基于半导体异质结构的光催化剂设计、合成及性质研究

发布时间：2018-03-01 00:23

本文关键词： 半导体异质结金属-到-配体的电荷转移跃迁 Z-型体系太阳能捕获助催化剂　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：能源与环境问题是当今全球范围关注的话题,开发新型环保可再生能源成为维持人类可持续发展的重要课题。太阳能和氢能被认为是21世纪最清洁的能源。光催化分解水过程是一种可以直接将太阳辐射能转化为氢能的途径。其中利用半导体实现光催化分解水是目前研究的热门之一,半导体在光催化中的反应过程包括:(i)吸收太阳光产生电子空穴对;(ii)光生电子空穴对分离并迁移至半导体表面;(iii)分解水产生氢气和氧气。但是单一的半导体在光催化分解水过程中吸收太阳光的能力是有限的,并且电子空穴对分离效率和电荷迁移率低,导致其无法获得高效率的光催化产氢效率。为了解决以上问题,本论文提出以下四种设计方案:1.为了拓宽半导体g-C_3N_4的吸收光谱范围,我们提出了将有机半导体g-C_3N_4作为配体与金属Pt形成配位结构,利用配位结构中金属-到-配体的电荷转移跃迁过程(MLCT)拓宽有机半导体g-C_3N_4的吸光范围,从而提高其光催化产氢性能。有机半导体g-C_3N_4光催化效率受到两方面因素的制约:(1)g-C_3N_4往往具有较宽的能带带隙(2.7eV),使其只能吸收波长小于460 nm的紫外光等短波太阳光;(2)高电子空穴复合率和低载流子迁移速率限制了 g-C_3N_4中光生电子的高效利用。为了更好地将g-C_3N_4应用于光催化分解水领域,需要从拓宽有机半导体g-C_3N_4吸收光谱范围和提高其载流子利用率两方面着手。我们通过实验证实,利用电子从金属离子Pt~(2+)到有机半导体g-C_3N_4之间的电荷转移跃迁(MLCT)可以很好地解决这两个问题。MLCT一般发生在配合物中的激发状态,它所需要的激发能量低于最高已占轨道(HOMO)-到-最低未占轨道(LUMO)的带间跃迁能量,弥补了有机半导体g-C_3N_4无法吸收长波长太阳光的不足,扩宽了吸收光谱。通过实验结果和理论模拟证明,HOMO-到-LUMO跃迁和MLCT跃迁能够在N共轭芳香单元相对独立地工作,避免了光催化过程中电子长距离转移所带来的电子空穴复合提高了光催化性能。2.针对半导体Ag_2S和TiO_2能带结构无法匹配的问题,我们提出在其界面引入痕量的金属Ag,抬高Ag_2S能带,从而形成Ag_2S-(Ag)-TiO_2 Z型结构体系。N-型半导体TiO_2具有稳定的高催化活性,但是较宽的能带带隙(3.2 eV)限制其只能在紫外区进行光吸收。Ag_2S是一种窄带隙(1.0eV)N-型半导体,能够吸收近红外区和可见区太阳光,因此Ag_2S与TiO_2形成复合结构在紫外至近红外区都具有光吸收。然而该两种半导体平行的能带结构却阻碍了 Z型结构体系的形成。为了解决该科学问题,我们在Ag_2S和TiO_2的界面上引入了痕量的金属Ag,实现了 Ag_2S能带的抬高,从而形成Ag_2S-(Ag)-TiO_2 Z型结构体系。这种Ag_2S-(Ag)-TiO2三元结构在全谱光照条件下表现出大幅度提高光催化制备氢气的性能。在光催化剂设计中,精准控制界面上的Ag层厚度是其难点。针对该挑战,我们设计了一种"生长-硫化"路线,首先在Ag纳米晶体表面生长TiO_2壳层,进而在TiO_2的壳层保护作用下对内核Ag单质完成硫化。通过Ag单质与TiO_2的强化学键合作用,避免界面上的单质Ag完全硫化,形成了独特的Ag_2S-(Ag)-TiO_2三元结构。3.针对表面等离激元金属热电子无法有效注入半导体并参与反应的问题,我们提出了利用TiO_2晶面异质结与助催化剂的协同作用,以提高金属热电子的有效利用。金属与半导体接触后会形成肖特基结,其电荷转移方向与等离激元金属热电子注入方向是相反的。同时等离激元金属热电子注入半导体时,缺乏热电子高效迁移的驱动力。为此,我们首先利用TiO_2纳米片晶面异质结作为等离激元金属热电子注入半导体过程的驱动力。半导体TiO_2纳米片由{001}和{101}两种晶面组成,我们的实验数据表明在晶面异质结作用诱导下的Au-TiO_2{001}样品具有更好的电荷分离能力。我们进一步在Au-TiO_2{001}体系中引入助催化剂Pt,最终获得了能够在全谱光照下高效产氢的Au-TiO_2{001}-Pt催化剂。4.针对TiO_2/石墨烯复体系中TiO_2上电子需要穿越其自身厚度达到石墨烯表面参加反应的问题,我们提出合成TiO_2纳米薄片减少电子消耗的方案,提高了光催化产氢效率。通常采用的TiO_2纳米线或者纳米球与石墨烯接触,不利于其界面上电子-空穴的分离。同时TiO_2上电子要穿过自身厚度到达与石墨烯结合的界面,在此过程中会有部分电子损耗。我们合成出TiO_2纳米薄片不但可以降低电子迁移过程中的损耗,同时也增加了 TiO_2纳米片与石墨烯接触的界面,最终提高TiO_2纳米片/石墨烯复合结构光催化产氢性能。我们系统研究了光生电子在TiO_2纳米片中以及TiO_2纳米片与石墨烯复合界面上的迁移路径。在此基础上,我们进行了 TiO_2纳米片/石墨烯复合结构上Pt助催化剂的集成,讨论了 Pt存在位置对TiO_2/石墨烯-Pt体系光催化性能的影响。
[Abstract]:The problem of energy and environment is a global concern, the development of new environmentally friendly renewable energy has become an important issue to maintain sustainable development. Solar energy and hydrogen energy twenty-first Century is considered the most clean energy. The photocatalytic decomposition of water is a kind of direct solar radiation energy into hydrogen way. Which use semiconductor photocatalytic implementation the decomposition of water is one of the most popular research at present. The reaction process of semiconductor photocatalysis including: (I) the absorption of solar light electron hole (II); photoinduced electron hole separation and moved to the surface of the semiconductor; (III) the decomposition of water to produce hydrogen and oxygen. But a single semiconductor is limited in the light