3d Band Research Articles

Mineral-Mediated Epitaxial Growth of CoO Nanoparticles for Efficient Electrochemical H2O2 Activation.

Solution-phase epitaxy is a versatile method to synthesize functional nanomaterials with customized properties, where supports play a central role as they not only serve as nucleation templates but also greatly affect the local electronic structures. However, developing functional supports remains a great challenge. Herein, inspired by the commonly observed epitaxy of minerals in the natural environment, we report using calcination-modified kaolinite as the support for the epitaxial growth of hexagonal CoO nanoparticles (h-CoO NPs), which enables over 40 times higher mass-specific activity toward H2O2 electrochemical activation than the counterpart without the support. High-resolution electron microscopy, magic-angle spinning nuclear magnetic resonance, and X-ray absorption fine structure results prove that the Al sites in kaolinite play a crucial role in the formation of h-CoO NPs. Moreover, the five-coordinate Al (AlV) sites produced by the dehydration of kaolinite are indispensable for forming the epitaxial interface. Theoretical calculations reveal that the local electron densities around AlV sites are lower than those of general six-coordinate Al sites, which render AlV sites with strong adsorption capability that facilitates the nucleation of h-CoO NPs. Also, the AlV sites induce the electron transfer from h-CoO to the kaolinite support that results in the upshift of the Co 3d band center and hence improve the H2O2 activation kinetics. Our results demonstrate the superiority of nanoclay as functional supports and could offer a more benign strategy to the solution-phase epitaxy production of functional nanomaterials for diverse applications.

Interfacial charge transfer and its impact on transport properties of LaNiO3/LaFeO3 superlattices.

Charge transfer or redistribution at oxide heterointerfaces is a critical phenomenon, often leading to remarkable properties such as two-dimensional electron gas and interfacial ferromagnetism. Despite studies on LaNiO3/LaFeO3 superlattices and heterostructures, the direction and magnitude of the charge transfer remain debated, with some suggesting no charge transfer due to the high stability of Fe3+ (3d5). Here, we synthesized a series of epitaxial LaNiO3/LaFeO3 superlattices and demonstrated partial (up to ~0.5 e-/interface unit cell) charge transfer from Fe to Ni near the interface, supported by density functional theory simulations and spectroscopic evidence of changes in Ni and Fe oxidation states. The electron transfer from LaFeO3 to LaNiO3 and the subsequent rearrangement of the Fe 3d band create an unexpected metallic ground state within the LaFeO3 layer, strongly influencing the in-plane transport properties across the superlattice. Moreover, we establish a direct correlation between interfacial charge transfer and in-plane electrical transport properties, providing insights for designing functional oxide heterostructures with emerging properties.

The Roles of Surface Hydrogen and Hydroxyl in Alkaline Hydrogen Oxidation on Ni-Based Electrocatalysts.

One important target for anion exchange membrane fuel cells (AEMFCs) is to enable the application of anode non-precious metal hydrogen oxidation reaction (HOR) catalyst. Nickel presents a promising candidate for alkaline HOR; yet, its practical application is hampered by the intrinsically sluggish activity and poor stability. Herein, a series of Ni-based metals (Ni5Mo, Ni25Co, Ni14W and Ni) are electrodeposited as model catalysts to systematically explore the alkaline HOR by considering the role of adsorbed hydroxyl (OHad). Spectroscopic studies together with density functional theory calculations shed light on the beneficial effect of transition metal M (M=Mo, Co, W) alloying/doping on HOR by introducing the charge transfer from M to Ni and down shifting Ni 3d band center. The HOR specific activities on Ni-based catalysts reveal a volcano-type relationship with the hydrogen binding energy (HBE). The strongly adsorbed OHad is proven to induce deactivation for Ni active sites, and the deactivation potential is OHad binding energy (OHBE) dependent. This study adds new insight into the HOR mechanism and stability of Ni-based electrocatalysts, providing a new avenue for the rational design of highly efficient and robust alkaline HOR catalysts.

