Thermally-induced color transformation of hematite: insight into the prehistoric natural pigment preparation

Since the prehistoric era, hematite has been known as a reddish color pigment on rock art, body paint, and decorating substances for objects discovered almost worldwide. Recently, studies about purple hematite used in prehistoric pigment have been done vigorously to investigate the origin of the purple pigment itself. These previous studies indicate that the differentiation of crystallinity, crystal size, morphology, and electronic structure can cause the color shift, resulting in purple hematite. In this study, we conducted a detailed study of the sintering temperature effects on the formation of hematite minerals. This study aims to reveal the structural, crystallography, and electronic transformation in hematite due to heating treatment at various temperatures. The hematite was synthesized using precipitation to imitate the primary method of hematite formation in nature. The sintering process was carried out with temperature variations from 600 °C to 1100 °C and then characterized by crystallographic and structural properties (XRD, Raman Spectroscopy, FTIR), particle size (TEM), as well as electronic properties (DRS, XANES). The crystallinity and particle size of hematite tend to increase along with higher sintering temperatures. Moreover, we noted that the octahedral distortion underwent an intensification with the increase in sintering temperature, which affected the electronic structure of hematite. Specifically, the 1s → 3d transition exhibited lower energy for hematite produced at a higher temperature. This induced a shift in the absorbed energy of the polychromatic light that led to a color shift within hematite, from red to purple. Our finding emphasizes the importance of electronic structure in explaining hematite pigment’s color change rather than relying on simple reasons, such as particle size and crystallinity. In addition, this might strengthen the hypothesis that the prehistoric human created a purple hematite pigment through heating.

Simultaneous roxarsone photocatalytic degradation and arsenic adsorption removal by TiO2/FeOOH hybrid

Roxarsone (3-nitro-4-hydroxyphenylarsonic acid) is an extensively used organoarsenic feed additive. The effective removal of arsenic from roxarsone degradation before discharging is of great importance for controlling artificial arsenic pollution in aquatic environment. In this study, a bifunctional TiO₂/ferrihydrite (TiO₂/FeOOH) hybrid was synthesized by a hydrothermal method for the simultaneously photocatalytic degradation of roxarsone and adsorption removal of released arsenic. The analysis of the prepared TiO₂/FeOOH by field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), Raman spectra, X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) confirmed the successful formation of the hybrid of crystalline TiO₂ and no crystalline FeOOH. TiO₂/FeOOH hybrid had better adsorption capacity for As(V) than roxarsone. Compared to TiO₂, the TiO₂/FeOOH hybrid exhibited much superior UV-driven photocatalytic activities for roxarsone degradation. After 12 h irradiation, more than 96% of roxarsone was degraded by 1:1 TiO₂/FeOOH hybrid, and the released As(V) was simultaneously removed from the solution. The residual As(V) concentration was lower than 0.02 mg L^-1. The reusability test indicated that TiO₂/FeOOH hybrid had excellent stability and reliability. The possible mechanism of roxarsone degradation and released inorganic arsenics removal by this hybrid was also proposed. These results clearly indicated that the TiO₂/FeOOH hybrid could be used for the removal of roxarsone and its degradation product.

Acid Red 66 Dye Removal from Aqueous Solution by Fe/C-based Composites: Adsorption, Kinetics and Thermodynamic Studies

The presence of synthetic dyes in water causes serious environmental issues owing to the low water quality, toxicity to environment and human carcinogenic effects. Adsorption has emerged as simple and environmental benign processes for wastewater treatment. This work reports the use of porous Fe-based composites as adsorbents for Acid Red 66 dye removal in an aqueous solution. The porous FeC and Fe/FeC solids were prepared by hydrothermal methods using iron sulfates and sucrose as precursors. The physicochemical properties of the solids were evaluated through X-ray diffraction (XRD), Scanning electron microscopy coupled with Energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared s (FTIR), Raman and Mössbauer spectroscopies, nitrogen adsorption–desorption isotherms, Electron Paramagnetic Resonance (EPR) and magnetic saturation techniques. Results indicated that the Fe species holds magnetic properties and formed well dispersed Fe₃O₄ nanoparticles on a carbon layer in FeC nanocomposite. Adding iron to the previous solid resulted in the formation of γ-Fe₂O₃ coating on the FeC type structure as in Fe/FeC composite. The highest dye adsorption capacity was 15.5 mg·g⁻¹ for FeC nanocomposite at 25 °C with the isotherms fitting well with the Langmuir model. The removal efficiency of 98.4% was obtained with a pristine Fe sample under similar experimental conditions.

