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Optical and electrical properties of ferroelectric Bi0.5Na0.5TiO3-NiTiO3 semiconductor ceramics
Materials Science in Semiconductor Processing 115 (2020) 105089
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Optical and electrical properties of ferroelectric Bi0.5Na0.5TiO3-NiTiO3 semiconductor ceramics Zexing Chen a, Changlai Yuan a, b, *, Xiao Liu a, b, Liufang Meng a, Shuai Cheng a, b, Jiwen Xu a, b, Changrong Zhou a, b, Jiang Wang a, b, Guanghui Rao a, b, c, ** a b c
College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, PR China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin, 541004, PR China Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: (1-x)BNT-xNTO Ferroelectric semiconductors Ferroelectric properties Optical band gap
(1-x)Bi0.5Na0.5TiO3-xNiTiO3 semiconducting ferroelectric ceramics (named (1-x)BNT-xNTO) were successfully prepared by using a solid state reaction method. All samples shows normal polarization-electric field hysteresis loops, and a maximum polarization Pmax is obtained in the x ¼ 0.06 composition. The diffuse factor γ decreases slightly from 1.732 to 1.571 with the addition of NiTiO3. In addition, Ni doping can substantially reduce the band gap of Bi0.5Na0.5TiO3 materials to ~2 eV. In a 0.94BNT–0.06NTO compound, the XPS spectra shows that the chemical formula is accurately [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ, Ti4þ]O2.25-0.06Ni[Ti3þ, Ti4þ]O2.67. Moreover, the short-circuit current (Jsc) in the composition is 1.36 nA/cm2, and the open-circuit voltaic (Voc) is 0.35 V for photovoltaic effects. The novel BNT system, which illustrates three narrow band gaps and relatively high po larization values, has broad application prospects for ferroelectric photovoltaic devices.
1 . Introduction Semiconductor ferroelectric ceramics with ABO3-type perovskite structure have been attracting considerable attention because of interest in their crystal structural behavior [1–3]. Researchers have been attempting to improve the ferroelectric and optical properties of perovskite-based oxides. The real potential of semiconductor ferroelec tric ceramics in photovoltaic applications has greatly promoted the exploration of new perovskite oxides with efficient visible light ab sorption [4]. Recently, with the realization of defect-driven ferroelec tricity and low bandgap states (~1.1 eV) in perovskites (KNbO3)1 x(BaNi0.5Nb0.5O3 δ)x(KN-BNN), many studies indicate that composite modification is an effective technique of simultaneously achieving ferroelectricity and low band gaps [5]. Green lead-free ferroelectric materials, especially Bi0.5Na0.5TiO3based materials, have been extensively developed and studied for their good ferroelectric properties [6]. Bi0.5Na0.5TiO3 materials exhibit a high Curie temperature of ~320 � C and a strong ferroelectric polarization value (P) of ~38 μC/cm2 [7,8]. However, conventional ABO3-type perovskite ferroelectrics are insulator or high-band-gap semiconductors
with band gaps greater than 3.0 eV. (e.g., Bi0.5Na0.5TiO3 materials have an optical band gap of 3.00–3.20 eV) [9,10]. Therefore, light absorption is primarily in the ultraviolet (UV) region, and thus further development is limited. Therefore, reducing the optical band gap of materials is the key to successful application in electronic devices. Ilmenite materials have a common formula of MTiO3, where M is the transition metal (M ¼ Fe, Co, Ni, Mn). The literature about the properties study of ilmenite materials influencing Bi0.5Na0.5TiO3 materials as a solid solution has rarely been reported. In recent years, NiTiO3, a widely known type of Ti-based ilmenite (MTiO3), has become a key research topic in basic physics and potential technology applications. Nickel titanate is a trigonal crystal system with an ilmenite structure. Ti and Ni atoms are spaced in the cationic layer of octahedral coordination. NiTiO3 is a bright candidate with a narrow band gap; it is an n-type semiconductor with a band gap of approximately 2.18 eV [11,12]. Despite the current theoretical and practical studies on Bi0.5Na0.5TiO3 materials, only a few works have been reported on the changes in the structure and in the optical and electrical properties of Bi0.5Na0.5TiO3 by a NiTiO3 dopant. Therefore, this work is the first to fabricate a NiTiO3-doped Bi0.5Na0.5TiO3 ferroelectric semiconductor material as a solid solution
* Corresponding author. College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, PR China. ** Corresponding author. College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, PR China. E-mail addresses: [email protected], [email protected] (C. Yuan), [email protected] (G. Rao). https://doi.org/10.1016/j.mssp.2020.105089 Received 4 January 2020; Received in revised form 8 March 2020; Accepted 23 March 2020 Available online 6 April 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1 . (a) XRD patterns of (1-x)BNT-xNTO (x ¼ 0.02–0.10) ceramics; (b) magnification of the XRD patterns (31� –34� ).
by applying the traditional solid-state reaction method. The optical band gap of the system is reduced to a low value, and strong ferroelectric properties are retained in the NiTiO3-doped Bi0.5Na0.5TiO3 materials.
using yttria-stabilized ZrO2 balls. The mixture was dried at 100 � C and then calcined in an aluminum oxide crucible at 850 � C for 2 h. The reacted powders were mixed with 5% polyvinyl alcohol (PVA) as a binder. The mixtures were ground into fine materials and pressed into a cylindrical disc with a diameter of 12.0 mm and a thickness of 1.0 mm under 2 MPa pressure. The PVA was burned at 600 � C for 2 h, and the mixture was directly sintered at 1125 � C for 2 h. Finally, silver electrodes were painted on the disc surfaces and fired at 590 � C for 30 min.
2 . Experimental procedures 2.1 . Preparation process (1-x)BNT-xNTO ceramics with x ¼ 0.02, 0.04, 0.06, 0.08, 0.10 were prepared by the conventional solid-state reaction. Oxides and carbonate powders such as Na2CO3 (>99%), NiO (>98%), Bi2O3 (>99%), and TiO2 (>99%), were used as raw materials. These powders were each dried in an oven at 100 � C for 24 h before being weighed. The raw powders were ball-milled for 24 h in ZrO2 pots, with ethanol as the mixing medium,
2.2 . Characterization The crystallographic structure of the ceramics was characterized using X-ray diffraction (XRD, Bruker, D8-2-Advance) with Cu Kα1 radi ation. Raman spectra were recorded on polished sintered pellets at 532
Fig. 2 . (a) Room temperature Raman spectra of (1-x)BNT-xNTO ceramics; (b) Raman deconvolution of x ¼ 0.02 and 0.08 samples. 2
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Fig. 3 . (a) Polarization hysteresis loops, (b) current-electric field loops and (c) bipolar strain curves of the (1-x)BNT-xNTO ceramics; (d) variations in Pmax, Smax and Imax.
nm excitation with a Jobin–Yvon LabRam HR800 (Horiba Jobin–Yvon Inc., Paris, France). Chemical valence was determined by using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB250Xi, USA). The strain electric field (S–E) and polarization electric field (P–E) in a silicone oil bath were measured by using a ferroelectric measuring system (TF Analyzer 2000HS, aixACCT Systems GmbH, Aachen, Germany) coupled with a fiber-optic sensor (MTI-2100, MTI Instruments Inc., American). A UV–visible–near infrared (UV–vis–NIR) spectrophotometer (Lambda 750S, PerkinElmer, USA) was used to measure the optical absorption data of the samples.
