Preparation And Properties Of Zn₂SnO₄ Based Photocatalyst

As is known,with the rapid development of economy,people are facing serious environmental pollution and energy shortage.How to solve these two problems which restrict the development of human beings has attracted the attention of the world.Since 1972,Fujishima first reported that when the battery which is composed of metal electrode and TiO2 semiconductor electrode is irradiated under UV light,the water can be decomposed to produce hydrogen.In various semiconductor materials,Zn2SnO4 is a typical inverse spinel structure,which has been widely used in the field of photochemical cell and photocatalysis due to its high electron mobility,high electricconductivity and good stability.However,the practical application of Zn2SnO4 photocatalyst is severely restrained due to its large band gap.The Zn2SnO4 semiconductor can only absorb UV light which occupies small part of the solar light.Therefore,it is necessary to adopt some effective ways to extend spectral responsive range of Zn2SnO4 in order to increase the absorption of visible light.In this paper,we concentrate on the regulation of morphology of Zn2SnO4,the modification of simple transition metal doping and the composition with narrow band gap semiconductor to enhance the photocatalytic performance of Zn2SnO4 under visible light.The main researched results are as follows:(1)The octahedral and polyhedral Zn2SnO4 nanopowders were successfully synthesized by a hydrothermal method.The influence of solution pH value on morphology during the hydrothermal process was investigated.The Zn2SnO4 photocatalyst prepared at different solution pH value was used to degrade Rhodamine B(RhB)solution.The photocatalytic performance was evaluated by the degradation efficiency of RhB solution.The experimental results indicate that octahedral Zn2Sn04 prepared at pH=11 exhibited the best photocatalytic performance(the degradation efficiency of RhB was 95%in 90 min).The enhanced photocatalytic performance of octahedral Zn2SnO4 can be attributed to its special surface morphology,which is formed by a number of densely hexagon nanoplates.These hexagon nanoplates connect with each other to compose interconnected network which can increase the transfer speed of photo-generated carriers from the interior to the surface.In addition,the lower recombination rate of electrons and holes of octahedral Zn2SnO4 prepared at pH=11 is another significant reason for the enhanced photocatalytic performance.(2)The Zn2SnO4 was modified by transition metal doping.In various of transition metal,the valence of Co2+is same as Zn2+andthe ion radius of Co2+(0.745 A)is similar to that of Zn2+(0.74 A),so we choose Co as the dopant.Various molar concentrations of Co-doped 2Zn2SnO4 nanoparticles were synthesized by a hydrothermal method.The photocatalytic performance of all prepared samples was evaluated by the degradation efficiency of RhB solution under visible light.The influences of Co doping content on photocatalytic performance of Zn2SnO4were investigated.The experiment results indicate that the maximum degradation rate of RhB was 93%in 120min when the Co2+ molar concentration was 2mol%.The enhanced photocatalytic performance of Co-doped Zn2SnO4 nanoparticles was due to the red shift of absorption edge and the reduction of band gap with the increasing of Co doping content,which leads to enhanced absorption of visible light.Furthermore,cdoping results in the formation of doping level between the conduction band and valence band of Zn2SnO4.Co2+ can act as the trap of photo-generated carriers to reduced recombination of electrons and holes.(3)The modification of Zn2SnO4 was achieved by compositing with narrow band gap semiconductor.The Bi2WO6/Zn2SnO4 photocatalyst was synthesized by establishingthe heterojunction.The photocatalytic performance of Bi2W06/Zn2SnO4 heterojunction was evaluated by the degradation efficiency of RhB solution under simulative sunlight.The influences of Bi2WO6 content on photocatalytic performance of Zn2SnO4were researched.The results show that the Bi2WO6/Zn2SnO4heterojunctionexhibited an excellent photocatalytic performance.The Bi2WO6/Zn2SnO4 heterojunction exhibited the best photocatalytic performance when Bi2WO6 molar concentration was 5mol%(the degradation efficiency of RhB was 96%in 80 min).The enhanced photocatalytic performance of Bi2WO6/Zn2SnO4 was due to the well-matched energy level structure and the formation of heterojunction between Bi2WO6 and Zn2Sn04.Due to the valence band potential of Zn2SnO4 is higher than that of Bi2WO6,the holes in the Zn2SnO4 valence band can easily migrate to the Bi2WO6 valence band,and the electrons in the Zn2SnO4 conduction band are difficult to migrate,which makes the electron-hole pairs effectively separated,therefore,enhanced the photocatalytic performance of Zn2SnO4.