J Mater Sci: Mater Electron (2014) 25:1423–1428 DOI 10.1007/s10854-014-1745-1
Enhanced ferroelectric and UV photocatalytic properties in a Bi4Ti3O12@ZnO core–shelled nanostructure X. B. Meng • J. Miao • Y. Zhao • S. Z. Wu X. G. Xu • S. G. Wang • Y. Jiang
•
Received: 30 October 2013 / Accepted: 18 January 2014 / Published online: 6 February 2014 Ó Springer Science+Business Media New York 2014
Abstract Composited Bi4Ti3O12@ZnO nanoparticles with a core of Bi4Ti3O12 (BIT) and a shell of ZnO have been synthesized by liquid chemical reaction method. Compared with pure BIT nanoparticles, the ferroelectricity in BIT@ZnO core–shell nanostructure was greatly enhanced. Moreover, the dielectric loss of BIT@ZnO is lower than that of BIT nanoparticles in a low frequency range. The band gap energy of BIT@ZnO core@shell nanostructure is larger than that of BIT, which formed as a type-II band alignment. Furthermore, the BIT@ZnO core– shell nanoparticles exhibit better UV photodegradation activity for organic contaminant. Such a BIT@ZnO core@shell nanostructure may have potential applications in microelectronics, photoelectronic, and photocatalytic of contamination.
1 Introduction Recently, ferroelectric bismuth titanate (Bi4Ti3O12, BIT) has received more attentions due to its relatively high dielectric constant, high Curie temperature and high breakdown strength [1, 2], which makes it a potential candidate for nonvolatile ferroelectric memory and field effect transistor devices [3, 4]. Furthermore, BIT nanoparticles have a
X. B. Meng J. Miao (&) Y. Zhao S. Z. Wu X. G. Xu Y. Jiang School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China e-mail:
[email protected] S. G. Wang State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
high photocatalytic ability in the field of the organic pollutants degrading [5–7]. On the other hand, the core–shell oxide nanostructure, such as CdSe@CsS, CdSe@ZnS and CdSe@TiO2 [8–10], exhibits better electrical and optical properties compared with that of single-component nanoparticles [11]. Particularly, due to its electronic, optical, and piezoelectric properties [12], ZnO has been used in core– shell nanostructures for metal@ZnO and BiFeO3@ZnO [13, 14]. However, to the best of our knowledge, no report of BIT@ZnO core@shell nanostructure has been carried out. In this work, BIT@ZnO core-shell nanoparticles were successfully synthesized. Very significantly, the ferroelectricity and photocatalytic properties was found to be enhanced in the BIT@ZnO nanostructure. Meanwhile, an separation process of electron–hole in BIT@ZnO nanostructure were confirmed in the PL spectra. The ZnO shell can reduce the leakage current and facilitate electron–hole separation of the nanocomposite. Such a BIT@ZnO core@shell nanostructure may have great potential in the microelectronics, high-efficient photoelectronic, and photocatalytic of contamination applications.
2 Experiment The Bi4Ti3O12@ZnO nanocomposited particles have been synthesized by liquid chemical reaction (LCR) method. The Bi4Ti3O12 nanoparticles were fabricated by a stoichiometric reaction of Bi(NO)3 and Ti(C4H9O)4 solutions. Bi(NO)3 and Ti(C4H9O)4 were dissolved in mixture of diluted nitric acid (HNO3) and ethanol (1:3). The pH value was adjusted to 8–9 by slowly (10 mL/ min) adding 2 wt% ammonia (NH3H2O), followed by aging 6 h at 80 °C. After that, the finial precipitate was
123
1424
J Mater Sci: Mater Electron (2014) 25:1423–1428
Fig. 1 a Morphology, b HRTEM and c SAED of BIT@ZnO nanostructure (before annealing)
filtered and washed with de-ionized water until the pH value of the sample was reduced to 7, and then calcined at 450 or 650 °C for 2 h [15]. To synthesis a core@shell nanostructure, 100 mg of BIT nanoparticles were dispersed in 100 mL de-ionized water by ultrasonication for more than 30 min [16]. 160 mg Zn(Ac)25H2O was introduced and the suspension was heated to 40 °C under stirring. A 20 mL of 5 wt% ammonia was added into the suspension in 0.5 h and the reaction was maintained at least 1 h. Then, the prepared solution was centrifuged and sintered 2 h at the same sintering temperature of BIT. Lastly, a desired BIT@ZnO core@shell nanoparticles was yielded. The morphology and crystallographic structure have been investigated by transmission electron microscopy (TEM) and X-ray diffraction methods. The UV–vis absorption spectra were measured by using an UV–vis spectrophotometer. The leakage current property was investigated by Keithley 2400-c. The dielectric measurement was conducted using HP4182a impendence precision analyzer. The ferroelectric property measurement was performed using TF-Analyzer 1,000 at room temperature.
