【作者】 陈礼平；

【导师】 王德军； 谢腾峰；

【作者基本信息】 吉林大学 ， 物理化学， 2014， 博士

【摘要】 利用太阳能解决能源和环境问题已经成为当今世界研究的一项重要课题，该项研究的本质是光能到化学能的转变，而半导体光催化剂是实现这种转变的最佳途径。半导体材料参与光催化和光化学反应主要涉及三个环节，一是吸收光能实现光生载流子的激发，二是光生电子空穴对在半导体材料表面和体相中发生分离，三是迁移到半导体表面的光生电子或空穴参与氧化或还原反应。对于每一个环节的研究都能帮助我们加深对半导体光催化剂的认识，其中第二个环节由于光生电荷分离、迁移及复合过程的复杂性，往往成为制约光催化和光化学反应效率的重要因素。光生电荷的各种行为一般发生于半导体表面或复合型半导体材料的界面空间电荷区，研究半导体表面或界面光电性质可以为提高光催化/光化学反应效率提供更加明确的指导作用。表面光电压（SPV）技术作为一种非直接接触型测量手段，具有灵敏度高、检测无损、快速简便等优点，适合研究半导体材料表面和界面的光生电荷行为。该技术中测量得到的相位谱与表面光电压谱同等重要，但相位谱却并未引起广大研究者的同等重视。结合光电压矢量模型，光电压相位谱能为复杂的光生电荷行为分析提供明确的解析方向。TiO2纳米管阵列（TNAs）作为一种一维纳米结构材料，其规整的取向有利于电子的便捷传输，是光催化和光电转化应用的理想材料；另一方面与同样作为一维纳米材料的纳米棒相比，TNAs的管状结构具有内、外双重表面，能够提供更大的比表面积，适合参与跟表面积密切相关的物理或化学反应。基于以上原因，本文采用TNAs及其复合材料作为研究对象，利用SPV谱、SPV相位谱和瞬态光电压（TPV）技术对单一和复合TNAs材料的表面与界面光生电荷激发、迁移和复合等行为进行研究，并对部分材料进行光（电）催化降解有机染料类污染物或还原重金属离子实验，力求寻找光生电荷行为和光催化活性之间的内在联系。围绕以上内容，本论文开展了以下四部分工作：1.以金属Ti片为原料，在0.3wt.%NH4F+2vol.%H2O+98vol.%乙二醇混合溶液中电化学阳极氧化制备TNAs，利用SPV谱、SPV相位谱和TPV技术对TNAs的表面光生电荷行为进行详细分析；SPV相位谱测量结果表明TNAs表面态激发后光生电荷分离和迁移受外加偏压影响较小，带带跃迁产生的光生电荷在负偏压和自建电场作用下迁移方向一致，而正偏压作用下光生电子向表面迁移、空穴向阵列底部方向迁移的光电压过程在整个光电压响应中的比重较大。2.在合成TNAs的基础上利用FeCl3和NaOH溶液交替浸泡处理得到负载α-Fe2O3的TNAs复合物，利用SPV谱、SPV相位谱和TPV技术分别对复合阵列可见光和紫外光激发下的光生电荷分离和迁移行为进行了研究。SPV相位谱结果表明，复合阵列表面的α-Fe2O3在持续可见光（400nm <λ <580nm，t>0.1ms）照射时，光生电荷在浓度梯度作用下会发生电子向表面迁移的光电压过程，而TPV结果显示在瞬态激光（λ=532nm，t=5ns）激发下会发生光生电子在界面电场作用下向复合材料的界面迁移的光电压过程；同时SPV相位谱测量结果还显示紫外光激发时TiO2中的光生电子向阵列底部迁移引起的光电压过程和向α-Fe2O3导带转移产生光电压过程同时存在。3.以金属Ti片为原料，在0.5wt.%NH4F+20vol.%H2O+80vol.%丙三醇混合溶液中利用电化学阳极氧化法制备TNAs，并采用Zn2+/Fe3+混合溶液处理制备了ZnFe2O4负载的TNAs复合材料，以SPV谱、SPV相位谱和TPV技术对其光生电荷行为进行了分析，并对比研究了TNAs和ZnFe2O4-TNAs复合材料的光催化还原Cr(VI)和光催化降解甲基橙（MO）的效果。SPV相位谱分析结果显示，ZnFe2O4-TNAs复合材料中无论是可见光还是紫外光激发，始终存在光生电子向表面ZnFe2O4迁移的光电压过程，该过程的存在是复合材料光催化还原Cr(VI)效率比TNAs高的主要原因；由于光生电子向表面ZnFe2O4迁移，增大了与表面光生空穴复合的几率，造成复合材料光催化降解MO效率弱于TNAs。4.利用电化学循环伏安法在TNAs管外表面进行了聚苯胺（PANi）复合，结合复合材料的合成机理，正向的SPV和TPV测量信号表明PANi-TNAs中TiO2与PANi的能级匹配，光照激发后产生了光生电子向PANi聚积、空穴向TiO2聚积的光电压过程，该过程中在TiO2上聚积的空穴有利于茜素红（AR）的光催化氧化降解。更多还原

