Thermal property improvement of polytetrafluoroethylene nanocomposites with graphene nanoplatelets

Thermal properties including the crystallization behavior, thermal stability and thermal conductivity for a series of graphene nanoplatelet (GNP)–polytetrafluoroethylene (PTFE) nanocomposites were studied. The GNP–PTFE nanocomposites were fabricated via solvent-assisted blending followed by cold-pressing and sintering. The results indicated that the GNP–PTFE nanocomposites retained the good thermal stability of the PTFE matrix, and possessed better crystallization and much higher thermal conductivity than pure PTFE. The thermal conductivity of PTFE nanocomposites with a GNP mass fraction of 20% could reach 4.02 W (m K)⁻¹, which was increased by 1300% compared with pure PTFE. Additionally, a theoretical model was proposed to analyze the thermal conductivity of GNP–PTFE nanocomposites. It is demonstrated that adding GNPs into PTFE homogeneously can effectively improve the thermal properties of the nanocomposites.

Adding GNPs into PTFE can significantly improve the thermal properties of nanocomposites.     

One-step nondestructive functionalization of graphene oxide paper with amines

Direct functionalization of prefabricated free-standing graphene oxide paper (GOP) is the only approach suitable for systematic tuning of its mechanical, thermal and electronic characteristics. However, the traditional liquid-phase functionalization can compromise physical integrity of the paper-like material up to its total disintegration. In the present paper, we attempted to apply an alternative, solvent-free strategy for facile and nondestructive functionalization of GOP with 1-octadecylamine (ODA) and 1,12-diaminododecane (DAD) as representatives of aliphatic amines, and with 1-aminopyrene (AP) and 1,5-diaminonaphthalene (DAN) as examples of aromatic amines. The functionalization can be carried out under moderate heating at 150–180 °C for 2 h in vacuum, and proceeds through both amidation and epoxy ring opening reactions. Comparative characterization of pristine and amine-modified GOP samples was carried out by means of Fourier-transform infrared, Raman, and X-ray photoelectron spectroscopy, thermogravimetric and differential thermal analysis, scanning electron and atomic force microscopy. In addition, we compared stability in water, wettability, electrical conductivity and elastic (Young''s) modulus of GOP samples before and after functionalization. The highest content of amine species was obtained in the case of GOP-ODA, followed by GOP-DAD, GOP-AP and GOP-DAN. The functionalization increased mechanical and thermal stability, as well as the electrical conductivity of GOP. The magnitude of each effect depends on the structure of amine employed, which allows for tuning a given GOP characteristic. Morphological characterization showed that, compared to pristine graphene oxide paper, amine-modified mats become relatively ordered layered structures, in which individual GO sheets are organized in a near-parallel fashion.

Solvent-free functionalization of graphene oxide paper with amines is facile, nondestructive, and attractive from economic and environmental points of view.     

The mechanochemical Scholl reaction as a versatile synthesis tool for the solvent-free generation of microporous polymers

Herein we report the mechanochemical Scholl polymerization of 1,3,5-triphenylbenzene in a high speed ball mill. The reaction is conducted solvent-free, solely using solid FeCl₃. The resulting porous polymer was obtained in >99% yield after very short reaction times of only 5 minutes and exhibits a high specific surface area of 658 m² g⁻¹, which could be further enhanced up to 990 m² g⁻¹ by liquid assisted grinding. Within this study we illuminate the origin of porosity by investigating the impact of various milling parameters and milling materials, temperature and pressure, and different liquids for LAG as well as post polymer milling. Finally we expand the procedure to different monomers and mills, to present the mechanochemical Scholl reaction as a versatile synthesis tool for porous polymers.

Herein we report the mechanochemical Scholl polymerization of 1,3,5-triphenylbenzene in a high speed ball mill.     

