Graphene: Powder, Flakes, Ribbons, and Sheets

Graphene’s unique physical and electrical properties (high tensile strength, Young’s modulus, electron mobility, and thermal conductivity) have led to its nickname of “super carbon.” Graphene research involves the study of several different physical forms of the material: powders, flakes, ribbons, and sheets and others not yet named or imagined. Within those forms, graphene can include a single layer, two layers, or ≤10 sheets of sp² carbon atoms. The chemistry and applications available with graphene depend on both the physical form of the graphene and the number of layers in the material. Therefore the available permutations of graphene are numerous, and we will discuss a subset of this work, covering some of our research on the synthesis and use of many of the different physical and layered forms of graphene.Initially, we worked with commercially available graphite, with which we extended diazonium chemistry developed to functionalize single-walled carbon nanotubes to produce graphitic materials. These structures were soluble in common organic solvents and were better dispersed in composites. We developed an improved synthesis of graphene oxide (GO) and explored how the workup protocol for the synthesis of GO can change the electronic structure and chemical functionality of the GO product. We also developed a method to remove graphene layers one-by-one from flakes. These powders and sheets of GO can serve as fluid loss prevention additives in drilling fluids for the oil industry.Graphene nanoribbons (GNRs) combine small width with long length, producing valuable electronic and physical properties. We developed two complementary syntheses of GNRs from multiwalled carbon nanotubes: one simple oxidative method that produces GNRs with some defects and one reductive method that produces GNRs that are less defective and more electrically conductive. These GNRs can be used in low-loss, high permittivity composites, as conductive reinforcement coatings on Kevlar fibers and in the fabrication of large area transparent electrodes.Using solid carbon sources such as polymers, food, insects, and waste, we can grow monolayer and bilayer graphene directly on metal catalysts, and carbon-sources containing nitrogen can produce nitrogen-doped graphene. The resulting graphene can be transferred to other surfaces, such as metal grids, for potential use in transparent touch screens for applications in personal electronics and large area photovoltaic devices. Because the transfer of graphene from one surface to another can lead to defects, low yields, and higher costs, we have developed methods for growing graphene directly on the substrates of interest. We can also produce patterned graphene to make GNRs or graphane/graphene superlattices within a single sheet. These superlattices could have multiple functions for use in sensors and other devices.This Account only touches upon this burgeoning area of materials chemistry, and the field will continue to expand as researchers imagine new forms and applications of graphene

Tuning Electrical Conductivity of Inorganic Minerals with Carbon Nanomaterials

Conductive powders based on Barite or calcium carbonate with chemically converted graphene (CCG) were successfully synthesized by adsorption of graphene oxide (GO) or graphene oxide nanoribbons (GONRs) onto the mineral surfaces and subsequent chemical reduction with hydrazine. The efficient adsorption of GO or GONRs on the surface of Barite and calcium carbonate-based mineral particles results in graphene-wrapped hybrid materials that demonstrate a concentration dependent electrical conductivity that increases with the GO or GONR loading

Graphene Oxide. Origin of Acidity, Its Instability in Water, and a New Dynamic Structural Model

The existing structural models of graphene oxide (GO) contradict each other and cannot adequately explain the acidity of its aqueous solutions. Inadequate understanding of chemical structure can lead to a misinterpretation of observed experimental phenomena. Understanding the chemistry and structure of GO should enable new functionalization protocols while explaining GO’s limitations due to its water instability. Here we propose an unconventional view of GO chemistry and develop the corresponding “dynamic structural model” (DSM). In contrast to previously proposed models, the DSM considers GO as a system, constantly changing its chemical structure due to interaction with water. Using potentiometric titration, ¹³C NMR, FTIR, UV–vis, X-ray photoelectron microscopy, thermogravimetric analysis, and scanning electron microscopy we show that GO does not contain any significant quantity of preexisting acidic functional groups, but gradually generates them through interaction with water. The reaction with water results in C–C bond cleavage, formation of vinylogous carboxylic acids, and the generation of protons. An electrical double layer formed at the GO interface in aqueous solutions plays an important role in the observed GO chemistry. Prolonged exposure to water gradually degrades GO flakes converting them into humic acid-like structures. The proposed DSM provides an explanation for the acidity of GO aqueous solutions and accounts for most of the known spectroscopic and experimental data

Meniscus-Mask Lithography for Narrow Graphene Nanoribbons

Described here is a planar top-down method for the fabrication of precisely positioned very narrow (sub-10 nm), high aspect ratio (>2000) graphene nanoribbons (GNRs) from graphene sheets, which we call meniscus-mask lithography (MML). The method does not require demanding high-resolution lithography tools. The mechanism involves masking by atmospheric water adsorbed at the edge of the lithography pattern written on top of the target material. The GNR electronic properties depend on the graphene etching method, with argon reactive ion etching yielding remarkably consistent results. The influence of the most common substrates (Si/SiO₂ and boron nitride) on the electronic properties of GNRs is demonstrated. The technique is also shown to be applicable for fabrication of narrow metallic wires, underscoring the generality of MML for narrow features on diverse materials

