Strategies for the Controlled Covalent Double Functionalization of Graphene Oxide

Graphene oxide (GO) is a versatile platform with unique properties that have found broad applications in the biomedical field. Double functionalization is a key aspect in the design of multifunctional GO with combined imaging, targeting, and therapeutic properties. Compared to noncovalent functionalization, covalent strategies lead to GO conjugates with a higher stability in biological fluids. However, only a few double covalent functionalization approaches have been developed so far. The complexity of GO makes the derivatization of the oxygenated groups difficult to control. The combination of a nucleophilic epoxide ring opening with the derivatization of the hydroxyl groups through esterification or Williamson reaction was investigated. The conditions were selective and mild, thus preserving the structure of GO. Our strategy of double functionalization holds great potential for different applications in which the derivatization of GO with different molecules is needed, especially in the biomedical field.     

One‐Step Double Covalent Functionalization of Reduced Graphene Oxide with Xanthates and Peroxides

Radical functionalization of reduced graphene oxide has been achieved by reaction with a xanthate in the presence of peroxide as a radical initiator. X‐ray photoelectron spectroscopy, bulk elemental analyses, and thermogravimetric analyses showed that the xanthate grafting is covalent and efficient. The synthesis and use of seven xanthates and three peroxides showed that the highest grafting yield is obtained when xanthate and peroxide are introduced in stoichiometric amounts. It also revealed that the peroxide used as radical initiator is grafted at the graphenic surface during the functionalization. The method presented in this contribution therefore allows bifunctionalized reduced graphene oxide samples to be easily obtained in one single step. This method leads to undamaged graphene sheets with higher dispersibility than the pristine sample.     

Covalent Modification of Graphene Oxide with Carbazole Groups for Laser Protection

In the past decades, significant effort has been invested into the research and development of optical limiting materials and processes in order to develop practical solutions for the protection from laser beams. In this study, a new soluble graphene oxide based material (GO–Cz) has been synthesized through the covalent modification of graphene oxide (GO) with a carbazole derivative (Cz). The formation of an amido bond between the Cz group and GO has been confirmed by X‐ray photoelectron and Fourier transform infrared spectroscopy. At the same concentration, both the nonlinear extinction coefficient and the imaginary third‐order susceptibility were increased by a factor of ≈6.93 at 532 nm and ≈6.07 at 1064 nm relative to those of GO, as a result of the covalent grafting of the Cz moieties onto the GO surface. The GO–Cz dispersions exhibit a much better optical limiting performance than GO and GO/Cz blends at both 532 and 1064 nm due to the possible intramolecular electron‐transfer between the GO and Cz moieties and the effective combination of the different nonlinear optical mechanisms.     

Visible-Light Assisted Covalent Surface Functionalization of Reduced Graphene Oxide Nanosheets with Arylazo Sulfones

We present an environmentally benign methodology for the covalent functionalization (arylation) of reduced graphene oxide (rGO) nanosheets with arylazo sulfones. A variety of tagged aryl units were conveniently accommodated at the rGO surface via visible-light irradiation of suspensions of carbon nanostructured materials in aqueous media. Mild reaction conditions, absence of photosensitizers, functional group tolerance and high atomic fractions (XPS analysis) represent some of the salient features characterizing the present methodology. Control experiments for the mechanistic elucidation (Raman analysis) and chemical nanomanipulation of the tagged rGO surfaces are also reported.     

The Covalent Functionalization of Graphene on Substrates

The utilization of grown or deposited graphene on solid substrates offers key benefits for functionalization processes, but especially to attain structures with a high level of control for electronics and “smart” materials. In this review, we will initially focus on the nature and properties of graphene on substrates, based on the method of preparation. We will then analyze the most relevant literature on the functionalization of graphene on substrates. In particular, we will comparatively discuss radical reactions, cycloadditions, halogenations, hydrogenations, and oxidations. We will especially address the question of how the reactivity of graphene is affected by its morphology (i.e., number of layers, defects, substrate, curvature, etc.).     