Observation of Cartesian light propagation through a three-dimensional cavity superlattice in a silicon photonic band gap crystal

We experimentally investigate peculiar light propagation inside a three-dimensional (3D) superlattice of resonant cavities that are confined within a 3D photonic band gap. To this end, we fabricated 3D diamondlike photonic crystals from silicon with a broad 3D band gap in the near-infrared and doped them with a periodic array of point defects. In position-resolved reflectivity and scattering microscopy, we observe narrow spectral features that match well with superlattice bands in band structures computed with the plane-wave expansion. The cavities are coupled in all three dimensions when they are closely spaced (aSL≤3a), and uncoupled when they are further apart. The superlattice bands correspond to light that hops in high-symmetry directions in 3D (“Cartesian light”) that opens applications in 3D photonic networks, 3D Anderson localization of light, and future 3D quantum photonic networks. Published by the American Physical Society 2024

The Effect of Combining Femtosecond Laser and Electron Irradiation on Silica Glass.

This study investigates the structural and optical responses of silica glass to femtosecond (fs) laser irradiation followed by high-energy electron (2.5 MeV, 4.9 GGy) irradiation. Using optical microscopy and spectroscopy techniques, we analyzed retardance, phase shifts, nanograting periodicity, and Raman D2 band intensity, which is an indicator of local glass densification. S-SNOM and nano-FTIR measurements further revealed changes in the Si-O-Si vibrational bands, indicating partial relaxation of the densified nanolayers under electron irradiation. Our findings reveal significant optical modifications due to subsequent electron irradiation, including reduced retardance and phase values, which are in agreement with the relaxation of the local densification. SEM analysis confirmed the preservation of nanogratings' morphology including their periodicity. Apart from revealing fundamental aspects related to glass densification within nanogratings, this study also underscores the potential of combined fs-laser and electron irradiation techniques in understanding silica glass behavior under high radiation conditions, which is crucial for applications in harsh environments.

Exploring the Synergistic Ni-Fe-W Interplay in Double Perovskites to Understand the Operando Electronic Transformations Driving High Oxygen Evolution Reaction Activity and Stability

Novel flame-spray synthesized perovskite oxides of the composition BaSrNixFeyWzO6 exhibit excellent oxygen evolution reaction (OER) activity under alkaline conditions. In particular, BaSrNi0.9Fe0.1WO6, with a catalytic turn over frequency (TOF) six-times that of NiO, displays a competitive Tafel slope of 41 mV/dec in 0.1 M KOH electrolyte. The double perovskite structure, with its tunable AA’BB’O6 formula, provides a unique opportunity to enhance the intrinsic activity of Ni sites by co-doping with Fe and W. Comparing the transformations of BaSrNi0.9Fe0.1WO6 (BSNFW) and BaSrNiWO6 (BSNW) using ex-situ and operando X-ray absorption spectroscopy (XAS) allows a deep exploration of the relationship between the electronic state of the B-site metals, structural changes induced by doping, and the mechanism of oxygen evolution. Ex-situ soft XAS at the Ni/Fe L2,3 -edge and O K-edge reveals that the electron-withdrawing W6+ stabilizes the Ni2+ state in the linear W-O-Ni chains in pristine BSN(F)W; however, cyclic voltammetry (CV) under OER conditions induces a surface transformation of Ni2+→Ni3+. In BSNW, Ni3+ formation facilitates the hybridization of Ni(eg) and O(2p) orbitals, activating the lattice oxygen for OER participation via the lattice oxygen mechanism (LOM).1 Henceforth, when the redox active Ni(3d) band donates electrons to the external circuit during OER catalysis, subsequent holes can be refilled by electrons from the O(2p) band, generating oxygen vacancies. Conversely, the presence of Fe3+ dramatically changes the post-OER O K-edge spectral features of BSNFW; pi-donation from O(2p) to Fe(t2g) orbitals induces a transformation of high to low-spin Fe3+ and prevents OER-induced Ni-O hybridization. Hence, it can be inferred that BSNFW favors the traditional adsorbate evolution mechanism (AEM) of OER catalysis.2 Ex-situ hard XAS at the W L3 -edge of BSNW shows a decrease in the post-OER white-line intensity, associated with surface W dissolution; this reflects the structural instability associated with lattice oxygen activation, a common problem also for the benchmark alkaline perovskite Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF).3 However, incorporation of Fe into to BSNW, leading to the BSNFW composition, results in high stability and durability under OER conditions, as revealed by W L3 -edge analysis, inductively coupled plasma optical emission spectroscopy (ICP-OES) and operando X-ray diffraction (XRD). Operando XAS at the Ni K-edge during cyclic voltammetry provides a unique insight into the influence of Fe on the electronic transformations of Ni during the OER (Figure 1). Ni2+ sites in BSNFW experience a strong irreversible oxidation to Ni3+ during the first CV cycle, followed by stable and reversible transformations between Ni2+ and Ni3+ (Figure 1a). Comparatively, BSNW requires a longer activation period and higher oxidative potentials before stable, reversible changes in the Ni absorption edge are observed (Figure 1b); this suggests that Ni-Fe charge transfer via pi-bonding orbitals increases the reversibility of the Ni electronic changes. Moreover, the incorporation of structural Fe induces a strong cathodic shift in the Ni redox peak potential of BSNFW; the lower overpotential for the Ni2+→Ni3+ transformation leads to a greater increase in the Ni absorption edge at high potentials. Indeed, a closer examination of the final CV cycle reveals that the oxidative transformation of Ni in BSNFW appears to be coupled with the Ni redox process, whereas for BSNW it occurs after the Ni redox (Figure 1c, d). Such operando characterizations provide a valuable insight into the synergistic Ni-Fe relationship regularly exploited in OER catalysis, though often poorly understood in complex multi-elemental systems. The highly competitive OER activity of BaSrNi0.9Fe0.1WO6 establishes this underexplored class of Ni-based double perovskites as a promising avenue for further, targeted exploration.References A. Grimaud, O. Diaz-Morales, B. Han, W. T. Hong, Y.-L. Lee, L. Giordano, K. A. Stoerzinger, M. T. M. Koper and Y. Shao-Horn, Nature Chemistry, 2017, 9, 457-465.C. E. Beall, E. Fabbri and T. J. Schmidt, ACS Catalysis, 2021, 11, 3094-3114.J.-W. Zhao, Z.-X. Shi, C.-F. Li, Q. Ren and G.-R. Li, ACS Materials Letters, 2021, 3, 721-737. Figure 1

