation of the nanowires could be tailored to produce up to 48.7% improvement over a random alignment [21]. Therefore, nanocomposites consisting of giant dielectric permittivity CCTO NWs may achieve higher dielectric permittivity and energy density for energy storage with tailorable properties depending on the morphology and orientation of the NWs. Various methods have been developed for the preparation of CCTO ceramics, powders, and thin films. These typically consist of solid-state reactions from metal oxides at high temperatures (typically 1000 1C for 20 h) with several intermediate grinding steps [22,23]. However, this method is tedious work requiring relatively long reaction times and high temperature conditions, which may result in unwanted secondary phases. Reduced reaction temperatures have been achieved by using organic gel-assisted citrate processes and polymeric citrate precursors [24,25]. However, these methods are still relatively complex and require long
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heat treatment times. Recently, Liu et al. prepared CCTO by pyrolyzing an organic solution containing stoichiometric amounts of the required metal cations, which was performed at relatively low temperatures using a shorter reaction time than the conventional solid-state reactions [26]. Additionally, Jin et al. prepared nano-ultrafine CCTO powders using the sol–gel method and the citrate autoignition method [27]. However, none of these methods can synthesize CCTO NWs, and there has been no reported method to prepare CCTO NWs. Therefore, it is necessary to develop a scalable method for CCTO NW synthesis. Here, we report the first method for large-scale synthesis of CCTO NWs by a two step-hydrothermal reaction. Initially, a hydrothermal reaction is performed to prepare the precursor hydrogen titanate (H2TiO3, HTO) NWs, which have an open structure for easy ion diffusion. Then, an optimized hydrothermal reaction is introduced to diffuse Cu and Ca ions into the precursor followed by a heat treatment to form CCTO NWs. Fang et al. utilized pulsed laser deposition to create multilayer SiO2 and CCTO thin films and showed the SiO2 provided an efficient barrier layer to reduce dielectric loss as compared to single layer CCTO thin films [28,29]. Here a layer of SiO2 is coated on the surface of the CCTO NWs by the Stöber Method to decrease the loss tangent which is a scalable solution based process that can be performed at ambient temperature. The resulting SiO2/ CCTO NWs are incorporated into a polyvinylidene fluoride (PVDF) matrix, yielding high-κ nanocomposites with low loss. It is demonstrated that nanocomposites with 40 vol% of CCTO NWs can reach a dielectric constant as high as 68 with low loss tangent (0.081). This report disseminates a state-of-the-art method for the large scale synthesis of CCTO NWs and the fabrication of high-κ nanocomposites.
Experimental section Nanowire synthesis The synthesis of CCTO NWs is approached through a two-step hydrothermal reaction. The first hydrothermal reaction synthesizes HTO NWs as the precursor for conversion to CCTO NWs. For this reaction, 1.88 g of anatase titanium dioxide powder (Sigma-Aldrich, ACS, 99%) was mixed with 91 mL of a 10 M sodium hydroxide (Fisher, ACS, 99%) aqueous solution. Then, the mixed solution was transferred into a 130 mL Teflon-lined stainless steel autoclave and stirred at 240 1C for 24 h. After the autoclave was cooled to room temperature, the resulting sodium titanate NWs were washed with water then soaked in a 0.2 M hydrochloric acid (HCl, Fisher, 37%) solution for 4 h to yield HTO NWs. The resulting powder was then washed with water six times through centrifugation until the pH reached around 7 with subsequent drying on a hotplate at 60 1C overnight. The HTO NWs were treated by a second hydrothermal reaction using 140 mL of a solution consisting of 0.396 g of copper nitrate hemi(pentahydrate) (Cu (NO3)2 2.5H2O, Alfa Aesar, 98%) and 1.17 g of calcium nitrate tetrahydrate (Ca(NO3)2 4H2O, Acros Organic, 99+%). The solution was then sonicated for 5 min, saturated with nitrogen, and transferred into a 200 mL Teflon-lined stainless steel autoclave. The reaction vessel was kept at 155 1C for 24 h.
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Fig. 1 SEM images and structure analysis of nanowires: (a) SEM image of HTO NWs, (b) SEM image of CCTO NWs, (c) EDS spectrum and (d) XRD patterns of CCTO NWs before and after heat treatment and acid wash.
After the hydrothermal process was complete, the precipitate was collected, washed with water until the pH reached around 7, and dried on a hotplate at 90 1C for 2 h. The powder was then heated at 800 1C for 2 h. Finally, the resulting powder was collected and sequentially washed with 0.2 M HCl aqueous solution, water and ethanol to yield CCTO powder.
temperature for 24 h, and peeled from the glass plate. In order to obtain smooth, void-free samples, the resulting films were further hot pressed at 170 1C for 1 h. Finally, gold was sputter coated on the top and bottom surfaces of the samples with a thickness of approximately 10 nm to act as the corresponding electrodes for electrical measurements.