Three-dimensional N-doped graphene aerogel-supported Pd nanoparticles as efficient catalysts for solvent-free oxidation of benzyl alcohol

Herein, three-dimensional (3D) nitrogen-doped graphene with large surface areas and abundant porous structures was prepared by a hydrothermal synthesis method, which served as a novel support to enhance the catalytic properties of noble metal catalysts for the solvent-free selective oxidation of benzyl alcohol. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and Brunauer–Emmett–Teller (BET) method. The results clearly showed that the introduced N-containing group prevented the aggregation of graphene sheets and provided more structural defects to maximize the number of exposed active sites. The three-dimensional structure can provide a unique porous structure and large specific surface area. Moreover, the three-dimensional structure makes the recycling and reuse of the catalyst easier. The combination of these properties results in the reduction of the average particle size of metal palladium to 3.2 nm; this significantly increases the catalytic activity of the catalyst. The three-dimensional N-doped graphene aerogel-supported Pd nanoparticle (3D Pd/NRGO) composites exhibit excellent catalytic activity for the solvent-free selective oxidation of benzyl alcohol to benzaldehyde by molecular oxygen at 90 °C for 3 hours under atmospheric pressure, resulting in a 72.2% conversion of benzyl alcohol with 94.5% selectivity for benzaldehyde. In addition, the catalytic efficiency shows no obvious loss even after six repeated cycles. Thus, 3D Pd/NRGO can be used as an efficient, easily separable, recyclable, and stable catalyst for the solvent-free selective oxidation of benzyl alcohol under relatively mild conditions.

3D Pd/NRGO with large surface areas and abundant porous structures was prepared, which served as a novel support to enhance the catalytic properties of noble metal catalysts for the solvent-free selective oxidation of benzyl alcohol.     

The chemical basis and specificity of the nitrosonaphthol reaction

1-Nitroso-2-naphthol reacts with various 5-hydroxyindoles as well as para-substituted phenols and guaiacols. Consequently, this reaction has long been used to measure tyrosine in tyrosinosis and tyrosinemia, homovanillic acid in neuroblastoma, and 5-hydroxy-3-indoleacetic acid in the carcinoid tumor. We report here on the specificity of this reaction, as well as our early observations with respect to the chemical basis of the reaction.     

Confined-based catalyst investigation through the comparative functionalization and defunctionalization of Zr-MOF

In metal–organic frameworks, confined space as a chemical nanoreactor is as important as organocatalysis or coordinatively unsaturated metal site catalysis. In the present study, a set of mixed-ligand structures with UiO-66 architecture have been prepared. To the best of our knowledge, for the first time, structures derived by the solvothermal mixing ligand method and ultrasonic-assisted linker exchange approaches have been compared. Additionally, the relationship between the preparation method, structural properties, and catalytic efficiency of the prepared materials in the Knoevenagel condensation of aldehydes has been investigated. The prepared catalyst is very stable and can be recovered and reused for at least ten periods.

In metal–organic frameworks, confined space as a chemical nanoreactor is as important as organocatalysis or coordinatively unsaturated metal site catalysis.     

The influence of graphene nanoplatelets (GNPs) on the semi-blunt puncture behavior of woven fabrics impregnated with shear thickening fluid (STF)

Fabrics are widely applied in various fields, such as body armor, aerospace industry and military equipment. This paper reports the semi-blunt puncture behavior of fabrics impregnated with a shear thickening fluid (STF), which is a dispersion system of nano silica/polyethylene glycol (SiO₂/PEG). The rheological properties and microstructure of the STF were tested to evaluate the influence of SiO₂ particle size and additives (graphene nanoplatelets) on the puncture behavior of the final fabrics. The surface characteristics of neat and STF-treated fabrics were separately measured by field emission scanning electron microscopy (FESEM), which helps study the damage morphology and mechanism of the fabric. The surface characteristics of the silica nanoparticles and additives were also measured by transmission electron microscopy (TEM). The results demonstrated that the puncture behavior of the woven fabric was significantly enhanced due to the presence of STF. Moreover, the presence of additive GNPs induced a relatively lower shear thickening phenomenon, while a significant enhancement in the semi-blunt puncture behavior of the woven fabric was observed.

