Bi3.25La0.75Ti3O12 Research Articles

Optoelectronic Properties of Ferroelectric Composites of Bi3.25La0.75Ti3O12 (BLT) and Co-Doped BLT Thin Films Modified by FeCo-Doped BLT

Driven by the growing demand for renewable and clean energy, the photovoltaic effect of various solar cells and materials was investigated for the conversion of light energy into electricity. We modified the Bi3.25La0.75Ti3O12 (BLT) and Co-doped BLT (Co–BLT) composites with Fe and Co-doped BLT (FeCo–BLT) films to narrow the bandgap and increase visible light absorption, thereby improving the efficiency of the photovoltaic reaction. In this study, BLT and Co–BLT thin films were fabricated by off-axis sputtering and then modified with FeCo–BLT thin films to produce dual-ferroelectric, thin-film composite materials that improved the photovoltaic power generation performance. Photoelectric test results showed that the modified double-ferroelectric, thin-film composites had superior optoelectronic properties. The current density was significantly enhanced by modifying the BLT films with doped Fe and Co. Therefore, this modification improved the efficiency of ferroelectric thin-film photovoltaic reactions.

Bi0.5Na0.5TiO3-Bi3.25La0.75Ti3O12 Lead-Free Thin Films for Energy Storage Applications through Nanodomain Design

Dielectric capacitors have received increasing attention due to their high power density. The Bi-based Aurivillius phase compound Bi3.25La0.75Ti3O12 (BLT) is considered a potential material in the field of energy storage due to its excellent ferroelectric properties and good fatigue resistance, and temperature stability. In this paper, 0.4Bi0.5Na0.5TiO3-0.6Bi3.25La0.75Ti3O12 (0.4NBT4BNT-0.6BLT)-thin films were prepared on Pt/Ti/SiO2/Si substrates with the sol-gel method. The addition of BNT destroys the long-range ferroelectric order of BLT and forms nanodomains. By increasing the BNT content, the BLT is transformed from a ferroelectric state to a relaxed state, and its application in the field of energy storage is realized. The recoverable energy density is 42.41 J/cm3, and the recoverable energy storage density is relatively stable in the range of 25–200 °C with good thermal stability. The energy storage efficiency is 75.32% at ~2663 kV/cm. The leakage current density at 300 kV/cm is 1.06 × 10−9 A/cm2.

Ferroelectric B-Site Modified Bismuth Lanthanum Titanate Thin Films for High-Efficiency PV Systems

Over the past decades, ferroelectric photovoltaic (FE-PV) systems, which use a homogenous ferroelectric material as a light-absorbing layer, have been studied using ferroelectric oxides. The PV activity of materials can be enhanced by adjusting the bandgap of materials, and it would have a large effect on the ferroelectric complex oxides. This phenomenon in epitaxial thin films of ferroelectric complex oxide, Bi3.25La0.75Ti3O12 (BLT), Fe- and Co-doped films were observed. Compared with undoped BLT, Co-(BLCT) doping and Fe and Co combined (BLFCT) doping resulted in the gradual reduction in the bandgap and efficient visible light absorption. The reduction in the bandgap to 11.4% and 18.1% smaller than the experimentally measured Eg of the bismuth titanate-based film using a simple Fe- and Co-doping method was performed, while maintaining ferroelectricity by analyzing the BLCT and BLFCT films based on polarization loops, and the temperature range of the out-of-plane lattice parameters and the photocurrent density of the BLFCT film was 32.2 times higher than that of the BLT film, which was caused by the decrease in the bandgap. This simple doping technique can be used to tune additional wide-bandgap complex oxides so that they can be used in photovoltaic energy conversion or optoelectronic devices.

