Spatial engineering of single-atom Fe adjacent to Cu-assisted nanozymes for biomimetic O2 activation.

The precise design of single-atom nanozymes (SAzymes) and understanding of their biocatalytic mechanisms hold great promise for developing ideal bio-enzyme substitutes. While considerable efforts have been directed towards mimicking partial bio-inspired structures, the integration of heterogeneous SAzymes configurations and homogeneous enzyme-like mechanism remains an enormous challenge. Here, we show a spatial engineering strategy to fabricate dual-sites SAzymes with atomic Fe active center and adjacent Cu sites. Compared to planar Fe-Cu dual-atomic sites, vertically stacked Fe-Cu geometry in FePc@2D-Cu-N-C possesses highly optimized scaffolds, favorable substrate affinity, and fast electron transfer. These characteristics of FePc@2D-Cu-N-C SAzyme induces biomimetic O2 activation through homogenous enzymatic pathway, resembling functional and mechanistic similarity to natural cytochrome c oxidase. Furthermore, it presents an appealing alternative of cytochrome P450 3A4 for drug metabolism and drug-drug interaction. These findings are expected to deepen the fundamental understanding of atomic-level design in next-generation bio-inspired nanozymes.

Electrocatalytic Activation in ReSe2-VSe2 Alloy Nanosheets to Boost Water-Splitting Hydrogen Evolution Reaction.

It is challenging to control the electronic structure of 2D transition metal dichalcogenides (TMD) for extended applications in renewable energy devices. Here, ReSe2-VSe2 (Re1- xVxSe2) alloy nanosheets over the whole composition range via a colloidal reaction is synthesized. Increasing x makes the nanosheets more metallic and induces a 1T″-to-1T phase transition at x = 0.5-0.6. Compared to the MoSe2-VSe2 and WSe2-VSe2 alloy nanosheets, ReSe2 and VSe2 are mixed more homogeneously at the atomic scale. The alloy nanosheets at x = 0.1-0.7 exhibit an enhanced electrocatalytic activity toward acidic hydrogen evolution reaction (HER). In situ X-ray absorption fine structure measurements reveal that alloying caused the Re and V atoms to be synergically more active in the HER. Gibbs free energy (ΔGH*) and density of state calculations confirm that alloying and Se vacancies effectively activate the metal sites toward HER. The composition dependence of HER performance is explained by homogenous atomic mixing with the increased Se vacancies. The study provides a strategy for designing new TMD alloy nanosheets with enhanced catalytic activity.

2H-2M Phase Control of WSe2 Nanosheets by Se Enrichment Toward Enhanced Electrocatalytic Hydrogen Evolution Reaction.

The phase control of transition metal dichalcogenides (TMDs) is an intriguing approach for tuning the electronic structure toward extensive applications. In this study, WSe2 nanosheets synthesized via a colloidal reaction exhibit a phase conversion from semiconducting 2H to metallic 2M under Se-rich growth conditions (i.e., increasing the concentration of Se precursor or lowering the growth temperature). High-resolution scanning transmission electron microscopy images are used to identify the stacking sequence of the 2M phase, which is distinctive from that of the 1T' phase. First-principles calculations employing various Se-rich models (intercalation and substitution) indicated that Se enrichment induces conversion to the 2M phase. The 2M phase WSe2 nanosheets with the Se excess exhibited enhanced electrocatalytic performance in the hydrogen evolution reaction (HER). In situ X-ray absorption fine structure studies suggested that the excess Se atoms in the 2M phase WSe2 enhanced the HER catalytic activity, which is supported by the Gibbs free energy (ΔGH* ) of H adsorption and the Fermi abundance function. These results provide an appealing strategy for phase control of TMD catalysts.

Orthogonal Dual Photocatalysis of Single Atoms on Carbon Nitrides for One-Pot Relay Organic Transformation.

