La2Ti2O7 Research Articles

Morphology and mesopores in photoelectrochemically active LaTiO2N single crystals

Abstract

Mechanism of enhanced photocatalytic activities on N-doped La2Ti2O7: An insight from density-functional calculations

Recently, a wide-band-gap layered perovskite compound La2Ti2O7 (LTO) has been reported to show a significant enhancement of both the visible and UV light photocatalytic activities after doping with N atoms. It is known that the doping sites often act as recombination centers of the photo-exited carriers and block the redox reactions. In this work, we investigated the origin of the enhancement in photocatalytic activity of N-doped LTO by using density functional theory (DFT) calculations. More than 40 models were constructed by considering ionic state of dopants, distance between dopants, doping concentrations, and formation of oxygen vacancy. The structure-properties relation of N-doped LTO was established. We found that the model of Sub3N–3Odv (with three dispersed substitutional N atoms at O sites and one oxygen vacancy in 87-atom LTO supercell) well explains both the shift-up of the valence bands and the narrowed band gap observed in experiment. The obtained band gap of 2.46 eV agrees well with the experimental value of 2.51 eV. For the models with interstitial N atoms, the impurity states are mainly localized at the higher-energy region of the band gap, which may trap the photo-excited carriers and decrease the photocatalytic activity. The work provides a potential implication for effective band-gap narrowing of wide-gap photocatalysts.

Room temperature multiferroic properties of Mn doped La2Ti2O7

Polycrystalline La2Ti2-xMnxO7 (LTMO) (x = 0−0.16) ceramics fabricated by the conventional solid-state reaction are utilized to investigate the effect of Mn doping on the ferroelectric (FE) and ferromagnetism (FM) systematically. Both the FE and FM properties strongly depend on the doping level of Mn atoms, which directly induces a structural phase transition from monoclinic to orthorhombic. Compared with pure La2Ti2O7 (LTO), remnant polarization decreases sharply with the doping level at first, and slightly increases when x ≥ 0.08, which is proposed to result from the evolution oxygen vacancies. All the doped samples exhibit weak FM property at room temperature, until the maximum can be observed for x = 0.08. Mn3+-Mn4+ double-exchange and anti-FM Mn3+-Mn3+ and Mn4+-Mn4+ super-exchange interactions are responsible for the different ferromagnetic behavior of LTMO.

Multiferroicity in vanadium-doped La2Ti2O7: insights from first principles

We explore an unconventional route in the search for multiferroicity investigating the magnetic doping of La2Ti2O7 (LTO), a ferroelectric layered perovskite, by density functional calculations. We find that substitution of Ti (in the 3d 0 configuration) by V (3d 1) produces: (i) robust ferromagnetic (FM) order, achieving multiferroicity; (ii) structural and magnetic anisotropy, as the dopants cluster in magnetic chains. Moreover, the FM V-chains possess antiferro orbital ordering. We show that the origin of the strong FM coupling lies in the fruitful host-dopant combination: the layered structure of the host prewires the orbitals t 2g with d xy and d xz lower in energy, the V dopants provide the full occupancy of the bonding levels. We further address the stability of chains vs. temperature and the role of oxygen vacancies in undoped as well as Sc-doped and V-doped LTO.

Piezoelectric and dielectric properties of Ce substituted La2Ti2O7 ceramics

The ferroelectric and dielectric properties of cerium (Ce) substituted La2Ti2O7 (LTO) have been investigated. Single phase, dense La2−xCexTi2O7 (x = 0.15, 0.25, 0.35) ceramics were prepared by spark plasma sintering. The solubility limit of Ce in La2−xCexTi2O7 was found to be between 0.35 and 0.5. The a-, b- and c-axes of the unit cell decrease with increasing Ce substitution. The Curie point (Tc) of La2−xCexTi2O7 (x = 0, 0.15, 0.25, 0.35) decreases and dielectric constant and loss increase with increasing Ce substitution. Cerium can increase the d33 of La2Ti2O7. The highest d33 was 3.9 ± 0.1 pC/N for La1.85Ce0.15Ti2O7 textured ceramic.

Photocatalytic generation of hydrogen with visible-light nitrogen-doped lanthanum titanium oxides

In order to generate hydrogen by splitting water under visible-light irradiation, nitrogen-doped La2Ti2O7 (LTO) nanosheets loaded with the Pt nanoparticles have been synthesized by hydrothermal processing, followed with wet impregnation and subsequent heat treatment in a NH3 flow. The light absorption of the pristine LTO nanosheets is cut off at 330nm while the N-doped LTO nanosheets exhibit the extended light absorption up to 560nm. Nitrogen doping has not only resulted in the visible-light-responsive photocatalytic water splitting but also enhanced the ultraviolet-light-responsive photocatalytic activity. Loading the Pt nanoparticles on the LTO surface further improved the photocatalytic activity due to the enhanced separation of charge carriers.

