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Luminescent carbon dots (CDs) have received increasing attention from many fields during the past decade. Unfortunately, the luminescent mechanisms of CDs remain unclear due to insufficient experimental and theoretical knowledge, which significantly hinders the development of CDs with desired optical properties. Currently, surface states of CDs, which are based on synergistic hybridization between the carbon backbones and the connected functional groups, have been considered as the dominant luminescence origins. This tutorial paper, thus, aims to offer an overview of the key features on the surface of CDs, such as particle size, surface functional groups, defects and heteroatom doping, and their influences on the photoluminescence of CDs. In addition, optical characteristics of surface state-derived luminescence emissions of CDs are also summarized. Finally, the potential approaches of characterizing surface states of CDs are introduced, followed by an outlook of synthesizing high-quality CDs through modulation of the surface states. [ABSTRACT FROM AUTHOR]

Mn3O4 is a promising cathode material for aqueous zinc ion batteries (ZIBs) which is a new type of low cost, eco‐friendly, high security energy storage system, while those previously reported electrochemical capacities of Mn3O4 are far from its theoretical value. In this work, Mn3O4 nanoparticles and nitrogen‐doped carbon dots (NCDs) are synthesized together through an in‐situ hydrothermal route, and then calcined to be a nanocomposite in which Mn3O4 nanoparticles are anchored on a nitrogen‐doped carbon skeleton (designated as Mn3O4/NCDs). Although the carbon content is only 3.9 wt.% in the Mn3O4/NCDs, the NCDs‐derived carbon skeleton provides an electrically conductive network and a stable structure. Such a special nanocomposite has a large specific surface area, plenty of active sites, excellent hydrophilicity and good electronic conductivity. Owing to these structural merits, the Mn3O4/NCDs electrode exhibits a preeminent specific capacity of 443.6 mAh g−1 and 123.3 mAh g−1 at current densities of 0.1 and 1.5 A g−1 in ZIBs, respectively, which are far beyond the bare Mn3O4 nanoparticles synthesized under the similar condition. The electrochemical measurement results prove that carbon dots, as a new type of carbon nanomaterials, have strong ability to modify and improve the performance of existing electrode materials, which may push these electrode materials forward to practical applications. [ABSTRACT FROM AUTHOR]

A series of polymer dots/graphene sheets composites with high densities are prepared and tested for supercapacitors. Polymer dots (PDs) are synthesized by one-step method at room temperature. They can effectively increase surface areas of the composites (almost 10 times), and the functional groups from PDs produce high pseudocapacitance, so that the samples exhibit high specific capacitances (e. g., 364.2 F cm −3 at 1 A g −1 ) and high cycling stability (e. g., more than 95% of the initial capacity retention over 10 000 cycles at different current densities). The optimal sample is employed to fabricate a symmetric supercapacitor, which exhibits an energy density up to 8 Wh L −1 and a power density up to 11 800 W L −1 , respectively. [ABSTRACT FROM AUTHOR]

Non-noble metal-based bifunctional electrocatalysts for both oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) are an essential component of high-performance rechargeable Zn-air batteries (ZABs). Herein, we report a novel and simple method for preparing Co 9 S 8 nanoparticles embedded in N and S codoped carbon materials with aid of carbon dots (CDs). CDs play a key role in distributing Co 9 S 8 nanoparticles in the matrix uniformly and enhancing the specific surface area and the electric conductivity simultaneously. The obtained Co 9 S 8 /CD@NSC exhibits an excellent ORR and OER bifunctional catalytic activity and a great long-term durability, with a half-wave potential of 0.84 V versus reversible hydrogen electrode (RHE) for the ORR and a low potential of 1.62 V versus RHE at 10 mA cm -2 , which outperform the popular Pt/C and RuO 2 commercial catalysts. Moreover, the Co 9 S 8 /CD@NSC catalyst also displays a superior activity and cycling stability as a cathode material in ZABs, which is far better than Pt/C + RuO 2 mixture catalysts. Such a ZAB shows a low charge/discharge voltage gap of 0.62 V and great cycling stability over 125 h at 10 mA cm -2 .

A series of carbon dots/NiCo 2 O 4 composites with various morphologies are prepared and tested for supercapacitors. These samples have good electrical conductivities and efficient ions transport paths, so they exhibit high specific capacitances, superior rate performances, and high cycling stabilities. The optimal composite for hybrid supercapacitor exhibits a high energy density up to 62.0 Wh kg -1 ., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)