Tailoring Catalysis of Encapsulated Pt Nanoparticles by Pore Wall Engineering of Covalent Organic Frameworks.

While supported metal nanoparticles (NPs) have shown significant promise in heterogeneous catalysis, precise control over their interaction with the support, which profoundly impacts their catalytic performance, remains a significant challenge. In this study, Pt NPs are incorporated into thioether-functionalized covalent organic frameworks (denoted COF-Sx), enabling precise control over the size and electronic state of Pt NPs by adjusting the thioether density dangling on the COF pore walls. Notably, the resulting Pt@COF-Sx demonstrate exceptional selectivity (>99%) in catalytic hydrogenation of p-chloronitrobenzene to p-chloroaniline, in sharp contrast to the poor selectivity of Pt NPs embedded in thioether-free COFs. Furthermore, the conversion over Pt@COF-Sx exhibits a volcano-type curve as the thioether density increases, due to the corresponding change of accessible Pt sites. This work provides an effective approach to regulating the catalysis of metal NPs via their microenvironment modulation, with the aid of rational design and precise tailoring of support structure.

Obtaining giant Rashba-Dresselhaus spin splitting in two-dimensional chiral metal-organic frameworks.

Two-dimensional (2D) nonmagnetic semiconductors with large Rashba-Dresselhaus (R-D) spin splitting at valence or conduction bands are attractive for magnetic-field-free spintronic applications. However, so far, the number of 2D R-D inorganic semiconductors has been quite limited, and the factors that determine R-D spin splitting as well as rational design of giant spin splitting, remain unclear. For this purpose, by exploiting 2D chiral metal-organic frameworks (CMOFs) as a platform, we theoretically develop a three-step screening method to obtain a series of candidate 2D R-D semiconductors with valence band spin splitting up to 97.2 meV and corresponding R-D coupling constants up to 1.37 eV Å. Interestingly, the valence band spin texture is reversible by flipping the chirality of CMOFs. Furthermore, five keys for obtaining giant R-D spin splitting in 2D CMOFs are successfully identified: (i) chirality, (ii) large spin-orbit coupling, (iii) narrow band gap, (iv) valence and conduction bands having the same symmetry at the Γ point, and (v) strong ligand field.

High-Throughput Computational Screening of Novel Two-Dimensional Covalent Organic Frameworks for Efficient Photocatalytic Overall Water Splitting.

The pursuit of efficient photocatalysts toward photocatalytic water splitting has attracted wide attention. However, the low efficiency of photocatalytic reactions due to the rapid electron-hole recombination and the time-consuming searching process hinder the development of high-performance photocatalysts. Here, we proposed a data-driven screening procedure for covalent organic frameworks (COFs) as overall solar water-splitting photocatalysts. Based on a COF database through assembling different Cores and Linkers, three COFs are predicted to be efficient photocatalysts for overall solar water splitting after high-throughput computational screening. We found that the photogenerated electrons and holes are well separated on single COF photocatalysts without material engineering, and both hydrogen and oxygen evolution reactions can occur spontaneously on the three screened COFs under visible light radiation. This kind of novel COF screened by a data-driven screening procedure offers new perspectives for advancing efficient photocatalysts.

Construction of an Electron Capture and Transfer Center for Highly Efficient and Selective Solar-Light-Driven CO2 Conversion.

Exploring high-efficiency photocatalysts for selective CO2 reduction is still challenging because of the limited charge separation and surface reactions. In this study, a noble-metal-free metallic VSe2 nanosheet was incorporated on g-C3N4 to serve as an electron capture and transfer center, activating surface active sites for highly efficient and selective CO2 photoreduction. Quasi in situ X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and femtosecond transient absorption spectroscopy (fs-TAS) unveiled that VSe2 could capture electrons, which are further transferred to the surface for activating active sites. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations revealed a kinetically feasible process for the formation of a key intermediate and confirmed the favorable production of CO on the VSe2/PCN (protonated C3N4) photocatalyst. As an outcome, the optimized VSe2/PCN composite achieved 97% selectivity for solar-light-driven CO2 conversion to CO with a high rate of 16.3 µmol·g-1·h-1, without any sacrificial reagent or photosensitizer. This work offers new insights into the photocatalyst design toward highly efficient and selective CO2 conversion.

Excitonic Effects of the Excited-State Photocatalytic Reaction at the Molecule/Metal Oxide Interface.

