Showing papers on "Valence (chemistry) published in 2020"

Lattice oxygen activation enabled by high-valence metal sites for enhanced water oxidation

Abstract: Anodic oxygen evolution reaction (OER) is recognized as kinetic bottleneck in water electrolysis. Transition metal sites with high valence states can accelerate the reaction kinetics to offer highly intrinsic activity, but suffer from thermodynamic formation barrier. Here, we show subtle engineering of highly oxidized Ni4+ species in surface reconstructed (oxy)hydroxides on multicomponent FeCoCrNi alloy film through interatomically electronic interplay. Our spectroscopic investigations with theoretical studies uncover that Fe component enables the formation of Ni4+ species, which is energetically favored by the multistep evolution of Ni2+→Ni3+→Ni4+. The dynamically constructed Ni4+ species drives holes into oxygen ligands to facilitate intramolecular oxygen coupling, triggering lattice oxygen activation to form Fe-Ni dual-sites as ultimate catalytic center with highly intrinsic activity. As a result, the surface reconstructed FeCoCrNi OER catalyst delivers outstanding mass activity and turnover frequency of 3601 A gmetal−1 and 0.483 s−1 at an overpotential of 300 mV in alkaline electrolyte, respectively. Electrocatalytic water oxidation is facilitated by high valence states, but these are challenging to achieve at low applied potentials. Here, authors report a multicomponent FeCoCrNi alloy with dynamically formed Ni4+ species to offer high catalytic activity via lattice oxygen activation mechanism.

Macroscopic Polarization Change via Electron Transfer in a Valence Tautomeric Cobalt Complex

Abstract: Polarization change induced by directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, we investigate electronic pyroelectricity in the crystal of a mononuclear complex, [Co(phendiox)(rac-cth)](ClO4)·0.5EtOH (1·0.5EtOH, H2phendiox = 9, 10-dihydroxyphenanthrene, rac-cth = racemic 5, 5, 7, 12, 12, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane), which undergoes a two-step valence tautomerism (VT). Correspondingly, pyroelectric current exhibits double peaks in the same temperature domain with the polarization change consistent with the change in dipole moments during the VT process. Time-resolved Infrared (IR) spectroscopy shows that the photo-induced metastable state can be generated within 150 ps at 190 K. Such state can be trapped for tens of minutes at 7 K, showing that photo-induced polarization change can be realized in this system. These results directly demonstrate that a change in the molecular dipole moments induced by intramolecular electron transfer can introduce a macroscopic polarization change in VT compounds. Polarization change from directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, the authors provide a proof-of-concept of electronic pyroelectricity induced by intramolecular electron transfer in the single crystal of a valence tautomeric compound.

Voltage- and time-dependent valence state transition in cobalt oxide catalysts during the oxygen evolution reaction

Abstract: The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li2Co2O4 under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co3+ to Co4+ accompanied by spontaneous delithiation, whereas the edge-shared Co-O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co4+ site, rather than the Co3+ site or the oxygen vacancy site, is mainly responsible for the high OER activity.

Observation of the fastest chemical processes in the radiolysis of water.

Abstract: Elementary processes associated with ionization of liquid water provide a framework for understanding radiation-matter interactions in chemistry and biology. Although numerous studies have been conducted on the dynamics of the hydrated electron, its partner arising from ionization of liquid water, H2O+, remains elusive. We used tunable femtosecond soft x-ray pulses from an x-ray free electron laser to reveal the dynamics of the valence hole created by strong-field ionization and to track the primary proton transfer reaction giving rise to the formation of OH. The isolated resonance associated with the valence hole (H2O+/OH) enabled straightforward detection. Molecular dynamics simulations revealed that the x-ray spectra are sensitive to structural dynamics at the ionization site. We found signatures of hydrated-electron dynamics in the x-ray spectrum.

