Construction And Modification Of ZnO/TiO₂ Nanotrees

TiO₂and ZnO have been shown to be the best photocatalysts for the redox degradation of organic compounds in many semiconductor oxides under ultraviolet(UV)irradiation.TiO₂has the advantages of a suitable band gap structure(3.2 e V),stable chemical properties,high corrosion resistance,non-toxicity,and low cost.While ZnO has a similar bandgap compared with TiO₂,the electron mobility is much higher than that of TiO₂.However,the rapid recombination of electron-hole pairs within a single semiconductor leads to a substantial decrease in its photocatalytic efficiency,while the construction of semiconductor heterojunctions can solve the problem of compounding of photogenerated electron-hole pairs,which leads to a significant increase in photocatalytic activity.In this paper,we constructed ZnO/TiO₂nanotree arrays by chemical methods and explored their applications in photocatalysis by further modifications.The main contents are as follows:A hydrothermal method was used to prepare the TiO₂nanorod arrays,and a ZnO seed layer was attached to the TiO₂nanorod surface using the sol-gel method,and then the seed layer was grown by secondary hydrothermal to form nanotree structure.The growth process was investigated.It is found that the ZnO/TiO₂heterojunction nanotree array has strong photocatalytic and photoelectrochemical properties,and the performance tends to strengthen and then weaken with the growth of the ZnO seed layer,and it shows the best performance at4 hours of growth.Its photocurrent density is 71?A/cm³,which is 5.5 times higher than that of the pure TiO₂nanorod array,and its photocatalytic rate constant(2.97×10^-3min-1)is 3times higher than that of the pure TiO₂nanorod array.This is mainly due to the heterojunction formed by TiO₂and ZnO which promotes charge transfer.And it decreases with further increase of time,mainly because the high density of ZnO plays a hindering role on the electron transfer channel.ZnO/TiO₂heterojunction nanotree arrays were first prepared and then modified with Ag quantum dots by quantum dot deposition.The test samples were characterized and performance tested,and the prepared Ag-modified ZnO/TiO₂heterojunction nanotree arrays were found to have enhanced photoelectrochemical and photocatalytic properties.Compared with pure TiO₂nanorod arrays,the Ag-modified ZnO/TiO₂heterojunction nanotree arrays showed a 7-fold increase in photocurrent,an increase in photocatalytic degradation rate from37%to 77%in 180 min,and a kinetic rate constant for the degradation of methyl orange that was three times higher than that of pure TiO₂nanorod arrays.The improved performance is partly due to the modification of Ag QD to produce localized surface plasmon resonance(LSPR)resulting in an increase in the intensity of absorbed light.Based on the ZnO/TiO₂nanotree arrays,the quaternary system of Ag₂S/Zn S co-modified ZnO/TiO₂nanotree arrays was obtained by sulfuration process and cation exchange method.the core-shell structure formed by the Zn S passivation layer and ZnO can not only prevent the decomposition of ZnO,but also enhance the light absorption caused by light scattering and promote the rapid transfer of photogenerated charges.In addition,modification by Ag₂S is a way to reduce the band gap width of the composites,which can lead to the red-shift of the light absorption edge and increase the light absorption intensity.The results show that the band gap width of the Ag₂S/Zn S/ZnO/TiO₂quaternary system is reduced to 1.95 e V.The instantaneous photocurrent density(0.15 m A/cm²)and photocatalytic rate constant(6.43×10^-3min^-1)of the Ag₂S/Zn S/ZnO/TiO₂quaternary system were 11 and 6.5 times higher than those of pure TiO₂,respectively.It is shown that the coordinated effect of Zn S and Ag₂S not only enhances the light absorption intensity,but also greatly promotes the photogenerated charge transfer and prevents the complexation of electron-hole pairs,thus improving the photoelectrochemical and photocatalytic performance.