59 research outputs found

    Electrocatalytically Active Graphene–Porphyrin MOF Composite for Oxygen Reduction Reaction

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    Pyridine-functionalized graphene (reduced graphene oxide) can be used as a building block in the assembly of metal organic framework (MOF). By reacting the pyridine-functionalized graphene with iron–porphyrin, a graphene–metalloporphyrin MOF with enhanced catalytic activity for oxygen reduction reactions (ORR) is synthesized. The structure and electrochemical property of the hybrid MOF are investigated as a function of the weight percentage of the functionalized graphene added to the iron–porphyrin framework. The results show that the addition of pyridine-functionalized graphene changes the crystallization process of iron–porphyrin in the MOF, increases its porosity, and enhances the electrochemical charge transfer rate of iron–porphyrin. The graphene–metalloporphyrin hybrid shows facile 4-electron ORR and can be used as a promising Pt-free cathode in alkaline Direct Methanol Fuel Cell

    Periodic Grain Boundaries Formed by Thermal Reconstruction of Polycrystalline Graphene Film

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    Grain boundaries consisting of dislocation cores arranged in a periodic manner have well-defined structures and peculiar properties and can be potentially applied as conducting circuits, plasmon reflectors and phase retarders. Pentagon-heptagon (5–7) pairs or pentagon-octagon-pentagon (5–8–5) carbon rings are known to exist in graphene grain boundaries. However, there are few systematic experimental studies on the formation, structure and distribution of periodic grain boundaries in graphene. Herein, scanning tunneling microscopy (STM) was applied to study periodic grain boundaries in monolayer graphene grown on a weakly interacting Cu(111) crystal. The periodic grain boundaries are formed after the thermal reconstruction of aperiodic boundaries, their structures agree well with the prediction of the coincident-site-lattice (CSL) theory. Periodic grain boundaries in quasi-freestanding graphene give sharp local density of states (LDOS) peaks in the tunneling spectra as opposed to the broad peaks of the aperiodic boundaries. This suggests that grain boundaries with high structural quality can introduce well-defined electronic states in graphene and modify its electronic properties

    Whisper Gallery Modes in Monolayer Tungsten Disulfide-Hexagonal Boron Nitride Optical Cavity

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    There are strong interests in constructing nanolasers using two-dimensional transition metal dichalcogenides (TMDs) due to their strong light–matter interactions and high optical gain. However, most cavity designs based on transfer of exfoliated TMDs on silicon oxide are not optimized since monolayer emitters are located far from where the photonic mode reaches maximum intensity. By taking advantage of the excellent dielectric properties of hexagonal boron nitride (h-BN), we design a new microdisk optical cavity fabricated from a van der Waals (VdW) stacked h-BN/WS<sub>2</sub>/h-BN. The heterostructure is patterned into microdisk cavities characterized by whispering gallery modes (WGMs). The emission intensity of the WS<sub>2</sub> trion is enhanced by 2.9 times that of exciton in the heterostructure, giving rise to whisper gallery modes with resonance intensities that show nonlinear power dependence. A Rayleigh scatterer directs the cavity emission to vertical collection. Such VdW heterostructure provides an atomically smooth interface that is ideal for low loss photon propagation, giving a <i>Q</i> factor of 1200

    Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer

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    In the short time since its discovery, perovskite solar cells (PSCs) have attained high power conversion efficiency but their lack of thermal stability remains a barrier to commercialization. Among the experimentally accessible parameter spaces for optimizing performance, identifying an electron transport layer (ETL) that forms a thermally stable interface with perovskite and which is solution-processable at low-temperature will certainly be advantageous. Herein, we developed a mesoporous graphene/polymer composite with these advantages when used as ETL in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> PSCs, and a high efficiency of 13.8% under AM 1.5G solar illumination could be obtained. Due to the high heat transmission coefficient and low isoelectric point of mesoporous graphene-based ETL, the PSC device enjoys good chemical and thermal stability. Our work demonstrates that the mesoporous graphene-based scaffold is a promising ETL candidate for high performance and thermally stable PSCs

    Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer

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    In the short time since its discovery, perovskite solar cells (PSCs) have attained high power conversion efficiency but their lack of thermal stability remains a barrier to commercialization. Among the experimentally accessible parameter spaces for optimizing performance, identifying an electron transport layer (ETL) that forms a thermally stable interface with perovskite and which is solution-processable at low-temperature will certainly be advantageous. Herein, we developed a mesoporous graphene/polymer composite with these advantages when used as ETL in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> PSCs, and a high efficiency of 13.8% under AM 1.5G solar illumination could be obtained. Due to the high heat transmission coefficient and low isoelectric point of mesoporous graphene-based ETL, the PSC device enjoys good chemical and thermal stability. Our work demonstrates that the mesoporous graphene-based scaffold is a promising ETL candidate for high performance and thermally stable PSCs

    Studying Edge Defects of Hexagonal Boron Nitride Using High-Resolution Electron Energy Loss Spectroscopy

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    Studying the phonons of hexagonal boron nitride (h-BN) is important for understanding its thermal, electronic, and imaging applications. Herein, we applied high-resolution electron energy loss spectroscopy (HREELS) to monitor the presence of edge defects in h-BN films. We observed an edge phonon at 90.5 meV with the initial formation of island-like domains on Ru(0001), which subsequently weakens with respect to the bulk phonon as the islands congregate into a film. The presence of a weak edge phonon peak even at full surface coverage of the h-BN film indicates the sensitivity of HREELS in detecting line defects. A shoulder peak at ∼160 meV assignable to sp<sup>3</sup> bonded modes was attributed to grain boundaries arising from misaligned domains. In addition, the strengths of substrate interaction and the rippling of the h-BN film can be judged from the shift in the phonon energy of the out-of-plane TO<sub>⊥</sub> mode

