Fabrication Of Two-dimensional Amorphous Molybdenum Oxides And Defective Molybdenum Oxide-based Heterostructures And Their Applications

Since 2004 when Novoselov et al successfully exfoliated graphene from graphite using Scotch tape,scientific researchers have developed a strong interest in two-dimensional materials,and at the same time,a variety of functional two-dimensional nanomaterials have emerged.Two-dimensional materials are sheet-like nanomaterials with an atomic thickness and a lateral dimension exceeding 100 nm.Two-dimensional nanomaterials can confine electrons in an ultra-thin two-dimensional region,and have strong in-plane covalent bonds and a high specific surface area.At the same time,the exposed atomic sites and defect vacancies on the surface can make the two-dimensional nanomaterials show excellent properties in the fields of electrochemistry,catalysis and mechanics.With the extension and development of science and technology,purely crystallized two-dimensional nanomaterials such as graphene can no longer meet the needs of scientific research exploration and production catalysis.Recently,amorphous nanomaterials,that is Amorphous Nanomaterials,due to their unique long-range disordered atoms arrangement,show high performance in the mechanics,catalysis and magnetic.Therefore,two-dimensional amorphous nanomaterials have opened a new way for the research of high-performance two-dimensional nanomaterials.Molybdenum oxide,a transition metal oxide,is a layered material combined with Van Der Waals force.Under the action of external force,it can become a two-dimensional nanomaterial with few layers or even single layer.Based on this,it is feasible to convert two-dimensional crystallized molybdenum oxide into two-dimensional amorphous nanomaterials,and at the same time,due to the long-range disorder structure,the oxygen vacancies and defect sites of amorphous nanomaterials are effective active catalytic sites.Combined with the plasma resonance effect of molybdenum oxide,the presence of oxygen vacancies can change the state and density of electron clouds around the active atoms in the system,which can reduce the material band gap and enhance the light absorption of molybdenum oxide in the visible and near-infrared regions.Thus,it is beneficial for the improvement of photocatalytic activity.Based on the above considerations,this paper studied the fabrication and regulation of the two-dimensional amorphous molybdenum oxide materials as well as their application performance.It also explored the construction of crystallized molybdenum dioxide and amorphous molybdenum oxide lateral heterojunctions,and the heterostructures can be applied in the fields of photocatalysis and electrocatalysis.（1）Ultrasonicated exfoliation of two-dimensional few-layered molybdenum oxide nanosheets and the study of their physicochemical propertiesSimply using only ultrasonicated exfoliation method,we prepared two-dimensional molybdenum oxide materials,and adjusted the number of layers and physicochemical properties of molybdenum oxide by changing the experimental conditions.Through experimental research and comparison,the number of layers of two-dimensional molybdenum oxide was significantly reduced in the mixed solution of ethanol/deionized water.The ultrasonic treatment can break the Van Der Waals force between layers,and with the extension of the ultrasonic time,molybdenum oxide materials would tend to be amorphous.The existence of some defects in molybdenum oxide expanded the light absorption range,and they provided the possibility for fabricating completely amorphous two-dimensional nanomaterials.（2）Amorphous Mo O_3-x nanosheets prepared by the reduction of crystalline Mo O₃by Mo metal for LSPR and photothermal conversionBased on the first work,we designed a simple liquid-phase metal ions intercalated molybdenum oxide（Mo O₃）reaction.Two-dimensional amorphous sub-stoichiometric molybdenum oxide nanosheets could be obtained via a hydrothermal reaction at 160℃for 48 h.The material aqueous solution was light blue,and the defect sites,including oxygen vacancies,were limited.By the irradiation of the external Xenon lamp,we found that the blue color of the sample solution deepened,and it showed a strong plasmon resonance effect in the visible and near-infrared regions.Due to the prolonged irradiation time of the Xenon lamp,the double absorption peaks of the plasmon resonance would obviously shift.The shifting of peaks position were the result that hydrogen ions inserted into the interlayers of molybdenum oxide.In this way,the content of defects of the final two-dimensional amorphous molybdenum oxide nanosheets increased,and the sample materials could be used in photocatalytic reaction.（3）Fabrication of novel lateral heterostructure of crystallized Mo O₂/amorphous Mo O_3-x for photothermal conversion and electrocatalytic nitrogen reductionWe can obtain lateral heterostructure of crystallized Mo O₂/amorphous Mo O_3-x by appropriately increasing the amount of Mo metal ions in the reaction system.More Mo ions could insert into the interlayers of Mo O₃ and further reduce Mo O₃ to Mo O₂.The heterojunction showed a broad absorption peak in the visible and near-infrared regions,which is the result of the synergistic interaction between the crystalline and amorphous phases.The heterostructures have some activity in the photothermal conversion and electrocatalytic nitrogen reduction.