the catalytic decomposition ability in the process of water absorption of sunlight, and the separation efficiency of electron hole pairs and charge transfer rate is low, resulting in its inability to obtain high efficiency for photocatalytic hydrogen production efficiency. To solve the above problems, this paper puts forward the following four schemes: 1. in order to widen the range of absorption spectra of semiconductor g-C_3N_4, we put forward the organic semiconductor g-C_3N_4 as ligand and metal Pt to form the coordination structure, the use of metal - ligand structure to ligand charge transfer transitions (MLCT) to broaden the range of organic light absorption the semiconductor g-C_3N_4, so as to improve its photocatalytic performance. The efficiency of organic semiconductor photocatalytic g-C_3N_4 is restricted by two factors: (1) g-C_3N_4 has wide band gap (2.7eV), ultraviolet shortwave solar so that it can only absorb wavelength less than 460 nm; (2) high electron hole composite rate and low carrier mobility rate limits the utilizing efficiency of photogenerated electrons in g-C_3N_4. In order to apply g-C_3N_4 in the photocatalytic decomposition of water absorption from the field, need to broaden the organic semiconductor g-C_3N_4 The scope and improve the utilization rate of the carrier in two aspects. We experimentally confirmed that the use of electrons from the metal ion Pt~ (2+) to the charge transfer between the organic semiconductor g-C_3N_4 (MLCT) can solve these two problems commonly occur in.MLCT complexes in the excited state, it needs the excitation energy below the highest occupied orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the interband transition energy, to make up for the lack of organic semiconductor g-C_3N_4 can not absorb the long wavelength of sunlight, broaden the absorption spectrum. The experimental results and theoretical simulation proved that the HOMO- to -LUMO transition and MLCT transition to the N conjugated aromatic unit relatively independently, to avoid the photocatalytic process of electronic long distance transfer caused by electron hole recombination and improve the photocatalytic performance of.2. for Ag_2S and TiO_2 semiconductor band structure can not match the problem, I The introduction of trace metal Ag in its interface, elevation Ag_2S band, thus forming Ag_2S- (Ag) -TiO_2 Z type structure of.N- type semiconductor TiO_2 with high catalytic activity and stability, but the wide band gap (3.2 eV) can only limit the light absorption of.Ag_2S is a narrow gap in the purple N- type semiconductor region (1.0eV), can absorb near-infrared and visible region of sunlight, so Ag_2S and TiO_2 to form a composite structure in the ultraviolet to the near infrared region has light absorption. However, the two parallel semiconductor band structure has hindered the formation of Z type structure system. In order to solve the problem of science. We introduce the metal Ag trace in the Ag_2S and TiO_2 interface, realize the elevation of the Ag_2S band, resulting in the formation of Ag_2S- (Ag) -TiO_2 Z type