Nanocrystalline Hematite Research Articles

Photoelectrocatalytic Activity of ZnO-Modified Hematite Films in the Reaction of Alcohol Degradation.

Thin-film nanocrystalline hematite electrodes were fabricated by electrochemical deposition and loaded with electrodeposited zinc oxide in various amounts. Under visible light illumination, these electrodes demonstrate high activity in the photoelectrochemical degradation of methanol, ethylene glycol and, in particular, glycerol. Results of intensity-modulated photocurrent spectroscopy show that the photoelectrocatalysis efficiency is explained by the suppression of the electron-hole pair recombination and an increase in the rate of photo-induced charge transfer. Thus, zinc oxide can be considered an effective modifying additive for hematite photoanodes.

Degradation of malachite green dye by capping polyvinylpyrrolidone and Azadirachta indica in hematite phase of Ni doped Fe2O3 nanoparticles via co-precipitation method

In the present research, a chemical co-precipitation approach has been used to approach the synthesis, characterization, and photocatalytic applicability of Ni-doped α-Fe2O3 (hematite) nanoparticles. Biosynthesized iron oxide nanoparticles (IONPs) were successfully synthesized using a non-toxic leaf extract of the Azadirachta indica (AI) plant (neem) as a reducing and stabilizing agent. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, FT-IR spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, and vibrating sample magnetometer (VSM) have all been used to examine the synthesized materials. All of the produced NPs contain only the nanocrystalline hematite phase, according to XRD measurements. The morphology studies of the Ni-doping hematite nanoparticles, as demonstrated by TEM and SEM. The phase purity and phonon modes of the prepared nanoparticles are confirmed by Raman spectroscopy. The UV-Vis absorption tests show also that value of the band gap increases together with the reduction in particle size, going from 2.26 eV for chemical α-Fe2O3 to 2.5 eV for green Ni-doped α-Fe2O3 nanoparticles. Additionally, it was clear from the magnetic characteristics that all of the samples behaved ferromagnetically at ambient temperatures. On the other side, malachite green (MG) dye was used as a surrogate industrial wastewater dye in order to study the photocatalytic efficiency of Ni-doped α-Fe2O3 particles. The pure/green Ni-doped α-Fe2O3 NPs showed that after 70 minutes of exposure, 92% of the MG had become discolored.

Photoelectrocatalytic Properties of a Ti-Modified Nanocrystalline Hematite Film Photoanode

Photoelectrocatalytic oxidation of methanol, ethylene glycol, glycerol, and 5,6,7,8-tetrahydro-2-naphthol on thin-film nanocrystalline hematite electrodes fabricated by electrochemical deposition and promoted with spin-coated titanium has been studied. It is shown that the modification of hematite transforms it into material exhibiting high activity in the photoelectrochemical process of substrate oxidation upon illumination with light in the visible region of the spectrum. The highest activity is observed in the reaction of photoelectrocatalytic oxidation of glycerol. Results of intensity-modulated photocurrent spectroscopy (IMPS) suggest that the effect is due to an increased rate of charge transfer in the process of photoelectro-oxidation and efficient suppression of the recombination of generated electron-hole pairs. Therefore, thin-film photoanodes based on modified hematite are promising for practical application in the photooxidation of glycerol, a by-product of biofuel production, as well as in the photoelectrochemical degradation of other organic pollutants, including those formed during the production of pharmaceuticals.

The photocatalytic antibacterial behavior of Cu-doped nanocrystalline hematite prepared by mechanical alloying

To improve the photocatalytic properties of hematite (Fe2O3), we take the merit of nanoscale and exploit a mechanical alloying (MA) method for Cu doping with various weight percentages (1, 3, 5, and 10). Our techniques to evaluate the phase and morphological characterization of the prepared nanoparticles were, respectively, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The elemental analysis and elemental distribution map were examined by energy dispersive X-ray spectroscopy (EDS). Furthermore, we utilized Fourier transform infrared (FTIR) analysis and ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) for appraisal of Fe–O and Cu–O bonds and band gaps. Then we investigated the photocatalytic activity based on methylene blue (MB) photodegradation under irradiation of 300 W Xenon lamp and finally explored the antibacterial behavior against Escherichia coli (E. coli). The UV–Vis DRS results indicated reduction of band gap in the Cu-doped hematite nanoparticles. For the samples considered here, the sample with 5 wt.% of the dopant showed the lowest band-gap, 1.57 eV. It also decomposed 92.5% of MB from the wastewater under the Xenon-lamp irradiation during 120 min, and had the highest rate of disinfection with > 99.99% of bacterial removal in 360 min. Low band gap, substantial sunlight absorption, efficient MB removal, and high antibacterial activity are some suitable photocatalytic properties specifically achieved in this work.

