Preparation, microstructure, and color tone of microtubule material composed of hematite/amorphous-silicate nanocomposite from iron oxide of bacterial origin

By heating an amorphous iron oxide produced by Leptothrix ochracea, an iron-oxidizing bacterium species, at 600-1100°C in air for 2 h, vivid red-colored powdered materials including α-Fe ₂O ₃ (hematite) and amorphous silicate with high thermostability were prepared which offer potential for use as overglaze enamels on porcelain. The precise color tone of the materials greatly depends on the heat-treatment temperature. The most strikingly beautiful sample, heat-treated at 800°C, is light yellowish-red in color (L* = 47.3, a* = 34.1, and b* = 34.6), has a unique microstructure, and does not fade in color even with reheating at 800°C, which is the firing temperature for overglaze enamel on porcelain. The sample primarily consists of crystalline hematite particles ∼40 nm in diameter with slightly longer axis unit-cell parameters than those of pure hematite. The particles are covered with amorphous silicate phase ∼5 nm in thickness and are intricately interconnected into microtubules with an average diameter of 1.26 μm. The attractive color of this material is due to the following structural features: small particle size (∼40 nm), nanocomposite of hematite and amorphous silicate, and a microtubule structure that inhibits aggregation of individual hematite particles and microtubules.