Fabrication Of BiVO₄ And ZnO Ceramics By Spark Plasma Sintering Method And Investigations Of Their Properties

Ceramic material is one of the most important inorganic non-metallic materials.Owing to its superior physical and chemical properties,ceramic materials have been widely used in various fields,such as aerospace,electronics,biomedicine,etc.Ceramic materials can be mainly divided into two groups:structural ceramics and functional ceramics.Thousands of ceramics have been developed and applicated in various fields based on their unique mechanical,optical and electrical properties up to now.As poly-crystalline materials,ceramics are similar as single crystals on one hand,which is highly stable and well crystallized.Furthermore,as ceramics are usually fabricated by inorganic non-metallic powders,the properties of ceramics can be easily modified and tuned by controlling the preparation of powders or nanomaterials.Thus,ceramics are expected to exhibit the merit of both single crystals and nanomaterials,which could be utilized in some new fields.Energy and environmental problems have become one of the most important issues for human society in the near future.In recent years,the exploration of renewable energies and the controlling of environmental pollution have become the hot spots for the scientists all over the world.As potential strategies to address the present energy and environmental problems,great progress has been made on photocatalysis and electrocatalysis.However,most of the electrodes or catalysts used in photocatalysis and electrocatalysis are made by nanomaterials.Although the activity of these electrodes or catalysts consisted of nanomaterials is almost comparative with noble metal materials,their stability is still far from the criteria for practical applications.Thus,to improve the stability of the electrodes or catalysts are very important.Based on the above analysis,ceramics could be the potential candidates to fabricate electrodes or catalysts for photocatalysis or electrocatalysis applications owing to its excellent stability.Furthermore,the investigations on ceramic electrodes or catalysts are quite few.Therefore,in this work,we proposed to fabricate ZnO and BiVO4 ceramics by spark plasma sintering method(SPS)and investigated their optical,electrical and photoelectrochemical properties systematically.And the purpose of this thesis is to explore the feasibility to employ metal oxide ceramics as stable electrodes or catalysts for photocatalysis,electrocatalysis or photoelectrochemical applications.This thesis contains six chapters.In chapter 1,we firstly reviewed the development and history of ceramic materials.Then,the categories of ceramics,the common methods to fabricate ceramic materials,the properties of ceramics are briefly reviewed.After that,we presented the introduction of spark plasma sintering method,including the history,development and mechanism,etc.At last,we focused on ZnO and BiVO4,which is the main materials that we investigated in this thesis,reviewed the development and present applications in the field of electronics,catalysis,etc.And based on the analysis of these two materials,we presented the main theme of this thesis.In chapter 2,we investigated the fabrication of BiVO4 ceramics through two different methods,namely,traditional high temperature annealing method and SPS method,respectively.And the experimental results indicated that the relative density of the BiVO4 ceramics obtained by traditional annealing method is lower than that obtained by SPS method.Then,we investigated the effects of annealing temperature,time and pressure during the SPS process,and optimized the parameters to fabricate BiVO4 ceramics.At last,we investigated the role of the morphologies,sizes of the BiVO4 nanoparticles on the quality of the BiVO4 ceramics,and further optimized the annealing parameters during SPS process.In chapter 3,we firstly synthesized Mo-doped BiVO4 nanopartilces with sol-gel method,and fabricated Mo-BiVO4 ceramics with high density and high crystallinity by SPS method.Then,we fabricated Mo-BiVO4 photoanode by the ceramics and investigated its photoelectrochemical properties.For comparison,we also fabricated Mo doped BiVO4 thin films on FTO substrate by spin-coating method.The experimental results showed that the Mo-BiVO4 ceramic and thin-film photoanodes exhibit photocurrent of 0.54 and 0.66 mA/cm2 at 1.23 V vs.RHE,respectively.Additionally,the Mo-BiVO4 ceramic photoanode exhibit superior stability comparing to Mo-BiVO4 films during long-term photoelectrochemical experiments.After series of characterizations including XRD,SEM,EDS,Raman and XPS measurements,the greatly improved stability could be attributed to the high crystallinity and dense structure of the Mo-BiVO4 ceramics,which indicates ceramic photoanode could be potentially used as a stable photoelectrode for photoelectrochemical water splitting.In chapter 4,we explored the fabrication parameter of ZnO ceramics by both traditional high temperature annealing and SPS methods with three different ZnO powders,namely,two commercial ZnO powders with different particle sizes and one ZnO nanoparticles obtained by co-precipitation method.Additionally,we investigated the role of particle sizes and morphologies on the relative density of ZnO ceramics.The experimental results indicated that the relative density of ZnO ceramics are greatly dependent on the particle sizes and morphologies of ZnO powders.In detail,larger ZnO powders are favorable for traditional high temperature annealing process,and smaller ZnO powders for SPS method.Furthermore,the number of oxygen vacancies in ZnO ceramics could be tuned by varying the annealing conditions,which provides us a simple way to controlling the defects in ZnO ceramics.In chapter 5,we investigated the fabrication of ZnO ceramics with different types and numbers of defects by SPS method.By varying the annealing conditions,four different ZnO ceramics with different colors were obtained.With systematic investigations through XRD,XPS,Raman,the types and numbers of defects in these ZnO ceramics could be obtained.And the different colors of the ZnO ceramics obtained under different annealing conditions could be attributed to the existence of different types of defects,such as oxygen vacancies(Vo),interstitial Zn defects(Zni)and Zn vacancies(VZn).And at last,we investigated the electrical and photoelectrochemical properties of these ZnO ceramics containing different defects.In chapter 6,we summarized th e main contents of this thesis,and analyzed the limitations of this work.We also proposed the plans for the future research on ceramic electrodes.