Graphene-Based Functional Architectures: Sheets Regulation and Macrostructure Construction toward Actuators and Power Generators.

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.

Size Fractionation of Graphene Oxide Nanosheets via Controlled Directional Freezing.

The properties and functions of graphene oxide (GO)-based materials strongly depend on the lateral size and size distribution of GO nanosheets; therefore, GO and its derivatives with narrow size distributions are highly desired. Here we report the size fractionation of GO nanosheets by controlled directional freezing of GO aqueous dispersions. GO nanosheets with a narrow size distribution can be obtained by controlling the growth rate of the freezing front. This interesting phenomenon can be explained by the adsorption of GO nanosheets on the ice crystal surface in combination with the stratification of GO nanosheets at the ice growth front. Such a convenient size fractionation approach will be essential for practical applications of chemically modified graphene, including GO, reduced GO, and their assemblies or composites.

Cobalt disulfide/graphite foam composite films as self-standing electrocatalytic electrodes for overall water splitting.

The development of ecofriendly electrocatalysts with earth-abundant metal elements for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for overall water splitting to generate clean and sustainable chemical energy. Here, we report a self-standing cobalt disulfide/graphite foam (CoS2/GF) electrocatalytic electrode for this purpose. It showed high catalytic activities for both the HER and OER, requiring only a cell voltage of 1.74 V to achieve a current density of 20 mA cm-2 for overall water splitting in an alkaline electrolyte. This three-dimensional microporous electrocatalytic electrode is cheap and available in a large area; thus, it is attractive for practical applications.

High-performance gas sensors based on a thiocyanate ion-doped organometal halide perovskite.

Thin films of a thiocyanate ion (SCN-)-doped organometal halide perovskite, CH3NH3PbI3-x(SCN)x, were used as a sensing material for developing high-performance gas sensors. The CH3NH3PbI3-x(SCN)x-based chemiresistor-type sensor can sensitively and selectively detect acetone and nitrogen dioxide (NO2) at room temperature with high sensitivities of 5.6 × 10-3 and 5.3 × 10-1 ppm-1. The limits of detection for acetone and NO2 were measured to be 20 ppm and 200 ppb. This sensor also exhibited excellent repeatability, and its environmental stability was greatly improved by doping the perovskite with SCN- ions.

Conducting Polymer-Based Catalysts.

Conducting polymers (CPs) have been widely applied as electrocatalysts and photocatalysts in energy-related systems, sensors, and environmental protection. This is mainly due to their promising catalytic activities, high conductivities, and unique electrochemical and optical properties. Furthermore, CPs can be cheaply and conveniently prepared in large scale via chemical or electrochemical approaches. In this Perspective, we review the recent advancements on the synthesis and applications of CP-based inherent and composite catalysts and CP-derived heteroatom-doped carbon catalysts. The mechanisms of catalysis will be introduced, and the challenges of developing CP-based catalysts with practical importance will be discussed.

Solution-Processed Ultraelastic and Strong Air-Bubbled Graphene Foams.

Solution-processed ultraelastic graphene foams are prepared via a convenient air-bubble-promoted synthesis. These foams can dissipate external compression through the ordered interconnecting graphene network between the bubbles without causing a local fracture and thus reliably show compressive stress of 5.4 MPa at a very high strain of 99%, setting a new benchmark for solution-processed graphene foams.

Self-Assembled Three-Dimensional Graphene Macrostructures: Synthesis and Applications in Supercapacitors.