Mineralogy of Vera Rubin Ridge From the Mars Science Laboratory CheMin Instrument

AbstractVera Rubin ridge (VRR) is an erosion‐resistant feature on the northwestern slope of Mount Sharp in Gale crater, Mars, and orbital visible/shortwave infrared measurements indicate it contains red hematite. The Mars Science Laboratory Curiosity rover performed an extensive campaign on VRR to study its mineralogy, geochemistry, and sedimentology to determine the depositional and diagenetic history of the ridge and constrain the processes by which the hematite could have formed. X‐ray diffraction (XRD) data from the CheMin instrument of four samples drilled on and below VRR demonstrate differences in iron, phyllosilicate, and sulfate mineralogy and hematite grain size. Hematite is common across the ridge, and its detection in a gray outcrop suggest localized regions with coarse‐grained hematite, which commonly forms from warm fluids. Broad XRD peaks for hematite in one sample below VRR and the abundance of FeOT in the amorphous component suggest the presence of nanocrystalline hematite and amorphous Fe oxides/oxyhydroxides. Well crystalline akaganeite and jarosite are present in two samples drilled from VRR, indicating at least limited alteration by acid‐saline fluids. Collapsed nontronite is present below VRR, but samples from VRR contain phyllosilicate with d(001) = 9.6 Å, possibly from ferripyrophyllite or an acid‐altered smectite. The most likely cementing agents creating the ridge are hematite and opaline silica. We hypothesize late diagenesis can explain much of the mineralogical variation on the ridge, where multiple fluid episodes with variable pH, salinity, and temperature altered the rocks, causing the precipitation and crystallization of phases that are not otherwise in equilibrium.

Acid-based geopolymers using waste fired brick and different metakaolins as raw materials

Three metakaolins and waste fired brick were used to explore the effects of iron oxide, amorphous silica and quartz in the raw materials on the compressive strength and the microstructural properties of acid-based geopolymers cured at room temperature and 60 °C. Quartz content in the metakaolin from Dibamba is about 22.0 wt% and each other samples contain around 8.0 wt%. Waste fired brick and metakaolin from Bangoua contain nanocrystalline hematite and have higher amorphous silica content. The higher quartz content in the metakaolin from Dibamba could prevent the incorporation of PO4 units in the networks. The compressive strengths of the acid-based geopolymers cured at room temperature are higher (35.3–56.4 MPa) compared to those cured at 60 °C (17.8–32.9 MPa). The higher amorphous silica and nanocrystalline hematite content in the starting materials could accelerate the hardening process. It can be concluded that iron oxide, amorphous silica and quartz in the starting material could affect the compressive strengths of acid-based geopolymers.

Retraction Note to: Synthesis and characterization of Ni-doped α-Fe2O3 nanoparticles through co-precipitation method with enhanced photocatalytic activities

In the present study, an attempt has been made for synthesis, characterization and photocatalytic application of pure and Ni-doped α-Fe2O3 (hematite) nanoparticles by chemical co-precipitation method. The synthesized products have been studied by Thermo Gravimetric Analysis (TGA), Differential Thermal Analysis (DTA), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red (FT-IR), Raman spectroscopy, Ultraviolet–Visible (UV–Vis) analysis and Vibrating Sample Magnetometer (VSM). The TGA showed three mass losses, whereas DTA resulted in three endothermic peaks. XRD measurements confirm that all the prepared nanocrystals consist only in nanocrystalline hematite phase. TEM and SEM show that the size of the nanoparticles decreases with Ni-doping. FTIR and Raman spectroscopies confirm the phase purity and the phonon modes of the synthesized nanoparticles. The UV–Vis absorption measurements confirm that the decrease of particle size is accompanied by a decrease in the band gap value from 2.02 eV for α-Fe2O3 down to 1.81 eV for 8 mol% Ni-doped α-Fe2O3. Furthermore, the magnetic properties demonstrated that all of the samples exhibited ferromagnetic behavior at room temperature. On the other part, the photocatalytic activity of Ni-doped α-Fe2O3 particles was studied using methylene blue (MB) as model organic pollutants. The 8 mol% Ni-doped α-Fe2O3 nanoparticles disclosed that the discoloration of MB reached 86% after irradiation of 140 min.

