Postsynthetic Exchanges of the Pillaring Ligand in Three-Dimensional Metal–Organic Frameworks

Metal–organic frameworks, [Ni­(HBTC)­(dabco)] (<b>2</b>) and [Ni₂(HBTC)₂(bipy)_0.6(dabco)_1.4] (<b>3</b>) (where H₃BTC is 1,3,5-benzenetricarboxylic acid, dabco is 1,4-diazabicyclo[2.2.2]­octane, and bipy is 4,4′-bipyridine), were prepared via postsynthetic ligand exchanges of [Ni­(HBTC)­(bipy)] (<b>1</b>). By controlling the concentration of dabco, we could obtain not only entropically favorable <b>2</b> with completely exchanged dabco but also enthalpically favorable <b>3</b> with selectively exchanged bipy/dabco in the alternating layers

Metal-Free Ketjenblack Incorporated Nitrogen-Doped Carbon Sheets Derived from Gelatin as Oxygen Reduction Catalysts

Electrocatalysts facilitating oxygen reduction reaction (ORR) are vital components in advanced fuel cells and metal-air batteries. Here we report Ketjenblack incorporated nitrogen-doped carbon sheets derived from gelatin and apply these easily scalable materials as metal-free electrocatalysts for ORR. These carbon nanosheets demonstrate highly comparable catalytic activity for ORR as well as better durability than commercial Vulcan carbon supported Pt catalysts in alkaline media. Physico-chemical characterization and theoretical calculations suggest that proper combination of graphitic and pyridinic nitrogen species with more exposed edge sites effectively facilitates a formation of superoxide, [O_2(ad)]⁻, via one-electron transfer, thus increasing catalytic activities for ORR. Our results demonstrate a novel strategy to expose more nitrogen doped edge sites by irregular stacked small sheets in developing better electrocatalysts for Zn-air batteries. These desirable architectures are embodied by an amphiphlilic gelatin mediated compatible synthetic strategy between hydrophobic carbon and aqueous water

Anomalous K-Point Phonons in Noble Metal/Graphene Heterostructure Activated by Localized Surface Plasmon Resonance

The metal/graphene interface has been one of the most important research topics with regard to charge screening, charge transfer, contact resistance, and solar cells. Chemical bond formation of metal and graphene can be deduced from the defect induced D-band and its second-order mode, 2D band, measured by Raman spectroscopy, as a simple and nondestructive method. However, a phonon mode located at ???1350 cm-1, which is normally known as the defect-induced D-band, is intriguing for graphene deposited with noble metals (Ag, Au, and Cu). We observe anomalous K-point phonons in nonreactive noble metal/graphene heterostructures. The intensity ratio of the midfrequency mode at ???1350 cm-1 over G-band (???1590 cm-1) exhibits nonlinear but resonant behavior with the excitation laser wavelength, and more importantly, the phonon frequency-laser energy dispersion is ???10-17 cm-1 eV-1, which is much less than the conventional range. These phonon modes of graphene at nonzero phonon wave vector (q ??? 0) around K points are activated by localized surface plasmon resonance and not by the defects due to chemical bond formation of metal/graphene. This hypothesis is supported by density functional theory (DFT) calculations for noble metals and Cr along with the measured contact resistances

Synthesis and Characterization of Patronite Form of Vanadium Sulfide on Graphitic Layer

With the exploding interest in transition metal chalcogenides, sulfide minerals containing the dianion S₂^2–, such as pyrite (FeS₂), cattierite (CoS₂), and vaesite (NiS₂), have recently attracted much attention for potential applications in energy conversion and storage devices. However, the synthesis of the patronite structure (VS₄, V⁴⁺(S₂^2–)₂) and its applications have not yet been clearly demonstrated because of experimental difficulties and the existence of nonstoichiometric phases. Herein, we report the synthesis of VS₄ using a simple, facile hydrothermal method with a graphene oxide (GO) template and the characterization of the resulting material. Tests of various templates such as CNT, pyrene, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and graphite led us to the conclusion that the graphitic layer plays a role in the nucleation during growth of VS₄. Furthermore, the VS₄/rGO hybrid was proved to be a promising functional material in energy storage devices

Catalytic Transparency of Hexagonal Boron Nitride on Copper for Chemical Vapor Deposition Growth of Large-Area and High-Quality Graphene

Graphene transferred onto h-BN has recently become a focus of research because of its excellent compatibility with large-area device applications. The requirements of scalability and clean fabrication, however, have not yet been satisfactorily addressed. The successful synthesis of graphene/h-BN on a Cu foil and DFT calculations for this system are reported, which demonstrate that a thin h-BN film on Cu foil is an excellent template for the growth of large-area and high-quality graphene. Such material can be grown on thin h-BN films that are less than 3 nm thick, as confirmed by optical microscopy and Raman spectroscopy. We have evaluated the catalytic growth mechanism and the limits on the CVD growth of high-quality and large-area graphene on h-BN film/Cu by performing Kelvin probe force microscopy and DFT calculations for various thicknesses of h-BN

Macroporous Inverse Opal-like Mo_<i>x</i>C with Incorporated Mo Vacancies for Significantly Enhanced Hydrogen Evolution

The hydrogen evolution reaction (HER) is one of the most important pathways for producing pure and clean hydrogen. Although platinum (Pt) is the most efficient HER electrocatalyst, its practical application is significantly hindered by high-cost and scarcity. In this work, an Mo_<i>x</i>C with incorporated Mo vacancies and macroporous inverse opal-like (IOL) structure (Mo_<i>x</i>C-IOL) was synthesized and studied as a low-cost efficient HER electrocatalyst. The macroporous IOL structure was controllably fabricated using a facile-hard template strategy. As a result of the combined benefits of the Mo vacancies and structural advantages, including appropriate hydrogen binding energy, large exposed surface, robust IOL structure and fast mass/charge transport, the synthesized Mo_<i>x</i>C-IOL exhibited significantly enhanced HER electrocatalytic performance with good stability, with performance comparable or superior to Pt wire in both acidic and alkaline solutions

Nitrogen-Doped Graphene Nanoplatelets from Simple Solution Edge-Functionalization for n‑Type Field-Effect Transistors

The development of a versatile method for nitrogen-doping of graphitic structure is an important challenge for many applications, such as energy conversions and storages and electronic devices. Here, we report a simple but efficient method for preparing nitrogen-doped graphene nanoplatelets <i>via</i> wet-chemical reactions. The reaction between monoketone (CO) in graphene oxide (GO) and monoamine-containing compound produces imine (Shiff base) functionalized GO (iGO). The reaction between α-diketone in GO and 1,2-diamine (<i>ortho</i>-diamine)-containing compound gives stable pyrazine ring functionalized GO (pGO). Subsequent heat-treatments of iGO and pGO result in high-quality, nitrogen-doped graphene nanoplatelets to be designated as hiGO and hpGO, respectively. Of particular interest, hpGO was found to display the n-type field-effect transistor behavior with a charge neutral point (Dirac point) located at around −16 V. Furthermore, hpGO showed hole and electron mobilities as high as 11.5 and 12.4 cm²V^–1s^–1, respectively

Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe₂/WSe₂ van der Waals Heterostructures

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe₂/WSe₂ vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe₂ and WSe₂ layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe₂ and WSe₂