Nanocrystalline Hematite Research Articles

The role of the domain size and titanium dopant in nanocrystalline hematite thin films for water photolysis.

Here we develop a novel technique for preparing high quality Ti-doped hematite thin films for photoelectrochemical (PEC) water splitting, through sputtering deposition of metallic iron films from an iron target embedded with titanium (dopants) pellets, followed by a thermal oxidation step that turns the metal films into doped hematite. It is found that the hematite domain size can be tuned from ∼10 nm to over 100 nm by adjusting the sputtering atmosphere from more oxidative to mostly inert. The better crystallinity at a larger domain size ensures excellent PEC water splitting performance, leading to record high photocurrent from pure planar hematite thin films on FTO substrates. Titanium doping further enhances the PEC performance of hematite photoanodes. The photocurrent is improved by 50%, with a titanium dopant concentration as low as 0.5 atom%. It is also found that the role of the titanium dopant in improving the PEC performance is not apparently related to the films' electrical conductivity which had been widely believed, but is more likely due to the passivation of surface defects by the titanium dopants.

Preparation and structural, optical, magnetic, and electrical characterization of Mn2+/Co2+/Cu2+ doped hematite nanocrystals

Pure and Mn2+ / Co2+ / Cu2+ doped (1 and 2 at.%) spherical hematite (α-Fe2O3)nanocrystals have been synthesized by a simple solvothermal method using a domestic microwave oven. XRD measurements confirm that all the seven nanocrystals prepared consist of nanocrystalline hematite phase without any other phases. The energy dispersive X-ray and Fourier transform infrared spectral analyses confirm the phase purity of the nanocrystals prepared. TEM analysis shows the average particle sizes within the range 33–51nm. Optical absorption measurements indicate that all the three dopants enhance the optical transmittance and reflectance. A red shift is observed in the bandgap energy values estimated from optical absorption and reflectance spectra. Results of magnetic measurements made at room temperature using a vibrating sample magnetometer indicate significant changes in the magnetic properties (coercivity, retentivity and saturationmagnetization) due to doping. Results of magnetic measurements indicate significant changes in the magnetic properties. Results of AC electrical measurements made at various temperatures in the range 40–130°C and frequencies in the range 100 Hz –1MHz indicate low dielectric constants and AC electrical conductivities and consequently show the occurrence of nanoconfined states.

Facile growth of hierarchical hematite (α-Fe2O3) nanopetals on FTO by pulse reverse electrodeposition for photoelectrochemical water splitting

Nanocrystalline hematite (α-Fe2O3) film was prepared on FTO using pulse reverse electrodeposition with hierarchical nano-petal structures (∼30 to 35 nm) that appear to protrude out from the agglomerated network. Photocurrent density of nano-petal morphology measured at E = 1.23 V vs. RHE in 1 M NaOH under 1 sun illumination was remarkably increased by 180% relative to that of Fe2O3 film produced by normal electrodeposition.

Dynamic Phenomenological Modeling of Pec Cells for Water Splitting Under Outdoor Conditions

A dynamic phenomenological model is proposed for better understanding the underlying mechanisms of photoelectrochemical (PEC) cells for water splitting. The main assumptions of the one-dimensional transient model are (1) that bulk recombination occurs between conduction band electrons and holes in the valence band; (2) that mobile charge transport takes place via diffusion and migration; and (3) that the effects of microscopic electric fields and screening in the cell are negligible. For modeling purposes, the photoanode was assumed to be a homogeneous nanocrystalline hematite structure, with thickness L, porosity ep and tortuosity x. The TCO/semiconductor interface was modeled as an ideal ohmic contact, while the electrolyte/platinised TCO interface was described by the Butler-Volmer equation. The continuity and transport governing equations were defined for the mobile species involved: electrons, holes and hydroxyl ions. The effect of outdoor conditions on the performance of the PEC cells, such as temperature and solar radiation variations during the day and for different seasons of the year, was simulated.