Structure, morphology and magnetic properties of hematite and maghemite nanopowders produced from rolling scale

The work is devoted to development of cost-efficient method of processing of metallurgical waste – oily rolling scale formed during hot-rolled steel strip mechanical cleaning in descaling mills. The most significant parameters of chemical metallurgical process for producing expensive and highly marketed products – α-Fe 2 O 3 and γ-Fe 2 O 3 nanopowders – have been experimentally determined. The properties of initial materials and nanodispersed products were studied by X-ray diffractometry, energy dispersive spectroscopy, scanning and transmission microscopy, and Mossbauer spectrometry. Temperature and field dependences of powders magnetization were built according to vibration magnetometer measurements. It is shown that rolling scale consists of three main phases: wustite, magnetite and hematite in a ratio of 6:8:7 by weight, respectively. The initial scale was activated in magnetic mill in stream of hydrogen and dissolved in mixture of hydrochloric and nitric acids. The resulting solutions were used to obtain α-Fe 2 O 3 nanocrystalline hematite by chemical-metallurgical method, the main stages of which were hydroxide precipitation with alkali at constant p H , washing, drying, and dehydration. γ-Fe 2 O 3 maghemite was obtained from hematite in two stages. At the first stage, hydrogen reduction was carried out, and at the second stage, the magnetite obtained was oxidized in air. Particles of synthesized nanodispersed oxide powders are in aggregated state. Particles of α-Fe 2 O 3 are spherical, and γ-Fe 2 O 3 are rod-shaped. According to Mossbauer spectroscopy, the lattices of both oxides contain magnesium, aluminum, silicon, chromium, and manganese that have passed from the initial scale. These elements determine magnetic properties of α-Fe 2 O 3 and γ-Fe 2 O 3 nanopowders. Set of properties of nanodispersed hematite and maghemite powders obtained from metallurgical waste (rolling scale) allows us to recommend their application as catalysts, in industrial wastewater heavy metal ions treatment systems, and in production of blood analysis markers.

Nanocrystalline hematite α-Fe2O3 synthesis with different precursors and their composites with graphene oxide

The facile wet chemical co-precipitation method was used for synthesis of hematite (α-Fe2O3) nanoparticles and nanocomposites with graphene oxide (GO) using ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) as different precursor materials. EDTA and DTPA acted as chelating agents to avoid multi nucleation and aggregation of nanoparticles in growth process. For nanocomposite, GO was considered as flexible material with theoretical specific surface area ~1000 m2/g and better surface functionalization due to presence of oxygen containing functional groups i.e. carboxylic, hydroxyl and epoxides groups at basal edge. The structural analysis of prepared nanoparticles and nanocomposites was conducted by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). XRD confirmed various cell parameters and crystalline structure of prepared particles. The crystallite size was found 5 nm–10 nm. After preparation of nanocomposites of hematite α-Fe2O3 (EDTA or DTPA) nanoparticles with GO, they were characterized by Scanning electron microscopy (SEM) and UV–Visible spectroscopy. Current-Voltage (I-V) measurements were also carried out to observe the decrease in resistivity values after mixing with GO. UV–Visible spectroscopy revealed the better photocatalytic degradation of methylene blue (MB) in visible light. Degradation of methylene blue was observed up to 67% with α- Fe2O3 (DTPA) @ GO and 86.06% for α- Fe2O3 (EDTA) @ GO greater than simple α- Fe2O3 (DTPA) 21.2% and α- Fe2O3 (EDTA) 36.8% nanoparticles. As a result, synergistic effect of α- Fe2O3 (EDTA) @ GO showed better photocatalytic action due to GO layer, it acted as electron acceptor and kept high adsorption properties. Electrostatic bonding in α- Fe2O3 (EDTA) @ GO with MB having different functional groups showed the stability of photocatalyst, not to be leached into water. For prolonged time, the charge carrier recombination was suppressed for improved degradation rate of MB in visible light in presence of α- Fe2O3 (EDTA) @ GO.

