Graphene Fragments Research Articles

π-Extended Doublet Open-Shell Graphene Fragments Exhibiting One-Dimensional Chain Stacking.

The hitherto elusive benzo[c]anthanthrenyl radical derivatives composed of seven fused six-membered rings are synthesized and isolated in the crystalline form, representing a laterally π-extended doublet open-shell graphene fragment compared to the phenalenyl and olympicenyl radical structures. X-ray crystallographic analysis revealed one-dimensional chain stacking with relatively close intermolecular contacts, which is an important precondition for achieving single-component conductors. The magnetic, optical, and redox properties are investigated in the solution phase. In combination with the good stability, such open-shell molecular systems have potentials as functional electronic materials.

The taming of Clar's hydrocarbon.

Triangulene is the smallest non-Kekulé graphene fragment known as Clar's hydrocarbon. Due to its open-shell electronic structure, triangulene is a promising molecular building block of carbon-based organic materials for spintronics and quantum molecular science. It comprises six benzenoid rings arranged in a triangular shape with two unpaired electrons delocalized over the entire conjugated core, making this molecule highly reactive. A triplet ground state is predicted for this hydrocarbon by Ovchinnikov's rule, or Lieb's theorem, in accord with Hund's rule. The pioneering work on triangulene was performed almost 70 years ago by Erich Clar, who attempted to prepare the pristine compound. Since then, several synthetic approaches to prepare this molecule have been exploited. The extreme reactivity of triangulene can be circumvented using on-surface techniques or by installation of sterically demanding substituents, which kinetically stabilize the diradical core against oligomerization in solution. The first two examples of a persistent derivative of triangulene were simultaneously and independently developed last year. This article presents a historical development in the synthesis of triangulene and its derivatives and outlines possible future applications in ferromagnetic materials, electrically conductive polymers or quantum computing.

Open-Shell Graphene Fragments for Scavenging Hazardous Sulfur Hexafluoride Components with Ultrafast Recovery Time: A First Principles Study

Open-Shell Graphene Fragments for Scavenging Hazardous Sulfur Hexafluoride Components with Ultrafast Recovery Time: A First Principles Study

Enhancement of SERS effect in Graphene-Silver hybrids

Surface enhanced Raman spectroscopy (SERS) is considered as one of the most intensively studied analytical technique. Here, Ag nanoparticles (NPs) are coated on graphene layers (GLs) to prepare a hybrid SERS substrate. GLs are known to contribute towards SERS based on chemical enhancement mechanism, and Ag NPs are based on electromagnetic enhancement mechanism. Thus, the synergetic effect of hybrid could be utilized to prepare excellent SERS substrates. Furthermore, effect of Ag NPs with varying size has been studied to check the improvement in SERS. Ten times remarkable enhancement in SERS intensities are observed from the GLs-Ag hybrid as compared to pristine GLs. Density functional theory (DFT) calculations have been carried out through small Ag clusters interaction with graphene fragments to support and understand the mechanism behind the observed phenomena. Strong adsorption of Ag3 cluster on graphene fragment reveals the substantial modification in electronic and vibrational properties of graphene fragment and considerable charge transfer from Ag3 cluster to graphene fragment. Such phenomena further result in G-band’s enhanced intensity in graphene fragment. Our experimental study and theoretical calculations set up basic understanding and open up a way to fabricate the high-performance SERS substrates for proficient sensing applications.

Facile synthesis of high surface area zeolite-templated carbons using divinylbenzene and propylene as carbon sources

Zeolite-templated carbon (ZTC) with a high surface area of 3860 m2 g−1 is readily synthesized from divinylbenzene (DVB) and propylene as carbon sources, by using NaY zeolite as a template. In the present method, the necessary amount of DVB can be reduced to the total pore volume of the zeolite. DVB can be directly mixed with previously dried zeolite, and each time for the adsorption and the thermal polymerization of DVB is shortened to only 1 h. The resulting NaY/polydivinylbenzene (PDVB) is subject to chemical vapor deposition (CVD) using propylene and the subsequent carbonization. In this process, DVB becomes graphene fragments that have a catalytic activity against propylene, and the ZTC framework with the three-dimensionally ordered and interconnected micropore structure is developed inside the zeolite pores during the CVD. Finally, zeolite is removed by HF etching, producing 0.3 g of ZTC per 1 g of zeolite. Instead of DVB, previous methods to prepare high surface area ZTCs use furfuryl alcohol that gradually polymerizes and strongly stacks on the zeolite particles, leading to a decrease in the surface area of ZTC due to the resulting nonporous carbon shell formed on the ZTC particles. The present method is much easier and more feasible than the previous methods, and does not require any reagents for polymerizing DVB or organic solvents.

