【作者】 徐凌；

【导师】 唐超群；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2010， 博士

【摘要】 纳米TiO2因为有合适的导带电位和价带电位、化学稳定性好、氧化还原能力强,无毒以及环境友好等优点而成为光催化和光电(特别是太阳能电池)领域研究的重要对象。但纯TiO2的带隙较宽,仅能吸收紫外光,从而影响了它在光催化和太阳能电池应用中的发展。因此扩大TiO2的光吸收范围,提高它的光催化活性成为当前研究热点问题之一。基于上述问题,本论文采用理论和实验相结合的方法对TiO2进行了系统深入的研究,首先在理论上用第一性原理方法深入研究了TiO2的晶体结构、电子结构、本征缺陷、掺杂改性等重要问题,并在此基础上对金属非金属共掺杂进行了理论计算和分析,发现合适的金属非金属共掺杂不但能够扩大TiO2的光吸收边,而且能有效抑制电子空穴对的复合。然后参考理论研究的结果在实验上制备了几种金属、非金属掺杂的TiO2纳米粉和膜,并对这些样品进行了性能测试与分析,发现掺杂改性特性与理论研究的结果相吻合。论文的主要工作如下。1、对7种不同晶相结构的TiO2进行了理论计算,获得了各种晶相的结构常数、总能量、禁带宽度等数据,并分析了这些晶相的物理特性。在进行第一性原理计算时,发现采用GGA近似中的PW91方法要优于其他方法；在TiO2的7种晶相中,萤石矿相TiO2的带隙最小,仅为1.13 eV(直接带隙),比锐钛矿相TiO2带隙要窄1.02eV,这与Ti受到立方晶体场作用而使eg(dz2)能级下移有关,价带顶附近空穴的低有效质量表明了萤石矿相TiO2的载流子有较高的电导率。2、采用第一性原理研究了TiO2各种本征点缺陷的几何结构和能带结构,发现本征缺陷的类型和浓度对材料性能有一定的影响。在富O条件下,容易形成VTi缺陷；在富Ti条件下,点缺陷Tii4+和Vo将大量出现,形成Schottky缺陷。对离子掺杂TiO2能带结构的模拟计算时,发现N和C单掺杂TiO2的缺陷形成能较高,使离子掺杂浓度降低,容易产生深能级杂质,成为电子和空穴的复合中心。3、针对单掺杂容易形成电子-空穴复合中心问题,论文进一步对金属和非金属共掺杂的TiO2进行了理论研究。结果表明,共掺杂一方面对几何结构的影响十分明显,非金属和金属杂质有团聚成键的趋势,另一方面对能带结构也有很大影响,对比单掺杂,N/V和C/Cr共掺杂的TiO2中的电子-空穴复合中心得到明显抑制,对此进一步分析了非金属和金属杂质的不同补偿作用(施主或受主),提出了共掺杂时可能存在协同效应,这种协同效应可以调整材料的能带结构,改善光催化性能。协同效应可能源自以下几方面：①金属离子掺杂有利于非金属离子掺杂浓度的提高；②对于C/Cr(N/V)共掺杂TiO2体系,导带底下方的施主杂质能级上的两个(一个)电子刚好补偿价带顶上方的受主杂质能级上的两个(一个)空穴,这可以有效减少电子和空穴的复合；③共掺杂作用使他们单掺杂引入过深的杂质能级变浅,减少了电子和空穴在杂质能级上的复合率。4、制备了N掺杂TiO2的样品。研究发现：掺N的TiO2样品发生从锐钛矿相到金红石相的相变温度在700℃,而纯TiO2的这种相变温度是600℃左右；N掺杂TiO2后,TiO2的光吸收边红移明显,在可见光区域的光吸收性能比纯TiO2明显好很多。还发现：N掺杂TiO2的可见光吸收性能与样品制备中的煅烧温度以及掺杂的浓度有关,从400℃到700℃,随着煅烧温度逐渐升高,N掺杂TiO2的可见光吸收性能逐渐降低；并且,随着N掺杂浓度的增大,TiO2在可见光区域的吸收能力先增大后减小,当氮掺杂浓度为3%光吸收能力达到最大。5、制备了不同浓度的V掺杂TiO2的薄膜样品。研究发现：掺杂前后TiO2薄膜在紫外-可见光范围内的光吸收峰位置变化不大,均在300nm左右；但V掺杂后,TiO2的光吸收边发生了红移,光吸收范围增大,与理论计算结果相吻合；掺杂浓度对光吸收效果也有影响,掺杂浓度为1.0%的TiO2薄膜的光吸收效果最佳。6、在制备N、V单掺杂样品并获得良好性能的基础上,进一步制备了N-V共掺杂TiO2的样品,研究了他们的光谱吸收等特性,发现N-V共掺杂样品的光吸收边红移范围比单掺杂样品明显大一些。上述几种掺杂样品的实验结果和前面的理论计算结果相一致。7、在前面工作基础上,尝试性设计制作出一种ITO/N-TiO2/V-TiO2/Au薄膜结构太阳能电池,测试了该电池的阻值以及光电特性,发现电池在无光照下有良好的整流特性,在光照下能产生一定的电流和电压,结果表明该电池具有整流特性和光生电压电流。论文研究工作取得的创新性成果有：(1)采用理论和实验相结合的研究方法,极大地加快了Ti02改性的探索方向和方法的确定。(2)发现了非金属和金属共掺杂Ti02能够明显抑制单掺杂Ti02中的电子-空穴复合中心,通过分析,提出了共掺杂可能存在协同效应的创新观点。(3)制备出了在可见光范围具有较好光吸收能力的Ti02纳米粉和膜,探索了制备条件和参数对Ti02光吸收边的影响；(4)创造性地制备出了具有平面P-N结结构的Ti02薄膜太阳能电池,经测试表明该电池已具有明显的整流特性和光生电压电流。更多还原

