title: Structure and Formation Mechanism of Black TiO2 Nanoparticles.
abstract: The remarkable properties of black TiO2 are due to its disordered surface shell surrounding a crystalline core. However, the chemical composition and the atomic and electronic structure of the disordered shell and its relationship to the core remain poorly understood. Using advanced transmission electron microscopy methods, we show that the outermost layer of black TiO2 nanoparticles consists of a disordered Ti2O3 shell. The measurements show a transition region that connects the disordered Ti2O3 shell to the perfect rutile core consisting first of four to five monolayers of defective rutile, containing clearly visible Ti interstitial atoms, followed by an ordered reconstruction layer of Ti interstitial atoms. Our data suggest that this reconstructed layer presents a template on which the disordered Ti2O3 layers form by interstitial diffusion of Ti ions. In contrast to recent reports that attribute TiO2 band-gap narrowing to the synergistic action of oxygen vacancies and surface disorder of nonspecific origin, our results point to Ti2O3, which is a narrow-band-gap semiconductor. As a stoichiometric compound of the lower oxidation state Ti(3+) it is expected to be a more robust atomic structure than oxygen-deficient TiO2 for preserving and stabilizing Ti(3+) surface species that are the key to the enhanced photocatalytic activity of black TiO2.
authors.affiliation: Department of Materials Science and Engineering, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: Mengkun
authors.initials: M
authors.lastname: Tian
authors.forename: Masoud
authors.initials: M
authors.lastname: Mahjouri-Samani
authors.forename: Gyula
authors.initials: G
authors.lastname: Eres
authors.forename: Ritesh
authors.initials: R
authors.lastname: Sachan
authors.forename: Mina
authors.initials: M
authors.lastname: Yoon
authors.forename: Matthew F
authors.initials: MF
authors.lastname: Chisholm
authors.forename: Kai
authors.initials: K
authors.lastname: Wang
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Puretzky
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Rouleau
authors.forename: David B
authors.initials: DB
authors.lastname: Geohegan
authors.affiliation: Department of Materials Science and Engineering, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: Gerd
authors.initials: G
authors.lastname: Duscher
keywords: TEM characterization; black TiO2; core−shell structure; nanoparticles; nonstoichiometry; reduced band-gap