BiOCl1-xIx及BiOBr1-xIx固体溶液的制备、表征及性能研究

Preparation, Characterization and Properties of BiOCl1-xIx and BiOBr1-xIx Solid Solution

在太阳光照射下,利用半导体光催化去除污染物是最绿色、有效的方法之一,其核心问题是获得高效光催化剂。目前研究最多的光催化剂是TiO 2和ZnO等,但由于其禁带宽度大故不能充分利用太阳光,从而限制了其实际使用。除了对TiO 2等改性以改进其可见光催化活性外,开发其他材料作为光催化剂也是解决的重要途径。铋基化合物半导体由于原材料丰富、种类多、太阳光响应性好及优良的光催化活性而成为重要研究对象。其中卤氧铋系化合物[BiOX, X=Cl, Br, I]由于层状结构特点而具有良好的光催化活性,但单独使用时光催化效率较低。研究表明, BiOX间能通过形成固体溶液(即彼此呈分子分散的固体混合物)进一步改善其光催化降解污染物的能力。本文利用低温湿化学法,以Bi 2O 3为铋源,在醋酸溶液中加入一定比例的KI/KBr或KI/KCl水溶液,室温下反应0.5 h,分别得到片状结构的BiOCl 1- xIx 和BiOBr 1- xIx 固体溶液。 X射线衍射(X-ray diffraction, XRD)分析结果表明,所合成的BiOCl 1- xIx 和BiOBr 1- xIx 样品结晶性良好,且能在 x =0~1的范围内形成固体溶液。透射电子显微镜(transmission electron microscope, TEM)测得所制备的固体溶液呈不规则的薄片状。 X射线光电子能谱(X-ray photoelectron spectroscopy, XPS)测试进一步证明了其表面元素组成及化学状态。紫外-可见漫反射光谱(diffuse reflectance spectroscopy, DRS)分析表明,随着碘元素含量的增加,固体溶液的吸收边界发生红移、禁带宽度减小,故可见光吸收能力增强、产生的载流子数目将增加。在可见光激发下对甲基橙(methyl orange, MO)降解的光催化性能研究表明, BiOCl 0.25 I 0.75 及BiOBr 0.25 I 0.75 拥有最高的光催化活性。循环实验表明, BiOCl 0.25 I 0.75 及BiOBr 0.25 I 0.75 都具有较高稳定性。光催化机理研究发现,这些卤氧铋样品光催化降解MO过程中的活性物种主要为空穴和超氧离子自由基。结合其能带结构,认为固体溶液的形成不但增加了可见光吸收能力,而且调变了其能带结构,相对于BiOI而言,固体溶液的形成降低了价带电位,提高了导带电位,因而增强了光生电子的还原能力及空穴的氧化能力,故催化性能提高。该工作的创新之处在于:采用的固体溶液制备方法,避免了高温水热法或加入表面活性剂等,而且所制备的BiOCl 1- xIx 和BiOBr 1- xIx 固体溶液,尤其是BiOCl 0.25 I 0.75 和BiOBr 0.25 I 0.75 ,在可见光激发下对MO具有良好的降解能力,且催化剂的重复使用及稳定性良好,有望在环境治理中得到应用。

The usage of semiconductor photocatalysts for removal of contaminants is one of the greenest and most effective methods under sunlight,whose core is obtaining high-efficient photocatalysts.The most widely studied photocatalysts are TiO 2,ZnO,etc.,but they cannot fully utilize sunlight due to their large band gapenergy,thus limiting their practical use.In addition to modifying TiO 2 to improve its visible light catalytic activity,the development of other materials as photocatalysts is also an important solution.Bismuth based compound semiconductors have become important research objects for their abundant raw materials,various types,good solar response and excellent photocatalytic activity.Bismuth oxyhalide compounds [BiOX,X=Cl,Br,I] exhibit excellent photocatalytic activity owning to layered structure,but they still have low photocatalytic efficiency when used alone.However,their photocatalytic degradation efficiency could be improved by preparing solid solutions (a mixture of solids that are molecularly dispersed with each other).In this work,a low-temperature wet chemical method can be used to obtain solid solutions BiOCl 1- x I x and BiOBr 1- x I x with sheet like structures,which is prepared by the reaction of a certain proportion of KI/KBr or KI/KCl aqueous solution with Bi 2O 3/HAc solution for half an hour at room temperature.X-ray diffraction (XRD) patterns showed that the synthesized BiOCl 1- x I x and BiOBr 1- x I x samples have good crystallinity and can form a solid solution in the range of x =0~1.The prepared solid solution was found to have an irregular sheetlike shape by a transmission electron microscope (TEM).X-ray photoelectron spectroscopy (XPS) tests further demonstrate their surface element composition and chemical state.Ultraviolet-visible diffuse reflectance spectroscopy (DRS) analysis indicates the red-shifted absorption edge of the solid solution and decreased band gap energy as the iodine content increased,so the visible light absorption capacity is enhanced and the number of generated carriers is agumented.The photocatalytic tests of MO degradation under visible light excitation manifest that BiOCl 0.25 I 0.75 and BiOBr 0.25 I 0.75 exhibit the highest photocatalytic activity.Cyclic experiments show that BiOCl 0.25 I 0.75 and BiOBr 0.25 I 0.75 have high stability.Photocatalytic mechanism studies show that the active species in the photocatalytic degradation of MO in these bismuth oxyhalide samples were holes and superoxide ion radicals.Combined with their energy band structures,it is believed that the formation of solid solution not only increases the visible light absorption capacity,but also modulates its energy band structure.Compared with BiOI,the formation of solid solution lowers the valence band position and raises the conduction band position.Therefore,the reducing ability of the photogenerated electrons and the oxidizing ability of the holes are enhanced,so that the catalytic performance is improved.The novelty of this work is low-temperature solid solution preparation,which avoids hydrothermal method or the addition of surfactants.Furthermore,the prepared BiOCl 1- x I x and BiOBr 1- x I x solid solutions,especially BiOCl 0.25 I 0.75 and BiOBr 0.25 I 0.75 ,have excellent photocatalytic degradation ability for MO under visible light excitation.Moreover,the catalysts have good stability,so it is expected to be applied in environmental management.