Shape-controlled Synthesis Of Hematite And Its Application In The Degradation Of Methylene Blue

Monodisperse hematite (α-Fe203) with special morphologies and uniform crystal size has shown extensive applications in the fields such as catalyst, photocatalyst, battery, magnetic and medical materials due to their excellent optical, magnetic, electronic and catalytic properties. Among various synthenic methods for iron oxides materials, the chemical precipitation method in the presence of Fe2+ is relatively simple and cheap and provides good limitations with respect to polydispersity, phase composition, and micro structure of the resultant products. But the disadvantage is that Fe2+ is an unstable reductant which can be oxidized easily when exposed to air. Therefore, this reaction has to be carried out under a rigorous N2 atmosphere which complicates the method. Besides, Fe2+ is unstable in solution because it can easily be precipitated at the high pH of the reaction system.In this work, the chemical precipitation method in the presence of reductant ascorbic acid (AA), oxalic acid (OA) or glucose (G) and uniform precipitation in the presence of urea collaborated with calcination pathway was used to prepare different shaped a-Fe2O3. Powder XRD, SEM, TEM, FT-IR and BET analysis were employed to characterize the crystal structure, phase components, particle size and morphology of the samples. As-preparedα-Fe2O3 samples with different shapes were selected to investigate their abilities for degradation of methylene blue (MB), and the influencing factors were also discussed. The main results are shown as follows:1. Iron oxides with different morphologies and particle sizes were obtained by the chemical precipitation in the presence of reductant AA, OA or G. The factors including pH, temperature and reaction time of the reaction system influenced the transformation rate of ferrihydrite to hematite, the phase components and crystallinity of the samples. The most suitable pH value for the transformation is about 7. The mineral phase transformed from lepidocrocite to goethite then to hematite with the increasing temperature. The crystallinity of the samples increased with prolonging reaction time.The initial concentration of Fe3+ and NaOH solution, to some extent, regulated the particle size of the samples by affecting the hydrolytic reaction of Fe3+. The lower concentration of Fe3+ and NaOH solution contributed to the formation of samples with smaller particle size.The amount of reductants added greatly influenced the phase components, crystallinity and morphologies of the products. As the molar ratio of AA/FeⅢincreased from 0.5 to 1%, the morphology changed from spherical particles with a diameter of 50-80 nm to ellipsoidal particles with a diameter of 50-80 nm and a length of 200-300 nm. When the ratio is further raised to 2%, spindle-type particles were formed with a similar diameter, but the lengths of the particles are increased to 400-500 nm. The molar ratio of OA/FeⅢhad a significant effect on the crystallinity but little influence on the morphologies of the samples. Hematite could be obtained when the molar ratio G/FeⅢwas between 0.25-2% and short ellipsoids were obtained with 2%G. Maghemite (y-Fe2O3) and magnetite (Fe3O4) came into being respectively when the molar ratio of G/FeⅢwere 4% and 10%.2. Hematite with a particle size of 20-30 nm and akaganeite (β-FeOOH) nano-rod with a particle diameter of 20-50 nm and a length of 150-300 nm were successfully synthesized via the uniform precipitation method in the presence of urea. The molar ratio of urea/Fe3+ and the initial conventration of Fe3+ solution controlled the crystallinity and particle size of the samples to some extents. Porous hematite with the similar morphology as akaganeite could be obtained by calcining the akaganeite predecessor.3. Hematite obtained in the presence of 2% AA performed best on the degradation of MB. The degradation dynamic curves of different shaped hematite on MB all fitted well with the first-order dynamical equation which proved that the degradation reaction could be affirmed to the Fenton-like reaction with the a-Fe2O3 as the Fe3+ provider. The degradation rate of MB by a-Fe2O3 increased with the increase of the percentage of H2O2 and the quality of mineral, but it increased firstly and then decreased with the increase of HC1 concentration.