Band gap modification and ferroelectric properties of Bi0.5(Na,K)0.5TiO3-based by LisubstitutionNgo Duc Quan, Vu Ngoc Hung, Nguyen Van Quyet, Hoang Vu Chung, and Dang Duc Dung

Citation: AIP Advances 4, 017122 (2014); doi: 10.1063/1.4863092 View online: http://dx.doi.org/10.1063/1.4863092 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An analysis of lead-free (Bi0.5Na0.5)0.915-(Bi0.5K0.5)0.05Ba0.02Sr0.015TiO3 ceramic for efficient refrigerationand thermal energy harvesting J. Appl. Phys. 115, 013505 (2014); 10.1063/1.4861031 Phase transitions, relaxor behavior, and large strain response in LiNbO3-modified Bi0.5(Na0.80K0.20)0.5TiO3lead-free piezoceramics J. Appl. Phys. 114, 044103 (2013); 10.1063/1.4816047 Structural and electrical properties of BKT rich Bi0.5K0.5TiO3-K0.5Na0.5NbO3 system AIP Advances 3, 032129 (2013); 10.1063/1.4796166 Piezoresponse and ferroelectric properties of lead-free [ Bi 0.5 ( Na 0.7 K 0.2 Li 0.1 ) 0.5 ] Ti O 3 thin films bypulsed laser deposition Appl. Phys. Lett. 92, 222909 (2008); 10.1063/1.2938364 Piezoelectric and ferroelectric properties of [ Bi 0.5 ( Na 1 x y K x Li y ) 0.5 ] Ti O 3 lead-free piezoelectricceramics Appl. Phys. Lett. 88, 062901 (2006); 10.1063/1.2171799

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AIP ADVANCES 4, 017122 (2014)

Band gap modification and ferroelectric propertiesof Bi0.5(Na,K)0.5TiO3-based by Li substitution

Ngo Duc Quan,1,2 Vu Ngoc Hung,2 Nguyen Van Quyet,3 Hoang Vu Chung,4

and Dang Duc Dung1,a

1Department of General Physics, School of Engineering Physics, Ha Noi Universityof Science and Technology, 1 Dai Co Viet road, Ha Noi, Viet Nam2International Training Institute for Materials Science, Hanoi University of Scienceand Technology, 1 Dai Co Viet road, Hanoi, Vietnam3Hanautech Co., Ltd., 832, Tamnip-dong, Yuseong-gu, Daejeon, Republic of Korea4Institute of Materials Science, Vietnam Academy of Science and Technology,18 Hoang Quoc Viet street, Hanoi, Vietnam

(Received 19 August 2013; accepted 6 January 2014; published online 23 January 2014)

We report on the reduction of band gap in Bi0.5(Na0.82-xLixK0.18)0.5(Ti0.95Sn0.05)O3

from 2.99 eV to 2.84 eV due to the substitutions of Li+ ions to Na+ sites. In addition,the lithium substitution samples exhibit an increasing of the maximal polarizationsfrom 21.8 to 25.7 μC/cm2. The polarization enhancement of ferroelectric and re-duction of the band gaps are strongly related to the Li substitution concentration asevaluated via the electronegative between A-site and oxygen and tolerance factor. Theresults are promising for photovoltaic and photocatalytic applications. C© 2014 Au-thor(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4863092]

