Role of morphology, composition, and structure on the optical response of nanostructured hematite thin films

Abstract

Nanostructured hematite films are promising for photoactive materials applications. One of the main challenges in this matter is to improve the understanding about the behavior of the optical properties affected by the nanostructure. This work presents a robust ellipsometric analysis (supported by AFM, XRD, and XPS characterization) of the optical response of nanostructured hematite films synthesized by an alternative methodology based on chemical bath deposition. The good match between the measured and calculated Δ and Ψ angles after the fitting processes is an evidence of the reliability of the ellipsometric model constructed here and leads to a well-agreement between the thicknesses obtained from ellipsometry and those from AFM and profilometry. The analysis of the fitted model parameters made possible to determine the existence of two types of porosity in the films. On a microscopic scale, cracks or gross porosity were quantified directly from the v% of air in an effective medium layer of the model. A fine porosity with nanometric size persisting among the solid-phase that constitutes the nanostructured hematite films, was inferred from their lower refractive index with respect to bulk hematite. The extinction coefficient reveled a strong electronic transition centered at ~550 nm associated with the optical bandgap of the films, determined in approx. 2 eV (620 nm), which correspond with bulk hematite. All results shown here reveal a complex relationship between the morphology and the optical response, which introduce relevant aspects about the nanostructured hematite films, like density and porosity, that could be used to propose performance improvements in their potential applications as photoactive material. • Nanostructured hematite thin films synthesized by a chemical-bath based procedure. • Morphological, structural, and chemical composition supports the optical analysis. • Ellipsometry describes complex correlation between morphology and optical response. • Development of an alternative route toward viable photocatalytic films systems.