123
3 Results and discussion Figure 1 shows the crystallography and microstructure of Bi4Ti3O12@ZnO nanoparticles (before annealed) by a high resolution TEM. It can be seen that BIT nanoparticles was coated by a ZnO shell and forming a core@shell nanostructured particles. Figure 1a shows the low magnification TEM image of BIT@ZnO nanoparticles. The microstructure of BIT@ZnO nanocomposite was characterized by a HRTEM (Fig. 1b). The BIT core part can be confirmed by ˚, interplanar spacing of (240) and (171) of 2.54 and 2.97 A respectively. Figure 1c exhibited selected area electron diffraction patterns of BIT@ZnO nanoparticles. It can be found polycrystalline diffractions with (171) and (240) lattice planes of BIT, respectively. This result indicates BIT@ZnO nanoparticles may perform unique ferroelectric and optical properties with a unique core@shell nanostructure. Figure 2 shows the XRD patterns of BIT and BIT@ZnO nanoparticles. As shown in figure, all reflections can be indexed by a wurtzite phase of ZnO (JCPDS 79-0205) and a layered perovskite phase of BIT (JCPDS 36-1486). Moreover, no other distinct peaks was found except the peaks of ZnO and BIT, indicating the absence of the
J Mater Sci: Mater Electron (2014) 25:1423–1428
1425
Fig. 4 Frequency dependence of er and tan d of BIT and BIT@ZnO nanostructures Fig. 2 XRD patterns of BIT and BIT@ZnO nanoparticles sintered at 650 °C, respectively
of a shell-part ZnO, which may act as more insulating layer to reduce the leakage current in nanocomposites. Similar phenomenon has been observed in a reported ZnO/BiFeO3PbTiO3 heterostructure film [19]. Figure 4a shows the curves of dielectric constant and dissipation factor versus frequency of BIT@ZnO nanoparticles in the frequency range of 104 Hz–1 MHz. As shown from figure, the dielectric constant er of BIT@ZnO nanoparticles reaches as large as 30 at 104 Hz, which was in the range of BIT bulk [20] and BIT nanostructure [21]. To describe dielectric behaviors of the BIT@ZnO nanoparticles, a Lichtenecker’s logarithmic expression in the composited system can be used for total property [22], e ¼ V1 ln e1 þ V2 ln e2 ;
Fig. 3 P–E curve of BIT and BIT@ZnO nanoparticles, respectively
impurity phase. As expected, the layered perovskite BIT may allow a spontaneous polarization rotating in a-b plane and possesses a small Pr value [17]. Figure 3 shows the electrical field dependence of polarizations (P-E) of the Bi4Ti3O12@ZnO core–shell and Bi4Ti3O12 as-prepared nanoparticles. It can be seen that the polarization of BIT nanoparticles were with 2Pr * 5 lC/ cm2 and Ec * 70 kV/cm under 10 Hz. Those values are comparable to that of BIT ceramics [18] (2Pr * 6.56 lC/ cm2). Interestingly, compared with BIT nanoparticles, the P–E loops of BIT@ZnO core–shell nanoparticles behave more saturated well. Moreover, the value of 2Pr of BIT@ZnO core–shell nanoparticles increases to 15 lC/ cm2. The enhancement of ferroelectricity in BIT@ZnO core–shell nanoparticles can be ascribed to the introduction
ð1Þ
where e, e1 and e2 are the dielectric constants of total system of phase 1 and phase 2, respectively; V1 and V2 are the volume fractions of the phase 1 and phase 2, respectively; and V1 ? V2 = 1. The higher dielectric constant in the BIT@ZnO nanoparticles at low frequency range may be ascribe to the relaxation of charged and localized charge in the structure [23]. On the other hand, the dielectric loss spectra of BIT@ZnO nanoparticles and BIT nanoparticles are different. The variations of dielectric loss tangent can be explained as following: (1) the reduction of host BIT may leads to less molecular dipoles and decreasing of dipole loss [24]. (2) An insulating part of ZnO shell can restrict the migration and accumulation of the space charges in the nanostructure, and lead to the decreasing of its dielectric loss [25]. Figure 5 shows the photoluminescence (PL) spectra of BIT@ZnO nanoparticles and BIT nanoparticles at room temperature. The PL spectrum of BIT@ZnO core–shell nanoparticle exhibits a dominant deep level emission band centered at 570 nm (yellow emission). The yellow
