Graphene-like Domains Research Articles

Harnessing the potential of coal-derived graphene oxide/epoxy nanocomposites: Enhancing thermal conductivity and fracture toughness

The choice of precursor material significantly influences the properties of graphene oxide (GO), thereby providing its adaptability across diverse applications. Coal, with its distinctive molecular structure characterized by graphene-like domains and aliphatic side chains, as well as abundant impurities and heteroatoms featuring specific functional groups, emerges as a promising precursor. Employing a recently developed facile one-pot process for synthesizing semianthracite coal-derived GO (AC-GO), we have, in this study, explored its potential as a nanofiller in an epoxy matrix, resulting in AC-GO-enriched nanocomposites. The main purpose of the project was to introduce AC-GO within the epoxy matrix to enhance its thermal conductivity and fracture resistance by enhancing interfacial interactions. Our findings indicate remarkable enhancements in thermal conductivity (0.646 Wm-1K-1 at 1.0 wt% loading) and fracture toughness (6.7 MPam1/2 at 0.8 wt% loading) within AC-GO loaded nanocomposites, with these improvements being 255 % and 215 %, respectively, over those for the epoxy matrix. Remarkably, they surpass those achieved with graphite-derived GO (Gr-GO) by approximately 112 % and 76 %, respectively. Enhancing thermal conductivity and fracture toughness in epoxy nanocomposites is vital for effective heat dissipation and durability, making them ideal for high-performance electronics, aerospace, and automotive applications. These improvements can be attributed to the enhanced interactions between oxygen moieties located at the periphery of AC-GO and the fundamental epoxy-amine hardener system within the resin formulation, fostering electron acceptor–donor interactions. Furthermore, we introduce a new figure of merit (FOM) that accurately captures the combined role of thermal conductivity and fracture toughness. It is shown that this FOM can serve as a useful design tool for selecting an optimal nanofiller loading for targeted applications. The findings of our paper highlight the versatility and unique attributes of AC-GO, offering promising opportunities for applications in the industrial engineering, owing to their outstanding interfacial properties.

Raman Spectroscopy Measurements Support Disorder-Driven Capacitance in Nanoporous Carbons.

Our recent study of 20 nanoporous activated carbons showed that a more disordered local carbon structure leads to enhanced capacitive performance in electrochemical double layer capacitors. Specifically, NMR spectroscopy measurements and simulations of electrolyte-soaked carbons evidenced that nanoporous carbons with smaller graphene-like domains have larger capacitances. In this study, we use Raman spectroscopy, a common probe of local structural disorder in nanoporous carbons, to test the disorder-driven capacitance theory. It is found that nanoporous carbons with broader D bands and smaller ID/IG intensity ratios exhibit higher capacitance. Most notably, the ID/IG intensity ratio probes the in-plane sizes of graphene-like domains and supports the findings from NMR that smaller graphene-like domains correlate with larger capacitances. This study supports our finding that disorder is a key metric for high capacitance in nanoporous carbons and shows that Raman spectroscopy is a powerful technique that allows rapid screening to identify nanoporous carbons with superior performance in supercapacitors.

Structural disorder determines capacitance in nanoporous carbons.

The difficulty in characterizing the complex structures of nanoporous carbon electrodes has led to a lack of clear design principles with which to improve supercapacitors. Pore size has long been considered the main lever to improve capacitance. However, our evaluation of a large series of commercial nanoporous carbons finds a lack of correlation between pore size and capacitance. Instead, nuclear magnetic resonance spectroscopy measurements and simulations reveal a strong correlation between structural disorder in the electrodes and capacitance. More disordered carbons with smaller graphene-like domains show higher capacitances owing to the more efficient storage of ions in their nanopores. Our findings suggest ways to understand and exploit disorder to achieve highly energy-dense supercapacitors.

Laser Photoreduction of Graphene Aerogel Microfibers: Dynamic Electrical and Thermal Behaviors.

