Synergistic role of MoS2 in gelation-induced fabrication of graphene oxide films.

Supporting materials for electrocatalysts must exhibit relative chemical inertness to facilitate unimpeded movement of gas, and demonstrate electrical conductivity to promote efficient electron transfer to the catalyst. Conventional catalyst electrodes, such as glassy carbon, carbon cloths, or Ni foam, are commonly employed. However, the challenge lies in the limited stability observed during testing due to the relatively weak adhesion between the catalyst and the electrode. Addressing this limitation is crucial for advancing the stability and performance of catalyst-electrode systems in various applications. Here, we suggest a novel fabrication method for a freestanding conducting film, accomplished through gelation, incorporating 1T-MoS2 and graphene oxide. 1T-MoS2 nanosheets play a crucial role in promoting the reduction of graphene oxide (GO) on the Zn foil. This contribution leads to accelerated film formation and enhanced electrical conductivity in the film. The synergistic effect also enhances the film's stability as catalyst supports. This study provides insights into the effective utilization of MoS2 and graphene oxide in the creating of advanced catalyst support systems with potential applications in diverse catalytic reaction.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts.

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

CNT-rGO Hydrogel-Integrated Fabric Composite Synthesized via an Interfacial Gelation Process for Wearable Supercapacitor Electrodes.

We demonstrate a flexible and stretchable supercapacitor assembled via straightforward interfacial gelation of reduced graphene oxide (rGO) with carbon nanotube (CNT) on a stretchable fabric surface. The difference between the redox potential of aqueous graphene oxide (GO) dispersion, prepared using a modified Hummers' method, and of a solid Zn plate, which was used as an external stimulus, induces a spontaneous reduction of GO flakes forming porous CNT-rGO hydrogel at the liquid-solid interface. With the aid of Zn, a macroporous and flexible CNT-rGO hydrogel was fabricated on a stretchable fabric platform using a facile fabrication method, and the CNT-rGO fabric composite was assembled into a supercapacitor to demonstrate its feasibility as a wearable electrode. The porous structure of the as-formed CNT-rGO fabric composite allows excellent electrolyte accessibility and ion transport that result in a fast charge/discharge rate up to 100 mV/s and a large areal capacity of 10.13 mF/cm2 at a discharge rate of 0.5 mA (0.1 mA/cm2). The inclusion of one-dimensional CNT as conductive bridges allows an excellent capacity retention of 95.2% after complete folding of the electrode and a capacity retention of 93.3% after 1000 bending cycles. Additional stretching test displayed a high capacity retention of 90.0% even at an applied strain as high as 50%, overcoming previous limitations of brittle graphene-based electrodes. This low-cost, lightweight, easy to synthesize, stretchable supercapacitor holds promise for next-generation wearable electronics and energy storage applications.

N-Dopant-Mediated Growth of Metal Oxide Nanoparticles on Carbon Nanotubes.

Metal oxide nanoparticles supported on heteroatom-doped graphitic surfaces have been pursued for several decades for a wide spectrum of applications. Despite extensive research on functional metal oxide nanoparticle/doped carbon nanomaterial hybrids, the role of the heteroatom dopant in the hybridization process of doped carbon nanomaterials has been overlooked. Here, the direct growth of MnOx and RuOx nanoparticles in nitrogen (N)-doped sites of carbon nanotubes (NCNTs) is presented. The quaternary nitrogen (NQ) sites of CNTs actively participate in the nucleation and growth of the metal nanoparticles. The evenly distributed NQ nucleation sites mediate the generation of uniformly dispersed <10 nm diameter MnOx and RuOx nanoparticles, directly decorated on NCNT surfaces. The electrochemical performance of the resultant hybridized materials was evaluated using cyclic voltammetry. This novel hybridization method using the dopant-mediated nucleation and growth of metal oxides suggests ways that heteroatom dopants can be utilized to optimize the structure, interface and corresponding properties of graphitic carbon-based hybrid materials.