Optimizing Cu 3d Bands with Nanotubular SnO2 to Boost Their Catalytic Transfer Hydrogenation Activity.

Catalytic transfer hydrogenation (CTH) using Cu nanocatalysts offers significant advantages over direct high-pressure hydrogenation. However, the active hydrogen (H*) in this process exhibits poor adsorption and tends to release H2 readily due to the fully occupied 3d states of Cu. To address this issue, a tubular SnO2 support with electron-accepting ability was selected to host Cu nanoparticles, aiming to optimize the Cu 3d bands. The Cu/SnO2 nanohybrids were prepared through an electrospinning technique, followed by hydrothermal synthesis. As evidenced by X-ray photoelectron spectroscopy (XPS) binding energy shifts and density functional theory (DFT) simulations, some electrons from Cu transferred to SnO2 in the Cu/SnO2 nanohybrids due to their different work functions. Such electron transfer enables the Cu 3d orbitals to lose electrons and alters its valence configuration from 3d10 to 3d10-x, which enhances the adsorption of active H* atoms and thereby inhibits undesirable H2 release. The 15 wt % Cu/SnO2 exhibits improved catalytic hydrogenation of 4-nitrophenol with NaBH4, with an optimal normalized rate constant of 56.98 mg-1 min-1 and a turnover frequency of 4.82 min-1, surpassing most reported catalysts. The enhanced activity is attributed to the optimized electronic states, improved hydrogen adsorption, and the tubular structure of the support. This work might shed light on developing more non-noble metal nanocatalysts for CTH by tuning their d bands with appropriate oxide supports.

Ferromagnetic ferroelectricity due to the Kugel-Khomskii mechanism of orbital ordering assisted by atomic Hund's second rule effects

The exchange interactions in insulators depend on the orbital state of magnetic ions, obeying certain phenomenological principles, known as Goodenough-Kanamori-Anderson rules. Particularly, the ferro order of alike orbitals tends to stabilize antiferromagnetic interactions, while the antiferro order of unlike orbitals favors ferromagnetic interactions. The Kugel-Khomskii theory provides a universal view on such coupling between spin and orbital degrees of freedom, based on the superexchange processes: namely, for a given magnetic order, the occupied orbitals tend to arrange in a way to further minimize the exchange energy. Then, if two magnetic sites are connected by the spatial inversion, the antiferro orbital order should lead to the ferromagnetic coupling break the inversion symmetry. This constitutes the basic idea of our work, which provides a pathway for designing ferromagnetic ferroelectrics: the rare but fundamentally and practically important multiferroic materials. After illustrating the basic idea on toy-model examples, we propose that such behavior can be indeed realized in the van der Waals ferromagnet VI3, employing for this analysis the realistic model derived from first-principles calculations for magnetic 3d bands. We argue that the intra-atomic interactions responsible for Hund's second rule, acting against the crystal field, tend to restore the orbital degeneracy of the ionic d2 state in VI3 and, thus, provide a necessary flexibility for activating the Kugel-Khomskii mechanism of the orbital ordering. In the honeycomb lattice, this orbital ordering breaks the inversion symmetry, stabilizing the ferromagnetic-ferroelectric ground state. The symmetry breaking leads to the canting of magnetization, which can be further controlled by the magnetic field, producing a huge change of electric polarization. Published by the American Physical Society 2024

