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Reverse micellar synthesis, structural characterization and dielectric properties of Sr-doped BaZrO3 nanoparticles
Materials Chemistry and Physics xxx (2016) 1e8
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Reverse micellar synthesis, structural characterization and dielectric properties of Sr-doped BaZrO3 nanoparticles Tokeer Ahmad a, *, Mohd Ubaidullah a, b, c, Mohd Shahazad a, Dinesh Kumar b, Omar A. Al-Hartomy d a
Nanochemistry Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, 110025, India Department of Chemistry, Banasthali University, Tonk, Rajasthan, 304022, India School of Science and Technology, Glocal University, Mirzapur, Saharanpur, 247121, Uttar Pradesh, India d Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia b c
h i g h l i g h t s Monophasic nanocrystalline Ba1xSrxZrO3 at low dopant concentration using reverse micelles for the first time. High surface area (104e244 m2 g1) with smaller particle size (40e65 nm). Extensive characterization using sophisticated techniques. Enhanced dielectric and impedance properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2016 Received in revised form 30 August 2016 Accepted 1 October 2016 Available online xxx
Sr-doped BaZrO3 nanoparticles with strontium content varying from 5 to 20 mol % were successfully synthesized by reverse micellar method at 900 C for the first time. Systematic studies have been carried out to establish the structural and electrical properties of the as prepared nanoparticles. These nanoparticles were characterized using powder X-ray diffraction, transmission electron microscopy, BET surface area and dielectric measurements. X-ray diffraction analysis showed the formation of monophasic and highly crystalline nanoparticles which could be indexed in cubic BaZrO3 with contraction of lattice on strontium substitution. A monotonic shift of diffraction pattern towards higher angel confirms the formation of solid solutions of Ba1xSrxZrO3 (0.05 x 0.20) which was corroborating well with lattice parameter studies. Transmission electron microscopic studies showed the formation of cubic, spherical and hexagonal nanoparticles with an average grain size of 40e65 nm. Energy dispersive X-ray spectroscopic studies confirmed the presence of dopant (Sr2þ) in the BaZrO3 matrix and estimated chemical species corroborate well with the loaded composition. Specific surface area of the solid solution comes out to be in the range of 104e244 m2 g-1. Smallest particle of size 40 nm shows highest surface area 244 m2 g-1 for 20 mol% Sr-doped BaZrO3. Dielectric and impedance studies were also carried out as a function of frequency and temperature to explore the electrical properties of Sr-doped BaZrO3. The dielectric constant of Ba1xSrxZrO3 (0.05 x 0.20) was found to be in the range of 13e25 for x ¼ 0.05 to x ¼ 0.20 with nearly similar dielectric loss of the order of 0.02. The conductance increases linearly with increase in frequency at room temperature, however the impedance has an inverse effect. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nanomaterials Oxides Powder X-ray diffraction Electron microscopy Surface area Dielectric properties
1. Introduction Nanoscience and Nanotechnology has revolutionized the world of science since last two decades and act as a bridge for
* Corresponding author. E-mail address: [email protected] (T. Ahmad).
hypothesis and reality because of the applications of nano objects as building blocks [1e4]. The materials can be designed at atomic level with high perfection which includes metals [5], semiconductors [6], core-shell nanostructures [7] and organic polymeric materials [8]. Oxide nanoparticles exhibit the improved properties such as mechanical hardness, thermal stability or chemical passivity [9]. ABO3 type of perovskite oxides gave a new
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T. Ahmad et al. / Materials Chemistry and Physics xxx (2016) 1e8
paradigm to materials for exploring the new possibilities in the field of electric [10], magnetic [11] and multiferroic [12,13] science. Researcher showed a keen interest to tailor new materials with atomic level accuracy because a number of moieties could fit in the arena of ABO3 perovskite structure for tuning the properties [14], viz. dielectric properties [15,16], optical properties (Electro optical modulator, laser host, switch) [17], ferroelectric/piezoelectric properties (Piezoelectric transducer, P.T.C thermistors, actuators) [18], magnetic properties (Magnetic bubble memory, Ferromagnet) [19] and ionic properties (Solid electrolyte, SOFC electrolyte, Hydrogen sensor) [20]. These nanoscale materials have been intensively studied in recent years on account of having excellent physico-chemical properties and broad range of applications from their bulk counterparts [21,22]. An immense interest has been shown by the scientists to synthesize AZrO3 perovskite materials (where A ¼ Ca, Pb, Sr, Ba etc) due to their unique electrical [23,24], optical [25], photoluminescence [26], radioluminsense [27], high mechanical hardness [28], low thermal expansion coefficient [29], substrate to grow high temperature superconductors [30], catalytic properties [31] and chemical stability for acidic gases [32]. Due to the easy vaporization of lead oxide in surroundings, its toxic effect to environment and human health, a general awareness for the development of lead free dielectric materials is required for the researchers to quest new alternate materials [33,34]. The doped form of barium zirconate also plays a very significant role as proton conductors [35], dielectric [15,16,36], photo catalytic [37] and photoluminescent [38] materials. High permittivity and low dielectric loss makes them fascinating in the field of communication and radar systems, as tunable microwave devices such as phase shifter and delay lines. Numerous well established physical and chemical methods like pulsed laser deposition [39], laser ablation [40], ball milling [41], aerosol [42], CVD [43], polymeric citrate precursor [16,36], microemulsion [15], hydrothermal [44], solvothermal [45], sonochemical [46] and sol-gel [47] methods are available in the literature for the synthesis of pure and doped barium zirconate nanosturstures. A very few reports have been found on Sr2þ doped BaZrO3 nanoparticles [47e49], however there is no report available in literature on Sr2þ doped barium zirconate nanoparticles using reverse micellar route. Reverse micellar method is considered as one of the best methods among all chemical routes, because a variety of nanoparticles can be produced with high degree of homogeneity and uniform size distribution [50,51]. In this paper, we report the synthesis of Sr2þ doped BaZrO3 dielectric oxide nanoparticles by reverse micellar method using tergitol as the surfactant at low dopant concentration for the first time. The Ba1xSrxZrO3 nanoparticles were characterized by powder X-ray diffraction, electron microscopic studies, BET surface area and dielectric studies.
