Shape anisotropy in iron oxide nanocrystals: ligand field and photocatalytic efficiency under tropical sunlight

Abstract

Recent efforts in structure–activity-based sustainable photocatalyst design for solar energy harvesting highlight the need for a deeper understanding of the photocatalytic process and structural control in synthetic methods. Also, the control over the nanocrystal structure and the shape anisotropy in Earth-abundant materials like iron (mineral) oxide (α-Fe₂O₃), particularly during bottom-up synthetic approaches, is of multiple significance yet poorly understood. Furthermore, the functional correlation of shape anisotropy with photocatalytic efficiency and its fundamental relevance in geochemical processes remains largely unexplored. This work investigated the comparative role of naturally relevant organic ligands from sugar press mud (PM) with chemical surfactants to induce shape anisotropy among iron oxide nanocrystals in an aqueous sol–gel (bottom-up) synthetic approach. Using electroanalytical tools, we further examine the dynamic link between the structure and activity of these nanocrystals during photocatalysis. Our results revealed that the transformation of hematite (α-Fe₂O₃) nanocrystals from spherical to sheet-and rod-like morphologies (∼24–44 nm) is broadly consistent with non-classical crystallization theory (NCCT), even in the presence of biogenic ligands (PM) as additives. Moreover, the differences in photocatalytic efficiency (rate constants, k ∼0.014–0.038 min⁻¹) are better explained by using a combined framework of ‘Langmuir–Hinshelwood (L–H) kinetic model and Marcus–Gerischer charge (e⁻/h⁺) transfer theory’ than solely by the traditional band gap (E_g ∼ 2.0 eV) and charge carrier (e⁻/h⁺) dynamics approach. These findings may provide insight into the rational design of sustainable photocatalysts for solar energy harvesting and contribute to understanding the fundamental geochemical (light-mineral interaction) processes in nature.