Modification Of Multi-shelled Hollow ZnO Microspheres And Their Application

In recent years,the photo-induced degradation of pollutants and production of clean energy has become one of the hotspots in the field of material science.Although nano-ZnO featured with excellent photoelectric properties and structural stability shows preferable photocatalytic activity,it still suffers from the drawbacks of the inadequate visible light response and the fast recombination rate of photogenerated carriers.Multi-shelled hollow structure,as a special hollow structure,has attracted much attentions due to its unique properties.However,the reports about ZnO-based composite photocatalysts with multi-shelled hollow structure are still very rare.In this thesis,a serious of hollow multi-shelled ZnO based photocatalysts were designed and prepared.By analyzing the photocatalytic performance and photoelectrochemical properties of as-synthesized samples,the structure-activity relationship was discussed.Moreover,the mechanism of this improved catalytic efficiency was proposed based on the experimental observations and theoretical calculation results.The main work is summarized as follows:（1）A series of multi-shelled un-doped and Fe,Sn doped ZnO hollow microspheres were synthesized by calcining carbon microspheres as sacrificial templates.The doping content could be adjusted by tuning the composition of the impregnating precursor.The MB and CIP photodegradation tests indicate that the multi-shelled ZnO sample exhibits a higher photodegradation rate compared to ZnO nanoparticles.Moreover,the activity could be further improved by doping modification.The·O₂^-and h⁺species are the predominant active radicals in MB and CIP photodegradation reaction,respectively.Combined with the experimental observations and DFT calculation results,effect of Fe and Sn doping on electronic structure was thoroughly discussed.Firstly,the Fe,Sn dopant could introduce Fe 3d and Sn 5s,Sn 5p states in the forbidden band of ZnO,leading to an increased density of states（DOS）at fermi level.Thus,the ZF and ZS samples obtain a stronger visible light absorption.In addition,these doping levels could regulate the band structure,resulting in an increased difference of transfer rate between photogenerated holes and electrons.Accordingly,the separation efficiency of photogenerated e-/h+pairs is facilitated,as demonstrated by PL and photocurrent characterizations.（2）A novel AgVO₃ nanoparticles decorated multi-shelled ZnO heterostructure was synthesized via calcining carbon spheres template with subsequent in-situ growth method.On the one hand,the multi-shelled ZnO supports a larger specific surface area for AgVO₃ deposition,and alleviate its photocorrosion phenomenon.On the other hand,the introduction of AgVO₃ could effectively extend visible light absorption and promote photogenerated charge transfer in composites.Significantly,the Z-scheme photocatalytic mechanism was verified according to radical capture experiments and theoretical calculations based on density functional theory（DTF）method.As a result,the photogenerated electrons and holes accumulates in the CB of AgVO₃ and the VB of ZnO,respectively,which provides a stronger redox ability of ZnO/AgVO₃ composites.Benefiting from the excellent synergistic effect,the prepared ZnO/AgVO₃ composites showed remarkably enhanced photocatalytic performance of CIP degradation and Cr（VI）reduction compared to the single-phase photocatalysts.Additionally,the intermediate products of CIP photodegradation over ZnO/AgVO₃ composite were proposed based on the LC-MS results.（3）In this chapter,a serious of CQD,N-CQD,P-CQD,NP-CQD decorated multi-shelled ZnO composites photocatalysts were synthesized via a simple electrostatic adsorption process.The photocatalytic activity towards MB degradation is improved after the introduction of CQDs,while 2 g is the optimal decoration content.Importantly,NP-CQD/ZnO composite exhibits highest photodegradation performance,indicating that the N,P co-doped carbon quantum dots have a superior promotion effect compared to single-doped/and un-doped CQDs.According to the UV-Vis DRS results,all the CQDs could enhance the visible light absorption and reduce the optical band gap of ZnO,while the NP-CQD exhibits the best promotion effect.Furthermore,the N-containing and P-containing groups could be introduced onto the surface of NP-CQD,which induce the opposite energy band bending.Accordingly,the NP-CQD/ZnO composites have lowest fluorescence intensity,indicating its least recombination rate of photogenerated carriers.