On the origin of an unusual dependence of (bio)chemical reactivity of ferric hydroxides on nanoparticle size

• Published in: Physical chemistry chemical physics : PCCP Vol. 12; no. 42; pp. 14045 - 14056
• Main Authors: CHERNYSHOVA, I. V, PONNURANGAM, S, SOMASUNDARAN, P
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 14.11.2010
• Subjects: Catalysis; Chemistry; Colloidal state and disperse state; Exact sciences and technology; Ferric Compounds - chemistry; General and physical chemistry; Iron - chemistry; Microscopy, Electron, Transmission; Nanoparticles - chemistry; Optical Phenomena; Oxygen - chemistry; Particle Size; Photoelectron Spectroscopy; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Spectrophotometry, Ultraviolet; Surface Properties; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Water - chemistry; X-Ray Diffraction; Hematite; Binary compound; Electronic properties; Catalytic reaction; Hydroxides; Iron oxide; Nanoparticle; Stabilization; In situ; Transition element compounds; X ray; Hybridization; Absorption spectrum; Phase transformation; Chemistry; Valence; Valence electron; Photoelectron spectrometry; Resonance; Chemical reactivity; Charge transfer; Lattice structure; X-ray photoelectron spectra; Ultraviolet visible spectrometry
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c0cp00168f

Application of in situ UV-Vis absorption spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) makes it possible to resolve the controversies about the electronic properties of hematite (α-Fe(2)O(3)) nanoparticles (NPs) and, on this basis, to rationalize the unusual dependence of aquatic (bio)chemistry of these NPs on NP size. 2-Line ferrihydrite (FH) is also included in the study as the end polymorph of the size-driven phase transformation of hematite NPs in aqueous media. It is shown that the absorption edge of all NPs studied is due to the direct O 2p-Fe 3d charge transfer (CT) process, while a manifold of weak bands superimposed onto two main p-d CT bands is attributed to the d-d ligand field transitions. The band gap decreases from 2.95 to 2.18 eV with increasing NP size from 7 nm to 120 nm. This effect is attributed to restoration of hematite lattice structure, which ultimately results in an increase in the O 2p-Fe 3d hybridization, stabilization of the valence band, and delocalization of valence electrons, as confirmed by XPS. Finally, we show that the optical effects such as the Mie resonance significantly distort absorption spectra of hematite NPs larger than ∼120 nm. Possible impacts of these findings on (photo)catalytic and biochemical properties of ferric (hydr)oxide NPs are discussed.