123
1426
J Mater Sci: Mater Electron (2014) 25:1423–1428
Fig. 5 PL spectras of BIT and BIT@ZnO nanostructures sintered at 650 °C, respectively
emission at 570 nm can be attributed to oxygen interstitial defects [26, 27] or hydroxyl groups [28–30]. Moreover, ZnO nanoparticles display a peak at 570 nm and a UV emission at 378 nm, which can be identified as near-bandedge emission (NBE). As known, the weakening of the NBE emission can be due to a type-II energy alignment, non-radiative recombination and a emission of energy photons [31]. Similar phenomenon has been found in ZnOSnO2 nanostructure [32]. Therefore, the BIT@ZnO core– shell nanoparticles can perform a better photocatalytic activity than that of pure ZnO. Figure 6a shows the absorbance edges of BIT@ZnO core–shell nanoparticles and BIT nanoparticles. Compared with samples sintered at 450 °C, the absorption edges of samples sintered at 650 °C exhibits a slight blue shift. The phenomenon is in accord with the report by Oliveira [33], which can be ascribed to the presence of Bi2O3. The absorption coefficient (a) as a function of photon energy can be expressed by the Tauc relation [34]. hm ¼ Cðhm Eg Þn ;
ð2Þ
where hm, C, and Eg are the photon energy, a constant, and the band gap energy, respectively; n is an index determined by the nature of the electron transition during the absorption process. Figure 6 (b) shows the plot of (Ahm)2 versus hm, which A is proportional to absorption coefficient a. Compared with BIT nanoparticle, the band gaps of BIT@ZnO are pronounced increasing from 3.09 eV to 3.13 eV@)450 °C, while increasing from 3.22 eV to 3.35 eV@)650 °C. Figure 7 exhibits the photocatalytic activities of BIT@ZnO core–shell nanoparticles and BIT nanoparticles under UV irradiation. We investigated the photocatalytic
123
Fig. 6 a UV–visible spectrum and b direct band gap of BIT and BIT@ZnO nanoparticles @ 450 °C. The insets shows the results @ 650 °C
activity by choosing methyl orange (MO) at 462 nm to monitor the absorption degradation process. As shown, the decomposition rate of BIT nanoparticles is very low (27 and 18 % sintered at 450 and 650 °C, respectively.). Interestingly, the BIT@ZnO core@shell nanocomposites exhibit significantly higher decomposition rates (96 and 64 % sintered at 450 and 650 °C, respectively.). Moreover, the decomposition rate of MO of ZnO powders degrades only 75 %. The enhanced photocatalytic activity of BIT@ZnO core@shell nanoparticles may be due to the enhanced photo absorption property, electron–hole recombination rate, and charges transfer rating [35]. This result is also consist with the absorbance spectra measurements (Fig. 6). Figure 8 illustrates the energy band diagram of ZnO and BIT in the Bi4Ti3O12@ZnO core@shell nanocomposite, which belongs to a type II of band alignment. Upon UV illumination, electrons in the valence band (VB) could be
J Mater Sci: Mater Electron (2014) 25:1423–1428
1427
photogenerated electrons recombination could lead to the increasing lifetime of interfacial charge carriers to directly decompose of organic compounds [38]. Therefore, the photocatalytic activity of the photocatalytic reactions could be enhanced in BIT@ZnO core@shell nanoparticle. That phenomenon has been also found in the photocatalytic reaction of BiFeO3@TiO2 nanostructure [35].
