Subnanometer vacancy defects introduced on graphene by oxygen gas.

The basal plane of graphene has been known to be less reactive than the edges, but some studies observed vacancies in the basal plane after reaction with oxygen gas. Observation of these vacancies has typically been limited to nanometer-scale resolution using microscopic techniques. This work demonstrates the introduction and observation of subnanometer vacancies in the basal plane of graphene by heat treatment in a flow of oxygen gas at low temperature such as 533 K or lower. High-resolution transmission electron microscopy was used to directly observe vacancy structures, which were compared with image simulations. These proposed structures contain C═O, pyran-like ether, and lactone-like groups.

Unraveling the Catalytic Graphitization Mechanism of Ni-P Electroless Plated Cokes via In Situ Analytical Approaches.

We elucidate the catalytic graphitization mechanism using in situ analytical approaches. Catalytic graphitization is achieved through a Ni-P electroless plating process at a relatively low temperature of 1600 °C, which allows for a high crystallinity of coke. We also employ an ultrasonic treatment during the Ni-P electroless plating stage to effectively form metal layers on the surface. The impact of the ultrasonic treatment on the Ni-P electroless plating is confirmed by field emission scanning electron microscopy images of the cross-section and an elemental composition analysis using energy dispersive X-ray spectroscopy mapping. Structural analysis of the graphitized cokes via X-ray diffraction (XRD) and Raman spectroscopy shows that Ni-P electroless plating significantly accelerates the graphitization process. Furthermore, we illuminate the graphitization behavior through in situ transmission electron microscopy and XRD analysis. Nickel layers on the coke surface facilitate graphite formation by encouraging the dissolution and precipitation of amorphous carbons, thus resulting in efficient graphitization at a relatively low temperature.

Structure Dependent Electrochemical Behaviors of Hard Carbon Anode Materials Derived from Natural Polymer for Next-Generation Sodium Ion Battery.

Hard carbons are one of the most promising anode materials for next-generation sodium-ion batteries due to their high reversible capacity, long cycle life, and low cost. The advantage in terms of price of hard carbons can be further improved by using cheaper resources such as biomass waste as precursors. Lignin is one of the richest natural bio-polymer in the earth which can be obtained from woods. As the lignin has three-dimensional amorphous polymeric structure, it is considered as good precursor for producing carbonaceous materials under proper carbonization processes for energy storage devices. In this study, structural properties of lignin-derived hard carbons such as interlayer spacing, degree of disorder and surface defects are controlled. Specifically, lignin-derived hard carbons were synthesized at 1000 °C, 1250 °C, and 1500 °C, and it was confirmed that the structure gradually changed from a disordered structure to ordered structure through X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Hard carbons exhibit sloping regions at high voltage and plateau region at low voltage during the electrochemical processes for sodium ions. As the heat treatment temperature increases, the contribution to the overall reversible capacity of the sloping region decreases and the contribution of the plateau region increases. This trend confirms that it affects reversible capacity, rate-capability, and cycling stability, meaning that an understanding of structural properties and related electrochemical properties is necessary when developing hard carbon as a negative electrode material for sodium ion batteries.

Regeneration of interfacial bonding force of waste carbon fibers by light: Process demonstration and atomic level analysis.

Although interest in recycling carbon fibers is rapidly growing, practical applications of recycled carbon fibers (rCFs) are limited owing to their poor wettability and adhesion. Surface modification of CFs was achieved through intense pulsed light (IPL) irradiation, which functionalizes surface of rCFs. Surface energy, chemical composition, morphology, and interfacial shear strength (IFSS) of rCFs before and after IPL irradiation were investigated. The rCF IPL-irradiated at 1,200 V improved both polar and dispersive components of surface energy, and the IFSS significantly increased by 2.93 times in relation to that of the pristine rCF and reached 95% of that of high-grade commercial CFs. We proposed a mechanism by which oxygen functional groups on the rCF surface enhance the molecular bonding force with HDPE, and the model was validated from molecular dynamics simulations. IPL irradiation is a rapid and effective surface treatment method that can be employed for the manufacture of rCF-reinforced composites.

