La2Ti2O7 Research Articles

Realization of ultra-high temperature stability and excellent electrical properties in LTO-based ceramics via A/B-site co-doing

La2Ti2O7 (LTO) ceramics, ideal for ultra-high temperature applications with a Curie temperature near 1460 °C, were previously limited by a low piezoelectric coefficient (d33) of 1.5 pC/N. In this work, the incorporation of Bi3+ and V5+ dopants have been employed to augment the piezoelectric properties of LTO. An enhanced d33 ∼ 4.0 pC/N is achieved in La1.91Bi0.09Ti1.97V0.03O7 (LBTV9) ceramics, while maintaining an ultra-high Curie temperature of 1437 °C. This improvement is attributed to local heterogeneity in the lattice structure induced by the doping. LBTV9 ceramics also exhibit excellent high-temperature stability and insulation properties, maintaining a d33 at 4.0 pC/N after annealing at 1000 °C for 48 h and stabilizing at 4.2 pC/N across temperatures from 200 °C to 1600 °C, with a DC resistivity of 1.7 × 106 Ω·cm at 1000 °C. These enhancements make LBTV9 ceramics suitable for extreme condition applications, thus unlocking new possibilities for their use in ultra-high temperature piezoelectric devices.

Ultra-high piezoelectric properties and ultra-high Curie temperature of Li/Ce-doped La2Ti2O7 ceramics.

The demand for ultra-high-temperature piezoelectric sensors in industrial applications has witnessed a rapid upsurge. In this study, the piezoelectric properties of La2Ti2O7 (LTO) piezoelectric ceramics with a perovskite-like layered structure were enhanced by doping with Li/Ce ions. It was found that a remarkable 300% enhancement in the piezoelectric constant (d33) value was achieved in Li/Ce-doped LTO ceramics compared to their pristine counterparts, reaching 6.4 pC N-1 at room temperature with an ultra-high Curie temperature of 1408 °C. After annealing at 500 °C, the d33 value of the samples can be further improved to 7.4 pC N-1. Moreover, temperature-dependent resistivity measurements indicate that even at 1000 °C, the ceramics exhibit a high resistivity of 8.9 × 105 Ω cm. By combining X-ray diffraction, Raman spectra, transmission electron microscopy and piezoresponse force microscopy data, the enhanced piezoelectricity of the ceramics is attributed to local heterogeneity induced by Li/Ce doping. Our results unequivocally demonstrate the suitability of modified LTO ceramics for ultra-high-temperature piezoelectric applications.

Oxygen Vacancies Defective La2Ti2O7 Nanosheets Enhanced Photocatalytic Activity of Hydrogen Evolution under Visible Light Irradiation.

A novel and efficient technique has been designed for the creation of oxygen vacancies on La2Ti2O7 (LTO) nanosheets. This is achieved via a controlled solid-state reaction between NaBH4 and LTO nanosheets. Transmission electron microscopy (TEM) analyses expose that these processed LTO specimens possess a unique crystalline core/amorphous shell structure, represented as La2Ti2O7@La2Ti2O7-x. According to X-ray photoelectron spectroscopy (XPS) observations, there is a notable correlation between the reaction time, temperature, and the concentration of oxygen vacancies. The concentration of these vacancies tends to increase along with the reaction time and temperature. Concurrently, UV-Visible spectra and photocatalytic tests reveal a significant impact of oxygen vacancies on the LTO surface on both light absorption and photocatalytic functionality. Most notably, the LTO nanosheets with engineered oxygen vacancies have demonstrated an exceptional photocatalytic capacity for hydrogen production under visible light. The maximal activity recorded was an impressive 149 μmol g-1 h-1, which is noticeably superior to the performance of the pristine La2Ti2O7.

