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Ni-doped SrBi2Nb2O9 – Perovskite oxides with reduced band gap and stable ferroelectricity for photovoltaic applications
Accepted Manuscript Ni-doped SrBi2Nb2O9 – Perovskite oxides with reduced band gap and stable ferroelectricity for photovoltaic applications Ming Wu, Xiaojie Lou, Tangyuan Li, Junning Li, Shaolan Wang, Wei Li, Biaolin Peng, Gaoyang Gou PII:
S0925-8388(17)31466-4
DOI:
10.1016/j.jallcom.2017.04.256
Reference:
JALCOM 41660
To appear in:
Journal of Alloys and Compounds
Received Date: 5 December 2016 Revised Date:
21 April 2017
Accepted Date: 24 April 2017
Please cite this article as: M. Wu, X. Lou, T. Li, J. Li, S. Wang, W. Li, B. Peng, G. Gou, Ni-doped SrBi2Nb2O9 – Perovskite oxides with reduced band gap and stable ferroelectricity for photovoltaic applications, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.256. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Ni-doped SrBi2Nb2O9 – Perovskite oxides with reduced band
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gap and stable ferroelectricity for photovoltaic applications
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Ming Wu a, Xiaojie Lou
a, *
, Tangyuan Li a, Junning Li a, Shaolan Wang a, Wei Li a,
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Biaolin Peng b and Gaoyang Gou a, *
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a
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Technology, and State Key Laboratory for Mechanical Behavior of Materials, Xi’an
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Jiaotong University, Xi’an 710049, China
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b
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Relativistic Astrophysics, Guangxi University, Nanning 530004, P. R. China
and Guangxi Key Laboratory for
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School of Physical Science & Technology
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Multi-disciplinary Materials Research Center, Frontier Institute of Science and
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Email addresses: [email protected] (X.J. Lou),
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and
[email protected] (G.Y.Gou)
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ABSTRACT In this work, Ni-doped SrBi2Nb2O9 (Sr1-xBi2+xNb2-xNixO9-x, or SBNN) ceramics
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were successfully fabricated by using a solid state reaction method, and their band
5
gaps were determined by UV-vis-NIR absorption spectrum. It is found that Ni doping
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could
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Sr0.91Bi2.09Nb1.91Ni0.09O8.91 (SBNN9) ceramics show the lowest band gap of 2.25 eV
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with a relatively high remanent polarization of 2.5 µC/cm2. The band-gap reducing
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effect upon Ni doping might be attributed to the formation of the intermediate gap
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states generated by the Ni2+ cations. Despite oxygen vacancies were unavoidably
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introduced into the systems after Ni doping, the remanent polarizations of the SBNN
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ceramics remain almost the same or even larger than the pure SrBi2Nb2O9. Our
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experimental results are also confirmed by the theoretical analysis based on the
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first-principle calculations. Novel ferroelectric photovoltaic effects were observed in
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these SBNN compounds. The switchable photovoltaic outputs induced by electric
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field were detected along different poling directions in the SBNN9 sample. The
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SBNN ceramics reported in the present work display narrow band gaps and relatively
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large polarizations, and show promise for ferroelectric photovoltaic device
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applications.
reduce
the
band
gap
of
SrBi2Nb2O9.
The
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Keywords: Bi-layered ferroelectrics, Ferroelectric photovoltaics, Band gap
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engineering.
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1. Introduction In the past few decades, ferroelectric photovoltaic (FE-PV) effect has attracted
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extensive research interests, due to their above-band-gap photovoltage[1, 2] [3] and
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switchable photocurrent under an external electric field[4] [5-8]. Spontaneous
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polarization and the induced internal bias field can effectively separate the
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photo-generated electron-hole pairs. This phenomena is different from the
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conventional photovoltaic mechanism observed in semiconductor p-n junctions [9].
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Recently, extensive studies have devoted to the investigations of FE-PV effect in a
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variety of ferroelectric systems (i.e., ceramics [10-12], single crystals[8] [13], thin
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films[4, 7] [14-17], polymers [18], and organic-inorganic ferroelectric composites
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[19]). Various models, such as bulk photovoltaic effect[20] [21], domain wall
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theory[22, 23] [3, 24], Schottky-junction effect [25] and depolarization field effect
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[26], have proposed to explain FE-PV effect. Based on the aforementioned models, it
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is widely believed that ferroelectric materials with relatively small energy band gap
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and large polarizations are promising for photovoltaic applications because of their
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capability in to absorb a wide range of solar spectrum and to separate photo-excited
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charge carriers [27].
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Unfortunately, traditional ABO3 structured perovskite ferroelectrics are insulators
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or high-band-gap semiconductors with band gap above 3.0 eV. Hence only the photon
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with energy beyond band gap can be absorbed by these materials. This results in low
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efficiency of the devices. High energy band gap of ABO3 perovskite oxides originated
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from the large difference in electron negativity between the B-site transition metal
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ACCEPTED MANUSCRIPT cations and oxygen anions [10, 28]. Band gap of these oxides can be effectively
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reduced via cation doping at the B-sites by other lower valence cations. Such doping
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procedure indeed leads to a decrease in band gap and also significantly reduces
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ferroelectric polarization as both metal dopant and oxygen vacancies are introduced
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into the lattice. For instance, after doping Ni into KNbO3, the resultant
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[KNbO3]1-x[BaNi1/2Nb1/2O3-δ]x solid solutions have a low band gap down to 1.18 eV,
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and the spontaneous polarizations are below 1.0 µC/cm2 [10] at room temperature. In this work,
we report Ni-doped
Aurivillius ferroelectric ceramics
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Sr1-xBi2+xNb2-xNixO9-x (abbreviated as SBNN), consisting of alternative bismuth layers
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(Bi2O2)2+ and two perovskite layers (SrNb2O7)2- [29](as shown in Fig. 1). Thanks to
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the bismuth layers, having a good toleration towards oxygen vacancies [30], we
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could introduce Ni cations into the lattice and keep the corner-sharing octahedral
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network almost undestroyed. (This will be discussed in detail later). Remarkably, both
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relatively large polarizations and narrow band gaps could be realized simultaneously
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in our designed SBNN ceramics. The band gaps of our SBNN samples range from
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2.25 eV to 2.70 eV as the Ni-doping concentration varies, and all the compositions
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exhibit considerably large remanent polarizations even at high temperatures (e.g., up
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to 120 °C), indicating that the SBNN ceramics can be used as FE-PV materials
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working efficiently in a wide temperature window.
