† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 11104202 and 51572193).
Multiferroic material as a photovoltaic material has gained considerable attention in recent years. Nanoparticles (NPs) La0.1Bi0.9−xSrxFeOy (LBSF, x = 0, 0.2, 0.4) with dopant Sr2+ ions were synthesized by the sol–gel method. A systematic change in the crystal structure from rhombohedral to tetragonal upon increasing Sr doping was observed. There is an obvious change in the particle size from 180 nm to 50 nm with increasing Sr substitution into LBFO. It was found that Sr doping effectively narrows the band gap from ∼ 2.08 eV to ∼ 1.94 eV, while it leads to an apparent enhancement in the electrical conductivity of LBSF NPs, making a transition from insulator to semiconductor. This suggests an effective way to modulate the conductivity of BiFeO3-based multiferroic materials with pure phase by co-doping with La and Sr at the A sites of BiFeO3.
Multiferroic materials which exhibit a coupling between magnetic and ferroelectric order parameters have been widely noted as materials promising for applications in multiple state memory elements, electric field controlled ferromagnetic resonance devices and transducers with magnetically modulated piezoelectricity.[1–4] Among all multiferroic materials studied so far, BiFeO3 (BFO), having the perovskite-type structure with space group R3C,[5] possesses a superior ferroelectric polarization (Pr ∼ 100 μC/cm2), a high Curie temperature (TC ∼ 1103 K), and a high Neel temperature (TN ∼ 643 K). In addition, BFO is the one with simultaneous A site ferroelectric and B site G-type antiferromagnetic nature, and has attracted more and more attention recently. Primarily, BFO-based materials are of interest as multiferroics that are used for the development of magnetoelectric and photovoltaic materials, including thin-film materials and nanostructure sand compounds with BFO nanosized blocks.[6–8] For example, BFO nanoparticles (NPs) were used for BFO-graphene nanohybrids, which may find application due to their photocatalytic properties.[9]
Recently, several studies on A-site substitution of isovalent (La3+, Nd3+, and Sm3+) or acceptor (Ca2+, Ba2+, and Pb2+) cations for Bi and B-site substitution of donors (V5+, Nb5+, Mn4+, or Ti4+) for Fe have been carried out in order to improve the magnetic and ferroelectric properties.[10–13] As reported earlier, it is known[14] that in Ca-doped BFO, oxygen vacancies are produced that act as donor impurities to compensate the calcium acceptors. In order to maintain the electrical neutrality of the system, the oxygen vacancies can modify the electronic structure and the electrical conductivity. Similarly, several researchers have reported that doping Sr2+ can also change the properties of BFO because it can affect the structural, optical, dielectric, and magnetic properties.[15,16]
Converting sunlight into electricity is one of the most promising approaches to generate renewable energy. It has also been observed that doping can improve the photovoltaic properties of BFO by tuning the multiferroic properties and the leakage current.[17] In order to make use of BFO, it is necessary to modulate the electrical conductivity of BFO, i.e., converting it from an insulator to a semiconductor even to a conductor. Such kind of transition may make BFO a more comprehensive multiferroic material with a wide adjustable range of conductivity. Meanwhile, SrFeO3 retains a metallic conductivity of ∼ 10−3 Ω/cm down to 4 K as reported by MacChesney et al.[18] So it is expected that Sr doping in BFO will increase the conductivity of the sample. There are only a few reported works on the substitution of divalent Sr ion at the trivalent Bi site of BFO NPs.[19,20] The reports demonstrated that the saturated magnetization of 20% Sr-doped BFO is 0.731 emu/g, which is higher than that of bulk BFO (0.229 emu/g), confirming that the substitution of Sr is an efficient way to enhance the magnetic properties of BFO.[21] In addition, the previously published works have focused on the substitution of low concentrations (below 20%) of Sr in BFO, and higher concentration Sr doping in NPs (up to 20%) has rarely been reported. Especially, there is no report on the effects of Sr doping on the leakage current for BFO NPs. In the present study, we focus on La0.1Bi0.9−xSrxFeOy (LBSF) NPs with Sr ions up to 40% and the effect of varying Sr concentration on the optical, leakage current, and magnetic properties.
