| Summary: | 碩士 === 國立成功大學 === 化學工程學系碩博士班 === 90 === In this study, palladium-silver (Pd-Ag) nanoparticles were synthesized by the reduction of Pd(NO3)2 and AgNO3 with formaldehyde in the presence of polyvinylpyrrolidone (PVP). The effects of reactant addition, reaction time, Pd/Ag molar ratio, and HCHO/metal ratio on the properties of resultant Pd-Ag nanoparticles were investigated. Furthermore, the formation mechanism of Pd-Ag particles was also studied by means of TEM, XRD, UV/Vis, and FT-IR analyses.
Experimental results show that the Pd-Ag particle size is decreased with increasing the metal ion concentration, HCHO/metal ratio, and PVP/metal ratio. As the Ag/Pd ratio is less than 70/30, it is found that the resultant particles are nanosized dispersion with Pdcore – Agshell structure. Their particle sizes are increased with increasing the Ag/Pd ratio. As the Ag/Pd is larger than 70/30, the particles are suggested to be the mixture of Pd and Ag nanoparticles which exhibit bimodal size distribution. In addition, there exists a minimum size when the PVP/metal ratio is about 10.
For the formation of Pd/Ag nanoparticles at Ag/Pd>70/30, the Pd and Ag particles are sequentially obtained via homogeneous nucleation. However, the growth of big Ag particles caused by the large Ag-Ag bond energy leads to the bimodal particle size distribution. At Ag/Pd<70/30, Ag grains are deposited on the Pd particle surface via heterogeneous nucleation, which a Pdcore-Agshell structure is formed. Additionally, a bimodal size distribution is also obtained by reduction of AgNO3 with preseeding 5.42 nm Pd nuclei. This again confirms that the Pd/Ag nanoparticles are formed by the sequential reduction mechanism under high Ag/Pd ratio.
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