aqueous phase. Ba2þ/Sr2þ were the constituents for microemulsion I and their stoichiometry were fixed as per the composition of final product, microemulsion II consists of Zr4þ while third microemulsion consists of precipitating agent NaOH. Two microemulsions (I, II) were mixed and then stirred for 2 h. After numerous collisions of micelles, the reactants are exchanged and make the desired nanoparticles inside the reverse micellar nanoreactors. Microemulsion III was added to this solution and continues for stirring until the whole solution became transparent. This was heated at 60 ± 5 C to evaporate the solvent. A white precipitate was obtained. The solution mixture was centrifuged and washed four times with acetone. The precipitate was then dried in oven at 50 C. To get pure phase of the desired nanoparticles; powders were annealed in air at 500 C for 20 h and then at 900 C for 12 h. Phase purity and structural studies of Ba1xSrxZrO3 nanoparticles were investigated by powder X-ray diffraction (PXRD) studies on Bruker D8 advance diffractometer using Ni-filtered Cu-Ka X-rays of wavelength (l) ¼ 1.54056 Å. The data were obtained at the scanning rate of 0.05 /s in the 2q range of 10 to 70 . The raw data was subjected to background correction and Ka2 reflections were stripped off by using normal stripping procedure. Transmission electron microscopic studies were carried out on FEI Technai G220 electron microscope with an accelerating voltage of 200 kV to estimate the size and shape of the as synthesized nanoparticles. The specimens were prepared by dispersing the small amount of the nanoparticles in absolute ethanol with the help of ultrasonicator. The drops of dispersed sample were then loaded on the porous carbon film coated copper grid. Texture and morphology of the nanoparticles were recorded on scanning electron microscope (SEM; FEI model No. Nova Nano SEM 450) which has very high resolution, magnification and large depth of field. The scanning electron microscopic studies were carried out on the powders sintered at 900 C to get the micrographs at various magnifications. In order to determine the elemental composition and doping effect of Sr2þ in BaZrO3 host lattice, EDX (Energy dispersive X-ray spectroscopy, Bruker, 127eV EDX detector) was performed which furnish the details of elements (Ba, Sr, Zr, O) present in the solid solution. The surface area studies of the solid solutions have been carried out at liquid nitrogen temperature (78 K) by using BET surface area analyzer (Nova 2000e, Quantachrome Instruments Limited). Approximately 0.26 g of the sample was placed in
2. Experimental Analytical grade reagents were used as received without further purification. 0.1 M solutions of Ba2þ, Sr2þ, Zr4þ and NaOH were prepared in double distilled water form barium acetate (Qualigens, India; 99.5%), strontium acetate (Aldrich, USA; 99.99%), zirconium oxychloride (CDH, India; 99%), sodium hydroxide (Qualigens, India; 97%). Tergitol NP9 (Aldrich, USA; 99.99%) used as a surfactant, 1octanol (LobaChemie, 99%) as co-surfactant, cyclohexane (Rankem, 99.5%) as non-polar solvent. To prepare Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) nanoparticles, three sets of microemulsions (I, II and III) were prepared. Each microemulsion consists of 23.4 mL tergitol NP-9 as a surfactant, 17.4 mL 1-octanol as a cosurfactant, 200 mL cyclohexane as non-polar solvent and 10 mL
Fig. 1. XRD patterns of Ba1xSrxZrO3 nanoparticles for x ¼ 0.05, 0.10, 0.15 and 0.20.
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To measure the dielectric and impedance properties, pellets were prepared by taking small amount of nanoparticles with 5% poly vinyl alcohol (PVA). The paste was allowed to dry in an oven at 100 C then pelletize at a pressure of 3 tons. The pellets were sintered at 900 C for 12 h. In order to check the capacitance and dissipation, both the faces of pellet were painted with colloidal silver paste. Thickness and diameters were measured to get the value of dielectric constant from the measured values of capacitance by using high frequency LCR meter (Model: 6505P, Make: Wayne Kerr Electronics, UK) as a function of frequency and temperature. AC Impedance and conductance studies were carried out in series mode with amplitude of 1000 mV in the frequency range of 20 kHz to 1 MHz at room temperature. Open and short calibration used before measurement to reduce the parasitic effect of the test fixture. The change in conductance and impedance with frequency at room temperature were also measured.
3. Results and discussion Fig. 2. The variation in lattice parameters with composition of Ba1xSrxZrO3 nanoparticles for x ¼ 0.05, 0.10, 0.15 and 0.20.
quartz sample cell for degassing in vacuum degassing mode for 3 h at 250 C to remove the adsorb gases and contaminants such as water vapour. Specific surface area was determined using multipoint BET equation and pore radius was investigated using the Dubinin-Astakhov (DA) method.
3.1. Powder X-ray diffraction studies The PXRD patterns of nanocrystalline Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) heated at 900 C in air are shown in Fig. 1. The diffraction patterns could be indexed to the pure cubic BaZrO3 (JCPDS No-74-1299). The diffraction pattern confirms that all the samples are monophasic in nature and the sharpness of the peaks indicate the higher degree of crystallinity of the nanoparticles. No other impurity phases were detected by XRD till 15 mol% Sr2þ
Fig. 3. TEM micrographs of Ba1xSrxZrO3 nanoparticles for (a) x ¼ 0.05, (b) x ¼ 0.10, (c) x ¼ 0.15 and (d) x ¼ 0.20.
