Nanoparticle Formation Kinetics and Mechanistic Studies Important to Mechanism-Based Particle-Size Control: Evidence for Ligand-Based Slowing of the Autocatalytic Surface Growth Step Plus Postulated Mechanisms

Ligands are known to affect the formation, stabilization, size, and size-dispersion control of transition-metal and other nanoparticles, yet the kinetic and mechanistic basis for such ligand effects remains to be elucidated and then coupled to predictions for improved particle size and narrower particle size distribution syntheses. Toward this broad goal, the effect of the added excess ligand (L) and the stabilizer, L = POM^9– (= the polyoxometalate, P₂W₁₅Nb₃O₆₂^9–) is studied for the formation of POM^9–-stabilized Ir(0)_n nanoparticles, {Ir(0)_n·(POM^9–)_m}^9m−, synthesized from an atomically characterized precatalyst (COD)­Ir·POM^8– under H₂. First, the balanced reaction stoichiometry and characterization of the nanoparticle products are established. Next, the kinetics of nanoparticle formation is analyzed initially by the FW 2-step minimum mechanism consisting of slow, continuous nucleation, A → B (rate constant k_1obs), and autocatalytic surface growth, A + B → 2B (rate constant k_2obs), where A is nominally (COD)­Ir·POM^8– and B is nominally the growing, average {Ir(0)_n·(POM^9–)_m}^9m− nanoparticle. The autocatalytic surface growth rate constant, k_2obs, was then studied as a function of the amount of added POM^9–. An inverse, quadratic-root-type dependence of k_2obs on the concentration of L = POM^9– is observed, which was then analyzed in terms of two main mechanisms. Specifically, the dependence of k_2obs on the [POM^9–] was analyzed in terms of (i) an A·L ⇌ A + L dissociative equilibrium, (COD)­Ir^I·POM^8– + 2 solvent ⇌ (COD)­Ir^I(solv)₂⁺ + POM^9–, and then (ii) this same A·L ⇌ A + L plus also a B + L ⇌ B·L nanoparticle-surface capping equilibrium, where B represents the average Ir(0)_n nanoparticle. Three other mechanisms were also considered. The high-resolution transmission electron microscopy of the parent nanoparticles when no excess POM^9– has been added is also provided as part of the Supporting Information. The results (a) provide the first evidence and resultant physical insight, for the prototype, well-studied {Ir(0)_n·(POM^9–)_m}^9m− nanoparticle formation system, that growth is a function of the amount of POM^9– ligand present and (b) provide compelling evidence that A = (COD)­Ir^I(solv)₂⁺ from the A·L dissociative equilibrium, (COD)­Ir^I·POM^8– + 2 solvent ⇌ (COD)­Ir^I(solv)₂⁺ + POM^9–, is the actual reactant in the FW 2-step formulation of the A + B → 2B autocatalytic growth step. The results also (c) support the 1-step more complex mechanism that adds a ligand-capping B + L ⇌ B·L step, namely, the mechanism consisting of the steps of A·L ⇌ A + L, then A → B, then A + B → 2B, and then B + L ⇌ B·L. Given the wide usage of the simpler FW 2-step mechanism, plus the fact that nanoparticle-stabilizing and -capping ligands are invariably present, one can anticipate a much broader applicability of mechanisms containing the A·L ⇌ A + L and the B + L ⇌ B·L steps to nanoparticle formation reactions.