Hematite Layer Research Articles

Effect of metal doping, doped structure, and annealing under argon on the properties of 30 nm thick ultrathin hematite photoanodes.

The doping of the whole hematite layer with W (9.4%) and the additional doping of the bottom half of the W-doped hematite layer with Sn (8.6%), and the subsequent annealing under argon at 600 °C give rise to large increases in the Fe(2+) concentration (by >∼200 times), carrier density (Cd, by ∼48 times) and current density (i(d), by ∼8 times at 1.23 V vs. RHE, under 1 sun) with respect to those of bare hematite photoanodes. The measured i(d) (0.9 mA cm(-2)) is the highest among those of the ultrathin hematite photoanodes and the measured Cd (3.8 × 10(22) cm(-3)) is the highest among those ever observed for hematite.

Photo-redox reactions of dicarboxylates and α-hydroxydicarboxylates at the surface of Fe(III)(hydr)oxides followed with in situ ATR-FTIR spectroscopy

Colloidal mineral-phases play an important role in the adsorption, transport and transformation of organic and inorganic compounds in the atmosphere and in aqueous environments. Artificial UV-light and sunlight can induce electron transfer reactions between metal ions of the solid phases and adsorbed compounds, leading to their transformation and degradation. To investigate different possible photo-induced oxidation pathways of dicarboxylates adsorbed on iron(III)(hydr)oxide surfaces, we followed UV-A induced photoreactions of oxalate, malonate, succinate and their corresponding α-hydroxy analogues tartronate and malate with in situ ATR-FTIR spectroscopy in immersed particle layers of lepidocrocite, goethite, maghemite and hematite at pH 4. UV-A light (365±5nm) lead to fast degradation of oxalate, tartronate and malate, while malonate and succinate were photo-degraded at much slower rates. Efficient generation of OH-radicals can be excluded, as this would lead to fast and indiscriminate degradation of all tested compounds. Rapid photo-degradation of adsorbed oxalate and the α-hydroxydicarboxylates must be induced by direct ligand-to-metal charge transfer (LMCT) or by selectively oxidizing valence band holes, both processes requiring inner-sphere coordination with direct ligand-to-metal bonds to enable efficient electron-transfer. The slow photo-degradation of malonate and succinate can be explained by low-yield production of OH-radicals at the surface of the iron(III)(hydr)oxides.

Gold Nanorod-Enhanced Light Absorption and Photoelectrochemical Performance of α-Fe2O3 Thin-Film Electrode for Solar Water Splitting

Surface plasmon-enhanced light absorption for photoelectrochemical water splitting at α-Fe2O3 thin film electrode coated with Au nanorods (NRs) is described. Au NRs are self-assembled onto a hematite thin film electrode using electrostatic force between their positively charged cetyltrimethylammonium bromide (CTAB) layer and negatively charged hematite surface functionalized with poly(4-styrenesulfonate). The photoelectrochemical reaction of the plasmon active substrate for water oxidation reaction is performed and compared at various hematite layer thicknesses. When the α-Fe2O3 electrode is biased at a small potential +0.2 V (vs Ag/AgCl), the photocurrent of water oxidation is reduced because of increased recombination of holes and electrons at the surface of the Au NRs. When the bias potential is increased to +0.5 V (vs Ag/AgCl), a plasmon enhancement of the photoelectrochemical reaction of water oxidation at wavelengths near the plasmon resonance condition is observed at the α-Fe2O3 thin film electrode...

Si-doped Fe2O3 nanotubular/nanoporous layers for enhanced photoelectrochemical water splitting

The present work reports the enhancement of the photoelectrochemical water splitting performance of in-situ silicon (Si)-doped nanotubular/nanoporous (NT/NP) layers. These layers were grown by self-organizing anodization on Fe-Si alloys of various Si content. The incorporation of Si is found to retard the layer growth rates, leads to a more pronounced nanotubular morphology, and most importantly, an improved photoelectrochemical behavior. By increasing Si content from 1, 2 to 5at.% in the iron oxide NT/NP photoanodes, the photocurrent onset potential shifts favorably to lower values. At 1.3V vs. RHE, hematite layer with 5at.% Si shows a 5-fold increase of the photocurrent, i.e. 0.5mAcm−2 in comparison to 0.1mAcm−2 for the undoped samples. The study also reveals that a suitable layer thickness is essential to achieve a beneficial effect of the Si doping.

