Preparation And Hydrogen Evolution Properties Of Molybdenum Boride Films Via Chemical Vapor Deposition

For elemental boron,the serious electron deficiency makes it have a complicated bonding mechanism with transition metals to easily form metal borides with unique structures and abundant performances,which has attracted widespread research interests.Theoretical predictions suggested that two-dimensional（2D）transition metal borides have unusual structures and exceptional properties.To date,however,2D transition metal borides have not been realized experimentally.In this thesis,we firstly perform the preparation and structural characterization as well as performance investigation of2D transition metal molybdenum borides.To obtain high-performance boron-based materials,we optimize thesynthesis conditions of 2D molybdenum borides by combining theory and experiment.Some important results have been obtained as follows:（1）2D atomic crystal Mo₃B films were prepared on molybdenum foils via chemical vapor deposition（CVD）.We adopt the FeCl₃ solution to corrode molybdenum foil,and then the 2D films were transferred onto the target substrates for facilitating structural and performance characterizations.Atomic force microscopy（AFM）measurement shows that the thickness of the Mo₃B film is around6.48 nm.The high resolution transmission electron microscopy（HR-TEM）image reveals that the Mo₃B film has a hexagonal lattice structure,in good agreement with the value obtained by first-principles calculations.Furthermore,the first-principles calculations suggest that the Mo₃B film is metallic.Therefore,both good conductivity and rich catalytic sites make the Mo₃B film being important in high-efficient electrocatalytic hydrogen evolution.（2）The electrocatalytic hydrogen evolution performances of the molybdenum boride thin films,which were synthesized under various experimental conditions,were compared,and the result shows that is superior to all the electrocatalytic hydrogen evolution performances of all the molybdenum boride materials reported so far.On the one hand,the Mo₃B thin films possess excellent electrocatalytic hydrogen evolution with minimal initial overpotential and Tafel slope of 52 mV/dec,which is better than all reported catalytic properties of molybdenum boride materials.In addition,the Mo₃B film has long-life characteristics in acid solution,which stems from the self-structural stability of Mo₃B film,the smallest charge transfer resistance and the largest effective active area.On the other hand,comparing with the electrocatalytic hydrogen evolution properties of the molybdenum boride films grown under different experimental conditions,the results show that good crystallinity and abundant active sites are responsible for the outstanding electrocatalytic properties of the films.The thinner the film is,the higher the HER activity become.（3）The 2D molybdenum boride thin film with the thickness of about 10 nm was obtained by an oxidation-boronization method,and its electrocatalytic hydrogen evolution performance was studied.The molybdenum oxide thin film is prepared by oxidizing the molybdenum foil to form a nanometer-scale molybdenum dioxide layer.Then,we directly boronize the molybdenum oxide layers to prepare molybdenum boride thin films by the CVD method.Comparing with the molybdenum boride thin films under different experimental conditions,it was found that the molybdenum boride thin films grown at 900 ^oC possess an excellent electrocatalytic hydrogen evolution with a minimal initial overpotential,Tafel slope and excellent stability,which stems from the its good crystallinity and abundant catalytically active sites on the surface.