Muhammad Habib

Electronic Structure Reconfiguration toward Pyrite NiS2 via Engineered Heteroatom Defect Boosting Overall Water Splitting

Developing highly active and low-cost heterogeneous catalysts toward overall electrochemical water splitting is extremely desirable but still a challenge. Herein, we report pyrite NiS2 nanosheets doped with vanadium heteroatoms as bifunctional electrode materials for both hydrogen- and oxygen-evolution reaction (HER and OER). Notably, the electronic structure reconfiguration of pyrite NiS2 is observed from typical semiconductive characteristics to metallic characteristics by engineering vanadium (V) displacement defect, which is confirmed by both experimental temperature-dependent resistivity and theoretical density functional theory calculations. Furthermore, elaborate X-ray absorption spectroscopy measurements reveal that electronic structure reconfiguration of NiS2 is rooted in electron transfer from doped V to Ni sites, consequently enabling Ni sites to gain more electrons. The metallic V-doped NiS2 nanosheets exhibit extraordinary electrocatalytic performance with overpotentials of about 290 mV for OER and about 110 mV for HER at 10 mA cm-2 with long-term stability in 1 M KOH solutions, representing one of the best non-noble-metal bifunctional electrocatalysts to date. This work provides insights into electronic structure engineering from well-designed atomic defect metal sulfide.

All-Carbon Ultrafast Supercapacitor by Integrating Multidimensional Nanocarbons.

Ultrafast and high capacity all-carbon supercapacitors with 3D porous aerogel electrode are realized by combining carbon nanostructures of various dimensionalities, including 0D carbon onions, 1D carbon nanotubes, and 2D graphene oxide. The synergistic effects from the different forms of nanocarbons render this hybrid outstanding capacitance with excellent stability, even at ultrafast charge-discharge rates.

A Ternary Alloy Substrate to Synthesize Monolayer Graphene with Liquid Carbon Precursor

Here we demonstrate a ternary Cu2NiZn alloy substrate for controllably synthesizing monolayer graphene using a liquid carbon precursor cyclohexane via a facile CVD route. In contrast with elemental metal or bimetal substrates, the alloy-induced synergistic effects that provide an ideal metallic platform for much easier dehydrogenation of hydrocarbon molecules, more reasonable strength of adsorption energy of carbon monomer on surface and lower formation energies of carbon chains, largely renders the success growth of monolayer graphene with higher electrical mobility and lower defects. The growth mechanism is systemically investigated by our DFT calculations. This study provides a selective route for realizing high-quality graphene monolayer via a scalable synthetic method by using economic liquid carbon supplies and multialloy metal substrates.

In situ growth of metallic 1T-WS2 nanoislands on single-walled carbon nanotube films for improved electrochemical performance

Layered tungsten disulfide (WS2) is a potential electrode material for electric double layer capacitance (EDLC) and hydrogen evolution reaction (HER). However, the electrochemical performance of WS2 has been hindered by the semiconducting nature and poor active sites. Herein, we have demonstrated a bottom-up hydrothermal approach to fabricate metallic 1T-WS2 nanoislands in situ grown on flexible single-walled carbon nanotube nonwovens (1T-WS2@SWCNT). The robust hybrids with a tight interface possess nanoscopic few-layered WS2 pieces with an abundance of exposed sites, along with a unique woven-architecture originating from the high conductive carbon nanotube network. The in situ-growing enhanced interface between metallic WS2 nanoislands and SWCNTs provides a relatively strong electrical coupling integrity, which facilitates charge transfer during electrochemical reactions. The merits of rich active sites, excellent conductivity and well bonding-interactions are significantly beneficial to improve the electrochemical performance. Particularly, in contrast to the pure material, the as-obtained hybrids are found to exhibit higher EDLC capacity (226 mF cm−2), almost 646-fold higher than pure 1T-WS2, smaller Tafel slope (57 mV per decade) and lower HER overpotential (∼25 mV) than any WS2-based materials reported so far.

Synthesis of Ni9S8/MoS2 heterocatalyst for Enhanced Hydrogen Evolution Reaction

We demonstrate a heterostructure Ni9S8/MoS2 hybrid with tight interface synthesized via an improved hydrothermal method. As compared to pure MoS2, the increased surface area and the shorten charge transport pathway in the layered hybrid significantly promote the photocatalytic efficiency for hydrogen evolution reaction (HER). In particularly, the optimized Ni9S8/MoS2 hybrid with 20 wt % Ni9S8 exhibits the highest photocatalytic activity with HER value of 406 μmolg-1h-1, which is enhanced by 70% compared to that of pure MoS2 nanosheets (285.0 μmolg-1h-1). Moreover, the value is 4 times more than the commercial MoS2 (92.0 μmolg-1h-1), indicating the high potential of the hybrid in the catalytic fields.