Morphological Changes of Panzhihua Ilmenite During Oxidation Treatment

Morphological changes with the phase transitions were investigated in detail for the oxidation roasting of Panzhihua ilmenite in air from 873 K to 1173 K (600 °C to 900 °C). It was found that a thin hematite layer of 1 to 2 μm formed rapidly at the initial stage of the oxidation process, independent of the oxidation temperature, on the surface of ilmenite particles, and the thickness of the hematite layer kept nearly constant with increasing time of oxidation. The morphology inside an ilmenite particle, however, changed with the oxidation temperature. Needlelike rutile network enwrapped by hematite grains was observed after oxidation below 1073 K (800 °C), and the structure can be well preserved during the oxidation process, although grain growth of rutile and hematite did occur with the extension of the oxidation time. The morphological changes at 1073 K (800 °C) and higher showed two distinct stages, where in the first stage ilmenite was oxidized to form rutile and hematite with morphology similar to that of oxidation below 1073 K (800 °C). The as-formed rutile and hematite were not stable and recombined to form pseudobrookite in the second stage, which caused significant morphology change; i.e., the needlelike rutile network gradually “dissolved” to form the final morphology of irregular rutile grains dispersed in the pseudobrookite matrix with some isolated hematite grains. Moreover, the oxidation temperature also has a great effect on the rate of morphological changes, i.e., the increased evolution rate with increasing the oxidation temperature.

Micromodel Investigation of Transport Effect on the Kinetics of Reductive Dissolution of Hematite

Reductive dissolution of hematite in porous media was investigated using a micromodel (8.1 × 4.5 × 0.028 mm) with realistic pore network structures that include distinctive advection domain, macropores and micropores created in silicon substrate. The micromodel pore surface was sputter deposited with a thin layer (230 nm) of hematite. The hematite in the micromodel was reduced by injecting pH-varying solutions (pH 5.0, 6.0, 7.0) containing a reduced form of flavin mononucleotide (FMNH2, 100 μM), a biogenic soluble electron transfer mediator produced by Shewanella species. The reduction kinetics was determined by measuring effluent Fe(II) (aq) concentration and by spectroscopically monitoring the hematite dissolution front in the micromodel. Batch experiment was also performed to estimate the hematite reduction rate under the well-mixed condition. Results showed significant spatial variation in local redox reaction rate that was controlled by the coupled transport and reaction. The overall rate of the redox reaction in the micromodel required a three-domain numerical model to effectively describe reaction kinetics either with distinctive apparent rate parameters or mass transfer coefficients in different pore domains. Results from this study demonstrated the feasibility of a domain-based modeling approach for scaling reaction rates from batch to porous media systems where reactions may be significantly limited by transport.

Effect of Steam Flow Rate and Sample Orientation on Steam Oxidation of Ferritic and Austenitic Steels at 650 and 700 °C

Steam oxidation of heat exchanger tubing is of growing interest as increasing the efficiencies of conventional pulverised fuel fired power plants requires higher steam temperatures and pressures. These new, more severe steam conditions result in faster steam oxidation reactions, which can significantly reduce the lifetime of boiler components. This paper reports some results from an investigation into the impact of steam flow rates and sample orientation on the steam oxidation of surface ground superheater tube materials. The results show that an increased steam flow rate not only causes faster oxidation rates but also a change in oxide scale morphology. In case of T23, it triggers formation of micro-layered inner oxide, whereas for T92 it promotes the formation of an outer haematite layer. For austenitic steels, the faster steam flow increases the formation of initially protective oxide scales, but also accelerates the growth of oxide nodules with prolonged exposure times.