Single-atom photocatalysis has shown potential in various single-step organic transformations, but its use in multistep organic transformations in one reaction systems has rarely been achieved. Herein, we demonstrate atomic site orthogonality in the M1/C3N4 system (where M = Pd or Ni), enabling a cascade photoredox reaction involving oxidative and reductive reactions in a single system. The system utilizes visible-light-generated holes and electrons from C3N4, driving redox reactions (e.g., oxidation and fluorination) at the surface of C3N4 and facilitating cross-coupling reactions (e.g., C-C and C-O bond formation) at the metal site. The concept is generalized to different systems of Pd and Ni, thus making the catalytic site-orthogonal M1/C3N4 system an ideal photocatalyst for improving the efficiency and selectivity of multistep organic transformations.

Unlocking Cu(I)-Mediated Catalytic Pathways for Efficient ROS Generation by Incorporating an Oxazole-Based Histidine Surrogate into Cu(II)-ATCUN Complexes.

The catalytic redox activity of Cu(II) bound to the amino-terminal copper and nickel (ATCUN) binding motif (Xxx-Zzz-His, XZH) is stimulating the development of catalytic metallodrugs based on reactive oxygen species (ROS)-mediated biomolecule oxidation. However, low Cu(I) availability resulting from the strong Cu(II) binding affinity of the ATCUN motif is regarded as a limitation to efficient ROS generation. To address this, we replaced the imidazole moiety (pKa 7.0) of Gly-Gly-His-NH2 (GGHa, a canonical ATCUN peptide) with thiazole (pKa 2.7) and oxazole (pKa 0.8), yielding GGThia and GGOxa, respectively. A newly synthesized amino acid, Fmoc-3-(4-oxazolyl)-l-alanine, served as a histidine surrogate featuring an azole ring with the lowest pKa among known analogues. Despite similar square-planar Cu(II)-N4 geometries being observed for the three Cu(II)-ATCUN complexes by electron paramagnetic resonance spectroscopy and X-ray crystallography, the azole modification enabled the Cu(II)-ATCUN complexes to exhibit significant rate enhancement for ROS-mediated DNA cleavage. Further analyses based on Cu(I)/Cu(II) binding affinities, electrochemical measurements, density functional theory calculations, and X-ray absorption spectroscopy indicated that the azole modification enhanced the accessibility of the Cu(I) oxidation state during ROS generation. Our oxazole/thiazole-containing ATCUN motifs provide a new design strategy for peptide ligands with modulated N donor ability, with potential applications in the development of ROS-mediated metallodrugs.

Importance of broken geometric symmetry of single-atom Pt sites for efficient electrocatalysis.

Platinum single-atom catalysts hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated PtII species on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous PtII catalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of PtII moieties on the single-atom catalysts beyond ideally four-coordinated PtII-N4. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated PtII species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on other d8 metal ions.

Chemical Influence of Carbon Interface Layers in Metal/Oxide Resistive Switches.

Thin layers introduced between a metal electrode and a solid electrolyte can significantly alter the transport of mass and charge at the interfaces and influence the rate of electrode reactions. C films embedded in functional materials can change the chemical properties of the host, thereby altering the functionality of the whole device. Using X-ray spectroscopies, here we demonstrate that the chemical and electronic structures in a representative redox-based resistive switching (RS) system, Ta2O5/Ta, can be tuned by inserting a graphene or ultrathin amorphous C layer. The results of the orbitalwise analyses of synchrotron Ta L3-edge, C K-edge, and O K-edge X-ray absorption spectroscopy showed that the C layers between Ta2O5 and Ta are significantly oxidized to form COx and, at the same time, oxidize the Ta layers with different degrees of oxidation depending on the distance: full oxidation at the nearest 5 nm Ta and partial oxidation in the next 15 nm Ta. The depth-resolved information on the electronic structure for each layer further revealed a significant modification of the band alignments due to C insertion. Full oxidation of the Ta metal near the C interlayer suggests that the oxygen-vacancy-related valence change memory mechanism for the RS can be suppressed, thereby changing the RS functionalities fundamentally. The knowledge on the origin of C-enhanced surfaces can be applied to other metal/oxide interfaces and used for the advanced design of memristive devices.

Na2ZrFe(PO4)3─A Rhombohedral NASICON-Structured Material: Synthesis, Structure and Na-Intercalation Behavior.

