Showing papers on "Hematite published in 2009"

Influence of Feature Size, Film Thickness, and Silicon Doping on the Performance of Nanostructured Hematite Photoanodes for Solar Water Splitting

Abstract: Photoanodes consisting of nanostructured hematite prepared by atmospheric pressure chemical vapor deposition (APCVD) have previously set a benchmark for solar water splitting. Here, we fully investigate this promising system by varying critical synthetic parameters and probing the photoanode performance to determine the major factors that influence operation. By varying the film thickness, we show film growth to be linear with an incubation time. We find no concern with electron transport for films up to 600 nm, but a higher recombination rate of photogenerated carriers in the hematite near the interface with the fluorine-doped tin oxide, as compared to the bulk section of the film. The mechanism for the formation of the thin film’s nanoporous dendritic structure is discussed on the basis of the results from varying the substrate growth temperate. The observed feature sizes of the film are found to depend strongly on this temperature and the presence of silicon dopant precursor (TEOS). Raman and Mossbauer...

A Silica-Supported Iron Oxide Catalyst Capable of Activating Hydrogen Peroxide at Neutral pH Values

Abstract: Iron oxides catalyze the conversion of hydrogen peroxide (H2O2) into oxidants capable of transforming recalcitrant contaminants. Unfortunately, the process is relatively inefficient at circumneutral pH values because of competing reactions that decompose H2O2 without producing oxidants. Silica- and alumina-containing iron oxides prepared by sol−gel processing of aqueous solutions containing Fe(ClO4)3, AlCl3, and tetraethyl orthosilicate efficiently catalyzed the decomposition of H2O2 into oxidants capable of transforming phenol at circumneutral pH values. Relative to hematite, goethite, and amorphous FeOOH, the silica−iron oxide catalyst exhibited a stoichiometric efficiency, defined as the number of moles of phenol transformed per mole of H2O2 consumed, which was 10−40 times higher than that of the iron oxides. The silica−alumina−iron oxide catalyst had a stoichiometric efficiency that was 50−80 times higher than that of the iron oxides. The significant enhancement in oxidant production is attributable t...

Band-gap measurements of bulk and nanoscale hematite by soft x-ray spectroscopy

Abstract: Chemical and photochemical processes at semiconductor surfaces are highly influenced by the size of the band gap, and ability to control the band gap by particle size in nanomaterials is part of their promise. The combination of soft x-ray absorption and emission spectroscopies provides band-gap determination in bulk and nanoscale itinerant electron semiconductors such as CdS and ZnO, but this approach has not been established for materials such as iron oxides that possess band-edge electronic structure dominated by electron correlations. We performed soft x-ray spectroscopy at the oxygen $K$-edge to reveal band-edge electronic structure of bulk and nanoscale hematite. Good agreement is found between the hematite band gap derived from optical spectroscopy and the energy separation of the first inflection points in the x-ray absorption and emission onset regions. By applying this method to two sizes of phase-pure hematite nanoparticles, we find that there is no evidence for size-driven change in the band gap of hematite nanoparticles down to around 8 nm.

Synthesis and microwave absorption of uniform hematite nanoparticles and their core-shell mesoporous silica nanocomposites

Abstract: Single-crystal α-iron oxide (denoted as FO) particles with uniform sub-micrometer size and polyhedron-like shape have been successfully fabricated by using polyvinylpyrrolidone (PVP) capping agent-mediated hydrolysis of iron nitrate under mild hydrothermal conditions (200 °C). The hematite products were characterized via combined techniques including scanning electronic microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The single-crystal hematite particles have relatively uniform sizes of 180–360 nm and octahedron-shaped structures with comparatively smooth surfaces. Furthermore, the as-made hematite particles can be used as cores to prepare core-shell mesoporous silica composites. The intermediate nonporous silica layer was coated first via a sol-gel process, and then the mesoporous silica structure was coated as the outer shell layer by a surfactant-assembly method, resulting in uniform core-shell mesoporous silica FO@nSiO2@mSiO2 composites. TEM images show that the FO@nSiO2@mSiO2 composites possess distinct two-layer coating core-shell structures with ordered hexagonal mesostructure in the outer silica shell layer. N2 sorption measurements show that the uniform accessible mesochannel size for the FO@nSiO2@mSiO2 nanocomposites is ∼2.10 nm, the surface area is as high as ∼445 m2/g, and the pore volume is as large as ∼0.29 cm3/g. Furthermore, the reflection loss (dB) spectra measured in the frequency range 2–18 GHz showed that the FO@nSiO2@ mSiO2 composites have improved electromagnetic interference (EMI) shielding effectiveness (SE) compared to that of pure hematite materials. This is mainly attributed to the better impedance match and multiple-interfacial polarization among the FO@nSiO2@mSiO2 nanocomposites.

