High catalytic activity and abundant active sites in M2C12 monolayer for nitrogen reduction reaction.

Developing highly efficient single-atom catalysts (SACs) for the nitrogen reduction reaction (NRR) to ammonia production has garnered significant attention in the scientific community. However, achieving high activity and selectivity remains challenging due to the lack of innate activity in most existing catalysts or insufficient active site density. This study delves into the potential of M2C12 materials (M = Cr, Ir, Mn, Mo, Os, Re, Rh, Ru, W, Fe, Cu, and Ti) with high transition metal coverage as SACs for NRR using first-principles calculations. Among these materials, Os2C12 exhibited superior catalytic activity for NRR, with a low overpotential of 0.39 V and an Os coverage of up to 72.53 wt%. To further boost its catalytic activity, a nonmetal (NM) atom doping (NM = B, N, O, and S) and C vacancy modification were explored in Os2C12. It is found that the introduction of O enables exceptional catalytic activity, selectivity, and stability, with an even lower overpotential of 0.07 V. Incorporating the O atom disrupted the charge balance of its coordinating C atoms, effectively increasing the positive charge density of the Os-d-orbit-related electronic structure. This promoted strong d-π* coupling between Os and N2H, enhancing N2H adsorption and facilitating NRR processes. This comprehensive study provides valuable insights into NRR catalyst design for sustainable ammonia production and offers a reference for exploring alternative materials in other catalytic reactions.

Large-Scale Synthesis of High-Loading Single Metallic Atom Catalysts by a Metal Coordination Route.

Single atom catalyst (SAC) is one of the most efficient and versatile catalysts with well-defined active sites. However, its facile and large-scale preparation, the prerequisite of industrial applications, has been very challenging. This dilemma originates from the Gibbs-Thomson effect, which renders it rather difficult to achieve high single atom loading (< 3 mol%). Further, most synthesizing procedures are quite complex, resulting in significant mass loss and thus low yields. Herein, a novel metal coordination route is developed to address these issues simultaneously, which is realized owing to the rapid complexation between ligands (e.g., biuret) and metal ions in aqueous solutions and subsequent in situ polymerization of the formed complexes to yield SACs. The whole preparation process involves only one heating step operated in air without any special protecting atmospheres, showing general applicability for diverse transition metals. Take Cu SAC for an example, a record yield of up to 3.565 kg in one pot and an ultrahigh metal loading 16.03 mol% on carbon nitride (Cu/CN) are approached. The as-prepared SACs are demonstrated to possess high activity, outstanding selectivity, and robust cyclicity for CO2 photoreduction to HCOOH. This research explores a robust route toward cost-effective, massive production of SACs for potential industrial applications.

Dynamic surface reconstruction of individual gold nanoclusters by using a co-reactant enables color-tunable electrochemiluminescence.

Here we report for the first time the phenomenon of continuously color-tunable electrochemiluminescence (ECL) from individual gold nanoclusters (Au NCs) confined in a porous hydrogel matrix by adjusting the concentration of the co-reactant. Specifically, the hydrogel-confined Au NCs exhibit strong dual-color ECL in an aqueous solution with triethylamine (TEA) as a co-reactant, with a record-breaking quantum yield of 95%. Unlike previously reported Au NCs, the ECL origin of the hydrogel-confined Au NCs is related to both the Au(0) kernel and the Au(i)-S surface. Surprisingly, the surface-related ECL of Au NCs exhibits a wide color-tunable range of 625-829 nm, but the core-related ECL remains constant at 489 nm. Theoretical and experimental studies demonstrate that the color-tunable ECL is caused by the dynamic surface reconstruction of Au NCs and TEA radicals. This work opens up new avenues for dynamically manipulating the ECL spectra of core-shell emitters in biosensing and imaging research.

Boosting CO2 piezo-reduction via metal-support interactions in Au/ZnO based catalysts.