Hematite-based Photo-oxidation of Water Using Transparent Distributed Current Collectors

High specific surface area transparent and conducting frameworks, fabricated by atomic layer deposition (ALD), were used as scaffolds for fabrication of equally high area, ALD-formed hematite structures for photo-oxidation of water to dioxygen. The frameworks offer high transparency to visible light and robust conductivity under harsh annealing and oxidizing conditions. Furthermore, they also make possible the spatially distributed collection of photocurrent from ultrathin coatings of hematite layers, enabling the formation of photoanodes featuring both large optical extinction and a hematite layer thickness nearly commensurate with the hole-collection distance. The distributed-current-collection approach increases the efficiency of water oxidation with hematite (by about a factor of 3 compared with an optimized flat electrode), is highly adaptable to future advances in thin film technology, and is further applicable to a multitude of nanostructures and optoelectronic applications that require ultrathin films without sacrificing optical thickness.

Influence of Au thickness on the performance of plasmonic enhanced hematite photoanodes

The surface plasmon effect of Au nanostructures placed on the surface of hematite has recently been used to enhance light absorption within its carrier collection distance of 10 nm from the water–hematite interface. Despite significant narrow band absorption enhancements in the visible region, the reported enhancements in the overall performance of the cell under standard AM1.5 sunlight illumination are not significant. We numerically design an array of Au stripes on the surface of an extremely thin layer of hematite to maximize the number of charge carriers that can reach the hematite–water interface and explore the optical processes involved. The lateral dimensions and in particular the thickness of the Au stripes turn out to be influential parameters for the overall enhancement that Au stripes provide. In general, the thickness of Au stripes for the optimized structure needs to be larger than 100 nm to obtain overall enhancements higher than 25% at incidence angles of up to 40°. The optimized structure is robust against edge bluntness, which is usually an artefact of the fabrication process. However, the presence of a 5 nm SiO2 insulating layer decreases the enhancement of the optimized configuration to 16%. These insights give important clues to design efficient plasmonic enhanced light harvesting structures for extremely thin layers of hematite.

Kinetics of leaching of a Chinese laterite containing maghemite and magnetite in sulfuric acid solutions

The kinetics of leaching of a Chinese laterite containing maghemite and magnetite in sulfuric acid solutions was studied in a batch autoclave at temperatures ranging from 230 to 270°C. Temperature, acid concentration and particle size had significant effects on the leaching rate while the influences of solid/liquid ratio and agitation speed greater than 450rpm were negligible. The leaching kinetics followed the shrinking core model, with surface chemical reaction as the rate-controlling step in the early and middle stages, changing to acid diffusion control in the later stage. The leaching of magnetite consumed sulfuric acid, causing the decrease of acid concentration in the early stage. After magnetite had completely dissolved, the leaching of maghemite did not consume acid and the acid concentration kept constant. This resulted in the change of kinetic model in the early and middle stages. The activation energy was 61.710kJ/mol for the early stage, 57.258kJ/mol for the middle stage and 11.174kJ/mol for the later stage. The variation of rate-controlling step was mainly due to the sheet hematite being deposited randomly in the early and middle stages and the formation of a dense hematite layer around the unreacted grains of maghemite in the later stage, as observed by scanning electron microscopy.

Alkali Induced High Temperature Corrosion of Stainless Steel: The Influence of NaCl, KCl and CaCl2

The influence of KCl, NaCl and CaCl2 on the oxidation of 304-type (Fe18Cr10Ni) stainless steel at 600 A degrees C in 5 %O-2 + 40 %H2O was investigated. Prior to exposure, a small amount of the preferred salt (cation equivalent: 1.35 mu mol/cm(2)) was deposited on the samples. Exposure time was 1-168 h. The oxidized samples were analyzed by SEM/EDX, XRD, FIB and IC. The presence of KCl and NaCl strongly accelerates high temperature corrosion of 304L. Corrosion attack is initiated by the formation of alkali chromate through the reaction of alkali with the protective oxide. Chromate formation is a sink for chromium in the oxide and leads to a loss of its protective properties. Subsequently a rapidly growing scale forms, consisting of an outer hematite layer with chromate particles on top and an inner spinel oxide layer. In contrast to NaCl and KCl, CaCl2 is not very corrosive. At temperature, CaCl2 is rapidly converted to CaO. Small amounts of CaCrO4 form where CaO is in direct contact with the scale. CaO also reacts with the scale to form Ca2Fe2O5.