In this work, we have synthesized high quality La0.1Bi0.9−xSrxFeOy (x = 0, 0.2, 0.4, as abbreviated to LBFO, LBSF-20, and LBSF-40, respectively) NPs via a facile sol–gel method. The optical, leakage, and magnetic properties of the NPs have been investigated. In the Sr-doped LBFO, the leakage current density of NPs is significantly increased and the largest leakage current density (∼ 16 mA/cm2) is observed in the LBSF-40 NPs. Thus the electrical conductivity of the LBSF NPs shows a clear signature of transition from an insulating state to a semiconducting state. The magnetic properties have been greatly improved and the saturated magnetization (MS) for LBSF-40 is 8.77 emu/g.
LBSF NPs were synthesized by the sol–gel method. High purity grade Bi(NO3)3·5H2O, Fe(NO3)3·9H2O, La(NO3)3·6H2O, and Sr(NO3)2 in an appropriate molar ratio were mixed in analytical grade ethylene glycol to prepare the precursor solution. Analytical grade tartaric acid was added to the solution in 1:1 molar ratio with respect to the precursors, and the mixture was stirred continuously at 70 °C for 3 h. The solution was a transparent, reddish-brown, and clear sol. Then it was placed in a drying oven at 90 °C for 4 h until the clear sol completely turned to a yellowish-brown gel. The gel was further dried at 140 °C for 3 h, then annealed at 400 °C for 2 h, and finally ground into powder. Subsequently the powder was calcined at 600 °C in air for 2 h to obtain NPs.
The crystalline structure analysis of the NPs was carried out by x-ray diffraction (XRD, Rigaku D/MAX-2500 diffractometer with Cu Kα radiation). The average particle size and microstructural properties of the NPs were investigated by a transmission electron microscope (TEM) (JEM-2100F). High resolution TEM (HRTEM) and the corresponding selected area electron diffraction (SAED) patterns were recorded by the same instrument. Magnetic characterization of the samples was done with a magnetic property measurement system (MPMS) (SQUID-VSM made by Quantum Design). The UV–vis absorption spectrum of our NPs was obtained by a Shimadzu UV-3600UV-VIS-NIR spectrophotometer. To study the electrical properties, the samples were pressed into pellets by compaction of powders in a uniaxial press, and the pellets were sintered for half an hour at 600 °C. The dielectric properties were measured by a precise impendence analyzer WK 6400 with an ac signal of 1000 mV. The leakage current curve was evaluated using an Axiacct model TF 2000 ferroelectric analyser. All the measurements were carried out at room temperature.
The observed, calculated, and difference XRD patterns for LBFO, LBSF-20, and LBSF-40 obtained from the Rietveld refinement are shown in Fig.
To investigate the morphology of the as-obtained NPs, we carried out transmission electron microscopy analysis. Figures
It is important to investigate the electronic structure of Sr2+ doped into LBFO because the heterovalent substitution-induced defects may cause high leakage current and narrow band gap. Figure
Figure
To completely understand the enhancement of the current density in La and Sr co-doped BFO NPs, one possible factor that controls the transport properties should be considered. It is well known that in the perovskite structure materials, the ionization process of the oxygen vacancy (VO) will create conducting electrons (e′), thus contributing to the leakage current. The process can be described as follows:
The ferroelectric hysteresis loops of the Sr-doped LBFO NPs at room temperature are shown in Fig.
The dielectric properties of the Sr-doped LBFO NPs measured at room temperature in the frequency range from 1 kHz to 1 MHz are shown in Fig.
Figure
Single-phase LBFO perovskite NPs and Sr-doped LBFO NPs have been successfully prepared by the sol–gel method. The average particle size decreases obviously with Sr doping. The M–H measurements reveal that the saturation magnetization of the NPs is distinctly influenced by the increase of the Sr doping concentration. Both dielectric constant and dielectric loss factors of all compositions decrease gradually with the increase of the frequency. Significant improvements, such as increased leakage current density and reduced band gap, are observed for the LBSF NPs. Such vast modulation of the conductivity may further enhance the application of the BFO structure from random access memories to solar cells and even electronic and photonic devices in a drastic fashion.
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