【Abstract】 It is an important research work to solve the energy and environmental problemsby using solar energy. The essence of the related research is the conversion of solarenergy to chemical energy. Among multiple solutions, using the semiconductorcatalysts is one of the best ways to achieve this goal. There are three stages for asemiconductor to take part in the photocatalytic or photochemical reaction: firstlysemiconductor material absorbs photon energy and electrons in the valance bands areexcited to the conduction bands; secondly photogenerated electrons and holes areseparated on the surface or in the bulk of the semiconductor and the recombination ofthe electrons and holes may also take place in those places; thirdly electrons or holesthat transferred to the surface of the semiconductor take part in the reduction oroxidation reaction. The study of each stage can help us to understand more about thesemiconductor catalysts. Among them the second stage about the separation, transferand recombination of the photogenerated charge carriers are much more complicatedthan the other two. For this reason the second stage becomes an important part inaffecting the efficiency of the photocatalytic or photochemical reaction. It is knownthat the behaviors of the photogenerated charge carriers mostly take place in the spacecharge area of the surface or interface of the semiconductor(s), so the research of thephotoelectric properties of the surface or interface of the semiconductor(s) can bringus a clear way for improving the efficiency of the photocatalytic or photochemical reaction. As a fast and convenient measurement, surface photovoltage (SPV)technique is one of the best way to study the behaviors of the photogenerated chargecarriers as it possesses a lot of merit such as non-contact, high sensitivity,non-destruction. In this technique, the phase spectrum is as important as the SPVspectrum but ignored by researchers for a long time, while it does bring us a newdirection for studying the complicated SPV processes when combined with the SPVvector model.Titania nanotube arrays (TNAs) is a one-dimensional nano-structure material.The precisely oriented nature of the nanotube arrays makes them excellent electronpercolation pathways for vectorial charge transfer between interfaces. Also the doublewall surfaces of the TNAs offer bigger area for the surface related physical orchemical reaction than nanorods or nanowires.Based on the above reasons, TNAs and modified TNAs were adopted for ourstudy and the SPV and transient photovoltage (TPV) measurements were used tostudy the behaviors of the photogenerated charge carriers of the TNAs and modifiedTNAs. Also the photocatalytic degradation of organic pollutant and reduction ofheavy metal pollutant experiments were conducted. Seeking the inherent relationshipbetween the behavior of the photogenerated charge carriers and the photocatalyticreactions is the main purpose in our studies. Based on this, we conducted thefollowing experiments and acquired several new results:1. TNAs were synthesized in a0.3wt.%NH4F+2vol.%H2O+98vol.%ethylene glycol mixed solution via potentiostatic anodization of titanium foil. SPVspectrum, SPV phase spectrum and TPV techniques were used for investigating thebehaviors of photogenerated charge carriers. The results showed that the external biascan barely affect the separation and transfer behavior of the surface states relatedphotogenerated charge carriers; the transfer direction of the photogenerated chargesunder the effect of negative bias was the same as that under the effect of build-inelectric field; while the photogenerated electrons transferred toward the surface and holes toward the bottom of the tubes caused SPV process under the effect of positivebias occupied a high proportion in the overall SPV process.2. α-Fe2O3modified TNAs were obtained by sinking the TNAs film in the FeCl3and NaOH solution alternately. SPV spectrum, SPV phase spectrum and TPVmeasurements were also conducted to investigate the behavior of the photogeneratedcharge carriers in the α-Fe2O3-TNAs. The results showed that when irradiated withconsecutive visible light (400nm <λ <580nm, duration time>0.1ms),photogenerated electrons will transfer toward the surface under the effect ofconcentration gradient while with the irradiation of transient laser pulse (λ=532nm, t=5ns), photogenerated electrons will transfer toward the interface between theα-Fe2O3and TiO2under the effect of interface electric field. Under the irradiation ofUV light, two SPV processes caused by photogenerated electrons in TiO2that transferto α-Fe2O3and toward the bottom of the tubes existed simultaneously.3. TNAs with large open entrances were synthesized in a0.5wt.%NH4F+20vol.%H2O+80vol.%glycerol mixed solution via potentiostatic anodization oftitanium foil, followed by sinking in the Zn2+and Fe3+mixed solution for preparingthe ZnFe2O4modified TNAs. SPV and TPV measurements were used for analyzingthe behaviors of photogenerated electrons and holes. Also the photocatalytic reductionof Cr(VI) and degradation of MO experiments were conducted. The results showedthat the SPV process caused by photogenerated electrons transferring to the surface ofZnFe2O4exits when irradiated with either visible light or UV light, which resulted inthe high efficiency of photocatalytic reduction of Cr(VI). And the lowered efficiencyof photodegradation of MO was also caused by the same reason that photogeneratedelectrons recombined with holes in the ZnFe2O4.4. PANi modified TNAs were synthesized by an electrochemical depositionmethod. It was confirmed by the deposition mechanism and SPV and TPV test resultsthat the conduction bands of TiO2and the HOMO level of PANi were aligned andwhen irradiated with visible or UV light, photogenerated electrons will accumulate on PANi and holes on TiO2, which benefited the photodegradation of alizarin red.更多还原