Investigation of the Effect of Structural Properties of a Vertically Standing CNT Cold Cathode on Electron Beam Brightness and Resolution of Secondary Electron Images.

Carbon nanotube (CNT)-based cold cathodes are promising sources of field emission electrons for advanced electron devices, particularly for ultra-high-resolution imaging systems, due to their high brightness and low energy spread. While the electron field emission properties of single-tip CNT cathodes have been intensively studied in the last few decades, a systematic study of the influencing factors on the electron beam properties of CNT cold cathodes and the resolution of the secondary electron images has been overlooked in this field. Here, we have systematically investigated the effect of the structural properties of a CNT cold cathode on the electron beam properties and resolution of secondary electron microscope (SEM) images. The aspect ratio (geometric factor) and the diameter of the tip of a vertically standing CNT cold cathode significantly affect the electron beam properties, including the beam size and brightness, and consequently determine the resolution of the secondary electron images obtained by SEM systems equipped with a CNT cold cathode module. Theoretical simulation elucidated the dependence of the structural features of CNT cold cathodes and electron beam properties on the contribution of edge-emitted electrons to the total field emission current. Investigating the correlations between the structural properties of CNT cold cathodes, the properties of the emitted electron beams, and the resolution of the secondary electron images captured by SEM equipped with CNT cold cathode modules is highly important and informative as a basic model.

Multidimensional Ti3C2Tx MXene Architectures via Interfacial Electrochemical Self-Assembly.

An effective pathway to build macroscopic scale functional architectures bearing diverse structural dimensions is one of the critical challenges in the two-dimensional (2D) MXene research area. Unfortunately, assembling MXene without adhesive binder is largely limited due to its innate brittle nature and the relatively weak inter-flake van der Waals contact, in contrast to other mechanically compliant 2D materials such as graphene. Herein, an electrochemical self-assembly of pure Ti3C2Tx MXenes is presented for functional multidimensional MXene structures, effectively driven by layer-by-layer spontaneous interfacial reduction at metal template surfaces and subsequent defunctionalization. A three-dimensional open porous aerogel as well as 2D highly stacked thin film structures could be readily obtained in this approach, along with largely enhanced electrical properties induced by spontaneous removal of charge-trapping oxygen functional groups. Accordingly, supercapacitors and electromagnetic interference shielding films based on the multidimensional assembly demonstrate excellent performances.

Intact Crystalline Semiconducting Graphene Nanoribbons from Unzipping Nitrogen-Doped Carbon Nanotubes.

Unzipping carbon nanotubes (CNTs) may offer a valuable route to synthesize graphene nanoribbon (GNR) structures with semiconducting properties. Unfortunately, currently available unzipping methods commonly rely on a random harsh chemical reaction and thereby cause significant degradation of the crystalline structure and electrical properties of GNRs. Herein, crystalline semiconducting GNRs are achieved by a synergistic, judiciously designed two-step unzipping method for N-doped CNTs (NCNTs). NCNTs are effectively unzipped by damage-minimized, dopant-specific electrochemical unzipping and subsequent sonochemical treatment into long ribbon-like nanostructures with crystalline basal planes. Owing to the nanoscale dimension originating from the dense nucleation of the unzipping reaction at highly NCNTs, the resultant GNRs demonstrate semiconducting properties, which can be exploited for chemiresistor-type gas-sensing devices and many other applications.

Ultrastable Graphene-Encapsulated 3 nm Nanoparticles by In Situ Chemical Vapor Deposition.

