Detection of Pb2+ traces in dispersion of Cs4PbBr6 nanocrystals by in situ liquid cell transmission electron microscopy

Abstract

The Cs₄PbBr₆ nanocrystals are often used as a starting material for the preparation of green-emitting CsPbBr₃ perovskite nanocrystals by means of chemical and physical transformations. Herein, we probe the Cs₄PbBr₆ nanocrystals dispersed in a solvent by liquid cell transmission electron microscopy (LCTEM). The nanocrystal dispersion in toluene is placed between two electron-transparent membranes separated by a gold spacer in a liquid cell and studied in a high angular annular dark-field scanning TEM mode with a fixed electron dose rate. We observe the spontaneous nucleation and growth of round and dendrite-shaped nanoparticles under electron beam illumination in the areas of solution where no Cs₄PbBr₆ nanocrystals are seen. These newly-formed nanoparticles show high contrast and contain Pb as the only heavy element, suggesting that they are made from metallic lead and indicating Pb²⁺-containing species in solution as their precursor. Also, a small amount of Au⁰ nanoparticles are formed, most likely due to the dissolution of the gold spacer by free Br-containing species in the nanocrystal dispersion and a subsequent reduction of the leached species under the electron beam. The analysis of the UV-Vis absorption spectra of Cs₄PbBr₆ nanocrystals and the supernatant isolated from the synthesis points to mixed lead(II) oleate/bromide species as the likely residue, corroborating LCTEM results. The identification of the residual precursors in Cs₄PbBr₆ nanocrystal samples after the post-synthetic isolation is an important task because the residues may alter the subsequent reactivity of the nanocrystals.