Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDCs) (e.g. TiSe2, NbSe2, TaS2, VS2, VSe2, etc.) are benchmark systems for exploring the intertwined electronic orders in solids, the superconductivity and charge-density waves (CDW), as well as for engineering versatile applications in advanced low-power electronics and high-performance catalysts. However, the batch production of such envisioned MTMDCs remains challenging, which has dramatically hindered the aforementioned explorations.
Based on their consecutive efforts in related fields, Prof. Yanfeng Zhang’s group have designed facile chemical vapor deposition (CVD) routes for the controllable syntheses of various functional 2D MTMDCs materials and explored their novel applications according to their different materials properties. They succeeded in achieving several nanometer-thick VS2 flakes on SiO2/Si, and developed a variety of applications such as contact electrodes for monolayer MoS2 with contact resistances of ~1/4 to that of Ni/Au metals, and as supercapacitor electrodes in aqueous electrolytes with specific capacitances as high as 650 F g-1 (Nano. Lett. 2017, 17, 4908). Meanwhile, they also accomplished the van der Waals epitaxial growth of 1T-phase, few-layer metallic VSe2 nanosheets on an atomically flat mica substrate via a “one-step” CVD route. The 2D VSe2 single crystals presented an excellent metallic feature with an extra-high electrical conductivity (~106 S m−1), which is 1–4 orders of magnitude higher than that of various conductive 2D materials (Adv. Mater. 2017, 29, 1702359).
Recently, Prof. Yanfeng Zhang’ group made a great progress in the direct synthesis of thickness-tunable 2H-TaS2 flakes or centimeter-sized ultrathin films on an electrode material of Au foil via a facile APCVD or LPCVD route, respectively. Through temperature-dependent Raman characterizations, they detected the CDW transition from nearly commensurate to commensurate phases on the ultrathin 2H-TaS2 flakes with reducing the thicknesses. More significantly, they also developed the direct application of as-grown 2H-TaS2 flakes as electrocatalysts in hydrogen evolution reaction (HER), followed with ultrahigh HER efficiency (Tafel slope ~33~42 mV/dec, exchange current density ~100~179.47 µA/cm2), which is even comparable to traditional Pt catalysts, owing to their abundant active sites concentrated at both edges and basal-planes, as well as the self-optimizing morphological change of 2H-TaS2 flakes. This work should evoke further efforts for exploring new efficient catalysts in the large materials family of MTMDCs, as well as exploiting their applications towards more versatile directions (Nature Commun. 2017, 8, 958).
Collaborators of these papers included Prof. Lin Gu (Institute of Physics, Chinese academy of sciences), Prof. Xiaosong Wu (School of Physics, Peking University), Prof. Xinfeng Liu (National Center for Nanoscience and Technology), Prof. Xina Wang (Faculty of Physics and Electronic Technology, Hubei University), Prof. Qing Zhang (College of Engineering, Peking University), and Prof. Zhongfan Liu (College of Chemistry and Molecular Engineering, Peking University). All these work are financial supported by the National Natural Science Foundation of China, the Ministry of Science and Technology of China, the Beijing Municipal Science, and the Open Research Fund Program of the State Key Laboratory of Low Dimensional Quantum Physics.
Left panel: Schematic illustration of the LPCVD growth process of 2H-TaS2 thin films on Au foils, the HER process of 2H-TaS2/Au foils, and the atomic-resolution TEM image of 2H-TaS2. Right panel: Optical microscopy image of VS2 flakes synthesized on SiO2/Si through an APCVD route, and the schematic model of VS2-contacted monolayer MoS2 device.