Co-O Bond Length Research Articles

Local compressive strain-induced anti-corrosion over isolated Ru-decorated Co3O4 for efficient acidic oxygen evolution

Enhancing corrosion resistance is essential for developing efficient electrocatalysts for acidic oxygen evolution reaction (OER). Herein, we report the strategic manipulation of the local compressive strain to reinforce the anti-corrosion properties of the non-precious Co3O4 support. The incorporation of Ru single atoms, larger in atomic size than Co, into the Co3O4 lattice (Ru-Co3O4), triggers localized strain compression and lattice distortion on the Co-O lattice. A comprehensive exploration of the correlation between this specific local compressive strain and electrocatalytic performance is conducted through experimental and theoretical analyses. The presence of the localized strain in Ru-Co3O4 is confirmed by operando X-ray absorption studies and supported by quantum calculations. This local strain, presented in a shortened Co-O bond length, enhances the anti-corrosion properties of Co3O4 by suppressing metal dissolutions. Consequently, Ru-Co3O4 shows satisfactory stability, maintaining OER for over 400 hours at 30 mA cm−2 with minimal decay. This study demonstrates the potential of the local strain effect in fortifying catalyst stability for acidic OER and beyond.

Degradation of bisphenol A by using perovskite LaCO3 as heterogeneous catalyst for activating peroxymonosulfate

Perovskite has attracted a lot of attention in the area of present-day environmental catalysis because of its cheap cost, excellent stability, great catalytic activity, varied structure, and significant transformation flexibility. In order to remove organic contaminants from wastewater, researchers have been experimenting with combining perovskite catalysts with advanced oxidation processes (AOPs). Herein present study, LaCoO3 was synthesized and calcined at 450, 650, and 850 °C (LC450, LC650, and LC850), and the connections between their surface features and crystalline assembly to catalytic performance were thoroughly explored to activate peroxymonosulfate (PMS) in order to degrade aqueous organic contaminants. Further, the catalysts were characterized by XRD, FTIR, FSEM, and TEM, and their catalytic efficiency was investigated. The Co-O bond length is recommended as a biomarker for PMS activation because it strongly influences the Co2+/Co3+ redox capability, regulating the total PMS activation. LC850/PMS has demonstrated the highest degradation efficiency (79%), with kapp value 0.102 min−1. Further modification of these types of perovskite catalysts might be very feasible and promising to degrading organic pollutants.

Constructing High-Performance Cobalt-Based Environmental Catalysts from Spent Lithium-Ion Batteries: Unveiling Overlooked Roles of Copper and Aluminum from Current Collectors.

Converting spent lithium-ion batteries (LIBs) cathode materials into environmental catalysts has drawn more and more attention. Herein, we fabricated a Co3O4-based catalyst from spent LiCoO2 LIBs (Co3O4-LIBs) and found that the role of Al and Cu from current collectors on its performance is nonnegligible. The density functional theory calculations confirmed that the doping of Al and/or Cu upshifts the d-band center of Co. A Fenton-like reaction based on peroxymonosulfate (PMS) activation was adopted to evaluate its activity. Interestingly, Al doping strengthened chemisorption for PMS (from -2.615 eV to -2.623 eV) and shortened Co-O bond length (from 2.540 Å to 2.344 Å) between them, whereas Cu doping reduced interfacial charge-transfer resistance (from 28.347 kΩ to 6.689 kΩ) excepting for the enhancement of the above characteristics. As expected, the degradation activity toward bisphenol A of Co3O4-LIBs (0.523 min-1) was superior to that of Co3O4 prepared from commercial CoC2O4 (0.287 min-1). Simultaneously, the reasons for improved activity were further verified by comparing activity with catalysts doped Al and/or Cu into Co3O4. This work reveals the role of elements from current collectors on the performance of functional materials from spent LIBs, which is beneficial to the sustainable utilization of spent LIBs.

Accelerating Oxygen Electrocatalysis Kinetics on Metal-Organic Frameworks via Bond Length Optimization.