Excitonic effects caused by the Coulomb interaction between electrons and holes play a crucial role in photocatalysis at the molecule/metal oxide interface. As an ideal model for investigating the excitonic effect, coadsorption and photodissociation of water and methanol molecules on titanium dioxide involve complex ground-state thermalcatalytic and excited-state photocatalytic reaction processes. Herein, we systemically investigate the excited-state electronic structures of the coadsorption of H2O and CH3OH molecules on a rutile TiO2(110) surface by linear-response time-dependent density functional theory calculations and probe the reaction path for generating HCOOH or CO2, from ground-state and excited-state perspectives. The reaction barriers in excited-state calculations are significantly different from those in ground-state calculations during three processes, with the largest decrease being 0.94 eV for the Ti5c-O-CH2-O-Ti5c formation process.

A stabilization rule for metal carbido cluster bearing µ3-carbido single-atom-ligand encapsulated in carbon cage.

Metal carbido complexes bearing single-carbon-atom ligand such as nitrogenase provide ideal models of adsorbed carbon atoms in heterogeneous catalysis. Trimetallic µ3-carbido clusterfullerenes found recently represent the simplest metal carbido complexes with the ligands being only carbon atoms, but only few are crystallographically characterized, and its formation prerequisite is unclear. Herein, we synthesize and isolate three vanadium-based µ3-CCFs featuring V = C double bonds and high valence state of V (+4), including VSc2C@Ih(7)-C80, VSc2C@D5h(6)-C80 and VSc2C@D3h(5)-C78. Based on a systematic theoretical study of all reported µ3-carbido clusterfullerenes, we further propose a supplemental Octet Rule, i.e., an eight-electron configuration of the µ3-carbido ligand is needed for stabilization of metal carbido clusters within µ3-carbido clusterfullerenes. Distinct from the classic Effective Atomic Number rule based on valence electron count of metal proposed in the 1920s, this rule counts the valence electrons of the single-carbon-atom ligand, and offers a general rule governing the stabilities of µ3-carbido clusterfullerenes.

Bipolar Rashba Semiconductors: A Class of Nonmagnetic Materials for Electrical Spin Manipulation.

The realization of the electrical control of spin is highly desirable. One promising approach is by regulating the Rashba spin-orbit coupling effect of materials through external electric fields. However, this method requires materials to possess either a high electric field response and a large Rashba constant or the simultaneous presence of Rashba splitting and ferroelectric polarization. These stringent requirements result in a scarcity of suitable materials. In order to surpass these limitations and exploit a new prospect for spin manipulation via the Rashba effect, a conceptual class of materials named bipolar Rashba semiconductors (BRS) is proposed, whose valence band and conduction band possess opposite spin texture directions when approaching the Fermi level. The unique electronic structure of BRS makes it feasible to reverse the spin precession by simply applying a gate voltage. The existence of BRS is confirmed through first-principles calculations on the two-dimensional (2D) material AlBiS3.

Data-Driven Discovery of a Covalent Organic Framework Heterojunction as Efficient Photocatalysts for Overall Solar Water Splitting.

Searching for highly efficient visible-light photocatalysts is a high-cost and time-consuming process in the water splitting field. The integration of data-driven screening based on the database and density functional theory calculations represents a promising approach. In this study, we first present a topologically assembled single-layer covalent organic framework (COF) that is used to build a COF heterojunction database via AA stacking. Then we propose a systematic search procedure for COF heterojunctions as overall solar water splitting photocatalysts, including suitable band gap (screen 1), appropriate band edge position (screen 2), spontaneous catalytic reactions for water splitting (screen 3), and efficient separation of photogenerated electrons and holes (proof). Finally, we successfully identify 1 heterojunction from the pool of 222 items as an efficient photocatalyst for overall solar water splitting. Clearly, this kind of data-driven screening procedure, based on a COF heterojunction database, opens up new avenues and inspires the development of high-performance photocatalysts.

Two-dimensional superhard silicon nitrides with widely tunable bandgap, high carrier mobility and hole-doping-induced robust magnetism.

