TiO₂纳米材料的光催化及光伏特性研究

【作者】 林彦明；

【导师】 姜振益； 胡晓云； 朱超原；

【作者基本信息】 西北大学 ， 凝聚态物理， 2014， 博士

【摘要】 随着世界经济的高速发展,人类面临着能源危机与环境污染的双重压力。自1972年日本科学家Fujishima等人发现了二氧化钛(TiO2)半导体的光催化现象以及1991年瑞士科学家Gratzel等人成功研制的第一块TiO2基染料敏化太阳能电池以来,TiO2纳米材料的光催化和光伏特性已被广泛的研究。然而,由于TiO2半导体的带隙较大,仅仅能吸收太阳光中5%的紫外光,而对大部分的可见光得不到很好的吸收,并且导带中的光生电子与价带中的空穴极易复合,从而导致它的光催化活性和光电转换效率不够高,这将严重限制了该材料在光催化和光伏领域中的广泛应用。因此,提高TiO2材料的光催化活性和光伏特性将成为本文研究的重要目标。本文采用基于平面波赝势方法的密度泛函理论(DFT),系统的研究了金属、金属与非金属、非金属与非金属元素对TiO2光催化材料的掺杂改性。并且在金属与非金属元素掺杂TiO2体系中,我们采用新颖的溶胶-凝胶溶剂热法制备了掺杂TiO2光催化剂以及表征了它的各种性能。另外,我们还采用类似的计算方法研究了染料敏化太阳能电池(DSSCs)中苯甲酸(BA)染料分子敏化TiO2光阳极的物理化学机制和光伏特性。研究获得的创新性成果如下：第一,在金属Ni掺杂TiO2体系中,我们的计算结果表明：Ni替位O掺杂的锐钛矿和金红石TiO2的禁带中出现了一系列的Ni3d杂质能级；对于富O生长条件下的Ni替位Ti掺杂锐钛矿TiO2,杂质Ni原子的3d电子态引起了体系的带隙减小。这些结果导致TiO2出现吸收光谱边缘红移和高效的可见光光催化活性。我们的理论结果首次揭示了人们在实验中观察到的Ni掺杂增强TiO2可见光光催化活性的物理化学起因。第二,为了研究金属与非金属掺杂TiO2的光催化活性,我们首次采用理论与实验结合的方式研究了Eu/Si共掺杂锐钛矿TiO2和Si&Fe共掺杂锐钛矿与金红石TiO2体系的光催化活性。研究结果表明：随着杂质Si的掺入,O2p态和Si3p态的杂化诱发了锐钛矿TiO2的带隙减小；Eu的掺入会使锐钛矿Ti02的禁带中产生Eu4f杂质能级；当Eu和Si共同掺入时,Eu和Si的协同效应会使得锐钛矿TiO2的带隙减小,并且在禁带中出现Eu4f的杂质能级,它们使得TiO2材料对紫外光和可见光的吸收能力同时增强,因而导致Eu/Si共掺杂锐钛矿TiO2具有高效的可见光光催化活性。而且我们认为：非金属Si的掺入具有抑制Eu掺杂锐钛矿TiO2体系中光生电子-空穴对复合的作用。对于Si&Fe共掺杂锐钛矿和金红石TiO2,随着Fe的掺入,TiO2的价带顶和导带底均出现了Fe3d杂质能级；Si和Fe的协同效应能使共掺杂TiO2的带隙减小,同时在它的价带顶和导带底也都出现Fe3d杂质能级,从而引起TiO2的吸收光谱边缘出现明显的红移现象,导致Si&Fe共掺杂TiO2具有很强的可见光光催化活性。我们的理论与实验结果吻合的很好,这些创新性的研究成果将进一步的推动了TiO2材料在光催化领域中的广泛应用。第三,为了研究非金属与非金属掺杂TiO2的光催化活性,我们系统的研究了C/B共掺杂锐钛矿和金红石TiO2的光催化活性。计算结果表明：C/B共掺杂能在锐钛矿TiO2体系的禁带中诱发出一些C2P和B2P的杂化态,并且带隙减小约0.8eV；在C/B共掺杂金红石TiO2中,C2P态、B2P态与价带之间会发生强烈的耦合作用,因而使得体系的带隙变窄。它们导致TiO2的光谱吸收边缘出现明显的红移现象以及降低光生电子-空穴对的复合率,从而增强了TiO2的光催化活性。此外,随着C/B杂质浓度的增大,C/B共掺杂TiO2的光吸收和光催化活性也逐渐的提高。这些理论研究首次揭示了科学家们在实验中观察到的C/B共掺杂增强TiO2光催化活性的物理化学机制。第四,为了研究BA染料分子敏化TiO2光阳极的敏化机制,我们借助于DFT计算获得了吸附BA染料分子的锐钛矿(101)和金红石(110)TiO2表面体系的最稳定吸附结构。BA染料分子中的O2P态与锐钛矿(101)Ti02表面中的O2p、Ti3d态间的杂化会引起吸附体系的价带顶出现上移,它导致了吸附BA染料分子的锐钛矿(101)TiO2表面体系的光吸收能力增强；在吸附BA染料分子的金红石(110)TiO2表面体系中,O2p、C2p和Ti3d间的强耦合作用诱发了体系的光谱吸收边缘出现红移现象,并且该吸附体系的吸收光谱在476.0nm和610.0nm处出现了两个强吸收峰。我们的计算结果表明：BA染料分子在TiO2表面的吸附行为能有效的增强DSSCs中TiO2光阳极的光伏特性,这将有助于提高TiO2-DSSCs的光电转换效率,以至于该光电池能够迅速的走向产业化市场。更多还原