Facile fabrication of tin-doped hematite photoelectrodes – effect of doping on magnetic properties and performance for light-induced water splitting

We present a new, easily scalable method for the deposition of nanocrystalline hematite photoelectrodes based on the spin-coating of a mixed solution containing tin(II) and iron(III) chlorides followed by thermal treatment. Our facile approach does not require any additional film-forming organic species and allows simple control of the photoelectrochemical performance of the electrode by adjusting the degree of tin doping. When annealed at 650 °C a strong increase in the water oxidation photocurrent is observed with increasing tin concentration. The maximum performance (0.45 mA cm−2 at 1.43 V vs. RHE) was found at the highest possible tin loading (20 : 100, Sn : Fe). The contrasting performance of electrodes annealed at 650 °C and 800 °C suggests different activation processes for dopant diffusion and activation. The doping of tin into the crystal structure of hematite thin films is directly evidenced by X-ray photoelectron spectroscopy and indirectly by changes in the intrinsic magnetic parameters (Morin temperature, Néel temperature) of the hematite films. The magnetization measurements thus represent a potential technique to quantify doping amounts in hematite.

Enhancing the photoelectrochemical performance of hematite (α-Fe2O3) electrodes by cadmium incorporation

Photoelectrochemical (PEC) water oxidation using hematite (α-Fe2O3) is of great interest in terms of solar fuels and artificial photosynthesis. In this study, Cd-incorporated nanocrystalline hematite films (Cd–Fe2O3) supported on conducting glass have been prepared via co-electrodeposition of aqueous Fe(III) and Cd(II) with varying Cd:Fe atomic ratios (up to 3.2at.%) and optimized for their PEC performances under a simulated solar light (AM 1.5-irradiation). Surface analysis indicates that the Cd co-deposition increases the hematite particle size from ca. 50nm to 70–100nm due to interparticle agglomeration and decreases the overall UV–Vis absorbance of hematite. X-ray photoelectron spectroscopic study also indicates that Cd incorporation shifts the binding energy of oxygen atoms to lower energy direction whereas it does not affect the binding energy of Fe 3d. This suggests that Cd exists mainly as CdO and/or Cd(OH)2 in the hematite surface. When an optimal level of Cd content (∼1at.%) is electrodeposited, the photocurrent of hematite film is significantly enhanced by a factor of ca. four at E=1.23VRHE under AM 1.5-irradiation and the photoactive spectral region is red-shifted. Electrochemical impedance spectroscopic analysis further reveals that the flat band potential of hematite is shifted by ca. −30mV to negative potential direction and the charge transfer resistance (Rct) is significantly reduced by Cd incorporation. Detailed surface analyses, optimization for preparation condition of hematite films, and discussion for PEC behaviors were described.

Photoelectrochemical and Photocatalytic Behaviors of Hematite-Decorated Titania Nanotube Arrays: Energy Level Mismatch versus Surface Specific Reactivity

Nanocrystalline hematite particles (α-Fe2O3) were electrodeposited on the TiO2 nanotube (TiNT) arrays that were fabricated via anodization of Ti foils. The short precontact time (1 h) of aqueous ferric ions (Fe3+) on TiNT resulted in formation of hematite particles selectively on the mouth surface of TiNT (hematite@1 h/TiNT), whereas the long precontact time (24 h) resulted in complete filling of the TiNT inside and an even full-covering of the TiNT top surface with the hematite particles (hematite@24 h/TiNT). For comparison, hematite particles were also electrodeposited on TiO2-nanoparticulate films obtained via oxidative annealing of Ti foil resulting in hematite fully covered TiO2 nanoparticles (hematite/TiNP). Photoelectrochemical (PEC) study with AM 1.5 light (UV + Vis) indicated that the PEC activity of TiNT decreased by ca. 40% and almost completely vanished when hematite covered the full surface of TiNT (hematite@ 24 h/TiNT) and loaded on the mouth surface of TiNT (hematite@1 h/TiNT), respectively...

Dynamics of photogenerated holes in nanocrystalline α-Fe2O3electrodes for water oxidation probed by transient absorption spectroscopy

Transient absorption spectroscopy on the μs-s time scale is used to monitor the yield and decay dynamics of photogenerated holes in nanocrystalline hematite photoanodes. In the absence of a positive applied bias, these holes are observed to undergo rapid electron-hole recombination. The application of a positive bias results in the generation of long-lived (3 ± 1 s lifetime) photoholes.