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2. Methodology section
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2.1 Materials and methods Polycrystalline Sr1-xBi2+xNb2-xNixO9-x (SBNN100x, x=0.00, 0.01, 0.03, 0.05, 0.07,
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0.09) ceramics were fabricated by using a traditional solid state reaction method, by
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which the raw materials reacted in high temperature process to synthesis the wanted
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matter. In order to maintain charge neutrality, content of bismuth should increase by
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the same amount as the nickel concentration. Stoichiometric quantities of SrCO3 (Alfa,
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99%), Bi2O3 (Alfa, 99.9995%), Nb2O5 (Alfa, 99.9985%), and NiO (Alfa, 99.998%)
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were mixed by ball milling in ethanol for 24h. Mixtures were dried and then calcined
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at 775 °C for 2h in Ar atmosphere in muffle furnace. Calcined powders were
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re-milled with ethanol for 24h, dried, ground with PVA, and pressed into pellets.
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Green pellets were firstly sintered at 500 °C for 5h in air in order to remove PVA then
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sintered at 975 °C in Ar atmosphere. Gold electrode was first made using an ion
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sputtering, and then annealed at 500 °C for 1h in Ar atmosphere as electrodes for
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electrical tests. ITO was sputtered by magnetron sputtering on both sides of the
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samples as transparent electrodes.
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Phase purity of grown ceramics was examined by powder X-ray diffraction
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(XRD; Shimadzu 7000) using Cu Kα radiation generated at 40 kV and 30 mA.
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Cross-section morphologies of ceramics were checked by using a scanning electron
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microscope (SEM; HITACHI SU-8010). Polarization-electric-field (P-E) hysteresis
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loops were recorded using a Radiant ferroelectric workstation. Phase purities and the
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oxygen vacancies were determined using an X-ray photoelectron spectroscopy (XPS) 5
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instrument.
absorption
experiments
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ultraviolet-visible-near-infrared
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UV3600) equipped with an integration sphere. A Xenon lamp was used to provide
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standard solar illumination (AM1.5). Photovoltaic effect was checked by the Keithley
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2400 source.
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2.2 First-principles calculations
(UV-Vis-NIR)
were
carried
out
spectrophotometer
using
an
(SHIMADZU
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Optical
First-principles calculations were performed within density-functional theory as
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implemented in the QUANTUM ESPRESSO code (QE) [31]. Nonlocal optimized
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norm-conserving pseudopotentials [32, 33] and a 60 Ry plane-wave energy cutoff
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were used for calculations. Partial core corrections (PCC) [34] were included in the Ni
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pseudopotential. All calculations were performed using PBEsol exchange-correlation
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functional [35] plus Hubbard U approach as implemented in QE [36], where an
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effective U value of 4.5 eV was applied on Ni [37]. PBEsol+U calculations can
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produce the correct semiconducting ground state for Ni-doped SrBi2Nb2O9. 8×8×4
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Monkhorst-Pack k-grids [38] were used to simulate pure and Ni-doped SrBi2Nb2O9.
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Both atomic positions and lattice parameters for SrBi2Nb2O9 systems were fully
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optimized until residual Hellmann-Feynman forces on the atoms were smaller than 1
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meV/Å and the stresses less than 0.1 kbar. Electronic contribution to the polarization
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was calculated following the Berry phase formalism [39].
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3. Results and discussion
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XRD patterns of SBNN ceramics are demonstrated in Fig. 2. All diffraction
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peaks can be indexed with the standard PDF card 49-0607 (space group of A21am 6
ACCEPTED MANUSCRIPT [40]). No additional peak of any other phase is observed, which indicates that all
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samples have a highly pure phase without secondary phases. The strongest diffraction
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peak (115) consistent with the (1 1 2m+1) formula [41] of the highest diffraction peak
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in the Aurivillius structure, where m stands for the repetitions of the perovskite
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substructure between bismuth layers. For our SBNN samples, m is equal to 2 (Fig. 1).
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Previous studies shown that the NbO6 octahedron has a high structural tolerance
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towards Ni doping, as Ni2+ and Nb5+ cations have the similar ionic radius (0.69 Å vs.
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0.64 Å) [42]. Previously, Ni-doped ferroelectric solid solutions were achieved in
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PbNi1/3Nb2/3O3 [43] and PbTi1-xNixO3-δ solid solutions [37, 44]. For our SBNN
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ceramics, the characteristic (115) peak shifts slightly lower angles with increasing
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Ni-content, as demonstrated in Fig. 2, which is attributed to the larger ionic radius of
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Ni2+ than that of Nb5+. Cross-section SEM images of the SBNN ceramics are shown
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in Fig. 3. All the specimens show a transcrystalline fracture, indicating good sintering
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properties of these ceramics. Characteristic lamellar grains are observed in all SBNN
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ceramics, which is originated from crystal growth dynamics of intrinsic bismuth layer
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lattice structure [45].
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Ferroelectric polarization is one of the essential properties for ferroelectric
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photovoltaic applications. Fig. 4a shows the ferroelectric hysteresis (P-E) loops
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measured at 120 °C and 10 Hz. Remanent polarization (Pr) of pure SBNN0 ceramic is
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~1.8 µC/cm2, consistent with those in previous studies [46]. With increasing
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Ni-doping concentration, the Pr value of the samples firstly decreases to 1.1 µC/cm2
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after that increases to 2.7µC/cm2, as depicted in Fig. 4b. The first decrease of Pr at 7
ACCEPTED MANUSCRIPT SBNN1 might originate from oxygen vacancies introduced upon Ni doping, which
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could pin the domain walls and cause difficulties in domain reversal [47]. With the
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further increase of Ni-doping concentration (x≥0.03), more Sr ions are substituted by
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Bi ions, leading to the enhancement in ferroelectricity of the SBNN ceramics, and the
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subsequent increase in Pr of the SBNN samples.
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To understand experimental results of Ni-doped SBNN ceramics, we further
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performed first-principles calculations to simulate the structural and ferroelectric
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properties of both the pure and Ni-doped SrBi2Nb2O9. We start our investigation by
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simulating the pure SrBi2Nb2O9. SrBi2Nb2O9 have an Aurivillius perovskite structure
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with orthorhombic A21am symmetry, which exhibits an a-a-c+ NbO6 octahedral
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rotation pattern in Glazer notation [48] and a spontaneous polarization along the
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in-plane pseudocubic [110] direction. Table 1‐shows calculated lattice parameters of
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SrBi2Nb2O9, similar results were also observed in previous studies [49]. The berry
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phase calculation predicts a spontaneous polarization of 30.8 µC/cm2 for SrBi2Nb2O9,
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which mainly comes from the off-center displacement of B-site Nb cations. It is well
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known that the strong texture effect in orthorhombic ferroelectric oxides significantly
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suppresses the realization of saturated polarization [50, 51]. Hence experimental
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spontaneous polarization value of SrBi2Nb2O9 is supposed to be much lower than that
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of theoretically predicted values.
