Ferromagnetic Insulating Ground-State Resolved in Mixed Protons and Oxygen Vacancies-Doped La0.67Sr0.33CoO3 Thin Films via Ionic Liquid Gating

The realization of ferromagnetic insulating ground state is a critical prerequisite for spintronic applications. By applying electric field-controlled ionic liquid gating (ILG) to stoichiometry La0.67Sr0.33CoO3 thin films, the doping of protons (H+) has been achieved for the first time. Furthermore, a hitherto-unreported ferromagnetic insulating phase with a remarkably high Tc up to 180 K has been observed which can be attributed to the doping of H+ and the formation of oxygen vacancies (VO). The chemical formula of the dual-ion migrated film has been identified as La2/3Sr1/3CoO8/3H2/3 based on combined Co L23-edge absorption spectra and configuration interaction cluster calculations, from which we are able to explain the ferromagnetic ground state in terms of the distinct magnetic moment contributions from Co ions with octahedral (Oh) and tetrahedral (Td) symmetries following antiparallel spin alignments. Further density functional theory calculations have been performed to verify the functionality of H+ as the transfer ion and the origin of the novel ferromagnetic insulating ground state. Our results provide a fundamental understanding of the ILG regulation mechanism and shed light on the manipulating of more functionalities in other correlated compounds through dual-ion manipulation.

Oxygen migration performance of LaFeO3 perovskite-type oxygen carriers with Sr doping

A perovskite-type metal oxide with the formula of ABO₃ exhibits excellent redox reaction properties for the chemical looping process, where A is usually an alkaline earth metal and B is a transition metal. The oxygen transfer property plays an important role in the performance of oxygen carriers. In this study, the difference in oxygen vacancy formation energy and oxygen migration barrier between the surface and the bulk in the LaFeO₃ (ABO₃ type) perovskite is investigated via density functional theory (DFT). The effect of Sr doping on the A-site is considered. The results show that the covalency of the interaction between Fe and O at the surface is higher than that in the bulk, and the Sr doping on the A-site enhances this covalency, leading to a lower oxygen vacancy formation energy. The oxygen migration barrier is also different with the distance from the surface. A smaller deviation of the surrounding lattice atoms from their original place in the crystal at a deeper layer leads to a higher oxygen migration energy. Meanwhile, Sr doping increases the lattice constant, leading to larger migration space thus lowering the migration barrier. In addition, Sr doping can weaken the difference in the oxygen migration barrier between near the surface and in the bulk.

Recent Status and Developments of Vacancies Modulation in the ABO3 Perovskites for Catalytic Applications

Perovskite oxides (ABO₃ ) have attracted comprehensive interest for wide range of functional applications (especially for chemical catalysis) due to their high design flexibility, controllable vacancies sites creation, abundant chemical properties, and stable crystal structure. Herein, the previous research and potential development of ABO₃ through adjusting the vacancy at different sites (A-site, B-site, and O-site) to enhance catalytic performance are systematically analyzed and generalized. Briefly, the ABO₃ with different vacancies sites prepared by multifarious direct and indirect methods, accompanied with the improved physical-chemical properties, endow them with distinct and intensified development of catalysis application. In addition, the impressive optimization proved by the vacancies sites adjustment over the ABO₃ is studied to continuously facilitate the advance in some common catalysis reactions, further expanding to other optimized functional applications. At last, the constructive suggestions for fine regulation and analysis of vacancies sites over ABO₃ are also put forward.

Novel visible-light-driven SrCoO3/Ag3PO4 heterojunction with enhanced photocatalytic performance for tetracycline degradation

The semiconductor photocatalytic technology has been considerably studied due to its excellent catalytic performance in water pollution control. Herein, in this study, novel SrCoO₃/Ag₃PO₄ composite materials with different SrCoO₃ content were synthesized via a simple hydrothermal synthesis method. The characteristics of the as-prepared samples were detected through SEM/HRTEM, XRD, UV-vis DRS, PL, ESR, FT-IR, and XPS techniques, and then, the photocatalytic performance of SrCoO₃/Ag₃PO₄ toward the degradation of tetracycline was investigated. When the mass ratio of SrCoO₃ and Ag₃PO₄ in the composite was 1:1.5, the degradation rate constant of tetracycline in SrCoO₃/Ag₃PO₄ (1:1.5) system is 0.0102 min^-1, which is 1.7 times that of the Ag₃PO₄, and 3.78 times that of the SrCoO₃. In addition, reactive species were also analyzed through the free radical trapping experiment and DMPO spin-trapping ESR spectra analysis, showing that OH^•, h⁺, and O₂^•-participated in the catalytic degradation process of tetracycline to varying degrees. Finally, the photocatalytic mechanism of SrCoO₃/Ag₃PO₄ was also proposed.