The search for new forms of the traditional bulk materials to enrich their interactions and properties is an attractive subject in two-dimensional (2D) materials. In this work, novel tetra-hexa-mixed coordinated 2D silicon nitrides (Si3N4) and their analogues are systematically investigated via density functional theory. The results show the global minimum 2D structure, Si3N4 (T-aa), is a highly chemically and thermally stable superhard semiconductor with a wide indirect bandgap (about 6.0 eV), which is widely adjustable under both biaxial strain and vertical electric field. It also possesses anisotropic high carrier mobility, up to 5490 cm2 V-1 s-1 at room temperature. Besides, its nitride analogues of group IVA (Si, Ge, Sn, and Pb) exhibit diverse electronic structures with regular bandgap distribution. Remarkably, some nitride analogues display linearly increasing robust magnetism with hole doping. The theoretical Curie temperatures of Si3N4 and Sn3N4 with hole doping (1h+ per unit cell) are 298 and 180 K, respectively. The Si3N4 (T-aa) and its analogues have a variety of excellent properties to be potentially applied in various fields, e.g., semiconductor electronics, spintronics, high-temperature structural materials, and superhard materials.

Gate-Tunable Spin Seebeck Effect and Pure Spin Current Generation in Molecular Junctions Based on Bipolar Magnetic Molecules.

Generating pure spin currents is very desirable in spintronics, as it provides a promising way to substantially reduce Joule heating and achieve ultrahigh integration density. However, to date, most spintronic devices exhibit spin currents that are accompanied by charge currents. The generation of pure spin currents on the nanoscale, particularly at the single-molecule level, remains challenging. Here, we propose that by exploiting our recently reported bipolar magnetic molecules (BMMs) as the core component of single-molecule devices, where the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) come from different spin channels, the generation of pure spin currents can be easily realized via the spin Seebeck effect (SSE) with applied temperature gradient. Moreover, the spin Seebeck coefficient can be modulated over a wide range by applying an external gate voltage. The proposal is verified through first-principles calculations on two BMM-based molecular junctions.

Dual Microenvironment Modulation of Pd Nanoparticles in Covalent Organic Frameworks for Semihydrogenation of Alkynes.

The chemical microenvironment modulation of metal nanoparticles (NPs) holds promise for tackling the long-lasting challenge of the trade-off effect between activity and selectivity in catalysis. Herein, ultrafine PdCu2 NPs incorporated into covalent organic frameworks (COFs) with diverse groups on their pore walls have been fabricated for the semihydrogenation of alkynes. The Cu species, as the primary microenvironment of Pd active sites, greatly improves the selectivity. The functional groups as the secondary microenvironment around PdCu2 NPs effectively regulate the activity, in which PdCu2 NPs encapsulated in the COF bearing -CH3 groups exhibit the highest activity with >99 % conversion and 97 % selectivity. Both experimental and calculation results suggest that the functional group affects the electron-donating ability of the COFs, which successively impacts the charge transfer between COFs and Pd sites, giving rise to a modulated Pd electronic state and excellent catalytic performance.

Self-Limited Embedding Alternating 585-Ringed Divacancies and Metal Atoms into Graphene Nanoribbons.

Because of their theoretically predicted intriguing properties, it is interesting to embed periodic 585-ringed divacancies into graphene nanoribbons (GNRs), but it remains a great challenge. Here, we develop an on-surface cascade reaction from periodic hydrogenated divacancies to alternating 585-ringed divacancies and Ag atoms via intramolecular cyclodehydrogenation in a seven-carbon-wide armchair GNR on the Ag(111) surface. Combining scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy combined with first-principles calculations, we in-situ-monitor the evolution of the distinct structural and electronic properties in the reaction intermediates. The observation of embedded Ag atoms and further nudged elastic band calculations provide unambiguous evidence for Ag adatom-mediated C-H activation in the intramolecular cyclodehydrogenation pathway, where the strain-induced self-limiting effect contributes to the formation of the GNR superlattice with alternating 585-ringed divacancies and Ag atoms, which shows a band gap of about 1.4 eV. Our findings open an avenue to introducing periodic impurities of single metal atoms and nonhexagonal rings in on-surface synthesis, which may provide a novel route for multifunctional graphene nanostructures.

Exclusive Co-N4 Sites Confined in Two-dimensional Metal-Organic Layers Enabling Highly Selective CO2 Electroreduction at Industrial-Level Current.

Metal-organic framework catalysts bring new opportunities for CO2 electrocatalysis. Herein, we first conduct density-functional theory calculations and predict that Co-based porphyrin porous organic layers (Co-PPOLs) exhibit good activity for CO2 conversion because of the low *CO adsorption energy at Co-N4 sites, which facilitates *CO desorption and CO formation. Then, we prepare two-dimensional Co-PPOLs with exclusive Co-N4 sites through a facile surfactant-assisted bottom-up method. The ultrathin feature ensures the exposure of catalytic centers. Together with large specific area, high electrical conductivity and CO2 adsorption capability, Co-PPOLs achieve a peak faradaic efficiency for CO production (FECO =94.2 %) at a moderate potential in CO2 electroreduction, accompanied with good stability. Moreover, Co-PPOLs reach an industrial-level current above 200 mA in a membrane electrode assembly reactor, and maintain near-unity CO selectivity (FECO >90 %) over 20 h in CO2 electrolysis.