【Abstract】 With the high speed development of world economy, mankind is faced with the dual pressures of the energy crisis and environmental pollution. Since1972, Japanese scientists Fujishima et al. discovered the photocatalytic phenomenon of TiO2semiconductor, and Swiss scientists Gratzel et al. developed successfully the first piece TiO2-based dye-sensitized solar cells in1991, the photocatalytic and photovoltaic properties of TiO2nanomaterials have been studied widely. However, due to the large band gap of TiO2semiconductor, it can only absorb5%ultraviolet(UV)-light and can’t absorb a big portion visible-light in sunlight, and that photogenergated electrons in the conduction band and holes in the valence band tend to recombine relatively easily, resulting in lower photocatalytic activity and photoelectric conversion efficiency of TiO2, which will limit severely its applications in photocatalysis and photovoltaic fields. Therefore, improving the photocatalytic activity and photovoltaic property of TiO2materials will become one of the most important goals in this paper.In this paper, doping TiO2materials by the metal, metal and nonmetal, nonmetal and nonmetal elements have been investigated systematically using the density functional theory (DFT) based on the plane-wave pseudopotential approach. Moreover, metal and nonmetal elements doped TiO2photocatalyst is prepared using a novel sol-gel solvothermal method and is characterized by a variety of laboratory instruments. In addition, we also studied the physical and chemical mechanism and photovoltaic property of benzoic acid (BA) dye molecule sensitized TiO2photoanode by the similar computational method in dye-sensitized solar cells (DSSCs). Obtaining the innovative research results are as follows:First, in the metal Ni-doped TiO2, our calculated results indicate that substitutional Ni to O-doped anatase and rutile TiO2have a series of Ni3d impurity levels appearing in the band gap. For substitutionally Ni to Ti-doped anatase TiO2under O-rich growth condition,3d electronic states of Ni atoms cause the band gap narrowing of the system. These results lead to TiO2appearing the redshift of absorption spectrum edge and the efficient visible-light photocatalytic activity. Our theoretical results reveal firstly the physical and chemical origin of Ni-doped enhanced the visible-light photocatalytic activity of TiO2in experiments.Second, to investigate the photocatalytic activity of the metal and nonmetal doped TiO2, we firstly investigate the photocatalytic activity of Eu/Si codoped anatase TiO2and Si&Fe codoped anatase and rutile TiO2systems by the combination of theory and experiment. The research results show that with the doping of impurity Si, the hybridizations of O2p state and Si3p state induce the band gap narrowing of anatase TiO2. Eu doping can make Eu4f impurity levels appearing in the forbidden gap of anatase TiO2. The synergistic effects of Eu and Si codoping may reduce the band gap of anatase TiO2and produce Eu4f impurity levels in the band gap of anatase TiO2, which makes TiO2materials enhance the absorption of the UV-and visible-light, resulting in Eu/Si-codoped anatase TiO2with the outstanding visible-light photocatalytic activity. Moreover, we believe that the doping of nonmetal Si can effectively inhibit the recombination of photogenerated electron-hole pairs in Eu-doped anatase TiO2system. For Si&Fe-codoped anatase and rutile TiO2, Fe3d impurity levels appear on the valence band top and conduction band bottom of TiO2with the doping of Fe. Synergistic effect of Si and Fe can further reduce the band gap of codoping TiO2while Fe3d impurity levels appear on the valence band top and conduction band bottom of TiO2, which causes an obvious redshift of the optical absorption edge in TiO2, resulting in a strong visible-light photocatalytic activity in Si&Fe-codoped TiO2. Our theoretical and experimental results agree well. These innovative researches will further promote the applications of TiO2nanomaterials in the photocatalysis field.Third, in order to study the photocatalytic activity of the nonmetal and nonmetal doped TiO2, we investigated systematically the photocatalytic activity of C/B-codoped anatase and rutile TiO2. The calculated results indicate that C/B codoping can induce some hybridized states of C2p and B2p appearing in the forbidden gap of anatase TiO2and the band gap has a narrowing about0.8eV. In C/B-codoped rutile TiO2, the couples of the valence band, C2p and B2p result in a band gap narrowing of the system. These results lead to an obvious redshift of the optical absorption edge and a low recombination rate of photogenerated electron-hole pairs in TiO2system, which enhances the photocatalytic activity of TiO2. In addition, we find that the optical absorption and photocatalytic activity of C/B-codoped TiO2improve gradually with the increase of C/B impurities concentration. Our theoretical research reveals firstly the physical and chemical mechanism of C/B-codoped enhancing the photocatalytic activity of TiO2in scientists’ experiments. Fourth, to investigate the sensitized mechanism of BA dye molecule sensitized TiO2photoanode, we obtain the most stable adsorption geometries of BA dye molecule adsorbed on anatase (101) and rutile (110) Ti02surfaces by the DFT calculations. The hybridization of between O2p states of BA molecule and O2p, Ti3d states of anatase (101) TiO2surface leads to an obvious rise of the valence band maximum, which causes the optical absorption enhancing of BA dye molecule adsorbed on anatase (101) TiO2surface. In BA dye molecule adsorbed on rutile (110) TiO2surface system, the coupling of among the O2p, C2p, and Ti3d induce a redshift of optical absorption edge, and the absorption spectrum of adsorption system appears two strong absorption peak at476.0nm and610.0nm. Our calculated results show that adsorption behavior of BA dye molecule on TiO2surfaces can effectively enhance the photovoltaic property of TiO2photoanode in DSSCs, which leads to an improved photoelectric conversion efficiency of TiO2-DSSCs, so that photoelectric cell can quickly take one’s place on the industrialization market.更多还原