Growth Mechanism of Dendritic Hematite via Hydrolysis of Ferricyanide

Abstract

ABSTRACT:The detailed process of the hydrolysis of ferricyanide into dendritic α-Fe2O3 (hematite) crystals with snowflake-like, feather-like and leaf-like morphologies has been investigated. [Fe(CN)6] 3-anions were found to polymerize into large, disordered soft matter aggregates at an early stage. The nucleation of hematite crystals took place near the surface of these aggregates via further hydrolysis. After the crystals grew to a certain size, branches started to appear. When the concentration of ferricyanide was low (i.e. 2 mM to 3.8 mM), growth was preferentially along the six equivalent <112 ̅ 0> directions, resulting in a flat snowflake-like shape, while high concentrations (i.e. 9 mM to 500 mM) of ferricyanide led to the growth of selective directions along the <101 ̅ 1> zone axes, forming a feather-like or leaf-like morphology. Highly selective adsorption and surface hydrolysis of [Fe(CN)6] 3-anions on α-Fe2O3 crystals was found to be a crucial process in the formation of these novel morphologies. It was found that the polymerisation of ferricyanide led to a reduction of pH value and that the formation of Fe2O3 increased pH value. The pH value of the solution at the point when the branches start to grow can significantly affect the distribution of Lewis acidic sites on different surfaces and, therefore, change the growth direction. The newly established mechanism is complementary to the classical theories of crystal growth. INTRODUCTIONControl over crystal growth orientation, exposure of selected crystal facets, and formation of novel crystal morphologies are hot topics in crystal engineering and in materials science, since the morphology and size of crystals often have a significant effect on their physio-chemical properties.

Second Harmonic Generation Response in Thermally reconstructed Multiferroic β′- Gd2(MoO4)3 Thin Films

“…Further research on thinner films (5 nm and 10 nm) showed that the films seem to follow a Pseudo-dendritic growth (Fig. S3 ), which has been already observed in some oxides 65 , 66 and molybdates materials 67 , 68 . Moreover, these studies allowed to determine the coalescence threshold of the films, with a critical thickness of ≈15 ± 3 nm and the similar dendritic behavior in thinner samples.…”

Three-Dimensional Microstructure of ε-Fe2O3 Crystals in Ancient Chinese Sauce Glaze Porcelain Revealed by Focused Ion Beam Scanning Electron Microscopy

“…As the temperature increased, the morphology of the dendritic fractals changed continuously from hyperbranched dendrites to slightly branched dendrites and, finally, nanorod-branched dendrites . When the concentration is low, growth was preferentially along six equivalent directions, resulting in a flat snowflake-like shape, while high concentrations led to the growth of selective directions forming a feather-like or leaf-like morphology . These results suggest that more comprehensive information, including the raw materials of the sauce glaze, the detailed operations during the manufacturing and firing processes, the local environment for the nucleation, the primary crystal growth and the branch growth, the temperature and time of different operations, and so on, is needed to investigate the detailed growth mechanism of ε-Fe 2 O 3 crystals in ancient Chinese sauce glaze porcelain.…”

“…Although a possible growth process has been proposed, the parameters of each step remain unclear. In recent years, dendritic fractals of several materials have been found and investigated, including Fe, Fe 3 O 4 , α-Fe 2 O 3 , γ-Fe 2 O 3 , Ag dendrites, and dendritic aluminum phosphate crystals. − Previous studies have suggested that many factors, such as the type and concentration of the precursor, the reaction temperature and time, and the environment for the crystal growth, could have influenced the morphology of dendritic fractals. Taking the dendritic α-Fe 2 O 3 for an example, the K 3 [Fe­(CN) 6 ] complex plays a crucial role in the formation of dendritic fractal structures .…”

“…47 When the concentration is low, growth was preferentially along six equivalent directions, resulting in a flat snowflake-like shape, while high concentrations led to the growth of selective directions forming a feather-like or leaf-like morphology. 51 These results suggest that more comprehensive information, including the raw materials of the sauce glaze, the detailed operations during the manufacturing and firing processes, the local environment for the nucleation, the primary crystal growth and the branch growth, the temperature and time of different operations, and so on, is needed to investigate the detailed growth mechanism of ε-Fe 2 O 3 crystals in ancient Chinese sauce glaze porcelain. If the above information can be clearly described, it would be possible to control and adjust the shape of the ε-Fe 2 O 3 crystals with high yield at micrometer scale by controlling the kinetic parameters of the manufacturing process.…”

Ultrafast Elemental and Oxidation-State Mapping of Hematite by 4D Electron Microscopy

“…Then the solution was subsequently sealed in an autoclave and maintained at 180 °C for 48 h, followed by cooling down to room temperature gradually. 28 The particles were then dispersed in ethanol, and a small drop of the suspension was deposited onto a graphene thin film-coated 2000 mesh copper grid. When dried in air, it was ready for the electron microscopic analysis.…”