LaCoO3 Research Articles

Strain-Induced Atomic-Scale Building Blocks for Ferromagnetism in Epitaxial LaCoO3.

The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.

Optimized Conductivity and Spin States in N-Doped LaCoO3 for Oxygen Electrocatalysis.

The spin state of antibonding orbital (eg) occupancy in LaCoO3 is recognized as a descriptor for its oxygen electrocatalysis. However, the Co(III) cation in typical LaCoO3 (LCO) favors low spin state, which is mediocre for absorbing oxygen-containing groups involved in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), thus hindering its further development in electrocatalysis. Herein, both experimental and theoretical results reveal the enhancement of bifunctional electrocatalytic activity in LaCoO3 by N doping. More specifically, electron energy loss spectroscopy and superconducting quantum interference devices magnetic analysis demonstrate that the Co(III) cation in N-doped LaCoO3 (LCON) achieves a moderate eg occupancy (≈1) compared with its low spin state in LaCO3. First-principle calculation results reveal that N dopants play a bifunctional role of tuning the spin-state transition of Co(III) cations and increasing the electrical conductivity of LCO. Thus, the optimized LCON exhibits an OER overpotential of 1.69 V at the current density of 50 mA/cm2 (1.94 V for pristine LCO) and yields an ORR limiting current density of 5.78 mA/cm2 (4.01 mA/cm2 for pristine LCO), which offers a new strategy to simultaneously modulate the magnetic and electronic structures of LCO to further enhance its electrocatalytic activity.

Two-dimensional magnetic interplay in the tensile-strained LaCoO3 thin films.

High-quality epitaxial LaCoO3 (LCO) thin films have been deposited on SrTiO3 (STO) substrates with pulsed laser deposition (PLD). We find that the LCO films undergo a typical ferromagnetic-paramagnetic (FM-PM) phase transition at ∼80 K. To understand the nature of magnetic phase transition, various methods, including the modified Arrott plot and critical isotherm analysis, were used to determine the critical exponents, which are β = 0.754(1) with a Curie temperature TC = 79.8(8) K and γ = 1.52(2) with TC = 79.9(2) K. The reliability of these critical exponents was confirmed using the Widom scaling relation and the scaling hypothesis. Further analysis revealed that the spin coupling within the LCO films exhibits two-dimensional (2D) long-range magnetic interaction and the magnetic exchange distance decays as J(r) ∼r-(3.46). Our critical behavior analysis may shed new light on the further understanding of the origin of FM and the relatively fixed TC in LCO thin films.

Growth of LaCoO3 crystals in molten salt: effects of synthesis conditions

Herein, several typical reaction media for the synthesis of LaCoO3 (LCO) crystals were examined by X-ray absorption spectroscopy (XAS).

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites.

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO3 (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO2 (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO6 octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 μB /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.

Charge‐Transfer‐Induced Multivalent States with Resultant Emergent Magnetism in Transition‐Metal Oxide Heterostructures

AbstractTransition‐metal oxide (TMO) heterostructures provide fertile grounds for creating and manipulating intriguing properties and functionalities. At the interface of TMO heterostructures, electronic reconstructions generally occur via charge transfer and lead to an extraordinary spectrum of emergent phenomena but unattainable in their bulk constituents. However, the basic mechanism of charge transfer at the interface is not fully determined or even understood in heterostructures, which may hide the underlying mechanisms and intriguing physics. Herein, an intrinsic charge transfer and resultant exotic ferromagnetism are unambiguously observed in the heterostructures between the nonmagnetic LaCoO3 (LCO) and SrTiO3 (STO). Combining element‐specific X‐ray absorption spectroscopy and atomic multiplet fitting, direct evidence of charge transfer‐induced multivalence of cobalt ions, interactions of which would contribute to the novel magnetism beyond the intuition, in concert with first‐principles density‐functional‐theory calculations, is demonstrated. Beyond LCO/STO system, a more broadly applicable principle for the heterostructures between 3d TMO and STO where charge transfer and resultant multivalence or conducting interfaces are coexistent is establish. This study represents an advance that the electronic reconstruction and the multiple electron configurations of 3d transition metal ions will constitute a powerful tool for the designs of functional materials and creations of unconventional physical properties.