Utilization of D2 molecular band emission for electron density measurement

D2 molecular band emission observed at a wavelength range of λ∼557−643nm is utilized to measure electron density, ne, in D plasmas of the PISCES-A and PISCES-RF linear plasma devices. The D2 band is divided at λ=593nm to make an intensity ratio, D2L (∼557–593nm)/D2R (∼593–643nm), where D2L consists predominantly of the g3Σg+3dσ, h3Σg+3sσ, i3Πg3dπ, j3Δg3dδ→c3Πu2pπ transitions, while D2R mainly includes the d3Πu3pπ→a3Σg+2sσ Fulcher band emission. It is experimentally found that D2L/D2R depends strongly on ne with little Te dependence in ranges of ne∼(0.031−6.1)×1018 m−3 and Te∼2.3−13.9eV. This observed trend is consistent with collisional-radiative model calculations using Yacora on the Web.

Effects of W alloying on the electronic structure, phase stability, and thermoelectric power factor in epitaxial CrN thin films

CrN-based alloy thin films are of interest as thermoelectric materials for energy harvesting. Ab initio calculations show that dilute alloying of CrN with 3 at. % W substituting Cr induce flat electronic bands and push the Fermi level EF into the conduction band while retaining dispersive Cr 3d bands. These features are conducive for both high electrical conductivity σ and high Seebeck coefficient α and, hence, a high thermoelectric power factor α2σ. To investigate this possibility, epitaxial CrWxNz films were grown on c-sapphire by dc-magnetron sputtering. However, even films with the lowest W content (x = 0.03) in our study contained metallic h-Cr2N, which is not conducive for a high α. Nevertheless, the films exhibit a sizeable power factor of α2σ ∼ 4.7 × 10−4 W m−1 K−2 due to high σ ∼ 700 S cm−1, and a moderate α ∼ − 25 μV/K. Increasing h-Cr2N fractions in the 0.03 < x ≤ 0.19 range monotonically increases σ, but severely diminishes α leading to two orders of magnitude decrease in α2σ. This trend continues with x > 0.19 due to W precipitation. These findings indicate that dilute W additions below its solubility limit in CrN are important for realizing a high thermoelectric power factor in CrWxNz films.

Optimizing the stability of NiFeOOH via oxyanion intercalation for water oxidation at large current densities

In alkaline water splitting, transition metals (Ni, Fe) have received extensive attention, and NiFe-oxyhydroxide (NiFeOOH) is regarded as an exceptionally active electrocatalysts for oxygen evolution reaction (OER). However, maintaining the long-term stability of NiFeOOH at high current densities is challenging due to Fe segregation and catalyst degradation. Herein, this study proposes an approach to enhancing the stability of the Ni/Fe-O covalent bond by intercalating oxyanions (NO3–, PO43-, SO42-, and SeO42-) into the NiFeOOH substrate, improving its resistance to bond breakage. And the NiFeOOH-NO3− electrocatalyst was found to be optimal, achieving an overpotential of 311 mV and stable performance at 1 A cm−2 for several hundred hours. Consequently, NiFeOOH-NO3– exhibited a significantly improved OER stability, with a mere 3.33 % stability attenuation after 100 h, compared to 13.19 % for pristine NiFeOOH. Notably, the presence of NO3– in NiFeOOH effectively mitigates Fe segregation, leading to a fourfold enhancement in long-term stability relative to that of NiFeOOH without NO3– modification. Theoretical calculations show that the introduction of NO3– effectively shifts metal 3d band centers of NiFeOOH closer to the Fermi level. It is suggested that the oxyanions lead to increased strength of the Ni/Fe-O bonds, thereby inhibiting the dissolution of Fe and enhancing the stability of NiFeOOH phase. This research represents a significant advance in controlling Fe segregation to stabilize NiFe-based electrocatalysts for high-current–density water oxidation.

