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

The continued interest in one of the ancient, well-known and earth-abundant materials, iron oxide, has been due to its fascinating magnetic, optical and redox properties. This perhaps allowed its intricate connection to the history of life on Earth. The recent spurt of interest in the properties of sustainable materials with reduced dimensions has put iron oxides again into intensive exploration for its catalytic, redox, and photoelectrochemical applications. However, the origin of the shape anisotropy in iron oxide nanocrystals and its functional link with the geochemical and photocatalytic processes remain unaddressed. We, in this work, attempted to understand their shape anisotropy induced by naturally relevant organic ligands from sugar press mud (PM) under the framework of non-classical crystallisation theory (NCCT). We further followed its emergent but dramatic behaviour during photocatalysis using an electro-analytical approach. We found in our experiment that the evolution of shape anisotropy (spheres to sheets and rods) in iron oxide nanocrystals (~24-44 nm) remains explainable using NCCT, even in the case of the PM ligands as an additive to the reaction solution. However, the differences in their photocatalytic abilities (rate constants ‘k’,0.14-0.34 min-1) could be better explained using the concept of electrochemical work function (Φ_ad^0), and electrochemically active surface area (ESCA) than merely relying on band gap (Eg) and charge (e-/h+) carrier dynamics. Moreover, the findings of this work may inform the understanding of the fundamental mineralisation process in nature as well as the rationale design of a sustainable photo-catalyst.