Asymmetric Cu-N-La Species Enabling Atomic-Level Donor-Acceptor Structure and Favored Reaction Thermodynamics for Selective CO2 Photoreduction to CH4.

Photocatalytic CO2 reduction into ideal hydrocarbon fuels, such as CH4 , is a sluggish kinetic process involving adsorption of multiple intermediates and multi-electron steps. Achieving high CH4 activity and selectivity therefore remains a great challenge, which largely depends on the efficiency of photogenerated charge separation and transfer as well as the intermediate energy levels in CO2 reduction. Herein, we construct La and Cu dual-atom anchored carbon nitride (LaCu/CN), with La-N4 and Cu-N3 coordination bonds connected by Cu-N-La bridges. The asymmetric Cu-N-La species enables the establishment of an atomic-level donor-acceptor structure, which allows the migration of electrons from La atoms to the reactive Cu atom sites. Simultaneously, intermediates during CO2 reduction on LaCu/CN demonstrate thermodynamically more favorable process for CH4 formation based on theoretical calculations. Eventually, LaCu/CN exhibits a high selectivity (91.6 %) for CH4 formation with a yield of 125.8 µmol g-1 , over ten times of that for pristine CN. This work presents a strategy for designing multi-functional dual-atom based photocatalysts.

P-Mediated Cu-N4 Sites in Carbon Nitride Realizing CO2 Photoreduction to C2 H4 with Selectivity Modulation.

Photocatalytic CO2 reduction to high value-added C2 products (e.g., C2 H4 ) is of considerable interest but challenging. The C2 H4 product selectivity strongly hinges on the intermediate energy levels in the CO2 reduction pathway. Herein, Cu-N4 sites anchored phosphorus-modulated carbon nitride (CuACs/PCN) is designed as a photocatalyst to tailor the intermediate energy levels in the the C2 H4 formation reaction pathway for realizing its high production with tunable selectivity. Theoretical calculations combined with experimental data demonstrate that the formation of the C-C coupling intermediates can be realized on Cu-N4 sites and the surrounding doped P facilitates the production of C2 H4 . Thus, CuACs/PCN exhibits a high C2 H4 selectivity of 53.2% with a yielding rate of 30.51 µmol g-1 . The findings reveal the significant role of the coordination environment and surrounding microenvironment of Cu single atoms in C2 H4 formation and offer an effective approach for highly selective CO2 photoreduction to produce C2 H4 .

Two-dimensional porphyrin covalent organic frameworks with tunable catalytic active sites for the oxygen reduction reaction.

Four novel two-dimensional porphyrin COFs (M-TP-COF, M = H2, Co, Ni and Mn) with donor-acceptor dyads were fabricated and served as electrocatalysts for the oxygen reduction reaction (ORR). The ORR catalytic activity of M-TP-COF was tuned by changing the M atom in the center of the porphyrin backbone. The experimental structure-function relationship was in accordance with the results of density functional theory calculations based on the O2-O2*-OOH*-O*-OH*-OH- route.

[Conception and framework of land ecological restoration for a new stage in China.] / 新时期我国国土空间生态修复理念与模式探讨.

Based on the theories of geography and landscape ecology, land ecological restoration is an important strategy to promote ecological civilization and build a beautiful China. Land ecological restoration in China has the characteristics of multi-disciplinary theoretical system, diverse work mode, integration of technology and methods, and diversified practice and exploration. The overall effectiveness of the work coexists with arduous tasks. Based on the new challenges in land ecological restoration, we summarized the overall framework and technical path of land ecological restoration, the working mechanism and mode of regional land ecological restoration, the content system and technical standards of land ecological restoration. In the new stage, the top-level design of land ecological restoration in China should focus on the work system, business boundary and institutional system, identify the responsibility boundary of different business processes, and realize the closed management of the whole work chain from the perspective of theory, system, engineering and technology. Rural settlement area, urban built-up area, industrial and mining gathering area, ecological function area and blue ocean area are the five major elements of regional scale land space, which correspond to five different ecological restoration modes, namely, comprehensive land improvement, urban renewal and double repair, mine geological environment restoration, mountain-river-forest-farmland-lake-grassland restoration, and blue bay restoration. It is necessary to comprehensively use the thinking of "Three Integrations" (information, technology, and process) to build a set of regional land ecological restoration work mode covering the integration of investigation, monitoring and evaluation, land space planning, project implementation, project acceptance and ecosystem monitoring and evaluation.

