Nucleation is Second Order: An Apparent Kinetically Effective Nucleus of Two for Ir(0)n Nanoparticle Formation from [(1,5-COD)IrI路P2W15Nb3

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• 作者：William W. Laxson ; Richard G. Finke
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：December 17, 2014
• 年：2014
• 卷：136
• 期：50
• 页码：17601-17615
• 全文大小：530K
• ISSN：1520-5126

文摘

Nucleation initiates phase changes across nature. A fundamentally important, presently unanswered question is if nucleation begins as classical nucleation theory (CNT) postulates, with n equivalents of monomer A forming a 鈥渃ritical nucleus鈥? A_n, in a thermodynamic (equilibrium) process. Alternatively, is a smaller nucleus formed at a kinetically limited rate? Herein, nucleation kinetics are studied starting with the nanoparticle catalyst precursor, [A] = [(Bu₄N)₅Na₃(1,5-COD)Ir^I路P₂W₁₅Nb₃O₆₂], forming soluble/dispersible, B = Ir(0)_鈭?00 nanoparticles stabilized by the P₂W₁₅Nb₃O₆₂^{9鈥?/sup> polyoxoanion. The resulting sigmoidal kinetic curves are analyzed using the 1997 Finke鈥揥atzky (hereafter FW) two-step mechanism of (i) slow continuous nucleation (A 鈫?B, rate constant k_1obs), then (ii) fast autocatalytic surface growth (A + B 鈫?2B, rate constant k_2obs). Relatively precise homogeneous nucleation rate constants, k_1obs, examined as a function of the amount of precatalyst, A, reveal that k_1obs has an added dependence on the concentration of the precursor, k_1obs = k_{1obs(bimolecular)}[A]. This in turn implies that the nucleation step of the FW two-step mechanism actually consists of a second-order homogeneous nucleation step, A + A 鈫?2B (rate constant, k_1obs(bimol)). The results are significant and of broad interest as an experimental disproof of the applicability of the 鈥渃ritical nucleus鈥?of CNT to nanocluster formation systems such as the Ir(0)_n one studied herein. The results suggest, instead, the experimentally-based concepts of (i) a kinetically effective nucleus and (ii) the concept of a first-observable cluster, that is, the first particle size detectable by whatever physical methods one is currently employing. The 17 most important findings, associated concepts, and conclusions from this work are provided as a summary.}