A NASICON-structured earth-abundant mixed transition metal (TM) containing Na-TM-phosphate, viz., Na2ZrFe(PO4)3, has been prepared via a sol-gel route using a low-cost Fe3+-based precursor. The as-prepared material crystallizes in the desired rhombohedral NASICON structure (space group: R3̅c) at room temperature. Synchrotron X-ray diffraction (XRD), transmission electron microscopy, X-ray absorption spectroscopy, etc., have been performed to determine the crystal structure, associated details, composition, and electronic structures. In light of the structural features, as one of the possible functionalities of Na2FeZr(PO4)3, Na-intercalation/deintercalation has been examined, which indicates the occurrence of reversible electrochemical Na-insertion/extraction via Fe2+/Fe3+ redox at an average potential of ∼2.5 V. The electrochemical data and direct evidences from operando synchrotron XRD indicate that the rhombohedral structure is preserved during Na-insertion/extraction, albeit within a certain range of Na-content (i.e., ∼2-3 p.f.u.), beyond which rhombohedral → monoclinic transformation takes place. Within this range, Na-insertion/extraction takes place via solid-solution pathway, resulting in outstanding cyclic stability, higher Na-diffusivity, and good rate-capability. To the best of the authors' knowledge, this represents the first in-depth structural, compositional, and electrochemical studies with Na2ZrFe(PO4)3, along with the interplay between those, which provide insights into the design of similar low-cost materials for various applications, including sustainable electrochemical energy storage systems.

Composition-Tuned (MoWV)Se2 Ternary Alloy Nanosheets as Excellent Hydrogen Evolution Reaction Electrocatalysts.

Ternary alloying of transition metal dichalcogenides (TMDs) has the potential for altering the electronic structure of materials to suit electrochemical applications. Herein, we synthesized (MoWV)Se2 nanosheets at various compositions via a colloidal reaction. The mole fraction of V atoms (xV) was successfully increased up to 0.8, producing a metallic phase that is highly durable against hydration. Furthermore, we synthesized (MoW)Se2 nanosheets over the entire composition range. The atomic mixing of the ternary alloys is more random than that of the constitutional binary alloys, as supported by first-principles calculations. Compared to binary alloying, ternary alloying more effectively enhanced the electrocatalytic activity for acidic hydrogen evolution reaction (HER). The HER performance increased upon increasing xV to 0.44, and thereafter, it declined at higher xV primarily owing to surface oxidation. The analysis of Gibbs free energy for H adsorption revealed that ternary alloying strongly activates the basal plane for the HER. VSe2 contains numerous sites favorable for H adsorption, facilitating the composition-dependent HER. These results provide a pioneering strategy for designing multicomponent TMD catalysts that maximize the advantages of each component.

Selective dealloying of chemically disordered Pt-Ni bimetallic nanoparticles for the oxygen reduction reaction.

Changes in electronic and compositional structures of Pt-Ni electrocatalysts with 44% of Ni fraction with repeated chemical dealloying have been studied. By comparing the Pt-enriched surfaces formed using hydroquinone and sulfuric acid as a leaching agent, we found that hydroquinone generated Pt-enriched surfaces exhibit the highest oxygen reduction reaction (ORR) activity after repeating the treatment twice. In particular, it was found that while sulfuric acid causes an uncontrollable dissolution of Ni clusters, the unique selectivity of hydroquinone allows the preferential dissolution of Ni atoms alloyed with Pt. Despite its wide usage in the field, the results show that traditional acid leaching is unsuitable for Pt-Ni alloys with a high Ni content and an incomplete alloying level. We finally proved that the unique and lasting selectivity of hydroquinone enables an incompletely alloyed Pt-Ni catalyst to obtain a highly ORR active Pt shell region without an extensive loss of Ni.

Tuning the C1 /C2 Selectivity of Electrochemical CO2 Reduction on Cu-CeO2 Nanorods by Oxidation State Control.