Electrical characterization of sol-gel La2Ti2O7 films for resistive random access memory applications

The bipolar resistive switching (BRS) devices were investigated for RRAM applications in a MIM structure based on sol-gel derived amorphous La2Ti2O7 (LTO) thin films. The influences on the resistive switching (RS) properties according to the film thickness, electrode, annealing temperature and post metal annealing (PMA) treatment were also discussed. Moreover, RS performance of the RRAMs is affected by the work functions of electrodes. The RS characteristics of the devices are controlled by oxygen vacancies and the content of oxygen vacancies was seriously reduced after annealing, which led to a significant decrease of the endurance. In addition, PMA treatment increased the content of oxygen vacancies attributed to the forming of AlOx interface layer and made Al ions diffuse into the films to form bridges, which improved the RS performance of the devices. Consequently, the sample with 300 °C PMA exhibits the optimized RS properties with maximum switching cycles of 2674, a Ron/Roff ratio of 102, low operating voltages (VSet/VReset = 1.32 V/−1.21 V), and a retention time of over 104 s at room temperature and 85 °C, indicating the promising potential for practical RRAM applications. Conduction mechanism of all devices with Al top electrode were described by trap-controlled space-charge limited current (SCLC) and ohmic conduction.

Self-rectifying and forming-free resistive switching behaviors in Pt/La2Ti2O7/Pt structure

In this work, we report the resistive switching behavior of an amorphous La2Ti2O7 (LTO) film as sandwiched between two Pt electrodes. The resistive switching is forming-free and highly uniform. Furthermore, it exhibits self-rectifying resistive switching behaviors owing to the Schottky contact and quasi-ohmic contact formed at the top and bottom interfaces, respectively. The mechanism of switching behavior in the device is attributed to the trapping/detrapping-mediated electronic bipolar resistance switching. By fitting the current-voltage characteristics, it indicates the coexistent conduction mechanisms of Schottky emission and space-charge-limited-conduction (SCLC), while the Schottky barrier modified by electron trapping/detrapping plays a dominating role in the resistive switching process.

Origin of the Enhanced Piezoelectricity of Vanadium-Doped La2Ti2O7 Ceramics

In recent years, La2Ti2O7 (LTO)-based piezoelectric materials, which belong to the perovskite-like layered structure (PLS) materials, have become candidates in the elevated temperature piezoelectri...

Plasmon enhanced photo-assisted electrochemical detection of bisphenol A based on Au decorated La2Ti2O7/RGO nanosheets

Ternary nanocomposite of gold nanoparticles, La2Ti2O7 (LTO) and reduced graphene oxide (RGO) was synthesized by a simple hydrothermal method. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectra (DRS) and X-ray diffraction (XRD) were applied in the characterization of the materials. The characterization results revealed that Au nanoparticles were dispersed well on the surface of LTO/RGO. Under light irradiation, the response signal of Au-LTO/RGO for BPA detection exhibited 1.91 times higher than that of dark condition thanks to the special SPR property of Au nanoparticles. Additionally, amperometry method was more beneficial for obtaining lower LOD compared with DPVs and photocurrent methods. The obtained linear work ranges from 0.1 to 21.9 μM and the LOD was 0.03 μM (S/N=3). Ultimately, the Au-LTO/RGO sensor also displayed satisfactory results to detect BPA in real samples system.

Hierarchically 1D CdS decorated on 2D perovskite-type La2Ti2O7 nanosheet hybrids with enhanced photocatalytic performance

In this paper, a heterojunction of hierarchically one-dimensional (1D) cadmium sulfide (CdS) nanowires decorated on the two-dimensional (2D) La2Ti2O7 (LTO) has been fabricated by a simple two-step hydrothermal process. The structure, morphology and surface chemical composition of CdS/LTO were studied by a variety of characterizations, in which the X-ray photoelectron spectroscopy (XPS) results showed a strong interaction between CdS and LTO through S–O bonds. The type II band alignment of CdS/LTO for favorable electron transfer from CdS to LTO and hole transfer from LTO to CdS was clarified by ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy and Mott–Schottky (M–S) plot measurements. In the absence of any co-catalyst, H2 production from water splitting and Rhodamine B (RhB) decomposition in aqueous solution were carried out to evaluate the photocatalytic activities, showing that the photocatalytic performance of CdS/LTO was superior to that of pure LTO and CdS under visible light and simulated sunlight. The optimal CdS content in the CdS/LTO system was determined to be 7.5 wt%. The present results show an effective pathway to design a 1D/2D heterojunction photocatalyst in the face of overwhelming problems of energy crisis and environmental pollution.

