δ-MnO2 is an important component of environmental minerals and is among the strongest sorbents and oxidants. The crystalline morphology of δ-MnO2 is one of the key factors affecting its reactivity. In this work, δ-MnO2 was initially synthesized and placed in an acidic environment to react with Mn2+ and undergo a crystalline transformation. During the transformation of crystalline δ-MnO2, kinetic sampling was conducted, followed by analyses of the structures and morphologies of the samples. The results showed that at pH 2.5 and 4, δ-MnO2 nanoflakes spontaneously self-assembled into nanoribbons via edge-to-edge assembly in the initial stage. Subsequently, these nanoribbons attached to each other to form primary nanorods through a face-to-face assembly along the c-axis. These primary nanorods then assembled along the (001) planes and lateral surfaces, achieving further growth and thickening. Since a lower pH is more favorable for the formation of vacancies in δ-MnO2, δ-MnO2 can rapidly adsorb Mn2+ directly onto the vacancies to form tunnel walls. At the same time, the rapid formation of the tunnel walls leads to a quick establishment of hydrogen bonding between adjacent nanoribbons, enabling the assembly of these nanoribbons into primary nanorods. Therefore, in a solution with the same concentration of Mn2+, the structure transformation and morphology evolution of δ-MnO2 to α-MnO2 occur faster at pH 2.5 than at pH 4. These findings provide insights into the mechanism for crystal growth from layer-based to tunnel-based nanorods and methods for efficient and controlled syntheses of nanomaterials.

中文翻译：

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酸性条件下Mn2+浓度对δ-MnO2晶体生长的影响


δ-MnO2是环境矿物质的重要组成部分，是最强的吸附剂和氧化剂之一。 δ-MnO2的晶体形貌是影响其反应活性的关键因素之一。在这项工作中，首先合成了δ-MnO2，并将其置于酸性环境中与Mn2+反应并发生结晶转变。在δ-MnO2晶体转变过程中，进行了动力学采样，并对样品的结构和形貌进行了分析。结果表明，在pH 2.5和4时，δ-MnO2纳米片在初始阶段通过边对边组装自发自组装成纳米带。随后，这些纳米带通过沿 c 轴面对面组装而彼此附着，形成初级纳米棒。然后，这些初级纳米棒沿着（001）平面和侧面组装，实现进一步的生长和增厚。由于较低的pH更有利于δ-MnO2空位的形成，δ-MnO2可以快速将Mn2+直接吸附到空位上形成隧道壁。同时，隧道壁的快速形成导致相邻纳米带之间氢键的快速建立，从而使这些纳米带组装成初级纳米棒。因此，在具有相同浓度 Mn2+ 的溶液中，δ-MnO2 向 α-MnO2 的结构转变和形貌演化在 pH 2.5 时比在 pH 4 时发生得更快。这些发现为从层状晶体生长的机制提供了见解。基于隧道的纳米棒以及高效、受控合成纳米材料的方法。