Etching Of Ternary Layered Transition Metal Boride/Carbide And Research On Properties Of The Products

Two-dimensional(2D)materials have attracted much attention since the discovery of graphene due to their unique properties.Two-dimensional(2D)transition metal carbide/nitride(MXene),obtained through etching of layered tertiary transition metal carbide/nitride,has been widely studied and applied in sensor,energy conversion/storage,catalysis and other fields,due to their atomic level thickness,adjustable structure and function,high conductivity and hydrophilicity.MAB phases are a type of ternary layered transition metal borides with rather similar layered structure as MAX phases.It is expected that two-dimensional layered transition metal borides can be obtained by etching MAB phases.However,the reseaech about the synthesis method and structure of MAB are quite rare,especially the successful etching of MoAlB to achieve 2D Mo B hasn't been reported.In this thesis,MoAlB was etched by fluoroboric acid,and materials with obvious layered structure was prepared.The number of Al atom layers was successfully reduced by adjusting the temperature and time.The etching product Mo₂Al B₂ showed excellent performance as electrocatalytic hydrogen evolution catalyst.Moreover,the structure of MAB phases were controlled by C or Ti doping,and the effect of doping amount on the structure of MAB phases and the electrochemical performance of the products was studied.Additionally,different preservation methods were explored for Ti₃C₂T_xdispersion and preservation time was effectively prolonged and its application scope was expanded.The thesis work could be summarized as following:Firstly,Mo₂Al B₂ was prepared by etching MoAlB with fluoroboric acid and tetramethylammonium hydroxide,which was a possible precursor of MBene.The effect of reaction temperature on the product was investigated,and the optimal reaction temperature was determined to be 80?.The mixture of MoAlB to Mo₂Al B₂ were obtained when the reaction temperature were 50?70?.The product etched at 90?contains Mo F₃ impurities.Compared with MILD method,the surface of the products etched by fluoroboric acid method at 80?contains less-F groups,and the hydrogen evolution activity is the best.When the current density is 10 m A cm^-2,the required over potential is the lowest,241mv,5 m V lower than that of MILD etching product.After 1000 cycles of cyclic test,there was no obvious change in over potential,indicating a good stability.Secondly,carbon doped ternary layered transition metal borides were prepared by introducing different content of C atoms into MoAlB precursors and calcinated at high temperature.When the doping content of C atom was low(<20%),the product of MoAlB with MAB phase structure was obtained.The mixture of Mo B and Mo₂C were obtained when the doping content of C atom increased to 30%.The characteristic peak of Mo₂Al C was observed when the doping ratio further increased to 50%.It was found that the ternary layered carbides of Mo₃Al C₂ were obtained when C content was 100.The calcined products with different content of C atom were etched by fluoroboric acid,and the hydrogen evolution performance of the etched products was inverstigated.It was found that the activation energy required for hydrogen evolution increased with the increase of C content.When the current density is 10 m A cm^-2,the over potential of the sample doped with 10%C is the lowest,which is 251 m V,while the over potential of the sample prepared with 100%C increased to298 m V.With the increase of C atom content,the specific capacitance of the sample increases gradually.Thirdly,Ti-doped ternary layered transition metal carboborides were prepared by adding different content of Ti into MoAlB precursors and calcination under high temperature.When the Ti doping content was low(<20%),the product of MoAlB was obtained.The product of Ti B₂ was observed when the doping content of Ti atom increased to 30%.The product of Ti B₂were obtained when the doping ratio of Ti atom was 100%,with Mo atom completely replaced by Ti atom.With the increase content of Ti atom,the activation energy for hydrogen evolution of the etching products increases.When the current density is 10 m A cm^-2,the over potential of sample doped with 10%Ti is 268 m V,and that of the sample doped with 100%Ti is 313 m V.After 1000 tests,the over potential of LSV curve at the current density of 10 m A cm^-2 has no obvious change,indicating that the sample has good stability.With the increase content of Ti,the charge discharge curves gradually become symmetrical.The specific capacitance of the etched products increases with the increase content of Ti,and the sample prepared with 100%Ti has the largest specific capacitance,172 F g^-1 at 1 A g^-1.When the current density is increased to 10 A g^-1,64.6%of its specific capacitance is retained,suggesting a good rate performance.Finally,Ti₃Al C₂ was etched and the storage stability of Ti₃C₂T_x dispersion was studied.Ti₃C₂T_x was stored in water and anhydrous isopropanol dispersions.Sodium thiosulfate and L-ascorbic acid were added to the aqueous dispersions,and butylated hydroxytoluene and triethyl phosphite were added to the anhydrous isopropanol dispersions.The samples were stored at room temperature and characterized regularly.The effects of different dispersions and antioxidants on the oxidation resistance and electrochemical performance of the samples were investigated.The results show that antioxidants can effectively inhibit the oxidation of Ti₃C₂T_x dispersion in aqueous solution.L-ascorbic acid could delay the oxidation of Ti₃C₂T_xdispersions for 75 days.The structure and properties of Ti₃C₂T_x remained good after being stored in anhydrous isopropanol for 200 days.The specific capacitance of the sample with triethyl phosphite is 204.1 F g^-1 at 1 A g^-1,which effectively prolongs the storage time of Ti₃C₂T_x dispersion and expands its application range.