    Low Power Consumption Ammonia Electrosynthesis Using Hydrogen-Nitrate Flow Electrolyzer

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    Ammonia synthesis through electrochemical nitrate reduction has emerged as a promising alternative to the conventional Haber–Bosch process. However, the use of a sluggish oxygen evolution reaction as the anode reaction leads to high energy consumption in nitrate reduction. In this study, we directly utilize hydrogen gas to synthesize ammonia by pairing the hydrogen oxidation reaction with the nitrate reduction reaction. A significantly lower cell voltage for ammonia synthesis was realized on a 16 cm2 flow electrolyzer. We achieved an impressive ammonia yield rate of 16.9 mmol h–1 at a cell voltage of 1.2 V cell voltage. Notably, this approach exhibits a low power consumption of 17 kWh kg–1 of NH3. The mechanism study shows hydroxyl ions generated from water splitting at the cathode cross the anion exchange membrane to react with protons generated from hydrogen oxidation at the anode. Through rigorous technical and economic analyses, this approach is found to be economically viable for industrial synthesis

    Analyzing Dirac Cone and Phonon Dispersion in Highly Oriented Nanocrystalline Graphene

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    Chemical vapor deposition (CVD) is one of the most promising growth techniques to scale up the production of monolayer graphene. At present, there are intense efforts to control the orientation of graphene grains during CVD, motivated by the fact that there is a higher probability for oriented grains to achieve seamless merging, forming a large single crystal. However, it is still challenging to produce single-crystal graphene with no grain boundaries over macroscopic length scales, especially when the nucleation density of graphene nuclei is high. Nonetheless, nanocrystalline graphene with highly oriented grains may exhibit single-crystal-like properties. Herein, we investigate the spectroscopic signatures of graphene film containing highly oriented, nanosized grains (20–150 nm) using angle-resolved photoemission spectroscopy (ARPES) and high-resolution electron energy loss spectroscopy (HREELS). The robustness of the Dirac cone, as well as dispersion of its phonons, as a function of graphene’s grain size and before and after film coalescence, was investigated. In view of the sensitivity of atomically thin graphene to atmospheric adsorbates and intercalants, ARPES and HREELS were also used to monitor the changes in spectroscopic signatures of the graphene film following exposure to the ambient atmosphere

    Energy Storage Studies on InVO<sub>4</sub> as High Performance Anode Material for Li-Ion Batteries

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    InVO<sub>4</sub> has attracted much attention as an anode material due to its high theoretical capacity. However, the effect of preparation methods and conditions on morphology and energy storage characteristic has not been extensively investigated and will be explored in this project. InVO<sub>4</sub> anode material was prepared using five different preparation methods: solid state, urea combustion, precipitation, ball-milling, and polymer precursor methods. Morphology and physical properties of InVO<sub>4</sub> were then analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), and Brunauer–Emmett–Teller (BET) surface area method. XRD patterns showed that orthorhombic phased InVO<sub>4</sub> was synthesized. Small amounts of impurities were observed in methods II, III, and V using XRD patterns. BET surface area ranged from 0.49 to 9.28 m<sup>2</sup> g<sup>–1</sup>. SEM images showed slight differences in the InVO<sub>4</sub> nanosized crystalline structures with respect to preparation methods and conditions. Energy storage studies showed that, among all the preparation methods, the urea combustion method produced the best electrochemical results, with negligible capacity fading between the 2nd and 50th cycles and high capacity of 1241 mA h g<sup>–1</sup> at the end of the 20th cycle, close to the theoretical capacity value. Precipitation method also showed good performance, with capacity fading (14%) and capacity of 1002 mA h g<sup>–1</sup> at the 20th cycle. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) was then used to determine the reaction mechanisms of InVO<sub>4</sub>

    Triple-State Liquid-Based Microfluidic Tactile Sensor with High Flexibility, Durability, and Sensitivity

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    We develop a novel triple-state liquid-based resistive microfluidic tactile sensor with high flexibility, durability, and sensitivity. It comprises a platinum-cured silicone microfluidic assembly filled with 2 μL liquid metallic alloy interfacing two screen-printed conductive electrodes on a polyethylene terephthalate (PET) film. This flexible tactile sensor is highly sensitive ((2–20) × 10<sup>–3</sup> kPa<sup>–1</sup>) and capable of distinguishing compressive loads with an extremely large range of pressure (2 to 400 kPa) as well as bending loads. Owing to its unique and durable structure, the sensor can withstand numerous severe mechanical load, such as foot stomping and a car wheel rolling over it, without compromising its electrical signal stability and overall integrity. Also, our sensing device is highly deformable, wearable, and able to differentiate and quantify pressures exerted by distinct bodily actions, such as a finger touch or footstep pressure. As a proof-of-concept of the applicability of our tactile sensor, we demonstrate the measurements of localized dynamic foot pressure by embedding the sensor inside the shoes and high heels. This work highlights the potential of the liquid-based microfluidic tactile sensing platform in a wide range of applications and can facilitate the realization of functional liquid-state sensing device technology with superior mechanical flexibility, durability, and sensitivity
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