Electrode protective barrier layers for molten carbonate confinement

Pure NiO or lithiated NiO with distinct Li/Ni atomic proportions (50/50—LN55 and 30/70—LN37) were tested as protective barrier layers (PBLs) to enhance the stability of LaCoO3 (LC) electrodes in cells with composite electrolytes (CEs) consisting of Gd-doped ceria (CGO) and a eutectic mixture of Na and Li carbonates (NLCs). PBLs and LC layers were sequentially deposited by screen printing before firing at 700°C and 550°C, respectively, to yield symmetrical cells (LC|PBL|CE|PBL|LC). Electrochemical testing involved impedance spectroscopy measurements in air in the 500°C to 650°C range. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM/EDS) revealed that distinct PBLs possess uneven wetting tendency, with impact on the role of LC layers. The best electrode performance (0.55 Ω cm2 electrode area specific resistance at 550°C in air) was observed using the LN37 PBL, stable throughout endurance tests up to 200 hours. Possible electrode mechanisms consistent with experimental evidence suggest an active role of the molten phase as an intermediate provider of O2− transport between the electrode and CGO.

The strain induced magnetic and anisotropic variations of LaCoO3 thin films

The emergence of ferromagnetism in tensile strained LaCoO3 (LCO) films is still being researched due to the lack of any uniform explanation. Herein, the magnetic anisotropy and the precise ratio of Co2+ ions in LCO films growth on different substrates are reported. The emergence of ferromagnetic state can be observed irrespective of the tensile or compressive strain on the LCO films. A small magnetization along the out-of-plane easy axis is exhibited in the LCO film under compressive strain. The maximum magnetization (1.11 μB/u.c.) is observed in the LCO film over tensile strained LSAT substrate along the in-plane easy axis, which is around four times compared to the magnetization of LCO film on compressive strain. In addition, the exact percentage of Co2+ ions in different LCO films deviated from 12.8% to 10.6% as the strain changed from tensile to compressive. This little variation has a relatively weak effect on the hugely different magnetization of LCO films. Subsequently, the magnetization of LCO film grown on STO buffer layer has a great increase compared to the film direct on LAO substrate. All these results support the proposition that the tensile strain should induce more IS (t2g5eg1) or HS (t2g4eg2) spin states of Co3+ ions, which plays the key role for enhancement of ferromagnetism.

Graphene mediated resistive switching and thermoelectric behavior in lanthanum cobaltate

Multifunctional materials are of utmost importance owing to the combination of several functionalities into a single device. We present the multifunctional properties, namely, thermoelectric (TE) for energy harvesting and resistive switching (RS) for memory storage in the LaCoO3 (LCO)–graphene nanocomposite system. The existence of individual phases of LCO and graphene is confirmed from x-ray diffraction and electron microscopy techniques. The x-ray photoelectron spectroscopy measurement reveals the formation of oxygen vacancies in the nanocomposite with the addition of graphene. The bipolar resistive switching behavior observed in a LCO–graphene nanocomposite is explained using space charge limited conduction mechanism and is found to evolve from a trap-limited to a trap-free region with an increase in the graphene volume fraction, which eventually ascribed to the ordering of oxygen vacancies in the nanocomposite system. The endurance plot of the nanocomposite shows a stable RS behavior for consecutive 1000 cycles. On the other hand, the creation of oxygen vacancies in the nanocomposite leads to change in the configurational entropy of charge states of cobalt, which tunes the Seebeck coefficient (α). The decrease in α leads to an increase in the hopping hole concentration (nh) estimated using the classical Heikes formula and is consistent with the increase in the electrical conductivity. The increase in oxygen vacancies leads to point-defect scattering, which further reduces the thermal conductivity of the nanocomposite. The optimized TE parameters show a figure-of-merit of 0.004±0.00048 at 300 K. The simultaneous observation of RS and TE properties in LCO–graphene nanocomposite provides new directions for multifunctional materials.

