【作者】 张潇；

【导师】 马玉臣；

【作者基本信息】 山东大学 ， 物理化学， 2022， 博士

【摘要】 随着人类社会的发展,全球变暖和能源危机已经成为我们急需解决的两大问题。人们想要寻找一种清洁可再生的能源来缓解日趋减少的化石燃料压力,太阳能因其具有这种特点得到了广泛地发展和利用。自从Fujishima和Honda两位教授首次发现二氧化钛(TiO2)单晶光催化分解水(H2O)产生氢气和氧气这一现象,利用半导体光催化将太阳能转化为其他可储存利用的能量吸引了越来越多的关注。TiO2因其高效、无毒、丰度高、化学稳定性高等优点,成为了最广泛应用的光催化剂。我们的模拟主要应用先进的多体格林函数理论方法,包括能够准确计算电子结构的GW方法和准确计算激发态性质的Bethe-Salpeter equation(BSE)。以下是本文的主要研究内容:一、板钛矿被认为是TiO2的一种活性相,在一些光催化反应中表现出了比锐钛矿和金红石更优越的活性。然而,对其电子性质以及缺陷的关键作用的研究和认识仍然很少。运用从头算多体格林函数理论,我们探究了板钛矿体相和其(210)表面中缺陷的准粒子结构,包括氧空位、钛填隙和羟基化。发现极化子可以产生一个深缺陷带和一个浅缺陷带,它们分别位于导带底以下约0.7和0.3 eV。在体相板钛矿中,氧空位只能产生一个低于导带底1.1 eV的深缺陷态,其由Ti 3d轨道之间形成的σ键诱导产生。这些特征与锐钛矿和金红石TiO2中的截然不同。我们还发现将羟基引入体相板钛矿中会使TiO2的带隙变窄至少0.4 eV,这可能有助于增强其对可见光的吸收。二、锐钛矿TiO2催化二氧化碳(CO2)光还原为有机分子已经引起了人们的极大关注,而其反应机理尚不清楚。一般认为,氧化还原反应是在激子解离后发生的,以前的研究仅仅考虑了孤立的光电子在还原反应中的作用。我们的第一性原理计算表明,界面电荷转移激子可能在还原反应中起关键作用。界面电荷转移激子不仅将之前提出的双电子过程(CO2→HCOOH和HCOOH→HCO)简化为单电子过程,而且大大降低了此前预测的难以克服的反应势垒。关键中间体自由基CO·-的稳定性也在激子的帮助下显著提高。更重要的是,我们发现CO2还原为HCOO-后剩下的空穴能够以极低的势垒促进H2O在桥接氧原子上的解离和氧化,这不仅为还原反应及时地提供质子,而且还导致了桥接氧空位的产生。这个氧空位可能成为H2O进一步解离或者CO2还原为CO的活性位点。我们的工作证明,在设计CO2还原的光催化剂时可能需要注意激子解离前的作用。三、单原子催化剂因其独特的活性位点和高的金属利用率等优点得到了广泛应用。我们模拟了单原子Cu在锐钛矿TiO2(101)表面不同的负载方式对CO2还原机理的影响。发现不同的负载方式拥有不同的电子结构,这些独特的电子结构和独特的Cu位点共同导致了 CO2吸附和还原路径的不同。其中Cu与两个桥接氧原子相连时(Cu/Obr体系)显示Cu1+价态,其在带隙中产生的靠近导带底的缺陷态稳定住了 CO2·-,使其倾向于生成HCOOH。对于Cu负载于氧空位的Cu/Ov体系在导带底存在极化子态,大大促进了CO2的解离和CO的生成。而当Cu替代表面一个五配位Ti时(Cu/Tiv体系),此结构中的Cu2+并不具有催化活性,即使引入了氧空位,弯曲CO2仍然不容易在此体系上生成。因此,Cu原子不同的负载方式将会使光催化CO2还原具有高度的选择性以及不同的效率。四、为了理解多相光催化的基本原理,人们对甲醇(CH3OH)在TiO2上的光氧化反应进行了广泛的研究。其中光生空穴的作用以及反应的能垒仍然存在争议。我们研究了金红石TiO2(110)表面CH3OH光氧化反应路径中激子的演变过程。解释了从CH3O到CH2O转换过程中,最终产物产生的Ti 3d缺陷态在降低势垒(约1.3 eV)和稳定产物方面的关键作用。随着氢从CH3O向TiO2转移,空穴逐渐从TiO2价带边缘的O 2p轨道向导带底附近的Ti 3d轨道迁移,并以带隙内空穴态局域在吸附物上作为桥接。我们发现空穴是否最初被反应物CH3O捕获不是CH3O发生氧化的关键,在光催化中空穴捕获可能只是发生在反应过程中的一个事件。此研究阐述的缺陷态的关键作用为理解金属氧化物表面的光化学反应提供了新的见解。五、单壁碳纳米管(single-walledcarbon nanotubes,SWCNTs)和有机分子之间以及SWCNTs之间的激发态能量转移(excitation energy transfer,EET)在实验中得到了广泛研究。基于多体格林函数理论的第一性原理计算表明,F(?)rster偶极-偶极耦合近似不适用于SWCNTs的EET,而更高的多极相互作用扮演着重要角色。这意味着,与SWCNTs的暗激子相关的供体-受体电子耦合有时可能比与亮激子相关的耦合强几个数量级,因此,暗激子可能主导着其EET。这与一般认为EET是通过亮激子之间的耦合来实现的观点相反。即使当供体和受体相距较近时,Dexter交换作用对SWCNTs的EET贡献很小。更多还原