Metal-organic frameworks (MOFs) have been developed as an ideal platform for exploration of the relationship between intrinsic structure and catalytic activity, but the limited catalytic activity and stability has hampered their practical use in water splitting. Herein, we develop a bond length adjustment strategy for optimizing naphthalene-based MOFs that synthesized by acid etching Co-naphthalenedicarboxylic acid-based MOFs (donated as AE-CoNDA) to serve as efficient catalyst for water splitting. AE-CoNDA exhibits a low overpotential of 260mV to reach 10mAcm-2 and a small Tafel slope of 62mV dec-1 with excellent stability over 100h. After integrated AE-CoNDA onto BiVO4, photocurrent density of 4.3mAcm-2 is achieved at 1.23V. Experimental investigations demonstrate that the stretched Co-O bond length was found to optimize the orbitals hybridization of Co 3d and O 2p, which accounts for the fast kinetics and high activity. Theoretical calculations reveal that the stretched Co-O bond length strengthens the adsorption of oxygen-contained intermediates at the Co active sites for highly efficient water splitting.

Spin-state regulation by secondary coordination sphere for improved oxygen evolution activity of LaCo1-xNixO3 perovskite

The evolution of oxygen evolution reaction (OER) remains a pivotal challenge in the realm of oxygen electrocatalysis. Recently, the proposition of regulating spin states has emerged as a novel avenue for enhancing the efficiency of electrocatalytic reactions. Presently, accurately modulating the metal active center into immediate spin (IS) remains a formidable challenge. Here, our research has achieved a breakthrough in precise spin state control and catalytic performance enhancement of Co3+ through the utilization of Ni-substituted LaCo1-xNixO3 perovskite. This achievement is primarily attributed to the fine regulation of both the secondary coordination sphere (SCS) and primary coordination sphere (PCS), i.e., Co6-y-[Co]-Niy (y = 0–6) and Co-O lengths, respectively. Our findings reveal that LaCo7/9Ni2/9O3 consisted of the SCS (y ≤ 2) with the eg1 fillings of IS Co3+ exhibits an optimal OER activity. Additionally, adjusting the Co-O bond length in PCS to approximately 1.89 Å proves to be more conducive to the transition of Co3+ spin states from high spin (HS) and low spin (LS) to IS. These discoveries present new approach to precisely modulating the spin state, offering promising prospects for the development of high-efficiency OER catalysts.

Thickness dependent strain effect on ferromagnetic insulating properties of LaCoO3 thin films

Epitaxial tensile-strain is an important factor to determine the superior ferromagnetic insulating property of LaCoO3 (LCO) epitaxial films. In this paper, the whole development of LCO ferromagnetic insulating properties modulated by thickness-dependent strain effect were studied and we found that the film strain decreased and the ferromagnetic property weakened with increasing film thickness. The measurements of x-ray magnetic circular dichroism confirm that the spin moment of Co3+ ions is significantly larger than its orbital magnetic moment and it also reduces with increasing thickness. The x-ray absorption spectroscopy revealed that the increasing film thickness is unfavorable for the presence of eg electrons, as well as for the intermediate-spin and high-spin states. Overall, the enhancement of film thickness leads to the decrease of the Co-O bond length and the increase of crystal field splitting energy, unfavorable for the electron transfer from t2g to eg orbitals, and in turn for the stability of intermediate-spin and high-spin states. The electronic transport behavior of LCO film was proved to be a good insulating properties and the temperature dependence of the resistivity is consistent with the 3D-Mott’s variable range hopping model. Our results not only demonstrated the feasibility of thickness-induced strain modulating the properties of LCO films, but also strengthen the understanding of the ferromagnetic insulating mechanism in LCO films.

Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO3 thin films

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO6 octahedral rotations throughout LaCoO3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO3 films while suggesting potential applications toward low-power spintronic devices.