Synchronous degradation of aqueous benzotriazole and bromate reduction in catalytic ozonation: Effect of matrix factor, degradation mechanism and application strategy in water treatment

Ozone-based technologies are used for micro-pollutants removal in wastewater treatment. However, the generation of the toxic by-product bromate (BrO3−) is of a great concern. LaCoO3 (LCO) catalytic ozonation has been used to overcome this significant drawback in the sole ozonation, achieving better BrO3− elimination efficiency. However, a key challenge is how to enhance micro-pollutant (benzotriazole, BZA) degradation efficiency and to eliminate formed BrO3− synchronously under various water qualities in drinking water or wastewater treatment. Therefore, the objective of this study is to propose a practical strategy of BZA removal and BrO3− reduction synchronously in water or wastewater treatment. In this study, important factors influencing BZA removal and BrO3− reduction were investigated, including [catalyst], [BZA], initial pH solution, [NH4+-N] and [(bi)carbonate alkalinity]. Based on the performance and mechanism of these effects, a practical strategy for BZA degradation and BrO3− elimination with and without Br− in the influent was developed. Additionally, the density functional theory (DFT) calculation successfully predicted the attack site on BZA by molecular ozone and formed hydroxyl radical (HO·) during LCO catalytic ozonation. Fukui indexes of f+ and f0 were calculated to forecast direct ozone molecule and HO· attack, respectively. Combination of DFT calculation with intermediates that identified through liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS), BZA degradation pathway was established more accurately. Additionally, four new intermediates were identified in this study. Overall, this study proposes a useful strategy for synchronous micro-pollutants degradation and BrO3− elimination, while also suggesting the feasibility of LCO catalytic ozonation for water or wastewater purification.

Engineering of the d-Band Center of Perovskite Cobaltite for Enhanced Electrocatalytic Oxygen Evolution.

Great efforts have been made to understand and upgrade the kinetically sluggish oxygen evolution reaction (OER). In this study, a series of V-doped LaCoO3 (V-LCO) OER electrocatalysts with optimized d-band centers are fabricated. When utilized as an electrode for the OER, as-formed LaCo0.8 V0.2 O3 (V-LCO-II) requires an overpotential of only 306 mV to drive a geometrical catalytic current density of 10 mA cm-2 . Furthermore, at a given overpotential of 350 mV, the OER current density of V-LCO-II is about 22 times that of pure LaCoO3 (LCO) nanoparticles. Tailoring of the d-band center by V doping facilitates the adsorption of OER intermediates and promotes the formation of amorphous active species on the surface of LCO through the exchange interaction between high-spin V4+ and low-spin Co2+ . This work may create new opportunities for developing other highly active OER catalysts through d-band center engineering.

Strain effects on the Co oxidation state and the oxygen dissociation activity in barium lanthanum cobaltite thin films on Y2O3 stabilized ZrO2

Tensile strain increases surface activity of cobaltite cathode oxides like Ba(La)CoO3 for solid oxide fuel cells and electrolysers (SOFCs/SOECs).

Structural, electronic, transport and magnetic studies of LaCo1 − xNixO3 (x = 0, 0.3) thin films

We present here a study on structural, electronic, transport, and magnetic properties of single-phase (001) oriented compressively strained thin films of LaCoO3 (LCO) and LaCo0.7Ni0.3O3 (LCNO). The films were deposited on the LaAlO3 (001) single crystal substrate using pulsed laser deposition. Electronic structure investigations by x-ray absorption spectroscopy (XAS) measurements at various edges (Co-L3,2, Ni-L3,2, and La-M5,4) confirm +3 charge states for Co, Ni, and La ions in all samples. The XAS study also reveals a transition in the spin state of Co+3 (low spin to intermediate/high spin), which is attributed to induced strain and Ni substitution. Electrical transport measurements suggest the semiconducting behavior for LCO and LCNO thin films in the measured temperature range. We found a drastic change in resistivity (ρ) and magnitude with Ni substitution. An in-depth analysis of the conduction mechanism governing the transport behavior in thin films has been done using various theoretical models. Different conduction mechanisms (Arrhenius law and Effros-Shklovski-type variable range hopping) have been observed to govern conduction in different temperature ranges, suggesting disorder controlled transport in these films. The magnetization measurements show a magnificent increase in saturation magnetization for LCNO and ferromagnetic ordered states, which persists even up to room temperature.

