Controlled Chemistry Approach to the Oxo‐Functionalization of Graphene

Graphene is the best‐studied 2D material available. However, its production is still challenging and the quality depends on the preparation procedure. Now, more than a decade after the outstanding experiments conducted on graphene, the most successful wet‐chemical approach to graphene and functionalized graphene is based on the oxidation of graphite. Graphene oxide has been known for more than a century; however, the structure bears variable large amounts of lattice defects that render the development of a controlled chemistry impossible. The controlled oxo‐functionalization of graphene avoids the formation of defects within the σ‐framework of carbon atoms, making the synthesis of specific molecular architectures possible. The scope of this review is to introduce the field of oxo‐functionalizing graphene. In particular, the differences between GO and oxo‐functionalized graphene are described in detail. Moreover analytical methods that allow determining lattice defects and functional groups are introduced followed by summarizing the current state of controlled oxo‐functionalization of graphene.     

Oxo‐Functionalized Graphene as a Cell Membrane Carrier of Nucleic Acid Probes Controlled by Aging

We applied a fluorescein‐containing oligonucleotide (ON) to probe surface properties of oxidized graphene (oxo‐G) and observed that graphene‐like patches are formed upon aging of oxo‐G, indicated by enhanced probe binding and by FTIR spectroscopic analysis. By using a recently developed fluorogenic endoperoxide (EP) probe, we confirmed that during the aging process the amount of EPs on the oxo‐G surface is reduced. Furthermore, aging was found to strongly affect cell membrane carrier properties of this material. In particular, freshly prepared oxo‐G does not act as a carrier, whereas oxo‐G aged for 28 days at 4 °C is an excellent carrier. Based on these data we prepared an optimized oxo‐G, which has a low‐defect density, binds ONs, is not toxic, and acts as cell membrane carrier. We successfully applied this material to design fluorogenic probes of representative intracellular nucleic acids 28S rRNA and β‐actin‐mRNA. The results will help to standardize oxidized graphene derivatives for biomedical and bioanalytical applications.     

Highly Intact and Pure Oxo‐Functionalized Graphene: Synthesis and Electron‐Beam‐Induced Reduction

Controlling the chemistry of graphene is necessary to enable applications in materials and life sciences. Research beyond graphene oxide is targeted to avoid the highly defective character of the carbon framework. Herein, we show how to optimize the synthesis of oxo‐functionalized graphene (oxo‐G) to prepare high‐quality monolayer flakes that even allow for direct transmission electron microscopy investigation at atomic resolution (HRTEM). The role of undesired residuals is addressed and sources are eliminated. HRTEM provides clear evidence for the exceptional integrity of the carbon framework of such oxo‐G sheets. The patchy distribution of oxo‐functionality on the nm‐scale, observed on our highly clean oxo‐G sheets, corroborates theoretical predictions. Moreover, defined electron‐beam irradiation facilitates gentle de‐functionalization of oxo‐G sheets, a new route towards clean graphene, which is a breakthrough for localized graphene chemistry.     

Identification of Semiconductive Patches in Thermally Processed Monolayer Oxo‐Functionalized Graphene

The thermal decomposition of graphene oxide (GO) is a complex process at the atomic level and not fully understood. Here, a subclass of GO, oxo‐functionalized graphene (oxo‐G), was used to study its thermal disproportionation. We present the impact of annealing on the electronic properties of a monolayer oxo‐G flake and correlated the chemical composition and topography corrugation by two‐probe transport measurements, XPS, TEM, FTIR and STM. Surprisingly, we found that oxo‐G, processed at 300 °C, displays C?C sp³‐patches and possibly C?O?C bonds, next to graphene domains and holes. It is striking that those C?O?C/C?C sp³‐separated sp²‐patches a few nanometers in diameter possess semiconducting properties with a band gap of about 0.4 eV. We propose that sp³‐patches confine conjugated sp²‐C atoms, which leads to the local semiconductor properties. Accordingly, graphene with sp³‐C in double layer areas is a potential class of semiconductors and a potential target for future chemical modifications.     

Selective Functionalization of Graphene at Defect‐Activated Sites by Arylazocarboxylic tert‐Butyl Esters

The development of versatile functionalization concepts for graphene is currently in the focus of research. Upon oxo‐functionalization of graphite, the full surface of graphene becomes accessible for C?C bond formation to introduce out‐of‐plane functionality. Herein, we present the arylation of graphene with arylazocarboxylic tert‐butyl esters, which generates aryl radicals after activation with an acid. Surprisingly, the degree of functionalization is related to the concentration of lattice vacancy defects in the graphene material. Consequently, graphene materials that are free from lattice defects are not reactive. The reaction can be applied to graphene dispersed in solvents and leads to bitopic functionalization as well as monotopic functionalization when the graphene is deposited on surfaces. As the arylazocarboxylic tert‐butyl ester moiety can be attached to various molecules, the presented method paves the way to functional graphene derivatives, with the density of defects determining the degree of functionalization.     