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With Ni substitution on Nb-site, oxygen vacancies are created and some of the
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A-site Sr ions are also replaced by extra Bi cations. Fig. 1 shows three
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Sr3Bi9Nb7NiO35 (SBNN) configurations where oxygen vacancy is located at different 8
ACCEPTED MANUSCRIPT inequivalent sites, for simulation of the Ni-doped SrBi2Nb2O9 solid solutions. After
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structural optimization, the SBNN-3 configuration where oxygen vacancy is located
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in the perovskite layers adjacent to the Bi2O2 layers is determined to be the
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energetically most stable one (Table 1), followed by the configuration where oxygen
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vacancy is the middle of NiO6 and NbO6 octahedral (SBNN-1). The SBNN-2
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configuration, in which the oxygen vacancy is located within the Bi2O2 layer, is the
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most unstable one with the highest ∆E (see Table 1). Both the SBNN-1 and SBNN-3
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configurations exhibit ferroelectric polarization smaller than that of the pure
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SrBi2Nb2O9, as their corner-sharing octahedral network within the perovskite layer is
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disrupted by oxygen vacancy (see Table 1 and Fig. 1). In contrast, the corner share
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octahedral network is well preserved in the SBNN-2 configuration, in which the
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oxygen vacancy is located within the Bi2O2 layer. Moreover, in comparison with
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polar-inactive Sr, the A-site Bi cation in SBNN-2 has a larger polar displacement,
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leading to an enhanced polarization for the SBNN-2 configuration. Note that there is
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only a little difference in total energy between these three SBNN configurations.
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Therefore, they most likely coexist in our SBNN solid solutions prepared. At high Ni
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doping concentrations, it is likely that oxygen vacancies are formed in the Bi2O2
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layers (corresponding to the SBNN-3 configuration). So, the SBNN ceramic samples
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with higher Ni doping levels may show a larger ferroelectric polarization than that of
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the pure SrBi2Nb2O9.
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Optical properties of the SBNN ceramics are presented in the Fig. 5a. With the
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increase of Ni-content, SBNN ceramics show an enhanced visible absorption. 9
ACCEPTED MANUSCRIPT Absorption edges of the samples exhibit a red-shift with the increased Ni-doping,
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indicating a reduction of band gap. In addition, absorption peak observed at ~700nm
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confirms the presence of Ni2+ [52-54]. To determine the type of optical transition and
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band gap energy, the Tuac’s plots of all samples are presented in the inset of Fig. 5b.
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Generally, the Tuac’s plots are defined by the energy of photon (hν) on the abscissa
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and (αhν) 1/n on the ordinate [55], where α is the absorption coefficient of the material.
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The value of n denotes the intrinsic type of band transition, and n=1/2, 3/2, 2, 3 stand
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for direct allowed transitions, direct forbidden transitions, indirect allowed transitions
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and indirect forbidden transitions, respectively. For SBNNs, the value of n is equal to
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1/2, demonstrating a direct allowed transition type. The band gaps calculated by the
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Tuac’s plots are shown in Fig. 5b, which reveals a tunable band gap ranging from
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2.25 eV to 2.7 eV. The band gap of SBNN0 is 2.7 eV, and the sample is palegreen, as
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shown in Fig. 5c. However, the reported SrBi2Nb2O9 is faint yellow, and its band gap
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is beyond 3.0 eV [56]. Since we sintered our samples at Ar atmosphere, the
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differences in sample’s color and band gap between our samples and those reported
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previously are attributed to different sintering conditions. It has been widely accepted
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that different sintering atmospheres, such as O2, N2, and Ar, have significant influence
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on the lattice constant, dielectric properties, ferroelectricity and optical properties of
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ferroelectric ceramics [57, 58]. For instance, Kang et al fabricated ferroelectric
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Sr0.5Ba0.5Nb2O6 ceramics in an atmosphere with different oxygen partial pressures
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[57] . With the decrease of the oxygen partial pressure, the transmittance of the
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samples decreases and the corresponding absorption edge shows a little red-shift. In
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ACCEPTED MANUSCRIPT our case, compared with air, the Ar sintering atmosphere has a lower oxygen partial
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pressure. Therefore, sintering of SBNN ceramics in Ar atmosphere could accelerate
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bismuth volatilization and subsequently generate more oxygen vacancies. The XPS
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spectra of SBNN0 ceramics sintered in Ar atmosphere are shown in Fig. 6. The XPS
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spectrum presented in Fig. 6a reveals the presence of Sr, Bi, Nb, and O without any
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other trace impurities except for a small amount of C, which is taken as a reference
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for the calibration of the banding energy [59]. Fig. 6b shows the narrow-scan
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spectrum of O 1s. The profile consists of three components: a main peak at 528.41 eV,
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corresponding to the bulk Nb-O cation-oxygen bonds. The two sub-peaks at 530.61
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eV and 532.41 eV indicate the existence of oxygen vacancies as well as the adsorbed
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surface H2O [60]. Oxygen vacancies lead to the formation of impurity energy levels in
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the band gap of the sample, giving rise to the decrease in the band gap measured
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experimentally. The above argument could also be a reasonable interpretation for the
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results in the work by Chen et al [61]. Band gap of the Bi5Ti3FeO15 ceramics in their
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work decreases from 3.39 eV to 3.22 eV, when the sintering time increases from 240
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mins to 360 mins. We believe that that is also caused by severe bismuth volatilization
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in the samples with longer sintering times. To further confirm our point, SBNN0
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ceramics were sintered in air atmosphere with the same annealing procedure. The
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as-fabricated SBNN ceramic is faint yellow, as shown in the inset of Fig. 5c,
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consistent with the results previously reported. Fig. 5d shows the comparison of the
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remanent polarization and band gap of the SBNN ceramics with some other reported
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materials. As one can see, all the ferroelectrics previously studied can be divided into
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ACCEPTED MANUSCRIPT two categories: one is the materials with a larger remanent polarization and a higher
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band gap, such as lead-based ferroelectrics [62], BiFeO3 and its solid solutions [21],
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and Bi4Ti3O12-based ferroelectrics [63]. The other is the materials with a smaller
4
remanent polarization and a lower band gap, such as KBiFe2O5 [13] and KBNNO [10].
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Therefore, for the first time, both a higher remanent polarization and a lower band gap,
6
essential for the photovoltaic applications [27], have been successfully achieved in
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our SBNN9 ceramics via Ni doping.
