Heteroatom doping can endow graphene with various new or improved electromagnetic, physicochemical, optical, and structural properties. This greatly extends the arsenal of graphene materials and their potential for a spectrum of applications. Considering the latest developments, we comprehensively and critically discuss the syntheses, properties and emerging applications of the growing family of heteroatom-doped graphene materials. The advantages, disadvantages, and preferential doping features of current synthesis approaches are compared, aiming to provide clues for developing new and controllable synthetic routes. We emphasize the distinct properties resulting from various dopants, different doping levels and configurations, and synergistic effects from co-dopants, hoping to assist a better understanding of doped graphene materials. The mechanisms underlying their advantageous uses for energy storage, energy conversion, sensing, and gas storage are highlighted, aiming to stimulate more competent applications.

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Heteroatom-doped graphene materials: syntheses, properties and applications

This article gives an overview of the various kinds of nanoparticles (NPs) that are widely used for purposes of fluorescent imaging, mainly of cells and tissues. Following an introduction and a discussion of merits of fluorescent NPs compared to molecular fluorophores, labels and probes, the article assesses the kinds and specific features of nanomaterials often used in bioimaging. These include fluorescently doped silicas and sol–gels, hydrophilic polymers (hydrogels), hydrophobic organic polymers, semiconducting polymer dots, quantum dots, carbon dots, other carbonaceous nanomaterials, upconversion NPs, noble metal NPs (mainly gold and silver), various other nanomaterials, and dendrimers. Another section covers coatings and methods for surface modification of NPs. Specific examples on the use of nanoparticles in (a) plain fluorescence imaging of cells, (b) targeted imaging, (c) imaging of chemical species, and (d) imaging of temperature are given next. A final section covers aspects of multimodal imaging (such as fluorescence/nmr), imaging combined with drug and gene delivery, or imaging combined with therapy or diagnosis. The electronic supplementary information (ESI) gives specific examples for materials and methods used in imaging, sensing, multimodal imaging and theranostics such as imaging combined with drug delivery or photodynamic therapy. The article contains 273 references in the main part, and 157 references in the ESI.

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An overview of nanoparticles commonly used in fluorescent bioimaging

Carbon quantum dots (CQDs) as a rising star of carbon nanomaterials, by virtue of their unique physicochemical, optical and electronic properties, have displayed tremendous momentum in numerous fields such as biosensing, bioimaging, drug delivery, optoelectronics, photovoltaics and photocatalysis. In particular, the rich optical and electronic properties of CQDs including efficient light harvesting, tunable photoluminescence (PL), extraordinary up-converted photoluminescence (UCPL) and outstanding photoinduced electron transfer have attracted considerable interest in different photocatalytic applications for the sake of full utilization of the solar spectrum. This review aims to demonstrate the recent progress in the synthesis, properties and photocatalytic applications of CQDs, particularly highlighting the fundamental multifaceted roles of CQDs in photoredox processes. Furthermore, we discuss the challenges and future direction of CQD-based materials in this booming research field, with a perspective toward the ultimate achievement of highly efficient and long-term stable CQD-based photocatalysts.

Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis

Graphene quantum dots (GQDs) are nanometer-sized fragments of graphene that show unique properties, which makes them interesting candidates for a whole range of new applications. This review article gives an overview of the synthesis, properties and applications of GQDs. Synthesis methods discussed include top-down and bottom-up approaches. Properties such as luminescence up- and down-conversion have been used in applications ranging from energy conversion to bio-analytics. This article provides an overview of the state-of-the-art and highlights promising findings as well as potential future directions of the research field.

Graphene Quantum Dots

Designing a highly active electrocatalyst with optimal stability at low cost is must and non-negotiable if large-scale implementations of fuel cells are to be fully realized. Zeolitic-imidazolate frameworks (ZIFs) offer rich platforms to design multifunctional materials due to their flexibility and ultrahigh surface area. Herein, an advanced Co–Nx/C nanorod array derived from 3D ZIF nanocrystals with superior electrocatalytic activity and stability toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) compared to commercial Pt/C and IrO2, respectively, is synthesized. Remarkably, as a bifunctional catalyst (Ej = 10 (OER) − E1/2 (ORR) ≈ 0.65 V), it further displays high performance of Zn–air batteries with high cycling stability even at a high current density. Such supercatalytic properties are largely attributed to the synergistic effect of the chemical composition, high surface area, and abundant active sites of the nanorods. The activity origin is clarified through post oxygen reduction X-ray photoelectron spectroscopy analysis and density functional theory studies. Undoubtedly, this approach opens a new avenue to strategically design highly active and performance-oriented electrocatalytic materials for wider electrochemical energy applications.