Aqueous Dispersions of Graphene from Electrochemically Exfoliated Graphite

A facile and environmentally friendly synthetic strategy for the production of stable and easily processable dispersions of graphene in water is presented. This strategy represents an alternative to classical chemical exfoliation methods (for example the Hummers method) that are more complex, harmful, and dangerous. The process is based on the electrochemical exfoliation of graphite and includes three simple steps: 1) the anodic exfoliation of graphite in (NH₄)₂SO₄, 2) sonication to separate the oxidized graphene sheets, and 3) reduction of oxidized graphene to graphene. The procedure makes it possible to convert around 30 wt % of the initial graphite into graphene with short processing times and high yields. The graphene sheets are well dispersed in water, have a carbon/oxygen atomic ratio of 11.7, a lateral size of about 0.5–1 μm, and contain only a few graphene layers, most of which are bilayer sheets. The processability of this type of aqueous dispersion has been demonstrated in the fabrication of macroscopic graphene structures, such as graphene aerogels and graphene films, which have been successfully employed as absorbents or as electrodes in supercapacitors, respectively.     

Brodie's or Hummers’ Method: Oxidation Conditions Determine the Structure of Graphene Oxide

Synthesis and studies of graphite oxide started more than 150 years ago and turned into a boom by the measurements of the outstanding physical properties of graphene. A series of preparation protocols emanated trying to optimize the synthesis of graphene oxide in order to obtain a less defective material, as source for graphene. However, over-oxidation of the carbon framework hampered establishing structure-property relationships. Here, the fact that two different synthetic methods for graphene oxide preparation lead to very similar types of graphene oxide with a preserved graphene lattice is demonstrated. Either sodium chlorate in nitric acid (similar to Brodie's method) or potassium permanganate in sulfuric acid (similar to Hummers’ method) treatment are possible; however, reaction conditions must be controlled. With a preserved carbon lattice analytical differences between the samples relate to the altered on-plane functionality. Consequently, terming preparation protocols “according to Brodie's/Hummers’ method” is not sufficient.     

Decomposition of Fluorinated Graphene under Heat Treatment

Fluorination modifies the electronic properties of graphene, and thus it can be used to provide material with on‐demand properties. However, the thermal stability of fluorinated graphene is crucial for any application in electronic devices. Herein, X‐ray photoelectron spectroscopy (XPS), temperature‐programmed desorption (TPD), and Raman spectroscopy were used to address the impact of the thermal treatment on fluorinated graphene. The annealing, at up to 700 K, caused gradual loss of fluorine and carbon, as was demonstrated by XPS. This loss was associated with broad desorption of CO and HF species, as monitored by TPD. The minor single desorption peak of CF species at 670 K is suggested to rationalize defect formation in the fluorinated graphene layer during the heating. However, fluorine removal from graphene was not complete, as some fraction of strongly bonded fluorine can persist despite heating to 1000 K. The role of intercalated H₂O and OH species in the defluorination process is emphasised.     

DFT investigation of molybdenum (oxo)carbide formation from MoO3

The present paper summarizes theoretical investigations of geometry and electronic structure of molybdenum (oxo)carbide, whose formation is modeled by systematic replacement of lattice oxygen atoms in MoO₃ by carbon atoms or by CH₂ groups. Both, in bulk and in the small surface cluster, the formation of molybdenum (oxo)carbide is accompanied by by-products observed in experiment, namely C₂ species and CO. The present theoretical studies reveal that these are formed without reaction barrier, even though in bulk the atom mobility is limited. The thermodynamic considerations based on the obtained DFT results indicate that the process of MoO₃ reduction to yield (oxo)carbides is endoenergetic and there is no synergy between the amount of carbon already introduced to the system and the energetic cost of replacing oxygen atoms by CH₂.     