Transient phenomenological modeling of photoelectrochemical cells for water splitting – Application to undoped hematite electrodes

A phenomenological model is proposed for a better understanding of the basic mechanisms of photoelectrochemical (PEC) cells. The main assumptions of the one-dimensional transient phenomenological model are: i) bulk recombination of the conduction band electrons with holes in the valence band; ii) the mobile charge transport takes place via diffusion, which arises from the concentration profiles, and migration, caused by a macroscopic electric field; iii) negligible effects of microscopic electric fields in the cell and screening effects, as well as negligible Helmholtz and diffuse layers. For modeling purposes, the photoanode was assumed to be a homogeneous nanocrystalline hematite structure, with thickness L, porosity ɛ p and tortuosity τ. The TCO/semiconductor interface was modeled as an ideal ohmic contact, while the electrolyte/platinized TCO interface was described by a Butler–Volmer approach. An alkaline electrolyte solution was used, allowing the transport of the ionic species from the counter-electrode to the photoanode. The continuity and transport governing equations are defined for the mobile species involved: electrons in the conduction band of the semiconductor, holes in the valence band and hydroxyl ions in the electrolyte. Simulated I– V characteristics were computed and the corresponding results compared with the experimental values. The simulated results were in straight agreement with the experimental data.

Direct electron-transfer conduits constructed at the interface between multicopper oxidase and nanocrystalline semiconductive Fe oxides

Herein, the electron-transfer reactions occurring at the interface between bilirubin oxidase (BOD) and nanocrystalline hematite (α-Fe 2O 3) were characterized. Cyclic voltammograms indicated that BOD has an affinity for hematite surfaces and establishes a direct electron-transfer (DET) conduit between the primary electron acceptor T1 site and the conduction band of α-Fe 2O 3. DET was also confirmed photo-electrochemically, as cathodic photocurrents were generated when a nanocomposite of BOD and α-Fe 2O 3 was illuminated under oxygenated conditions. A proline residue displayed a high-binding affinity for hematite surfaces and is therefore likely part of an orientation-controlled motif which serves to locate BOD at the T1 site at a suitable distance for DET to α-Fe 2O 3.

A nanocrystalline hematite film prepared from iron(III) chloride precursor

This paper deals with a simple and low-cost method developed to deposit hematite (α-Fe 2O 3) layers on a fluorine-doped tin oxide (FTO/F:SnO 2) substrate by thermal decomposition of solid iron(III) chloride hexahydrate (FeCl 3⋅ 6H 2O). Deposition procedure takes place through chemical intermediate iron(III) oxide chloride (FeOCl) film. A crucial influence of atmosphere dynamics involved in the calcination process of FeOCl has been observed. As-deposited films were characterized by means of Conversion Electron Mössbauer Spectroscopy (CEMS), Grazing Angle X-Ray Diffractometry (GAXRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis. Final nanocrystalline hematite film with a cactus-field-like design consists of 20 nm thick porous crystal plates. A process of hematite doping by tin atoms from substrate coating is also discussed.

Facile Fabrication of 3D-Ordered Macroporous Nanocrystalline Iron Oxide Films with Highly Efficient Visible Light Induced Photocatalytic Activity

A facile approach of layer-by-layer depositing and hydrolysis of FeCl3 is developed to fabricate 3D-ordered Fe2O3 film. The 3D-ordered Fe2O3 film was characterized by SEM, XRD, and DRUV−vis. It has 3D-ordered interconnecting macropores (340 nm) with nanocrystalline hematite Fe2O3 walls (27.2 nm). The 3D-ordered macroporous nanocrystalline Fe2O3 film exhibits 2.4 times larger photocatalytic activity for the photodegradation of dye in the presence of H2O2 under visible irradiation than the nanocrystalline α-Fe2O3 film without macropores and very good photostability. The much higher photocatalytic activity of the 3D-ordered macroporous nanocrystalline Fe2O3 film than that of the reference Fe2O3 film is attributed to the unique nanostructure and architecture of the 3D-ordered Fe2O3 film, which result in the much greater light harvesting efficiency and efficient mass transport in the former than in the latter due to the existence of 3D-ordered interconnecting macropores. The effect of photonic stop band on the...

