Identification of intrinsic vacancies and polarization effect on ternary halo-sulfur-bismuth compounds for efficient CO2 photoreduction under near-infrared light irradiation

Ternary halo-sulfur bismuth compound Bi₁₉X₃S₂₇ (X = Cl, Br, I) with distinct electronic structure and full-spectrum light-harvesting properties show great application potential in the CO₂ photoreduction field. However, the relationship between photocatalytic CO₂ reduction performance and the function of halogens in Bi₁₉X₃S₂₇ is still poorly understood. Herein, a series of Bi₁₉X₃S₂₇ nanorod photocatalysts with intrinsic X and S dual vacancies were developed, which showed significant near-infrared (NIR) light responses. The types and concentrations of intrinsic vacancies were confirmed and quantified by positron annihilation spectrometry and electron spin resonance spectroscopy. Experimental results showed that Br atoms and intrinsic vacancies (dual Br-S) in Bi₁₉Br₃S₂₇ could greatly enhance the internal polarized electric field and improve the transfer and separation of photogenerated carriers compared with Bi₁₉Cl₃S₂₇ and Bi₁₉I₃S₂₇. Theoretical calculations revealed that Br atoms in Bi₁₉Br₃S₂₇ could facilitate CO₂ adsorption and activation and decrease the formation energy of reactive hydrogen. Among Bi₁₉X₃S₂₇ nanorods, Bi₁₉Br₃S₂₇ nanorods revealed the highest CO₂ photoreduction activity with CO yield rate of 28.68 and 2.28 μmol g_catalyst⁻¹ h⁻¹ with full-spectrum and NIR lights, respectively. This work presents an atomic understanding of the intrinsic vacancies and halogen-mediated CO₂ photoreduction mechanism.