Tuning the Surface Chemistry of Chiral Cu(531)^<i>S</i> for Enhanced Enantiospecific Adsorption of Amino Acids

Amino acids are important bioorganic compounds composed of amine and carboxylic acid because they are the main building blocks of many biomolecules. All of them are chiral except glycine. Thus, they have two enantiomers which provide dramatically different biological effects, thereby requiring their separation. High Miller index metal surfaces often define intrinsically chiral structures. A number of previous studies have proved the enantiospecific adsorption difference of chiral molecules on those surfaces. To further enhance the enantiospecificity, step decoration, which is doping the kink site of chiral metal surface with a second metal, can be one route. It may induce one enantiomer adsorbed on the surface to become more stable than the other, inducing the larger enantiospecific energy difference. In this study, we performed density functional theory (DFT) calculations to systemically examine the adsorption geometries and energetics of each enantiomer of alanine, serine, and cysteine, and their enantiospecific energy differences on pure, Pd-, Pt-, and Au-decorated Cu(531)^<i>S</i>, respectively. By decorating the kinked site with an Au atom, the enantiospecificity of adsorbed cysteine was meaningfully enhanced by 0.08 eV, in the case when the side chain has a high affinity with the surface. Our results provide useful insight of how to tune chiral metal surfaces to enlarge the enantiospecificity of chiral molecules

Surface Electronic Structure Transitions at High Temperature on Perovskite Oxides: The Case of Strained La_0.8Sr_0.2CoO₃ Thin Films

In-depth probing of the surface electronic structure on solid oxide fuel cell (SOFC) cathodes, considering the effects of high temperature, oxygen pressure, and material strain state, is essential toward advancing our understanding of the oxygen reduction activity on them. Here, we report the surface structure, chemical state, and electronic structure of a model transition metal perovskite oxide system, strained La_0.8Sr_0.2CoO₃ (LSC) thin films, as a function of temperature up to 450 °C in oxygen partial pressure of 10^–3 mbar. Both the tensile and the compressively strained LSC film surfaces transition from a semiconducting state with an energy gap of 0.8–1.5 eV at room temperature to a metallic-like state with no energy gap at 200–300 °C, as identified by in situ scanning tunneling spectroscopy. The tensile strained LSC surface exhibits a more enhanced electronic density of states (DOS) near the Fermi level following this transition, indicating a more highly active surface for electron transfer in oxygen reduction. The transition to the metallic-like state and the relatively more enhanced DOS on the tensile strained LSC at elevated temperatures result from the formation of oxygen vacancy defects, as supported by both our X-ray photoelectron spectroscopy measurements and density functional theory calculations. The reversibility of the semiconducting-to-metallic transitions of the electronic structure discovered here, coupled to the strain state and temperature, underscores the necessity of in situ investigations on SOFC cathode material surfaces

New Insights into the Strain Coupling to Surface Chemistry, Electronic Structure, and Reactivity of La_0.7Sr_0.3MnO₃

Effects of strain on the surface cation chemistry and the electronic structure are important to understand and control for attaining fast oxygen reduction kinetics on transition-metal oxides. Here we demonstrate and mechanistically interpret the strain coupling to Sr segregation, oxygen vacancy formation, and electronic structure on the surface of La_0.7Sr_0.3MnO₃ (LSM) thin films as a model system. Our experimental results from X-ray photoelectron spectroscopy and scanning tunneling spectroscopy are discussed in light of our first principles-based simulations. A stronger Sr enrichment tendency and a more facile oxygen vacancy formation prevail for the tensile-strained LSM surface. At 500 °C in 10⁻³ mbar oxygen, both LSM film surfaces exhibit a metallic-like tunneling conductance, with a higher density of electronic states near the Fermi level on the tensile-strained LSM surface, contrary to the behavior at room temperature. Our findings illustrate the potential role and mechanism of lattice strain in tuning the reactivity of perovskite transition-metal oxides with oxygen in solid oxide fuel cell cathodes

Hybrid MnO₂ Film with Agarose Gel for Enhancing the Structural Integrity of Thin Film Supercapacitor Electrodes

