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Showing papers on "Hematite published in 2012"


Journal ArticleDOI
TL;DR: The utility of impedance spectroscopy in investigations of hematite electrodes is demonstrated to provide key parameters of photoelectrodes with a relatively simple measurement, and new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation.
Abstract: Hematite (α-Fe2O3) constitutes one of the most promising semiconductor materials for the conversion of sunlight into chemical fuels by water splitting. Its inherent drawbacks related to the long penetration depth of light and poor charge carrier conductivity are being progressively overcome by employing nanostructuring strategies and improved catalysts. However, the physical–chemical mechanisms responsible for the photoelectrochemical performance of this material (J(V) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by atomic layer deposition to study the photoelectrochemical properties of this material under water-splitting conditions. We employed impedance spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity, and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. T...

831 citations


Journal ArticleDOI
TL;DR: It was found that under illumination, the Co-Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode and produce increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film.
Abstract: Uniform thin films of hematite (α-Fe(2)O(3)) deposited by atomic layer deposition (ALD) coated with varying amounts of the cobalt phosphate catalyst, "Co-Pi," were investigated with steady-state and transient photoelectrochemical measurements and impedance spectroscopy. Systematic studies as a function of Co-Pi thickness were performed in order to clarify the mechanism by which Co-Pi enhances the water-splitting performance of hematite electrodes. It was found that under illumination, the Co-Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode. This charge separation reduces surface state recombination which results in increased water oxidation efficiency. It was also found that thicker Co-Pi films produced increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film. These combined results provide important new understanding of the enhancement and limitations of the Co-Pi catalyst coupled with semiconductor electrodes for water-splitting applications.

594 citations


Journal ArticleDOI
TL;DR: In this article, the authors provide an overview of recent progress on the synthesis and characterization of nanostructured hematite, with an emphasis on the charge carrier dynamics and photoelectrochemical properties.
Abstract: As one of the most prevalent metal oxides on Earth, iron oxide, especially α-Fe2O3 or hematite, has been the subject of intense research for several decades. In particular, the combination of a relatively small bandgap and related visible light absorption, natural abundance, low cost, and stability under deleterious chemical conditions has made it ideal for many potential applications. However, the short charge carrier lifetime or diffusion length has limited its applicability. Nanostructures of hematite offer the possibility of overcoming some of the limitations through control of the structures and thereby its optical and electronic properties. In this review, we provide an overview of recent progress on the synthesis and characterization of nanostructured hematite, with an emphasis on the charge carrier dynamics and photoelectrochemical properties. Both current challenges and future opportunities are also discussed.

467 citations


Journal ArticleDOI
TL;DR: This work performs periodic density functional theory + U calculations for the water oxidation reaction on the fully hydroxylated hematite (0001) surface and shows that moderately charged O anions give rise to smaller overpotentials.
Abstract: In photoelectrochemical cells, sunlight may be converted into chemical energy by splitting water into hydrogen and oxygen molecules. Hematite (α-Fe(2)O(3)) is a promising photoanode material for the water oxidation component of this process. Numerous research groups have attempted to improve hematite's photocatalytic efficiency despite a lack of foundational knowledge regarding its surface reaction kinetics. To elucidate detailed reaction mechanisms and energetics, we performed periodic density functional theory + U calculations for the water oxidation reaction on the fully hydroxylated hematite (0001) surface. We investigate two different concentrations of surface reactive sites. Our best model involves calculating water oxidation mechanisms on a pure (1×1) hydroxylated hematite slab (corresponding to 1/3 ML of reactive sites) with an additional overlayer of water molecules to model solvation effects. This yields an overpotential of 0.77 V, a value only slightly above the 0.5-0.6 V experimental range. To explore whether doped hematite can exhibit an even lower overpotential, we consider cation doping by substitution of Fe by Ti, Mn, Co, Ni, or Si and F anion doping by replacing O on the fully hydroxylated surface. The reaction energetics on pure or doped hematite surfaces are described using a volcano plot. The relative stabilities of holes on the active O anions are identified as the underlying cause for trends in energetics predicted for different dopants. We show that moderately charged O anions give rise to smaller overpotentials. Co- or Ni-doped hematite surfaces give the most thermodynamically favored reaction pathway (lowest minimum overpotential) among all dopants considered. Very recent measurements (Electrochim. Acta 2012, 59, 121-127) reported improved reactivity with Ni doping, further validating our predictions.