Piezoelectric polarization promoted separation of photogenerated carriers in Bi3.25La0.75Ti3O12 with different micro-morphologies for efficient elimination of 2,4-Dichlorophenol and tetracycline

Piezoelectric photocatalysis has been considered as a fascinating solution to promote the effective separation of photogenerated carriers for the efficient removal of organic pollutants. Herein, we report that micro-morphology plays a decisive role in determining piezo potential to enhance photocatalytic activities. The high deformation capacity of Bi3.25La0.75Ti3O12 (BLT) nanowires leads to superior piezo-photocatalytic activity compared to BLT nanosheets and BLT nanospheres for efficient removal of refractory 2,4-Dichlorophenol (2,4-DCP) and tetracycline (TC). One-dimensional materials exhibit extremely high elasticity due to their greatly reduced size, allowing for a large degree of mechanical deformation. Under light and ultrasonic, the removal rates of 2,4-DCP and TC by BLT nanowires reached 93.04% and 96.30% after 60 min and 12 min, respectively, which were 2.97 and 2.96 times higher than those by light irradiation only. Furthermore, the ultrasonic power and frequency are important factors to optimize the piezoelectric catalytic and piezo-photocatalytic performance. Modulating the morphology of the material can regulate the piezoelectric effect of the material to suit different practical needs. Moreover, it is simpler to obtain efficient piezoelectric effects through morphology optimization than to design and develop new materials for a wider range of applications.

Creation of oxygen vacancies to activate lanthanum-doped bismuth titanate nanosheets for efficient synchronous photocatalytic removal of Cr(VI) and methyl orange

Bi3.25La0.75Ti3O12 (BLTO) nanosheets were synthesized via a simple hydrothermal process, followed by an economically facile vacuum-deoxidized treatment to fabricate Bi3.25La0.75Ti3O12-x (BLTO12-x) nanosheets with abundant surface oxygen vacancies. Relative ratio of surface and bulk oxygen vacancy, as evidenced with positron annihilation lifetime and X-ray photoelectron spectra, was controllable via adjusting vacuum-deoxidized temperature and time. Photocatalytic performance of the pristine and deoxidized nanosheets for synchronous photocatalytic removal of Cr(VI) and methyl orange (MO) were thoroughly investigated. Control experimental results show that the photocatalytic activities of the BLTO12-x nanosheets significantly relied on vacuum-deoxidized temperature and time. The optimal deoxidized nanosheets as activated at 330 °C for 3 h, which has the largest relative concentration of surface oxygen vacancy, shows highest activity under visible light irradiation. Moreover, the synergetic effect for synchronous enhanced photocatalytic degradation of Cr(VI) and MO were observed over all BLTO12-x nanosheets. The detailed synergistic reduction−oxidation removal mechanism is proposed. The surface oxygen vacancy led to rise of valence band maximum, which is responsible for both widening of valence band and decreasing of band gap. The former enhances separation efficiency of photoinduced e−/h + pairs, and the latter extends photoresponse range into visible light region. As a result, compared with pristine BLTO, the visible-light catalytic activity of deoxidization-activated BLTO12-x was significantly enhanced. The vacuum-deoxidized strategy proposed in this work can introduce oxygen vacancy into the matrix of catalyst, which provides an effective way to design and prepare high-efficiency oxides based photocatalyst.

Reversible magnetoelectric switching in multiferroic three-dimensional nanocup heterostructure films

Self-assembled nanocomposite films containing ferroelectric and ferromagnetic phases have attracted enormous research interest because they are the most promising candidates for practical multiferroic applications. However, obtaining a genuine magnetoelectric (ME) coupling effect is still challenging in this research area. To substantially improve the ME effect, new heterostructure designs with efficient strain control between two phases are urgently needed. Herein, a novel three-dimensional (3D) nanocup architecture of a heterostructure film is developed. To establish the unique architecture, a heavily Co, Fe-doped ferroelectric Bi3.25La0.75Ti3O12 (BLT) target was used during the growth of BLT thin films via pulsed laser deposition. Consequently, 3D nanocup-structured CoFe2O4 (CFO) particles formed inside the BLT via spontaneous nucleation and agglomeration. The 3D nanocup BLT-CFO film exhibited magnetically controlled reversible dielectric switching, which is direct evidence of strong ME coupling caused by the efficient interfacial strain coupling and low leakage of the novel nanocup architecture. The obtained results strongly suggest that the 3D nanocup heterostructure film significantly improves the ME coupling effect. In addition, we propose a new paradigm in the architecture design of self-assembled nanocomposite films for diverse multifunctional devices.