I. INTRODUCTION

Ferroelectric perovskite (ABO3) oxides are used in a wide range of applications that includetransducers, sonar, and nonvolatile random access memory.1 In addition, they exhibit the applicationsin the photovoltaic and photocatalytic.2 Very different from traditional semiconductor solar cells,the photovoltaic effect in ferroelectric is relied on the polarization-induced internal electric fieldinstead of a p-n or Schottky junctions, which can not only improve the separation and migration oflight-generated electron-hole pairs but also reduce the cost cell fabrication.2 Various requirementsfor high efficiency ferroelectric ABO3 are i) low band gap (Eg), ii) high spontaneous saturation (PS),iii) eco-friendly and iv) low poling electrical field (EC) etc. Based on researching, Bennett et al.point out that most solid oxide ferroelectrics have a Eg of above 3 eV.3 First-principle calculationspredicted that doping the TiO6 network with an oxygen-vacancy-stabilized d8 M2+ (M = Ni, Pt,Pt, and Ce) into PbTiO3 may lower Eg to below 2.0 eV.3 Recently, Zhang et al. obtained the bandgap of ferroelectric KBiFe2O5 of 1.59 eV which was lower than band gap of BiFeO3 of 2.6 eV.2, 4

However, those exhibited low the spontaneous polarizations, 3.73 and 30 μC/cm2 for KBiFe2O5

and BiFeO3, respectively, which were smaller compared to that of Pb-based.2, 5 However, the Pb-based ferroelectric has been recently retracted due to hazard environment and human body causeof Pb content approximately of 60 wt.% in PZT.6, 7 Among lead-free ferroelectric, orthorhombicBi0.5Na0.5TiO3 (BNT) is thought to be an excellent lead-free ferroelectric ceramics candidate becauseof its large remnant polarization (Pr ∼ 38 μC/cm2) and high Curie temperature (TC ∼ 320 ◦C).8

The large coercive field EC ∼ 73 kV/cm can cause problems in the poling process, and thus limitsits practical application. Such a barrier was found to be overcome by forming solid solution withtetragonal perovskites Bi0.5K0.5TiO3 (BKT), especially, near the morphotropic phase boundary,where the BKT concentration ranges from 16 to 20%.8 Recently, the enhancement ferroelectric in

aCorresponding author: dung.dangduc@hust.edu.vn

2158-3226/2014/4(1)/017122/7 C© Author(s) 20144, 017122-1

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017122-2 Quan et al. AIP Advances 4, 017122 (2014)

Bi0.5(Na,K)0.5TiO3 was reported for various element substitutions in A-site and B-site such as Sb-,Nb-, CaZrO3-.9–11 However, other dopants such as Ta-, Y-, BiAlO3-, and co-doped (Li,Ta)-, werefound to be decreased the ferroelectric properties result from phase transition from ferroelectric toparaelectric.12–15 The effect of dopants on the ferroelectric properties of BNKT-based was stronglyrelated to the tolerance factor (t), first introduced by Goldschmidt, that decreasing t in turn makesthe ferroelectric order unstable resulting in phase transition from ferroelectric to paraelectric.11, 15–17

In addition, Li et. al. found that the octahedral factor, rB/rO, is as important as the tolerance factor,(rA + rO)/[

√2(rB + rO)], with regards to the formability of perovskite. This is assigned to the

fact that in perovskites, the octahedron BO6 is the basic unit, if ion B is too small, this unit maybecome unstable.18 In contrary to the Lee’ report, Han and colleagues reported that the substitutionof Sn, Zr, Ta, Cu, Nb (larger ion size than Ti ion) results in more unstable than without doping.16

However, the effect of A-site was not carried out to evaluate the stability of ABO3 cubic perovskite.In addition, the tolerance factor and octahedral factor were evaluated for most of B-site substitution.In fact, the A-site substitution was also strongly affective to the properties of ABO3 perosvkite,i.e., Bi0.5Na0.5TiO3-based. Yan et. al. found that the ferroelectric was strongly dependent on thedifferences of the ionic size, atomic weight, and electric negative of the A-site and B-site ions ofthe ABO3 perovskite (Bi1-xNax)TiO3-based solid solution.19 In addition, the substitution the B-siteof Pb-based perovskites with elements whose bonds with oxygen are less ionic and more covalentshould reduce the band gap.20 However, there were few reports on the effect of A-site substation inBNKT-based on ferroelectric and optical properties.

