Yoong Ahm Kim

Raman spectroscopy of boron-doped single-layer graphene

The introduction of foreign atoms, such as nitrogen, into the hexagonal network of an sp(2)-hybridized carbon atom monolayer has been demonstrated and constitutes an effective tool for tailoring the intrinsic properties of graphene. Here, we report that boron atoms can be efficiently substituted for carbon in graphene. Single-layer graphene substitutionally doped with boron was prepared by the mechanical exfoliation of boron-doped graphite. X-ray photoelectron spectroscopy demonstrated that the amount of substitutional boron in graphite was ~0.22 atom %. Raman spectroscopy demonstrated that the boron atoms were spaced 4.76 nm apart in single-layer graphene. The 7-fold higher intensity of the D-band when compared to the G-band was explained by the elastically scattered photoexcited electrons by boron atoms before emitting a phonon. The frequency of the G-band in single-layer substitutionally boron-doped graphene was unchanged, which could be explained by the p-type boron doping (stiffening) counteracting the tensile strain effect of the larger carbon-boron bond length (softening). Boron-doped graphene appears to be a useful tool for engineering the physical and chemical properties of graphene.

Graphene: preparation and structural perfection

Three basic processes for the production of monolayer graphene are reviewed: cleavage of graphite crystals, exfoliation of graphite intercalation compounds and chemical vapor deposition. The relationship of the structural perfection of the obtained thin flakes to the preparation method is discussed.

Formation of Nitrogen-Doped Graphene Nanoribbons via Chemical Unzipping

In this work, we carried out chemical oxidation studies of nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) using potassium permanganate in order to obtain nitrogen-doped graphene nanoribbons. Reaction parameters such as oxidation reaction, reaction time, the oxidizer to nanotube mass ratio, and the temperature were varied, and their effect was carefully analyzed. The presence of nitrogen atoms makes CNx-MWCNTs more reactive toward oxidation when compared to undoped multiwalled carbon nanotubes (MWCNTs). High-resolution transmission electron microscopy studies indicate that the oxidation of the graphitic layers within CNx-MWCNTs results in the unzipping of large diameter nanotubes and the formation of a disordered oxidized carbon coating on small diameter nanotubes. The nitrogen content within unzipped CNx-MWCNTs decreased as a function of the oxidation time, temperature, and oxidizer concentration. By controlling the degree of oxidation, the N atomic % could be reduced from 1.56% in pristine CNx-MWCNTs down to 0.31 atom % in nitrogen-doped oxidized graphene nanoribbons. A comparative thermogravimetric analysis reveals a lower thermal stability of the (unzipped) oxidized CNx-MWCNTs when compared to MWCNT samples. The oxidized graphene nanoribbons were chemically and thermally reduced and yielded nitrogen-doped graphene nanoribbons (N-GNRs). The thermal reduction at relatively low temperature (300 °C) results in graphene nanoribbons with 0.37 atom % of nitrogen. This method represents a novel route to preparation of bulk quantities of nitrogen-doped unzipped carbon nanotubes, which is able to control the doping level in the resulting reduced GNR samples. Finally, the electrochemical properties of these materials were evaluated.

Rice Husk‐Derived Graphene with Nano‐Sized Domains and Clean Edges

A new synthetic method is demonstrated for transforming rice husks into bulk amounts of graphene through its calcination and chemical activation. The bulk sample consists of crystalline nano-sized graphene and corrugated individual graphene sheets; the material generally contains one, two, or a few layers, and corrugated graphene domains are typically observed in monolayers containing topological defects within the hexagonal lattice and edges. Both types of graphenes exhibit atomically smooth surfaces and edges.

Importance of open, heteroatom-decorated edges in chemically doped-graphene for supercapacitor applications

Chemically doped graphene has been actively investigated as an electrode material for achieving high-performance electrochemical systems. However, the stability of pure-carbon-rich edges and/or heteroatom-decorated edges, and their effect on the electrochemical performance remain largely unexplored. We found that in a high temperature thermal doping process, the functionalized graphene edges were structurally stable at 1200 °C, whereas the edges at 1500 °C were unstable and coalesced into loops through covalent bond formation between adjacent graphene edges. Interestingly, boron and nitrogen co-doped graphene prepared at 1200 °C showed the largest capacitance in both acidic and alkaline media due to the presence of the BNO moieties along the edge sites. The doped material also showed the best rate capability due to the largely enhanced electrical conductivity originating from the substitutionally doped boron and nitrogen atoms. Our findings regarding the stability of heteroatom-decorated edges without loop formation can now be utilized as a guideline for maximizing the electrochemical activity of graphene in various electrochemical systems.

The synergistic effect of the combined thin multi-walled carbon nanotubes and reduced graphene oxides on photothermally actuated shape memory polyurethane composites

We evaluated the synergistic effect of the hybrid-type nanocarbon, consisting of 1D thin-walled carbon nanotubes (TWNTs) and 2D reduced graphene oxide (RGO), on the shape memory performance of hyperbranched polyurethane composites. The shape recovery of the resulting composites was activated via a photothermal process using a near-infrared laser. The best laser-induced shape recovery performance was achieved for the composites with a 7/3 of TWNT/RGO ratio and a 1wt.% of nanocarbon content. Such result can be explained by good dispersion of TWNTs and RGO in the hyperbranched polymer as well as three-dimensionally enhanced interconnection between carbon nanotubes and graphenes. The optically active TWNTs with a high optical absorption section exhibited high ability of transferring laser-induced thermal energy to polymer matrix whereas RGO provided a high mechanical property to polymer matrix. The tensile modulus and electrical conductivity of the composites also showed a similar dependence on the TWNT/RGO composition ratio as the photothermal shape recovery. Our study demonstrated an effective conversion from light, thermal to mechanical work by irradiating shape memory polymer composite containing hybrid-type fillers using a near-infrared laser.