【Abstract】 Titanium dioxide (TiO2) has received much attention as a important material for photocatalysis and photoelectricity (fields of Solar Cell), which is relatively appropriate conduction band and valance band potential, wide in band gap, chemically stable, strongly reductive and oxidative, non-poisonous, and environmentally friendly. Therefore, TiO2 is wide application in the photocatalystic reaction and the Solar Cell.However, the wide band gap requiring ultraviolet irradiation limits the practical application; Increase TiO2’s light absorption region and enhance the photocatalystic active have became one of the hot spot problem.Aim at the problem, the paper adopted the theoretical and experiment method to study TiO2.Firstly, study the electron structure property, intrinsic point defects and doping et al important problems by first principles calculation in this dissertation, on these base study the metal and nonmetal codoped TiO2, and found the metal and nonmetal codoped TiO2 are not only can broaden the visible light absorption region of TiO2, but also reduce the electrons and holes recombination ratio. Secondly, according to the result of theory, N-doped and V-doped and N/V codoped TiO2 samples were prepared by a sol-gel technology, measure and analyse these samples properties, the experimental results are in agreement with the theoretical results. The conclusions have outlined as follows:1、Seven known TiO2 polymorphs crystal structure are established and the structural optimizations are performed to obtain the equilibrium structural parameters, total energy, and electron energy band structure, et al, and further analyze their energy band, density of states and difference charge density. We have carried out the first-principles calculations for TiO2 polymorphs crystal structure, and find the PW91 project of GGA is excelled other projects. In these calculations, the energy band structure shows that fluorite-type TiO2 has a narrow band gap (only is 1.13eV) and high electrical conductivity for potential photocatalytic applications under visible light.2、The geometry structure and stability of intrinsic point defects for TiO2 are studied by first principles calculations. The results show the defect types and defect concentrations are related to the nonequilibrium growth condition.In general, under the O-rich condition, VTi would form spontaneously, and under the Ti-rich condition, Tij4+ and Vo easily appear in Schottky defects. To obtain highly effective photocatalysts, the incorporation of impurities in TiO2 has been made to modify the energy band structure. It is found that the formation energy of N-or C-doped TiO2 is high, resulting in the lower p-type dopant concentration,Easily appear a hight impurity band and male it become the center of electrons and holes recombination.3、In order to solve the electrons and holes recombination of doping TiO2, the codoped TiO2 with nonmetals and transition metals is more studied. We find that one codoping have great influence on the original geometry, namely, TM/NM defect pairs tend to bind to each, the other the energy band structures of codoped-TiO2 show that N/V and C/Cr codoping is superior to other codoped systems, due to the passivated donor-acceptor codoping on anatase TiO2 photocatalysts. Our results may help understanding synergistic effect of codoping approach for improving photoelectrochemical activity of anatase TiO2. The synergistic effect is come from as follows:first, metal ion doped TiO2 is advantaged to enhancing the concentration of nonmetal ion. Second, for the N/V and C/Cr codoping systems, the electrons on the impurity bands below the conduction band bottom are right compensate the holes above the valence band, which can availably reduce the electrons and holes recombination ratio. Third, the codoping make the impurity band to become shallow, this is also reduce the recombination ratio.4、N-doped TiO2 samples have prepared, we find that pure TiO2 normally undergoes anatase-to-rutile phase transformation in the temperature range of 600℃, but the rutile phase appeared is 700℃when N-doped TiO2, compared with the pure anatase TiO2, The absorbency of N-doped TiO2 in the visible range is more better, it’s absorption band red shift is obviously. The visible light absorbency of the N-doped TiO2 will reduce form 400℃to 700℃by increasing the calcined temperature, the optimum temperature in our experiments is 400℃.The results indicate that the optical absorption edge obviously shifts to the visible light range with increasing N content. The N-doped TiO2 showed a significant absorption from 380 to 450nm compared with the non-doped TiO2. However, with further increasing N content, when the N content is larger than 3%,the absorption edges become blue shift,so the optimum N content is 3%.5、TiO2 film with different V contents were obtained by changing the addition of th Triethylamine, we find the absorb peak value is about 300nm for doping or non-doping TiO2 by testing with UV-Vis. The spectral responses of TiO2 powders shift to the visible light region after doping, and the absorption range increased, which confirm the theoretical results. The optimum doping concentration of vanadium is 1%.6、For the sake of comparing, N-, V-doped and N-V codoped nanosized TiO2 powder were prepared by a sol-gel technology in experiment. Further study their spectral absorption characteristics,we find that the spectral responses of TiO2 powders shift to the visible light region after doping, and the absorption region of N-V codoped TiO2 enlarge mostly,which confirm the theoretical results.7、Finally, we study a film structure Solar Cell.We measure its property of U-I, and find the cell have a good current property without light intensity, the Cell can produce a litte current and pressure with light.The innovative conclusions have outlined as follows:(1)Adopted the theoretical and experiment method to study TiO2 is very quickly confirm the aim and mean of doping TiO2; (2) Found the metal and nonmetal codoped can availably reduce the electrons and holes recombination ratio, advanced synergistic effect of codoping approach for improving photoelectrochemical activity of anatase TiO2 is great; (3)Prepared some TiO2’s powders and films that they can absorb by the visible light, and explored the law that some parameter how to affect the light absorption region; (4) Prepared an double deck film structure Solar Cell, studied how to enhance the photoelectric conversion efficiency.更多还原