in Fig. 2(b). The deconvoluted individual components of the total Raman spectrum are located at 60, 130, 274, 521, 580, 715 and 859 cm 1. The Raman peaks correspond to four main regions. The first re gion at 40–200 cm 1 is controlled by the A1 mode assigned to the change in the A-site cation, and it is sensitive to phase transitions in the change in A-site symmetry. The Raman active mode is closely associated to distort octahedral [BiO6] and [NaO6] clusters in this range [16,17]. Above 200 cm 1, the bands are mainly associated with Ti-O stretching vibrations (analogous to the BO6 octahedron). In particular, the 274 cm 1 mode involves O-Ti-O bending motion only, which is also sensitive to phase transitions [18,19]. The next mode is associated with the vi bration of the BO6 octahedron around 450–700 cm 1. Near 580 cm 1, the TO3 mode is associated with the (-O-Ti-O-) tensile symmetric vi bration of octahedral [TiO6] clusters [20]. Finally, the region above 700 cm 1 indicates the overlap of the longitudinal optical of A1 (longitudinal optics) and E (longitudinal optics). The vibration mode and the position of the NiO6 octahedron near 715 cm 1 are strongly correlated. The LO3 mode is due to the position of the distorted octahedral TiO6 clusters in the rhombohedral lattice near 859 cm 1 [21]. Therefore, the XRD and Raman analysis results are consistent with the reported solubility of NiTiO3 in Bi0.5Na0.5TiO3 materials. The polarization-electric field hysteresis loops of the (1-x)BNT-xNTO ceramics with varying Ni contents, measured at 1 Hz and 100 kV/cm, are shown in Fig. 3(a). The corresponding polarization current and bi polar strain curves are illustrated in Fig. 3(b) and (c), respectively. The maximum polarization (Pmax) values, maximum current (Imax), and maximum strain Smax (Smax is the difference between the maximum and minimum strains) of the (1-x)BNT-xNTO ceramics, concluded from the P-E loops, I-E loops and S-E loops, are depicted in Fig. 3(d). All samples exhibit normal polarization-electric field hysteresis loops, and the bi polar strain curve shows a symmetric butterfly strain hysteresis curve and a large negative strain, which are typical features of ferroelectric/
3 . Results and discussion Fig. 1 shows the XRD patterns of (1-x)BNT-xNTO ceramics with different NiTiO3 contents (0.02–0.1) in a Bragg angle (2θ) range of 20� –80� . The XRD patterns show that the main phase is perovskite structure (according to JCPDS card No.46–0001), and no noticeable secondary phase is observed. Moreover, all the ceramics have a rhom bohedral crystal structure at room temperature. This finding indicates that NiTiO3 can be used as a solid solution in Bi0.5Na0.5TiO3 materials [13]. The XRD patterns are magnified within the 2θ range of 31� –34� in Fig. 2(b). The (101) settle peak position shifts to a high angle, thereby compressing the lattice parameter. However, the Ni ions in perovskites have various ionic states, such as Ni2þ and Ni3þ [14]. The ionic radii of Ni2þ and Ni3þ are 0.69 and 0.58 Å, respectively, which are considerably smaller than those of Naþ and Bi3þ (1.18 and 1.17 Å, respectively) [15]. Therefore, in our study, the peak position of the Bi0.5Na0.5TiO3 materials shifts to high angles with an increased concentration of NiTiO3. Fig. 2(a) shows the Raman spectra of the NiTiO3-doped Bi0.5Na0.5 TiO3 obtained at room temperature in a wide wave number range of 40–1000 cm 1. The measured spectra (x ¼ 0.02 and 0.08) were fitted by using Gaussian peak functions (using the software Origin 8.0), as shown 3
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rhombohedral (R) phase to the tetragonal (T) phase. The broad peak is similar with Curie temperature, which is expressed as Tm. The dielectric loss shows obscure peaks above Tm. Both the maximum temperature Tm exhibit opposite variation trends with maximum permittivity εm versus x, as shown in the inset of Fig. 4. Tm initially increases with the Ni content, reaches the maximum at x ¼ 0.06, and finally decreases with a continuous increase in x. This finding shows the relaxor behavior due to the increase of the rhombohedral–tetragonal (R–T) transition in correspondence. The relaxor behavior is characterized to describe quantitatively the relaxor behavior of the diffusion phase transition of the (1-x)BNT-xNTO system. Fig. 5(a–e) present the reciprocal permittivity values of the (1-x) BNT-xNTO ceramics with temperature. The ε values above Tc will follow the Curie-Weiss law for a normal ferroelectric material, as described by [25]. 1
ε
Fig. 4 . Temperature dependence of εr and tanδ of (1-x)BNT-xNTO ceramics at 200 kHz. The inset shows variations in εm and Tm.
¼
T
T0 C
;
(1)
where C is the Curie–Weiss constant, which reflects the ferroelectric phase transition properties [26], and T0 is the Curie–Weiss temperature. ΔT ¼ Tm-T06¼0 can also exclude the possibility of a second-order ferro electric transition [27]. From Fig. 5(a–e), the εr values of the ceramics clearly deviate from the Curie-Weiss law above the Curie temperature, and the deviation ΔTm is can be defined as [25].
piezoelectric materials (rhombohedral [R] and tetragonal [T] phases) [22,23]. The x ¼ 0.06 composition exhibit a reduced maximum strain Smax with a high current Imax and maximum polarization Pmax. The current peaks Imax in the I-E loops can be attributed to the ferroelectric domain switch in the coercive field. Moreover, the Ec values increase gradually with the addition of Ni, as shown in Fig. 3(a) [24]. An enhanced clamping effect in the domain wall motion, which causes difficulties in achieving a saturated polarization state, increases the coercive field. Fig. 4 shows the relative permittivity (εr) and dielectric loss (tanδ) of the (1-x)BNT-xNTO ceramics and its variation with temperature at 200 kHz. The measured temperature range is 30 � C–500 � C. All samples show a dielectric anomaly, resulting from the transition from the
ΔTm ¼ Tcw-Tm,
(2)
where Tcw is the temperature at which the dielectric constant begins to deviate from the Curie–Weiss law, and Tm is the temperature at which εr assumes its maximum value. The ideal linear fittings above Tcw are obtained and the best fitting parameters are listed in Table 1. For the (1x)BNT-xNTO ceramics, ΔTm decreases gradually with the change in the x values. In general, the phase transition diffusion of a relaxed ferro electric with a diffuse phase transition follows a modified Curie–Weiss
Fig. 5 . (a)–(e) Inverse permittivity as a function of temperature for (1-x)BNT-xNTO ceramics as a function of x ¼ 0.02–0.10; (f1–f5) plot of log(1/εr-1/εm) as a function of log(T-Tm) for (1-x)BNT-xNTO ceramics as a function of x ¼ 0.02–0.10. 4
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between log(1/εr-1/εm) and log(T-Tm) are plotted in Fig. 5(f1–f5). A good linearity is observed for all compositions. The calculated γ values of the (1-x)BNT-xNTO ceramics are in the range of 1.571–1.732, which con firms the relaxor behavior. It can be seen that γ decreases gradually from 1.732 to 1.571 with the addition of NiTiO3, implying that the (1-x) BNT-xNTO ceramics change from relaxor ferroelectrics to typical ferroelectrics. XPS measurement was performed to study the chemical state of the selected composition with x ¼ 0.06 (i.e., 0.94BNT-0.06NTO). The XPS spectra of Bi 4f, Na 1s, Ni 2p, Ti 2p, and O 1s of all the tested BNT ce ramics are shown in Fig. 6(a–f). The C 1s peak is at 284.6 eV and used as a reference standard in the XPS study. Fig. 6(b) shows that the binding energies of the Ni 2p3/2, Ni 2p3/2 state, Ni 2p1/2, and Ni 2p1/2 satellite are 853.3, 854.4, 860.4, 864.1, 871.9, and 879.1 eV, respectively, which are close to previously reported values [29]. This finding also supports the conclusion that Ni ions enter the cation vacancies in the A-site of BNT perovskite structures (i.e., the Ni ions is in þ2 valence state) [30, 31]. Fig. 6(c) shows that the binding energy of Na 1s in the 0.94BNT-0.06NTO ceramics is 1710.4 eV. The locations of the Ti 2p1/2 spectrum at 461.1 eV are attributed to the Ti4þ oxidation state, and the binding energies at 455.3 and 463.0 eV correspond to Ti3þ in Fig. 6(e)
Table 1 Characteristic parameters of Curie-Weiss of (1-x)BNT-xNTO ceramics at 200 kHz. Composition
T0(� C)
C� 105(� C)
Tcw(� C)
Tm(� C)
ΔTm ¼ TcwTm(� C)
γ
x ¼ 0.02 x ¼ 0.04 x ¼ 0.06 x ¼ 0.08 x ¼ 0.10
230 217 223 40 12
2.403 2.592 3.398 8.829 8.604
371 361 355.2 348.3 336
325 325 333.2 328.3 324
36 26 22 20 12
1.732 1.715 1.635 1.579 1.571
law as depicted by [28]. 1
1
ε
εm
¼
ðT
Tm Þγ ; C
(3)
where εr and γ (1 � γ � 2) represent the maximum relative permittivity and diffusion coefficients, respectively. Usually, γ ¼ 1 and γ ¼ 2 repre sent the Curie-Weiss behavior of normal ferroelectrics and typical relaxor ferroelectrics, respectively [28]. On the basis of the εr-T curves of the (1-x)BNT-xNTO ceramics with 0.02 � x � 0.10, the relations
Fig. 6 . XPS spectra of 0.94BNT-0.06NTO ceramic: (a) survey scan, (b) Ni 2p, (c) Na 1s, (d) Bi 4f, (e) Ti 2p, and (f) O 1s. 5
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Fig. 7 . Band gap (Eg) determination of (1-x)BNT-xNTO with absorption data and Tauc’s relations. The inset of Fig. 7 shows UV–vis–NIR absorption spectra of (1-x) BNT-xNTO ceramics.
