尖晶石型金属氧化物的制备及光催化有机污染物降解:综述

Preparation of Metal Spinel Oxides for Photocatalytic Degradation of Organic Pollutants:a Review

有机染料已成为当今水质污染的重要来源,而大多有机染料对光稳定、不易降解,因此,利用光催化技术解决水源污染问题是目前研究的热点。尖晶石型金属氧化物半导体材料被证明是一种能有效利用太阳光催化降解有机污染物的催化剂。尖晶石型金属氧化物具有禁带较窄、可吸收可见光的特点。它们在光的照射下能激发产生电子和空穴,实现对有机染料的催化降解。如何调控尖晶石型金属氧化物的光催化性能,是当今国内外学者的研究重点。众多研究表明,影响尖晶石型金属氧化物催化性能的主要因素包括以下三个方面:(1)制备方法。其制备方法繁多,如热处理法(包括热分解)、水热-溶剂热法、溶胶-凝胶法、微波法、自燃烧法、共沉淀法、固态合成法、静电纺丝法等。制备方法不同可影响所制备材料的形貌、表面积、颗粒尺寸及禁带宽度等,从而影响材料的光催化性能。如对于ZnAl2O4,采用共沉淀法得到的材料的表面积为94.4m^2/g,而采用微波-水热法得到的材料的表面积大幅提高到279.7m^2/g;又如,采用金属硝酸盐和金属氯化物所制备的尖晶石型NiFe2O4的禁带宽度不同,分别为2.7eV和1.6~1.8eV。(2)掺杂第三种金属。对于禁带较宽的尖晶石型金属氧化物,可通过掺杂第三种金属来降低其禁带宽度。如金属Mg的掺杂使CoFe2O4的禁带宽度由2.4eV降至1.8eV,Ce^3+的掺杂可使ZnAl2O4的禁带宽度由3.8eV降至2.8eV,少量Bi的掺杂可使CoFe2O4的禁带宽度由1.4eV降至1.3eV。(3)光生电子和空穴的有效分离。光催化剂的活性取决于光生电子或空穴的有效量。为改善尖晶石型光催化剂的光催化活性,可通过掺杂导电性的材料(如石墨烯)或具有合适电位的金属氧化物(如ZnO)、限域等办法实现光生电子和空穴的有效分离,降低二者的复合率,从而提高催化剂的催化活性。目前报道的尖晶石型金属氧化物的种类很多,本文按金属酸根对其进行了分类,分别对尖晶石型铁酸盐、铝酸盐等材料的制备及光催化有机染料的降解机理进行了分析与总结,期望对开发新型的、具有优良催化性能的尖晶石型光催化剂提供一定的指导。

Most organic dyes are difficult to be photo-degraded due to their light stability.And organic dyes have been considered as an important source of water pollution now.Therefore,it is imminent to use photocatalytic technology to solve the problem of water pollution.Fortunately,metal spinel oxides semiconductor materials have been demonstrated to be an effective catalyst for the degradation of organic pollutants by using sunlight.The spinel-type metal oxides have the characteristics of narrow band gap and absorption of visible light,and can generate electrons and holes under the irradiation of light to realize photocatalytic degradation of the organic dyes.How to regulate the photocatalytic performance of metal spinel oxides is the focus of current international and domestic research.Numerous studies have shown that the main factors affecting their catalytic performances include the following three aspects:i.Preparation method.Various preparation methods,such as heat treatment(including thermal decomposition),hydrothermal-solvent heat,sol-gel,microwave method,self-combustion method,coprecipitation method,solid state synthesis method,electrospinning method,etc.Diffe-rent preparation methods can affect the morphology,surface area,particle size and band gap of the prepared materials,thus affecting the photocatalytic properties of the materials.For example,the surface area of ZnAl2O4 prepared by coprecipitation method and microwave-hydrothermal method was 94.4 m^2/g and 279.7 m^2/g,respectively;the forbidden band width of spinel NiFe2O4 prepared by using metal nitrate and metal chloride corresponded to 2.7 eV,1.6—1.8 eV.ⅱ.Doping the third metal.For spinel-type metal oxides with a wider band gap,the forbidden band width can be reduced by doping the third me-tal.For example,the doping of metal Mg can reduce the band gap of CoFe2O4 from 2.4 eV to 1.8 eV,and the doping of Ce^3+reduces the band gap of ZnAl2O4 from 3.8 eV to 2.8 eV.The doping of a small amount of Bi makes the band gap of CoFe2O4 drops from 1.4 eV to 1.3 eV.ⅲ.Effective separation of photoinduced electrons and holes.In order to improve the photocatalytic activity of the spinel photocatalyst,it can be realized by doping a conductive material(such as graphene)and a metal oxide with a suitable potential(such as ZnO).The photoinduced electrons and the holes can be effectively separated to reduce the recombination rate of electron-hole pairs and to improve the catalytic activity of the photocatalyst.There are many kinds of reported spinel-type metal oxides so far.This review classifies spinel-type metal oxides according to their metal acids.The preparation of spinel-type ferrites and aluminates,and the research progress of performance and mechanism of photocatalytic degradation of organic dyes are analyzed and summarized respectively,and the future development direction has also been prospected.It is expected that this review will play an important role in the development of a series of novel spinel photocatalysts with excellent catalytic performances.