Oxygen Coordination Environment Research Articles

Efficient pathways for photogenerated charge transfer induced by Co dopants in WO3/TiO2 nanorod arrays

Modifying the energy band structure in heterostructured photocatalysts enhances charge separation efficiency and improves photoelectrochemical (PEC) performance by decreasing the charge transfer barrier. In this study, cobalt doping into WO3/TiO2 core/shell heterojunction nanorod arrays introduces versatile valence states of cobalt altering the oxygen coordination environment around W atoms in WO3, resulting in an increase in W5+ ions and oxygen vacancy defects in WO3 lattice, facilitating the water splitting reaction. Photogenerated electrons transfer easily from the WO3:Co shell to the TiO2 core due to the lower conduction band minimum (CBM) of WO3:Co shell. Moreover, photogenerated holes transfer from the TiO2 core to the WO3:Co shell efficiently due to the higher valence band maximum (VBM) of TiO2 core. The heterostructure has a high photogenerated carrier density (9.89 × 1018 cm-3), improving photoconversion efficiency (2.55 mA cm-2 at 1.23 V vs. RHE) and reducing charge recombination rates (6.51 × 10–4 s-1). Co doping increases the -OH bond on WO3/TiO2 surface, improves its hydrophilicity, and is more conducive to the reaction in aqueous electrolyte. Additionally, the nanorod array structure facilitates PEC reaction kinetics by providing open spaces for mass exchange. This work proposes a feasible strategy for improving photogenerated charge transport and enhancing PEC by combining regulation of the band structure of WO3/TiO2 heterostructures with morphology design.

Regulating oxygen vacancies and coordination environment of manganese dioxide for enhanced high-mass-loading energy storage

This work presents a conceptual strategy that the optimized Mn coordination environment with oxygen vacancy could induce a local built-in electric field and additional active sites, achieving excellent ionic-transport and pseudocapacitive capacity.

Structural and computational studies on a quenchable high-temperature polymorph of magnesium tungstate (MgWO4-II)

Single-crystals of a quenchable high-temperature polymorph of magnesium tungstate (MgWO4-II) have been grown using the flux method. Polycrystalline material of the same compound could be obtained from solid-state reactions performed at 1200 °C. Basic crystallographic data of the previously unknown modification are as follows: space group P1‾, a = 6.5525(6) Å, b = 7.5883(7) Å, c = 7.6976(6) Å, α = 119.064(9)°, β = 95.545(7)°, γ = 107.645(8)°, V = 304.84(5)3 Å and Z = 4. The crystal structure was solved from single-crystal diffraction using direct methods and refined to a residual value of R1 = 2.16%. Both cation species exhibit an octahedral oxygen coordination environment. By sharing common edges and corners, the octahedra form a three-dimensional network, which can be built from infinite rod-like elements running along [010] having a 2 × 2 octahedra wide cross section. MgWO4-II is topologically equivalent to the VO2(HT) structure-type. A detailed analysis of the relationships with other ABO4-compounds is presented. Solid-state characterization has been supplemented by micro-Raman spectroscopy. Interpretation of the spectroscopic data, including the allocation of the bands to certain vibrational species, has been aided by DFT-calculations. The thermal expansion tensor between ambient temperature and about 700 °C has been determined. The calculations indicate quasi-two-dimensional expansion behavior.

Single-atom copper embedded in two-dimensional MXene toward peroxymonosulfate activation to generate singlet oxygen with nearly 100% selectivity for enhanced Fenton-like reactions

Singlet oxygen (1O2)-dominated advanced oxidation processes has drawn widespread attention for selective oxidation of organic pollutants in complex water environments. However, the high efficiency and selectivity of 1O2 generation remains challenging. Herein, we develop single-atom copper anchored on Ti3C2Tx MXene nanosheets (Cu-SA/MXene) by molten salt etching for the generation of 1O2 via peroxymonosulfate (PMS) activation. Particularly, it exhibits a high selectivity of 99.71% toward 1O2 generation by activating PMS, which shows outstanding catalytic activity for multiple pollutants, and strong resistance to inorganic anions. Experimental and theoretical results reveal that the Cu single atoms with three oxygen coordination environments (Cu-O3) are more favorable for PMS absorption and selectively adsorb the terminal oxygen of PMS to promote the generation of SO5.-, resulting in the generation of 1O2. A continuous-flow wastewater system by dispersing catalysts on poly (vinylidene fluoride) membrane exhibit stable catalytic activity for polycarbonate plant wastewater treatment over 8 h.

