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纳米材料——晶体成核问题
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有两个问题: 1、为什么要快速成核？作者说 If nucleation is allowed to proceed over an extended period, reactants are unevenly depleted from solution leading to variations in growth rate for seed particles formed at different reaction times. 2、文中所说的临界浓度应该是金属原子的浓度吧。但是作者说 Fast nucleation is typically achieved by slowly building up the concentration of metal ions in solution through serial injections of the precursor materials until nucleation occurs. 文献：small 2008, 4, No. 3, 310 – 325 In homogeneous nucleation, seed formation proceeds according to the LaMer model (see Figure 4a), where reduction of metal ions occurs to generate a critical composition of atomic species in solution. Above this critical concentration, nucleation results in a rapid depletion of the reactants such that all subsequent growth occurs on the pre-existing nuclei. As long as the concentration of reactants is kept below the critical level, further nucleation is discouraged. This is particularly important for shape control: to obtain a highly shape-monodisperse yield of nanocrystals, nucleation must occur rapidly and instantaneously. This is exemplified by the synthesis of Ag nanocrystalline octahedra, whose reaction rate can be monitored by UV/vis spectroscopy (see Figure 4b). In other words, only a single nucleation event can occur. If nucleation is allowed to proceed over an extended period, reactants are unevenly depleted from solution leading to variations in growth rate for seed particles formed at different reaction times. Fast nucleation is typically achieved by slowly building up the concentration of metal ions in solution through serial injections of the precursor materials until nucleation occurs. However, preventing the fast reduction of metal ions – especially for colloidal syntheses carried out at reflux temperatures – is necessary to accumulate enough reactant species to reach the critical nucleation concentration. This accumulation may be aided by modifying the reactivity of the metal precursor. One possible method is through the formation of metal–surfactant or metal–polymer complexes in the reaction solution, which hinder the reduction reaction even under highly favorable conditions. |
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2楼2008-08-31 18:44:17