Characterization of highly (117)-oriented Bi3.25La0.75Ti3O12 thin films prepared by rf-magnetron sputtering technique

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were deposited on Pt(111)/Ti/SiO2/Si substrates at 400 °C using rf-magnetron sputtering method. The microstructures were studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometer. The as-deposited thin film is amorphous, while transformed to microcrystalline and well crystalline states after annealing at 650 °C and 750 °C, respectively. After annealing at 750 °C, the polycrystalline BLT thin film showed plated-like grains all with (117)-preferred orientation. Analyses of ferroelectric properties indicated that, comparing with the as-deposited thin film, the highly (117)-oriented crystalline thin film exhibited well-saturated hysteresis loops with a superior remnant polarization (2Pr) of 30.7 μC/cm2.

Comparison between the electrical properties of bismuth layer-structured and intergrowth bismuth layer-structured ferroelectric ceramics

Bismuth layer-structured ferroelectric (BLSF)-type Bi4Ti3O12 (BiT), Bi3.25La0.75Ti3O12 (BLaT), and Bi3.1Nd0.9Ti3O12 (BNdT) ceramics and intergrowth BLSF-type BiT-CaBi4Ti4O15 (CBTO), BLaT-CBTO, and BNdT-CBTO ceramics were prepared using the solid-state reaction method. The electrical and the high-temperature properties of the intergrowth BLSF ceramics were compared with those of BLSF ceramics to investigate the possibility of using the former for high-temperature applications. The X-ray diffraction (XRD) analysis revealed well-formed stable structures in all BLSF and intergrowth BLSF ceramics without any second-phase formation. When their electrical properties were examined, the intergrowth BLSF ceramics BiT-CBTO, BLaT-CBTO, and BNdT-CBTO were found to outperform the BLSF ceramics BiT, BLaT, and BNdT, respectively, in terms of the remanent polarization and piezoelectric coefficient values. Among the intergrowth BLSF ceramics, those doped with rare-earth ions La and Nd, i.e., BLaT-CBTO and BNdT-CBTO ceramics, were found to have improved electrical properties compared to the BiT-CBTO ceramics. In particular, the dielectric constants and the piezoelectric coefficients of the BNdT-CBTO ceramics were observed to be as high as 146 and 15.4 pC/N, respectively, which were 28% and 10% higher than those of BNdT ceramics. In the thermal depoling behavior measured to examine high-temperature stability, the intergrowth BLSF ceramics, compared to the BLSF ceramics, demonstrated improvements in the thermal depoling temperature ranging from 100 to 300 °C.

Effect of Annealing Temperature on Bi3.25La0.75Ti3O12 Powders for Humidity Sensing Properties

Bi3.25La0.75Ti3O12 (BLT) powders have been synthesized via the metal–organic decomposition method with annealing of the BLT precursor solution at 350°C, 450°C, 550°C, 650°C or 750°C. The crystalline structure and morphology of the BLT powders were characterized by x-ray diffraction analysis, field-emission scanning electron microscopy, energy-dispersive x-ray spectroscopy, and specific surface and pore size analyses. The humidity sensing properties of the BLT powders annealed at the five temperatures were investigated to determine the effect of annealing temperature. The annealing temperature strongly influenced the grain size, pore size distribution, and specific surface area of the BLT powders, being largely correlated to their humidity sensing properties. The specific surface area of the BLT powder annealed at 550°C was 68.2 m2/g, much larger than for the other annealing temperatures, and the majority of the pores in the BLT powder annealed at 550°C were mesoporous, significantly increasing the adsorption efficiency of water vapor onto the surface of the material. The impedance of the BLT powder annealed at 550°C varied by more than five orders of magnitude over the whole humidity range at working frequency of 100 Hz, being approximately five times greater than for BLT powders annealed at other temperatures. The response time was about 8 s, with maximum hysteresis of around 3% relative humidity. The BLT powder annealed at 550°C exhibited the best humidity sensing properties compared with the other annealing temperatures. We expect that these results will offer useful guidelines for preparation of humidity sensing materials.

Large enhancement of the photovoltaic effect in ferroelectric complex oxides through bandgap reduction.

