| Ever since the1970s, steadily worsening environmental pollution and energy shortages have raised awareness of a potential global crisis. Among the wide variety of green earth and renewable energy projects underway, semiconductor photocatalysis has emerged as one of the most promising technologies because it represents an easy way to utilize the energy of natural sunlight. Based on the problems of semiconductor photocatalytic technology that existed in practical application, this thesis took the semiconductor band engineering as the guiding ideology to design and synthesize efficient visible-light photocatalysts. On the one hand, we used the charge compensation effect in the donor-acceptor pairs to control the band structures of wide-bandgap semiconductors. The microscopic mechanism of tailoring the band structure by codoping and the relationship between codoping atoms, codoping forms and photocatalytic activities were studied by first-principles density function theory calculations. On the other hand, we fabricated heterojunction photocatalysts by combining two different semicondutors and investigated the microscopic mechanisms of interface interaction and interface carriers transfer by a combination of theoretical calculations and experimental techniques. Based on this, the influences of effective hetero-interfaces and transfers of separated electrons and holes on the photocatalytic activity of heterojunctions were also studied to provide theoretical guidance for the design of new efficient visible-light photocatalysts. The main researches are listed as follows:The first chapter introduced the research background of this thesis, including the development and research status of semiconductor photocatalytic technology and semiconductor band engineering, and the applications of codoping and fabricating heterojunctions in improving the photocatalytic activity of semiconductors. In this chapter, it put forward revealing the relationship between codoping atoms, codoping forms and photocatalytic activities from the micro level, the microscopic mechanisms of interface interaction and interface carriers transfer, and the influences of effective hetero-interfaces and transfers of separated electrons and holes on the photocatalytic activity of heterojunctions. In addition, the research ideas and content of this thesis were briefly introduced.In the second chapter, we have introduced the basic theoretical methods of density functional theory. The main line was the development of exchange-correlation functional, including Local Density Approximation (LDA), Generalized Gradient Approximation (GGA) and self-consistent field theory. Then, the software packages used in this thesis were introduced.Part I contained the chapter3,4and5. This part took the band-gap graphics engineering as the guiding ideology. The charge compensation effect in the donor-acceptor pairs was used to passivate the partially occupied states in the monodoping systems, thus improving the visible-light photocatalytic activity of semiconductors.In the third chapter, we performed first-principles density function theory calculations to study the geometric and electronic structures and photoactivity of C, N, and F monodoped and pairwise codoped ZnWO4. The photon transition energy could be decreased to varying degrees by momodoping, while the partially occupied states in induced by the impurity were located in the gap, which may act as recombination centers and weaken the photocatalytic activity. By analyzing the defect wave function character, we proposed several pairwise codoped ZnW04systems, such as CS+2FS-, Ns+Fs-, Ci+2NS-, and Ni+Fs-codoped ZnW04, to passivate the partially occupied states in the monodoping systems by the charge compensation effect in the donor-acceptor pairs, resulting in occupied states in the gap and reducing the formation energy compared with the monodoping systems. All of these four codoping forms can decrease the transition energy to some extent, and the Ci+2Ns and Ni+Fs codoping in the ZnWO4can red shift the transition energy of photoexcited electrons to the ideal visible-light region.In the fourth chapter, the electronic properties of mono N-and F-doped and (N, F)-codoped ZnW04(010) surfaces were studied by means of first-principles calculations. In the monodoping surfaces, the N and F doping got unsatisfactory redshifting of absorption edge and introduced partially occupied states in the band gap, which would act as recombination centers. The electronic structures of (N,F)-codoped ZnWO4(010) surfaces showed that the related partially occupied defect bands in the monodoped surfaces were passivated by the synergetic effect of N-F recombination. However, the details of compensation mechanism and effects of enhancing photoactivity for these four codoped surfaces were different from each other:in the NsFs-codoped surface, the electron on the W5d1state formed by the single Fs donor passivated the hole on the single Ns acceptor; in the NadFs-codoped surface, the Fs acted as a single donor and the extra electron on the W5d1state passivated the hole on the NadObπ*state; in the NsFad-codoped surface, the Ns-Os species, acting as a single donor, transfered the electron on a*to Fad impurity to fill its2p orbital; in the NadFad-codoped surface, the Fad impurity as a single acceptor obtained the electron from Nad-Ob Nad-Ob π*orbital to passivate the hole on its2p states. All these four codoping forms can decrease the transition energy of ZnW04(010) surface to some extent, and the NadFs and NadFad codoping can red shift the absorption edge to visible-light region. Due to the electrons only transferring between the Nad-Ob π*states, the NadFad codoping had little sense to photocatalytic process of ZnWO4(010) surface.In the fifth chapter, the interaction between implanted La, substitutional N, and an oxygen