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Fig. 4. SEM micrographs of Ba1xSrxZrO3 for x ¼ (a) 0.05 (b) 0.10, (c) 0.15 and (d) 0.20 nanoparticles. Inset shows the high resolution SEM images of corresponding solid solution.
Fig. 5. EDX patterns of Ba1xSrxZrO3 for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 nanoparticles.
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3.2. TEM, SEM and EDAX analysis TEM and SEM studies were carried out to investigate the shape, size, surface texture and morphology of the nanoparticles. Fig. 3 shows the TEM micrographs of Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20). It is apparent from micrographs that mono-dispersed mixed shape (spherical, cubical and hexagonal) particles were formed in nano regime with nominal agglomeration after heating the powder at 900 C in air. It may be associated to the fact that SrO is usually used to enhance density as seen in the micrographs due to the formation of liquid phase at the grain boundaries during sintering. Fig. 3a shows the TEM image of Ba0.95Sr0.05ZrO3 nanoparticles in which hexagonal nanoparticles could be seen along with spherical particles; however grain boundaries of the particles are very clear. For 10 mol % Sr-doped BaZrO3, slight agglomeration can be seen which is due to the fusion or overlapping of the smaller particles. The TEM micrographs reveal the average particle size was found in the range of 40e65 nm. Some bigger particles are also seen in the micrographs, which may be due to the aggregation of smaller particles. The decrease in the grain size from 65 to 40 nm on increasing the Sr2þ concentration in BaZrO3 host lattice probably be associated to the small ionic radius of Sr2þ (1.18 Å) as compared with Ba2þ (1.35 Å). Morphology and texture of the nanoparticles have been studied by scanning electron microscopy. Fig. 4(aed) shows the SEM images of Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20). SEM micrographs show the formation of densely packed nearly spherical and hexagonal particles and corroborate to the TEM studies. Inset of Fig. 4 shows the high resolution SEM images, which demonstrate the morphology and texture of the particles. Fig. 5 shows the EDX (energy dispersive X-ray spectroscopy) micrographs of Sr-doped BaZrO3 which confirms the incorporation of Sr2þ in the BaZrO3 host lattice. The presence of peaks of Ba, Sr, Zr, and O affirm the formation of Ba1xSrxZrO3 solid solution. The loaded chemical compositions of all elements were found to be in close agreement to the experimentally calculated composition. 3.3. BET surface area studies Fig. 6. (a) BET surface area and (b) DA pore size distribution plots of Ba1xSrxZrO3 nanoparticles.
concentration, showing the incorporation of Sr2þ at Ba2þ lattice site. On increasing the dopant (Sr2þ) concentration, a monotonic shift in diffraction pattern towards the higher angle observed, which clearly indicates the formation of solid solution Ba1xSrxZrO3. The refined unit cell parameter “a” was found to be smaller than undoped BaZrO3. It can be clearly seen from Fig. 2 that on increasing the dopant concentration from x ¼ 0.05 to 0.20, the lattice parameter decreases monotonically, which may be due to the marginal contraction in BaZrO3 lattice. The lattice contraction of host lattice is associated to the smaller ionic radius of Sr2þ (1.18 Å) as compared with Ba2þ (1.35 Å) [52], which may lead to the substantial solubility of Sr2þ ions in BaZrO3 host lattice. The careful analysis of X-ray diffraction pattern of (Ba0.80Sr0.20)ZrO3 show the appearance of very small reflections of low intensity (~1%) at 2q values of around 28, 32 and 50 which are attributed to the intermittent unreacted phases of SrO2 (JCPDS 73-1740) and SrCO3 (JCPDS 84-1778). The appearance of minor impurity phases in 20 mol% Srdoped BaZrO3 suggests that the solubility limit of Sr2þ ions in A-site of BaZrO3 perovskites may be less than 20% at 900 C. It is hard to find the report on solubility limit of Sr2þ in the A-site of BaZrO3 to the best of our knowledge, however the solubility of its group analog Ca2þ in BaZrO3 increases from a few percent at 1400 C to about 30% at 1650 C [53].
Brunauer-Emmett-Teller (BET) gas adsorption method [54] was used for the determination of the surface area and pore size of Sr2þ doped BaZrO3. Specific surface areas of the nanoparticles have been determined in the P/P0 range of 0.05e0.30 at 20 points using multipoint BET equation as shown in Fig. 6a. The values of surface area comes out to be 104, 161, 190 and 244 m2 g-1 respectively which were found to be higher than the earlier reported values so far [49]. Highest surface area of 244 m2 g-1 was found for x ¼ 0.20 composition which has smallest particle size of 40 nm, however low surface area of 104 m2 g-1 was found for Ba0.95Sr0.05ZrO3 which has largest particle size of 65 nm. The BET particle sizes have also been estimated by using BET equation DBET ¼ 6000/(r.Sw) where the symbols DBET, r, and Sw represents the average diameter of the spherical particle in nm, theoretical density in gm/cm3 and specific surface area in m2/g of the samples respectively [55]. The average particle size calculated by BET method comes out to be 9.32, 6.03, 5.17 and 4.06 nm respectively. The BET sizes were found to be smaller than the measured values of TEM analysis. The smaller particles using BET studies may be due to the fact that most of the particles are not smooth and spherical in shape [55]. DubininAstakhov (DA) pore size distribution plots of solid solution Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) are shown in Fig. 6b. DA pore radius plots for x ¼ 0.15 and x ¼ 0.20 composition overlap which suggest nearly the same pore radius of these compositions. The surface area and pore size studies showed good agreement with TEM particle size because as the particle size decreases from
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Fig. 7. The variation of dielectric constant and dielectric loss with frequency of Ba1xSrxZrO3 nanoparticles for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 at room temperature.