The high activity of Co-Mn-based solid solution catalysts for lean methane combustion

Developing non-noble metal-based catalysts for methane combustion with high catalytic activity and stability is important in the new context of a changing global energy landscape. In this study, we reported an Co5Mn1-O catalyst achieved 90% conversion of methane to CO2 at 305 °C, which was about 60 ℃ lower than the catalysts reported. The synthetic strategy via lattice distortion that inhibits the deep oxidation of Mn2+ with prepared a highly catalytic reactive Co-Mn-based solid solution material. The results show that introduction of Mn atoms into the Co3O4 spinel structure caused lattice distortion, which modulated the oxygen coordination environment and oxygen vacancies of the catalyst, and the Co5Mn1-O (Optimum) catalyst prepared under optimized conditions obtained excellent methane activation capacity and lattice oxygen mobility. The higher lattice oxygen mobility of the Co5Mn1-O catalyst facilitated the adsorption, activation, and oxidation of methane, especially the rate-determining step of methane oxidation, the conversion of CH4 to CH3-. The conversion rate decreases slightly in the presence of 5 vol% or 10 vol% H2O, but it also still maintained good catalytic activity and stability under high temperatures and long-term reactions. In addition, in situ infrared specra was used to explore the reaction mechanism of methane combustion under real reaction conditions.

Interfacial oxygen coordination environment regulation towards high-performance Li-rich layered oxide cathode

Lithium-rich layered oxides (LLO) have drawn increasing attention as one of the most promising cathodes for next-generation high-energy–density lithium-ion batteries (LIBs). However, they are plagued by low initial Coulombic efficiency (ICE) and terrible cyclic stability due to their intrinsic irreversible oxygen release. Herein, to enhance the comprehensive electrochemical performance of LLO, an effective strategy is brought up to regulate the interfacial oxygen coordination environment via lithium deficiencies, Na+ ion doping, as well as the induced Li+/Ni2+ antisite defects and in-situ epitaxial grown spinel phase. The density functional theory calculations (DFT) confirm that the existence of lithium deficiencies and Na+ ions doping can decrease the diffusion energy barrier of Li+ ions. Meanwhile, the regulated oxygen coordination environment results in an increase in the oxygen vacancy formation energy, which is beneficial for the improvement of the lattice oxygen stability in the deeply charged state. As a result, the modified sample exhibits high initial coulombic efficiency of 91.2% and good capacity retention of 95.3% after 400 cycles at 1C (1C = 250 mA g−1). This work offers a new idea for designing advanced LLO cathodes, which could promote their practical applications in high-energy–density LIBs.

