Wide-range tunable bandgap in Bi1−xCaxFe1−yTiyO3−δ nanoparticles via oxygen vacancy induced structural modulations at room temperature
11 Sep 2015-Vol. 2, Iss: 9, pp 095012
TL;DR: In this article, the authors demonstrate that oxygen vacancies produced by aliovalent (Ca2+) doping in BiFeO3 (BCFO) and associated structural changes due to VO ordering result in systematic alteration of the bandgap (Eg) over a wide range from 1.5 eV to 2.3 eV.
Abstract: We demonstrate that oxygen vacancies (VO) produced by aliovalent (Ca2+) doping in BiFeO3 (BCFO) and associated structural changes due to VO ordering result in systematic alteration of the bandgap (Eg) over a wide range from 1.5 eV to 2.3 eV. By contrast, the change in the bandgap of a Ca2+ and Ti4+ co-doped BiFeO3 (BCFTO) system, wherein the VO formation is suppressed, is negligible. These contrastive results strongly confirm the role of oxygen vacancies in altering the bandgap of BCFO. Irrespective of doping, microstrain, which is found to be large (0.3 to 1.2%) below a critical size (dc ~ 60 nm) also produces a small, yet linear change in the bandgap (Eg from 2.0 to 2.3 eV). The cubic phase stabilizes gradually in BCFO for x > 0.1 through an orthorhombic phase (for 0.05 0.1 in BCFTO. This change in BCFO at 300 K suggests a high-pressure-like (or high-temperature-like) effect of the oxygen vacancies and dopants on the structure. Systematic variations in the relative intensities and peak positions of Fe d–d transitions in BCFO reveal the local changes in Fe–O–Fe coordination. These results along with XANES and HRTEM studies substantiate the observed structural changes.
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TL;DR: In this article, a singe phase bismuth ferrite doped by yttrium (Bi1−xYxFeO3, x = 0, 0.05, 1, 0., 0.15, 1.2 and 0.25) was synthesized by solid-state reaction followed by sintering.
31 citations
TL;DR: In this article, the compressive microstrain of 0.024 and 0.016 was estimated for CoFe2O4 samples H120 and H180 respectively using Williamson-Hall plot analysis assuming uniform deformation model.
25 citations
TL;DR: In this article, the optical response of multiferroic Bi1−xSrxFeO3 (0,≤,x,≤ 0.45) samples is studied in the spectral range from 1 eV to 4 eV.
24 citations
TL;DR: This work demonstrates oxygen vacancies (VO), present within the lattice and at grain boundary (GB), can explicitly be controlled to achieve high JSC and VOC simultaneously and demonstrates the efficiency ~ 0.22 % can be achieved in solid state BFO solar cells under AM 1.5 one Sun illumination.
Abstract: Designing solid-state perovskite oxide solar cells with large short circuit current (JSC) and open circuit voltage (VOC) has been a challenging problem. Epitaxial BiFeO3 (BFO) films are known to exhibit large VOC (>50 V). However, they exhibit low JSC (≪μA/cm2) under 1 Sun illumination. In this work, taking polycrystalline BiFeO3 thin films, we demonstrate that oxygen vacancies (VO) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high JSC and VOC simultaneously. While aliovalent substitution (Ca2+ at Bi3+ site) is used to control the lattice VO, Ca and Ti cosubstitution is used to bring out only GB-VO. Fluorine-doped tin oxide (FTO)/Bi1-xCaxFe1-yTiyO3-δ/Au devices are tested for photovoltaic characteristics. Introducing VO increases the photocurrent by four orders (JSC ∼ 3 mA/cm2). On the contrary, VOC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO3. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-VO. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in JSC. The cosubstitution, following a charge compensation mechanism, decreases the lattice VO concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve VOC (3-8 V) much larger than that of BiFeO3 (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice VO concentration dominates GB-VO concentration. Notwithstanding, both lattice and GB-VO contribute to the increase in JSC; the former weakens ferroelectricity, and as a consequence, undesirably, VOC is lowered well below 0.5 V. Using optimum JSC and VOC, we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.
