Structural Basis And Photogenerated Carrieer Behavior And Properties Of Silver-based Solid-solution Heterojunctions

Environmental pollution has become a major concern in today’s society,and semiconductor photocatalysis technology has become an important technology to solve environmental pollution because of its efficient treatment and no secondary pollution.Traditional photocatalytic materials have problems such as low absorption efficiency of visible light and poor separation efficiency of photogenerated electron-hole pairs,therefore,new photocatalytic materials have received wide attention.Silver-based solid solution as a new photocatalytic material has a narrower band gap to absorb visible light,and its unique electronic structure can effectively improve the electron-hole separation efficiency.However,silver-based solid solution still suffers from the problem of easy electron-hole recombination.The construction of heterojunctions becomes an effective method to improve the properties of silver-based solid solutions,which can provide a rich coupling interface and effectively enhance the driving force of photogenerated carriers.This can be useful for enhancing the photocatalytic performance.In this thesis,new heterogeneous photocatalytic materials were constructed by the strategy of mixed-phase heterojunctions and piezoelectric heterojunctions with silver-based solid solution.The phase,morphology,absorption spectrum,photogenerated carrier behavior and photocatalytic performance were systematically investigated as follows.（1）α/β-Ag Al_0.4Ga_0.6O₂ mixed-phase heterojunctions were obtained in situ by phase transformation under hydrothermal conditions.The microstructure of the mixed-phase heterojunction indicates that the sample contains bothαandβphases with high crystallinity.The cut-off edge of visible light absorption can be tuned between 500 and 575 nm by the energy band overlap.The degradation rate of tetracycline can reach 87%in 90 min under visible light irradiation,which is higher than the degradation rates ofαandβphases.The effects of tetracycline concentration,photocatalyst dosage,p H and coexisting anions on the photocatalytic performance of the tetracycline degradation process were investigated.The photochemical studies verify the improvement of carriers separation and migration efficiency.Among them,the photocurrents of the mixed-phase heterojunction are 5 and 7 times higher than those ofβandαphases,respectively.In addition,their recyclability,oxidation actives,and photocatalytic mechanisms were investigated.（2）BaTiO₃/AgAlO₂ ferroelectric heterojunctions were constructed by sol-gel method combined with hydrothermal treatment to improve the photocatalytic performance of the samples using the polarization effect.The results of microstructure tests show that the BaTiO₃ particles are dispersed on the surface of lamellar AgAlO₂ with uniform size.The PFM test results show a large change in the local phase angle of the sample at 1.5 V,which confirms the polarization switching process.The butterfly-shifted voltage hysteresis confirms the local piezoelectric response originating from BaTiO₃.The UV-Vis absorption cut-off edge of the heterojunction is between 415 and 545 nm due to the overlap of energy bands of the two phases.The photocatalytic experimental results show that the photodegradation efficiency of the ferroelectric heterojunction for methyl blue is increased by 22%with the assistance of polarization induction by ultrasound.The photoelectrochemical analysis show that the polarization effect improves the separation efficiency of photogenerated carriers of the samples,especially the photocurrent intensity of BaTiO₃/AgAlO₂ is 4 and 6 times higher than that of BaTiO₃ and AgAlO₂,respectively.In addition,the photocurrent intensity of BaTiO₃/AgAlO₂ after ultrasonic polarization is 1.6 times higher than that before polarization,and piezoelectric polarization can effectively enhance the photogenerated carrier separation efficiency.（3）ZnO/AgAlO₂ heterojunctions were prepared using the solid-state reaction.The phase and microstructure analysis of the ZnO/AgAlO₂ heterojunction show that the high crystallinity pure ZnO and pure AgAlO₂ are combined with each other and uniformly distributed.The UV-Vis absorption edge of this heterojunction can be controlled between 390～440 nm by overlapping the energy bands of the two phases.In addition,the ZnO/AgAlO₂ heterojunction has good visible photocatalytic performance with 91.8%degradation of methyl blue in 90 min,which is 1.24 and1.37 times of the degradation rates of pure ZnO and pure AgAlO₂.The separation and transfer efficiencies of photogenerated electron-hole pairsare investigated by photoelectrochemical tests.Among them,the PL spectral intensity of ZnO/AgAlO₂heterojunction is much lower than that of pure ZnO and pure AgAlO₂,which proves that the heterojunction has a lower electron-hole recombination rate.In addition,the transient photocurrent and EIS spectra also verify the above results.