Single-crystal-like hematite colloidal nanocrystal clusters: synthesis and applications in gas sensors, photocatalysis and water treatment

Abstract: A facile and efficient one-pot solvothermal synthetic route based on a simplified self-assembly is proposed to fabricate spherical hematite colloidal nanocrystal clusters (CNCs) of uniform shape and size. The as-prepared hematite CNCs are composed of numerous nanocrystals of approximately 20 nm in size, and present a single-crystal-like characteristic. A possible formation process based on the nucleation–oriented aggregation–recrystallization mechanism is proposed. Our experiments demonstrated that both the surfactant and the mixed solvent play very critical roles in controlling the size of primary nanocrystals and the final morphology of single-crystal-like spherical CNCs. Compared with other hematite nanostructures, the spherical hematite CNCs show outstanding performance in gas sensing, photocatalysis and water treatment due to their large surface area and porous structure. In addition, interesting tertiary CNCs formed by further assembly of secondary spherical CNCs were observed for the first time.

Predicting the Band Structure of Mixed Transition Metal Oxides: Theory and Experiment

Abstract: Fuel production from sunlight using mixed metal oxide photocatalysts is a promising route for harvesting solar energy. While photocatalytic processes can operate with high efficiency using UV light, it remains a challenge of paramount importance to drive them with visible light. Engineering the electronic energy band structure of mixed metal oxides through judicious control of atomic composition is a promising route to increasing visible light photoresponse. The goal of this paper is to develop a simple mixed metal oxide band structure (MMOBS) method to predict the electronic band structure of mixed metal oxides. Several materials in the Ti−Fe−O system that span the composition spectrum were considered in this study: anatase TiO2, Fe-doped anatase TiO2, ilmenite TiFeO3, Ti-doped hematite α-Fe2O3, and hematite α-Fe2O3. The predictions by the MMOBS method for the Ti−Fe−O system were tested and confirmed using first-principles density functional theory (DFT) calculations and experimental UV−visible absorptio...

Nanostructured anodic iron oxide film as photoanode for water oxidation

Abstract: Two different configurations of photoanodes based on anodic iron oxide were investigated for photoelectrochemical water oxidation. A self-ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in an ethylene glycol based electrolyte containing 0.1M NH4F + 3 vol% water (EGWF solution) at 50 V for 15 min. Annealing of the oxide nanotubes in a hydrogen environment at 500 °C for 1 h resulted in a predominantly hematite phase. The second type of photoanode was obtained by a two-step anodization procedure. This process resulted in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. This electrode configuration combined the better photocatalytic properties of the nano-dendritic iron oxide and better electron transportation behaviour of vertically oriented nano-channels. Annealing of these double anodized samples in an acetylene environment at 550 °C for 10 min resulted in a mixture of maghemite and hematite phases. Photocurrent densities of 0.74 mA cm−2 at 0.2 VAg/AgCl and 1.8 mA cm−2 at 0.5 VAg/AgCl were obtained under AM 1.5 illumination in 1M KOH solution. The double anodized samples showed high photoconductivity and more negative flat band potential (−0.8 VAg/AgCl), which are the properties required for promising photoanode materials.

The role of monomeric iron during the selective catalytic reduction of NOx by NH3 over Fe-BEA zeolite catalysts

Abstract: A series of highly active Fe-BEA iron zeolite catalysts for selective catalytic reduction (SCR) of NOx with ammonia have been prepared by incipient wetness impregnation and aqueous ion exchange with Fe(NO3)3 solutions. While the catalysts were stable during short term hydrothermal ageing, extended periods of ageing did reduce activity. The catalysts were investigated by diffuse reflectance UV–vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, ammonia temperature programmed desorption (NH3-TPD), X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) both ex-situ and in-situ with online gas analysis to measure catalytic activity. UV–vis, EPR and EXAFS showed that low iron loadings (≤1.2 wt.% Fe) resulted in mostly iron monomers, especially for the ion exchanged samples. In contrast a mixture of monomers, oligomers and hematite particles was formed at intermediate to high loadings (2.5–5.1 wt.% Fe). Linear combination analysis of UV–vis spectra, with real spectra and one Gaussian as basis, was used for quantifying the three iron species. A good correlation between iron monomer content and NO conversion was observed. In-situ XANES and EXAFS showed that heating in oxygen atmosphere removes water from the iron coordination sphere and the resultant vacant positions are occupied by NO, NH3 or H2O upon exposing the catalyst to a gas mixture of NO, NH3 and O2 in He. Heating in this gas mixture gave a distinct correlation between the catalytic performance and the oxidation state of iron, which is more pronounced in the catalysts where mostly iron monomers are present. Iron was more partially reduced when the SCR activity increased. These results support a mechanism where the oxidation of iron or NO is the rate-determining step.

