Dynamic Electrosorption Analysis as an Effective Means to Characterise the Structure of Bulk Graphene Assemblies

Restacking of graphene sheets to a graphite‐like structure is a prevailing problem that is known to compromise the performance of individual graphene sheets in an assembled bulk form. To address this common problem efficiently and monitor the structure and quality of graphene products comprehensively, it is highly desirable to develop reliable metrology techniques for characterising graphene‐based materials on a bulk assembly level and in a quantitative manner. Here, by revisiting the physicochemical principle of electrosorption, we propose a simple electrochemical approach, namely dynamic electrosorption analysis (DEA), as an easily accessible and effective technique for evaluation of the self‐stacking behaviour of graphene. Taking multilayered chemically converted graphene films as a model, we demonstrate that the DEA technique can effectively reveal very subtle variation in accessible surface area and pore size of graphene assemblies in the liquid phase and thus can provide useful insights to the experimental design relating to restacking control. This work also reveals the huge effect some routine processing conditions, such as heat treatment and drying, can have on the structure and performance of graphene‐based bulk materials, providing useful guidance for future manufacturing of this class of materials.     

A benchmark quantum chemical study of the stacking interaction between larger polycondensed aromatic hydrocarbons

Large-scale electronic structure calculations were performed for the interaction energy between coronene, C₂₄H₁₂ with circumcoronene, C₅₄H₁₈, and between two circumcoronene molecules, in order to get a picture of the interaction between larger graphene sheets. Most calculations were performed at the SCS-MP2 level but we have corrected them for higher-order correlation effects using a calculation on the coronene-circumcoronene system at the quadratic CI, QCISD(T) level. Our best estimate for the interaction energy between coronene and circumcoronene is 32.1?kcal/mol. We estimate the binding of coronene on a graphite surface to be 37.4 or 1.56?kcal/mol per carbon atom (67.5?meV/C atom). This is also our estimate for the exfoliation energy of graphite. It is higher than most previous theoretical estimates. The SCS-MP2 method which reproduces the CCSD(T) and QCISD(T) values very well for smaller aromatic hydrocarbons, e.g., for the benzene dimer, increasingly overestimates dispersion as the bandgap (the HOMO-LUMO separation) decreases. The barrier to the sliding motion of coronene on circumcoronene is 0.45?kcal/mol, and for two circumcoronene molecules 1.85?kcal/mol (0.018 and 0.034?kcal/mol per C atom, respectively). This means that larger graphenes cannot easily glide over each other.     

Surface‐Confined Self‐Assembled Janus Tectons: A Versatile Platform towards the Noncovalent Functionalization of Graphene

A general strategy for simultaneously generating surface‐based supramolecular architectures on flat sp²‐hybridized carbon supports and independently exposing on demand off‐plane functionality with controlled lateral order is highly desirable for the noncovalent functionalization of graphene. Here, we address this issue by providing a versatile molecular platform based on a library of new 3D Janus tectons that form surface‐confined supramolecular adlayers in which it is possible to simultaneously steer the 2D self‐assembly on flat C(sp²)‐based substrates and tailor the external interface above the substrate by exposure to a wide variety of small terminal chemical groups and functional moieties. This approach is validated throughout by scanning tunneling microscopy (STM) at the liquid–solid interface and molecular mechanics modeling studies. The successful self‐assembly on graphene, together with the possibility to transfer the graphene monolayer onto various substrates, should considerably extend the application of our functionalization strategy.     

Adsorption characteristic of self‐assembled corrole dimers on HOPG

Our study first focus on two types of corrole dimers oxidized and reduced forms on highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS) and contact angle measurement (CAM) were used to investigate the self‐assembled monolayers of corrole dimers adsorbed on HOPG surfaces at room temperature in air. XPS and CAM results have confirmed both two molecules adsorbed on an HOPG surface and formed self‐assembled films, and STM experiments found that the corrole dimers adsorbed on HOPG surfaces form similar lobes. The different stable space structure of the oxidized form molecule (OFM) and reduced form molecule (RFM), led to the diversity of the tetramer structural dimensions. The occurrence of molecular aggregations and assembly was controlled by the interactions between molecular–molecular and molecule–substrate. The electrostatic interactions between the molecules control the geometrical sizes and molecule–substrate interactions determine topographical shapes of the self‐assembled corrole dimers on HOPG surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Facile Synthesis of Nitrogen‐Doped Graphene Aerogels for Electrode Materials in Supercapacitors

