Edge-Terminated MoS2 Nanoassembled Electrocatalyst via In Situ Hybridization with 3D Carbon Network

Anion Extraction-Induced Polymorph Control of Transition Metal Dichalcogenides

Controlled phase conversion in polymorphic transition metal dichalcogenides (TMDs) provides a new synthetic route for realizing tunable nanomaterials. Most conversion methods from the stable 2H to metastable 1T phase are limited to kinetically slow cation insertion into atomically thin layered TMDs for charge transfer from intercalated ions. Here, we report that anion extraction by the selective reaction between carbon monoxide (CO) and chalcogen atoms enables predictive and scalable TMD polymorph control. Sulfur vacancy, induced by anion extraction, is a key factor in molybdenum disulfide (MoS₂) polymorph conversion without cation insertion. Thermodynamic MoS₂-CO-CO₂ ternary phase diagram offers a processing window for efficient sulfur vacancy formation with precisely controlled MoS₂ structures from single layer to multilayer. To utilize our efficient phase conversion, we synthesize vertically stacked 1T-MoS₂ layers in carbon nanofibers, which exhibit highly efficient hydrogen evolution reaction catalytic activity. Anion extraction induces the polymorph conversion of tungsten disulfide (WS₂) from 2H to 1T. This reveals that our method can be utilized as a general polymorph control platform. The versatility of the gas-solid reaction-based polymorphic control will enable the engineering of metastable phases in 2D TMDs for further applications.

3D Carbon Foam Supported Edge-Rich N-Doped MoS2 Nanoflakes for Enhanced Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS₂ ) is one of the most promising alternatives to the Pt-based electrocatalysts for the hydrogen evolution reaction (HER). However, its performance is currently limited by insufficient active edge sites and poor electron transport. Hence, enormous efforts have been devoted to constructing more active edge sites and improving conductivity to obtain enhanced electrocatalytic performance. Herein, the 3D carbon foam (denoted as CF) supported edge-rich N-doped MoS₂ nanoflakes were successfully fabricated by using the commercially available polyurethane foam (PU) as the 3D substrate and PMo₁₂ O₄₀ ^3- clusters (denoted as PMo₁₂ ) as the Mo source through redox polymerization, followed by sulfurization. Owing to the uniform distribution of nanoscale Mo sources and 3D carbon foam substrate, the as-prepared MoS₂ -CF composite possessed well-exposed active edge sites and enhanced electrical conductivity. Systematic investigation demonstrated that the MoS₂ -CF composite showed high HER performance with a low overpotential of 92 mV in 1.0 m KOH and 155 mV in 0.5 m H₂ SO₄ at a current density of 10 mA cm^-2 . This work offers a new pathway for the rational design of MoS₂ -based HER electrocatalysts.

Solvent-Exchange Strategy toward Aqueous Dispersible MoS2 Nanosheets and Their Nitrogen-Rich Carbon Sphere Nanocomposites for Efficient Lithium/Sodium Ion Storage

Major challenges in developing 2D transition-metal disulfides (TMDs) as anode materials for lithium/sodium ion batteries (LIBs/SIBs) lie in rational design and targeted synthesis of TMD-based nanocomposite structures with precisely controlled ion and electron transport. Herein, a general and scalable solvent-exchange strategy is presented for uniform dispersion of few-layer MoS₂ (f-MoS₂ ) from high-boiling-point solvents (N-methyl-2-pyrrolidone (NMP), N,N-dimethyl formaldehyde (DMF), etc.) into low-boiling-point solvents (water, ethanol, etc.). The solvent-exchange strategy dramatically simplifies high-yield production of dispersible MoS₂ nanosheets as well as facilitates subsequent decoration of MoS₂ for various applications. As a demonstration, MoS₂ -decorated nitrogen-rich carbon spheres (MoS₂ -NCS) are prepared via in situ growth of polypyrrole and subsequent pyrolysis. Benefiting from its ultrathin feature, largely exposed active surface, highly conductive framework and excellent structural integrity, the 2D core-shell architecture of MoS₂ -NCS exhibits an outstanding reversible capacity and excellent cycling performance, achieving high initial discharge capacity of 1087.5 and 508.6 mA h g^-1 at 0.1 A g^-1 , capacity retentions of 95.6% and 94.2% after 500 and 250 charge/discharge cycles at 1 A g^-1 , for lithium/sodium ion storages, respectively.

