Article pubs.acs.org/accounts
Graphene-Based Functional Architectures: Sheets Regulation and Macrostructure Construction toward Actuators and Power Generators Huhu Cheng,†,§ Yaxin Huang,† Gaoquan Shi,§ Lan Jiang,†,∥ and Liangti Qu*,†,‡ †
Key Laboratory for Advanced Materials Processing Technology, Ministry of Education of China; State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China ‡ Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China § Department of Chemistry, Tsinghua University, Beijing 100084, PR China ∥ Laser Micro-/Nano-Fabrication Laboratory, Beijing Institute of Technology, Beijing, 100081, P. R. China CONSPECTUS: Graphene, with large delocalized π electron cloud on a two-dimensional (2D) atom-thin plane, possesses excellent carrier mobility, large surface area, high light transparency, high mechanical strength, and superior flexibility. However, the lack of intrinsic band gap, poor dispersibility, and weak reactivity of graphene hinder its application scope. Heteroatom-doping regulation and surface modification of graphene can effectively reconstruct the sp2 bonded carbon atoms and tailor the surface chemistry and interfacial interaction, while microstructure mediation on graphene can induce the special chemical and physical properties because of the quantum confinement, edge effect, and unusual mass transport process. Based on these regulations on graphene, series of methods and techniques are developed to couple the promising characters of graphene into the macroscopic architectures for potential and practical applications. In this Account, we present our effort on graphene regulation from chemical modification to microstructure control, from the morphology-designed macroassemblies to their applications in functional systems excluding the energy-storage devices. We first introduce the chemically regulative graphene with incorporated heteroatoms into the honeycomb lattice, which could open the intrinsic band gap and provide many active sites. Then the surface modification of graphene with functional components will improve dispersibility, prevent aggregation, and introduce new functions. On the other hand, microstructure mediation on graphene sheets (e.g., 0D quantum dots, 1D nanoribbons, and 2D nanomeshes) is demonstrated to induce special chemical and physical properties. Benefiting from the effective regulation on graphene sheets, diverse methods including dimension-confined strategy, filtration assembly, and hydrothermal treatment have been developed to assemble individual graphene sheets to macroscopic graphene fibers, films, and frameworks. These rationally regulated graphene sheets and well-constructed assemblies present promising applications in energy-conversion materials and device systems focusing on actuators that can convert different energy forms (e.g., electric, chemical, photonic, thermal, etc.) to mechanical actuation and electrical generators that can directly transform environmental energy to electric power. These results reveal that graphene sheets with surface chemistry and microstructure regulations as well as their rationally designed assemblies provide a promising and abundant platform for development of diverse functional devices. We hope that this Account will promote further efforts toward fundamental research on graphene regulation and the wide applications of advanced designed assemblies in new types of energy-conversion materials/devices and beyond. less controlled morphologies.1,3,4 However, the practical applications of graphene are largely limited by its intrinsic zero band gap and weak chemical activity. To address this problem, graphene sheets need to be modified on their surfaces and microstructures to tailor their intrinsic physical and chemical properties.2,4−6,11,12 Furthermore, rational assembly of graphene sheets into macroscopic architectures is desirable to integrate the excellent characters of individual graphene
1. INTRODUCTION Graphene is a two-dimensional (2D) atom-thin sheet of sp2bonded carbon atoms. It has high intrinsic carrier mobility, large specific surface area, high mechanical strength, and light transparency, as well as superior flexibility, showing a considerable potential for the applications in a variety of fields.1−10 The diversified preparation methods (e.g., mechanical/liquid exfoliation, epitaxial growth, chemical vapor deposition (CVD), chemical oxidation, and reduction of graphite) provide abundant types of graphene and derivatives despite their imperfect structures, inhomogeneous sizes, and © 2017 American Chemical Society
Received: March 17, 2017 Published: June 28, 2017 1663
DOI: 10.1021/acs.accounts.7b00131 Acc. Chem. Res. 2017, 50, 1663−1671
Article
Accounts of Chemical Research
Scheme 1. Schematic of Graphene Based Architectures from the Chemically Regulative and Microstructure-Tailored Graphene Nanosheets to the Macroassemblies of Fibers, Films, and Frameworks, and Their Applications in Actuators and Generators
Figure 1. Scheme of chemically doped graphene (a), graphene/polymer (b), and graphene/metal (c) hybrids. (d) Piece of N-graphene film. Reproduced with permission from ref 24. Copyright 2010 American Chemical Society. (e) Transmission electron microscopy (TEM) image of graphene/PPy sheet. Reproduced with permission from ref 25. Copyright 2013 Wiley-VCH. (f) TEM image of shape-defined Pt nanostructures on graphene sheet. Reproduced with permission from ref 27. Copyright 2012 Wiley-VCH.
