Fabrication,Photocatalytic and Gas Sensing Properties of ZnO and TiO2 Materials

Author:Cui Shu

Supervisor:li xiao tian


Degree Year:2019





ZnO is one of the most important oxide semiconductors in the fields of photocatalysis and gas sensor,which has received wide attention for its excellent physical and chemical properties.However,ZnO also has some shortcomings need to be urgently improved,such as low photocatalytic efficiency,poor selectivity and low sensitivity.In this thesis,starting from the synthesis of ZnO materials,a series of new ZnO materials were prepared,and the photocatalytic efficiency and gas sensitivity were effectively improved through morphology control,element doping,and material recombination.The main contents and conclusions are as follows:In chapter Ⅱ,a ZnO material that is simple to synthesize and of good photocatalytic and gas-sensitive properties was obtained by adjusting experimental conditions.Pure ZnO particles were prepared via sol-gel and hydrothermal method in a water-ethylene glycol binary solvent system.By controlling the concentration of the zinc source,ZnO particles with three morphologies were prepared:porous particles,peanut-like and grenade-like particles.Comparative analysis of photocatalytic properties revealed that the porous ZnO particles with smallest size(100-200 nm)showed the highest photocatalytic efficiency.The degradation rate was 0.054/min,and the cycle property was stable.The gas sensitivity test to ethanol gas showed that all samples performed well in cycling stability and response-recovery time.Samples with the porous granular morphology exhibited a higher response to ethanol gas(12.5,15.6for two samples,respectively),and the porous ZnO particles with the smallest particle size showed preferential selectivity for ethanol gas.It is found that the small size and porous characteristics provide more channels and space for gas diffusion and reaction,which improves the adsorption ability to oxygen in air and enlarges the electron depletion layer,thereby improving its gas sensing performance.This work provides a new strategy for simple synthesis of ZnO with good photocatalytic and gas sensitive properties,and provides a research reference for ethanol detection gas sensor.In chapter Ⅲ,based on the synthesis parameters of porous granulated ZnO mentioned above,the photocatalytic and gassensing properties of the materials were effectively improved by rare-earth Nd doping.A series of Nd/ZnO materials with different doping concentrations were prepared by introducing the doping source NdN3O9·6H2O,where Nd elements were successfully mixed into ZnO lattice and existed in Nd3+state.Nd doping can effectively inhibit the growth of ZnO crystallitesand make porous particles smaller and more uniform.The valence stateof Nd ion in the material changed flexibly between+3 and+2 valence during the photocatalytic process,capturing electrons in the conduction band,reducing the spontaneous recombination efficiency of photogenerated electron-hole pairs,effectively improving the photocatalytic performance,and raising the degradation rate to 2.7 times of that before doping.In terms of gas sensing properties,Nd doping increased the proportion of oxygen vacancies and adsorbed oxygen in the material,so as to enhance the adsorption ability of Nd/ZnO samples to gas.Consequently,the electron depletion layer in Nd/ZnO samples was thicker in air and thinner in ethanol gas,resulting in an increase in the gas sensitivity response of 36.02(100 ppm),which was 3.5 times higher than that before doping.Such multifunctional Nd/ZnO material is simple to synthesize,with superior performance and wider potential application,especially in the detection of ethanol gas,which is expected to be developed as an economical,highly sensitive and stable gas sensor for ethanol detection.In chapter Ⅳ,Fe3O4@SiO2@ZnO and Fe3O4@SiO2@Nd/ZnO core-shell composites with different morphologies were synthesized to improve the recovery performance and cycle stability of ZnO photocatalyst.The Fe3O4@SiO2 core was first synthesized step by step via template method,and then two photocatalysts were obtained by loading ZnO or Nd/ZnO on the core surface.The magnetic core acted as a template for the formation of the core-shell structure,which made the material have the magnetic recovery function and improved the practicability of the catalyst.In Fe3O4@SiO2@ZnO,ZnO grew uniformly on the core surface of Fe3O4@SiO2 spheres with nano-flake morphology,and the specific surface area of the material was up to68.43 cm2/g.This special structural characteristic made the photocatalyst exhibit strong adsorption performance,and the RB adsorption rate was up to 40%.The ZnO on the surface of Fe3O4@SiO2@Nd/ZnO exists in particle morphology,and the larger the doping concentration is,the smaller the size of ZnO particles will be.This proves that Nd doping plays an obvious modulation role on the surface ZnO morphology.The flexible change of the valence state of the rare earth ion Nd3+in the photocatalytic process improves the separation efficiency of photogenerated electronhole pair in the photocatalyst,and finally improves the photocatalytic performance of the material.This study provides a design idea for the synthesis of composite photocatalysts with good photocatalytic performance,practical magnetic recovery function and stable recycling efficiency.In chapter Ⅴ,starting from the structural design and response expansion in visible region of photocatalyst,we prepared a new m-TiO2@CdS visible photocatalyst with mesoporous characteristics.TiO2 mesoporous spheres were first prepared by combining the sol-gel and hydrothermal methods.Then,CdS nanoparticles were loaded on the surface of mesoporous spheres by hydrothermal method to obtain the m-TiO2@CdS heterogeneous photocatalyst.The large specific surface area(107.9m2/g)and rich pores of m-TiO2 framework effectively controlled the size and distribution of CdS nanoparticles(about 7 nm)and facilitated close contaction between TiO2 and CdS semiconductors.The structural design and the wide/narrow bandgap semiconductor composite broadened the response of m-TiO2@CdS photocatalyst to visible light and improved photocatalytic efficiency.Comparative experiments revealed that the degradation rate of m-TiO2@CdS material was up to21.7 times and 2.0 times higher than m-TiO2 and pure CdS.The improved photocatalytic performance was attributed to the synergistic interaction between TiO2and CdS,which effectively inhibited the recombination of photo-generated electron-hole pair.Featured by large specific surface area,strong visible light response ability and high photocatalytic efficiency,such m-TiO2@CdS material can be used to remove the organic pollutants in industrial wastewater.