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/cm² and power density of 17.79 mW/cm² 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

Self-Size-Limiting Nanoscale Perforation of Graphene for Dense Heteroatom Doping

A scalable and controllable nanoscale perforation method for graphene is developed on the basis of the two-step thermal activation of a graphene aerogel. Different resistance to the thermal oxidation between graphitic and defective domains in the weakly reduced graphene oxide is exploited for the self-limiting nanoscale perforation in the graphene basal plane via selective thermal degradation of the defective domains. The resultant nanoporous graphene with a narrow pore-size distribution addresses the long-standing challenge for the high-level doping of graphene with lattice-mismatched large-size heteroatoms (S and P). Noticeably, this novel heteroatom doping strategy is demonstrated to be highly effective for oxygen reduction reaction (ORR) catalysis. Not only the higher level of heteroatom doping but also favorable spin and charge redistribution around the pore edges leads to a strong ORR activity as supported by density functional theory calculations

Perovskite Light-Emitting Diodes via Laser Crystallization: Systematic Investigation on Grain Size Effects for Device Performance

Owing to unique potential for high color purity luminance based on low-cost solution processes, organic/inorganic hybrid perovskite light-emitting diodes (PeLEDs) are attracting a great deal of research attention. For high performance PeLEDs, optimum control of the perovskite film morphology is a critical parameter. Here, we introduce a reliable and well-controllable PeLED crystallization process based on beam-damage-free near-infrared laser (λ = 808 nm) irradiation. Morphology of the CH₃NH₃PbBr₃ films can be precisely controlled by laser irradiation condition parameters: power density and beam scan rate. We systematically investigate the perovskite film morphology and device performance of the PeLEDs under different processing conditions. In the optimum power density and high beam scan rate (30 W cm^–2, 0.1 mm s^–1), a dense and smooth perovskite film is attained with a small crystal grain size. The critical relationship between the crystal grain size and LED efficiency is established while attaining the device performance of 0.95 cd A^–1 efficiency and 1784 cd m^–2

High Performance Organic Photovoltaics with Plasmonic-Coupled Metal Nanoparticle Clusters

Performance enhancement of organic photovoltaics using plasmonic nanoparticles has been limited without interparticle plasmon coupling. We demonstrate high performance organic photovoltaics employing gold nanoparticle clusters with controlled morphology as a plasmonic component. Near-field coupling at the interparticle gaps of nanoparticle clusters gives rise to strong enhancement in localized electromagnetic field, which led to the significant improvement of exciton generation and dissociation in the active layer of organic solar cells. A power conversion efficiency of 9.48% is attained by employing gold nanoparticle clusters at the bottom of the organic active layer. This is one of the highest efficiency values reported thus far for the single active layer organic photovoltaics

One-Dimensional RuO₂/Mn₂O₃ Hollow Architectures as Efficient Bifunctional Catalysts for Lithium–Oxygen Batteries

Rational design and massive production of bifunctional catalysts with fast oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics are critical to the realization of highly efficient lithium–oxygen (Li–O₂) batteries. Here, we first exploit two types of double-walled RuO₂ and Mn₂O₃ composite fibers, i.e., (i) phase separated RuO₂/Mn₂O₃ fiber-in-tube (RM-FIT) and (ii) multicomposite RuO₂/Mn₂O₃ tube-in-tube (RM-TIT), by controlling ramping rate during electrospinning process. Both RM-FIT and RM-TIT exhibited excellent bifunctional electrocatalytic activities in alkaline media. The air electrodes using RM-FIT and RM-TIT showed enhanced overpotential characteristics and stable cyclability over 100 cycles in the Li–O₂ cells, demonstrating high potential as efficient OER and ORR catalysts

Application of N‑Doped Three-Dimensional Reduced Graphene Oxide Aerogel to Thin Film Loudspeaker

