Local Valence Research Articles

Amorphous Ru-based metallene with monometallic atomic interfaces for electrocatalytic hydrogen evolution in anion exchange membrane electrolyzer

Amorphous metallene is a prospective catalyst due to disordered atomic arrangement and abundant defects/edges. However, challenges persist in preparing amorphous metallene with atomic interface, primarily due to thermodynamic impediment of unconventional phase and difficulty in precise controlling of atomic interface. Here, we synthesized a new class of amorphous RuGa metallene (A-RuGa metallene) with abundant atomic interface by introducing trace atomic-dispersed Ga. The atomic interface between Ga-coordinated Ru and Ru0 achieves monometallic synergism during alkaline hydrogen evolution reaction. The turnover frequency value of A-RuGa0.06 metallene reaches 67 s−1 at −0.1 V vs. RHE. Meanwhile, the mass activity of A-RuGa0.06 metallene in anion exchange membrane electrolyzer reaches 5.2 A mgRu−1 at 2.0 V, firstly surpassing commercial Pt/C (1.0 A mgPt−1). The trace atomic dispersed Ga could change the local valence state of monometallic Ru, thus promoting water adsorption and dissociation, optimizing H* adsorption and accelerating H* transport at the atomic level.

Local valence analysis of 316L austenitic stainless steel produced by laser powder bed fusion

Ultra-rapid cooling of laser powder bed fusion (LPBF) generates a cellular solidification microstructure with submicron-sized periodicity accompanied by non-negligible segregation. In 316L austenitic stainless steel, an important corrosion-resistant alloy, the effect of segregation, particularly variations in Cr concentration, on the corrosion resistance of the LPBF product is unknown. Local valence analysis of the LPBF-produced 316L by electron energy-loss spectroscopy revealed no obvious changes in the energy-loss near-edge structures of Cr and Fe measured within the solidification cellular microstructure, at the cell boundary, or at the melt-pool boundary. This result indicates that solidification segregation in the LPBF-produced 316L is unlikely to affect the corrosion resistance of the material.

An inorganic-blended p-type semiconductor with robust electrical and mechanical properties

Inorganic semiconductors typically have limited p-type behavior due to the scarcity of holes and the localized valence band maximum, hindering the progress of complementary devices and circuits. In this work, we propose an inorganic blending strategy to activate the hole-transporting character in an inorganic semiconductor compound, namely tellurium-selenium-oxygen (TeSeO). By rationally combining intrinsic p-type semimetal, semiconductor, and wide-bandgap semiconductor into a single compound, the TeSeO system displays tunable bandgaps ranging from 0.7 to 2.2 eV. Wafer-scale ultrathin TeSeO films, which can be deposited at room temperature, display high hole field-effect mobility of 48.5 cm2/(Vs) and robust hole transport properties, facilitated by Te-Te (Se) portions and O-Te-O portions, respectively. The nanosphere lithography process is employed to create nanopatterned honeycomb TeSeO broadband photodetectors, demonstrating a high responsibility of 603 A/W, an ultrafast response of 5 μs, and superior mechanical flexibility. The p-type TeSeO system is highly adaptable, scalable, and reliable, which can address emerging technological needs that current semiconductor solutions may not fulfill.

Quasi-degenerate extension of local N-electron valence state perturbation theory with pair-natural orbital method based on localized virtual molecular orbitals.