Confronting the challenge of climate change necessitates innovative approaches for the reduction of CO2 emissions. Metal-support interaction has been widely demonstrated to enable greatly improved performances in thermal-catalytic, photocatalytic and electrocatalytic CO2 reduction. However, its applicability and specifically its role in the emerging piezo-electrocatalytic CO2 reduction are unknown, severely hampering the utilizations of piezo-electrocatalysis in CO2 conversion. Herein, by adopting Au particles supported on ZnO (Au/ZnO) as a paradigm, it is found that the metal-support interaction can remarkably improve the separation and transfer of piezo-carriers and enhance CO2 adsorption. As a result, Au/ZnO demonstrates a substantially boosted activity for piezo-electrocatalytic CO2 reduction and the optimal sample exhibits a 37.3% increase in CO yield compared to the pristine ZnO. The integration of metal-support interactions opens a new avenue to the design of advanced piezo-electrocatalysts for CO2 reduction.

Engineering Symmetry-Breaking Centers and d-Orbital Modulation in Triatomic Catalysts for Zinc-Air Batteries.

Unraveling the configuration-activity relationship and synergistic enhancement mechanism (such as real active center, electron spin-state, and d-orbital energy level) for triatomic catalysts, as well as their intrinsically bifunctional oxygen electrocatalysis, is a great challenge. Here we present a triatomic catalyst (TAC) with a trinuclear active structure that displays extraordinary oxygen electrocatalysis for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), greatly outperforming the counterpart of single-atom and diatomic catalysts. The aqueous Zn-air battery (ZAB) equipped with a TAC-based cathode exhibits extraordinary rechargeable stability and ultrarobust cycling performance (1970 h/3940 cycles at 2 mA cm-2, 125 h/250 cycles at 10 mA cm-2 with negligible voltage decay), and the quasi-solid-state ZAB displays outstanding rechargeability and low-temperature adaptability (300 h/1800 cycles at 2 mA cm-2 at -60 °C), outperforming other state-of-the-art ZABs. The experimental and theoretical analyses reveal the symmetry-breaking CoN4 configuration under incorporation of neighboring metal atoms (Fe and Cu), which leads to d-orbital modulation, a low-shift d band center, weakened binding strength to the oxygen intermediates, and decreased energy barrier for bifunctional oxygen electrocatalysis. This rational tricoordination design as well as an in-depth mechanism analysis indicate that hetero-TACs can be promisingly applied in various electrocatalysis applications.

Strongly Coupled Ag/Sn-SnO2 Nanosheets Toward CO2 Electroreduction to Pure HCOOH Solutions at Ampere-Level Current.

Electrocatalytic reduction of CO2 converts intermittent renewable electricity into value-added liquid products with an enticing prospect, but its practical application is hampered due to the lack of high-performance electrocatalysts. Herein, we elaborately design and develop strongly coupled nanosheets composed of Ag nanoparticles and Sn-SnO2 grains, designated as Ag/Sn-SnO2 nanosheets (NSs), which possess optimized electronic structure, high electrical conductivity, and more accessible sites. As a result, such a catalyst exhibits unprecedented catalytic performance toward CO2-to-formate conversion with near-unity faradaic efficiency (≥ 90%), ultrahigh partial current density (2,000 mA cm-2), and superior long-term stability (200 mA cm-2, 200 h), surpassing the reported catalysts of CO2 electroreduction to formate. Additionally, in situ attenuated total reflection-infrared spectra combined with theoretical calculations revealed that electron-enriched Sn sites on Ag/Sn-SnO2 NSs not only promote the formation of *OCHO and alleviate the energy barriers of *OCHO to *HCOOH, but also impede the desorption of H*. Notably, the Ag/Sn-SnO2 NSs as the cathode in a membrane electrode assembly with porous solid electrolyte layer reactor can continuously produce ~ 0.12 M pure HCOOH solution at 100 mA cm-2 over 200 h. This work may inspire further development of advanced electrocatalysts and innovative device systems for promoting practical application of producing liquid fuels from CO2.