Covalent Functionalization of Strained Graphene

An enhancement of the chemical activity of graphene is evidenced by first‐principles modelling of the chemisorption of hydrogen, fluorine, oxygen and hydroxyl groups on strained graphene. For the case of negative strain or compression, chemisorption of the single hydrogen, fluorine or hydroxyl group is energetically more favourable than those of their pairs on different sublattices. This behaviour stabilizes the magnetism caused by the chemisorption being against its destruction by the pair formations. Initially flat, compressed graphene is shown to buckle spontaneously right after chemisorption of single adatoms. Unlike hydrogenation or fluorination, the oxidation process turns from the endothermic to exothermic for all types of the strain and depends on the direction of applied strains. Such properties will be useful in designing graphene devices utilizing functionalization as well as mechanical strains.     

Low‐Temperature Solid‐State Microwave Reduction of Graphene Oxide for Transparent Electrically Conductive Coatings on Flexible Polydimethylsiloxane (PDMS)

Microwaves (MWs) are applied to initialize deoxygenation of graphene oxide (GO) in the solid state and at low temperatures (～165 °C). The Fourier‐transform infrared (FTIR) spectra of MW‐reduced graphene oxide (rGO) show a significantly reduced concentration of oxygen‐containing functional groups, such as carboxyl, hydroxyl and carbonyl. X‐ray photoelectron spectra confirm that microwaves can promote deoxygenation of GO at relatively low temperatures. Raman spectra and TGA measurements indicate that the defect level of GO significantly decreases during the isothermal solid‐state MW‐reduction process at low temperatures, corresponding to an efficient recovery of the fine graphene lattice structure. Based on both deoxygenation and defect‐level reduction, the resurgence of interconnected graphene‐like domains contributes to a low sheet resistance (～7.9×10⁴ Ω per square) of the MW‐reduced GO on SiO₂‐coated Si substrates with an optical transparency of 92.7 % at ～547 nm after MW reduction, indicating the ultrahigh efficiency of MW in GO reduction. Moreover, the low‐temperature solid‐state MW reduction is also applied in preparing flexible transparent conductive coatings on polydimethylsiloxane (PDMS) substrates. UV/Vis measurements indicate that the transparency of the thus‐prepared MW‐reduced GO coatings on PDMS substrates ranges from 34 to 96 %. Correspondingly, the sheet resistance of the coating ranges from 10⁵ to 10⁹ Ω per square, indicating that MW reduction of GO is promising for the convenient low‐temperature preparation of transparent conductors on flexible polymeric substrates.     

An Effective Method for Bulk Obtaining Graphene Oxide Solids

We report an effective method for bulk obtaining exfoliated graphene oxide (GO) solids from their aqueous solutions, which were prepared from nature graphite by an oxidation method. Tyndall effect proved that GO solution has a colloidal nature. Different flocculants were used to coagulate GO colloidal, and it was found that NaOH had the most obvious coagulation effect to GO. Transmission electron microscopy, X‐ray diffraction and atomic force microscopy analysis demonstrated that there were a large number of complete few‐layer GO sheets with thickness of about 0.8 nm, and the surfaces were very smooth, almost free of impurities. Liquid state ¹³C NMR and Fourier transformation infrared spectra showed the presence of abundant benzene carboxylic, hydroxyl and epoxide groups in the basal planes of GO. The graphene materials reduced from GO solids had good electrical conductivity. Our work explored a simple and effective route to extract GO from their solution, which is the most important to GO and graphene researches and applications.     

Covalent Functionalization of Graphene by Nucleophilic Addition Reaction: Synthesis and Optical‐Limiting Properties

Covalent functionalization of reduced graphene oxide (rGO) was performed by using conjugated polymers with different monomers through nucleophilic addition of nitrogen anions to rGO. Three conjugated polymers containing tetraphenylethylene, carbazole, and phenyl groups were used, and as a result of π–π interactions and the “polymer‐wrapping” effect, the dispersion stability of rGO was improved. Even if the reaction site in the polymers was the same, there were great differences in the reactivities of the polymers, the dispersion stabilities of the resultant composites, and also the optical limiting (OL) performances of the resultant composites. The differences may be attributed to the π‐conjugated structure and steric hindrance of the moiety in the polymer skeleton, which has scarcely been reported. Besides, the resultant rGO‐P1 and rGO‐P3 materials both showed excellent OL responses, even at 4 μJ. This behavior should enable their potential application in photonic and optoelectronic devices to protect human eyes or optical sensors from damage by intense laser irradiation.     