Nanowire coating
Characterization
In order to decrease the loss tangent of the CCTO NWs, the surface was coated with a SiO2 thin film via the Stöber method. Approximately 0.3 g of CCTO NWs was dispersed into a mixture of 500 mL 2-propanol and 100 mL water. Under continuous magnetic stirring, 12 mL of 30% ammonium hydroxide solution and 0.4 g of TEOS (Sigma Aldrich) were consecutively added to this system. After the reaction had proceeded for 2 h, the resulting powder was washed with water six times through centrifugation until the pH approached 7 with subsequent drying in the oven at 100 1C overnight.
The morphology and crystalline structure of the NWs were characterized using a scanning electron microscope (FE-SEM; 6335F, JEOL) and an X-ray diffractometer (XRD, PANalytical X'Pert Powder) with Cu Kα radiation, respectively. The elemental composition of the nanowires was studied using energy dispersive X-ray spectroscopy (EDS). The morphology and crystal structure of individual nanowires were further studied using a high-resolution transmission electron microscope (HRTEM, JEOL TEM-1011) operated at 200 kV. Frequencydependent capacitance constant and loss tangent (dissipation factor) were measured using an Agilent 4980A LCR meter with a frequency range from 1000 Hz to 1 MHz at 1 Vrms with a parallel equivalent circuit. The dielectric constant was calculated from the measured capacitance by
Nanocomposite fabrication The CCTO NWs and SiO2 coated CCTO NWs were dispersed in a 7 wt% PVDF (Arkema, Kynars 301F) in dimethylformamide (DMF) solution through one hour of bath sonication. The solution was then cast onto a glass plate and dried at 90 1C under vacuum overnight to obtain thin films. The as-cast films were heated at 200 1C for 10 min, dried at room
εr ¼ Cd=ε0 A
ð1Þ
where εr is the dielectric constant of the capacitor, C is the capacitance (Farads), d is the thickness (m) of the samples, ε0 is the permittivity of free space (8.854 10 12
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric
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Fig. 2 (a) TEM image of hierarchical CCTO NWs for nanocomposite fabrication; (b) SEM image of a 30 vol% CCTO NWs in PVDF nanocomposite.
F m 1), and A is the surface area of the capacitor's electrode (m2).
Results and discussion The CCTO NWs were synthesized by a two-step hydrothermal reaction. The first hydrothermal reaction synthesized HTO NWs, which have been widely investigated and offers an easily controllable morphology [17,30–33]. More importantly, HTO is a layered titanate, which is an ideal precursor for soft chemical synthesis because its open structure is compatible with ion exchange reactions [30,34]. As shown in Fig. 1(a), the first reaction resulted in high aspect ratio, free-standing HTO NWs with a length around 14 mm and a diameter around 200 nm. The second hydrothermal process was optimized to maintain the morphology of the precursor nanowires and diffuse the Cu and Ca ions into precursor to produce CCTO NWs. It should be mentioned that it was critically important to control the reaction temperature because the Cu ions reacted with OH- to form a Cu(OH)2 precipitate when the temperature was higher than 175 1C. Additionally, it was hard to diffuse the Cu ions into hydrogen titanate at low temperature. Therefore, the reaction temperature was set as 155 1C to ensure the diffusion of Cu into the HTO NW while maintaining its morphology. After this second hydrothermal reaction, the resulting powders were heat treated at 800 1C for two hours. The resulting yellow powder was washed with hydrochloric acid (HCl) to remove the unwanted copper (II) oxide (CuO), which forms as a by-product of the reaction. Fig. 1(b) shows the CCTO NWs after these conversion steps and illustrates the morphology preservation. The successful conversion of CCTO NWs was further confirmed by the presence of Cu, Ca, Ti and O peaks as shown in the energy dispersive X-ray spectroscopy (EDS) spectrum in Fig. 1(c). The X-ray diffraction (XRD) patterns of the resultant NWs (Fig. 1(d)) show the presence of CuO (JCPDS 80-0076) before the heat treatment and HCl wash procedure along with a small amount of