Selectivity of Electrochemical Ion Insertion into Manganese Dioxide Polymorphs

The ion insertion redox chemistry of manganese dioxide has diverse applications in energy storage, catalysis, and chemical separations. Unique properties derive from the assembly of Mn-O octahedra into polymorphic structures that can host protons and nonprotonic cations in interstitial sites. Despite many reports on individual ion-polymorph couples, much less is known about the selectivity of electrochemical ion insertion in MnO2. In this work, we use density functional theory to holistically compare the electrochemistry of AxMnO2 (where A = H+, Li+, Na+, K+, Mg2+, Ca2+, Zn2+, Al3+) in aqueous and nonaqueous electrolytes. We develop an efficient computational scheme demonstrating that Hubbard-U correction has a greater impact on calculating accurate redox energetics than choice of exchange-correlation functional. Using PBE+U, we find that for nonprotonic cations, ion selectivity depends on the oxygen coordination environments inside a polymorph. When H+ is present, however, the driving force to form hydroxyl bonds is usually stronger. In aqueous electrolytes, only three ion-polymorph pairs are thermodynamically stable within water's voltage stability window (Na+ and K+ in α-MnO2, and Li+ in λ-MnO2), with all other ion insertion being metastable. We find Al3+ may insert into the δ, R, and λ polymorphs across the full 2-electron redox of MnO2 at high voltage; however, electrolytes for multivalent ions must be designed to impede the formation of insoluble precipitates and facilitate cation desolvation. We also show that small ions coinsert with water in α-MnO2 to achieve greater coordination by oxygen, while solvation energies and kinetic effects dictate water coinsertion in δ-MnO2. Taken together, these findings explain reports of mixed ion insertion mechanisms in aqueous electrolytes and highlight promising design strategies for safe, high energy density electrochemical energy storage, desalination batteries, and electrocatalysts.

Quantifying the Anomalous Local and Nanostructure Evolutions Induced by Lattice Oxygen Redox in Lithium-Rich Cathodes.

Due to their accessible lattice oxygen redox (l-OR) at high voltages, Li-rich layered transition metal (TM) oxides have shown promising potential as candidate cathodes for high-energy-density Li-ion batteries. However, this l-OR process is also associated with unusual electrochemical issues such as voltage hysteresis and long-term voltage decay. The structure response mechanism to the l-OR behavior also remains unclear, hindering rational structure optimizations that would enable practical Li-rich cathodes. Here, this study reveals a strong coupling between l-OR and structure dynamic evolutions, as well as their effects on the electrochemical properties. Using the technique of neutron total scattering with pair distribution function analysis and small-angle neutron scattering, this study quantifies the local TM migration and formation of nanopores that accompany the l-OR. These experiments demonstrate the causal relationships among l-OR, the local/nanostructure evolutions, and the unusual electrochemistry. The TM migration triggered by the l-OR can change local oxygen coordination environments, which results in voltage hysteresis. Coupled with formed oxygen vacancies, it will accelerate the formation of nanopores, inducing a phase transition, and leading to irreversible capacity and long-cycling voltage fade.

Energy transfer dynamics and photoluminescence properties of sol-gel synthesized dense-packed Ca3−(x+y)TbxSmyMgSi2O8 phosphor

The Ca3−(x+y)TbxSmyMgSi2O8 (x = 0–0.15, y = 0–0.04 mol), dense packed merwinite type crystalline phosphors were studied for color tunability via Tb+3 → Sm+3 energy transfer. All the phosphor samples were prepared by acid catalyzed sol-gel reaction route followed by high temperature annealing. The structural and optical characteristics of the phosphors were ascertained from X-ray diffraction, scanning electron microscopy, Fourier transforms infrared and photoluminescence spectroscopy. The synthesized phosphors were devoid of any detectable impurities or phase separation and belonged to phase pure monoclinic crystal system in the P121/a1 space group. Tb+3 doped samples demonstrated intense green emission from 5D4 → 7FS (S = 6, 5) upon f-d (243 nm) and f-f excitation (378 nm). Sm+3 doped samples did not exhibit charge transfer excitation and however, strong orange-red emission was realized due to the 4G5∕2 → 6HS emission upon 404 nm (f-f) excitation. The onset of concentration quenching occurred at 0.07 for Tb+3 and 0.02 mol for Sm+3 corresponding to the Tb+3-Tb+3 and Sm+3-Sm+3 atomic distance of 11.4 and 17.4 Å respectively. Both Tb+3 and Sm+3 exhibited biexponential decay with τavg of 2.23 (Tb+3) and 2.16 ms (Sm+3), which is explained on the basis of their two different oxygen coordination environment in the lattice. The occurrence of Tb+3 → Sm+3 energy transfer (ET) was established from the analysis of excitation and emission spectra, and luminescence decay characteristics of Tb+3 and Tb+3-Sm+3 doped phosphors. Tb+3 → Sm+3 ET mechanism was further illustrated from the energy level diagram, and emission decay curves. The Tb+3 → Sm+3 ET efficiency and ET probability were observed to be maximum for the Tb0.07Sm0.02 codoped phosphor and determined to be 52.9% and 0.504 × 10−3·s−1, respectively. The Dexter hypothesis explicated the dominant energy transfer mechanism to be Tb-Sm dipole-dipole interaction. The Tb+3 → Sm+3 ET lead to the emission colour tunability with discrete CIE coordinates.