Chemical phenomena involving near-degenerate electronic states, such as conical intersections or avoided crossing, can be properly described using quasi-degenerate perturbation theory. This study proposed a highly scalable quasi-degenerate second-order N-electron valence state perturbation theory (QD-NEVPT2) using the local pair-natural orbital (PNO) method. Our recent study showed an efficient implementation of the PNO-based state-specific NEVPT2 method using orthonormal localized virtual molecular orbitals (LVMOs) as an intermediate local basis. This study derived the state-coupling (or off-diagonal) terms to implement QD-NEVPT2 in an alternative manner to enhance efficiency based on the internally contracted basis and PNO overlap matrices between different references. To facilitate further acceleration, a local resolution-of-the-identity (RI) three-index integral generation algorithm was developed using LMOs and LVMOs. Although the NEVPT2 theory is considered to be less susceptible to the intruder-state problem (ISP), this study revealed that it can easily suffer from ISP when calculating high-lying excited states. We ameliorated this instability using the imaginary level shift technique. The PNO-QD-NEVPT2 calculations were performed on small organic molecules for the 30 lowest-lying states, as well as photoisomerization involving the conical intersection of 1,1-dimethyldibenzo[b,f] silepin with a cis-stilbene skeleton. These calculations revealed that the PNO-QD-NEVPT2 method yielded negligible errors compared to the canonical QD-NEVPT2 results. Furthermore, we tested its applicability to a large photoisomerization system using the green fluorescent protein model and the ten-state calculation of the large transition metal complex, showcasing that off-diagonal elements can be evaluated at a relatively low cost.

The effects of V doping on the intrinsic properties of SmFe10Co2 alloys: A theoretical investigation

The present study focuses on the intrinsic properties of the SmFe10Co2-xVx (x = 0–2) alloys, which includes the SmFe10Co2 alloy, one of the most promising permanent magnets with the ThMn12 type of structure due to its large saturation magnetization (µ0Ms = 1.78 T), high Curie temperature (Tc = 859 K), and anisotropy field (µ0Ha = 12 T) experimentally obtained. Unfortunately, its low coercivity (<0.4 T) hinders its use in permanent magnet applications. The effect of V-doping on magnetization, magnetocrystalline anisotropy energy, and Curie temperature is investigated by electronic band structure calculations. The spin-polarized fully relativistic Korringa-Kohn-Rostoker (SPR-KKR) band structure method, which employs the coherent potential approximation (CPA) to deal with substitutional disorder, has been used. The Hubbard-U correction to local spin density approximation (LSDA + U) was used to account for the large correlation effects due to the 4f electronic states of Sm. The computed magnetic moments and magnetocrystalline anisotropy energies were compared with existing experimental data to validate the theoretical approach’s reliability. The exchange-coupling parameters from the Heisenberg model were used for obtaining the mean-field estimated Curie temperature. The magnetic anisotropy energy was separated into contributions from transition metals and Sm, and its relationships with the local environment, interatomic distances, and valence electron delocalization were analyzed. The suitability of the hypothetical SmFe10CoV alloy for permanent magnet manufacture was assessed using the calculated anisotropy field, magnetic hardness, and intrinsic magnetic properties.

Oxygen vacancy induction strategy to regulate the evolution of valence- and free-electrons for boosting visible photodegradation of tetracycline hydrochloride

The generation, migration and reaction paths of electrons are the key steps for photodegradation of pollutants. However, efficient operation of the above pathways is still challenging. Herein, by strong coordination and slow pyrolysis, we constructed a narrow band Zn-Mn bimetallic photoactive core-shell material (Mn@Zn-N-C, Eg = 3.38 eV) with abundant oxygen vacancies for enhancing the above electronic paths. The photodegradation experiments of tetracycline hydrochloride (TCH) showed that the formation and transfer of vacancy-induced free electrons in the synthesized Mn@Zn-N-C was the key to improve the photocatalytic activity. The DFT calculation results revealed that the photogenerated electrons can transfer along the Mn-O-Zn bridge in Mn@Zn-N-C, and promote the formation of MnIV, which directly capture the free electrons and reset itself to MnII site. In this case, the introduction of Mn would enhance the separation of h+ and e-. The adjacent vacancies and defects then also trapped the above free electrons and hinder the recombination of photogenerated carriers. Simultaneously, the localized valence electron transfer between the above redox pairs (Mn4+/Mn2+ and Zn2+/Zn0) also promoted the long-term stability of the photocatalytic process. In summary, using vacancy induction strategy to regulate the evolution of valence- and free-electrons is a promising method to improve the production-transfer-utilization efficiency of photogenerated carriers.

Temperature-dependent work function, thermionic emission and bulk modulus of TiC: a study on the identification of free valence electrons and localized valence electrons and their roles played in carbides.