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The band-gap narrowing effect observed in this work could be attributed to the
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gap states introduced by the Ni dopants [44, 45]. The valance and conduction band of
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SrBi2Nb2O9 are composed of hybridized O 2p and Nb 4d states. After Ni doping, Ni
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cations introduce both filled and unfilled Ni 3d gap states into the SBNN systems,
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giving a rise to direct optical transition from the filled to unfilled Ni-3d states.
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Therefore Ni doping plays a crucial part in lowering the energy band gap.
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For present work, SBNN9 ceramic is found to possess relatively higher remanent
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polarization and a smaller band gap. Hence photovoltaic measurements done on
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SBNN9 ceramics and obtained results are demonstrated in Fig. 7. As shown in Fig. 7a,
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standard AM1.5 illumination causes an obvious photovoltaic response in the
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as-fabricated samples. The short circuit current (Jsc) is 3.5 nA/cm2 and the open
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circuit voltaic (Voc) is -2.3 V. Fig. 7b shows the dependence of photovoltaic response
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upon polarization switching. In order to switch the polarization direction, sample was
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poled under an electric field of 50kV/cm at 100
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show an enhanced photovoltaic response, with Jsc of 7.0 nA/cm2, and Voc of -3.5 V. It
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. Samples poled in the UP direction
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as-fabricated one. Samples poled in the DOWN direction show a reversed response.
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Since both bottom and top electrodes are ITO films and the same, the switching of the
4
photovoltaic responses observed in the SBNN9 samples must originate from the
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switching of ferroelectric polarizations under electrical poling. Also, we notice that
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samples exhibit a very low Jsc and a relatively high Voc. It is well accepted that there
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are many grain boundaries and pores in ceramics, which block the migration of
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photo-generated carriers and thus reduce photocurrent. Remarkably, our SBNN
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samples show a high Voc of several V, in comparison with the Voc of several hundred
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mV in most of the thin films reported previously. Difference in photovoltaic response
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of thin films and bulk materials (e.g, ceramics, or single crystals) has been noticed for
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a long time. For example, Qin et al [26] reported a low Voc of 0.7 V in PLZT thin
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films. However, a high Voc of 7 V was also reported in PLZT ceramics of the same
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composition by Zhang et al [62]. This difference could be explained by using the
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method developed by Yang et al [3]. The hypothesis made by Yang et al [3] is that
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each nanometer-scale domain wall can generate a potential step, and all these
17
potential steps are in series, sum of which determines the magnitude of output
18
photovoltage. Since ceramics have large volume to react than that of thin films as a
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result ceramic samples are supposed to have more domain walls connected in series.
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Therefore, it is reasonable to observe a higher Voc in our SBNN9 ceramic samples.
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4. Conclusions
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In summary, pure phase Ni-doped strontium bismuth niobate ceramics are
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and a considerable remanent polarization (2.5 µC/cm2) at 120 °C. In addition, UP
3
poled SBNN9 ceramic shows an excellent photovoltaic response (Jsc of 7.0 nA/cm2,
4
and Voc of -3.5 V). Photovoltaic output can be switched reversibly by applying an
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external UP/Down electric DC field. The SBNN9 ceramic developed in this work
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demonstrates superior photovoltaic properties among all the FE-PV candidate
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materials because of lower band gap and higher remanent polarization. Our work
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implies that modifying ferroelectrics with a bismuth-layered perovskite structure
9
using transition metal (Ni in this work) may be a good strategy to achieve
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ferroelectrics with a lower band gap and a large remnant polarization, both of which
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are crucial for obtaining superior FE-PV properties. Therefore, this work opens a new
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avenue for ferroelectric photovoltaic and photocatalytic applications.
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Acknowledgments
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This work was supported by the National Science Foundation of China (NSFC
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No. 51372195, NO.51402196 and No. 11574244), the China Postdoctoral Science
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Foundation (Grants 2014M552229 and 2015T80915), the CSS project (Grant No.
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YK2015-0602006), the Fundamental Research Funds for the Central Universities
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(2013JDGZ03), and Program for Innovative Research Team in University of Ministry
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of Education of China (IRT13034). X.J. Lou would like to thank the “One Thousand
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Youth Talents” program for support. G. Y. Gou acknowledges the computational
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support from National supercomputer center (NSCC) in Tianjin.
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Fig. 1: Three symmetrically inequivalent configurations representing Ni-doped SrBi2Nb2O9, where oxygen vacancy is located between perovskite NiO6 and NbO6 octahedral, within Bi2O2 layers and in the perovskite layers adjacent to the Bi2O2
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Fig. 2: Powder XRD patterns of the polycrystalline SBNN ceramics. The right inset
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Fig. 3: SEM images of the SBNN ceramics with Ni-doping concentration at (a) x=0,
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(b) x=0.01, (c) x=0.03, (d) x=0.05, (e) x=0.07, (f) x=0.09.
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tested at 120
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Fig. 4: (a) Polarization-electric field (P-E) hysteresis loops of the SBNN ceramics and 10 Hz. (b) Remanent polarization of the SBNN ceramics as a
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Fig. 5: (a)UV-Vis-NIR absorption spectrums of SBNN ceramics. (b) Band gaps of the
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SBNN ceramics as a function of Ni-doping concentration. The inset shows the Tauc’s
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Fig. 6: (a) XPS spectrum of SBNN0 ceramics sintered in Ar. (b) The narrow-scan
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Fig. 7: J-V characteristics in the SBNN9 ceramics. (a) The as-fabricated samples with and without illumination. (b) The I-V characteristics with illumination at different
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ACCEPTED MANUSCRIPT Table 1: Our calculated structural parameters and ferroelectric polarization for pure SrBi2Nb2O9 and three configurations representing Ni-doped SrBi2Nb2O9. Relative
b/Å
c/Å
P(μC/cm2)
SrBi2Nb2O9
5.59
5.57
24.90
30.8
SBNN-1
5.51
5.50
25.59
25.0
SBNN-2
5.63
5.60
24.54
33.5
SBNN-3
5.54
5.50
24.68
27.8
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E/(eV/f. u.)
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a/Å
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energies of three SBNN configurations are also given for comparison.
—
0.22 0.29 0.00
ACCEPTED MANUSCRIPT Highlights:
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Synthesis of Ni-doped SrBi2Nb2O9 ceramics. Stable ferroelectrcity (Pr=2.5 µC/cm2) and decreased optical band gap (2.25 eV). The band gaps are dependent on the sintering atmosphere. Photovoltaic effects can be switched by the applied external field.