Ceria (CeO2 ) is one of the most extensively used rare earth oxides. Recently, it has been used as a support material for metal catalysts for electrochemical energy conversion. However, to date, the nature of metal/CeO2 interfaces and their impact on electrochemical processes remains unclear. Here, a Cu-CeO2 nanorod electrochemical CO2 reduction catalyst is presented. Using operando analysis and computational techniques, it is found that, on the application of a reductive electrochemical potential, Cu undergoes an abrupt change in solubility in the ceria matrix converting from less stable randomly dissolved single atomic Cu2+ ions to (Cu0 ,Cu1+ ) nanoclusters. Unlike single atomic Cu, which produces C1 products as the main product during electrochemical CO2 reduction, the coexistence of (Cu0 ,Cu1+ ) clusters lowers the energy barrier for C-C coupling and enables the selective production of C2+ hydrocarbons. As a result, the coexistence of (Cu0 ,Cu1+ ) in the clusters at the Cu-ceria interface results in a C2+ partial current density/unit Cu weight 27 times that of a corresponding Cu-carbon catalyst under the same conditions.

Ni single atoms on carbon nitride for visible-light-promoted full heterogeneous dual catalysis.

Visible-light-driven organic transformations are of great interest in synthesizing valuable fine chemicals under mild conditions. The merger of heterogeneous photocatalysts and transition metal catalysts has recently drawn much attention due to its versatility for organic transformations. However, these semi-heterogenous systems suffered several drawbacks, such as transition metal agglomeration on the heterogeneous surface, hindering further applications. Here, we introduce heterogeneous single Ni atoms supported on carbon nitride (NiSAC/CN) for visible-light-driven C-N functionalization with a broad substrate scope. Compared to a semi-heterogeneous system, high activity and stability were observed due to metal-support interactions. Furthermore, through systematic experimental mechanistic studies, we demonstrate that the stabilized single Ni atoms on CN effectively change their redox states, leading to a complete photoredox cycle for C-N coupling.

A Versatile Strategy for Achieving Fast-Charging Batteries via Interfacial Engineering: Pseudocapacitive Potassium Storage without Nanostructuring.

The rapid transport of alkali ions in electrodes is a long-time dream for fast-charging batteries. Though electrode nanostructuring has increased the rate-capability, its practical use is limited because of the low tap density and severe irreversible reactions. Therefore, development of a strategy to design fast-charging micron-sized electrodes without nanostructuring is of significant importance. Herein, a simple and versatile strategy to accelerate the alkali ion diffusion behavior in micron-sized electrode is reported. It is demonstrated that the diffusion rate of K+ ions is significantly improved at the hetero-interface between orthorhombic Nb2 O5 (001) and monoclinic MoO2 (110) planes. Lattice distortion at the hetero-interface generates an inner space large enough for the facile transport of K+ ions, and electron localization near oxygen-vacant sites further enhances the ion diffusion behavior. As a result, the interfacial-engineered micron-sized anode material achieves an outstanding rate capability in potassium-ion batteries (KIBs), even higher than nanostructured orthorhombic Nb2 O5  which is famous for fast-charging electrodes. This is the first study to develop an intercalation pseudocapacitive micron-sized anode without nanostructuring for fast-charging and high volumetric energy density KIBs. More interestingly, this strategy is not limited to K+ ion, but also applicable to Li+ ion, implying the versatility of interfacial engineering for alkali ion batteries.

Unraveling the Simultaneous Enhancement of Selectivity and Durability on Single-Crystalline Gold Particles for Electrochemical CO2 Reduction.

Electrochemical carbon dioxide reduction is a mild and eco-friendly approach for CO2 mitigation and producing value-added products. For selective electrochemical CO2 reduction, single-crystalline Au particles (octahedron, truncated-octahedron, and sphere) are synthesized by consecutive growth and chemical etching using a polydiallyldimethylammonium chloride (polyDDA) surfactant, and are surface-functionalized. Monodisperse, single-crystalline Au nanoparticles provide an ideal platform for evaluating the Au surface as a CO2 reduction catalyst. The polyDDA-Au cathode affords high catalytic activity for CO production, with >90% Faradaic efficiency over a wide potential range between -0.4 and -1.0 V versus RHE, along with high durability owing to the consecutive interaction between dimethylammonium and chloride on the Au surface. The influence of polyDDA on the Au particles, and the origins of the enhanced selectivity and stability are fully investigated using theoretical studies. Chemically adsorbed polyDDA is consecutively affected the initial adsorption of CO2 and the stability of the *CO2 , *COOH, and *CO intermediates during continuous CO2 reduction reaction. The polyDDA functionalization is extended to improving the CO Faradaic efficiency of other metal catalysts such as Ag and Zn, indicating its broad applicability for CO2 reduction.