Developing Ni single-atom sites in carbon nitride for efficient photocatalytic H2O2 production.

Photocatalytic two-electron oxygen reduction to produce high-value hydrogen peroxide (H2O2) is gaining popularity as a promising avenue of research. However, structural evolution mechanisms of catalytically active sites in the entire photosynthetic H2O2 system remains unclear and seriously hinders the development of highly-active and stable H2O2 photocatalysts. Herein, we report a high-loading Ni single-atom photocatalyst for efficient H2O2 synthesis in pure water, achieving an apparent quantum yield of 10.9% at 420 nm and a solar-to-chemical conversion efficiency of 0.82%. Importantly, using in situ synchrotron X-ray absorption spectroscopy and Raman spectroscopy we directly observe that initial Ni-N3 sites dynamically transform into high-valent O1-Ni-N2 sites after O2 adsorption and further evolve to form a key *OOH intermediate before finally forming HOO-Ni-N2. Theoretical calculations and experiments further reveal that the evolution of the active sites structure reduces the formation energy barrier of *OOH and suppresses the O=O bond dissociation, leading to improved H2O2 production activity and selectivity.

Single-Fe-Atom Catalyst for Sensitive Electrochemical Detection of Caffeic Acid.

A single-atom catalyst (Fe SAs/-N-C) with excellent stability and conductivity was strategically fabricated via high-temperature calcination using the NiFe layered double hydroxide (LDH)/ZIF-8 composite as precursors. With the help of Ni as a catalyst, a great number of carbon nanotubes were produced whereby the isolated carbon bulks were interconnected to form an "island-bridge"-like 3D network structure, which greatly enhanced the exposure of active sites and the electron transfer. Accordingly, caffeic acid (CA) with versatile biological and pharmacological activities was chosen as the model analyte. The Fe SAs/-N-C with Fe-N4 as the catalytic active site was employed to establish the electrochemical sensing of CA with satisfactory sensitivity, selectivity, and long-term stability. This work expands the application range of single-atom catalysts and contributes a significant reference for the synthesis of hybrid double-atom catalysts.

NiO Matrix Decorated by Ru Single Atoms: Electron-Rich Ru-Induced High Activity and Selectivity toward Electrochemical N2 Reduction.

Developing a single-atom catalyst with electron-rich active sites is a promising strategy for catalyzing the electrochemical N2 reduction reaction (NRR). Herein, we choose NiO(001) as a model template and deposit a series of single transition metal (TM) atoms with higher formal charges to create the electron-rich active centers. Our first-principles calculations show that low-valent Ru (+2) on NiO(001) can significantly activate N2, with its oxidation states varying from +2 to +4 throughout the catalytic cycle. The Ru/NiO(001) catalyst exhibits the best activity with a relatively low limiting potential of -0.49 V. Furthermore, under NRR operating conditions, the Ru site is primarily occupied by *N2 rather than *H, indicating that NRR overwhelms the hydrogen evolution reaction and thus exhibits excellent selectivity. Our work highlights the potential of designing catalysts with electron-rich active sites for NRR.

Band Position-Independent Piezo-Electrocatalysis for Ultrahigh CO2 Conversion.