Confined Synthesis of Oriented Two-Dimensional Ni3(hexaiminotriphenylene)2 Films for Electrocatalytic Oxygen Evolution Reaction.

Metal-organic frameworks (MOFs) can provide atomically dispersed metal active coordination sites (M-NX, M-SX, and M-OX) for electrocatalytic reactions. Among them, MOFs with motif M-NX or analogues are expected to be promising active electrode materials for oxygen evolution reaction (OER). Contrary to bulk MOFs, two-dimensional (2D) MOFs usually have high surface areas, fully exposed active sites, and specific electrical properties. Herein, we constructed 2D Ni3(hexaiminotriphenylene)2 [Ni3(HITP)2] films on the electrode surface by utilizing the bottom-up liquid/liquid/gel tri-phase interface system and explored their potential applications in electrocatalytic OER. The thickness of the 2D Ni3(HITP)2 films can be controlled to be about 5 nm. The prepared 2D Ni3(HITP)2 films had oriented polycrystalline character and showed excellent performance in OER. A current density of 10 mA cm-2 for 3-layer Ni3(HITP)2 film electrodes was obtained at 1.62 V, which was 20 mV lower than that for the commercial IrO2 catalyst. Electrochemical tests and electrochemical impedance spectroscopy showed that better OER performance of 3-layer Ni3(HITP)2 films was ascribed to their high electrochemically active surface area, better kinetic process, and fast ion diffusion and transport.

One-step ultrasonic synthesis of Co/Ni-catecholates for improved performance in oxygen reduction reaction.

The inherent periodically arranged M-NX, M-SX and M-OX units (M are usually Fe, Co, Ni, etc.) in metal-organic frameworks (MOFs) can be promising active centers in electrocatalysis. In previous studies, MOFs were usually constructed by energy-consuming hydro- or solvo-thermal reactions. Ultrasonic synthesis is a rapid and environment-friendly technique when envisaging MOFs' industrial applications. In addition, different synthetic pathways for MOFs may lead to difference in their microstructure, resulting in different electrocatalytic performance. Nevertheless, only a handful of MOFs were successfully prepared by ultrasonic synthesis and few were applied in electrochemical catalysis. Herein, we constructed Ni/Co-catecholates (Ni/Co-CATs) synthesized by one-step ultrasonic method (250 W, 40 KHz, 25 W/L, Ultrasonic clearing machine) and compared their performance in oxygen reduction reaction (ORR) with that of Ni/Co-CATs synthesized by hydrothermal method. Ni-CAT and Co-CAT prepared by ultrasonic showed the half-wave potential of -0.196 V and -0.116 V (vs. Ag/AgCl), respectively. The potentials were more positive than those prepared by hydro-thermal method. And they showed excellent electrochemical stability in neutral solution. The latter was only 32 mV lower than that of commercial Pt/C. The improved performance in ORR was attributed to higher specific surface area and mesopore volume as well as more structural defects generated in the ultrasonic synthesis process, which could facilitate their exposure of electrocatalytic active sites and their mass transport. This work gives some perspective into cost-effective synthetic strategies of efficient MOFs-based electrocatalysts.

Tailored Polyimide-Graphene Nanocomposite as Negative Electrode and Reduced Graphene Oxide as Positive Electrode for Flexible Hybrid Sodium-Ion Capacitors.

