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Formation Mechanisms of Nanocrystalline MnO2 Polymorphs under Hydrothermal Conditions

机译:水热条件下纳米晶MNO <亚烷> 2 多晶型物的形成机制

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摘要

Understanding and controlling the polymorphism of manganese dioxide (MnO_(2)) is of vital importance in many nanoscale applications. Here in situ powder X-ray diffraction (PXRD) in combination with in situ total X-ray scattering are used to reveal the formation mechanism as well as polymorph evolution of MnO_(2) under hydrothermal synthesis conditions. A “PXRD invisible” amorphous phase with a local structure resembling α-MnO_(2) (denoted α-MnO_(2)(A)) is observed at all reaction stages, and it never fully disappears from the reaction solution. The MnO_(2) phase evolution involves initial formation of δ-MnO_(2), which transforms to α-MnO_(2), and then subsequently to β-MnO_(2). The phase transformations between different polymorphs do not involve dissolution–recrystallization, but they occur via solid-state mechanisms. However, the amorphous α-MnO_(2)(A) phase plays a key role since it is consumed in growing both the α- and β-MnO_(2) polymorphs. Overall, the polymorphism of the crystalline product can be controlled through reaction time and temperature to form either nanocrystalline and disordered δ-MnO_(2), nanocrystalline α-MnO_(2), or nanocrystalline β-MnO_(2). At the lowest temperature (200 °C) the very early growth of α-MnO_(2) appears to be by oriented attachment along (110) crystal planes of primary nanorods, but this is quickly followed by rapid growth along the c -direction supported by consumption of α-MnO_(2)(A).
机译:理解和控制二氧化锰的多态性(MNO_(2))在许多纳米级应用中具有至关重要的重要性。这里,原位粉末X射线衍射(PXRD)与原位总X射线散射相结合用于露出水热合成条件下的形成机制以及MNO_(2)的多晶型化演化。在所有反应阶段观察到具有类似α-MnO_(2)的局部结构的“PXRD隐形”非晶相(表示为α-MNO_(2)),并且从不完全消失在反应溶液中。 MnO_(2)相进化涉及α-MnO_(2)的初始形成,其转化为α-mnO_(2),然后随后转化为β-mnO_(2)。不同多晶型物之间的相变不涉及溶解 - 再结晶,但它们通过固态机制发生。然而,无定形α-MnO_(2)(a)相起着关键作用,因为它在生长α-和β-mnO_(2)多晶型物中消耗。总体而言,可以通过反应时间和温度来控制结晶产物的多态性,以形成纳米晶和无序的δ-mnO_(2),纳米晶α-MnO_(2),或纳米晶β-MnO_(2)。在最低温度(200°C)处,α-mnO_(2)的非常早期的生长似乎是沿着(110)主纳米棒的晶体平面的附着,但是沿着支撑的C型C型快速增长。通过消耗α-mnO_(2)(a)。

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  • 来源
    《Crystal growth & design》 |2018年第2期|共12页
  • 作者单位

    Center for Materials Crystallography Department of Chemistry and iNANO Aarhus University DK-8000 Aarhus C Denmark;

    Center for Materials Crystallography Department of Chemistry and iNANO Aarhus University DK-8000 Aarhus C Denmark;

    Center for Materials Crystallography Department of Chemistry and iNANO Aarhus University DK-8000 Aarhus C Denmark;

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  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 晶体学;
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