Hybrid 1D/2D nanocarbon-based conducting polymer nanocomposites for high-performance wearable electrodes.

A low interfacial contact resistance is a challenge in polymer nanocomposites based on conductive nanomaterials for high-performance wearable electrode applications. Herein, a polydimethylsiloxane (PDMS)-based flexible nanocomposite incorporating high-conductivity 1D single-walled carbon nanotubes (SWCNTs) and 2D reduced graphene oxide (r-GO) was developed for high-performance electrocardiogram (ECG) wearable electrodes. A PDMS-SWCNT (P-SW; type I) nanocomposite containing only SWCNTs (2 wt%), exhibited rough and non-uniform surface morphology owing to the strong bundling effect of as-grown SWCNTs and randomly entangled aggregate structures and because of inefficient vacuum degassing (i.e., R P-SW = 1871 Ω). In contrast, owing to the hybrid structure of the SWCNTs (1 wt%) and r-GO (1 wt%), the PDMS-SWCNTs/r-GO (P-SW/r-GO; type II) nanocomposite exhibited uniform surface characteristics and low contact resistance (i.e., R P-SW/r-GO = 63 Ω) through the formation of hybrid and long conducting pathways. The optimized nanocomposite (P-SW/r-GO/f; type III) possessed a fabric-assisted structure that enabled tunable and efficient vacuum degassing and curing conditions. Additionally, a long and wide conducting pathway was formed through more uniform and dense interconnected structures, and the contact resistance was drastically reduced (i.e., R P-SW/r-GO/f = 15 Ω). The performance of the electrodes fabricated using the optimized nanocomposites was the same or higher than that of commercial Ag/AgCl gel electrodes during real-time measurement for ECG Bluetooth monitoring. The developed high-performance hybrid conducting polymer electrodes are expected to contribute significantly to the expansion of the application scope of wearable electronic devices and wireless personal health monitoring systems.

Bromination Reactivity of Oxygen-Terminated Edges of Graphene.

The bromination reactivity of various types of polycyclic aromatic hydrocarbons (PAHs) with oxygen atoms and graphene with oxygen atoms was estimated by density functional theory calculation and experimentally clarified by analyzing bromination of PAHs using gas chromatography-mass spectrometry. In the experimental and theoretical bromination reactivity of PAHs, the presence of hydroxyl group increased the reactivity of PAHs because of electron-donating nature of the hydroxyl group but the other oxygen-containing functional groups such as lactone, ether, and ketone decreased the reactivity due to the electron-withdrawing nature of those groups. These effects of functional groups on the reactivity were also confirmed in graphene. The tendency of theoretical bromination reactivity of graphene was graphene with hydroxyl group > graphene with no group > graphene with lactone group > graphene with ether group > graphene with ketone group. Our study on the estimation of bromination reactivity of graphene edges provides the groundwork for the bromination of graphene edges.

Heptagons in the Basal Plane of Graphene Nanoflakes Analyzed by Simulated X-ray Photoelectron Spectroscopy.

The performance of graphene-based electronic devices depends critically on the existence of topological defects such as heptagons. Identifying heptagons at the atomic scale is important to completely understand the electronic properties of these materials. In this study, we report an atomic-scale analysis of graphene nanoflakes with two to eight isolated or connected heptagons, using simulated C 1s X-ray photoelectron spectroscopy (XPS) to estimate the XPS profiles depending on the density and the position of the heptagons. The introduction of up to 24% of isolated heptagons shifted the peak position toward high binding energies (284.0 to 284.3 eV), whereas the introduction of up to 39% of connected heptagons shifted the calculated peak position toward low binding energies (284.0 to 283.5 eV). The presence of heptagons also influenced the full width at half-maximum (FWHM). The introduction of 24% of isolated heptagons increased the FWHMs from 1.25 to 1.50 eV. However, the introduction of connected heptagons did not increase the FWHMs above 1.40 eV. The FWHMs increased to 1.40 eV for 19% of connected heptagons, but did not increase further as the percentage of connected heptagons increased to 39%. Based on the calculated results, the XPS profiles of graphene nanoflakes containing heptagons with different densities and positions can be obtained. Our precise identification of heptagons in graphene nanoflakes by XPS lays the groundwork for the analysis of graphene.