By analyzing the projected states of valence electrons (fatband structures), the localized valence electrons and the free valence electrons of TiC were identified, respectively. After defining the volumes and the magnitudes of localized valence electrons and free valence electrons, the influences of the temperature, including the thermal expansion and the atomic thermal vibration, on the localized valence electron density and the free valence electron density were investigated, respectively. Based on the metallic plasma model (MPM), the temperature-dependent work functions and the thermionic emission current densities of TiC were calculated in terms of temperature-dependent free valence electron densities. The results were in good agreement with experimental results. Furthermore, as it was observed, the linear dependence of the bulk modulus on the localized valence electron density demonstrated that the bulk modulus of TiC was determined by the localized valence electron density. The different roles played by the free valence electrons and the localized valence electrons in the work function and the bulk modulus of TiC could be attributed to their different contributions to the kinetic energy density of valence electrons. The influences of the temperature on the work function, thermionic emission and bulk modulus of TiC indicated that the transition metal carbides with lower free valence electron density, higher localized valence electron density and heavier atomic mass were desired to achieve lower work function, higher current density and higher stability.

Unravelling the interplay of local structure and valence transitions in Ce-doped CaYAlO4 luminescent materials.

Cerium has been widely used as a dopant in luminescent materials due to its unique electronic configurations. It is generally anticipated that the luminescence properties of rare-earth-doped materials are closely related to the local environment of activators, especially for Ce3+ . In addition, it is convenient to modulate its emission wavelength by adjusting the composition and structure. In this study, we systematically analyzed the microstructure of the Ce-doped CaYAlO4 system at atomic resolution. The quantitive results indicated that the structure distortion greatly influenced the valence state of the Ce dopant, which is critical to its luminescence efficiency. In addition, valence variations also exist from surface to inner structure due to the big distortion area around the surface. Our results unravel the interplay of local structure and valence transitions in Ce-doped aluminate phosphors, which has the potential to be applied in other luminescent materials.

Crystal structures of biocompatible Mg-, Zn-, and Co-whitlockites synthesized via one-step hydrothermal reaction

Abstract Large-scale single crystals of Ca9Mg(PO4)6(PO3OH), Ca9Zn(PO4)6(PO3OH), and Ca9Co(PO4)6(PO3OH) were synthesized using hydrothermal technique, and turned out to be similar to natural bone whitlockite. The hexagonal single crystals about 1 mm with high-quality were obtained with this method for the first time. The crystals were of sufficiently good quality for the precision X-ray structural investigation. The compounds crystallize in usual for this structural type trigonal space group R3c. Presence of hydrogen atom in the structure was confirmed by means of infra-red (IR) spectroscopy, differential scanning calorimetry (DSC) and differential thermogravimetry (DTG) methods. Based on the analysis of the local bond valence sum (BVS), a conclusion on the localization of H atoms was made. The formation of O–H groups and hydrogen bonds H⋯O in vicinity of PO4 tetrahedra was shown and similar to bone whitlockite. This research provides new data on possibility of using hydrothermal technique for obtaining doped bone whitlockites. Hydrogen-containing doped whitlockites can combine bioactive properties and improve biocompatibility due to similarity to natural bond. New structural data are useful for finding the ways to better biocompatibility of whitlockite-based materials.

Mesoporous Manganese Oxides with High-Valent Mn Species and Disordered Local Structures for Efficient Oxygen Electrocatalysis.

Active and nonprecious-metal bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital components of clean energy conversion devices such as regenerative fuel cells and rechargeable metal-air batteries. Porous manganese oxides (MnOx) are promising electrocatalyst candidates because of their high surface area and the abundance of Mn. MnOx catalysts exhibit various oxidation states and crystal structures, which critically affect their electrocatalytic activity. These effects remain elusive mainly because the synthesis of oxidation-state-controlled porous MnOx with similar structural properties is challenging. In this work, four different mesoporous manganese oxides (m-MnOx) were synthesized and used as model catalysts to investigate the effects of local structures and Mn valence states on the activity toward oxygen electrocatalysis. The following activity trends were observed: m-Mn2O3 > m-MnO2 > m-MnO > m-Mn3O4 for the ORR and m-MnO2 > m-Mn2O3 > m-MnO ≈ m-Mn3O4 for the OER. These activity trends suggest that high-valent Mn species (Mn(III) and Mn(IV)) with disordered atomic arrangements induced by nanostructuring significantly influence electrocatalysis. In situ X-ray absorption spectroscopy was used to analyze the changes in the oxidation states under the ORR and OER conditions, which showed the surface phase transformation and generation of active species during electrocatalysis.