Three‐dimensional porous nitrogen‐doped graphene aerogels (NGAs ) were synthesized by using graphene oxide (GO ) and chitosan (CS ) via a self‐assembly process by one‐pot hydrothermal method. The morphology and structure of the as‐prepared materials were characterized by means of scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, XPS spectroscopy, Raman spectroscopy, nitrogen adsorption/desorption measurement and Fourier transform infrared spectroscopy. The electrochemical performance of NGAs was studied by cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. The microstructure, surface area and capacitance of NGAs could be facilely controlled by adding different amounts of chitosan. The prepared NGA ‐4 showed a specific capacitance of 148.0 F/g at the discharge current density of 0.5 A/g and also retained 95.3% of the initial capacitance after 5000 cycles at the scan rate of 10 mV /s. It provided a possible way to obtain graphene based materials with high surface area and capacitance.     

Effect of Head‐Group Chemistry on Surface‐Mediated Molecular Self‐Assembly

Surface molecular self‐assembly is a fast advancing field with broad applications in sensing, patterning, device assembly, and biochemical applications. A vast number of practical systems utilize alkane thiols supported on gold surfaces. Whereas a strong Au? S bond facilitates robust self‐assembly, the interaction is so strong that the surface is reconstructed, leaving etch pits that render the monolayers susceptible to degradation. By using different head group elements to adcust the molecule–surface interaction, a vast array of new systems with novel properties may be formed. In this paper we use a carefully chosen set of molecules to make a direct comparison of the self‐assembly of thioether, selenoether, and phosphine species on Au(111). Using the herringbone reconstruction of gold as a sensitive readout of molecule–surface interaction strength, we correlate head‐group chemistry with monolayer (ML) properties. It is demonstrated that the hard/soft rules of inorganic chemistry can be used to rationalize the observed trend of molecular interaction strengths with the soft gold surface, that is, P>Se>S. We find that the structure of the monolayers can be explained by the geometry of the molecules in terms of dipolar, quadrupolar, or van der Waals interactions between neighboring species driving the assembly of distinct ordered arrays. As this study directly compares one element with another in simple systems, it may serve as a guide for the design of self‐assembled monolayers with novel structures and properties.     

Concentration induced the interfacial self‐assembly polymorphism of 4, 4′‐dihexadecyloxy‐benzophenon by scanning tunneling microscopy

The concentration effect on a two‐dimensional (2D) self‐assembly of 4, 4′‐dihexadecyloxy‐benzophenon (DHB) has been investigated by scanning tunneling microscopy. The self‐assembly of DHB at the phenyloctane/graphite interface was concentration dependent. Under low concentration, the DHB molecules were adsorbed intactly on the graphite surface. With the increasing of concentration, one of side chains connecting the conjugated moiety stretched into the liquid phase. The coexistence of two self‐assembled structures was observed in a moderate concentration. The result indicated that the van der Waals interactions between the molecules and the graphite lattice were decreasing with the increasing concentration. After the samples were placed in ambient conditions over 24 h, a different self‐assembled structure was obtained on the gas/solid interface, in which the DHB molecules were adsorbed on the surface with only one of the side chains. Both the benzophenon core and the other side chain were extended to the gas phase. The results demonstrated that concentration played an important role in forming the 2D molecular self‐assembly and provided an efficient approach for the control of the DHB molecular nanostructure. Copyright © 2013 John Wiley & Sons, Ltd.     

From 2D Graphene Nanosheets to 3D Graphene‐based Macrostructures

The combination of exceptional functionalities offered by 3D graphene‐based macrostructures (GBMs) has attracted tremendous interest. 2D graphene nanosheets have a high chemical stability, high surface area and customizable porosity, which was extensively researched for a variety of applications including CO₂ adsorption, water treatment, batteries, sensors, catalysis, etc. Recently, 3D GBMs have been successfully achieved through few approaches, including direct and non‐direct self‐assembly methods. In this review, the possible routes used to prepare both 2D graphene and interconnected 3D GBMs are described and analyzed regarding the involved chemistry of each 2D/3D graphene system. Improvement of the accessible surface of 3D GBMs where the interface exchanges are occurring is of great importance. A better control of the chemical mechanisms involved in the self‐assembly mechanism itself at the nanometer scale is certainly the key for a future research breakthrough regarding 3D GBMs.     

