Controllable Preparation And Photocatalytic Performance Of Bismuth-Based Semiconductor Heterojunction Micro-/Nanostructures

Semiconductor photocatalytic technology is an effective,green and most promising way to solve both the problems of energy shortage and environmental pollution,and the most important part is the study of photocatalys.Among numerous semiconductor photocatalytic materials,bismuth-based semiconductor photocatalysts have became a very important new type of photocatalytic material after TiO2 due to their controllable structure,environmental friendliness,abundant raw materials,good photocatalytic performance and stability.Especially,the bismuth-based photocatalysts that belong to Sillen family usually show excellent photocatalytic performance because of their unique layered structure that enables efficient transfer and separation of photogenerated charge carriers between layers.However,some of these semiconductors,such as Bi2O2CO3 and BiOCOOH,have large band gap energy that leads to the low utilization of solar light and greatly limits their practical application.To solve these problems of Bi2O2CO3 and BiOCOOH,in this paper,the simple product synthesis routes were designed to modify Bi2O2CO3 and BiOCOOH through morphology control and heterostructure construction that can effectively enhance the absorption of visible light and reduce the recombination rate of photogenerated carriers,and thereby greatly improved their photocatalytic performance.The structure,composition,and optical properties of the as-prepared bismuth-based semiconductor photocatalysts were characterized and analyzed by SEM,TEM,HRTEM,XRD,XPS,and UV-Vis DRS.The formation mechanism of heterojunction micro-/nanostructures and the photocatalytic mechanisms were deeply studied.The main contents of this work are as follows:1.A flower-like hierarchical Bi-containing precursor was synthesized using a composite soft template,which was constructed from DL-aspartic acid and nonionic amphiphilic triblock copolymer F127.The morphology,phase structure,composition,effect of reactant concentration,and possible formation mechanism were systematically studied,and the effect of the composite soft template was elucidated.Then,the p-n type β-Bi2O3/Bi2O2CO3 heterojunction was constructed through controlling the calcination temperature to generate β-Bi2O3 in situ on the Bi2O2CO3 by using the flower-like hierarchical precursor as the self-sacrificial template,and the effect of calcination temperature on the morphology and composition of product was investigated.Under simulated sunlight irradiation,the as-prepared flower-like β-Bi2O3/Bi2O2CO3 heterojunction obtained after calcination at 290℃ showed excellent photocatalytic performance in degradation of tetracycline(TC)and rhodamine B(RhB)compared with pure Bi2O2CO3,β-Bi2O3 and heterojunctions obtained at other calcination temperatures.And it was attributed to the constructed p-n type heterojunction that enhanced the absorption of visible light,promoted the separation and transfer of photogenerated electron-hole pairs,and the flower-like hierarchical structure that endowed the material with large surface area and mesoporous structure.Furthermore,the recyclable reuse performance of the material was investigated.According to the experimental results of active species capture and the band structure of semiconductors,the photocatalytic mechanism based on the p-n type heterojunction structure was proposed.2.Although the p-n type heterojunction structure has many advantages,its carrier migration mechanism makes photocatalyst have low redox ability.The metal Bi nanoparticles also have the surface plasmon resonance(SPR)effect that is similar to the noble metals such as Au and Ag.The heterojunction structure constructed with metal can maintain the redox ability of the material.Moreover,Bi nanoparticles can be in situ reduced from bismuth-based semiconductor materials.Therefore,with the assistance of formamide,Bi/Bi2O2CO3 heterojunction nanosheets were obtained by in situ loading Bi on Bi2O2CO3 through a simple one-step hydrothermal method at high temperature(170℃).The effect of hydrothermal time on the morphology,phase and composition of the as-prepared materials were investigated.The results revealed that the initial in situ growth of Bi nanoparticles on BiOCOOH(an intermediate formed during the decomposition of formamide)was followed by the reaction of BiOCOOH with CO32-to afford Bi2O2CO3 nanosheets.Meanwhile,the content of Bi nanoparticles will increase at the same time,thus constructing Bi/Bi2O2CO3 heterojunction with different amount of Bi nanoparticles.In the process of preparation,formamide was used as carbon source and alkali source,and the formic acid produced by hydrolysis of formamide played the role of reductant.Under simulated sunlight irradiation,the