Fig. 8. The variation of dielectric constant and dielectric loss with temperature of Ba1xSrxZrO3 nanoparticles for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 at 500 kHz frequency.
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65 to 40 nm with composition, surface area increase from 104 to 244 m2 g-1 and the pore radius marginally decreases from 15 to 14 Å. 3.4. Electrical properties The electrical (dielectric, impedance/conductance) behavior of barium strontium zirconate nanoparticles with general formula Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) has been examined as a function of frequency and temperature on sintered disks. The room temperature dielectric characteristics are shown in Fig. 7(aed) which provides the details of dielectric constant and dielectric loss in the frequency range of 20 kHz to 1 MHz. The room temperature dielectric constant and dielectric loss were found to be nearly stable with frequency which might be associated to the inability of electric dipoles to follow up the fast variation of alternating applied electric field [56]. The dielectric constant was found to be 13.5, 15, 15.6 and 25.5 for Ba0.95Sr0.05ZrO3, Ba0.90Sr0.10ZrO3, Ba0.85Sr0.15ZrO3, and Ba0.80Sr0.20ZrO3 nanoparticles respectively with nearly same dielectric losses ranges from 0.023 to 0.021. The values of dielectric constant and dielectric loss at lower frequency were found to be high because the dipoles follow up the slow variation of alternating applied electric field easily as compared to high frequency. As frequency increases, the net polarization of materials decreases because each polarization mechanism contributes the phenomenon, and hence the dielectric constant decreases. The variation of dielectric constant (ε) and dielectric loss (D) of Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) nanoparticles have also been studied with temperature in the range of 50 Ce400 C at 500 kHz frequency as shown in Fig. 8(aed). Both the dielectric constant and dielectric loss remain stable till 150 C, thereafter a linear increase could be seen with increase of the temperature. As the temperature increases, the molecules have more thermal
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energy and therefore the amplitude of random thermal motion is high. This shows the range of deviation from a perfect alignment with the field is higher; therefore the molecules are less closely aligned with each other and hence results in the increase of dielectric constant. The increasing value of dielectric loss with temperature may be due to the polarization lag behind the applied field causing an interaction between the field and dielectric polarization which arises on heating. Impedance spectroscopy is considered as the absolute source of information for grains and grain boundary structures that is associated to the conductance of materials [57,58]. The conductance and impedance of the solid solutions were also measured as a function of frequency as shown in Fig. 9. It is evident from the figures that conductance increases linearly with increase in frequency; however the impedance has an inverse effect at room temperature. It is observed that the initial value of impedance is higher at low frequency but it decrease gradually with increasing frequency which may be associated to the increase in conductance. 4. Conclusions Nanocrystalline Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) ceramics have been successfully prepared by reverse micellar route at 900 C for the first time. Monophasic and highly crystalline nature of nanoparticles have been established by the X-ray diffraction studies. The slight decrease in lattice parameter confirms the marginal contraction in barium zirconate host lattice which is due to the incorporation of Sr2þ (1.18 Å) on Ba2þ (1.35 Å) lattice site. TEM studies showed the formation of mixed shaped particles of sizes 40e65 nm which corroborates well with BET surface area studies. High dielectric constant and low dielectric loss was found for 40 nm sized 20 mol% Sr-doped BaZrO3 nanoparticles. The large surface area, high dielectric constant and low dielectric loss of as-
Fig. 9. The variation of impedance and conductance with frequency of Ba1xSrxZrO3 nanoparticles for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 at room temperature.
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prepared doped nanoparticles make them a potential candidate for electronic applications. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements TA and MU thank to CSIR Delhi, Govt. of India and Tabuk University, Govt of Saudi Arabia for financial support. References [1] L.N. Lewis, Chem. Rev. 93 (1993) 2693e2730. [2] D.L. Feldheim, C.D. Keating, Chem. Soc. Rev. 27 (1998) 1e12. [3] G.A.D. Vries, M. Brunnbauer, Y. Hu, A.M. Jackson, B. Long, B.T. Neltner, O. Uzun, B.H. Wunsch, F. Stellacc, Science 315 (2007) 358e360. [4] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115e2117. [5] T. Ahmad, I.A. Wani, I.H. Lone, A. Ganguly, N. Manzoor, A. Ahmad, J. Ahmed, A.S. Al-Shihri, Mater. Res. Bull. 48 (2013) 12e20. [6] T. Trindade, P.O. Brien, N.L. Pickett, Chem. Mater 13 (2001) 3843e3858. [7] S.C. Farmer, T.E. Patten, Chem. Mater 13 (2001) 3920e3926. [8] X.J. Xu, P.Y. Chow, L.M. Gan, J. Nanosci. Nanotechnol. 2 (2002) 61e65. [9] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 10 (2003) 668e677. [10] R.A. De Souza, Phys. Chem. Chem. Phys. 11 (2009) 9939e9969. [11] R.V. Shpanchenko, V.V. Chernaya, A.A. Tsirlin, P.S. Chizhov, D.E. Sklovsky, E.V. Antipov, E.P. Khlybov, V. Pomjakushin, A.M. Balagurov, J.E. Medvedeva, E.E. Kaul, C. Geibel, Chem. Mater 16 (2004) 3267e3273. [12] T. Ahmad, I.H. Lone, M. Ubaidullah, RSC Adv. 5 (2015) 58065e58071. [13] T. Ahmad, I.H. Lone, New J. Chem. 40 (2016) 3216e3224. [14] M.S. Islam, J. Mater. Chem. 10 (2000) 1027e1038. [15] T. Ahmad, A.K. Ganguli, J. Am. Ceram. Soc. 89 (2006) 3140e3146. [16] M. Ubaidullah, I.H. Lone, O.A. Al-Hartomy, D. Kumar, T. Ahmad, Adv. Sci. Lett. 20 (2014) 1354e1359. [17] E. Longo, E. Orhan, F.M. Pontes, C.D. Pinheiro, E.R. Leite, J.A. Varela, P.S. Pizani, T.M. Boschi, F. Lanciotti Jr., A. Beltran, J. Andres, Phys. Rev. B 69 (2004) 125115. [18] T. Furuta, K. Miura, Solid. State. Commun. 150 (2010) 2350e2353. [19] W.R. Gemmill, M.D. Smith, H.C.Z. Loye, J. Solid. State. Chem. 177 (2004) 3560e3567. [20] M. Shirpour, B. Rahmati, W. Sigle, P.A.V. Aken, R. Merkle, J. Maier, J. Phys. Chem. C 116 (2012) 2453e2461. [21] H. Weller, Adv. Mater 5 (1993) 88e95. [22] R.A.M. Qasem, A. Gadenken, J. Colloid Interface Sci. 248 (2005) 489e494. [23] M. Shirpour, R. Merkle, C.T. Lin, J. Maier, Phys. Chem. Chem. Phys. 14 (2012) 730e740. [24] T. Ahmad, A.K. Ganguli, Mater. Lett. 60 (2006) 3660e3663.
