Phase-controlled Synthesis And Electrochemical Performances Of Binary Transition Metal Nitrides

Nanostructured transition metal nitrides have promising applications in the field of functional materials due to their unique electronic,optical and electrochemical properties.However,it is usually synthesized by solid-phase method,which suffers from uncontrollable solid-phase reaction,complex products,and harsh preparation conditions.In addition,transition metal nitrides are interstitial non-integer ratio compounds,and their variable chemical composition,crystal structure and valence state make them have rich phase structure,which further increases the difficulty of their preparation.Therefore,the phase-controlled synthesis of transition metal nitrides is still facing challenges,and the research and application of their phase-dependent physical properties have not been fully explored.In view of this,the phase evolution of Co,Mo,Fe,W,Nb and V-based nitrides during the synthesis by thermal nitridation was investigated in this paper with the design idea of the selection of precursors（hydrox ides,oxides,metal elements）and the control of reaction parameters（reaction temperature,reaction time,reaction atmosphere）.By precisely adjusting the degree of nitridation,the phase-controlled synthesis of Co,Fe,and Mo-based nitrides was achieved,and single-phase Co₂N（simple orthorhombic phase）,Co₃N（simple hexagonal phase）,Co₄N（face-centered cubic phase）,and Mo N（simple hexagonal phase）,Mo₂N（simple cubic phase）,Fe₃N（simple hexagonal phase）,and Fe₄N（simple cubic phase）materials were synthesized.Simultaneously the controllable synthesis of single-phaseβ-WN（simple cubic phase）,Nb₄N₅（body-centered tetragonal phase）,VN（face-centered cubic phase）were realized.It was found that Co,Mo,and Fe-based nitrides have different types of precursors and control reaction parameters for phase control.Mo-based nitrides use MoO₃ as the precursor to achieve phase control by changing the reaction atmosphere,while Fe and Co-based nitrides achieve phase control by changing the reaction temperature.The difference of controllable phase synthesis between Co and Fe-based nitrides is that Fe-based nitrides use metallic Fe as the precursor,while Co-based nitrides use Co（OH）₂ as a precursor.In addition,based on this design idea,controlled preparation of various types of mixed phases（oxide with different components,oxide/nitride,nitride with different components,nitride/metal）can be realized.According to the dependent evolution of the final product phase with the reaction time,temperature and atmosphere,the phase evolution path of a single precursor was determined.It is found that the initial precursor phase is not directly transformed into the final product phase in one step,but gradually transformed through several intermediate phases.That is,the formation of nitride is the result of gradual phase transformation,and the whole phase transformation path includes multi-step solid phase transformation reactions.It is found that the general rule of the evolution path of hydroxide phase is:hydroxide→high-valence oxide（oxygen-rich phase）→low-valence oxide（oxygen-poor phase）→high-valence nitride（nitrogen-rich phase）→low-valence state nitride（nitrogen-poor phase）→metal element.The metal element is:metal element→low-valent nitride（nitrogen-poor phase）→high-valent nitride（nitrogen-rich phase）→low-valent nitride（nitrogen-depleted phase）→metal element.It is considered in this paper that the reason for the difference of the above two phase evolution paths is caused by the different valence states of transition metal elements.The transition metal valence in the hydroxide is positive and in oxidation state,while the metal valence in the elemental metal is zero and in the reduction state.During nitriding,the valence of the former decreases gradually,while the valence of the latter increases first and then decreases gradually.The effects of precursor types and reaction parameters on the nitridation process were elucidated.The precursor type determines the phase evolution path of the initial precursor phase in the nitridation process.The reaction temperature is a thermodynamic factor that provides the driving force for the phase transition reaction.The reaction tim e is a kinetic factor that determines the degree of completion of the phase transition reaction.While the reaction atmosphere mainly affects the kinetics process of the phase transition reaction.Introducing inert N₂into the reaction atmosphere reduces the concentration of active N source and H₂ concentration,which slows down the kinetics processes of nitridation and reduction reactions.Moreover,increasing the reaction temperature enhances the kinetics processes of nitridation and reduction reactions.A mechanism of nitride formation based on the control of competitive chemical reactions is proposed.The NH₃-mediated gas-solid phase nitridation process is essentially a competitive process between nitridation and reduction reaction.The high temperature promotes the dissociation of NH₃ into active N and reductive H₂.The active N drives the nitridation reaction,which is accompanied by the nitriding process,while active H ₂drives the reduction reaction,which is accompanied by the dehydrogenation,deoxygenation,and denitrification processes.The thermodynamic conditions,kinetic process speed and product phase formation energy required by the nitridation and the reduction reaction are different,and they are competitive with each other.Since the competi tive reaction controls the nucleation and growth rate,the dominant reaction in the nitridation process determines the phase of the final product.Therefore,under the condition of random experimental,the competitive chemical reactions occurring in the nitriding process leads to uncontrollable phase of the final product,and it is easy to obtain incompletely transformed mixed-phase materials.Further,the electrochemical properties of Co _XN（X=2,3,4）,Mo _XN（X=1,2）,and Fe _XN（X=3,4）as anode materials for lithium-ion batteries were evaluated.It was found that all seven nitrides showed excellent rate performance and cycle stability.After cycling stability,the nitrogen-poor Co-based nitrides have higher reversible specific capacities than the nitrogen-rich phases,while Fe and Mo-based nitrogen-rich phases have higher reversible specific capacities than the nitrogen-poor phases.It was found that seven nitrides exhibit a unique behavior of increasing reversible specific capacity with cycling in the middle of the cycle.This paper demonstrates the feasibility of phase-controlled synthesis of nitrides based on precursor selection and reaction parameters control,which provides a certain experimental basis and reference value for the phase-controlled synthesis of nitrides.In addition,the obtained nitrides with controllable components and phase structure will provide the possibility for exploring the structure-activity relationship of phase-dependent properties.