In Situ Synthesis of Efficient Water Oxidation Catalysts in Laser-Induced Graphene

NiFe-based catalysts are highly active for the oxygen evolution reaction (OER) in alkaline electrolytes. These catalysts are generally synthesized by solution-based methods. We present an in situ synthesis method for NiFe-based OER catalysts through a laser-induced graphene (LIG)-assisted process. By loading the metal precursor on a preformed LIG surface followed by laser scribing, we synthesized the NiFe/LIG catalysts via a solid phase transition that did not require utilization of CVD or typical solution-based reactions. The catalysts showed high OER activity and durability. The overpotential at 10 mA cm^–2 is as low as 240 mV with a Tafel slope of 32.8 mV dec^–1 in 1 M KOH. Additionally, this method worked well on a carbon fiber paper substrate, providing a convenient approach for the preparation of a free-standing catalytic electrode. This method provides a potential route to the facile synthesis of a variety of catalysts on a conductive surface

Influence of the Substrate on the Mobility of Individual Nanocars

We monitored the mobility of individual fluorescent nanocars on three surfaces: plasma cleaned, reactive ion etched, and amine-functionalized glass. Using single-molecule fluorescence imaging, the percentage of moving nanocars and their diffusion constants were determined for each substrate. We found that the nanocar mobility decreased with increasing surface roughness and increasing surface interaction strength

Reversible Formation of Ammonium Persulfate/Sulfuric Acid Graphite Intercalation Compounds and Their Peculiar Raman Spectra

Graphite intercalation compounds (GICs) can be considered stacks of individual doped graphene layers. Here we demonstrate a reversible formation of sulfuric acid-based GICs using ammonium persulfate as the chemical oxidizing agent. No covalent chemical oxidation leading to the formation of graphite oxide occurs, which inevitably happens when other compounds such as potassium permanganate are used to charge carbon layers. The resulting acid/persulfate-induced stage-1 and stage-2 GICs are characterized by suppression of the 2D band in the Raman spectra and by unusually strong enhancement of the G band. The G band is selectively enhanced at different doping levels with different excitations. These observations are in line with recent reports for chemically doped and gate-modulated graphene and support newly proposed theories of Raman processes. At the same time GICs have some advantageous differences over graphene, which are demonstrated in this report. Our experimental observations, along with earlier reported data, suggest that at high doping levels the G band cannot be used as the reference peak for normalizing Raman spectra, which is a commonly used practice today. A Fermi energy shift of 1.20–1.25 eV and ∼1.0 eV was estimated for the stage-1 and stage-2 GICs, respectively, from the Raman and optical spectroscopy data

Efficient Electrocatalytic Oxygen Evolution on Amorphous Nickel–Cobalt Binary Oxide Nanoporous Layers

Nanoporous Ni–Co binary oxide layers were electrochemically fabricated by deposition followed by anodization, which produced an amorphous layered structure that could act as an efficient electrocatalyst for water oxidation. The highly porous morphologies produced higher electrochemically active surface areas, while the amorphous structure supplied abundant defect sites for oxygen evolution. These Ni-rich (10–40 atom % Co) binary oxides have an increased active surface area (roughness factor up to 17), reduced charge transfer resistance, lowered overpotential (∼325 mV) that produced a 10 mA cm^–2 current density, and a decreased Tafel slope (∼39 mV decade^–1). The present technique has a wide range of applications for the preparation of other binary or multiple-metals or metal oxides nanoporous films. Fabrication of nanoporous materials using this method could provide products useful for renewable energy production and storage applications

Flexible Three-Dimensional Nanoporous Metal-Based Energy Devices

A flexible three-dimensional (3-D) nanoporous NiF₂-dominant layer on poly­(ethylene terephthalate) has been developed. The nanoporous layer itself can be freestanding without adding any supporting carbon materials or conducting polymers. By assembling the nanoporous layer into two-electrode symmetric devices, the inorganic material delivers battery-like thin-film supercapacitive performance with a maximum capacitance of 66 mF cm^–2 (733 F cm^–3 or 358 F g^–1), energy density of 384 Wh kg^–1, and power density of 112 kW kg^–1. Flexibility and cyclability tests show that the nanoporous layer maintains its high performance under long-term cycling and different bending conditions. The fabrication of the 3-D nanoporous NiF₂ flexible electrode could be easily scaled

Chemical Mass Production of Graphene Nanoplatelets in ∼100% Yield

Successful application of graphene is hampered by the lack of cost-effective methods for its production. Here, we demonstrate a method of mass production of graphene nanoplatelets (GNPs) by exfoliation of flake graphite in the tricomponent system made by a combination of ammonium persulfate ((NH₄)₂S₂O₈), concentrated sulfuric acid, and fuming sulfuric acid. The resulting GNPs are tens of microns in diameter and 10–35 nm in thickness. When in the liquid phase of the tricomponent media, graphite completely loses its interlayer registry. This provides a ∼100% yield of GNPs from graphite in 3–4 h at room temperature or in 10 min at 120 °C