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γ-Al2O3 nanorods with tuneable dimensions – a mechanistic understanding of their hydrothermal synthesis

机译：尺寸可调节的γ-Al 2 O 3 纳米棒–对它们水热合成的机理理解

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This paper reports for the first time the size control of well-defined and morphologically pure alumina (γ-Al2O3) nanorods, presenting an economic and reproducible route for the manufacture of these materials with tuneable sizes for useful applications, for example serving as adsorbents, catalysts and catalyst supports. A detailed understanding of the different steps taking place during the hydrothermal synthesis has been deduced herein. Understanding the effect of temperature on the relative rates of these steps is essential for achieving size and morphology selectivity, but has often been overlooked in the literature. This systematic study identifies six distinct steps taking place during the synthesis: (1) formation of Al(OH)3, (2) dissolution of Al(OH)3 into hexameric based fragments (3) thermolysis at temperatures ≥ 170 °C into soluble AlOOH (boehmite) building blocks (4) formation of lamellar AlOOH sheets (5) scrolling into nanorod crystallites and subsequent oriented attachment into high aspect nanorods and (6) growth by Ostwald ripening to low aspect nanorods. The obtained AlOOH nanorods are converted into γ-Al2O3 with conservation of morphology by calcination at 500 °C. Nanorod formation (step 5) can only be achieved at temperatures ≥ 180 °C (after 20 hours). At 180 °C, growth of the rods (step 6) takes place simultaneously with their slow formation (step 5) leading to two distinct nanorod products with different aspect ratios. At higher temperatures (200 °C), the rate of formation (step 5) is fast, quickly reaching completion, allowing for substantial growth of the nanorods and resulting in a monomodal size distribution. Thus, we have identified that γ-Al2O3 rods with high aspect ratio can be selectively synthesised at 180 °C for ≥20 hours, while low aspect ratios are produced at 200 °C for ≥10 hours. In all cases, the average size of the nanorods increases linearly with prolonged reaction time due to their continuous growth.
机译：本文首次报道了形态明确的纯氧化铝（γ-Al 2 O 3 <纳米棒，为生产这些具有可调节尺寸的材料提供了经济且可重复的途径，以用于有用的应用，例如用作吸附剂，催化剂和催化剂载体。本文已经推导了对在水热合成过程中发生的不同步骤的详细理解。了解温度对这些步骤的相对速率的影响对于实现尺寸和形态选择性是必不可少的，但是在文献中经常被忽略。这项系统的研究确定了合成过程中发生的六个不同步骤：（1）Al（OH） 3 的形成，（2）Al（OH）的溶解 3 变成基于六聚体的片段（3）在≥170°C的温度下热分解成可溶性AlOOH（勃姆石）构件（4）形成薄片AlOOH片（5）滚动成纳米棒微晶并随后定向附着到高纵横比纳米棒中，以及（6）通过奥斯特瓦尔德熟化生长到低纵横比纳米棒。将获得的AlOOH纳米棒通过500°C煅烧转化为γ-Al 2 O 3 C。仅在温度≥180°C（20小时后）时才能形成纳米棒（步骤5）。在180°C下，棒的生长（步骤6）与缓慢形成（步骤5）同时发生，从而导致两种不同的长宽比不同的纳米棒产品。在较高的温度（200°C）下，形成速率（步骤5）快速，迅速达到完成，从而使纳米棒大量生长并导致单峰尺寸分布。因此，我们已经确定，可以高选择性地合成高纵横比的γ-Al 2 O 3 180°C≥20小时，而低长宽比在200°C≥10小时产生。在所有情况下，由于纳米棒的连续生长，其平均尺寸随反应时间的延长而线性增加。

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