[32]. Fig. 6(f) presents the O 1s peaks of all the measured BNT ceramics. The peaks at 527.2, 528.7, and 529.9 eV, belong to oxygen in the perovskite lattice [33]. Fig. 6(d) shows that the corresponding peaks for metallic Bi (0) and Bi (þ3) in BNT are 156.4 and 161.5 eV, respectively [34,35]. The raw material of bismuth is Bi2O3, so bismuth is initially measured as þ3 and is partially converted to 0 valence after the (and/or period) sintering stage. Therefore, the chemical formula for this ceramic can be written as 0.94 [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ,Ti4þ]O3-δ-0.06Ni[Ti3þ, Ti4þ]O3-δ’. According to the fitting result, the area ratio of Bi3þ and Bi is approximately 4:5, and the area ratio of Ti4þand Ti3þ is approximately 1:2. The chemical formula is thus determined to be 0.94 [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ,Ti4þ]O2.25-0.06Ni[Ti3þ,Ti4þ]O2.67 [36]. The optical properties of the thick ceramics were studied by using the UV–vis–NIR spectra. All the Ni-mediated samples show light absorption from visible light range(the absorbance data is shown in the inset of Fig. 7), even reaching the near-infrared (NIR) range (λ < 1500 nm), as shown in the inset of Fig. 7; this result has never been observed/reported for metal-halide perovskite materials [37]. The optical band gap values (Eg) of the NiTiO3-doped Bi0.5Na0.5TiO3 samples were obtained by using the Kubelka-Tauc method, and the equation is (αhν)n ¼ A(hν-Eg), where n represents the constants of different types of electronic transitions (n ¼ 1/2 and 2 denote the indirect and direct band gaps, respectively) [38]. According to theoretical prediction, Bi0.5Na0.5TiO3 materials show a direct transition [39]. Therefore, we used n ¼ 2 to explain the Eg values of the NiTiO3-doped Bi0.5Na0.5TiO3 materials. Fig. 7 shows the band gap of the (1-x)BNT-xNTO (0.02 � x � 0.10) ceramics. The (1-x)BNT-xNTO series have three absorption edges (~1900, ~1500, and ~700 nm), which correspond to three gap states, as shown in Fig. 7. These ab sorption edges exhibit a red shift with increasing Ni concentration; this shift should be related to the random distribution of polyvalent Ni ions at octahedral site and/or promotion from oxygen vacancies. Hence, all the Ni2þ-mediated samples show similarly large variations among the three gap states. The two left band gaps are in the NIR region, and the right band gaps are in the UV vis region. In this region, the NiTiO3-doped
Bi0.5Na0.5TiO3 materials show band-gaps of 2.11, 2.10, 2.04, 1.99, and 1.98 eV for x ¼ 0.02-0.10, respectively. Therefore, proper doping of NiTiO3 can significantly reduce the band gap and improve optical absorption. In the present work, the 0.94BNT-0.06NTO ceramics have a rela tively high polarization and a small band gap. Tentative photovoltaic measurements on the 0.94BNT-0.06NTO ceramics and the obtained re sults are shown in Fig. 8(a). As shown in Fig. 8(b), a significant photo voltaic response was studied by using standard AM1.5 illumination (100 mW/cm2) in the as-fabricated samples. Under this condition, the short circuit-current (Jsc) is 1.36 nA/cm2 and the open-circuit voltaic (Voc) is 0.35 V. Then, we measured Voc and Jsc at standard AM1.5 illu mination after 3.0 kV poling for 10 min. The poling curves are appar ently better than unpolarized curves under the same conditions. The measured Voc and Jsc are approximately 5.11 nA/cm2 and 0.44 V, respectively, which verify the ferroelectricity of the 0.94BNT-0.06NTO ceramics. A stable and remarkable light-dark response with a maximum current density of ~1.36 nA/cm2 can be obtained while turning on/off the light in Fig. 8(b). Furthermore, the samples exhibits a low residual Jsc and a relatively high Voc. Ceramics have many grain boundaries and pores, which can hinder the migration of photo generated carriers and thus reduce the photocurrent [40]. 4 . Conclusions In this study, (1-x)BNT-xNTO (x ¼ 0.02, 0.04, 0.06, 0.08, 0.10) ce ramics were prepared through traditional solid-state reactions. The structure, relaxation behaviors, ferroelectric properties, and optical properties of the (1-x)BNT-xNTO ceramics were investigated. The main phase of the perovskite structure in the (1-x)BNT-xNTO ceramics was observed by XRD. The (1-x)BNT-xNTO ceramics become normal ferro electrics gradually with the decreasing diffuse factor γ from 1.732 to 1.571. The optical band gap also decreases gradually from 2.11 eV to 1.98 eV for x ¼ 0.02-0.10, which is shows a typical characteristic of 6
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CRediT authorship contribution statement Zexing Chen: Investigation, Writing - original draft. Changlai Yuan: Conceptualization, Methodology, Funding acquisition, Data curation, Writing - review & editing. Xiao Liu: Validation. Liufang Meng: Validation, Resources. Shuai Cheng: Resources, Supervision. Jiwen Xu: Writing - review & editing, Resources. Changrong Zhou: Supervision. Jiang Wang: Writing - review & editing. Acknowledgements This study was financially supported by Guangxi Natural Science Foundation (Grants No. 2018GXNSFAA294039) and Guangxi Key Lab oratory of Information Materials (Grants No. 151003-Z). Appendix A . Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.105089. References [1] A. Bhatnagar, A.R. Chaudhuri, Y.H. Kim, D. Hesse, M. Alexe, Role of domain walls in the abnormal photovoltaic effect in BiFeO3, Nat. Commun. 4 (2013) 1–8, 365391. [2] S.Y. Yang, J. Seidel, S.J. Byrnes, P. Shafer, C.H. Yang, M.D. Rossell, P. Yu, Y. H. Chu, J.F. Scott, J.W. Ager, Above-bandgap voltages from ferroelectric photovoltaic devices, Nat. Nanotechnol. 5 (2010) 143–147. [3] Z.X. Wang, C.L. Yuan, B.H. Zhu, Q. Feng, F. Liu, J.W. Xu, C.R. Zhou, G.H. Chen, Complex impedance spectroscopy of perovskite microwave dielectric ceramics with high dielectric constant, J. Am. Ceram. Soc. 102 (2019) 1852–1865. [4] C.S. Tu, C.M. Hung, V.H. Schmidt, R.R. Chien, M.D. Jiang, J. Anthoninappen, The origin of photovoltaic responses in BiFeO3 multiferroic ceramics, J. Phys. Condens. Matter 24 (2012) 1–6, 495902. [5] I. Grinberg, D.V. West, M. Torres, G. Gou, D.M. Stein, L. Wu, G. Chen, E.M. Gallo, A.R. Akbashev, P.K. Davies, Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials, Nature 503 (2013) 509–512. [6] N.D. Quan, L.H. Bac, D.V. Thiet, V.N. Hung, D.D. Dang, Current development in lead-free Bi0.5(Na,K)0.5TiO3-based piezoelectric, Ann. Mater. Sci. Eng. 2014 (2014) 1–13, 365391. [7] Y. Hiruma, H. Nagata, T. Takenaka, Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics, J. Appl. Phys. 105 (2009) 1–9, 084112. [8] D.K. Lee, H. Vu, J.G. Fisher, Growth of (Na0.5Bi0.5)TiO3-Ba(Ti1-xZrx)O3 single crystals by solid state single crystal growth, J. Electroceram. 