掺杂二氧化钛纳米颗粒膜的制备及其可见光催化特性和机理研究

【作者】 糜岚；

【导师】 王培南；

【作者基本信息】 复旦大学 ， 光学， 2007， 博士

【摘要】 二氧化钛具备无毒害性和长期的稳定性，成为光催化材料的研究热点。但是由于二氧化钛的吸收边小于380 nm，因此只能在紫外光的照射下发生光催化作用，无法充分利用廉价的太阳能而增加了使用成本，影响了它的应用推广。近年来，理论和实验研究发现，非金属掺杂的TiO₂，特别是掺氮TiO₂，在可见区域的光学吸收会有明显的增强，因此极大地提高了使用太阳光进行光催化反应的效率，使其成为光催化反应的理想材料，因而得到了广泛的关注。本文的主要工作是分别用脉冲激光沉积（PLD），离子注入和煅烧等方法，制备了掺氮浓度较高、可见光催化效率较好的掺氮氧化钛纳米颗粒膜。分别用原子力显微镜（AFM），表面轮廓仪，拉曼光谱（Raman），X射线光电子能谱（XPS），紫外—可见光分光光度计（UV-Vis）等手段对掺氮氧化钛纳米颗粒膜进行了表征。重点研究了薄膜的吸收光谱和光催化特性，并探讨了掺氮的形式和氮氢共掺杂对可见光催化活性的影响。同时结合理论计算的结果，对掺氮和氮氢共掺杂氧化钛的能带结构和可见光吸收机理进行讨论。主要结果如下：1．利用激光烧蚀成膜（PLD）方法分别在O₂，N₂／O₂和NH₃／N₂／O₂混合气体的气氛下，合成了纯TiO₂和掺氮TiO₂纳米颗粒膜。通过对激光烧蚀等离子体的光谱诊断，发现加入少量氨气能有效促进氮气解离，对于提高TiO₂的掺氮量有很好的促进作用。对薄膜的表征结果表明，在我们使用的制备参数条件下，PLD方法制备的TiO₂纳米颗粒膜均为锐钛矿相结构，颗粒大小分布为20-40 nm，在N₂／O₂和NH₃／N₂／O₂中制备的薄膜氮含量分别为2.0％和4.4％，都具有可见光催化能力。其中，在NH₃／N₂／O₂中制备的薄膜的可见光吸收和可见光催化都有明显的增强。2．掺氮TiO₂膜的多次使用会严重降低其光催化效率。我们首次采用了温和加热方法对已经使用过的薄膜进行后处理，结果发现其光催化效率几乎可以完全恢复，这对于材料的实际应用具有重要意义。3．使用低能氮离子注入方法对TiO₂薄膜进行了氮的注入掺杂。结果发现，通过低能离子注入可以有效地掺氮而同时保持原有的锐钛矿相的结构不变。注入掺氮会减小薄膜的颗粒尺寸，去除薄膜表面的一部分松散结合的大颗粒。低能氮离子注入得到的是填隙型掺N，含氮量可以达到3.4％，其可见光催化能力比纯TiO₂明显增强，表明填隙型掺N同样能够提高可见光催化能力。4．采用在流动氮气和氨气中煅烧TiO₂纳米颗粒膜的方法制备了掺氮和氮氢共掺杂的TiO₂膜。在煅烧过程中先在高温下短时间煅烧，后在较低温下长时间煅烧，既减少了锐钛矿相向金红石相的转化，又得到了较高的掺氮量。通过对薄膜的表征，发现在氨气中煅烧比在氮气中煅烧有更好的掺杂效率；在掺氮量一样的情况下，氮氢共掺杂与掺氮相比有明显的吸收边的红移，因此具有更强的可见光吸收和催化效率。5．基于第一性原理，计算了纯TiO₂、掺氮TiO₂和氮氢共掺杂TiO₂的能带结构和态密度，对可见光吸收机理进行了分析，发现掺氮会在能隙间产生孤立的能级，对能隙的变窄并没有很大的贡献，而N和H同时掺入会导致N能级的降低，使N能级更靠近价带从而引起N2p态和O2p态的充分耦合，导致带隙变窄和吸收边红移。这些计算结果，可以很好地解释实验中氮氢共掺杂引起的吸收边的红移，而掺氮TiO₂的可见光催化则是那些孤立能级的贡献，因此实验中氮氢共掺杂比掺氮TiO₂具有更强的可见光催化效率。更多还原

【Abstract】 TiO₂ as a photocatalytic material with attractive characteristics of long-term stability and nontoxicity has been widely studied in the past years. However, TiO₂ with its absorption edge below 380 nm has photoactivity only under ultraviolet （UV） light. Thus, only a small portion of the solar energy can be utilized, which increases its application cost seriously. This is the main reason why TiO₂ has not been widely used commercially. How to effectively utilize sunlight as the light source is one of the important subjects for the wide application of TiO₂. In recent years, theoretical and experimental studies have indicated that using non-metal main group dopants, especially nitrogen dopant, can greatly enhance the photoactivity of TiO₂ in the visible spectral range. Hence, great efforts have been made for synthesizing the N-doped TiO₂.In this thesis, N-doped TiO₂ nanoparticulate films with high nitrogen dopant concentrations and high visible-light photocatalytic abilities were prepared by pulsed laser deposition （PLD）, ion implantation and calcination, respectively. The films were characterized by atomic force microscope （AFM）, surface profiler, Raman Spectroscopy, X-ray photoelectron Spectroscopy （XPS） and ultraviolet-visible （UV-Vis） absorption Spectroscopy. The photoabsorption and photocatalysis activities of these films in the UV and visible region were investigated. The effects of different N-doping and N:H co-doping structures on the photoactivities were studied. Moreover, the mechanism of visible-light photoactivity of N:H co-doped TiO₂ films was discussed on the basis of theoretical calculation. The results are as follows:1. Bare and N-doped TiO₂ nanoparticulate films were prepared by laser ablation of titanium target in O₂, N₂/O₂ and NH₃/N₂/O₂ atmospheres, respectively. It was found by spectral diagnostics that the dissociation of nitrogen molecules was enhanced when adding a small amount of ammonia into the nitrogen gas, which favored the incorporation of nitrogen into the TiO₂ matrix. All the prepared films were in anatase phase, and the size of the particles in the films was 20- 40 nm. The nitrogen concentrations were 2.0 % and 4.4 % for the films deposited in N₂/O₂ and NH₃/N₂/O₂, respectively. They both exhibited enhanced photocatalytic abilities in the visible-light region as compared with the undoped TiO₂, especially the one deposited in NH₃/N₂/O₂.2. The photocatalytic ability declined rapidly with the operating time. The method of mild-heating was applied for the first time for the regeneration of photocatalytic films. It was found that the photocatalytic ability of the used film was fully recovered by this treatment, which is of particular importance for the practical applications.3. N-doped TiO₂ films were prepared by implantation of low energy nitrogen ions into TiO₂ films. It was found that low energy implantation did not change the anatase phase of TiO₂ films. The ion bombardment reduced the particle size and removed some loosely bonded particles. The nitrogen species implanted in the films could be attributed to the interstitial nitrogen, and the dopant concentration was up to 3.4 %. The visible light photoactivities of the N-doped TiO₂ films were greatly improved as compared with the undoped TiO₂, indicating that the interstitial nitrogen could also enhance the visible-light photocatalytic ability.4. Calcination of TiO₂ nanoparticulate films under flowing N₂ and NH₃ were applied to prepare N-doped and N:H co-doped TiO₂ films, respectively. The calcination were carried out under high temperature for a short time and then under lower temperature for a long time to achieve high dopant concentrations and avoid the transformation of anatase phase. Characterizations of the produced films showed that the N-doping was more efficient when calcined in ammonia. With the same nitrogen dopant concentration, the films calcined in ammonia showed remarkable redshifts of the photoabsorption edges and higher visible-light photocatalysis efficiencies.5. Based on the first principles, the band structures and density of states of the bare, N-doped and N:H co-doped TiO₂ were calculated, respectively. The mechanism of the photocatalysis in the visible-light region was discussed. Isolated N 2p states were found just above the valence-band maximum of N-doped TiO₂, which might be the reason for the photocatalytic activity in the visible light region. However, for N:H co-doped TiO₂, hydrogen may contribute to the lowering of the energy levels of nitrogen, bringing the N states closer to the valence band, therefore enhancing the mixing of N 2p states with the O 2p states in the valence band, and leading to a real band gap narrowing and consequently a redshift of the optical absorption edge. The computational results agree with our experimental data.更多还原