Defect Engineering of Titania and Hematite Thin Films By Advanced Plasma Deposition Triggers High Photoelectrochemical Water Splitting Activity

The development of highly efficient energy conversion/storage devices has attracted attention from scientific and technological researchers due to the increasing concerns about the environment and the depletion of fossil fuels. In this context, the development of cost-competitive materials capable of producing fuels or electricity directly from the energy harvested from sunlight offers a desirable approach towards fulfilling the need of clean energy. Semiconductor metal oxides (e.g., TiO2, α-Fe2O3, WO3 or ZnO) are the most widely adopted materials for the conversion of solar energy into storable and transportable chemical energy such as hydrogen (H2) via the photo-electrochemical water splitting (PEC-WS), where they serve as so-called photoanodes. Especially hematite (α-Fe2O3) and titania (TiO2) based photoanodes have attracted great amount of interest in the field of PEC-WS, however their overall efficiency for solar-driven applications still remain rather low for practical application. The defect engineering (DE) has become an important research direction for improving the optical and electronic properties of these materials towards highly efficient PEC processes. For example the discovery of black TiO2 by Chen and Mao in 2011 [1] with its substantially enhanced solar absorption has recently triggered a worldwide research interest. In spite of their remarkable enhancement in visible light absorption black TiO2 has demonstrated expected photocatalytic and PEC performance. The introduction of oxygen vacancies has been shown as an effective approach to improve the activity of α-Fe2O3 photoelectrodes [2].The main limitation related to the current DE approaches is that they are predominantly realized via a high-pressure high-temperature gas reduction. However, this methodology severely reduces the possibility to influence the DE process itself and the final properties of the material in a very fine manner, which is undoubtedly the key to precisely understand all the induced electronic changes and related phenomena. In the presented work, we address this significant drawback by utilizing a novel deposition method based on high-impulse magnetron sputtering (HiPIMS) for the deposition of titania and hematite films under the DE conditions. This method offers the desired capability of high process controllability due to the unique deposition plasma properties. By adjusting the deposition conditions, we can regulate the extent of induced defects and under significantly reduced temperature. [3, 4] Hematite, α-Fe2O3, is considered as one of the most promising materials for PEC-WS with a theoretical solar-to-hydrogen efficiency of 17%. However, the poor electrical conductivity and short diffusion length of photogenerated holes are substantial limitations reducing hematite’s efficiency in real experimental conditions. Here, we report on addressing these two drawbacks by deposition of very thin nanocrystalline hematite films (~ 30 nm) HiPIMS. Despite of computing models suggesting that the electrical conductivity is extremely anisotropic, revealing up to four orders of magnitude higher electron transport with conduction along the (110) hematite crystal plane, synthetic approaches allowing the sole growth in that direction have not been reported yet. We present a new strategy based on HiPIMS for tuning the crystal orientation of 2D thin hematite films by carefully controlling the energy of particles bombarding the substrate during the plasma assisted deposition procedure. [3] The PEC activity can be even boosted when commonly used fluorine doped tin oxide (FTO) conductive substrate is substituted by platinized substrate. Oriented platinum substrate can induce crystalline defects in hematite film during its growth under very high-energy bombardment, which are thermodynamically difficult to obtain. Such anomalies further considerably increase final PEC activity of hematite photoanodes. The enhanced photoactivity of α-Fe2O3@Pt is attributed to several synergistically contributing features ;namely: (i) a higher electrical conductivity due to self-doping oxygen vacancies; (ii) the formation of an intrinsic heterojunction between the distorted and typical hematite phases that ensures highly efficient separation of photogenerated charge carriers, and (iii) increased optical absorption due to enhance intensity in metal-to-ligand charge-transfer (MLCT). According to our best knowledge, the achieved photocurrent density of 1.1 mA/cm2 at 1.5 V vs. RHE represents the champion values among ultrathin undoped hematite based photoanodes. Acknowledgments The authors gratefully acknowledge the support by the Operational Programme Research, Development and Education - European Regional Development Fund, project no. CZ.02.1.01/0.0/0.0/15_003/0000416 of the Ministry of Education, Youth and Sports of the Czech Republic. References Chen, L. Liu, P. Y. Yu, S. S. Mao, Science, 2011, 331, 746-750.Forster, R. J. Potter, Y. Ling, Y. Yan et al., Chem. Sci., 2015, 6, 4009-4016.Kment, P. Schmuki, Z. Hubicka, et al., ACS Nano, 2015, 9, 7113-7123.Kment, Z. Hubicka, et al., Appl. Catal. B-Environ., 2015, 165, 344-350.

