Lanthanum-based Oxides Research Articles

Unveiling Structural Defects by 139La NMR and Raman Spectroscopies at the Origin of Surface Stability for the Design of Cerium-Based Catalysts

Cerium-based complex oxides are essential materials well-suited for numerous applications related to heterogeneous catalysis and electrocatalysis. However, the impact of structural defects on the thermal stability of the surface has still to be elucidated. Probing lanthanum environments by 139La NMR and analyzing, for the first time in parallel, the associated defects by Raman spectroscopy allow for a better view of the structural defects at the surface and in the bulk for samples annealed at T = 600 and 1200 °C, respectively. Moreover, we propose another indexing of the Raman spectra of cerium-based compounds with a fluorite structure by considering two tetrahedral environments around the cationic defects. We evidence the existence of an unusual isolated La pseudo-cubic site, in a series of cerium-, zirconium-, and lanthanum-based oxides. It is found to be stabilized at the subsurface farther from surface’s Zr atoms and oxygen vacancies. La3+ ions in the bulk are preferentially associated with bulk Zr4+ cations as in the La2Zr2O7 ternary compound. When this peculiar La3+ environment surrounded exclusively by Ce4+ cubic sites is thermally stable, the specific surface area remains interesting for catalytic application at high temperatures. However, enhancing the La and Zr contents tends to increase the association of La3+ and Zr4+ ions in clusters and induce a loss of surface area.

Synthesis and characterization of the flower-like LaxCe1−xO1.5+δ catalyst for low-temperature oxidative coupling of methane

Lanthanum-based oxides are promising candidates for low-temperature oxidative coupling of methane (OCM). To further lower the OCM reaction temperature, the Ce doped flower-like La2O2CO3 microsphere catalysts were synthesized, achieving a significantly low reaction temperature (375 °C) while maintaining high C2+ hydrocarbon selectivity (43.0%). Doping Ce into the lattice of La2O2CO3 created more surface oxygen vacancies and bulk lattice defects, which was in favor of the transformation and migration of oxygen species at 350–400 °C. The designed H2 temperature-programmed reduction (H2-TPR) experiments provided strong evidence that the low reaction temperature of LaxCe1−xO1.5+δ can be attributed to the transformation and migration of oxygen species, which dynamically generated surface oxygen vacancies for continuous oxygen activation to selectively convert methane. Moreover, designed temperature-programmed surface reaction (TPSR) clarified that two kinds of surface oxygen species in LaxCe1−xO1.5+δ catalysts were concerned with catalytic performance, that is, the surface chemisorbed oxygen species for the activation of CH4 and the formation of CH3• intermediates, surface La-Ce-O lattice oxygen species that caused the excessive oxidation of CH3• intermediates. Finally, the factors affecting the transformation and migration of oxygen species were explored.

La2Ni0.5Li0.5O4 Modified Single Polycrystalline Particles of NMC622 for Improved Capacity Retention in High-Voltage Lithium-Ion Batteries

Ni-rich lithium nickel manganese cobalt oxides have been extensively studied as high capacity and low cost positive electrode materials for lithium-ion batteries. In order to improve their electrochemical properties, especially cycle capability, we develop a surface modification on single polycrystalline particles of LiNi0.6Mn0.2Co0.2O2 (NMC622) powder with a lanthanum-based oxide. A wet-chemical process and following heat-treatment at 750 °C produce K2NiF4-type La2Ni0.5Li0.5O4 nanoparticles on the surface of NMC622 particles. The La2Ni0.5Li0.5O4-modification improves capacity retention of NMC622 during long-term charge-discharge cycling in the wide voltage range of 3.0–4.5 V at room temperature. The capacity retention at the 150th cycle is about 86% (from 182 to 157 mAh g−1) which is higher than 78% (from 176 to 135 mAh g−1) obtained for the unmodified NMC622 electrode. Further improvements by the La2Ni0.5Li0.5O4 modification are confirmed in the capacity retention cycled at 60 °C and in the discharging rate capability. The La2Ni0.5Li0.5O4-modification is highly effective on the single polycrystalline particles of Ni-rich NMC for long cycle-life and high-rate lithium-ion batteries.

In-depth understanding of the crystal-facet effect of La2O2CO3 for low-temperature oxidative coupling of methane

Facet effect of lanthanum-based oxides on low-temperature oxidative coupling of methane (OCM) reaction have attracted great attention. In the present work, La2O2CO3 nanoparticles in the morphologies of triangular nanoplate and triangular nanocube with different ratio of {001} and {100} facets were prepared by facile hydrothermal method. The {001} facets exhibited higher OCM activity than {100} facets at 410 °C due to larger number of La3+–O2− sites, which could adsorb CO2 to form bidentate carbonate, and then reacting with oxygen to form adsorbed oxygen species. However, the higher methyl radical adsorption energy and the lower free energy of methane successive dehydrogenation of La2O2CO3{001} surface caused the excessive dissociation and deep oxidation of methyl radical, thus {001} facets show poor selectivity than {100} facets. Our findings will get a better understanding the facet effect of lanthanum-based oxides and provide guidance for the design of efficient low-temperature OCM catalysts.

Ab Initio Calculation of Surface Thermochemistry for Popular Solid Transition Metal-Based Species.

