Fe(II)-induced transformation from ferrihydrite to lepidocrocite and goethite

The transformation of Fe(II)-adsorbed ferrihydrite was studied. Data tracking the formation of products as a function of pH, temperature and time is presented. The results indicate that trace of Fe(II) adsorbed on ferrihydrite can accelerate its transformation obviously. The products are lepidocrocite and/or goethite and/or hematite, which is different from those without Fe(II). That is, Fe(II) not only accelerates the transformation of ferrihydrite but also leads to the formation of lepidocrocite by a new path. The behavior of Fe(II) is shown in two aspects—catalytic dissolution-reprecipitation and catalytic solid-state transformation. The results indicate that a high temperature and a high pH(in the range from 5 to 9) are favorable to solid-state transformation and the formation of hematite, while a low temperature and a low pH are favorable to dissolution-reprecipitation mechanism and the formation of lepidocrocite. Special attentions were given to the formation mechanism of lepidocrocite and goethite.     

The transformation of ferrihydrite in the presence of trace Fe(II): The effect of the ammonia, amine and the coordination ions of Fe(III)

This work examined Fe(II)-induced transformation of ferrihydrite in the presence of ammonia, amine and the coordination ions of Fe(III). Our earlier results showed that ferrihydrite transformed into the mixture of lepidocrocite, goethite and/or hematite in the presence of trace Fe(II) and absence of ammonia and similar species. However, the formation of lepidocrocite was restrained when using ammonia as precipitants. When introducing some amines (e.g. ethanolamine and diethanolamine) and some coordination ions (e.g. F⁻ and ions) into the reaction system, a similar effect on the transformation of ferrihydrite was found. Probably, the complexes formed between Fe(III) and those additives favor the formation of goethite. At the same time, the introduction of these additives hinders Fe(II) from interacting with ferrihydrite, which makes the catalytic dissolution of ferrihydrite be limited, thus, the formation of lepidocrocite be restrained.     

Infrared spectra of oxalate, malonate and succinate adsorbed on the aqueous surface of rutile, anatase and lepidocrocite measured with in situ ATR-FTIR

The adsorption of oxalate, malonate and succinate on anatase, rutile and lepidocrocite, was studied by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) at aqueous concentrations of 200 μM between pH 9 and 3. Clear spectral differences between the aqueous species and the surface adsorbed species for all three dicarboxylates are taken as strong evidence for inner-sphere adsorption. The characteristically different spectra on each oxide reveal surface specific interactions and could be used as a diagnostic tool, e.g., to probe the relative abundance of anatase and rutile on the surface of TiO₂ samples. Spectral changes between pH 7.0 and 3.0 show that two to three different surface complexes of oxalate and one to three surface complexes of malonate and succinate are formed on each of the three surfaces. While the exact structures of each complex can currently not be derived, important differences between the dicarboxylates can be identified. Only adsorbed oxalate exhibits two strong bands above 1670 cm⁻¹, as expected for a five- (bidentate chelating) or six-membered (bidentate bridging) ring structure with one oxygen of each carboxylic group coordinated to surface sites and two CO double bonds pointing away from the surface. The absence of clear CO double bond vibrations above 1620 cm⁻¹ show that malonate and succinate adsorb differently, with one or both of the carboxylic groups independently forming monodentate hydrogen bonded, bidentate chelating (four-ring) or bidentate bridging (five-ring) structures. Oxalate is the only one of the three dicarboxylates that formed additional surface complexes at low pH on rutile and anatase and lead to rapid dissolution of lepidocrocite below pH 5.0.     

纤铁矿型TiO_2纳米片层结构的稳定性与电子结构研究

采用平面波超软赝势方法研究了纤铁矿型TiO_2纳米片层结构的稳定性和电子结构.结果显示该结构具有较高的稳定性,其带隙比锐钛矿型TiO_2要大0.59 e V,带隙内没有出现表面态.通过对空位缺陷形成能的比较,结果显示这在还原性气氛下纤铁矿型TiO_2纳米片层表面Ti空位的形成能明显低于O空位的形成能,确定出最容易出现的缺陷是-4价的Ti空位,该空位缺陷的出现会使带隙中产生表面缺陷态.与体相内缺陷不同,表面缺陷态可以促进电子和空穴的分离,这些发现可以合理的解释最近的实验结果 .     

Studies on the controllable transformation of ferrihydrite

Ferrihydrite was prepared by two different procedures. Ferrihydrite-1 was prepared by dropping NaOH solution into Fe(III) solution. Ferrihydrite-2 was prepared by adding Fe(III) and NaOH solutions into a certain volume of water simultaneously. Our earlier results obtained at ∼100 °C have shown that the structure of ferrihydrite-2 favors its solid state transformation mechanism. Further research reveals that the structure of ferrihydrite-2 favors its dissolution re-crystallization mechanism at a temperature of ≤60 °C. Based on the transformation mechanism of ferrihydrite at different temperatures, the controllable transformation from ferrihydrite to various iron (hydr)oxides such as lepidocrocite, goethite, hematite and magnetite can be achieved by adjusting the pH, transformation temperature, transformation time, the amount of Fe(II) as well as the preparation procedures of ferrihydrite. The results in the present paper give a nice example that the transformation of a precursor can be controlled with the help of mechanism.     

Electrochemical fabrication and characterization of lepidocrocite (γ-FeOOH) nanowire arrays

We report on the fabrication of γ-phase iron oxyhydroxide (γ-FeOOH, lepidocrocite) nanowire (nw) arrays within the alumina pores by electrodeposition. An aqueous solution, friendly to alumina matrix, was generated and applied in this study for uniform deposition of γ-FeOOH nw arrays directly through the alumina barrier layer using an alternating current (ac) mode. As-deposited nanowired products were characterized using ⁵⁷Fe Mössbauer spectroscopy (MS), atomic absorption spectrophotometry analysis, field-emission scanning electron microscopy, UV-vis transmission spectroscopy, transmission electron microscopy and X-ray diffraction. The formation of pure lepidocrocite nw arrays in the alumina pores with the average Ø_pore of 45 and 150 nm was verified by transmission MS at cryogenic temperatures.