Self-driven carbon atom implantation into fullerene embedding metal-carbon cluster.

Hundreds of members have been synthesized and versatile applications have been promised for endofullerenes (EFs) in the past 30 y. However, the formation mechanism of EFs is still a long-standing puzzle to chemists, especially the mechanism of embedding clusters into charged carbon cages. Here, based on synthesis and structures of two representative vanadium-scandium-carbido/carbide EFs, VSc2C@Ih (7)-C80 and VSc2C2@Ih (7)-C80, a reasonable mechanism-C1 implantation (a carbon atom is implanted into carbon cage)-is proposed to interpret the evolution from VSc2C carbido to VSc2C2 carbide cluster. Supported by theoretical calculations together with crystallographic characterization, the single electron on vanadium (V) in VSc2C@Ih (7)-C80 is proved to facilitate the C1 implantation. While the V=C double bond is identified for VSc2C@Ih (7)-C80, after C1 implantation the distance between V and C atoms in VSc2C2@Ih (7)-C80 falls into the range of single bond lengths as previously shown in typical V-based organometallic complexes. This work exemplifies in situ self-driven implantation of an outer carbon atom into a charged carbon cage, which is different from previous heterogeneous implantation of nonmetal atoms (Group-V or -VIII atoms) driven by high-energy ion bombardment or high-pressure offline, and the proposed C1 implantation mechanism represents a heretofore unknown metal-carbon cluster encapsulation mechanism and can be the fundamental basis for EF family genesis.

Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies.

The bottom-up approach through on-surface synthesis of porous graphene nanoribbons (GNRs) presents a controllable manner for implanting periodic nanostructures to tune the electronic properties of GNRs in addition to bandgap engineering by width and edge configurations. However, owing to the existing steric hindrance in small pores like divacancies, it is still difficult to embed periodic divacancies with a nonplanar configuration into GNRs. Here, we demonstrate the on-surface synthesis of atomically precise eight-carbon-wide armchair GNRs embedded with periodic divacancies (DV8-aGNRs) by utilizing the monatomic step edges on the Au(111) surface. From a single molecular precursor correspondingly following a trans- and cis-coupling, the DV8-aGNR and another porous nanographene are respectively formed at step edges and on terraces at 720 and 570 K. Combining scanning tunneling microscopy/spectroscopy, atomic force microscopy, and first-principles calculations, we determine the out-of-plane conformation, wide bandgap (∼3.36 eV), and wiggly shaped frontier orbitals of the DV8-aGNR. Nudged elastic band calculations further quantitatively reveal that the additional steric hindrance effect in the cyclodehydrogenative reactions has a higher barrier of 1.3 eV than that in the planar porous nanographene, which also unveils the important role played by the monatomic Au step and adatoms in reducing the energy barriers and enhancing the thermodynamic preference of the oxidative cyclodehydrogenation. Our results provide the first case of GNRs containing periodic pores as small as divacancies with a nonplanar configuration and demonstrate the strategy by utilizing the chemical heterogeneity of a substrate to promote the formation of novel carbon nanomaterials.

Bipolar Magnetic Molecules for Spin-Polarized Electric Current in Molecular Junctions.

Electrical control of spin transport at single molecule level is highly desired for molecular nanospintronics. By exploiting magnetic bistability of spin crossover complexes, the magnitude of spin polarization can be modulated. However, efficiently controlling the direction of spin polarization at single molecule level is still challenging. Here, we propose a general method to flip the transport electron spin simply by electrical gating in single molecule devices based on bipolar magnetic molecules (BMMs), of which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) come from different spin channels. Thus, when the device's Fermi level is reversibly adjusted approaching either HOMO or LUMO by changing the polarity of the applied voltage gate, a 100 % spin polarized current with switchable spin direction is achievable. The proposed method is verified by the calculated electronic and transport properties of 9 potential transition metal coordination BMMs.

C7N6/Sc2CCl2 Weak van der Waals Heterostructure: A Promising Visible-Light-Driven Z-Scheme Water Splitting Photocatalyst with Interface Ultrafast Carrier Recombination.