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DOI:
10.1016/j.jallcom.2017.04.256
Reference:
JALCOM 41660
To appear in:
Journal of Alloys and Compounds
Received Date: 5 December 2016 Revised Date:
21 April 2017
Accepted Date: 24 April 2017
Please cite this article as: M. Wu, X. Lou, T. Li, J. Li, S. Wang, W. Li, B. Peng, G. Gou, Ni-doped SrBi2Nb2O9 – Perovskite oxides with reduced band gap and stable ferroelectricity for photovoltaic applications, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.256. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Ni-doped SrBi2Nb2O9 – Perovskite oxides with reduced band
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gap and stable ferroelectricity for photovoltaic applications
3
Ming Wu a, Xiaojie Lou
a, *
, Tangyuan Li a, Junning Li a, Shaolan Wang a, Wei Li a,
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Biaolin Peng b and Gaoyang Gou a, *
5
a
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Technology, and State Key Laboratory for Mechanical Behavior of Materials, Xi’an
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Jiaotong University, Xi’an 710049, China
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b
9
Relativistic Astrophysics, Guangxi University, Nanning 530004, P. R. China
and Guangxi Key Laboratory for
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School of Physical Science & Technology
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Multi-disciplinary Materials Research Center, Frontier Institute of Science and
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Email addresses: [email protected] (X.J. Lou),
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and
[email protected] (G.Y.Gou)
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1
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ABSTRACT In this work, Ni-doped SrBi2Nb2O9 (Sr1-xBi2+xNb2-xNixO9-x, or SBNN) ceramics
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were successfully fabricated by using a solid state reaction method, and their band
5
gaps were determined by UV-vis-NIR absorption spectrum. It is found that Ni doping
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could
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Sr0.91Bi2.09Nb1.91Ni0.09O8.91 (SBNN9) ceramics show the lowest band gap of 2.25 eV
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with a relatively high remanent polarization of 2.5 µC/cm2. The band-gap reducing
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effect upon Ni doping might be attributed to the formation of the intermediate gap
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states generated by the Ni2+ cations. Despite oxygen vacancies were unavoidably
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introduced into the systems after Ni doping, the remanent polarizations of the SBNN
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ceramics remain almost the same or even larger than the pure SrBi2Nb2O9. Our
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experimental results are also confirmed by the theoretical analysis based on the
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first-principle calculations. Novel ferroelectric photovoltaic effects were observed in
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these SBNN compounds. The switchable photovoltaic outputs induced by electric
16
field were detected along different poling directions in the SBNN9 sample. The
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SBNN ceramics reported in the present work display narrow band gaps and relatively
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large polarizations, and show promise for ferroelectric photovoltaic device
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applications.
reduce
the
band
gap
of
SrBi2Nb2O9.
The
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Keywords: Bi-layered ferroelectrics, Ferroelectric photovoltaics, Band gap
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engineering.
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1. Introduction In the past few decades, ferroelectric photovoltaic (FE-PV) effect has attracted
3
extensive research interests, due to their above-band-gap photovoltage[1, 2] [3] and
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switchable photocurrent under an external electric field[4] [5-8]. Spontaneous
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polarization and the induced internal bias field can effectively separate the
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photo-generated electron-hole pairs. This phenomena is different from the
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conventional photovoltaic mechanism observed in semiconductor p-n junctions [9].
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Recently, extensive studies have devoted to the investigations of FE-PV effect in a
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variety of ferroelectric systems (i.e., ceramics [10-12], single crystals[8] [13], thin
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films[4, 7] [14-17], polymers [18], and organic-inorganic ferroelectric composites
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[19]). Various models, such as bulk photovoltaic effect[20] [21], domain wall
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theory[22, 23] [3, 24], Schottky-junction effect [25] and depolarization field effect
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[26], have proposed to explain FE-PV effect. Based on the aforementioned models, it
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is widely believed that ferroelectric materials with relatively small energy band gap
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and large polarizations are promising for photovoltaic applications because of their
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capability in to absorb a wide range of solar spectrum and to separate photo-excited
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charge carriers [27].
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Unfortunately, traditional ABO3 structured perovskite ferroelectrics are insulators
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or high-band-gap semiconductors with band gap above 3.0 eV. Hence only the photon
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with energy beyond band gap can be absorbed by these materials. This results in low
21
efficiency of the devices. High energy band gap of ABO3 perovskite oxides originated
22
from the large difference in electron negativity between the B-site transition metal
3
ACCEPTED MANUSCRIPT cations and oxygen anions [10, 28]. Band gap of these oxides can be effectively
2
reduced via cation doping at the B-sites by other lower valence cations. Such doping
3
procedure indeed leads to a decrease in band gap and also significantly reduces
4
ferroelectric polarization as both metal dopant and oxygen vacancies are introduced
5
into the lattice. For instance, after doping Ni into KNbO3, the resultant
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[KNbO3]1-x[BaNi1/2Nb1/2O3-δ]x solid solutions have a low band gap down to 1.18 eV,
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and the spontaneous polarizations are below 1.0 µC/cm2 [10] at room temperature. In this work,
we report Ni-doped
Aurivillius ferroelectric ceramics
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Sr1-xBi2+xNb2-xNixO9-x (abbreviated as SBNN), consisting of alternative bismuth layers
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(Bi2O2)2+ and two perovskite layers (SrNb2O7)2- [29](as shown in Fig. 1). Thanks to
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the bismuth layers, having a good toleration towards oxygen vacancies [30], we
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could introduce Ni cations into the lattice and keep the corner-sharing octahedral
13
network almost undestroyed. (This will be discussed in detail later). Remarkably, both
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relatively large polarizations and narrow band gaps could be realized simultaneously
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in our designed SBNN ceramics. The band gaps of our SBNN samples range from
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2.25 eV to 2.70 eV as the Ni-doping concentration varies, and all the compositions
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exhibit considerably large remanent polarizations even at high temperatures (e.g., up
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to 120 °C), indicating that the SBNN ceramics can be used as FE-PV materials
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working efficiently in a wide temperature window.
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2. Methodology section
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2.1 Materials and methods Polycrystalline Sr1-xBi2+xNb2-xNixO9-x (SBNN100x, x=0.00, 0.01, 0.03, 0.05, 0.07,
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0.09) ceramics were fabricated by using a traditional solid state reaction method, by
5
which the raw materials reacted in high temperature process to synthesis the wanted
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matter. In order to maintain charge neutrality, content of bismuth should increase by
7
the same amount as the nickel concentration. Stoichiometric quantities of SrCO3 (Alfa,
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99%), Bi2O3 (Alfa, 99.9995%), Nb2O5 (Alfa, 99.9985%), and NiO (Alfa, 99.998%)
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were mixed by ball milling in ethanol for 24h. Mixtures were dried and then calcined
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at 775 °C for 2h in Ar atmosphere in muffle furnace. Calcined powders were
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re-milled with ethanol for 24h, dried, ground with PVA, and pressed into pellets.
