Evolution of Shape Anisotropy in Iron Oxides Nanocrystals: Biogenic Ligand Field, and Photocatalytic Efficiency under Tropical Sunlight

Iron oxides, one of the most ancient and earth-abundant materials, have long been studied for their remarkable magnetic, optical, and redox properties—attributes that have intricately linked them to the history of life on Earth. The renewed interest in sustainable materials with reduced dimensions has once again put iron oxides into intensive exploration, particularly for their catalytic, redox, and photoelectrochemical applications. However, the origin of the shape anisotropy in iron oxide nanocrystals and its functional significance in geochemical and photocatalytic processes remains largely unexplored. In this work, we investigated the role of naturally relevant organic ligands from sugar press mud (PM) in inducing shape anisotropy in iron oxide nanocrystals within the framework of non-classical crystallization theory (NCCT). Using an electro-analytical approach, we further examine the dynamic behaviour of these nanocrystals during photocatalysis. Our results reveal that the transformation from spherical to sheet- and rod-like morphologies (~24–44 nm) is consistent with NCCT, even in the presence of PM ligands as additives. However, differences in photocatalytic efficiency (rate constants, k ~ 0.014–0.038 min⁻¹) are better explained by electrochemical work function (Φ_ad^0) and electrochemically active surface area (ECSA) rather than solely by band gap (Eg) and charge carrier (e⁻/h⁺) dynamics. These findings not only provide insight into the fundamental mineralization processes in nature but also contribute to the rational design of sustainable photocatalysts.