A New Member of the Graphene Family: Graphene Acid

A new member of the family of graphene derivatives, namely, graphene acid with a composition close to C₁(COOH)₁, was prepared by oxidation of graphene oxide. The synthetic procedure is based on repeated oxidation of graphite with potassium permanganate in an acidic environment. The oxidation process was studied in detail after each step. The multiple oxidations led to oxidative removal of other oxygen functional groups formed in the first oxidation step. Detailed chemical analysis showed only a minor amount of other oxygen‐containing functional groups such as hydroxyl and the dominant presence of carboxyl groups in a concentration of about 30 wt %. Further oxidation led to complete decomposition of graphene acid. The obtained material exhibits unique sorption capacity towards metal ions and carbon dioxide. The highly hydrophilic nature of graphene acid allowed the assembly of ultrathin free‐standing membranes with high transparency.     

Investigation of the Thermal Stability of the Carbon Framework of Graphene Oxide

In this study, we use our recently prepared graphene oxide (GO) with an almost intact σ‐framework of carbon atoms (ai‐GO) to probe the thermal stability of the carbon framework for the first time. Ai‐GO exhibits few defects because CO₂ formation is prevented during synthesis. Ai‐GO was thermally treated before chemical reduction and the resulting defect density in graphene was subsequently determined by statistical Raman microscopy. Surprisingly, the carbon framework of ai‐GO is stable in thin films up to 100 °C. Furthermore, we find evidence for an increase in the quality of ai‐GO upon annealing at 50 °C before reduction. The carbon framework of GO prepared according to the popular Hummers’ method (GO‐c) appears to be less stable and decomposition starts at 50 °C, which is qualitatively indicated by CO₂‐trapping experiments in μm‐thin films. Information about the stability of GO is important for storing, processing, and using GO in many applications.     

Strongly Veined Carbon Nanoleaves as a Highly Efficient Metal‐Free Electrocatalyst

Effective integration of one‐dimensional carbon nanofibers (CNF) and two‐dimensional carbon sheets into three‐dimensional (3D) conductive frameworks is essential for their practical applications as electrode materials. Herein, a novel “vein‐leaf”‐type 3D complex of carbon nanofibers with nitrogen‐doped graphene (NG) was prepared through a simple thermal condensation of urea and bacterial cellulose. During the formation of the 3D complex CNF@NG, the graphene species was tethered to CNF via carbon–carbon bonds. Such an interconnected 3D network facilitates both the electron transfer and mass diffusion for electrochemical reactions.     

Formation of Stone–Wales edge: Multistep reconstruction and growth mechanisms of zigzag nanographene

Although the existence of Stone–Wales (5‐7) defect at graphene edge has been clarified experimentally, theoretical study on the formation mechanism is still imperfect. In particular, the regioselectivity of multistep reactions at edge (self‐reconstruction and growth with foreign carbon feedstock) is essential to understand the kinetic behavior of reactive boundaries but investigations are still lacking. Herein, by using finite‐sized models, multistep reconstructions and carbon dimer additions of a bared zigzag edge are introduced using density functional theory calculations. The zigzag to 5‐7 transformation is proved as a site‐selective process to generate alternating 5‐7 pairs sequentially and the first step with largest barrier is suggested as the rate‐determining step. Conversely, successive C₂ insertions on the active edge are calculated to elucidate the formation of 5‐7 edge during graphene growth. A metastable intermediate with a triple sequentially fused pentagon fragment is proved as the key structure for 5‐7 edge formation. © 2017 Wiley Periodicals, Inc.     

Ultralight,Ultraflexible, Anisotropic,Highly Thermally Conductive Graphene Aerogel Films

Graphene aerogels have attracted much attention as a promising material for various applications. The unusually high intrinsic thermal conductivity of individual graphene sheets makes an obvious contrast with the thermal insulating performance of assembled 3D graphene materials. We report the preparation of anisotropy 3D graphene aerogel films (GAFs) made from tightly packed graphene films using a thermal expansion method. GAFs with different thicknesses and an ultimate low density of 4.19 mg cm⁻³ were obtained. GAFs show high anisotropy on average cross-plane thermal conductivity (K_⊥) and average in-plane thermal conductivity (K_||). Additionally, uniaxially compressed GAFs performed a large elongation of 11.76% due to the Z-shape folding of graphene layers. Our results reveal the ultralight, ultraflexible, highly thermally conductive, anisotropy GAFs, as well as the fundamental evolution of macroscopic assembled graphene materials at elevated temperature.     