vacancy at TiO2anatase (101) surface was investigated by means of first-principles density function theory calculations. Our calculations suggested that both the adsorptive and substitutional La-doped TiO2(101) surfaces were probably defective configurations in experiments. The h-Cave-adsorbed La doping decreased the formation energy for the substitutional N implantation and vice versa, while the charge compensation effects did not take effect between the adsorptive La and substitutional N dopants, resulting in some partially occupied states in the band gap acting as traps of the photoexcited electrons. The Ti5c-substituted La doping decreased the energy required for the substitutional N implantation, and the substitutional La and N codoping promoted the formation of an oxygen vacancy, which migrated from the Osb-3c site at the inner layer toward the surface Ob site. For the substitutional La/N-codoped (Ti55c_O3c-down) surface, the charge compensation between the substitutional La and substitutional N led to the formation of two isolated occupied Ns-O π*impurity levels in the gap. After further considering an oxygen vacancy on the Ti5c_O3c-down surface, the two electrons on the double donor levels (Ob vacancy) passivated the same amount of holes on the acceptor levels (substitutional La and N), forming the acceptor-donor-acceptor compensation pair, which provided a reasonable mechanism for the enhanced visible-light photocatalytic activity of La/N codoped TiO2anatase.Part Ⅱ contained the chapter6,7and8. This part took the band structure engineering as the guiding ideology. Based on the lattice match and band match, the narrow-band-gap semiconductors were composited with the wide-band-gap semicondutors to fabricate the heterojunction photocatalysts. The visible-light response of heterojunction photocatalysts was achieved by the narrow-band-gap semiconductors, and the excited electrons transfered from the narrow-band-gap semiconductors into the attached wide-band-gap semiconductors in the case of proper conduction band potentials, which favored the separation of photoinduced electrons and holes and thus improved the visible-light photocatalytic efficiency of semiconductor heterojunctions dramatically.In the sixth chapter, we presented a systematic investigation of the microscopic mechanism of interface interaction, charge transfer and separation, as well as their influence on the photocatalytic activity of heterojunctions by a combination of theoretical calculations and experimental techniques for the g-C3N4ZnWO4composite. HRTEM results and DFT calculations mutually validated each other to indicate the reasonable existence of g-C3N4(001)/ZnWO4(010) and g-C3N4(001)/ZnWO4(011) interfaces. The g-C3N4/ZnWO4heterojunctions showed higher photocatalytic activity for degradation of MB than pure g-C3N4and ZnW04under visible-light irradiation. Moreover, the heterojunctions significantly enhanced the oxidation of phenol in contrast to pure g-C3N4, the phenol oxidation capacity of which was weak, clearly demonstrating a synergistic effect between g-C3N4and ZnWO4. Based on the theoretical calculations, we found that electrons in the upper valence band can be directly excited from g-C3N4to the W5d orbital of ZnWO4, under visible-light irradiation, which should yield well-separated electron-hole pairs.In the seventh chapter, efficient g-C3N4/Zn2Ge04photocatalysts with effective interfaces were designed by controlling the surface charges of the two individual materials inside the same aqueous dispersion medium, making use of the electrostatic attraction between oppositely charged particles. The g-C3N4/Zn2Ge04heterojunction with opposite surface charge (OSC) showed higher visible-light photocatalytic activity for degradation of methylene blue than those of pure g-C3N4, pure Zn2GeO4, and the g-C3N4/Zn2GeO4with identical surface charge (ISC). The investigation of the light absorption spectrum, adsorption ability, and photocurrent responses revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the OSC g-C3N4/Zn2Ge04sample’s photocatalytic activity. By combining with theoretical calculations, the microscopic mechanisms of interface charge transfer was that the photogenerated electrons in the g-C3N4N2p states directly excited into the Zn4s and Ge4s hybrid states of Zn2GeO4.In the eighth chapter, we designed and synthesized two models of BiOI/BiOCl heterojunction photocatalysts with different interfaces, denoted as BiOI(001)/BiOCl(001) and BiOI(001/BiOCl(010), via the combination of heterojunction nanostructure construction and crystal facet engineering. Due to the formation of heterojunctions that can significantly reduce the recombination and speed up the separation rate of photogenerated charge carriers, both of the BiOI(001/BiOCl(001) and BiOI(001)/BiOCl(010) heterojunctions were photocatalytically more active than the three individual components. Though BiOI(001)/BiOCl(001) had the better lattice match, the visible-light photocatalytic activity of BiOI(001)/BiOCl(010) was superior to that of BiOI(001)/BiOCl(001) heterojunctions. The BiOI(001)/BiOCl(001) showed the same ηsep with the BiOI(001)/BiOCl(010), because of the similar band-match situations. While, since the self-induced internal electric fields of BiOCl slabs in BiOI(001)/BiOCl(001) and BiOI(001/BiOCl(010) heterojunctions were perpendicular and parallel to these two heterojunctions, respectively, the ηinj of BiOI(001)/BiOCl(010) was higher than that of BiOI(001)/BiOCl(001) by optimizing the separated electrons transfer pathway. This was the main factor answering for the higher visible-light photocatalytic activity of BiOI(001/BiOCl(010)heterojunction.In the last chapter, we summarized the conclusions and innovative points of this dissertation, and preview the further studies. |
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