Mechanism Behind the Enhanced Ferromagnetic Contributions Upon Ru Substitution in Ca3Mn2O7 from X-Ray Absorption Spectroscopies.

We have studied the local structure and electronic and magnetic properties of hybrid improper ferroelectric Ca3Mn2O7 upon Ru substitution at the Mn site by a combination of atomic-selective X-ray absorption spectroscopies in the soft and hard X-ray energy regimes. Ru substitution enhances the macroscopic ferromagnetic contributions, whose origin is here elucidated. In particular, soft X-ray magnetic circular dichroism (XMCD) data indicate that the spin moments of Mn and Ru are aligned in opposite directions, with the effective magnetic moments of Ru being about 1 order of magnitude smaller than for Mn. The XMCD results also demonstrate the presence of an intrinsic ferromagnetic component in the Mn sublattice for Ru contents x ≥ 0.3, although the values of the net Mn magnetic moment are much smaller than expected for a fully saturated Mn4+ magnetic sublattice. Soft and hard X-ray absorption spectroscopy measurements show that Mn keeps a +4 valence state independently of the Ru content, whereas Ru is in an intermediate valence state, ranging between +4.7 (x ≤ 0.1) and +4.4 (x ≥ 0.7). Analysis of the extended X-ray absorption fine structure (EXAFS) signals at the Mn and Ru K-edges suggests the lack of a complete solid solution in the local structure across the entire range of compositions. These findings disclose the existence of charge transfer between Mn and Ru atoms, so the weak ferromagnetic component is attributed to a canting of the antiferromagnetically ordered Mn4+ spins caused by the oxygen-mediated hybridization between localized Mn4+ 3d and itinerant intermediate valence Ru 4d bands, in analogy to the mechanism proposed for ferromagnetism in ordered doubled perovskites with 3d and 4d/5d transition-metal ions. In the present case, disorder prevents the formation of long-range ferromagnetism.

Roles of Impurity Levels in 3d Transition Metal-Doped Two-Dimensional Ga2O3.

Doping engineering is crucial for both fundamental science and emerging applications. While transition metal (TM) dopants exhibit considerable advantages in the tuning of magnetism and conductivity in bulk Ga2O3, investigations on TM-doped two-dimensional (2D) Ga2O3 are scarce, both theoretically and experimentally. In this study, the detailed variations in impurity levels within 3d TM-doped 2D Ga2O3 systems have been explored via first-principles calculations using the generalized gradient approximation (GGA) +U method. Our results show that the Co impurity tends to incorporate on the tetrahedral GaII site, while the other dopants favor square pyramidal GaI sites in 2D Ga2O3. Moreover, Sc3+, Ti4+, V4+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+, Cu2+, and Zn2+ are the energetically favorable charge states. Importantly, a transition from n-type to p-type conductivity occurs at the threshold Cu element as determined by the defect formation energies and partial density of states (PDOS), which can be ascribed to the shift from electron doping to hole doping with respect to the increase in the atomic number in the 3d TM group. Moreover, the spin configurations in the presence of the square pyramidal and tetrahedral coordinated crystal field effects are investigated in detail, and a transition from high-spin to low-spin arrangement is observed. As the atomic number of the 3d TM dopant increases, the percentage contribution of O ions to the total magnetic moment significantly increases due to the electronegativity effect. Additionally, the formed 3d bands for most TM dopants are located near the Fermi level, which can be of significant benefit to the transformation of the absorbing region from ultraviolet to visible/infrared light. Our results provide theoretical guidance for designing 2D Ga2O3 towards optoelectronic and spintronic applications.

Stealthy and hyperuniform isotropic photonic band gap structure in 3D.

