Xuan-He Liu

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.

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.

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.

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.

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.

Screening for fractions of Oxytropis falcata Bunge with antibacterial activity

Preliminary studies with the four extracts of Oxytropis falcate Bunge exhibited that the chloroform and ethyl acetate extracts showed stronger antibacterial activities against the nine tested Gram-positive and Gram-negative bacteria. The HPLC-scanned and bioassay-guided fractionation led to the isolation and identification of the main flavonoid compounds, i.e. rhamnocitrin, kaempferol, rhamnetin, 2′,4′-dihydroxychalcone and 2′,4′,β-trihydroxy-dihydrochalcon. Except 2′,4′,β-trihydroxy-dihydrochalcon, four other compounds had good antibacterial activities. The minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) of the four compounds ranged between 125 and 515 µg mL−1. Staphylococcus aureus was the most susceptible to these compounds, with MIC and MBC values from 125 to 130 µg mL−1. This is the first report of antibacterial activity in O. falcate Bunge. In this study, evidence to evaluate the biological functions of O. falcate Bunge is provided, which promote the rational use of this herb.

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.

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.

Development of low-temperature desulfurization performance of a MnO2/AC composite for a combined SO2 trap for diesel exhaust

Growing concern about the removal of sulfur dioxide (SO2) from combustion exhaust has resulted in the development of desulfurization materials for a SO2 trap. In this study, a manganese dioxide/activated carbon (MnO2/AC) composite was proposed as a low temperature desulfurization material for a combined SO2 trap. The MnO2/AC composite was synthesized using a redox deposition method and characterized using scanning electron microscopy (SEM), nitrogen adsorption, X-ray fluorescence spectrometry (XRF) and Fourier transform infrared (FTIR) spectroscopy. The SO2 adsorption capacity of the composites was measured using thermogravimetry and the SO2 adsorption characteristics were also investigated. In the low temperature region (50–200 °C), the MnO2/AC composite exhibits good SO2 trap performance and the MnO2 conversion of the composite is significantly improved. It was found that the SO2 adsorption on the MnO2/AC composite is a chemisorption process. The experimental data for SO2 adsorption on the MnO2/AC composite could fit the Freundlich model well. Changes in the thermodynamic parameters were determined. The calculated values of ΔG0 and ΔH0 indicate that the SO2 adsorption on the MnO2/AC composite is spontaneous and thermodynamically favorable.

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.