In this work, we report the effect of lithium ion substitution into A-site ofBi0.5(Na0.82K0.18)0.5(Ti0.95Sn0.05)O3. The band gaps were reduced from 2.99 to 2.84 eV as theLi amount is increased. Interestingly, the spontaneous polarizations were increased from 21.8 to25.7 μC/cm2. This finding makes this material a promising candidate for use as ferroelectric sub-strate for solar conversion devices.

II. EXPERIMENT

The Bi0.5(Na(0.82-x)K0.18Lix)0.5(Ti0.95Sn0.05)O3 (BNKTS-100xLi) (x = 0.00, 0.01, 0.02, 0.03,0.04, and 0.05) ceramics were prepared by a conventional solid state reaction route. The raw materialswere powders composed of Bi2O3, K2CO3, SnO2, TiO2, Li2CO3 (99.9%, Kojundo Chemical) andNa2CO3 (99.9%, Ceramic Specialty Inorganics). At first, the powders were weighted according totheir chemical formulas and then ball-milled for 24 h in anhydrous ethanol with zirconia balls. Theslurry was dried and calcined at 850 ◦C for 2 h. The calcined powder was added with polyvinylalcohol as a binder and uniaxially pressed into circular disks with a diameter of 12 mm at 98 MPa.The green compacts were sintered in a covered alumina crucible at 1150 ◦C for 2 h in ambientcondition. Electrical measurements were carried out after screen-printing Ag paste on both sides ofa disk-shaped specimen and subsequent firing at 700 ◦C for 30 min. The surface morphology wasobserved with a field emission scanning electron microscope (FE-SEM). The crystalline structuresof the samples were characterized by X-ray diffraction (XRD). The polarization-electric fields (P-E)were measured in silicon oil using a modified Sawyer–Tower circuit. The optical properties werestudied by UV-VIS spectroscopy. The vibrational and rotational modes in samples were characterizedby Raman spectroscopy.

III. RESULTS AND DISCUSSION

Fig. 1 shows FE-SEM micrographs of the BNKTS-100xLi ceramics with x = 0.00, 0.01, 0.02,0.03, 0.04 and 0.05. A dense microstructure with some distinct pores is observed for the BNKTSceramic, as seen in Fig. 1(a). The appearances of small grains were obtained when Li added. Inaddition, the average grain size is increased as increasing the content of Li substitution, as seen inFig. 1(b)–1(f).

Fig. 2 shows XRD patterns of BNKTS-100xLi ceramics. All the samples exhibited a single-phase perovskite structure, without any traces of secondary phases. This indicates that Li+ wassuccessfully substituted on the Na-site of BNKTS-100xLi ceramics and formed a solid solution.

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017122-3 Quan et al. AIP Advances 4, 017122 (2014)

FIG. 1. FE-SEM images of the BNKTS-100xLi ceramics with (a) x = 0.00, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03,(e) x = 0.04, and (f) x = 0.05.

FIG. 2. (a) X-ray diffraction pattern of Li doped BNKTS ceramics as a function of Li doping level x, (b) the magnified XRDpatterns in the 2θ ranges of 45◦–48◦.

The single (200) peaks at a 2θ of around 46.5◦ were obtained as evident for pseudocubic sym-metry. However, the (200) peak was split to (002)/(200) as increasing the Li concentration up tox = 0.05, indicating the presence of phase transition from pseudocubic to tetragonal phase due toLi+ substitution into Na-site. In the other word, the tetragonal phase increases in the pseudocubicmatrix of BNKTS-100xLi ceramics by increasing Li concentration.