Unravelling the Nature of the Intrinsic Complex Structure of Binary-Phase Na-Layered Oxides.

The layered sodium transition metal oxide, NaTMO2 (TM = transition metal), with a binary or ternary phases has displayed outstanding electrochemical performance as a new class of strategy cathode materials for sodium-ion batteries (SIBs). Herein, an in-depth phase analysis of developed Na1-x TMO2 cathode materials, Na0.76 Ni0.20 Fe0.40 Mn0.40 O2 with P2- and O3-type phases (NFMO-P2/O3) is offered. Structural visualization on an atomic scale is also provided and the following findings are unveiled: i) the existence of a mixed-phase intergrowth layer distribution and unequal distribution of P2 and O3 phases along two different crystal plane indices and ii) a complete reversible charge/discharge process for the initial two cycles that displays a simple phase transformation, which is unprecedented. Moreover, first-principles calculations support the evidence of the formation of a binary NFMO-P2/O3 compound, over the proposed hypothetical monophasic structures (O3, P3, O'3, and P2 phases). As a result, the synergetic effect of the simultaneous existence of P- and O-type phases with their unique structures allows an extraordinary level of capacity retention in a wide range of voltage (1.5-4.5 V). It is believed that the insightful understanding of the proposed materials can introduce new perspectives for the development of high-voltage cathode materials for SIBs.

Harnessing Strong Metal-Support Interaction to Proliferate the Dry Reforming of Methane Performance by In Situ Reduction.

The strong bonding at the interface between the metal and the support, which can inhibit the undesirable aggregation of metal nanoparticles and carbon deposition from reforming of hydrocarbon, is well known as the classical strong metal-support interaction (SMSI). SMSI of nanocatalysts was significantly affected by heat treatment and reducing conditions during catalyst preparation.the heat treatment and reduction conditions during catalyst preparation. SMSI can be weakened by the decrement of metal-doped sites in the supporting oxide and can often deactivate catalysts by the encapsulation of active sites through these processes. To retain SMSI near the active sites and to enhance the catalytic activity of the nanocatalyst, it is essential to increase the number of surficial metal-doped sites between nanometal and the support. Herein, we propose a mild reduction process using dry methane (CH4/CO2) gas that suppresses the aggregation of nanoparticles and increases the exposed interface between the metal and support, Ni and cerium oxide. The effects of mild reduction on the chemical state of Ni-cerium oxide nanocatalysts were specifically investigated in this study. As a result, mild reduction led to form large amounts of the Ni3+ phase at the catalyst surface of which SMSI was significantly enhanced. It can be easily fabricated while the dry reforming of methane (DRM) reaction is on stream. The superior performance of the catalyst achieved a considerably high CH4 conversion rate of approximately 60% and stable operation up to 550 h at a low temperature, 600 °C.

Interspersing CeOx Clusters to the Pt-TiO2 Interfaces for Catalytic Promotion of TiO2-Supported Pt Nanoparticles.

We propose an interface-engineered oxide-supported Pt nanoparticle-based catalyst with improved low-temperature activity toward CO oxidation. By wet-impregnating 1 wt % Ce on TiO2, we synthesized hybrid oxide support of CeOx-TiO2, in which dense CeOx clusters formed on the surface of TiO2. Then, the Pt/CeOx-TiO2 catalyst was synthesized by impregnating 2 wt % Pt on the CeOx-TiO2 supporting oxide. Pt-CeOx-TiO2 triphase interfaces were eventually formed upon impregnation of Pt on CeOx-TiO2. The Pt-CeOx-TiO2 interfaces open up the interface-mediated Mars-van Krevelen CO oxidation pathway, thus providing additional interfacial reaction sites for CO oxidation. Consequently, the specific reaction rate of Pt/CeOx-TiO2 for CO oxidation was increased by 3.2 times compared with that of Pt/TiO2 at 140 °C. Our results demonstrate a widely applicable and straightforward method of catalytic activation of the interfaces between metal nanoparticles and supporting oxides, which enabled fine-tuning of the catalytic performance of oxide-supported metal nanoparticle classes of heterogeneous catalysts.

