Characterization of the sol-gel formation of iron(III) oxide/hydroxide nanonetworks from weak base molecules

This work deals with the synthesis of iron(III) oxide/hydroxide xerogels from a hydrated ferric nitrate precursor salt (Fe(NO ₃) ₃·9H ₂O) through the addition of different weak base chemicals (e.g., propylene oxide, pyridine, and tetrahydrofuran) as gelation agents. The dried xerogel samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). XPS and XRD data show Fe(2p _3/2) and Fe(2p _1/2) peaks at values corresponding to that of an amorphous Fe ₂O ₃/FeO(OH) material. The morphology of the iron oxide/hydroxide xerogels obtained from TEM and AFM images indicates the formation of a "nanonetwork" assembly, with base particles of ∼5 nm in diameter aggregated into a network of clusters ∼30 nm in size. In addition to sample characterization, a mechanistic study was performed to probe the formation of the iron oxide/hydroxide xerogel using the three different weak base gelation agents. Previous work, in which propylene oxide was used as the gelation agent, has proposed that a "proton scavenging" mechanism was responsible for the formation of the xerogel. In the current study, this mechanistic view has been further investigated and generalized to encompass a broader class of weak bases through pH studies, XPS, and infrared spectroscopy.