Fig. 3(a) shows the Raman spectra of BNKTS-100xLi ceramics at room temperature in thewave number range of 100–1000 cm−1. The Raman bands for BNKTS-100Li are relatively broadresult from disorder on the A site and also from overlapping Raman modes, that making it difficult todistinguish different modes. For a close inspection and reliable comparison, the spectral propertiesof all compositions are fitted to an equal number of Lorentzian peaks and obtained fitting parametersa plotted as a function of Li concentration doping, as showed in Fig. 3(b) in selection peaks.The Na-O vibration mode at ∼ 138 cm−1 of Bi0.5Na0.5TiO3, instead of ∼ 182 cm−1 for BNKTSsamples without Li dopant. The bands were shifted 44 cm−1 because of the substitution of K+ ion to

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017122-4 Quan et al. AIP Advances 4, 017122 (2014)

FIG. 3. (a) Raman spectra of the BNKTS-100xLi ceramics at room temperature in the frequency range of 100–1000 cm−1,and (b) the selected frequencies peaks position as function of Li dopant in BNKTS-100xLi ceramics.

FIG. 4. (a) Room temperature P–E hysteresis loops of the BNKTS–100xLi ceramics and (b) maximum polarization (Pm),and coercive field (EC) as functions of Li amount in the BNKTS–100xLi ceramics.

Na+-site.21 The peaks were shifted to lower frequency via Li substation with Li concentration up to3 mol.% then increased, indicating that Li substituted at Na-site and phase transition occurred toform with XRD results. In addition, the Li radii 0.98 Å, smaller than other ions at A-sites (Na:1.18 Å, K: 1.33 Å, Bi: 1.17 Å), results in that the [TiO6] octahedral will tilt in order to fill thefree space.22, 23 However, the frequencies have trended to reverse when the Li concentration werefurther added above 3 mol.%. We suggest that the opposite frequency trend were obtained in Ramanspectroscopy relative with multisite occurred in BNKTSn-100xLi ceramics.24–26

Fig. 4(a) shows the polarization-electric field (P-E) hysteresis loops of BNKTS-100Li ceramicsat room temperature. All samples show well saturated P-E hysteresis loops. The variations of themaximum polarization (Pm) and coercive field (EC) with Li concentration are shown in Fig. 4(b).It is strong evident that the ferroelectric properties of BNKTS ceramics have significantly beenaffected by doping with Li, as shown in Fig. 4(a). The maximum polarization increased with theincreasing of Li concentration up to 3 mol.% Li, then decreased by further addition of Li dopantconcentration. Pm reaches a maximum value of 25.7 μC/cm2 when Li contents are 3 mol.% which ishigher than that of 21.8 μC/cm2 for BNKTS. EC was gradually decreased from 12.21 to 9.2 kV/cm.Enhanced spontaneous polarization and decreased coercive field show that ferroelectric propertiesof the BNKTS-100xLi ceramics have been improved with the addition of Li. The effectiveness ofthe dopants on the ferroelectric properties of BNKT-based has been reported. The replacement of

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017122-5 Quan et al. AIP Advances 4, 017122 (2014)

FIG. 5. (a) UV-vis absorption spectra of the BNKTS ceramics as a function of Li dopant, and (b) The (αhγ )2 proposal withphoton energy (hγ ) of the BNKTS ceramics as function of Li dopant. The inset of (b) shows the band gap Eg of the BNKTSceramics as function of Li dopant.

Ti4+ by Zr4+, Sn4+ and Hf4+ was found to be reduced the ferroelectric properties.16, 27, 28 Li et al.reported that the ferroelectric properties of BaTi0.94Sn0.06O3 were enhanced due to replacement ofBa by Ca as dopants.29 Chen et al. reported that Li substitution at the A-site in BNKT ceramicssuppresses formation of the second phase and Ti valence transition result from a strong bond fromA, B-site cations and oxygen to stabilize the structure due to Li substitution.30 We can suggest thatthe Li addition to BNKTS has an effect on strengthening the bonding of the perovskite structureresult in the enhancement of the ferroelectric properties. Theoretical predicted that the B-cationdisplacement highest in case of Ti which were expected for strong ferroelectric active.31 Thus, wesuggest that the ions Li+ replaced in Ti4+ then hindered the displacement B-site, even the tetragonalstructure were existed, result in reduction the maxima polarization.

