Plasmon absorption band

Michaelis and Henglein [131] prepared Pd-core/Ag-shell bimetallic nanoparticles by the successive reduction of Ag ions on the surface of Pd nanoparticles (mean radius 4.6 nm) with formaldehyde. The core/shell nanoparticles, however, became larger and deviated from spherical with an increase in the shell thickness. The Pd/Ag bimetallic nanoparticles had a surface plasmon absorption band close to 380 nm when more than 10-atomic layer of Ag are deposited. When the shell thickness is less than 10-atomic layer, the absorption band is located at shorter wavelengths and the band disappears below about three-atomic layer. 

In a more simple and cheap way, silver clusters can be prepared in aqueous solutions of commercially available polyelectrolytes, such as poly(methacrylic acid) (PM A A) by photo activation using visible light [20] or UV light [29]. Ras et al. found that photoactivation with visible light results in fluorescent silver cluster solutions without any noticeable silver nanoparticle impurities, as seen in electron microscopy and from the absence of plasmon absorption bands near 400 nm (F = 5-6%). It was seen that using PMAA in its acidic form, different ratios Ag+ MAA (0.15 1-3 1) lead to different emission bands, as discussed in the next section (Fig. 12) [20]. When solutions of PMAA in its sodium form and silver salt were reduced with UV light (365 nm, 8 W), silver nanoclusters were obtained with emission band centered at 620 nm and [Pg.322]

Adsorption of ions or molecules on metal clusters markedly affects their optical properties. It was shown that the intensity and the shape of the surface plasmon absorption band of silver nanometric particles, which is close to 380 nm, change upon adsorption of various substances [125]. The important damping of the band generally observed is assigned to the change of the electron density of the thin surface layer of the... 

Irradiation with 7-rays was also used to synthesize bimetallic nanoparticles. Remita et al. synthesized poly(vinyl alcohol) (PVA)-stabilized Ag/Pt bimetallic nanoparticles by radiolysis of an aqueous mixture of Ag2S04 and K2PtCl4 at a concentration of 10-4 mol dm-3. A typical Ag plasmon absorption band is observed at —410 nm with only low intensity at the mole ratio of Ag Pt = 60 40, indicating the formation of Ag/Pt bimetallic nanoparticles. Polyfacrylic acid) was also used as the stabilizer, although the resulting UV-Vis spectra were quite different. 

The reaction described by Eq. (25) occurs in the millisecond time scale and results in an exponential increase in the conductivity of the solution and a parallel decrease of the absorbance at 440 nm. The net result of the reaction was both a blue shift and a narrowing of the silver plasmon absorption band (see 0 -> a change in Fig. 83) [506]. 

Fig. 83. Surface plasmon absorption band of a 1 x 10 4moldm-3 silver sol (O). Changes in absorption after electron donation (a) and positive hole injection (fc) by free radicals [506]...

Hole injection into the silver particles was accomplished by allowing OH (formed in the pulse radiolysis of N20-saturated, aqueous, 3.0-nm-diameter Ag particle solution (Eqs. 22,23) in the absence of the -OH scavenger, 2-propanol) to extract electrons from the surface of colloidal, metallic silver particles. The process resulted in a red shift, broadening, and a decrease in intensity of the silver plasmon absorption band (see 0 - b change in Fig. 83) [506]. Addition of silver ions to metallic silver colloids elicited a similar change in the absorption spectrum [506]. 

Alteration of the silver plasmon band spectrum upon electron and hole injection has been rationalized in terms of changes in the density, Ne, and conductivity, o, of the electron gas in the metal particles as described by Eqs. (16)—(18) [506]. Thus, a decrease in Ne by electron extraction from the metallic silver particles increases Xc (Eq. 16) and thereby shifting the absorption maximum (Eq. 15) of the plasmon band to a longer wavelength (Fig. 83). A decrease in Ne also decreases a (Eq. 18), which leads, in turn, to an increase of w (Eq. 17) that is, to an increase in the bandwidth of the plasmon band absorption (Fig. 83). Similarly, the increase in Ne by electron transfer to the silver colloids is paralleled by a decrease in Xc (Eq. 16) and, hence, by a decrease in Xm (Eq. 15), as seen by the shift of the plasmon absorption band to a shorter wavelength (Fig. 83). Electron donation to the silver particles also causes an increase in cr (Eq. 18)... 

Electron transfer was mediated by metallic silver colloids whose surfaces contained either a strong (SH ) or a weak (CN ) nucleophile [531]. The former case is illustrated by changes in the absorption spectrum of a 1.0 x 10 4 M, deaerated solution of metallic silver particles, subsequent to the consecutive addition of 2.0 x 10 4 M NaSH and 3.0 x 10-4 M anthracene quinone sulfonic acid, AQS (Fig. 85) [506]. The origin of the intensity decrease and the broadening of the silver plasmon absorption band upon the addition of nucleophilic SH is incompletely understood. However, that an absorption... 

Fig. 85. Plasmon absorption band before and after addition of NaSH, and after subsequent addition of AQS. The solution was finally exposed to air [506]...

Annealing the particulate film (transferred to a quartz substrate) at 140 °C for 10 min led to the development of colloidal gold particles showing a characteristic plasmon absorption band with a maximum at 560 nm (Fig. 94) [110]. 

Nanosized particles (5-10 nm in diameter for silver) possessing metallic properties, ie. conducting electricity and having sharp plasmon absorption bands. 

Formation of zero-valent copper clusters with glutathione has been studied anaerobically. The ligand has been reported to provide a snbstantial degree of surface passivation. Rednction of a Cu(II)-GSH complex prodnced nanoparticles with a plasmon resonance band at 363 nm. These nanoparticles possessed a diameter of 9.7 4.3nm as demonstrated by TEM analysis. Under aerobic conditions, the nanoparticles oxidatively degrade as evinced by the loss of the plasmon absorption band over time. 

Silver hydrosols (10 ml) with chloride anions were filtered by using ANODISC alumina filters (0.02 mm diameter) and a membrane filter holder supplied by Whatman Intern. Ltd. (England). After two successive filtrations, the liquid was free of metal particles, as detected by the total absence of plasmon absorption bands in the UV-vis region. The filter surface resulted coated by a layer of colloidal silver. 

The curves 1-3 in Fig. 1 evidently show that for the two-layer silver-copper systems there is a strong optical density Increase over the spectral range X. 600-1200nm that overlaps the copper surface plasmon absorption band. 

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