462 citations


Journal ArticleDOI
TL;DR: This is the first demonstration of highly photoactive hematite nanowire arrays at a relatively low activation temperature without a dopant element and shows substantially enhanced photoactivity compared to the pristine hematites prepared in air.
Abstract: A promising photoelectrode material for solar-driven water splitting, hematite (a-Fe2O3) is non-toxic, abundant, chemically stable, low-cost, and has a bandgap of approximately 2.1 eV, which accounts for a maximum theoretical solar-tohydrogen (STH) efficiency of 15%. This last property compares favorably with the most studied metal oxide materials for photoeletrochemical (PEC) water splitting, including TiO2, [6–10] ZnO, and WO3. [12–15] However, the reported STH efficiencies of hematite photoelectrodes are substantially lower than the theoretical value, owing to several limiting factors such as poor conductivity, short excited-state lifetime (< 10 ps), poor oxygen evolution reaction kinetics, low absorption coefficient, short diffusion length for holes (2–4 nm), and lower flat-band potential in energy for water splitting. Enormous efforts have been made to overcome these limitations of hematite, including the incorporation of oxygen evolving catalysts to reduce the kinetic barrier for water oxidation on the hematite surface, the development of nanostructures to increase the effective surface area and to reduce diffusion length for carriers, as well as the development of element-doped hematite for improving electrical conductivity and/or light absorption. Recently, we demonstrated that TiO2 nanowires thermally treated in hydrogen showed increased donor density and PEC performance as a result of the formation of oxygen vacancies. We anticipated that creating oxygen vacancy (VO), and thereby Fe, sites in hematite could significantly increase the conductivity of the material through a polaron hopping mechanism. Although VO can be created by sintering hematite in a reductive atmosphere such as hydrogen, it may introduce hydrogen as a dopant into the structure. Additionally, hematite can be easily reduced in hydrogen to produce magnetite (Fe3O4), which is photo-inactive. [27] Herein, we report an alternative method for the preparation of highly conductive and photoactive hematite through thermal decomposition of b-FeOOH in an oxygen-deficient atmosphere (N2+ air). The resulting hematite sample showed substantially enhanced photoactivity compared to the pristine hematite prepared in air. The oxygen content during thermal activation significantly affects the formation of VO and thereby the photoactivity of hematite nanowires for water oxidation. This is the first demonstration of highly photoactive hematite nanowire arrays at a relatively low activation temperature without a dopant element. Akaganeite nanowires were prepared through the hydrolysis of FeCl3 (0.15m) in an environment with a high ionic strength (1m NaNO3) and low pH value (pH 1.5, adjusted by HCl) at 95 8C for 4 h. The resulting yellow film on a fluorine-doped tin oxide (FTO) substrate was covered with nanowire arrays with an average diameter and length of 70 nm and 700 nm, respectively (Figure 1a). X-ray diffraction

346 citations


Journal ArticleDOI
TL;DR: The p-type layer was found to create a built-in field that could be used to assist photoelectrochemical water splitting reactions, suggesting that it is possible to achieve desired energetics for solar water splitting directly on metal oxides through advanced material preparations.
Abstract: Mg-doped hematite (α-Fe2O3) was synthesized by atomic layer deposition (ALD). The resulting material was identified as p-type with a hole concentration of ca. 1.7 × 1015 cm–3. When grown on n-type hematite, the p-type layer was found to create a built-in field that could be used to assist photoelectrochemical water splitting reactions. A nominal 200 mV turn-on voltage shift toward the cathodic direction was measured, which is comparable to what has been measured using water oxidation catalysts. This result suggests that it is possible to achieve desired energetics for solar water splitting directly on metal oxides through advanced material preparations. Similar approaches may be used to mitigate problems caused by energy mismatch between water redox potentials and the band edges of hematite and many other low-cost metal oxides, enabling practical solar water splitting as a means for solar energy storage.