Enhanced lubricity of CVD diamond films by in-situ synthetization of top-layered graphene sheets

In the present study, we demonstrated a novel process of in-situ fabrication of graphene sheets by adopting the chemical vapor deposition (CVD) method. Fabricating such a layer of vertical graphene sheets (VGs) on the surface of microcrystalline diamond (MCD) films could decrease the stable coefficient of friction (COF) by 20–30%. Besides, such lubricity showed excellent load capacity and durability. With normal loads ranging from 3 N to 9 N (Hertz contact pressure of 2.1–3.1 GPa), the MCD-VGs films exhibited COFs stabilized at 0.105 and wear rates less than 5 × 10−6 mm3 N−1 m−1. Additionally, the produced VGs displayed an excellent service lifetime with 72,000 sliding cycles. During the friction tests, an equilibrium graphene self-mated contact formed on the sliding interface as graphene fragments adhered onto the counterpart ball and resulted in the reduction of friction and low wear rates. Additionally, the mixtures of Si3N4/SiO2 particles and graphene sheets residual within the wear tracks would promote the transformation of the sliding regime to three-body abrasion, beneficial for friction reduction. The findings presented in this study provide a novel and effective approach to enhance the lubricity of any engineering diamond surface.

Correlating Local Structure and Sodium Storage in Hard Carbon Anodes: Insights from Pair Distribution Function Analysis and Solid-State NMR.

Hard carbons are the leading candidate anode materials for sodium-ion batteries. However, the sodium-insertion mechanisms remain under debate. Here, employing a novel analysis of operando and ex situ pair distribution function (PDF) analysis of total scattering data, supplemented by information on the local electronic structure provided by operando 23Na solid-state NMR, we identify the local atomic environments of sodium stored within hard carbon and provide a revised mechanism for sodium storage. The local structure of carbons is well-described by bilayers of curved graphene fragments, with fragment size increasing, and curvature decreasing with increasing pyrolysis temperature. A correlation is observed between the higher-voltage (slope) capacity and the defect concentration inferred from the size and curvature of the fragments. Meanwhile, a larger lower-voltage (plateau) capacity is observed in samples modeled by larger fragment sizes. Operando PDF data on two commercially relevant hard carbons reveal changes at higher-voltages consistent with sodium ions stored close to defective areas of the carbon, with electrons localized in the antibonding π*-orbitals of the carbon. Metallic sodium clusters approximately 13-15 Å in diameter are formed in both carbons at lower voltages, implying that, for these carbons, the lower-voltage capacity is determined by the number of regions suitable for sodium cluster formation, rather than by having microstructures that allow larger clusters to form. Our results reveal that local atomic structure has a definitive role in determining storage capacity, and therefore the effect of synthetic conditions on both the local atomic structure and the microstructure should be considered when engineering hard carbons.

Iridaaromatics via Methoxy(alkenyl)carbeneiridium Complexes.

This review describes the development of a versatile methodology to synthesize polycyclic metallaaromatic hydrocarbons based on iridium, as well as the studies that helped us to determine and understand what is required in order to broaden the scope and the selectivity of the methodology and stabilize the complexes obtained. This methodology aims to open the door to new materials based on graphene fragments.

Lithium and Sodium Ion Binding Mechanisms and Diffusion Rates in Lignin-Based Hard Carbon Models.

Hard carbons are the primary candidate for the anode of next-generation sodium-ion batteries for large-scale energy storage, as they are sustainable and can possess high charge capacity and long cycle life. These properties along with diffusion rates and ion storage mechanisms are highly dependent on nanostructures. This work uses reactive molecular dynamics simulations to examine lithium and sodium ion storage mechanisms and diffusion in lignin-based hard carbon model systems with varying nanostructures. It was found that sodium will preferentially localize on the surface of curved graphene fragments, while lithium will preferentially bind to the hydrogen dense interfaces of crystalline and amorphous carbon domains. The ion storage mechanisms are explained through ion charge and energy distributions in coordination with snapshots of the simulated systems. It was also revealed that hard carbons with small crystalline volume fractions and moderately sized sheets of curved graphene will yield the highest sodium-ion diffusion rates at ∼10–7 cm2/s. Self-diffusion coefficients were determined by mean square displacement of ions in the models with extension through a confined random walk theory.

