Preparation And Properties Of Two-dimensional Transition Metal Chalcogenide Nanostructures

In the low-dimensional physical systems,two-dimensional(2D)nanostructures have attracted great attention owing to their unique structures,outstanding properties and potential use in broad fields.At the same time,the emergence of new systems about two-dimensional nanostructures as well as the novel phenomenan and mechanisms appeared in these systems has also injected new vitality into the development of 2D nanomaterials.Among them,the two-dimensional transition metal chalcogenides exhibit unique physical and chemical properties due to their abundant elemental compositions and special electronic structures,which have great potential in many fields such as optoelectronic devices,catalysis,energy conversion and storage.Benefited from the unique two-dimensional confinement effect of the two-dimensional crystal,the electronic structure,optical,thermal and magnetic properties of the transition metal chalcogenides would be developed by simply adjust the crystal structure and element composition,to realize the functional properties of materials in various fields.It provides a good material research platform for the design and performance optimization of two-dimensional nanomaterials.Based on the analysis of the advantages and limitations of the transition metal chalcogenide in the field of photo-and electro-catalysis,in this thesis,a series of 2D transition metal chalcogenide nanostructures were constructed.Moreover,the photoelectrical and electrocatalytic properties and performance of these novel materials were studied.The purpose of this work is to design and regulate the crystal structure and the component of the 2D transition metal chalcogenide system,which are based on the unique electronic properties of the chalcogenide compound and the structural advantages of the two-dimensional nanomaterials.We aim to achieve more efficient electrocatalytic activity and energy conversion efficiency with this 2D system.This thesis mainly includes the following aspects:1.Based on the research requirement on the two-dimensional transition metal chalcogenide nanostructures,a new method for large-scale synthesis of two-dimensional nanosheets of transition metal chalcogenide compounds has been developed.The ultrathin molybdenum disulfide and molybdenum diselenide nanosheets were successfully prepared by a convenient hydrothermal intercalation/exfoliation method.The purity of the products was high,and the thickness of the as-prepared samples was only four nm with a smooth surface.This method is universally applicable and can be extended to other layered transition metal chalcogenides.With the as-prepared ultrathin molybdenum disulfide nanosheets,a series of tests and studies have shown that the ultrathin molybdenum disulfide nanosheets have excellent photoelectrochemical properties,good photoresponse,and outstanding electrochemical energy storage performance.This work will greatly facilitate the application of two-dimensional transition metal chalcogenide compounds in the fields of photodetectors,sensors,and energy storage.2.A series of highly efficient electrocatalytic hydrogen evolution electrode were prepared by electrophoretic deposition using the two-dimensional transition metal chalcogenide nanoplatelets in a liquid-phase system.The electrode preparation method is simple and effective.The experimental results show that the disulfide(selenide)molybdenum nanoplatelets are tightly anchored on carbon cloth,and the charge transfer between the electrocatalytic material and the electrode carrier is very efficient.Secondly,by selecting three-dimensional porous carbon fiber cloth,the ultra-thin nanosheet structure is effectively loaded onto the carbon fiber surface,and the catalytic active site of the transition metal chalcogenide compound is exposed to a great extent.It is confirmed through a series of tests that the hydrogen evolution electrode exhibits excellent efficiency and stability of the electrolysis of water hydrogen.This work proposed an effective approach to increase the active site of the catalyst and improve the conductance of the hydrogen evolution electrode,which is of great significance for the large-scale application of two-dimensional transition metal chalcogenide electrocatalyst.3.The non-lamellar transition metal chalcogenides have good electrocatalytic activity.However,it is not easy to form two-dimensional nanosheets because their crystal structures are different from layered transition metal chalcogenides such as molybdenum disulfide.Here,the ultra-thin structure of cobalt diselenide nanosheets was prepared by hydrothermal method for the first time by combining the ion-exchange route and selecting the intermediate transition products with proper morphologies as precursors.The prepared cobalt selenide nanosheets were grown on the carbon cloth composed of carbon fiber and contact very well with the carbon fiber,which could be directly used as the electrode for electrolysis of water.The cobalt selenide nanosheets prepared here have uniform distribution on the carbon fiber and good crystallinity,and the thickness is only 1.8 nm.Through a series of electrocatalytic activity tests,it was found that in the acidic system,the electrocatalytic performance of the ultra-thin cobalt selenide nanosheets was better than that of cobalt selenide nanoparticles.Our results further reveal that the two-dimensional structure is beneficial to expose more catalytic active interfaces,so we can prepare more non-layered transition metal chalcogenide two-dimensional nanostructures in the future.The successful design of highly efficient electrocatalytic active materials provides a strong reference for further study of their catalytic reaction mechanism.4.Generally,catalytic properties of catalytic materials are determined by the electronic properties.Benefited from unique bonding and electronic structure,the electronic properties of the transition metal chalcogenides should be varied by adjusting components.Based on the above characteristics,a series of binary transition metal chalcogenides NixCo1-xSe2 nanosheets were prepared by two-step method.In view of the strategy for preparing cobalt selenide nanosheets,we add the multi-metal elements into the precursors,and finally the binary nickel-cobalt selenide nanostructures were successfully prepared through ion-exchange with a hydrothermal reaction.The different metal elements will have an effect on the electronic properties and catalytic activity of the transition metal chalcogenide.We also fabricated binary selenide nanosheets with different ratios of cobalt to nickel,and investigated the characteristics in electrocatalytic hydrogen evolution.It was found that the binary selenide nanostructures were composed of NiSe2 and CoSe2.The results show that the overpotential and tafel slope of cobalt-nickel binary selenide are better than that of pure NiSe2 and CoSe2 in the acidic system.Nevertheless,the content of cobalt and nickel in different proportions will influence the catalytic activity and stability of the electrocatalyst.The purpose of this work is to optimize the electrocatalytic activity and to explore the mechanism of the catalytic activity by preparing multi-metal chalcogenides and adjusting the proportions of different metal components based on the two-dimensional nanostructures.The idea provides a new way to design the future catalysts and study the catalytic activity mechanism of two-dimensional materials.