Fabrics are widely applied in various fields, such as body armor, aerospace industry and military equipment.     

Facial fabrication of few-layer functionalized graphene with sole functional group through Diels–Alder reaction by ball milling

The widespread use of graphene as a next-generation material in various applications requires developing an environmentally friendly and efficient method for fabricating functionalized graphene. Chemically, graphene can be used as an electron donor or attractor. Here, graphite was successfully exfoliated, and an in situ Diels–Alder reaction (D–A) was carried out to fabricate functionalized graphene with sole functional groups via mechanochemical ball milling. The reactivities of graphene acting as a diene or a dienophile were investigated. Few-layer (≤2 layers) graphene specimens were obtained by wet ball milling, heating in a nitrogen atmosphere, and solvent ultrasonic treatment. The ball-milling method was more effective than heating in a nitrogen atmosphere, and the [2 + 4] D–A of graphene was more dominant than the [4 + 2] D–A in the ball-milling process. The surface tension of functionalized graphene decreased, which provided a theoretical basis for the dispersion and exfoliation of graphite in a suitable solvent. Functionalized graphene still had a high electrical conductivity, which has far-reaching significance for functionalized graphene to be applied in electronic semiconductors and related applications. Meanwhile, functionalized graphene was applied to polymer composite fibers, the tensile strength and the Young''s modulus could reach 780 MPa and 19 GPa. The volume resistivity was two orders of magnitude lower than that of pure fiber. Thus, the use of ball milling to efficiently exfoliate and in situ functionalize graphite will help to develop a strategy that can be widely used to manufacture nanomaterials for various application fields.

The widespread use of graphene as a next-generation material in various applications requires developing an environmentally friendly and efficient method for fabricating functionalized graphene.     

Cu oxidation kinetics through graphene and its effect on the electrical properties of graphene

The oxidation kinetics of Cu through graphene were evaluated from the surface coverage of Cu oxide (F_ox) by varying the oxidation time (t_ox = 10–360 min) and temperature (T_ox = 180–240 °C) under an air environment. F_ox, as a function of time, well followed the Johnson–Mehl–Avrami–Kolmogorov equation; thus, the activation energy of Cu oxidation was estimated as 1.5 eV. Transmission electron microscopy studies revealed that Cu₂O formed on the top of the graphene at grain boundaries (G-GBs), indicating that Cu₂O growth was governed by the out-diffusion of Cu through G-GBs. Further, the effect of Cu oxidation on graphene quality was investigated by measuring the electrical properties of graphene after transferring. The variation of the sheet resistance (R_s) as a function of t_ox at all T_ox was converted into one curve as a function of F_ox. R_s of 250 Ω sq⁻¹ was constant, similar to that of as-grown graphene up to F_ox = 15%, and then increased with F_ox. The Hall measurement revealed that the carrier concentration remained constant in the entire range of F_ox, and R_s was solely related to the decrease in the Hall mobility. The variation in Hall mobility was examined according to the graphene percolation probability model, simulating electrical conduction on G-GBs during Cu₂O evolution. This model well explains the constant Hall mobility within F_ox = 15% and drastic F_ox degradation of 15–50% by the concept that the electrical conduction of graphene is disconnected by Cu₂O formation along with the G-GBs. Therefore, we systematically developed the oxidation kinetics of Cu through graphene and simultaneously examined the changes in the electrical properties of graphene.

The oxidation kinetics of Cu through graphene were evaluated from the surface coverage of Cu oxide (F_ox) by varying the oxidation time (t_ox = 10–360 min) and temperature (T_ox = 180–240 °C) under an air environment.     