Second hyperpolarizability of hexagonal graphene quantum dots: Effects of size and structural defects

Graphene quantum dots (GQDs) are nanosized fragments of graphene containing a bandgap caused by edge effects and quantum confinement. These features enable GQDs to have attractive photonics applications. During the preparation of graphene nanostructures, defects are unavoidable as they influence the behavior of π-electron transfer, which in turn impact the nonlinear optical (NLO) response. Herein, density functional theory was used to determine the electronic and three-order NLO properties of hexagonal GQDs with structural defects. The size of GQDs and the edge effects on their NLO properties were also investigated. Results showed with increased molecule size, the second hyperpolarizability increased for zigzag- and armchair-edged GQDs. This finding was due to the decrease in their frontier molecular-orbital energy gaps. Moreover, the introduction of point vacancy limited the π-electron transport in the zigzag-edged GQD, thereby widening the frontier molecular-orbital energy gap compared with the perfect zigzag-edged GQDs. However, in the presence of zigzag-edged GQDs having Stone–Wales defects, the energy gap decreased; consequently, the NLO response improved. All these findings provided new insights into the design of novel graphene-based NLO materials that may help expand the applications of GQDs in optoelectronic nanodevices.

Metal-free π-conjugated hybrid g-C3N4 with tunable band structure for enhanced visible-light photocatalytic H2 production

Development of low-cost and efficient photocatalytic materials with visible-light response is of urgent need for solving energy and environmental problems. Here, a metal-free two-dimensional (2D) π-conjugated hybrid g-C3N4 photocatalyst with tunable band structure was prepared by a novel one-pot bottom-up method based on a supersaturated precipitation process of urea and triethanolamine (TEOA) solution. The microstructure of the hybrid g-C3N4 is revealed to be a compound of periodic tri-s-triazine units grafted with N-doped graphene (GR) fragments. From experimental evidence and theoretical calculations, the two different π-conjugated fragments in the hybrid g-C3N4 material are proved to construct a 2D in-plane junction structure, thereby expanding the light absorption range and accelerating the interface charge transfer. The π-conjugated electron coupling in the 2D photocatalyst eliminates the grain boundary effect, and the coupled highest occupied molecular orbital (HOMO) effectively promotes the separation of photo-induced charge carriers. Compared with the g-C3N4 prepared by the conventional method, the visible-light H2 production activity of the optimized sample is enhanced by 253 %. This work provides a new strategy of constructing metal-free g-C3N4 hybrids for efficient photocatalytic water splitting.

Thermal Reductive Perforation of Graphene Cathode for High‐Performance Aluminum‐Ion Batteries

AbstractControlling the structure of graphene‐based materials with improved ion intercalation and diffusivity is crucial for their applications, such as in aluminum‐ion batteries (AIBs). Due to the large size of AlCl4− ions, graphene‐based cathodes have specific capacities of ≈60 to 148 mAh g−1, limiting the development of AIBs. A thermal reductive perforation (TRP) strategy is presented, which converts three‐layer graphene nanosheets to surface‐perforated graphene materials under mild temperature (400 °C). The thermal decomposition of block copolymers used in the TRP process generates active radicals to deplete oxygen and create graphene fragments. The resultant material has a three‐layer feature, in‐plane nanopores, >50% expanded interlayer spacing, and a low oxygen content comparable to graphene annealed at a high temperature of ≈3000 °C. When applied as an AIB cathode, it delivers a reversible capacity of 197 mAh g−1 at a current density of 2 A g−1 and reaches 92.5% of the theoretical capacity predicted by density‐functional theory simulations.

Optical Absorption of C32H24 and C32H36 Hybrid Molecular Clusters Based on Diamondene–Graphene Fragments

Optical Absorption of C32H24 and C32H36 Hybrid Molecular Clusters Based on Diamondene–Graphene Fragments

On the Clar number of graphene fragment

A graphene fragment is a benzenoid graph that its dualist graph is a unicyclic graph. In particular, when the dualist graph of a benzenoid graph is a circle, it is called cyclofusene. In this paper, we determine the Clar number of a cyclofusene graph, and prove a bound for the Clar number of the graphene fragment. Moreover, we construct the graphene fragment which can attain this bound. More precisely, it is shown that the Clar number of the graphene fragment with n hexagons is at most $$[\frac{2n}{3}]$$ .

High-density monolithic pellets of double-sided graphene fragments based on zeolite-templated carbon

High-density and highly porous graphene-based pellets with anomalous gas densification property and glass-like hardness have been fabricated by using zeolite-templated carbon and reduced graphene oxide.

Influence of cerium doping on Cu-Ni/activated carbon low-temperature CO-SCR denitration catalysts.

