Temperature-dependent Crystallization of MoS2 Nanoflakes on Graphene Nanosheets for Electrocatalysis

Oxygen evolution reaction on MoS2/C rods-robust and highly active electrocatalyst

Recently, water oxidation or oxygen evolution reaction (OER) in electrocatalysis has attracted huge attention due to its prime role in water splitting, rechargeable metal-air batteries, and fuel cells. Here, we demonstrate a facile and scalable fabrication method of a rod-like structure composed of molybdenum disulfide and carbon (MoS2/C) from parent 2D MoS2. This novel composite, induced via the chemical vapor deposition (CVD) process, exhibits superior oxygen evolution performance (overpotential = 132 mV at 10 mA cm-2and Tafel slope = 55.6 mV dec-1) in an alkaline medium. Additionally, stability tests of the obtained structures at 10 mA cm-2during 10 h followed by 20 mA cm-2during 5 h and 50 mA cm-2during 2.5 h have been performed and clearly prove that MoS2/C can be successfully used as robust noble-metal-free electrocatalysts. The promoted activity of the rods is ascribed to the abundance of active surface (ECSA) of the catalyst induced due to the curvature effect during the reshaping of the composite from 2D precursor (MoS2) in the CVD process. Moreover, the presence of Fe species contributes to the observed excellent OER performance. FeOOH, Fe2O3, and Fe3O4are known to possess favorable electrocatalytic properties, including high catalytic activity and stability, which facilitate the electrocatalytic reaction. Additionally, Fe-based species like Fe7C3and FeMo2S5offer synergistic effects with MoS2, leading to improved catalytic activity and durability due to their unique electronic structure and surface properties. Additionally, turnover frequency (TOF) (58 1/s at the current density of 10 mA cm-2), as a direct indicator of intrinsic activity, indicates the efficiency of this catalyst in OER. Based onex situanalyzes (XPS, XRD, Raman) of the electrocatalyst the possible reaction mechanism is explored and discussed in great detail showing that MoS2, carbon, and iron oxide are the main active species of the reaction.

Black Si Photocathode with a Conformal and Amorphous MoSx Catalytic Layer Grown Using Atomic Layer Deposition for Photoelectrochemical Hydrogen Evolution

We demonstrated how the photoelectrochemical (PEC) performance was enhanced by conformal deposition of an amorphous molybdenum sulfide (a-MoS_x) thin film on a nanostructured surface of black Si using atomic layer deposition (ALD). The a-MoS_x is found to predominantly consist of an octahedral structure (S-deficient metallic phase) that exhibits high electrocatalytic activity for the hydrogen evolution reaction with a Tafel slope of 41 mV/dec in an acid electrolyte. The a-MoS_x has a smaller work function (4.0 eV) than that of crystalline 2H-MoS₂ (4.5 eV), which induces larger energy band bending at the p-Si surface, thereby facilitating interface charge transfer. These features enabled us to achieve an outstanding kinetic overpotential of ∼0.2 V at 10 mA/cm² and an onset potential of 0.27 V at 1 mA/cm². Furthermore, the a-MoS_x layer provides superior protection against corrosion of the Si surface, enabling long-term PEC operation of more than 50 h while maintaining 87% or more performance. This work highlights the remarkable advantages of the ALD a-MoS_x layer and leads to a breakthrough in the architectural design of PEC cells to ensure both high performance and stability.

A ternary hybrid of Zn-doped MoS2-RGO for highly effective electrocatalytic hydrogen evolution

Modification of MoS₂-based catalysts is effective in solving the overdependence of hydrogen evolution reactions (HERs) on noble metal catalysts. In this work, a Zn-doped molybdenum disulfide-reduced graphene oxide (Zn-MoS₂-RGO) hybrid was synthesized in one step employing a hydrothermal method. By substituting the position of Mo, uniform doping with Zn improved the catalytic activity of MoS₂ for HER. The interlayer spacing of MoS₂ increased from 0.65 to 0.75 nm, demonstrating RGO effectively interpolate into MoS₂ nanosheets. This prevented aggregation and exposed more edge active sites of MoS₂. According to density functional theory (DFT) calculations, the layered structure of the MoS₂ nanosheets doped with Zn and intercalated with RGO promoted charge transfer and resulted in outstanding hydrogen evolution activity. Compared with MoS₂ (6.86 eV), the Zn-MoS₂-RGO hybrid (5.47 eV) with a considerably lower energy level value exhibited excellent electrocatalytic performance. Under optimal conditions, at a potential of -0.3 V vs. RHE, the current density reached -169 mA cm^-2 in a 0.5 M H₂SO₄ solution, 4.78 μmol of H₂ was produced in 6 h, and the Faraday efficiency reached 92%. The results obtained herein indicated that Zn-MoS₂-RGO was a promising candidate for application in electrocatalytic HER.

