水热条件下纳米晶MnO <sub>2</sub>多晶型物的形成机理,Crystal Growth & Design

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水热条件下纳米晶MnO ₂多晶型物的形成机理
Crystal Growth & Design ( IF 3.2 ) Pub Date : 2018-01-22 00:00:00 , DOI: 10.1021/acs.cgd.7b01304
Steinar Birgisson ₁ , Dipankar Saha ₁ , Bo B. Iversen ₁

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在许多纳米级应用中，了解和控制二氧化锰（MnO ₂）的多态性至关重要。在这里，原位粉末X射线衍射（PXRD）与原位总X射线散射相结合，揭示了在水热合成条件下MnO ₂的形成机理以及多晶型物的演化。A“PXRD隐形”无定形与本地结构类似于α-MnO的相₂（表示为α-MnO的₂（A））在所有的反应阶段中观察到的，它从不完全从反应溶液中消失。MnO的₂相进化涉及初始形成δ-的MnO ₂，其变换以α-MnO的，₂，并随后到β-MnO的₂。不同多晶型物之间的相变不涉及溶解-重结晶，而是通过固态机制发生的。然而，无定形α-MnO的₂，因为它是在生长两个α-和β-MnO的消耗（A）相起着关键的作用₂多晶型物。总体而言，该结晶产物的多态性可以通过反应时间和温度来控制，以形成纳米晶体要么和无序δ-的MnO ₂，纳米晶α-MnO的₂，或纳米晶体β-MnO的₂。在最低温度（200℃）的很早期生长α-MnO的₂似乎是由沿着初级纳米棒的（110）晶面取向的附着，但是这接着很快沿快速增长Ç由α-MnO的消耗支持-方向₂（A）。

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Formation Mechanisms of Nanocrystalline MnO2 Polymorphs under Hydrothermal Conditions

Understanding and controlling the polymorphism of manganese dioxide (MnO₂) 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 MnO₂ under hydrothermal synthesis conditions. A “PXRD invisible” amorphous phase with a local structure resembling α-MnO₂ (denoted α-MnO₂(A)) is observed at all reaction stages, and it never fully disappears from the reaction solution. The MnO₂ phase evolution involves initial formation of δ-MnO₂, which transforms to α-MnO₂, and then subsequently to β-MnO₂. The phase transformations between different polymorphs do not involve dissolution–recrystallization, but they occur via solid-state mechanisms. However, the amorphous α-MnO₂(A) phase plays a key role since it is consumed in growing both the α- and β-MnO₂ polymorphs. Overall, the polymorphism of the crystalline product can be controlled through reaction time and temperature to form either nanocrystalline and disordered δ-MnO₂, nanocrystalline α-MnO₂, or nanocrystalline β-MnO₂. At the lowest temperature (200 °C) the very early growth of α-MnO₂ 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-MnO₂(A).

更新日期：2018-01-22

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