Enhanced catalytic performance of hierarchical Zn/ZSM-5 with balanced acidities synthesized utilizing ZIF-14 as porogen and Zn source in methanol to aromatics

A series of Zn loaded ZSM-5 (Zn/ZSM-5) zeolites were prepared utilizing either Zn(NO3)2 or Zn-based MOFs (ZIF-61, ZIF-8 and ZIF-14) as Zn sources to explore their effects on the catalysts’ physiochemical properties through characterizations and simulations. Results revealed that hierarchical structures containing meso- to macro- pores of 10∼80 nm aside from the intrinsic micropores were constructed profiting from the use of Zn-based MOFs. On the other side, lager proportions of medium acid and Lewis acid sites were generated by more Zn(OH)+, whose content was proven to be inextricably linked to the binding energy between Zn and ligand as well as local charge valence state of Zn. Benefitting from hierarchical structures and balanced acidities, Zn/Z5(ZIF-14) employing ZIF-14 as Zn source demonstrated higher aromatic selectivity and longer catalyst lifetime (50.9% and 26 h) than the microporous counterpart synthesized from zinc nitrate hexahydrate (42.1% and 20 h) in methanol to aromatics (MTA) reaction.

Effective Fragment Potentials for Microsolvated Excited and Anionic States.

The effective fragment potential (EFP) approach is a sophisticated hybrid approach that allows the inclusion of solvation effects when describing properties and reactivity in the condensed phase, without using empirical parameters. This work examines the performance of the EFP method when describing microsolvation in electronically excited states of neutrals and anions. The examples selected include both localized valence states, as well as diffuse nonvalence states, which represent greater challenges to conventional electronic structure methods. The equation-of-motion coupled cluster with singles and doubles (EOM-XX-CCSD) methodology has been used to provide the quantum chemical description of both the full microsolvated clusters, and the chromophoric moiety in mixed quantum/EFP calculations. We find that, when averaging over multiple configurations of microsolvated clusters, the differences between QM/EFP and full quantum results are minimal, although individual configurations often have larger errors. As expected, diffuse states have somewhat larger errors, although not significantly so. The close proximity of states leading to mixing can make QM/EFP less accurate because a change of ordering of states can occur. Other properties, such as photoelectron images and lifetimes of metastable states, are very well described for the monohydrated clusters investigated.

Induced Hyperfine Field in Doped Chalcopyrite Compounds Cu(In&lt;sub&gt;1-x&lt;/sub&gt;A&lt;sub&gt;x&lt;/sub&gt;)S&lt;sub&gt;2&lt;/sub&gt; (A = V and Cr)

Hyperfine field and electronic states of doped chalcopyrite compounds Cu(In1-xAx)S2 with A = V and Cr are calculated by Korringa- Kohn-Rostoker Green’s function method, where x denotes the concentration of A atoms. The V and Cr doped compounds are ferromagnetically stable in terms of energy minimization and their electronic states are half metallic. The hyperfine fields in the stable ferromagnetic (FM) state are calculated with their valence and core contributions. In addition, site preference local magnetic moments are calculated for the FM and disordered spin moment (DSM) states. Density of states (DOS) exhibit the spintronic property of half metallicity. The local core and valence field polarizations are found in the doped chalcopyrite systems, where the s-orbital hybridization with the local d-shell is explained in terms of some exchange mechanisms. Dhaka Univ. J. Sci. 69(3): 149-153, 2022 (June)

Band Bending and Trap Distribution along the Channel of Organic Field-Effect Transistors from Frequency-Resolved Scanning Photocurrent Microscopy

