Page 154 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 8
8.3 Results and Discussions
Figure 8.2(a) shows a typical overall spectrum recorded during the plasma-electrochemical synthesis of Ag nanoparticles from the Tollens’ reagent. In the spectrum the main features correspond to the argon atomic transitions (Ar I) between highly excited electronic states (4p→4s) in a wavelength region of 680-1000 nm. Diatomic emission band of OH radicals with a steep peak starting at 305 nm and falling off towards 325 nm is also clearly detected. In addition, emission lines of atomic hydrogen (657 nm) and oxygen (777 nm) radicals are observed, suggesting the splitting of water molecules and the generation of reactive species in the plasma. It should be noted that, less intensive lines appear at the wavelength of 328 nm and 338 nm, which can be attributed to the resonances peaks of Ag atoms, suggesting the reduction of Ag(NH3)2+ to Ag atoms. Figure 8.2(b) shows a high-resolution emission spectrum in the wavelength range of 270-350 nm. The dominant spectral feature is the OH transition (3064 Å system) related to the dissociation of H2O molecules. Sharp emission lines of Ag atoms (Ag I) are also clearly observed, which further confirms the generation of Ag atoms.
Figure 8.2 (a) Overall and (b) high-resolution optical emission spectra recorded during the synthesis of Ag nanoparticles
After plasma exposure, both solutions of Ag(NH3)2+ and Ag+ precursors showed the appearance of the surface plasmon resonance peaks of spherical Ag nanoparicles around 400 nm in the UV-Vis absorbance spectra (Figure 8.3). The solution with dispersant shows higher absorbance. Additionally, the absorbance of Ag nanoparicles from Ag(NH3)2+ was much higher than that from Ag+, indicating more Ag nanoparicles were generated at the same reaction time. This could be explained by the difference of their reaction kinetics. According to the literature, solvated electrons (e-eq) supplyed from plasma column are consumed by two main competing reactions, namely the direct reduction of positive ions Ag(NH3)2+/Ag+ and dissosiative attachement to water molecules, as it is shown below:10
Ag(NH3)2+ + e-eq → Ag + 2NH3 Ag+ + e-eq → Ag
k1a=8×1010M/s (8.1a) k1b=3.3×1010M/s (8.1b)
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