Preparation,characterization Of Co₃O₄ And Their Composites,as Well As Applications In Surface Modification Of Cement-based Materials

In this thesis,without using templates and surfactants,various morphologies of Co₃O₄ were synthesized via hydrothermal method and a subsequent annealing process,the formation mechanism of Co₃O₄ with different morphologies was proposed,the relationship between microstructure and properties of materials was explored;A novel architecture of Co₃O₄ / reduced graphene oxide（RGO）composite was created,the effect of graphene on the growth and electrochemical performance of Co₃O₄ materials was explored;To expand the application field of Co₃O₄ material,we prepared SiO₂@Co₃O₄ compounds,and modified the cement-based materials with them,we explored the influences of SiO₂@Co₃O₄ materials on properties of cement-based materials.The specific research contents are as follows:1.Enhanced CO catalytic oxidation of flower-like Co₃O₄ composed of small nanoparticles.Three dimensional（3D）flower-like Co₃O₄ composed of nanoparticles（NPs）was synthesized via a hydrothermal route and a subsequent calcination process.Calcination system was the key process for controlling the size of the Co₃O₄ NPs and their catalytic performance: high temperatures resulted in the formation of heterogeneous and large particles;the NPs in resulting samples packed together or aggregated,and their sizes decreased with an increasing heating rate.CO catalytic oxidation investigation indicated that the flower-like Co₃O₄ calcined at 400 °C with a heating rate of 1 °C min-1 exhibited the highest catalytic performance with a 100% conversion ratio at 93 °C.It was suggested that the enhanced catalytic activity was due to the increasing number of Co3+ active sites and adsorbed active oxygen species at the surface of flower-like Co₃O₄ that was calcined under optimum conditions,meanwhile,the increasing specific surface area also provides more reaction sites for CO catalytic oxidation.2.A precursor-derived morphology-controlled synthesis method for mesoporous Co₃O₄ nanostructures towards supercapacitor application.The controlled synthesis of multifarious nanostructures is valuable for understanding the properties associated with the surface structure and for exploring potential applications.Herein,various morphological Co₃O₄ precursors were synthesized hydrothermally for fabricating mesoporous Co₃O₄ nanostructures,importantly,the morphology,size,and crystalline phase of precursors were independently tuned by controlling the components in solution（e.g.anions,cations,and hydrolysis agents）,the microstructure of Co₃O₄ precursors has obviously changed from porous hexagons to nanoneedles by changing the anions in solution.After calcination,as-prepared precursors were converted into mesoporous Co₃O₄ nanostructures,and their morphology and size were well preserved.Electrochemical tests showed that the 3D hierarchical flower-like Co₃O₄ assembled by hexagonal nanosheets exhibited higher specific capacitance（327.3 F g-1 at 0.5 A g-1）with excellent retention（96.07 % at 5 A g-1）after 10 000 cycles.It is believed that this anion-assisted synthetic approach can be applied into the preparation of other transition metal oxides.3.Ultrathin and highly crystalline Co₃O₄ nanosheets in situ grown on graphene toward enhanced supercapacitor performance.Effective design of an electrode material is essential for energy storage and conversion of supercapacitor.Herein,a well-designed architecture of ultrathin and highly crystalline Co₃O₄ nanosheets（NSs）with a thickness of 2–3 nm were grown in situ on RGO via a hydrothermal route and a subsequent calcination process,in the process of preparation,the exsitence of graphene oxide（GO）affected precursors’ growth.The results indicated that Co₃O₄/RGO-2 electrode（the mass ratio of CoCl₂.6H₂O to GO is 10:1）had the best performance with a specific capacitance of 3344.1 F g-1 at a current density of 1.25 A g-1,fine rate capability,and retention capacity of 87.9 % even after 6000 cycles at 10 A g-1.Such excellent performance was ascribed to the synergistic effects of ultrathin and highly crystalline Co₃O₄ NSs and its interacting model with RGO.This ultrathin sheet–sheet architecture will provide a promising for electrochemical fields.4.Preparation of SiO₂@Co₃O₄ compounds and the study on surface modification of cement based materials.SiO₂ nanoparticles were coated on the as-prepared Co₃O₄ samples by sol-gel method,and the synthesized SiO₂@Co₃O₄ composite was first used to modify the cement paste and mortar.In this part,the microstructure of SiO₂@Co₃O₄ modified cement paste indicated that Si O2@Co₃O₄ composites could be combined with cement matrix.The results of performance test showed that the water absorption and microwave absorbing properties of SiO₂@Co₃O₄ modified cement-based materials were improved to a great extent.