Amorphous and nanocrystalline hematite photocatalysts synthesized in ferric chloride-choline chloride acting as a green and reactive synthesis medium

Ferric chloride-choline chloride deep eutectic mixture was successfully employed to synthesize α-Fe2O3 (hematite) nanoparticles. The characterization results confirmed the formation of amorphous α-Fe2O3 nanoparticles having a spherical morphology and the size ranged from 25 to 75 nm, which were highly crystallized upon calcination at 500 ℃. The UV absorption-reflection study on the α-Fe2O3 nanoparticles revealed the decrease of UV absorption capability, enhancement of band gap energy from 2.2 to 2.3 eV, and raise of UV reflectance from 11 to 26% upon calcination of the nanoparticles resulted from increased crystallinity or decreased crystal defects in the nanoparticle. The photocatalytic performance of the α-Fe2O3 nanoparticles was evaluated in the decomposition of Rhodamine Blue (RhB) organic dye. The RhB decomposition rates of 55% and 47% within the exposure time of 75 min were obtained for as-synthesized and calcined α-Fe2O3 nanoparticles, respectively.

RETRACTED ARTICLE: Synthesis and characterization of Ni-doped α-Fe2O3 nanoparticles through co-precipitation method with enhanced photocatalytic activities

In the present study, an attempt has been made for synthesis, characterization and photocatalytic application of pure and Ni-doped α-Fe2O3 (hematite) nanoparticles by chemical co-precipitation method. The synthesized products have been studied by Thermo Gravimetric Analysis (TGA), Differential Thermal Analysis (DTA), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red (FT-IR), Raman spectroscopy, Ultraviolet–Visible (UV–Vis) analysis and Vibrating Sample Magnetometer (VSM). The TGA showed three mass losses, whereas DTA resulted in three endothermic peaks. XRD measurements confirm that all the prepared nanocrystals consist only in nanocrystalline hematite phase. TEM and SEM show that the size of the nanoparticles decreases with Ni-doping. FTIR and Raman spectroscopies confirm the phase purity and the phonon modes of the synthesized nanoparticles. The UV–Vis absorption measurements confirm that the decrease of particle size is accompanied by a decrease in the band gap value from 2.02 eV for α-Fe2O3 down to 1.81 eV for 8 mol% Ni-doped α-Fe2O3. Furthermore, the magnetic properties demonstrated that all of the samples exhibited ferromagnetic behavior at room temperature. On the other part, the photocatalytic activity of Ni-doped α-Fe2O3 particles was studied using methylene blue (MB) as model organic pollutants. The 8 mol% Ni-doped α-Fe2O3 nanoparticles disclosed that the discoloration of MB reached 86% after irradiation of 140 min.

Facet-Dependent Selective Adsorption of Mn-Doped α-Fe2O3 Nanocrystals toward Heavy-Metal Ions

Engineering exposed active facets by doping impurities can dramatically modify the morphology and physicochemical properties of nanocrystalline hematite. In the work described in this paper, through the combination of laser ablation in liquid and hydrothermal treatment techniques, faceted Mn-doped α-Fe2O3 nanocrystals (NCs) were prepared by adjusting the doping level of elemental Mn. With the increase of Mn doping level, the hematite crystal evolved sequentially from isotropic polyhedral nanoparticles (NPs) to {116}-faceted saucer-shaped nanosheets (NSs), and then to {001}-faceted hexagonal NSs. Electrochemical stripping tests revealed that the Mn-doped α-Fe2O3 NCs show a facet-dependent adsorption ability toward Pb(II), Cd(II), and Hg(II) heavy-metal ions; that is, {001}-faceted hexagonal NSs exhibit high and selective adsorption toward Pb2+ ions, while {116}-faceted saucer-shaped NSs present strong and selective adsorption toward Cd2+ and Hg2+ ions. Density functional theory (DFT) calculations found tha...

