Transition Metal Chalcogenides:Electronic Property Regulation And Catalytic Application

It is of great importance to conduct research in electronic properties of inorganic nanomaterials and to determine the specific relation between the electronic structures and their functions for the discipline of solid-state chemistry. Transition metal dichalcogenides (TMDs) is i particular eye-catching because of its abundancy and its unique electronic structures which promise to bring a whole new platform for regulation of intrinsic properties of inorganic materials. By tuning the intrinsic electronic behaviors in TMDs, we have improved significantly the application of such materials in catalysis, and at the same time, have explored new route for designs in novel catalysts.The goal of this dissertation is to design and develop new methods for the tuning of intrinsic electronic behaviors in TMDs based on the analysis of their pristine crystal structures as well as their electronic structures, and to unveil the close relationship between the electronic structures and their functions. With tremendous methods such as intralayer doping, strain engineering, surface regulation, the fine modulation of electronic properties in TMDs was successfully realized, with the catalytic activities obviously enhanced, which also provide the ideal model for deep analysis of the close relationship between electronic behavior and functionalities from atomic level. The details are summarized briefly as follows:1. We unveiled the important role of atomic-level structural evolutions across the charge-density-wave (CDW) transition process. As is known, CDW is an electronic transition process, during which the subtle structure disturbance is usually hard to be observed, hindering the understanding of underlying physical nature of electron-lattice interactions. We observed the clear CDW transition process in pristine VS2system by temperature-dependent in-situ x-ray absorption fine spectroscopy (XAFS) coupled with low temperature high resolution transmission electron microscopy (HRTEM) for the first time. This structural phase transition can be definitely characterized by the formation of local V trimers with the contracted V-V bond length of3.11A, which constantly existed as the temperature decreases apart from TCDW. Meanwhile, it was clearly observed that the electronic properties were also modulated with a saltation on temperature dependent resistivity curves due to the fluctuation of atomic lattice. Based on the DFT calculation, it is conducted that the formation of vanadium trimers in the VS2system will break the pristine symmetry and influence the a1g band, thus leading to the reduction of conductivity. These results reveal a clear correlation between the dynamics of the lattice structure and electronic properties in VS2system, paving the new way for deep analysis of the nature of electronic properties of VS2across the CDW transition.2. As for the development of next-generation nano-spintronic devices, low dimensional ferromagnetic semiconductors are eagerly needed. And in consideration of the rich information of spin and electronic structures, transition metal dichalcogenides provide the new platform for the realization of ferromagnetic semiconductor. After the exfoliation process, semiconducting VS2nanosheet was successfully synthesized, along with which the ferromagnetic property was realized as well. And in consideration of the coexistence of ferromagnetic and semiconducting behaviors, ultrathin VS2nanosheet was demonstrated to be the first two-dimensional pristine room temperature ferromagnetic semiconductor in transition metal dichalcogenides. The realization of ferromagnetic semiconducting VS2nanosheet via exfoliation process in this work provides the new avenue for understanding the close relationship between crystal structure and intrinsic electronic properties in transition metal dichalcogenides.3. MoS2, as the prototype Mott-insulator, its relatively low conductivity and carrier concentration still greatly hamper its wide applications for hydrogen evolution reaction. The enhancement of conductivity is of great significance for the improvement of catalytic activity. We highlighted a novel intralayer vanadium doping strategy to produce VxMo1-xS2(VMS) ultrathin nanosheets with less than5S-(V, Mo)-S atomic layers, as a new concept of two dimensional material. The heterogeneous ions incorporated in the intralayer lattice of2D S-Mo-S layer rather than the interlayer spacing, ensuring the intriguing chemical stability as well as the ease of exfoliation. Unlike Mott-insulating behavior in pure MoS2nanosheets, the VxMo1-xS2ultrathin nanosheet exhibits the intriguing semimetal behavior with significantly enhanced conductivity of about1.7×103S/m and an increment of carrier concentration of about7×1017cm-3, which are about40times and20times larger than that of pure MoS2system (40S/m and3.5×1016cm-3), respectively. The semimetal VMS ultrathin nanosheet catalyst behaves an advanced catalytic efficiency with overpotential of0.13V and a Tafel slope of60mV/dec, exhibiting significantly enhanced catalytic performance than that of pure Mott-insulator MoS2system. The enhanced catalytic performance of this new intralayer-doped nanosheet provides a new avenue to expand the design space of2D material and to make high performance catalysts of2D materials.4. The author has firstly realized novel designs for organic catalysis by utilizing the interfacial transfer of electron in hetero-nanosturctures to manipulate the electron affinity of the transition metal dichalcogenides to activate the reaction much more efficiently. A facile strategy for synthesizing2D hetero-nanostructures grown epitaxially on the substrate MoS2has been successfully implemented. In the interface between the hetero-nanostructure of Cu2S/MoS2, the transfer of electron from Cu2S to MoS2has initiated a higher electronpositivity in Cu+, making the Ullmann reaction highly favorable under relatively lower temperatures, with a yield of92%. The stability of the catalyst (with a yield of89%after5cycles) is also guaranteed due to the stabilization provided from the substrate and the domain-matching growth mechanism. By tuning the electronic structure via creating2D hetero-nanostructures to adjust the electrophilicity/nucleophilicity of the catalyst, authors have demonstrated a new model for understanding and utilizing the interfacial electron transfer for intriguing design for catalysis in organic reactions.