This work reports the dynamic behaviors of graphene aerogel (GA) microfibers during and after continuous wave (CW) laser photoreduction. The reduction results in one-order of magnitude increase in the electrical conductivity. The experimental results reveal the exact mechanisms of photoreduction as it occurs: immediate photochemical removal of oxygen functional groups causing a sharp decrease in electrical resistance and subsequent laser heating that facilitates thermal rearrangement of GO sheets towards more graphene-like domains. X-ray and Raman spectroscopy analysis confirm that photoreduction removes virtually all oxygen and nitrogen containing functional groups. Interestingly, a dynamic period immediately following the end of laser exposure shows a slow, gradual increase in electrical resistance, suggesting that a proportion of the electrical conductivity enhancement from photoreduction is not permanent. A two-part experiment monitoring the resistance changes in real-time before and after photoreduction is conducted to investigate this critical period. The thermal diffusivity evolution of the microfiber is tracked and shows an improvement of 277 % after all photoreduction experiments. A strong linear coherency between thermal diffusivity and electrical conductivity is also uncovered. This is the first known work to explore both the dynamic electrical and thermal evolution of a GO-based aerogel during and after photoreduction.

Ab initio simulation of amorphous BC3

We report the structural and electrical properties of an amorphous BC3 model based on ab initio molecular dynamics simulations. The amorphous network is achieved from the melt and has a layer-like structure consisting of mainly hexagonal (six membered) rings as in the crystal. However, the distribution of boron atoms in the noncrystalline configuration appears to differ significantly from that of boron atoms in the crystal. The network is a solid solution and has randomly distributed nanosized graphene-like domains at each layer. Boron atoms have a tendency to form more overcoordinated defects involving with boron-boron homopolar bond(s). The mean coordination of boron and carbon atoms is 3.2 and 3.0, respectively. Interestingly the amorphous configuration is found to have a slightly higher density and bulk modulus than the crystal, which are attributed to the existence of overcoordinated units in the amorphous state. Based on the localization of the band tail states, noncrystalline BC3 is speculated to be a semiconducting material.

The evolution of hydrogen induced defects and the restoration of [formula omitted]-plasmon as a monitor of the thermal reduction of graphene oxide

In this article, we study the modification of the optical, chemical and electronic properties of graphene oxide (GO) during thermal reduction in ultra-high-vacuum by combining the results of several electron spectroscopies. We find that the fraction of oxygen moieties on the surface, as deduced from the evolution of C 1s core level in photoemission, is progressively reduced upon increasing the annealing temperature from 150 to 650 °C. The intensity of the CH stretching mode, associated with CH defects on GO surface and measured in the low energy region of electron energy loss spectra (EELS), decreases as a function of the annealing temperature. The removal or the reduction of such hydrogen or oxygen defects induces a restoration of sp2 carbon hybridization. The presence of such hybridization is confirmed by the capability to excite π-plasmon as observed in the EELS spectra. In particular we find a critical annealing temperature (Tann = 300 °C) at which π-plasmon excitation via electron scattering is accessible suggesting the formation of graphene-like domains with size comparable with the plasmon wavelength (λp~5 nm). The linear dispersion of π-band close to Fermi level, as measured in UPS, confirms the formation of graphene-like domains.

Reagentless fabrication of a porous graphene-like electrochemical device from phenolic paper using laser-scribing

This study fabricated a portable, high-performance, and reagentless electrochemical devices using CO2 laser-scribing process, which allowed localized carbonization of a non-conductive and low-cost polymer platform, i.e., phenolic-paper. The carbonized material was extensively characterized by Raman spectroscopy, XPS, XRD, SEM, and electrochemical impedance spectroscopy. The carbon-based electrodes were obtained from the photothermal process induced by CO2 laser radiation and subsequently subjected to electrochemical treatment to fabricate a functional material with excellent conductivity and low charge-transfer resistance. Additionally, the laser-scribed electrodes presented a porous structure with graphene-like domains, thus indicating both potential for on-site electroanalytical applications and better performance than conventional carbon electrodes.

How Reliable Are Raman Spectroscopy Measurements of Graphene Oxide?

