【作者】 蔡芳共；

【导师】 赵勇； 杨峰；

【作者基本信息】 西南交通大学 ， 材料学， 2013， 博士

【摘要】 能源危机和环境问题是当前人类社会所面临的两个重大问题,利用可再生资源制备清洁、可持续的可再生能源是解决上述问题的有效途径之一。二氧化钛(Ti02)是一种重要的宽禁带半导体材料,在能源转换利用领域引起了广泛的关注,例如光催化降解有机污染物、光电化学裂解水制氢以及太阳能电池等。随着纳米科学的进步,研究人员开发出了许多不同形态的Ti02纳米材料。相比于传统多孔Ti02薄膜,高度有序的Ti02纳米管阵列(TiO2NAs)具有取向性更好、比表面积更大等特点,同时可为光生载流子提供一个快速有效的输运通道。然而,由于Ti02存在带隙宽、量子效率低等缺点,限制其在太阳能电池领域的实际应用。锐钛矿型Ti02的禁带宽度乓达到3.2eV,以致于Ti02只能利用太阳光谱中占极少部分的紫外光。因此,寻找一种改性Ti02的有效途径,拓宽其光谱响应范围,从而提高其在太阳能电池领域的应用是本论文研究的主要目的。该论文工作的主要内容如下：1、以含有NH4F的乙二醇溶液为电解液,在宽温度范围内(10～70℃)对纯Ti表面进行阳极氧化,制得形貌可控的Ti02纳米结构。随着电解液温度的变化,纳米Ti02的形貌得到控制,可形成Ti02纳米管阵列及纳米管阵列／纳米线复合结构,温度40～50℃为转折温区。在该温度区间内,Ti02开始由非晶态向锐钛矿发生转变。重点了考察电解液温度在阳极氧化过程中的作用。2、与窄禁带半导体复合是拓展Ti02光响应范围的重要方式。Bi2S3的禁带宽度只有1.3eV,且具有大光吸收系数,是Ti02光敏剂的一种理想选择。选择非晶态a-Ti02NAs和锐钛矿型TiO2NAs薄膜作为初始材料,通过传统水热法在薄膜上沉积纳米Bi2S3,形成Bi2S3敏化的TiO2NAs,考察不同晶体结构TiO2NAs对水热合成Bi2S3/Ti02NAs异质结构光电性能的影响。结果表明,虽然Bi2S3/Ti02NAs中Bi2S3的含量更少,造成其光吸收和表面光电压响应均弱于Bi2S3/a-TiO2NAs,但Bi2S3/Ti02NAs中Bi2S3的颗粒更细小,且集中在管内部及管隙之间,未覆盖在纳米管阵列表面,使得Bi2S3/TiO2界面电场更强,从而获得更高的光电化学性能。Bi2S3/Ti02NAs的短路电流密度Jsc及光电转换效率η分别为4.54mA/cm2和1.86%。3、通过改变水热反应前驱体溶液的浓度,调控纳米结构Bi2S3在锐钛矿型TiO2NAs中的负载量。相比于其他情况,当Bi2S3将Ti02纳米管刚好完全填充满的时候,即前驱体溶液(Bi(NO3)3)的浓度为5mmol/L时,Bi2S3敏化TiO2NAs的光吸收及表面光电压响应均达到最强,光电化学性能得到进一步的提高,此时光电转换效率η达到2.65%。4、由于PbS能够充分利用太阳光,其常被用于太阳能电池。通过调控PbS的尺寸,其第一激子峰的吸收波长易拓展至红外区。采用连续离子层吸附与反应方法在TiO2NAs中沉积PbS纳米颗粒(PbS/TiO2NAs),并在沉积过程中引入超声辅助(PbS(u)/TiO2NAs),比较两种不同工艺制得的PbS敏化TiO2NAs的形貌、结构及光电化学性能。TiO2NAs经PbS纳米颗粒改性后,光吸收范围拓宽至可见光区甚至近红外区。与无超声辅助情况下形成大尺寸纳米颗粒将纳米管阵列覆盖的PbS/TiO2NAs不同,引入超声辅助制得的PbS纳米颗粒尺寸更小,且只分布在Ti02纳米管内部和管隙之间。最终使PbS(u)/TiO2NAs具有更强的光生载流子分离效率,从而获得更高的光电化学性能。5、利用超声辅助连续离子层吸附与反应方法制备PbS纳米颗粒改性TiO2NAs, PbS纳米颗粒的负载量及尺寸可通过改变连续离子层吸附与反应的循环次数来调控。当循环次数为15次时,PbS纳米颗粒完全将Ti02纳米管填充满,且未形成大颗粒堆积在纳米管表面。此时PbS纳米颗粒的粒径分布是不均匀的,从小于4nm直至25nm。紫外-可见光漫反射光谱和表面光电压谱的结果表明,经PbS纳米颗粒改性的Ti02NAs的光吸收范围被拓宽至可见光区,PbS纳米颗粒与TiO2NAs之间形成异质结,都有助于其光电化学性能的提升。当循环次数为15次时,即Ti02纳米管完全被PbS纳米颗粒填充满时,PbS纳米颗粒敏化的TiO2NAs表现出最好的光电化学性能,其短路电流密度Jsc、开路电压Voc和光电转换效率η分别达到9.55mA/cm2,0.95V和2.83%。6、尝试利用无机半导体PbS和Bi2S3共敏化TiO2NAs,最终获得的光电转换效率η仅为1.13%。更多还原

