【作者】 赵明；

【导师】 叶金花； 许华；

【作者基本信息】 天津大学 ， 材料学， 2018， 博士

【摘要】 世界人口激增所引起的能源危机与环境恶化是实时悬在人类头顶的利剑。为了控制事态进一步恶化,研究者们竭尽全力在寻找可替代的清洁能源以及降低环境污染的方法。光催化技术是目前最具前景的能够同时解决能源与环境两大问题的技术手段之一。而光催化反应的最终阵地位于光催化材料的表面,因此,光催化材料的表/界面调控一直备受关注。本论文以经典光催化材料TiO₂为研究对象,对其表/界面进行调控,从表面电子结构和反应环境角度出发,围绕电子激发、转移和利用过程研究光催化材料的表/界面性质对光催化活性的影响。本文的具体研究内容如下:（1）调控板钛矿TiO₂晶面,研究其表面电子结构对光催化性能的影响。通过调节前驱体溶液的碱度,分别得到了主要暴露{121}和{211}晶面的板钛矿TiO₂,记为T₁₂₁和T₂₁₁。理论计算表明:相较于{211}晶面而言,{121}晶面含有较多不饱和配位的Ti原子,并拥有较高的表面能。实验结果表明:T₁₂₁具备更正的价带,这种优势让T₁₂₁在光催化降解RhB的实验中表现出更优异的性能。此外,态密度计算分析显示:暴露{121}表面的TiO₂,电子更倾向于分布在体相材料中,不易迁移向表面发生还原产氢反应,这也是T₁₂₁还原能力较弱的另一个原因。相反,T₂₁₁表现出更负的导带电位,导致其光催化水分解产氢的速率最高(801?mol h^-1)。因此,板钛矿TiO₂的{121}晶面有利于光催化氧化半反应,而{211}晶面有利于促进光催化还原半反应。（2）构建Au@TiO₂体系,采用能带结构存在差异的板钛矿TiO₂(T₁₂₁和T₂₁₁)作为基底,研究TiO₂的表面电子结构对Au@TiO₂热电子转移的影响。沿用第（1）部分结论,T₂₁₁具有更负的导带位置,导致Au@T₂₁₁的肖特基势垒比Au@T₁₂₁的势垒更大。在不同的光照强度下,表面电子结构对热电子的转移会产生不同的影响。强光条件下(可见光,?（29）400 nm,光强338 mW cm^-2),热电子在Au颗粒上被激发并聚积,造成Au的费米能级负向移动,降低了Au/TiO₂界面处的肖特基势垒,从而有利于热电子从Au颗粒向TiO₂导带的连续注入,此时T₂₁₁较负的导带电位和较快的电子转移速率成为产氢速率的控制因素,导致Au@T₂₁₁在可见光下的产氢速率是Au@T₁₂₁的2.5倍。而在弱光条件下(单色光,光强2²0 mW cm^-2),热电子的聚积程度较低,肖特基势垒在一定程度上阻碍热电子转移,使Au@T₂₁₁和Au@T₁₂₁表观量子效率之间的差异远远小于2.5倍。因此,TiO₂的表面电子结构能够影响热电子在Au/TiO₂界面的转移以及在TiO₂表面的转移,来进一步影响Au@TiO₂的光催化性能。（3）调节反应环境,研究碱环境对光催化产氢活性的促进机制。实验发现,在强碱性环境下（pH=13.7）,Au@TiO₂体系的析氢速率是中性环境下的280倍。为了研究碱性环境与TiO₂之间的界面关系,探究碱环境促进产氢活性的内在机制（表面电位负移机制或甲醇氧化机制）,构建了Au@TiO₂体系。这种Au@TiO₂结构在可见光下,能够将质子还原反应（发生在TiO₂表面,仅与表面电位移动过程相关）和电子供体氧化反应（发生在Au表面,仅与甲醇氧化过程相关）在物理空间上进行分离。为了研究碱环境促进产氢活性的主导因素,设计了两个研究路线:构建Au@ST01体系（ST01是一种商用锐钛矿相TiO₂）,通过电化学法半定量的研究表面电位负移和甲醇氧化两种作用在光催化水分解产氢中的贡献比。Au@ST01体系的开路电压表明,当pH<13.5时,表面电位负移对光催化产氢活性提高的贡献比高达80%以上;即使碱度达到pH≥13.5时,贡献比也超过50%。电化学阻抗谱（EIS）和光响应电流的结果也进一步证实了这个结论;构建Au@T₁₀₀和Au@T₁₀₁体系(T₁₀₀和T₁₀₁是暴露{100}和{101}晶面的锐钛矿相TiO₂),通过调控甲醇氧化作用在Au@T₁₀₀和Au@T₁₀₁中相同,来研究表面电位负移作用对产氢活性的影响。在实验中,产氢活性比(rH₂(Au@T₁₀₀)/rH₂(Au@T₁₀₁))呈现出不规律的波动,说明表面电位负移作用对促进光催化产氢起到主导作用。通过两条研究路线的综合研究,阐明了碱环境对光催化产氢活性的促进机制:表面电位负移作用为主导,甲醇氧化作用辅助。更多还原

