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Growth of potassium niobate micro-hexagonal tablets with monoclinic phase and its excellent piezoelectric property
Journal of Crystal Growth 354 (2012) 9–12
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of potassium niobate micro-hexagonal tablets with monoclinic phase and its excellent piezoelectric property Zhong Chen, Jingyun Huang n, Ye Wang, Yefeng Yang, Yongjun Wu, Zhizhen Ye State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 22 March 2012 Received in revised form 27 April 2012 Accepted 14 May 2012 Communicated by T. Nishinaga Available online 23 May 2012
Potassium niobate micro-hexagonal tablets were synthesized through hydrothermal reaction with KOH, H2O and Nb2O5 as source materials by using a polycrystalline Al2O3 as substrate. X-ray diffraction, Raman spectra and selected area electron diffraction analysis results indicated that the tablets exhibit monoclinic phase structure and are highly crystallized. Meanwhile, piezoelectric property of the microhexagonal tablets was investigated. The as-synthesized tablets exhibit excellent piezoactivities in the experiments, and an effective piezoelectric coefficient of around 80 pm/V was obtained. The tablets have huge potential applications in micro/nano-integrated piezoelectric and optical devices. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Micro-structures B1. Niobates B1. Potassium compounds B2. Piezoelectric materials
1. Introduction Recently, micro-/nano-structural niobate-based materials have received considerable attention because of their excellent electric and optical properties [1–5]. Potassium niobate is one of the most important and typical compounds among niobate-based materials, and it exhibits outstanding dielectric, ferroelectric [6], piezoelectric [7], photorefractive, photocatalytic [8], acoustic-optical, electrooptical, and nonlinear optical properties [9], as well as superior mechanical and chemical stability. In addition, it has been widely used in many electro-optic devices and surface acoustic devices. In the phase diagram of the system K2O–Nb2O5, various different compounds have been reported: K2O Nb2O5 (KNbO3), 2K2O 3Nb2O5 (K4Nb6O17), 5.75K2O 10.85Nb2O5 (K5.75Nb10.85O30), 4K2O 9Nb2O5 (K8Nb18O49), 3K2O 7Nb2O5 (K3Nb7O19), K2O 3Nb2O5 (KNb3O8), K2O 4Nb2O5 (K2Nb8O21), K2O 7Nb2O5 (KNb7O18), K2O 13Nb2O5 (KNb13O33) [10–14]. Among these compounds, the research works of nano-structural KNbO3 materials have been widely reported, while the amount of others is limited. For the synthesis of potassium niobate nanostructured materials, hydrothermal treatment has often been applied, which is a suitable synthetic route for material preparation at low temperature. Depending on hydrothermal reaction, various kinds of potassium niobate nanostructured materials have been produced, such as nanoparticles [15,16], nanorods [17], and nanowires
n
Corresponding author. Tel.: þ86 571 87952118; fax: þ 86 571 87952625. E-mail address: [email protected] (J. Huang).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.05.018
[8–9,18]. Besides, some other chemical methods have been employed to prepare nanosized potassium niobate, including sol–gel method, glycothermal method, molten salt reaction [13], and polymerized complex (PC) method [19]. There have been many studies on investigation of the piezoelectric properties of micro-/nano-structural niobate-based materials [3,7,20–21]. Compared to lead-containing piezoceramics, niobate-based materials are much more environmentally friendly. Hence, they will be potential substitutes for the traditional lead zirconium titanate (PZT) [22]. In this paper, we focus on the synthesis of K3Nb7O19 micro-hexagonal tablets by hydrothermal reaction and the investigation of their piezoelectric properties due to their potential applications in such devices as nanoelectromechanical systems (NEMS). The piezoresponse of clamped KNbO3 nanowires has been reported [7]. However, to the best of our knowledge, this is the first time it is reported that K3Nb7O19 micro-hexagonal tablets have been synthesized, and the related research on the piezoelectric properties of the micro-hexagonal structural K3Nb7O19 prepared by hydrothermal method is very limited.