4 Conclusions
Fig. 7 Normalized optical absorbance measurements a absence of catalysts; BIT @ b 650 °C and c 450 °C, respectively; BIT@ZnO sintered at @ d 650 °C and f 450 °C, respectively
A core@shelled nanostructure of BIT@ZnO has been successfully prepared by LCR. Compared with pure BIT, the ferroelecrtricity with a 2Pr * 15 lC/cm2 was enhanced in BIT@ZnO nanostructure. Moreover, the dielectric loss of BIT@ZnO nanocomposites was much lower from 10 kHz to 1 MHz. More interestingly, the BIT@ZnO nanostructure presents an excellent UV-light photocatalytic activity property, which can be ascribed to the increasing of the charge’s separation. Such a BIT@ZnO core@shell nanostructure may bring a new insight into the microelectronics, high-efficient photocatalysts and others potential applications. Acknowledgments This work was supported by National Basic Research Program of China (No. 2012CB932702), Beijing Municipal Natural Science Foundation (No. 2122037), NCET, the NSFC (Nos. 11174031, 51371024, 51325101), PCSIRT, and Fundamental Research Funds for the Central Universities.
References
Fig. 8 Demonstrated nanoparticles
energy
levels
diagrams
of
BIT@ZnO
excited to the conduction band (CB) with a concomitant formation of holes. The CB and VB potentials of BIT@ZnO can be estimated by an empirical equation [36], 1 EVB ¼X Ee þ Eg ; 2
ð3Þ
where EVB is the edge potential of VB; X is the geometric mean of the electronegativity of the constituent atoms [35]; Ee is the energy of free electrons on the hydrogen scale (*4.5 eV), Eg is the band gap energy of the semiconductor, and ECB can be determined by ECB = EVB - Eg. The electrons can be collected by the BIT nanoparticles and the holes by ZnO shells as transferred to stable electronic state. That is, downward for electron transfer from ZnO to BIT, and upward for holes to a more stable bonding from BIT to ZnO [37]. The subdued ability of the
1. K. Shoji, Y. Uehara, K. Sakata, Jpn. J. Appl. Phys. 35B, 5126 (1956) 2. Q.B. Yang, Y.X. Li, Q.R. Yin, P.L. Wang, Y.B. Cheng, J. Eur. Ceram. Soc. 23, 131 (2003) 3. J.F. Scott, C.A. Arauzo, Science 246, 1400 (1989) 4. H. Buhay, S. Sinharoy, W.H. Kasner, M.H. Francombe, D.R. Lampe, E. Stepke, Appl. Phys. Lett. 58, 1470 (1991) 5. W.F. Yao, X.H. Xu, H. Wang, J.T. Zhou, X.N. Yang, Y. Zhang, S.X. Shang, B.B. Huang, Appl. Catal. B Environ 52, 109 (2004) 6. Z.Z. Wang, Y.J. Qi, H.Y. Qi, C.J. Lu, S.M. Wang, J. Mater. Sci.Mater. Electron. 