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Green pellets were firstly sintered at 500 °C for 5h in air in order to remove PVA then
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sintered at 975 °C in Ar atmosphere. Gold electrode was first made using an ion
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sputtering, and then annealed at 500 °C for 1h in Ar atmosphere as electrodes for
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electrical tests. ITO was sputtered by magnetron sputtering on both sides of the
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samples as transparent electrodes.
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Phase purity of grown ceramics was examined by powder X-ray diffraction
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(XRD; Shimadzu 7000) using Cu Kα radiation generated at 40 kV and 30 mA.
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Cross-section morphologies of ceramics were checked by using a scanning electron
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microscope (SEM; HITACHI SU-8010). Polarization-electric-field (P-E) hysteresis
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loops were recorded using a Radiant ferroelectric workstation. Phase purities and the
22
oxygen vacancies were determined using an X-ray photoelectron spectroscopy (XPS) 5
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instrument.
absorption
experiments
2
ultraviolet-visible-near-infrared
3
UV3600) equipped with an integration sphere. A Xenon lamp was used to provide
4
standard solar illumination (AM1.5). Photovoltaic effect was checked by the Keithley
5
2400 source.
6
2.2 First-principles calculations
(UV-Vis-NIR)
were
carried
out
spectrophotometer
using
an
(SHIMADZU
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Optical
First-principles calculations were performed within density-functional theory as
8
implemented in the QUANTUM ESPRESSO code (QE) [31]. Nonlocal optimized
9
norm-conserving pseudopotentials [32, 33] and a 60 Ry plane-wave energy cutoff
10
were used for calculations. Partial core corrections (PCC) [34] were included in the Ni
11
pseudopotential. All calculations were performed using PBEsol exchange-correlation
12
functional [35] plus Hubbard U approach as implemented in QE [36], where an
13
effective U value of 4.5 eV was applied on Ni [37]. PBEsol+U calculations can
14
produce the correct semiconducting ground state for Ni-doped SrBi2Nb2O9. 8×8×4
15
Monkhorst-Pack k-grids [38] were used to simulate pure and Ni-doped SrBi2Nb2O9.
16
Both atomic positions and lattice parameters for SrBi2Nb2O9 systems were fully
17
optimized until residual Hellmann-Feynman forces on the atoms were smaller than 1
18
meV/Å and the stresses less than 0.1 kbar. Electronic contribution to the polarization
19
was calculated following the Berry phase formalism [39].
20
3. Results and discussion
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XRD patterns of SBNN ceramics are demonstrated in Fig. 2. All diffraction
22
peaks can be indexed with the standard PDF card 49-0607 (space group of A21am 6
ACCEPTED MANUSCRIPT [40]). No additional peak of any other phase is observed, which indicates that all
2
samples have a highly pure phase without secondary phases. The strongest diffraction
3
peak (115) consistent with the (1 1 2m+1) formula [41] of the highest diffraction peak
4
in the Aurivillius structure, where m stands for the repetitions of the perovskite
5
substructure between bismuth layers. For our SBNN samples, m is equal to 2 (Fig. 1).
6
Previous studies shown that the NbO6 octahedron has a high structural tolerance
7
towards Ni doping, as Ni2+ and Nb5+ cations have the similar ionic radius (0.69 Å vs.
8
0.64 Å) [42]. Previously, Ni-doped ferroelectric solid solutions were achieved in
9
PbNi1/3Nb2/3O3 [43] and PbTi1-xNixO3-δ solid solutions [37, 44]. For our SBNN
10
ceramics, the characteristic (115) peak shifts slightly lower angles with increasing
11
Ni-content, as demonstrated in Fig. 2, which is attributed to the larger ionic radius of
12
Ni2+ than that of Nb5+. Cross-section SEM images of the SBNN ceramics are shown
13
in Fig. 3. All the specimens show a transcrystalline fracture, indicating good sintering
14
properties of these ceramics. Characteristic lamellar grains are observed in all SBNN
15
ceramics, which is originated from crystal growth dynamics of intrinsic bismuth layer
16
lattice structure [45].
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Ferroelectric polarization is one of the essential properties for ferroelectric
17 18
photovoltaic applications. Fig. 4a shows the ferroelectric hysteresis (P-E) loops
19
measured at 120 °C and 10 Hz. Remanent polarization (Pr) of pure SBNN0 ceramic is
20
~1.8 µC/cm2, consistent with those in previous studies [46]. With increasing
21
Ni-doping concentration, the Pr value of the samples firstly decreases to 1.1 µC/cm2
22
after that increases to 2.7µC/cm2, as depicted in Fig. 4b. The first decrease of Pr at 7
ACCEPTED MANUSCRIPT SBNN1 might originate from oxygen vacancies introduced upon Ni doping, which
2
could pin the domain walls and cause difficulties in domain reversal [47]. With the
3
further increase of Ni-doping concentration (x≥0.03), more Sr ions are substituted by
4
Bi ions, leading to the enhancement in ferroelectricity of the SBNN ceramics, and the
5
subsequent increase in Pr of the SBNN samples.
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To understand experimental results of Ni-doped SBNN ceramics, we further
7
performed first-principles calculations to simulate the structural and ferroelectric
8
properties of both the pure and Ni-doped SrBi2Nb2O9. We start our investigation by
9
simulating the pure SrBi2Nb2O9. SrBi2Nb2O9 have an Aurivillius perovskite structure
10
with orthorhombic A21am symmetry, which exhibits an a-a-c+ NbO6 octahedral
11
rotation pattern in Glazer notation [48] and a spontaneous polarization along the
12
in-plane pseudocubic [110] direction. Table 1‐shows calculated lattice parameters of
13
SrBi2Nb2O9, similar results were also observed in previous studies [49]. The berry
14
phase calculation predicts a spontaneous polarization of 30.8 µC/cm2 for SrBi2Nb2O9,
15
which mainly comes from the off-center displacement of B-site Nb cations. It is well
16
known that the strong texture effect in orthorhombic ferroelectric oxides significantly
17
suppresses the realization of saturated polarization [50, 51]. Hence experimental
18
spontaneous polarization value of SrBi2Nb2O9 is supposed to be much lower than that
19
of theoretically predicted values.
