Regulating The Spin And Electrical Behavior Of Inorganic Graphene Analogues For Electrocatalysis Application

This dissertation aims to function-oriented synthesis of inorganic graphene analogues, regulate the spin and electrical behavior of inorganic graphene analogues and realize controlled preparation of inorganic graphene analogues. It also preliminarily explored the correlation between electronic structure and properties of two-dimensional (2D) inorganic graphene analogues. In this dissertation, we chiefly researched the interrelationship between dimension and electrical property of materials, in addition, we developed several2D inorganic graphene analogues which possess room temperature ferromagnetism. Through the liquid exfoliation method, we synthesized the first2D transition metal dichalcogenide with room temperature ferromagnetism and charge-density-wave phase transition behavior. We synthesized ferromagnetic2D ultrathin nanosheets via a topochemical transformation strategy, which possesses the higher saturation magnetization value compared with that of other previous reported ferromagnetic2D nanomaterials. We achieved magnetic couple modulation of2D metal-free materials by introducing hydrogen dangling bonds to modify2D metal-free materials. In addition, we developed a new strategy to synthesize nitrides similar to graphene by precursor morphology-oriented method, and carried out deep theoretical and experimental investigation on metallic nickel nitride nanosheets, then explored the intrinsic reason why metallic nickel nitride nanosheets could act as highly efficient oxygen evolution electrocatalysts. Detailed content includes the following several points:1. By analyzing the crystal structure of VSe2, the author thinks that2D VSe2nanosheets may simultaneously possess room temperature ferromagnetism and Charge-Density-Wave phase transition behavior. Theoretical calculation shows that interaction between VSe2layers is weak, which means we can obtain VSe2ultrathin nanosheets by liquid-phase exfoliation. The author synthesized VSe2in aqueous phase for the first time through simple hydrothermal method and applied this method to preparations of other selenides. Meanwhile, the author initially reported2D VSe2ultrathin nanosheets experimentally, by liquid-phase exfoliation strategy, using methane amide as solvent. In addition, with characterizations of electronic transport and magnetic properties, the author verified that2D VSe2ultrathin nanosheets could simultaneously possess room temperature ferromagnetism and Charge-Density-Wave phase transition behavior experimentally. Theory calculation further demonstrated that VSe2ultrathin nanosheets are intrinsic ferromagnetic. To our best knowledge, as an inorganic graphene analogue, VSe2ultrathin nanosheet is the first one which simultaneously possesses room temperature ferromagnetism and Charge-Density-Wave phase transition behavior to be reported. Based on the room temperature ferromagnetism and Charge-Density-Wave phase transition behavior of2D VSe2ultrathin nanosheets, they are promising to be applied to the next generation nano electronic devices.2. Feroxyhyte (δ-FeOOH)ultrathin nanosheets were obtained by a topochemical transformation strategy, and the thickness of δ-FeOOH ultrathin nanosheets are about1.2nm. Further magnetic study indicated that the δ-FeOOH ultrathin nanosheets possess the robust room ferromagnetic behavior with a saturation magnetization value of7.5emu/g. To the best of knowledge, the saturation magnetization value of δ-FeOOH ultrathin nanosheets is far higher than other previous reported ferromagnetic2D nanomaterials. Meanwhile, the δ-FeOOH ultrathin nanosheets present a semiconductor character with a direct band gap of2.2eV. Owing to the advantages of robust room temperature ferromagnetic behavior in semiconductors, feroxyhyte nanosheets are promising for the construction of next-generation spintronics.3. Concerning the problem that most metal-free materials usually lack intrinsic spin ordering structure, taking graphite carbon nitride (g-C3N4) as an example, the author introduced hydrogen dangling bonds into2D g-C3N4ultrathin nanosheets, with a function-oriented strategy, and for the first time, experimentally verified that hydrogen dangling bonds could realize magnetic coupling regulation in two-dimensional g-CsN4ultrathin nanosheets. The saturation magnetization value of2D g-C3N4ultrathin nanosheets with hydrogen dangling bonds at room temperature was as high as0.015emu g. In view of that ferromagnetic metal-free materials usually possess only an s/p electronic configuration with weak spin-orbit coupling and a large spin relaxation time, ferromagnetic2D g-C3N4ultrathin nanosheets would play an important role in constructing next generation spintronic devices. We anticipate that this strategy, introducing hydrogen dangling bonds into structure of2D nanomaterials, could pave a new way to engineer and optimize the intrinsic physical properties in other2D nanomaterials.4. Concerning that the oxygen evolution reaction (OER) efficiency for most of the developed Ni-based electrocatalysts has been intrinsically limited due to their low electrical conductivity and poor active site exposure yield, the author synthesized2D metallic M3N ultrathin nanosheets with sheet thickness between2.15nm and2.95nm, and systematically investigated the correlation between their intrinsic physical properties and electrocatalytic oxygen evolution. The first-principles calculations and electrical property measurements unravel that the M3N is intrinsically metallic, and the carrier concentration can be remarkably improved with dimensional confinement. The EXAFS spectra provides solid evidence that the N13N nanosheets have disordered structure resultant of dimensional reduction, which then could provide more active sites for OER. Benefiting from enhanced electrical conductivity with metallic behavior and atomically disordered structure, the M3N nanosheets realize intrinsically improved OER activity compared with bulk M3N and NiO nanosheets. This work provided large amount of useful information for designing high efficient OER electrocatalysts based on metallic nitrides, as well as inspires more application of2D metallic nitrides nanosheets in the field of energy storage and conversion.