Doping- and Strain-Dependent Electrolyte-Gate-Induced Perovskite to Brownmillerite Transformation in Epitaxial La1-xSrxCoO3-δ Films

Much recent attention has focused on the voltage-driven reversible topotactic transformation between the ferromagnetic metallic perovskite (P) SrCoO_3-δ and oxygen-vacancy-ordered antiferromagnetic insulating brownmillerite (BM) SrCoO_2.5. This is emerging as a paradigmatic example of the power of electrochemical gating (using, e.g., ionic liquids/gels), the wide modulation of electronic, magnetic, and optical properties generating clear application potential. SrCoO₃ films are challenging with respect to stability, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P → BM transformations across almost the entire La_1-xSr_xCoO₃ phase diagram (0 ≤ x ≤ 0.70), under both tensile and compressive epitaxial strain. Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish that voltage-induced P → BM transformations are possible at essentially all x, including x ≤ 0.50, where both P and BM phases are highly stable. Under small compressive strain, the transformation threshold voltage decreases from approximately +2.7 V at x = 0 to negligible at x = 0.70. Both larger compressive strain and tensile strain induce further threshold voltage lowering, particularly at low x. The P → BM threshold voltage is thus tunable, via both composition and strain. At x = 0.50, voltage-controlled ferromagnetism, transport, and optical transmittance are then demonstrated, achieving Curie temperature and resistivity modulations of ∼220 K and at least 5 orders of magnitude, respectively, and enabling estimation of the voltage-dependent Co valence. The results are analyzed in the context of doping- and strain-dependent oxygen vacancy formation energies and diffusion coefficients, establishing that it is thermodynamic factors, not kinetics, that underpin the decrease in the threshold voltage with x, that is, with increasing formal Co valence. These findings substantially advance the practical and mechanistic understanding of this voltage-driven transformation, with fundamental and technological implications.

Oxygen vacancy mediated room-temperature ferromagnetism and band gap narrowing in DyFe0.5Cr0.5O3 nanoparticles

We report on the magnetic and optical properties of DyFe0.5Cr0.5O3 nanoparticles synthesized by a sol-gel method. Rietveld refinement of a powder X-ray diffraction (XRD) pattern confirms the formation of an orthorhombic disordered phase with the Pnma space group. The formation of nano-sized particles, with an average size of 42(±12) nm, was approximated by the transmission electron microscopy (TEM) image analysis. X-ray photoelectron spectroscopy (XPS) of this compound reveals the presence of Fe2+/Fe3+ and Cr2+/Cr3+ mixed-valence states as a consequence of oxygen vacancies present at the surface of nanoparticles. The temperature-dependent magnetization (M-T) shows a finite non-zero magnetization up to 300 K and the field-dependent magnetization (M-H) curve exhibits a weak ferromagnetic (WFM) nature at 300 K with a clear hysteresis loop, which is quite appealing compared to that of the previously reported micron-sized DyFe0.5Cr0.5O3. These observations indicate that the large concentration of uncompensated surface spin of nanoparticles could be responsible for the observed room-temperature ferromagnetism. Moreover, DyFe0.5Cr0.5O3 nanoparticles show a significantly narrow band gap (Eg ∼ 2.0 eV). Meanwhile, the oxygen vacancies may generate shallow trap energy levels within the band gap as observed from photoluminescence (PL) spectroscopy. The observed band gap narrowing by Fe doping and the effect of oxygen vacancies on the band gap are consistent with the predictions of density functional theory (DFT) calculations. The evidence of room-temperature ferromagnetism in DyFe0.5Cr0.5O3 nanoparticles compared to their bulk counterparts and the significantly narrow band gap in the visible range manifest the potential of this material in spintronic and optical applications.

