Neodymium-decorated graphene as an efficient electrocatalyst for hydrogen production.

Rare earth (RE) materials such as neodymium (Nd) and others consist of unique electronic configurations which result in unique electronic, electrochemical, and photonic properties. The high temperature (>1100 °C) growth and low active surface areas of REs hinder their use as an efficient electrocatalyst. Herein, different morphologies of Nd were successfully fabricated in situ on the surface of graphene using a double-zone chemical vapor deposition (CVD) method. The morphology of the Nd material on graphene is controlled, which results in the significant enhancement of the large specific surface area and electrochemical active area of the composite material due to the spatial morphology of Nd, thereby improving the hydrogen evolution reaction (HER) performance in an alkaline medium. The significantly enhanced HER activity with an overpotential of 75 mV and a Tafel slope of 95 mV dec-1 at a current density of 10 mA cm-2 is observed in Nd-GF. Mainly, a high specific surface area of ∼2217 cm2 g-1 and the porosity of graphene play major roles in the enhancement of activity. Thus, the present work provides a new strategy for the neodymium engineering synthesis of efficient rare earth-graphene composite electrocatalysts with a high electrochemical active area.

Unusual magnetotransport properties in graphene fibers.

Graphene, purely sp2-hybridized, has already been extensively studied for magnetoelectronics, however, the magnetotransport properties of graphene fibers (GrFib) have not been explored very well to date. Herein, unique magnetotransport properties of graphene fibers are detected. All the GrFib-samples show the highest positive magnetoresistance (MR ∼ 60%) at room temperature (300 K) that gradually decreases (MR ∼ 37%) at low temperature (5 K), indicating quite different behavior for a graphene derivative. The MR of three different morphologies are compared: single graphene sheet (60-100% at 300 K and 100-110% at 5 K under an applied magnetic field of 5 T), graphene foam (GF-100% at 300 K and 158% at 5 K under an applied magnetic field of 5 T), and graphene fiber (60% at 300 K and 37% at 300 K under an applied magnetic field of 5 T), and found that each morphology has a different magnitude of MR under similar magnitude of magnetic field and temperature. Unlike graphene and GF, GrFib shows a decreasing trend of MR at low temperatures, violating commonly used weak anti-localization phenomena in graphene. Technologically, each morphology of graphene has a unique set of magnetotransport properties that can be considered for particular magnetoelectronic devices depending upon the mechanical, electrical, and magnetotransport properties.

Nitrogen-Doped Oxygenated Molybdenum Phosphide as an Efficient Electrocatalyst for Hydrogen Evolution in Alkaline Media.

Phosphides of transition metals (TMPs) are a developing class of materials for hydrogen evolution reaction (HER) as an alternative to expensive noble metals to produce clean energy. Herein, the nitrogen-doped molybdenum oxide (MoOx) is developed via a facile and simple hydrothermal method, followed by annealing in the N2 atmosphere and phosphorization to form a nitrogen-doped oxygenated molybdenum phosphide (N-MoP) sphere-shaped structure. The developed N-doped phosphide structure depicts enhanced HER activity by reaching a current density of 10 mA cm-2 at a very low overpotential of only 87 mV, which is much better than annealed nitrogen-doped molybdenum oxide (A-MoOx) 138 mV in alkaline medium. N-MoP is a highly efficient electrocatalyst for HER attributed to a more exposed surface, large electrode/electrolyte interface and appropriate binding energies for reactants. This study extends the opportunity of developing nitrogen-doped TMPs, which can display exceptional properties as compared to their oxides.

Recent Progress, Challenges, and Prospects in Two-Dimensional Photo-Catalyst Materials and Environmental Remediation.

The successful photo-catalyst library gives significant information on feature that affects photo-catalytic performance and proposes new materials. Competency is considerably significant to form multi-functional photo-catalysts with flexible characteristics. Since recently, two-dimensional materials (2DMs) gained much attention from researchers, due to their unique thickness-dependent uses, mainly for photo-catalytic, outstanding chemical and physical properties. Photo-catalytic water splitting and hydrogen (H2) evolution by plentiful compounds as electron (e-) donors is estimated to participate in constructing clean method for solar H2-formation. Heterogeneous photo-catalysis received much research attention caused by their applications to tackle numerous energy and environmental issues. This broad review explains progress regarding 2DMs, significance in structure, and catalytic results. We will discuss in detail current progresses of approaches for adjusting 2DMs-based photo-catalysts to assess their photo-activity including doping, hetero-structure scheme, and functional formation assembly. Suggested plans, e.g., doping and sensitization of semiconducting 2DMs, increasing electrical conductance, improving catalytic active sites, strengthening interface coupling in semiconductors (SCs) 2DMs, forming nano-structures, building multi-junction nano-composites, increasing photo-stability of SCs, and using combined results of adapted approaches, are summed up. Hence, to further improve 2DMs photo-catalyst properties, hetero-structure design-based 2DMs' photo-catalyst basic mechanism is also reviewed.

Mixed-dimensional heterostructures of hydrophobic/hydrophilic graphene foam for tunable hydrogen evolution reaction.

The synergetic effect of hydrophilic and hydrophobic carbon can be used to obtain tunable hydrogen evolution reaction (HER) at the interface. Herein, graphene oxide (GO-Hummers method) was coated on graphene foam (GF) synthesized via chemical vapor deposition to develop mixed-dimensional heterostructure for the observation of HER. The porosity of GF not only provides an optimized diffusion coefficient for better mass transport but also modified surface chemistry (GF/GO-hydrophobic/hydrophilic interface), which results in an onset potential 50 mV and overpotential of 450 mV to achieve the current density 10 mA/cm2. The surface analysis shows that inherent functional groups at the surface played a key role in tuning the activity of hybrid, providing a pathway to introduce non-corrosive electrodes for water splitting.