In photonic crystals, the propagation of light is governed by their photonic band structure, an ensemble of propagating states grouped into bands, separated by photonic band gaps. Due to discrete symmetries in spatially strictly periodic dielectric structures their photonic band structure is intrinsically anisotropic. However, for many applications, such as manufacturing artificial structural color materials or developing photonic computing devices, but also for the fundamental understanding of light-matter interactions, it is of major interest to seek materials with long range nonperiodic dielectric structures which allow the formation of isotropic photonic band gaps. Here, we report the first ever 3D isotropic photonic band gap for an optimized disordered stealthy hyperuniform structure for microwaves. The transmission spectra are directly compared to a diamond pattern and an amorphous structure with similar node density. The band structure is measured experimentally for all three microwave structures, manufactured by 3D laser printing for metamaterials with refractive index up to . Results agree well with finite-difference-time-domain numerical investigations and a priori calculations of the band gap for the hyperuniform structure: the diamond structure shows gaps but being anisotropic as expected, the stealthy hyperuniform pattern shows an isotropic gap of very similar magnitude, while the amorphous structure does not show a gap at all. Since they are more easily manufactured, prototyping centimeter scaled microwave structures may help optimizing structures in the technologically very interesting region of infrared.

Optimization of d-p band centers as efficient active sites for solar energy conversion into H2 by tuning surface atomic arrangement

The lowered reaction energy barrier and accelerated dynamic behavior for photocatalytic H2O overall splitting (HOS) involving oriented chemisorption, activation and conversion of *H and oxyhydrogen intermediates are crucial for solar energy conversion into H2 (STH). Herein, the localized heterojunction (Cd-S-Ni) composed of NiS and CdS via. tuning surface atomic arrangement with S atoms as the shared ligands has been constructed to synchronously elevate and optimize Ni 3d (Ni ε3d) and S 2p (S ε2p) band centers as efficient active sites for chemisorption of oxyhydrogen and *H intermediates with a declined Cd 4d band center (Cd ε4d) to suppress reverse reaction. A sustainable STH of 3.21 % under AM 1.5 G has been completed over Cd-S-Ni with a decreased activation energy for H2 evolution, verified by fs-TAS, in situ DRIFTS and dynamic DFT. These results devote to solving the reaction energy barrier and dynamical bottleneck for HOS by optimizing εd and εp.

Tailoring the electron redistribution of RuO2 by constructing a Ru-O-La asymmetric configuration for efficient acidic oxygen evolution

Stabilizing the highly active RuO2 electrocatalyst for the oxygen evolution reaction (OER) is critical for the application of proton exchange membrane water electrolysis, but this remains challenging due to the inevitable over-oxidation of Ru in harsh oxidative environments. Herein, we describe constructing Ru-O-La asymmetric configurations into RuO2 via a facile sol-gel method to tailor electron redistribution and thereby eliminate the over-oxidation of Ru centers. Specifically, the as-prepared optimal La0.1Ru0.9O2 shows a low overpotential of 188 ​mV at 10 ​mA ​cm−2, a high mass activity of 251 A gRu−1 at 1.6 ​V vs. reversible hydrogen electrode (RHE), and a long-lasting durability of 63 ​h, far superior to the 8 ​h achieved by standard RuO2. Experiments and density functional theory calculations jointly reveal that the Ru-O-La asymmetric configuration could trigger electron redistribution in RuO2. More importantly, electron transfer from La to Ru via the Ru-O-La configuration could lead to increased electron density around Ru, thus preventing the over-oxidation of Ru. In addition, electron redistribution tunes the Ru 4d band center’s energy level, which optimizes the adsorption and desorption of oxygen intermediates. This work offers an effective strategy for regulating electronic structure to synergistically boost the activity and stability of RuO2-based acidic OER electrocatalysts.

Transient absorption of warm dense matter created by an X-ray free-electron laser

Warm dense matter is at the boundary between a plasma and a condensed phase and plays a role in astrophysics, planetary science and inertial confinement fusion research. However, its electronic structure and ionic structure upon irradiation with strong laser pulses remain poorly understood. Here, we use an intense and ultrafast X-ray free-electron laser pulse to simultaneously create and characterize warm dense copper using L-edge X-ray absorption spectroscopy over a large irradiation intensity range. Below a pulse intensity of 1015 W cm−2, an absorption peak below the L edge appears, originating from transient depletion of the 3d band. This peak shifts to lower energy with increasing intensity, indicating the movement of the 3d band upon strong X-ray excitation. At higher intensities, substantial ionization and collisions lead to the transition from reverse saturable absorption to saturable absorption of the X-ray free-electron laser pulse, two nonlinear effects that hold promise for X-ray pulse-shaping. We employ theoretical calculations that combine a model based on kinetic Boltzmann equations with finite-temperature real-space density-functional theory to interpret these observations. The results can be used to benchmark non-equilibrium models of electronic structure in warm dense matter.

