| Energy crisis and environmental pollution have threatened the sustainable development of society being. Semiconductor photocatalysis has attracted great interest because it can utilize the energy of natural sunlight to solve energy supply and environmental pollution problems. Heterojunction photocatalysts have potential application in the photocatalysis field because it can improve the low quantum efficiency and poor visible light response of single semiconductor photocatalyst. The successful separation and migration of photogenerated charge carriers in the interfaces of heterojunctions determine the activity of heterojunction photocatalysts, while the structure and property of heterojunction interfaces is a key factor that influence the charge transport behavior. Therefore, enhanced photocatalytic performance can be achieved by reasonably and effectively controlling the surface/interface of heterojunction photocatalysts. Our work was performed around the surface/interface control, aiming at constructing efficient heterojunction photocatalysts. Firstly, efficient heterojunction photocatalyst with large interface area was prepared by controlling the surface charge of semiconductors and the effect of dispersants on the effective interface area and charge transport in heterojunction interfaces was invetigated. Secondly, efficient phase junction photocatalyst was prepared by introducing graphic carbon in the phase interface and the effect of carbon-modification on charge separation was invetigated. Finally, efficient heterojunction photocatalyst was prepared by controlling the crystal facet orientation of polar semiconductors and the effect of polar fields on seperated charge’transport was invetigated. The main researches are listed as follows:The first chapter mainly introduced the research background and significance of our work, including the principle, the main questions and solving strategy of semiconductor photocatalysis, and the present situation and deficiency of the study on the surface/interface control of heterojunction photocatalysts. On this basis, we briefly introduced the research ideas and content of this thesis.In the second chapter, efficient BiOI/TiO2 p-n junction photocatalysts with large interface area successfully prepared via surface charge control. The results showed that the BiOI nanosheets are uniformly covered with TiO2 nanosheets, making use of the electrostatic attraction between oppositely charged particles. The photocatalytic experimental results showed that, BiOI/TiO2 heterojunctions obtained in formic acid (BiOI/TiO2-1) and acetic acid (BiOI/TiO2-2) displayed higher visible-light photocatalytic activity than the sample obtained in water (BiOI/TiO2-O) towards the degradation of MO and phenol solutions, while BiOI/TiO2 heterojunction obtained in APTES solutions (BiOI/TiO2-3) have poor photocatalytic performance. First-principles calculations are used to obtain the work function of BiOI (001) facet and TiO2 (001) facet, and the bandalignment between the two constituent samples was investigate. The Fermi level of BiOI is lower than that of TiO2, because the work function of BiOI is larger. When the p-n junction was constructed, the CBM potential of BiOI is more active than that of TiO2, the photogenerated electrons would easily migrate from BiOI to the CB of TiO2, leaving the holes on the BiOI valence band. Moreover, the built-in electric field could further facilitate the separation of the electron-hole pairs. The investigation of the photocurrent responses revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the BiOI/TiO2-1 and BiOI/TiO2-2 sample’s photocatalytic activity, which benefitted from more effective interfaces. However, separation of photogenerated carriers is low in BiOI/TiO2-1 sample, which indicating that APTES adversely affects the formation of BiOI/TiO2 p-n junction, though it can controls the surface charge of two constituent samples and make TiO2 nanosheets prefer to distribute uniformly on the BiOI surfaces. Therefore, efficient p-n junctions with large interface area can be successfully constructed via surface charge control with appropriate dispersants, which can facilitate uniform composite and the formation of p-n junctions. Our work is beneficial to design and develop more effcient p-n junction photocatalysts.In the third chapter, the charge seperation and photocatalytic activity of carbon-modified SnO2 phase junction photocatalyst were investigated. The tetragonal phase SnO2 (t-SnO2) and mixed phases SnO2 (to-SnO2) nanorods were obtained by calcinations of SnC2O4 synthesized with a chemical precipitation method, then the carbon-modified SnO2 samples (t-SnO2-C and to-SnO2-C) were prepared by solvothermal method combined with a post-calcination under a nitrogen atmosphere. The photocatalytic experimental results showed that, to-SnO2 sample displayed slightly higher photocatalytic activity than t-SnO2 sample, and the reaction rate constant of to-SnO2 is 1.4 times larger than t-SnO2 sample in MO photocatalytic degradation experiment. In particular, the photocatalytic activity of to-SnO2-C sample was significantly improved by the introduction of graphic carbon. The reaction rate constant of to-SnO2-C sample was 3.4 times and 3 times larger than that of t-SnOo-C and to-SnO2 samples in MO photocatalytic degradation experiment. Photoelectrochemical measurements indicated that the enhancement of photoactivity over to-SnO2 was attributed to the improved the charge separation. The investigation of photocurrent responses and electrochemical impedance spectroscopy revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the to-SnO2-C sample’ photocatalytic activity, indicating that the introduction of graphic carbon facilitated the transport and seperation of photogenerated charges in to-SnO2-C sample. First-principles calculations are used to investigate band alignment between the tetragonal SnO2 and orthorhombic SnO2 in to-SnO2-C sample, the CBM potential of t-SnO2 is more active than that of o-SnO2, the photogenerated electrons would easily migrate from t-SnO2 to the CB of o-SnO2, leaving the holes on the o-SnO2 valence band. Moreover, the introduction of graphic carbon could further facilitate the separation of the electron-hole pairs. The formation of tetragonal-orthorhombic phase junction could improve the separation of the electron-hole pairs and the outstanding electroconductivity of graphic carbon could facilitate the transport and separation of photogenerated charges in the interfaces. Our work is beneficial to design and develop more effcient phase junction photocatalysts.In the fourth chapter, photocatalysts with heterojunctions are constructed by loading g-C3N4 nanoparticles onto BiOCl nanosheets with different exposed facets (BOC-001 and BOC-010). The g-C3N4 nanoparticles with decreasing size and increasing zeta potential could induce stronger coupling and scattering in the heterojunction. The relationship between the crystal facet orientation in the BiOCl nanosheets and charge separation/effective migration behaviours of the materials is investigated. The visible light photocatalytic activity of the composites is evaluated by methyl orange (MO) and phenol degradation experiments, and the results show that ng-CN/BOC-010 heterojunction exhibit higher photocatalytic performance than that of ng-CN/BOC-001 heterojunction. Both photoelectrochemical and fluorescence emission measurements indicate that the different exposed facets in ng-CN/BiOCl heterojunctions could induce the migration of the photogenerated electrons in different ways, but do not significantly alter the separation effciencies. The separated electrons in ng-CN/BOC-010 heterojunction undergo a shorter transport distance than that of ng-CN/BOC-001 heterojunction to reach the surface reactive sites. Based on the calculated results, the effective mass of electrons along the [001] and [010] directions is about 0.576m0 and 1.053mo (mo is free-electron mass), respectively, indicating a higher mobility of electrons along the [001] direction. This means that the transfer of separated electrons is within the BOC-001 bulk in ng-CN/BOC-001 heterojunction, but along the BOC-010 surface in ng-CN/BOC-010 heterojunction. Our study may suggest that the crystal facet orientation in polar semiconductors is a critical factor for designing highly efficient heterojunction photocatalysts.In the last chapter, we summarized the conclusions and innovative points of this dissertation, and preview the further studies. |
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