Magnetic nanocomposite of self‐doped polyaniline–graphene as a novel sorbent for solid‐phase extraction

This work is the first study on the extraction efficiency of self‐doped polyaniline that is immobilized on the graphene‐modified magnetic nanoparticles. The new material was used as a sorbent for the magnetic solid‐phase extraction of methyl‐, propyl‐, and butylparabens. The use of graphene provides a high surface area and prevents aggregation of the nanoparticles. The self‐doped polyaniline also provides multifunctionality, high extraction capacity, and chemical stability even in the basic medium. The parabens were acetylated for determination by gas chromatography with flame ionization detection. The effects of monomer ratio, extraction solvent, sorbent amount, sample volume, desorption solvent volume, adsorption and desorption times, and sample ionic strength were optimized. Preconcentration factors obtained were from 190 to 310. The detection limits of the method were <2.8 μg/L. Linear ranges of the method were 5–2000 μg/L for propyl and butyl parabens, and 10–2000 μg/L for methyl paraben. The method was applied for the determination of the parabens in cosmetic products and extraction recoveries were 89–101% with RSDs ≤7.9%.     

In situ self‐assembled reduced graphene oxide aerogel embedded with nickel oxide nanoparticles for the high‐efficiency separation of ovalbumin

A three‐dimensional reduced graphene oxide aerogel with embedded nickel oxide nanoparticles was prepared by a one‐step self‐assembly reaction in a short time. The nanoparticles could be captured into the interior of reduced graphene oxide network during the formation of the three‐dimensional architecture. The composite exhibited porosity, good biocompatibility, and abundant metal affinity binding sites. The aerogel was used to isolate ovalbumin selectively from egg white, and favorable adsorption was achieved at pH 3. An adsorption efficiency of 90.6% was obtained by using 1 mg of the composite for adsorbing 70 μg/mL of ovalbumin in 1.0 mL of sample solution, and afterwards a recovery of 90.7% was achieved by using an eluent of 1.0 mL Britton–Robinson buffer solution at pH 5. After the adsorption/desorption, ovalbumin showed no change in the conformation. The adsorption behavior of ovalbumin on the reduced graphene oxide composite well fitted to the Langmuir adsorption model, and a corresponding theoretical maximum adsorption capacity was 1695.2 mg/g. A sodium dodecyl sulfate polyacrylamide gel electrophoresis assay demonstrated that the aerogel could selectively isolate ovalbumin from chicken egg white.     

Photochemical Reactions in Self‐Assembled Organic Monolayers Characterized by using Scanning Tunneling Microscopy

Research on the supramolecular self‐assembly behavior at interfaces is of great importance to improving the performance of nanodevices that are based on optical functional materials. In this Minireview, several photoinduced isomerization and polymerization reactions in self‐assembled organic monolayers on surfaces are discussed. Typical organic molecules contain azobenzene, alkynyl, or olefins groups. The feature surface base is a highly oriented pyrolytic graphite (HOPG) surface or a gold surface. Scanning tunneling microscopy (STM) is used as a strong tool to characterize new species’ structures before and after illumination.     

Dense or Porous Packing? Two‐Dimensional Self‐Assembly of Star‐Shaped Mono‐, Bi‐, and Terpyridine Derivatives

The self‐assembly behavior of five star‐shaped pyridyl‐functionalized 1,3,5‐triethynylbenzenes was studied at the interface between an organic solvent and the basal plane of graphite by scanning tunneling microscopy. The mono‐ and bipyridine derivatives self‐assemble in closely packed 2D crystals, whereas the derivative with the more bulky terpyridines crystallizes with porous packing. DFT calculations of a monopyridine derivative on graphene, support the proposed molecular model. The calculations also reveal the formation of hydrogen bonds between the nitrogen atoms and a hydrogen atom of the neighboring central unit, as a small nonzero tunneling current was calculated within this region. The title compounds provide a versatile model system to investigate the role of multivalent steric interactions and hydrogen bonding in molecular monolayers.     

