摘要
纳米二氧化钛具有良好的光催化特性,在污染物降解、自清洁涂层等领域有广泛应用。但TiO_2的禁带宽度较大,需要以紫外光为激发光源,限制了其实际应用。将TiO_2的光吸收波长红移至可见光区,可以利用太阳光作为激发光源,节能环保。因此,扩展TiO_2光响应范围,使其在太阳光照射下催化降解有机物备受研究者关注。
掺杂非金属是制备可见光响应TiO_2的重要技术途径之一。本论文以四氯化钛为原料,采用沸腾回流法,以柠檬酸为碳源,制备了碳掺杂纳米TiO_2。利用水热法,分别以硝酸铵和氟化钠为添加剂,制备了氮掺杂和氟掺杂纳米TiO_2。利用XRD、TEM、SEM、UV-Vis、XPS和X射线吸收精细结构(XAFS)等表征方法,研究了纳米TiO_2的结构、形貌、光吸收性质等。通过对甲基橙溶液的降解来测试掺杂TiO_2样品的可见光催化活性,探讨了三种元素的最佳掺杂量,分析了可见光响应机理。
研究结果表明:(1)随着柠檬酸添加量的增大,产物中锐钛矿含量增加,球型颗粒变大,分散程度提高。未添加柠檬酸时,TiO_2的比表面积为92.232m2·g~(-1);添加柠檬酸后,样品比表面积增大到187.747m2·g~(-1)。比表面积的增大,有利于提高样品的光催化性能。(2)在反应体系中加入NH4NO3,产物为金红石相,形貌为纳米棒状颗粒组成的微球。
未掺杂TiO_2的比表面积为41.479m2·g~(-1),当NH4NO3添加量为0.4%时,比表面积为50.645 m2·g~(-1)。
(3)在反应体系中加入少量NaF所制备样品为金红石相,是由纳米棒状颗粒组成的微球。添加2.0%NaF所得样品为混晶,锐钛矿相含量为20.7%,金红石相含量为79.3%,比表面积为40.355 m~2·g~(-1)。
(4)与未掺杂样品和P25相比,掺杂C、N、F元素所制备TiO_2样品的吸收波长发生红移,在可见光区有吸收。以12W主波长460nm的蓝色LED灯为光源,甲基橙溶液为模拟污染物,测试样品的光催化性能。实验结果表明掺杂样品在光催化降解的过程中表现出良好的可见光活性,最佳样品的降解结果为:2.0%C-TiO_2光照3h降解率约为90%,0.5%N-TiO_2和0.4%F-TiO_2样品在光照5h时降解率约为62%,72%。
(5)利用XAFS分析了非金属元素掺杂TiO_2中Ti原子的局域结构,获得了配位数、键长等信息,研究结果表明,随着非金属元素N和F掺杂量的增大,所得样品的配位数和Ti-O键的键长变大。XAFS和XPS结果表明C和F原子主要以替代氧或钛原子的形式掺入二氧化钛晶格中。非金属掺杂改变了二氧化钛的能带结构,增强了可见光催化活性。
Due to excellent photocatalytic properties, titanium dioxide has been widely used in many fields, such as degradation of pollutants, self-cleanning coating, and so on. However, more practical applications have been limited by its wide band gap, which require ultraviolet light as the optical excitation source. Shifting the optical response region of TiO_2 to visible spectral range means that the sun light can be used as the optical excitation source. It is helpful for saving energy and environmental protection. So many researchers are focused on enlarging the optical response region of TiO_2 to degrade organics under the sun radiation.
It is an important way to prepare TiO_2 with visible light response by doping nonmental. In this paper, a series of TiO_2 photocatalysts were prepared using titanium tetrachloride as raw material. C-TiO_2 was synthesized using citric acid as carbon source by a simple hydrolysis method. N-TiO_2 and F-TiO_2 with visible light response were prepared using NH4NO3 and NaF as doped sources by hydrothermal method. The structure, morphology and light adsorption of such photocatalysts were characterized by XRD, TEM, SEM, UV-Vis, XPS, X-ray absorption fine structure(XAFS), etc. The photocatalytic degradation of methyl-orange was used as model reaction to evaluate the visible light activity of TiO_2 samples. It suggested that a best doping concentration existed for every dopant and the mechanism of doped TiO_2 for the visible-light activity was discussed.
The research results were suggested as following:
(1)The content of anatase phase of the sample rised with increasing amount of citric acid. The photocatalysts were well dispersed and their particle sizes increase with increasing amount of citric acid. The experimental result showed that the surface area of pure TiO_2 was 92.232m2·g~(-)1. After adding citric acid, the surface area of sample increased to 187.747m2·g~(-)1. The increase of surface area was helpful to improve their photocatalytic properties.
(2)With the addition of NH4NO3, the samples were rutile phase composed of nanorods. The surface area of undoped TiO_2 was 41.479m2·g~(-)1. When the addition amount of NH4NO3 was 0.4%, its surface area was 50.645 m2·g~(-)1.
(3)The TiO_2 samples prepared with small amount of NaF were rutile phase with the morphology of the microspheres composed of nanorods. When the mole percentage of NaF to titanium ions was 2.0%, the anatase phase could be observed in the TiO_2 powder. The content of anatase phase was 20.7% and the rutile phase was 79.3%, with the surface area of 40.355 m2·g-1.
(4)The UV–Vis adsorption spectroscopy showed that C、N、F-doped TiO_2 appeared a certain visible light adsorption and the optical band edge exhibited a certain red-shift compared with that of undoped TiO_2 and Degussa P25. The photocatalytic activity of prepared samples were tested under 12W LED light with a major emission wavelength at 460nm as visible light source and methyl-orange as model reaction. The research suggested that the final degradation rate of the optimum samples were as following: 2.0%C-TiO_2 was found to be nearly 90% within 3h. 0.5%N-TiO_2, 0.4%F-TiO_2 were about 62% and 72% within 5h.
(5)The XAFS technique was employed to analyse the change of the local structure around Ti atom. It showed the structure information of doping atoms, such as coordination number, bond distance, etc. With the increasing of N and F dopant, the coordination number and Ti-O bond become bigger. XPS and XAFS analysis indicated that C and F atoms had been doped in the lattice of crystalline TiO_2 by substituting the oxygen or Ti atom. Non-metal doping changed the energy band of TiO_2 and improved photocatalytic activity under visible light irradiation.
引文
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