2. Experiment Hydrothermal reaction is often employed to prepare KNbO3. However, in our experiments we found that K3Nb7O19 microhexagonal tablets could be often synthesized on the surface of polycrystalline Al2O3 substrate when the concentration of KOH is lower than that for the synthesis of KNbO3 in Ref. [8]. The
10
Z. Chen et al. / Journal of Crystal Growth 354 (2012) 9–12
measured pH value of the precursor solution and after hydrothermal reaction is over 14.05, beyond measurement range. The typical synthesis of K3Nb7O19 micro-hexagonal tablets is presented as follows, hydrothermal reaction was carried out with a Teflon-lined autoclave (50 ml). 0.3 wt% Nb2O5 (0.092 g), 29.7 wt% KOH, and 70 wt% H2O mixture, was stirred for approximately 40 min, and then poured into Teflon vessels. A piece of polycrystalline Al2O3 substrate which had been cleaned in acetone and deionized water sequentially were obliquely placed at the bottom of Teflon inserts. The hydrothermal reaction lasted at 150 1C for 5 days, followed by natural cooling to room temperature. After the synthesis, the products were removed from the surface of Al2O3 substrates in acetone by ultrasonication, and the hexagonal tablet-like products were separated from the resulting mixture acetone suspension through simple centrifugation by CT14D high-speed centrifuge at rotation speed of 1000 rpm for 5 min, and then dried in air at 80 1C overnight. After this, phase composition of as-prepared samples was determined by X-ray diffraction (XRD) on a Bede D1 system with high Cu Ka radiation. The morphologies and structures were investigated by JEOL JSM 6300F field emission scanning electron microscopy (SEM) and
Fig. 1. (a) SEM image of the hexagonal tablet-like products obtained on the surface of the the polycrystalline Al2O3 substrate, (b) SEM image of the separated K3Nb7O19 micro-hexagonal tablets, and (c) XRD patterns of the K3Nb7O19 microhexagonal tablets.
JEOL 2010 transmission electron microscopy (TEM) study. Ramanscattering data were collected in the frequency range of 100– 1000 cm 1 using a micro-Raman spectrometer with an argon ion laser operating at an excitation wavelength of 514 nm (inVia plus, Reinshaw, England).
3. Results and discussion The hexagonal tablet-like products as well as some micro-/ nano-particles can be obtained on the surface of the polycrystalline Al2O3 substrate through the hydrothermal reaction, as shown in Fig. 1a. The hexagonal tablet-like products are separated from the mixture products through simple centrifugation at rotation speed of 1000 rpm for 5 min. Finally approximately 35 wt% hexagonal tablet-like products against total products could be obtained, and Fig. 1b presents the SEM image of the separated micro-hexagonal tablets. It is obvious that the as-synthesized micro-hexagonal tablets have a size of several micrometers and a thickness of around 1–5 mm. The crystal phase of the separated micro-hexagonal tablets is determined by XRD, and the XRD pattern is presented in Fig. 1c. As can be seen from the XRD pattern, the major phase of the products is crystalline K3Nb7O19, with diffraction peaks assigned to the monoclinic phase of K3Nb7O19 (JCPDS 78-1395) with lattice parameters of ˚ b¼6.431 A, ˚ c ¼18.897 A, ˚ and b ¼98.061. The detailed a¼13.777 A, morphology and microstructure of a typical individual K3Nb7O19 micro-hexagonal tablet are further characterized by the TEM, as illustrated in Fig. 2a. A single free-standing tablet with a hexagonal shape is displayed in the image. The corresponding selected area electron diffraction (SAED) pattrens shown in Fig. 2b confirm that the obtained K3Nb7O19 micro-hexagonal tablets are single-crystalline, and the diffraction spots can be indexed to the monoclinic K3Nb7O19. Raman spectra have provided an effective way to confirm phase composition of alkaline niobate as supplementary means, because each alkaline niobate has its own characteristic Raman spectrum due to their special crystal structure. A micro-Raman spectrum of a typical K3Nb7O19 micro-hexagonal tablet at room temperature is shown in Fig. 3. K3Nb7O19 crystallizes in the monoclinic structure, and it is clearly seen from Fig. 3 that the main Raman peaks are observed at 226, 283, 542, 837, and 900 cm 1. According to the results of previous studies on niobates materials [23–24], 226 cm 1 can be assigned of the Nb–O framework. 283 cm 1 can be associated with the deformation in the NbO6 framework. The peak at 542 cm 1 involves large contributions of the longer Nb O bonds, while the shorter Nb O bonds contribute to the bands in the 837–900 cm 1 range.