Determining the adsorption energies of small molecules with the intrinsic properties of adsorbates and substrates

Abstract: Adsorption is essential for many processes on surfaces; therefore, an accurate prediction of adsorption properties is demanded from both fundamental and technological points of view. Particularly, identifying the intrinsic determinants of adsorption energy has been a long-term goal in surface science. Herein, we propose a predictive model for quantitative determination of the adsorption energies of small molecules on metallic materials and oxides, by using a linear combination of the valence and electronegativity of surface atoms and the coordination of active sites, with the corresponding prefactors determined by the valence of adsorbates. This model quantifies the effect of the intrinsic properties of adsorbates and substrates on adsorbate–substrate bonding, derives naturally the well-known adsorption-energy scaling relations, and accounts for the efficiency and limitation of engineering the adsorption energy and reaction energy. All involved parameters are predictable and thus allow the rapid rational design of materials with optimal adsorption properties. Adsorption of molecules at surfaces is at the basis of many processes in chemistry. Here the authors propose an approach to determine the adsorption energies of different chemical species on a variety of solid surfaces based on fundamental and accessible properties of adsorbate and surface atoms.

Laser picoscopy of valence electrons in solids

Abstract: Valence electrons contribute a small fraction of the total electron density of materials, but they determine their essential chemical, electronic and optical properties. Strong laser fields can probe electrons in valence orbitals1–3 and their dynamics4–6 in the gas phase. Previous laser studies of solids have associated high-harmonic emission7–12 with the spatial arrangement of atoms in the crystal lattice13,14 and have used terahertz fields to probe interatomic potential forces15. Yet the direct, picometre-scale imaging of valence electrons in solids has remained challenging. Here we show that intense optical fields interacting with crystalline solids could enable the imaging of valence electrons at the picometre scale. An intense laser field with a strength that is comparable to the fields keeping the valence electrons bound in crystals can induce quasi-free electron motion. The harmonics of the laser field emerging from the nonlinear scattering of the valence electrons by the crystal potential contain the critical information that enables picometre-scale, real-space mapping of the valence electron structure. We used high harmonics to reconstruct images of the valence potential and electron density in crystalline magnesium fluoride and calcium fluoride with a spatial resolution of about 26 picometres. Picometre-scale imaging of valence electrons could enable direct probing of the chemical, electronic, optical and topological properties of materials. Laser-generated high-harmonic emission is used to image the valence potential and electron density in magnesium fluoride and calcium fluoride at the picometre scale, enabling direct probing of material properties.

Facile Top-Down Strategy for Direct Metal Atomization and Coordination Achieving a High Turnover Number in CO2 Photoreduction.

Abstract: Developing unique single atoms as active sites is vitally important to boosting the efficiency of photocatalytic CO2 reduction, but directly atomizing metal particles and simultaneously adjusting the configuration of individual atoms remain challenging. Herein, we demonstrate a facile strategy at a relatively low temperature (500 °C) to access the in situ metal atomization and coordination adjustment via the thermo-driven gaseous acid. Using this strategy, the pyrolytic gaseous acid (HCl) from NH4Cl could downsize the large metal particles into corresponding ions, which subsequently anchored onto the surface defects of a nitrogen-rich carbon (NC) matrix. Additionally, the low-temperature treatment-induced C═O motifs within the interlayer of NC could bond with the discrete Fe sites in a perpendicular direction and finally create stabilized Fe-N4O species with high valence status (Fe3+) on the shallow surface of the NC matrix. It was found that the Fe-N4O species can achieve a highly efficient CO2 conversion when accepting energetic electrons from both homogeneous and heterogeneous photocatalysts. The optimized sample achieves a maximum turnover number (TON) of 1494 within 1 h in CO generation with a high selectivity of 86.7% as well as excellent stability. Experimental and theoretical results unravel that high valence Fe sites in Fe-N4O species can promote the adsorption of CO2 and lower the formation barrier of key intermediate COOH* compared with the traditional Fe-N4 moiety with lower chemical valence. Our discovery provides new points of view in the construction of more efficient single-atom cocatalysts by considering the optimization of the atomic configuration for high-performance CO2 photoreduction.

Interaction Mediator Assisted Synthesis of Mesoporous Molybdenum Carbide: Mo-Valence State Adjustment for Optimizing Hydrogen Evolution.

