Rodríguez-Sánchez M Luisa, Rodríguez M José, Blanco M Carmen, Rivas José, López-Quintela M Arturo
Laboratory of Magnetism and Nanotechnologies, Institute of Technology, Department of Physical Chemistry, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain.
J Phys Chem B. 2005 Jan 27;109(3):1183-91. doi: 10.1021/jp046056n.
The formation of Ag nanoparticles by electrochemical techniques has been investigated through a time-resolved UV-vis spectroscopy study. The formation of Ag(4)(2+) clusters is suggested as the main precursors to the particle formation. The mechanism also considers the electrodeposition which occurs as a parallel process in the electrochemical particle formation. Experiments at different current densities show that the electrodeposition is more important at low current densities. From the fittings of the change of the plasmon (lambda approximately 430 nm) and the cluster (lambda = 250 nm) bands to the proposed mechanism, the kinetic constants of the formation and disappearance of the Ag(4)(2+) cluster are derived. The kinetic fittings also allowed an estimation of the Ag(4)(2+) cluster extinction coefficient (epsilon(250) = 1.0 x 10(4) M(-1) cm(-1)). It is observed that the plasmon bandwidth (fwhm) follows the theoretical predicted 1/R law only for particles with sizes d approximately >3 nm, but the law is broken for the smallest particles (d < 2.5 nm). The break is associated with the existence of single-electron (SE) transitions which are activated by the plasmon decay for the smallest nanoparticles. From the broken 1/R law, a limit relaxation time of about 4 fs is derived for the plasmon deactivation. Below this limit, the plasmon seems to decay mainly through a nonradiative channel with the formation of electron-hole (e-h) pairs. By comparison of the 1/R broken law with other literature results, it is concluded that large interactions of the Ag nanoparticles with the used capping molecule (tetrabutylammonium acetate) facilitate the e-h plasmon deactivation.
通过时间分辨紫外可见光谱研究，对电化学技术制备银纳米颗粒的过程进行了研究。研究表明，Ag(4)(2+)簇的形成是颗粒形成的主要前驱体。该机制还考虑了在电化学颗粒形成过程中作为并行过程发生的电沉积。不同电流密度下的实验表明，在低电流密度下电沉积更为重要。通过将等离子体（λ约430nm）和簇（λ = 250nm）带的变化拟合到所提出的机制中，得出了Ag(4)(2+)簇形成和消失的动力学常数。动力学拟合还允许估计Ag(4)(2+)簇的消光系数（ε(250)= 1.0 x 10(4) M(-1) cm(-1)）。观察到，仅对于尺寸d约> 3nm的颗粒，等离子体带宽（半高宽）遵循理论预测的1/R定律，但对于最小的颗粒（d <2.5nm）该定律不成立。这种不成立与单电子（SE）跃迁的存在有关，对于最小的纳米颗粒，单电子跃迁由等离子体衰减激活。从破坏的1/R定律中，得出等离子体失活的极限弛豫时间约为4fs。低于此极限，等离子体似乎主要通过形成电子 - 空穴（e-h）对的非辐射通道衰减。通过将1/R破坏定律与其他文献结果进行比较，得出结论：银纳米颗粒与所用封端分子（醋酸四丁铵）的大相互作用促进了e-h等离子体失活。
