Research On ZnO Photocatalyst Used For Indoor Air Purification

Indoor air pollution is becoming more and more serious,which does great harm to people's health.Photocatalytic technology is one of the most potential indoor air purification technologies at the moment and ZnO is a kind of indispensable purification material of photocatalytic system for its inexpensive,non-toxic characteristics.As a result,using ZnO photocatalyst for indoor air purification is of great significance.In this paper,compositions of photocatalysts used for indoor air purification based on the structure and performance requirements of indoor purification materials were designed.ZnO,Nd-ZnO,Dy-ZnO,CdS-ZnO,CdS-Nd-ZnO,CdS-Dy-ZnO and CdS-Nd-Dy-ZnO samples were prepared by microwave hydrothermal method using zinc acetate and sodium hydroxide as the main starting materials.Samples'microstructure and performance were characterized by XRD,FE-SEM,SEM,UV-Vis spectrophotometer,XPS and photocatalytic degradation of methylene blue and toluene.The influence of reaction temperature,microwave treatment time and ultrasonication time during the preparation process on particle size,morphology and properties of ZnO were discussed.Also,the optimal preparation technology and technological parameters were selected.What's more,the mechanism of ZnO doped with rare earth elements and semiconductor composite photocatalysts for indoor air purification were explored.ZnO prepared by microwave hydrothermal method was of hexagonal prism shape with a wurtzite structure.The influence law of reaction temperature,microwave treatment time and ultrasonication time during the preparation process on particle size,morphology and properties of ZnO were discussed.Also,photocatalytic performance of samples was evaluated with the degradation of methylene blue and toluene which is a kind of typical indoor air pollutant.The results showed that as reaction temperature increased,particle size of ZnO had a tendency to decrease and microwave treatment time had little impact on the particle size of ZnO products,but longer reaction time was good for the crystallization of ZnO.Ultrasonic treatment accelerated mass transfer process and was beneficial to grain refinement.Order of influence of reaction conditions on ZnO photocatalytic performance as follows:reaction temperature>microwave treatment time>ultrasonication time.The optimum technological parameters as follows:reaction temperature at 160?,microwave treatment time of 60 min,and ultrasonication time of 20 min.Sample A19prepared under the above reaction conditions achieved the optimal photocatalytic performance,which made the degradation rate of methylene blue reach 89.69%under UV irradiation for 4 h,the degradation rate of methylene blue reach 87.89%under sunlight for 4 h,and the degradation rate of toluene reach 83.60%under UV irradiation for 4 h.In order to improve the photocatalytic performance of ZnO,rare earth element Nd was doped and Nd-ZnO photocatalyst samples were prepared.Photocatalytic performance of samples was evaluated with the degradation of methylene blue and toluene which is a kind of typical indoor air pollutant.Also,the mechanism of Nd-ZnO photocatalyst was explored.The results showed that Nd-ZnO samples were of hexagonal prism shape with a wurtzite structure.The doping of Nd did not change the crystal structure of ZnO,but led to the lattice distortion and increase of cell parameters,and also promoted the growth of ZnO crystals.Doping of Nd introduced impurity level which led to redshift of UV-Vis absorption spectra and increased light utilization.What's more,it caused lattice distortion and formed lattice defects which worked as electron capture traps to inhibit the recombination of electrons and holes.With the content of Nd increased,photocatalytic performance of Nd-ZnO presented a reduction trend after a period of increase.When n_Nd/n_Zn=0.20 mol%,Nd-ZnO sample B4 had the highest photocatalytic activity,which made the degradation rate of methylene blue reach 91.01%under UV irradiation for 4 h,the degradation rate of methylene blue reach 89.10%under sunlight for 4 h,and the degradation rate of toluene reach 85.40%under UV irradiation for 4 h,indicated that it is expected to be used for indoor air purification.In order to improve the photocatalytic performance of ZnO,rare earth element Dy was doped and Dy-ZnO photocatalyst samples were prepared.Photocatalytic performance of samples was evaluated with the degradation of methylene blue and toluene which is a kind of typical indoor air pollutant.Also,the mechanism of Dy-ZnO photocatalyst was explored.The results showed that Dy-ZnO samples were of hexagonal prism shape with a wurtzite structure.The doping of Dy did not change the crystal structure of ZnO,but led to the lattice distortion and increase of cell parameters and cell volume,and also promoted the growth of ZnO crystals.On the one hand,the introduction of Dy formed the impurity level,thus reduced the band gap of ZnO,increased absorption threshold,and widened the scope of light absorption of samples.On the other hand,doping of Dy caused lattice distortion and formed lattice defects which worked as electron capture traps.4f electron transformation of rare earth element Dy promoted the separation of electrons and holes.With the content of Dy increased,photocatalytic performance of Dy-ZnO presented a reduction trend after a period of increase.When n_Dy/n_Zn=0.40 mol%,Dy-ZnO sample C4 had the highest photocatalytic activity,which made the degradation rate of methylene blue reach 91.13%under UV irradiation for 4 h,the degradation rate of methylene blue reach 89.47%under sunlight for 4 h,and the degradation rate of toluene reach86.00%under UV irradiation for 4 h,indicated that it is expected to be used for indoor air purification.In order to improve the photocatalytic performance of ZnO,ZnO was composited with CdS semiconductor and CdS-ZnO,CdS-Nd-ZnO,CdS-Dy-ZnO,and CdS-Nd-Dy-ZnO samples were prepared.Photocatalytic performance of samples was evaluated with the degradation of methylene blue and toluene which is a kind of typical indoor air pollutant.Also,the mechanism of CdS-ZnO photocatalyst was explored.The results showed that ZnO was of hexagonal wurtzite structure and CdS was of cubic sphalerite structure.CdS attached to the ZnO crystal surface in the form of flocculent particles,and compounding with Cd S promoted the growth of ZnO crystals while an excess of CdS was unfavorable to the crystallization of ZnO.Compounding of semiconductors broadened light excitation energy range,and energy level difference between semiconductors promoted the separation of the carrier,reduced the recombination of electrons and holes.With the content of CdS increased,photocatalytic performance of composite materials presented a reduction trend after a period of increase.When n_CdS/n_ZnO=0.30 mol%,composite materials achieved the optimal photocatalytic performance,the corresponding samples of CdS-ZnO,CdS-Nd-ZnO,CdS-Dy-ZnO,and CdS-Nd-Dy-ZnO materials refered to D3,E3,F3and G3,made the degradation rate of methylene blue reach 90.38%,92.74%,92.90%and 93.42%respectively under UV irradiation for 4 h,made the degradation rate of methylene blue reach 89.37%,90.17%,90.66%,91.61%respectively under sunlight for 4 h,and made the degradation rate of toluene reach 84.80%,86.60%,87.40%,86.60%respectively under UV irradiation for 4 h.Sample G3 referimg to CdS-Nd-Dy-ZnO(n_CdS/n_ZnO=0.30 mol%,n_Nd/n_Zn=0.20 mol%,n_Dy/n_Zn=0.40 mol%)achieved the highest photocatalytic activity which indicated it can be used for indoor air purification.The reason of its improved photocatalytic peformance as follows:On the one hand,doping of rare earth elements introduced impurity level which led to redshift of UV-Vis absorption spectra and increased light utilization.What's more,it caused lattice distortion and formed lattice defects which worked as electron capture traps to inhibit the recombination of electrons and holes.On the other hand,compounding of semiconductors broadened energy range of light excitation,and energy level difference between semiconductors promoted the separation of the carrier which reduced the recombination of electronbs and holes.