(Digital Presentation) Multiscale Understanding of Local Structure-Dependent Hydrogen Incorporation in TiO2

Hydrogen interaction with metal oxides is an important phenomenon that affects all vital areas of industry such as aerospace, transportation, and commercial applications. The metal oxide provides a naturally forming protective layer against dissolution of the underlying metal. It has been reported that hydrogen is still able to percolate through this protective layer traveling all the way to the metal to form a corrosive brittle metal hydride. There have been many studies looking to understand the interaction of hydrogen at the surface but an in depth look at the surface to bulk transition of hydrogen through a metal oxide for all representative phases of an oxide is minimally represented in the literature. For the focus of our study, we employed a multi-scale approach combined with experimental studies to explore the different phases of titania (TiO2) which includes rutile, anatase, and an amorphous phase. A thermodynamic analysis of the stability for hydrogen was considered using density functional theory (DFT) starting at the low energy surface facets to the bulk. For the amorphous phase, the binding energy was analyzed as a function of the hydrogen content and oxygen coordination environment. Temperature programmed desorption (TPD) experiments provided a direct comparison with theory. Additionally, NMR simulations validified the generated amorphous phase which agreed well with experimental data. The material properties computed using DFT were used in combination with experimental results to parameterize a mesoscale model. Several environmental conditions were analyzed consisting of grain size, temperature, and grain boundary properties. Further work investigated crystalline titanium (Ti) in the bulk and at several grain boundaries to elucidate possible initiation stages of hydride formation.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Cobalt Single Atoms Anchored on Oxygen‐Doped Tubular Carbon Nitride for Efficient Peroxymonosulfate Activation: Simultaneous Coordination Structure and Morphology Modulation

AbstractSimultaneous regulation of the coordination environment of single‐atom catalysts (SACs) and engineering architectures with efficient exposed active sites are efficient strategies for boosting peroxymonosulfate (PMS) activation. We isolated cobalt atoms with dual nitrogen and oxygen coordination (Co−N3O1) on oxygen‐doped tubular carbon nitride (TCN) by pyrolyzing a hydrogen‐bonded cyanuric acid melamine–cobalt acetate precursor. The theoretically constructed Co−N3O1 moiety on TCN exhibited an impressive mass activity of 7.61×105 min−1 mol−1 with high 1O2 selectivity. Theoretical calculations revealed that the cobalt single atoms occupied a dual nitrogen and oxygen coordination environment, and that PMS adsorption was promoted and energy barriers reduced for the key *O intermediate that produced 1O2. The catalysts were attached to a widely used poly(vinylidene fluoride) microfiltration membrane to deliver an antibiotic wastewater treatment system with 97.5 % ciprofloxacin rejection over 10 hours, thereby revealing the suitability of the membrane for industrial applications.

Cobalt Single Atoms Anchored on Oxygen‐Doped Tubular Carbon Nitride for Efficient Peroxymonosulfate Activation: Simultaneous Coordination Structure and Morphology Modulation

Simultaneous regulation of the coordination environment of single-atom catalysts (SACs) and engineering architectures with efficient exposed active sites are efficient strategies for boosting peroxymonosulfate (PMS) activation. We isolated cobalt atoms with dual nitrogen and oxygen coordination (Co-N3 O1 ) on oxygen-doped tubular carbon nitride (TCN) by pyrolyzing a hydrogen-bonded cyanuric acid melamine-cobalt acetate precursor. The theoretically constructed Co-N3 O1 moiety on TCN exhibited an impressive mass activity of 7.61×105 min-1 mol-1 with high 1 O2 selectivity. Theoretical calculations revealed that the cobalt single atoms occupied a dual nitrogen and oxygen coordination environment, and that PMS adsorption was promoted and energy barriers reduced for the key *O intermediate that produced 1 O2 . The catalysts were attached to a widely used poly(vinylidene fluoride) microfiltration membrane to deliver an antibiotic wastewater treatment system with 97.5 % ciprofloxacin rejection over 10 hours, thereby revealing the suitability of the membrane for industrial applications.