Integrating the cationic engineering and hollow structure engineering into perovskites oxides for efficient and stable electrocatalytic oxygen evolution

Designing highly efficient oxygen evolution reaction (OER) electrocatalysts from nanoscale level has sparked great interest in several important catalytic reactions, such as water splitting, metal-air batteries, and regenerative fuel cells. Herein, we have reported an effective strategy for designing one-dimensional perovskite oxides with unique hollow nanostructures by combining an electrospinning technology and high temperature carbonization and oxidation. The LaCoO3 (LC) was used as the model catalyst and the La0.6Sr0.4CoO3 (LSC) and La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) hollow nanofibers were synthesized through the A-site and AB-site doping engineering. The building block perovskite oxides nanoparticles connected with each other along one direction and formed a one-dimensional (1D) hollow nanofibers with hierarchical architectures. The one-dimensional perovskite oxide nanofibers can be tailored from porous hollow structures to solid nanofibers by adjusting the temperatures and the A, B site doping. Compared with the LC (444 mV) and the LSC (426 mV), the LSCF demonstrated the superior OER activity with overpotential of 353 mV (10 mA cm−2), suggesting the enhanced activity by the AB-site doping engineering. A series of LSCF electrocatalysts were prepared at different carbonization and oxidation conditions and the La0.6Sr0.4Co0.8Fe0.2O3 prepared at 700 °C obtained the best catalytic performance and exceptional durability. This strategy provides an effective approach to design advanced perovskite oxides catalysts with superior activity and stability.

Catalytic ozonation of emerging pollutant and reduction of toxic by-products in secondary effluent matrix and effluent organic matter reaction activity

In this study, the performance of LaCoO3 (LCO) catalytic ozonation was evaluated comprehensively, including the degradation efficiency of benzotriazole (BZA) as a typical emerging pollutant, toxic bromate reduction and the disinfection by-products (DBPs) precursors removal ability in effluent organic matter (EfOM), as well as EfOM reactive activity in catalytic ozonation. Additionally, the reduction of toxic halogenated by-products in (catalytic) ozonation was reported, which was not focused on previous researches before. Results showed that LCO catalytic ozonation improved the removal efficiency of BZA, UV254 and SUVA via enhanced HO· formation. Interestingly, LCO catalytic ozonation showed the ability on the reduction of aldehydes and toxic halogenated organic by-products. Moreover, the formed [trichloromethane (TCM)], [bromochloroacetonitrile (BCAN)] and [dichloroacetamide (DCAcAm)] decreased significantly in catalytic ozonation. Catalytic ozonation was also able to remove DBPs precursors to decline the formation of DBPs, such as TCM, bromodichloromethane (BDCM), trichloroacetonitrile (TCAN) and trichloronitromethane (TCNM). This process was involved in the transformation of EfOM in catalytic ozonation, which was confirmed by multi-spectrum methods, two-dimensional correlation spectroscopy (2D-COS) and hetero-spectral 2D-COS. In summary, LCO was shown to be an effective catalyst to improve the performance of the sole ozonation on the removal of emerging contaminants and DBPs precursors, as well as toxic by-products reduction. Additionally, the strategy of toxic by-products reduction in catalytic ozonation was proposed. Results indicated this technology was an important contribution on removal of refractory organics and formation of toxic by-products in water supply and wastewater treatment industry.

Thermoelectric Properties of (1 − x)LaCoO3.(x)La0.95Sr0.05CoO3 composite

Thermopower in cobalt oxides has been a rich area of interest due to the existence of the different charge states along-with different spin states. In this report, we have systematically studied the structural and thermal transport properties of (1 − x)LaCoO3.(x)La0.95Sr0.05CoO3 composite. The Seebeck coefficient (α) values for the composite increases at high temperatures compared to the LaCoO3 (LCO) and La0.95Sr0.05CoO3 (LSCO) systems. The electrical conductivity (σ) decreases with the increase in the LSCO fraction which may be attributed to the localization of charge carriers due to intersite diffusion. All the samples show increase in the value of σ with increase in temperature. The thermal conductivity (κ) values decrease with the increase of LSCO content in the composite and the phonon thermal conductivity dominates over the total thermal conductivity. We observe a maximum value of figure of merit (ZT) ∼ 0.06 at 640 K for x = 0.05.

