Controllable Synthesis Of Hierarchical ZnO Microspheres And Photocatalytic CO₂ Reduction Modification

The severe greenhouse effect and energy shortage urges the development of low-carbon and carbon-free renewable sources of energy.Among all the candidates,solar energy is the most attractive one due to its big storage,non-pollution and universality.Recently,transforming solar energy to hydrocarbon fuel for storage and utilization through photocatalytic strategy has become a promising way.Zinc oxide（ZnO）,as one of the effective photocatalysts,has been popularly studied.However,the photocatalytic performance of ZnO is largely limited due to its broad band gap,low quantum yield and easy inactivation.In this thesis,hierarchical ZnO microspheres were successfully synthesized via a mild and non-pollution method.Graphitic carbon nitride and dopamine were used to modify the ZnO microspheres,and the photocatalytic performance was promoted.The main contents are listed as follows:（1）（Zn（NO₃）₂·6H₂O）was used as the source of zinc,deionized water was used as solvent,（CO（NH₂）₂）and（Na₃C₆H₅O₇·2H₂O）were added in the solution.After full stirring,the solution was transferred into a Teflon-lined autoclave and maintained at120℃for 6 h.（Zn₃（OH）₄CO₃·H₂O）can be obtained by this hydrothermal method.Then the sample was calcined to synthesize ZnO.（CO（NH₂）₂）was calcined to synthesize graphitic carbon nitride.Herein,an electrostatic self-assembly method was exploited to prepare g-C₃N₄/ZnO composite microsphere.This method simply utilized the opposite surface charge of each component,achieving a hierarchical structure with intimate contact between them.The hierarchical structure greatly enhances the light utilization by multi-scattering effect.The optimal capacity of g-C₃N₄ was proved to be 5%,and the obtained g-C₃N₄/ZnO nanocomposites upgraded the photocatalytic CO₂ reduction activity by 4 times.A direct Z-scheme mechanism was proposed and confirmed,which was responsible for the improvement of photocatalytic performance.（2）As an organic compound,dopamine is a promising surface modification with a wide range of application.It has a lot of chemical groups,such as amidogen and hydroxy,which can be used for effective CO₂ absorption.The hydroxy can bond with Zn²⁺on the surface of ZnO,which results in an interaction between them.It also can promote the transfer of photo-generated charge carriers.Herein,we synthesized dopamine-modified ZnO microspheres with enhanced spectral utilization.The photocatalytic CO₂ reduction experiment indicated that the addition of dopamine benefited the photocatalytic performance,the optimal capacity of dopamine was proved to be 2%.Analysis indicates that the addition of dopamine can broaden spectral response region of ZnO,and also constructs a Z-scheme charge transfer mechanism with ZnO,which is the main reason for the promotion of photocatalytic performance of ZnO.