The scanning photocurrent microscopy (SPCM) method is applied to pentacene field-effect transistors (FETs). In this technique, a modulated laser beam is focused and scanned along the channel of the transistors. The resulting spatial photocurrent profile is attributed to extra free holes generated from the dissociation of light-created excitons after their interaction with trapped holes. The trapped holes result from the local upward band bending in the accumulation layer depending on the applied voltages. Thus, the photocurrent profile along the conducting channel of the transistors reflects the pattern of the trapped holes and upward band bending under the various operating conditions of the transistor. Moreover, it is found here that the frequency-resolved SPCM (FR-SPCM) is related to the interaction of free holes via trapping and thermal release from active probed traps of the first pentacene monolayers in the accumulation layer. The active probed traps are selected by the modulation frequency of the laser beam so that the FR-SPCM can be applied as a spectroscopic technique to determine the energy distribution of the traps along the transistor channel. In addition, a crossover is found in the FR-SPCM spectra that signifies the transition from empty to partially empty probed trapping states near the corresponding trap quasi-Fermi level. From the frequency of this crossover, the energy gap from the quasi-Fermi Etp level to the corresponding local valence band edge Ev, which is bent up by the gate voltage, can be estimated. This allows us to spatially determine the magnitude of the band bending under different operation conditions along the channel of the organic transistors.

Breaking the Limitation of Elevated Coulomb Interaction in Crystalline Carbon Nitride for Visible and Near-Infrared Light Photoactivity.

Most near‐infrared (NIR) light‐responsive photocatalysts inevitably suffer from low charge separation due to the elevated Coulomb interaction between electrons and holes. Here, an n‐type doping strategy of alkaline earth metal ions is proposed in crystalline K+ implanted polymeric carbon nitride (KCN) for visible and NIR photoactivity. The n‐type doping significantly increases the electron densities and activates the n→π* electron transitions, producing NIR light absorption. In addition, the more localized valence band (VB) and the regulation of carrier effective mass and band decomposed charge density, as well as the improved conductivity by 1–2 orders of magnitude facilitate the charge transfer and separation. The proposed n‐type doping strategy improves the carrier mobility and conductivity, activates the n→π* electron transitions for NIR light absorption, and breaks the limitation of poor charge separation caused by the elevated Coulomb interaction.

N–TiO2 crystal seeds incorporated in amorphous matrix for enhanced solar hydrogen generation: Experimental & first-principles analysis

We present here a combined study on the photoelectrochemical activity of highly active Nitrogen doped titanium dioxide thin-film using experiments and First principle density (DFT) based calculation. Hybridization of N 2p with O 2p and localized valence band upshifting leads to the reduction in band-gap of N–TiO2. To validate theoretical findings, the role of nitrogen in TiO2 is revisited with a focus on partial crystallinity. The best-case photoelectrode, nanostructured partially crystalline nitrogen-doped titanium dioxide (PCNDTO) offered photocurrent density of 24.3 mA/cm2 at 1 V versus saturated calomel electrode (SCE). The absence of well-defined peaks and long-range order in XRD pattern and Raman spectrum respectively suggests partially crystallinity. High-resolution transmission electron microscopy (HR-TEM) images confirm the presence of TiO2 crystals in the amorphous matrix. High photoelectrochemical response can be attributed to the abundance of hydroxyl groups, high electrochemical active surface area, reduced charge transfer resistance, and reduced charge carrier recombination rate.

Structure Determination, Mechanical Properties, Thermal Stability of Co2MoB4 and Fe2MoB4.

The precise determination of atomic position of materials is critical for understanding the relationship between structure and properties, especially for compounds with light elements of boron and single or multiple transition metals. In this work, the single crystal X-ray diffraction is employed to analyze the atomic positions of Co2MoB4 and Fe2MoB4 with a Ta3B4-type structure, and it is found that the lengths of B-B bonds connecting the two zig-zag boron chains are 1.86 Å and 1.87 Å, but previously unreported 1.4 Å. Co and Fe atoms occupy the same crystallographic position in lattice for the doped samples and the valence is close to the metal itself, and Co/Fe K-edge X-ray Absorption Fine Structure(XAFS) spectra of borides with different ratios of Co to Fe are collected to detect the local environment and chemical valence of Co and Fe. Vickers hardness and nano indentation measurements are performed, together with the Density Functional Theory (DFT) calculations. Finally, Co2MoB4 possess better thermal stability than Fe2MoB4 evaluated by Thermogravimetric Differential Thermal Analysis (TG-DTA) results.