Oxyhydroxide of metallic nanowires in a molecular H2O and H2O2 environment and their effects on mechanical properties

To avoid unexpected environmental mechanical failure, there is a strong need to fully understand the details of the oxidation process and intrinsic mechanical properties of reactive metallic iron (Fe) nanowires (NWs) under various aqueous reactive environmental conditions. Herein, we employed ReaxFF reactive molecular dynamics (MD) simulations to elucidate the oxidation of Fe NWs exposed to molecular water (H2O) and hydrogen peroxide (H2O2) environment, and the influence of the oxide shell layer on the tensile mechanical deformation properties of Fe NWs. Our structural analysis shows that oxidation of Fe NWs occurs with the formation of different iron oxide and hydroxide phases in the aqueous molecular H2O and H2O2 oxidizing environments. We observe that the resulting microstructure due to pre-oxide shell layer formation reduces the mechanical stress via increasing the initial defect sites in the vicinity of the oxide region to facilitate the onset of plastic deformation during tensile loading. Specifically, the oxide layer of Fe NWs formed in the H2O2 environment has a relatively significant effect on the deterioration of the mechanical properties of Fe NWs. The weakening of the yield stress and Young modulus of H2O2 oxidized Fe NWs indicates the important role of local oxide microstructures on mechanical deformation properties of individual Fe NWs. Notably, deformation twinning is found as the primary mechanical plastic deformation mechanism of all Fe NWs, but it is initially observed at low strain and stress level for the oxidized Fe NWs.

Spectral and morphological characteristics of synthetic nanophase iron (oxyhydr)oxides

Nanophase iron (oxyhydr)oxides are ubiquitous on Earth, globally distributed on Mars, and likely present on numerous other rocky solar system bodies. They are often structurally and, therefore, spectrally distinct from iron (oxyhydr)oxide bulk phases. Because their spectra vary with grain size, they can be difficult to identify or distinguish unless multiple analysis techniques are used in tandem. Yet, most literature reports fail to use multiple techniques or adequately parameterize sample morphology, making it difficult to understand how morphology affects spectral characteristics across techniques. Here, we present transmission electron microscopy, Raman, visible and near-infrared, and mid-infrared attenuated total reflectance data on synthetic, nanophase akaganéite, lepidocrocite, goethite, hematite, ferrihydrite, magnetite, and maghemite. Feature positions are tabulated and compared to those for bulk (oxyhydr)oxides and other nanophase iron (oxyhydr)oxides from the literature. The utility and limitations of each technique in analyzing nanophase iron (oxyhydr)oxides are discussed. Raman, mid-infrared, and visible near-infrared spectra show broadening, loss of some spectral features, and shifted positions compared to bulk phases. Raman and mid-infrared spectroscopies are useful in identifying and distinguishing akaganéite, lepidocrocite, goethite, and hematite, though ferrihydrite, magnetite, and maghemite have overlapped band positions. Visible near-infrared spectroscopy can identify and distinguish among ferrihydrite, magnetite, and maghemite in pure spectra, though akaganéite, lepidocrocite, and goethite can have overlapping bands. It is clear from this work that further understanding of variable spectral features in nanophase iron (oxyhydr)oxides must await additional studies to robustly assess effects of morphology. This study establishes a template for future work.

