Supercritical CO₂-assisted Construction Of Functional Transition Metal Compound-based Two-dimensional Materials And Their Applications

How to deal with the increasingly prominent environmental and energy issues is a major challenge facing humanity today.The development of new energy and environmental materials is one of the most important means to solve environmental pollution and energy consumption.Nanomaterials,especially two-dimensional（2D）layered materials,due to their unique structure and excellent physical and chemical properties,have a wide range of applications in electricity,catalysis,biology,energy and materials science,and have attracted great attentions.Therefore,the use of green chemistry to construct new 2D layered nanomaterials and study their structure-performance relationship has important scientific and theoretical significance as well as practical application value.However,there are still many key issues to be solved in the practical application of 2D nanomaterials,how to reliably and accurately exfoliate and fabricate,how to realize structural control and composite assembly,how to improve harsh preparation conditions,and as well as reduce the difficulty of post-processing.As an effective “field”,supercritical CO₂（SC CO₂）has unique low viscosity,high diffusivity and high mass transfer ability.It can not only effectively intercalate and exfoliate 2D layered materials,but also induce phase transitions and chemical reactions.And surprising results have been achieved in the construction of 2D heterostructures and the modification of 2D materials.Based on these,this paper selects typical 2D layered nanomaterials WS2 and MoS2 as research objects,and studies the influences of SC CO₂ system on their defects formation,construction of heterostructures,atomic structure regulation and phase transitions.Moreover,the application of 2D transition metal compound-based nanomaterials in the field of photoelectrochemistry including catalytic water splitting and degradation of organics is also studied.The main research contents are as follows:（1）CO₂-induced defect engineering: A new protocol by doping oxygen defects in 2D heterostructures for enhanced visible-light photoelectrocatalysisDefect engineering has emerged as an efficient and promising performance regulation strategy in the field of semiconductor materials,while assembling controllable defect structure and 2D heterostructures into together is still challenging.In this work,2D heterostructures of WS2/WO3·H2O doped with oxygen defects have been controllable synthesized using SC CO₂ as an effective field.The presence of defects can not only facilitate exposure of surface atoms to increase the catalytic active sites,but also generate a new defect level,which effectively promote the generation and separation of the electron-hole pairs,and then the performance of photoelectrocatalytic water splitting is significantly enhanced.（2）CO₂-assisted selective stacking of 2D WS2-WO3·H2O and 1T-2H MoS2 to desirable multi-heterostructuresCompared to the limitations of 2D binary heterostructures in photoelectrocatalytic applications,2D multinary heterostructures have shown great advantages.Precise control of the lateral and vertical integration of 2D heterostructures is particularly exciting for novel electronic and photovoltaic applications.However,fabrication of such high quality heterostructures with desirable compositions and sequences is a great challenge.In this work,we successfully prepared a 2D complex heterostructures of WS2-WO3·H2O/1T-2H MoS2 by skillfully integration of lateral and vertical heterostructures in selective stacking in the unique pickering microemulsion system constructed by SC CO₂.First,SC CO₂ was used as the reaction medium to build 2D lateral heterostructures of WS2-WO3·H2O and 1T-2H MoS2,respectively.Second,and more importantly,thanks to the unique properties of SC CO₂ that help to construct a unique solvent system,vertically stacked bilayers of 2D WS2-WO3·H2O and 1T-2H MoS2 with 2H-WS2/2H-MoS2 as the preferred van der Waals heterojunctions can be formed.A superior photoelectrochemical activity toward hydrogen evolution based on such the quaternary heterostructures is achieved due to their strong interlayer electronic coupling as well as the lateral epitaxy of WO3·H2O and 1T-MoS2 in the heterostructures,which increase the proton diffusion coefficient,conductivity and active sites of the heterostructures effectively.（3）Building of peculiar heterostructures of Ag/two-dimensional fullerene structure-WO3-x and its application in photoelectrocatalysisPhotoelectrochemical（PEC）water splitting has been an effective approach for conversion of solar to chemical energy to meet the clean energy demand.Design and fabrication of high-quality photoelectrode for water splitting with enhanced light absorption efficiency,as well as efficient charge separation and transport are challenging.Herein,2D WO3-x nanosheets with unique fullerene structure are prepared with assistance of SC CO₂.Then a novel plasmonic photoanode heterostructures composed of plasmonic Ag and WO3-x is synthesized.The unique co-existence of amorphous and crystalline structure of WO3-x can lead to increase the active site density and improve the electron transport,and Ag nanoparticles can be uniformly distributed in the amorphous regions.Femtosecond time-resolved IR absorption spectrum analysis indicates the localized surface plasmonic resonance（LSPR）effect of Ag nanoparticles can mediate efficient electron transfer to WO3-x nanosheets.Based on the synergistic effects of the special microstructures of plasmonic Ag and 2D fullerene structure-WO3-x,high-efficiency PEC water splitting and photooxidation degradation of methyl orange can be achieved.（4）Two-dimensional amorphous heterostructures of Ag/a-WO3-x for high-efficiency photoelectrocatalytic performanceSynergistic photocatalysis is an important concept for designing the high-efficiency catalyst for fundamental research and technical applications.In this study,a well-defined synergistic photocatalysis system is realized by the 2D amorphous heterostructures（2DAHs）Ag/a-WO3-x,which are constructed by uniformly loading Ag nanoparticles on 2D amorphous tungsten oxide（a-WO3-x）nanosheets fabricated via SC CO₂ method.We demonstrate that the oxygen evolution reactions（OER）,characterized by photocurrent response,have been dramatically improved in Ag/a-WO3-x than those of both single a-WO3-x and Ag/WO3 systems.Such an enhanced photoelectrochemical performance attributes to the superposition effect of amorphous structure and LSPR effect.More importantly,the density-functional theory（DFT）calculations reveal that the amorphous effect catalysis derives from the unique d-d tail states coupling of both Ag and W atoms in the 2DAHs.The research work of this dissertation provides a reference for the development of function-oriented design and controllable new methods of 2D nanomaterials,and it also provides a certain design concept and theoretical basis for exploring the application of novel 2D heterostructures in the field of photoelectrocatalysis.