【作者】 张晓茹；

【导师】 王德军； 谢腾峰；

【作者基本信息】 吉林大学 ， 物理化学， 2010， 博士

【摘要】 太阳能的研究和利用是当今世界所面临的重大课题,利用太阳能进行光催化一直是人们研究的热点,光催化技术由于其价格便宜,环境友好,稳定性高等优点,在环境污染控制领域具有广泛的应用前景。半导体光催化是近年来国内外最活跃的研究领域之一,尤其是随着纳米技术的飞速发展,光催化纳米材料通过吸收太阳光,可以直接分解环境污染物,无二次污染,所以深入研究纳米半导体光催化剂对于从根本上解决环境污染问题具有重大的意义。通常,光催化过程包括三步,（1）光催化材料吸收一定的光能（hν> Eg）,（2）在光激发下产生的光生载流子（电子、空穴）迁移到催化剂表面,（3）光生载流子与吸附在催化剂表面的物质发生氧化还原反应。在这三步过程中,可能影响光催化过程量子效率的重要因素有:光生载流子的分离效率和复合效率,以及光生载流子向表面迁移过程中载流子的迁移率等。因此深入研究光催化纳米材料的光催化活性与光生电荷行为的内在关系,对于光催化剂材料的开发和应用至关重要。研究光催化纳米结构材料的表面与界面光生电荷行为,是探讨其光催化机理的基础,为进一步优化光催化体系的性能提供理论依据。以TiO₂为代表的传统光催化纳米材料,存在着量子效率低以及不能有效利用太阳光等缺点,影响了它的广泛应用,因此,开发高效利用太阳光、高量子效率的光催化纳米结构材料具有重大意义。近年来的研究发现,非金属掺杂的TiO₂,特别是N掺杂TiO₂,能够拓展TiO₂的响应光谱范围,进而提高其对太阳光的利用率。因此,深入研究以N掺杂TiO₂为中心的光催化材料的光电性质,能够使我们更好的理解新一代光催化纳米结构材料的光催化机制。本文工作主要是利用表面光电压技术,瞬态光电压技术以及Kelvin探针技术研究了N掺杂TiO₂纳米材料,N-TiO₂ / TiO₂复合材料,TiO₂混晶结构纳米材料的表面与界面光生电荷行为,以及光电性质与光催化活性之间的关系,为进一步优化光催化体系的性能提供理论依据。得到了以下创新性成果:1.以尿素为氮源,采用水热法制备了不同N掺杂量的TiO₂ （N- TiO₂）光催化剂.利用X射线衍射（XRD）、紫外-可见漫反射光谱（UV-Vis DRS）、X射线光电子能谱（XPS）以及荧光光谱（PL）等技术对其进行了系统的表征,以罗丹明B （RhB）和甲基橙（MO）溶液的脱色降解为模型反应,分别考察了不同N含量的样品在紫外和可见光区的光催化活性.利用表面光电压（SPV）和瞬态光电压（TPV）技术研究了N掺杂TiO₂纳米粒子表面光生电荷的产生和传输机制,并探讨了光生电荷与光催化活性之间的关系.结果显示,随着N含量的增大, TiO₂表面光伏响应阈值发生红移,可见光部分光电压响应强度逐渐增强,同时瞬态光伏响应达到最大值的时间亦有着不同程度的延迟.这表明适量的N掺杂能够提高TiO₂纳米粒子中光生载流子的分离效率,相应地延长了载流子的传输时间,从而增加光生电荷的寿命,而过量的N掺杂则增加了TiO₂纳米粒子中光生载流子的复合中心,同时亦改变了TiO₂的能级结构,影响了光生空穴的氧化能力,从而导致了光催化活性的下降。2.在N掺杂TiO₂纳米粒子的基础上,制备了不同质量比（N-TiO₂ vs. TiO₂）的TiO₂包覆N-TiO₂的具有异质结构的N-TiO₂ / TiO₂复合纳米粒子,并对其进行了光催化降解甲基橙及亚甲基蓝的实验。结果表明,无论在紫外光还是可见光的照射下,当N-TiO₂与TiO₂的质量比为8: 2时,复合粉体的光催化活性都是最好的。通过Kelvin探针测试结果可知N-TiO₂的表面功函低于TiO₂, N-TiO₂与TiO₂之间形成的界面能够使光生电子从TiO₂向N-TiO₂转移,我们通过SPV和TPV的结果可以得到光生电荷的微观运动行为,N-TiO₂ / TiO₂复合纳米粒子在光的激发下,由于界面场的存在促进了光生电子和空穴的有效分离,并抑制其复合。3.通过对N-TiO₂ / TiO₂复合纳米粒子的研究,我们发现异质结构纳米粒子作为光催化剂具有更好的光催化活性,因此我们对混晶TiO₂ （锐钛矿/金红石）纳米粒子晶相间界面结的形成,以及光生电荷行为与光催化反应之间的关系进行了研究。我们制备了不同质量比（锐钛矿vs.金红石）的TiO₂纳米粒子,并对其进行了紫外光催化降解罗丹明B实验。随着金红石含量的逐渐增多,TiO₂光催化活性先增大后减小。应用Kelvin探针技术研究了混晶TiO₂纳米粒子的表面功函,利用表面光电压和瞬态光电压表征了光生电荷的运动行为。混晶TiO₂的表面功函介于纯锐钛矿和纯金红石相TiO₂之间,锐钛矿相TiO₂的表面功函高于金红石相,所以可以认为在混晶TiO₂中,锐钛矿和金红石之间的界面结有形成的可能,SPV和TPV的结果进一步证明了在紫外光照射下,锐钛矿和金红石之间形成的界面结影响着体系的光生电荷的运动行为,金红石相的含量也影响着界面电场作用的大小,从而导致光催化活性的不同。更多还原

【Abstract】 Development and utilization of solar energy is a major issue which is facing the world today, and photocatalysis using solar energy has attracted an enormous amount of research interest. The photocatalytic technology has been widely applied in the field of environmental pollution control because of cheap, environmentally friendly, and high stability advantages. Semiconductor Photocatalysis is one of the most active areas of research in recent years at home and abroad, especially with the rapid development of nanotechnology, photocatalytic nano-materials can break down environmental pollutants, and no secondary pollution. Therefore, it is meaningful to study the nano- photocatalyst for the fundamental solution to environmental problems. Tiypically, the photocatalytic process includes three steps, （1） photocatalytic materials absorb photoenergy （hν> Eg）, （2） photogenerated carriers （electron, hole） migration to the catalyst surface under the excitation, （3） redox reactions occurred on the catalyst surface. In this three-step process, the important factors which may affect the quantum efficiency of photocatalytic are: the separation efficiency and recombination efficiency of photogenerated carriers, the carrier mobility of the photogenerated carriers that migrate to the surface and so on. Therefore, it is essential to study the intrinsic relationship between photoinduced charge behavior and photocatalytic activity of nano-materials for the development and application of photocatalytic materials. The behaviors of the photoinduced charges at the surface and interface of the photoactive systems are important to explore the basis of the photocatalytic mechanism, and further optimize the performance of photocatalytic system.TiO₂ is the tradition photocatalytic nanomaterials, but its application is limited by the dissatisfactory quantum efficiency, the neglectable utilization of solar energy and other shortcomings. So it is meaningful to develop the nano-structured photocatalytic materials which can use sunlight efficiently. In recent years, many studies have found that non-metal-doped TiO₂, especially the N-doped TiO₂, could expand the range of the response spectrum, thereby increasing the utilization of solar energy. Therefore, it is essential to study the N-doped TiO₂ photocatalytic material, and we can better understand the new generation of nano-structured materials, and the photocatalytic mechanism.In this thesis, we mainly studied the photoelectric properties of N doped TiO₂ nano-materials, N-TiO₂ / TiO₂ composite materials, TiO₂ with mixed crystal structure by the surface photovoltage technique, transient photovoltaic technology and Kelvin probe technique, and the relationship between the photocatalytic activity and the behaviors of photoinduced charges at the interface and surface of nano-materials was also discussed. Innovative achievements as following:1. Nitrogen-doped TiO₂ （N-TiO₂） photocatalysts with different N doping were successfully synthesized by hydrothermal method using urea as the nitrogen source. The samples were characterized by X-ray diffraction （XRD）, UV-Vis diffuse reflectance spectroscopy （UV-Vis DRS）, X-ray photoelectron spectroscopy （XPS） and photo luminescence spectroscopy （PL）. The photodegradation of Rhodamine B （RhB） and Methyl Orange （MO） solution was used to evaluate the photocatalytic activity of the catalysts under UV and visible light irradiation. The surface photovoltage （SPV） and transient photovoltage （TPV） techniques were applied to investigate the separation and transport mechanism of the photo-generated charge carriers of N-doped TiO₂ nanoparticles. The relationship between the photo-generated charge carriers and the photocatalytic activity was also discussed. The results show that the SPV threshold values were shifted to the visible region, and stronger