Graphitic carbon nitride, g-C3N4, can be made by polymerization of cyanamide, dicyandiamide or melamine. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities are obtained. The firstly formed polymeric C3N4 structure, melon, with pendant amino groups, is a highly ordered polymer. Further reaction leads to more condensed and less defective C3N4 species, based on tri-s-triazine (C6N7) units as elementary building blocks. High resolution transmission electron microscopy proves the extended two-dimensional character of the condensation motif. Due to the polymerization-type synthesis from a liquid precursor, a variety of material nanostructures such as nanoparticles or mesoporous powders can be accessed. Those nanostructures also allow fine tuning of properties, the ability for intercalation, as well as the possibility to give surface-rich materials for heterogeneous reactions. Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions, such as for the activation of benzene, trimerization reactions, and also the activation of carbon dioxide. Model calculations are presented to explain this unusual case of heterogeneous, metal-free catalysis. Carbon nitride can also act as a heterogeneous reactant, and a new family of metal nitride nanostructures can be accessed from the corresponding oxides.

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Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts

Graphitic carbon nitrides (g-C3N4) are becoming increasingly significant due to the theoretical prediction of their unusual properties and promising applications ranging from photocatalysis, heterogeneous catalysis, to fuel cells. Recently, a variety of nanostructured and nanoporous g-C3N4 materials have been developed for a wide range of new applications. This feature article gives, at first, an overview on the synthesis of g-C3N4 nanomaterials with controllable structure and morphology, and secondly, presents and categorizes applications of g-C3N4 as multifunctional metal-free catalysts for environmental protection, energy conversion and storage. A special emphasis is placed on the potential applications of nanostructured g-C3N4 in the areas of artificial photocatalysis for hydrogen production, oxygen reduction reaction (ORR) for fuel cells, and metal-free heterogeneous catalysis. Finally, this perspective highlights crucial issues that should be addressed in the future in the aforementioned exciting research areas.

Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis

Sodium-ion batteries have captured widespread attention for grid-scale energy storage owing to the natural abundance of sodium. The performance of such batteries is limited by available electrode materials, especially for sodium-ion layered oxides, motivating the exploration of high compositional diversity. How the composition determines the structural chemistry is decisive for the electrochemical performance but very challenging to predict, especially for complex compositions. We introduce the "cationic potential" that captures the key interactions of layered materials and makes it possible to predict the stacking structures. This is demonstrated through the rational design and preparation of layered electrode materials with improved performance. As the stacking structure determines the functional properties, this methodology offers a solution toward the design of alkali metal layered oxides.

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Rational design of layered oxide materials for sodium-ion batteries

Towards better Li metal anodes: Challenges and strategies

In this communication, we demonstrate for the first time that ultrathin graphitic carbon nitride (g-C3N4) nanosheets can serve as a low-cost, green, and highly efficient electrocatalyst toward the reduction of hydrogen peroxide. We further demonstrate its application for electrochemical glucose biosensing in both buffer solution and human serum medium with a detection limit of 11 μM and 45 μM, respectively.

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Ultrathin graphitic carbon nitride nanosheets: a low-cost, green, and highly efficient electrocatalyst toward the reduction of hydrogen peroxide and its glucose biosensing application.

A Stable Layered Oxide Cathode Material for High-Performance Sodium-Ion Battery

Li metal anodes, which have attracted much attention for their high specific capacity and low redox potential, face a great challenge in realizing their practical application. The fatal issue of dendrite formation gives rise to internal short circuit and safety hazards and needs to be addressed. Here we propose a rational strategy of trapping Li within microcages to confine the deposition morphology and suppress dendrite growth. Microcages with a carbon nanotube core and porous silica sheath were prepared and proved to be effective for controlling the electrodeposition behavior. In addition, the insulative coating layer prevents concentrated electron flow and decreases the possibility of “hot spots” formation. Because of the Li trapper and uniform electron distribution, the electrode with delicate structure exhibits a dendrite-free morphology after plating 2 mA h cm–2 of Li. As the dendrite growth is suppressed, the as-obtained electrode maintains a high plating/stripping efficiency of 99% over 200 cycles...

Trapping Lithium into Hollow Silica Microspheres with a Carbon Nanotube Core for Dendrite-Free Lithium Metal Anodes

Delivery of high-energy density with long cycle life is facing a severe challenge in developing cathode materials for rechargeable sodium-ion batteries (SIBs). Here a composite Na0.6MnO2 with layered-tunnel structure combining intergrowth morphology of nanoplates and nanorods for SIBs, which is clearly confirmed by micro scanning electron microscopy, high-resolution transmission electron microscopy as well as scanning transmission electron microscopy with atomic resolution is presented. Owing to the integrated advantages of P2 layered structure with high capacity and that of the tunnel structure with excellent cycling stability and superior rate performance, the composite electrode delivers a reversible discharge capacity of 198.2 mAh g(-1) at 0.2C rate, leading to a high-energy density of 520.4 Wh kg(-1). This intergrowth integration engineering strategy may modulate the physical and chemical properties in oxide cathodes and provide new perspectives on the optimal design of high-energy density and high-stable materials for SIBs.

A Layered–Tunnel Intergrowth Structure for High-Performance Sodium-Ion Oxide Cathode

Layered O3-type sodium oxides (NaMO2 , M=transition metal) commonly exhibit an O3-P3 phase transition, which occurs at a low redox voltage of about 3 V (vs. Na+ /Na) during sodium extraction and insertion, with the result that almost 50 % of their total capacity lies at this low voltage region, and they possess insufficient energy density as cathode materials for sodium-ion batteries (NIBs). Therefore, development of high-voltage O3-type cathodes remains challenging because it is difficult to raise the phase-transition voltage by reasonable structure modulation. A new example of O3-type sodium insertion materials is presented for use in NIBs. The designed O3-type Na0.7 Ni0.35 Sn0.65 O2 material displays a highest redox potential of 3.7 V (vs. Na+ /Na) among the reported O3-type materials based on the Ni2+ /Ni3+ couple, by virtue of its increased Ni-O bond ionicity through reduced orbital overlap between transition metals and oxygen within the MO2 slabs. This study provides an orbital-level understanding of the operating potentials of the nominal redox couples for O3-NaMO2 cathodes. The strategy described could be used to tailor electrodes for improved performance.

An Abnormal 3.7 Volt O3-Type Sodium-Ion Battery Cathode.

A route to prepare nitrides, such as GaN, VN, and other nitrides, is reported. The reaction pathway involves a two-step process by using the as-synthesized a-C3N3.69 as precursor. The route is so potent that a series of nitrides can be directly synthesized from their oxides at moderate temperatures. A striking feature of this method lies in that a-C3N3.69 is found to play double roles as both carbonizing and nitridizing agent in these reactions. These results will greatly deepen our understandings of the mechanism for solid-state metathesis reactions.

Xinan Yang

Papers

A Stable Layered Oxide Cathode Material for High-Performance Sodium-Ion Battery

Trapping Lithium into Hollow Silica Microspheres with a Carbon Nanotube Core for Dendrite-Free Lithium Metal Anodes

A Layered–Tunnel Intergrowth Structure for High-Performance Sodium-Ion Oxide Cathode

An Abnormal 3.7 Volt O3-Type Sodium-Ion Battery Cathode.

Route to GaN and VN assisted by carbothermal reduction process.