Self-Assembled TMD Nanoparticles on N-Doped Carbon Nanostructures for Oxygen Reduction Reaction and Electrochemical Oxygen Sensing Thereof

Here, we report on a universal carbothermal reduction strategy for the synthesis of well-dispersed WS₂ nanoparticles (∼1.7 nm) supported on a N-doped carbon (N_xC) nanostructure and the electrocatalytic activity toward oxygen reduction reaction (ORR). Bulk WS₂ powder (2 μm) is the source for WS₂ nanoparticles, and dicyandiamide is the source for N_xC and carbothermal reduction. Interestingly, WS₂/N_xC serves the purpose of innovative and robust active sites for ORR through an efficient four-electron transfer process with excellent durability. Remarkably, WS₂/N_xC suppresses the peroxide generation due to the dominating inner-sphere electron transfer mechanism where the direct adsorption of the desolvated O₂ molecule on the electroactive centers takes place. The mass activity (at 0.4 and 0.85 V vs RHE) of WS₂/N_xC outperforms the previously reported transition metal based electrocatalysts. The study further establishes a correlation between the work function and the ORR activity. We have also exploited WS₂/N_xC for electrochemical oxygen sensing, and there exists a direct correlation between oxygen sensing and ORR as both depend on the oxygen adsorption ability. Finally, the carbothermal reduction strategy has been extended for the synthesis of other TMDs/N_xC such as MoS₂/N_xC, MoSe₂/N_xC, and WSe₂/N_xC.

Boron vacancy: a strategy to boost the oxygen reduction reaction of hexagonal boron nitride nanosheet in hBN-MoS2 heterostructure

The incorporation of vacancies in a system is considered a proficient method of defect engineering in general catalytic modulation. Among two-dimensional materials, the deficiency of surface active sites and a high band gap restrict the catalytic activity of hexagonal boron nitride (hBN) material towards the oxygen reduction reaction (ORR), which hinders its applicability in fuel cells. A bane to boon strategy has been introduced here by coupling two sluggish ORR materials (hBN & MoS2) by a probe-sonication method to form a heterostructure (termed HBPS) which fosters four electron pathways to assist the reduction of oxygen. Theoretical and experimental studies suggest the kinetically and thermodynamically favorable formation of boron vacancies (B-vacancies) in the presence of MoS2, which act as active sites for oxygen adsorption in HBPS. B-vacancy induced uneven charge distribution together with band gap depression promote rapid electron transfer from the valance band to the conduction band which prevails over the kinetic limitation of pure hBN nanosheets towards ORR kinetics. The formed B-vacancy induced HBPS further exhibits a low Tafel slope (66 mV dec-1), and a high onset potential (0.80 V vs. RHE) with an unaltered electrochemically active surface area (ESCA) after long-term cycling. Thus, vacancy engineering in hBN has proved to be an efficient approach to unlock the potential of catalytic performance enhancement.

Recent Advances in Metal Chalcogenides (MX; X = S, Se) Nanostructures for Electrochemical Supercapacitor Applications: A Brief Review

Supercapacitors (SCs) have received a great deal of attention and play an important role for future self-powered devices, mainly owing to their higher power density. Among all types of electrical energy storage devices, electrochemical supercapacitors are considered to be the most promising because of their superior performance characteristics, including short charging time, high power density, safety, easy fabrication procedures, and long operational life. An SC consists of two foremost components, namely electrode materials, and electrolyte. The selection of appropriate electrode materials with rational nanostructured designs has resulted in improved electrochemical properties for high performance and has reduced the cost of SCs. In this review, we mainly spotlight the non-metallic oxide, especially metal chalcogenides (MX; X = S, Se) based nanostructured electrode materials for electrochemical SCs. Different non-metallic oxide materials are highlighted in various categories, such as transition metal sulfides and selenides materials. Finally, the designing strategy and future improvements on metal chalcogenide materials for the application of electrochemical SCs are also discussed.

