【作者】 王辉；

【导师】 谢毅； 张晓东；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2017， 博士

【摘要】 近年来,基于太阳能开发利用的光催化反应,例如光分解水、二氧化碳还原以及污染物处理等,以其在解决环境污染与能源紧缺等问题上的迷人前景而受到广泛关注。作为一种重要的光催化反应载体,半导体材料以其独特的电子结构以及材料物性吸引了研究人员的目光。然而,光生载流子之间相互作用对半导体材料催化性能的影响被长期忽略,成为深入理解材料光催化机制、提升材料光催化性能以及拓展催化反应类型的重要制约因素。从激子效应的角度对半导体材料的光催化过程开展系统而深入的研究工作,不仅有助于理解光催化机制,同时也为设计具有高效、特异光催化性能的催化材料提供新思路。本论文旨在揭示激子效应在低维半导体材料光激发过程的重要作用,并通过探索新策略,对半导体材料中激子过程进行调控,最终实现半导体材料光催化性能的优化。在本论文中,作者聚焦几种典型半导体材料(黑磷、聚合物氮化碳、溴氧化铋),利用超快光谱与发光光谱等技术手段,并结合密度泛函理论计算,研究材料激子和载流子行为特性,揭示材料的结构因素(包括结构限域、维度、微结构等)对其光激发过程的影响,从而实现相关光催化性能的调控与优化。本论文主要包括以下几方面的内容:1.基于黑磷晶体独特的电子结构,作者首次选择水作为溶剂,通过液相剥离得到厚度约为2 nm、表面洁净、具有延伸至红外区的广泛光吸收谱带的超薄黑磷纳米片。作者利用原子分辨显微分析技术观测到制备的黑磷纳米片表面原子排布的不均匀性,首次证明了通过液相剥离得到的超薄黑磷纳米片存在结构扭曲。黑磷纳米片的表面原子排布扭曲不仅提升了结构稳定性同时也增强的激子效应,从而获取了高效单线态氧产生,使其在光动力学治疗和催化有机反应领域具有潜在的应用价值。该研究拓展了黑磷纳米片在光催化领域的应用前景。2.针对具有共轭结构的聚合物氮化碳材料,作者通过发光光谱首次证实了结构中强烈的激子效应和延迟荧光现象,提出三线态一三线态激子湮灭过程是限制材料量子效率提升的关键因素,并通过时间分辨的超快光谱技术构建完整的光激发过程模型,为研究其光催化过程提供了新视角;基于上述实验结果,作者提出通过向聚合物氮化碳中引入羰基来提升结构的自旋—轨道耦合,增从而增强其系间窜越过程,实现体系三线态激子浓度显著提升,使其在光照下呈现出显著增强的单线态氧产生;在理解材料结构特点和激子过程的基础上,首次提出了通过有序—无序界面策略调控聚合物光催化剂激子过程,从而提升热载荷子相关光催化反应性能,理论模拟显示有序聚合物链较无序链具有更低的最高占有轨道能级和最低空轨道能级,使得激子解离产生的电子和空穴分别向有序链和无序链注入,发光光谱和电化学结果显示半结晶氮化碳具有明显提升的激子解离效率和热电子浓度,苄醇氧化实验显示半结晶样品相较无定型样品具有大约3—6倍的相应醛产率。3.针对溴氧化铋材料,作者提出其特殊的层状结构能够对光生载流子产生结构限域作用,利于激子的形成。理论模拟显示材料的导带边和价带边在铋原子上具有明显交叠,使得材料具有较大的激子结合能;据此,作者以BiOBr为例研究了晶面对材料激子过程的影响,超快光谱和发光光谱结果显示,BiOBr材料的{001}晶面较{010}晶面更易产生激子,从而实现晶面工程调控材料光催化性能。该工作不仅为深入理解激子效应在光催化过程中的作用提供指导,同时还为设计具有强激子效应无机半导体材料指明了新方向。更多还原

【Abstract】 Given their promising potentials for solving the energy crisis and preventing environmental pollution,great efforts have been devoted to the studies of photocatalysts with efficient solar-driven catalytic processes.As one of the most important photocatalysts,semiconductors show intriguing prospects by virtue of their unique electronic structures.Photocatalytic studies have long been limited to photogenerated charge carrier aspects,whereas the attractive Coulomb interactions between electrons and holes have rarely been realized,which inevitably hinders further researches.As compared to bulk counterparts,low-dimensional materials tend to possess reduced screening effects and enhanced confinement effects,thus leading to significant electron-hole interactions.In this case,it is quite necessary to take the potential excitonic effects in consideration when dealing with the photoexcitation processes of low-dimensional materials.The interrogation of excitonic aspects in photoexcited semiconductors would be helpful for comprehensive understanding on the involved photocatalytic mechanisms,and thus pave the way for designing effective photocatalysts through excitonic engineering.The goal of this dissertation is to reveal the excitonic aspects of the photoexcitation processes in low-dimensional materials.Here by taking a series of typical semiconductors as examples,we systematically investigated the photoexcitation processes involved in these materials by combining photoluminescence spectroscopy and ultrafast transient absorption spectroscopy,highlighting the influence of excitonic aspects on the photocatalytic performance.On this basis of these investigations,we further regulated the excitonic processes via different strategies such as dimensional control,defect-and facet-engineering,for different dominating photoexcited species generation,and thus optimized photocatalytic performances.The details of this dissertation are summarized briefly as follows:1.In view of its layered structures,we first proposed the preparation of ultrathin black phosphorus nanosheets through water-assisted exfoliation process.Benefiting from the intriguing electronic structure,ultrathin black phosphorus nanosheets possess extremely high singlet oxygen generation,which renders their great potentials in photodynamic therapy and photocatalysis.This work greatly extends the applications of black phosphorus,and provides an efficient photocatalysts for singlet oxygen generation.2.Due to their low dielectric properties,polymeric photocatalysts tend to possess much strong excitonic effects that would greatly impact their photocatalytic applications.Here by taking polymeric carbon nitride as an example,we systematically investigated the photoexcitation processes involved in these materials by combining photoluminescence spectroscopy and ultrafast transient absorption spectroscopy and establish novel excitonic insights into the photocatalytic performance,thus offering various strategies to regulate the photocatalytic activities.We highlighted the robust triplet-triplet annihilation process,carbonyl group induced promoted singlet oxygen generation,and order-disorder interface induced exciton dissociation in the well-known polymeric photocatalyst.3.By taking bismuth oxyhalide as examples,we demonstrate that giant electron-hole interactions would be expected in confined layered structures,thus raising facet engineering strategy for the regulation of excitonic process.Theoretical calculations indicate that the simultaneous localizations of conduction and valence band edge states on Bi atomic sites,implying potential enormous electron-hole interactions.Further photoluminescence measurements verify the giant electron-hole interactions and long-lived excitonic states in these materials,in which the facet-dependent feature clearly suggests the dominating contributions of structural confinement.Photocatalytic molecular oxygen activation tests suggested that {001} facet favored photocatalytic singlet oxygen generation via energy transfer process,thus promising its potentials in the selective sulfoxidation reaction.This study not only provides an excitonic prospective on photocatalytic processes,but also paves a new approach for pursuing robust excitonic materials.更多还原