Graphene quantum dots synthesized by green method regulate electron transport on the surface of hollow spherical NiCo2S4 for efficient photocatalytic H2 evolution

Crystallization Engineering of CuNi2 S4 Ultra-Fine Nanocrystals with Optimized Band Structures for Efficient Photocatalytic Pollutant Degradation and Hydrogen Production

The mono-dispersed cubic siegenite CuNi2 S4 ultra-fine (≈5 nm) nanocrystals are fabricated through crystallization engineering under hot injection. The strong hydroxylation on mostly exposed CuNi2 S4 (220) surface leads to the formation of multi-valence (Cu+ , Cu2+ , Ni2+ , Ni3+ ) species with unsaturated hybridization and coordination micro-environments, which can induce rich redox reactions to optimize interfacial kinetics for the adsorbed reaction intermediates. The as-synthesized CuNi2 S4 nanocrystals with ultra-small particle size and the characteristics of being highly dispersed can increase specific surface area and hydroxylated active sites, which considerably contribute to the improvement of photocatalytic activities. Experimental and theoretical studies indicate that the CuNi2 S4 with unique surface condition can properly modulate the charge density distribution and the electronic band structure, thus achieving an optimal band gap for enhancing visible light absorption. Additionally, the strong hydroxylation on CuNi2 S4 (220) surface can not only make the photocatalytic process stable in alkaline environment but also bring about an impurity level between conduction and valence band, which facilitates the separation of photo-induced charge carriers by suppressing the rapid re-combination of exited electrons and holes. The optimization of band structure should be the intrinsic reason for the efficient photocatalytic pollutant degradation and hydrogen production under visible light illumination.

Solvent-free synthesis of NiCo2S4 having the metallic nature

Nickel-cobalt sulfide (NiCo2S4) is a prominent member of bimetallic transition metal sulfides. It is being widely used for a variety of applications such as electrode material, photocatalysis, and energy storage devices (like pseudo capacitors, supercapacitors, solar cells, and fuel cells) due to its better electronic conductivity, manageable morphology, and high capacitance. This work presents the one-step solventless synthesis of NiCo2S4 sheet-like nanostructures and then explores their metallic nature. Scanning electron microscopy (SEM) and transmission electron microscopic (TEM) analysis show the sheet-like grown morphology. Few nanorods are also seen. Except for a recent study (Xia et al. 2015) that shows metallic behavior, most of the reports show that NiCo2S4 is a semiconductor with claimed bandgap between 1.21 and 2.4 eV. In this study, we observe from UV-Vis and diffuse reflectance spectroscopy (DRS) that NiCo2S4 has a specific band gap value between 2.02 and 2.17 eV. However, IV characteristics in the temperature range of 300–400 K show that NiCo2S4 is a metal with a positive temperature coefficient of resistance consistent with a recent report. Furthermore, we see the ohmic conduction mechanism. The Arrhenius plot is drawn, and the activation energy is calculated to be 3.45 meV. The metallic nature is attributed to the coupling of two metal species (nickel and cobalt), which accounts for its superior conductivity and performance in a variety of essential applications.

Micro-controlling of the grain boundaries in NiCo2O4/NiCo2S4 nanowires for enhanced charge storage

In this work, the micro-controlling of the grain boundaries in NiCo₂O₄/NiCo₂S₄ nanowire arrays was achieved by exposing them to a sulfur containing atmosphere at 300 °C with different exposure times. The results showed that the capacitance of the optimal NiCo₂S₄ sample reached 400 μA h cm^-2 at 30 mA cm^-2 with a current retention rate of 58.6%. The assembled NiCo₂O₄/NiCo₂S₄//activate carbon electrode delivered a capacitance of 299.2 μA h cm^-2 and 241.7 μA h cm^-2 at a current density of 3 mA cm^-2 and 30 mA cm^-2, respectively. The cycling tests showed a good current retention rate of 88.1% after 8000 cycles. Besides, the NiCo₂O₄/NiCo₂S₄ composite exhibited a power density of 480.1 W kg^-1 at an energy density of 47.87 W h kg^-1, and 4773.66 W kg^-1 at 38.67 W h kg^-1, which indicated its potential for practical applications.