Boron carbide composites with highly aligned graphene nanoplatelets: light-weight and efficient electromagnetic interference shielding materials at high temperatures

B₄C-based ceramic composites containing 0–2 vol% highly aligned graphene nanoplatelets (GNPs) are fabricated. The electromagnetic interference (EMI) shielding properties of the obtained composites are investigated at X-band (8.2–12.4 GHz) frequency range from room-temperature up to 800 °C. All composites exhibit outstanding EMI shielding properties with satisfactory frequency- and thermal-stability. The shielding effectiveness (SE) of GNP/B₄C composites increases monotonically with increasing GNP loading. Superior room-temperature SE close to 40 dB is achieved with only 2 vol% GNPs and high SE around 35 dB still persists at 800 °C. Considering their relatively low density, GNP/B₄C composites possess a high specific shielding effectiveness (SSE) of 16 dB cm³ g⁻¹ which is among the highest values in reported ceramic-based shielding composites. Especially, the GNP/B₄C composite with 2 vol% GNPs exhibits the highest SSE/t (SSE divided by thickness) values at temperatures above 200 °C for all reported shielding composites, indicating that GNP/B₄C composites belong to the most promising high-temperature shielding composites. The excellent shielding properties of GNP/B₄C composites arise mainly from the high electrical conductivity, high dielectric loss and the multiple reflections by the highly aligned and large-sized GNP layers.

The incorporation of a small amount of highly aligned graphene nanoplatelets into boron carbide leads to light-weight and efficient high-temperature electromagnetic interference shielding composites.     

Direct chemical vapor deposition synthesis of large area single-layer brominated graphene

Graphene and its derivatives such as functionalized graphene are considered to hold significant promise in numerous applications. Within that context, halogen functionalization is exciting for radical and nucleophilic substitution reactions as well as for the grafting of organic moieties. Historically, the successful covalent doping of sp² carbon with halogens, such as bromine, was demonstrated with carbon nanotubes. However, the direct synthesis of brominated graphene has thus far remained elusive. In this study we show how large area brominated graphene with C–Br bonds can be achieved directly (i.e. a single step) using hydrogen rich low pressure chemical vapor deposition. The direct synthesis of brominated graphene could lead to practical developments.

In this study we present the first direct synthesis of large area, single layer, crystalline graphene with covalently doped bromine.     

The solvent-free mechano-chemical grinding of a bifunctional P25–graphene oxide adsorbent–photocatalyst and its configuration as porous beads

Owing to their use in water-cleaning technology, titanium-dioxide-based nanomaterials have dominated the photocatalysis scene, with so-called Degussa (P25) being the most promising under UV light. However, this is not the case under visible light, where it is necessary to combine titanium dioxide with other photosensitising nanomaterials. Unfortunately, most of the strategies aimed in this direction are chemically non-facile, energy-intensive, economically expensive, and not suitable for large-scale production. We herein describe a straightforward solvent-free approach for accessing visible-light-activated titanium-dioxide-based photocatalysts via the mechanochemical grinding of Degussa P25 with a second solid partner. Upon comparing several solid-material benchmarks, P25–graphene oxide is the best combination. The resulting material showed efficient performance for the adsorption and photodegradation of different dye pollutants, namely methylene blue, malachite green, Congo red, and methyl orange. The recorded performance was nearly comparable to that reached using sol–gel materials, with the ultimate advantage of being more sustainable and industrially scalable. The recyclability can be improved through a porous-bead configuration using biomass waste chitosan hydrogel, an approach that can further fulfill the requirement for more sustainable photocatalyst designs.

Grinding of P25 in the presence of graphene oxide and shaping as porous beads enable access to recyclable photocatalysts.     