An effect of iron(III) oxides crystallinity on their catalytic efficiency and applicability in phenol degradation—A competition between homogeneous and heterogeneous catalysis

Catalytic efficiency, stability and environmental applicability of five iron(III) oxide nanopowders differing in surface area and crystallinity were tested in degradation of concentrated phenolic aqueous solutions (100 g/L) at mild temperature (30 °C), initially almost neutral pH and equimolar ratio of hydrogen peroxide and phenol. The catalyst properties were easily controlled by varying in reaction time during isothermal treatment of ferrous oxalate dihydrate in air at 175 °C. Although the catalytic efficiency clearly increases with the surface area of the nanopowders, it is not due to the solely heterogeneous catalytic mechanism as would be expected. The amorphous Fe 2O 3 nanopowders possessing the largest surface areas (401 m 2 g −1, 386 m 2 g −1) are the most efficient catalysts evidently due to their highest susceptibility to leaching in acidic environment arising as a consequence of phenol degradation products. Thus, these amorphous samples act partially as homogeneous catalysts, which was confirmed by a high concentration of leached Fe(III) ions in the solution (∼19 ppm). The crystalline hematite (α-Fe 2O 3) samples, varying in surface area between 337 m 2 g −1 and 245 m 2 g −1, are generally less efficient when compared to the amorphous powders, however their catalytic action is almost exclusively heterogeneous as only 3 ppm of leached Fe(III) was found in the reaction systems catalyzed by nanohematite samples. A significant difference in relative contributions of heterogeneous and homogenous catalysis was definitely established in buffered reaction systems catalyzed by amorphous Fe 2O 3 and nanocrystalline hematite. The nanohematite sample exhibiting the highest heterogeneous action was tested at decreased initial phenol concentration (10 g/L), which is closer to the real contents of phenol in waste waters, and at different hydrogen peroxide/phenol molar ratios to consider its environmental applicability. At the hydrogen peroxide/phenol ratio equal to 5, no traces of the leached iron were detected and the phenol conversion of 84% was reached. Moreover, such a high degree of conversion is accompanied by a decrease of the chemical oxygen demand (COD) from the initial value of 11.23 g/L to 4.22 g/L after 125 min. This fact indicates that the considerable fraction of primary reaction products was totally degraded.

Anisotropic photocatalytic properties of hematite

Hematite is a semiconducting mineral with a role in natural photoelectrochemical processes, and has been studied from the viewpoint of solar energy utilization. Hematite is an anisotropic conductor, with faster conduction parallel to (001) planes. Flatband potentials and photocurrent onset potentials for natural hematite single crystals and synthetic nanocrystalline hematite films are similar, and show Nernstian behavior within error. At pH7, the flatband potential of single crystals is -0.25 ± 0.1 V vs. Ag/AgCl. Photocurrent onset potential is 0.02 to 0.03 V more negative for crystal faces than for crystal edges. Photocurrent density is a factor of 5 to 10 higher for crystal edges than for crystal (001) faces, presumably because of more rapid charge separation for the edges. Falling photocurrent transients decay more slowly for edges than for faces, consistent with more rapid removal of conduction band electrons into the bulk and therefore reduced availability of such electrons for back reaction. Rising photocurrent transients occur at higher potential, and have the same rise time for both faces and edges. This suggests that the rising transients are due to slow conduction through bulk hematite. The transition from falling to rising transients occurs at a more positive potential for edges than for faces, which is also consistent with more rapid charge transport away from edge surfaces and with Fermi level pinning at edges.