Electrical Properties of La2−xCexTi2O7 (x = 0–0.1) Ceramics

As an important functional material, the applications of piezoelectric materials have shifted from convenient use to service in extreme environments. Herein, a new layered high‐temperature ceramic La2Ti2O7 (LTO) with the substitution of La3+ ions by Ce3+ ions at A sites is designed and synthesized using the conventional solid‐state sintering technology. The effects of dopant Ce3+ ions on the microstructure, phase structure, and electrical properties of LTO are investigated methodically. Moderate dopant Ce3+ ions decrease the concentration of thermions in LTO ceramics, but excess dopant Ce3+ ions can lead to cell distortions, which have some relationships with defects. Through analysis, the La1.95Ce0.05Ti2O7 (LCTO‐0.05) ceramics with a pure monoclinic phase possess optimal electrical properties. At 650 °C, the enhanced fitted direct current (DC) resistivity of the LCTO‐0.05 ceramics is 1.5 × 106 Ω cm. The activation energy of conductivity and relaxation activation energy of imaginary impedance Z″ for LCTO‐0.05 ceramics are 1.12 and 1.16 eV, respectively, in the high‐temperature regime. The piezoelectric coefficient d33 of LCTO‐0.05 ceramics is up to 3.6 pCN−1. It is believed that this ceramic system promotes the development of LTO‐based high‐temperature piezoelectric ceramics.

Improved Electrical Properties of Layer Structured La2Ti1.96V0.04O7 Ceramics

La2Ti1.96V0.04O7 (LTVO) ceramics with Ti4+ ions partially substituted by V5+ ions at B sites show excellent electrical properties. The dopant V5+ ions do not result in collapse of the cell layered structure, but they lead to the enhanced distortions of BO6 oxygen octahedrons. Impedance spectroscopy reveals that a certain number of defects are formed due to the substitution of Ti4+ ions by V5+ ions. Meanwhile, the concentration of oxygen vacancies is decreased compared to pure La2Ti2O7 (LTO) ceramics. The direct current resistivity of LTVO ceramics obtained from alternating current (AC) impedance fitting at 600°C is 1.3 × 106 Ω cm, which is more than five times of that of the pure LTO at the same temperature (2.2 × 105 Ω cm). The substitution of Ti4+ ions by V5+ ions greatly enhances the piezoelectric coefficient, d33 = 4.8 pC/N. Therefore, the doping of V5+ ions in the B sites of LTO ceramics should be a good choice for enhancing their piezoelectric properties and resistivity at high temperature regimes.

Ti3 C2 : An Ideal Co-catalyst?

How 2D Ti3 C2 enhances photocatalytic efficiency remains unclear. Now, it is shown that it is graphene quantum dots (GQDs) derived from Ti3 C2 , rather than 2D Ti3 C2 itself, that play the role of co-catalyst for La2 Ti2 O7 /Ti3 C2 (LTC) composites during the photocatalytic reaction. After modification of Ti3 C2 derivatives, the photocatalytic efficiency of La2 Ti2 O7 is enhanced 16 times over pure La2 Ti2 O7 . Solid-state NMR, Raman, and HRTEM results confirm the existence of GQDs in Ti3 C2 and LTC composites. The GQDs are formed during the chemical change from Ti3 AlC2 to Ti3 C2 via HF etching, as Ti atoms are removed and unsaturated carbon bonds are left, which react with each other to form sp2 π-conjugation GQDs. 2D Ti3 C2 is completely oxidized to COx modified TiOx species, causing Ti3 C2 to lose its electrical conductivity and the role as co-catalyst. GQDs largely suppress the photogenerated charge recombination of La2 Ti2 O7 , as revealed by the photoluminescence (PL) and transient photocurrent.