Tuning the bandgap in ferroelectric complex oxides is a possible route for improving the photovoltaic activity of materials. Here, we report the realization of this effect in epitaxial thin films of the ferroelectric complex oxide Bi3.25La0.75Ti3O12 (BLT) suitably doped by Fe and Co. Our study shows that Co (BLCT) doping and combined Fe, Co (BLFCT) doping lead to a reduction of the bandgap by more than 1 eV compared to undoped BLT, accompanied by a surprisingly more efficient visible light absorption. Both BLCT and BLFCT films can absorb visible light with a wavelength of up to 500 nm while still exhibiting ferroelectricity, whereas undoped BLT only absorbs UV light with a wavelength of less than 350 nm. Correlated with its bandgap reduction, the BLFCT film shows a photocurrent density enhanced by 25 times compared to that of BLT films. Density functional theory calculations indicate that the bandgap contraction is caused by the formation of new energy states below the conduction bands due to intermixed transition metal dopants (Fe, Co) in BLT. This mechanism of tuning the bandgap by simple doping can be applied to other wide-bandgap complex oxides, thereby enabling their use in solar energy conversion or optoelectronic applications.

Humidity sensing and dielectric properties of mesoporous Bi3.25La0.75Ti3O12 nanorods

Bi3.25La0.75Ti3O12 (BLT) nanorods are synthesized via electrospinning technique, and mesoporous structures are formed in the BLT nanorods via nanocasting method. The mesoporous BLT (M-BLT) nanorods are characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and energy dispersive spectrometer, and then coated on the ceramic substrates with Ag-Pd interdigitated electrodes to form the humidity sensitive device. The humidity sensitive properties of M-BLT nanorods humidity sensitive device are investigated at room temperature within the relative humidity (RH) ranges of 11–95%. The impedance properties of M-BLT nanorods are effected by frequencies. The M-BLT nanorods have good sensing properties at low frequency. The humidity sensitive device is of high sensitivity at 100Hz, and the impedance variations are over six orders of magnitude within the humidity ranges from 11% to 95% RH. The response time and recovery time of the M-BLT nanorods humidity sensitive device are about 8s and 250s, and the maximum hysteresis is about 4% RH. The results indicate that the electrospinning technique combining with nanocasting method would be one of important and effective approaches for preparing high-performance humidity-sensing material, and the M-BLT nanorods are the promising materials to fabricate high sensitivity humidity sensor.

Effect of mesoporous structure on Bi3.25La0.75Ti3O12 powder for humidity sensing properties

Bi3.25La0.75Ti3O12 (BLT) powder is synthesized via a metal-organic decomposition (MOD) method, and mesoporous structure is formed in the BLT powder via nanocasting method. The humidity sensing properties of BLT and mesoporous BLT (M-BLT) humidity sensors are investigated at room temperature within the relative humidity (RH) range of 11–95%. The BET surface area of M-BLT powder (10.8m2/g) is remarkably bigger than it of BLT powder (0.4m2/g). The optimum working frequency of BLT and M-BLT humidity sensors are 100Hz, and the impedance variation of M-BLT humidity sensor is about six orders of magnitude within the whole humidity range from 11% to 95% RH, which is six times more than it of BLT humidity sensor (only about one order of magnitude). Linearity of the M-BLT humidity sensor is obviously better than it of the BLT humidity sensor, and its maximum linear error is only 2% at 75% RH. The response time and recovery time of M-BLT humidity sensor are about 32s and 68s at 100Hz. The results indicate that mesoporous structure is beneficial for enhancing the humidity sensing properties of materials, and nanocasting method would be one of important and effective approach for preparing high-performance humidity-sensing material.

Effects of 60Co γ-ray irradiation on microstructure and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films

Bi3.25La0.75Ti3O12 (BLT) thin films were deposited on Pt/Ti/SiO2/Si substrates through sol–gel method. Films underwent 60Co γ-ray irradiation with different doses; 0, 50, 100 and 150kGy, respectively. Impacts of γ-ray on the microstructure, ferroelectric properties, leakage current density and fatigue characteristic were studied in detail. The results of SEM images show that grain patterns become irregular. Remnant polarization (2Pr) and coercive field (2Ec) decrease with irradiation dose increase. C–V curves reveal obvious asymmetry along y-axis. The irradiated thin films display lower leakage current density and fatigue endurance up to more than 1010 switching cycles. These results suggest that radiation can improve the film performance in some areas.