Fig. 2. (a) TEM image of the K3Nb7O19 micro-hexagonal tablet and (b) the typical SAED pattern obtained from the K3Nb7O19 micro-hexagonal tablet.
Z. Chen et al. / Journal of Crystal Growth 354 (2012) 9–12
11
(d33–V) loops (see Fig. 1d) can be determined via Eq. (1) of piezoelectric effect [25–26]. d33 ¼
Fig. 3. Micro-Raman spectra of a single K3Nb7O19 micro-hexagonal tablet.
DDI VV I
ð1Þ
In Eq. (1), D is the measured values of piezoelectric deformation, V is the applied voltage for each point on the D–V curve, D is the piezoelectric deformation and V is the applied voltage of intersection. The values of d33 calculated from the D–V ‘‘butterfly’’ curves may usually deviate from the real value for many factors. For the instance, AFM tip could not always keep the upright position during the PFM measurements, and the measured D–V ‘‘butterfly’’ curve often includes the electrostrictive effect information of the tested material. However, piezoelectric properties can be determined from estimated d33 values to some extent. From our experiment results, the estimated max d33 value of the individual K3Nb7O19 micro-hexagonal tablet can reach approximately 80 pm/V. Therefore, K3Nb7O19 micro-hexagonal tablet may be a very important environment-friendly lead-free piezoelectric materials with a special micro-/nano-structural shape.
4. Conclusion In summary, a hydrothermal route has been employed for the synthesis of the K3Nb7O19 micro-hexagonal tablets, and the obtained K3Nb7O19 micro-hexagonal tablets on the surface of polycrystalline Al2O3 substrate have good crystallinity. The max d33 value can reach approximately 80 pm/V, which indicates K3Nb7O19 micro-hexagonal tablet is an excellent environmentfriendly lead-free piezoelectric material with a special shape. It is a promising candidate in applications like integrated piezoelectric sensors, resonators, and NEMS related devices.
Acknowledgment Fig. 4. (a) A schematic diagram of the PFM experiment on a typical K3Nb7O19 micro-hexagonal tablet, (b) topographic image of a single K3Nb7O19 microhexagonal tablet, (c) the typical D–V ‘‘butterfly’’ curve, and (d) shows the corresponding d33–V loop.
In this work, piezoelectric property of individual K3Nb7O19 micro-hexagonal tablet was characterized by a scanning probe microscope (SPM) (Dimension 3100 V, Veeco, USA) under the model of PFM, and a Pt/SiO2/Si substrate was used as a bottom electrode. Prior to the piezoelectric measurement, a small amount of micro-hexagonal tablet containing K3Nb7O19 powders were dispersed in alcohol by ultrasonication, and a driblet of the resulting suspension was deposited on the surface of the bottom electrode and dried. In order to improve adhesion of the nanowires and micro-hexagonal tablets to Pt-coated surface, the substrate was then annealed at 500 1C for 1 h. During the measurement, a small alternate current (AC) voltage was applied between the bottom electrode and the conductive tip (force constant K ¼0.2 N/m, Cr/Pt coated). The displacement–voltage (D–V) ‘‘butterfly’’ curve was recorded by applying a AC voltage from 10 V to 10 V. The schematic diagram