Radiochemistry Division, Bhabha Atomic Research Centre , Trombay, Mumbai 400 085, India.
Langmuir. 2014 Mar 11;30(9):2460-9. doi: 10.1021/la4048787. Epub 2014 Feb 26.
We demonstrate for the first time the intrinsic role of nanoconfinement in facilitating the chemical reduction of metal ion precursors with a suitable reductant for the synthesis of metal nanoparticles, when the identical reaction does not occur in bulk solution. Taking the case of citrate reduction of silver ions under the unusual condition of [citrate]/[Ag(+)] ≫ 1, it has been observed that the silver citrate complex, stable in bulk solution, decomposes readily in confined nanodomains of charged and neutral matrices (ion-exchange film and porous polystyrene beads), leading to the formation of silver nanoparticles. The evolution of growth of silver nanoparticles in the ion-exchange films has been studied using a combination of (110m)Ag radiotracer, small-angle X-ray scattering (SAXS) experiments, and transmission electron microscopy (TEM). It has been observed that the nanoconfined redox decomposition of silver citrate complex is responsible for the formation of Ag seeds, which thereafter catalyze oxidation of citrate and act as electron sink for subsequent reduction of silver ions. Because of these parallel processes, the particle sizes are in the bimodal distribution at some stages of the reaction. A continuous seeding with parallel growth mechanism has been revealed. Based on the SAXS data and radiotracer kinetics, the growth mechanism has been elucidated as a combination of continuous autoreduction of silver ions on the nanoparticle surfaces and a sudden coalescence of nanoparticles at a critical number density. However, for a fixed period of reduction, the size, size distribution, and number density of thus-formed Ag nanoparticles have been found to be dependent on physical architecture and chemical composition of the matrix.
我们首次证明了纳米限域在促进具有合适还原剂的金属离子前体的化学还原方面的固有作用，而在本体溶液中相同的反应不会发生。以[柠檬酸盐]/[Ag(+)]>>1 条件下柠檬酸还原银离子为例，我们观察到在带电荷和中性基质（离子交换膜和多孔聚苯乙烯珠）的纳米限域区域中，稳定存在于本体溶液中的银柠檬酸配合物容易分解，导致银纳米颗粒的形成。使用(110m)Ag 放射性示踪剂、小角 X 射线散射 (SAXS) 实验和透射电子显微镜 (TEM) 的组合，研究了银纳米颗粒在离子交换膜中的生长演变。我们观察到银柠檬酸配合物的纳米限域氧化还原分解负责形成 Ag 种子，然后这些种子催化柠檬酸的氧化，并作为随后还原银离子的电子阱。由于这些平行过程，在反应的某些阶段颗粒尺寸呈双峰分布。揭示了连续成核和并行生长机制。基于 SAXS 数据和放射性示踪动力学，阐明了生长机制是纳米颗粒表面上银离子的连续自还原与纳米颗粒在临界密度下突然聚结的组合。然而，对于固定的还原时间，所形成的 Ag 纳米颗粒的尺寸、尺寸分布和数密度取决于基质的物理结构和化学组成。