Designing and characterizing high-efficiency direct Z-scheme photocatalysts both from theoretical and experimental aspects still remain a big challenge. Here, based on extensive first-principles calculations combined with excited state dynamics simulations, we report that a weak van der Waals (vdW) C7N6/Sc2CCl2 heterostructure is a mediator-free direct Z-scheme photocatalyst for solar water splitting. Theoretical results clearly reveal that this heterostructure displays a nice light-harvesting performance extending to the near-infrared region. The relatively strong interfacial non-adiabatic coupling accelerates the interlayer carrier recombination in the time scale of sub-picoseconds. The well separated electrons and holes with a strong redox capacity make the hydrogen evolution reaction spontaneously occur on the C7N6 surface and the oxygen evolution reaction on the Se-doped Sc2CCl2 surface, respectively, when the proposed heterostructure is radiated with solar light.

Non-Collinear Orbital-induced Planar Quantum Anomalous Hall Effect.

Based on the 2D star lattice with noncollinear orbital, we present an intriguing six-band tight-binding model for planar quantum anomalous Hall effect (PQAHE) with in-plane magnetization. Depending on the orientation of the noncollinear orbital, the characterized Dirac-, Kagome-, and Four-band are all realizable. Moreover, including the intrinsic spin-orbital coupling, a topological phase diagram versus angle of both in-plane magnetization and noncollinear orbital is mapped out, showing a tunable PQAHE with Chern number of either +1 or -1. Most remarkably, using first-principles calculations, this exact model of PQAHE is identified in an experimentally synthesized 2D metal-organic framework (MOF) of Pr2(C6O4Cl2)3. These notable results not only demonstrate the significance of noncollinear orbital for designing PQAHE but also provide a MOF platform for its realization, greatly enriching the fundamental topological physics and extending the candidate topological materials.

A Highly Efficient Metal-Free Electrocatalyst of F-Doped Porous Carbon toward N2 Electroreduction.

N2 electroreduction into NH3 represents an attractive prospect for N2 utilization. Nevertheless, this process suffers from low Faraday efficiency (FE) and yield rate for NH3 . In this work, a highly efficient metal-free catalyst is developed by introducing F atoms into a 3D porous carbon framework (F-doped carbon) toward N2 electroreduction. At -0.2 V versus reversible hydrogen electrode (RHE), the F-doped carbon achieves the highest FE of 54.8% for NH3 , which is 3.0 times as high as that (18.3%) of pristine carbon frameworks. Notably, at -0.3 V versus RHE, the yield rate of F-doped carbon for NH3 reaches 197.7 µgNH3 mg-1 cat. h-1 . Such a value is more than one order of magnitude higher than those of other metal-free electrocatalysts under the near-ambient conditions for NH3 product to date. Mechanistic studies reveal that the improved performance in N2 electroreduction for F-doped carbon originates from the enhanced binding strength of N2 and the facilitated dissociation of N2 into *N2 H. F bonding to C atom creates a Lewis acid site due to the different electronegativity between the F and C atoms. As such, the repulsive interaction between the Lewis acid site and proton H suppresses the activity of H2 evolution reaction, thus enhancing the selectivity of N2 electroreduction into NH3 .

Irreversible accumulated SERS behavior of the molecule-linked silver and silver-doped titanium dioxide hybrid system.

In recent years, surface-enhanced Raman scattering (SERS) of a molecule/metal-semiconductor hybrid system has attracted considerable interest and regarded as the synergetic contribution of the electromagnetic and chemical enhancements from the incorporation of noble metal into semiconductor nanomaterials. However, the underlying mechanism is still to be revealed in detail. Herein, we report an irreversible accumulated SERS behavior induced by near-infrared (NIR) light irradiating on a 4-mercaptobenzoic acid linked with silver and silver-doped titanium dioxide (4MBA/Ag/Ag-doped TiO2) hybrid system. With increasing irradiation time, the SERS intensity of 4MBA shows an irreversible exponential increase, and the Raman signal of the Ag/Ag-doped TiO2 substrate displays an exponential decrease. A microscopic understanding of the time-dependent SERS behavior is derived based on the microanalysis of the Ag/Ag-doped TiO2 nanostructure and the molecular dynamics, which is attributed to three factors: (1) higher crystallinity of Ag/Ag-doped TiO2 substrate; (2) photo-induced charge transfer; (3) charge-induced molecular reorientation.