of the PFM experiment is exhibited in Fig. 4a. The K3Nb7O19 micro-hexagonal tablet can be found in Fig. 4b, and a flat local region of the surface of the micro-hexagonal tablet where the sample protrudes 3 mm out of the surface is chosen for PFM measurement. As can be seen from the typical D–V ‘‘butterfly’’ curve shown in Fig. 4c, the K3Nb7O19 micro-hexagonal tablet exhibits excellent piezoelectric response. Every point on D–V ‘‘butterfly’’ loop contains information about piezoelectric deformation under corresponding applied voltage. From the corresponding D–V ‘‘butterfly’’ curves, the d33–voltage
This work was supported by the New Century 151 Excellent Person of Zhejiang Province and Doctoral Fund of Ministry of Education of China under Grant no. 2011010110013. We thank Yanping Wei working at Ningbo Institute of Material Technology and Engineering belonging to Chinese Academy of Sciences for the SPM measurements. References [1] H. Zhu, Z. Zheng, X. Gao, Y. Huang, Z. Yan, J. Zou, H. Yin, Q. Zou, S.H. Kable, J. Zhao, Y. Xi, W.N. Martens, R.L. Frost, Journal of the American Chemical Society 128 (2006) 2373. [2] A.D. Handoko, G.K.L. Goh, Green Chemistry 12 (2010) 680. [3] C. Yan, L. Nikolova, A. Dadvand, C. Harnagea, A. Sarkissian, D.F. Perepichka, D. Xue, F. Rosei, Advanced Materials 22 (2010) 1741. [4] R. Grange, J.W. Choi, C.L. Hsieh, Y. Pu, A. Magrez, R. Smajda, L. Forro, D. Psaltis, Applied Physics Letters 95 (2009) 143105. [5] J. Huang, Z. Chen, Z. Zhang, C. Zhu, H. He, Z. Ye, G. Qu, L. Tong, Applied Physics Letters 98 (2011) 093102. [6] L. Louis, P. Gemeiner, I. Ponomareva, L. Bellaiche, G. Geneste, W. Ma, N. Setter, B. Dkhil, Nano Letters 10 (2010) 1177. [7] J. Wang, C. Stampfer, C. Roman, W.H. Ma, N. Setter, C. Hierold, Applied Physics Letters 93 (2008) 223101. [8] Q. Ding, Y. Yuan, X. Xiong, R. Li, H. Huang, Z. Li, T. Yu, Z. Zou, S. Yang, Journal of Physical Chemistry C 112 (2008) 18846. [9] Y. Nakayama, P.J. Pauzauskie, A. Radenovic, R.M. Onorato, R.J. Saykally, J. Liphardt, P.D. Yang, Nature 447 (2007) 1098. [10] G.D. Fallon, B.M. Gatehouse, L. Guddat, Journal of Solid State Chemistry 61 (1986) 181. [11] E. Irle, R. Blachnik, B. Gather, Thermochimica Acta 179 (1991) 157. [12] J. Liu, X. Li, Y. Li, Journal of Crystal Growth 247 (2003) 419. [13] L. Li, J. Deng, J. Chen, X. Sun, R. Yu, G. Liu, X. Xing, Chemistry of Materials 21 (2009) 1207. [14] L. Li, J. Deng, R. Yu, J. Chen, X. Wang, X. Xing, Inorganic Chemistry 49 (2010) 1397.
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[15] S. Wohlrab, M. Weiss, H. Du, S. Kaskel, Chemistry of Materials 18 (2006) 4227. [16] H. Hayashi, Y. Hakuta, Y. Kurata, Journal of Materials Chemistry 14 (2004) 2046. [17] G. Wang, S.M. Selbach, Y. Yu, X. Zhang, T. Grandea, M. Einarsrud, CrystEngComm 11 (2009) 1958. [18] A. Magrez, E. Vasco, J.W. Seo, C. Dieker, N. Setter, L. Forro´, Journal of Physical Chemistry B 110 (2006) 58. [19] I. Priboˇsicˇ, D. Makovec, M. Drofenik, Chemistry of Materials 17 (2005) 2953. [20] T. Ke, H. Chen, H. Sheu, J. Yeh, H. Lin, C. Lee, H. Chiu, Journal of Physical Chemistry C 112 (2008) 8827.