Tailoring the pore structure of carbon nanofibers for achieving ultrahigh-energy-density supercapacitors using ionic liquids as electrolytes

The low energy density of commercially available activated carbon-based supercapacitors has limited their widespread applications. In the current work, we demonstrated fabrication of carbon nanofiber-based supercapacitors that exhibited ultra-high energy density by rationally tailoring their pore structure in an ionic liquid system. To gain control on the pore structure, three different methods were employed for the synthesis of an electrospinning-derived freestanding carbon nanofiber web. They are incorporation of a pore generator (i.e., tetraethyl orthosilicate) in the electrospinning step, physical activation (e.g., H2O or CO2), and hydrogen treatment. We observed finely tuned pore sizes ranging from 0.734 to 0.831 nm and accompanying changes in BET surface areas ranging from 1160 to 1624 m2 g−1. The entrapped TEOS within the electrospun organic nanofiber web provided high tuning ability of the pore structure in the following carbonization step, and decreased the activation energy of the pore formation. Both high specific capacitance (161 F g−1) and ultra-high energy density (246 W h kg−1) were achieved when the pore size on the surface of carbon nanofibers matched with the ionic size of the electrolyte. Our results demonstrate the importance of a finely tuned pore structure to secure high-temperature operable carbon nanofiber-based supercapacitors with ultrahigh energy density using ionic liquids as electrolytes.

Boron-doped onion-like carbon with enriched substitutional boron: the relationship between electronic properties and catalytic performance

Incorporating heteroatoms into onion-like carbon (OLC, average size ∼5 nm) with a fullerene-like structure and investigating its peculiar properties are fascinating challenges. Here, we present straightforward fabrication of doped OLC samples with a high concentration of boron (0.63–4.57 at%) via a high-temperature thermal diffusion method. The highest proportion of substitutional boron of the boron species reaches 29%, which far exceeds most of the reported boron-doped carbon materials. The influence of boron on the fullerene-like layers and electronic properties of OLC is systematically investigated using Raman spectroscopy with different excitation energies (1.58–3.8 eV) and ultraviolet photoelectron spectroscopy (UPS). The as-prepared boron-doped OLC samples exhibit a perfect four-electron process for the oxygen reduction reaction (ORR), which is similar to commercial Pt/C. It is worth noting that the intrinsic relationship between the electronic properties and catalytic performance of doped samples is explored based on experimental studies instead of theoretical calculations. The results indicate that the lower work function, lower valence band edge and higher density of states (DOSs) of doped OLC are crucial to improve the catalytic performance. Our work can provide valuable information on the design of doped metal-free materials and give new evidence for enhanced ORR activity associated with heteroatom doping and electronic properties.

Soluble conducting polymer-functionalized graphene oxide for air-operable actuator fabrication

An effective route for the preparation of a processable, conducting polymer-functionalized graphene oxide for actuator applications is investigated. First, graphene oxide (GO) is covalently functionalized with a 3-thiophene acetic acid (TAA) monomer by an esterification reaction. Then, the TAA-functionalized GO is self-polymerized by chemical oxidative polymerization to yield poly(3-thiophene acetic acid)-functionalized GO (GO-f-PTAA). Further, the GO-f-TAA is also copolymerized with thiophene (Th) to yield GO-f-PTAA-co-PTh. The synthesis of GO-f-PTAA and GO-f-PTAA-co-PTh composites is confirmed by Fourier transform infrared, 1H-nuclear magnetic resonance, and X-ray photoelectron spectroscopies. The composites show better electrochemical properties than pure PTAA and superior solubility in organic solvents compared to pure GO. Using the soluble GO-f-PTAA and GO-f-PTAA-co-PTh composites, air-operable actuators are fabricated and their actuation performance is investigated. The copolymer-functionalized GO actuator exhibits good electroactive actuation behavior between 2 and 4 V, mainly because of the enhanced electrochemical performance of the composites, whereas the pure PTAA and GO-f-PTAA actuators do not show actuation under the applied voltage. The soluble conducting polymer-functionalized graphene composites developed in this study have potential applications in the fabrication of actuators that can be operated in air.

Rationally engineered surface properties of carbon nanofibers for the enhanced supercapacitive performance of binary metal oxide nanosheets

The hybridization of an electrochemically active metal oxide with electrically conductive carbon nanofibers (CNFs) has been utilized as a solution to overcome the energy density limitation of carbon-based supercapacitors as well as the poor cyclic stability of metal oxides. Herein, we have demonstrated the growth of binary metal oxide nanosheets on the engineered surface of CNFs to fully exploit their electrochemical activity. Metal oxide nanosheets were observed to grow vertically from the surface of CNFs. The high structural toughness of the CNF–metal oxide composite under strong sonication indicated strong interfacial binding strength between the metal oxide and the CNFs. The rationally designed porous CNFs presented a high specific surface area and showed high capacity for adsorbing metal ions, where the active edge sites acted as anchoring sites for the nucleation of metal oxides, thereby leading to the formation of a well dispersed and thin layer structure of binary metal oxide nanosheets. Excellent electrochemical performance (e.g., specific capacitance of 2894.70 F g−1 and energy density of 403.28 W h kg−1) was observed for these binary metal oxide nanosheets, which can be attributed to the large increase in the accessible surface area of the electrochemically active metal oxide nanosheets due to their homogeneous distribution on porous CNFs, as well as the efficient charge transfer from the metal oxide to the CNFs facilitated the improvement in the performance.