Elucidating "screw dislocation"-driven film formation of sodium thiosulphate with complex hierarchical molecular assembly

Sodium thiosulphate (Na₂S₂O₃) films were synthesized on carbon steel substrates through solution deposition, and a film formation growth mechanism is delineated in detail herein. Dislocation-driven film formation took place at the lower concentration of Na₂S₂O₃ (0.1 M) studied, where screw dislocation loops were identified. Interestingly, we observed the co-existence of screw dislocation spiral loops and hierarchically-ordered molecular assembly in the film, and showed the importance of hierarchical morphology in the origin of screw dislocation. The screw dislocation loops were, however, distorted at the higher studied concentration of Na₂S₂O₃ (0.5 M), and no hierarchical structures were formed. The mechanisms of film formation are discussed in detail and provide new insights into our understanding regarding morphology of the hierarchical molecular assembly, screw dislocation loop formation, and the role of chemical elements for their development. The main crystalline and amorphous phases in the surface films were identified as pyrite/mackinawite and magnetite. As sodium thiosulphate is widely used for energy, corrosion inhibition, nanoparticle synthesis and catalysis applications, the knowledge generated in this study is applicable to the fields of corrosion, materials science, materials chemistry and metallurgy.

Metal doped mesoporous FeOOH nanorods for high performance supercapacitors

In the present study, the effect of doping of foreign atoms on the parent atoms and the application of the resultant material for energy storage are successfully investigated.

Spectroscopic characterization of iron ores formed in different geological environments using FTIR, XPS, Mössbauer spectroscopy and thermoanalyses

Application of thermoanalyses, FTIR, XPS and Mössbauer spectroscopic methods can differentiate between iron ores formed in different geological environments. Two types of iron ore are formed in shallow marine environments in the Bahariya Depression, Egypt, yellowish brown ooidal ironstones (type 1) and black mud and fossiliferous ironstones (type 2). Both types were subjected to subaerial weathering, producing a dark brown lateritic (pedogenic) iron ore (type 3). Microscopic investigation indicates goethite is the main mineral in types 1 and 3, while hematite is the main mineral in type 2 and also occurs in type 3. Thermoanalyses indicated the dehydroxylation endothermic peak of goethite of type 1 occurs between 329 and 345°C, while in type 3 occurs between 284 and 330°C. This variation can be attributed to the nanocrystalline nature of the pedogenic goethite. The presence of an exothermic peak at 754°C in type 3 is probably attributed to goethite-hematite phase transformation. FTIR spectroscopy indicated that goethite of type 1 is characterized by the presence of the δ-OH band between 799 and 802cm(-1), the γ-OH between 898 and 904cm(-1) and the bulk hydroxyl stretch between 3124 and 3133cm(-1). Goethite of type 3 is characterized by the absence of the bulk hydroxyl stretch band and the δ-OH and γ-OH are shifted to higher Wavenumbers that can attributed to a relative Al-for Fe-substitution. Hematite is identified by two IR bands; the first is between 464 and 475cm(-1) and at the second is between 540 and 557cm(-1). Quartz is identified in all iron ore types, nitrates are identified in types 1 and 2, but absent in type 3 and Kaolinite is identified in type 2. The Mössbauer spectrum of type 1 is fitted with one magnetic sextet corresponding to goethite with an isomer shift (IS)=0.374mms(-1), a quadruple splitting (QS)=-0.27mms(-1) and a hyperfine magnetic field (BHF)=∼37. The Mössbauer spectrum of type 2 is fitted with one magnetic sextet corresponding to hematite with IS=0.363mms(-1), QS=-0.23mms(-1) and BHF=∼50. The Mössbauer spectrum of type 3 is best fitted with a single doublet corresponding to ferrihydrite and one sextet corresponding to hematite. The XPS survey scans and the high resolution of the Fe 2p3/2 can differentiate between the yellowish-brown and green ooidal laminae of type 1. The XPS survey scans indicate the presence of Fe, O, C, N, Na, Cl, Ca and Si in all laminae, while S, Zn, Ti and P are only restricted to the green laminae. The high resolution of the Fe 2p3/2 indicates that Fe is linked to OH(-) ligand in the yellowish-brown laminae that correspond to goethite, while Fe is linked to SO4(2-) ligand in the green laminae. The XPS survey scans of types 2 and 3 indicate that Fe is linked to O(2-) ligand that corresponds to hematite.

High coercivity α-Fe2O3 nanoparticles prepared by continuous spray pyrolysis

Surfactant free growth of high coercivity (H_C) α-Fe₂O₃ nanoparticles by continuous spray pyrolysis (CoSP).