unconverted titanium oxide. After the heat treatment and acid wash, the CuO is not present and the characteristic peaks of CCTO (JCPDS 75-2188) appear with small amounts of titanium oxide still remaining. This confirms the diffusion of the Cu and Ca ions into the precursor HTO nanowires. In order to demonstrate the application of these CCTO NWs, nanocomposites were fabricated by dispersing them in a PVDF matrix. Nanocomposites combining an ultra-high dielectric permittivity ceramic filler offers significant improvement of dielectric permittivity [6,7,9,23]. However, while CCTO NWs offer a high dielectric permittivity, this comes at the expense of the loss tangent, which limits the ultimate performance of the nanocomposite [23,35–37]. In order to capture the giant dielectric permittivity and avoid high loss of the CCTO NW, the NWs were coated with a thin layer of SiO2 through an additional hydrolysis reaction thus creating a core-shell structure as shown in the TEM image in Fig. 2(a). The thin SiO2 films have been employed to isolate the high loss tangent from CCTO NWs, since SiO2 has ultra low dielectric loss (0.00002). To increase compatibility and improve dispersion of the fillers in the matrix, the SiO2/CCTO NWs were surface functionalized by ethylenediamine [9]. Fig. 2(b) shows the top surface of the nanocomposite with 30 vol% CCTO NWs. It indicates that the fillers functionalized with amine groups are homogeneously dispersed in the PVDF matrix without voids in the film. To effectively demonstrate the effect of the core–shell structure on the dielectric properties of the fabricated nanocomposites, CCTO NWs without the SiO2 coating were fabricated. Fig. 3(a) and (b) show the comparison of dielectric constant and loss tangent between nanocomposites with CCTO NWs and nanocomposites with SiO2/CCTO NWs, respectively. It is clearly demonstrated that by creating the core–shell structure, the nanocomposites with SiO2/CCTO NWs showed a significant decrease in the loss tangent while maintaining the high dielectric permittivity and a high resistance of up to 45 MΩ. For the nanocomposites with 40 vol% NWs, the loss tangent of 0.35 was reduced to 0.081 by utilizing the SiO2 coated NWs, which is a 432% reduction in loss tangent, while the dielectric constant only decreased from 73 to 68 or only a
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Fig. 3 (a) Comparison of measured dielectric constant (at 1 kHz) (b) loss tangent of nanocomposites with different fillers: CCTO NWs and SiO2/CCTO NWs; (c) dielectric constant and (d) loss tangent of the nanocomposites with different volume fractions of CCTO NWs in PVDF from 1 kHz to 1 MHz.
6.8% reduction. It should be noted that the loss tangent of nanocomposites with the hierarchical CCTO NWs was around 0.081 at 1 kHz, which is significantly lower than dielectric materials prepared by other methods but comparable to previous research on multilayered thin films with CCTO/SiO2/ CCTO layering [20,28,29,32,34,35]. This low loss tangent was attributed to the hierarchical nanostructure, where the SiO2 shell isolates the high loss tangent of the CCTO core. The dielectric constant of the nanocomposite with 40 vol% CCTO NWs reached 68, which was 6.9 times higher than the PVDF matrix (9.8) and 70% larger than a similar nanocomposite with PZT NWs at the same volume fraction (40) [7]. This improvement in dielectric constant can be further enhanced using NWs of various morphologies and orientations which provides a route to tailor the nanocomposite properties and enhance dielectric constant further [21,28,29,33]. The high dielectric permittivity and low loss tangent created by the hierarchical SiO2/CCTO NWs have the capability to produce higher energy density nanocomposites with higher energy storage efficiency, which has the potential to replace existing materials in the energy storage field. The dielectric constant and loss tangent of the nanocomposites with various volume fractions of NWs over the frequency range from 1 kHz to 1 MHz are shown in
Fig. 3(c) and (d), respectively. The dielectric constant decreased with increasing frequency due to the dipole mobility, which is not sufficiently mobile to displace as the frequency of the applied electric field exceeds the relaxation frequency [9]. The loss tangent of the nanocomposites increases with increasing volume fraction of CCTO NWs and frequency.
Conclusions A method for the scalable synthesis of CCTO NWs has been successfully developed using a two-step hydrothermal reaction. CCTO has a relatively high loss tangent along with its giant dielectric permittivity thus requiring a core–shell structure to decrease the loss tangent while maintaining its high dielectric permittivity. By dispersing these hierarchical NWs into a PVDF matrix, nanocomposites were fabricated with dielectric constants reaching as high as 68, which is 6.9 times higher than the PVDF matrix (9.8). The loss tangent of the nanocomposites with 40% SiO2/CCTO NWs is as low as 0.081, or 432% lower than those with uncoated CCTO NWs (0.35). The scalable synthesis method proposed
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric here provides the first route to synthesize CCTO NWs thus allowing for the expansion of future research on understanding the physical mechanism of these CCTO NWs and developing new applications for this high performance dielectric material.