Water Adsorption Behavior on a Highly Dense Single-Walled Carbon Nanotube Film with an Enhanced Interstitial Space.

In this study, we describe the adsorption behavior of water (H2O) in the interstitial space of single-walled carbon nanotubes (SWCNTs). A highly dense SWCNT (HD-SWCNT) film with a remarkably enhanced interstitial space was fabricated through mild HNO3/H2SO4 treatment. The N2, CO2, and H2 adsorption isotherm results indicated remarkably developed micropore volumes (from 0.10 to 0.40 mL g-1) and narrower micropore widths (from 1.5 to 0.9 nm) following mild HNO3/H2SO4 treatment, suggesting that the interstitial space was increased from the initial densely-packed network assembly structure of the SWCNTs. The H2O adsorption isotherm of the HD-SWCNT film at 303 K showed an increase in H2O adsorption (i.e., by ∼170%), which increased rapidly from the critical value of relative pressure (i.e., 0.3). Despite the remarkably enhanced adsorption capacity of H2O, the rates of H2O adsorption and desorption in the interstitial space did not change. This result shows an adsorption behavior different from that of the fast transport of H2O molecules in the internal space of the SWCNTs. In addition, the adsorption capacities of N2, CO2, H2, and H2O molecules in the interstitial space of the HD-SWCNT film showed a linear relationship with the kinetic diameter, indicating an adsorption behavior that is highly dependent on the kinetic diameter.

Analysis of Shape Memory Behavior and Mechanical Properties of Shape Memory Polymer Composites Using Thermal Conductive Fillers.

Shape memory polymers (SMPs) are attracting attention for their use in wearable displays and biomedical materials due to their good biocompatibility and excellent moldability. SMPs also have the advantage of being lightweight with excellent shape recovery due to their low density. However, they have not yet been applied to a wide range of engineering fields because of their inferior physical properties as compared to those of shape memory alloys (SMAs). In this study, we attempt to find optimized shape memory polymer composites. We also investigate the shape memory performance and physical properties according to the filler type and amount of hardener. The shape memory composite was manufactured by adding nanocarbon materials of graphite and non-carbon additives of Cu. The shape-recovery mechanism was compared, according to the type and content of the filler. The shape fixation and recovery properties were analyzed, and the physical properties of the shape recovery composite were obtained through mechanical strength, thermal conductivity and differential scanning calorimetry analysis.

Pentagons and Heptagons on Edges of Graphene Nanoflakes Analyzed by X-ray Photoelectron and Raman Spectroscopy.

Identifying pentagons and heptagons in graphene nanoflake (GNF) structures at the atomic scale is important to completely understand the chemical and physical properties of these materials. Herein, we used X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to analyze the spectral features of GNFs according to the position of pentagons and heptagons introduced onto their zigzag and armchair edges. The XPS peak maxima were shifted to higher binding energies by introducing the pentagons or heptagons on armchair rather than zigzag edges, and the structures could be distinguished depending on the positions of the introduced pentagons or heptagons. Raman spectroscopic analyses also revealed that the position of edges with introduced pentagons or heptagons could also be identified using Raman spectroscopy, with characteristic bands appearing at 800-1200 cm-1, following the introduction of either pentagons or heptagons on armchair edges. This precise spectroscopic identification of pentagons and heptagons in GNFs provides the groundwork for the analysis of graphene-related materials.