Mixed 1T-2H Phase MoS2/Reduced Graphene Oxide as Active Electrode for Enhanced Supercapacitive Performance

A hybrid aerogel, composed of MoS₂ sheets of 1T (distorted octahedral) and 2H (trigonal prismatic) phases, finely mixed with few layers of reduced graphene oxide (rGO) and obtained by means of a facile environment-friendly hydrothermal cosynthesis, is proposed as electrode material for supercapacitors. By electrochemical characterizations in three- and two-electrode configurations and symmetric planar devices, unique results have been obtained, with specific capacitance values up to 416 F g^-1 and a highly stable capacitance behavior over 50000 charge-discharge cycles. The in-depth morphological and structural characterizations through field emission scanning electron microscopy, Raman, X-ray photoelectron spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller, and transmission electron microscopy analysis provides the proofs of the unique assembly of such 3D structured matrix. The unpacked MoS₂ structure exhibits an excellent distribution of 1T and 2H phase sheets that are highly exposed to interaction with the electrolyte, and so available for surface/near-surface redox reactions, notwithstanding the quite low overall content of MoS₂ embedded in the reduced graphene oxide (rGO) matrix. A comparison with other "more conventional" hybrid rGO-MoX₂ electrochemically active materials, synthesized in the same conditions, is provided to support the outstanding behavior of the cosynthesized rGO-MoS₂.

Synergistically Enhanced Electrocatalytic Performance of an N-Doped Graphene Quantum Dot-Decorated 3D MoS2-Graphene Nanohybrid for Oxygen Reduction Reaction

Nitrogen-doped graphene quantum dots (N-GQDs) were decorated on a three-dimensional (3D) MoS2-reduced graphene oxide (rGO) framework via a facile hydrothermal method. The distribution of N-GQDs on the 3D MoS2-rGO framework was confirmed using X-ray photoelectron spectroscopy, energy dispersive X-ray elemental mapping, and high-resolution transmission electron microscopy techniques. The resultant 3D nanohybrid was successfully demonstrated as an efficient electrocatalyst toward the oxygen reduction reaction (ORR) under alkaline conditions. The chemical interaction between the electroactive N-GQDs and MoS2-rGO and the increased surface area and pore size of the N-GQDs/MoS2-rGO nanohybrid synergistically improved the ORR onset potential to +0.81 V vs reversible hydrogen electrode (RHE). Moreover, the N-GQDs/MoS2-rGO nanohybrid showed better ORR stability for up to 3000 cycles with negligible deviation in the half-wave potential (E 1/2). Most importantly, the N-GQDs/MoS2-rGO nanohybrid exhibited a superior methanol tolerance ability even under a high concentration of methanol (3.0 M) in alkaline medium. Hence, the development of a low-cost metal-free graphene quantum dot-based 3D nanohybrid with high methanol tolerance may open up a novel strategy to design selective cathode electrocatalysts for direct methanol fuel cell applications.

Fabrication of zero to three dimensional nanostructured molybdenum sulfides and their electrochemical and photocatalytic applications

Transition metal dichalcogenides (TMDs) are emerging as promising materials, particularly for electrochemical and photochemical catalytic applications, and among them molybdenum sulfides have received tremendous attention due to their novel electronic and optoelectronic characteristics. Several review articles have summarized the recent progress on TMDs but no critical and systematic summary exists about the nanoscale fabrication of MoS₂ with different dimensional morphologies. In this review article, first we will summarize the recent progress on the morphological tuning and structural evolution of MoS₂ from zero-dimension (0D) to 3D. Then the different engineering methods and the effect of synthesis conditions on structure and morphology of MoS₂ will be discussed. Moreover, the corresponding change in the electronic and physicochemical properties of MoS₂ induced by structure tuning will also be presented. Further, the applications of MoS₂ in various electrochemical systems e.g. hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and supercapacitors as well as photocatalytic hydrogen evolution will be highlighted. The review article will also critically focus on challenges faced by researchers to tune the MoS₂ nanostructures and the resulting electrochemical mechanism to enhance their performances. At the end, concluding remarks and future prospects for the development of better MoS₂ based nanostructured materials for the aforementioned applications will be presented.

