【作者】 孙立鸣；

【导师】 赵显； 樊唯镏；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 环境问题和能源问题是21世纪人类可持续发展面临的两大挑战。能利用洁净太阳能资源的半导体光催化技术成为应对这两大挑战的重要手段之一。本论文针对半导体光催化技术实际研究中存在的问题,以半导体能带工程为指导思想,以设计及合成高效可见光催化材料为研究目标,采用理论和实验相结合的研究手段,一方面利用共掺杂对间的电荷补偿效应对宽禁带半导体光催化材料进行改性修饰,探索共掺杂对宽禁带半导体材料能带结构剪裁的微观机制,揭示共掺杂原子种类、掺杂形式及光催化活性间的构效关系；另一方面,通过两种半导体材料的复合构筑异质结光催化材料,揭示界面相互作用及界面光生载流子迁移的微观机制,探讨有效异质界面及分离后的光生电子和空穴的迁移对材料光催化活性的影响,为新型高效可见光催化材料的设计及合成提供理论指导。主要研究内容及结果如下：第一章绪论介绍了半导体光催化技术的发展及研究现状,及以半导体能带工程中带隙图形工程为指导思想的掺杂改性手段和以半导体能带工程中的能带结构工程为指导思想的构筑异质结手段在改善半导体光催化材料活性及探索新型可见光催化材料中的应用,提出从微观层面上揭示共掺杂原子种类、掺杂形式与光催化活性间的构效关系,以及异质界面相互作用及界面光生载流子迁移机制,探索有效异质界面及分离后光生电子和空穴迁移对异质结光催化材料活性的影响,并简要介绍了本论文的研究内容。第二章简要介绍了密度泛函理论的基本理论方法及其发展和应用,以交换相关能量泛函的发展为主线,介绍了局域密度近似、广义梯度近似等泛函形式以及自洽场计算流程,并在此基础上,介绍了本论文使用的计算软件包。上篇中包含第三章、第四章和第五章三章研究内容。本部分以半导体能带工程中的带隙图形工程为指导思想,通过两种或两种以上的共掺杂异质原子间的电荷补偿效应钝化体系带隙中因单一掺杂引起的半充满杂质态,在减小半导体带隙的同时,消除因掺杂产生的电子-空穴复合中心,从而提高半导体材料的可见光催化活性。第三章中,我们采用密度泛函理论计算的方法,研究了C、N和F单一掺杂和两两共掺杂ZnW04体块的几何结构、电子结构及光活性。虽然单一掺杂能在一定程度上减小体系的电子激发能,但是同时会在体系的带隙中引入半充满的杂质态,这些杂质态会成为光生电子-空穴对的复合中心,不利于光催化活性的提升。基于缺陷波函数特征的分析,我们设计了Cs+2Fs、Ns+Fs、Ci+2Ns和Ni+Fs四种共掺杂对,利用施主-受主对间的电荷补偿效应钝化带隙中由单一掺杂引起的半充满杂质态,同时还能减小ZnWO4共掺杂体系的形成能。这四种共掺杂对均在一定程度上减小了ZnWO4体系的电子激发能,其中Ci+2NS和Ni+Fs共掺杂成功将ZnWO4的光吸收边红移至可见光区。第四章中,我们采用密度泛函理论计算的方法,研究了N和F单一掺杂以及N,F)共掺杂ZnWO4(010)表面的电子性质。N和F的单一掺杂并不能满足体系吸收边红移的需求,并且会在带隙中引入半充满的杂质态,形成光生电子-空穴对的复合中心。N,F)共掺杂ZnW04(010)表面的电子结构显示,N-F共掺杂对间的协同效应会钝化体系中因单一掺杂引起的半充满杂质态,但是,不同的共掺杂形式对应着不同的电荷补偿机制,对体系光活性的影响也不同。四种共掺杂形式对应的电荷补偿机制分别为：在NsFs共掺杂ZnWO4(010)表面中,单施主Fs形成的W5d’态上的电子钝化单受主Ns上的空穴；在NadFs共掺杂ZnWO4(010)表面中,单施主Fs形成的W5d’态上的电子钝化NadOb π*反键轨道上的空穴；在NsFad共掺杂ZnWO4(010)表面中,Ns-Ob作为单施主,其σ*反键上的电子迁移至Fad,填充Fad2p轨道；在NadFad共掺杂表面中,Fad作为单受主从Nad-Ob π*反键轨道上捕获电子填充其2p轨道。这四种共掺杂形式均能减小ZnWO4(010)表面的电子激发能,其中NadFs和NadFad共掺杂能将ZnWO4(010)表面的光吸收边红移至可见光区。但是由于NadFad共掺杂表面中的电子仅在Nad-Ob π*反键轨道间迁移,这使得其对改善ZnWO4(010)表面光催化活性的实际意义不大。第五章中,我们采用密度泛函理论计算的方法,研究了锐钛矿相TiO2(101)表面的掺杂La、取代掺杂N与一个氧空位之间的相互作用。计算结果显示,La吸附掺杂和取代掺杂的Ti02(101)表面都是实验上可能存在的缺陷构型。