Bimetallic electron-induced phase transformation of CoNi LDH-GO for high oxygen evolution and supercapacitor performance

Bifunctional energy materials in energy conversion and storage have a significant research value. Herein, bifunctional α-phase Co2Ni1 layered double hydroxide (LDH)-graphene oxide (GO) was prepared through bimetallic electronic regulation and GO assistance. Without GO addition, Co2Ni1 LDH bulk is dominated by the β phase. Compared with the β-phase type, the α-phase Co2Ni1 LDH-GO shows a larger interlayer spacing, which shortens the ion movement path and time and thus improves the electrochemical performance. As an electrocatalyst for oxygen evolution in energy conversion, the ultrathin α-phase Co2Ni1 LDH-GO possesses the lowest overpotential η10 of 218.5 mV at a current density of 10 mA cm−2. It also exhibits excellent stability mainly due to the Co-O bond by the cooperative electronic regulation of Ni2+. With the increase in Co-O bond length, the adsorbed OH− further oxidizes Co3+ to Co4+. As an electrode in a three-electrode supercapacitor for energy storage, the specific capacitance of Co2Ni1 LDH-GO can reach 3317.5 F g−1. When used in an asymmetric capacitor (Co2Ni1 LDH-GO//activated carbon), Co2Ni1 LDH-GO shows a remarkable capacitance of 328.75 F g−1 and excellent cycling stability. This work provides a phase transformation approach to improve the oxygen evolution and supercapacitor performance of multifunctional materials.

An intrinsic descriptor of perovskite cobaltites for catalytic peroxymonosulfate activation toward water remediation

A series of strontium cobaltite perovskite oxides with various dopants (SrCo0.95M0.05O3-δ, M=Fe, Sc, Co, Zn, Gd) are designed for catalytic peroxymonosulfate (PMS) activation to degrade aqueous organic pollutants and the correlations between their crystalline structure and surface properties to catalytic activity were comprehensively investigated. SrCo0.95M0.05O3-δ displays three crystalline structures depending on the dopant metals and exhibits different catalytic activities. Among the structures and properties, Co-O bond length significantly affects the lattice oxygen diffusivity and Co2+/Co3+ redox capacity, governing the overall PMS activation, and is suggested as a descriptor for PMS activation. This study provides new insight to the reaction pathways and the structure-activity correlation for new design of effective perovskite oxides for PMS-based oxidation process toward wastewater treatment and other catalytic processes.

Band topology resisted spin-state evolution of perovskite ACoO[formula omitted] (A = Ca, Sr) under pressure

There has been a debate on the theoretical explaining of the spin states in the perovskite ACoO3, especially SrCoO3. The aim of this work was to investigate the magnetic phase stability and to study the evolution of the spin states of the cubic perovskite ACoO3 (A = Ca, Sr) under pressure up to 40 GPa by using the density functional theory and the Hubbard +U parameters (DFT+U). Both CaCoO3 and SrCoO3 exhibit the ferromagnetic and metallic properties under pressure. Our results of the total magnetization and the net-spin density corroborate that a change of the magnetic behaviors on ACoO3 mostly depends on the value of Co-O bond length (dCo−O). We discovered a down-stepwise evolution of the total magnetization as a relationship with the dCo−O decreasing. This stepwise behavior occurs prior to the spin crossover from intermediate spin (IS) state to low spin (LS) state. The total magnetization values non-monotonically decreased from 2.49 to 2.16 μB/f.u. between the dCo−O = 1.88 and 1.80 Å. From our evidence, we indicated that the spin state of Co-3d states resembles the d7L̲2 character with t2g3↑,2↓eg2−x↑,x↓L̲2 state, where x≈0.5 and L̲ denotes the oxygen’s charge transfer from the oxygen ligands. Indeed, we discovered that the stepwise evolution of the total magnetization is associated with the changes in the band topology of the Co-3deg∗ band.

Structure Recovery and Recycling of Used LiCoO2 Cathode Material.