Nanoscale materials offer enormous opportunities for catalysis, sensing, energy storage, and so on, along with their superior surface activity and extremely large surface area. Unfortunately, their strong reactivity causes severe degradation and oxidation even under ambient conditions and thereby deteriorates long-term usability. Here superlative stable graphene-encapsulated nanoparticles with a narrow diameter distribution prepared via in situ chemical vapor deposition (CVD) are presented. The judiciously designed CVD protocol generates 3 nm size metal and ceramic nanoparticles intimately encapsulated by few-layer graphene shells. Significantly, graphene-encapsulated Co3 O4 nanoparticles exhibit outstanding structural and functional integrity over 2000 cycles of lithiation/delithiation for Li-ion battery anode application, accompanied by 200% reversible volume change of the inner core particles. The insight obtained from this approach offers guidance for utilizing high-capacity electrode materials for Li-ion batteries. Furthermore, this in situ CVD synthesis is compatible with many different metal precursors and postsynthetic treatments, including oxidation, phosphidation, and sulfidation, and thus offers a versatile platform for reliable high-performance catalysis and energy storage/conversion with nanomaterials.

Graphene oxide liquid crystals: a frontier 2D soft material for graphene-based functional materials.

Graphene, despite being the best known strong and electrical/thermal conductive material, has found limited success in practical applications, mostly due to difficulties in the formation of desired large-scale highly organized structures. Our discovery of a liquid crystalline phase formation in graphene oxide dispersion has enabled a broad spectrum of highly aligned graphene-based structures, including films, fibers, membranes, and mesoscale structures. In this review, the current understanding of the structure-property relationship of graphene oxide liquid crystals (GOLCs) is overviewed. Various synthetic methods and parameters that can be optimized for GOLC phase formation are highlighted. Along with the results from different characterization methods for the identification of the GOLC phases, the typical characteristics of different types of GOLC phases introduced so far, including nematic, lamellar and chiral phases, are carefully discussed. Finally, various interesting applications of GOLCs are outlined together with the future prospects for their further developments.

Omnidirectional Deformable Energy Textile for Human Joint Movement Compatible Energy Storage.

Omnidirectional deformability is an unavoidable basic requirement for wearable devices to accommodate human daily motion particularly at human joints. We demonstrate omnidirectionally bendable and stretchable textile-based electrochemical capacitor that retains high power performance under complex mechanical deformation. Judicious synergistic hybrid structure of woven elastic polymer yarns with carbon nanotubes and conductive polymers offers reliable electrical and electrochemical activity even under repeated cycles of severe complex deformation modes. The textile-based electrochemical capacitors exhibit omnidirectional stretchability with 93% of capacitance retention under repeated 50% omnidirectional stretching condition while demonstrating excellent specific capacitance (412 mF cm-2) and cycle stability (>2000 stretch). The wearable power source stably powers red LED under omnidirectional stretching that accompanies human elbow joint motion.

Interface-Confined High Crystalline Growth of Semiconducting Polymers at Graphene Fibers for High-Performance Wearable Supercapacitors.

We report graphene@polymer core-shell fibers (G@PFs) composed of N and Cu codoped porous graphene fiber cores uniformly coated with semiconducting polymer shell layers with superb electrochemical characteristics. Aqueous/organic interface-confined polymerization method produced robust highly crystalline uniform semiconducting polymer shells with high electrical conductivity and redox activity. When the resultant core-shell fibers are utilized for fiber supercapacitor application, high areal/volume capacitance and energy densities are attained along with long-term cycle stability. Desirable combination of mechanical flexibility, electrochemical properties, and facile process scalability makes our G@PFs particularly promising for portable and wearable electronics.

Supramolecular Nanotubules as a Catalytic Regulator for Palladium Cations: Applications in Selective Catalysis.

Despite the recent development of highly efficient and stable metal catalysts, conferral of regulatory characteristics to the catalytic reaction in heterogeneous systems remains a challenge. Novel supramolecular nanotubules were prepared by alternative stacking from trimeric macrocycles, which was found to be able to coordinate with Pd cations. The Pd complexes exhibited a high catalytic performance for C-C coupling reaction. Notably, the tubular catalyst was observed to be controlled by supramolecular reversible assembly and showed superior heterogeneous catalytic activity, which was maintained for a number of cycles or reuse under an aerobic environment. Furthermore, the supramolecular catalyst showed unprecedented selectivity for the multifunctional coupling reaction and was able to serve as a new constructor of asymmetrical compounds.

