Ultrathin two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their unique properties and great potential in a wide range of applications. Great efforts have been devoted to the preparation of novel-structured TMD nanosheets by engineering their intrinsic structures at the atomic scale. Until now, a lot of new-structured TMD nanosheets, such as vacancy-containing TMDs, heteroatom-doped TMDs, TMD alloys, 1T′/1T phase and in-plane TMD crystal-phase heterostructures, TMD heterostructures and Janus TMD nanosheets, have been prepared. These materials exhibit unique properties and hold great promise in various applications, including electronics/optoelectronics, thermoelectrics, catalysis, energy storage and conversion and biomedicine. This review focuses on the most recent important discoveries in the preparation, characterization and application of these new-structured ultrathin 2D layered TMDs.

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Novel structured transition metal dichalcogenide nanosheets

Transition metal oxides (TMOs) based on conversion reactions are attractive candidate anode materials for lithium-ion batteries (LIBs) because of their high theoretical capacity and safety characteristics. In this review, we have summarized recent progress in the rational design and efficient synthesis of TMOs with controllable morphologies, compositions, and micro-/nanostructures, along with their Li storage behaviors. Single metal oxides of manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), ruthenium (Ru), chromium (Cr), molybdenum (Mo), and tungsten (W) and their common binary metal oxides have been discussed in this review. Finally, the less well-known merits of conversion reactions are put forward, and the design of metal oxide electrodes making full use of these merits has been proposed.

Recent progress in conversion reaction metal oxide anodes for Li-ion batteries

Recently, hexavalent chromium (Cr(VI)) in wastewater has become a threat to the ecosystem and human health. The synthesis of high-performance and recyclable photocatalysts still remains a challenge. The photoreduction efficiency of Cr(VI) is inhibited by the high rate of recombination of electron-hole pairs and the low adsorption capacity of Cr(VI). In this study, three-dimensional (3D) MoO3/ZIF-8 core-shell nanorod composite photocatalysts were prepared via a facile two-step method and applied to the reduction of Cr(VI). The chemical state of the elements, microstructure, surface elements, and optical properties of the MoO3/ZIF-8 core-shell nanorods were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectroscopy. The as-prepared MoO3@ZIF-8 catalysts displayed superior photocatalytic activity for Cr(VI) reduction under visible light, compared to the pure ZIF-8 and MoO3 nanowires. Further, MoO3@ZIF-8 (with 15% of ZIF-8) exhibited the best photocatalytic activity, and promoted 100% reduction of Cr(VI) (15 mg L−1) within 45 min. The 3-D core-shell structure not only provided a large surface area, but also separated the electron-hole pairs effectively. The photocatalytic activity of the composite remained almost unchanged after four cycles. The mechanism of Cr(VI) reduction is also discussed in detail.

Facile construction of MoO3@ZIF-8 core-shell nanorods for efficient photoreduction of aqueous Cr (VI)

Photo/Electrochemical Applications of Metal Sulfide/TiO2 Heterostructures

Hierarchical Co and Nb dual-doped MoS2 nanosheets shelled micro-TiO2 hollow spheres as effective multifunctional electrocatalysts for HER, OER, and ORR

One dimensional (1D) hierarchical core–shell structures (MoO3@MoS2) constructed by perpendicular growth of few-layered MoS2 nanosheets on MoO3 nanowires were successfully fabricated by a direct anion-exchange reaction of the inorganic MoO3 nanowire precursor. The morphology and structure of the products can be controlled by adjusting the hybrid solvent in the anion-exchange process, and the contents of MoS2 and MoO3 in the core–shell nanowires can be easily tuned by adding a varied amount of thiourea. Although the organic–inorganic precursor was not used, the 1D structure of the inorganic MoO3 nanowire was still maintained. An amazing feature of the MoO3@MoS2 nanowire was its 1D hierarchical core–shell structure integrated with two-dimensional (2D) ultrathin MoS2 nanosheets and 1D MoO3 nanowires. The numerous few-layered MoS2 nanosheets grew perpendicularly on the MoO3 nanowires, providing plenty of diffusion channels for Li+ during the insertion/extraction process. Due to the abundant hardly stacked MoS2 nanosheets and the synergistic effects between MoO3 nanowires and MoS2 nanosheets, this unique MoO3@MoS2 nanowire showed greatly improved Li+ storage properties when evaluated as an anode material for lithium ion batteries (LIBs).

Perpendicular growth of few-layered MoS2 nanosheets on MoO3 nanowires fabricated by direct anion exchange reactions for high-performance lithium-ion batteries

Cost-effective and high-performance electrocatalysts are vital factors that affect the development of the oxygen evolution reaction (OER). Herein, SnS2 nanosheets (SnS2 NSs)-modified TiO2 nanobelts (TiO2 NBs) (TiO2/SnS2) electrocatalyst with significant enhancement of the OER activity was successfully synthesized by the hydrothermal method. This was achieved by taking advantage of the fast radial transfer of the one-dimensional (1-D) TiO2 NBs, large areas of contact with the electrolyte and the stability of the two-dimensional (2-D) SnS2 NSs. The experimental results indicate that the change in the valence bond environment due to the formation of the heterojunction and the independent exposed crystal facets could distinctly reduce the charge-transfer resistance and the adsorption barrier of the H2O molecules in electrocatalytic water splitting, which are favourable for improving the OER activity. The synergistic effect of the valence bond environment and exposed crystal facets of the TiO2/SnS2 heterojunction catalyst have led to remarkable OER performance with an overpotential of 570 mV at the current density of 10 mA cm−2, a low Tafel slope of 107 mV dec−1 and long-time durability in alkaline solution. This non-precious TiO2/SnS2 heterojunction catalyst was designed based on the concept of combining valance factors and crystal facet engineering, which sheds light on a new pathway for the design of new, promising and highly efficient OER electrocatalysts.

Synergistic effect of the valence bond environment and exposed crystal facets of the TiO2/SnS2 heterojunction for achieving enhanced electrocatalytic oxygen evolution

Meng Ding

Papers

Perpendicular growth of few-layered MoS2 nanosheets on MoO3 nanowires fabricated by direct anion exchange reactions for high-performance lithium-ion batteries

Synergistic effect of the valence bond environment and exposed crystal facets of the TiO2/SnS2 heterojunction for achieving enhanced electrocatalytic oxygen evolution

Three-dimensional elastic ultrathin reduced graphene oxide coating SnS2 hierarchical microsphere as lithium ion batteries anode materials

Constructing of hierarchical yolk-shell structure Li4Ti5O12-SnO2 composites for high rate lithium ion batteries

In-situ constructing of hierarchical Li4Ti5O12-TiO2 microspheres assembled by nanosheets for lithium-ion batteries