Abstract: To overcome inherent limitations of molybdenum carbide (MoxC) for hydrogen evolution reaction (HER), i.e., low density of active site and nonideal hydrogen binding strength, we report the synthesis of valence-controlled mesoporous MoxC as a highly efficient HER electrocatalyst. The synthesis procedure uses an interaction mediator (IM), which significantly increases the density of active site by mediating interaction between PEO-b-PS template and Mo source. The valence state of Mo is tuned by systematic control of the environment around Mo by controlled heat treatment under air before thermal treatment at 1100 °C. Theoretical calculations reveal that the hydrogen binding is strongly influenced by Mo valence. Consequently, MoxC achieves a significant increase in HER activity (exceeding that of Pt/C at high current density ∼35 mA/cm2 in alkaline solution). In addition, a volcano-type correlation between HER activity and Mo valence is identified with various experimental indicators. The present strategies can be applied to various carbide and Mo-based catalysts, and the established Mo valence and HER relations can guide development of highly active HER electrocatalysts.

Evidence for a pressure-induced antiferromagnetic quantum critical point in intermediate-valence UTe2.

Abstract: UTe2 is a recently discovered unconventional superconductor that has attracted much interest because of its potentially spin-triplet topological superconductivity. Our ac calorimetry, electrical resistivity, and x-ray absorption study of UTe2 under applied pressure reveals key insights on the superconducting and magnetic states surrounding pressure-induced quantum criticality at Pc1 = 1.3 GPa. First, our specific heat data at low pressures, combined with a phenomenological model, show that pressure alters the balance between two closely competing superconducting orders. Second, near 1.5 GPa, we detect two bulk transitions that trigger changes in the resistivity, which are consistent with antiferromagnetic order, rather than ferromagnetism. Third, the emergence of magnetism is accompanied by an increase in valence toward a U4+ (5f2) state, which indicates that UTe2 exhibits intermediate valence at ambient pressure. Our results suggest that antiferromagnetic fluctuations may play a more substantial role on the superconducting state of UTe2 than previously thought.

Enhancing Luminescence and Controlling the Mn Valence State of Gd3Ga5-x-δAlx-y+δO12:yMn Phosphors by the Design of the Garnet Structure.

Abstract: Gd3Ga5–x–δAlx–y+δO12:yMn solid solutions with improving luminescence properties were prepared via cation substitution and a controllable Mn valence state. The abnormal autoreduction from Mn4+ to Mn...

Analysis of the Fe 2p XPS for hematite α Fe2O3: Consequences of covalent bonding and orbital splittings on multiplet splittings.

Abstract: The origins of the complex Fe 2p X-Ray Photoelectron Spectra (XPS) of hematite (α-Fe2O3) are analyzed and related to the character of the bonding in this compound. This analysis provides a new and novel view of the reasons for XPS binding energies (BEs) and BE shifts, which deepens the current understanding and interpretation of the physical and chemical significance of the XPS. In particular, many-body effects are considered for the initial and the final, 2p-hole configuration wavefunctions. It is shown that a one-body or one-configuration analysis is not sufficient and that the many-body, many-determinantal, and many-configurational character of the wavefunctions must be taken into account to describe and understand why the XPS intensity is spread over an extremely large number of final 2p-hole multiplets. The focus is on the consequences of angular momentum coupling of the core and valence open shell electrons, the ligand field splittings of the valence shell orbitals, and the degree of covalent mixing of the Fe(3d) electrons with the O(2p) electrons. Novel theoretical methods are used to estimate the importance of these various terms. An important consequence of covalency is a reduction in the energy separation of the multiplets. Although shake satellites are not considered explicitly, the total losses of intensity from the angular momentum multiplets to shake satellites is determined and related to the covalent character of the Fe-O interaction. The losses are found to be the same for Fe 2p1/2 and 2p3/2 ionization.