Structural Insights into Multi-Metal Spinel Oxide Nanoparticles for Boosting Oxygen Reduction Electrocatalysis.

Multi-metal oxide (MMO) materials have significant potential to facilitate various demanding reactions by providing additional degrees of freedom in catalyst design. However, a fundamental understanding of the (electro)catalytic activity of MMOs is limited because of the intrinsic complexity of their multi-element nature. Additional complexities arise when MMO catalysts have crystalline structures with two different metal site occupancies, such as the spinel structure, which makes it more challenging to investigate the origin of the (electro)catalytic activity of MMOs. Here, uniform-sized multi-metal spinel oxide nanoparticles composed of Mn, Co, and Fe as model MMO electrocatalysts are synthesized and the contributions of each element to the structural flexibility of the spinel oxides are systematically studied, which boosts the electrocatalytic oxygen reduction reaction (ORR) activity. Detailed crystal and electronic structure characterizations combined with electrochemical and computational studies reveal that the incorporation of Co not only increases the preferential octahedral site occupancy, but also modifies the electronic state of the ORR-active Mn site to enhance the intrinsic ORR activity. As a result, nanoparticles of the optimized catalyst, Co0.25 Mn0.75 Fe2.0 -MMO, exhibit a half-wave potential of 0.904 V (versus RHE) and mass activity of 46.9 A goxide -1 (at 0.9 V versus RHE) with promising stability.

Flash Bottom-Up Arc Synthesis of Nanocarbons as a Universal Route for Fabricating Single-Atom Electrocatalysts.

Despite considerable development in the field of single-atom catalysts (SACs) on carbon-based materials, the reported strategies for synthesizing SACs generally rely on top-down approaches, which hinder achieving both simple and universal synthesis routes that are simultaneously applicable to various metals and nanocarbons. Here, a universal strategy for fabricating nanocarbon based-SACs using a flash bottom-up arc discharge method to mitigate these issues is reported. The ionization of elements and their recombination process during arc discharge allows the simultaneous incorporation of single metal atoms (Mn, Fe, Co, Ni, and Pt) into the crystalline carbon lattice during the formation of carbon nanohorns (CNHs) and N-doped arc graphene. The coordination environment around the Co atoms of Co1 /CNH can be modulated by a mild post-treatment with NH3 . As a result, Co1 /CNH exhibits good oxygen reduction reaction activity, showing a 1.92 times higher kinetic current density value than the commercial Pt/C catalyst in alkaline media. In a single cell experiment, Co1 /CNH exhibits the highest maximum power density of 472 mW cm-2 compared to previously reported nonprecious metal-based SACs.

Oxygen-Vacancy-Driven Orbital Reconstruction at the Surface of TiO2 Core-Shell Nanostructures.

Oxygen vacancies and their correlation with the electronic structure are crucial to understanding the functionality of TiO2 nanocrystals in material design applications. Here, we report spectroscopic investigations of the electronic structure of anatase TiO2 nanocrystals by employing hard and soft X-ray absorption spectroscopy measurements along with the corresponding model calculations. We show that the oxygen vacancies significantly transform the Ti local symmetry by modulating the covalency of titanium-oxygen bonds. Our results suggest that the altered Ti local symmetry is similar to the C3v, which implies that the Ti exists in two local symmetries (D2d and C3v) at the surface. The findings also indicate that the Ti distortion is a short-range order effect and presumably confined up to the second nearest neighbors. Such distortions modulate the electronic structure and provide a promising approach to structural design of the TiO2 nanocrystals.