Poly(vinylalcohol) (PVA) Assisted Sol-Gel Fabrication of Porous Carbon Network-Na3V2(PO4)3 (NVP) Composites Cathode for Enhanced Kinetics in Sodium Ion Batteries.

Na3V2(PO4)3 is regarded as one of the promising cathode materials for next-generation sodium ion batteries, but its undesirable electrochemical performances due to inherently low electrical conductivity have limited its direct use for applications. Motivated by the limit, this study employed a porous carbon network to obtain a porous carbon network-Na3V2(PO4)3 composite by using poly(vinylalcohol) assised sol-gel method. Compared with the typical carbon-coating approach, the formation of a porous carbon network ensured short ion diffusion distances, percolating electrolytes by distributing nanosized Na3V2(PO4)3 particles in the porous carbon network and suppressing the particle aggregation. As a result, the porous carbon network-Na3V2(PO4)3 composite exhibited improved electrochemical performances, i.e., a higher specific discharge capacity (~110 mAh g-1 at 0.1 C), outstanding kinetic properties (~68 mAh g-1 at 50 C), and stable cyclic stability (capacity retention of 99% over 100 cycles at 1 C).

Distinguishing Zigzag and Armchair Edges on Graphene Nanoribbons by X-ray Photoelectron and Raman Spectroscopies.

Graphene nanoribbons (GNRs) have recently emerged as alternative 2D semiconductors owing to their fascinating electronic properties that include tunable band gaps and high charge-carrier mobilities. Identifying the atomic-scale edge structures of GNRs through structural investigations is very important to fully understand the electronic properties of these materials. Herein, we report an atomic-scale analysis of GNRs using simulated X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Tetracene with zigzag edges and chrysene with armchair edges were selected as initial model structures, and their XPS and Raman spectra were analyzed. Structurally expanded nanoribbons based on tetracene and chrysene, in which zigzag and armchair edges were combined in various ratios, were then simulated. The edge structures of chain-shaped nanoribbons composed only of either zigzag edges or armchair edges were distinguishable by XPS and Raman spectroscopy, depending on the edge type. It was also possible to distinguish planar nanoribbons consisting of both zigzag and armchair edges with zigzag/armchair ratios of 4:1 or 1:4, indicating that it is possible to analyze normally synthesized GNRs because their zigzag to armchair edge ratios are usually greater than 4 or less than 0.25. Our study on the precise identification of GNR edge structures by XPS and Raman spectroscopy provides the groundwork for the analysis of GNRs.

In Situ Spectroscopic and Computational Studies on a MnO2-CuO Catalyst for Use in Volatile Organic Compound Decomposition.

In situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations were conducted to demonstrate the decomposition mechanism of propylene glycol methyl ether acetate (PGMEA) on a MnO2-CuO catalyst. The catalytic activity of MnO2-CuO was higher than that of MnO2 at low temperatures, although the pore properties of MnO2 were similar to those of MnO2-CuO. In addition, whereas the chemical state of MnO2 remained constant following PGMEA dosing at 150 °C, MnO2-CuO was reduced under identical conditions, as confirmed by in situ NEXAFS spectroscopy. These results indicate that the presence of Cu in the MnO2-CuO catalyst enables the release of oxygen at lower temperatures. More specifically, the released oxygen originated from the Mn-O-Cu moiety on the top layer of the MnO2-CuO structure, as confirmed by calculation of the oxygen release energies in various oxygen positions of MnO2-CuO. Furthermore, the spectral changes in the in situ NEXAFS spectrum of MnO2-CuO following the catalytic reaction at 150 °C corresponded well with those of the simulated NEXAFS spectrum following oxygen release from Mn-O-Cu. Finally, after the completion of the catalytic reaction, the quantities of lactone and ether functionalities in PGMEA decreased, whereas the formation of C=C bonds was observed.