Growing and Etching MoS2 on Carbon Nanotube Film for Enhanced Electrochemical Performance

In this work we directly synthesized molybdenum disulfide (MoS2) nanosheets on carbon nanotube film (MoS2@CNT) via a two-step chemical vapor deposition method (CVD). By etching the obtained MoS2@CNT into 10% wt HNO3, the morphology of MoS2 decorated on CNT bundles was modulated, resulting in more catalytic active MoS2 edges being exposed for significantly enhanced electrochemical performance. Our results revealed that an 8 h acid etching sample exhibited the best performance for the oxygen evolution reaction, i.e., the current density reached 10 mA/cm2 under 375 mV over-potential, and the tafel slope was as low as 94 mV/dec. The enhanced behavior was mainly originated from the more catalytic sites in MoS2 induced by the acid etching treatment and the higher conductivity from the supporting CNT films. Our study provides a new route to produce two-dimensional layers on CNT films with tunable morphology, and thus may open a window for exploring its promising applications in the fields of catalytic-, electronic-, and electrochemical-related fields.

Light scattering by subwavelength Cu2O particles

Novel metamaterials with new capabilities to manipulate light may be used by considering basic building blocks with new optical properties. This is the case with resonant magneto-dielectric particles. In this work, the resonant response of a high-dielectric Cu2O subwavelength particle is analyzed, both analytically and experimentally. The emergence of electric and magnetic resonances and their interferential effects, producing directional behaviors, can be used in a new generation of metamaterials, as well as new integrated optical devices.

Low temperature CVD growth of ultrathin carbon films

We demonstrate the low temperature, large area growth of ultrathin carbon films by chemical vapor deposition under atmospheric pressure on various substrates. In particularly, uniform and continuous carbon films with the thickness of 2-5 nm were successfully grown at a temperature as low as 500 oC on copper foils, as well as glass substrates coated with a 100 nm thick copper layer. The characterizations revealed that the low-temperature-grown carbon films consist on few short, curved graphene layers and thin amorphous carbon films. Particularly, the low-temperature grown samples exhibited over 90% transmittance at a wavelength range of 400-750 nm and comparable sheet resistance in contrast with the 1000oC-grown one. This low-temperature growth method may offer a facile way to directly prepare visible ultrathin carbon films on various substrate surfaces that are compatible with temperatures (500-600oC) used in several device processing technologies.

Ferromagnetism in CVT Grown Tungsten Diselenide Single Crystals with Nickel Doping

Two dimensional (2D) single crystal layered transition materials have had extensive consideration owing to their interesting magnetic properties, originating from their lattices and strong spin-orbit coupling, which make them of vital importance for spintronic applications. Herein, we present synthesis of a highly crystalline tungsten diselenide layered single crystal grown by chemical vapor transport technique and doped with nickel (Ni) to tailor its magnetic properties. The pristine WSe2 single crystal and Ni-doped crystal were characterized and analyzed for magnetic properties using both experimental and computational aspects. It was found that the magnetic behavior of the 2D layered WSe2 crystal changed from diamagnetic to ferromagnetic after Ni-doping at all tested temperatures. Moreover, first principle density functional theory (DFT) calculations further confirmed the origin of room temperature ferromagnetism of Ni-doped WSe2, where the d-orbitals of the doped Ni atom promoted the spin moment and thus largely contributed to the magnetism change in the 2D layered material.

Engineering the optical properties of dielectric nanospheres by resonant modes

Recent progress in nanoscale optical physics is associated with the development of a new branch of nanophotonics exploring strong Mie resonances in dielectric nanoparticles with high refractive index (HRI). The high-index resonant dielectric nanostructures form building blocks for novel photonic meta-devices with low losses and advanced functionalities. In this work, we investigate the size effect of an HRI cuprous oxide (Cu2O) nanosphere on the optical properties of the structure, such as, scattering and absorption spectrum. We also experimentally demonstrate that the scattering field can be significantly engineered by tuning the radius of Cu2O. It is found that the resonant eigenmodes supported by the nanospheres play the dominant role in the absorption and scattering characteristic of the structure. From the perspective of eigenmodes, we can immediately find the right structure parameters to realize strong absorption (scattering) at either single wavelength or broadband wavelength. Furthermore, the multipole expansion method has been applied to explore the physical nature (i.e. electric mode or magnetic mode) of the eigenmode as well as contributions from different modes.