Graphene and its derivatives are versatile building blocks for bottom-up assembly of advanced functional materials. In particular, with exceptionally large specific surface area, excellent electrical conductivity, and superior chemical/electrochemical stability, graphene represents the ideal material for various electrochemical energy storage devices including supercapacitors. However, due to the strong π-π interaction between graphene sheets, the graphene flakes tend to restack to form graphite-like powders when they are processed into practical electrode materials, which can greatly reduce the specific surface area and lead to inefficient utilization of the graphene layers for electrochemical energy storage. The self-assembly of two-dimensional graphene sheets into three-dimensional (3D) framework structures can largely retain the unique properties of individual graphene sheets and has recently garnered intense interest for fundamental investigations and potential applications in diverse technologies. In this Account, we review the recent advances in preparing 3D graphene macrostructures and exploring them as a unique platform for supercapacitor applications. We first describe the synthetic strategies, in which reduction of a graphene oxide dispersion above a certain critical concentration can induce the reduced graphene oxide sheets to cross-link with each other via partial π-π stacking interactions to form a 3D interconnected porous macrostructure. Multiple reduction strategies, including hydrothermal/solvothermal reduction, chemical reduction, and electrochemical reduction, have been developed for the preparation of 3D graphene macrostructures. The versatile synthetic strategies allow for easy incorporation of heteroatoms, carbon nanomaterials, functional polymers, and inorganic nanostructures into the macrostructures to yield diverse composites with tailored structures and properties. We then summarize the applications of the 3D graphene macrostructures for high-performance supercapacitors. With a unique framework structure in which the graphene sheets are interlocked in 3D space to prevent their restacking, the graphene macrostructures feature very high specific surface areas, rapid electron and ion transport, and superior mechanical strength. They can thus be directly used as supercapacitor electrodes with excellent specific capacitances, rate capabilities, and cycling stabilities. We finally discuss the current challenges and future opportunities in this research field. By regarding the graphene as both a single-atom-thick carbon sheet and a conjugated macromolecule, our work opens a new avenue to bottom-up self-assembly of graphene macromolecule sheets into functional 3D graphene macrostructures with remarkable electrochemical performances. We hope that this Account will promote further efforts toward fundamental investigation of graphene self-assembly and the development of advanced 3D graphene materials for their real-world applications in electrochemical energy storage devices and beyond.

Mildly reduced less defective graphene oxide/sulfur/carbon nanotube composite films for high-performance lithium-sulfur batteries.

The microstructures and properties of the carbonaceous matrices in the cathodes of lithium-sulfur (Li-S) batteries have strong effects on their performances. We prepared a ternary composite cathode of mildly reduced less defective graphene oxide (mrLGO), sulfur, and carbon nanotubes (CNTs) by filtration for Li-S batteries. This battery showed a high initial specific capacity of 1219 mA h g(-1) at 0.2 C and a stable specific capacity of around 1000 mA h g(-1) after 200 cycles with a coulombic efficiency of 99%. Its excellent performance is mainly attributed to the good conductivity and residual oxygen containing groups of mrLGO, and the three-dimensional (3D) framework constructed using mrLGO sheets and CNTs.

N,P-Codoped Carbon Networks as Efficient Metal-free Bifunctional Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions.

The high cost and scarcity of noble metal catalysts, such as Pt, have hindered the hydrogen production from electrochemical water splitting, the oxygen reduction in fuel cells and batteries. Herein, we developed a simple template-free approach to three-dimensional porous carbon networks codoped with nitrogen and phosphorus by pyrolysis of a supermolecular aggregate of self-assembled melamine, phytic acid, and graphene oxide (MPSA/GO). The pyrolyzed MPSA/GO acted as the first metal-free bifunctional catalyst with high activities for both oxygen reduction and hydrogen evolution. Zn-air batteries with the pyrolyzed MPSA/GO air electrode showed a high peak power density (310 W g(-1) ) and an excellent durability. Thus, the pyrolyzed MPSA/GO is a promising bifunctional catalyst for renewable energy technologies, particularly regenerative fuel cells.

An introduction to the chemistry of graphene.

Pristine graphene and chemically modified graphenes (CMGs, e.g., graphene oxide, reduced graphene oxide and their derivatives) can react with a variety of chemical substances. These reactions have been applied to modulate the structures and properties of graphene materials, and to extend their functions and practical applications. This perspective outlines the chemistry of graphene, including functionalization, doping, photochemistry, catalytic chemistry, and supramolecular chemistry. The mechanisms of graphene related reactions will be introduced, and the challenges in controlling the chemical reactions of graphene will be discussed.

Porphyrin-based graphene oxide frameworks with ultra-large d-spacings for the electrocatalyzation of oxygen reduction reaction.