34 (2015) 150–157. [9] D.D. Dung, D.V. Thiet, D. Odkhuu, L.V. Cuong, N.H. Tuan, S. Cho, Roomtemperature ferromagnetism in Fe-doped wide band gap ferroelectric Bi0.5K0.5TiO3 nanocrystals, Mater. Lett. 156 (2015) 129–133. [10] L.T.H. Thanh, N.B. Doan, N.Q. Dung, L.V. Cuong, L.H. Bac, N.A. Duc, P.Q. Bao, D. D. Dung, Origin of room temperature ferromagnetism in Cr-doped lead-free ferroelectric Bi0.5Na0.5TiO3 materials, J. Electron. Mater. 46 (2017) 3367–3372. [11] M.A. Ruiz-Preciado, A. Kassiba, A. Gibaud, A. Morales-Acevedo, Comparison of nickel titanate (NiTiO3) powders synthesized by sol-gel and solid state reaction, Mater. Sci. Semicond. Process. 37 (2015) 171–178. [12] T.T. Pham, S.G. Kang, E.W. Shin, Optical and structural properties of Mo-doped NiTiO3 materials synthesized via modified Pechini methods, Appl. Surf. Sci. 411 (2017) 18–26. [13] G. Shirane, S.J. Pickart, Y. Ishikawa, Neutron diffraction study of antiferromagnetic MnTiO3 and NiTiO3, J. Phys. Soc. Jpn. 14 (1959) 1352–1360. [14] N.H. Tuan, D.V. Thiet, D. Odkhuu, L.H. Bac, P.V. Binh, D.D. Dung, Defect induced room temperature ferromagnetism in lead-free ferroelectric Bi0.5K0.5TiO3 materials, Phys. B Condens. Matter 532 (2018) 108–114. [15] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A: Found. of Crystallogr. 32 (1976) 751–767. [16] H. Xie, Y.Y. Zhao, J.W. Xu, L. Yang, C.R. Zhou, H.B. Zhang, X.W. Zhang, W. Qiu, H. Wang, Structure, dielectric, ferroelectric, and field-induced strain response properties of (Mg1/3Nb2/3)4þ complex-ion modified Bi0.5(Na0.82K0.18)0.5TiO3 leadfree ceramics, J. Alloys Compd. 743 (2018) 73–82. [17] N.V. Prasad, V.S. Puli, D.K. Pradhan, S.M. Gupta, G. Prasad, R.S. Katiyar, G. S. Kumar, Impedance and Raman spectroscopic studies on La-modified BLSF ceramics, Ferroelectrics 474 (2015) 29–42. [18] J. Kreisel, A.M. Glazer, P. Bouvier, G. Lucazeau, High-pressure Raman study of a relaxor ferroelectric: the Na0.5Bi0.5TiO3 perovskite, Phys. Rev. B 17 (2001) 1–10, 174106. [19] M.K. Niranjan, T. Karthik, S. Asthana, J. Pan, U.V. Waghmare, Theoretical and experimental investigation of Raman modes, ferroelectric and dielectric properties of relaxor Na0.5Bi0.5TiO3, J. Appl. Phys. 113 (2013) 1–8, 194106.
Fig. 8 . (a) Current-voltage (J–V) curves of the 3.0 kV polarization/unpoled 0.94BNT-0.06NTO ceramic under simulated AM 1.5 sunlight of 100 mW/cm2 irradiance, and in the dark; (b) typical light–dark response photocurrent density of 0.94BNT-0.06NTO.
semiconductors. Moreover, a high polarization value and low permit tivity are observed in the ceramic with x ¼ 0.06. The Ni ions at x ¼ 0.06 are in þ2 valence state and the normal chemical formula is [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ,Ti4þ]O2.25-0.06Ni[Ti3þ,Ti4þ]O2.67, as determined by XPS measurements. The light photovoltaic response was studied by standard AM1.5 illumination. The short circuit-current (Jsc) is 1.36 nA/ cm2 and the open-circuit voltaic (Voc) is 0.35 V and the photovoltaic effects was improved after 3.0 kV poling. These findings are helpful in understanding the properties, including electrical and optical proper ties, of perovskite ferroelectric semiconductor ceramics. Declaration of competing interest The follow is the declaration that we would like to state: 1. I confirm that this manuscript is the author’s original work; 2. The article has been written by the stated authors who are all aware of its content and approve its submission; 3. The article has not been published previously; 4. The article is not under consideration for publication elsewhere; 5. No conflict of interest exists; 6. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher. 7
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[30] H.W. Nesbitt, D. Legrand, G.M. Bancroft, Interpretation of Ni2p XPS spectra of Ni conductors and Ni insulators, Phys. Chem. Miner. 27 (2000) 357–366. [31] D. Leinen, A. Fern� andez, J.P. Espin� os, A.R. Gonz� alez-Elipe, XPS and ISS study of NiTiO3 and PbTiO3 subjected to low-energy ion bombardment. I. Influence of the type of ion (Arþ and O2þ), Surf. Interface Anal. 20 (1993) 941–948. [32] B. Inceesungvorn, T. Teeranunpong, J. Nunkaew, S. Suntalelat, D. Tantraviwat, Novel NiTiO3/Ag3VO4 composite with enhanced photocatalytic performance under visible light, Catal. Commun. 54 (2014) 35–38. [33] X. Xu, Y. Xie, N. Shuang, A.K. Azad, T. Cao, Photocatalytic H2 production from spinels ZnGa2-xCrxO4 (0�x�2) solid solutions, J. Solid State Chem. 230 (2015) 95–101. [34] N. Ertas, O.Y. Ataman, S. Suzer, XPS characterization of Bi and Mn collected on atom-trapping silica for AAS, Appl. Spectrosc. 53 (1999) 479–482. [35] G. Zhang, J. Yang, S. Zhang, Q. Xiong, B. Huang, J. Wang, W. Gong, Preparation of nanosized Bi3NbO7 and its visible-light photocatalytic property, J. Hazard Mater. 172 (2009) 986–992. [36] L.F. Meng, C.L. Yuan, Z.X. Wang, Q. Feng, J.W. Xu, C.R. Zhou, G.H. Chen, Microwave dielectric properties of Sr0.7Ce0.2TiO3-Sr(Mg1/3Nb2/3)O3 ceramics, J. Mater. Sci. Mater. Electron. 29 (2018) 2668–2675. [37] M.A. Green, H.-B. Anita, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photon. 8 (2014) 506–514. [38] D.L. Wood, J. Tauc, Weak absorption tails in amorphous semiconductors, Phys. Rev. B 5 (1972) 3144–3151. [39] M. Zeng, S.W. Or, H.L.W. Chan, Giant resonance frequency tunable magnetoelectric effect in a device of Pb(Zr0.52Ti0.48)O3 drum transducer, NdFeB magnet, and Fe-core solenoid, Appl. Phys. Lett. 96 (2010) 1–4, 203502. [40] S.Y. Yang, J. Seidel, S.J. Byrnes, P. Shafer, C.-H. Yang, M.D. Rossell, P. Yu, Y.H. Chu, J.F. Scott, J.W. Ager, Above-bandgap voltages from ferroelectric photovoltaic devices, Nat. Nanotechnol. 5 (2010) 143–147.