Self‐Healing Behavior in a Thermo‐Mechanically Responsive Cocrystal during a Reversible Phase Transition

The molecular‐level motions of a coronene‐based supramolecular rotator are amplified into macroscopic changes of crystals by co‐assembly of coronene and TCNB (1,2,4,5‐tetracyanobenzene) into a charge‐transfer complex. The as‐prepared cocrystals show remarkable self‐healing behavior and thermo‐mechanical responses during thermally‐induced reversible single‐crystal‐to‐single‐crystal (SCSC) phase transitions. Comprehensive analysis of the microscopic observations as well as differential scanning calorimetry (DSC) measurements and crystal habits reveal that a thermally‐reduced‐rate‐dependent dynamic character exists in the phase transition. The crystallographic studies show that the global similarity of the packing patterns of both phases with local differences, such as molecular stacking sequence and orientations, should be the origin of the self‐healing behavior of these crystals.     

Bilayer Molecular Assembly at a Solid/Liquid Interface as Triggered by a Mild Electric Field

The construction of a spatially defined assembly of molecular building blocks, especially in the vertical direction, presents a great challenge for surface molecular engineering. Herein, we demonstrate that an electric field applied between an STM tip and a substrate triggered the formation of a bilayer structure at the solid–liquid interface. In contrast to the typical high electric‐field strength (10⁹ V m^?1) used to induce structural transitions in supramolecular assemblies, a mild electric field (10⁵ V m^?1) triggered the formation of a bilayer structure of a polar molecule on top of a nanoporous network of trimesic acid on graphite. The bilayer structure was transformed into a monolayer kagome structure by changing the polarity of the electric field. This tailored formation and large‐scale phase transformation of a molecular assembly in the perpendicular dimension by a mild electric field opens perspectives for the manipulation of surface molecular nanoarchitectures.     

Coadsorption-induced reconstruction of supramolecular assembly characteristics.

A host supramolecular structure consisting of bis-(2,2':6',2"-terpyridine)-4'-oxyhexadecane (BT-O-C16) is shown to respond to coadsorbed molecules in dramatic ways, as observed by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface under ambient conditions. Interestingly, the lattice parameter of the triphenylene-filled complex differs significantly from that of the coronene-filled one, although the triphenylene and coronene molecules are nearly the same size. The STM study and density functional theory calculations reveal that intermolecular hydrogen-bond interactions play an essential role in forming the assembly structures. The different electronic properties of coronene and triphenylene molecules are responsible for the difference in lattice parameters and consequently for the difference in filling behaviors in the coronene/BT-O-C16 and triphenylene/BT-O-C16 binary systems.     

Surface chemical modifications induced on high surface area graphite and carbon nanofibers using different oxidation and functionalization treatments

Two graphitic carbon materials with different edge to basal plane ratio, high surface area graphite (HSAG) and graphitized carbon nanofibers (CNFs), were oxidized by two methods, aqueous-HNO(3) wet oxidation and oxygen plasma oxidation. Characterization of the materials by temperature-programmed desorption, thermogravimetry and X-ray photoelectron and Raman spectroscopies indicated that the amount and nature of oxygen surface groups introduced depended on the oxidation method and on the structure of the original material. While surface sites within the layers were only oxidized by oxygen plasma, surface sites at the edges of graphene layers were oxidized by both treatments being the wet oxidation more effective. Modification of the oxidized materials with a diamine or a triamine molecule resulted in the formation of ammonium carboxylate salt species on the carbon surface.     

Reversible Hydrogenation of Graphene on Ni(111)—Synthesis of “Graphone”

Understanding the adsorption and reaction between hydrogen and graphene is of fundamental importance for developing graphene‐based concepts for hydrogen storage and for the chemical functionalization of graphene by hydrogenation. Recently, theoretical studies of single‐sided hydrogenated graphene, so called graphone, predicted it to be a promising semiconductor for applications in graphene‐based electronics. Here, we report on the synthesis of graphone bound to a Ni(111) surface. We investigate the formation process by X‐ray photoelectron spectroscopy (XPS), temperature‐programmed desorption (TPD), and density‐functional theory calculations, showing that the hydrogenation of graphene with atomic hydrogen indeed leads to graphone, that is, a hydrogen coverage of 1 ML (4.2 wt %). The dehydrogenation of graphone reveals complex desorption processes that are attributed to coverage‐dependent changes in the activation energies for the associative desorption of hydrogen as molecular H₂.     

