Sn/Be Sequentially co-doped Hematite Photoanodes for Enhanced Photoelectrochemical Water Oxidation: Effect of Be²⁺ as co-dopant

For ex-situ co-doping methods, sintering at high temperatures enables rapid diffusion of Sn⁴⁺ and Be²⁺ dopants into hematite (α-Fe₂O₃) lattices, without altering the nanorod morphology or damaging their crystallinity. Sn/Be co-doping results in a remarkable enhancement in photocurrent (1.7 mA/cm²) compared to pristine α-Fe₂O₃ (0.7 mA/cm 2), and Sn⁴⁺ mono-doped α-Fe₂O₃ photoanodes (1.0 mA/cm²). From first-principles calculations, we found that Sn⁴⁺ doping induced a shallow donor level below the conduction band minimum, which does not contribute to increase electrical conductivity and photocurrent because of its localized nature. Additionally, Sn⁴⁺ -doping induce local micro-strain and a decreased Fe-O bond ordering. When Be²⁺ was co-doped with Sn⁴⁺ -doped α-Fe₂O₃ photoanodes, the conduction band recovered its original state, without localized impurities peaks, also a reduction in micro-strain and increased Fe-O bond ordering is observed. Also the sequence in which the ex-situ co-doping is carried out is very crucial, as Be/Sn co-doping sequence induces many under-coordinated O atoms resulting in a higher micro-strain and lower charge separation efficiency resulting undesired electron recombination. Here, we perform a detailed systematic characterization using XRD, FESEM, XPS and comprehensive electrochemical and photoelectrochemical studies, along with sophisticated synchrotron diffraction studies and extended X-ray absorption fine structure.