Structural, optical and morphological characterization of Cu-doped α-Fe2O3 nanoparticles synthesized through co-precipitation technique

Pure and copper (Cu concentration varying from 2 to 8%) doped hematite (α-Fe2O3) nanocrystals were synthesized through co-precipitation method using simple equipment. X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red (FT-IR), Raman spectroscopy, Differential Thermal Analysis (DTA), Thermo Gravimetric Analysis (TGA) and Ultraviolet–Visible (UV–Vis) techniques were used to characterize the synthesized samples. XRD measurements confirm that all the prepared nanocrystals consist only in nanocrystalline hematite phase. These results along with TEM and SEM show that the size of the nanoparticles decreases with Cu-doping down to 21 nm. FT-IR confirm the phase purity of the nanoparticles synthesized. The Raman spectroscopy was used not only to prove that we synthesized pure and Cu-doped hematite but also to identify their phonon modes. The TGA showed three mass losses, whereas DTA resulted in three endothermic peaks. The UV–Vis absorption measurements confirm that the decrease of particle size is accompanied by a decrease in the band gap value from 2.12 eV for pure α-Fe2O3 down to 1.91 eV for 8% Cu-doped α-Fe2O3. 8% Cu-doped hematite had the smallest size, the best crystallinity and the lowest band gap.

Arsenic Mobilization Is Enhanced by Thermal Transformation of Schwertmannite

Fires in iron-rich seasonal wetlands can thermally transform Fe(III) minerals and alter their crystallinity. However, the fate of As associated with thermally transformed Fe(III) minerals is unclear, as are the consequences for As mobilization during subsequent reflooding and reductive cycles. Here, we subject As(V)-coprecipitated schwertmannite to thermal transformation (200, 400, 600 and 800 °C) followed by biotic reductive incubation (150 d) and examine aqueous- and solid-phase speciation of As, Fe and S. Heating to >400 °C caused transformation of schwertmannite to a nanocrystalline hematite with greater surface area and smaller particle size. Higher temperatures also caused the initially structurally incorporated As to become progressively more exchangeable, increasing surface-complexed As (AsEx) by up to 60-fold, thereby triggering enhanced As mobilization during incubation (∼70-fold in the 800 °C treatment). Although more As was mobilized in biotic treatments than controls (∼3-20×), in both cases it was directly proportional to initial AsEx and mainly due to abiotic desorption. Higher transformation temperatures also drove divergent pathways of Fe and S biomineralization and led to more As(V) and SO4 reduction relative to Fe(III) reduction. This study reveals thermal transformation of schwertmannite can greatly increase As mobility and has major consequences for As/Fe/S speciation under reducing conditions. Further research is warranted to unravel the wider implications for water quality in natural wetlands.

Synthesis and characterization of Sn-doped hematite as visible light photocatalyst

Sn-doped hematite nanoparticles are prepared by solution combustion synthesis. The products are characterized with various analytical and spectroscopic techniques to determine their structural, morphological, light absorption and photocatalytic properties. The results reveal that all the samples consist of nanocrystalline hematite with mesoporous structures, and Sn has the ability to inhibit the growth of hematite particle. Compared to pure hematite, the doped hematite samples with appropriate amount of Sn show better activities for degradation of methylene blue under visible light irradiation. The highest activity is observed for 5% Sn doped hematite and this product has long-term stability and no selectivity for dye degradation. The enhanced performance of 5% Sn doped hematite is ascribed to the smaller particle size, increased ability to absorb in visible light, efficient charge separation as well as improved e− transfer associated with the effects of appropriate amount of Sn doped sample.

Complementary use of monochromatic and white-beam X-ray micro-diffraction for the investigation of ancient materials

Archaeological artefacts are often heterogeneous materials where several phases coexist in a wide grain size distribution. Most of the time, retrieving structure information at the micrometre scale is of great importance for these materials. Particularly, the organization of different phases at the micrometre scale is closely related to optical or mechanical properties, manufacturing processes, functionalities in ancient times and long-term conservation. Between classic X-ray powder diffraction with a millimetre beam and transmission electron microscopy, a gap exists and structure and phase information at the micrometre scale are missing. Using a micrometre-size synchrotron X-ray beam, a hybrid approach combining both monochromatic powder micro-diffraction and Laue single-crystal micro-diffraction was deployed to obtain information from nanometre- and micrometre-size phases, respectively. Therefore providing a way to bridge the aforementioned gap, this unique methodology was applied to three different types of ancient materials that all show a strong heterogeneity. In Romanterra sigillata, the specific distribution of nanocrystalline hematite is mainly responsible for the deep-red tone of the slip, while the distribution of micrometre-size quartz in ceramic bodies reflects the change of manufacturing process between pre-sigillataand high-qualitysigillataperiods. In the second example, we investigated the modifications occurring in Neolithic and geological flints after a heating process. By separating the diffracted signal coming from the nano- and the micrometre scale, we observed a domain size increase for nanocrystalline quartz in geological flints and a relaxation of the residual strain in larger detritic quartz. Finally, through the study of a Roman iron nail, we showed that the carburation process to strengthen the steel was mainly a surface process that formed 10–20 µm size domains of single-crystal ferrite and nanocrystalline cementite.