A large number of lithium batteries have been retiring from the market of energy storage. Thus, recycling of the used electrode materials is becoming urgent. In this study, an industrial machinery processing was used to recover the crystal structure of the waste LiCoO2 materials with the combination of small-scale equipment repair technology. The results show that the crystal parameters of the repaired LiCoO2 material become small, the unit cell volume is reduced, and the crystal structure tends to be stable. The Co-O bond length of 1.9134 nm, O-Co-O bond angle of 94.72°, the (003) interplanar spacing of 0.467 nm indicate the excellent recovery level of the repaired LiCoO2 . In addition, the electrochemical performance of the repaired LiCoO2 material is greatly improved, compared with the waste material. The capacity of the repaired electrode material can be maintained at 120 mAh g-1 after 100 cycles at the current density of 0.2C. The promising rate performance of the repaired electrode material demonstrates the stable structure. This research work provides a large-scale method for the direct recovery of LiCoO2 with the reduction of unnecessary energy and reagent consumption, which will be beneficial to the environmental protection.

Engineering of B-Metal Exsolution for High Performance Anode Materials in Solid Oxide Fuel Cells

B-metal exsolution has great potential to synthesize well-distributed metal nanoparticles (NPs) on the surface of oxide support materials. In previous works, we found the different exsolution tendencies of different kinds of transition metals on double-layered PrBaMn2O5+δ (L-PBMO). In particular, the exsolved NPs showed higher catalytic activity and stability for fuel oxidation, such as dry reforming of methane. To further engineer that concept, we have explored the possibility and detailed mechanism for various kinds of B-metal exsolution on L-PBMO. Based on our density functional theory (DFT) results, we will introduce (1) a possible driving force for B-metal NPs exsolution, (2) thermodynamically favorable mechanism for alloy NPs exsolution, and (3) how to boost B-metal exsolution to further improve cell performance SOFCs with the experimental verifications.In addition to L-PBMO, we also introduce the bond length of B-metal (Co) with lattice oxygen in perovskite structure (Co-doped SrTiO3, STC) as a key descriptor to represent the degree of Co exsolution on STC. Interestingly, we found that the Co exsolution can be facilitated under biaxial tensile strain (larger bond length of Co-O). By examining the bond length of Co-O under different strain states, we demonstrated that the weaker bond strength of Co-O is originated from the larger bond length of Co-O under tensile strain. This results in easier bond breaking between Co and O, thereby facilitating Co exsolution. To confirm our hypothesis (controlling Co-O bond length), we doped the larger ionic radius of A-site cation (Ba) into Sr in STC to induce tensile strain around Co. As we expected, the exsolution of Co is more preferred than than of Ba on STO, which is a more practical way to induce tensile strain than the mechanically applied strategy. As an application of exsolved Co NPs, we showed enhanced catalytic activity for CO oxidation.

Electrochemical Properties and Analysis of High Performance Nano-LiCoO2

The long-term cycle behavior of nano-LiCoO2 cathodes over 500 cycles at high rates is investigated. The post-mortem analysis of the cycled nano-LiCoO2 cathodes using a combination of X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) shed light on the capacity decay mechanisms. It is found that crystalline nano-LiCoO2 after 500 charge/discharge cycles at 10C becomes amorphous, exhibits Co2+ species rather than the expected Co3+ at the fully lithiated condition, has longer Co-O bond length than pristine nano-LiCoO2 and micro-LiCoO2, and shows 53% specific capacity loss over 500 cycles at 10C. In addition, slow Li-ion intercalation and de-intercalation at the electrode/electrolyte interface are found to be the rate-limiting step for nano-LiCoO2 during discharge and charge, respectively. Thus, to achieve the high rate capability of LiCoO2, not only LiCoO2 particle sizes should be reduced to nanoscales, but also the Li-ion intercalation and de-intercalation rates at the electrode/electrolyte interface need to be enhanced.

Off to the Races: Comparison of Excited State Dynamics in Vitamin B12 Derivatives Hydroxocobalamin and Aquocobalamin.