Piezo-electrocatalysis as an emerging mechano-to-chemistry energy conversion technique opens multiple innovative opportunities and draws great interest over the past decade. However, the two potential mechanisms in piezo-electrocatalysis, i.e., screening charge effect and energy band theory, generally coexist in the most piezoelectrics, making the essential mechanism remain controversial. Here, for the first time, the two mechanisms in piezo-electrocatalytic CO2 reduction reaction (PECRR) is distinguished through a narrow-bandgap piezo-electrocatalyst strategy using MoS2 nanoflakes as demo. With conduction band of -0.12 eV, the MoS2 nanoflakes are unsatisfied for CO2 -to-CO redox potential of -0.53 eV, yet they achieve an ultrahigh CO yield of ≈543.1 µmol g-1  h-1 in PECRR. Potential band position shifts under vibration are still unsatisfied with CO2 -to-CO potential verified by theoretical investigation and piezo-photocatalytic experiment, further indicating that the mechanism of piezo-electrocatalysis is independent of band position. Besides, MoS2 nanoflakes exhibit unexpected intense "breathing" effect under vibration and enable the naked-eye-visible inhalation of CO2 gas, independently achieving the complete carbon cycle chain from CO2 capture to conversion. The CO2 inhalation and conversion processes in PECRR are revealed by a self-designed in situ reaction cell. This work brings new insights into the essential mechanism and surface reaction evolution of piezo-electrocatalysis.

A carbon allotrope with twisted Dirac cones induced by grain boundaries composed of pentagons and octagons.

The grain boundaries (GBs) composed of pentagons and octagons (558 GBs) have been demonstrated to induce attractive transport properties such as Van Hove singularity (VHS) and quasi-one-dimensional metallic wires. Here, we propose a monolayer carbon allotrope which is formed from the introduction of periodic 558 GBs to decorate intact graphene, termed as PHO-graphene. The calculated electronic properties indicate that PHO-graphene not only inherits the previously superior characteristics such as Van Hove singularity and quasi-one-dimensional metallic wires, but also possesses two twisted Dirac cones near the Fermi level. Further calculation finds that the Berry phase is quantized to ± π at the two Dirac points, which is consistent with the distribution of the corresponding Berry curvature. The parity argument uncovers that PHO-graphene hosts a nontrivial band topology and the edge states connecting the two Dirac points are clearly visible. Our findings not only provide a reliable avenue to realize the abundant and extraordinary properties of carbon allotropes, but also offer an attractive approach for designing all carbon-based devices.

The surface charge induced high activity of oxygen reduction reaction on the PdTe2 bilayer.

Developing transition metal dichalcogenides as electrocatalysts has attracted great interest due to their tunable electronic properties and good thermal stabilities. Herein, we propose a PdTe2 bilayer as a promising electrocatalyst candidate towards the oxygen reduction reaction (ORR), based on extensive investigation of the electronic properties of PdTe2 thin films as well as atomic-level reaction kinetics at explicit electrode potentials. We verify that under electrochemical reducing conditions, the electron emerging on the electrode surface is directly transferred to O2 adsorbed on the PdTe2 bilayer, which greatly reduces the dissociation barrier of O2, and thereby facilitates the ORR to proceed via a dissociative pathway. Moreover, the barriers of the electrochemical steps in this pathway are all found to be less than 0.1 eV at the ORR limiting potential, demonstrating fast ORR kinetics at ambient conditions. This unique mechanism offers excellent energy efficiency and four-electron selectivity for the PdTe2 bilayer, and it is identified as a promising candidate for fuel cell applications.

Engineering Support and Distribution of Palladium and Tin on MXene with Modulation of the d-Band Center for CO-resilient Methanol Oxidation.

The efficiency of direct methanol fuel cell (DMFC) is largely determined by the activity and durability of methanol oxidation reaction (MOR) catalysts. Herein, we present a CO-resilient MOR catalyst of palladium-tin nano-alloy anchored on Se-doped MXene (PdSn0.5 /Se-Ti3 C2 ) via a progressive one-step electrochemical deposition strategy. MOR mass activity resulting from Pd/Se-Ti3 C2 catalyst (1046.2 mA mg-1 ) is over 2-fold larger than that of Pd/Ti3 C2 , suggesting that the introduction of Se atoms on MXene might accelerate the reaction kinetics. PdSn0.5 /Se-Ti3 C2 with Se-doping progress of MXene and the cooperated Pd-Sn sites has a superior MOR mass activity (4762.8 mA mg-1 ), outperforming many other reported Pd-based catalysts. Both experimental results and theoretical calculation reveal that boosted electron interaction of metal crystals with Se-doped MXene and optimized distribution of Pd-Sn sites can modulate the d band center, reduce adsorption energies of CO* at Pd site and enhance OH* generation at Sn site, resulting in highly efficient removal of CO intermediates by reaction with neighboring OH species on adjacent Sn sites.