Redox-active polyimide materials hold a great promise for electrochemical energy storage applications, especially for flexible energy storage devices. However, the low utilization efficiency due to poor electrical conductivity of the materials remains one of the greatest challenges. In this work, we designed and prepared polyimide-graphene composite materials and tested their electrochemical properties in sodium-ion capacitors. By manipulating the interfacial chemistry and interactions between the polyimide and graphene, composite electrode materials with different polyimide particle sizes and morphologies were obtained. Sodium-ion storage capacity was significantly improved, from ∼50 mAh g-1 for pure polyimide to 225 mAh g-1 for a polyimide-graphene composite. A hybrid sodium-ion capacitor fabricated with freestanding polyimide-graphene composite as the negative electrode and reduced graphene oxide as the positive electrode delivered energy densities of 55.5 and 21.5 Wh kg-1 at power densities of 395 and 3400 W kg-1, respectively. A flexible sodium-ion capacitor with outstanding mechanical properties was also demonstrated.

Self-template construction of nanoporous carbon nanorods from a metal-organic framework for supercapacitor electrodes.

The morphologies and structures of nanostructured carbons generally influence their catalysis, electrochemical performance and adsorption properties. Metal-organic framework (MOF) nanocrystals usually have various morphologies, and can be considered as a template to construct nanostructured carbons with shaped nanocubes, nanorods, and hollow particles by thermal transformation. However, thermal carbonization of MOFs usually leads to collapse of MOF structures. Here, we report shape-preserved carbons (termed as CNRods) by thermal transformation of nickel catecholate framework (Ni-CAT) nanorods. Supercapacitors of CNRods treated at 800 °C were demonstrated to have enhanced performance due to their structural features that facilitate electron conduction and ion transport as well as abundant O content benefiting the wettability of the carbon materials. This may provide a potential way to explore novel carbon materials for supercapacitors with controllable morphologies and high capacitive performance.

Concentration-Directed Polymorphic Surface Covalent Organic Frameworks: Rhombus, Parallelogram, and Kagome.

Polymorphic single-layered covalent organic frameworks (sCOFs) via on-surface synthesis have been investigated by employing the tetradentate monomer 1,3,6,8-tetrakis(p-formylphenyl)pyrene with D2h symmetry and ditopic linear diamine building blocks. Three kinds of well-ordered sCOFs, including rhombus, parallelogram, and Kagome networks, are observed on the graphite surface by scanning tunnel microscopy. The pore size and periodicity of sCOFs are tunable by employing diamine monomers with different lengths. Statistical analysis reveals that two types of quadrate networks are preferred at high concentration, whereas the occupancy of Kagome networks increases at low concentration. This trend can be understood by the differences in the network density of three kinds of networks. The reversibility and the self-sorting ability of the dynamic covalent reaction make it possible to control the polymorphic distribution similar to the principle demonstrated in supramolecular self-assembly.

Well-Defined Metal-O6 in Metal-Catecholates as a Novel Active Site for Oxygen Electroreduction.

Metal-nitrogen coordination sites, M-Nx (M = Fe, Co, Ni, etc.), have shown great potential to replace platinum group materials as electrocatalysts for oxygen reduction reaction (ORR). However, the real active site in M-Nx is still vague to date due to their complicated structure and composition. It is therefore highly desirable but challenging to develop ORR catalysts with novel and clear active sites, which could meet the needs of comprehensive understanding of structure-function relationships and explore new cost-effective and efficient ORR electrocatalysts. Herein, well-defined M-O6 coordination in metal-catecholates (M-CATs, M = Ni or Co) is discovered to be catalytically active for ORR via a four-electron-dominated pathway. In view of no pyrolysis involved and unambiguous crystalline structure of M-CATs, the M-O6 octahedral coordination site with distinct structure is determined as a new type of active site for ORR. These findings extend the scope of metal-nonmetal coordination as an active site for ORR and pave a way for bottom-up design of novel electrocatalysts containing M-O6 coordination.

The intramolecular H-bonding effect on the growth and stability of Schiff-base surface covalent organic frameworks.

