Cluster calculation of electronic structure and hyperfine interactions for α-Fe2O3

The MSX_α cluster technique has been used to study the electronic structure of hematite α-Fe₂O₃, where iron is formally in a 3d₅⁶S state. The calculated energy levels are compared with X-ray emission and photoelectron spectra. The wave functions have been used to compute the charge distribution, as well as hyperfine parameters such as quadrupole coupling constant, isomer shift and magnetic hyperfine field. The results indicate a considerable influence of chemical bonding on these parameters due to charge transfer and covalency. From the calculated field gradient and the measured quadrupole coupling constant a nuclear quadrupole moment for ^57mTe of about 0.11b is deduced. This value is smaller than the most recent estimates of 0.15b based on the ionic model but not as small as the value of 0.082b obtained from first principles calculations on iron dihalides.     

固相法制备氧化铁纳米粒子

采用固相法制备了氧化铁的纳米粒子，并对其进行了XRD谱图表征，发现产物氧化铁的晶型与灼烧温度及灼烧时间有关，随着灼烧温度的升高和灼烧时间的延长，氧化铁的晶型从γ型向α型转变。同时讨论了温度、时间等条件对产品粒径、产率等因素的影响。     

Improved flotation performance of hematite fines using citric acid as a dispersant

In this study, citric acid was used as a dispersant to improve the flotation performance of hematite fines. The effect and mechanism of citric acid on the reverse flotation of hematite fines were investigated by flotation tests, sedimentation experiments, scanning electron microscopy (SEM), zeta-potential measurements, and X-ray photoelectron spectroscopy (XPS). The results of SEM analysis and flotation tests reveal that a strong heterocoagulation in the form of slime coating or coagulation in hematite fine slurry affects the beneficiation of hematite ores by froth flotation. The addition of a small amount of citric acid (less than 300 g/t) favorably affects the reverse flotation of hematite fines by improving particle dispersion. The results of sedimentation experiments, zeta-potential measurements, and XPS measurements demonstrate that citric acid adsorbs onto hematite and quartz surfaces via hydrogen bonding, thereby reducing the zeta potentials of mineral surfaces, strengthening the electrical double-layer repulsion between mineral particles, and dispersing the pulp particles.     

Acid Treatment Enables Suppression of Electron–Hole Recombination in Hematite for Photoelectrochemical Water Splitting

We report a strategy for efficient suppression of electron–hole recombination in hematite photoanodes. Acid‐treated hematite showed a substantially enhanced photocurrent density compared to untreated samples. Electrochemical impedance spectroscopy studies revealed that the enhanced photocurrent is partly due to improved efficiency of charge separation. Transient absorption spectroscopic studies coupled to electrochemical measurements indicate that, in addition to improved bulk electrochemical properties, acid‐treated hematite has significantly decreased surface electron–hole recombination losses owing to a greater yield of the trapped photoelectrons being extracted to the external circuit.     

Controlled Aqueous Growth of Hematite Nanoplate Arrays Directly on Transparent Conductive Substrates and Their Photoelectrochemical Properties

Two‐dimensional (2D) hematite nanoplate arrays were synthesized directly on fluorine‐doped tin oxide (FTO)‐coated glass by using a facile and novel hydrothermal method. High‐temperature annealing retained the morphology of the nanoplate arrays while simultaneously introducing porosity. The thickness and length of the nanoplates could be tailored by adjusting the precursor composition. Photoelectrochemical (PEC) measurements showed that the photocurrent generated with bare hematite nanoplate photoelectrode under backside illumination was about four times of that under frontside illumination in the entire bias range used, which suggested that slow electron transport was a limiting factor for its PEC performance. Upon Sn doping and Co‐Pi co‐catalyst addition, the photocurrent increased significantly owing to the enhancement of electron conductivity and oxidation kinetics. Electrochemical impedance spectroscopy (EIS) measurements were conducted to understand the surface properties of the nanoplate arrays. Since this strategy is simple, cost‐effective, and highly reproducible, it provides exciting opportunities for the large‐scale growth of porous 2D metal oxide photoelectrodes for a variety of photoelectrochemical and photocatalytic applications.     

