Autocatalytic surface growth

Using the well-defined system of polyoxoanion/Bu4N -stabilized iridium nanoparticles [9, 29] as a model for the studies, Finke and coworkers [30] proposed a method that attempted to explain the formation and growth of transition-metal nanoparticles. This indirect method is based on an autocatalytic mechanism that considers a nudcation step in which a precursor A is converted to a zero-valent nuclei B with a rate constant fej, and a second step that considers the autocatalytic surface growth of the metal nanoparticles where species B catalyzes its own formation with a rate constant tc2 (Scheme 15.5). 

Morris, A.M., Watzky, M.A., Agar, J.N., Finke, R.G. (2008) Fitting neurological protein aggregation kinetic data via a 2-step, minimal/ Ockham s razor model The Finke-Watzky mechtmism of nucleation followed by autocatalytic surface growth. Biochemistry, 47 (8), 2413-2427. 

The hydrogenation kinetics of cydohexene catalyzed by Pt2(dba)3 dispersed in BMI.PFg, BMI.BF4 and BMl,OTf are shown in Fig. 6.3. The kinetics curves were treated using the pseudo-elementary step and fitted (Eq. (6.1) by the following integrated rate equation for metal-salt decomposition (A —> B, hi) and autocatalytic nanoduster surface growth (A + B — 2B, 2). For a more detailed description of the use of the pseudo-elementary step for the treatment of hydrogenation kinetic data and derivation of the kinetic equations see elsewhere [81-83]. 

The formation of the nanoparticles from the first two methods apparently follows the autocatalytic mechanism (Besson et al, 2005 Besson et al., 2005) that basically involves two steps nucleation and surface growth. In various cases these ionic liquids colloidal mixtures can be used directly as catalysts or they may be isolated and used as powders in solventless conditions (the substrates/ products are per definition the solvent) or re-dispersed in the ionic liquids. 

When initiator is first added the reaction medium remains clear while particles 10 to 20 nm in diameter are formed. As the reaction proceeds the particle size increases, giving the reaction medium a white milky appearance. When a thermal initiator, such as AIBN or benzoyl peroxide, is used the reaction is autocatalytic. This contrasts sharply with normal homogeneous polymerizations in which the rate of polymerization decreases monotonicaHy with time. Studies show that three propagation reactions occur simultaneously to account for the anomalous auto acceleration (17). These are chain growth in the continuous monomer phase chain growth of radicals that have precipitated from solution onto the particle surface and chain growth of radicals within the polymer particles (13,18). 

The decomposition mechanisms are difficult to understand because (i) the surface is not homogeneous with respect to its morphology and chemical composition and (ii) these features evolve continuously during the deposition process. Moreover, as has been clearly demonstrated for noble metals, autocatalytic phenomena can occur, dramatically increasing the growth rate while decreasing the nucleation rate. 

Abstract A growing tendency in chemical vapour deposition is to produce ultra-thin films or nano-objects as particles, tubes or wires. Such an objective addresses the question of a better control of the main parameters which govern the nucieation and growth steps of the deposit. This chapter focuses on the interfacial phenomena that occur at both the solid surface and the gaseous phase levels. The role of surface defects, surface reactive groups, and autocatalytic phenomena on the nucieation step are discussed by means of representative examples from the literature. In an attempt to clarify gas-phase properties, the influence of the supersaturation parameter on the nucieation step is also described. 

The direct autocatalytic multiplication of both caprylate and oleic acid vesicles received a simplified kinetic analysis [52], It was shown, in agreement with elementary reasoning that the catalytic effect is due to large growth of the reaction surface. 

Figure 6.15. Schematic of the four-step mechanism for transition metal (e.g., Pt) nanocluster formation. Shown are (i) nucleation to a desired cluster size (ii) autocatalytic growth onto the cluster surface (hi) diffusive agglomerative growth of two nanoclusters and (iv) autocatalytic agglomeration into bulk metal particulates. Reproduced with permission from Besson, C. Finney, E. E. Einke, R. G. J. Am. Chem. Soc. 2005,127, an9. Copyright 2005 American Chemical Society.

The growth of film or increase in thickness of the deposit is influenced by the autocatalytic activity of the surface on which a metal is deposited. 

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