Isothermal oxidation of plasma-nitrided hot-work tool steel at 750°C

Isothermal oxidation of plasma-nitrided tool steel at 750°C, typical of steel thixoforming process, was investigated. A multi-layered oxide forms on the surface of plasma nitrided X32CrMoV33 hot work tool steel when held at 750°C in an air furnace for 6 hours. The hematite layer at the surface is replaced by magnetite in the middle. The innermost layer is an iron –chromium spinel with approximately 4 at% Cr. These features are typical of oxidised low-Cr steels and suggest that nitriding does not affect the response to thermal exposure of hot work tool steels at elevated temperatures. It is thus concluded that hot-work tool steels are inadequate for tooling in the steel thixoforming process even when plasma-nitrided. The substrate tool material must contain higher levels of Cr to assure a slowly growing, adherent Cr2O3 layer to reduce the overall oxidation rate.

Role of Hematite Formation on Blister Generation during High Temperature Oxidation of Steel

Blistering is a phenomenon in which oxide scale swells during oxidation at high temperatures. Blistered scale causes surface defects in steel products during hot-rolling processes. The present study investigated the effect of oxygen and the scale layer structure on blister initiation when steel is oxidized at high temperatures, and the mechanism of blister formation is discussed. The following conclusions are drawn. Blisters are not formed when the scale is a wustite mono-layer, but start to nucleate when the scale layer structure changes from the wustite mono-layer to a three-layered scale comprised of hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO). The compressive stress in the oxide scale that is applied due to the oxide scale growth is largely released when scale layer structure changes from the wustite mono-layer to the three-layered scale. This result suggests that the compressive stress is not the main factor for blister initiation. It is deduced that the pressure of the CO, CO2, and N2 gases generated at the scale/steel interface is the main factor for blister formation. The hematite layer on the top surface of scale presumably acts as a barrier of gas permeability.

A Ga2O3underlayer as an isomorphic template for ultrathin hematite films toward efficient photoelectrochemical water splitting

Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination because of the short diffusion lengths of photoexcited carriers. However, poor crystallinity caused by structural interaction with a substrate negates the potential of ultrathin hematite photoanodes. This study demonstrates that ultrathin Ga2O3 underlayers, which were deposited on conducting substrates prior to hematite layers by atomic layer deposition, served as an isomorphic (corundum-type) structural template for ultrathin hematite and improved the photocurrent onset of PEC water splitting by 0.2 V. The benefit from Ga2O3 underlayers was most pronounced when the thickness of the underlayer was approximately 2 nm. Thinner underlayers did not work effectively as a template presumably because of insufficient crystallinity of the underlayer, while thicker ones diminished the PEC performance of hematite because the underlayer prevented electron injection from hematite to a conductive substrate due to the large conduction band offset. The enhancement of PEC performance by a Ga2O3 underlayer was more significant for thinner hematite layers owing to greater margins for improving the crystallinity of ultrathin hematite. It was confirmed that a Ga2O3 underlayer was applicable to a rough conducting substrate loaded with Sb-doped SnO2 nanoparticles, improving the photocurrent by a factor of 1.4. Accordingly, a Ga2O3 underlayer could push forward the development of host-guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.

Characterisation and Pickling Behaviour of Thermal Oxide Scale on Low Carbon Steel Produced from a Thin Slab

Thin slab interested in this work was the one with the thickness of ca. 50 mm and mainly made from recycled steel. Chemical composition of the studied steel strip produced from such slab was Fe with 0.077 wt% C, 0.233 wt% Mn, 0.191 wt% Si, 0.159 wt% Cu and 0.052 wt% Ni. Scale retained on that steel after hot rolling was studied. The hot-rolled sample was pickled in 10%v/v HCl aqueous solution at 80 °C. Weight loss and relative XRD peaks of hematite-per-iron and magnetite-per-iron were measured at different pickling periods of time. It was observed that the as-received scale was crack-free. Hematite-and-iron ratio approached zero at the pickling time of 3 seconds. Magnetite-per-iron ratio gradually decreased with increased pickling time and approached zero later. These results indicated that pickling solution attacked the outermost hematite layer resulting in removing of this layer first. Sublayers of scale consisting of magnetite were completely pickled later. Pickling behaviour was not merely volumetric since scale was crack-free. Mechanical adhesion of scale on steel substrate was additionally investigated by tensile test to help characterise the oxide scale.