21, 523 (2010) 7. X. Lin, Appl. Surf. Sci. 258, 7146 (2012) 8. X. Peng, M.C. Schlamp, A.V. Kadavanchi, A.P. Alivisatos, J. Am. Chem. Soc. 119, 7019 (1997) 9. B.O. Dabbousi, K.F. Jensen, M.G. Bawendi, J. Phys. Chem. B 101, 9463 (1998) 10. Q. Shen, T. Sato, M. Hasimoto, C. Chen, T. Toyoda, Thin Solid Film 499, 299 (2006) 11. H. Wang, L.Y. Chen, Y.H. Feng, H.Y. Chen, Acc. Chem. Res. 46(7), 1636 (2013) 12. Z.L. Wang, J. Phys. Condens. Matter 16R, 829 (2004) 13. Z. Li, G.Z. Wang, Q.H. Yang, Z.B. Shao, Y. Wang, Nanoscale Res. Lett. 7, 316 (2012) 14. S.W. Chen, C.C. Lee, M.T. Chen, J.M. Wu, Nanotechnology 22, 115605 (2011) 15. M.S. Islam, J. Kano, F. Shikanai, S. Kojima, Integr. Ferroelectr. 100, 146 (2008)
123
1428 16. S.F. Si, C.H. Li, X. Wang, Q. Peng, Y.D. Li, Sens. and Actuators, B 119, 52 (2006) 17. T. Takenaka, K. Sakata, Jpn. J. Appl. Phys. 19, 31 (1980) 18. H.B. Chen, B. Shen, J.B. Xu, J.W. Zhai, J. Alloys Comp. 551, 92 (2013) 19. S.W. Yu, R. Chen, G.J. Zhang, J.R. Cheng, Z.R. Meng, Appl. Phys. Lett. 89, 212906 (2006) 20. Z.Y. Fan, J.G. Lu, J. Nanosci. Nanotechnol. 5, 1561 (2005) 21. Y. Yang, W. Guo, X.Q. Wang, Z.Z. Wang, J.J. Qi, Y. Zhang, Nano Lett. 12, 1919 (2012) 22. I. Bunget, M. Popescu, Physics of solid dielectrics (Elsevier, Romania, 1984), p. 85 23. P. Pandit, S. Satapathy, P.K. Gupta, V.G. Sathe, J. Appl. Phys. 106, 114105 (2009) 24. A.K. Jonscher, Nature 267, 673 (1977) 25. X.Y. Huang, C.Y. Zhi, P.K. Jiang, D. Golberg, Y. Bando, T. Tanaka, Nanotechnology 23, 455705 (2012) 26. Z.W. Chen, Y. Yu, J.Q. Hu, A. Shui, X.H. He, J. Ceram. Soc. Jpn. 117(3), 264 (2009) 27. L.P. Li, X.Q. Qiu, G.S. Li, Appl. Phys. Lett. 87, 124101 (2005) 28. K.H. Tam, C.K. Cheung, Y.H. Leung, A.B. Djurisic, C.C. Ling, C.D. Beling, S. Fung, W.M. Kwok, W.K. Chan, D.L. Phillips, L. Ding, W.K. Ge, J. Phys. Chem. B 110, 20865 (2006)
123
J Mater Sci: Mater Electron (2014) 25:1423–1428 29. J.J. Qiu, X.M. Li, W.Z. He, S.J. Park, H.K. Kim, Y.H. Hwang, J.H. Lee, Y.D. Kim, Nanotechnology 20, 155603 (2009) 30. N.S. Norberg, D.R. Gamelin, J. Phys. Chem. B 109, 20810 (2005) 31. J.P. Xu, P. Liu, S.B. Shi, X.S. Zhang, L.S. Wang, Z.R. Ren, L. Ge, L. Li, Appl. Surf. Sci. 258, 7118 (2012) 32. V. Kuzhalosai, B. Subash, A. Senthilraja, P. Dhatshanamurthi, M. Shanthi, Spectrochim. Acta. A Mol. Biomol. Spectrosc. 115, 876 (2013) 33. R.C. Oliveira, L.S. Cavalcante, J.C. Sczancoski, E.C. Aguiar, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo, J. Alloys Comp. 478, 661 (2009) 34. Y.F. Liu, Y.N. Lu, S.H. Dai, J. Alloys Comp. 484, 801 (2009) 35. S. Li, Y.H. Lin, B.P. Zhang, J.F. Li, C.-W. Nan, J. Appl. Phys. 105, 054310 (2009) 36. A.H. Nethercot, Phys. Rev. Lett. 33, 1088 (1974) 37. M.A. Butler, D.S. Ginley, J. Electrochem. Soc. 125, 228 (1978) 38. Y. Kim, S.J. Atherton, E.S. Brigham, T.E. Mallouk, J. Phys. Chem. 97, 11802 (1993)