【Abstract】 Nowadays, the world is facing serious energy and environmental problems, so it is very urgent to produce a clean, sustainable and renewable energy source to solve such problems. Titanium dioxide (TiO2) is one of the most important wide gap semiconductors and has been attracted the attention in energy conversion, such as photocatalytic degradation of pollutants in water, hydrogen production from photoelectrocatalytic water splitting, and solar cells. With advances in nanoscience, development of various forms of TiO2nanomaterials has made great progress. Compared with the traditional mesoporous TiO2film, the highly ordered TiO2nanotube arrays (TiO2NAs) has better alignment characteristics and larger specific surface area. Simltaneously, TiO2NAs allows a fast and efficient transfer of the photogenerated charge carriers. Unfortunately, TiO2is restricted in the application as solar cell materials due to its wide band gap and low quantum efficiency. The band gap of anatase TiO2is3.2eV, which is too large for efficient absorption of energy from sunlight. Therefore, we try to seek an efficiently approach of modifying TiO2to improve the overlap of the absorption spectrum with the solar spectrum. The main content in this work is as follows:1. Morphology controllable TiO2nanostructures were fabricated on the Ti substrate in an ethylene glycol solution of0.25wt%NH4F via anodic oxidation method at different temperatures (10～70℃). The morphology of TiO2nanostructures can be controlled by varying temperature of the electrolyte, such as TiO2nanotube array or nanotube/nanowire composite film. Temperature between40℃and50℃is the turning area of changing nanotube to nanowire. In the above-mentioned temperature range, the amorphous TiO2begins to change into anatase. The effect of temperature of the electrolyte during the anodic oxidation process was mainly studied.2. Compositing with narrow bandgap semiconductor is an important way to expand the TiO2light response range. Bismuth sulfide (Bi2S3) has a narrow band gap (1.3eV) and a large absorption coefficient. It is an ideal candidate for the photosensitization of TiO2. Amorphous a-TiO2NAs and anatase TiO2NAs were chose as templates to synthesis nanocomposite. A novel heterostructure of nanoscale Bi2S3-sensitized TiO2NAs was fabricated by a conventional hydrothermal method. The result shows that the coverage of Bi2S3in Bi2S3/a-TiO2NAs is larger than corresponding coverage in Bi2S3/TiO2NAs, leading to stronger light absoption and surface photovoltage response are obtained from Bi2S3/a-TiO2NAs than Bi2S3/TiO2NAs. However, in the case of Bi2S3/TiO2NAs, the Bi2S3distributed both the inside and outside rather than the top surface of TiO2nanotubes, and the size of Bi2S3is much smaller than that in Bi2S3/a-TiO2NAs. Thus, the interfacial electric field in Bi2S3/Ti02NAs is stronger than that in the case of Bi2S3/a-TiO2NAs. The results demonstrate that photoelectrochemical solar cells based on Bi2S3/TiO2NAs has short-circuit current JSc of4.54mA/cm2and photoelectric conversion efficiency η of1.86%.3. The coverage of Bi2S3in TiO2NAs can be tuned by concentration of precursor solution in hydrothermal process. Bi2S3fill the TiO2nanotubes in and do not accumulate on the top of nanotubes when the concentration of precursor solution (Bi(NO3)3) is5mmol/L. Compared with other cases, its light absoption and surface photovoltage response are strongest. And then the photoelectric conversion efficiency η of2.65%is obtained.4. Lead sulfide (PbS) is a good candidate for solar cells, because it can be made to overlap the solar spectrum optimally. By controlling its size, the absorption wavelength of the first exciton peak can easily be extended into the infrared. PbS-sensitized TiO2heterostructure nanotube arrays were synthesised by Successive Ionic Layer Adsorption and Reaction (SILAR) method. Ultrasound (PbS(u)/TiO2NAs) has an important function for the formation of PbS nanoparticles. Compared with non-ultrasound PbS/TiO2NAs, PbS(u)/TiO2NAs have small PbS nanoparticles uniformly distributed on both the outside and inside rather than the top surface of TiO2NAs. A stronger separate efficiency of photogenerated charge carriers is found in PbS(u)/TiO2NAs than that in PbS/TiO2NAs, resulting in a better photoelectrochemical property is obtained.5. PbS nanoparticles as an efficient sensitizer for TiO2nanotube arrays (TiO2NAs) were fabricated by the SILAR method under ultrasound. The coverage and size of PbS nanoparticles can be tuned by changing the repeated cycles (n) of SILAR process. UV-vis diffuse-reflectance spectra and surface photovoltage spectra were used to investigate the light absorption properties and the transfer behavior of photogenerated charges in PbS-modified TiO2NAs heterostructures. The results show that the absorption range of TiO2NAs is widened from ultraviolet to the visible region by PbS nanoparticles modifying. And a heterojunction is formed between PbS nanoparticles and TiO2NAs, which facilitates the separation of photogenerated charge carriers. TiO2NAs can be fully covered with PbS NPs with size from below4nm to25nm and large aggregates inside and outside of nanotubes when the repeated cycles (n) reach15, which exhibits the best photoelectrochemical performance in all PbS-sensitized TiO2NAs electrodes. With AM1.5G illumination at100mW/cm2, its short-circuit current density Jsc, open-circuit voltage Foe and photoelectric conversion efficiency η is9.55mA/cm2,0.95V and2.83%, respectively.6. PbS and Bi2S3co-sensitized TiO2NAs electrode was fabricated. The photoelectric conversion efficiency η is only1.13%.更多还原