From 3D ZIF Nanocrystals to Co-N x /C Nanorod Array Electrocatalysts for ORR, OER, and Zn-Air Batteries

Doping of low-dimensional graphitic materials, including graphene, graphene quantum dots and single-wall carbon nanotubes with nitrogen, sulfur or boron can significantly change their properties. We report that simple fluorination followed by annealing in a dopant source can superdope low-dimensional graphitic materials with a high level of N, S or B. The superdoping results in the following doping levels: (i) for graphene, 29.82, 17.55 and 10.79 at% for N-, S- and B-doping, respectively; (ii) for graphene quantum dots, 36.38 at% for N-doping; and (iii) for single-wall carbon nanotubes, 7.79 and 10.66 at% for N- and S-doping, respectively. As an example, the N-superdoping of graphene can greatly increase the capacitive energy storage, increase the efficiency of the oxygen reduction reaction and induce ferromagnetism. Furthermore, by changing the degree of fluorination, the doping level can be tuned over a wide range, which is important for optimizing the performance of doped low-dimensional graphitic materials.

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Elemental superdoping of graphene and carbon nanotubes

Fluorinated graphene quantum dots (F-GQDs) were synthesized by cutting fluorinated graphene through a hydrothermal approach. The F-GQDs with oxygen-rich functional groups have a F/C atomic ratio of ca. 23.68% and diameter of 1-7 nm. The photoluminescence (PL) properties of the F-GQDs were investigated. The results showed that besides exhibiting bright blue PL, the F-GQDs display a clear upconversion PL.

Synthesis and photoluminescence of fluorinated graphene quantum dots

The long spin diffusion length makes graphene very attractive for novel spintronic devices, and thus has triggered a quest for integrating the charge and spin degrees of freedom. However, ideal graphene is intrinsic non-magnetic, due to a delocalized π bonding network. Therefore, synthesis of ferromagnetic graphene or its derivatives with high magnetization is urgent due to both fundamental and technological importance. Here we report that N-doping can be an effective route to obtain a very high magnetization of ca. 1.66 emu/g, and can make graphene oxide (GO) to be ferromagnetism with a Curie-temperature of 100.2 K. Clearly, our findings can offer the easy realization of ferromagnetic GO with high magnetization, therefore, push the way for potential applications in spintronic devices.

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Realization of ferromagnetic graphene oxide with high magnetization by doping graphene oxide with nitrogen

Fluorination was confirmed to be the most effective route to introduce localized spins in graphene. However, adatoms clustering in perfect graphene lead to a low efficiency. In this study, we report experimental evidence of the generation of localized spin magnetic moments on defective graphene (reduced graphene oxide) through fluorination. More interstingly, the result shows that defects help increase the efficiency of the fluorination with regard to the density of magnetic moments created. Fluorinated reduced graphene oxide can have a high magnetic moment of 3.187 × 10–3 μB per carbon atom and a high efficiency of 8.68 × 10–3 μB per F atom. It may be attributed to the many vacancies, which hinder the clustering of F atoms, and introduce many magnetic edge adatoms.

Obtaining high localized spin magnetic moments by fluorination of reduced graphene oxide.

Highly oxidative debris (OD) was obtained by aqueous ammonia wash of as-prepared graphene oxide (GO) which composed of OD and lightly oxidative GO sheets. The magnetic properties of OD and GO were studied. The results showed that OD has a low magnetization of 0.16 emu/g, and the magnetization of GO can be increased from 0.38 to 0.42 emu/g by discarding low-magnetization OD. Thus, this study provided a reliable method to increase the magnetization of GO. Hydroxyl groups were proposed to be the magnetic source.

Fuchi Liu

Papers

Elemental superdoping of graphene and carbon nanotubes

Synthesis and photoluminescence of fluorinated graphene quantum dots

Realization of ferromagnetic graphene oxide with high magnetization by doping graphene oxide with nitrogen

Obtaining high localized spin magnetic moments by fluorination of reduced graphene oxide.

Identifying the magnetic properties of graphene oxide