15 citations
TL;DR: In this article, photo-response of nanoparticle-segregated grain boundary (BFO-AP) and clean grain boundary(BFO -AA) samples was investigated on spark plasma sintered BiFeO3 samples with two contrasting morphologies.
Abstract: Photoconductivity studies on spark plasma sintered BiFeO3 samples with two contrasting morphologies, viz., nanoparticle-segregated grain boundary (BFO-AP) and clean grain boundary (BFO-AA), show that their photo-response is largely influenced by the grain boundary defects. Impedance analyses at 300 K and 573 K clearly demarcate the contributions from grain, grain-boundary, and the nanoparticle-segregated grain-boundary conductivities. I-V characteristics under 1 sun illumination show one order of higher conductivity for BFO-AP, whereas conductivity decreases for BFO-AA sample. Larger photocurrent in BFO-AP is attributed to the extra conduction path provided by oxygen vacancies on the nanoparticle surfaces residing at the grain boundaries. Creation of photo-induced traps under illumination and the absence of surface conduction channels in BFO-AA are surmised to result in a decreased conductivity on illumination.
15 citations
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TL;DR: In this paper, the evolution of transport properties with temperature is discussed in relation to structure, revealing a series of phase transitions between ambient temperature and 1000 °C, which is confirmed by measurement of water vapour transfer in a cell in which the perovskite is exposed to wet hydrogen on both sides.
331 citations
TL;DR: In this article, a single-phase polycrystalline BiFeO3 was synthesized from ultrapure starting oxides, and techniques for reducing the fraction of the secondary phases in the reaction products were developed.
Abstract: Studies of a BiFeO3 synthesis were performed to identify reasons for the appearance of secondary phases, the Bi25FeO39- and Bi2Fe4O9-type phases, in the reaction product. X-ray diffraction and microstructural analyses, performed on samples with different concentrations of impurities, showed that the impurities in the starting material crucially influence the phase composition of the reaction product. A fraction of the generated secondary phases strongly depends on the nature and concentration of the impurities. The experimental results can be explained by the theoretical consideration of ternary phase relations between Bi2O3, Fe2O3, and an impurity oxide. Single-phase polycrystalline BiFeO3 was synthesized from ultrapure starting oxides. To avoid using the expensive ultrapure oxides, techniques for reducing the fraction of the secondary phases in the reaction products were developed.
319 citations
TL;DR: In this paper, the effect of heterovalent Ca, Sr, Pb, and Ba substitution on the crystal structure, dielectric, local ferroelectric, and magnetic properties of the BiFeO3 multiferroic perovskite was studied.
Abstract: In this work, we studied the effect of heterovalent Ca, Sr, Pb, and Ba substitution on the crystal structure, dielectric, local ferroelectric, and magnetic properties of the BiFeO3 multiferroic perovskite. Ceramic solid solutions with the general formula Bi0.7A0.3FeO3 (A is a doping element) were prepared and characterized by x-ray diffraction, dielectric, piezoresponse force microscopy (PFM), and magnetic measurements. It is shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence was confirmed by the PFM data. Magnetic properties of the solid solutions are determined by the ionic radius of the substituting element. Experimental results suggest that the increase in the radius of the A-site ion leads to the effective suppression of the spiral spin structure of BiFeO3, resulting in the appearance of net magnetization.
312 citations
TL;DR: In this paper, a modified Pechini method was used to reduce the rhombohedral distortion from cubic structure by decreasing particle size accompanied by decreasing polarization inferred from atomic displacements found by Rietveld refinement of X-ray powder diffraction data, and the Neel temperature was correlated with the volume of the crystallites and the polar displacements of cations.
Abstract: BiFeO3 nanoparticles prepared by a modified Pechini method display strong size-dependent properties. The rhombohedral distortion from cubic structure is reduced by decreasing particle size accompanied by decreasing polarization inferred from atomic displacements found by Rietveld refinement of X-ray powder diffraction data. With decreasing crystallite size, the Neel temperature decreased and the magnetic transition was increasingly diffuse. The Neel temperature was shown to correlate with the volume of the crystallites and the polar displacements of cations.
300 citations
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272 citations