超声辅助Hummers法制备氧化石墨烯

采用超声辅助Hummers法制备氧化石墨烯,单片层厚～1 nm。本法首先在Hummers法的低温、中温反应阶段加入超声振荡,以此来分别提高石墨的插层效率和氧化程度,然后在高温反应开始时,采用把含有残留浓硫酸的混合液缓慢滴入低温去离子水中再加热的方式,以此减少硫酸分子等插入物因为局部温度过高从石墨层间脱出,最后通过低速离心得到氧化石墨。使用超声辅助Hummers法制备氧化石墨烯既方便快捷,又能有效地增大氧化石墨的层间距,且随着超声功率的提高,所得氧化石墨的层间距呈扩大趋势。     

The Role of Oxidative Debris on Graphene Oxide Films

We study the effect of oxidative impurities on the properties of graphene oxide and on the graphene oxide Langmuir–Blodgett films (LB). The starting material was grupo Antolín nanofibers (GANF) and the oxidation process was a modified Hummers method to obtain highly oxidized graphene oxide. The purification procedure reported in this work eliminated oxidative impurities decreasing the thickness of the nanoplatelets. The purified material thus obtained presents an oxidation degree similar to that achieved by chemical reduction of the graphite oxide. The purified and non‐purified graphene oxides were deposited onto silicon by means of a Langmuir–Blodgett (LB) methodology. The morphology of the LB films was analyzed by field emission scanning microscopy (FE‐SEM) and micro‐Raman spectroscopy. Our results show that the LB films built by transferring Langmuir monolayers at the liquid‐expanded state of the purified material are constituted by close‐packed and non‐overlapped nanoplatelets. The isotherms of the Langmuir monolayer precursor of the LB films were interpreted according to the Volmer’s model.     

Surface and Interface Engineering of Graphene Oxide Films by Controllable Photoreduction

We report herein the engineering of the surface/interface properties of graphene oxide (GO) films by controllable photoreduction treatment. In our recent works, typical photoreduction processes, including femtosecond laser direct writing (FsLDW), laser holographic lithography, and controllable UV irradiation, have been employed to make conductive reduced graphene oxide (RGO) microcircuits, hierarchical RGO micro‐nanostructures with both superhydrophobicity and structural color, as well as moisture‐responsive GO/RGO bilayer structures. Compared with other reduction protocols, for instance, chemical reduction and thermal annealing, the photoreduction strategy shows distinct advantages, such as mask‐free patterning, chemical‐free modification, controllable reduction degree, and environmentally friendly processing. These works indicate that the surface and interface engineering of GO through controllable photoreduction of GO holds great promise for the development of various graphene‐based microdevices.     

Octahedral Cuprous Oxide Decorated Flexible Reduced Graphene Oxide Paper for Food Sensing Application

This work presents a simple method to fabricate an octahedral cuprous oxide (Cu₂O) decorated two-dimensional (2D) flexible rGOP electrode with filtration and electrodeposition strategies. The characteristic of the Cu₂O/rGOP electrodes was recorded by SEM, EDX, XPS, XRD, and Raman spectroscopy. The results clearly showed that Cu₂O was successfully electrodeposited on the surface of rGOP by controlling the electrodeposition potential without the introduction of any template or surfactant. The electrochemical characterizations of the Cu₂O/rGOP exhibited high electrocatalytic activity toward the reduction of H₂O₂. The linear detection range for the Cu₂O/rGOP flexible sensor was 5.0 μM to 5.5 mM, with a limit of detection of 1.27 μΜ. Subsequently, the developed flexible rGOP sensor was extended for H₂O₂ detection in milk samples for avoiding milk spoilage_. Such judicial preparation of rGOP as a sensing device will certainly pave the way for various other sensing applications including environmental and biomedical applications.     