Hybrid excitation mechanism of upconversion fluorescence in hollow La2Ti2O7: Tm3+/Yb3+ submicron fibers

High-crystalline, hollow-mesh-like Tm3+/Yb3+-co-doped La2Ti2O7 (LTO) submicron fibers are prepared by electrospinning technique and identified as monoclinic structure. The LTO matrix fibers and the Tm3+/Yb3+-co-doped fibers exhibit different frequency upconversion luminescence. The fluorescence of the matrix at the 487 and 542 nm is ascribed to the two-photon absorption and the cross-relaxation processes caused by the defect center at 977 nm excitation, respectively. The upconversion luminescence intensity enhances when the rare-earth ions are incorporated into LTO fibers. The emissions of Tm3+ in co-doped LTO membranes at 479 and 789 nm under the excitation of 977 nm indicate the effectiveness of the three- and two-photon absorption processes, respectively. The pristine LTO fibers have the potential to be employed for water purification as a laser-excited photocatalytic material because the LTO materials are conducive to absorbing the highly penetrating NIR laser. Furthermore, the Tm3+/Yb3+ ions play a positive role in further promoting the laser-absorption capacity, and the hybrid excitation mechanism in the Tm3+/Yb3+-co-doped LTO composite fibers provides a new perspective for the development of anti-laser inorganic materials.

Electrical properties of La2-xPrxTi2O7 (x = 0–0.3) ceramics

The electrical properties of La2Ti2O7 (LTO) ceramics have been enhanced through the substitution of La3+ ions by Pr3+ ions. Almost all doped Pr3+ ions will get at A - site without causing a change on monoclinic phase of LTO. The average grain size is 17.8 μm for La1.9Pr0.1Ti2O7 ceramics. The relaxation activation energy which is contributed by defect dipoles that are formed from TiO6 oxygen octahedrons’ distortions in grains is 1.6 eV for La1.8Pr0.2Ti2O7 ceramics. This kind of defects will be activated from 520 °C and completely be activated until 650 °C. The piezoelectric coefficient d33 = 3.0 pC/N of La2-xPrxTi2O7 ceramics maintains stable when the Pr3+ doping content x ranging from 0.1 to 0.3.

The effect of second phase La0.67TiO2.87 on the phase structure and impedance spectroscopy of La2Ti2(1 + x)O7 piezoelectric ceramics

Abstract The Ti excess La2Ti2 (1+x) O7 (x = 0, 0.005, 0.01, 0.02, 0.05, 0.1) piezoelectric ceramics have been prepared by sol-gel technology and solid state synthesis method. Through refinement analysis, the phase structure of the ceramics varies with Ti content. Most monoclinic phase (∼93%) and a handful of orthogonal phase (∼7%) coexist in La2Ti2 (1+0) O7 ceramics. Pure monoclinic phase La2Ti2O7 with space group P21 appears in La2Ti2 (1+0.005) O7 and La2Ti2 (1+0.01) O7 ceramics. Monoclinic phase La2Ti2 O7 and a certain proportion of tetragonal phase La0.67TiO2.87 coexist in La2Ti2 (1+0.02) O7, La2Ti2 (1+0.05) O7 and Ti2 (1+0.1) O7 ceramics. With the excess of Ti content, the monoclinic phase ratio and distortion angles in a-b projection plane of the ceramics increase first and then decrease, which is consistent with the variation tendency of piezoelectric constant d33. The excellent piezoelectric constant for Ti2 (1+0.01) O7 ceramics is 2.8 pC/N. Impedance analysis shows that the conductive mechanisms of all samples include both grain and grain boundary conductivity at temperature range T ≥ 500 °C. The formation of tetragonal phase La0.67TiO2.87 derives from Ti excess in pure monoclinic phase La2Ti2O7. The existence of tetragonal phase La0.67TiO2.87 can obviously increase the capacitance of ceramics at x ≥ 0.05. All prepared piezoelectric La2Ti2 (1+x) O7 ceramics have highly frequency stability and are candidates for ultrahigh temperature piezoelectric application.