ˇ [21] H. Urˇsicˇ, A. Bencˇan, M. Skarabot, M. Godec, M. Kosec, Journal of Applied Physics 107 (2010) 033705. [22] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84. [23] M. Maczka, M. Ptak, A. Majchrowski, J. Hanuza, Journal of Raman Spectroscopy 42 (2011) 209. [24] J. Zhan, D. Liu, W. Du, Z. Wang, P. Wang, H. Cheng, B. Huang, M. Jiang, Journal of Crystal Growth 318 (2011) 1121. [25] Y.Q. Chen, X.J. Zheng, X. Feng, Nanotechnology 21 (2010) 055708. [26] Y.C. Yang, C. Song, X.H. Wang, F. Zeng, F. Pan, Applied Physics Letters 92 (2008) 012907.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of potassium niobate micro-hexagonal tablets with monoclinic phase and its excellent piezoelectric property Zhong Chen, Jingyun Huang n, Ye Wang, Yefeng Yang, Yongjun Wu, Zhizhen Ye State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 22 March 2012 Received in revised form 27 April 2012 Accepted 14 May 2012 Communicated by T. Nishinaga Available online 23 May 2012
Potassium niobate micro-hexagonal tablets were synthesized through hydrothermal reaction with KOH, H2O and Nb2O5 as source materials by using a polycrystalline Al2O3 as substrate. X-ray diffraction, Raman spectra and selected area electron diffraction analysis results indicated that the tablets exhibit monoclinic phase structure and are highly crystallized. Meanwhile, piezoelectric property of the microhexagonal tablets was investigated. The as-synthesized tablets exhibit excellent piezoactivities in the experiments, and an effective piezoelectric coefficient of around 80 pm/V was obtained. The tablets have huge potential applications in micro/nano-integrated piezoelectric and optical devices. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Micro-structures B1. Niobates B1. Potassium compounds B2. Piezoelectric materials
1. Introduction Recently, micro-/nano-structural niobate-based materials have received considerable attention because of their excellent electric and optical properties [1–5]. Potassium niobate is one of the most important and typical compounds among niobate-based materials, and it exhibits outstanding dielectric, ferroelectric [6], piezoelectric [7], photorefractive, photocatalytic [8], acoustic-optical, electrooptical, and nonlinear optical properties [9], as well as superior mechanical and chemical stability. In addition, it has been widely used in many electro-optic devices and surface acoustic devices. In the phase diagram of the system K2O–Nb2O5, various different compounds have been reported: K2O Nb2O5 (KNbO3), 2K2O 3Nb2O5 (K4Nb6O17), 5.75K2O 10.85Nb2O5 (K5.75Nb10.85O30), 4K2O 9Nb2O5 (K8Nb18O49), 3K2O 7Nb2O5 (K3Nb7O19), K2O 3Nb2O5 (KNb3O8), K2O 4Nb2O5 (K2Nb8O21), K2O 7Nb2O5 (KNb7O18), K2O 13Nb2O5 (KNb13O33) [10–14]. Among these compounds, the research works of nano-structural KNbO3 materials have been widely reported, while the amount of others is limited. For the synthesis of potassium niobate nanostructured materials, hydrothermal treatment has often been applied, which is a suitable synthetic route for material preparation at low temperature. Depending on hydrothermal reaction, various kinds of potassium niobate nanostructured materials have been produced, such as nanoparticles [15,16], nanorods [17], and nanowires
n
Corresponding author. Tel.: þ86 571 87952118; fax: þ 86 571 87952625. E-mail address: [email protected] (J. Huang).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.05.018
[8–9,18]. Besides, some other chemical methods have been employed to prepare nanosized potassium niobate, including sol–gel method, glycothermal method, molten salt reaction [13], and polymerized complex (PC) method [19]. There have been many studies on investigation of the piezoelectric properties of micro-/nano-structural niobate-based materials [3,7,20–21]. Compared to lead-containing piezoceramics, niobate-based materials are much more environmentally friendly. Hence, they will be potential substitutes for the traditional lead zirconium titanate (PZT) [22]. In this paper, we focus on the synthesis of K3Nb7O19 micro-hexagonal tablets by hydrothermal reaction and the investigation of their piezoelectric properties due to their potential applications in such devices as nanoelectromechanical systems (NEMS). The piezoresponse of clamped KNbO3 nanowires has been reported [7]. However, to the best of our knowledge, this is the first time it is reported that K3Nb7O19 micro-hexagonal tablets have been synthesized, and the related research on the piezoelectric properties of the micro-hexagonal structural K3Nb7O19 prepared by hydrothermal method is very limited.