Engineering NiFe layered double hydroxide by valence control and intermediate stabilization toward the oxygen evolution reaction

Abstract: NiFe layered double hydroxide (LDH) is one of the most promising candidates for oxygen evolution reaction (OER) electrocatalysts. However, it is still challenging to design NiFe LDH with both high activity and good stability. Herein, NiFe LDH-Ni(III)Li is synthesized by a facile approach through valence control of Ni ions and selective doping of Li. Ni(III) ion percentage is significantly increased from less than 60% to 72.5% in the LDH thanks to the combined benefits of electrochemical oxidation, Mo-leaching and Li-doping. The catalytic activity of NiFe LDH-Ni(III)Li is greatly promoted because the eg electron occupancy of Ni3+ approaches one, producing optimal bond strength with O. Moreover, density functional theory calculations reveal that Li-doping reduces the adsorption energy difference between *OOH and *OH from 3.16 eV to 2.79 eV by the stabilization of *OOH, decreasing the energy barrier of the rate-determining step by 0.29 eV. At the same time, higher covalency of Ni–O bonds results from the enlarged overlap between Ni 3d and O 2p orbitals, further enhancing the OER kinetics. Consequently, NiFe LDH-Ni(III)Li exhibits excellent OER performance (overpotential of 248 mV at 10 mA cm−2 on a flat current collector). This work provides a promising technique for engineering the electronic structure of LDH toward outstanding OER properties.

Cobalt-Salen-Based Porous Ionic Polymer : The Role of Valence on Cooperative Conversion of CO2 to Cyclic Carbonate

Abstract: Cobalt–salen-based porous ionic polymers, which are composed of cobalt and halogen anions decorated on the framework, effectively catalyze the CO2 cycloaddition reaction of epoxides to cyclic carbo...

Coherent spin control of s-, p-, d- and f-electrons in a silicon quantum dot

Abstract: Once the periodic properties of elements were unveiled, chemical behaviour could be understood in terms of the valence of atoms. Ideally, this rationale would extend to quantum dots, and quantum computation could be performed by merely controlling the outer-shell electrons of dot-based qubits. Imperfections in semiconductor materials disrupt this analogy, so real devices seldom display a systematic many-electron arrangement. We demonstrate here an electrostatically confined quantum dot that reveals a well defined shell structure. We observe four shells (31 electrons) with multiplicities given by spin and valley degrees of freedom. Various fillings containing a single valence electron—namely 1, 5, 13 and 25 electrons—are found to be potential qubits. An integrated micromagnet allows us to perform electrically-driven spin resonance (EDSR), leading to faster Rabi rotations and higher fidelity single qubit gates at higher shell states. We investigate the impact of orbital excitations on single qubits as a function of the dot deformation and exploit it for faster qubit control. Quantum dots are often referred to as “artificial atoms” as they create zero-dimensional traps for electrons, with characteristic atom-like spectra. Leon et al. demonstrate that higher shell and orbital states of a multi-electron silicon quantum dot with better control fidelity than single electron dots.

The chemical and physical properties of tetravalent lanthanides: Pr, Nd, Tb, and Dy

Abstract: The fundamental redox chemistry and valence electronic structure of the lanthanides in molecular complexes and extended solids continues to be a fertile area of research. The contemporary understanding of the accessible oxidation states of the lanthanide elements and the variability in their electronic structure is the result of several paradigm shifts. While the lanthanide elements have already found widespread use in technical and consumer applications, the continued reevaluation of basic redox properties is a central chemical concern to establish a more complete description of periodic properties. This fundamental understanding of valence electronic structure as it is derived from oxidation state and coordination environment is essential for the continued development of lanthanides in quantum information science and quantum materials research. This review presents the chemical and physical properties of tetravalent lanthanide ions in extended solids and molecules with a focus on the elements apart from cerium: praseodymium, neodymium, terbium, and dysprosium.

Ultralow Ru-Induced Bimetal Electrocatalysts with a Ru-Enriched and Mixed-Valence Surface Anchored on a Hollow Carbon Matrix for Oxygen Reduction and Water Splitting.

Abstract: Rational design of trifunctional electrocatalysts with robust efficiency used for oxygen reduction, oxygen evolution, and hydrogen evolution reactions (ORR, OER, and HER) is of significance to rene...

