光催化合成金属Ag纳米颗粒的生长机制——晶核密度控制的生长模式转换

利用AgNO3水溶液,通过严格控制TiO2薄膜的化学活性,系统研究了在TiO2表面光催化合成金属Ag纳米颗粒的生长行为。研究发现,光催化合成金属Ag纳米颗粒存在着两个完全不同的生长机制,分别对应着金属Ag纳米颗粒的各向同性和各向异性生长。当溶液浓度较低时,Ostwald熟化(OR)机制主导着金属Ag纳米颗粒的长大过程;当溶液浓度较高时,取向附生(OA)机制决定着金属Ag纳米颗粒长大成纳米片。原位消光光谱分析表明,OR机制和OA机制生长的前期具有相近消光特征,决定金属Ag纳米颗粒生长模式的关键是AgNO3溶液的浓度,更准确地说是金属Ag初级晶核的局域密度。在此基础上提出了有关光催化合成金属Ag纳米颗粒的生长模型。

Transition of Growth Mode Controlled by the Density of Nuclei in Photocatalytic Synthesis of Ag Nanoparticles

Using the aqueous solution of AgNO₃, the growth behavior of Ag nanoparticles synthesized by photocatalytic method is studied by controlling the chemical activity of TiO₂ film. It is found that the Ag nanoparticles can grow up with two distinct growth modes, which correspond to the isotropic and anisotropic growth, respectively. When the concentration of AgNO₃ is low, Ostwald ripening (OR) dominates the growth of Ag nanoparticles. If the concentration of AgNO₃ exceeds a critical level, oriented attachment (OA) determines the Ag nanoparticles growing up into Ag nanoplates. The in-situ extinction spectra suggest that the Ag nuclei have the very similar features in the stage of nucleation before the growth of Ag nanoparticles is dominated by OR or OA mechanism. The transition of growth behavior is determined by the concentration of AgNO₃ in the solution, or the local density of Ag primary nuclei. On the basis of experimental results and theoretical analysis, a physical model is suggested for the growth of Ag nanoparticles in the photocatalytic synthesis.