Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Covalent organic frameworks (COFs) represent an exciting new type of porous organic materials, which are ingeniously constructed with organic building units via strong covalent bonds. The well-defined crystalline porous structures together with tailored functionalities have offered the COF materials superior potential in diverse applications, such as gas storage, adsorption, optoelectricity, and catalysis. Since the seminal work of Yaghi and co-workers in 2005, the rapid development in this research area has attracted intensive interest from researchers with diverse expertise. This critical review describes the state-of-the-art development in the design, synthesis, characterisation, and application of the crystalline porous COF materials. Our own opinions on further development of the COF materials are also presented for discussion (155 references).

Covalent organic frameworks (COFs): from design to applications

Covalent organic frameworks (COFs) are a class of crystalline porous polymers that allow the atomically precise integration of organic units to create predesigned skeletons and nanopores. They have recently emerged as a new molecular platform for designing promising organic materials for gas storage, catalysis, and optoelectronic applications. The reversibility of dynamic covalent reactions, diversity of building blocks, and geometry retention are three key factors involved in the reticular design and synthesis of COFs. This tutorial review describes the basic design concepts, the recent synthetic advancements and structural studies, and the frontiers of functional exploration.

Covalent organic frameworks

Conversion of carbon dioxide (CO2) to carbon monoxide (CO) and other value-added carbon products is an important challenge for clean energy research. Here we report modular optimization of covalent organic frameworks (COFs), in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. The catalysts exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000, with initial turnover frequency of 9400 hour(-1)) at pH 7 with an overpotential of -0.55 volts, equivalent to a 26-fold improvement in activity compared with the molecular cobalt complex, with no degradation over 24 hours. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.

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Covalent organic frameworks comprising cobalt porphyrins for catalytic CO2 reduction in water

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Covalent Organic Frameworks: Design, Synthesis, and Functions.

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 possib...

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

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.

Graphene‐Like Single‐Layered Covalent Organic Frameworks: Synthesis Strategies and Application Prospects

Construction and repair of highly ordered 2D covalent networks by chemical equilibrium regulation.

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.

Cui-Zhong Guan

Papers

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

Graphene‐Like Single‐Layered Covalent Organic Frameworks: Synthesis Strategies and Application Prospects

Construction and repair of highly ordered 2D covalent networks by chemical equilibrium regulation.

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

Electrospray soft-landing for the construction of non-covalent molecular nanostructures using charged droplets under ambient conditions