【Abstract】 With the explosion of world population,the energy crisis and environment issues are the swords that hang over the top of human beings.To prevent further deterioration of these problems,researchers have tried their best to find alternative clean energy and depress environmental pollution.Photocatalysis is one of the environmentally friendly technologies to convert solar energy into chemical energy for the promising applications of the photodegradation of target pollutants and photocatalytic splitting water into hydrogen.Since photocatalytic redox reactions take place on the surface of photocatalyst,it is of great importance to study the surface/interface of the semiconductor photocatalyst.This study focuses on the surface/interface modulation of the classical photocatalyst TiO₂,and mainly explores the effects of surface electronic structure and reaction environment on photocatalytic performance.The research contents of this study mainly include:（1）The facet engineering of brookite TiO₂ was carried out to investigate the effect of surface electronic structure on photocatalytic reactivity.The brookite TiO₂nanocrystals mainly exposed with{121}and{211}facet were obtained by adjusting the alkalinity of the precursor solution,which were recorded as T₁₂₁ and T₂₁₁,respectively.Theoretical calculations reveal that the{121}surface contains more undercoordinated Ti atoms and a higher surface energy than that of the{211}surface,and the experimental results show that brookite TiO₂ nanorods exposed with majority{121}facet(T₁₂₁)have a more positive valence band potential;those above mentioned superior properties enable T₁₂₁ to show excellent performance in RhB photodegradation.Nevertheless,the electronic structure analyzed from the Density of State plots revealed that the electron prefers to be dispersed in the bulk for TiO₂covered with a{121}surface,indicating that the electrons might be more reluctant to migrate from bulk to surface,which might be the reason for the poor H₂ productivity of T₁₂₁.In contrast,brookite TiO₂ nanosheets exposed with dominant{211}facets(T₂₁₁)exhibited a more negative conduction band potential resulting in a much higher H₂ evolution rate(801?mol h^-1)in photocatalytic water splitting.Accordingly,combining the analyses of the surface atomic structure and electronic band structure,it is indicated that,for brookite TiO₂,the{121}surface is beneficial for photocatalytic oxidation reactions while the{211}surface can facilitate the photocatalytic reduction process.（2）Based on the brookite TiO₂(T₁₂₁ and T₂₁₁)with adjustable band structure,the Au@TiO₂ system was constructed to study the effect of the surface electronic structure of TiO₂ on the hot electrons transfer process.On the basis of the conclusion in part（1）,T₂₁₁ exhibits a much higher conduction band potential.Thus,the Au@T₂₁₁shows a larger Schottky barrier than Au@T₁₂₁.The effect of the surface electronic structure of TiO₂ on the hot electrons transfer is different under various light irradiation.Under high-intensity photoexcitation(visible light,?（29）400 nm,338 mW cm^-2),the accumulation of hot-electrons will negatively shift the E_F of Au and ensure the consecutive injection to the conduction band of TiO₂,thus hot-electrons over T₂₁₁with more reductive potential and better charge transfer will be more promising.Hence,the photocatalytic H₂ production of Au@T₂₁₁ is about 2.5 times higher than that of Au@T₁₂₁ under visible light irradiation.However,under low-intensity light excitation(monochromic light,about 2²0 mW cm^-2),a higher Schottky barrier will impede the hot-electron transfer to some extent due to the low accumulation of hot electrons.Consequently,the discrepancy of AQE measured upon the monochromatic light irradiation between Au@T₂₁₁ and Au@T₁₂₁ is smaller than 2.5 times.Therefore,the surface electronic structure of TiO₂ affects the transfer of hot electrons at the interface of Au/TiO₂ and on the surface of TiO₂ which further impacts on the photocatalytic H₂ evolution reactivity.（3）The reaction environment was modulated to explore the mechanism of the alkali-induced enhancement of H₂ evolution.Experimentally,The H₂ evolution efficiency over Au@TiO₂ measured at pH=13.7 is 280 times higher than that at pH=6.0.In order to study the relationship between the TiO₂ surface and alkaline environment,we constructed the Au@TiO₂ system and investigate the mechanism of alkali-enhanced photocatalytic H₂ evolution（surface potential shift（SPS）or methanol oxidation（MO））.Under visible light irradiation,the architecture of Au@TiO₂ can easily separate the reduction process（occurred on TiO₂ surface,only related to SPS）and the oxidation process（occurred on Au surface,only related to MO）in physical space.To explore the dominant factor for the H₂ evolution enhancement,two investigation schemes are designed.For Au@ST01 system（ST01,a commercial anatase TiO₂）,the dominant role of the SPS on the H₂ evolution enhancement is semiquantitatively calculated via the open-circuit potential test（more than 80%at pH（27）13.5 and exceeds 50%at pH?13.5）.This conclusion is further proved by EIS and photoresponse current tests.For Au@T₁₀₀ and Au@T₁₀₁ system(T₁₀₀ and T₁₀₁,facet-optimized anatase TiO₂),the ratio of H₂ evolution rates(rH₂(Au@T₁₀₀)/rH₂(Au@T₁₀₁))exhibits a irregular fluctuation,which manifests the decisive role of SPS.Evidenced by these two investigation schemes,this work clarifies that the enhancement of H₂ evolution is dominated by the alkali-induced negative shift of the surface potential and meanwhile is complemented by the accelerated methanol oxidation.更多还原