General Liquid-Driven Coaxial Flow Focusing Preparation of Novel Microcapsules for Rechargeable Magnesium Batteries

Magnesium batteries have been considered promising candidates for next-generation energy storage systems owing to their high energy density, good safety without dendrite formation, and low cost of magnesium resources. However, high-performance cathodes with stable capacity, good conductivity, and fast ions transport are needed, since many conventional cathodes possess a low performance and poor preparation controllability. Herein, a liquid-driven coaxial flow focusing (LDCFF) approach for preparing a novel microcapsule system with controllable size, high loading, and stable magnesium-storage performance is presented. Taking the MoS2-infilled microcapsule as a case study, the magnesium battery cathode based on the microcapsules displays a capacity of 100 mAh g-1 after 100 cycles. High capacity retention is achieved at both low and high temperatures of -10, ‒5, and 45 °C, and a stable rate-performance is also obtained. The influences of the liquid flow rates on the size and shell thickness of the microcapsules are investigated; and electron and ion diffusion properties are also studied by first-principle calculations. The presented LDCFF method is quite general, and the high performance of the microcapsules enables them to find broad applications for making emerging energy-storage materials and secondary battery systems.

Instrumental Techniques for Characterization of Molybdenum Disulphide Nanostructures

The excellent chemical and physical properties of materials (nanomaterials) with dimensions of less than 100 nm (nanometers) resulted in researchers and industrialists to have great interest in their discovery and applications in various systems/applications. As their sizes are reduced to nanoscale, these nanomaterials tend to possess exceptional properties differing from those of their bulk counterparts; hence, they have found applications in electronics and medicines. In order to apply them in those applications, there is a need to synthesise these nanomaterials and study their structural, optical, and electrochemical properties. Among several nanomaterials, molybdenum disulphide (MoS2) has received a great interest in energy applications due to its exceptional properties such as stability, conductivity, and catalytic activities. Hence, the great challenge lies in finding the state-of-the-art characterization techniques to reveal the different properties of MoS2 nanostructures with great accuracy. In this regard, there is a need to study and employ several techniques to accurately study the surface chemistry and physics of the MoS2 nanostructures. Hence, this review will comprehensively discuss a detailed literature survey on analytical techniques that can be used to study the chemical, physical, and surface properties of MoS2 nanostructures, namely, ultraviolet-visible spectroscopy (UV-vis), photoluminescence spectroscopy (PL), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, time-of-flight secondary ion mass spectroscopy (TOF-SIMS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopies (SEM and TEM), atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDS/X), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and electroanalytical methods which include linear sweep (LSV) and cyclic (CV) voltammetry and electrochemical impedance spectroscopy (EIS).

Fabrication of Graphene/Molybdenum Disulfide Composites and Their Usage as Actuators for Electrochemical Sensors and Biosensors

From the rediscovery of graphene in 2004, the interest in layered graphene analogs has been exponentially growing through various fields of science. Due to their unique properties, novel two-dimensional family of materials and especially transition metal dichalcogenides are promising for development of advanced materials of unprecedented functions. Progress in 2D materials synthesis paved the way for the studies on their hybridization with other materials to create functional composites, whose electronic, physical or chemical properties can be engineered for special applications. In this review we focused on recent progress in graphene-based and MoS2 hybrid nanostructures. We summarized and discussed various fabrication approaches and mentioned different 2D and 3D structures of composite materials with emphasis on their advances for electroanalytical chemistry. The major part of this review provides a comprehensive overview of the application of graphene-based materials and MoS2 composites in the fields of electrochemical sensors and biosensors.

NiFe Alloy Nanotube Arrays as Highly Efficient Bifunctional Electrocatalysts for Overall Water Splitting at High Current Densities

A bubble-releasing assisted pulse electrodeposition method was developed to create metallic alloy, NiFe, nanotube arrays in one step. The NiFe alloy nanotube array exhibited excellent bifunctional electrolytic activities, achieving low overpotentials of 100 mV for the hydrogen evolution reaction and 236 mV for the oxygen evolution reaction at 10 mA cm^-2, both in 1 M KOH at room temperature. For overall water splitting, the NiFe alloy nanotube array delivered 10 mA cm^-2 at an ultralow cell voltage of 1.58 V, among the top tier of the state-of-the-art bifunctional electrocatalysts. The NiFe alloy nanotube array also exhibited ultrastability at high current densities, experiencing only a minor chronoamperometric decay of 6.5% after a 24 h operation at 400 mA cm^-2. The success of the present binder-free nanotube array-based electrode can be attributed to the much enlarged reaction surface area, one-dimensionally guided charge transport and mass transfer offered by the nanotube structure, and improved product crystallinity provided by the pulse current electrodeposition. The nanotube array structure proves to be a promising new architecture design for electrocatalysts.

XPS experimental and DFT investigations on solid solutions of Mo1-xRexS2 (0 < x < 0.20)

The synthesis, characterization, experimental X-ray photoelectron spectra (XPS) and density-functional theory (DFT) investigations on solid solutions of Mo1-xRexS2 (x = 0.05, 0.10, 0.15 and 0.20) are reported herein. It is shown that even at a low concentration of dopant Re atoms, clustering occurs. At an Re concentration of 5% the formation of dimer-like impregnations is observed. An increase in the dopant concentration leads to an increase in the amount of clustered rhenium atoms and to the formation of rhombic clusters. The absence of magnetism within the studied Mo1-xRexS2 solid solutions allowed us to suggest a mechanism for the distribution of rhenium inside molybdenum disulphide through the initial formation of rhenium disulphide and its subsequent spreading.