A combined study by XRD, FTIR, TG and HRTEM on the structure of delaminated Fe-intercalated/pillared clay

Fe-PILC samples were synthesized by the reaction between Na +- and/or Ca 2+-montmorillonite (Mt) and base-hydrolyzed solutions of Fe(III) nitrate. Different from the known usual microporous pillared structure, a meso–microporous delaminated structure containing intercalated or pillared fragments was found in the respective resulting Fe-intercalated or -pillared clays. XRD patterns of Na +-Mt-based Fe-intercalated/pillared clays show one large d-spacing above 6.4 nm corresponding to the mesoporous delaminated part, whereas another d-spacing of ca. 1.5 nm was indicative of the microporous pillared part. Fe-intercalated/pillared clays based on Ca 2+-Mt lead to similar results, but with a d-spacing less than 6 nm and a second low intense d-spacing less than 1.5 nm. In the delaminated Fe-intercalated clays, NO − 3 anions were retained even after thorough washing process. They play as counterions to neutralize the positive-charged iron aggregates in the delaminated structure, and can be exchanged by heteropolyanions as [PW 12O 40] 3−. The delaminated Fe-pillared clays show good thermal stability at 500 °C and exhibit at this temperature dramatically higher specific surface area and porosity than the starting montmorillonites. However, calcination at a higher temperature leads to the formation of nanocrystalline hematite. Air-drying after ethanol extraction (EAD) method has an advantage over air-drying (AD) method in preserving the delaminated structure.

Fe oxidation processes at Meridiani Planum and implications for secondary Fe mineralogy on Mars

Fe oxidation processes may have occurred during groundwater‐mediated diagenesis in Meridiani Planum sediments. To address this question, melanterite oxidation experiments were conducted at epsomite saturation as a function of pH. Results show that schwertmannite is initially formed from acidic Fe oxidation and that its formation and aging to mixtures of jarosite and nanocrystalline goethite is strongly controlled by pH over the range ∼2.0–4.0. The pH is controlled in turn by Fe oxidation and Fe3+ hydrolysis. In one 77‐d oxidation experiment, nanocrystalline hematite was tentatively identified by Mössbauer spectroscopy. Accordingly, aging experiments with synthetic nanocrystalline goethite were conducted (1) to further resolve the formation mechanisms of Fe‐phases identified from oxidation experiments and (2) to test whether low water activity (aw) controls the thermodynamically favored goethite to hematite transition at low temperature. Mössbauer spectroscopy and total X‐ray scattering show no observable changes after 4 months of aging, and instead, these results point to a jarosite precursor for the tentatively identified hematite. On the basis of these results, we suggest that the oxidation and maturation of initially formed Fe2+‐bearing saline minerals may account in large part for the distribution of secondary Fe minerals at the Martian surface, contributing to the association of Fe oxides and Mg/Ca sulfates observed from orbital surface analyses. We hypothesize that oxidation of Fe2+ sulfates at low temperature could account for sustained diagenetic acidity in addition to much of the observed Fe mineralogy in Meridiani Planum outcrop rocks. The origin of the gray crystalline hematite at Meridiani, however, is deserving of further experimental work to test this mechanism.

Limited iron isotope variations in recent lateritic soils from Nsimi, Cameroon: Implications for the global Fe geochemical cycle

Laterites are oxidized Fe-rich soils covering one third of the continents and are drained by half of the continental waters. They therefore represent a key component of the iron geochemical cycle at the Earth's surface, yet no iron isotope study has been conducted on recent laterites so far. Building on previous integrated morpho-pedological studies of soils located in an equatorial rainforest, Southern Cameroon, we have undertaken a mineralogical, elemental and isotopic study of iron in two lateritic profiles. One is a borehole 3620 cm deep going to the parent granodioritic rock, located at the top of a hill, whereas the other is 675 cm deep and is located downhill. Mössbauer spectroscopy reveals that typically ≥ 96% of the soil's iron is held in nanocrystalline hematite and goethite. Iron isotope measurements performed by plasma source mass spectrometry show that Fe isotopic equilibrium was rarely reached between these iron oxide and hydroxide despite their small size. Overall, it is found that most samples display iron isotope signatures very close to the mean crustal value, with a maximum range of 0.2‰ in δ 57Fe. Given that lateritic soils evolve over millions of years, show large variations in Fe concentrations and mineralogical abundances, this range is surprisingly small when compared to other Fe isotope studies of soils from different climatological contexts, that can easily show δ 57Fe ranges in excess of 1‰ at the bulk sample scale. The most likely explanation of this finding is that despite notable vertical and lateral Fe mobility in the studied laterites from Cameroon, as computed using the open-system mass fraction transport function, τ Fe,w, iron remained mostly in the oxidized form as shown by Mössbauer pectroscopy. This probably results from the strong bioturbation and pedoturbation of these lateritic soils that lead them to remain porous over a large thickness and facilitated the flow of oxygenated waters from the surface down to the saprolite horizon. This study therefore reveals that soils that remained as an open system for iron do not necessarily show large Fe isotopic variations. It is suggested that the Fe drained from such lateritic soils should have δ 57Fe values within ~ 0.1‰ of that of the continental crust. This is in contrast with previous soil studies, notably from sites located at higher latitudes, that imply that these soils should release isotopically more variable Fe to surface waters. Such possible contrasted isotopic signatures from different surface waters will likely lead to an isotopically heterogeneous ocean given the short residence time of Fe in seawater.