Tune the chemical activity of graphene via the transition metal substrate

To achieve the chemical modification of graphene efficiently is desirable and essential to promote the technological applications of graphene. In this study, the density functional theory (DFT) calculations have been carried out to investigate the hydrogenation and fluorination activities of graphene on Ni(111), Re(0001) and Pt(111). The calculation results indicate that the chemical activity of graphene is related to both the characteristics of the graphene–substrate interfacial interaction and the local atomic stacking, namely the chemical activity of graphene is position-dependent. The strong covalently interacting substrates Ni(111) and Re(0001) will remarkably enhance the chemical activity of graphene, while the modulation effects from the weak van der Waals interacting substrate Pt(111) is trivial. Electronic structure studies reveal that the intensive graphene–substrate interfacial interaction can gain in chemical energy to offset the strain energy caused by the C atom sp²–sp³ transition and stabilize the absorbing state.

The chemical activity of graphene can be tuned by the transition metal substrate.     

Polydopamine-coated graphene nanosheets as efficient electrocatalysts for oxygen reduction reaction

Polydopamine-modified graphene (G-PDA) materials were synthesized by in situ polymerization of a dopamine monomer on the surface of graphene oxide. X-ray photoelectron spectroscopy (XPS) has confirmed that new N-containing functional groups are formed during the synthesis process, which result in the excellent electrocatalytic activity of the composite towards ORR in terms of onset potential, number of electron transferred and limiting current density. The electrocatalytic activity of the optimized G-PDA sample is better than N-doped graphene and comparable to the commercial 20 wt% Pt/C catalyst. Furthermore, compared with the Pt-based catalysts, the G-PDA showed superior stability and methanol resistance, which favored its practical applications in fuel cells.

Polydopamine-coated graphene nanosheets show excellent electrocatalytic activity towards oxygen reduction reaction.     

Effect of nitrogen-doping configuration in graphene on the oxygen reduction reaction

In this study, we investigate the oxygen reduction reaction (ORR) reactivity of nitrogen-doped graphene by using density functional theory (DFT), a computational quantum mechanical technique. Four doping configurations and five models were comprehensively studied: quaternary nitrogen (NQ), pyrrolic nitrogen (N5), two forms of pyridinic nitrogen (N6, N6nH) and three-pyridinic nitrogen (3N6). Models for possible sites during each step of ORR were set up and visualized to provide a platform to calculate the free energy of the reaction pathway to determine the suitability of each doping scenario. Associative mechanisms were displayed by all models except N5, which showed dissociative mechanism. The calculated free energy pathways demonstrate that the ranking of the reactivity for ORR by different nitrogen configurations from most reactive to least reactive is N6, NQ, N6nH, 3N6, and N5. Spin density and charge density aid in describing levels of reactivity.

The oxygen reduction reaction (ORR) reactivity of various nitrogen-doped graphene configurations are probed in detail using density functional theory (DFT) calculations.     

Metal-free and solvent-free synthesis of m-terphenyls through tandem cyclocondensation of aryl methyl ketones with triethyl orthoformate

Reported here is a novel cyclocondensation of aryl methyl ketones and triethyl orthoformate for the simple synthesis of m-terphenyls. In the presence of a catalytic amount of TfOH, alkyl- and chloro-substituted acetophenones produced a series of terphenyls through a tandem reaction which merged six steps into a one-pot procedure. Moreover, the corresponding ester products were obtained when using other substituted acetophenones as the starting materials under the same reaction conditions.

A facile metal-free and solvent-free benzannulation was developed for the construction of m-terphenyl derivatives starting from aryl methyl ketones and triethyl orthoformate. This is a tandem reaction which merged six steps into one-pot procedure.     

