Growth Mechanism of Dendritic Hematite via Hydrolysis of Ferricyanide

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• 作者：Alice E. Green ; Chang-Yang Chiang ; Heather F. Greer ; Ashleigh Waller ; Aron Ruszin ; James Webster ; Ziyin Niu ; Katherine Self ; Wuzong Zhou
• 刊名：Crystal Growth & Design
• 出版年：2017
• 出版时间：February 1, 2017
• 年：2017
• 卷：17
• 期：2
• 页码：800-808
• 全文大小：629K
• ISSN：1528-7505

文摘

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 ⟨112̅0⟩ 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 ⟨101̅1⟩ 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 polymerization of ferricyanide led to a reduction of pH value and that the formation of Fe₂O₃ increased the 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.