Neodymium (Nd) based oxide perovskite nanostructures for photocatalytic and photoelectrochemical water splitting functions

Neodymium (Nd) based perovskite (Nd_1-xCo_xFeO₃) nanostructures were processed to address the rising energy and environment crisis through offering solutions by photocatalytic and photoelectrochemical (PEC) water splitting reactions. The impact of cobalt (Co) ions on the physicochemical properties of Nd-perovskites were studied using X-ray diffraction (XRD), Raman and electron microscopic instruments. The interaction of metal ions was studied in depth via X-ray photoelectron spectroscopy (XPS). Absorption and photoluminescence signals inferred the optical band gap to be lowered and defect states to increase upon Co substitution. Improved photocatalytic efficacy in Nd_1-xCo_xFeO₃ was evaluated by comparative studies using NdFeO₃. Secondly, the enhanced conductivities in Nd_1-xCo_xFeO₃ studied via Nyquist plot was found to be advantageous in photoelectrode fabrication for PEC functions. Time-dependent photocurrent density results affirmed the stability in processed devices. Co ions were also inferred to boost the separation of charge carriers effectively. The improved performance in Nd_1-xCo_xFeO₃ nanostructures were well justified to the successful incorporation of Co ions that sway the Nd-O, Co-O and Co-Fe-O bondings and boost the photon absorption and electronic conductivity to facilitate the observed performance.

Assessing the Effects of Temperature and Oxygen Vacancy on Band Gap Renormalization in LaCrO3-δ: First-Principles and Experimental Corroboration

Understanding the temperature dependence of functional properties in high-temperature gas sensors is vital for applications in combustion environments. Temperature effect on the electronic structure due to electron-phonon coupling is a key property of interest as this influences other responses of sensors. In this work, we assess the impact of temperature on band gap renormalization of pristine and oxygen-vacant LaCrO_3-δ perovskite employing Allen-Heine-Cardona theory with first-principles simulations and corroborate with experimental observation. Antiferromagnetic cubic LaCrO₃ shows a direct ground-state band gap of 2.62 eV that is reduced by over 1 eV due to the presence of oxygen vacancies, which can form endothermically. We find excellent agreement in temperature-dependent band gap shift in LaCrO₃ between theory and an in-house experiment, proving that the theory can adequately predict renormalization on the band gap in a magnetic system. Band gaps in cubic LaCrO_3-δ are found to monotonically narrow by 1.13 eV in pristine and by around 0.62 eV in oxygen-vacant structures as temperature increases from 0 to 1500 K. The predicted band gap variations are rationalized using an analytical model. The experimental zero-temperature band gaps are extracted from the model fits that can provide useful insights on the simulated band gaps.

Recent advances in perovskite oxides as electrode materials for supercapacitors

Owing to the high power density and ultralong cycle life, supercapacitors represent an alternative to electrochemical batteries in energy storage applications. However, the relatively low energy density is the main challenge for supercapacitors in the current drive to push the entire technology forward to meet the benchmark requirements for commercialization. To effectively solve this issue, it is crucial to develop electrode materials with excellent electrochemical performance since the electrode used is closely related to the specific capacitance and energy density of supercapacitors. With the unique structure, compositional flexibility, and inherent oxygen vacancy, perovskite oxides have attracted wide attention as promising electrode materials for supercapacitors. In this review, we summarize the recent advances in perovskite oxides as electrode materials for supercapacitors. Firstly, the structures and compositions of perovskite oxides are critically reviewed. Following this, the progress in various perovskite oxides, including single perovskite and derivative perovskite oxides, is depicted, focusing on their electrochemical performance. Furthermore, several optimization strategies (i.e., modulating the stoichiometry of the anion or cation, A-site doping, B-site doping, and constructing composites) to improve their electrochemical performance are also discussed. Finally, the significant challenges facing the advancement of perovskite oxide electrodes for supercapacitor applications and future outlook are proposed.