Highly dispersed spherical Bi3.25La0.75Ti3O12 nanocrystals via topotactic crystallization of aggregation-free gel particles from an effective inverse miniemulsion sol–gel approach

Aggregation-free spherical lanthanum-doped bismuth titanate (Bi3.25La0.75Ti3O12, BLT) gel particles with an average size of about 150 nm were successfully obtained from an inverse miniemulsion sol–gel process, with Span-80 acting as surfactant, n-butanol as co-surfactant, cyclohexane as continuous phase, and submicro-droplets of aqueous solution containing Bi3+, La3+ and Ti4+ ions as dispersed phase, and then topotactically transformed into highly dispersed spherical BLT nanocrystals after an in situ crystallization at 600 °C for 8 h. It has been found that the BLT gel particles can be obtained via a moderate sol–gel reaction inside the miniemulsion droplets at 65 °C, but their morphology and aggregation degree are strongly affected by the relative amounts of Span-80 and n-butanol. The perfect spherical BLT gel particles with no aggregation can be achieved only under the condition of 3 wt% n-butanol relative to the mass of cyclohexane, with excessive amount of n-butanol leading to the formation of ill-gelled particles with irregular shapes, while insufficient addition of n-butanol resulting in terrible aggregation of gel particles. To understand the formation of aggregation-free spherical BLT gel particles, a tentative mechanism is proposed and discussed, which reveals that a well-coordinated oil–water interfacial film made up of Span-80 and n-butanol molecules and the appropriately enhanced evaporation of water from such interfaces should be responsible for the formation of aggregation-free spherical BLT gel particles. Aggregation-free spherical BLT (Bi3.25La0.75Ti3O12) gel particles can be prepared from an effective inverse miniemulsion sol–gel process, and subsequently topotactically transformed into spherical BLT nanocrystals through an in situ crystallization.

Effects of Bi2O3, TiO2, and Bi4Ti3O12 Seeding Layers on the Structural and Electrical Properties of Bi3.25La0.75Ti3O12 Thin Films Grown by a Sol–Gel Method

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films with Bi2O3, TiO2, and Bi4 Ti3O12 (BTO) seeding layers were prepared on Pt/TiO2/SiO2/Si (100) substrate by a sol–gel technique. The effects of the different seeding layers on the structural and electrical properties of the BLT films were investigated. X-ray diffraction indicated that the predominant orientation of the BLT thin film on the BTO seeding layer was (117). Growth of films with the Pt/BLT/Bi2O3/Pt structure was oriented differently from that of films with the Pt/BLT/TiO2/Pt and Pt/BLT/Pt structures. In addition, in comparison with the BLT film deposited directly on Pt, the TiO2 layer substantially enhanced the leakage current resistance of the BLT film. The ferroelectric nature of the BLT thin film was substantially improved by use of a BTO seeding layer. In an applied field of 750 kV/cm, the remnant polarization (2P r) of the Pt/BLT/BTO/Pt capacitor was 61.5 μC/cm2. After 1010 switching cycles, 2P r of the BLT, BLT/Bi2O3, BLT/TiO2, and BLT/BTO films was degraded by approximately 13, 11, 1, and 2%, respectively, indicating that all the capacitors with the different seeding layers had good polarization fatigue characteristics.

Mechanical stress effect on leakage current in Bi3.25La0.75Ti3O12 thin films

Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films were prepared on Pt/Ti/SiO2/Si substrates by metalorganic decomposition method. Mechanical stress effect on leakage current was investigated. The results demonstrate that the leakage current increases with increasing electric field. Under mechanical tensile or compressive stress, the increase trend of the leakage current with electric field shows negligible change in low field region (below 75kV/cm), while that gets weakened in high field region (above 75kV/cm) compared with that at free state (zero stress). At a certain electric field (above 75kV/cm), the leakage current decreases with increasing tensile or compressive stress. The effect of domain reorientation induced by stress combined with the contribution from charge injected into films is found to be reasonable for the explanation of the leakage current in BLT thin films under different stress.

