Ligancy-Driven Controlling of Covalency and Metallicity in a Ruthenium Two-Dimensional System

The homopolar network and conjugation in <i>d</i>-block single elements can materialize a highly anisotropic and robust structure of a noble-metal system. Here, we have prepared ruthenium (Ru) atomic monolayers of a nonmetallic hexagonal lattice, and determined their layering scheme and metallization. The two-dimensional (2D) network is retained at the first stacking of the monolayer, while maintaining the nonmetallic features. We find out that the <i>atop</i> (AA) related stacking structure of bilayered Ru nanosheets occurs due to the ligancy-driven covalency, and the inception of the metallic electronic states is from trilayered stacking. These results indicate that the metallic states can be separated from covalent-bonding linkage and unpaired electrons in <i>spd</i> hybrid orbital systems. Our approach enables the molecular structure of noble-metal atoms to be induced via controlling the ligancy of <i>d</i>-block atomic bonds

Enhancement in Kinetics of the Oxygen Reduction Reaction on a Nitrogen-Doped Carbon Catalyst by Introduction of Iron via Electrochemical Methods

The iron (Fe) electrodeposition–electrochemical dissolution has been employed on nitrogen-doped carbon material (P-PI) prepared via multi-step pyrolysis of a polyimide precursor to achieve the introduction of Fe species, and its influence on the oxygen reduction reaction (ORR) is investigated by cyclic and rotating ring-disk electrode voltammetry in 0.5 M H₂SO₄. After the electrochemical treatment, the overpotential and H₂O₂ production percentage of ORR on the P-PI are decreased and the number of electrons transferred is increased in the meanwhile. In combination with the results of X-ray absorption fine structure spectra, the presence of Fe–N_<i>x</i> sites (Fe ions coordinated by nitrogen) is believed to be responsible for the improved ORR performance. Further kinetic analysis indicates that a two-electron reduction of O₂ is predominant on the untreated P-PI with coexistence of a direct four-electron transformation of O₂ to H₂O, while the introduction of Fe species leads to a larger increase in the rate constant for the four-electron reduction than that for the two-electron process, being in good agreement with the view that Fe–N_<i>x</i> sites are active for four-electron ORR

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X‑ray Emission Spectroscopy

We combine Mn <i>L</i>_2,3-edge X-ray absorption, high resolution Mn 2p–3d–2p resonant X-ray emission, and configuration–interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn²⁺ low-spin state separately from that of the Mn²⁺ high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn²⁺ low spin state having a stronger ligand-to-metal charge-transfer effect between Mn <i>t</i>_2g and CN π orbitals than the Mn²⁺ high spin state. The d–d-excitation peak nearest to the elastic scattering was ascribed to the Mn²⁺ LS state by the CIFM calculation, indicating that the Mn²⁺ LS state with a hole on the <i>t</i>_2g orbital locates near the Fermi level

In Situ Hard X‑ray Photoelectron Study of O₂ and H₂O Adsorption on Pt Nanoparticles

To improve the efficiency of Pt-based cathode catalysts in polymer electrolyte fuel cells, understanding of the oxygen reduction process at surfaces and interfaces in the molecular level is essential. In this study, H₂O and O₂ adsorption and dissociation as the first step of the reduction process were investigated by in situ hard X-ray photoelectron spectroscopy (HAXPES). Pt 5d valence band and Pt 3d, Pt 4f core HAXPES spectra of Pt nanoparticles upon H₂O and O₂ adsorption revealed that H₂O adsorption has a negligible effect on the electronic structure of Pt, while O₂ adsorption has a significant effect, reflecting the weak and strong chemisorption of H₂O and O₂ on the Pt nanoparticle, respectively. Combined with ab initio theoretical calculations, it is concluded that Pt 5d states responsible for Pt–O₂ bonding reside within 2 eV from the Fermi level

Epitaxially Stabilized EuMoO₃: A New Itinerant Ferromagnet

Synthesizing metastable phases often open new functions in materials, but it is a challenging topic. Thin film techniques have advantages to form materials which do not exist in nature since nonequilibrium processes are frequently utilized. In this study, we successfully synthesize an epitaxially stabilized new compound of perovskite Eu²⁺Mo⁴⁺O₃ as a thin film form by a pulsed laser deposition. The analogous perovskite SrMoO₃ is a highly conducting paramagnetic material, but Eu²⁺ and Mo⁴⁺ are not compatible in equilibrium, and a previous study found that the more stable pyrochlore Eu₂³⁺Mo₂⁴⁺O₇ prefers to form. By using isostructural perovskite substrates, the gain of the interface energy between the film and the substrate stabilizes the matastable EuMoO₃ phase. This compound exhibits high conductivity and large magnetic moment, originating from Mo 4d² electrons and Eu 4f⁷ electrons, respectively. Our result indicates the epitaxial stabilization is effective not only to stabilize crystallographic structures but also to form a new compound which contains unstable combinations of ionic valences in bulk form

Elucidation of Rh-Induced In-Gap States of Rh:SrTiO₃ Visible-Light-Driven Photocatalyst by Soft X‑ray Spectroscopy and First-Principles Calculations

The occupied and unoccupied in-gap electronic states of a Rh-doped SrTiO₃ photocatalyst were investigated by X-ray emission spectroscopy and X-ray absorption spectroscopy for different Rh impurity valence states and doping levels. An unoccupied midgap Rh⁴⁺ acceptor state was found 1.5 eV below the SrTiO₃ conduction band minimum. Both Rh⁴⁺ and Rh³⁺ dopants were found to have an occupied donor level close to the valence band maximum of SrTiO₃. The density of states obtained from first-principles calculations show that all observed spectral features can be assigned to electronic states of substitutional Rh at the Ti site and that Rh:SrTiO₃ is an unusual titanate compound with a characteristic p-type electronic structure. The Rh doping results in a large decrease of the bandgap energy, making Rh:SrTiO₃ an attractive material for use as a visible-light-driven H₂-evolving photocatalyst in a solar water splitting reaction