High-Performance Solid Oxide Fuel Cell with an Electrochemically Surface-Tailored Oxygen Electrode.

State-of-the-art cathodes for solid oxide fuel cells (SOFCs), such as (La,Sr)MnO3 -(Y2 O3 )0.08 (ZrO2 )0.92 (LSM-YSZ), suffer from sluggish oxygen reduction reaction (ORR) kinetics at reduced temperatures, leading to a significant decline in their performance. Herein, we report a tailored SOFC cathode with high ORR activity at intermediate temperatures using a simple but effective approach based on "electrochemical" surface modification. The proposed process involves chemically assisted electrodeposition (CAED) of a metal hydroxide (LaCo(OH)x ) on LSM-YSZ surfaces followed by in situ thermal conversion of LaCo(OH)x to perovskite-type LaCoO3 (LCO) nanoparticles during the SOFC startup. This method facilitates easy loading of the LCO nanoparticles with a precisely controlled morphology without the need for repeated deposition/annealing processes. An anode-supported SOFC with the LCO-tailored LSM-YSZ electrode exhibits a remarkably increased power density, approximately 180 % at 700 °C, compared with an SOFC with the pristine electrode as well as excellent long-term stability, which are attributed to the beneficial role of the CAED-derived LCO nanoparticles in enlarging the active areas for ORR and promoting oxygen adsorption/diffusion. This work demonstrates that controlled surface tailoring of the cathode by CAED could be an effective approach for improving the performance of SOFCs at reduced temperatures.

Hydrothermal synthesis of sandwich interspersed LaCO3OH/Co3O4/graphene oxide composite and the enhanced catalytic performance for methane combustion

Novel structured LaCO3OH/Co3O4/graphene oxide composite was synthesized through a hydrothermal process using a certain ratio of urea (N), LaCoO3 (LC) and graphene oxide (GO) as starting materials. Morphology characterizations showed that as-prepared composite had sandwich structure interspersed on graphene oxide sheets. The LaCO3OH nano-sheets were self-organized into stratiform nano-structures, where spherical Co3O4 nano-particles with a diameter of 17–31 nm were homogeneously distributed between two adjacent LaCO3OH nano-sheets, forming sandwich structure. At the same time, the sandwich structure inlaid on the surface of two-dimensional GO, forming sandwich structure interspersed morphology. The results indicated that the amount of urea additive (m(N)) played a key role in the formation of the neoteric morphology. The LaCO3OH/Co3O4/graphene oxide composite showed optimized catalytic performance for methane combustion as 50% methane conversion at 315 °C and 90% at 420 °C when m(N)/m(LC) = 5, which can match the activity of noble metal catalysts. The enhanced catalytic performance was attributed to its unique loaded structure with high surface area and more exposed active sites, which showed enhanced application in methane combustion as a new type of Pt-group metals free catalyst. In addition, stability performance was systematically studied, indicating that the synthesized catalyst has great potential for industrial applications.

Strain effect on the magnetic and transport properties of LaCoO3 thin films

LaCoO3 (LCO) has attracted much attention due to the unique magnetic transition and spin transition of Co3+ ions. Epitaxial LCO film exhibits an unexpected ferromagnetism, in contrast to the non-magnetism of bulk LCO. An in-depth study on the property of strained LCO film is of great importance. We have fabricated 30 nm LCO films on various substrates and studied the magnetic and transport properties of films in different strain states (compressed strain for LCO/LaAlO3, tensile strain for LCO/(LaAlO3)0.3(Sr2TaAlO6)0.35, SrTiO3). The in-plane tensiled LCO films exhibit ferromagnetic ground state at 5K and magnetic transition with TC around 85K, while compressed LCO/LaAlO3 film has a negligibly small moment signal. Our results reveal that in-plane tensile strain and tetragonal distortion are much more favorable for stabilizing the FM order in LCO films.

High-performance nanofibrous LaCoO3 perovskite cathode for solid oxide fuel cells fabricated via chemically assisted electrodeposition

In this study, a new method is developed for the fabrication of nanofibrous LaCoO3 (LCO) perovskites as cathodes (oxygen electrodes) for solid oxide fuel cells (SOFCs).