First-principles exploration of oxygen vacancy impact on electronic and optical properties of ABO3-δ (A = La, Sr; B = Cr, Mn) perovskites

ABO3-δ perovskites are utilized in many applications including optical gas sensing for energy systems. Understanding the opto-electronic properties allows rational selection of the perovskite-based sensors from a diverse family of ABO3-δ perovskites, associated with the choices of A and B cations and range of oxygen concentrations. Herein, we assess the impact of oxygen vacancies on the electronic structure and optical response of pristine and oxygen-vacant ABO3-δ (A = La, Sr; B = Cr, Mn) perovskites via first-principles calculations. The endothermic formation energy for oxygen vacancies shows that the generation of ABO3-δ defect structures is thermodynamically possible. LaCrO3 and LaMnO3 have direct and indirect ground-state band gaps, respectively, whereas SrCrO3 and SrMnO3 are metallic. In the presence of an oxygen mono-vacancy, however, the band gap decreases in LaCrO3-δ and vanishes in LaMnO3-δ. In contrast to the decrease in the band gaps, the oxygen vacancies in ABO3-δ are found to increase optical absorption in the visible to near-infrared wavelength regime, and thus lower the onset energy of absorption compared with the pristine materials. Our assessments emphasize the role of the oxygen vacancy, or other possible oxygen non-stoichiometry defects, in perovskite oxides with respect to the opto-electronic performance parameters that are of interest for optical gas sensors for energy generation process environments.

The optimal co-doping of SrFe1-xCoxO3-δ oxygen carriers in redox applications

Although the oxygen carrier SrCoO₃ has higher redox activity than SrFeO₃, cobalt is both more expensive and scarcer than iron, which would hinder the wide implementation of SrCoO₃. For these reasons, doping SrFeO₃ with Co is a potential compromise, benefitting the redox properties of SrFeO₃, while still limiting the overall amount of cobalt being used. To find the optimal level of Co-doping, density functional theory calculations were performed to investigate the Co-doping effect on the oxygen vacancy formation and oxygen migration in SrFe_1-xCo_xO_3-δ (x = 0, 0.125, 0.25, 0.375, 0.5). Our findings show that the oxygen vacancy formation energies (E_f) decrease with the increase of Co content resulting from the increased composition of the O-2p band at the Fermi level upon Co doping. In particular, the E_f decreases nearly 0.5 eV between the x = 0 and x = 0.25 samples while E_f only decreases 0.1 eV further as Co content is increased to x = 0.5. We obtain that x = 0.25 is an optimal cost/benefit ratio for Co doping, which is preserved at both low oxygen vacancy concentrations (δ = 0.0625 values listed above) and at high concentrations of δ = 0.1875 and 0.375. Kinetically, the oxygen migration barrier has slight change upon Co doping due to the similar size of Co and Fe. Therefore, considering both redox activity and economics in reversible oxygen storage applications, x = 0.25 is suggested as the optimal Co-doping value in SrFe_1-xCo_xO_3-δ.

Theoretical study of the optical and thermodynamic properties of LaxSr1-xCo1-yFeyO3-δ (x/y = 0.25, 0.5, 0.75) perovskites

The performance of LaxSr1-xCo1-yFeyO3-δ perovskite systems in applications such as solid oxide fuel cells and catalysis is related to the proportion of substitution atoms. Using a density functional theory method, we investigate the doping effect on the electronic, optical, and thermodynamic properties of LaxSr1-xCo1-yFeyO3-δ (x/y = 0.25, 0.5, 0.75). Our results show that La doping introduces an empty state and pushes the Fermi level upwards. The doping Fe derived states locate away from the Fermi level as compared with Co states. From the results of optical absorption, the peak at 200-300 nm is enhanced and experiences a blue-shift with increasing La concentration. The corresponding peak at 400-700 nm also shows a blue-shift induced by both La and Fe doping, and it could be enhanced by Fe doping while being suppressed by La doping. And the peak above 1500 nm is enhanced by the cooperation of La and Fe doping. From thermodynamic calculations via an Ellingham diagram, it is found that the parent SrCoO3 is the most favorable composition for releasing O2, with both La and Fe doping hampering the reduction reaction. Therefore, the optical and thermodynamic properties of LaxSr1-xCo1-yFeyO3-δ could be adjusted by special doping values.