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Reverse micellar synthesis, structural characterization and dielectric properties of Sr-doped BaZrO3 nanoparticles Tokeer Ahmad a, *, Mohd Ubaidullah a, b, c, Mohd Shahazad a, Dinesh Kumar b, Omar A. Al-Hartomy d a
Nanochemistry Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, 110025, India Department of Chemistry, Banasthali University, Tonk, Rajasthan, 304022, India School of Science and Technology, Glocal University, Mirzapur, Saharanpur, 247121, Uttar Pradesh, India d Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia b c
h i g h l i g h t s Monophasic nanocrystalline Ba1xSrxZrO3 at low dopant concentration using reverse micelles for the first time. High surface area (104e244 m2 g1) with smaller particle size (40e65 nm). Extensive characterization using sophisticated techniques. Enhanced dielectric and impedance properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2016 Received in revised form 30 August 2016 Accepted 1 October 2016 Available online xxx
Sr-doped BaZrO3 nanoparticles with strontium content varying from 5 to 20 mol % were successfully synthesized by reverse micellar method at 900 C for the first time. Systematic studies have been carried out to establish the structural and electrical properties of the as prepared nanoparticles. These nanoparticles were characterized using powder X-ray diffraction, transmission electron microscopy, BET surface area and dielectric measurements. X-ray diffraction analysis showed the formation of monophasic and highly crystalline nanoparticles which could be indexed in cubic BaZrO3 with contraction of lattice on strontium substitution. A monotonic shift of diffraction pattern towards higher angel confirms the formation of solid solutions of Ba1xSrxZrO3 (0.05 x 0.20) which was corroborating well with lattice parameter studies. Transmission electron microscopic studies showed the formation of cubic, spherical and hexagonal nanoparticles with an average grain size of 40e65 nm. Energy dispersive X-ray spectroscopic studies confirmed the presence of dopant (Sr2þ) in the BaZrO3 matrix and estimated chemical species corroborate well with the loaded composition. Specific surface area of the solid solution comes out to be in the range of 104e244 m2 g-1. Smallest particle of size 40 nm shows highest surface area 244 m2 g-1 for 20 mol% Sr-doped BaZrO3. Dielectric and impedance studies were also carried out as a function of frequency and temperature to explore the electrical properties of Sr-doped BaZrO3. The dielectric constant of Ba1xSrxZrO3 (0.05 x 0.20) was found to be in the range of 13e25 for x ¼ 0.05 to x ¼ 0.20 with nearly similar dielectric loss of the order of 0.02. The conductance increases linearly with increase in frequency at room temperature, however the impedance has an inverse effect. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nanomaterials Oxides Powder X-ray diffraction Electron microscopy Surface area Dielectric properties
1. Introduction Nanoscience and Nanotechnology has revolutionized the world of science since last two decades and act as a bridge for
* Corresponding author. E-mail address: [email protected] (T. Ahmad).