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Optical and electrical properties of ferroelectric Bi0.5Na0.5TiO3-NiTiO3 semiconductor ceramics Zexing Chen a, Changlai Yuan a, b, *, Xiao Liu a, b, Liufang Meng a, Shuai Cheng a, b, Jiwen Xu a, b, Changrong Zhou a, b, Jiang Wang a, b, Guanghui Rao a, b, c, ** a b c
College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, PR China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin, 541004, PR China Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: (1-x)BNT-xNTO Ferroelectric semiconductors Ferroelectric properties Optical band gap
(1-x)Bi0.5Na0.5TiO3-xNiTiO3 semiconducting ferroelectric ceramics (named (1-x)BNT-xNTO) were successfully prepared by using a solid state reaction method. All samples shows normal polarization-electric field hysteresis loops, and a maximum polarization Pmax is obtained in the x ¼ 0.06 composition. The diffuse factor γ decreases slightly from 1.732 to 1.571 with the addition of NiTiO3. In addition, Ni doping can substantially reduce the band gap of Bi0.5Na0.5TiO3 materials to ~2 eV. In a 0.94BNT–0.06NTO compound, the XPS spectra shows that the chemical formula is accurately [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ, Ti4þ]O2.25-0.06Ni[Ti3þ, Ti4þ]O2.67. Moreover, the short-circuit current (Jsc) in the composition is 1.36 nA/cm2, and the open-circuit voltaic (Voc) is 0.35 V for photovoltaic effects. The novel BNT system, which illustrates three narrow band gaps and relatively high po larization values, has broad application prospects for ferroelectric photovoltaic devices.
1 . Introduction Semiconductor ferroelectric ceramics with ABO3-type perovskite structure have been attracting considerable attention because of interest in their crystal structural behavior [1–3]. Researchers have been attempting to improve the ferroelectric and optical properties of perovskite-based oxides. The real potential of semiconductor ferroelec tric ceramics in photovoltaic applications has greatly promoted the exploration of new perovskite oxides with efficient visible light ab sorption [4]. Recently, with the realization of defect-driven ferroelec tricity and low bandgap states (~1.1 eV) in perovskites (KNbO3)1 x(BaNi0.5Nb0.5O3 δ)x(KN-BNN), many studies indicate that composite modification is an effective technique of simultaneously achieving ferroelectricity and low band gaps [5]. Green lead-free ferroelectric materials, especially Bi0.5Na0.5TiO3based materials, have been extensively developed and studied for their good ferroelectric properties [6]. Bi0.5Na0.5TiO3 materials exhibit a high Curie temperature of ~320 � C and a strong ferroelectric polarization value (P) of ~38 μC/cm2 [7,8]. However, conventional ABO3-type perovskite ferroelectrics are insulator or high-band-gap semiconductors
with band gaps greater than 3.0 eV. (e.g., Bi0.5Na0.5TiO3 materials have an optical band gap of 3.00–3.20 eV) [9,10]. Therefore, light absorption is primarily in the ultraviolet (UV) region, and thus further development is limited. Therefore, reducing the optical band gap of materials is the key to successful application in electronic devices. Ilmenite materials have a common formula of MTiO3, where M is the transition metal (M ¼ Fe, Co, Ni, Mn). The literature about the properties study of ilmenite materials influencing Bi0.5Na0.5TiO3 materials as a solid solution has rarely been reported. In recent years, NiTiO3, a widely known type of Ti-based ilmenite (MTiO3), has become a key research topic in basic physics and potential technology applications. Nickel titanate is a trigonal crystal system with an ilmenite structure. Ti and Ni atoms are spaced in the cationic layer of octahedral coordination. NiTiO3 is a bright candidate with a narrow band gap; it is an n-type semiconductor with a band gap of approximately 2.18 eV [11,12]. Despite the current theoretical and practical studies on Bi0.5Na0.5TiO3 materials, only a few works have been reported on the changes in the structure and in the optical and electrical properties of Bi0.5Na0.5TiO3 by a NiTiO3 dopant. Therefore, this work is the first to fabricate a NiTiO3-doped Bi0.5Na0.5TiO3 ferroelectric semiconductor material as a solid solution
* Corresponding author. College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, PR China. ** Corresponding author. College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, PR China. E-mail addresses: [email protected], [email protected] (C. Yuan), [email protected] (G. Rao). https://doi.org/10.1016/j.mssp.2020.105089 Received 4 January 2020; Received in revised form 8 March 2020; Accepted 23 March 2020 Available online 6 April 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1 . (a) XRD patterns of (1-x)BNT-xNTO (x ¼ 0.02–0.10) ceramics; (b) magnification of the XRD patterns (31� –34� ).
by applying the traditional solid-state reaction method. The optical band gap of the system is reduced to a low value, and strong ferroelectric properties are retained in the NiTiO3-doped Bi0.5Na0.5TiO3 materials.
using yttria-stabilized ZrO2 balls. The mixture was dried at 100 � C and then calcined in an aluminum oxide crucible at 850 � C for 2 h. The reacted powders were mixed with 5% polyvinyl alcohol (PVA) as a binder. The mixtures were ground into fine materials and pressed into a cylindrical disc with a diameter of 12.0 mm and a thickness of 1.0 mm under 2 MPa pressure. The PVA was burned at 600 � C for 2 h, and the mixture was directly sintered at 1125 � C for 2 h. Finally, silver electrodes were painted on the disc surfaces and fired at 590 � C for 30 min.
