Studies On Nanostructure Construction Of Visible-light-responsive N-type Semiconductors And Their Photocatalytic And Photoelectrochemical Performance

Semiconductor-based photo(electro)catalysis(ie.Photocatalysis and photoelectrocatalysis),which can directly harvest and convert the renewable solar energy to chemical energy,have been considered as one of the most promising routes to address the current global energy crisis and environmental issues.To fulfil this challenging target,the development of visible-light-responsive,highly active,robust and cost-effective photo(electro)catalysts and the investigation of photo(electro)catalytic mechanism is the key technological point and the important scientific foundation,respectively.To this end,in this thesis,two kinds of heterojunction photocatalysts(sheet-to-sheet architectured m-BiVO4/(001)TiO2 and sponge-like porous ZnFe2O4/TiO2)with highly improved photocatalytic performance and a nanorods array ZnFe2O4 photoanode with a record solar water oxidation photocurrent on this material were designed and fabricated by adopting the combination strategy of heterojunction construction,facet engineering and nanostructure self-assembling.The photo(electro)catalytic performances were investigated from applications of both organic degradation and water splitting.Specifically,this thesis consists of seven chapters as followed:Chapter 1 is the introduction,which started with a short history of photochemical conversion techniques.Subsequently,the basic concepts and theories of semiconductor physics were briefly introduced,including the solid energy band structure and the formation of the semiconductor/solution interface.Thereafter,the research progresses of photo(electro)catalysis based on semiconductors,involving fundamental principles,general experimental set-ups,activity evaluation parameters,development of semiconductor photo(electro)catalytic materials,main factors influencing the activity,and the common strategies applied for improving the performance were reviewed.Finally,the main research contents and significance of this thesis were expounded.The second chapter described the experimental methods used in this thesis in detail,including material synthesis methods,physical and chemical properties characterization techniques,electrochemical investigation methods and testing methods of photo(electro)catalytic activity.Chapter 3 is the study on the preparation of a sheet-to-sheet architectured m-BiVO4/(001)TiO2 heterojunction photocatalyst and its photocatalytic performance of degradation of methylene blue(MB)under visible light irradiation.The m-BiVO4/(001)TiO2 heterojunction photocatalyst with different morphologies were successfully prepared by hydrothermal method.Results have shown the m-BiVO4/(001)TiO2 heterojunction photocatalyst synthesized from the precursor with pH=8 was oriented to form a unique shuriken shape.The TiO2 nanosheets with dominant(001)facets adhered uniformly on the surface of m-BiVO4,forming the heterojunction structure.By tracing the phase composition and morphology evolution of the sample during its growth process,the possible growth mechanism was deduced.The obtained m-BiVO4/(001)TiO2 showed about 1.4 times and 8.9 times higher photocatalytic activity for degrading MB under visible light than that of the bare BiVO4 and physical mixture m-BiVO4-(001)TiO2,respectively.It’s attributed to the formation of heterojunction between two semiconductors.Without retarding the visible light absorption ability of m-BiVO4,the incorporation of(001)TiO2 improved the specific surface area,suggesting a better adsorption ability for pollutants.More importantly,the formed heterojunction between m-BiVO4 and(001)TiO2 significantly improved the separation efficiency of the photogenerated electron-hole pairs.In addition,the kinetics and a possible mechanism of photocatalytic degradation of MB on m-BiVO4/(001)TiO2 heterojunction under visible light were studied as well.According to the results in Chapter 3,in Chapter 4,we further studied the effects of hydrothermal conditions(the pH of precursor and the blend ratio R of two components)on the phase composition,morphology,optical property and photocatalytic performance for oxygen evolution from water splitting on the sheet-to-sheet architectured m-BiVO4/(001)TiO2 heterojunction.The results showed that both the pH of the precursor and the blend ratio existed an optimal value for the photocatalytic oxygen production from water splitting.The sample prepared under the neutral precursor possessed smaller particle size,larger specific surface area and better crystallinity,showing lower probability of photo-carrier recombination and exhibiting the optimal photocatalytic activity.When the blend ratio was 0.3,the BiVO4/(001)TiO2 heterojunction possessed the largest two-phase contact area,and the photocatalyst carrier separation was enhanced by the formation of more heterojunction interface,exhibiting the highest photocatalytic activity.The oxygen evolution of 1600 μmol/g/h was about 5.7 times of that on the bare m-BiVO4 under the same conditions.This work further confirmed that photocatalytic activity can be significantly improved through a multi-method combination of heterojunction construction and facet engineering.In Chapter 5,sponge-like porous ZnFe2O4/TiO2 heterojunction photocatalysts with high visible-light-responsive photocatalytic activity were fabricated by a simple solution combustion method and the photocatalytic performance of degradating MB was investigated.Based on the charactrization of the morphology,surface structure and other properties,we believe that the significant increase on the photocatalytic degradation activity can be attributied to two aspects.On one hand,the spongy porous morphology facilitated the light harvesting and absorption/desorption of reactants/products;on the other hand,the formation of heterojunction can effectively suppress the recombination of charge carriers.With these synergistic effects,the degradation rate of MB on ZnFe2O4/TiO2 was up to 93.2%under visible light irradiation and remained stable even after five consecutive reaction runs.Additionally,a possible photocatalytic mechanism was proposed.This work focused on photocatalytic degradation of MB,and the activity of the photocatalyst was significantly improved by constructing a multi-method combination of constructing a porous structure and a heterojunction,which provided a very simple synthesis of magnetically recoverable porous heterojunction photocatalysts with great potential for wastewater treatment.In chapter 6,nanorods array ZnFe2O4 photoanodes were constructed using a solution-assisted solution synthesis process and the effects of synthesis temperature on the photoelectrochemical performance were studied.The synthesis temperature can effectively control the cations distribution and the crystallinity in the spinel structure without significantly changing the structure and size of the ZnFe2O4 nanorods array.As the synthesis temperature increased from 500 ℃ to 800 ℃,the crystallinity of the sample increased while the degree of cation disorder was reduced.Compared to the ZnFe2O4 photoanodes with higher crystallinity and lower cation disorder degree,the ZnFe2O4 photoanodes with relatively lower crystallinity and higher cation disorder degree showed more excellent electron transport properties in the bulk but lower catalytic ability on the surface.By optimizing the separation efficiency of photogenerated carriers in the bulk of the photoelectrode and the efficiency of the injection at the semiconductor/solution interface,we have successfully obtained a ZnFe2O4 nanorods array photoanode with a new record solar water oxidation photocurrent under simulated the sunlight irradiation(AM 1.5G,100 mW/cm2).The photocurrent density on the optimal ZnFe2O4 photoanode with NiFeOx co-catalyst modification was 1 mA/cm2 at 1.23 V vs.RHE.The influence of this structural disorder in spinel structure on their photoelectrochemical performance was first proposed,which provided a new insight into the factors important to the photoelectrochemical performance of the spinel ferrites and suggested a path to further improvement.In chapter 7,the research work of this dissertation was summarized and the prospects for the future development of this research field were given.