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[32] S. Kang, H. Jang, K. Kim, B.H. Park, M. Jung, Y. Kim, Mater. Res. Bull. 43 (2008) 996–1003. [33] H. Tang, Z. Zhou, H.A. Sodano, ACS Appl. Mater. Interfaces 6 (2014) 5450–5455. [34] C. Jiang, K. Kiyofumi, Y. Wang, K. Koumoto, Cryst. Growth Des. 7 (2007) 2713–2715. [35] M. Arbatti, X. Shan, Z. Cheng, Adv. Mater. 19 (2007) 1369–1372. [36] C. Mu, P. Liu, Y. He, J. Zhou, H. Zhang, J. Alloy. Compd. 471 (2009) 137–141. [37] P. Thomas, K. Varughese, K. Dwarakanath, K. Varma, Compos. Sci. Technol. 70 (2010) 539–545. Dr. Haixiong Tang is a Research and Development Engineer at Powdermet Inc. He received his Ph.D. in Materials Science and Engineering from the University of Florida in 2013. Dr. Tang's research work is published in 33 technical publications, 24 journal papers and 9 conference proceeding papers. Dr. Tang specializes in nanostructured materials with an emphasis on their synthesis and integration into nanocomposites for sensors, energy storage and energy harvesting. He possesses extensive knowledge in the areas of nanomaterials synthesis, advance structural materials, multifunctional materials, ferroelectric and dielectric materials, carbon materials and energy storage, especially high-energy density capacitors. Dr. Zhi Zhou is a Research Scientist in Research and Development at Sonavation Inc. He received his Ph.D. in Materials Science and Engineering from the University of Florida in 2014. Dr. Zhou published 14 journal papers and 2 conference papers. His research involved the development of multifunctional composites utilizing ferroelectric nanowires and films.
Christopher C. Bowland is currently a Ph.D. candidate in Materials Science and Engineering at the University of Florida. He received his B.S. and M.S. degrees in Materials Science and Engineering from the University of Tennessee (2012) and the University of Florida (2013), respectively. His research interests consist of nanomaterial synthesis on the surface of fibers for developing multifunctional composites. Prof. Henry A. Sodano is an Associate Professor in the Aerospace Engineering Department at the University of Michigan with a joint position in the Materials Science and Engineering Department. He received his Ph.D. in Mechanical Engineering from Virginia Tech in 2005. He has published 195 technical articles, including 96 journal articles, 6 book chapters, and 93 proceedings, and has made over 100 national and international presentations. His research group focuses on multifunctional materials, nanomaterial synthesis, self-healing polymers, nanocomposites, energy harvesting, and hybrid composites.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric constant nanocomposites Haixiong Tanga, Zhi Zhoua, Christopher C. Bowlanda, Henry A. Sodanob,c,n a
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United States b Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI 48109, United States c Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, United States Received 25 May 2015; received in revised form 29 August 2015; accepted 3 September 2015 Available online 11 September 2015
KEYWORDS
Abstract
Calcium copper titanate; Nanowires; Nanocomposites; Dielectric constant; Silicon dioxide
The discovery of a giant dielectric constant of 4105 in calcium copper titanate (CCTO) has generated significant interest for application in electronic devices. Over the past decade, many methods have been developed to synthesize CCTO particles in an effort to capitalize upon the outstanding dielectric properties. Polymer film capacitors are one application where CCTO could greatly increase the typically low dielectric constant of the polymer. Recent work has shown that the aspect ratio of the dispersed filler can greatly affect the dielectric constant of the polymer with high aspect ratio nanowires (NWs) producing greater dielectric constant and energy density than spherical particles. While CCTO NWs hold great potential to improve the properties of nanocomposite capacitors, no method currently exists for their preparation and their integration is challenged by the barrier layer capacitance which leads to high dielectric loss. Here, a novel two-step hydrothermal reaction is developed for the large-scale synthesis of CCTO NWs and an insulating barrier layer is applied to greatly reduce the loss while preserving the high dielectric constant. & 2015 Published by Elsevier Ltd.
n Corresponding author at: Department of Aerospace Engineering and Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, United States. E-mail address: [email protected] (H.A. Sodano).
http://dx.doi.org/10.1016/j.nanoen.2015.09.002 2211-2855/& 2015 Published by Elsevier Ltd.