2 . Experimental procedures 2.1 . Preparation process (1-x)BNT-xNTO ceramics with x ¼ 0.02, 0.04, 0.06, 0.08, 0.10 were prepared by the conventional solid-state reaction. Oxides and carbonate powders such as Na2CO3 (>99%), NiO (>98%), Bi2O3 (>99%), and TiO2 (>99%), were used as raw materials. These powders were each dried in an oven at 100 � C for 24 h before being weighed. The raw powders were ball-milled for 24 h in ZrO2 pots, with ethanol as the mixing medium,
2.2 . Characterization The crystallographic structure of the ceramics was characterized using X-ray diffraction (XRD, Bruker, D8-2-Advance) with Cu Kα1 radi ation. Raman spectra were recorded on polished sintered pellets at 532
Fig. 2 . (a) Room temperature Raman spectra of (1-x)BNT-xNTO ceramics; (b) Raman deconvolution of x ¼ 0.02 and 0.08 samples. 2
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Fig. 3 . (a) Polarization hysteresis loops, (b) current-electric field loops and (c) bipolar strain curves of the (1-x)BNT-xNTO ceramics; (d) variations in Pmax, Smax and Imax.
nm excitation with a Jobin–Yvon LabRam HR800 (Horiba Jobin–Yvon Inc., Paris, France). Chemical valence was determined by using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB250Xi, USA). The strain electric field (S–E) and polarization electric field (P–E) in a silicone oil bath were measured by using a ferroelectric measuring system (TF Analyzer 2000HS, aixACCT Systems GmbH, Aachen, Germany) coupled with a fiber-optic sensor (MTI-2100, MTI Instruments Inc., American). A UV–visible–near infrared (UV–vis–NIR) spectrophotometer (Lambda 750S, PerkinElmer, USA) was used to measure the optical absorption data of the samples.
in Fig. 2(b). The deconvoluted individual components of the total Raman spectrum are located at 60, 130, 274, 521, 580, 715 and 859 cm 1. The Raman peaks correspond to four main regions. The first re gion at 40–200 cm 1 is controlled by the A1 mode assigned to the change in the A-site cation, and it is sensitive to phase transitions in the change in A-site symmetry. The Raman active mode is closely associated to distort octahedral [BiO6] and [NaO6] clusters in this range [16,17]. Above 200 cm 1, the bands are mainly associated with Ti-O stretching vibrations (analogous to the BO6 octahedron). In particular, the 274 cm 1 mode involves O-Ti-O bending motion only, which is also sensitive to phase transitions [18,19]. The next mode is associated with the vi bration of the BO6 octahedron around 450–700 cm 1. Near 580 cm 1, the TO3 mode is associated with the (-O-Ti-O-) tensile symmetric vi bration of octahedral [TiO6] clusters [20]. Finally, the region above 700 cm 1 indicates the overlap of the longitudinal optical of A1 (longitudinal optics) and E (longitudinal optics). The vibration mode and the position of the NiO6 octahedron near 715 cm 1 are strongly correlated. The LO3 mode is due to the position of the distorted octahedral TiO6 clusters in the rhombohedral lattice near 859 cm 1 [21]. Therefore, the XRD and Raman analysis results are consistent with the reported solubility of NiTiO3 in Bi0.5Na0.5TiO3 materials. The polarization-electric field hysteresis loops of the (1-x)BNT-xNTO ceramics with varying Ni contents, measured at 1 Hz and 100 kV/cm, are shown in Fig. 3(a). The corresponding polarization current and bi polar strain curves are illustrated in Fig. 3(b) and (c), respectively. The maximum polarization (Pmax) values, maximum current (Imax), and maximum strain Smax (Smax is the difference between the maximum and minimum strains) of the (1-x)BNT-xNTO ceramics, concluded from the P-E loops, I-E loops and S-E loops, are depicted in Fig. 3(d). All samples exhibit normal polarization-electric field hysteresis loops, and the bi polar strain curve shows a symmetric butterfly strain hysteresis curve and a large negative strain, which are typical features of ferroelectric/
3 . Results and discussion Fig. 1 shows the XRD patterns of (1-x)BNT-xNTO ceramics with different NiTiO3 contents (0.02–0.1) in a Bragg angle (2θ) range of 20� –80� . The XRD patterns show that the main phase is perovskite structure (according to JCPDS card No.46–0001), and no noticeable secondary phase is observed. Moreover, all the ceramics have a rhom bohedral crystal structure at room temperature. This finding indicates that NiTiO3 can be used as a solid solution in Bi0.5Na0.5TiO3 materials [13]. The XRD patterns are magnified within the 2θ range of 31� –34� in Fig. 2(b). The (101) settle peak position shifts to a high angle, thereby compressing the lattice parameter. However, the Ni ions in perovskites have various ionic states, such as Ni2þ and Ni3þ [14]. The ionic radii of Ni2þ and Ni3þ are 0.69 and 0.58 Å, respectively, which are considerably smaller than those of Naþ and Bi3þ (1.18 and 1.17 Å, respectively) [15]. Therefore, in our study, the peak position of the Bi0.5Na0.5TiO3 materials shifts to high angles with an increased concentration of NiTiO3. Fig. 2(a) shows the Raman spectra of the NiTiO3-doped Bi0.5Na0.5 TiO3 obtained at room temperature in a wide wave number range of 40–1000 cm 1. The measured spectra (x ¼ 0.02 and 0.08) were fitted by using Gaussian peak functions (using the software Origin 8.0), as shown 3
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rhombohedral (R) phase to the tetragonal (T) phase. The broad peak is similar with Curie temperature, which is expressed as Tm. The dielectric loss shows obscure peaks above Tm. Both the maximum temperature Tm exhibit opposite variation trends with maximum permittivity εm versus x, as shown in the inset of Fig. 4. Tm initially increases with the Ni content, reaches the maximum at x ¼ 0.06, and finally decreases with a continuous increase in x. This finding shows the relaxor behavior due to the increase of the rhombohedral–tetragonal (R–T) transition in correspondence. The relaxor behavior is characterized to describe quantitatively the relaxor behavior of the diffusion phase transition of the (1-x)BNT-xNTO system. Fig. 5(a–e) present the reciprocal permittivity values of the (1-x) BNT-xNTO ceramics with temperature. The ε values above Tc will follow the Curie-Weiss law for a normal ferroelectric material, as described by [25]. 1
ε
Fig. 4 . Temperature dependence of εr and tanδ of (1-x)BNT-xNTO ceramics at 200 kHz. The inset shows variations in εm and Tm.
¼
T
T0 C
;
(1)
where C is the Curie–Weiss constant, which reflects the ferroelectric phase transition properties [26], and T0 is the Curie–Weiss temperature. ΔT ¼ Tm-T06¼0 can also exclude the possibility of a second-order ferro electric transition [27]. From Fig. 5(a–e), the εr values of the ceramics clearly deviate from the Curie-Weiss law above the Curie temperature, and the deviation ΔTm is can be defined as [25].