morphology-designed macroassemblies, then to their applications in sensitive actuators and generators (Scheme 1). First, we will introduce the chemical modification of graphene with heteroatoms or other functional components, followed by outlining graphene sheets with special microstructures such as quantum dots and nanomeshes. Then, we will discuss the preparation of macroscopic functional graphene assemblies including fibers, films and frameworks and their applications in actuators and electric generators.
sheets into macrostructures, improving the performances of energy-related materials and devices.4−6,13−18 Stimulus-sensitive graphene materials have attracted a tremendous attention in recent years because of their wide applications in actuators, generators, and so forth.7−9 Graphene-based smart devices are expected to be superior to those of the counterparts based on other stimuli-responsive materials such as metal alloys or polymers, mainly due to the unique structure and excellent electrical, thermal, and mechanical properties of graphene. Many reviews have focused on the fabrication of graphene composites, macroscopic graphene assemblies, and their applications in energy storage.2,4,6,16,19,20 A systematic discussion from graphene nanosheet modification to special macrostructure construction toward stimuli-responsive energy-conversion devices is still absent. In this Account, we will present our effort in constructing graphene-based functional architectures, starting from chemically regulative and microstructure-tailored nanosheets to
2. GRAPHENE SHEET REGULATION: HETEROATOM DOPING AND SURFACE MODIFICATION The properties of graphene strongly depend on its atomic composition and surface chemistry. The intentional incorporation of special heteroatoms of different size and electronegativity compared to carbon atom into the honeycomb lattices of graphene can effectively reconstruct its sp2 bonded state and the density of electron, opening the intrinsic band gap and providing many active sites, thus greatly affecting the 1664
DOI: 10.1021/acs.accounts.7b00131 Acc. Chem. Res. 2017, 50, 1663−1671
Article
Accounts of Chemical Research
Figure 2. Scheme of GQDs (a), graphene nanoribbon (b) and nanomesh (c). (d) TEM image of electrochemically prepared GQDs. Reproduced with permission from ref 31. Copyright 2011 Wiley-VCH. (e) TEM image of GO nanoribbon. Reproduced with permission from ref 34. Copyright 2017 Wiley-VCH. (f) Scanning transmission electron microscopy image of graphene nanomesh with an enlarged nanohole. Reproduced with permission from ref 37. Copyright 2014 Royal Society of Chemistry.
regard, we have very early prepared N-doped graphene containing pyridine-like and pyrrolic N atoms (Figure 1a and d) through a straightforward CVD method. Its effective electrocatalytic behavior toward ORR has been proved.24 Previous studies showed that low temperature benefited the formation of graphitic N.12,23 However, we found that three types of N were coexisting in N-doped graphene prepared under laser treatment (500 °C).24 The alternative heteroatoms include boron, phosphor, sulfur, fluorine, and so on.12,23 They provide a broad scope of regulation on intrinsic characters of graphene.
localized electronic nature and planar structure of graphene (Figure 1a).6,9,12 On the other hand, the modification of graphene with functional components will modulate its surface and interface behaviors, and induce new functions by synergetic effect between them (Figure 1b and c).2,6,21 As a result, characteristics such as conductivity, dispersibility, and reaction activity of graphene can be well regulated and controlled through surface modification. 2.1. Heteroatom Doping
The incorporation of heteroatoms can rearrange the atomic configuration on the graphene plane, and induce the polarization of electron density at carbon−heteroatom bonds, which greatly influences the electrical properties and chemical activity of graphene. For example, graphene oxide (GO) is not electrically conductive, because the incorporation of oxygen distorts the planar structure of graphene by transfer of carbon atoms from sp2 to sp3 hybridized states in carbon−oxygen bonding sites,22 accordingly with the increased thickness of GO sheet (∼1.0 nm) compared to that of graphene sheet (∼0.34 nm). The abundant oxygen-containing groups (e.g., carboxylic acid on the edge, phenol hydroxyl and epoxide groups at the basal plane) of GO make it hydrophilic and well dispersible in aqueous solvents.21,22 Meanwhile, many defects are introduced into the graphene plane during the GO preparation process, providing more active sites and further changing the conductivity of GO.22 GO can be produced in industrial scale and is important as a