van Rijt Mark M J, Oosterlaken Bernette M, Friedrich Heiner, de With Gijsbertus
Laboratory of Physical Chemistry, Centre for Multiscale Electron Microscopy, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology P. O. Box 513 Eindhoven 5600 MB The Netherlands
CrystEngComm. 2021 Mar 24;23(18):3340-3348. doi: 10.1039/d1ce00222h.
Hexamethylenetetramine (HMTA) is commonly used as a base releasing agent for the synthesis of ZnO under mild aqueous conditions. HMTA hydrolysis leads to gradual formation of a base during the reaction. Use of HMTA, however, does have limitations: HMTA hydrolysis yields both formaldehyde and ammonia, it provides no direct control over the ammonia addition rate or the total amount of ammonia added during the reaction, it results in a limited applicable pH range and it dictates the accessible reaction temperatures. To overcome these restrictions, this work presents a direct base titration strategy for ZnO synthesis in which a continuous base addition rate is maintained. Using this highly flexible strategy, wurtzite ZnO can be synthesized at a pH >5.5 using either KOH or ammonia as a base source at various addition rates and reaction pH values. pH measurements suggest a similar reaction mechanism to HMTA-based synthesis, independent of the varied conditions. The type and concentration of the base used for titration affect the reaction product, with ammonia showing evidence of capping behaviour. Optimizing this strategy, we are able to influence and direct the crystal shape and significantly increase the product yield to 74% compared to the ∼13% obtained by the reference HMTA reaction.
六亚甲基四胺（HMTA）通常用作在温和水相条件下合成氧化锌的碱释放剂。HMTA水解会在反应过程中导致碱的逐渐形成。然而，使用HMTA确实存在局限性：HMTA水解会产生甲醛和氨，它无法直接控制氨的添加速率或反应过程中添加的氨的总量，它导致适用的pH范围有限，并且它决定了可达到的反应温度。为了克服这些限制，这项工作提出了一种用于氧化锌合成的直接碱滴定策略，其中保持连续的碱添加速率。使用这种高度灵活的策略，可以在pH>5.5的条件下，以氢氧化钾或氨作为碱源，在各种添加速率和反应pH值下合成纤锌矿型氧化锌。pH测量表明，与基于HMTA的合成具有相似的反应机制，与各种条件无关。用于滴定的碱的类型和浓度会影响反应产物，氨显示出封端行为的证据。通过优化该策略，我们能够影响和控制晶体形状，并将产物产率显著提高至74%，相比之下，参考HMTA反应的产率约为13%。