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With Ni substitution on Nb-site, oxygen vacancies are created and some of the
21
A-site Sr ions are also replaced by extra Bi cations. Fig. 1 shows three
22
Sr3Bi9Nb7NiO35 (SBNN) configurations where oxygen vacancy is located at different 8
ACCEPTED MANUSCRIPT inequivalent sites, for simulation of the Ni-doped SrBi2Nb2O9 solid solutions. After
2
structural optimization, the SBNN-3 configuration where oxygen vacancy is located
3
in the perovskite layers adjacent to the Bi2O2 layers is determined to be the
4
energetically most stable one (Table 1), followed by the configuration where oxygen
5
vacancy is the middle of NiO6 and NbO6 octahedral (SBNN-1). The SBNN-2
6
configuration, in which the oxygen vacancy is located within the Bi2O2 layer, is the
7
most unstable one with the highest ∆E (see Table 1). Both the SBNN-1 and SBNN-3
8
configurations exhibit ferroelectric polarization smaller than that of the pure
9
SrBi2Nb2O9, as their corner-sharing octahedral network within the perovskite layer is
10
disrupted by oxygen vacancy (see Table 1 and Fig. 1). In contrast, the corner share
11
octahedral network is well preserved in the SBNN-2 configuration, in which the
12
oxygen vacancy is located within the Bi2O2 layer. Moreover, in comparison with
13
polar-inactive Sr, the A-site Bi cation in SBNN-2 has a larger polar displacement,
14
leading to an enhanced polarization for the SBNN-2 configuration. Note that there is
15
only a little difference in total energy between these three SBNN configurations.
16
Therefore, they most likely coexist in our SBNN solid solutions prepared. At high Ni
17
doping concentrations, it is likely that oxygen vacancies are formed in the Bi2O2
18
layers (corresponding to the SBNN-3 configuration). So, the SBNN ceramic samples
19
with higher Ni doping levels may show a larger ferroelectric polarization than that of
20
the pure SrBi2Nb2O9.
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Optical properties of the SBNN ceramics are presented in the Fig. 5a. With the
22
increase of Ni-content, SBNN ceramics show an enhanced visible absorption. 9
ACCEPTED MANUSCRIPT Absorption edges of the samples exhibit a red-shift with the increased Ni-doping,
2
indicating a reduction of band gap. In addition, absorption peak observed at ~700nm
3
confirms the presence of Ni2+ [52-54]. To determine the type of optical transition and
4
band gap energy, the Tuac’s plots of all samples are presented in the inset of Fig. 5b.
5
Generally, the Tuac’s plots are defined by the energy of photon (hν) on the abscissa
6
and (αhν) 1/n on the ordinate [55], where α is the absorption coefficient of the material.
7
The value of n denotes the intrinsic type of band transition, and n=1/2, 3/2, 2, 3 stand
8
for direct allowed transitions, direct forbidden transitions, indirect allowed transitions
9
and indirect forbidden transitions, respectively. For SBNNs, the value of n is equal to
10
1/2, demonstrating a direct allowed transition type. The band gaps calculated by the
11
Tuac’s plots are shown in Fig. 5b, which reveals a tunable band gap ranging from
12
2.25 eV to 2.7 eV. The band gap of SBNN0 is 2.7 eV, and the sample is palegreen, as
13
shown in Fig. 5c. However, the reported SrBi2Nb2O9 is faint yellow, and its band gap
14
is beyond 3.0 eV [56]. Since we sintered our samples at Ar atmosphere, the
15
differences in sample’s color and band gap between our samples and those reported
16
previously are attributed to different sintering conditions. It has been widely accepted
17
that different sintering atmospheres, such as O2, N2, and Ar, have significant influence
18
on the lattice constant, dielectric properties, ferroelectricity and optical properties of
19
ferroelectric ceramics [57, 58]. For instance, Kang et al fabricated ferroelectric
20
Sr0.5Ba0.5Nb2O6 ceramics in an atmosphere with different oxygen partial pressures
21
[57] . With the decrease of the oxygen partial pressure, the transmittance of the
22
samples decreases and the corresponding absorption edge shows a little red-shift. In
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ACCEPTED MANUSCRIPT our case, compared with air, the Ar sintering atmosphere has a lower oxygen partial
2
pressure. Therefore, sintering of SBNN ceramics in Ar atmosphere could accelerate
3
bismuth volatilization and subsequently generate more oxygen vacancies. The XPS
4
spectra of SBNN0 ceramics sintered in Ar atmosphere are shown in Fig. 6. The XPS
5
spectrum presented in Fig. 6a reveals the presence of Sr, Bi, Nb, and O without any
6
other trace impurities except for a small amount of C, which is taken as a reference
7
for the calibration of the banding energy [59]. Fig. 6b shows the narrow-scan
8
spectrum of O 1s. The profile consists of three components: a main peak at 528.41 eV,
9
corresponding to the bulk Nb-O cation-oxygen bonds. The two sub-peaks at 530.61
10
eV and 532.41 eV indicate the existence of oxygen vacancies as well as the adsorbed
11
surface H2O [60]. Oxygen vacancies lead to the formation of impurity energy levels in
12
the band gap of the sample, giving rise to the decrease in the band gap measured
13
experimentally. The above argument could also be a reasonable interpretation for the
14
results in the work by Chen et al [61]. Band gap of the Bi5Ti3FeO15 ceramics in their
15
work decreases from 3.39 eV to 3.22 eV, when the sintering time increases from 240
16
mins to 360 mins. We believe that that is also caused by severe bismuth volatilization
17
in the samples with longer sintering times. To further confirm our point, SBNN0
18
ceramics were sintered in air atmosphere with the same annealing procedure. The
19
as-fabricated SBNN ceramic is faint yellow, as shown in the inset of Fig. 5c,
20
consistent with the results previously reported. Fig. 5d shows the comparison of the
21
remanent polarization and band gap of the SBNN ceramics with some other reported
22
materials. As one can see, all the ferroelectrics previously studied can be divided into
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ACCEPTED MANUSCRIPT two categories: one is the materials with a larger remanent polarization and a higher
2
band gap, such as lead-based ferroelectrics [62], BiFeO3 and its solid solutions [21],
3
and Bi4Ti3O12-based ferroelectrics [63]. The other is the materials with a smaller
4
remanent polarization and a lower band gap, such as KBiFe2O5 [13] and KBNNO [10].
5
Therefore, for the first time, both a higher remanent polarization and a lower band gap,
6
essential for the photovoltaic applications [27], have been successfully achieved in
7
our SBNN9 ceramics via Ni doping.
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The band-gap narrowing effect observed in this work could be attributed to the
9
gap states introduced by the Ni dopants [44, 45]. The valance and conduction band of
10
SrBi2Nb2O9 are composed of hybridized O 2p and Nb 4d states. After Ni doping, Ni
11
cations introduce both filled and unfilled Ni 3d gap states into the SBNN systems,
12
giving a rise to direct optical transition from the filled to unfilled Ni-3d states.
