Size-dependent bandgap of nanogoethite

Rod-shaped goethite nanoparticles with average particle sizes (equivalent spherical diameters) of between ∼9 and 38 nm were synthesized via reaction of ferric nitrate with potassium/sodium hydroxide in aqueous solutions. We deconvoluted the UV-vis spectra into individual absorption bands for each of the nanogoethite samples and determined the particle size dependence for each band. As the particle size decreases, the charge transfer band is slightly red-shifted, whereas five other bands, including the electron pair transition that determines the absorption edge, are blue-shifted. Spectra were also used to determine bandgap energies as a function of particle size via Tauc-Mott plots. Over different photon energy ranges, nanogoethite appears to exhibit direct bandgap (2.5-3.1 eV) and indirect bandgap (1.6-2.1 eV) behaviors. The bandgap widens as particle size decreases, an effect that can be described by the Kayanuma equation, from which the reduced mass of an exciton in nanogoethite was found to be ∼3-4% the mass of a rest electron. The existence of an indirect bandgap at relatively lower energy as compared to the direct bandgap and altered redox properties due to shifts and opening of the bandgap as particle size decreases may partially explain size-dependent chemical and photochemical reactivity of goethite.