hypothesis and reality because of the applications of nano objects as building blocks [1e4]. The materials can be designed at atomic level with high perfection which includes metals [5], semiconductors [6], core-shell nanostructures [7] and organic polymeric materials [8]. Oxide nanoparticles exhibit the improved properties such as mechanical hardness, thermal stability or chemical passivity [9]. ABO3 type of perovskite oxides gave a new
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paradigm to materials for exploring the new possibilities in the field of electric [10], magnetic [11] and multiferroic [12,13] science. Researcher showed a keen interest to tailor new materials with atomic level accuracy because a number of moieties could fit in the arena of ABO3 perovskite structure for tuning the properties [14], viz. dielectric properties [15,16], optical properties (Electro optical modulator, laser host, switch) [17], ferroelectric/piezoelectric properties (Piezoelectric transducer, P.T.C thermistors, actuators) [18], magnetic properties (Magnetic bubble memory, Ferromagnet) [19] and ionic properties (Solid electrolyte, SOFC electrolyte, Hydrogen sensor) [20]. These nanoscale materials have been intensively studied in recent years on account of having excellent physico-chemical properties and broad range of applications from their bulk counterparts [21,22]. An immense interest has been shown by the scientists to synthesize AZrO3 perovskite materials (where A ¼ Ca, Pb, Sr, Ba etc) due to their unique electrical [23,24], optical [25], photoluminescence [26], radioluminsense [27], high mechanical hardness [28], low thermal expansion coefficient [29], substrate to grow high temperature superconductors [30], catalytic properties [31] and chemical stability for acidic gases [32]. Due to the easy vaporization of lead oxide in surroundings, its toxic effect to environment and human health, a general awareness for the development of lead free dielectric materials is required for the researchers to quest new alternate materials [33,34]. The doped form of barium zirconate also plays a very significant role as proton conductors [35], dielectric [15,16,36], photo catalytic [37] and photoluminescent [38] materials. High permittivity and low dielectric loss makes them fascinating in the field of communication and radar systems, as tunable microwave devices such as phase shifter and delay lines. Numerous well established physical and chemical methods like pulsed laser deposition [39], laser ablation [40], ball milling [41], aerosol [42], CVD [43], polymeric citrate precursor [16,36], microemulsion [15], hydrothermal [44], solvothermal [45], sonochemical [46] and sol-gel [47] methods are available in the literature for the synthesis of pure and doped barium zirconate nanosturstures. A very few reports have been found on Sr2þ doped BaZrO3 nanoparticles [47e49], however there is no report available in literature on Sr2þ doped barium zirconate nanoparticles using reverse micellar route. Reverse micellar method is considered as one of the best methods among all chemical routes, because a variety of nanoparticles can be produced with high degree of homogeneity and uniform size distribution [50,51]. In this paper, we report the synthesis of Sr2þ doped BaZrO3 dielectric oxide nanoparticles by reverse micellar method using tergitol as the surfactant at low dopant concentration for the first time. The Ba1xSrxZrO3 nanoparticles were characterized by powder X-ray diffraction, electron microscopic studies, BET surface area and dielectric studies.
aqueous phase. Ba2þ/Sr2þ were the constituents for microemulsion I and their stoichiometry were fixed as per the composition of final product, microemulsion II consists of Zr4þ while third microemulsion consists of precipitating agent NaOH. Two microemulsions (I, II) were mixed and then stirred for 2 h. After numerous collisions of micelles, the reactants are exchanged and make the desired nanoparticles inside the reverse micellar nanoreactors. Microemulsion III was added to this solution and continues for stirring until the whole solution became transparent. This was heated at 60 ± 5 C to evaporate the solvent. A white precipitate was obtained. The solution mixture was centrifuged and washed four times with acetone. The precipitate was then dried in oven at 50 C. To get pure phase of the desired nanoparticles; powders were annealed in air at 500 C for 20 h and then at 900 C for 12 h. Phase purity and structural studies of Ba1xSrxZrO3 nanoparticles were investigated by powder X-ray diffraction (PXRD) studies on Bruker D8 advance diffractometer using Ni-filtered Cu-Ka X-rays of wavelength (l) ¼ 1.54056 Å. The data were obtained at the scanning rate of 0.05 /s in the 2q range of 10 to 70 . The raw data was subjected to background correction and Ka2 reflections were stripped off by using normal stripping procedure. Transmission electron microscopic studies were carried out on FEI Technai G220 electron microscope with an accelerating voltage of 200 kV to estimate the size and shape of the as synthesized nanoparticles. The specimens were prepared by dispersing the small amount of the nanoparticles in absolute ethanol with the help of ultrasonicator. The drops of dispersed sample were then loaded on the porous carbon film coated copper grid. Texture and morphology of the nanoparticles were recorded on scanning electron microscope (SEM; FEI model No. Nova Nano SEM 450) which has very high resolution, magnification and large depth of field. The scanning electron microscopic studies were carried out on the powders sintered at 900 C to get the micrographs at various magnifications. In order to determine the elemental composition and doping effect of Sr2þ in BaZrO3 host lattice, EDX (Energy dispersive X-ray spectroscopy, Bruker, 127eV EDX detector) was performed which furnish the details of elements (Ba, Sr, Zr, O) present in the solid solution. The surface area studies of the solid solutions have been carried out at liquid nitrogen temperature (78 K) by using BET surface area analyzer (Nova 2000e, Quantachrome Instruments Limited). Approximately 0.26 g of the sample was placed in
2. Experimental Analytical grade reagents were used as received without further purification. 0.1 M solutions of Ba2þ, Sr2þ, Zr4þ and NaOH were prepared in double distilled water form barium acetate (Qualigens, India; 99.5%), strontium acetate (Aldrich, USA; 99.99%), zirconium oxychloride (CDH, India; 99%), sodium hydroxide (Qualigens, India; 97%). Tergitol NP9 (Aldrich, USA; 99.99%) used as a surfactant, 1octanol (LobaChemie, 99%) as co-surfactant, cyclohexane (Rankem, 99.5%) as non-polar solvent. To prepare Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) nanoparticles, three sets of microemulsions (I, II and III) were prepared. Each microemulsion consists of 23.4 mL tergitol NP-9 as a surfactant, 17.4 mL 1-octanol as a cosurfactant, 200 mL cyclohexane as non-polar solvent and 10 mL
Fig. 1. XRD patterns of Ba1xSrxZrO3 nanoparticles for x ¼ 0.05, 0.10, 0.15 and 0.20.
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To measure the dielectric and impedance properties, pellets were prepared by taking small amount of nanoparticles with 5% poly vinyl alcohol (PVA). The paste was allowed to dry in an oven at 100 C then pelletize at a pressure of 3 tons. The pellets were sintered at 900 C for 12 h. In order to check the capacitance and dissipation, both the faces of pellet were painted with colloidal silver paste. Thickness and diameters were measured to get the value of dielectric constant from the measured values of capacitance by using high frequency LCR meter (Model: 6505P, Make: Wayne Kerr Electronics, UK) as a function of frequency and temperature. AC Impedance and conductance studies were carried out in series mode with amplitude of 1000 mV in the frequency range of 20 kHz to 1 MHz at room temperature. Open and short calibration used before measurement to reduce the parasitic effect of the test fixture. The change in conductance and impedance with frequency at room temperature were also measured.