13
Therefore Ni doping plays a crucial part in lowering the energy band gap.
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For present work, SBNN9 ceramic is found to possess relatively higher remanent
15
polarization and a smaller band gap. Hence photovoltaic measurements done on
16
SBNN9 ceramics and obtained results are demonstrated in Fig. 7. As shown in Fig. 7a,
17
standard AM1.5 illumination causes an obvious photovoltaic response in the
18
as-fabricated samples. The short circuit current (Jsc) is 3.5 nA/cm2 and the open
19
circuit voltaic (Voc) is -2.3 V. Fig. 7b shows the dependence of photovoltaic response
20
upon polarization switching. In order to switch the polarization direction, sample was
21
poled under an electric field of 50kV/cm at 100
22
show an enhanced photovoltaic response, with Jsc of 7.0 nA/cm2, and Voc of -3.5 V. It
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. Samples poled in the UP direction
ACCEPTED MANUSCRIPT can be seen that Jsc value of poled sample is approximately twice that of the
2
as-fabricated one. Samples poled in the DOWN direction show a reversed response.
3
Since both bottom and top electrodes are ITO films and the same, the switching of the
4
photovoltaic responses observed in the SBNN9 samples must originate from the
5
switching of ferroelectric polarizations under electrical poling. Also, we notice that
6
samples exhibit a very low Jsc and a relatively high Voc. It is well accepted that there
7
are many grain boundaries and pores in ceramics, which block the migration of
8
photo-generated carriers and thus reduce photocurrent. Remarkably, our SBNN
9
samples show a high Voc of several V, in comparison with the Voc of several hundred
10
mV in most of the thin films reported previously. Difference in photovoltaic response
11
of thin films and bulk materials (e.g, ceramics, or single crystals) has been noticed for
12
a long time. For example, Qin et al [26] reported a low Voc of 0.7 V in PLZT thin
13
films. However, a high Voc of 7 V was also reported in PLZT ceramics of the same
14
composition by Zhang et al [62]. This difference could be explained by using the
15
method developed by Yang et al [3]. The hypothesis made by Yang et al [3] is that
16
each nanometer-scale domain wall can generate a potential step, and all these
17
potential steps are in series, sum of which determines the magnitude of output
18
photovoltage. Since ceramics have large volume to react than that of thin films as a
19
result ceramic samples are supposed to have more domain walls connected in series.
20
Therefore, it is reasonable to observe a higher Voc in our SBNN9 ceramic samples.
21
4. Conclusions
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In summary, pure phase Ni-doped strontium bismuth niobate ceramics are
22 13
ACCEPTED MANUSCRIPT synthesized successfully. SBNN9 ceramic exhibits a relatively low band gap (2.25 eV)
2
and a considerable remanent polarization (2.5 µC/cm2) at 120 °C. In addition, UP
3
poled SBNN9 ceramic shows an excellent photovoltaic response (Jsc of 7.0 nA/cm2,
4
and Voc of -3.5 V). Photovoltaic output can be switched reversibly by applying an
5
external UP/Down electric DC field. The SBNN9 ceramic developed in this work
6
demonstrates superior photovoltaic properties among all the FE-PV candidate
7
materials because of lower band gap and higher remanent polarization. Our work
8
implies that modifying ferroelectrics with a bismuth-layered perovskite structure
9
using transition metal (Ni in this work) may be a good strategy to achieve
10
ferroelectrics with a lower band gap and a large remnant polarization, both of which
11
are crucial for obtaining superior FE-PV properties. Therefore, this work opens a new
12
avenue for ferroelectric photovoltaic and photocatalytic applications.
13 14
Acknowledgments
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This work was supported by the National Science Foundation of China (NSFC
16
No. 51372195, NO.51402196 and No. 11574244), the China Postdoctoral Science
17
Foundation (Grants 2014M552229 and 2015T80915), the CSS project (Grant No.
18
YK2015-0602006), the Fundamental Research Funds for the Central Universities
19
(2013JDGZ03), and Program for Innovative Research Team in University of Ministry
20
of Education of China (IRT13034). X.J. Lou would like to thank the “One Thousand
21
Youth Talents” program for support. G. Y. Gou acknowledges the computational
22
support from National supercomputer center (NSCC) in Tianjin.
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Fig. 1: Three symmetrically inequivalent configurations representing Ni-doped SrBi2Nb2O9, where oxygen vacancy is located between perovskite NiO6 and NbO6 octahedral, within Bi2O2 layers and in the perovskite layers adjacent to the Bi2O2
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layers.
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Fig. 2: Powder XRD patterns of the polycrystalline SBNN ceramics. The right inset
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Fig. 3: SEM images of the SBNN ceramics with Ni-doping concentration at (a) x=0,
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(b) x=0.01, (c) x=0.03, (d) x=0.05, (e) x=0.07, (f) x=0.09.
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tested at 120
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Fig. 4: (a) Polarization-electric field (P-E) hysteresis loops of the SBNN ceramics and 10 Hz. (b) Remanent polarization of the SBNN ceramics as a
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Fig. 5: (a)UV-Vis-NIR absorption spectrums of SBNN ceramics. (b) Band gaps of the
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SBNN ceramics as a function of Ni-doping concentration. The inset shows the Tauc’s
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Fig. 6: (a) XPS spectrum of SBNN0 ceramics sintered in Ar. (b) The narrow-scan
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Fig. 7: J-V characteristics in the SBNN9 ceramics. (a) The as-fabricated samples with and without illumination. (b) The I-V characteristics with illumination at different
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poling directions.
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ACCEPTED MANUSCRIPT Table 1: Our calculated structural parameters and ferroelectric polarization for pure SrBi2Nb2O9 and three configurations representing Ni-doped SrBi2Nb2O9. Relative
b/Å
c/Å
P(μC/cm2)
SrBi2Nb2O9
5.59
5.57
24.90
30.8
SBNN-1
5.51
5.50
25.59
25.0
SBNN-2
5.63
5.60
24.54
33.5
SBNN-3
5.54
5.50
24.68
27.8
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E/(eV/f. u.)
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a/Å
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energies of three SBNN configurations are also given for comparison.
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0.22 0.29 0.00
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Synthesis of Ni-doped SrBi2Nb2O9 ceramics. Stable ferroelectrcity (Pr=2.5 µC/cm2) and decreased optical band gap (2.25 eV). The band gaps are dependent on the sintering atmosphere. Photovoltaic effects can be switched by the applied external field.
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