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- Nanoparticle formation (3) (entfernen)
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The discussion of the surface-enhanced Raman scattering (SERS) spectra of p-aminothiophenol (PATP) and of ist photocatalytic reaction product 4,4′-dimercaptoazobenzene (DMAB) is important for understanding plasmon-supported spectroscopy and catalysis. Here, SERS spectra indicate that DMAB forms also in a nonphotocatalytic reaction on silver nanoparticles. Spectra measured at low pH, in the presence of the acids HCl, H2SO4, HNO3, and H3PO4, show that DMAB is reduced to PATP when both protons and chloride ions are present. Moreover, the successful reduction of DMAB in the presence of other, halide and nonhalide, ligands suggests a central role of these species in the reduction. As discussed, the ligands increase the efficiency of hot-electron harvesting. The pH-associated reversibility of the SERS spectrum of PATP is established as an Observation of the DMAB dimer at high pH and of PATP as a product of its hot-electron reduction at low pH, in the presence of the appropriate ligand.
Although metallic nanoparticles play an important role in the area of nanotechnology, a coherent mechanistic explanation for the evolution of the particles during their chemical synthesis has not yet been provided in many cases. To gain a profound understanding of the growth mechanism of colloidal nanoparticles, new approaches using Small Angle X-Ray Scattering (SAXS) combined with X-ray absorption near-edge structure (XANES) are presented. This combination allows for insights into two prominent syntheses routes of gold nanoparticles (GNP): The 'slow' reaction using sodium citrate (30-90 min) as a reducing agent and the 'fast' reaction employing NaBH4 (within few seconds). In the first case data derived with the coupled XANES and SAXS suggests a four-step particle formation mechanism. For the second system a time resolution in the order of 100-200 ms was achieved by coupling a common laboratory SAXS instrument with a microstructured mixer, which allows data acquisition in a continuous-flow mode. The results indicate a coalescence driven growth process. Based on the capabilities to deduce the size, number and polydispersity of the particles, the results of both methods enable the development of mechanistic schemes explaining the different phases of particle formation and growth, thus providing a basis for improved control over the synthesis processes.
In this paper experimental temperature and density maps of the laser induced plasma in water during Pulsed Laser ablation in Liquid (PLAL) for the production of metallic nanoparticles (NPs) has been determined. A detection system based on the simultaneous acquisition of two emission images at 515 and 410 nm has been constructed and the obtained images have been processed simultaneously by imaging software. The results of the data analysis show a variation of the temperature between 4000 and 7000 K over the plasma volume. Moreover, by the study of the temperature distribution and of the number densities along the plasma expansion axis it is possible to observe the condensation zone of the plasma where NPs can be formed.
Finally, the time associated to the electron processes is estimated and the plasma charging effect on NPs is demonstrated. The set of observations retrieved from these experiments suggests the importance of the plasma phase for the growth of NPs and the necessity of considering the spatial distribution of plasma parameters for the understanding of one of the most important issues of the PLAL process, that is the source of solid material in the plasma phase.