3. Results and discussion Fig. 2. The variation in lattice parameters with composition of Ba1xSrxZrO3 nanoparticles for x ¼ 0.05, 0.10, 0.15 and 0.20.
quartz sample cell for degassing in vacuum degassing mode for 3 h at 250 C to remove the adsorb gases and contaminants such as water vapour. Specific surface area was determined using multipoint BET equation and pore radius was investigated using the Dubinin-Astakhov (DA) method.
3.1. Powder X-ray diffraction studies The PXRD patterns of nanocrystalline Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) heated at 900 C in air are shown in Fig. 1. The diffraction patterns could be indexed to the pure cubic BaZrO3 (JCPDS No-74-1299). The diffraction pattern confirms that all the samples are monophasic in nature and the sharpness of the peaks indicate the higher degree of crystallinity of the nanoparticles. No other impurity phases were detected by XRD till 15 mol% Sr2þ
Fig. 3. TEM micrographs of Ba1xSrxZrO3 nanoparticles for (a) x ¼ 0.05, (b) x ¼ 0.10, (c) x ¼ 0.15 and (d) x ¼ 0.20.
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Fig. 4. SEM micrographs of Ba1xSrxZrO3 for x ¼ (a) 0.05 (b) 0.10, (c) 0.15 and (d) 0.20 nanoparticles. Inset shows the high resolution SEM images of corresponding solid solution.
Fig. 5. EDX patterns of Ba1xSrxZrO3 for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 nanoparticles.
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3.2. TEM, SEM and EDAX analysis TEM and SEM studies were carried out to investigate the shape, size, surface texture and morphology of the nanoparticles. Fig. 3 shows the TEM micrographs of Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20). It is apparent from micrographs that mono-dispersed mixed shape (spherical, cubical and hexagonal) particles were formed in nano regime with nominal agglomeration after heating the powder at 900 C in air. It may be associated to the fact that SrO is usually used to enhance density as seen in the micrographs due to the formation of liquid phase at the grain boundaries during sintering. Fig. 3a shows the TEM image of Ba0.95Sr0.05ZrO3 nanoparticles in which hexagonal nanoparticles could be seen along with spherical particles; however grain boundaries of the particles are very clear. For 10 mol % Sr-doped BaZrO3, slight agglomeration can be seen which is due to the fusion or overlapping of the smaller particles. The TEM micrographs reveal the average particle size was found in the range of 40e65 nm. Some bigger particles are also seen in the micrographs, which may be due to the aggregation of smaller particles. The decrease in the grain size from 65 to 40 nm on increasing the Sr2þ concentration in BaZrO3 host lattice probably be associated to the small ionic radius of Sr2þ (1.18 Å) as compared with Ba2þ (1.35 Å). Morphology and texture of the nanoparticles have been studied by scanning electron microscopy. Fig. 4(aed) shows the SEM images of Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20). SEM micrographs show the formation of densely packed nearly spherical and hexagonal particles and corroborate to the TEM studies. Inset of Fig. 4 shows the high resolution SEM images, which demonstrate the morphology and texture of the particles. Fig. 5 shows the EDX (energy dispersive X-ray spectroscopy) micrographs of Sr-doped BaZrO3 which confirms the incorporation of Sr2þ in the BaZrO3 host lattice. The presence of peaks of Ba, Sr, Zr, and O affirm the formation of Ba1xSrxZrO3 solid solution. The loaded chemical compositions of all elements were found to be in close agreement to the experimentally calculated composition. 3.3. BET surface area studies Fig. 6. (a) BET surface area and (b) DA pore size distribution plots of Ba1xSrxZrO3 nanoparticles.
concentration, showing the incorporation of Sr2þ at Ba2þ lattice site. On increasing the dopant (Sr2þ) concentration, a monotonic shift in diffraction pattern towards the higher angle observed, which clearly indicates the formation of solid solution Ba1xSrxZrO3. The refined unit cell parameter “a” was found to be smaller than undoped BaZrO3. It can be clearly seen from Fig. 2 that on increasing the dopant concentration from x ¼ 0.05 to 0.20, the lattice parameter decreases monotonically, which may be due to the marginal contraction in BaZrO3 lattice. The lattice contraction of host lattice is associated to the smaller ionic radius of Sr2þ (1.18 Å) as compared with Ba2þ (1.35 Å) [52], which may lead to the substantial solubility of Sr2þ ions in BaZrO3 host lattice. The careful analysis of X-ray diffraction pattern of (Ba0.80Sr0.20)ZrO3 show the appearance of very small reflections of low intensity (~1%) at 2q values of around 28, 32 and 50 which are attributed to the intermittent unreacted phases of SrO2 (JCPDS 73-1740) and SrCO3 (JCPDS 84-1778). The appearance of minor impurity phases in 20 mol% Srdoped BaZrO3 suggests that the solubility limit of Sr2þ ions in A-site of BaZrO3 perovskites may be less than 20% at 900 C. It is hard to find the report on solubility limit of Sr2þ in the A-site of BaZrO3 to the best of our knowledge, however the solubility of its group analog Ca2þ in BaZrO3 increases from a few percent at 1400 C to about 30% at 1650 C [53].
