Synthesis Of Two Dimensional Organic-inorganic Hybrid Perovskite Nanosheets And Their Degradation Under Illumination Studied By Transmission Electron Microscopy

Two-dimensional organic-inorganic hybrid perovskite materials have emerged rapidly,and attracted much attention due to their excellent photovoltaic,photoelectronic properties and their broad application prospects in nano-optoelectronic devices.However,little is known about the photostability and degradation mechanisms of two-dimensional perovskite nanosheets,although two-dimensional perovskite photovoltaic devices are exposed to harsh environments and long-term light illumination.Therefore,based on the successful synthesis of?C₄H₉NH₃?₂PbBr₄?BA₂PbBr₄?two-dimensional perovskite nanosheets,transmission electron microscopy?TEM?,X-ray diffraction?XRD?,atomic force microscopy?AFM?,photoluminescence spectroscopy?PL?and UV-Vis spectroscopy?UV-Vis?were used to study the morphology,optical properties,especially microstructure changes of BA₂PbBr₄ nanosheets under ultraviolet?UV?and white light irradiation,and we proposed the degradation mechanism of BA₂PbBr₄ nanosheets under illumination.1,Two-dimensional organic-inorganic hybrid perovskite nanosheets were successfully obtained by solution synthesis.By optimizing the synthesis conditions,the obtained BA₂PbBr₄ nanosheets showed regular square shape,high crystallinity,excellent optical properties.2,The effect of light illumination on the morphology of BA₂PbBr₄ nanosheets were studied by TEM.?a?During UV irradiation,the degradation of BA₂PbBr₄ nanosheets begins at the edge and gradually develops toward the center;while white light is more likely to introduce defects at the edge and inside of the nanosheets at the same time.?b?BA₂PbBr₄ nanosheets are more sensitive to UV illumination.The nanosheets have been damaged or became bulk particles after 5 minutes'UV light irradiation;however,white light irradiation showed significant degradation after 30 minutes.?c?Toluene could reduce the light illumination effect of the nanosheets.When there was toluene and UV light irradiation for 1 minute,the BA₂PbBr₄ nanosheets only degraded at the edge and still showed excellent crystallinity.However,when toluene was absent,the morphology of the BA₂PbBr₄ nanosheets was completely destroyed and became amorphous when exposed to UV light for 1 minute.3,Based on the in-depth analysis of electron diffraction and structural simulation,the changes of nanostructure and the adjustments of structural symmetry of BA2PbBr4 during the illumination were studied,and the degradation mechanism was proposed.?a?The?2n+1,k,0?and?0,2n+1,0?diffraction spots presents in the electron diffraction patterns when BA₂PbBr₄ nanosheets are exposed to UV light or white light for a short period of time.As the illumination time increases,the intensity of the?2n+1,k,0?and?0,2n+1,0?diffraction spots gradually decrease,while the?2n+1,2n+1,0?diffraction spots appear and enhance gradually.The symmetry of the two-dimensional BA₂PbBr₄ nanosheets was further altered.?b?Combined with the structural simulation,we confirmed that the appearance of the?2n+1,k,0?and?0,2n+1,0?diffraction spots related to the partial disorder of the organic parts of the BA₂PbBr₄ nanosheets.Due to the weak bond between the organic parts and the inorganic layers,the illumination causes a transition of the organic parts from an ordered arrangement to a partially disordered arrangement firstly.?c?The organic parts gradually change to disorder,and[PbBr₆]^4-octahedron is newly rearranged and tilted along the<110>direction under continuous illumination,resulting in a further change about crystal symmetry.Two-dimensional organic-inorganic hybrid perovskite nanosheets are one of the emerging nano-photoelectricl materials.However,they are extremely sensitive to electron beam.Therefore,there is no TEM study to further characterize its structure and structural transformation.In this paper,TEM and electron diffraction were used to reveal the illumination degradation mechanism of two-dimensional nanosheets.This paper enriches the understanding of two-dimensional perovskite nanosheets and their photostability,and has important guiding significance for design and applications of two-dimensional perovskite-based optoelectronic devices.