School of Materials Science and Engineering, Georgia Institute of Technology , Atlanta, Georgia 30332, United States.
ACS Nano. 2018 Jan 23;12(1):298-307. doi: 10.1021/acsnano.7b06353. Epub 2017 Dec 21.
We report a facile synthesis of Ag nanocubes with concave side faces and Au-Ag alloy frames, namely Ag@Au-Ag concave nanocrystals, by titrating HAuCl solution into an aqueous mixture of Ag nanocubes, ascorbic acid (HAsc), NaOH, and cetyltrimethylammonium chloride (CTAC) at an initial pH of 11.6 under ambient conditions. Different from all previous studies involving poly(vinylpyrrolidine), the use of CTAC at a sufficiently high concentration plays an essential role in carving away Ag atoms from the side faces through galvanic replacement. Concurrent co-deposition of Au and Ag atoms via chemical reduction at orthogonal sites on the surface of Ag nanocubes leads to the generation of Ag@Au-Ag concave nanocrystals with well-defined and controllable structures. Specifically, in the presence of CTAC-derived Cl ions, the titrated HAuCl is maintained in the AuCl species, enabling its galvanic replacement with the Ag atoms located on the side faces of nanocubes. The released Ag ions can be retained in the soluble form of AgCl by complexing with the Cl ions. Both the AuCl and AgCl in the solution are then reduced by ascorbate monoanion, a product of the neutralization reaction between HAsc and NaOH, to Au and Ag atoms for their preferential co-deposition onto the edges and corners of the Ag nanocubes. Compared with Ag nanocubes, the Ag@Au-Ag concave nanocrystals exhibit much stronger SERS activity at an excitation of 785 nm, making it feasible to monitor the Au-catalyzed reduction of 4-nitrothiophenol by NaBH in situ. When the Ag cores are removed, the concave nanocrystals evolve into Au-Ag nanoframes with controllable ridge thicknesses.
我们报告了一种简便的方法，通过在初始 pH 值为 11.6 的条件下，将 HAuCl 溶液滴定到 Ag 纳米立方、抗坏血酸 (HAsc)、NaOH 和十六烷基三甲基氯化铵 (CTAC) 的水溶液中，合成具有凹面侧和 Au-Ag 合金框架的 Ag 纳米立方的 Ag@Au-Ag 凹纳米晶体。与之前所有涉及聚（乙烯基吡咯烷酮）的研究不同，在足够高的浓度下使用 CTAC 通过电置换从侧面去除 Ag 原子起着至关重要的作用。Au 和 Ag 原子通过在 Ag 纳米立方表面的正交位置上通过化学还原同时共沉积，导致生成具有明确定义和可控结构的 Ag@Au-Ag 凹纳米晶体。具体而言，在 CTAC 衍生的 Cl 离子存在下，滴定的 HAuCl 保持在 AuCl 物种中，使其能够与位于纳米立方侧面的 Ag 原子发生电置换。释放的 Ag 离子可以通过与 Cl 离子络合而以 AgCl 的可溶形式保留在溶液中。溶液中的 AuCl 和 AgCl 都被抗坏血酸单阴离子还原，抗坏血酸单阴离子是 HAsc 和 NaOH 之间中和反应的产物，用于将 Au 和 Ag 原子优先共沉积到 Ag 纳米立方的边缘和角上。与 Ag 纳米立方相比，Ag@Au-Ag 凹纳米晶体在 785nm 的激发下表现出更强的 SERS 活性，从而可以实现原位监测 Au 催化的 4-硝基硫酚的还原反应。当 Ag 核被去除时，凹纳米晶体演变成具有可控脊厚度的 Au-Ag 纳米框架。