340 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report on the use of synthetic and natural Fe (hydr)oxides as catalysts in environmental remediation procedures using an advanced oxidation process, more specifically the Fentonlike system, which is highly efficient in generating reactive species such as hydroxyl radicals, even at room temperature and under atmospheric pressure.
Abstract: Iron is the fourth most common element by mass in the Earth’s crust and forms compounds in several oxidation states. Iron (hydr)oxides, some of which form inherently and exclusively in the nanometre-size range, are ubiquitous in nature and readily synthesized. These facts add up to render many Fe (hydr)oxides suitable as catalysts, and it is hardly surprising that numerous studies on the applications of Fe (hydr)oxides in catalysis have been published. Moreover, the abundant availability of a natural Fe source from rocks and soils at minimal cost makes the potential use of these as heterogeneous catalyst attractive. Besides those Fe (hydr)oxides that are inherently nanocrystalline (ferrihydrite, Fe5HO8·4H2O, and feroxyhyte, δ′-FeOOH), magnetite (Fe3O4) is often used as a catalyst because it has a permanent magnetization and contains Fe in both the divalent and trivalent states. Hematite, goethite and lepidocrocite have also been used as catalysts in their pure forms, doped with other cations, and as composites with carbon, alumina and zeolites among others. In this review we report on the use of synthetic and natural Fe (hydr)oxides as catalysts in environmental remediation procedures using an advanced oxidation process, more specifically the Fenton-like system, which is highly efficient in generating reactive species such as hydroxyl radicals, even at room temperature and under atmospheric pressure. The catalytic efficiency of Fe (hydr)oxides is strongly affected by factors such as the Fe oxidation state, surface area, isomorphic substitution of Fe by other cations, pH and temperature.

311 citations


Journal ArticleDOI
TL;DR: This work addresses a critical challenge of using hematite for PEC water splitting, namely, the fact that the band-edge positions are too positive for high-efficiency water splitting.
Abstract: Hematite (α-Fe(2)O(3)) was grown on vertically aligned Si nanowires (NWs) using atomic layer deposition to form a dual-absorber system. Si NWs absorb photons that are transparent to hematite (600 nm < λ < 1100 nm) and convert the energy into additional photovoltage to assist photoelectrochemical (PEC) water splitting by hematite. Compared with hematite-only photoelectrodes, those with Si NWs exhibited a photocurrent turn-on potential as low as 0.6 V vs RHE. This result represents one of the lowest turn-on potentials observed for hematite-based PEC water splitting systems. It addresses a critical challenge of using hematite for PEC water splitting, namely, the fact that the band-edge positions are too positive for high-efficiency water splitting.

303 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of thermal annealing on uncoated and coated magnetite nano-particles has been investigated using transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, infrared (IR) and Raman spectrographic analysis (TGA).

283 citations


Journal ArticleDOI
TL;DR: A 2-nm thick Nb(2)O(5) underlayer deposited by atomic layer deposition increases the charge separation efficiency and the photovoltage of ultrathin hematite films by suppressing electron back injection.
Abstract: Reference EPFL-ARTICLE-176405doi:10.1002/adma.201104868View record in Web of Science Record created on 2012-04-25, modified on 2017-05-12

268 citations


Journal ArticleDOI
TL;DR: In this paper, a study was conducted to evaluate hematite, magnetite, goethite, and iron rich laterite soil as arsenic adsorbents, and the results showed that arsenic adsorption occurred over the entire pH range tested (pH 4 − 11).

Journal ArticleDOI
TL;DR: In this article, it was shown that a significant amount of iron (> 0.5%) can only be solubilized in the dust when pH is lower than 4, suggesting that acid processing rather than cloud processing might be a prime mechanism to cause an increase in FIFO solubility during transport.

Journal ArticleDOI
TL;DR: A simple hydrothermal process for fabrication of hematite (α-Fe2O3) nanostructures with narrow size distribution was developed by using PVP as surfactant and NaAc as precipitation agent.
Abstract: A simple hydrothermal process for fabrication of hematite (α-Fe2O3) nanostructures with narrow size distribution was developed by using PVP as surfactant and NaAc as precipitation agent. The influence of experimental parameters including the concentration of the precursor, precipitation agent, stabilizing agent, and reaction time was systematically investigated to study the possible formation mechanism of α-Fe2O3. Finally, the electrochemical properties of the obtained hematite particles were studied using cyclic voltammetry and galvanostatic charge–discharge measurement by a three-electrode system. The results reveal that their specific capacitances are related to their sizes. By virtue of large surface area, the as-prepared hematite nanoparticles can present the highest capacitance (340.5 F·g–1) and an excellent long cycle life within the operated voltage window (−0.1 to 0.44 V), demonstrating that the as-prepared hematite nanoparticles can serve as one of the most excellent electrode materials for supe...