A Comparative Study of CO Oxidation on Nitrogen‐ and Phosphorus‐Doped Graphene

The geometry, electronic structure, and catalytic properties of nitrogen‐ and phosphorus‐doped graphene (N‐/P‐graphene) are investigated by density functional theory calculations. The reaction between adsorbed O₂ and CO molecules on N‐ and P‐graphene is comparably studied via Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms. The results indicate that a two‐step process can occur, namely, CO+O₂→CO₂+O_ads and CO+O_ads→CO₂. The calculated energy barriers of the first step are 15.8 and 12.4 kcal mol^?1 for N‐ and P‐graphene, respectively. The second step of the oxidation reaction on N‐graphene proceeds with an energy barrier of about 4 kcal mol^?1. It is noteworthy that this reaction step was not observed on P‐graphene because of the strong binding of O_ads species on the P atoms. Thus, it can be concluded that low‐cost N‐graphene can be used as a promising green catalyst for low‐temperature CO oxidation.     

A Universal Graphene Quantum Dot Tethering Design Strategy to Synthesize Single-Atom Catalysts

A general graphene quantum dot-tethering design strategy to synthesize single-atom catalysts (SACs) is presented. The strategy is applicable to different metals (Cr, Mn, Fe, Co, Ni, Cu, and Zn) and supports (0D carbon nanosphere, 1D carbon nanotube, 2D graphene nanosheet, and 3D graphite foam) with the metal loading of 3.0–4.5 wt %. The direct transmission electron microscopy imaging and X-ray absorption spectra analyses confirm the atomic dispersed metal in carbon supports. Our study reveals that the abundant oxygenated groups for complexing metal ions and the rich defective sites for incorporating nitrogen are essential to realize the synthesis of SACs. Furthermore, the carbon nanotube supported Ni SACs exhibits high electrocatalytic activity for CO₂ reduction with nearly 100 % CO selectivity. This universal strategy is expected to open up new research avenues to produce SACs for diverse electrocatalytic applications.     

PtPd-nanoparticles supported by new carbon materials

Nanocomposites based on PtPd nanoparticles with chemical ordering like disordered solid solution on surface of multilayer graphene have been prepared through thermal shock of mechanically obtained mixture of double complex salt [Pd(NH₃)₄][PtCl₆] and different carbon materials–exfoliated graphite, graphite oxide and graphite fluoride. An effect of original carbon precursors on formation of PtPd bimetallic nanoparticles was studied using X-ray absorption spectroscopy (XAFS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was shown that the distribution of bimetallic nanoparticles over the multilayer graphene surface as well as the particles size distribution is controlled by the graphene precursors. For all nanocomposites, the surface of the nanoparticles was found to be Pd-enriched. In case when the thermal exfoliated graphite and graphite oxide were used as the graphene precursors a thin graphitized layer covered the nanoparticles surface. Such a graphitized layer was not observed in the nanocomposite, which used the fluorinated graphite as the precursor.     

Hydroboration of Graphene Oxide: Towards Stoichiometric Graphol and Hydroxygraphane

Covalently functionalized graphene materials with well‐defined stoichiometric composition are of a very high importance in the research of 2D carbon material family due to their well‐defined properties. Unfortunately, most of the contemporary graphene‐functionalized materials do not have this kind of defined composition and, usually, the amount of heteroatoms bonded to graphene framework is in the range of 1–10 at. %. Herein, we show that by a well‐established hydroboration reaction chain, which introduces ?BH₂ groups into the graphene oxide structure, followed by H₂O₂ or CF₃COOH treatment as source of ?OH or ?H, we can obtain highly hydroxylated compounds of precisely defined composition with a general formula (C₁O_0.78H_0.75)_n, which we named graphol and highly hydroxylated graphane (C₁(OH)_0.51H_0.14)_n, respectively. These highly functionalized materials with an accurately defined composition are highly important for the field of graphene derivatives. The enhanced electrochemical performance towards important biomarkers as well as hydrogen evolution reaction is demonstrated.     

石墨烯和铂/石墨烯的合成及其表征

以天然鳞状石墨为原料,采用化学氧化法合成氧化石墨,在此基础上采用低温热解膨胀结合微波加热乙二醇还原法合成石墨烯(Gr)以及铂/石墨烯(Pt/Gr)复合材料。SEM和TEM显示所制备的石墨烯为层状结构的半透明薄膜。采用X射线光电子能谱(XPS)和傅立叶转换红外光谱(FTIR)分别确定氧化石墨、膨胀石墨及石墨烯表面含氧官能团的数量和性质。以所制备的碳氧原子比5.94的石墨烯作为载体制备出可用于质子交换膜燃料电池的高负载量的Pt/Gr催化剂,在铂载量高达60%时,表面铂粒子依然具有高分散性,平均粒径为3.8 nm。