Crystal Chemistry, Band-Gap Red Shift, and Electrocatalytic Activity of Iron-Doped Gallium Oxide Ceramics.

Abstract: This work for the first time unfurls the fundamental mechanisms and sets the stage for an approach to derive electrocatalytic activity, which is otherwise not possible, in a traditionally known wide band-gap oxide material. Specifically, we report on the tunable optical properties, in terms of wide spectral selectivity and red-shifted band gap, and electrocatalytic behavior of iron (Fe)-doped gallium oxide (β-Ga2O3) model system. X-ray diffraction (XRD) studies of sintered Ga2-x Fe x O3 (GFO) (0.0 ≤ x ≤ 0.3) compounds provide evidence for the Fe3+ substitution at Ga3+ site without any secondary phase formation. Rietveld refinement of XRD patterns reveals that the GFO compounds crystallize in monoclinic crystal symmetry with a C2/m space group. The electronic structure of the GFO compounds probed using X-ray photoelectron spectroscopy data reveals that at lower concentrations, Fe exhibits mixed chemical valence states (Fe3+, Fe2+), whereas single chemical valence state (Fe3+) is evident for higher Fe content (x = 0.20-0.30). The optical absorption spectra reveal a significant red shift in the optical band gap with Fe doping. The origin of the significant red shift even at low concentrations of Fe (x = 0.05) is attributed to the strong sp-d exchange interaction originated from the 3d5 electrons of Fe3+. The optical absorption edge observed at ≈450 nm with lower intensity is the characteristic of Fe-doped compounds associated with Fe3+-Fe3+ double-excitation process. Coupled with an optical band-gap red shift, electrocatalytic studies of GFO compounds reveal that, interestingly, Fe-doped Ga2O3 compound exhibits electrocatalytic activity in contrast to intrinsic Ga2O3. Fe-doped samples (GFO) demonstrated appreciable electrocatalytic activity toward the generation of H2 through electrocatalytic water splitting. An onset potential and Tafel slope of GFO compounds include ∼900 mV, ∼210 mV dec-1 (x = 0.15) and ∼1036 mV, ∼290 mV dec-1 (x = 0.30), respectively. The electrocatalytic activity of Fe-doped Ga-oxide compounds is attributed to the cumulative effect of different mechanisms such as doping resulting in new catalytic centers, enhanced conductivity, and electron mobility. Hence, in this report, for the first time, we explored a new pathway; the electrocatalytic behavior of Fe-doped Ga2O3 resulted due to Fe chemical states and red shift in the optical band gap. The implications derived from this work may be applicable to a large class of compounds, and further options may be available to design functional materials for electrocatalytic energy production.

Evidence for a pressure-induced antiferromagnetic quantum critical point in intermediate valence UTe2

Abstract: UTe$_2$ is a recently discovered unconventional superconductor that has attracted much interest due to its many intriguing properties - a large residual density-of-states in the superconducting state, re-entrant superconductivity in high magnetic fields, and potentially spin-triplet topological superconductivity. Our ac calorimetry, electrical resistivity, and x-ray absorption study of UTe$_2$ under applied pressure reveals key new insights on the superconducting and magnetic states surrounding pressure-induced quantum criticality at P$_{c1}$ = 1.3 GPa. First, our specific heat data at low pressures, combined with a phenomenological model, show that pressure alters the balance between two closely competing superconducting orders. Second, near 1.5 GPa we detect two bulk transitions that trigger changes in the resistivity which are consistent with antiferromagnetic order, rather than ferromagnetism. The presence of both bulk magnetism and superconductivity at pressures above P$_{c2}$ = 1.4 GPa results in a significant temperature difference between resistively and thermodynamically determined transitions into the superconducting state, which indicates a suppression of the superconducting volume fraction by magnetic order. Third, the emergence of magnetism is accompanied by an increase in valence towards a U$^{4+}$ (5f2) state, which indicates that UTe$_2$ exhibits intermediate valence at ambient pressure. Our results suggest that antiferromagnetic fluctuations may play a more significant role on the superconducting state of UTe$_2$ than previously thought.