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric
Introduction High dielectric constant (high-κ) materials are widely used in semiconductor manufacturing processes to replace traditional dielectric layer materials (silicon dioxide, silicon nitride) [1,2]. The relatively low κ of SiO2 (3.9) limits its use in transistors as gate lengths scale down to tens of nanometers [1–4]. This has motivated the development of high-κ dielectrics to allow further miniaturization of microelectronic components [3,4]. Ferroelectric materials with a perovskite-type structure, such as BaTiO3 [5,6], PbxZr1 xTiO3 (PZT),[7,8] and BaxSr1 xTiO3,[9,10] are widely applied in microelectronics because of their high dielectric constants. Recently, the discovery of calcium copper titanate, CaCu3Ti4O12 (CCTO), has generated intense research interests because of its giant dielectric permittivity (104–105) [11,12]. This high dielectric constant makes it an ideal candidate for application in microelectronic components, such as advanced transistors and energy storage capacitors [1,2,9,10]. Additionally, the dielectric properties of CCTO are independent of temperature over a wide range, 100– 600 K, as a result of its stable cubic structure [11,13]. Traditional high dielectric ferroelectric materials, such as PZT and BaTiO3, are limited to low temperature applications due to the instability of their dielectric constant as a function of temperature especially above their Curie temperatures, where the dielectric permittivity changes sharply [12]. Therefore, CCTO can provide a high performance dielectric material that is suitable for high temperature applications, which provides significant improvements over existing high-κ materials and traditional dielectric layers. Recently, nanowires (NWs) have been widely studied due to their unique properties and their potential for fabrication into high density nano-scale devices including capacitors [6,7,9,14,15], sensors [16,17] and electronics [18,19]. For example, NWs with high dielectric permittivity have been widely utilized in the fabrication of nanocomposites for capacitor and energy storage applications [6,7,9]. Recent modeling and experimental efforts by Andrews et al. and Tang et al., respectively, have shown that the aspect ratio of filler impacts the effective properties of the nanocomposite, such as dielectric constant and energy density [7,20]. Tang et al. showed that nanocomposites with high aspect ratio PZT and BaTiO3 NWs had higher dielectric constants and energy densities as compared to low aspect ratio nanoparticles [6,7]. The authors later showed that the orientation of the nanowires could be tailored to produce up to 48.7% improvement over a random alignment [21]. Therefore, nanocomposites consisting of giant dielectric permittivity CCTO NWs may achieve higher dielectric permittivity and energy density for energy storage with tailorable properties depending on the morphology and orientation of the NWs. Various methods have been developed for the preparation of CCTO ceramics, powders, and thin films. These typically consist of solid-state reactions from metal oxides at high temperatures (typically 1000 1C for 20 h) with several intermediate grinding steps [22,23]. However, this method is tedious work requiring relatively long reaction times and high temperature conditions, which may result in unwanted secondary phases. Reduced reaction temperatures have been achieved by using organic gel-assisted citrate processes and polymeric citrate precursors [24,25]. However, these methods are still relatively complex and require long
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heat treatment times. Recently, Liu et al. prepared CCTO by pyrolyzing an organic solution containing stoichiometric amounts of the required metal cations, which was performed at relatively low temperatures using a shorter reaction time than the conventional solid-state reactions [26]. Additionally, Jin et al. prepared nano-ultrafine CCTO powders using the sol–gel method and the citrate autoignition method [27]. However, none of these methods can synthesize CCTO NWs, and there has been no reported method to prepare CCTO NWs. Therefore, it is necessary to develop a scalable method for CCTO NW synthesis. Here, we report the first method for large-scale synthesis of CCTO NWs by a two step-hydrothermal reaction. Initially, a hydrothermal reaction is performed to prepare the precursor hydrogen titanate (H2TiO3, HTO) NWs, which have an open structure for easy ion diffusion. Then, an optimized hydrothermal reaction is introduced to diffuse Cu and Ca ions into the precursor followed by a heat treatment to form CCTO NWs. Fang et al. utilized pulsed laser deposition to create multilayer SiO2 and CCTO thin films and showed the SiO2 provided an efficient barrier layer to reduce dielectric loss as compared to single layer CCTO thin films [28,29]. Here a layer of SiO2 is coated on the surface of the CCTO NWs by the Stöber Method to decrease the loss tangent which is a scalable solution based process that can be performed at ambient temperature. The resulting SiO2/ CCTO NWs are incorporated into a polyvinylidene fluoride (PVDF) matrix, yielding high-κ nanocomposites with low loss. It is demonstrated that nanocomposites with 40 vol% of CCTO NWs can reach a dielectric constant as high as 68 with low loss tangent (0.081). This report disseminates a state-of-the-art method for the large scale synthesis of CCTO NWs and the fabrication of high-κ nanocomposites.