Hierarchical core-shell Ce-doped NiO@MoO2 architecture with Ni 3d-band center modulation for enhanced high-current-density oxygen evolution

Ce-doped NiO catalyst supported on MoO₂ nanorods (Ce-NiO@MoO₂) was constructed as a high-performance oxygen evolution reaction (OER) electrocatalyst. The introduction of Ce induced the formation of nanoflower-like NiO on the MoO₂ nanorod substrate. Ce doping not only created numerous unsaturated Ni coordination sites with higher oxidation states, serving as active sites, but also strengthened the adsorption of oxygen intermediates by upshifting the Ni 3d band center, thereby reducing the energy barrier of the rate-determining OH* → O* step. Consequently, the Ce-NiO@MoO₂ catalyst exhibited remarkable OER activity, with a low overpotential of 313 mV and outstanding 200-hour stability at 1000 mA cm⁻², which is attributed to unsaturated Ni active sites and promoted charge and mass transfer facilitated by the unique hierarchical structure. These findings highlight the crucial role of Ce doping in improving electrocatalytic performance and offer insights into developing efficient and cost-effective catalysts for large-scale hydrogen production via water electrolysis.

Synergistic Assistance of Ir Clusters and NiCo2O4 Nanosheets Interfaces in Direct O-O Coupling for High-Efficiency Alkaline Oxygen Evolution.

Adopting noble metals on non-noble metals is an effective strategy to balance the cost and activity of electrocatalysts. Herein, a thorough analysis of the synergistic OER is conducted at the heterogeneous interface formed by Ir clusters and NiCo2O4 based on DFT calculations. Specifically, the electrons spontaneously bring an eg occupancy of interfacial Ir close to unity after the absorbed O, providing more transferable electrons for the conversion of the absorbed O-intermediates. Besides, the diffuse distribution of electrons in the Ir 5d orbital fills the antibonding orbital after O is absorbed, avoiding the desorption difficulties caused by the stronger Ir-O bonds. The electrons transfer from Ir to Co atoms at the heterogeneous interface and fill the Co 3d band near the Fermi level, stimulating the interfacial Co to participate in the direct O-O coupling (DOOC) pathway. Experimentally, the ultrathin-modulated NiCo2O4 nanosheets are used to support Ir clusters (Ircluster-E-NiCo2O4) by the electrodeposition method. The as-synthesized Ircluster-E-NiCo2O4 catalyst achieves a current density of 10 mA cm-2 at an ultralow overpotential of 238 mV and works steadily for 100 h under a high current of 100 mA cm-2, benefiting from the efficient DOOC pathway during the OER.

Symmetries and wave functions of photons confined in three-dimensional photonic band gap superlattices

We perform a computational study of confined photonic states that appear in a three-dimensional (3D) superlattice of coupled cavities, resulting from a superstructure of intentional defects. The states are isolated from the vacuum by a 3D photonic band gap, using a diamondlike inverse woodpile crystal structure, and they exhibit “Cartesian” hopping of photons in high-symmetry directions. We investigate the confinement dimensionality to verify which states are fully 3D-confined, using a recently developed scaling theory to analyze the influence of the structural parameters of the 3D crystal. We create confinement maps that trace the frequencies of 3D-confined bands for select combinations of key structural parameters, namely the pore radii of the underlying regular crystal and of the defect pores. We find that a certain minimum difference between the regular and defect pore radii is necessary for 3D-confined bands to appear, and that an increasing difference between the defect pore radii from the regular radii supports more 3D-confined bands. In our analysis, we find that their symmetries and spatial distributions are more varied than electronic orbitals known from solid-state physics. We surmise that this difference occurs since the confined photonic orbitals derive from global Bloch states governed by the underlying superlattice structure, whereas single-atom orbitals are localized. Based on this realization, we suggest that the extent symmetries of “photonic orbitals” could possibly translate to novel macroscopic behaviors of “photonic solid-state matter,” never before seen in the standard electronic solid-state systems. We also discover pairs of degenerate 3D-confined bands with p-like orbital shapes and mirror symmetries matching the symmetry of the superlattice. Finally, we investigate the enhancement of the local density of optical states for cavity quantum electrodynamics applications. We find that donorlike superlattices, i.e., where the defect pores are smaller than the regular pores, provide greater enhancement in the air region than acceptorlike structures with larger defect pores, and thus offer better prospects for doping with quantum dots and ultimately for 3D networks of single photons steered across strongly coupled cavities. Published by the American Physical Society 2024