Journal ArticleDOI
TL;DR: The ease and scalability of the conversion from hydrated fluoride NWs to oxide NWs suggest a potentially versatile and low-cost strategy to make NWs of other useful iron-based compounds that may enable their large-scale renewable energy applications.
Abstract: We report for the first time the facile solution growth of α-FeF(3)·3H(2)O nanowires (NWs) in large quantity at a low supersaturation level and their scalable conversion to porous semiconducting α-Fe(2)O(3) (hematite) NWs of high aspect ratio via a simple thermal treatment in air. The structural characterization by transmission electron microscopy shows that thin α-FeF(3)·3H(2)O NWs (typically 100 nm in diameter) become polycrystalline porous α-Fe(2)O(3) NWs. We further demonstrated the photoelectrochemical (PEC) application of the nanostructured photoelectrodes prepared from these converted hematite NWs. The optimized photoelectrode with a ~400 nm thick hematite NW film yielded a photocurrent density of 0.54 mA/cm(2) at 1.23 V vs reversible hydrogen electrode potential after modification with cobalt catalyst under standard conditions (AM 1.5 G, 100 mW/cm(2), pH = 13.6, 1 M NaOH). The low cost, large quantity, and high aspect ratio of the converted hematite NWs, together with the resulting simpler photoelectrode preparation, can be of great benefit for hematite-based PEC water splitting. Furthermore, the ease and scalability of the conversion from hydrated fluoride NWs to oxide NWs suggest a potentially versatile and low-cost strategy to make NWs of other useful iron-based compounds that may enable their large-scale renewable energy applications.

Journal ArticleDOI
TL;DR: A p-n junction photoanode has been fabricated by depositing p-type NiO nanoparticles on the n-type hematite thin film, which facilitates the extraction of accumulated holes fromHematite via the p-N junction, but also lowers the barrier for oxygen evolution reaction.

Journal ArticleDOI
TL;DR: The promise, problems and progress of utilizing hematite for photocatalyzed water oxidation and issues which require further research are presented.
Abstract: This article focuses on the promise, problems and progress of utilizing hematite for photocatalyzed water oxidation. Issues which require further research are also presented.

Journal ArticleDOI
07 Sep 2012-Science
TL;DR: In this paper, pump-probe spectroscopy was used to study the dynamics of electrons introduced into iron(III) (oxyhydr)oxide nanoparticles via ultrafast interfacial electron transfer.
Abstract: Electron mobility within iron (oxyhydr)oxides enables charge transfer between widely separated surface sites. There is increasing evidence that this internal conduction influences the rates of interfacial reactions and the outcomes of redox-driven phase transformations of environmental interest. To determine the links between crystal structure and charge-transport efficiency, we used pump-probe spectroscopy to study the dynamics of electrons introduced into iron(III) (oxyhydr)oxide nanoparticles via ultrafast interfacial electron transfer. Using time-resolved x-ray spectroscopy and ab initio calculations, we observed the formation of reduced and structurally distorted metal sites consistent with small polarons. Comparisons between different phases (hematite, maghemite, and ferrihydrite) revealed that short-range structural topology, not long-range order, dominates the electron-hopping rate.

Journal ArticleDOI
TL;DR: In this paper, the authors reported a strategy to perform in situ incorporation of oxygen evolution catalyst, Co3O4, during hydrothermal growth of Fe2O3 nanorod arrays.
Abstract: In this Article, we report a strategy to perform in situ incorporation of oxygen evolution catalyst, Co3O4, during hydrothermal growth of Fe2O3 nanorod arrays. It was found that the highest photocurrent increase and onset potential shift was observed with 5% Co2+. The photocurrent density increases from 0.72 for the pristine Fe2O3 nanorod to 1.20 mA/cm2 at 1.23 V versus RHE (i.e., 67% improvement) with 5% Co2+ added. Concomitant with this improvement was a shift in the onset potential by ∼40 mV and improvements in incident-photon-to-current efficiencies and oxygen evolution. Hematite photoanodes with in situ deposition of Co3O4 nanoparticles showed better performance than those prepared by ex situ procedures because of high surface roughness, larger Co3O4/hematite interfacial area, and smaller Co3O4 particle size.