Direct hydrothermal synthesis of single-crystalline hematite nanorods assisted by 1,2-propanediamine

Abstract: Uniform alpha-Fe2O3 nanorods with high aspect ratios were synthesized in large scale by a simple and direct 1,2-propanediamine-assisted hydrothermal method. The resultant products were characterized by x-ray diffraction scanning electron microscopy, and transmission electron microscopy. The as-synthesized alpha-Fe2O3 nanorods were single crystalline and uniform, with an average aspect ratio greater than 10. The effects of various experimental parameters on the morphology of products, such as 1,2-propanediamine content, pH value, concentration of FeCl3 and reaction temperature, were studied. In the formation process of alpha-Fe2O3 nanorods, the 1,2- propanediamine not only provides OH- but also plays a role for retaining the rod-like morphology of hematite. The magnetic properties including Morin transition and coercivity of the samples with different synthesis conditions and aspect ratios were also investigated in detail.

Mössbauer spectroscopy and magnetic properties of hematite/magnetite nanocomposites

Abstract: A thermal reduction method has been developed to prepare magnetite/hematite nanocomposites and pure magnetite nanoparticles targeted for specific applications. The relative content of hematite α-Fe2O3 and magnetite Fe3O4 nanoparticles in the product was ensured by maintaining proper conditions in the thermal reduction of α-Fe2O3 powder in the presence of a high boiling point solvent. The structural, electronic, and magnetic properties of the nanocomposites were investigated by F57e-Mossbauer spectroscopy, x-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at every step of the chemical and thermal treatment. It is established that not all iron ions in the octahedral B-sites of magnetite nanoparticles participate in the electron hopping Fe2+⇄Fe3+ above the Verwey temperature TV, and that the charge distribution can be expressed as (Fe3+)tet[Fe1.852.5+Fe0.153+]octO4. It is shown that surface effects, influencing the electronic states of iron ions, dominat...

Photocatalytic degradation of organics in water in the presence of iron oxides: Influence of carboxylic acids

Abstract: The effects of four carboxylic acids: malic, citric, tartaric and oxalic acids on the leaching of iron from two commercial iron oxides (hematite, α-Fe2O3, and magnetite, Fe3O4) have been investigated. The variables studied were the doses of iron oxides and carboxylic acids used as well as aqueous pH, temperature and the presence of hydrogen peroxide and/or UV-A radiation. On the whole, Fe3O4 led to higher amounts of leached iron than α-Fe2O3, and oxalic acid was the most effective carboxylic acid used. The importance of iron leaching has been considered to explain the photodegradation of bisphenol A (BPA) by UV-A/iron oxides systems. The influence of the presence of hydrogen peroxide and/or titania on the efficiency of these oxidation systems was also investigated. At the conditions tested, advanced oxidation with the UV-A/iron oxide/oxalic acid/H2O2/TiO2 system led to the lowest BPA half life (<15 min) among those processes studied.

Preparation of Monodisperse Iron Oxide Nanoparticles via the Synthesis and Decomposition of Iron Fatty Acid Complexes

Abstract: Iron fatty acid complexes (IFACs) are prepared via the dissolution of porous hematite powder in hot unsaturated fatty acid The IFACs are then decomposed in five different organic solvents under reflux conditions in the presence of the respective fatty acid The XRD analysis results indicate that the resulting NPs comprise a mixture of wustite, magnetite, and maghemite phases The solvents with a higher boiling point prompt the formation of larger NPs containing wustite as the major component, while those with a lower boiling point produce smaller NPs with maghemite as the major component In addition, it is shown that unstable NPs with a mixed wustite–magnetite composition can be oxidized to pure maghemite by extending the reaction time or using an oxidizing agent

High temperature stability of natural maghemite: a magnetic and spectroscopic study

Abstract: SUMMARY A combined magneto-mineralogical approach is used to diagnose maghemitization in magnetic grains of basaltic rock fragments from sand dunes in the Namibian desert in SW Africa. Data were obtained from static magnetic analysis, ferromagnetic resonance (FMR) spectroscopy, micro-Raman spectroscopy and electron microscopy. Micro-Raman spectroscopy showed that the magnetic grains in the lithic fragments form oxidative solid solution series with magnetite and maghemite as end-members. The five active Raman modes at 712, 665, 507, 380 and 344 cm −1 indicate that maghemite in the magnetic grains has well-defined structural properties. The FMR spectral analysis provides evidence for long-range dipolar coupling, which suggests intergrowth of the magnetic phases of the oxidative solid solution series. Thermomagnetic experiments and hysteresis measurements reveal a Curie temperature of about 890 K for this maghemite. Upon heating to 970 K part of the maghemite is altered to thermodynamically more stable hematite. After selective thermal decomposition of the maghemite in a protected atmosphere, the remaining magnetic phase has a Curie temperature of 850 K, characteristic for magnetite. The unique thermal stability of this natural maghemite above its Curie temperature is explained by the well-defined mineral structure, which formed during slow oxidative alteration of magnetite under arid climate conditions.