Confining Pd Nanoparticles and Atomically Dispersed Pd into Defective MoO3 Nanosheet for Enhancing Electro- and Photocatalytic Hydrogen Evolution Performances

Interface engineering of two-dimensional (2D) transition-metal composites for activating plane and edge sites is a significant yet step challenging in boosting their performance for hydrogen evolution reaction (HER). Herein, two-dimensional (2D) MoO₃ with petal-shaped nanosheets confining Pd nanoparticles (Pd@MoO₃ heterostructure) was prepared via an efficient solvothermal and subsequently hydrogen reduction processes. The atomically dispersed Pd-substituted sites in the interface of Pd nanoparticles and 2D MoO₃ lattices significantly play an important role in enhancing the electrocatalytic and photocatalytic performances of the Pd@MoO₃ heterostructure. As a result, the Pd@MoO₃ heterostructure exhibits a high HER catalytic activity with an overpotential (η) of 71 mV to achieve a current density of 10 mA cm^-2 and an extremely low Tafel slope of 42.8 mV dec^-1 in 0.5 M H₂SO₄ solution. Furthermore, the photoresponse of the Pd@MoO₃ heterostructure is about 3 times higher than that of the MoO₃ nanosheets. This work highlighted a strategy of interface engineering for highly efficient cost-effective catalyst for hydrogen evolution by electric and solar energy conversion.

Active Pore-Edge Engineering of Single-Layer Niobium Diselenide Porous Nanosheets Electrode for Hydrogen Evolution

Two-dimensional transition-metal dichalcogenides (TMDs) possess interesting catalytic properties for the electrochemical-assisted hydrogen-evolution reaction (HER). We used niobium diselenide (NbSe₂) as a representative TMD, and prepared single-layer NbSe₂ porous nanosheets (PNS) by a double-sonication liquid-phase exfoliation, with H₂O₂ as a pore-forming agent. The single-layer NbSe₂ PNS were drop-cast on carbon foam (CF) to fabricate a three-dimensional robust NbSe₂ PNS/CF electrode. The NbSe₂ PNS/CF electrode exhibits a high current density of -50 mA cm^-2 with an overpotential of 148 mV and a Tafel slope of 75.8 eV dec^-1 for the HER process. Little deactivation is detected in continuous CV testing up to 20,000 cycles, which suggests that this novel NbSe₂ PNS/CF is a promising catalytic electrode in the HER application. The porous structure of single-layer NbSe₂ nanosheets can enhance the electrochemical performance compared with that of pore-free NbSe₂ nanosheets. These findings illustrate that the single-layer NbSe₂ PNS is a potential electrocatalytic material for HER. More importantly, the electrochemical performance of the NbSe₂ PNS/CF expands the use of two-dimensional TMDs in electrocatalysis-related fields.

Mirror twin grain boundaries in molybdenum dichalcogenides

Mirror twin grain boundaries (MTBs) exist at the interface between two grains of 60° rotated hexagonal transition metal dichalcogenides (TMDC). These grain boundaries form a regular atomic structure that extends in one dimension and thus may be described as a one-dimensional (1D) lattice embedded in the 2D TMDC. In this review, the different atomic structures and compositions of these MTBs are discussed. The obvious formation of MTBs is by coalescence of two twinned grains. In addition, however, in MoSe₂ and MoTe₂ a different formation mechanism has been revealed for the formation of Mo-rich MTBs. It has been shown that excess Mo can be incorporated into the TMDC lattices. These excess Mo atoms can then reorganize into closed, triangular MTB-loops that can grow in size by adding more Mo atoms to them. This mechanism allows the formation of dense MTB networks in MoSe₂ and MoTe₂. Such MTB networks have been observed in samples grown by molecular beam epitaxy (MBE) and consequently their presence needs to be considered in understanding the properties of MBE grown MoSe₂ and MoTe₂. Density functional theory as well as photoemission spectroscopy of MTB networks have shown that MTBs exhibit dispersing 1D-bands that intersect the Fermi-level, thus suggesting that these are 1D electron systems. Consequently, experimental data have been interpreted to reveal a charge density wave (or Peierls) instability, as well as a Tomonaga-Luttinger liquid behavior for electrons confined in 1D. We discuss these observations and the controversies that remain in the interpretation of some data. The metallic properties of the MTBs and their formation in dense networks also sparked the potential use of such crystal modifications for making metallic contacts to MoTe₂ or MoSe₂. Moreover, these crystal modifications may also boost the catalytic properties of these materials.