Regulate the microstructure and band gap of La2Ti2O7

La2Ti2O7 (LTO) dry gel was obtained according to sol–gel technology. Considering that processing temperature could have effect on the microstructure and band gap of LTO, four temperature points, 500 °C, 850 °C, 1000 °C and 1200 °C were selected to dispose gel to acquire LTO powder. The results indicate that except the sample disposed at 500 °C, other three samples are univocal monoclinic structure with space group P21 which corresponds to the standard card 1950-ICSD. The crystallinity of LTO powders increases with the increase of processing temperature. The distortions of the LTO unit cell at a–b projection plane first decrease and then increase. Adopting the UV–visible reflection spectrums analysis, the band gaps of LTO powders disposed at 500 °C, 850 °C, 1000 °C and 1200 °C are 3.82 eV, 3.99 eV, 4.08 eV and 3.94 eV, respectively.

Synergistic Effect of Doping and Compositing on Photocatalytic Efficiency: A Case Study of La2Ti2O7.

Charge generation and separation are two key issues in developing a high-efficiency semiconductor for the visible-light-driven photocatalysis. Here, we use the layered perovskite-type wide-gap semiconductor La2Ti2O7 (LTO) as a model to systematically explore the synergistic effect of doping (with sulfur or nitrogen) and heterojunction (with graphitic C3N4) on improving visible light absorption and photoexcited charge separation by means of density functional theory calculations. It is found that the anion (N or S) doping into the LTO(010) surface can not only shift the optical absorption edge to the visible region, but also creates some partially occupied or unoccupied states in the band gap that would facilitate the formation of recombination centers. For the purpose of promoting electron-hole separation, the (N or S-doped) LTO(010) surfaces were hybridized with the monolayer g-C3N4. Interestingly, we found that the (S-doped) LTO/g-C3N4 heterostructure forms a type-II heterojunction, with the valence band maximum residing in the (S-doped) LTO and the conduction band minimum in g-C3N4, respectively. This band alignment feature facilitates efficient electron-hole separation. Moreover, we found that the S-doped LTO/g-C3N4 composite has a short interfacial distance (about 2.1 Å), implying that the interfacial interaction of this composite might be a chemical bond rather than a weak van der Walls interaction. The chemical bonding can enhance charge separation. Our theoretical findings provide design principles for optimizing the photocatalytic performance of the wide-gap photocatalysts and demonstrate that the S-doped LTO/g-C3N4 composite would be a potential candidate for the photocatalysis of water splitting.

Black phosphorus-CdS-La2Ti2O7 ternary composite: Effective noble metal-free photocatalyst for full solar spectrum activated H2 production

Wide-bandgap semiconductor photocatalysts such as La2Ti2O7 (LTO) are stable but can only work under ultraviolet (UV) light, while narrow-bandgap semiconductor photocatalysts such as CdS and black phosphorus (BP) with broad-wavelength range absorption generally have low stability and high electron-hole pairs recombination rate. In this work, ternary heterostructure composing of BP quantum dots (QDs), CdS nanoparticles (NPs) and LTO nanosteps (NSP) is developed for the first time to make full use of their respective advantages, which offers a promising approach to achieving desired stability and solar energy harvesting. As an effective noble metal-free photocatalyst, BP-CdS-LTO composite generates H2 from Na2S/Na2SO3 aqueous solution with a rate of 0.96 mmol g−1 h−1 under solar light irradiation, and 0.26 mmol g−1 h−1 in the near-infrared (NIR) range. In this system, BP serves as a NIR photosensitizer; CdS not only contributes to visible light absorption but also bonds with BP to promote the transmission of photogenerated charge carriers; LTO absorbs UV light as well as provides reaction sites for photoinduced electrons. Femtosecond transient absorption spectroscopy is used to elucidate the kinetics of the injection of photogenerated electrons from BP QDs to CdS NPs and finally to LTO. This work provides deep insight into charge transfer between semiconductors with different band alignments, which can open a new avenue for more rationally designing heterostuctured photocatalysts for H2 production.

