本研究以硝酸銀、硝酸鈀為前驅鹽，甲醛為還原劑，PVP為保護劑，利用化學還原法製備銀鈀奈米微粉，文中探討反應液添加順序、反應時間、前驅鹽濃度、銀鈀比例、保護劑比例與還原劑比例等製備變因對銀鈀微粉之影響，並以TEM，XRD，UV/Vis及FT-IR等特性分析，探討統銀鈀微粉之生成機制。

實驗結果顯示，金屬前驅鹽、還原劑及PVP保護劑之濃度增加時，所得之微粉粒徑會有降低之趨勢。當銀鈀比例Ag/Pd <70/30時，所得為單佈性Pdcore-Agshell型奈米級微粉，且隨銀比例之增高，平均粒徑增大；但當Ag/Pd >70/30時，所得為雙佈性之銀鈀混合微粉。另外，當PVP/金屬比例為10時，所得微粉之粒徑為最小。

在銀鈀微粉之共還原機制方面，由於銀離子還原電位較鈀為小且鍵結能較高，因此在高銀鈀比例Ag/Pd >70/30時，鈀銀分別依序析出，且因銀易聚集成較大微粒，而形成雙佈性之微粉。當銀鈀比例Ag/Pd <70/30時，溶液中之銀離子便以先行析出之鈀奈米微粒為核種，包覆其上而生成核-殼型鈀銀奈米微粉。另外，以5.42 nm之鈀奈米微粒為核種，再加入硝酸銀等反應物，結果發現所得亦為雙佈性之微粉。因此，可印證鈀銀在高銀鈀比例時，係依序還原而出。

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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