【Abstract】 With the development of human society,global warming and energy crisis have become two major problems which we need to face and solve.People devote to finding one kind of clean and renewable energy to alleviate the pressure of decreasing fossil fuels.Solar energy has been widely developed and utilized because of above characteristics.Since Fujishima and Honda discovered the phenomenon of hydrogen and oxygen generation from photocatalytic water splitting by the titanium dioxide(TiO2)single crystal,the use of semiconductor photocatalysis to convert solar energy into other energy that can be stored has attracted more and more attention.TiO2 is widely used as a photocatalyst because of its high efficiency,nontoxicity,abundance,high chemical stability,etc.Our simulation is mainly based on the state-of-the-art many-body Green’s function theory,including the GW method which can accurately calculate the electronic structures and the Bethe-Salpeter equation(BSE)which can accurately calculate the properties of excited-states.The followings are the main research contents:(1)Brookite is now recognized as an active phase of TiO2,which exhibits superior activities compared with anatase and rutile in some photocatalytic reactions.However,there is little research and knowledge on its electron properties as well as the crucial role of defects in brookite yet.Using the ab initio many-body Green’s function theory,we examined the quasiparticle structures of defects,including oxygen vacancies,Ti interstitials,and hydroxyl groups,in the bulk and the(210)surface of brookite.We discovered that small polarons may generate a deep defect band and a shallow one which are 0.7 eV and 0.3 eV approximately below the conduction band minimum(CBM).In brookite bulk,oxygen vacancy can only create a deep defect band which is 1.1 eV below CBM and induced by the σ bonds formed between Ti 3d orbitals.These features are quite distinct from those in anatase and rutile.We also found that introducing hydroxyl groups into brookite bulk would make the band gap narrow by 0.4 eV at least,which may help to enhance its visible light absorption.(2)Photoreduction of CO2 into organic molecules under the catalysis of anatase TiO2 has aroused great attention.Its reaction mechanism is still far from understood.It is generally thought that the redox reactions take place after excitons dissociation,and hence previous investigations only consider the role of isolated photoelectrons in the reduction reactions.Our first-principles calculations identify that interfacial charge-transfer(CT)excitons might play a crucial role in the reduction reactions.CT excitons cannot only simplify the previous proposed two-electron processes in the reduction of CO2→ HCOOH and HCOOH→HCO to oneelectron processes,but also cut down substantially the reaction barriers which have been predicted to be insurmountable.Stability of the key intermediate radical CO2·-can also be greatly enhanced with the help of the CT exciton.We also discover that the remaining holes after the reduction of CO2 to HCOO-can promote the dissociation and the oxidation of H2O at the bridging oxygen atom with an extremely low barrier.This cannot only provide timely supply of protons for the reduction reactions,but also lead to the formation of the bridging oxygen vacancy.The oxygen vacancy could be the active site for either further H2O dissociation or the reduction of CO2 to CO.Our work reveals the synergism between the reduction and the oxidation half-reactions,and demonstrates that the role of excitons before their dissociation might need to be paid attention to in designing photocatalysts for CO2 reduction.(3)Single atom catalysts have been widely used due to their unique active sites and high metal utilization.We simulated different loading modes of single atom Cu on the anatase TiO2(101)surface to explore its influence on the mechanism of CO2 reduction.It is found that different loading methods possess diverse electronic structures,and these unique electronic structures and Cu sites together lead to the differences in CO2 adsorption configuration and reduction pathways.When Cu is connected with two bridging oxygen atoms(Cu/Obr system),it shows Cu1+valence state.And the defect state near the bottom of the conduction band stabilizes CO2·-,making the reduction tend to generate HCOOH.For the system where Cu is supported on oxygen vacancies(Cu/Ov),a polaron state exists at the CBM,which greatly promotes the dissociation of CO2 and the generation of CO.When Cu replaces a fivecoordinated Ti on the surface(Cu/Tiv system),the Cu2+in this structure does not have catalytic activity,and even if one oxygen vacancie is introduced,bent CO2 is still not easily generated on this system.Therefore,various loading of Cu atoms will make photocatalytic CO2 reduction with high selectivity and distinct efficiencies.(4)Photo-oxidation of methanol(CH3OH)on TiO2 has been extensively studied in order to understand the fundamental principles of heterogeneous photocatalysis.However,the role of the photogenerated hole and the energy barrier in this reaction are still controversial.We examine the evolution of excitons along the reaction path of CH3OH photo-oxidation on the rutile TiO2(110)surface.We reveal the critical role of the Ti 3d defect state produced by the final product in lowering the energy barrier(by 1.3 eV)and stabilizing the product in the conversion from CH3O to CH2O.With the transfer of hydrogen from CH3O to TiO2,the hole migrates gradually from the O 2p orbitals of TiO2 at the valence band edge to the Ti 3d orbitals near the conduction band bottom,with the band-gap hole state localized on the adsorbate as the bridge.We find that whether the hole is initially trapped by the reactant CH3O is not a crucial issue for the oxidation of CH3O to occur.Hole trapping required in photocatalytic reactions might just be an event happening in the reaction process.The critical role of defect state clarified in this work provide new insights for understanding photochemical reactions at metal oxides surfaces.(5)Excitation energy transfer(EET)between single-walled carbon nanotubes(SWCNTs)and organic molecules and that between S WNCTs have been extensively studied in experiments.Our first-principles calculations based on many-body Green’s function theory show that the F(?)rster’s dipole-dipole coupling approximation fails for the EET of SWNCTs,while higher multipole interactions play a major role.This makes that donor-acceptor electronic coupling related to the dark excitons of S WNCTs sometimes may become several orders of magnitude stronger than that related to the bright ones,and therefore dark excitons might govern the EET of SWNCTs.This is in contrast to the general viewpoint that EET is realized through the coupling between bright excitons.Dexter exchange interactions contribute little to the EET of SWNCTs even when the donor and acceptor are in close proximity to each other.更多还原