Tuning piezoelectric properties through epitaxy of La2Ti2O7 and related thin films

Current piezoelectric sensors and actuators are limited to operating temperatures less than ~200 °C due to the low Curie temperature of the piezoelectric material. Strengthening the piezoelectric coupling of high-temperature piezoelectric materials, such as La2Ti2O7 (LTO), would allow sensors to operate across a broad temperature range. The crystalline orientation and piezoelectric coupling direction of LTO thin films can be controlled by epitaxial matching to SrTiO3(001), SrTiO3(110), and rutile TiO2(110) substrates via pulsed laser deposition. The structure and phase purity of the films are investigated by x-ray diffraction and scanning transmission electron microscopy. Piezoresponse force microscopy is used to measure the in-plane and out-of-plane piezoelectric coupling in the films. The strength of the out-of-plane piezoelectric coupling can be increased when the piezoelectric direction is rotated partially out-of-plane via epitaxy. The strongest out-of-plane coupling is observed for LTO/STO(001). Deposition on TiO2(110) results in epitaxial La2/3TiO3, an orthorhombic perovskite of interest as a microwave dielectric material and an ion conductor. La2/3TiO3 can be difficult to stabilize in bulk form, and epitaxial stabilization on TiO2(110) is a promising route to realize La2/3TiO3 for both fundamental studies and device applications. Overall, these results confirm that control of the crystalline orientation of epitaxial LTO-based materials can govern the resulting functional properties.

Graphitic-C3N4 hybridized N-doped La2Ti2O7 two-dimensional layered composites as efficient visible-light-driven photocatalyst

Perovskite-type La2Ti2O7 (LTO), having a layered structure and the separated H2 and O2 evolution sites, is attractive as an efficient photocatalyst. However, the photocatalytic activity is often limited by the poor electron mobility. This problem can be conquered by hybridization with materials having efficient properties for the visible light absorption and charge carrier transport. Here, we report a two-dimensional (2D) layered composite hybridized by approximately 2nm thick graphitic C3N4 nanosheets (g-C3N4) and 7nm thick nitrogen doped LTO nanosheets (NLTO) (g-C3N4/NLTO), in which g-C3N4 and NLTO act as hole receptor and electron conductor, respectively. g-C3N4/NLTO exhibited high photocatalytic activities for H2 production via water splitting and dye degradation under the UV and visible light irradiation, due to the interfacial charge transfer between g-C3N4 and NLTO. The electrochemical measurement showed the type II band alignment with favorable charge transfer from g-C3N4 to NLTO. The 2D architecture with a maximized interfacial area allows efficient charge separation with a short interfacial distance. This efficient interfacial charge transfer is further elucidated from monitoring of charge separation and trapping processes using the femtosecond time-resolved diffused reflectance (TDR) measurement.

Enhanced Solar Hydrogen Generation by a Heterojunction of Perovskite-type La2 Ti2 O7 Nanosheets Doped with CdS Quantum Dots.

A heterojunction of a perovskite-type La2 Ti2 O7 (LTO)/CdS quantum dot (QD) nanocomposite was prepared by the chemical bath deposition method. The solar-light-driven H2 generation results show that this CdS QDs/LTO heterojunction exhibits great improvement in the photocatalytic H2 production and its photoelectrochemical properties in comparison with pure CdS QDs and LTO nanosheets. A possible enhanced mechanism of photoinduced interfacial charge transfer between CdS and LTO is proposed for the heterojunction. The shift of the CdS conduction band (CB) edge can enlarge the CB potential difference between CdS QDs and LTO, enhancing transferred driven force of the interfacial electrons and finally improving the two components' separation activity of photogenerated carriers. This study provides a promising method for constructing an efficient photoinduced interfacial charge transfer over LTO and CdS QD heterojunctions for photocatalytic hydrogen production.