Investigation of microstructure in ferroelectric lead-free La2Ti2O7 thin film grown on (001)-SrTiO3 substrate

(012)-Oriented La2Ti2O7 (LTO) thin films with perovskite layered/monoclinic structure (a = 7.825 Å, b = 5.532 Å, c = 12.938 Å, β = 98.0°) have been deposited on (001)-oriented doped Nb:SrTiO3 (STO) substrates by a sol–gel method coupled to the spin-coating technique. Rocking curves measurements evidence the existence of four crystallographic domains in these films, the (012)LTO planes for each domain being slightly disoriented from 2.3° with respect to the plane of the substrate, as confirmed by reciprocal space map performed around the (002) reflection of the substrate. Pole figure measurements permit the conclusion that the crystallographic relationships between the film and substrate are [001]STO//[100]LTO, [010]STO//[01]LTO and (100)STO//(012)LTO. Using high-resolution X-ray diffraction, the (025)LTO-reflection can be distinguished on 2θ/ω-scan pattern recorded in parallel beam geometry. As highlighted by both rocking-curve measurements and reciprocal space map recording, the (025)LTO planes for each of the four populations are disoriented from 4.1° with respect to the plane of the substrate. As demonstrated, this angle's value is a signature of the existence of the fourfold disoriented crystalline domain structure. From piezoresponse force microscopy experiments, intermediate directions of the polarization vectors between the normal and the plane of the substrate are revealed. On the other hand, piezoloops recorded on nanoscale unambiguously confirm the ferroelectric behavior of these films.

Ab initiostudy of proper topological ferroelectricity in layered perovskite La2Ti2O7

We present a first-principles investigation of ferroelectricity in layered perovskite oxide La2Ti2O7 (LTO), one of the compounds with highest Curie temperature known (1770 K). Our calculations reveal that LTO's ferroelectric transition results from the condensation of two soft modes that have the same symmetry and are strongly coupled anharmonically. Further, the leading instability mode essentially consists of rotations of the oxygen octahedra that are the basic building block of the perovskite structure; remarkably, because of the particular topology of the lattice, such O6 rotations give raise to a spontaneous polarization in LTO. The effects discussed thus constitute an example of how nano-structuring -- provided here by the natural layering of LTO -- makes it possible to obtain a significant polar character in structural distortions that are typically non-polar. We discuss the implications of our findings as regards the design of novel multifunctional materials. Indeed, the observed proper ferroelectricity driven by O6 rotations provides the ideal conditions to obtain strong magnetoelectric effects.

Microstructure and Nanoscale Piezoelectric/Ferroelectric Properties in La2Ti2O7 Thin Films Grown on (110)‐Oriented Doped Nb:SrTiO3 Substrates

Abstract(00l)‐Oriented La2Ti2O7 (LTO) thin films with monoclinic perovskite‐layer structure [a = 7.806(2) Å, b = 5.552(3) Å, c = 13.015(5) Å, β = 98.62(2)°] have been grown by a sol–gel route on conducting (110)‐oriented doped Nb:SrTiO3 (STO) substrates. The narrow rocking curves (0.24° width for 004LTO peak) demonstrate the sharp mosaïcity of the films. Using high‐resolution X‐ray diffraction (HR‐XRD), epitaxial relationships between the LTO, and the STO substrate are given. In addition, HR‐XRD evidences the existence of (212)‐oriented crystallites 1.5° disoriented with respect to the plane of the substrate. We confirm, by DFT calculations, that the polarization vector lies in the b‐axis of the LTO cell and consequently, the existence of these (212)‐oriented crystallites enables to explain the origin of the various contrasts observed both on the in‐plane and out‐of‐plane images when collected by piezoresponse force microscopy. Finally, both successful poling experiments performed via the tip of atomic force microscope and the existence of local piezoloops within the domains, unambiguously confirm the ferroelectric state of the films at the nanoscale level. Once again, this study demonstrates that a clear understanding of nanoscale piezoelectric/ferroelectric phenomena in oriented thin films passes through a carefully structural analysis as performed by HR‐XRD.

Density functional calculations of the structural, electronic, and ferroelectric properties of high-k titanate Re2Ti2O7 (Re=La and Nd)

We have studied the structural, electronic, and ferroelectric properties of La2Ti2O7 (LTO) and Nd2Ti2O7 (NTO) by first-principles density functional theory calculations. The computed structural parameters are found to be in good agreement with experimental findings. In particular, the P21 phase is confirmed to be energetically preferred over the other configurations for both titanates. The calculations revealed the possible existence of an unidentified phase, namely, the P21/m paraelectric structure. From the modern theory of polarization the spontaneous polarization of LTO and NTO was calculated to be 7.72 μC/cm2 and 7.42 μC/cm2, respectively, in accordance with experimental findings. Its origin is ascribed to the displacement of the rare earth (Re) cations in the [100] cleavage plane and parallel to the b axis. Electronic charge density distributions and Bader’s topological analysis indicates that the bonding interactions between Re and O as well as Ti and O are not purely ionic, a noticeable covalent bonding is identified between Ti and O. Band structure calculations using a generalized gradient approximation (GGA) approach predicted insulating ground state for LTO with band gap energy of about 2.84 eV. Conversely, a half-metallic ground state was predicted for NTO. Using GGA+U approach, an appropriate insulating ground state was found with band gap energy of 1.63 eV. Density of states analysis suggest that the localized Nd 4f levels act as charge-trapping sites, explaining the reduced photocatalytic activity of NTO.

Bi3.25La0.75Ti3O12 films on La2Ti2O7 thin films prepared by chemical solution deposition

We report the synthetic and physical procedures for obtaining the Lanthanum-substituted Bi4Ti3O12 (BLT) ferroelectric films, Bi3.25La0.75Ti3O12, with its polarisation axis (a) oriented perpendicular to the surface of the electrode by employing partially oriented (001) polycrystalline thin films of La2Ti2O7 (LTO) as a buffer onto Si-wafers. The LTO thin film was achieved by dip coating and annealing at high temperatures while the BLT film was deposited using RF magnetron sputtering and annealing at a temperature as low as 650°C. Furthermore, the dependence of the thickness, grain size and orientation of the LTO films on the withdrawal speed of the dip-coating and annealing temperature is reported.