ConspectusHydrogen evolution by the electrolysis of water is an important energy conversion process, especially for some intermittent energy systems. However, its practical applicability is limited by its low energy-conversion efficiency and the need for expensive electrocatalysts. In response, carbon-based nanocomposites have gained substantial interest as promising alternatives to the currently used precious metal catalysts. Among them, carbon dots (CDs) have exhibited many outstanding features, including flexible composition, high conductivity, good dispersion, and strong metal coordination. The past decade has seen remarkable advances in CD-based electrocatalysts for hydrogen evolution, where CDs and their derived hybrids have shown impressive performance and indispensable prospects for future hydrogen utilization. However, the widespread use of CD-based electrocatalysts still requires the development of high-quality CDs and advanced synthesis strategies. New assessment techniques are also required to elucidate the unique functions of CDs in composition regulation, structure fabrication, surface modification, and host–guest interactions in electrocatalysts and ultimately to establish the relationships among structure, composition, and activity.This Account is based on previous studies by our group and focuses on the synthesis of CDs and the types of CDs that make excellent electrocatalysts for hydrogen evolution. It draws on evidence from a range of advanced characterization techniques such as aberration-corrected scanning transmission electron microscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, in situ Fourier transform infrared spectroscopy, and atomic force microscopy. As a result, a general route to fabricating efficient CD-based electrocatalysts for hydrogen evolution is reported, and the structure–activity correlations of CD-based electrocatalysts are demonstrated. Heteroatomic modification of electrocatalysts via composition regulation of CDs is also investigated. For example, first-principles density functional theory is used to investigate the unique properties of molybdenum phosphide modified with nitrogen-doped CDs: the introduction of nitrogen atoms is shown to generate carbon vacancies, thus reducing the coordination number of reaction sites and enhancing their electrocatalytic activity. The effect of the CDs’ spatial confinement due to self-assembly is also elucidated, where the splicing of CDs and novel CD–metal interfacial effects also greatly stabilize the metal active components and improve their catalytic efficiency. Finally, we offer some of our knowledge and insights on the current challenges and future research directions in this field from the perspectives of CD growth mechanisms, electrocatalyst synthesis, performance optimization, and expanding applicability. 

Carbon-Dot-Enhanced Electrocatalytic Hydrogen Evolution

Resistance to aggregation-caused quenching: chitosan-based solid carbon dots for white light-emitting diode and 3D printing

As a relatively new type of fluorescent carbon-based nanomaterials, multicolor carbon dots (MCDs) have attracted much attention because of their excellent biocompatibility, tunable photoluminescence (PL), high quantum yield, and unique electronic and physicochemical properties. The multicolor emission characteristics of carbon dots (CDs) obviously depend on the carbon source precursor, reaction conditions, and reaction environment, which directly or indirectly determines the multicolor emission characteristics of CDs. Therefore, this review is the first systematic classification and summary of multiple regulation methods of synthetic MCDs and reviews the recent research progress in the synthesis of MCDs from a variety of precursor materials such as aromatic molecules, small organic molecules, and natural biomass, focusing on how different regulation methods produce corresponding MCDs. This review also introduces the innovative applications of MCDs in the fields of biological imaging, light-emitting diodes (LEDs), sensing, and anti-counterfeiting due to their excellent PL properties. It is hoped that by selecting appropriate adjustment methods, this review can inspire and guide the future research on the design of tailored MCDs, and provide corresponding help for the development of multifunctional MCDs.

The Emerging Development of Multicolor Carbon Dots.

The challenges of toxicity and stability of traditional long-wavelength emitting luminescent materials undoubtedly restrain their widespread applications. Some remedial proposals do not fundamentally solve these problems. So, the development of new types of long-wavelength emitting luminescent materials has become a new trend. Herein, orange emissive carbon quantum dots (O-CQDs) are successfully prepared by a simple one-step solvothermal approach using 1-amino-2-naphthol hydrochloride and citric acid (CA). Interestingly, O-CQDs can always be achieved by the regulation of different reaction conditions (e.g., CA concentration, temperature, and reaction times), which is closely related to the fixed structure of O-CQDs inherited from the used precursor. An O-CQD displays its homogeneous size distribution with an average size of 2.5 nm and obtains a high-graphitoid sp2 hybrid architecture. Impressively, O-CQDs exhibit remarkable photoluminescent (PL) properties, including excitation-independent behavior, thermostability, photostability, and the robust non-solvatochromic effect, in different solvents due to their efficient π-conjugated system. These excellent properties lay the optical foundation of O-CQDs for the construction of high-performance white-light-emitting diode (WLED) devices. More importantly, O-CQDs are employed as a light regulator combined with yellow emissive CQDs to form the phosphor, and then CQD phosphor-based all-type WLEDs with a high color rendering index (CRI, ≥81) are realized. This work provides a new avenue for the exploration of low-cost, environment-friendly, and high-performance CQD phosphor-based all-type WLEDs using long-wavelength emitting CQDs as a light regulator. 