Inner structure and properties of diamond-shaped and spherical α-Fe2O3 particles

Uniform diamond-shaped and spherical -Fe₂O₃ particles, prepared by a forced hydrolysis of FeCl₃–HCl solutions, were characterized by various means. Electron microscope and x-ray diffraction measurements indicated that these particles are formed by recrystallization of -FeOOH to -Fe₂O₃ accompanying the dissolution of -FeOOH. Ultramicropores were formed in spherical particles with outgassing in vacuo above 150 °C by dehydration of inner OH groups, proving that the particles are polycrystalline. On the other hand, the highly crystallized diamond-shaped particles showed a less microporosity and were thermally stable against outgassing up to 400 °C. These results are compatible with those obtained for the monodispersed cubic and spherical -Fe₂O₃ particles reported in our previous paper [J Chem Soc Faraday Trans 87: 2241 (1991)].     

回流水热合成均分散纳米α-Fe2O3单晶粒子

系统地研究了以Fe(NO₃)₃为原料，回流水热合成均分散α-Fe₂O₃纳米粒子。实验结果显示:当pH=1时，随着水热时间的增加，由透射电镜测试可知，多孔性纺锤形α-Fe₂O₃单晶转变成粒径在45 nm左右的均分散菱形单晶;而Fe(NO₃)₃用氨水中和形成Fe(OH)₃凝胶后，随着初始pH值的增加，完全转变成α-Fe₂O₃所需的时间增加、粒径减小，但产物的结构更为完整。另外通过实验发现，反应体系中存在的电解质对α-Fe₂O₃晶体的生成有促进作用。同时对初始pH值及电解质对α-Fe₂O₃生成速率的影响机理进行了分析。     

中性油同矿粒间的相互作用

研究了中性油乳浊液同矿粒间的相互作用。从热力学角度讨论了油滴同矿粒作用的条件。通过对粒子间的范氏作用力、静电力和疏水作用力的定量计算和化学作用力的分析,解释了油滴同矿粒间作用的选择性。最后,在物理模型的基础上,说明了“空间效应”的产生。     

The Iron Behaviour in Aluminosilicate Gel-Derived Materials

Electron spin resonance (ESR) measurements have been carried out on a 10Fe₂O₃ · 10Al₂O₃ · 80SiO₂ gel heat-treated at different temperatures in air and under reducing conditions. ESR spectra were obtained at 300, 50 and 5 K. The effective g value (g = h/H), linewidth (H_pp) and ESR amplitude (A) depend on heat-treatment temperature of the gel-derived samples. ESR spectra exhibit different magnetic characteristics as a function of heat-treatment temperature and atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM) and a.c. susceptibility (_a.c.) analyses were used to better understand the ESR results. The results show that in the samples heat-treated in air, up to 700°C, Fe³⁺ ions are incorporated in the glass network in tetrahedral and/or octahedral co-ordinations. In the samples heat-treated between 250 and 700°C was not detected, by ESR, the presence of iron oxide aggregates. However, the formation of hematite particles was observed by XRD and SEM. The presence of iron oxide aggregates was detected (by ESR) in the samples heat-treated at temperatures higher than 700°C. These aggregates are formed, at 1200 and 1300°C, by hematite and magnetite particles as proved by XRD. The ESR spectra and a.c. susceptibility, of the samples heat-treated at 250°C (under reducing conditions), show a behaviour characteristic of small magnetite particles presence. The sample heat-treated at 500°C (under reducing conditios) contains magnetite particles (XRD). In the ESR spectra of the sample heat-treated at 1000°C, under reducing conditions, was not detected any resonance signal.     

Strontium Sorption on Hematite at Elevated Temperatures

Acid–base reactions and surface complexation of Sr(II) at the hematite/water interface have been studied by means of potentiometric titrations at three different temperatures: 25, 50, and 75°C. Equilibrium measurements were performed in 0.1 M NaCl. In the evaluation of equilibrium models for the acid–base reactions and complexation reactions in the three-component system H⁺ ---(FeOH)---Sr²⁺, the constant capacitance model was applied. During the titrations with Sr, aliquots of the suspension were sampled at in several points. The aqueous concentrations of Sr were analyzed by atomic absorption spectrometry. Treatment of data included tests for formation of both inner-sphere and outer-sphere complexes of different stoichiometric composition. The proposed equilibrium model consists of the following surface complexes of inner sphere type: FeOHSr²⁺ and FeOSrOH. Besides the stability constants for the surface complexes, the thermodynamic parameters ΔH and ΔS were evaluated. The combined effect of a decrease in pH_pzc with increasing temperature and positive enthalpies of surface complex formation favors adsorption of Sr at elevated temperatures.