Redox Species-Based Electrolytes for Advanced Rechargeable Lithium Ion Batteries

Seeking high-capacity cathodes has become an intensive effort in lithium ion battery research; however, the low energy density still remains a major issue for sustainable handheld devices and vehicles. Herein, we present a new strategy of integrating a redox species-based electrolyte in batteries to boost their performance. Taking the olivine LiFePO₄-based battery as an example, the incorporation of redox species (i.e., polysulfide of Li₂S₈) in the electrolyte results in much lower polarization and superior stability, where the dissociated Li⁺/S_<i>x</i>^2– can significantly speed up the lithium diffusion. More importantly, the presence of the S₈^2–/S^2– redox reaction further contributes extra capacity, making a completely new LiFePO₄/Li₂S_<i>x</i> hybrid battery with a high energy density of 1124 Wh kg_cathode^–1 and a capacity of 442 mAh g_cathode^–1. The marriage of appropriate redox species in an electrolyte for a rechargeable battery is an efficient and scalable approach for obtaining higher energy density storage devices

Scalable Patterning of MoS₂ Nanoribbons by Micromolding in Capillaries

In this study, we report a facile approach to prepare dense arrays of MoS₂ nanoribbons by combining procedures of micromolding in capillaries (MIMIC) and thermolysis of thiosalts ((NH₄)₂MoS₄) as the printing ink. The obtained MoS₂ nanoribbons had a thickness reaching as low as 3.9 nm, a width ranging from 157 to 465 nm, and a length up to 2 cm. MoS₂ nanoribbons with an extremely high aspect ratio (length/width) of ∼7.4 × 10⁸ were achieved. The MoS₂ pattern can be printed on versatile substrates, such as SiO₂/Si, sapphire, Au film, FTO/glass, and graphene-coated glass. The degree of crystallinity of the as-prepared MoS₂ was discovered to be adjustable by varying the temperature through postannealing. The high-temperature thermolysis (1000 °C) results in high-quality conductive samples, and field-effect transistors based on the patterned MoS₂ nanoribbons were demonstrated and characterized, where the carrier mobility was comparable to that of thin-film MoS₂. In contrast, the low-temperature-treated samples (170 °C) result in a unique nanocrystalline MoS_<i>x</i> structure (<i>x</i> ≈ 2.5), where the abundant and exposed edge sites were obtained from highly dense arrays of nanoribbon structures by this MIMIC patterning method. The patterned MoS_<i>x</i> was revealed to have superior electrocatalytic efficiency (an overpotential of ∼211 mV at 10 mA/cm² and a Tafel slope of 43 mV/dec) in the hydrogen evolution reaction (HER) when compared to the thin-film MoS₂. The report introduces a new concept for rapidly fabricating cost-effective and high-density MoS₂/MoS_<i>x</i> nanostructures on versatile substrates, which may pave the way for potential applications in nanoelectronics/optoelectronics and frontier energy materials

Scalable Patterning of MoS₂ Nanoribbons by Micromolding in Capillaries

In this study, we report a facile approach to prepare dense arrays of MoS₂ nanoribbons by combining procedures of micromolding in capillaries (MIMIC) and thermolysis of thiosalts ((NH₄)₂MoS₄) as the printing ink. The obtained MoS₂ nanoribbons had a thickness reaching as low as 3.9 nm, a width ranging from 157 to 465 nm, and a length up to 2 cm. MoS₂ nanoribbons with an extremely high aspect ratio (length/width) of ∼7.4 × 10⁸ were achieved. The MoS₂ pattern can be printed on versatile substrates, such as SiO₂/Si, sapphire, Au film, FTO/glass, and graphene-coated glass. The degree of crystallinity of the as-prepared MoS₂ was discovered to be adjustable by varying the temperature through postannealing. The high-temperature thermolysis (1000 °C) results in high-quality conductive samples, and field-effect transistors based on the patterned MoS₂ nanoribbons were demonstrated and characterized, where the carrier mobility was comparable to that of thin-film MoS₂. In contrast, the low-temperature-treated samples (170 °C) result in a unique nanocrystalline MoS_<i>x</i> structure (<i>x</i> ≈ 2.5), where the abundant and exposed edge sites were obtained from highly dense arrays of nanoribbon structures by this MIMIC patterning method. The patterned MoS_<i>x</i> was revealed to have superior electrocatalytic efficiency (an overpotential of ∼211 mV at 10 mA/cm² and a Tafel slope of 43 mV/dec) in the hydrogen evolution reaction (HER) when compared to the thin-film MoS₂. The report introduces a new concept for rapidly fabricating cost-effective and high-density MoS₂/MoS_<i>x</i> nanostructures on versatile substrates, which may pave the way for potential applications in nanoelectronics/optoelectronics and frontier energy materials

Three-Dimensional Heterostructures of MoS₂ Nanosheets on Conducting MoO₂ as an Efficient Electrocatalyst To Enhance Hydrogen Evolution Reaction

Molybdenum disulfide (MoS₂) is a promising catalyst for hydrogen evolution reaction (HER) because of its unique nature to supply active sites in the reaction. However, the low density of active sites and their poor electrical conductivity have limited the performance of MoS₂ in HER. In this work, we synthesized MoS₂ nanosheets on three-dimensional (3D) conductive MoO₂ via a two-step chemical vapor deposition (CVD) reaction. The 3D MoO₂ structure can create structural disorders in MoS₂ nanosheets (referred to as 3D MoS₂/MoO₂), which are responsible for providing the superior HER activity by exposing tremendous active sites of terminal disulfur of S₂^–2 (in MoS₂) as well as the backbone conductive oxide layer (of MoO₂) to facilitate an interfacial charge transport for the proton reduction. In addition, the MoS₂ nanosheets could protect the inner MoO₂ core from the acidic electrolyte in the HER. The high activity of the as-synthesized 3D MoS₂/MoO₂ hybrid material in HER is attributed to the small onset overpotential of 142 mV, a largest cathodic current density of 85 mA cm^–2, a low Tafel slope of 35.6 mV dec^–1, and robust electrochemical durability

van der Waals Epitaxy of MoS₂ Layers Using Graphene As Growth Templates

We present a method for synthesizing MoS₂/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports,, a much lower growth temperature of 400 °C is required for this procedure. The chemical vapor deposition of MoS₂ on the graphene surface gives rise to single crystalline hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temperature, which was then followed by post annealing. At an elevated temperature, the precursor self-assembles to form MoS₂ flakes epitaxially on the graphene surface via thermal decomposition. With higher amount of precursor delivered onto the graphene surface, a continuous MoS₂ film on graphene can be obtained. This simple chemical vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface functionalization of graphene. The synthesized two-dimensional MoS₂/Graphene hybrids possess great potential toward the development of new optical and electronic devices as well as a wide variety of newly synthesizable compounds for catalysts

Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe₂ by Hydrohalic Acid Treatment

Atomically thin two-dimensional transition-metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe₂ monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap-state emission and promote the neutral exciton and trion emission in defective MoSe₂ monolayers through the <i>p</i>-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe₂ layers. Our results suggest that the HBr treatment not only reduces the <i>n</i>-doping in MoSe₂ but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs