【作者】 高远鹏；

【导师】 方鹏飞；

【作者基本信息】 武汉大学 ， 材料学， 2013， 博士

【摘要】 光催化技术的关键问题之一是制备稳定高效的光催化材料。TiO2基光催化材料因其原料丰富,成本低廉,对环境友好,化学稳定性高,光催化性能优良等优点,得到世界各国科技工作者的广泛关注。有关TiO2基光催化材料的研究已经有数十年的历史,但其在实际应用方面i仍然有诸多不足之处,譬如：低太阳光利用率和低光量子效率所导致的光催化活性不够高；以及使用过程中光催化活性的稳定性较差等。因此,通过合适的制备方法和掺杂改性途径以开发具有更高催化活性和使用稳定性的TiO2基光催化材料仍然是富有挑战性的课题。本论文针对性地从两方面入手：一是制备具有高比表而积的二维TiO2基纳米片结构,并分别通过N掺杂和Ag负载对其进行改性,获得具有高比表面积和高可见光活性的光催化材料；二是针对N掺杂TiO2光催化剂的光催化性能的稳定性,利用Ag表面修饰来改变N-TiO2的表面电子结构,提高光催化剂在制备以及使用过程中的稳定性。具体的研究内容如下：通过简单的“碱热-质子交换法”合成了具有介孔结构和高比表面积的TiO2基纳米片,这种纳米片具有锐钛矿/钛酸异质结结构,表现出优异的光催化性能。本工作采用原位Raman和XRD等方法,调查了该纳米片结构随温度升高的结构和性能变化情况。结果表明,随着温度的升高,纳米片中的钛酸成分会因脱水而逐渐向锐钛矿相转变。在温度低于300℃时,锐钛矿相基本上没有生长；300℃之后锐钛矿迅速生长。温度的升高,一方面会提高光催化剂的结晶度,提高催化剂内光生载流子的分离、迁移速率,从而改善其光催化性能；另一方面也会使纳米片因脱水而导致比表面积和孔结构迅速减少,而且促进了钛酸成分向锐钛矿相转变,进而破坏锐钛矿/钛酸异质结构构,对光催化性能造成了不利的影响。这两方面因素对气相和液相催化反应造成了不同的影响：随着温度上升,样品对RhB的光催化活性迅速下降；而对苯的光催化降解则以450℃为转折点,其光催化活性先上升后下降。以上述合成的具有高比表面积的锐钛矿/钛酸纳米片为原料,采用简单的固相法制备出比表而积相对较高的N掺杂TiO2。可见光气相催化降解苯的实验结果表明,所制备的N掺杂TiO2催化剂具有良好的可见光催化活性。研究了煅烧温度和投料比(尿素/TiO2摩尔比)对N掺杂TiO2结构和性能的影响,结果表明N掺杂最会随着煅烧温度的升高而降低,可能是因为随着温度的升高,热运动加剧,N元素留在TiO2晶格中的可能性会变得更低。另外温度太低会造成尿索分解不完全,所以N掺杂的最佳温度应为450℃。N掺杂量也会随着投料比的提高而增加,但是当投料比超过2：1时继续提高尿素加入量也再难以提高N的掺杂量,可能是因为此时N掺杂已经趋于饱和。在循环实验中,N掺杂TiO2的光催化活性持续下降,说明N掺杂TiO2的光催化活性稳定性并不理想,这是因为光催化反应中产生的光生空穴会将掺杂进入TiO2晶格的N氧化,掺杂N的损失使N掺杂TiO2的光催化活性逐渐丧失。所以应该设法提高掺杂N元素的稳定性。采用一步法首次直接合成了具有较高比表面积的Ag纳米颗粒负载层状钛酸纳米片,并提出了这种独特的Ag负载层状纳米片结构的形成机理。与未负载的样品相比,Ag负载纳米片在液相和气相反应中显示出优良的可见光催化活性可见光催化活性的产生是基于Ag的表面等离子体共振(SPR)效应：可见光下Ag的光生电子被激活后,可能克服其与钛算纳米片之间的肖特基势垒而进入钛酸的导带,因此八g负载能使钛酸的吸收边带红移,增强其在可见光波段的吸收。另外,样品的光催化活性随着Ag负载量的增加首先会逐渐增强,超过一定量之后反而会逐渐较低。这可能是由于过量的八g聚集在一起会形成载流子复合中心,同时还可能会影响光辐射的吸收。因此,寻找最佳的Ag负载量对于获得最优的光催化性能是很有必要的。通过水热处理,成功的将不同含量的Ag纳米颗粒引入到N掺杂的TiO2光催化材料表面上。有效地提高了光催化剂的可将光催化活性。同时发现掺杂的N元素在水热处理过程中的流失量会随着Ag负载量的增加而减少,说明Ag负载能增强TiO2晶格中掺杂N元素的稳定性;此外循环光催化实验的结果也表明Ag负载还可以提高光催化反应过程中,的掺杂N元素的稳定性,使N的掺杂浓度在重复使用过程中保持稳定,从而提高了光催化剂的循环使用性能。Ag负载的这种稳定化作用可以归结为电子从Ag的5s轨道向掺杂N元素的2p轨道的转移。另外光催化活性随着Ag负载量的增加首先呈增加趋势,Ag负载量超过一定值后光催化活性反而下降。因此也需找到合适的Ag负载量来获得最佳的光催化活性。更多还原

【Abstract】 One of the key points of photocatalysis technology is to achieve high-performance photocatalysts. TiO2-derived photocatalysts have been attracting the worldwide attentions due to their low cost, low toxicity, and outstanding physical, chemical properties. TiO2-derived photocatalysts have been widely researched for several decades for applications in different fields including environmental depollution and energy conversion. However, there are still some disadvantages for TiO2-derived photocatalysts, hindering their practical applications, such as low utilization of sunlight, poor quantum efficiency and lack of photocatalytic stability. Therefore, it is essential to develop TiO2-derived photocatalysts with higher photocatalytic activity and stability. This research work is pertinently progressing from two aspects:one is to improve visible-light photocatalytic activity of TiO2-derived materials with Ag loading and/or N doping; the other is to enhance the photocatalytic stability of these photocatalysts with Ag, N co-modification.In this research work, series of Ag-loaded, N-doped and Ag-N co-modified TiO2-derived photocatalysts have been successfully synthesized. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS), fluorescence spectroscopy (FL), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and N2adsorption-desorption analysis are employed to characterize the as-prepared materials. Rhodamine B (RhB) and benzene are used as the degradation objects to evaluate the liquid-phase and gas-phase photocatalytic activities of the as-prepared samples. Some achievements have been made in this research work.Protonated anatase/titanate nanosheets with a high specific surface area of378m2/g were synthesized through an alkaline hydrothermal method with subsequent acid washing. In situ Raman, XRD and other techniques are employed to investigate the structural transformation influenced by thermal treatment. During the thermal treatment, dehydration of the nanosheets resulted in the complete transformation from titanate phase to anatase phase and the destroying of the laminated structure. The anatase phase has not been found to grow at temperatures below300℃. Increasing the temperature can lead to crystallinity improvement and the decrease of the surface area and the pore volume. These conversions influenced liquid-phase and gaseous-phase photocatalytic activity of the materials in different ways. With temperature increasing, the liquid-phase photocatalytic activity for RhB of the materials exhibited a downward trend, while the gaseous-phase photocatalytic activity for benzene firstly increased and then decreased when the temperature exceeded450℃.The aforementioned protonatcd anatase/titanate nanosheets are used as raw material to prcapare N-doped TiO2with high specific surface area. The as-prepared N-doped TiO2shows high visible-light photocatalytic activity for benzene. Influences of calcination temperature and mole feed ratio on the structure and properties of N-doped TiO2are also studied. It is found that the optimal calcination temperature is450℃. Lower temperature would lead to incomplete decomposition of urea, while higher temperature would hinder N dopant remaining in the TiO2lattice because of the excessive thermal vibration. N doping content will increase with the increasing mole feed ratio. However, the excess urea make no contribution to increase the N doping content when the mole feed ratio is above2:1. It is also found that the photocatalytic stability of N-doped TiO2is unsatisfactory because the photocatalytic activity of N-doped TiO2gradually decreases during the cycle experiment. That is because the N dopants in the TiO2lattice would be oxidized by the photogenerated holes. The photocatalytic activity of N-doped TiO2will decrease with the loss of N dopants. Therefore, it is necessary to improve the stability of the N dopants in the TiO2lattice.A facile one-pot hydrothermal method is proposed for synthesis of layered protonated titanate nanoshects (LPTNs) loaded with highly dispersed Ag nanoparticles, and the formation mechanism of this unique structure is also proposed. The as-synthesized samples are confirmed to be H2T2O5·H2O structure with mesoporous of3-4nm in diameter and high surface area of about200m2/g. The Ag nanoparticles loaded on the nanosheets are4-6nm in diameter and mainly exist in the form of zero-valence, which shifts the absorption edges toward longer wavelengths and enhances the visible-light absorption for the nanosheets due to the SPR of Ag. Ag loading remarkably enhances both the liquid-phase and gas-phase photoeatalytic activities of the titanate nanosheets under visible-light irradiation. An alternative possible mechanism for the enhancement of the visible-light photocalalylic activity is proposed. Moreover, the photocatalytic activity increases gradually with increasing Ag loading content first, and then decreases after maximizing at the optimal Ag/Ti molar ratio (2.87mol.%for photocatalytic degradation of RhB and1.57mol.%for photocatalytic mineralization of benzene, respectively). It can be attributed to that the excess Ag may aggregate to act as the recombination site and prevent light absorption of the LPNTs host by covering its surface. Therefore, the optimal Ag loading content should be explored for achieving the highest photocatalytic activity.Various contents of Ag nanoparticles were successfully introduced into the N-doped TiO2photocatalysts via a hydrothermal procedure. The photocatalysts were uniform particles and the adherent Ag mainly existed in the form of zero-valence. The existence of Ag restrained the escape of N dopants from the oxide during hydrothermal procedure. Such stabilization may be attributable to an electron transfer from the Ag5s orbitals towards the2p orbitals of the implanted N. The dependence of the photocatalytic activity on Ag content was also investigated by degradation of RhB under visible light irradiation. The photocatalytic activity increases gradually with increasing Ag content first, and then decreases after maximizing at the optimal Ag/Ti molar ratio of0.92mol.%. Therefore, the photocatalytic activity of Ag loaded N-doped TiO2photocatalysts can be adjusted by the Ag content and the optimal Ag loading content should be explored for achieving the highest photocatalytic activity.更多还原