Published: August 09, 2011 r2011 American Chemical Society 17704 dx.doi.org/10.1021/jp205192a | J. Phys. Chem. C 2011, 115, 17704–17710 ARTICLE pubs.acs.org/JPCC Size-Dependent Bandgap of Nanogoethite Hengzhong Zhang,* ,† Meredith Bayne, † Sandra Fernando, † Benjamin Legg, † Mengqiang Zhu, ‡ R. Lee Penn, § and Jillian F. Banfield †,‡ † Department of Earth and Planetary Science, University of California, Berkeley, California 94720, United States ‡ Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Chemistry, University of Minnesota À Twin Cities, Minneapolis, Minnesota 55455, United States ’ INTRODUCTION Goethite (α-FeOOH) is a semiconductor (bandgap 2.10À2.50 eV 1,2 ). It is abundant at and near Earth’s surface and forms as nanoparticles as the result of mineral weathering and neutraliza- tion of acid mine drainage. During nucleation, crystal growth, and aggregation, nanogoethite particles can adsorb oxyanions, toxic metal cations, and organic molecules. In sunlight, nanogoethite can photochemically oxidize adsorbed organic molecules, leading to its reductive dissolution. 3 The goethite band positions and bandgap determine the generation of electrons and holes and, thus, the chemical/photochemical redox behavior. Usually, the bandgap of semiconductor nanomaterials (such as nano-CdSe 4 and ZnS 5 ) increases with decreasing particle size due to quantum size effects. 6 For example, goethite nanorods have shorter UVÀvis absorption wavelengths than do micrometer-scale rods, 3 implying a blue-shift in the bandgap. In contrast, it was also reported that the bandgap of a nanogoethite is smaller than that of bulk goethite. 7 To investigate this apparent inconsistency, we systematically explored the particle size dependence of the bandgap of goethite. The size-dependence of the goethite bandgap predicts the size- dependence of its reactivity in the environment 3,8 and is also relevant for technical applications (e.g., lithium battery electrodes and solar energy devices 9,10 ). ’ EXPERIMENTAL SECTION Syntheses of Nanogoethite. The 8.7 nm goethite nanopar- ticles (size obtained from Rietveld analysis, as described below) were synthesized using the method in ref 11. Eighteen milliliters of 2 M Fe(NO 3 ) 3 3 9H 2 O was mixed with 70 mL of 1 M NaOH, and then with 70 mL of deionized (DI) water. The reacted mixture was aged at room temperature for 49 days. The suspen- sion was centrifuged to separate the precipitates from the solution. Precipitates were redispersed in DI water to remove salts and then recovered by centrifugation. The centrifugation/ washing cycle was repeated six times. The final product was dried at 40 °C overnight. The 10.1 nm goethite sample was synthesized as follows. Five milliliters of 5 M KOH was mixed with 250 mL of 0.1 M Fe(NO 3 ) 3 3 9H 2 O in a beaker under magnetic stirring. The mixture was aged at 60 °C for 70 h. Next, the cooled mixture was centrifuged and washed, as above, and the product was dried at 80 °C for 4 h, then at 50 °C for 2 h. The 16.6 nm goethite sample was synthesized by reacting 30 mL of 0.5 M Fe(NO 3 ) 3 3 9H 2 O with 125 mL of 2.5 M KOH, followed by aging at 60 °C for 100 h. 11 The aged suspension was treated by repeated centrifugation/washing and then dried at 40 °C overnight to obtain the final product. The 26.8 nm and the 38.2 nm goethite samples were synthe- sized by reacting 20 mL of 5 M KOH with 200 mL of 0.1 M Fe(NO 3 ) 3 3 9H 2 O, followed by aging at 40 °C for 42 or 72 h, respectively. The aged suspensions were centrifuged, washed, and dried as above. Received: June 2, 2011 Revised: August 5, 2011 ABSTRACT: 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 UVÀvis 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 TaucÀMott plots. Over different photon energy ranges, nanogoethite ap- pears to exhibit direct bandgap (2.5À3.1 eV) and indirect bandgap (1.6À 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 ∼3À4% 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.