Journal ArticleDOI
TL;DR: In this paper, a remarkable plateau photocurrent density of 3.76 mA/cm2 has been observed for Ti-doped hematite nanostructures under standard illumination conditions in 1'M NaOH electrolyte, which is 2.5 times higher than that for undoped nano-structures.
Abstract: Ti-doped hematite nanostructures have been synthesized for efficient solar water splitting by adding TiCN as the Ti precursor in a hydrothermal method. Ti-doped hematite nanostructures show an urchin-like morphology with nano feature size, which increases the effective surface area compared to undoped nanostructures. A remarkable plateau photocurrent density value of 3.76 mA/cm2 has been observed for Ti-doped nanostructures under standard illumination conditions in 1 M NaOH electrolyte, which is 2.5 times higher than that for undoped nanostructures (1.48 mA/cm2). The photocurrent at 1.23 V vs. RHE (1.91 mA/cm2) is also enhanced to be over 2 times higher than that for undoped nanostructures (0.87 mA/cm2). X-ray photoelectron spectroscopy and x-ray absorption spectroscopy have been used to investigate the electronic structure of Ti-doped hematite, which suggest the increased donor density of hematite by Ti doping. The remarkable plateau current density in Ti-doped hematite nanostructures can be attributed t...

Journal ArticleDOI
TL;DR: In this article, the effects of pH and temperature on jarosite initial dissolution rates and iron oxide reaction products were determined in the laboratory at initial pH 1−10 and temperatures from 277 to 323 K.

Journal ArticleDOI
TL;DR: In this article, the oxidation of Fe in pure oxygen between 400 and 600 C was investigated in order to obtain Fe oxidation in the presence of pure oxygen, and it was shown that Fe oxidizes at a rate between 400 to 600 C.
Abstract: a b s t r a c t The oxidation of Fe in pure oxygen between 400 ◦ C and 600 ◦ C has been investigated in order to obtain

Journal ArticleDOI
TL;DR: The results demonstrate that structurally incorporated trace elements are mobilized from iron oxides into fluids without abiotic or microbial net iron reduction.
Abstract: Electron transfer and atom exchange (ETAE) between aqueous Fe(II) and Fe(III) oxides induces surface growth and dissolution that affects trace element fate and transport. We have recently demonstrated Ni(II) cycling through goethite and hematite (adsorbed Ni incorporates into the mineral structure and preincorporated Ni releases to solution) during Fe(II)-Fe(III) ETAE. However, the chemical parameters affecting net trace element release remain unknown. Here, we examine the chemical controls on Ni(II) and Zn(II) release from Ni- and Zn-substituted goethite and hematite during reaction with Fe(II). Release follows a rate law consistent with surface reaction limited mineral dissolution and suggests that release occurs near sites of Fe(III) reductive dissolution during Fe(II)-Fe(III) ETAE. Metal substituent type affects reactivity; Zn release is more pronounced from hematite than goethite, whereas the opposite trend occurs for Ni. Buildup of Ni or Zn in solution inhibits further release but this resumes upon fluid exchange, suggesting that sustained release is possible under flow conditions. Mineral and aqueous Fe(II) concentrations as well as pH strongly affect sorbed Fe(II) concentrations, which directly control the reaction rates and final metal concentrations. Our results demonstrate that structurally incorporated trace elements are mobilized from iron oxides into fluids without abiotic or microbial net iron reduction. Such release may affect micronutrient availability, contaminant transport, and the distribution of redox-inactive trace elements in natural and engineered systems.

