Role of Cation–Water Disorder during Cation Exchange in Small-Pore Zeolite Sodium Natrolite

By combining X-ray diffraction with oxygen K-edge absorption spectroscopy we track changes occurring during the K⁺–Na⁺ cation exchange of Na-natrolite (Na-NAT) as tightly bonded Na⁺ cations and H₂O molecules convert into a disordered K⁺–H₂O substructure and the unit cell expands by ca. 10% after 50% cation exchange. The coordination of the confined H₂O and nonframework cations change from a tetrahedral configuration, similar in ice <i>I</i>_<i>h</i>, with Na⁺ near the middle of the channels in Na-NAT to two-bonded configuration, similar in bulk water, and K⁺ located near the walls of the framework in K-NAT. This is related to the enhanced ion-exchange properties of K-NAT, which, in marked contrast to Na-NAT, permits the exchange of K⁺ by a variety of uni-, di-, and trivalent cations

Spontaneous Ionic Polarization in Ammonia-Based Ionic Liquid

Ionic liquids and gels have attracted attention for a variety of energy storage applications, as well as for high-performance electrolytes for batteries and supercapacitors. Although the electronic structure of ionic electrolytes in these applications is of practical importance for device design and improved performance, the understanding of the electronic structure of ionic liquids and gels is still at an early stage. Here we report soft X-ray spectroscopic measurements of the surface electronic structure of a representative ammonia-based ionic gel (DEME-TFSI with PS-PMMA-PS copolymer). We observe that, near the outermost surface, the area of the anion peak (1s N^– core level in TFSI) is relatively larger than that of the cation peak (N⁺ in DEME). This spontaneous ionic polarization of the electrolyte surface, which is absent for the pure ionic liquid without copolymer, can be directly tuned by the copolymer content in the ionic gel, and further results in a modulation in work function. These results shed new light on the control of surface electronic properties of ionic electrolytes, as well as a difference between their implementation in ionic liquids and gels

Atomically Engineered Metal–Insulator Transition at the TiO₂/LaAlO₃ Heterointerface

We demonstrate that the atomic boundary conditions of simple binary oxides can be used to impart dramatic changes of state. By changing the substrate surface termination of LaAlO₃ (001) from AlO₂ to LaO, the room-temperature sheet conductance of anatase TiO₂ films are increased by over 3 orders of magnitude, transforming the intrinsic insulating state to a high mobility metallic state, while maintaining excellent optical transparency

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron–proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co­(II), with the remainder in octahedral coordination. Oxygen-mediated 4<i>p</i>–3<i>d</i> hybridization through Co–O–Co bonding was detected by RXES and the intersite <i>dd</i> excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO₂-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance