Morphology evolution of NaTaO3 submicrometer single-crystals: from cubes to quasi-spheres

Abstract

Surface structure control of functional nano-/micro-crystallites has attracted great attention because many important physicochemical properties depend on their surface. Guided by the supersaturation-dependent surface structure evolution strategy we proposed recently, NaTaO3 submicrometer crystals with morphologies of cubes, corner truncated cubes, edge and corner truncated cubes, and quasi-spheres can be synthesized by changing the volume ratio of ethylene glycol to water and the amount of NaOH in the composite solvent. Under low supersaturation condition, NaTaO3 cubic crystals with low energy {100} facets were obtained. As the supersaturation increases, the corners and edges of NaTaO3 cubic crystals, which possess higher surface energy, were gradually truncated. Surprisingly, quasi-sphere crystallites formed under extremely high supersaturation condition, which is difficult to be explained by the classical crystal growth theories. By analyzing the formation work of two-dimension crystal nuclei, we concluded that the crystal growth tend to be isotropic at extremely high supersaturation, which well explained the formation of the quasi-sphere crystallites.

Crystal facet tailoring arts in perovskite oxides

“…6). 78 It seems that EG plays an essential role in the shape evolution of NaTaO 3 by tailoring the growth speed in the {110} and {111} planes due to the capping effect of polar groups. For hydrothermally prepared LiTaO 3 crystals, however, the microcube shapes are highly distorted with the crystal space group R3c, because the smallest metal cation size of the Li-cation changed the unit cell into a hexagonal crystal system.…”

“…6 SEM images of NaTaO 3 crystals obtained with 10 mmol NaOH and different volume ratios of EG to water: (a) 1/13, (b) 3/11, (c) 5/9, and (d) 11/3. Insets are the corresponding SEM images and schematic models 78. Copyright Springer and Science China Press.…”

Surface structure-controlled synthesis of nanocrystals through supersaturation dependent growth strategy

“…即使不使用晶面调节剂, 仅通过 调节良溶剂与不良溶剂的比例或反应物NaOH浓度, 就可以得到不同表面结构的NaTaO3微晶 [24] . 例如, 随 [24] . 二维晶核在不同晶面上的成核功Whkl反映了晶体 在不同晶面上的生长位垒 [27] .…”

“…如前所述, 晶体生长过程中, 生长基元的 过饱和度会随着纳米晶的生长逐步下降, 在反应之初, 过饱和度比较高, 因此产物也有可能暴露高表面能晶 面. 在合成MOF-5过程中, 王训课题组 [48] 就可调节过饱和度 [22] ; 在贵金属纳米晶的合成过程中, 生长基元为金属原子, 因此提高金属前驱体的还原速 度是提高过饱和度的关键, 在合成过程中可以通过提 高反应温度、增大反应物的浓度、使用强的还原剂 等方式来实现 [22,40] ; 在氧化物纳米晶的生长过程中, 由 于其金属-氧化学键以及晶体结构的多样性, 使得如 何提高其生长基元的过饱和度成为难点, 目前较为有 效的方法是通过调节体系的溶剂(良溶剂与不良溶剂 的选择)等 [23,24] ; 类似的, 在生长MOFs过程中, 通过溶 剂、额外添加剂的选择, 或提高有机配体的浓度, 都 可以一定程度上促进生长基元过饱和度的提高, 进而 得到具有高表面能晶体裸露的晶体 [45,47,48] . 值得注意 的是, 体系的过饱和度不能够一直地提高, 当生长基 元过饱和度达到或超过临界成核浓度时, 二次成核现 象将发生.…”

“…值得注意 的是, 体系的过饱和度不能够一直地提高, 当生长基 元过饱和度达到或超过临界成核浓度时, 二次成核现 象将发生. 例如, 在Au纳米晶合成过程中, 过高的过 饱和度会导致二次成核, 伴随着大量小颗粒纳米晶生 成 [22] ; 另外, 生长基元过饱和度过高时, 各个晶面上二 维成核功函可能趋于0, 晶体在各个方向的生长几率 趋于一致, 容易形成球形或准球形外形的晶体 [24] . http://engine.scichina.com/doi/10.1360/N032016-00208…”