Tunable Magnetic Anisotropy in Patterned SrRuO3 Quantum Structures: Competition between Lattice Anisotropy and Oxygen Octahedral Rotation

AbstractArtificial perovskite oxide nanostructures possess intriguing magnetic properties due to their tailorable electron–electron interactions, which are extremely sensitive to the oxygen coordination environment. To date, perovskite oxide nanodots with sizes below 50 nm have rarely been reported. Furthermore, the oxygen octahedral distortion and its relation to magnetic properties in perovskite oxide nanodots remain unexplored thus far. Here, the magnetic anisotropy in patterned SrRuO3 (SRO) nanodots as small as 30 nm are studied. The constituent elements, in particular oxygen ions, are directly visualized via performing atomic resolution electron microscopy and spectroscopy. It is observed that the magnetic anisotropy and RuO6 octahedra distortion in SRO nanodots are both nanodot size‐dependent but remain unchanged in the first 3‐unit‐cell interfacial SRO monolayers regardless of the dots’ size. Combined with first principle calculations, a unique structural mechanism behind the nanodots’ size‐dependent magnetic anisotropy in SRO nanodots is unraveled, suggesting that the competition between lattice anisotropy and oxygen octahedral rotation mediates anisotropic exchange interactions in SRO nanodots. These findings demonstrate a new avenue toward tuning magnetic properties of correlated perovskite oxides and imply that patterned nanodots could be a promising playground for engineering emergent functional behaviors.

Interfacial Oxygen Coordination Environment Regulation Towards High-Performance Li-Rich Layered Oxide Cathode

Interfacial Oxygen Coordination Environment Regulation Towards High-Performance Li-Rich Layered Oxide Cathode

Template Engineering of Metal-to-Insulator Transitions in Epitaxial Bilayer Nickelate Thin Films.

Understanding metal-to-insulator phase transitions in solids has been a keystone not only for discovering novel physical phenomena in condensed matter physics but also for achieving scientific breakthroughs in materials science. In this work, we demonstrate that the transport properties (i.e., resistivity and transition temperature) in the metal-to-insulator transitions of perovskite nickelates are tunable via the epitaxial heterojunctions of LaNiO3 and NdNiO3 thin films. A mismatch in the oxygen coordination environment and interfacial octahedral coupling at the oxide heterointerface allows us to realize an exotic phase that is unattainable in the parent compound. With oxygen vacancy formation for strain accommodation, the topmost LaNiO3 layer in LaNiO3/NdNiO3 bilayer thin films is structurally engineered and it electrically undergoes a metal-to-insulator transition that does not appear in metallic LaNiO3. Modification of the NdNiO3 template layer thickness provides an additional knob for tailoring the tilting angles of corner-connected NiO6 octahedra and the linked transport characteristics further. Our approaches can be harnessed to tune physical properties in complex oxides and to realize exotic physical phenomena through oxide thin-film heterostructuring.

A Review of Transition Metal Oxygen-Evolving Catalysts Decorated by Cerium-Based Materials: Current Status and Future Prospects

Non-noble metal catalysts are suitable for the oxygen evolution reaction (OER) owing to their original oxidation states and oxygen coordination environments, which can regulate the adsorption of OH...