2. Experiment Hydrothermal reaction is often employed to prepare KNbO3. However, in our experiments we found that K3Nb7O19 microhexagonal tablets could be often synthesized on the surface of polycrystalline Al2O3 substrate when the concentration of KOH is lower than that for the synthesis of KNbO3 in Ref. [8]. The
10
Z. Chen et al. / Journal of Crystal Growth 354 (2012) 9–12
measured pH value of the precursor solution and after hydrothermal reaction is over 14.05, beyond measurement range. The typical synthesis of K3Nb7O19 micro-hexagonal tablets is presented as follows, hydrothermal reaction was carried out with a Teflon-lined autoclave (50 ml). 0.3 wt% Nb2O5 (0.092 g), 29.7 wt% KOH, and 70 wt% H2O mixture, was stirred for approximately 40 min, and then poured into Teflon vessels. A piece of polycrystalline Al2O3 substrate which had been cleaned in acetone and deionized water sequentially were obliquely placed at the bottom of Teflon inserts. The hydrothermal reaction lasted at 150 1C for 5 days, followed by natural cooling to room temperature. After the synthesis, the products were removed from the surface of Al2O3 substrates in acetone by ultrasonication, and the hexagonal tablet-like products were separated from the resulting mixture acetone suspension through simple centrifugation by CT14D high-speed centrifuge at rotation speed of 1000 rpm for 5 min, and then dried in air at 80 1C overnight. After this, phase composition of as-prepared samples was determined by X-ray diffraction (XRD) on a Bede D1 system with high Cu Ka radiation. The morphologies and structures were investigated by JEOL JSM 6300F field emission scanning electron microscopy (SEM) and
Fig. 1. (a) SEM image of the hexagonal tablet-like products obtained on the surface of the the polycrystalline Al2O3 substrate, (b) SEM image of the separated K3Nb7O19 micro-hexagonal tablets, and (c) XRD patterns of the K3Nb7O19 microhexagonal tablets.
JEOL 2010 transmission electron microscopy (TEM) study. Ramanscattering data were collected in the frequency range of 100– 1000 cm 1 using a micro-Raman spectrometer with an argon ion laser operating at an excitation wavelength of 514 nm (inVia plus, Reinshaw, England).
3. Results and discussion The hexagonal tablet-like products as well as some micro-/ nano-particles can be obtained on the surface of the polycrystalline Al2O3 substrate through the hydrothermal reaction, as shown in Fig. 1a. The hexagonal tablet-like products are separated from the mixture products through simple centrifugation at rotation speed of 1000 rpm for 5 min. Finally approximately 35 wt% hexagonal tablet-like products against total products could be obtained, and Fig. 1b presents the SEM image of the separated micro-hexagonal tablets. It is obvious that the as-synthesized micro-hexagonal tablets have a size of several micrometers and a thickness of around 1–5 mm. The crystal phase of the separated micro-hexagonal tablets is determined by XRD, and the XRD pattern is presented in Fig. 1c. As can be seen from the XRD pattern, the major phase of the products is crystalline K3Nb7O19, with diffraction peaks assigned to the monoclinic phase of K3Nb7O19 (JCPDS 78-1395) with lattice parameters of ˚ b¼6.431 A, ˚ c ¼18.897 A, ˚ and b ¼98.061. The detailed a¼13.777 A, morphology and microstructure of a typical individual K3Nb7O19 micro-hexagonal tablet are further characterized by the TEM, as illustrated in Fig. 2a. A single free-standing tablet with a hexagonal shape is displayed in the image. The corresponding selected area electron diffraction (SAED) pattrens shown in Fig. 2b confirm that the obtained K3Nb7O19 micro-hexagonal tablets are single-crystalline, and the diffraction spots can be indexed to the monoclinic K3Nb7O19. Raman spectra have provided an effective way to confirm phase composition of alkaline niobate as supplementary means, because each alkaline niobate has its own characteristic Raman spectrum due to their special crystal structure. A micro-Raman spectrum of a typical K3Nb7O19 micro-hexagonal tablet at room temperature is shown in Fig. 3. K3Nb7O19 crystallizes in the monoclinic structure, and it is clearly seen from Fig. 3 that the main Raman peaks are observed at 226, 283, 542, 837, and 900 cm 1. According to the results of previous studies on niobates materials [23–24], 226 cm 1 can be assigned of the Nb–O framework. 283 cm 1 can be associated with the deformation in the NbO6 framework. The peak at 542 cm 1 involves large contributions of the longer Nb O bonds, while the shorter Nb O bonds contribute to the bands in the 837–900 cm 1 range.
