Au,Ag Nanostructure And Crystal Growth Mechanism In Solution

Noble metal (Au, Ag, Pt, Pd) nanocrystals have recently attracted intensive research interest because of their important applications in catalysis, biological labeling, optics, and microelectronics. Above properties of noble metals are determined by its size, shapes/morphologies, structures characteristics and so on. Therefore the size/ shape/morphology - controlled synthesis of noble metals has become one of the hot topics in the area of nanomaterials preparations. The controllable growth of nanomaterials and the manipulations in size, shape/morphology, compositions, crystalline structures, physical and chemical properties will bring a meaningful influence on the further studies on the relationships between nanostructures and properties, and finally realize the design of new functional materials as desired.In recent years, as the development of electron microscopy technique, some novel phenomena have been found by the means of in situ observation during the crystals growth. The traditional crystals theories for the solution system such as surface energy balance theory, 2D nucleation mode, screw dislocation growth mechanisms, stacking faults mechanism, twin mechanism, and the anionic coordination polyhedron growth unit model et al., are failed in explanations on these phenomena. For example, the oriented attachment mechanism, the mesoscopic transformation, and the double interface growth mode reported by our group recently where the amorphous cluster plays as transient phase during the crystal growth. These are the new phenomena, new questions, and new law in the liquid-phase synthesis. The understanding of these phenomena and developing of related growth mechanisms and crystal growth theories will play important roles in recognizing of experimental results, optimizing experimental processes, and increasing the control of materials structures and properties.In this study, several of Au or Ag nanostructures including nanoplate, dendrite, spindle-like, nanospheres, octahedron, high density of nanoparticle arrays and flower-like fractal patterns on Si substrate et al. were synthesized within electroless deposition and electrodeposition systems. Using the SEM,TEM,HRTEM,XRD,UV spectrum and SERS characterization methods, the compositional, structural, morphological, optical and SERS characteristics and properties for above several nanostructures obtained here were measured and investigated in detail. The Monte Carlo simulation, concentration field model, finite difference time-domain method (FDTD) simulation, and mocelular dyniamc(MD)simulation were carried out to study the growth processes and corresponding crystal growth mechanism for above Au,Ag nanostructures. The main research contents and achievements are as follows:(1)The Au,Ag mesocrystals and mesoscopic transformation. Mesocrystals are defined as oriented superstructures or colloidal crystals composed of individual nanocrystals that are aligned in a common crystallographic fashion The mesoscopic transformation is a non-classical crystallization process based on the self-assembly of nanoparticles building units and is quite different with the classical crystallization pathway started from the primary building blocks like atoms, ions, molecules, or forming clusters. Within Sn/AgNO₃ replacement reaction,the plate-like Ag mesocrystals aggregated by numerous nanoparticles was, for the first time, reported and the process of mesoscopic transformations was investigated in detail. The two-dimensional layer-by-layer growth mode was also carefully studied. It was found that initial orientation nucleations followed by the grains rotation and realignment mechanism drive the growth of Ag mesocrystals with plate-like morphology. Within Zn/HAuCl₄ reaction system, the mesoscopic transformation was also observed. By the adjustment of reaction conditions, the Ag mesocrystals with plate-like and dendritic morphologies, and Au mesocrystals with dendritic and spindle-like morphologies were synthesized, respectively. The self-assembly ability of nanoparticle building units and the transition behavior from mesocrystals to single crystals were also studies in detail by changing of concentration and reaction time. These phenomena provide a directed evidence for the mesoscopic transformation which not only exists in the biomimetic mineralization process and copolymer mediated reactions, but also is a common pathway for the liquid-phase growth of metallic crystals. The growth mechanism related studies offer considerable insight into the oriented aggregation-based crystallization of nanoparticle building units, meanwhile, provide theoretical preparations for the further researches of physical and chemical properties of mesocrystals.( 2 ) The Ag,Au amorphous phase formation in galvanic replacement and the double-interface mode. Within Zn/AgNO₃,Fe/HAuCl₄ and Zn/HAuCl₄ replacement reaction, it was found that the Ag and Au amorphous phase were the common phenomenon. Namely, there were two interfaces in the front of the depositing Ag or Au crystals. One was the interface of solution and amorphous phase; the other was the interface of amorphous phase and crystal. Within the two interfaces, amorphous phase and the nanoparticles in situ crystallized can be observed. These indicated that the reduced Ag,Au atoms deposited as the amorphous feature. Then the unstable amorphous phase in situ crystallized and experienced a grain rotation and realignment process, finally formed to be the single crystal of Ag or Au. Based on above observations, we put forward a novel double interface growth mode. The mocelular dyniamc(MD)simulation indicated that the coordinately operations originated from lattice mismatch between two elements and water interactions with silver atoms were responsible for the formation of silver amorphous precursor phase. The interactions among different crystallized nuclei contributed to the above grains rotation and realignment processes and also the crystallization of surrounding amorphous phase. Thus finally formed to be the single crystal. This study not only enriches the classical crystal growth theories, but also provides a significant theoretical reference for further adjusting nanomaterials structures and designing the disordered structures materials as desired.(3)The high density Au,Ag nanoparticles arrays with <10nm interparticle gap. By the use of great over-potential (>50V), for the first time, the nucleation and coarsening of Au and Ag nanoislands were controlled and the nanoparticles arrays were prepared on the surface of silicon. Under the applied potential of 60¹00V, the high density Au nanoparticle arrays (>2.0×1011 cm-2) with particle size of 10nm and interparticles gap < 10nm were obtained. Under the applied potential of 100V, the Ag nanoparticle arrays with particle size of 20-50nm and interparticles gap < 10nm were synthesized. The uniformity for above results is very good. Due to the"hot-spots"effect, the nanoparticles arrays with sub-10nm spacing could contribute a very significant application to the SERS properties.(4)The concentration field model. Using the basic principle of fluid dynamics and Fick's law, a concentration field model used for the electroless deposition system was developed. The measurement of concentration distribution and diffusion layer thickness for the Zn/AgNO₃ system was carried out and the theoretical results was very consistent with the experimental data. Comparing with the previous model, the model developed here can describe the growth speed at a wide scale. In addition, using the current model, the transition in macroscopic scale from fractal to DBM to dendritic pattern was explained. A protuberance effect was adopted to explain above morphological transition.(5)The SERS characteristic of flower-like Ag nanostructures. The flower-like Ag nanostructures was synthesized by changing the electrodeposition conditions. By means of the FDTD simulation and SERS measurement, the electromagnetic enhancement and SERS characteristic were investigated for the nanoparticles arrays and flower-like pattern. The FDTD simulations indicate that the high density of Ag flower-like pattern could provide a preferable electromagnetic enhancement character due to the strong"hot-spots"effect. The SERS results also validated this argument. Therefore, above results are the important reference for the study of enhancement mechanism and increasing the enhancement factor of SERS.