Data analysis method to achieve sub-10 pm spatial resolution using extended X-ray absorption fine-structure spectroscopy

Tailoring Oxygen Reduction Reaction Pathway on Spinel Oxides via Surficial Geometrical-Site Occupation Modification Driven by the Oxygen Evolution Reaction

The oxygen reduction reaction (ORR) has been demonstrated as a critical technology for both energy conversion technologies and hydrogen peroxide intermediate production. Herein, an in situ oxygen evolution reaction (OER) surface evolution strategy is applied for changing the surface structure of MnCo₂ O₄ oxide with tetrahedral and octahedral cations vacancies to realize reaction pathway switching from 2e^- ORR and 4e^- ORR. Interestingly, the as-synthesized MnCo₂ O₄ -pristine (MnCo₂ O₄ -P) with the highest surficial Mn/Co octahedron occupation favors two electrons reaction routes exhibiting high H₂ O₂ selectivity (≈80% and reaches nearly 100% at 0.75 V vs RHE); after surface atoms reconstruction, MnCo₂ O₄ -activation (MnCo₂ O₄ -A) with the largest Mn/Co tetrahedron occupation present excellent ORR performance through the four-electron pathway with an ultrahigh onset potential and half-wave potential of 0.78 and 0.92 V, ideal mass activity (MA), and turnover frequencies (TOF) values. Density functional theory (DFT) calculations reveal the concurrent modulations of both Co and Mn by the surface reconstructions, which improve the electroactivity of MnCo₂ O₄ -A toward the 4e^- pathway. This work provides a new perspective to building correlation of OER activation-ORR property, bringing detailed understating for reaction route transformation, and thus guiding the development of certain electrocatalysts with specific purposes.

Bridging Structural Inhomogeneity to Functionality: Pair Distribution Function Methods for Functional Materials Development

The correlation between structure and function lies at the heart of materials science and engineering. Especially, modern functional materials usually contain inhomogeneities at an atomic level, endowing them with interesting properties regarding electrons, phonons, and magnetic moments. Over the past few decades, many of the key developments in functional materials have been driven by the rapid advances in short-range crystallographic techniques. Among them, pair distribution function (PDF) technique, capable of utilizing the entire Bragg and diffuse scattering signals, stands out as a powerful tool for detecting local structure away from average. With the advent of synchrotron X-rays, spallation neutrons, and advanced computing power, the PDF can quantitatively encode a local structure and in turn guide atomic-scale engineering in the functional materials. Here, the PDF investigations in a range of functional materials are reviewed, including ferroelectrics/thermoelectrics, colossal magnetoresistance (CMR) magnets, high-temperature superconductors (HTSC), quantum dots (QDs), nano-catalysts, and energy storage materials, where the links between functions and structural inhomogeneities are prominent. For each application, a brief description of the structure-function coupling will be given, followed by selected cases of PDF investigations. Before that, an overview of the theory, methodology, and unique power of the PDF method will be also presented.

Interaction of Copper Phthalocyanine with Nitrogen Dioxide and Ammonia Investigation Using X-ray Absorption Spectroscopy and Chemiresistive Gas Measurements

The interaction site of phthalocyanine (Pc) with nitrogen dioxide (NO₂) has been characterized using different methods and found to be conflicting. By knowing the interaction site, the Pc molecule can be better customized to improve the gas sensitivity. In this article, the interaction sites of copper phthalocyanine (CuPc) with oxidizing NO₂ or with reducing gas (ammonia, NH₃) were identified using in situ X-ray absorption spectroscopy (XAS). The sensitivity of CuPc to sub-ppm levels of the tested gases was established in the CuPc chemoresistive gas sensors. The analyte-sensor interaction sites were identified and validated by monitoring the Cu K-edge XAS before and during gas exposure. From the X-ray absorption near-edge structure and its first derivative, a low or lack of axial coordination on the Cu metal center of CuPc is evident. Using the extended X-ray absorption fine structure with molecular orbital information of the involved molecules, the macrocycle interaction between CuPc and NO₂ or NH₃ was proposed to be the dominant sensing mechanism on CuPc sensors.

Superexchange Effects on Oxygen Reduction Activity of Edge-Sharing [Cox Mn1-x O6 ] Octahedra in Spinel Oxide

Mn-Co containing spinel oxides are promising, low-cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn-Co spinels or to strongly coupled hybrids (e.g., MnCo₂ O₄ /N-doped-rmGO) to maximize the mass efficiency. The lack of analyses by metal oxide intrinsic activity (activity normalized to catalysts' surface area) hinders the development of fundamental understanding of the physicochemical principles behind the catalytic activities. A systematic study on the composition dependence of ORR in ZnCo_x Mn_2-x O₄ (x = 0.0-2.0) spinel is presented here with special attention to the role of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the spinel structure. The ORR specific activity of ZnCo_x Mn_2-x O₄ spans across a potential window of 200 mV, indicating an activity difference of ≈3 orders of magnitude. The curve of composition-dependent ORR specific activity as a function of Co substitution exhibits a volcano shape with an optimum Mn/Co ratio of 0.43. It is revealed that the modulated e_g occupancy of active Mn cations (0.3-0.9), as a consequence of the superexchange effect between edge sharing [CoO₆ ] and [MnO₆ ], reflects the ORR activity of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the ZnCo_x Mn_2-x O₄ spinel oxide. These findings offer crucial insights in designing spinel oxide catalysts with fine-tuned e_g occupancy for efficient catalysis.