Cobalt-Based Active Species Molecularly Immobilized on Carbon Nanotubes for the Oxygen Reduction Reaction.

Hybrid systems in which molecule-based active species are combined with nanoscale materials may offer valuable routes to enhance catalytic performances for electrocatalytic reactions. The development of rationally designed, cost-effective, efficient catalysts for the oxygen reduction reaction (ORR) is a crucial challenge for applications in fuel cells and metal-air batteries. A new hybrid ORR catalyst has been synthesized through a well-defined reaction between Co-based organometallic molecules and N-doped multiwalled carbon nanotubes (MWCNTs) at room temperature. The hybrid ORR catalyst shows excellent catalytic performance with an onset potential of 0.95 V [vs. the reversible hydrogen electrode (RHE)], superior durability, and good methanol tolerance. Chemical and structural characterization after many reaction cycles reveals that the Co-based organometallic species maintained the original structure of cobalt(II) acetylacetonate with coordination to the heteroatoms of the MWCNTs. A thorough electrochemical investigation indicates that the major catalytically active site is Co-O4 -NCNT .

Flash Light Millisecond Self-Assembly of High χ Block Copolymers for Wafer-Scale Sub-10 nm Nanopatterning.

One of the fundamental challenges encountered in successful incorporation of directed self-assembly in sub-10 nm scale practical nanolithography is the process compatibility of block copolymers with a high Flory-Huggins interaction parameter (χ). Herein, reliable, fab-compatible, and ultrafast directed self-assembly of high-χ block copolymers is achieved with intense flash light. The instantaneous heating/quenching process over an extremely high temperature (over 600 °C) by flash light irradiation enables large grain growth of sub-10 nm scale self-assembled nanopatterns without thermal degradation or dewetting in a millisecond time scale. A rapid self-assembly mechanism for a highly ordered morphology is identified based on the kinetics and thermodynamics of the block copolymers with strong segregation. Furthermore, this novel self-assembly mechanism is combined with graphoepitaxy to demonstrate the feasibility of ultrafast directed self-assembly of sub-10 nm nanopatterns over a large area. A chemically modified graphene film is used as a flexible and conformal light-absorbing layer. Subsequently, transparent and mechanically flexible nanolithography with a millisecond photothermal process is achieved leading the way for roll-to-roll processability.

Highly tunable refractive index visible-light metasurface from block copolymer self-assembly.

The refractive index of natural transparent materials is limited to 2-3 throughout the visible wavelength range. Wider controllability of the refractive index is desired for novel optical applications such as nanoimaging and integrated photonics. We report that metamaterials consisting of period and symmetry-tunable self-assembled nanopatterns can provide a controllable refractive index medium for a broad wavelength range, including the visible region. Our approach exploits the independent control of permeability and permittivity with nanoscale objects smaller than the skin depth. The precise manipulation of the interobject distance in block copolymer nanopatterns via pattern shrinkage increased the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1,000 nm wavelength bandwidth. Spatially graded and anisotropic refractive indices are also obtained with the design of transitional and rotational symmetry modification.

High Energy Density All Solid State Asymmetric Pseudocapacitors Based on Free Standing Reduced Graphene Oxide-Co3O4 Composite Aerogel Electrodes.

Modern flexible consumer electronics require efficient energy storage devices with flexible free-standing electrodes. We report a simple and cost-effective route to a graphene-based composite aerogel encapsulating metal oxide nanoparticles for high energy density, free-standing, binder-free flexible pseudocapacitive electrodes. Hydrothermally synthesized Co3O4 nanoparticles are successfully housed inside the microporous graphene aerogel network during the room temperature interfacial gelation at the Zn surface. The resultant three-dimensional (3D) rGO-Co3O4 composite aerogel shows mesoporous quasiparallel layer stack morphology with a high loading of Co3O4, which offers numerous channels for ion transport and a 3D interconnected network for high electrical conductivity. All solid state asymmetric pseudocapacitors employing the composite aerogel electrodes have demonstrated high areal energy density of 35.92 µWh/cm(2) and power density of 17.79 mW/cm(2) accompanied by excellent cycle life.