Ultrafast time-resolved spectroscopy was used to study the photochemistry of hydroxocobalamin (HOCbl) and aquocobalamin (H2OCbl+) in solution. Spectroscopic measurements and TD-DFT simulations provide a consistent picture of the spectroscopy and photochemistry. Excitation of H2OCbl+ results in formation of an excited state followed by rapid internal conversion to the ground state (0.35 ± 0.15 ps) through an S1/S0 seam at a slightly elongated Co-O bond length and a significantly elongated Co-NIm bond length. In contrast, the initial elongation of the axial bonds in HOCbl is followed by contraction to an excited state minimum with bonds slightly shorter than those in the ground state. Internal conversion to the ground state follows on a picosecond time scale (5.3 ± 0.4 ps). For both compounds, photodissociation forming cob(II)alamin and hydroxyl radicals (∼1.5% yield) requires excitation to highly excited states. Dissociation is mediated by competition between internal conversion to the S1 surface and prompt bond cleavage.

Cobalt spin states investigation of Ruddlesden-Popper La2−xSrxCoO4, using X-ray diffraction and infrared spectroscopy

Ruddlesden-Popper (RP) La2−xSrxCoO4 (x = 0.7, 0.9, 1.1, 1.3) were synthesized by a modified sol-gel route. The synthesized samples were characterized by XRD, FE-SEM technique, iodometric titration, FT-FIR spectroscopy and VSM analysis. The variation of Co-O bond length obtained from Rietveld refinements and the related peak shifts in the FT-FIR spectra showed that Co3+ has both LS and IS spin states. It is found that the population of Co3+ in the LS state decreases and IS state increases with increasing Sr concentration. Jahn-Teller effect was identified in the FT-FIR absorption bands, corresponding to the vibration modes of Co bonds with the apical and equatorial oxygen atoms. The ferromagnetic characteristic of La0.7Sr1.3CoO4, obtained by VSM, revealed the increase of Co3+ IS population by Sr doping, in agreement with XRD and FT-FIR results. The shift and split of the IR absorption bands with the change in the structural parameters are proposed to be an applicable tool to probe the spin states and Jahn-Teller effect in La2−xSrxCoO4 compounds.

Studies on the structural stability of Co2P2O7 under pressure

The crystal structural evolution of Co2P2O7 was studied by using in situ high pressure angle dispersive x-ray diffraction with synchrotron radiation. The results demonstrate that the α phase of Co2P2O7 goes through a partially irreversible structural transformation to β phase under pressure. The pressure is conductive to reduce the longest CoO bond length of the α phase, and then more uniform CoO bonds and regular hexagonal arrangement of CoO6 octahedra of the β phase are favored. According to the Birch-Murnaghan equation, the fitted bulk modulus B0 is 158.1(±5.6) GPa for α phase and 276.5(±6.5) GPa for β phase. Furthermore, the first-principles calculations show that these two phases of Co2P2O7 have almost equal total energies, and also have similar band structures and spin-polarized density of states at their ground states. This may be the reason why these two phases of Co2P2O7 can coexist in the pressure released state. It is found that the band gap energies decrease with increasing pressure for both phases.

Correlated oxygen displacements and phonon mode changes in LaCoO3 single crystal

X-ray diffraction and inelastic X-ray scattering studies have been performed across the spin (~100K) and semiconductor-metal (~500K) transitions in LaCoO3 single crystals. The quadratic increase with temperature of the oxygen displacement parameters parallel and perpendicular to the Co-O bond has been correlated with softening of the TO2 and hardening of the TO1 phonon branches along the [0 ξ ξ] high symmetry direction. The latter effect can be associated with the weakening of the Co-O bond strength derived from the increase of Co-O bond length and angle as expected upon increasing the high spin state population of the system with temperature.

Novel visible-light-driven photoconductive properties of LaCoO3 epitaxial nano-thin films

LaCoO3 nano-thin films were grown on (001), (110) and (111) oriented SrTiO3 substrates by the polymer assisted deposition method, respectively. All the LaCoO3 films are in epitaxial growth in accordance with the orientation of STO, with biaxial tensile strain. The films, especially for (111) oriented film, exhibit excellent visible-light photoconductive properties. The strain-induced increase of Co-O bond length may contribute to reduce the t2g-eg gap of Co3+ and further increase the number of photoexcited eg itinerant electrons. The different photoconductive activity of LaCoO3 epitaxial films may be attributed to the different surface energies of crystal planes. It suggests that the LaCoO3 epitaxial film can be considered as a potential candidate for visible-light photodetector.

