Controllable Synthesis Of Low Dimensional Manganese Oxides And Research On Its Properties

By virtue of the low cost,environmentally friendly,biocompatibility and excellent redox activity,manganese dioxide(Mn02)is deemed as one promising material for energy storage and wastewater treatment.Generally,MnO2 is used as the electrode material for supercapacitor(SC).Nevertheless,the poor electrical conductivity and easy decomposition of MnO2 in the charge/discharge process greatly confine its output capacitance and durability,especially for the condition of high charge/discharge rates or mass loading.In terms of wastewater treatment,small surface area of MnO2 with limited active sites usually hinder its catalytic oxidation performance.Until now on,the researchers realize that the morphology and phase structure of MnO2 have a significant impact on the performance for energy storage or catalytic oxidation.Thus,controllable preparation and modification of MnO2 with different morphology and phase structures have been drawing great attention.This doctoral dissertation is based on the solution of the problems for MnO2 in the applications of energy storage and wastewater treatment.Firstly,the different crystal structures of MnO2,the influence of different preparation methods for the morphology structure of MnO2 and the background in energy storage and water treatment are summarized.Then,we focus on how to improve the electron and ion transport for MnO2 by modification of its phase structure or tailoring its morphology,and optimize its electrochemical and catalytic oxidation performance.The main results are as following:(1)Metal ion doping improved the electron/ion transport of ?-MnO2 Firstly,we build the metal ion doping models to reveal the possible doping configurations based on the special tunnel structure of ?-MnO2 with first principle calculation.The density of states(DOS)data indicate that electrical conductivity of ?-MnO2 was improved by metal ion doping.Based on the first principle calculation,we fabricated Al-doped ?-MnO2 by hydrothermal method.The different Al3+ doping content could be easily controlled by adding different dopant amount during the synthesis process.The relationship was discussed between doping content and morphology of a-Mn02.Using four-probe method to measure the conductivity of ?-MnO2 with different Al3+ doping content,the result illustrate that the improved conductivity followed the trends of Al3+ doping amount.When Al3+doped ?-MnO2 was used as SC electrode material to investigate the electrochemical performance,this material shows high capacitance value and long cycle life under mass loading of 4 mg cm-2.Furthermore,we also fabricated V5+ doped 2D ?-MnO2 nanosheet in the similar hydrothermal system.In the synthesis process,NH4VO3 act as not only dopant but also "template" to guide the formation of 2D morphology.As a result,the electronic conductivity of ?-MnO2 is enhanced and the ion transport channel be shortened through V5+doping.(2)Molten salt method(MSM)was developed to controllable synthesis of MnO2 with 2D morphology by using.MnSO4 precursor was introduced into low melting point nitrate(molten salt)to carry out large-scale synthesis of 2D ion-intercalated Mn02 nanosheet.The 2D ion-intercalated MnO2 including 2D Na0.55Mn2O4·1.52O,2D K0.27MnO2·0.54H2O,2D Ba2Mn14O27·xH2O and 2D Cs4Mn14O27·xH2O were synthesized by selecting different molten salt.The synthesis mechanism is discussed in detail in this system.Based on the structure properties of this 2D MnO2,we explored the effect on the stability of the morphology and structure with two types of metal ion doping.Moreover,we expand this synthesis system to produce other 2D materials,containing 2D ion-intercalated metal oxides(Li2WO4 and Na2W4O13)and 2D ion-intercalated metal hydroxides(Zn5(OH)8(NO3)2·2H2O and Cu(OH)3NO3).This result illustrates the universality of the synthesis system.Finally,the 2D K0.27MnO2·0.54H2O shows good catalytic oxidation performance in the treatment of Congo Red wastewater.(3)Salt template method was used to further control the morphology of MnO2.First,Mn precursors were coated on the surface of water-soluble salt(KCl)template.The as-obtained composite was further annealed under the air atmosphere and then KCl template was removed by deionized water to obtain 2D mesoporous MnO2.The 2D Mn3O4 and 2D mesoporous Mn3O4 could be prepared by means of altering the manganese precursors and reaction temperature.A possible formation mechanism of mesoporous structure was proposed through analyzing the structure and composition of precursors before annealing.We have chosen 2D mesoporous MnO2 nanosheets as a candidate to evaluate the catalytic activity,and discussed the influence factor on catalytic activity from kind of dyes,concentration,pH values and so on.The results show that 2D mesoporous MnO2 presents the high-efficiency catalytic behavior as well as excellent structure stability.This superb performance benefits from the unique structure advantages for high reactivity,abundant and accessible active sites from the large specific surface area.Meanwhile,we tested the structure of MnO2 after degradation process,and the results show that the structure stability is the key factor to realize long cycle life.