Brunauer-Emmett-Teller (BET) gas adsorption method [54] was used for the determination of the surface area and pore size of Sr2þ doped BaZrO3. Specific surface areas of the nanoparticles have been determined in the P/P0 range of 0.05e0.30 at 20 points using multipoint BET equation as shown in Fig. 6a. The values of surface area comes out to be 104, 161, 190 and 244 m2 g-1 respectively which were found to be higher than the earlier reported values so far [49]. Highest surface area of 244 m2 g-1 was found for x ¼ 0.20 composition which has smallest particle size of 40 nm, however low surface area of 104 m2 g-1 was found for Ba0.95Sr0.05ZrO3 which has largest particle size of 65 nm. The BET particle sizes have also been estimated by using BET equation DBET ¼ 6000/(r.Sw) where the symbols DBET, r, and Sw represents the average diameter of the spherical particle in nm, theoretical density in gm/cm3 and specific surface area in m2/g of the samples respectively [55]. The average particle size calculated by BET method comes out to be 9.32, 6.03, 5.17 and 4.06 nm respectively. The BET sizes were found to be smaller than the measured values of TEM analysis. The smaller particles using BET studies may be due to the fact that most of the particles are not smooth and spherical in shape [55]. DubininAstakhov (DA) pore size distribution plots of solid solution Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) are shown in Fig. 6b. DA pore radius plots for x ¼ 0.15 and x ¼ 0.20 composition overlap which suggest nearly the same pore radius of these compositions. The surface area and pore size studies showed good agreement with TEM particle size because as the particle size decreases from
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Fig. 7. The variation of dielectric constant and dielectric loss with frequency of Ba1xSrxZrO3 nanoparticles for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 at room temperature.
Fig. 8. The variation of dielectric constant and dielectric loss with temperature of Ba1xSrxZrO3 nanoparticles for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 at 500 kHz frequency.
Please cite this article in press as: T. Ahmad, et al., Reverse micellar synthesis, structural characterization and dielectric properties of Sr-doped BaZrO3 nanoparticles, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.10.001
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65 to 40 nm with composition, surface area increase from 104 to 244 m2 g-1 and the pore radius marginally decreases from 15 to 14 Å. 3.4. Electrical properties The electrical (dielectric, impedance/conductance) behavior of barium strontium zirconate nanoparticles with general formula Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) has been examined as a function of frequency and temperature on sintered disks. The room temperature dielectric characteristics are shown in Fig. 7(aed) which provides the details of dielectric constant and dielectric loss in the frequency range of 20 kHz to 1 MHz. The room temperature dielectric constant and dielectric loss were found to be nearly stable with frequency which might be associated to the inability of electric dipoles to follow up the fast variation of alternating applied electric field [56]. The dielectric constant was found to be 13.5, 15, 15.6 and 25.5 for Ba0.95Sr0.05ZrO3, Ba0.90Sr0.10ZrO3, Ba0.85Sr0.15ZrO3, and Ba0.80Sr0.20ZrO3 nanoparticles respectively with nearly same dielectric losses ranges from 0.023 to 0.021. The values of dielectric constant and dielectric loss at lower frequency were found to be high because the dipoles follow up the slow variation of alternating applied electric field easily as compared to high frequency. As frequency increases, the net polarization of materials decreases because each polarization mechanism contributes the phenomenon, and hence the dielectric constant decreases. The variation of dielectric constant (ε) and dielectric loss (D) of Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) nanoparticles have also been studied with temperature in the range of 50 Ce400 C at 500 kHz frequency as shown in Fig. 8(aed). Both the dielectric constant and dielectric loss remain stable till 150 C, thereafter a linear increase could be seen with increase of the temperature. As the temperature increases, the molecules have more thermal
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energy and therefore the amplitude of random thermal motion is high. This shows the range of deviation from a perfect alignment with the field is higher; therefore the molecules are less closely aligned with each other and hence results in the increase of dielectric constant. The increasing value of dielectric loss with temperature may be due to the polarization lag behind the applied field causing an interaction between the field and dielectric polarization which arises on heating. Impedance spectroscopy is considered as the absolute source of information for grains and grain boundary structures that is associated to the conductance of materials [57,58]. The conductance and impedance of the solid solutions were also measured as a function of frequency as shown in Fig. 9. It is evident from the figures that conductance increases linearly with increase in frequency; however the impedance has an inverse effect at room temperature. It is observed that the initial value of impedance is higher at low frequency but it decrease gradually with increasing frequency which may be associated to the increase in conductance. 4. Conclusions Nanocrystalline Ba1xSrxZrO3 (x ¼ 0.05, 0.10, 0.15 and 0.20) ceramics have been successfully prepared by reverse micellar route at 900 C for the first time. Monophasic and highly crystalline nature of nanoparticles have been established by the X-ray diffraction studies. The slight decrease in lattice parameter confirms the marginal contraction in barium zirconate host lattice which is due to the incorporation of Sr2þ (1.18 Å) on Ba2þ (1.35 Å) lattice site. TEM studies showed the formation of mixed shaped particles of sizes 40e65 nm which corroborates well with BET surface area studies. High dielectric constant and low dielectric loss was found for 40 nm sized 20 mol% Sr-doped BaZrO3 nanoparticles. The large surface area, high dielectric constant and low dielectric loss of as-
Fig. 9. The variation of impedance and conductance with frequency of Ba1xSrxZrO3 nanoparticles for x ¼ (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.20 at room temperature.
Please cite this article in press as: T. Ahmad, et al., Reverse micellar synthesis, structural characterization and dielectric properties of Sr-doped BaZrO3 nanoparticles, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.10.001
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Please cite this article in press as: T. Ahmad, et al., Reverse micellar synthesis, structural characterization and dielectric properties of Sr-doped BaZrO3 nanoparticles, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.10.001