Preparation And Photoelectric Properties Research Of Two Dimensional Amorphous Molybdenum Oxide Nanosheets Assisted By Supercritical Carbon Dioxide

The discovery and rise of two-dimensional materials have brought a new era with various possibilities for material science and nanotechnology.Compared with large-scale three-dimensional materials,two-dimensional materials have unique electronic structure,better mechanical properties,larger specific surface area and higher atomic activity due to the electronic limitation and super strong in-plane covalent bond introduced by atomic thickness and flake structure.Materials can be divided into three structural forms:crystalline,quasicrystal and amorphous according to their atomic order and arrangement.Among them,amorphous materials only have local short-range order instead of long-range atomic order.Compared with crystalline two-dimensional materials,the long-range disorder of amorphous materials makes the materials themselves have more defects and active sites,which can show better photocatalytic and electrocatalytic properties.Therefore,two-dimensional amorphous materials with both two-dimensional and amorphous properties introduce a new direction for the development of high-performance two-dimensional materials.At present,it has been found that a variety of two-dimensional materials can form two-dimensional amorphous state under certain conditions,such as amorphous graphene,amorphous boron nitride,amorphous transition metal oxides and so on.However,compared with two-dimensional crystal materials,the research progress of two-dimensional amorphous materials is much slower,especially the understanding of their preparation methods and synthesis mechanism is not deep enough.As we all know,the performance of two-dimensional materials largely depends on the structure,which mainly includes size,shape,composition and phase.Therefore,finding a suitable preparation method to regulate the properties of two-dimensional materials is considered to be a challenge that must be overcome to realize the application of nanotechnology.However,it is very difficult to adjust the structure,especially to control the morphology of two-dimensional amorphous materials.Due to the complex internal structure of two-dimensional amorphous materials,the growth process of two-dimensional amorphous materials is usually irregular.As a result,irregular nanoparticles are often formed by using conventional methods for the preparation of nanomaterials,such as liquid phase synthesis or gas phase synthesis.Therefore,the proposal of a simple and general method for the preparation of two-dimensional amorphous materials has become a key and difficult problem in the development of two-dimensional amorphous materials at the present stage.Based on the above research background,this thesis attempts to introduce supercritical carbon dioxide fluid into the preparation of two-dimensional amorphous materials.This research method not only expands the preparation method of two-dimensional amorphous materials,but also plays a role in regulating the electronic structure and surface state of the main materials,and effectively improves the optical properties of the main materials.The research focus of this paper is to select the appropriate main material and explore the influence of critical carbon dioxide fluid on the morphology and lattice structure of the host material in the reaction process.In general,molybdenum based functional materials with different morphologies,sizes and crystallinity were obtained by using molybdenum trioxide as the main material,assisted by supercritical carbon dioxide fluid,from top-down and bottom-up paths,and applied to photothermal conversion,biomacromolecule detection and surface Raman enhancement.The main contents are as follows:(1)Supercritical carbon dioxide assisted bottom-up synthesis of two-dimensional amorphous molybdenum oxide nanosheet and its application in the field of photothermal conversion.In this work,Mo O3-x nanosheet with adjustable structure were successfully synthesized by bottom-up experimental route using supercritical carbon dioxide for the first time,and two-dimensional amorphous Mo O3-x nanosheet were prepared under supercritical environment by pressure control.With the assistance of supercritical fluid,the adsorption of carbon dioxide molecules on the surface of the precursor effectively suppresses the growth of the precursor crystal nucleus and blocks the crystallization behavior of the material.Based on the excellent optical absorption properties of amorphous two-dimensional molybdenum oxide nanosheet,this paper effectively uses it in photothermal conversion applications,and successfully increases the photothermal conversion efficiency of molybdenum oxide materials to 69.3%,indicating that it has excellent photothermal treatment effect.(2)Study on supercritical carbon dioxide assisted stripping of two-dimensional molybdenum oxide nanosheet from top-down and its amorphization process.In this work,the stripping of layered molybdenum trioxide was realized with the aid of supercritical carbon dioxide fluid,and two-dimensional small-layer molybdenum oxide nanosheet were prepared.In the adjustment of the experimental conditions,we found the effect mechanism of supercritical carbon dioxide on the lattice destruction of crystal materials,and drew the temperature-time-phase transition diagram.Through the fitting of the transformation from crystalline to amorphous in the phase diagram,the critical conditions for the amorphization of materials assisted by supercritical carbon dioxide under different conditions are determined,which plays a guiding role in the preparation of two-dimensional amorphous materials assisted by supercritical carbon dioxide in the future.(3)Study on the application of superior optical absorption properties based on two-dimensional porous amorphous molybdenum oxide nanosheets.In this work,based on the phase diagram of the previous work,using the optimized reaction conditions,two-dimensional porous amorphous molybdenum oxide nanosheet with excellent optical absorption properties were prepared by supercritical carbon dioxide fluid assistance from top-down.Then it is effectively used in biological macromolecule detection and surface enhanced Raman spectroscopy detection system.The detection of ultra-low concentration(0.2 mg m L^-1)biological macromolecular protein(bovine serum albumin)and highly sensitive detection of small organic molecule(methylene blue)were realized,and the detection limit of 10^-9M and ultra-high enhancement factor of 4.9x10⁵ were achieved successfully.It provides a new application prospect for two-dimensional amorphous molybdenum oxide nanosheet with excellent optical absorption properties.