proper amount of metal Bi nanoparticles could greatly improve the photocatalytic performance in degradation of RhB and TC,and it was ascribed to improvements of the visible-light harvesting and electron-hole separation efficiency that caused by the SPR effect of Bi nanoparticles.Furthermore,the recyclable reuse performance of the material was investigated.According to the experimental results of active species capture and the band structure of semiconductor,the photocatalytic mechanism based on the enhanced SPR effect of Bi nanoparticles was proposed.3.On the basis of the results of the second part,Z-scheme Bi/Bi2O2CO3/g-C3N4 ternary heteroj unction composites were prepared by a simple hydrothermal method at high temperature(170℃).Under simulated sunlight irradiation,the effect of loading amount of g-C3N4 on the photocatalytic performance in degradation of RhB and TC was also investigated.The results showed that the sample loaded with 20 wt.%g-C3N4 exhibited the best photocatalytic performance,and it was because that the SPR effect of Bi nanoparticles enhanced the absorption of visible light and quantum yield.Meanwhile,the direct Z-scheme charge carrier transfer mechanism between Bi2O2CO3 and g-C3N4 can greatly improve transfer and separation of the electron-hole pairs while maintaining the maximum redox ability of the composites.Furthermore,the recyclable reuse performance of the material was investigated.The active species in the photocatalytic process were determined by active species capture experiments,and the synergistic effect of direct Z-scheme heterojunction carrier transfer process and the SPR effect of Bi nanoparticles was proposed to elucidate the photocatalytic mechanism.4.The above studies showed that formamide can act as the carbon source and alkali source to obtain BiOCOOH.Moreover,the conduction band potential of BiOCOOH is more negative than that of Bi2O2CO3,which leads the stronger reduction ability of electrons on the conduction band.Therefore,a series of BiOCOOH and Bi/BiOCOOH heteroj unction micro-/nanostructures with various morphologies have been successfully synthesized by one-step hydrothermal process at 120℃ and changing the amount of formamide.The effects of the increase in the amount of formamide on the morphology,structure and composition of as-prepared materials were systematically studied.The results revealed that the formamide played multiple roles in the synthesis of the BiOCOOH and Bi/BiOCOOH heterojunctions.Under simulated sunlight irradiation,the well-defined BiOCOOH flower-like hierarchical structure showed higher photocatalytic performace in degradation of RhB and reduction of K2Cr2O7 than other BiOCOOH samples,which could be attributed to its larger specific surface area and stronger adsorption capacity.Meanwhile,suitable amount of Bi nanoparticles can also improve the photocatalytic activity of BiOCOOH,which was ascribed to the in situ reduced Bi nanoparticle that can enhance the absorbance of visible light and efficiently promote the separation and transfer of photogenerated electrons and holes.Furthermore,according to experimental results of active species capture and the band structure of semiconductor,the photocatalytic mechanism based on the synergistic effect of indirect dye photosensitization and SPR effect of Bi nanoparticles was also proposed5.The wide band gap of BiOCOOH limits its absorption of visible light and it is an effective way to optimize the band structure and promote the absorption of visible light by ion doping.Therefore,the BiOCOOH with flower-like hierarchical structure was used as precursor,and I-doped "heterojunction-like" BiOCOOH materials were obtained by a simple ion-exchange method.The UV-Vis DRS and Mott-Schottky test were used to investigate the effects of the amount of doped I-on the optical properties and the band structure of BiOCOOH.Under simulated sunlight and visible light irradiation,the photocatalytic performance in degradation of RhB by as-prepared samples were investigated,respectively.It was found that the the proper amount of I-doped "heteroj unction-like" BiOCOOH showed higher photocatalytic degradation activity compared with pure BiOCOOH、BiOI and P25.The introduction of I-maked the absorption of BiOCOOH extend to the visible region and form a series of "heterogeneous energy levels" that optimized the band structure of the BiOCOOH,and a proper amount of I-can be used as a hole trap to promote the effective separation of photogenerated carriers.Furthermore,the recyclable reuse performance of the material and the photocatalytic degradation pathway of RhB were investigated.The active species in the photocatalytic process were determined by active species capture experiments.The photocatalytic mechanism based on the optimization of "heterogeneous energy level" was proposed.