The introduction of intramolecular H-bonding by adding -OH functionalities adjacent to the Schiff base centers is considered to be a useful strategy to enhance the stability and crystallinity of bulk covalent organic frameworks (COFs). However, the influence of intramolecular H-bonding on the synthesis of surface COFs (SCOFs) have been barely explored. Herein, SCOFs based on the Schiff-base reaction between 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehydes with symmetry or asymmetrically substituted hydroxyl functional groups are designed. In the absence of a solvent, hydroxyl substituents can be easily oxidized; thus argon protection is required to obtain high-quality SCOFs. Besides, an extended network with uniform pores can be achieved in spite of the symmetry of substituents. Both experimental results and theoretical calculations show that the influence of intramolecular hydrogen bonding on surface synthesis is not as important as that in bulk phase synthesis because the substrate itself can lead to the complanation of adsorbed molecules. The existence of intramolecular H-bonding can enhance the stability of the network in both acid and alkali environments.

The on-surface synthesis of imine-based covalent organic frameworks with non-aromatic linkage.

A pair of isomeric imine-based covalent organic frameworks with non-aromatic linkage has been fabricated at the graphite surface, which extends the structural diversity of surface covalent organic frameworks.

Formation of Halogen Bond-Based 2D Supramolecular Assemblies by Electric Manipulation.

Halogen bonding has attracted much attention recently as an important driving force for supramolecular assembly and crystal engineering. Herein, we demonstrate for the first time the formation of a halogen bond-based open porous network on a graphite surface using ethynylpyridine and aryl-halide based building blocks. We found that the electrical stimuli of a scanning tunneling microscopy (STM) tip can induce the formation of a binary supramolecular structure on the basis of halogen bond formation between terminal pyridyl groups and perfluoro-iodobenzene. This electrical manipulation method can be applied to engineer a series of linear or porous structures by selecting halogen bond donor and acceptor fragments with different symmetries, as the directional interactions ultimately determine the structural outcome.

Molecular engineering of Schiff-base linked covalent polymers with diverse topologies by gas-solid interface reaction.

The design and construction of molecular nanostructures with tunable topological structures are great challenges in molecular nanotechnology. Herein, we demonstrate the molecular engineering of Schiff-base bond connected molecular nanostructures. Building module construction has been adopted to modulate the symmetry of resulted one dimensional (1D) and two dimensional (2D) polymers. Specifically, we have designed and constructed 1D linear and zigzag polymers, 2D hexagonal and chessboard molecular nanostructures by varying the number of reactive sites and geometry and symmetry of precursors. It is demonstrated that high-quality conjugated polymers can be fabricated by using gas-solid interface reaction. The on-demanding synthesis of polymeric architectures with diverse topologies paves the way to fabricate molecular miniature devices with various desired functionalities.

Bilayer molecular assembly at a solid/liquid interface as triggered by a mild electric field.

The construction of a spatially defined assembly of molecular building blocks, especially in the vertical direction, presents a great challenge for surface molecular engineering. Herein, we demonstrate that an electric field applied between an STM tip and a substrate triggered the formation of a bilayer structure at the solid-liquid interface. In contrast to the typical high electric-field strength (10(9)  V m(-1) ) used to induce structural transitions in supramolecular assemblies, a mild electric field (10(5)  V m(-1) ) triggered the formation of a bilayer structure of a polar molecule on top of a nanoporous network of trimesic acid on graphite. The bilayer structure was transformed into a monolayer kagome structure by changing the polarity of the electric field. This tailored formation and large-scale phase transformation of a molecular assembly in the perpendicular dimension by a mild electric field opens perspectives for the manipulation of surface molecular nanoarchitectures.

Isomeric routes to Schiff-base single-layered covalent organic frameworks.

With graphene-like topology and designable functional moieties, single-layered covalent organic frameworks (sCOFs) have attracted enormous interest for both fundamental research and application prospects. As the growth of sCOFs involves the assembly and reaction of precursors in a spatial defined manner, it is of great importance to understand the kinetics of sCOFs formation. Although several large families of sCOFs and bulk COF materials based on different coupling reactions have been reported, the synthesis of isomeric sCOFs by exchanging the coupling reaction moieties on precursors has been barely explored. Herein, a series of isomeric sCOFs based on Schiff-base reaction is designed to understand the effect of monomer structure on the growth kinetics of sCOFs. The distinctly different local packing motifs in the mixed assemblies for the two isomeric routes closely resemble to those in the assemblies of monomers, which affect the structural evolution process for highly ordered imine-linked sCOFs. In addition, surface diffusion of monomers and the molecule-substrate interaction, which is tunable by reaction temperature, also play an important role in structural evolutions. This study highlights the important roles of monomer structure and reaction temperature in the design and synthesis of covalent bond connected functional nanoporous networks.