Fabrication and characterization of ferroelectric-gate thin-film transistors with an amorphous oxide semiconductor, amorphous In–Ga–Zn–O

We fabricated and characterized ferroelectric-gate thin-film transistors (FeTFTs) with an amorphous In–Ga–Zn–O (a-IGZO) channel and an oxide ferroelectric (Bi,La)4Ti3O12 (BLT) gate insulator, and examined the fabrication conditions beforehand for realizing ferroelectric hysteresis in their drain current versus gate voltage (ID–VG) characteristics. First, the optimal annealing conditions for BLT-thin-film formation by the sol–gel method were considered in terms of chemical phenomena and crystallization mechanism. As a result, the leakage current density of the film was decreased by more than two orders of magnitude. Second, sputtering deposition conditions and postdeposition annealing temperatures of a-IGZO thin films were investigated to prepare the a-IGZO channel with good electrical properties. A small charge-injection-type hysteresis width of 0.2 V was obsereved in ID–VG characteristics of a-IGZO/SiO2 TFTs. Finally, a-IGZO/BLT FeTFTs were fabricated using the above results, and the ferroelectric hysteresis with a width of 0.4 V was observed.

Dielectric functions and energy band gap variation studies of manganese doped Bi3.25La0.75Ti3O12 thin films using spectroscopic ellipsometry

Single phase polycrystalline Mn-modified Bi3.25La0.75Ti3O12 (BLT) thin films were prepared by chemical solution deposition method using spin coating technique on Pt/Ti/SiO2/Si (100) substrates. Raman spectroscopy of these films shows that Mn3+ is well substituted at Ti4+ site. The optical properties of BLT and Mn modified BLT thin films were investigated at room temperature by using spectroscopic ellipsometry (SE) in the energy range 0.72–6.2eV. A double Tauc–Lorentz (DTL) dispersion relation was successfully used to model the dielectric functions of these films where a shift to the lower energy side with Mn doping is seen. The full width at half maxima (FWHM) (Г) of dielectric function is found to increase with Mn doping. This increase in FWHM may be attributed to the increase in the trap density in forbidden band which consequently decreases the value of direct optical band gap (Egd). The direct optical band gap (Egd) is found to decrease with increase in Mn content in the studied composition range. This decrease in Egd with doping may be attributed to the variation in the defect concentration present in the structure.

Mechanism for leakage current conduction in manganese doped Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films

Bi3.25La0.75Ti3O12 (BLT) is one of the prospective candidate materials for data storage devices application. In order to understand the leakage current conduction mechanism in BLT we have studied Mn doped Bi3.25La0.75Ti3O12 (BLT) thin films with varying concentration of Mn. These films were prepared by chemical solution deposition technique. The structure and phase analysis of these films was performed at room temperature by X-ray diffraction, and scanning electron microscope was used to investigate the surface morphology. Different leakage current conduction mechanisms, e.g., Schottky emission, Modified Schottky, Poole–Frenkel emission and space charge limited (Lampert’s theory) conduction were examined to explain the true nature of charge transport phenomena in BLT and Mn doped BLT thin films. It was found that BLT and Mn doped BLT thin films leakage current conduction mechanism follows the Lampert’s theory of space charge limited conduction in an insulator with traps.

Effects of annealing ambient on ferroelectric properties and surface chemistry of sol–gel derived Bi3.25La0.75Ti3O12 thin films

In this work, Bi3.25La0.75Ti3O12 (BLT) thin films were prepared by sol-gel coatings followed by rapid thermal annealing in Ar or O-2 ambient. The correlation among annealing ambient, ferroelectric characteristics and surface chemistry of the BLT thin films were investigated. The BLT thin film annealed in Ar showed weaker crystallization, less dense surface and smaller polarization value than that annealed in O-2. After 10(9) cycles, the remnant polarization of the BLT film annealed in Ar decreased to 83.5 % of the initial value while it remained 89.5 % for the sample annealed in O-2. X-ray photoelectron spectroscopy results indicated the inferior fatigue characteristics of the sample annealed in Ar was the comprehensive result of oxygen vacancies vicinity to Bi and Ti ion in the thin film.