Graphene oxide (GO) has garnered attention for its tunable chemical, electrical, and optical properties. An integral part of the efforts to manipulate and improve the performance of GO is the ability to reliably characterize its complex structure. Raman spectroscopy and confocal Raman mapping are widely used for insight into the extent of GO’s nanoscale graphene-like domains, the degree of lattice order, and its sheet stacking structure. It has also been reported, however, that laser sources, similar to those used for Raman spectroscopy, can be used to intentionally reduce and ablate GO. In light of this, it is unclear how invasive Raman measurements of GO are and how reliable published Raman data is. In this study, we employ Raman laser doses spanning 4 orders of magnitude to investigate the impact of Raman measurements on GO structure. We find that GO undergoes reduction at all practical laser doses, with the degree of reduction increasing with dose. Lattice damage and ablation dominate at high laser do...

Carbonisation of biomass-derived chars and the thermal reduction of a graphene oxide sample studied using Raman spectroscopy

Chars and carbonised chars were produced from three different oxygen-rich precursors (Pinus radiata wood, Phormium tenax leaf fibres, and sucrose crystals). These non-graphitisable carbons were analysed with Raman spectroscopy in order to study the nanostructural development which occurs with increasingly severe heat treatments up to approximately 1000°C. The thermal reduction of a graphene oxide sample was similarly studied, as this is considered to involve the development of nanometre-scale graphene-like domains within a different oxygen-rich precursor. Increasing the heat treatment temperatures used in the charring and carbonisation processes, led to significant changes in a number of parameters measured in the Raman spectra. Correlations based on these parameter changes could have future applications in evaluating various char samples and estimating the heat treatment temperatures employed during their manufacture. After production heat treatment temperatures exceeded 700°C, the Raman spectra of the carbonised chars appeared to be largely precursor independent. The spectra of these carbonised chars were similar to the spectra obtained from thermally-reduced graphene oxides, especially when compared to a wide range of other carbonaceous materials analysed using this particular methodology. Partial reduction of a graphene oxide sample due to reasonably mild laser exposures during Raman analysis was also observed.

Synthesis of conducting graphene/Si3N4 composites by spark plasma sintering

Graphene/silicon nitride (Si3N4) composites with high fraction of few layered graphene are synthesized by an in situ reduction of graphene oxide (GO) during spark plasma sintering (SPS) of the GO/Si3N4 composites. The adequate intermixing of the GO layers and the ceramic powders is achieved in alcohol under sonication followed by blade mixing. The reduction of GO occurs together with the composite densification in SPS, thus avoiding the implementation of additional reduction steps. The materials are studied by X-ray photoelectron and micro-Raman spectroscopy, revealing a high level of recovery of graphene-like domains. The SPS graphene/Si3N4 composites exhibit relatively large electrical conductivity values caused by the presence of reduced graphene oxide (∼1 S cm−1 for ∼4 vol.%, and ∼7 S cm−1 for 7 vol.% of reduced-GO). This single-step process also prevents the formation of highly curved graphene sheets during the thermal treatment as the sheets are homogeneously embedded in the ceramic matrix. The uniform distribution of the reduced GO sheets in the composites also produces a noticeable grain refinement of the silicon nitride matrix.

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 (∼7.9×10(4) Ω per square) of the MW-reduced GO on SiO(2) -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 10(5) to 10(9) Ω per square, indicating that MW reduction of GO is promising for the convenient low-temperature preparation of transparent conductors on flexible polymeric substrates.

Polarization-resolved magneto-Raman scattering of graphenelike domains on natural graphite

The micro-Raman scattering response of a graphene-like location on the surface of bulk natural graphite is investigated both at $T=\unit{4.2}{K}$ and at room temperature in magnetic fields up to 29 T. Two different polarization configurations, co-circular and crossed-circular, are employed in order to determine the Raman scattering selection rules. Several distinct series of electronic excitations are observed and we discuss their characteristic shapes and amplitudes. In particular, we report a clear splitting of the signals associated with the inter-Landau level excitations $-n\rightarrow+n$. Furthermore, we observe the pronounced interaction of the zone-center E$_{\text{2g}}$-phonon with three different sets of electronic excitations. Possible origins for these graphene-like inclusions on the surface of bulk graphite are discussed.