It has been found that not only inorganic anions such as sulfate and phosphate, but also organic reagents such as dihydroxybenzenes, dihydroxynaphthalenes, ethylenediamine tetraacetic acid (EDTA), and nitrilotriacetic acid (NTA) can be used as a shape controller for forming ellipsoidal hematite (α-Fe2O3) particles in the gel-sol system. Specifically, this paper focuses on the formation process of monodisperse ellipsoidal hematite particles with hydroxybenzenes. Interestingly, while 1,3-dihydroxybenzene had no effect on the shape of hematite, 1,4- and 1,2- dihydroxybenzenes could produce ellipsoidal particles. However, 1,4- dihydroxybenzene (hydroquinone) itself turned out to have no ability of adsorption to hematite. It was finally concluded that hydroquinone was oxidatively polymerized to form a kind of humic compound during aging at 100°C for the growth of hematite particles, controlling the anisotropic growth by adsorption of the polymerized species. In addition, a considerable amount of the adsorbed polymer was found to be retained in the internal grain boundaries of the subcrystals, as revealed from XPS, XRD, and FT-IR analyses. Lastly, the mechanism of shape control by 1,2-dihydroxy-3,5-benzenedisulfonic acid disodium salt (Tiron) which is directly adsorbed to growing hematite particles is also discussed.
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