Date Published: March 02, 2016
Publisher: John Wiley and Sons Inc.
Author(s): Wei Feng, Peng Long, Yiyu Feng, Yu Li.
Fluorinated graphene, an up‐rising member of the graphene family, combines a two‐dimensional layer‐structure, a wide bandgap, and high stability and attracts significant attention because of its unique nanostructure and carbon–fluorine bonds. Here, we give an extensive review of recent progress on synthetic methods and C–F bonding; additionally, we present the optical, electrical and electronic properties of fluorinated graphene and its electrochemical/biological applications. Fluorinated graphene exhibits various types of C–F bonds (covalent, semi‐ionic, and ionic bonds), tunable F/C ratios, and different configurations controlled by synthetic methods including direct fluorination and exfoliation methods. The relationship between the types/amounts of C–F bonds and specific properties, such as opened bandgap, high thermal and chemical stability, dispersibility, semiconducting/insulating nature, magnetic, self‐lubricating and mechanical properties and thermal conductivity, is discussed comprehensively. By optimizing the C–F bonding character and F/C ratios, fluorinated graphene can be utilized for energy conversion and storage devices, bioapplications, electrochemical sensors and amphiphobicity. Based on current progress, we propose potential problems of fluorinated graphene as well as the future challenge on the synthetic methods and C‐F bonding character. This review will provide guidance for controlling C–F bonds, developing fluorine‐related effects and promoting the application of fluorinated graphene.
Since Andre Geim and Kostya Novoselov first isolated high‐quality few‐atom‐thick nanosheets (including single‐layer) from graphite, the ability to prepare graphene and its derivatives have triggered intense research in two‐dimensional nanomaterials all over the world.1 Subsequently, graphene‐based materials receive much attention in nanotechnology because of their extraordinary properties, such as an ultrahigh theoretical specific surface area (2630 m2 g−1), exceptional charge carrier mobility (200 000 cm2 V−1 s−1), high thermal conductivity (≈5000 W m−1 K−1), high optical transmittance (≈97.7%).2 Despite these aforementioned superiorities, pristine graphene suffers from several shortcomings including structural defects, chemical inertness and a zero bandgap. Thus, many functionalization methods such as chemical bonding, loading or generating functional groups or free radicals on graphene (or its derivatives) have been utilized to improve structural integrity, surface activity and processability.3 The functionalization not only inherits unique carbon conjugated structures but also brings about a promise to alter the graphene’s properties including dispersion, orientation, interaction and electronic properties.4
The methods for synthesizing fluorinated graphene or fluorographene are mainly classified into two groups: fluorination and exfoliation methods. Fluorination mainly include direct gas‐fluorination,[[qv: 4a]],[[qv: 12a]],[[qv: 13b]],[[qv: 21a]],22 plasma fluorination,23 hydrothermal fluorination,24 and photochemical/electrochemical synthesis.25 Exfoliation methods includes sonochemical exfoliation,[[qv: 9a]],26 modified Hummer’s exfoliation,20,[[qv: 21b]] and thermal exfoliation.27 F/C ratios of fluorinated graphene prepared by different methods are summarized in Table2.
C‐F bonding character including C‐F bonds, F/C ratio, and configuration largely determines the chemical (electrochemical), electrical, electronic, optical, magnetic structures, stability and hydrophobicity of fluorinated graphene. Thus, the deep understanding of fluoro‐carbon structure is fundamental to control the properties and design the application of fluorinated graphene. In this section, we discusse the C‐F structural characteristics controlled by a variety of methods or technologies to offer a strategy for tuning C‐F bonds precisely and uniformly.
Fluorinated graphene shows many excellent properites such as wide bandgap of 3.1 eV, the highest theoretical specific capacity (865 mA h g−1), good thermal stability below 400 °C, distinct nonlinear feature and high hydrophobicity. In this section, we discuss a variety of properties including bandgap, absorption or luminescence, stability, electronic conductivity, dspersibility, magnetic properties, tribological properties, mechanical or micromechanical properties, and thermal conductivity. These properties are significantly important for the application of fluorinated graphene.
In this review, we have given an overview of synthetic methods, structures and properties of fluorinated graphene that can be utilized for applications in high‐energy storage, unique biological response and magnetic resonance imaging, fluorinated graphene quantum dots, supercapacitors, electrochemistry and amphiphobicity. We have emphasized the importance and significance of controlling C‐F bonding characters, F/C ratios and configurations of fluorinated graphene, fluorographene and F‐GQDs by fluorination (gas or liquid phase) or exfoliation. The selective fluorination enables graphene with different two‐dimensional configurations for various properties including wide bandgap, blue luminescence, excellent electrochemistry, high stability and self‐lubricating. For example, an increase in the F/C ratio enlarges the bandgap of fluorographene, while a low F/C ratio usually ensures charge transport based on π‐conjugated structures. The covalent C‐F bonds in gas fluorination are crucial for thermal and chemical stability, while semi‐ionic and ionic bonds endow fluorinated graphene with a higher discharge potential for lithium batteries. Moreover, thermal conductivity, magnetic properties and luminescence of fluorinated graphene are not well developed because of a complicated fluoro‐carbon structure.