Metallorganic Routes to Nanoscale Iron and Titanium Oxide Particles Encapsulated in Mesoporous Alumina: Formation, Physical Properties, and Chemical Reactivity

Abstract

Iron and titanium oxide nanoparticles have been synthesized in parallel mesopores of alumina by a novel organometallic "chimie douce" approach that uses bis(toluene)iron(0) (1) and bis(toluene)titanium(0) (2) as precursors. These complexes are molecular sources of iron and titanium in a zerovalent atomic state. In the case of 1, core shell iron/iron oxide particles with a strong magnetic coupling between both components, as revealed by magnetic measurements, are formed. Mössbauer data reveal superparamagnetic particle behavior with a distinct particle size distribution that confirms the magnetic measurements. The dependence of the Mössbauer spectra on temperature and particle size is explained by the influence of superparamagnetic relaxation effects. The coexistence of a paramagnetic doublet and a magnetically split component in the spectra is further explained by a distribution in particle size. From Mössbauer parameters the oxide phase can be identified as low-crystallinity ferrihydrite oxide. In agreement with quantum size effects observed in UV-visible studies, TEM measurements determine the size of the particles in the range 5-8 nm. The particles are mainly arranged alongside the pore walls of the alumina template. TiO2 nanoparticles are formed by depositing 2 in mesoporous alumina template. This produces metallic Ti, which is subsequently oxidized to TiO2 (anatase) within the alumina pores. UV-visible studies show a strong quantum confinement effect for these particles. From UV-visible investigations the particle size is determined to be around 2 nm. XPS analysis of the iron- and titania- embedded nanoparticles reveal the presence of Fe2O3 and TiO2 according to experimental binding energies and the experimental line shapes. Ti4+ and Fe3+ are the only oxidation states of the particles which can be determined by this technique. Hydrogen reduction of the iron/iron-oxide nanoparticles at 500 degrees C under flowing H2/N2 produces a catalyst, which is active towards formation of carbon nanotubes by a CVD process. Depending on the reaction conditions, the formation of smaller carbon nanotubes inside the interior of larger carbon nanotubes within the alumina pores can be achieved. This behavior can be understood by means of selectively turning on and off the iron catalyst by adjusting the flow rate of the gaseous carbon precursor in the CVD process.

Magnetic Core/Shell and Quantum-Confined Semiconductor Nanoparticles via Chimie Douce Organometallic Synthesis

“…When using this approach within the mesospace provided by porous alumina films, strongly magnetically coupled metal/metal oxide systems as well as pure single-phase metal oxides are accessible. [14] The homogeneous and tunable size, as well as the aqueous acidic atmosphere of the alumina mesopores, have proven to be unique as a reaction partner offering both confinement and special reaction conditions for metal oxide formation in their interior. Incorporation of organometallics, followed by subsequent reaction of a defined molecular organometallic entity, has so far not been reported for this type of host.…”

“…The subsequent formation of a two-layer-like particle, comprising an iron oxide-/hydroxide-rich shell near the pore surface with an inner, metallic-iron core, can be deduced from magnetic, Mössbauer, extended X-ray absorption fine structure (EX-AFS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements. [14] From such studies, the oxide phase can be identified as low-crystallinity ferrihydrite oxide with low-ordering and substantial defect sites. [14] Defects in nanocrystals have already been recognized to be important in many aspects of their surface chemistry.…”

“…[14] From such studies, the oxide phase can be identified as low-crystallinity ferrihydrite oxide with low-ordering and substantial defect sites. [14] Defects in nanocrystals have already been recognized to be important in many aspects of their surface chemistry. According to transmission electron microscopy (TEM) the c/s particles are mainly arranged alongside the mesoporous host matrix (Fig.…”

Organometallic Nanoparticles of Metals or Metal Oxides

“…Thus, the impregnation of mesoporous alumina by a solution of bis(toluene) titanium leads to a black metallic deposit which, after oxidation at 400°C, leads to the growth of TiO 2 nanoparticles [87]. In organic solution bis(cyclooctatetraene) titanium complex (Ti(C 8 H 8 ) 2 ) reacts with dry DMSO to yield a precipitate of amorphous TiO 2 .…”

Electrochemical Synthesis of TiO2 Photocatalyst with Anodic Porous Alumina

“…On the other hand, the absorption edge was shorter than that of the bulk TiO 2 because the blue shift is caused by the quantum size effect, which has already been explained. [16][17][18][19] The size of TiO 2 is sufficiently small because it was immobilized into nanopores (pore diameter: approx. 10 nm) of the anodic porous alumina, so it is possible that the quantum size effect is manifested in TiO 2 /Al 2 O 3 .…”