In this study, to evaluate the effects of two methods for activation of nitric acid, air thermal oxidation and Ce doping were applied to a Cu–Ni/activated carbon (AC) low-temperature CO-SCR denitration catalyst. The Cu–Ni–Ce/AC0,1 catalyst was prepared using the ultrasonic equal volume impregnation method. The physical and chemical structures of Cu–Ni–Ce/AC0,1 were studied using scanning electron microscopy, Brunauer–Emmett–Teller analysis, Fourier-transform infrared spectroscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, CO-temperature programmed desorption (TPD) and NO-TPD characterisation techniques. It was found that the denitration efficiency of 6Cu–4Ni–5Ce/AC1 can reach 99.8% at a denitration temperature of 150 °C, a GHSV of 30 000 h−1 and 5% O2. Although the specific surface area of the AC activated by nitric acid was slightly lower than that activated by air thermal oxidation, the pore structure of the AC activated by nitric acid was more developed, and the number of acidic oxygen-containing functional groups was significantly increased. Ce metal ions were inserted into the graphite microcrystalline structure of AC, splitting it into smaller graphene fragments, whereby the dispersibility of Cu and Ni was improved. In addition, many reaction units were formed on the catalyst surface, which could adsorb more CO and NO reaction gases. With the increase in Ce doping, the relative proportions of Cu2+/Cun+, Ni3+/Nin+ and surface adsorbed oxygen (Oα) in the Cu–Ni–Ce/AC0,1 catalyst increased. In addition, after the introduction of Ce into Cu–Ni/AC, the amount of weak and medium acids significantly increased. This may be due to the Ce species or its influence on the Cu/Ni species. Further, the active sites of the acid were more exposed. According to the results of the study, a composite metal oxide CO-SCR denitration mechanism is proposed. Through the oxidation–reduction reaction between the metals, the reaction gas of CO and NO is adsorbed and the incoming O2 is converted into (Oα), which promotes the conversion of NO into NO2. The CO-SCR reaction is accelerated, and the rate of low-temperature denitration was increased. Overall, the results of this study will provide theoretical support for the research and development of low-temperature denitration catalysts for sintering flue gas in iron and steel enterprises.

Charge transport properties of open-shell graphene fragments: a computational study of the phenalenyl tilings.

Thinking outside the box of the phenalenyl radical: a systematic structure design strategy, phenalenyl tiling, is found to benefit the electron transport properties of open-shell graphene fragments with one free radical. Compared with the closed-shell species, phenalenyl-based π-radicals exhibit smaller intramolecular reorganization energies and larger intermolecular electronic couplings. However, the on-site Coulomb repulsion can be too strong and impedes the charge transport efficiency of such materials. The repulsion can be weakened in radical species by spin delocalization. In this paper, the extended π-radicals we studied are categorized into three types of open-shell structures: the zigzag, the armchair and the discotic odd alternant hydrocarbons. The latter two belong to phenalenyl tilings. We found that the phenalenyl tilings fully inherit the desirable features of the singly occupied molecular orbital of the phenalenyl radical in a predictable and delocalized fashion, and their on-site Coulomb repulsion is effectively reduced. The zigzag π-radicals are less satisfactory. Therefore, the phenalenyl tilings are favorable candidates for charge transporting materials.

Оптическое поглощение гибридных молекулярных кластеров С-=SUB=-32-=/SUB=-Н-=SUB=-24-=/SUB=-, С-=SUB=-32-=/SUB=-Н-=SUB=-36-=/SUB=- на основе фрагментов даймондена и графена

Using the density functional theory (DFT), the geometry optimised for molecular clusters C32H24 and C32H36, constructed with a hybrid of fragments of diamondene and graphene with the dangling bonds passivated with hydrogen. The time-dependent DFT allowed calculating molar extinction spectra of the clusters. The comparison of the calculated spectra with the results of astrophysical observations evidence on a possible contribution of the clusters to the extinction spectrum of light by the carbon-based constituent of the interstellar medium.

Edge-Functionalized Nanographenes.

Nanographenes (NGs) have recently emerged as new carbon materials. Their nanoscale size results in a size-dependent quantum confinement effect, opening the band gap by a few eV. This energy gap allows NGs to be applied as optical materials. This property has attracted researchers across multiple scientific fields. The photophysical properties of NGs can be manipulated by introducing organic groups onto their basal planes and/or into their edges. In addition, the integration of organic functional groups into NGs results in NG-based hybrid materials. These features make the post-synthetic modification of NGs an active research area. As obtainable information on chemically functionalized NGs is limited owing to their nonstoichiometry and structural uncertainty, their structural characterization requires a combination of multiple spectroscopic methods. Therefore, information on the characterization procedures of recently published chemically functionalized NGs is of value for advancing the field of NG-based hybrid materials. The present review focuses on the structural characterization of chemically functionalized NGs. It is hoped that this review will help to advance this field.

Study on the tensile and wear properties of laser-cladded IN718 superalloy reinforced by carbon nanoproducts transformed from carbon nanotubes

Abstract