Graphene oxide frameworks (GOFs) have attracted a great deal of attention because of their unique functional building blocks, and tunable structures and properties. Herein, a series of porphyrin-based GOFs with crystalline lamellar structures were synthesized via esterification between boronic acid groups of porphyrins and hydroxyl groups of GO sheets. These GOFs have ultra-large d-spacings of up to 26.0 Å, and they were reduced by facile electrochemical reduction. The resulting reduced GOFs (rGOFs) can be used as catalysts for oxygen reduction reaction (ORR). Electrochemical reduction improved the conductivities of GOFs, accelerating the charge transfer of ORR. The rGOF with Co-porphyrin showed the most positive onset potential of ORR at 0.89 V (vs. RHE, reversible hydrogen electrode), while the rGOF with Fe-porphyrin exhibited the highest catalytic efficiency through an approximately four-electron process. This study provides a new insight for the development of GOFs using multi-functional macrocyclic molecules, revealing their promising applications in electrocatalysts.

A three-dimensional interpenetrating electrode of reduced graphene oxide for selective detection of dopamine.

Electrochemical detection of dopamine plays an important role in medical diagnosis. In this paper, we report a three-dimensional (3D) interpenetrating graphene electrode fabricated by electrochemical reduction of graphene oxide for selective detection of dopamine. This electrochemically reduced graphene oxide (ErGO) electrode was used directly without further functionalization or blending with other functional materials. This electrode can efficiently lower the oxidation potential of ascorbic acid; thus, it is able to selectively detect dopamine in the presence of ascorbic acid and uric acid. The ErGO-based biosensor exhibited a linear response towards dopamine in the concentration range of 0.1-10 µM with a low detection limit of 0.1 µM. Furthermore, this electrode has good reproducibility and environmental stability, and can be used to analyse real samples.

Picomolar detection of mercury (II) using a three-dimensional porous graphene/polypyrrole composite electrode.

A polypyrrole (PPy)-functionalized three-dimensional (3D) porous electrode of electrochemically reduced graphene oxide (ErGO) has been prepared by electrochemical deposition. This PPy-modified 3D-ErGO electrode was used for the electrochemical detection of Hg(2+) ions, and it exhibited high sensitivity and selectivity. Furthermore, the limit of detection (LOD) was measured to be as low as 0.03 nM (30 ppt), and this value is much lower than the guideline value of 2 ppb for drinking water given by the World Health Organization.

A high-performance platinum electrocatalyst loaded on a graphene hydrogel for high-rate methanol oxidation.

Platinum (Pt)-based catalysts used in direct methanol fuel cells (DMFCs) usually suffer from low catalytic activity, slow kinetics of methanol oxidation and poor electrochemical stability. This is mainly due to the toxic effect of carbon monoxide and inefficient use of the Pt catalysts. To address these problems, we immobilized Pt nanoparticles with diameters of 4-6 nm onto the three-dimensional (3D) interpenetrating graphene networks (graphene hydrogel or G-Gel) deposited in the micropores of nickel foam (NF). In this Pt/G-Gel/NF composite catalyst, nearly all the Pt nanoparticles are accessible to methanol and can be efficiently used for electrocatalyzation. It showed excellent electrochemical stability and an activity 2.6 times that of a conventional Pt/reduced graphene oxide (Pt/rGO) composite catalyst. Furthermore, the rate of methanol electro-oxidation at the Pt/G-Gel/NF catalyst can be about 27 times that at the Pt/rGO catalyst, making it applicable for fabricating DMFCs with high current and/or power outputs.

Graphitic carbon nitride nanoribbons: graphene-assisted formation and synergic function for highly efficient hydrogen evolution.

The development of new promising metal-free catalysts is of great significance for the electrocatalytic hydrogen evolution reaction (HER). Herein, a rationally assembled three-dimensional (3D) architecture of 1D graphitic carbon nitride (g-C3N4) nanoribbons with 2D graphene sheets has been developed by a one-step hydrothermal method. Because of the multipathway of charge and mass transport, the hierarchically structured g-C3N4 nanoribbon-graphene hybrids lead to a high electrocatalytic ability for HER with a Tafel slope of 54 mV decade(-1), a low onset overpotential of 80 mV and overpotential of 207 mV to approach a current of 10 mA cm(-2), superior to those non-metal materials and well-developed metallic catalysts reported previously. This work presents a great advance for designing and developing highly efficient metal-free catalyst for hydrogen evolution.