Magnetism and the spin state in cubic perovskite CaCoO3 synthesized under high pressure

Cubic $\mathrm{SrCo}{\mathrm{O}}_{3}$ with an intermediate spin state can only be stabilized by high pressure and high temperature (HPHT) treatment. It is metallic and ferromagnetic with the highest Curie temperature of the transition-metal perovskites. The chemical substitution by Ca on Sr sites would normally lower crystal symmetry from cubic to orthorhombic as seen in the perovskite family of $\mathrm{Ca}M{\mathrm{O}}_{3}$ ($M={M}^{4+}$ of transition metals, $\mathrm{G}{\mathrm{e}}^{4+}, \mathrm{S}{\mathrm{n}}^{4+}$, and $\mathrm{Z}{\mathrm{r}}^{4+}$) at room temperature. This structural change narrows the bandwidth, so as to further enhance the Curie temperature as the crossover to the localized electronic state is approached. We report a successful synthesis of the perovskite $\mathrm{CaCo}{\mathrm{O}}_{3}$ with a HPHT treatment. Surprisingly, $\mathrm{CaCo}{\mathrm{O}}_{3}$ crystallizes in a simple cubic structure that remains stable down to 20 K, the lowest temperature in the structural study. The new perovskite has been thoroughly characterized by a suite of measurements including transport, magnetization, specific heat, thermal conductivity, and thermoelectric power. Metallic $\mathrm{CaCo}{\mathrm{O}}_{3}$ undergoes two successive magnetic transitions at 86 K and 54 K as temperature decreases. The magnetization at 5 K is compatible with the intermediate spin state ${t}^{4}{e}^{1}$ of $\mathrm{C}{\mathrm{o}}^{4+}$ at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population of high spin state ${t}^{3}{e}^{2}$ increases for $Tg100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The shortest Co-O bond length in cubic $\mathrm{CaCo}{\mathrm{O}}_{3}$ is responsible for delocalizing electrons in the ${\ensuremath{\pi}}^{*}$-band and itinerant-electron ferromagnetism at $Tl54\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. A comprehensive comparison between $\mathrm{SrCo}{\mathrm{O}}_{3}$ and $\mathrm{CaCo}{\mathrm{O}}_{3}$ and the justification of their physical properties by first-principles calculation have also been made in this report. Partially filled ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ bands would make $\mathrm{CaCo}{\mathrm{O}}_{3}$ suitable to study the Hund's coupling effect in a metal.

Relation between the Co-O bond lengths and the spin state of Co in layered Cobaltates: a high-pressure study

The pressure-response of the Co-O bond lengths and the spin state of Co ions in a hybrid 3d-5d solid-state oxide Sr2Co0.5Ir0.5O4 with a layered K2NiF4-type structure was studied by using hard X-ray absorption and emission spectroscopies. The Co-K and the Ir-L3 X-ray absorption spectra demonstrate that the Ir5+ and the Co3+ valence states at ambient conditions are not affected by pressure. The Co Kβ emission spectra, on the other hand, revealed a gradual spin state transition of Co3+ ions from a high-spin (S = 2) state at ambient pressure to a complete low-spin state (S = 0) at 40 GPa without crossing the intermediate spin state (S = 1). This can be well understood from our calculated phase diagram in which we consider the energies of the low spin, intermediate spin and high spin states of Co3+ ions as a function of the anisotropic distortion of the octahedral local coordination in the layered oxide. We infer that a short in-plane Co-O bond length (<1.90 Å) as well as a very large ratio of Co-Oapex/Co-Oin-plane is needed to stabilize the IS Co3+, a situation which is rarely met in reality.