piezoelectric materials (rhombohedral [R] and tetragonal [T] phases) [22,23]. The x ¼ 0.06 composition exhibit a reduced maximum strain Smax with a high current Imax and maximum polarization Pmax. The current peaks Imax in the I-E loops can be attributed to the ferroelectric domain switch in the coercive field. Moreover, the Ec values increase gradually with the addition of Ni, as shown in Fig. 3(a) [24]. An enhanced clamping effect in the domain wall motion, which causes difficulties in achieving a saturated polarization state, increases the coercive field. Fig. 4 shows the relative permittivity (εr) and dielectric loss (tanδ) of the (1-x)BNT-xNTO ceramics and its variation with temperature at 200 kHz. The measured temperature range is 30 � C–500 � C. All samples show a dielectric anomaly, resulting from the transition from the
ΔTm ¼ Tcw-Tm,
(2)
where Tcw is the temperature at which the dielectric constant begins to deviate from the Curie–Weiss law, and Tm is the temperature at which εr assumes its maximum value. The ideal linear fittings above Tcw are obtained and the best fitting parameters are listed in Table 1. For the (1x)BNT-xNTO ceramics, ΔTm decreases gradually with the change in the x values. In general, the phase transition diffusion of a relaxed ferro electric with a diffuse phase transition follows a modified Curie–Weiss
Fig. 5 . (a)–(e) Inverse permittivity as a function of temperature for (1-x)BNT-xNTO ceramics as a function of x ¼ 0.02–0.10; (f1–f5) plot of log(1/εr-1/εm) as a function of log(T-Tm) for (1-x)BNT-xNTO ceramics as a function of x ¼ 0.02–0.10. 4
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between log(1/εr-1/εm) and log(T-Tm) are plotted in Fig. 5(f1–f5). A good linearity is observed for all compositions. The calculated γ values of the (1-x)BNT-xNTO ceramics are in the range of 1.571–1.732, which con firms the relaxor behavior. It can be seen that γ decreases gradually from 1.732 to 1.571 with the addition of NiTiO3, implying that the (1-x) BNT-xNTO ceramics change from relaxor ferroelectrics to typical ferroelectrics. XPS measurement was performed to study the chemical state of the selected composition with x ¼ 0.06 (i.e., 0.94BNT-0.06NTO). The XPS spectra of Bi 4f, Na 1s, Ni 2p, Ti 2p, and O 1s of all the tested BNT ce ramics are shown in Fig. 6(a–f). The C 1s peak is at 284.6 eV and used as a reference standard in the XPS study. Fig. 6(b) shows that the binding energies of the Ni 2p3/2, Ni 2p3/2 state, Ni 2p1/2, and Ni 2p1/2 satellite are 853.3, 854.4, 860.4, 864.1, 871.9, and 879.1 eV, respectively, which are close to previously reported values [29]. This finding also supports the conclusion that Ni ions enter the cation vacancies in the A-site of BNT perovskite structures (i.e., the Ni ions is in þ2 valence state) [30, 31]. Fig. 6(c) shows that the binding energy of Na 1s in the 0.94BNT-0.06NTO ceramics is 1710.4 eV. The locations of the Ti 2p1/2 spectrum at 461.1 eV are attributed to the Ti4þ oxidation state, and the binding energies at 455.3 and 463.0 eV correspond to Ti3þ in Fig. 6(e)
Table 1 Characteristic parameters of Curie-Weiss of (1-x)BNT-xNTO ceramics at 200 kHz. Composition
T0(� C)
C� 105(� C)
Tcw(� C)
Tm(� C)
ΔTm ¼ TcwTm(� C)
γ
x ¼ 0.02 x ¼ 0.04 x ¼ 0.06 x ¼ 0.08 x ¼ 0.10
230 217 223 40 12
2.403 2.592 3.398 8.829 8.604
371 361 355.2 348.3 336
325 325 333.2 328.3 324
36 26 22 20 12
1.732 1.715 1.635 1.579 1.571
law as depicted by [28]. 1
1
ε
εm
¼
ðT
Tm Þγ ; C
(3)
where εr and γ (1 � γ � 2) represent the maximum relative permittivity and diffusion coefficients, respectively. Usually, γ ¼ 1 and γ ¼ 2 repre sent the Curie-Weiss behavior of normal ferroelectrics and typical relaxor ferroelectrics, respectively [28]. On the basis of the εr-T curves of the (1-x)BNT-xNTO ceramics with 0.02 � x � 0.10, the relations
Fig. 6 . XPS spectra of 0.94BNT-0.06NTO ceramic: (a) survey scan, (b) Ni 2p, (c) Na 1s, (d) Bi 4f, (e) Ti 2p, and (f) O 1s. 5
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Materials Science in Semiconductor Processing 115 (2020) 105089
Fig. 7 . Band gap (Eg) determination of (1-x)BNT-xNTO with absorption data and Tauc’s relations. The inset of Fig. 7 shows UV–vis–NIR absorption spectra of (1-x) BNT-xNTO ceramics.
[32]. Fig. 6(f) presents the O 1s peaks of all the measured BNT ceramics. The peaks at 527.2, 528.7, and 529.9 eV, belong to oxygen in the perovskite lattice [33]. Fig. 6(d) shows that the corresponding peaks for metallic Bi (0) and Bi (þ3) in BNT are 156.4 and 161.5 eV, respectively [34,35]. The raw material of bismuth is Bi2O3, so bismuth is initially measured as þ3 and is partially converted to 0 valence after the (and/or period) sintering stage. Therefore, the chemical formula for this ceramic can be written as 0.94 [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ,Ti4þ]O3-δ-0.06Ni[Ti3þ, Ti4þ]O3-δ’. According to the fitting result, the area ratio of Bi3þ and Bi is approximately 4:5, and the area ratio of Ti4þand Ti3þ is approximately 1:2. The chemical formula is thus determined to be 0.94 [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ,Ti4þ]O2.25-0.06Ni[Ti3þ,Ti4þ]O2.67 [36]. The optical properties of the thick ceramics were studied by using the UV–vis–NIR spectra. All the Ni-mediated samples show light absorption from visible light range(the absorbance data is shown in the inset of Fig. 7), even reaching the near-infrared (NIR) range (λ < 1500 nm), as shown in the inset of Fig. 7; this result has never been observed/reported for metal-halide perovskite materials [37]. The optical band gap values (Eg) of the NiTiO3-doped Bi0.5Na0.5TiO3 samples were obtained by using the Kubelka-Tauc method, and the equation is (αhν)n ¼ A(hν-Eg), where n represents the constants of different types of electronic transitions (n ¼ 1/2 and 2 denote the indirect and direct band gaps, respectively) [38]. According to theoretical prediction, Bi0.5Na0.5TiO3 materials show a direct transition [39]. Therefore, we used n ¼ 2 to explain the Eg values of the NiTiO3-doped Bi0.5Na0.5TiO3 materials. Fig. 7 shows the band gap of the (1-x)BNT-xNTO (0.02 � x � 0.10) ceramics. The (1-x)BNT-xNTO series have three absorption edges (~1900, ~1500, and ~700 nm), which correspond to three gap states, as shown in Fig. 7. These ab sorption edges exhibit a red shift with increasing Ni concentration; this shift should be related to the random distribution of polyvalent Ni ions at octahedral site and/or promotion from oxygen vacancies. Hence, all the Ni2þ-mediated samples show similarly large variations among the three gap states. The two left band gaps are in the NIR region, and the right band gaps are in the UV vis region. In this region, the NiTiO3-doped
