Defect-engineered BiOI competitive to BiOI composites for photoelectrochemical sensing and photocatalytic removal of various water pollutants

This study underscores the significance of dopant selection and defect engineering of BiOI in maximizing its photoelectrochemical (PEC) sensing and photocatalytic activity (PCA), all without the necessity of incorporating it into advanced composites. Undoped and Al- or Sn-doped BiOI urchin nanostructures were synthesized via an ethylene glycol assisted solvothermal method, followed by analysis with XRD, XPS, SEM, TEM, BET, DRS, DFT, ROS, Raman spectroscopy, and electrochemical measurements. Al³⁺, with a remarkably smaller ionic radius than Bi³⁺ and very low solubility in BiOI, induced significant crystal deformation, reduced interplanar distances and lattice constants, particularly along the c-axis. However, Sn²⁺, with a close ionic radius to Bi³⁺ and different valency (aliovalent), generated oxygen vacancies (OVs) and narrowed the bandgap (E_g) by 0.10 eV compared to 0.03 eV for Al³⁺. The optimal BiOI:Sn (1 % Sn) showed superior PCA, with rate constants 1.7–1.9 times higher than BiOI:Al for Congo red (CR) and Doxycycline (DOX) degradation (30 × 10⁻⁶ and 12 × 10⁻⁶ W⁻¹ min⁻¹), which rivals advanced BiOI composites, demonstrating effective defect engineering without complex architectures. While E_g reduction did not add to the PCA enhancement, Sn doping created OVs, evidenced by Raman spectroscopy, and reduced charge transfer resistance (R_ct) to 39 Ω, enhancing charge separation and PCA. Moreover, the optimal BiOI:Sn demonstrated exceptional PEC sensing of Cr(VI) with a low detection limit of 0.13 μM and stability over 800 s, which rivals that of advanced BiOI composites. The superior performance of Sn-doped BiOI with respect to the Al-doped BiOI and BiOI composites is attributed to the choice of an aliovalent dopant (Sn²⁺) and close-in-size to the host atom (Bi³⁺) that creates balanced OVs and doping defect states. This work demonstrates that precise defect engineering as a standalone approach can elevate simple semiconductors to compete with advanced composites for photophysical applications, offering facile cost-effective alternatives and even further boosting current composites.