Superior Magnetoresistance Performance of Hybrid Graphene Foam/Metal Sulfide Nanocrystal Devices.

Interfaces between metals and semiconducting materials can inevitably influence the magnetotransport properties, which are crucial for technological applications ranging from magnetic sensing to storage devices. By taking advantage of this, a metallic graphene foam is integrated with semiconducting copper-based metal sulfide nanocrystals, i.e., Cu2ZnSnS4 (copper-zinc-tin-sulfur) without direct chemical bonding and structural damage, which creates numerous nanoboundaries that can be basically used to tune the magnetotransport properties. Herein, the magnetoresistance of a graphene foam is enhanced from nearly 90 to 130% at room temperature and under the application of 5 T magnetic field strength due to the addition of Cu2ZnSnS4 nanocrystals in high densities. We believe that the enhancement of magnetoresistance in hybrid graphene foam/Cu2ZnSnS4 nanocrystals is due to the evolution of the mobility fluctuation mechanism, triggered by the formation of nanoboundaries. Incorporating Cu2ZnSnS4 nanocrystals into a graphene foam not only provides an effective way to further enhance the magnitude of magnetoresistance but also opens a suitable window to achieve efficient and highly functional magnetic sensors with a large, linear, and controllable response.

Structure optimization of perovskite quantum dot light-emitting diodes.

Although all-inorganic perovskite light emitting diodes (PeLED) have satisfactory stability under an ambient atmosphere, producing devices with high performance is challenging. A device architecture with a reduced energy barrier between adjacent layers and optimized energy level alignment in the PeLED is critical to achieve high electroluminescence efficiency. In this study, we report the optimization of a CsPbBr3-based PeLED device structure with Li-doped TiO2 nanoparticles as the electron transport layer (ETL). Optimal Li doping balances charge carrier injection between the hole transport layer (HTL) and ETL, leading to superior performance in both devices. The turn-on voltages for devices with Li-doped TiO2 nanoparticles were significantly reduced from 7.7 V to 4.9 V and from 3 V to 2 V in the direct and inverted PeLED structures, respectively. The low turn-on voltage for green emission is one of the lowest values among the reported CsPbBr3-based PeLEDs. Further investigations show that the device with an inverted structure is superior to the device with a direct structure because the energy barrier for carrier injection was minimized. The inverted structure devices exhibited a current efficiency of 5.6 cd A-1 for the pristine TiO2 ETL, while it was 15.2 cd A-1 for the Li-doped TiO2 ETL, a factor of ∼2.7 enhancement at 5000 cd m-2.

Large, Linear, and Tunable Positive Magnetoresistance of Mechanically Stable Graphene Foam-Toward High-Performance Magnetic Field Sensors.

Here, we present the first observation of magneto-transport properties of graphene foam (GF) composed of a few layers in a wide temperature range of 2-300 K. Large room-temperature linear positive magnetoresistance (PMR ≈ 171% at B ≈ 9 T) has been detected. The largest PMR (∼213%) has been achieved at 2 K under a magnetic field of 9 T, which can be tuned by the addition of poly(methyl methacrylate) to the porous structure of the foam. This remarkable magnetoresistance may be the result of quadratic magnetoresistance. The excellent magneto-transport properties of GF open a way toward three-dimensional graphene-based magnetoelectronic devices.

Synthesis of scalable and tunable slightly oxidized graphene via chemical vapor deposition.

Semiconducting, large sheets of carbon as an active material in optoelectronic research are missing and reduced graphene oxide (rGO) can be a good candidate. However, chemical synthesis cannot produce large sheets of rGO (i.e. maximum: 20-30µm) as well as high quality rGO due to the restraints of fabrication method. Thus, a novel strategy for the synthesis of large sheets of semiconducting rGO is urgently required. Large area slightly oxidized graphene (SOG) is fabricated at the interface of silicon dioxide (SiO2) and silicon via Chemical Vapor Deposition (CVD) method, herein for the first time. Carbon atoms bond with oxygen functionalities (i.e. CO, COH) at the time of diffusion in SiO2 allowing for C/O ratios from 7 to 10 adjustable by the variation of SiO2 thickness, indicating the tunable oxidation. Moreover, electronic structure and morphology of SOG are similar to the chemically grown rGO. The fabrication mechanism of SOG is also investigated.

Flexible, Low Cost, and Platinum-Free Counter Electrode for Efficient Dye-Sensitized Solar Cells.

A platinum-free counter electrode composed of surface modified aligned multiwalled carbon nanotubes (MWCNTs) fibers was fabricated for efficient flexible dye-sensitized solar cells (DSSCs). Surface modification of MWCNTs fibers with simple one step hydrothermal deposition of cobalt selenide nanoparticles, confirmed by scanning electron microscopy and X-ray diffraction, provided a significant improvement (∼2-times) in their electrocatalytic activity. Cyclic voltammetry and electrochemical impedance spectroscopy suggest a photoelectric conversion efficiency of 6.42% for our modified fibers, higher than 3.4% and 5.6% efficeincy of pristine MWCNTs fiber and commonly used Pt wire, respectively. Good mechanical and performance stability after repeated bending and high output voltage for in-series connection suggest that our surface modified MWCNTs fiber based DSSCs may find applications as flexible power source in next-generation flexible/wearable electronics.