Exploring the Built-in Electric Field in Single Core–Shell Microcrystals for Exceptional Iodine-131 Uptake

Creating a built-in electric field (BIEF) can effectively modulate the performance of functional materials. However, the effect of BIEF on adsorbents at the single-particle level has not been explored. Here, we employ in situ dark-field optical microscopy (DFM) imaging to track the reaction of single Cu₂O microcrystals and Cu₂O@BiOBr core-shell structures with I⁻. At high concentrations of I⁻, selective etching occurs in the Cu₂O core, while the pristine Cu₂O remains inactive. This illustrates the enhanced reactivity of p-type Cu₂O when coupled with the n-type BiOBr shell. Mechanistic studies reveal that the BIEF in the p-n interface region drives directional migration of I⁻, accelerating mass transfer and increasing local concentration, thus significantly enhancing the reactivity of Cu₂O@BiOBr. The reactivity improvement is further validated by batch adsorption at low I⁻ concentration, and Cu₂O@BiOBr is also employed for μCi-level radioactive ¹³¹I⁻ uptake. These findings highlight the critical role of the BIEF in adsorbent activity and may provide a general design principle for high-performance adsorbents.