The mechanism and activity of oxygen reduction reaction on single atom doped graphene: a DFT method

Heteroatom doped graphene as a single-atom catalyst for oxygen reduction reaction (ORR) has received extensive attention in recent years. In this paper, the ORR activity of defective graphene anchoring single heteroatom (IIIA, IVA, VA, VIA and VIIA) was systematically investigated using a dispersion-corrected density functional theory method. For all of the 34 catalysts, 14 of which were further analyzed, and the Gibbs free energy of each elementary reaction was calculated. According to the scaling relationship between ΔG_OOH* and ΔG_OH*, we further analyzed the rate-determining step of the remaining 20 catalysts. The results show that when the ORR reaction proceeds in the path O₂ → OOH → O → OH → H₂O, the reaction energy barriers are lower than 0.8 eV for Te-SV, Sb-DV, Pb-SV, Pb-DV, As-SV, As-DV, B-SV, Sn-SV and N-SV. Our result provides a theoretical basis for further exploration of carbon-based single-atom catalysts for ORR.

Heteroatom doped graphene as a single-atom catalyst for oxygen reduction reaction (ORR) has received extensive attention in recent years.     

Oxygen reduction reaction activity of an iron phthalocyanine/graphene oxide nanocomposite

Electrocatalysts with metal–nitrogen–carbon (M–N–C) sites have recently attracted much attention as potential catalysts for the oxygen reduction reaction (ORR), and a hybrid of iron phthalocyanine (FePc) and reduced graphene oxide (rGO) is one of the promising candidates. Herein, a FePc/GO nanocomposite was synthesized by electrostatic deposition on the electrode. The electrochemically reduced FePc/GO nanocomposite (ER(FePc/GO)) contained Fe²⁺ centers in well reduced graphene sites without agglomeration. The ER(FePc/GO) exhibited high ORR activity with an ORR onset (E_onset) and half-wave potential (E_1/2) of 0.97 and 0.86 V, respectively. Furthermore, the ORR activity successfully improved by adding an electrolyte such as KCl or KNO₃. The small H₂O₂ yield of 2%, superior tolerance to methanol addition and high-durability indicate that the ER(FePc/GO) is a promising electrocatalyst. Theoretical studies, indicating that the presence of Cl⁻ and NO₃⁻ ions lowered the conversion energy barrier, strongly supported the experimental results.

We demonstrate from both experimental and theoretical viewpoints that an iron phthalocyanine/graphene oxide nanocomposite directly synthesized on electrode exhibits high ORR activity.     

Reversible functionalization and exfoliation of graphite by a Diels–Alder reaction with furfuryl amine

Furfuryl amine-functionalized few-layered graphene was prepared via a mechanochemical process by a [4 + 2] cycloaddition under solvent-free conditions. By employing ball milling, active sites are merged mostly at the edge of the graphene sheets which makes them prone to Diels–Alder click reactions (D–A) in the presence of a diene precursor. Consequently, one-pot grafting with furfuryl amine onto the graphene sheets, exfoliates pristine graphite resulting in functionalized few-layered graphene which is soluble in organic solvents. Thereafter, the cleavage of the bonds in the adduct can occur by exposure to an external stimulus like temperature, to initiate a retro-Diels–Alder reaction. The success of the thermoreversible functionalization of the few-layered graphene was confirmed by Raman spectroscopy, TGA, XPS, EDX, contact angle and XRD analysis. The morphology of the samples was investigated by scanning electron microscopy and AFM. The latter was utilized to estimate graphene thickness. The results showed that functionalization proceeded under nitrogen with dry ball milling and mild temperatures efficiently.

Furfuryl amine-functionalized few-layered graphene was prepared via a mechanochemical process by a [4 + 2] cycloaddition under solvent-free conditions.     

Correction: Direct chemical vapor deposition synthesis of large area single-layer brominated graphene

Correction for ‘Direct chemical vapor deposition synthesis of large area single-layer brominated graphene’ by Maria Hasan et al., RSC Adv., 2019, 9, 13527–13532.

In the Acknowledgements section, the attribution “the Czech Republic from ERDF “Institute of Environmental Technology – Excellent Research” (No. CZ.02.1.01/0.0/0.0/15_019/0000853), should read “the Czech Republic from ERDF OP RDE project No. CZ.02.1.01/0.0/0.0/16_019/0000853”.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.