photovoltaic response in the visible region were observed with the increase of N doping. Simultaneously, it is found that the maximum values of TPV response time are different for N-TiO₂. These results indicate that the photo-induced charge carriers are separated efficiently with appropriate amount of N doping, and accordingly the transmission time is extended, which imply the lifetime of photo-induced charge carriers is increased. While an excessive amount of N act as the recombination centers for the photo-induced electrons and holes, they reduce the photocatalytic activity though they contribute to the visible light absorbance.2. Based on N doped TiO₂ nanoparticles, we prepared the N-TiO₂ / TiO₂ composite nanoparticles with different mass ratio of N-TiO₂ and TiO₂. The N-TiO₂ nanoparticles were coated with TiO₂ which had the heterogeneous structure. And we tested the photocatalytic activity by degrading methyl orange and methylene blue. The results showed that under both the UV and visible light irradiation, the photocatalytic activity of the composite powders （the mass ratio of N-TiO₂ and TiO₂ is 8:2） is the best. Kelvin probe results indicated that the surface work function of N-TiO₂ was less than that of TiO₂, the interface between N-TiO₂ and TiO₂ could be formed and the photo-electrons might transfer from TiO₂ to N-TiO₂. The SPV and TPV results revealed the behavior of photo-generated charges, we could observe that the photo-generated charges were easier to separate and difficult to recombination under the hetero-interface in the N-TiO₂ / TiO₂ composite nanoparticles.3. Through the N-TiO₂ / TiO₂ composite nanoparticles study, we found that the nanoparticles with heterogeneous structure have better photocatalytic activity, so we studied the mixed crystal TiO₂ （anatase / rutile）nanopowders. We prepared the mixed crystal TiO₂ （anatase / rutile） nanoparticles via hydrolyze method. The influence of the interface junction between anatase and rutile TiO₂ on the photogenerated charge carriers properties were studied by Kelvin probe （KP）, surface photovoltage （SPV） and transient photovoltage （TPV） techniques. The nanopowders of anatase, rutile and mixed-phase TiO₂ were synthesized through hydrolysis method. The prepared nanoparticles were well characterized by XRD, TEM, HRTEM, and UV-vis diffuse reflectance spectroscopy. And the photocatalytic activities of these samples were evaluated on the degradation of RhB under UV-light irradiation. The KP results revealed that the difference of the surface work function between anatase and rutile may cause the built-in field at the interface in the mixed-phase TiO₂. The formation of the interface junction was further revealed through the red shift of the SPV response peak and the time retardation of the TPV response in the mixed-phase TiO₂. The relationship between enhanced photocatalytic activity of mixed-phase TiO₂ and the characteristics of photogenerated charge carriers was also discussed.更多还原