Bi0.5Na0.5TiO3 materials show band-gaps of 2.11, 2.10, 2.04, 1.99, and 1.98 eV for x ¼ 0.02-0.10, respectively. Therefore, proper doping of NiTiO3 can significantly reduce the band gap and improve optical absorption. In the present work, the 0.94BNT-0.06NTO ceramics have a rela tively high polarization and a small band gap. Tentative photovoltaic measurements on the 0.94BNT-0.06NTO ceramics and the obtained re sults are shown in Fig. 8(a). As shown in Fig. 8(b), a significant photo voltaic response was studied by using standard AM1.5 illumination (100 mW/cm2) in the as-fabricated samples. Under this condition, the short circuit-current (Jsc) is 1.36 nA/cm2 and the open-circuit voltaic (Voc) is 0.35 V. Then, we measured Voc and Jsc at standard AM1.5 illu mination after 3.0 kV poling for 10 min. The poling curves are appar ently better than unpolarized curves under the same conditions. The measured Voc and Jsc are approximately 5.11 nA/cm2 and 0.44 V, respectively, which verify the ferroelectricity of the 0.94BNT-0.06NTO ceramics. A stable and remarkable light-dark response with a maximum current density of ~1.36 nA/cm2 can be obtained while turning on/off the light in Fig. 8(b). Furthermore, the samples exhibits a low residual Jsc and a relatively high Voc. Ceramics have many grain boundaries and pores, which can hinder the migration of photo generated carriers and thus reduce the photocurrent [40]. 4 . Conclusions In this study, (1-x)BNT-xNTO (x ¼ 0.02, 0.04, 0.06, 0.08, 0.10) ce ramics were prepared through traditional solid-state reactions. The structure, relaxation behaviors, ferroelectric properties, and optical properties of the (1-x)BNT-xNTO ceramics were investigated. The main phase of the perovskite structure in the (1-x)BNT-xNTO ceramics was observed by XRD. The (1-x)BNT-xNTO ceramics become normal ferro electrics gradually with the decreasing diffuse factor γ from 1.732 to 1.571. The optical band gap also decreases gradually from 2.11 eV to 1.98 eV for x ¼ 0.02-0.10, which is shows a typical characteristic of 6
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CRediT authorship contribution statement Zexing Chen: Investigation, Writing - original draft. Changlai Yuan: Conceptualization, Methodology, Funding acquisition, Data curation, Writing - review & editing. Xiao Liu: Validation. Liufang Meng: Validation, Resources. Shuai Cheng: Resources, Supervision. Jiwen Xu: Writing - review & editing, Resources. Changrong Zhou: Supervision. Jiang Wang: Writing - review & editing. Acknowledgements This study was financially supported by Guangxi Natural Science Foundation (Grants No. 2018GXNSFAA294039) and Guangxi Key Lab oratory of Information Materials (Grants No. 151003-Z). Appendix A . Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.105089. References [1] A. Bhatnagar, A.R. Chaudhuri, Y.H. Kim, D. Hesse, M. Alexe, Role of domain walls in the abnormal photovoltaic effect in BiFeO3, Nat. Commun. 4 (2013) 1–8, 365391. [2] S.Y. Yang, J. Seidel, S.J. Byrnes, P. Shafer, C.H. Yang, M.D. Rossell, P. Yu, Y. H. Chu, J.F. Scott, J.W. Ager, Above-bandgap voltages from ferroelectric photovoltaic devices, Nat. Nanotechnol. 5 (2010) 143–147. [3] Z.X. Wang, C.L. Yuan, B.H. Zhu, Q. Feng, F. Liu, J.W. Xu, C.R. Zhou, G.H. Chen, Complex impedance spectroscopy of perovskite microwave dielectric ceramics with high dielectric constant, J. Am. Ceram. Soc. 102 (2019) 1852–1865. [4] C.S. Tu, C.M. Hung, V.H. Schmidt, R.R. Chien, M.D. Jiang, J. Anthoninappen, The origin of photovoltaic responses in BiFeO3 multiferroic ceramics, J. Phys. Condens. Matter 24 (2012) 1–6, 495902. [5] I. Grinberg, D.V. West, M. Torres, G. Gou, D.M. Stein, L. Wu, G. Chen, E.M. Gallo, A.R. Akbashev, P.K. Davies, Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials, Nature 503 (2013) 509–512. [6] N.D. Quan, L.H. Bac, D.V. Thiet, V.N. Hung, D.D. Dang, Current development in lead-free Bi0.5(Na,K)0.5TiO3-based piezoelectric, Ann. Mater. Sci. Eng. 2014 (2014) 1–13, 365391. [7] Y. Hiruma, H. Nagata, T. Takenaka, Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics, J. Appl. Phys. 105 (2009) 1–9, 084112. [8] D.K. Lee, H. Vu, J.G. Fisher, Growth of (Na0.5Bi0.5)TiO3-Ba(Ti1-xZrx)O3 single crystals by solid state single crystal growth, J. Electroceram. 34 (2015) 150–157. [9] D.D. Dung, D.V. Thiet, D. Odkhuu, L.V. Cuong, N.H. Tuan, S. Cho, Roomtemperature ferromagnetism in Fe-doped wide band gap ferroelectric Bi0.5K0.5TiO3 nanocrystals, Mater. Lett. 156 (2015) 129–133. [10] L.T.H. Thanh, N.B. Doan, N.Q. Dung, L.V. Cuong, L.H. Bac, N.A. Duc, P.Q. Bao, D. D. Dung, Origin of room temperature ferromagnetism in Cr-doped lead-free ferroelectric Bi0.5Na0.5TiO3 materials, J. Electron. Mater. 46 (2017) 3367–3372. [11] M.A. Ruiz-Preciado, A. Kassiba, A. Gibaud, A. Morales-Acevedo, Comparison of nickel titanate (NiTiO3) powders synthesized by sol-gel and solid state reaction, Mater. Sci. Semicond. Process. 37 (2015) 171–178. [12] T.T. Pham, S.G. Kang, E.W. Shin, Optical and structural properties of Mo-doped NiTiO3 materials synthesized via modified Pechini methods, Appl. Surf. Sci. 411 (2017) 18–26. [13] G. Shirane, S.J. Pickart, Y. Ishikawa, Neutron diffraction study of antiferromagnetic MnTiO3 and NiTiO3, J. Phys. Soc. Jpn. 14 (1959) 1352–1360. [14] N.H. Tuan, D.V. Thiet, D. Odkhuu, L.H. Bac, P.V. Binh, D.D. Dung, Defect induced room temperature ferromagnetism in lead-free ferroelectric Bi0.5K0.5TiO3 materials, Phys. B Condens. Matter 532 (2018) 108–114. [15] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A: Found. of Crystallogr. 32 (1976) 751–767. [16] H. Xie, Y.Y. Zhao, J.W. Xu, L. Yang, C.R. Zhou, H.B. Zhang, X.W. Zhang, W. Qiu, H. Wang, Structure, dielectric, ferroelectric, and field-induced strain response properties of (Mg1/3Nb2/3)4þ complex-ion modified Bi0.5(Na0.82K0.18)0.5TiO3 leadfree ceramics, J. Alloys Compd. 743 (2018) 73–82. [17] N.V. Prasad, V.S. Puli, D.K. Pradhan, S.M. Gupta, G. Prasad, R.S. Katiyar, G. S. Kumar, Impedance and Raman spectroscopic studies on La-modified BLSF ceramics, Ferroelectrics 474 (2015) 29–42. [18] J. Kreisel, A.M. Glazer, P. Bouvier, G. Lucazeau, High-pressure Raman study of a relaxor ferroelectric: the Na0.5Bi0.5TiO3 perovskite, Phys. Rev. B 17 (2001) 1–10, 174106. [19] M.K. Niranjan, T. Karthik, S. Asthana, J. Pan, U.V. Waghmare, Theoretical and experimental investigation of Raman modes, ferroelectric and dielectric properties of relaxor Na0.5Bi0.5TiO3, J. Appl. Phys. 113 (2013) 1–8, 194106.
Fig. 8 . (a) Current-voltage (J–V) curves of the 3.0 kV polarization/unpoled 0.94BNT-0.06NTO ceramic under simulated AM 1.5 sunlight of 100 mW/cm2 irradiance, and in the dark; (b) typical light–dark response photocurrent density of 0.94BNT-0.06NTO.
semiconductors. Moreover, a high polarization value and low permit tivity are observed in the ceramic with x ¼ 0.06. The Ni ions at x ¼ 0.06 are in þ2 valence state and the normal chemical formula is [Bi0þ, Bi3þ]0.5Na0.5[Ti3þ,Ti4þ]O2.25-0.06Ni[Ti3þ,Ti4þ]O2.67, as determined by XPS measurements. The light photovoltaic response was studied by standard AM1.5 illumination. The short circuit-current (Jsc) is 1.36 nA/ cm2 and the open-circuit voltaic (Voc) is 0.35 V and the photovoltaic effects was improved after 3.0 kV poling. These findings are helpful in understanding the properties, including electrical and optical proper ties, of perovskite ferroelectric semiconductor ceramics. Declaration of competing interest The follow is the declaration that we would like to state: 1. I confirm that this manuscript is the author’s original work; 2. The article has been written by the stated authors who are all aware of its content and approve its submission; 3. The article has not been published previously; 4. The article is not under consideration for publication elsewhere; 5. No conflict of interest exists; 6. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher. 7
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