Journal ArticleDOI
TL;DR: Large-scale cultivation effort using various Fe(III) oxides and carbon substrates along a dilution gradient to enrich for microbial populations capable of reducing Fe oxides spanning a wide range of crystallinities and reduction potentials highlights the need for further targeted investigations into the composition and activity of speciation-directed metal-reducing populations within natural environments.
Abstract: Iron (Fe) oxides exist in a spectrum of structures in the environment, with ferrihydrite widely considered the most bioavailable phase. Yet, ferrihydrite is unstable and rapidly transforms to more crystalline Fe(III) oxides (e.g., goethite, hematite), which are poorly reduced by model dissimilatory Fe(III)-reducing microorganisms. This begs the question, what processes and microbial groups are responsible for reduction of crystalline Fe(III) oxides within sedimentary environments? Further, how do changes in Fe mineralogy shape oxide-hosted microbial populations? To address these questions, we conducted a large-scale cultivation effort using various Fe(III) oxides (ferrihydrite, goethite, hematite) and carbon substrates (glucose, lactate, acetate) along a dilution gradient to enrich for microbial populations capable of reducing Fe oxides spanning a wide range of crystallinities and reduction potentials. While carbon source was the most important variable shaping community composition within Fe(III)-reducing enrichments, both Fe oxide type and sediment dilution also had a substantial influence. For instance, with acetate as the carbon source, only ferrihydrite enrichments displayed a significant amount of Fe(III) reduction and the well known dissimilatory metal reducer Geobacter sp. was the dominant organism enriched. In contrast, when glucose and lactate were provided, all three Fe oxides were reduced and reduction coincided with the presence of fermentative (e.g. Enterobacter spp.) and sulfate-reducing bacteria (e.g. Desulfovibrio spp.). Thus, changes in Fe oxide structure and resource availability may shift Fe(III)-reducing communities between dominantly metal-respiring to fermenting and/or sulfate-reducing organisms which are capable of reducing more recalcitrant Fe phases. These findings highlight the need for further targeted investigations into the composition and activity of speciation-directed metal-reducing populations within natural environments.

Journal ArticleDOI
TL;DR: A two-step method involving high temperature nucleation followed by growth at low temperature is shown to produce a highly dense and uniform coverage of nanowire arrays.
Abstract: Undoped hematite nanowire arrays grown using plasma oxidation of iron foils show significant photoactivity (?0.38?mA?cm?2 at 1.5?V versus reversible hydrogen electrode in 1?M KOH). In contrast, thermally oxidized nanowire arrays grown on iron exhibit no photoactivity due to the formation of a thick (>7??m?Fe1?xO) interfacial layer. An atmospheric plasma oxidation process required only a few minutes to synthesize hematite nanowire arrays with a 1?5??m interfacial layer of magnetite between the nanowire arrays and the iron substrate. An amorphous oxide surface layer on hematite nanowires, if present, is shown to decrease the resulting photoactivity of as-synthesized, plasma grown nanowire arrays. The photocurrent onset potential is improved after removing the amorphous surface on the nanowires using an acid etch. A two-step method involving high temperature nucleation followed by growth at low temperature is shown to produce a highly dense and uniform coverage of nanowire arrays.

Journal ArticleDOI
TL;DR: These findings demonstrate that, by doping hematite and by engineering the interface between the hematites and the electrolyte, charge separation can be effectively promoted and photocurrent density can be dramatically increased.
Abstract: Iron(III) oxide photoelectrodes show promise in water oxidation applications. In this study, micro-nano-structured hematite films are synthesized, and Ti ions are doped to improve photoelectric conversion efficiency. The photocurrent increases for enhanced electrical conductivity. Further enhanced photocurrent is achieved for Fe2O3:Ti/ZnFe2O4 heterojunction electrodes. Cyclic voltammograms combined with optical absorbance examinations demonstrate that the conduction and valence band edges of ZnFe2O4 shift from those of Ti doped Fe2O3 to the negative direction, which facilitates the efficient separation of electron–hole pairs at the Fe2O3:Ti/ZnFe2O4 interface. These findings demonstrate that, by doping hematite and by engineering the interface between the hematite and the electrolyte, charge separation can be effectively promoted and photocurrent density can be dramatically increased.