One-Pot Synthesis of Orange Emissive Carbon Quantum Dots for All-Type High Color Rendering Index White Light-Emitting Diodes

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Triple-emission nitrogen and boron co-doped carbon quantum dots from lignin: Highly fluorescent sensing platform for detection of hexavalent chromium ions.

Tunable emission carbonized polymer dots (CPDs) are highly desirable for the preparation of optoelectronic devices, especially white light-emitting diodes (WLEDs). In most available studies, polychromatic CPDs are synthesized using aromatic molecules as precursors. However, few studies report the successful synthesis of polychromatic CPDs using two or more unconjugated precursors. In this work, we prepare multicolor fluorescent CPDs from a single unconjugated precursor, glucose, via a hydrothermal reaction. By controlling the particle size and degree of graphitization of the synthesized CPDs, their emission wavelength can be tuned in the range 440-625 nm (i.e., almost the entire visible region). Furthermore, the CPDs can be used to construct LEDs of varying colors, including WLEDs (CIE coordinates: 0.34, 0.36) with the correlated color temperature and color rendering index of 4997 K and 92.69, respectively. In brief, the strategy proposed in this study successfully converts unconjugated glucose into high-performance LEDs with great application potential.

Facile Synthesis of Water-Stable Multicolor Carbonized Polymer Dots from a Single Unconjugated Glucose for Engineering White Light-Emitting Diodes with a High Color Rendering Index.

Polymeric semiconductors are crucial candidates for the construction of next-generation flexible and printable electronic devices. By virtue of the successful preparation of monodispersed colloidal solution in orthogonal solvent, poly(3-hexylthiophene) (P3HT) nanofibers are developed into versatile building blocks for nanoelectronics and their compatibilities are verified with photolithographic lift-off technology. Then, the joint efforts from both the bottom-up hierarchical self-assembly and top-down self-alignment technology have led to the realization of lateral asymmetric heterojunctions with resolution better than 1 µm. As a result, planar photovoltaic devices incorporating N,N'-dioctyl-3,4,9,10-perylenedicarboximide and P3HT supramolecular nanowires as active components are constructed with the cathode-to-anode distance being tuned from ≈0.1 to 1-2 µm. Based on such a novel device configuration, an interesting phenomenon of channel-length-dependent photovoltaic efficiency is observed for the first time, strongly suggesting the impact of near-field light intensity on the performance of nanophotonic devices. 

Construction of Laterally Asymmetric Heterojunctions with Sub‐Micrometer Resolution by Hierarchical Self‐Assembly of Polythiophene Nanofibers

The mutual conversion between light and electricity lies at the heart of optoelectronic and photonic applications. Maximization of the photoelectric conversion is a long‐term goal that can be pursued via the fabrication of devices with ad‐hoc architectures. In this framework, it is of utter importance to harvest and transform light irradiation into high electric potential in specific area for driving functional dielectrics that respond to pure electric field. Here, a nano‐fabrication technology has been devised featuring double self‐alignment that is applied to construct zebra‐like asymmetric heterojunction arrays. Such nanostructured composite, which covers a surface area of 5 × 4 mm2 and contains 500 periodic repeating units, is capable of photo generating voltages as high as 140 V on a flexible substrate. This approach represents a leap over the traditional functionalization process based on simply embedding materials into devices by demonstrating the disruptive potential of integrating oriented nanoscale device components into meta‐material.

Cailing Ou

Papers

Facile Synthesis of Water-Stable Multicolor Carbonized Polymer Dots from a Single Unconjugated Glucose for Engineering White Light-Emitting Diodes with a High Color Rendering Index.

Construction of Laterally Asymmetric Heterojunctions with Sub‐Micrometer Resolution by Hierarchical Self‐Assembly of Polythiophene Nanofibers

A Flexible, High‐Voltage (>100 V) Generating Device Based on Zebra‐Like Asymmetrical Photovoltaic Cascade