precursor for reduced graphene oxide (rGO) and other graphene-based derivatives. Unlike oxygen atoms that easily form covalent functional groups on graphene, nitrogen atoms can be incorporated into carbon hexagonal rings due to the similar lengths between C− N (1.41 Å) and C−C (1.42 Å) bonds.12,23 Generally, N atoms exist in the forms of sp2 hybridized graphitic, pyridinic, and sp3 hybridized pyrrolic N. Graphitic and pyridinic N have a slight influence on the graphene plane structure, while pyrrolic N disturbs it. Graphitic N inner carbon hexagonal rings benefit the conductivity of graphene, while pyridinic and pyrrolic N at defect sites provide active sites on graphene in favor of chemical or electrochemical process in supercapacitors, solar cells, and oxygen reduction reaction (ORR) in fuel cells.12,23 In this
2.2. Surface Modification with Functional Components
Surface-modified graphene hybrids with organic, inorganic, and polymer constituents are important for various applications.2,6,21 The modification of graphene sheets with polymers, especially conducting polymers (Figure 1b), is a good route to improve the mechanical and electrical properties.25,26 For example, polypyrrole (PPy) can uniformly coat along the graphene surface and form a PPy/graphene/PPy sandwiched structure because its conjugated structure with electron-rich N atom induced the H-bonding or π−π interaction with graphene (Figure 1e), which largely reduces the aggregation between graphene sheets and reinforces the self-supporting nature of graphene framework.25,26 Up to now, graphene hybrids with various polymers have been fabricated, including polyaniline and poly(3,4-ethylenedioxythiophene), poly(vinyl alcohol), poly(styrene), and poly(vinylidene fluoride). These modification greatly improves the mechanical, electrical, persistent property of graphene due to the synergism of polymers and graphene.21 The decoration of graphene with functional metal or metal oxide nanocomponents (Figure 1c)6,27 further broadens the application in various areas. For example, shape-defined Pt nanostructures on graphene sheets (Figure 1f) effectively prevented graphene aggregation and increased its electroactivity for oxidation of fuel molecules. 27 Meanwhile, the Pt consumption is largely reduced due to the large surface area of graphene. Diverse functional nanoparticles have been combined with graphene, including magnetic Fe3O4, light sensitive TiO2, graphitic C3N4, and metal−organic frameworks, 1665
DOI: 10.1021/acs.accounts.7b00131 Acc. Chem. Res. 2017, 50, 1663−1671
Article
Accounts of Chemical Research
Figure 3. Strategies toward graphene assembled fiber, film, and framework. (a) Graphene sheet. (b) Dimension-confined assembly of graphene in pipeline with GO precursor solution. (c) Graphene fiber. Reproduced with permission from ref 38. Copyright 2012 Wiley-VCH. (d) Graphene film formed by vacuum filtration. (e) Flexible graphene film. (f) Graphene framework formed by hydrothermal method. (g) Photo of graphene frameworks.
which provide rich sorts of surface-modified graphene for specific applications.17,28,29
And the character of GQDs can be further regulated by size control, heteroatoms doping, or preparation method.4,32 3.2. Graphene Nanoribbons
3. MICROSCOPIC STRUCTURE MEDIATION OF GRAPHENE: 0D QUANTUM DOTS (GQDs), 1D NANORIBBONS, AND 2D NANOMESHES Graphene sheets often exhibits inhomogeneous size from hundreds of nanometers to tens of micrometers. Beyond surface modification, microstructure mediation on graphene shape/size will induce the quantum confinement, edge effect, and unusual mass transport process.4,11,30 Typical examples include 0D GQDs, 1D nanoribbons, and 2D nanomeshes (Figure 2a−c).
Graphene nanoribbons are graphene sheets with nanoscale width (Figure 2b). As quasi-one-dimensional nanomaterials, graphene nanoribbons have nonzero band gap originating from quantum confinement or staggered sublattice potential related to armchair or zigzag edges.11,33 Graphene nanoribbons can be prepared by many methods include lithographic patterning, chemical treatment, direct CVD, ultrasonic, or solvent oxidation exfoliation of carbon nanotubes (CNTs).4 However, they face the poor dispersibility and need further modification at the atomic level or surface. Oxidized graphene nanoribbons are dispersible in aqueous phase, which provides a versatile platform for broad applications, as we presented by cutting CNTs under strong acid (Figure 2e).34
3.1. GQDs
GQD has a reduced size of graphene in nanometer level, and the carriers in it are confined in a nanosized domain. Thus, the quantum confinement and edge effect will occur and affect the chemical/physical properties of graphene.4,11,30 Graphene sheet with a desired geometry (