Dirac Fermions in the Boron Nitride Monolayer with a Tetragon.

Two-dimensional (2D) boron nitride (BN) is a promising candidate for aerospace materials due to its excellent mechanical and thermal stability properties. However, its unusually prominent band gap limits its application prospects. In this work, we report a gapless monolayer BN, t-BN, which has four anisotropic Dirac cones in the first Brillouin zone exactly at the Fermi level. To further confirm the semimetallic character, the nontrivial topological properties are proven through the topologically protected edge states and the invariant non-zero Z2. Additionally, the Young's modulus and Poisson ratio characterize the strong mechanical strength of t-BN. Our theoretical predictions provide more possibilities for exploring the Dirac cone in BN, which will enhance the 2D boron derivative materials.

Isotype Heterojunction-Boosted CO2 Photoreduction to CO.

Photocatalytic conversion of CO2 to high-value products plays a crucial role in the global pursuit of carbon-neutral economy. Junction photocatalysts, such as the isotype heterojunctions, offer an ideal paradigm to navigate the photocatalytic CO2 reduction reaction (CRR). Herein, we elucidate the behaviors of isotype heterojunctions toward photocatalytic CRR over a representative photocatalyst, g-C3N4. Impressively, the isotype heterojunctions possess a significantly higher efficiency for the spatial separation and transfer of photogenerated carriers than the single components. Along with the intrinsically outstanding stability, the isotype heterojunctions exhibit an exceptional and stable activity toward the CO2 photoreduction to CO. More importantly, by combining quantitative in situ technique with the first-principles modeling, we elucidate that the enhanced photoinduced charge dynamics promotes the production of key intermediates and thus the whole reaction kinetics.

Single-atom sites on perovskite chips for record-high sensitivity and quantification in SERS.

Surface enhanced Raman scattering (SERS) is a rapid and nondestructive technique that is capable of detecting and identifying chemical or biological compounds. Sensitive SERS quantification is vital for practical applications, particularly for portable detection of biomolecules such as amino acids and nucleotides. However, few approaches can achieve sensitive and quantitative Raman detection of these most fundamental components in biology. Herein, a noble-metal-free single-atom site on a chip strategy was applied to modify single tungsten atom oxide on a lead halide perovskite, which provides sensitive SERS quantification for various analytes, including rhodamine, tyrosine and cytosine. The single-atom site on a chip can enable quantitative linear SERS responses of rhodamine (10-6-1 mmol L-1), tyrosine (0.06-1 mmol L-1) and cytosine (0.2-45 mmol L-1), respectively, which all achieve record-high enhancement factors among plasmonic-free semiconductors. The experimental test and theoretical simulation both reveal that the enhanced mechanism can be ascribed to the controllable single-atom site, which can not only trap photoinduced electrons from the perovskite substrate but also enhance the highly efficient and quantitative charge transfer to analytes. Furthermore, the label-free strategy of single-atom sites on a chip can be applied in a portable Raman platform to obtain a sensitivity similar to that on a benchtop instrument, which can be readily extended to various biomolecules for low-cost, widely demanded and more precise point-of-care testing or in-vitro detection. Electronic Supplementary Material: Supplementary material is available for this article at 10.1007/s40843-022-1968-5 and is accessible for authorized users.

Ultrahigh Photocatalytic CO2 Reduction Efficiency and Selectivity Manipulation by Single-Tungsten-Atom Oxide at the Atomic Step of TiO2.

