Characterization of BP during the reaction. (a) Raman spectra of BP in 20 μmol/L Cu²⁺ solution at 0 and 2 h. (b) XPS spectra of BP exposed to Cu²⁺. The morphology of the BP sheet at 0 h (c) and 2 h (d) after reaction with Cu²⁺. The white tip indicates the complete edge of the BP sheet. The yellow tip indicates the rounded edge after partial oxidation, and the red circle indicates the black spots loaded on the surface of BP. (e) In situ AFM images acquired with Cu²⁺ at different times, showing the morphology evolution of a BP sheet. (f) Height profiles of the BP nanoflake acquired along the dashed lines in panel (e).