Effect of Firing Temperature and Reducing Gas Composition during Low-Temperature Reduction of Nanocrystalline Fe2O3

Pure nanocrystalline hematite (40 to 100 nm) compacts were prepared and sintered at various temperatures (300 °C to 600 °C) and then reduced with 100 pct H2 at 500 °C. On the other hand, fired compacts at 500 °C were reduced with a H2-Ar gas mixture containing different concentration of hydrogen (100, 75, 50, and 25 pct) at 500 °C using thermogravimetric techniques. Nanocrystalline Fe2O3 compacts were characterized before and after reduction with X-ray diffraction, scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and reflected light microscope. It was found that the fired compacts at 400 °C to 600 °C have relatively faster reaction behaviors compared to that at lower firing temperature 300 °C. By decreasing the firing temperature to 300 °C, partial sintering with grain growth was observed clearly during reduction. Also, it was found that the reduction rate increased with increasing hydrogen content in the reducing gas. Comparatively, grain growth and partial coalescence took place during reduction with 25 pct H2 due to long reaction time.

Microstructure characterization of mechanically activated hematite using XRD line broadening

The effect of dry milling in a vibratory mill on the structural changes and microstructural characteristics of hematite using different methods was investigated. We have described the line profile analysis (LPA) to extract the size of coherently diffracting domains and the lattice strain of activated hematite in a vibratory mill. The Warren–Averbach and Williamson–Hall methods were used as the main tools for characterization. The changes in the particle size, surface area and new phase formation of hematite concentrate were also investigated. It was concluded that the breakage and agglomeration of particles take place mainly at lower and higher levels of specific energy input, respectively. The pores in agglomerates remain accessible for the nitrogen gas. Milling of hematite increased specific surface area up to 18.4 m 2/g. The hematite milled under various levels of specific energy input did not undergo a significant reaction or phase transformation during milling. The Williamson–Hall method confirms its merit for a rapid overview of the line broadening effects and possible understanding of the main causes. The anisotropic character of line broadening for deformed hematite as a function of specific energy input was revealed. Higher level of specific energy input favors the generation of small crystallite size, higher microstrain, BET surface area, amorphization and line breadth. The Warren–Averbach method suggested that the nanocrystalline hematite with grain sizes of 73.5–12.2 nm was formed by mechanical treatment using different milling intensities in the vibratory mill. The root mean square strain (RMSS) at L = 10 nm varies between 1.7 × 10 − 3 and 4.0 × 10 − 3 depending on the level of energy input. Limits in the applicability of Williamson–Hall method and reliability of the results are discussed in detail.

An inelastic neutron scattering study of hematite nanoparticles

We have studied the magnetic dynamics in nanocrystalline hematite by inelastic neutron scattering at the high-resolution time-of-flight spectrometer IRIS at ISIS. Compared to previous inelastic neutron scattering experiments an improvement of the resolution function is achieved and more detailed information is obtained. We have performed measurements at several temperatures ranging from 5 to 325 K. The particles are canted antiferromagnetic (weakly ferromagnetic) at temperatures at least down to 5 K. We study both superparamagnetic relaxation and collective magnetic excitations of the whole particle moment at the antiferromagnetic Bragg reflection. We have studied different weightings of the particle size distribution. The data and their temperature dependence can with good agreement be interpreted on the basis of the Néel-Brown theory for superparamagnetic relaxation and a model for the collective magnetic excitations.