Journal ArticleDOI
TL;DR: 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 and could push forward the development of host-guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.
Abstract: 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 02 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 14 Accordingly, a Ga2O3 underlayer could push forward the development of host–guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers

Journal ArticleDOI
TL;DR: It is demonstrated that pH and reaction time influence cell-mineral interactions, implying that these parameters play an important role in determining cell mobility and biofilm formation in aqueous geochemical environments.
Abstract: Attachment of live cells of Shewanella putrefaciens strain CN-32 to the surface of hematite (α-Fe(2)O(3)) was studied with in situ ATR-FTIR spectroscopy at variable pH (4.5-7.7) and contact times up to 24 h. The IR spectra indicate that phosphate based functional groups on the cell wall play an important role in mediating adhesion through formation of inner-sphere coordinative bonds to hematite surface sites. The inner-sphere attachment mode of microbial P groups varies with pH, involving either a change in protonation or in coordination to hematite surface sites as pH is modified. At all pH values, spectra collected during the early stages of adhesion show intense IR bands associated with reactive P-groups, suggestive of preferential coordination of P-moieties at the hematite surface. Spectra collected after longer sorption times show distinct frequencies from cell wall protein and carboxyl groups, indicating that bacterial adhesion occurring over longer time scales is to a lesser degree associated with preferential attachment of P-based bacterial functional groups to the hematite surface. The results of this study demonstrate that pH and reaction time influence cell-mineral interactions, implying that these parameters play an important role in determining cell mobility and biofilm formation in aqueous geochemical environments.

Journal ArticleDOI
TL;DR: In this paper, a supported hematite α-Fe2O3 (hematite supported on Algerian clay) by impregnation is successfully used for the chromate reduction under visible light.

Journal ArticleDOI
TL;DR: In this article, the authors used a CSIRO Hy-Logging™ system from 180 reverse circulation and 14 diamond drill cores spanning a depth from surface to 55 m below surface, intersecting the Rocklea deposit.
Abstract: Visible-near to shortwave infrared reflectance spectra (VNIR-SWIR—400–2,500 nm wavelength region) provide quantitative measurements of mineral abundances and mineral physicochemistries from drill core samples of channel iron ore. The infrared spectral reflectance measurements of drill core samples from the Rocklea channel iron deposit, located in the Hamersley Basin of Western Australia, were validated against X-ray flouresence (XRF) and X-ray diffraction (XRD). The reflectance data were collected using a CSIRO Hy-Logging™ system from 180 reverse circulation and 14 diamond drill cores spanning a depth from surface to 55 m below surface, intersecting the Rocklea deposit. The mineralogy of this channel iron deposit comprises chiefly goethite (both vitreous and ochreous forms) with lesser amounts of hematite and variable amounts of quartz, kaolinite, smectite (both dioctahedral and trioctahedral varieties), and carbonate (both dolomite and calcite). Iron ore and waste rock mineralogy were extracted from the infrared spectroscopic reflectance data using the geometry (depth/wavelength) of continuum-removed reflectance spectra, with depths of absorption features proportional to mineral abundance and wavelength positions proportional to chemical composition. For any one mineral, a number of its diagnostic spectral features are used to determine its mineral abundance and composition, in order to avoid complications with minerals that spectrally overlap with part of the diagnostic spectral features of the mineral of interest. This method that combines multiple spectral features to identify and quantify minerals is transferable to all types of infrared reflectance spectroscopic data collected from drill core to satellites. Final products include: iron (oxyhydr-)oxide content (root mean square error (RMSE) 9.1 wt % Fe); Al clay content (RMSE 3.9 wt % Al 2 O 3 ); hematite/goethite ratio (RMSE 9.0 wt % goethite); vitreous versus ochreous goethite (not easily measured using other analytical techniques); clay mineral type (kaolinite, montmorillonite and nontronite); and carbonate composition (dolomite vs. calcite). The Rocklea infrared reflectance spectroscopy-based mineral abundance and composition results have been used in an associated study to characterize the architecture of the Rocklea channel iron deposit, with implications for exploration, mining, and ore genesis.

Journal ArticleDOI
TL;DR: A very thin ZnO overlayer was deposited on top of a thin film of hematite and found to increase the photocurrent and reduce the onset potential for generating oxygen from water.
Abstract: A simple and inexpensive method to form a hematite photoanode for efficient water oxidation is reported. A very thin ZnO overlayer was deposited on top of a thin film of hematite and found, compared with non-treated hematite, to increase the photocurrent and reduce the onset potential for generating oxygen from water. After 3 cycles of ZnAc treatment, the photocurrent increased more than 40% to 1.08 mA cm−2 at 0.23 V vs. Ag/AgCl and onset potential for water oxidation shifted by −170 mV. It is proposed that the ZnO overlayer changes the flat band potential of hematite and reduces the surface defects.