Nanostructured MoS2/BiVO4 Composites for Energy Storage Applications

We report the optimized synthesis and electrochemical characterization of a composite of few-layered nanostructured MoS₂ along with an electroactive metal oxide BiVO₄. In comparison to pristine BiVO₄, and a composite of graphene/BiVO₄, the MoS₂/BiVO₄ nanocomposite provides impressive values of charge storage with longer discharge times and improved cycling stability. Specific capacitance values of 610 Fg⁻¹ (170 mAhg⁻¹) at 1 Ag⁻¹ and 166 Fg⁻¹ (46 mAhg⁻¹) at 10 Ag⁻¹ were obtained for just 2.5 wt% MoS₂ loaded BiVO₄. The results suggest that the explicitly synthesized small lateral-dimensioned MoS₂ particles provide a notable capacitive component that helps augment the specific capacitance. We discuss the optimized synthesis of monoclinic BiVO₄, and few-layered nanostructured MoS₂. We report the discharge capacities and cycling performance of the MoS₂/BiVO₄ nanocomposite using an aqueous electrolyte. The data obtained shows the MoS₂/BiVO₄ nanocomposite to be a promising candidate for supercapacitor energy storage applications.

Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides

The discovery of graphene marks a major event in the physics and chemistry of materials. The amazing properties of this two-dimensional (2D) material have prompted research on other 2D layered materials, of which layered transition metal dichalcogenides (TMDCs) are important members. Single-layer and few-layer TMDCs have been synthesized and characterized. They possess a wide range of properties many of which have not been known hitherto. A typical example of such materials is MoS2 In this article, we briefly present various aspects of layered analogues of graphene as exemplified by TMDCs. The discussion includes not only synthesis and characterization, but also various properties and phenomena exhibited by the TMDCs.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Phosphoric acid-assisted synthesis of layered MoS2/graphene hybrids with electrolyte-dependent supercapacitive behaviors

An electrolyte-dependent behavior (greatly differ in pseudocapacitance) was investigated in the modified freestanding layered MoS₂/graphene hybrid.

Porous Hybrid Composites of Few-Layer MoS2 Nanosheets Embedded in a Carbon Matrix with an Excellent Supercapacitor Electrode Performance

Porous hierarchical architectures of few-layer MoS2 nanosheets dispersed in carbon matrix are prepared by a microwave-hydrothermal method followed by annealing treatment via using glucose as C source and structure-directing agent and (NH4 )2 MoS4 as both Mo and S sources. It is found that the morphology and size of the secondary building units (SBUs), the size and layer number of MoS2 nanosheets as well as the distribution of MoS2 nanosheets in carbon matrix, can be effectively controlled by simply adjusting the molar ratio of (NH4 )2 MoS4 to glucose, leading to the materials with a low charge-transfer resistance, many electrochemical active sites and a robust structure for an outstanding energy storage performance including a high specific capacitance (589 F g(-1) at 0.5 A g(-1) ), a good rate capability (364 F g(-1) at 20 A g(-1) ), and an excellent cycling stability (retention 104% after 2000 cycles) for application in supercapacitors. The exceptional rate capability endows the electrode with a high energy density of 72.7 Wh kg(-1) and a high power density of 12.0 kW kg(-1) simultaneously. This work presents a facile and scalable approach for synthesizing novel heterostructures of MoS2 -based electrode materials with an enhanced rate capability and cyclability for potential application in supercapacitor.

Comparative Study of Potential Applications of Graphene, MoS2, and Other Two-Dimensional Materials in Energy Devices, Sensors, and Related Areas

Novel properties of graphene have been well documented, whereas the importance of nanosheets of MoS2 and other chalcogenides is increasingly being recognized over the last two to three years. Borocarbonitrides, BxCyNz, with insulating BN and conducting graphene on either side are new materials whose properties have been attracting attention. These two-dimensional (2D) materials contain certain common features. Thus, graphene, MoS2, and borocarbonitrides have all been used in supercapacitor applications, oxygen reduction reactions (ORRs), and lithium-ion batteries. It is instructive, therefore, to make a comparative study of some of the important properties of these layered materials. In this article, we discuss properties related to energy devices at length. We examine the hydrogen evolution reaction facilitated by graphene, MoS2, and related materials. We also discuss gas and radiation sensors based on graphene and MoS2 as well as gas storage properties of graphene and borocarbonitrides. The article should be useful in making a judicious choice of which 2D material to use for a particular application.