Experimental section Nanowire synthesis The synthesis of CCTO NWs is approached through a two-step hydrothermal reaction. The first hydrothermal reaction synthesizes HTO NWs as the precursor for conversion to CCTO NWs. For this reaction, 1.88 g of anatase titanium dioxide powder (Sigma-Aldrich, ACS, 99%) was mixed with 91 mL of a 10 M sodium hydroxide (Fisher, ACS, 99%) aqueous solution. Then, the mixed solution was transferred into a 130 mL Teflon-lined stainless steel autoclave and stirred at 240 1C for 24 h. After the autoclave was cooled to room temperature, the resulting sodium titanate NWs were washed with water then soaked in a 0.2 M hydrochloric acid (HCl, Fisher, 37%) solution for 4 h to yield HTO NWs. The resulting powder was then washed with water six times through centrifugation until the pH reached around 7 with subsequent drying on a hotplate at 60 1C overnight. The HTO NWs were treated by a second hydrothermal reaction using 140 mL of a solution consisting of 0.396 g of copper nitrate hemi(pentahydrate) (Cu (NO3)2 2.5H2O, Alfa Aesar, 98%) and 1.17 g of calcium nitrate tetrahydrate (Ca(NO3)2 4H2O, Acros Organic, 99+%). The solution was then sonicated for 5 min, saturated with nitrogen, and transferred into a 200 mL Teflon-lined stainless steel autoclave. The reaction vessel was kept at 155 1C for 24 h.
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Fig. 1 SEM images and structure analysis of nanowires: (a) SEM image of HTO NWs, (b) SEM image of CCTO NWs, (c) EDS spectrum and (d) XRD patterns of CCTO NWs before and after heat treatment and acid wash.
After the hydrothermal process was complete, the precipitate was collected, washed with water until the pH reached around 7, and dried on a hotplate at 90 1C for 2 h. The powder was then heated at 800 1C for 2 h. Finally, the resulting powder was collected and sequentially washed with 0.2 M HCl aqueous solution, water and ethanol to yield CCTO powder.
temperature for 24 h, and peeled from the glass plate. In order to obtain smooth, void-free samples, the resulting films were further hot pressed at 170 1C for 1 h. Finally, gold was sputter coated on the top and bottom surfaces of the samples with a thickness of approximately 10 nm to act as the corresponding electrodes for electrical measurements.
Nanowire coating
Characterization
In order to decrease the loss tangent of the CCTO NWs, the surface was coated with a SiO2 thin film via the Stöber method. Approximately 0.3 g of CCTO NWs was dispersed into a mixture of 500 mL 2-propanol and 100 mL water. Under continuous magnetic stirring, 12 mL of 30% ammonium hydroxide solution and 0.4 g of TEOS (Sigma Aldrich) were consecutively added to this system. After the reaction had proceeded for 2 h, the resulting powder was washed with water six times through centrifugation until the pH approached 7 with subsequent drying in the oven at 100 1C overnight.
The morphology and crystalline structure of the NWs were characterized using a scanning electron microscope (FE-SEM; 6335F, JEOL) and an X-ray diffractometer (XRD, PANalytical X'Pert Powder) with Cu Kα radiation, respectively. The elemental composition of the nanowires was studied using energy dispersive X-ray spectroscopy (EDS). The morphology and crystal structure of individual nanowires were further studied using a high-resolution transmission electron microscope (HRTEM, JEOL TEM-1011) operated at 200 kV. Frequencydependent capacitance constant and loss tangent (dissipation factor) were measured using an Agilent 4980A LCR meter with a frequency range from 1000 Hz to 1 MHz at 1 Vrms with a parallel equivalent circuit. The dielectric constant was calculated from the measured capacitance by
Nanocomposite fabrication The CCTO NWs and SiO2 coated CCTO NWs were dispersed in a 7 wt% PVDF (Arkema, Kynars 301F) in dimethylformamide (DMF) solution through one hour of bath sonication. The solution was then cast onto a glass plate and dried at 90 1C under vacuum overnight to obtain thin films. The as-cast films were heated at 200 1C for 10 min, dried at room
εr ¼ Cd=ε0 A
ð1Þ
where εr is the dielectric constant of the capacitor, C is the capacitance (Farads), d is the thickness (m) of the samples, ε0 is the permittivity of free space (8.854 10 12
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric
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Fig. 2 (a) TEM image of hierarchical CCTO NWs for nanocomposite fabrication; (b) SEM image of a 30 vol% CCTO NWs in PVDF nanocomposite.
F m 1), and A is the surface area of the capacitor's electrode (m2).