We report on the fabrication of a robust hybrid film containing MnO₂ for achieving large areal capacitances. An agarose gel, as an ion-permeable and elastic layer coated on a current collector, plays a key role in stabilizing the deposited pseudocapacitive MnO₂. Cyclic voltammetry and electrochemical impedance spectroscopy data indicate that the hybrid electrode is capable of exhibiting a high areal capacitance up to 52.55 mF cm^–2, with its superior structural integrity and adhesiveness to the current collector being maintained, even at a high MnO₂ loading

Chemisorption of NH₃ on Monomeric Vanadium Oxide Supported on Anatase TiO₂: A Combined DRIFT and DFT Study

V/TiO₂ catalysts are used in various reactions, including oxidative dehydrogenation, partial oxidation of ethanol, and selective catalytic reduction of NO_<i>x</i> with NH₃. In this work, we investigated the effect of supported monomeric vanadium oxide (VO₃) on the acidity of anatase TiO₂(101) surface by using density functional theory calculations combined with in situ diffuse reflectance infrared Fourier transform (DRIFT) experiments. The hydrogenation of TiO₂ to form hydroxyl groups on the surface was energetically more favorable in the presence of the supported monomeric vanadium oxide. Charge transfer between TiO₂ support and VO₃ was considered as an origin of −OH stabilization, which made Brønsted acid sites more abundant on the V/TiO₂ surface than on TiO₂. Moreover, it was observed that the cationic vanadium center in VO₃ can act as much weaker Lewis acid sites than the titanium center in TiO₂. Such weakened acidity of Lewis acid sites in the presence of monomeric vanadium oxide was consistently observed in in situ DRIFT results, which could explain the higher reactivity of NH₃ adsorbed on Lewis acid sites of V/TiO₂ than those of TiO₂ in the NH₃-selective catalytic reduction reaction

Hybrid MnO₂ Film with Agarose Gel for Enhancing the Structural Integrity of Thin Film Supercapacitor Electrodes

We report on the fabrication of a robust hybrid film containing MnO₂ for achieving large areal capacitances. An agarose gel, as an ion-permeable and elastic layer coated on a current collector, plays a key role in stabilizing the deposited pseudocapacitive MnO₂. Cyclic voltammetry and electrochemical impedance spectroscopy data indicate that the hybrid electrode is capable of exhibiting a high areal capacitance up to 52.55 mF cm^–2, with its superior structural integrity and adhesiveness to the current collector being maintained, even at a high MnO₂ loading

Kinetic and Mechanistic Insights into the All-Solid-State Z‑Schematic System

An all-solid-state Z-schematic system, CdS/Au/TiO_1.96C_0.04, has been reported for the efficient H₂ generation from water under visible-light irradiation. However, a kinetic and mechanistic study of the directional charge transfer at the interfaces has not been done. In this study, electron pathways were constructed on the basis of steady-state photoluminescence (PL) spectral data, and the rate constants for charge transfer were calculated from time-resolved PL spectra. The PL results revealed that Au core played an important role in capturing the photoexcited electrons in the conduction band (CB) of TiO_1.96C_0.04 and accelerating the electron transfer to the valence band (VB) of CdS, leading to an efficient quenching of the holes left in the VB of CdS shell. The minimum energy pathways for H₂ production on the surfaces of TiO_1.96C_0.04(101) and CdS(101) were elucidated through first-principles calculations, indicating that the CdS shell has a lower energy barrier (2.81 eV) for the surface reaction than that (3.34 eV) of TiO_1.96C_0.04. Consequently, CdS/Au/TiO_1.96C_0.04 showed a vectorial electron transfer of TiO_1.96C_0.04 → Au → CdS in the form of the letter Z, which allowed the photoexcited electrons to be shuttled to a higher energy level, thereby producing a substantial level of H₂ on the CdS(101) surface

Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li⁺ Adsorbents

Crown ether (CE)-based Li⁺ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly­(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g^–1 MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE–GA–PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, ¹³C cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N₂ adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li⁺ adsorption. At their saturated states, the MFs were able to use 90–99% CEs for 1:1 Li⁺ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Li⁺. The MFs were highly Li⁺-selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na⁺ and K⁺, whereas the dibenzo CE was best in discriminating divalents Mg²⁺ and Ca²⁺. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their “size-match” Li⁺ selectivity