Large-Area Buckled MoS2 Films on the Graphene Substrate.

In this study, a novel buckled structure of edge-oriented MoS2 films is fabricated for the first time by employing monolayer graphene as the substrate for MoS2 film growth. Compared to typical buckling methods, our technique has several advantages: (1) external forces such as heat and mechanical strain are not applied; (2) uniform and controllable buckling over a large area is possible; and (3) films are able to be transferred to a desired substrate. Dual MoS2 orientation was observed in the buckled film where horizontally aligned MoS2 layers of 7 nm thickness were present near the bottom graphene surface and vertically aligned layers dominated the film toward the outer surface, in which the alignment structure was uniform across the entire film. The catalytic ability of the buckled MoS2 films, measured by performing water-splitting tests in acidic environments, shows a reduced onset potential of -0.2 V versus reversible hydrogen electrode (RHE) compared to -0.32 V versus RHE for pristine MoS2, indicating that the rough surface provided a higher catalytic activity. Our work presents a new method to generate a buckled MoS2 structure, which may be extended to the formation of buckled structures in various 2D materials for future applications.

Subnanometer Cobalt-Hydroxide-Anchored N-Doped Carbon Nanotube Forest for Bifunctional Oxygen Catalyst.

Electrochemical oxygen redox reactions are the crucial elements for energy conversion and storage including fuel cells and metal air batteries. Despite tremendous research efforts, developing high-efficient, low-cost, and durable bifunctional oxygen catalysts remains a major challenge. We report a new class of hybrid material consisting of subnanometer thick amorphous cobalt hydroxide anchored on NCNT as a durable ORR/OER bifunctional catalyst. Although amorphous cobalt species-based catalysts are known as good OER catalysts, hybridizing with NCNT successfully enhanced ORR activity by promoting a 4e reduction pathway. Abundant charge carriers in amorphous cobalt hydroxide are found to trigger the superior OER activity with high current density and low Tafel slope as low as 36 mV/decade. A remarkably high OER turnover frequency (TOF) of 2.3 s(-1) at an overpotential of 300 mV was obtained, one of the highest values reported so far. Moreover, the catalytic activity was maintained over 120 h of cycling. The unique subnanometer scale morphology of amorphous hydroxide cobalt species along with intimate cobalt species-NCNT interaction minimizes the deactivation of catalyst during prolonged repeated cycles.

Dopant-specific unzipping of carbon nanotubes for intact crystalline graphene nanostructures.

Atomic level engineering of graphene-based materials is in high demand to enable customize structures and properties for different applications. Unzipping of the graphene plane is a potential means to this end, but uncontrollable damage of the two-dimensional crystalline framework during harsh unzipping reaction has remained a key challenge. Here we present heteroatom dopant-specific unzipping of carbon nanotubes as a reliable and controllable route to customized intact crystalline graphene-based nanostructures. Substitutional pyridinic nitrogen dopant sites at carbon nanotubes can selectively initiate the unzipping of graphene side walls at a relatively low electrochemical potential (0.6 V). The resultant nanostructures consisting of unzipped graphene nanoribbons wrapping around carbon nanotube cores maintain the intact two-dimensional crystallinity with well-defined atomic configuration at the unzipped edges. Large surface area and robust electrical connectivity of the synergistic nanostructure demonstrate ultrahigh-power supercapacitor performance, which can serve for AC filtering with the record high rate capability of -85° of phase angle at 120 Hz.

3D Tailored Crumpling of Block-Copolymer Lithography on Chemically Modified Graphene.

Novel 3D self-assembled nanopatterning is presented via tailored crumpling of chemically modified graphene. Block-copolymer self-assembly formed on a layer of chemically modified graphene provides highly dense and uniform 2D nanopatterns, and the controlled crumpling of the chemically modified graphene by mechanical instabilities realizes the controlled 3D transformation of the self-assembled nanopatterns.