Covalent Reactions on Chemical Vapor Deposition Grown Graphene Studied by Surface‐Enhanced Raman Spectroscopy

Graphene is a material of unmatched properties and eminent potential in disciplines ranging from physics, to chemistry, to biology. Its advancement to applications with a specific function requires rational design and fine tuning of its properties, and covalent introduction of various substituents answers this requirement. We challenged the obstacle of non‐trivial and harsh procedures for covalent functionalization of pristine graphene and developed a protocol for mild nucleophilic introduction of organic groups in the gas phase. The painstaking analysis problem of monolayered materials was addressed by using surface‐enhanced Raman spectroscopy, which allowed us to monitor and characterize in detail the surface composition. These deliverables provide a toolbox for reactivity of fluorinated graphene under mild reaction conditions, providing structural freedom of the species to‐be‐grafted to the single‐layer graphene.     

Superionic Conductivity in Hybrid of 3‐Hydroxypropanesulfonic Acid and Graphene Oxide

Insertion of 3‐hydroxypropanesulfonicacid (HPS) in the graphene oxide (GO) interlayer results in high proton conductivity (10⁻²–10⁻¹ S cm⁻¹), owing to an improvement in oxygen content, interlayer distance and water absorbing capacity. This result indicates that hydroxyalkylsulfonicacids can be perfect guest molecules for improving the proton conductivity of carbon materials.     

Flexible Magnetic Nanoparticles–Reduced Graphene Oxide Composite Membranes Formed by Self‐Assembly in Solution

A facile and robust route for the pre‐synthesized Fe₃O₄ nanoparticles (NPs) exclusively assembled on both sides of reduced graphene oxide (RGO) sheets with tunable density forming two‐dimensional NPs composite membranes is developed in solution. The assembly is driven by electrostatic attraction, and the nanocomposite sheets display considerable mechanical robustness, such as it can sustain supersonic and solvothermal treatments without NPs falling off, also, can freely float in solution and curl into a tube. The obtained two‐dimensional composite grain membranes exhibit superparamagnetic behavior at room temperature but responds astutely to an external magnetic field. In addition, these magnetic composite membranes show an enhanced absorption capability for microwaves. The grain sheets are attractive for biomedical, sensors, environmental applications and electric‐magnetic devices benefited from large surfaces, high magnetization moment, and superparamagnetic properties. The effective integration of oxide nanocrystals on RGO sheets provides a new way to design semiconductor–carbon nanocomposites for nanodevices or catalytic applications.     

Covalent Surface Modification of Oxide Surfaces

The modification of surfaces by the deposition of a robust overlayer provides an excellent handle with which to tune the properties of a bulk substrate to those of interest. Such control over the surface properties becomes increasingly important with the continuing efforts at down‐sizing the active components in optoelectronic devices, and the corresponding increase in the surface area/volume ratio. Relevant properties to tune include the degree to which a surface is wetted by water or oil. Analogously, for biosensing applications there is an increasing interest in so‐called “romantic surfaces”: surfaces that repel all biological entities, apart from one, to which it binds strongly. Such systems require both long lasting and highly specific tuning of the surface properties. This Review presents one approach to obtain robust surface modifications of the surface of oxides, namely the covalent attachment of monolayers.     

BCN: A Graphene Analogue with Remarkable Adsorptive Properties

A new analogue of graphene containing boron, carbon and nitrogen (BCN) has been obtained by the reaction of high‐surface‐area activated charcoal with a mixture of boric acid and urea at 900 °C. X‐ray photoelectron spectroscopy and electron energy‐loss spectroscopy reveal the composition to be close to BCN. The X‐ray diffraction pattern, high‐resolution electron microscopy images and Raman spectrum indicate the presence of graphite‐type layers with low sheet‐to‐sheet registry. Atomic force microscopy reveals the sample to consist of two to three layers of BCN, as in a few‐layer graphene. BCN exhibits more electrical resistivity than graphene, but weaker magnetic features. BCN exhibits a surface area of 2911 m² g^?1, which is the highest value known for a B_xC_yN_z composition. It exhibits high propensity for adsorbing CO₂ (≈100 wt %) at 195 K and a hydrogen uptake of 2.6 wt % at 77 K. A first‐principles pseudopotential‐based DFT study shows the stable structure to consist of BN₃ and NB₃ motifs. The calculations also suggest the strongest CO₂ adsorption to occur with a binding energy of 3.7 kJ mol^?1 compared with 2.0 kJ mol^?1 on graphene.