Graphene-like single-layered covalent organic frameworks: synthesis strategies and application prospects.

Two-dimensional (2D) nanomaterials, such as graphene and transition metal chalcogenides, show many interesting dimension-related materials properties. Inspired by the development of 2D inorganic nanomaterials, single-layered covalent organic frameworks (sCOFs), featuring atom-thick sheets and crystalline extended organic structures with covalently bonded building blocks, have attracted great attention in recent years. With their unique graphene-like topological structure and the merit of structural diversity, sCOFs promise to possess novel and designable properties. However, the synthesis of sCOFs with well-defined structures remains a great challenge. Herein, the recent development of the bottom-up synthesis methods of 2D sCOFs, such as thermodynamic equilibrium control methods, growth-kinetics control methods, and surface-assisted covalent polymerization methods, are reviewed. Finally, some of the critical properties and application prospects of these materials are outlined.

Adaptive reorganization of 2D molecular nanoporous network induced by coadsorbed guest molecule.

The ordered array of nanovoids in nanoporous networks, such as honeycomb, Kagome, and square, provides a molecular template for the accommodation of "guest molecules". Compared with the commonly studied guest molecules featuring high symmetry evenly incorporated into the template, guest molecules featuring lower symmetry are rare to report. Herein, we report the formation of a distinct patterned superlattice of guest molecules by selective trapping of guest molecules into the honeycomb network of trimesic acid (TMA). Two distinct surface patterns have been achieved by the guest inclusion induced adaptive reconstruction of a 2D molecular nanoporous network. The honeycomb networks can synergetically tune the arrangement upon inclusion of the guest molecules with different core size but similar peripherals groups, resulting in a trihexagonal Kagome or triangular patterns.

Surface tectonics of nanoporous networks of melamine-capped molecular building blocks formed through interface Schiff-base reactions.

Control over the assembly of molecules on a surface is of great importance for the fabrication of molecule-based miniature devices. Melamine (MA) and molecules with terminal MA units are promising candidates for supramolecular interfacial packing patterning, owing to their multiple hydrogen-bonding sites. Herein, we report the formation of self-assembled structures of MA-capped molecules through a simple on-surface synthetic route. MA terminal groups were successfully fabricated onto rigid molecular cores with 2-fold and 3-fold symmetry through interfacial Schiff-base reactions between MA and aldehyde groups. Sub-molecular scanning tunneling microscopy (STM) imaging of the resultant adlayer revealed the formation of nanoporous networks. Detailed structural analysis indicated that strong hydrogen-bonding interactions between the MA groups persistently drove the formation of nanoporous networks. Herein, we demonstrate that functional groups with strong hydrogen-bond-formation ability are promising building blocks for the guided assembly of nanoporous networks and other hierarchical 2D assemblies.

On-surface synthesis of single-layered two-dimensional covalent organic frameworks via solid-vapor interface reactions.

Surface covalent organic frameworks (SCOFs), featured by atomic thick sheet with covalently bonded organic building units, are promised to possess unique properties associated with reduced dimensionality, well-defined in-plane structure, and tunable functionality. Although a great deal of effort has been made to obtain SCOFs with different linkages and building blocks via both "top-down" exfoliation and "bottom-up" surface synthesis approaches, the obtained SCOFs generally suffer a low crystallinity, which impedes the understanding of intrinsic properties of the materials. Herein, we demonstrate a self-limiting solid-vapor interface reaction strategy to fabricate highly ordered SCOFs. The coupling reaction is tailored to take place at the solid-vapor interface by introducing one precursor via vaporization to the surface preloaded with the other precursor. Following this strategy, highly ordered honeycomb SCOFs with imine linkage are obtained. The controlled formation of SCOFs in our study shows the possibility of a rational design and synthesis of SCOFs with desired functionality.