We built a thermoacoustic loudspeaker employing N-doped three-dimensional reduced graphene oxide aerogel (N-rGOA) based on a simple template-free fabrication method. A two-step fabrication process, which includes freeze-drying and reduction/doping, was used to realize a three-dimensional, freestanding, and porous graphene-based loudspeaker, whose macroscopic structure can be easily modulated. The simplified fabrication process also allows the control of structural properties of the N-rGOAs, including density and area. Taking advantage of the facile fabrication process, we fabricated and analyzed thermoacoustic loudspeakers with different structural properties. The anlayses showed that a N-rGOA with lower density and larger area can produce a higher sound pressure level (SPL). Furthermore, the resistance of the proposed loudspeaker can be easily controlled through heteroatom doping, thereby helping to generate higher SPL per unit driving voltage. Our success in constructing an array of optimized N-rGOAs able to withstand input power as high as 40 W demonstrates that a practical thermoacoustic loudspeaker can be fabricated using the proposed mass-producible solution-based process

Application of N‑Doped Three-Dimensional Reduced Graphene Oxide Aerogel to Thin Film Loudspeaker

We built a thermoacoustic loudspeaker employing N-doped three-dimensional reduced graphene oxide aerogel (N-rGOA) based on a simple template-free fabrication method. A two-step fabrication process, which includes freeze-drying and reduction/doping, was used to realize a three-dimensional, freestanding, and porous graphene-based loudspeaker, whose macroscopic structure can be easily modulated. The simplified fabrication process also allows the control of structural properties of the N-rGOAs, including density and area. Taking advantage of the facile fabrication process, we fabricated and analyzed thermoacoustic loudspeakers with different structural properties. The anlayses showed that a N-rGOA with lower density and larger area can produce a higher sound pressure level (SPL). Furthermore, the resistance of the proposed loudspeaker can be easily controlled through heteroatom doping, thereby helping to generate higher SPL per unit driving voltage. Our success in constructing an array of optimized N-rGOAs able to withstand input power as high as 40 W demonstrates that a practical thermoacoustic loudspeaker can be fabricated using the proposed mass-producible solution-based process

Systematic Study on the Sensitivity Enhancement in Graphene Plasmonic Sensors Based on Layer-by-Layer Self-Assembled Graphene Oxide Multilayers and Their Reduced Analogues

The use of graphene in conventional plasmonic devices was suggested by several theoretic research studies. However, the existing theoretic studies are not consistent with one another and the experimental studies are still at the initial stage. To reveal the role of graphenes on the plasmonic sensors, we deposited graphene oxide (GO) and reduced graphene oxide (rGO) thin films on Au films and their refractive index (RI) sensitivity was compared for the first time in SPR-based sensors. The deposition of GO bilayers with number of deposition L from 1 to 5 was carried out by alternative dipping of Au substrate in positively- and negatively charged GO solutions. The fabrication of layer-by-layer self-assembly of the graphene films was monitored in terms of the SPR angle shift. GO-deposited Au film was treated with hydrazine to reduce the GO. For the rGO-Au sample, 1 bilayer sample showed a higher RI sensitivity than bare Au film, whereas increasing the rGO film from 2 to 5 layers reduced the RI sensitivity. In the case of GO-deposited Au film, the 3 bilayer sample showed the highest sensitivity. The biomolecular sensing was also performed for the graphene multilayer systems using BSA and anti-BSA antibody

Au–Ag Core–Shell Nanoparticle Array by Block Copolymer Lithography for Synergistic Broadband Plasmonic Properties

Localized surface plasmon resonance of metallic nanostructures receives noticeable attention in photonics, electronics, catalysis, and so on. Core–shell nanostructures are particularly attractive due to the versatile tunability of plasmonic properties along with the independent control of core size, shell thickness, and corresponding chemical composition, but they commonly suffer from difficult synthetic procedures. We present a reliable and controllable route to a highly ordered uniform Au@Ag core–shell nanoparticle array <i>via</i> block copolymer lithography and subsequent seeded-shell growth. Size-tunable monodisperse Au nanodot arrays are generated by block copolymer self-assembly and are used as seed layers to grow Ag shells with variable thickness. The resultant Au@Ag core–shell nanoparticle arrays exhibit widely tunable broadband enhancement of plasmonic resonance, greatly surpassing single-element nanoparticle or homogeneous alloy nanoparticle arrays. Surface-enhanced Raman scattering of the core–shell nanoparticle arrays showed an enhancement factor greater than 270 from Au nanoparticle arrays