KCl-Induced High Temperature Corrosion of the Austenitic Stainless Steel 304L – The Influence of SO<sub>2</sub>

The effect of SO2 on the oxidation of alloy 304L in O2+H2O and O2+H2O+KCl environment has been investigated at 600°C. Exposure time was 1-168 hours. The exposed samples were analyzed by SEM/EDX, XRD and IC. In dry O2, a protective and chromium-rich corundum-type oxide forms. In the presence of H2O(g), chromium is volatilized in the form of CrO2(OH)2(g). The corresponding chromium depletion of the protective oxide triggers a partial loss of protective properties resulting in the formation of oxide islands on the alloy grain centers. The oxide islands consist of an outward growing hematite layer and an inward growing FeCrNi spinel layer. By coating the samples with KCl the chromia depletion of the protective oxide dramatically increases due to the formation of K2CrO4. This leads to breakaway corrosion, a rapidly growing scale forming all over the surface. The resulting thick scale has a similar structure as the oxide islands formed in the absence of KCl. The addition of 300 ppm SO2 to the O2+H2O and O2+H2O+KCl environments results in a drastic reduction of corrosion rate. In O2+H2O environment the effect of SO2 is attributed to the formation of a thin sulphate film on the oxide surface that impedes chromium volatilization and decreases the rate of oxygen reduction on the oxide surface. In O2+H2O+KCl environment the corrosion mitigating effect of SO2 is mainly attributed to the rapid conversion of KCl to K2SO4. In contrast to KCl, K2SO4 does not deplete the protective oxide in chromium by forming K2CrO4.

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe–Cr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe–Cr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite–spinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N2 gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.

Relationship between the size distribution of mineral pigments and color saturation

The saturation and the lightness of hematite layers are quantitatively related to the size distribution of the pigments, when applied on skin or a perfect white substrate. The optical properties, reflectance spectra, and colorimetric coordinates are calculated from the complex refractive index of hematite and by use of the radiative transfer equation. Monodisperse pigments are investigated first and the maximum of saturation is calculated as a function of the pigment radius. Polydisperse pigments are then investigated with a log-normal size distribution. The maximum of saturation is then calculated as a function of the width of the pigment distribution, for different mean radii. This modeling can be extended to any mineral pigments.

The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600 °C in 5% O2 + 40% H2O. Prior to exposure the samples were sprayed with KCl, K2CO3 or K2SO4, the amount of salt corresponding to 1.35 μmol K+/cm2. For reference, salt-free samples were exposed in 5% O2 + 40% H2O and in 5% O2 (N2 was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K2CO3 strongly accelerate the corrosion of 304L while K2SO4 has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K2CO3 react with the chromium-rich oxide on the sample surface, forming K2CrO4. The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO2 and SO3).

INFLUENCE OF PREHEATING PARAMETERS ON THE PLASMA-JET OXIDATION OF A LOW-CARBON STEEL

The adhesion/cohesion (A/C) of coatings onto metallic substrates is an important property for most service conditions. Preheating the substrate before spraying the powder is a way to improve its A/C. It results in a surface oxidation of the substrate due to the high temperature of the gas close to the substrate surface and consequently to the high level of atomic oxygen. This study is devoted to the estimation of the influence of this oxide layer onto the A/C values. A precise monitoring of the preheating process by presetting values of the heating rate, temperature level, and heating time allows producing oxide layers with controlled composition and thickness. In the studied case of low carbon steel a specific choice in the heating rate and temperature leads to the production of an oxide layer with a low or high content of hematite layers in a duplex layer of hematite and magnetite iron oxides.