| Due to the continuous consumption of fossil energy, we are confronted with twomajor problems: energy shortage and environmental pollution. Recently,photocatalysis has become a research focus because it is low-cost, environmentalfriendly, and may be a promising solution for both the energy and environmentalissues simultaneously. Benefit from the advanced nanotechnique and characterizationmethods, significant progress has been made in the field of photocatalysis. Thousandsof photocatalysts have been developed and some of them have already been utilized inpractical applications such as commercial air purifier, self-cleaning coating material,antibacterial materials, etc. Nevertheless, the efficiency of photocatalysis is still toolow to meet the requirement in many specific situations, including solar watersplitting, artificial photosynthesis, and degradation the pollution in air and water. Sothe further enhancement of efficiency in the photocatalysis is necessary anddeveloping high efficiency photocatalyst remains to be a pressing task and hugechallenge.In a typical photocatalytic reaction, there are three main steps:(1) the absorptionof photons,(2) the separation and transportation of photoinduced electron-hole pairs,(3) the reaction at surface active sites. Among them the separation and transportationof photoinduced electron-hole pairs is the key step, which has an important effect onphotocatalytic performance. However, due to the restriction of carrier diffusion length,most of the photoinduced charges will be recombined during the migration from bulkto surface of the materials. So restraining the recombination during the migration and improving the separation efficiency are the keys for designing and constructing highefficiency photocatalyst. The charge recombination during the transportation can berestrained by constructing a heterojunction or homojunction and leads to enhancementof the charge separation efficiency. For either heterojunction or homojunction, there isa built-in electric field at the interface and the photoinduced charge carriers will beseparated directionally. The direction of the built-in electric field determines theseparation direction of the photoinduced charge, and the magnitude of the built-inelectric field determines the separation efficiency. Thus the properties of the junctioncould be modulated by tuning the Fermi levels of the two materials, andcorrespondingly the behaviors of photoinduced charge to obtain high chargeseparation efficiency.In this thesis, we selected ZnO/Cu2O heterojunction and p-n Cu2O homojunctionphotocatalytic materials as the models to investigate the effect of the properties ofjunction on the photoinduced charge by surface photovoltage andphotoelectrochemical methods. The photocatalytic performance of materials withdifferent charge behaviors was discussed, the relationship among the properties ofjunction, the behaviors of photoinduced charge and the corresponding photocatalyticperformance were revealed and finally utilized to direct the design and synthesis ofmaterials theoretically and experimentally. Moreover, we elucidated the mechanism ofphotophysics process in photocatalysis by studying the separation and transportationof photoinduced electron-hole pairs. The thesis contains the four parts below:1. Study on the influence of the interface properties upon the photoinduced chargeseparation direction in Cu2O/ZnO heterojunction film: to reveal the separationdirection in heterojunction photocatalysts, the Cu2O/ZnO heterojunction film wasdesigned and employed as a model to investigate the photoinduced charge behaviorsand analyze the charge separation direction using surface photovoltage method. It wasfound that when irradiated by UV light the photons were absorbed by outside Cu2Olayer and the photoinduced charge carriers were separated by the downward bandbending of Cu2O; when irradiated by visible light the photons could penetrate the whole Cu2O/ZnO heterojunction film. Under the effect of interface band bending thephotoinduced electrons in Cu2O transferred to ZnO and the photoinduced holesaccumulated in Cu2O, i.e. the interface built-in electric field had a dominant effect onthe photoinduced charge transfer process. When changes the sequence of the growthof ZnO and Cu2O, the direction of interface electric field changes and the separationdirection of photoinduced charge carriers changed accordingly. The type ofphotoinduced charge migrated to the surface of the photocatalyst determined theselectivity of reaction. The study of separation direction of photoinduced charge couldprovide a theoretical basis for the design of photocatalytic materials for some specificchemical reactions.2. Study on the influence of the interface properties upon the photoinduced chargeseparation efficiency in Cu2O/ZnO heterojunction nanorod arrays electrode: to furtherinvestigate the effect of the interface built-in electric field on the photoinduced chargebehaviors we designed Cu2O/ZnO heterojunction nanorod arrays and studied thephotoinduced charge behaviors using surface photovoltage method by regulating theFermi level of Cu2O. The influence factors of separation efficiency and the rationalityof the design of the materials were analyzed. It was found that when the pH ofelectrolyte was11.0, the Cu2O presented the highest carrier concentrations,3.01×1017cm-3, relating to the lowest Fermi level and the largest difference of Fermi levelwith ZnO. For Cu2O (pH11.0)/ZnO, the surface photovoltege arised within thefollowing80ns and its magnitude became the highest,0.28mV. The fastest andhighest surface photovoltege indicated that the charge separation efficiency was raisedto the highest, which would lead to the highest photocatalytic performance. Suchmodulation of the magnitude of interface electric field is helpful to improve thecharge separation efficiency, which provides a theoretical basis for the design ofphotocatalytic materials.3. The relationship between the interface properties and photocatalytic performance inp-n Cu2O homojunction film: to avoid the lattice mismatch in heterojunction, wedesigned the p-n Cu2O homojunction film and employed it as a model to investigate the photoinduced charge separation direction and efficiency by surface photovoltagemethod. It was found that when the pH of the electrolyte was9.0, the Cu2O exhibitedthe highest carrier concentrations,5.51×1016cm-3, which indicated the largestinterface electric field after constructing a homojunction with n-Cu2O prepared underpH4.9. From the Mott-Schottky plots, the p-n Cu2O (pH9.0+pH4.9) had the mostpositive flat band potential,0.92V, which indicated the largest interface electric field.From the surface photovoltage results for p-n Cu2O (pH9.0+pH4.9), thephotoinduced electrons transfer to the surface and the quantity was the largest. It wasdemonstrated that the largest interfacial electric field results in the highest chargeseparation efficiency, which is contributed to the highest activity in photocatalyticperformance. Construction of homojunction is helpful to improve the separationefficiency, which provides an idea for preparing interface type photocatalyticmaterials with high efficiency.4. Fe2O3/Ti-Fe2O3homojunction nanorod arrays electrodes for photoelectrochemicalwater splitting: to increase the diffusion length of photoinduced charge carriers inFe2O3, a homojunction nanorod arrays between Ti-doped Fe2O3and pure Fe2O3wasdesigned. The surface photovoltage method was used to investigate the photoinducedcharge behaviors. The transient photovoltage measurements revealed that therecombination of photoinduced charge carriers is refrained and the charge separationefficiency is enhanced, which improves the performance of photoelectrochemicalwater slitting. |
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