Spin‐Übergänge in supramolekularen Strukturen. Für Speicherbausteine von morgen?

We reported about octahedrally coordinated Fe2+‐complexes, which are able to switch between two stable spin states (LS and HS) with different magnetic properties. This phenomenon is called spin crossover (SCO). The interaction between metal ion and ligand determines the actual spin state and whether an extern stimulus can trigger a spin crossover. Due to this fact it is possible for the chemist through the choice of the ligands to manipulate the character and the temperature region of the SCO. Some metal complexes assemble into highly ordered structures on graphite by molecular self assembly. The substitution of the metal complexes with alkyl chains and the interaction of these chains with the highly ordered graphite is crucial for a periodic arrangement of the complexes on the surface. For the future we are curious to see whether through the cooperative effort of coordination chemistry (SCO phenomenon) and surface science (self assembly of SCO complexes on a surface) the vision of a molecular memory will turn into a reality.     

Synthesis of polydopamine‐coated graphene–polymer nanocomposites via RAFT polymerization

Graphene‐polymer nanocomposites have significant potential in many applications such as photovoltaic devices, fuel cells, and sensors. Functionalization of graphene is an essential step in the synthesis of uniformly distributed graphene‐polymer nanocomposites, but often results in structural defects in the graphitic sp² carbon framework. To address this issue, we synthesized graphene oxide (GO) by oxidative exfoliation of graphite and then reduced it into graphene via self‐polymerization of dopamine (DA). The simultaneous reduction of GO into graphene, and polymerization and coating of polydopamine (PDA) on the reduced graphene oxide (RGO) surface were confirmed with XRD, UV–Vis, XPS, Raman, TGA, and FTIR. The degree of reduction of GO increased with increasing DA/GO ratio from 1/4 to 4/1 and/or with increasing temperature from room temperature to 60 °C. A RAFT agent, 2‐(dodecylthiocarbonothioylthio)?2‐methylpropionic acid, was linked onto the surface of the PDA/RGO, with a higher equivalence of RAFT agent in the reaction leading to a higher concentration of RAFT sites on the surface. Graphene‐poly(methyl methacrylate), graphene‐poly(tert‐butyl acrylate), and graphene‐poly(N‐isopropylacrylamide) nanocomposites were synthesized via RAFT polymerization, showing their characteristic solubility in several different solvents. This novel synthetic route was found facile and can be readily used for the rational design of graphene‐polymer nanocomposites, promoting their applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3941–3949     

Self‐assembly of well‐defined amphiphilic polymeric miktoarm stars,dendrons, and dendrimers in water: The effect of architecture

Five polymeric architectures with a systematic increase in architectural complexity were synthesized by “click” reactions from a toolbox of functional linear polymers and small molecule linkers. The amphiphilic architectures ranged from a simple 3‐miktoarm star block copolymer to the more complex third generation dendrimer‐like block copolymer, consisting of polystyrene (PSTY) and polyacrylic acid (PAA). Micellization of these architectures in water at a pH of 7 under identical ionic strength gave spherical micelles ranging in size from 9 to 30 nm. Subsequent calculations of the PSTY core density, average surface area per PAA arm on the corona‐core interface, and the relative stretching of the PAA arms provided insights into the effect of architecture on the self‐assembly processes. A particular trend was observed that with increased architectural complexity the hydrodynamic diameter, radius of the core in the dry state and the aggregation number also increased with the exception of the third generation dendrimer. On the basis of these observations, we postulate that thermodynamic factors controlling self‐assembly were the entropic penalty of forming PSTY loops in the core counterbalanced by the reduction in repulsive forces through chain stretching. This results in a greater number of aggregating unimers and consequently larger micelle sizes. The junction points within the architecture also play an important role in controlling the self‐assembly process. The G3 dendrimer showed results contradictory to the aforementioned trend. We believe that the self‐assembly process of this architecture was dominated by the increased attractive forces due to stretching of the PSTY core chains to form a more compact core. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6292–6303, 2009