Furthermore, the change of the band gap of specimens due to the Li substitution in A-site ofBNKTS has been studied. Fig. 5(a) shows the UV-VIS absorbance spectrum of BNKTS-100Li as thefunction of Li substation. The shift to the higher wavelengths was obtained as the Li concentrationincreased, indicating the reduction of band gap. The band gap energy (Eg) was associated with theabsorbance and photon energy by following equation hγα ∝ (hγ -Eg)n, where α is the absorbance,h the Planck constant, γ the frequency, Eg the optical band gap and n a constant associated withdifferent types of electronic transition (n = 1/2, 2, 3/2 or 3 for direct allowed, indirect allowed, directforbidden and indirect forbidden transitions, respectively).32 Thus, the Eg values of BNKTS-100xLiceramics were evaluated by extrapolating the linear portion of the curve or tail. Our results indicatedthat an indirect transition which occurred because of n = 2, as shown in Fig. 5(b). The values ofEg decreased from 2.99 to 2.84 eV by adding 5 mol.% Li access, shown as an inset in Fig. 4(b).Parija et al. pointed out that the obtained Eg value of 2.94 eV for the Bi0.5Na0.5TiO3 ceramics canbe associated to a structural order-disorder effect into the lattice due to a symmetry break betweenthe O-Ti-O bonds and/or distortions on the (TiO6) clusters.32 Lee et al. reported the strong relationbetween band gap and the tolerance factor in various solid solution ABO3-A’B’O3.33, 34 Levin et al.suggested that the dominant effect of the local displacements on band-gap values due to strong O2p- Ti 3d hybridization.35 Jan et al. obtained the covalent Pb-O bonding strongly affects O 2p-Ti3d hybridization in the TiO6 octahedron while the ionic Ba-O bonding does not substantially affectO 2p –Ti 3d hybridization results in Pb-induced tetragonal distortion in the TiO6 octahedron andineffectiveness in Ba substitution to Sr-site in SrTiO3.36 The covalence or ionic bonds were stronglyrelated to the difference of electro-negativity between A- and B-site with O in perovskite.37

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017122-6 Quan et al. AIP Advances 4, 017122 (2014)

FIG. 6. The tolerance factor and difference between electronegative of average ions at <A> site and oxygen of theBNKTS–100xLi ceramics as function of Li-added concentration.

To further understanding the mechanism of enhancement in spontaneous and reduction of theband gap due to lithium substitution in A-site BNKT-based, the tolerance factor and differenceaverage electronegativity between A-site with O as function of lithium dopants are analyzed andshown in the Fig. 6. The tolerance factor decreased via increasing the Li amount due to small radiiof Li compared to radii of Na, and K ions. The reduction of the tolerance factor results in the phasetransition from pseudocubic (nonpolar) to tetragonal (polar). The phase transition related to thedistortion tolerance factor was report in BNKT-modified at A- and B-site.11, 15–17 The increasing ofthe polar phase in BNKTS-100xLi indicated the enhancement in maximal polarization. The averagedifferences between electro-negativity A-O were decreased, indicating that the bonding was morecovalence than ionic due to Li substitution at Na-site. We suggested that the reduction of band gapis strong related to the covalence bonding between A-site and oxygen.

IV. CONCLUSION

The enhancement of ferroelectric in BNKTS ceramics by Li substitution at A-site has beenreported. The Pm increased from 21.8 to 25.7 μC/cm2 results from the growth of tetragonal phase inpseudocubic phase, as the reduction of the tolerance factor led to the phase transition. The reductionof band gap was obtained from 2.99 to 2.84 eV and it is assigned to the fact that from the morecovalence bonding than ionic via Li substitution at A-site perovskite BNKTS. The results werepromising for enhancement the efficiency of photovoltaic and photocatalytic applications by usingthe visible light.

ACKNOWLEDGMENT

This research is funded by Vietnam National Foundation for Science and Technology Develop-ment (NAFOSTED) under grant number 103.02-2012.62

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