The photocatalytic CO2 reduction reaction is a sustainable route to the direct conversion of greenhouse gases into chemicals without additional energy consumption. Given the vast amount of greenhouse gas, numerous efforts have been devoted to developing inorganic photocatalysts, e.g., titanium dioxide (TiO2 ), due to their stability, low cost, and environmentally friendly properties. However, a more efficient TiO2 photocatalyst without noble metals is highly desirable for CO2 reduction, and it is both difficult and urgent to produce selectively valuable compounds. Here, a novel "single-atom site at the atomic step" strategy is developed by anchoring a single tungsten (W) atom site with oxygen-coordination at the intrinsic steps of classic TiO2 nanoparticles. The composition of active sites for CO2 reduction can be controlled by tuning the additional W5+ to form W5+ -O-Ti3+ sites, resulting in both significant CO2 reduction efficiency with 60.6 µmol g- 1 h- 1 and selectivity for methane (CH4 ) over carbon monoxide (CO), which exceeds those of pristine TiO2 by more than one order of magnitude. The mechanism relies on the accurate control of the single-atom sites at step with 22.8% coverage of surface sites and the subsequent excellent electron-hole separation along with the favorable adsorption-desorption of intermediates at the sites.

Dirac Fermions in Graphene with Stacking Fault Induced Periodic Line Defects.

The exploration of carbon phases with intact massless Dirac fermions in the presence of defects is critical for practical applications to nanoelectronics. Here, we identify by first-principles calculations that the Dirac cones can exist in graphene with stacking fault (SF) induced periodic line defects. These structures are width (n)-dependent to graphene nanoribbon and are thus termed as (SF)n-graphene. The electronic properties reveal that the semimetallic features with Dirac cones occur in (SF)n-graphene with n = 3m + 1, where m is a positive integer, and then lead to a quasi-one-dimensional conducting channel. Importantly, it is found that the twisted Dirac cone in the (SF)4-graphene is tunable among type-I, type-II, and type-III through a small uniaxial strain. The further stability analysis shows that (SF)n-graphene is thermodynamic stable. Our findings provide an artificial avenue for exploring Dirac Ffermions in carbon-allotropic structures in the presence of defects.

The Dirac cone in two-dimensional tetragonal silicon carbides: a ring coupling mechanism.

The exploration of novel two-dimensional semimetallic materials is always an attractive topic. We propose a series of two-dimensional silicon carbides with a tetragonal lattice. The band structure of silicon carbides with tetragonal carbon rings and silicon rings exhibits Dirac cones. Interestingly, the Dirac cone of tetragonal SiC originates from a "ring coupling" mechanism. This mechanism refers to the mutual coupling between the four carbon atoms in the tetragonal C ring, and the same coupling in the tetragonal Si ring. Additionally, the "ring coupling" mechanism is applicable to other group IV binary compounds such as monolayer GeC and SnC. This work provides reliable evidence for the argument that two-dimensional tetragonal materials can produce Dirac cones.

Developing Proton-Conductive Metal Coordination Polymer as Highly Efficient Electrocatalyst toward Oxygen Reduction.

Developing earth-abundant transition metal (TM)-based electrocatalysts toward oxygen reduction reaction (ORR) is significant in overcoming the high cost of fuel cells. Herein, using an as-synthesized proton-conductive coordination polymer (termed TM-DHBQ) as a template, we investigate the ORR performance of a series of such TM-DHBQs via screening 3d, 4d, and 5d TMs. We find that most 3d TM-DHBQs exhibit distinguished durability under ORR turnover conditions. The formation energies of these TM-DHBQs and adsorption free energies of ORR intermediates show a good correlation with the number of outer electrons of TM ions in TM-DHBQs, enabling the formation energy as a robust ORR activity descriptor. The Sabatier-type volcano plot and microkinetic modeling coidentify Fe- and Co-DHBQs as two promising alternatives to Pt-based ORR electrocatalysts. For those TM-DHBQs showing strong bonding to oxygen species, the ORR intermediate is found to combine with the TM ion serving as the active center.