structure. The Ag_2S- (Ag) -TiO2 three element structure in full spectrum light conditions exhibit greatly improved photocatalytic The preparation of hydrogen performance. In the light of catalyst design, precise control of Ag layer thickness on the interface is difficult. To solve these challenges, we design a "growth - vulcanization" line, first growth of TiO_2 shell on the surface of Ag nanocrystals, and then complete the vulcanization on kernel Ag elemental in the shell under the protection of TiO_2 through the strong chemical bonds. Ag and TiO_2, to avoid the interface of elemental Ag completely curing, forming a unique Ag_2S- (Ag) -TiO_2 three element structure.3. for surface plasmon metal hot electron injection semiconductor can not effectively and participate in response to the problem, we propose a synergistic effect with TiO_2 crystal surface heterogeneity with the catalyst, in order to improve the effective utilization of metal hot electrons. Metal and semiconductor contact will form a Schottky junction, the direction of charge transfer and plasmonic metal hot electron injection is the opposite direction. At the same time, plasmon Metal hot electron injection semiconductor, lack of driving force of hot electron migration efficiently. To this end, we first use the TiO_2 nanosheets as heterojunction surface plasmon metal hot electron injection semiconductor process drive. Semiconductor nano TiO_2 is composed of {001} and {101} two kinds of crystal surface, our experimental data indicated that the charge in the induction of crystal heterostructures under the action of Au-TiO_2{001} sample has better separation ability. We further introduce the promoter of Pt in Au-TiO_2{001} system, obtained Au-TiO_2{001} -Pt.4. catalyst to hydrogen production in full spectrum light for TiO_2/ graphene complex system TiO_2 through its own electronic thickness reaches the surface of graphite in response to the problem of graphene, we proposed the synthesis of TiO_2 nano flakes to reduce the consumption of electronic solutions, improve the photocatalytic efficiency. The nano TiO_2 Noodle or nanoparticles and graphene contact, not conducive to the interface of electron hole separation. At the same time on the TiO_2 through its thickness to combine electronic and graphene interface, in this process there will be some electronic loss. We synthesize TiO_2 nanosheets can not only reduce the loss of the electron transfer process, while also increasing TiO_2 nanosheets and graphene contact interface, and ultimately improve the nano TiO_2 / graphene composite structure of photocatalytic hydrogen production performance. We studied the photogenerated electrons in TiO_2 nanosheets and TiO_2 nanosheets and graphene composite migration path interface. On this basis, we conducted TiO_2 nanosheets Pt / graphene composite catalyst on the structure of integration, discusses the existence of Pt effect on the TiO_2/ graphene -Pt photocatalytic system performance.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O643.36

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