Supercritical CO₂-Induced Phase Transition For Controllable Fabrication Of Metastable Molybdenum Oxides And Their Functional Applications

The urgent demands of environmental and energy fields are driving people to continuously explore the methods of preparing new functional nanomaterials.Among them,it is of great theoretical and practical significance to use green chemical means to design and construct functional nanostructures.Molybdenum trioxide(MoO3)is a kind of typical metal oxide with multiple crystal phases.Metastable phases with unstable thermodynamics,due to their unique electronic structure,abundant defects and surface-active sites,play an irreplaceable role in energy conversion,green catalysis,biological applications,materials science and other fields.Specially,two-dimensional amorphous metastable materials possess the characteristics of both two-dimensional materials with high specific surface area and multiple active sites and amorphous semiconductors.They have stronger localized electrons,which can improve the catalytic performance and energy conversion efficiency of materials.Therefore,it is of great guiding significance to explore an effective green chemical method for controllable preparations of metastable molybdenum oxide and carry out the in-depth study of the relationships between their different crystal structures and properties.Supercritical CO2(SC CO2),as a widely recognized green solvent system,has low density and viscosity,high diffusivity and excellent mass transfer ability,which makes it play an important physical and chemical role in the confined space between layers for realizing intercalation,exfoliation and modification of layered materials.And chemical reactions usually are happened in these processes.Therefore,this paper designs the controllable preparation strategies from MoS2 to metastable hexagonal molybdenum oxide and 2D molybdenum oxide with amorphous structure with the assistance of SC CO2.The phase transition mechanism,functional structure design,free carrier concentration control and electronic state information in amorphous structure are explored,and the functional applications of molybdenum oxide with different structures in photoelectrocatalytic water splitting,electrocatalytic hydrogen evolution(HER),biosensing and electrocatalytic nitrogen reduction reaction(NRR)are also carried out.The main research contents are as follows:(1)Metastable h-MoO3 with local surface plasmon resonance(LSPR)effect was prepared by using supercritical CO2 technology because of phase transition,which has strong absorption and high photothermal conversion efficiency in visible-near infrared region.At the same time,molybdenum oxide with Mo5+ valence can effectively reduce silver nitrate,thus obtaining Ag/h-MoO3 heterostructure.The combination of metastable h-MoO3 and Ag can enhance light absorption and inhibit the recombination of photogenerated electrons and holes,improve the efficiency of charge separation,and effectively improve the photoelectrochemical(PEC)performance.(2)Metastable h-MoO3-x with disordered structure and oxygen defects can be prepared with the assistance of supercritical CO2.Defects in disordered structure can be used as anchor points for metal atoms.The loading of single Pt atoms on molybdenum oxide substrate is realized by in-situ reduction because of abundant carriers of h-MoO3-x,and the Pt1/MoO3-x single atom catalyst is obtained.The introduction of single Pt atoms can greatly improve the electrocatalytic activity.The combination of Pt1/MoO3-x and carbon black can further improve the utilization of catalyst.For the catalyst with only 2 wt%Pt content,the catalytic effect is comparable to that of commercial 20 wt%Pt/C.This study provides a new idea for expanding the applications of molybdenum oxide-based catalysts in electrocatalysis.(3)A new strategy of obtaining two-dimensional amorphous metastable.molybdenum oxide(MoO3-x)nanosheets with the assistance of supercritical CO2 is successfully designed.It is found that SC CO2 can interact with different atomic arrangement areas on the material surface,and the difference of interaction strength between these areas can introduce local stress field,thus promoting atomic rearrangement.Under the excitation of external light,MoO3-x can realize controllable LSPR effect in the visible-near infrared region.More importantly,compared with thermodynamically stable crystalline MoO3 nanosheets,2D amorphous MoO3-x nanosheets have the unique biological detection capability and the best responsiveness.The strategy of constructing amorphous structure by phase transition provides a new route for the development of new molybdenum oxide-based biosensor.(4)The atomic and electronic structures of the 2D amorphous metastable MoO3-x nanosheets are further investigated by means of experimental and theoretical calculations.and it is proved that amorphous MoO3-x can exhibit excellent electrocatalytic performance for NRR.Theoretical calculation results show that the ideal NRR performance is attributed to the existence of discrete and local Anderson tail electrons in the band gap,which can effectively interact with HNNH intermediate products in the NRR reaction path,effectively reduce the reaction barrier,and enhance the NRR activity.