Results and discussion The CCTO NWs were synthesized by a two-step hydrothermal reaction. The first hydrothermal reaction synthesized HTO NWs, which have been widely investigated and offers an easily controllable morphology [17,30–33]. More importantly, HTO is a layered titanate, which is an ideal precursor for soft chemical synthesis because its open structure is compatible with ion exchange reactions [30,34]. As shown in Fig. 1(a), the first reaction resulted in high aspect ratio, free-standing HTO NWs with a length around 14 mm and a diameter around 200 nm. The second hydrothermal process was optimized to maintain the morphology of the precursor nanowires and diffuse the Cu and Ca ions into precursor to produce CCTO NWs. It should be mentioned that it was critically important to control the reaction temperature because the Cu ions reacted with OH- to form a Cu(OH)2 precipitate when the temperature was higher than 175 1C. Additionally, it was hard to diffuse the Cu ions into hydrogen titanate at low temperature. Therefore, the reaction temperature was set as 155 1C to ensure the diffusion of Cu into the HTO NW while maintaining its morphology. After this second hydrothermal reaction, the resulting powders were heat treated at 800 1C for two hours. The resulting yellow powder was washed with hydrochloric acid (HCl) to remove the unwanted copper (II) oxide (CuO), which forms as a by-product of the reaction. Fig. 1(b) shows the CCTO NWs after these conversion steps and illustrates the morphology preservation. The successful conversion of CCTO NWs was further confirmed by the presence of Cu, Ca, Ti and O peaks as shown in the energy dispersive X-ray spectroscopy (EDS) spectrum in Fig. 1(c). The X-ray diffraction (XRD) patterns of the resultant NWs (Fig. 1(d)) show the presence of CuO (JCPDS 80-0076) before the heat treatment and HCl wash procedure along with a small amount of
unconverted titanium oxide. After the heat treatment and acid wash, the CuO is not present and the characteristic peaks of CCTO (JCPDS 75-2188) appear with small amounts of titanium oxide still remaining. This confirms the diffusion of the Cu and Ca ions into the precursor HTO nanowires. In order to demonstrate the application of these CCTO NWs, nanocomposites were fabricated by dispersing them in a PVDF matrix. Nanocomposites combining an ultra-high dielectric permittivity ceramic filler offers significant improvement of dielectric permittivity [6,7,9,23]. However, while CCTO NWs offer a high dielectric permittivity, this comes at the expense of the loss tangent, which limits the ultimate performance of the nanocomposite [23,35–37]. In order to capture the giant dielectric permittivity and avoid high loss of the CCTO NW, the NWs were coated with a thin layer of SiO2 through an additional hydrolysis reaction thus creating a core-shell structure as shown in the TEM image in Fig. 2(a). The thin SiO2 films have been employed to isolate the high loss tangent from CCTO NWs, since SiO2 has ultra low dielectric loss (0.00002). To increase compatibility and improve dispersion of the fillers in the matrix, the SiO2/CCTO NWs were surface functionalized by ethylenediamine [9]. Fig. 2(b) shows the top surface of the nanocomposite with 30 vol% CCTO NWs. It indicates that the fillers functionalized with amine groups are homogeneously dispersed in the PVDF matrix without voids in the film. To effectively demonstrate the effect of the core–shell structure on the dielectric properties of the fabricated nanocomposites, CCTO NWs without the SiO2 coating were fabricated. Fig. 3(a) and (b) show the comparison of dielectric constant and loss tangent between nanocomposites with CCTO NWs and nanocomposites with SiO2/CCTO NWs, respectively. It is clearly demonstrated that by creating the core–shell structure, the nanocomposites with SiO2/CCTO NWs showed a significant decrease in the loss tangent while maintaining the high dielectric permittivity and a high resistance of up to 45 MΩ. For the nanocomposites with 40 vol% NWs, the loss tangent of 0.35 was reduced to 0.081 by utilizing the SiO2 coated NWs, which is a 432% reduction in loss tangent, while the dielectric constant only decreased from 73 to 68 or only a
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Fig. 3 (a) Comparison of measured dielectric constant (at 1 kHz) (b) loss tangent of nanocomposites with different fillers: CCTO NWs and SiO2/CCTO NWs; (c) dielectric constant and (d) loss tangent of the nanocomposites with different volume fractions of CCTO NWs in PVDF from 1 kHz to 1 MHz.