Persistent photoconductivity in a-IGZO thin films induced by trapped electrons and metastable donors

Amorphous In–Ga–Zn–O (a-IGZO) thin films are prepared by pulsed laser deposition and fabricated into thin film transistors (TFTs). The concentration of oxygen vacancies in a-IGZO thin films is determined by the deposition oxygen pressure, as characterized by in situ x-ray photoelectron spectroscopy. The oxygen vacancies could induce persistent photoconductivity (PPC) and thus the negative shift of threshold voltage of the TFTs under illumination. The PPC in a-IGZO is quantitatively described by the time constant (τ) of decay current. The continuous illumination could cause a fast decay (τ ∼ 0.1 s) and a slow decay (τ ∼ 100 s); however, the pulsed laser (20 ns duration) just results in the fast decay (τ ∼ 0.1 s). The fast decay is temperature independent and can be ascribed to the transition of trapped electrons at 0.035–0.75 eV below the conduction band. The slow decay occurs at 210 K or above, resulting from the generation of metastable donors at 0.9 and 19.3 meV below the conduction band. The thermal activation energies required for the generation and annihilation of the metastable donors are 2.2 and 375 meV, respectively. The spectrum response of photocurrent (600–300 nm) and density functional theory calculation illustrate that the metastable donors are transformed from the neutral oxygen vacancies at a highly localized valence band tail.

Simple hexagonal structured gold with eight-coordination formed with ordered structural vacancies

Phase engineering of nanomaterials attracts increasing research interest, as crystal structures of metals often determine their physical and chemical properties. Metals tend to form close-packed crystal structures to maximize their space filling and atomic coordination numbers due to the isotropic metallic bonding. Hitherto no metals yet crystallize with the simple hexagonal packing which has low coordination number and packing fraction. Here, we fabricate a novel simple hexagonal structured gold through in situ selective etching on ordered Cu-Au intermetallic compound, where the new crystal structure has only 8 nearest neighbors for each atom and a low packing fraction of 0.60. The simple hexagonal structured Au demonstrates an electrical conductivity of 389 S/m, approximately 105 times lower than that of face-centered cubic structured gold, according to first principles calculation, associated with localized valence electrons. The discovery of simple hexagonal structure suggests that metals can form more diverse crystal structures by changing effective atomic volume, which provides a pathway to engineer new structures of metals for novel physical properties and applications.

Shifts in valence states in bimetallic MXenes revealed by electron energy-loss spectroscopy (EELS)

MXenes are an emergent class of two-dimensional materials with a very wide spectrum of promising applications. The synthesis of multiple MXenes, specifically solid-solution MXenes, allows fine tuning of their properties, expands their range of applications, and leads to enhanced performance. The functionality of solid-solution MXenes is closely related to the valence state of their constituents: transition metals, oxygen, carbon, and nitrogen. However, the impact of changes in the oxidation state of elements in MXenes is not well understood. In this work, three interrelated solid-solution MXene systems (Ti2−y Nb y CT x , Nb2−y V y CT x , and Ti2−y V y CT x ) were investigated with scanning transmission electron microscopy and electron energy-loss spectroscopy to determine the localized valence states of metals at the nanoscale. The analysis demonstrates changes in the electronic configuration of V upon modification of the overall composition and within individual MXene flakes. These shifts of oxidation state can explain the nonlinear optical and electronic features of solid-solution MXenes. Vanadium appears to be particularly sensitive to modification of the valence state, while titanium maintains the same oxidation state in Ti–Nb and Ti–V MXenes, regardless of stoichiometry. The study also explains Nb’s influential role in the previously observed electronic properties in the Nb–V and Nb–Ti systems.