This work reports the thermochemistry calculations for solid-phase periodic models of ten popular transition metal-based species. These model structures were refined to stable geometry by geometric optimization along with calculating the thermodynamic properties including enthalpy, entropy, heat capacity at constant pressure, and Gibbs free energy by DMol3 package via first-principles ab initio calculations. The temperature-dependent thermochemistry values were converted to a NASA seven-polynomial format. The behavior of different thermodynamic parameters based on temperature was investigated and their comparative analysis was done. A higher number of atoms tends to show higher thermodynamic values. Moreover, these thermodynamic values agree reasonably well with previously reported experimental and computational values. Metal copper shows higher thermodynamic values as compared to its oxide. The thermodynamic properties of lanthanum-based oxides have been newly calculated through the ab initio method. Amorphous structures reveal higher thermodynamic values compared to their crystalline counterparts. A comparison between different transition metal-based species gives a better understanding of the different crystalline structures and their surface sites. These calculated thermodynamic data and polynomials can be used for a variety of thermodynamic calculations and kinetic modeling.

Solid-state synthesis of lanthanum-based oxides Co-coated LiNi0.5Co0.2Mn0.3O2 for advanced lithium ion batteries

Abstract The high voltage application of the LiNi0.5Co0.2Mn0.3O2 cathode is still hindered by its unstable surficial structure and poor capacity retention. Surface coating has been proved to be an effective strategy to deal with these problems. However, previous multistep surface coating approaches greatly complicate the synthesis procedure. In this work, a simple and large-scale solid-state approach is developed to prepare lanthanum-based oxides co-coated LiNi0.5Co0.2Mn0.3O2 cathode materials. Electrochemical tests indicate that the co-coated samples exhibit superior cycle life and higher rate performance than those of the pristine sample. Further studies reveal that the enhanced interface stability and facilitated electron transportation can be attributed to the synergetic contribution provided by the lanthanum-based oxides co-coating layer. The present work demonstrates that our approach can achieve cathode synthesis and surface coating synchronously, and shows great potential to apply on a large scale and extent to all Ni-rich cathodes.

Effect of structure and composition on the electronic excitation induced amorphization of La2Ti2\u2212xZrxO7 ceramics

Understanding the response of ceramics operating in extreme environments is of interest for a variety of applications. Ab initio molecular dynamic simulations have been used to investigate the effect of structure and B-site (=Ti, Zr) cation composition of lanthanum-based oxides (La2B2O7) on electronic-excitation-induced amorphization. We find that the amorphous transition in monoclinic layered perovskite La2Ti2O7 occurs for a lower degree of electronic excitation than for cubic pyrochlore La2Zr2O7. While in each case the formation of O2-like molecules drives the structure to an amorphous state, an analysis of the polyhedral connection network reveals that the rotation of TiO6 octahedra in the monoclinic phase can promote such molecule formation, while such octahedral rotation is not possible in the cubic phase. However, once the symmetry of the cubic structure is broken by substituting Ti for Zr, it becomes less resistant to amorphization. A compound made of 50% Ti and 50% Zr (La2TiZrO7) is found to be more resistant in the monoclinic than in the cubic phase, which may be related to the lower bandgap of the cubic phase. These results illustrate the complex interplay of structure and composition that give rise to the radiation resistance of these important functional materials.

Calculated and experimental electron energy-loss spectra of La2O3, La(OH)3, and LaOF nanophases in high permittivity lanthanum-based oxide layers

Using first principles methods, the O K energy-loss near-edge structure of cubic and hexagonal La2O3, La(OH)3, and LaOF phases have been calculated. These calculations support the identification of nanocrystalline phases evidenced experimentally by electron energy-loss spectroscopy (EELS) performed in a transmission electron microscope. The nanocrystals have been observed in atomic layer deposited La2O3 thin films developed for advanced metal-oxide-semiconductor field effect transistor applications. The presence of the nanophases can be explained by the hygroscopicity and the particular reactivity with fluorine of La2O3. These calculations provide a set of EELS fingerprints which will be useful for local phase identification in La2O3-based materials.

Properties of LaAlO Film after Waterless Process Using Organic Solvent Containing Anhydrous Hydrofluoric Acid

Lanthanum (La)-based oxide films have been studied as high-k (high dielectric constant) gate dielectrics. However, moisture absorption is a serious problem for oxide films containing La. We have attempted to use waterless solutions instead of water-based solutions to remove high-k films to suppress the moisture absorption of the lanthanum aluminate (LaAlO) film. We report the effect of an anhydrous hydrofluoric acid (AHF) and isopropyl alcohol (IPA) mixed solution as an etching solution and hydrofluoro-ether (HFE) as a rising solution on the properties of LaAlO films. We have succeeded in suppressing the moisture absorption of LaAlO films by using waterless solutions for a front end of line (FEOL) process. In addition, the selectivity (LaAlO/SiO2), the etching ratio of LaAlO to SiO2, was improved using this process. It is considered that this technology will be useful for the next-generation devices with lanthanum-based oxide films.

Micropatterning of lanthanum-based oxide thin film on self-assembled monolayers

We studied the direct micropatterning of a lanthanum-based thin film on a template of self-assembled monolayers in an aqueous solution at 80 °C. The template composed of silanol and octadecyl areas was prepared by UV-modified octadecyltrichlorosilane SAMs through a photomask. The amorphous La 2O(CO 3) 2· xH 2O thin films were selectively deposited in the silanol regions. Crystallized La 2O 3 was obtained after heating at 800 °C in air.