6.8% reduction. It should be noted that the loss tangent of nanocomposites with the hierarchical CCTO NWs was around 0.081 at 1 kHz, which is significantly lower than dielectric materials prepared by other methods but comparable to previous research on multilayered thin films with CCTO/SiO2/ CCTO layering [20,28,29,32,34,35]. This low loss tangent was attributed to the hierarchical nanostructure, where the SiO2 shell isolates the high loss tangent of the CCTO core. The dielectric constant of the nanocomposite with 40 vol% CCTO NWs reached 68, which was 6.9 times higher than the PVDF matrix (9.8) and 70% larger than a similar nanocomposite with PZT NWs at the same volume fraction (40) [7]. This improvement in dielectric constant can be further enhanced using NWs of various morphologies and orientations which provides a route to tailor the nanocomposite properties and enhance dielectric constant further [21,28,29,33]. The high dielectric permittivity and low loss tangent created by the hierarchical SiO2/CCTO NWs have the capability to produce higher energy density nanocomposites with higher energy storage efficiency, which has the potential to replace existing materials in the energy storage field. The dielectric constant and loss tangent of the nanocomposites with various volume fractions of NWs over the frequency range from 1 kHz to 1 MHz are shown in
Fig. 3(c) and (d), respectively. The dielectric constant decreased with increasing frequency due to the dipole mobility, which is not sufficiently mobile to displace as the frequency of the applied electric field exceeds the relaxation frequency [9]. The loss tangent of the nanocomposites increases with increasing volume fraction of CCTO NWs and frequency.
Conclusions A method for the scalable synthesis of CCTO NWs has been successfully developed using a two-step hydrothermal reaction. CCTO has a relatively high loss tangent along with its giant dielectric permittivity thus requiring a core–shell structure to decrease the loss tangent while maintaining its high dielectric permittivity. By dispersing these hierarchical NWs into a PVDF matrix, nanocomposites were fabricated with dielectric constants reaching as high as 68, which is 6.9 times higher than the PVDF matrix (9.8). The loss tangent of the nanocomposites with 40% SiO2/CCTO NWs is as low as 0.081, or 432% lower than those with uncoated CCTO NWs (0.35). The scalable synthesis method proposed
Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric here provides the first route to synthesize CCTO NWs thus allowing for the expansion of future research on understanding the physical mechanism of these CCTO NWs and developing new applications for this high performance dielectric material.
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[32] S. Kang, H. Jang, K. Kim, B.H. Park, M. Jung, Y. Kim, Mater. Res. Bull. 43 (2008) 996–1003. [33] H. Tang, Z. Zhou, H.A. Sodano, ACS Appl. Mater. Interfaces 6 (2014) 5450–5455. [34] C. Jiang, K. Kiyofumi, Y. Wang, K. Koumoto, Cryst. Growth Des. 7 (2007) 2713–2715. [35] M. Arbatti, X. Shan, Z. Cheng, Adv. Mater. 19 (2007) 1369–1372. [36] C. Mu, P. Liu, Y. He, J. Zhou, H. Zhang, J. Alloy. Compd. 471 (2009) 137–141. [37] P. Thomas, K. Varughese, K. Dwarakanath, K. Varma, Compos. Sci. Technol. 70 (2010) 539–545. Dr. Haixiong Tang is a Research and Development Engineer at Powdermet Inc. He received his Ph.D. in Materials Science and Engineering from the University of Florida in 2013. Dr. Tang's research work is published in 33 technical publications, 24 journal papers and 9 conference proceeding papers. Dr. Tang specializes in nanostructured materials with an emphasis on their synthesis and integration into nanocomposites for sensors, energy storage and energy harvesting. He possesses extensive knowledge in the areas of nanomaterials synthesis, advance structural materials, multifunctional materials, ferroelectric and dielectric materials, carbon materials and energy storage, especially high-energy density capacitors. Dr. Zhi Zhou is a Research Scientist in Research and Development at Sonavation Inc. He received his Ph.D. in Materials Science and Engineering from the University of Florida in 2014. Dr. Zhou published 14 journal papers and 2 conference papers. His research involved the development of multifunctional composites utilizing ferroelectric nanowires and films.
Christopher C. Bowland is currently a Ph.D. candidate in Materials Science and Engineering at the University of Florida. He received his B.S. and M.S. degrees in Materials Science and Engineering from the University of Tennessee (2012) and the University of Florida (2013), respectively. His research interests consist of nanomaterial synthesis on the surface of fibers for developing multifunctional composites. Prof. Henry A. Sodano is an Associate Professor in the Aerospace Engineering Department at the University of Michigan with a joint position in the Materials Science and Engineering Department. He received his Ph.D. in Mechanical Engineering from Virginia Tech in 2005. He has published 195 technical articles, including 96 journal articles, 6 book chapters, and 93 proceedings, and has made over 100 national and international presentations. His research group focuses on multifunctional materials, nanomaterial synthesis, self-healing polymers, nanocomposites, energy harvesting, and hybrid composites.