Large scale preparation of graphene quantum dots from graphite with tunable fluorescence properties.

We report an improved Hummers method for synthesizing graphene quantum dots (GQDs) by directly oxidizing and etching graphite powders. The yield of GQDs is as high as 63 ± 7% (by weight, wt%), suggesting this technique is suitable for producing GQDs on a large scale. The GQDs are nanocrystals with lateral dimensions in the range of 2-4 nm and an average thickness of around 1.3 nm. The emission peaks of as-prepared GQDs can be tuned in the range of 440 to 510 nm by varying the reaction conditions. Their fluorescence quantum yields were tested to be around 1%, which could be further increased to about 3% by hydrothermal treatment. These GQDs have low cytotoxicity and excellent biocompatibility, indicating that they are promising for biological applications.

Nitrogen-doped graphene quantum dots with oxygen-rich functional groups.

Graphene quantum dots (GQDs) represent a new class of quantum dots with unique properties. Doping GQDs with heteroatoms provides an attractive means of effectively tuning their intrinsic properties and exploiting new phenomena for advanced device applications. Herein we report a simple electrochemical approach to luminescent and electrocatalytically active nitrogen-doped GQDs (N-GQDs) with oxygen-rich functional groups. Unlike their N-free counterparts, the newly produced N-GQDs with a N/C atomic ratio of ca. 4.3% emit blue luminescence and possess an electrocatalytic activity comparable to that of a commercially available Pt/C catalyst for the oxygen reduction reaction (ORR) in an alkaline medium. In addition to their use as metal-free ORR catalysts in fuel cells, the superior luminescence characteristic of N-GQDs allows them to be used for biomedical imaging and other optoelectronic applications.

Synthesis of gold@carbon dots composite nanoparticles for surface enhanced Raman scattering.

Gold@carbon dots composite nanoparticles (Au@CDs) with ultrathin carbon dot (CD) shells of ca. 2 nm were prepared by reducing HAuCl(4) with CDs at 100 °C. By adjusting the feeding mass ratio of HAuCl(4) to CDs, the average diameters of Au@CDs can be modulated from 8 to 44 nm. The suspension of Au@CDs with an average diameter of ca. 24 nm was applied as a substrate for surface enhanced Raman scattering (SERS) and it exhibited a higher SERS effect for rhodamine 6G (Rh6G) than the suspension of pure Au nanoparticles with nearly the same size. The excellent SERS effect of Au@CDs is mainly attributed to their improved capability of adsorbing the aromatic probe molecules.

A versatile, ultralight, nitrogen-doped graphene framework.

Graphene lite: a density of (2.1 ± 0.3) mg cm(-3), the lowest to date for a graphene framework architecture, is achieved by preparing an ultralight, N-doped, 3D graphene framework (see photo of a block of the material balancing on a dandelion). Its adsorption capacity for oils and organic solvents is much higher than that of the best carbonaceous sorbents, and it is a promising electrode material for supercapacitors (484 F g(-1)) and as a metal-free catalyst for the oxygen reduction reaction.

Size fractionation of graphene oxide sheets by pH-assisted selective sedimentation.

Graphene oxide (GO) sheets prepared by Hummers' method have been separated into two portions with large (f1) or small (f2) lateral dimensions from their aqueous dispersion. This method is based on the selective precipitation of GO sheets with lateral dimensions mostly (>90%) larger than 40 µm(2) at a pH value of 4.0 because of their larger hydrophobic planes and fewer hydrophilic oxygenated groups. The hydrazine reduced Langmuir-Blodgett (LB) films of f1 showed much higher conductivities than those of f2. Furthermore, the thin film of f1 prepared by filtration exhibited a smaller d-space and much higher tensile strength and modulus than those of f2 films. The one-step size fractionation method reported here is simple, cheap, efficient, and environmentally friendly, which can be used for the size fractionation of GO sheets in large scale.