Study On The Properties Of Photocatalytic CO₂ Reduction Of Bismuth Based Lead Free Perovskite Nanocrystal

In response to the environmental pollution caused by the excessive consumption of traditional fossil fuels,many researchers are devoted to the development of clean energy for a long time.Photocatalytic CO₂ reduction can not only effectively alleviate the problem of greenhouse effect,but also can convert CO₂ into clean chemical fuel,which has attracted widespread attention.The design and development of photocatalysts with high cost-effective is the key to promoting the industrial application of this technology.In recent years,lead halide perovskites have been widely applied in the field of photocatalytic CO₂ reduction,due to their excellent photochemical properties and low-cost fabrication techniques.However,the toxicity of lead-based perovskite is a difficult problem,which limits its commercial application.Therefore,it is very important to develop alternative lead-free metal halide perovskite materials.Cs₃Bi₂I₉ is a lead-free perovskite material with low toxicity and high stability.However,this semiconductor material displays very low photocatalytic activity.In order to improve the photocatalytic CO₂ reduction activity of Cs₃Bi₂I₉,the following two works were carried out in this thesis:（1）Considering that the photogenerated holes in the valence band of Cs₃Bi₂I₉nanocrystals cannot effectively drive the water oxidation reaction,we have modulated the energy band structure of Cs₃Bi₂I₉ nanocrystals by doping Br.A series of Cs₃Bi₂Br_xI_9-x（X=0～9）nanocrystalline materials were prepared by changing Br content based on ligand assisted recrystallization method,and the photocatalytic CO₂ reduction performance with H₂O as an electron source was also investigated for these photocatalysts.The results show that increasing the Br content can decrease the light absorption capacity of Cs₃Bi₂Br_xI_9-x,but meanwhile bring forth obvious increasement of thermodynamic driving force for water oxidation.For photocatalytic CO₂ reduction system with undoped Cs₃Bi₂I₉ nanocrystal as photocatalyst,the yield of CO product is only 21μmol g^-1 after 15 h of irradiation.Under the same condition of irradiation,the yield of CO for Cs₃Bi₂I₆Br₃ nanocrystal reaches up to 70μmol g^-1,exhibiting an over3-fold increase of photocatalytic activity compared to Cs₃Bi₂I₉ counterpart.Further¹³CO₂ and H₂¹⁸O isotopic labeling experiments confirmed that CO was generated by CO₂ reduction,and the required electrons originated from H₂O oxidation.（2）In order to improve the separation efficiency of photogenerated carriers in Cs₃Bi₂I₉ nanocrystals,we have prepared Cs₃Bi₂I₉/Bi₂WO₆ heterojunction by the in-situ growth of Cs₃Bi₂I₉ nanocrystal on the surface of ultrathin Bi₂WO₆ nanosheet with the co-sharing of Bi atoms.Co-sharing of the Bi atom can enable intimate contact and strong electron coupling between Cs₃Bi₂I₉ and Bi₂WO₆,which are beneficial to interfacial charge transfer.In-situ irradiated X-ray photoelectron spectroscopy（XPS）studies demonstrated that the pathway of photogenerated carrier separation in this heterojunction complies with Z-scheme mode,resulting in a well-preserved redox ability.The Cs₃Bi₂I₉/Bi₂WO₆ heterojunction presents a significant improvement of photocatalytic performance for the CO₂-to-CO conversion with water as the electron source,being 65μmol g^-1 after 9 h of irradiation,which is over 4 times higher than that of pristine Cs₃Bi₂I₉ nanocrystals.This work provides an effective strategy for the construction of Z-scheme heterojunctions based on lead-free halide perovskite.