Fig. 2. (a) TEM image of the K3Nb7O19 micro-hexagonal tablet and (b) the typical SAED pattern obtained from the K3Nb7O19 micro-hexagonal tablet.
Z. Chen et al. / Journal of Crystal Growth 354 (2012) 9–12
11
(d33–V) loops (see Fig. 1d) can be determined via Eq. (1) of piezoelectric effect [25–26]. d33 ¼
Fig. 3. Micro-Raman spectra of a single K3Nb7O19 micro-hexagonal tablet.
DDI VV I
ð1Þ
In Eq. (1), D is the measured values of piezoelectric deformation, V is the applied voltage for each point on the D–V curve, D is the piezoelectric deformation and V is the applied voltage of intersection. The values of d33 calculated from the D–V ‘‘butterfly’’ curves may usually deviate from the real value for many factors. For the instance, AFM tip could not always keep the upright position during the PFM measurements, and the measured D–V ‘‘butterfly’’ curve often includes the electrostrictive effect information of the tested material. However, piezoelectric properties can be determined from estimated d33 values to some extent. From our experiment results, the estimated max d33 value of the individual K3Nb7O19 micro-hexagonal tablet can reach approximately 80 pm/V. Therefore, K3Nb7O19 micro-hexagonal tablet may be a very important environment-friendly lead-free piezoelectric materials with a special micro-/nano-structural shape.
4. Conclusion In summary, a hydrothermal route has been employed for the synthesis of the K3Nb7O19 micro-hexagonal tablets, and the obtained K3Nb7O19 micro-hexagonal tablets on the surface of polycrystalline Al2O3 substrate have good crystallinity. The max d33 value can reach approximately 80 pm/V, which indicates K3Nb7O19 micro-hexagonal tablet is an excellent environmentfriendly lead-free piezoelectric material with a special shape. It is a promising candidate in applications like integrated piezoelectric sensors, resonators, and NEMS related devices.
Acknowledgment Fig. 4. (a) A schematic diagram of the PFM experiment on a typical K3Nb7O19 micro-hexagonal tablet, (b) topographic image of a single K3Nb7O19 microhexagonal tablet, (c) the typical D–V ‘‘butterfly’’ curve, and (d) shows the corresponding d33–V loop.
In this work, piezoelectric property of individual K3Nb7O19 micro-hexagonal tablet was characterized by a scanning probe microscope (SPM) (Dimension 3100 V, Veeco, USA) under the model of PFM, and a Pt/SiO2/Si substrate was used as a bottom electrode. Prior to the piezoelectric measurement, a small amount of micro-hexagonal tablet containing K3Nb7O19 powders were dispersed in alcohol by ultrasonication, and a driblet of the resulting suspension was deposited on the surface of the bottom electrode and dried. In order to improve adhesion of the nanowires and micro-hexagonal tablets to Pt-coated surface, the substrate was then annealed at 500 1C for 1 h. During the measurement, a small alternate current (AC) voltage was applied between the bottom electrode and the conductive tip (force constant K ¼0.2 N/m, Cr/Pt coated). The displacement–voltage (D–V) ‘‘butterfly’’ curve was recorded by applying a AC voltage from 10 V to 10 V. The schematic diagram of the PFM experiment is exhibited in Fig. 4a. The K3Nb7O19 micro-hexagonal tablet can be found in Fig. 4b, and a flat local region of the surface of the micro-hexagonal tablet where the sample protrudes 3 mm out of the surface is chosen for PFM measurement. As can be seen from the typical D–V ‘‘butterfly’’ curve shown in Fig. 4c, the K3Nb7O19 micro-hexagonal tablet exhibits excellent piezoelectric response. Every point on D–V ‘‘butterfly’’ loop contains information about piezoelectric deformation under corresponding applied voltage. From the corresponding D–V ‘‘butterfly’’ curves, the d33–voltage
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