La在h-Cave位点的吸附掺杂能促进N的取代掺杂,反之亦然,然而电荷补偿效应在吸附掺杂La与取代掺杂N之间并未生效,这导致在带隙中引入半充满的杂质态。取代Ti5c的La掺杂能促进N的取代掺杂,且La和N的取代共掺杂会促进氧空位的形成,所形成的氧空位从内层Osb-3c位点迁移至表面Ob位点。在La和N分别取代Ti5c和O3c-down的共掺杂表面,取代La和取代N间的电荷补偿在带隙中形成两条孤立的占据Ns-O π*反键杂质能级。进一步考虑在该共掺杂表面上有氧空位存在的情况,氧空位作为双施主,取代La和取代N分别作为单受主,形成受主-施主-受主补偿对,双施主能级的两个电子分别钝化两个单受主能级上的空穴,形成与价带连续的占据态,使得体系带隙减小了0.17eV,与实验测试的数据吻合,这为实验上La/N共掺杂锐钛矿相Ti02增强其可见光催化活性提供了一个合理的微观机制解释。下篇中包含第六章、第七章和第八章三章研究内容。本部分以半导体能带工程中的能带结构工程为指导思想,基于晶格匹配和能带匹配原理,将窄带隙半导体与宽禁带半导体复合构筑异质结材料,利用窄带隙半导体吸收可见光,同时利用两半导体间的能带势差促进光生电子-空穴对的分离,从而获得高效的可见光催化材料。第六章中,我们采用理论与实验相结合的方法,系统研究了g-C3N4/ZnWO4异质结中的界面相互作用、电荷迁移及分离的微观机制,以及其对体系光催化活性的影响。高分辨透射电子显微镜(HRTEM)和密度泛函理论(DFT),计算的结果相互验证,表明体系中g-C3N4(001)/ZnWO4(010)和g-C3N4(001)/ZnWO4(011)界面存在的合理性。g-C3N4/ZnWO4异质结可见光下降解甲基蓝的活性优于纯相g-C3N4和ZnWO4。此外,与纯相g-C3N4对苯酚微弱的氧化能力相比,异质结对苯酚的氧化能力明显增强,表明g-C3N4和ZnWO4间存在协同效应。在可见光照射下,g-C3N4/ZnWO4异质结的电子微观迁移路径为,异质结价带顶的电子直接由g-C3N4跃迁至ZnWO4导带底的W5d轨道,从而实现光生电子和空穴的分离。第七章中,我们通过调控两种单体材料在同一分散溶液中的表面电荷,利用带相反电荷的颗粒间的静电吸引力,设计并制备了具有高有效异质界面率的高效g-C3N4/Zn2GeO4异质结光催化材料。单体材料带相反表面电荷(OSC)的g-C3N4/Zn2GeO4异质结在可见光下对亚甲基蓝的降解活性优于纯g-C3N4、纯Zn2Ge04和单体材料带相同表面电荷(ISC)的g-C3N4/Zn2Ge04异质结。所制备样品的光吸收、吸附能力和光电流响应的测试结果表明,OSC g-C3N4/Zn2Ge04异质结光催化材料的高活性主要是由于其有效异质界面率高,能显著提高光生电子-空穴对的分离率。我们的理论计算揭示了g-C3N4/Zn2Ge04异质结可见光下的电子微观迁移路径：g-C3N4的N2p态上的电子受激发直接跃迁至Zn2Ge04的Zn4s和Ge4s杂化轨道上。第八章中,我们通过异质结纳米结构构筑与晶面工程相结合的方法,设计并合成了两种不同界面组成的BiOI/BiOCl异质结光催化剂,分别为BiOI(001)/BiOCl(001)和BiOI(001)/BiOCl(010)异质结。BiOI(001/BiOCl(001)和BiOI(001/BiOCl(010)异质结的可见光催化活性均高于BiOCl和BiOI单体材料。这是由于形成异质结能显著减弱光生载流子复合,提高光生载流子的分离效率。虽然BiOI(001)/BiOCl(001)具有更高的晶格匹配度,但是BiOI(001/BiOCl(010)的可见光催化活性优于BiOI(001)/BiOCl(001)。由于BiOI(001)/BiOCl(001)和BiOI(001)/BiOCl(010)异质结的能带匹配情况相似,所以二者的ηsep相同。然而,因为BiOCl的自发内电场平行于BiOI(001)/BiOCl(010)异质结,优化了分离后的光生电子的迁移路径,从而使得BiOI(001)/BiOCl(010)异质结的ηinj高于BiOI(001)/BiOCl(001)异质结。这是BiOI(001)/BiOCl(010)异质结具有更优可见光催化活性的主要原因。第九章对本论文进行了总结,归纳了创新点,并对今后拟开展的研究工作进行了展望。更多还原

【Abstract】 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.更多还原