Direct visualization of lattice oxygen evolution and related electronic properties of Li1.2Ni0.2Mn0.6O2 cathode materials

Li-rich layered manganese oxide-based cathodes have drawn much attention for the next-generation lithium-ion batteries due to the large discharge capacities and low cost. However, the lattice oxygen release and subsequent surface densification during cycling inevitably lead to their instability, which incurs capacity failure at high voltage. Herein, we have systematically explored the oxygen oxidation mechanisms in Li1.2Ni0.2Mn0.6O2 (003) surface by considering five possible different local oxygen coordination environments and investigate their oxidation process during the different degrees of delithiation. Based on the density functional theory calculations, we identify the triggering factors for the oxygen dimerization at the cathode surface. Our study reveals that the oxygen atoms linearly coordinated with two Li-ions (LiOLi geometries) are unstable and proceed oxidation via O2− to O− to O22− to O2− and finally O2 evolution occurred on the cathode surface at a higher degree of delithiation. Based on our theoretical results, we expect that the oxygen molecule release can be suppressed by modifying the surface's oxygen coordination environment. Such a comprehensive understanding is essential for developing the novel complex Li-rich layered manganese oxide cathodes.

Field-induced single-ion magnet based on a quasi-octahedral Co(II) complex with mixed sulfur-oxygen coordination environment.

Synthesis and characterization of structure and magnetic properties of the quasi-octahedral complex (pipH2)[Co(TDA)2] 2H2O (I), (pipH22+ = piperazine dication, TDA2- = thiodiacetic anion) are described. X-ray diffraction studies reveal the first coordination sphere of the Co(II) ion, consisting of two chelating tridentate TDA ligands with a mixed sulfur-oxygen strongly elongated octahedral coordination environment. SQUID magnetometry, frequency-domain Fourier-transform (FD-FT) THz-EPR spectroscopy, and high-level ab initio SA-CASSCF/NEVPT2 quantum chemical calculations reveal a strong "easy-plane" type magnetic anisotropy (D ≈ +54 cm-1) of complex I. The complex shows field-induced slow relaxation of magnetization at an applied DC field of 1000 Oe.

Growth of Titanium Oxide Nanostructures on γ-Аl2О3 by Atomic Layer Deposition

This paper presents results on control over the surface composition, surface structure, and pore texture of core/shell materials, as exemplified by the growth of conformal titanium oxide nanocoatings on γ‑Аl2О3 by atomic layer deposition via sequential and alternating exposure of the alumina to TiCl4 and H2O vapor. The alumina surface and growing titanium oxide layer are shown to influence the characteristics of the forming two-phase material. Increasing the amount of titanium via an increase in the number of deposition cycles leads to a systematic decrease in specific surface area, pore volume, and pore size, which points to conformal pore filling in the starting matrix by a titanium oxide layer. The composition and structure of the titanium oxide coating are influenced by its thickness and the nature of the starting matrix. The coordination state of the titanium oxide in monolayer structures is characteristic of the titanium oxide polyhedra in aluminum titanate. As the distance from the top monolayer to the surface of the matrix (coating thickness) increases, an X-ray amorphous layer is formed in which the oxygen coordination environment of the titanium is similar to that in an anatase-like phase of titanium dioxide.

High Pressure Effect on Structural and Electrochemical Properties of Anionic Redox-Based Lithium Transition Metal Oxides

Summary The richness of anionic redox chemistry in the solid state offers new opportunities and a possible paradigm shift in energy storage. The excess capacity that goes beyond conventional theoretical values is attributed to the anionic redox in Li-rich transition metal oxide cathodes for Li-ion batteries. Their electrochemical behavior is thermodynamically determined by structural evolution. To better understand the electrochemical dependence on structural factors, we have induced structural modifications in pristine and electrochemically activated Li1.144Ni0.136Co0.136Mn0.544O2 through high-pressure treatment. A unique cyclical change of structural reordering is observed in the anionic redox-activated material during operando pressure sweep, characterized by a periodic evolution of superlattice peak intensity in synchrotron X-ray diffraction patterns. During the structural reordering period, the bulk compressibility of the material decreases, even becoming negative. These insights elucidate the structural flexibility and metastability of anionic redox-based materials, which can undergo large compressions and structural modifications while delivering good electrochemical properties.