Mineral transformations and textural evolution during roasting of bog iron ores

The processes occurring during roasting of bog iron ores were characterized using TG–DTG–DTA–QMS, XRD, FTIR and specific surface analysis. Removal of physically adsorbed water is followed by dehydroxylation of iron oxyhydroxides and oxidation of organic matter at 200–600 °C. The main product of the transformations is disordered nanocrystalline (proto)hematite or hematite/maghemite mixture, depending on organic matter content and heating conditions. The conversion of iron oxyhydroxides to hematite occurs at temperatures different than those reported for pure compounds. At higher temperatures, protohematite undergoes recrystallization to the stoichiometric hematite, and manganese oxides are partially reduced. At 1000 °C, the roasting products consist of hematite and cristobalite together with Mn–Fe spinels if the initial ore contained Mn oxides. The admixtures of various secondary silicates were encountered as well. Low- to moderate-temperature roasting slightly affects the specific surface area and lowers volume of micropores. The high-temperature transformations lead to decrease in the specific surface area and to the destruction of porous texture of the bog iron ores. Although the general course of the processes during roasting was similar in all the samples, some of their details as well as mineralogy and properties of the products are highly dependent on the composition of the initial material.

The glassy behaviour of poorly crystalline Fe2O3 nanorods obtained by thermal decomposition of ferrous oxalate

Nanorod ferrous oxalate dihydrate (FeC2O4 × 2H2O) which had been synthesized by the microemulsion method, was used as a precursor in the thermal decomposition process performed in air atmosphere. The formation of nanocrystalline hematite as the final product was preceded by the appearence of an intermediate product. Comprehensive study comprising several complementary techniques (x-ray diffraction, transmission electron microscopy, selected area electron diffraction, thermogravimetric/differential thermal analyses and SQUID magnetometry) confirmed that the intermediate product corresponds to the poorly crystalline Fe2O3. Due to the specific nanorod shape and poorly crystalline structure, the investigated Fe2O3 showed high coercive field value of at Special attention in this study was devoted to the peculiar magnetic properties of poorly crystalline Fe2O3, which were thoroughly investigated by employing sophisticated experimental procedures such as relaxation of thermoremanent magnetization for different cooling fields, zero field and field cooled memory effects as well as aging experiments for different waiting times. At low temperatures and weak applied magnetic fields, the investigated system behaves similarly to spin glasses, manifesting slow, collective relaxation dynamics of magnetic moments through memory, rejuvenation and aging effects.

Goethite as a potential source of magnetic nanoparticles in sediments

Goethite is an important iron-bearing mineral found in nearly all types of soils and sediments that typically occurs as particles of nanoscale crystallites. Here we show that poorly crystalline nanophase goethite can undergo dehydroxylation and alteration to produce strongly magnetic Fe-oxide nanoparticles. After moderate reductive heating, synthetic oriented aggregates of nanogoethite were converted to stoichiometric sub-micron magnetite. In contrast, oxidative heating produced only nanocrystalline hematite. We present a novel characterization of the products of nanogoethite alteration, which represents an important pathway for the production of both superparamagnetic and stable single-domain magnetic particles in a wide variety of sedimentary environments, including soils and lacustrine and marine settings in which goethite is commonly present. In nature, the mechanism of magnetite formation observed in our experiments could contribute to the widespread phenomenon of magnetic enhancement in many soil types, and could be responsible for secondary magnetizations often observed to accompany diagenesis in sedimentary rocks. Thus, nanogoethite may be an important precursor of fine magnetic particles and magnetic remanence carriers in the environment, particularly in soils and catchment areas affected by wildfire as well as in sediments subjected to deep burial diagenesis and low-grade metamorphism.