Ultrathin High-Entropy Fe-Based Spinel Oxide Nanosheets with Metalloid Band Structures for Efficient Nitrate Reduction toward Ammonia.

Spinel oxides with tunable chemical compositions have emerged as versatile electrocatalysts, however their performance is greatly limited by small surface area and low electron conductivity. Here, ultrathin high-entropy Fe-based spinel oxides nanosheets are rationally designed (i.e., (Co0.2Ni0.2Zn0.2Mg0.2Cu0.2)Fe2O4; denotes A5Fe2O4) in thickness of ≈4.3 nm with large surface area and highly exposed active sites via a modified sol-gel method. Theoretic and experimental results confirm that the bandgap of A5Fe2O4 nanosheets is significantly smaller than that of ordinary Fe-based spinel oxides, realizing the transformation of binary spinel oxide from semiconductors to metalloids. As a result, such A5Fe2O4 nanosheets manifest excellent performance for the nitrate reduction reaction (NO3 -RR) to ammonia (NH3), with a NH3 yield rate of ≈2.1 mmol h-1 cm-2 at -0.5 V versus Reversible hydrogen electrode, outperforming other spinel-based electrocatalysts. Systematic mechanism investigations reveal that the NO3 -RR is mainly occurred on Fe sites, and introducing high-entropy compositions in tetrahedral sites regulates the adsorption strength of N and O-related intermediates on Fe for boosting the NO3 -RR. The above findings offer a high-entropy platform to regulate the bandgap and enhance the electrocatalytic performance of spinel oxides.

Branched DNA-Based Electrochemical Biosensor for Sensitive Nucleic Acids Analysis with Gold Nanoparticles as Amplifier.

A branched DNA-based electrochemical biosensor was designed to sensitively detect specific nucleic acids. On this platform, novel a branched DNA with three sticky ends could be used as a biosensor to sensitively and specifically detect nucleic acids. Meanwhile, we also employed branched DNA-modified AuNPs as a signal amplifier to further improve the sensitivity. Branched DNA sensors, target DNA, and DNA-modified AuNPs formed a sandwich structure to produce an electronic signal for target DNA detection. The reaction primarily involved DNA hybridization without bulky thermal cyclers and enzymes. We proved that the hybridization reaction easily occurred under different conditions, such as the NaCl concentration, reaction time, pH, and temperature, except for a pH lower than 4. The limit of detection could go as low as 0.09 pM (S/N = 3) with excellent specificity and selectivity. There was a correlation curve relationship between the peak current and the logarithm of the target DNA concentration (0.10 pM to 10 nM). The correlation coefficient reached 0.987. The electrochemical platform enables a branched DNA nanostructure to determine nucleic acids for disease diagnosis.

Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation.

Atomically dispersed metals on metal oxide supports are a rapidly growing class of catalysts. Developing an understanding of where and how the metals are bonded to the supports is challenging because support surfaces are heterogeneous, and most reports lack a detailed consideration of these points. Herein, we report two atomically dispersed CO oxidation catalysts having markedly different metal-support interactions: platinum in the first layer of crystalline MgO powder and platinum in the second layer of this support. Structural models have been determined on the basis of data and computations, including those determined by extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies, infrared spectroscopy of adsorbed CO, and scanning transmission electron microscopy. The data demonstrate the transformation of surface to subsurface platinum as the temperature of sample calcination increased. Catalyst performance data demonstrate the lower activity but greater stability of the subsurface platinum than of the surface platinum.

A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts.

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically dispersed platinum supported on crystalline MgO (chosen for its well-defined bonding sites) as a prototypical example, we demonstrate how systematic density functional theory calculations for assessing all the potentially stable platinum sites, combined with automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, leads to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging and EXAFS and high-energy resolution fluorescence detection X-ray absorption near edge spectroscopy. The proposed platinum sites are in agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active site than what is currently possible with conventional EXAFS analyses. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

Dimensionality Control of Electrocatalytic Activity in Perovskite Nickelates.

The dimensionality of the crystal structure plays an important role in the electronic structures of materials. Ruddlesden-Popper perovskite oxides offer an attractive platform for studying this role due to dimensional flexibility. The effects of dimensionality on physical properties in those oxides have been widely reported. However, the study of dimensional dependence on the chemical properties is still lacking. Here, we synthesized a series of Ruddlesden-Popper perovskite nickelates LanSrNinO3n+1 (n = 1, 2, 3, and ∞) to explore the role of dimensionality on oxygen-evolution reaction (OER) performance. As the dimensionality increased with n, the nickelates exhibited an enhanced OER activity. We found that the weakening of electron correlations among Ni 3d electrons by increasing the dimensionality induced an insulator-to-metal transition and a strengthened Ni-O hybridization, both of which accelerated the OER kinetics. This work sets up a bridge between the dimensionality and electrocatalysis, which provides guidance for designing highly efficient oxygen-evolving catalysts.

Intercalated Iridium Diselenide Electrocatalysts for Efficient pH-Universal Water Splitting.

Developing bifunctional catalysts for both hydrogen and oxygen evolution reactions is a promising approach to the practical implementation of electrocatalytic water splitting. However, most of the reported bifunctional catalysts are only applicable to alkaline electrolyzer, although a few are effective in acidic or neutral media that appeals more to industrial applications. Here, a lithium-intercalated iridium diselenide (Li-IrSe2 ) is developed that outperformed other reported catalysts toward overall water splitting in both acidic and neutral environments. Li intercalation activated the inert pristine IrSe2 via bringing high porosities and abundant Se vacancies for efficient hydrogen and oxygen evolution reactions. When Li-IrSe2 was assembled into two-electrode electrolyzers for overall water splitting, the cell voltages at 10 mA cm-2 were 1.44 and 1.50 V under pH 0 and 7, respectively, being record-low values in both conditions.

Electron Correlations Engineer Catalytic Activity of Pyrochlore Iridates for Acidic Water Oxidation.

The development of highly efficient oxygen-evolving catalysts compatible with powerful proton-exchange-membrane-based electrolyzers in acid environments is of prime importance for sustainable hydrogen production. In this field, understanding the role of electronic structure of catalysts on catalytic activity is essential but still lacking. Herein, a family of pyrochlore oxides R2 Ir2 O7 (R = rare earth ions) is reported as acidic oxygen-evolving catalysts with superior-specific activities. More importantly, it is found that the intrinsic activity of this material significantly increases with the R ionic radius. Electronic structure studies reveal that the increased R ionic radius weakens electron correlations in these iridate oxides. This weakening induces an insulator-metal transition and an enhancement of IrO bond covalency, both of which promote oxygen evolution kinetics. This work demonstrates the importance of engineering the electron correlations to rationalize the catalytic activity toward water oxidation in strongly correlated transition-metal oxides.