Nanoparticles silver

Nie S and Emory S R 1997 Near-field surface-enhanced Raman spectroscopy on single silver nanoparticles Anal. Chem. 69 2631-5... 

Fig. 17. Variation of conductivity of Ag-starch nanocomposites with temperature. Inset shows variation with concentration of silver nanoparticles.

FIG. 9 Silver nanoparticles capped by 4-carboxythiophenol electrostatically adsorbed to positively charged octadecylamine monolayers, (a) Mass uptake versus number of layers at subphase pH 12 and pH 9 the inset shows the contact angle of water versus the number of layers, (b) Absorbance spectra as a function of the number of layers transferred (left), with the inset showing the plasmon absorbance at 460 nm versus the number of layers. Thickness versus number of layers as determined by optical interferometry is shown on the right. (Reprinted with permission from Ref. 103. Copyright 1996 American Chemical Society.)... 

The production of fatty acid-capped silver nanoparticles by a heating method has been reported [115]. Heating of the silver salts of fatty acids (tetradecanoic, stearic, and oleic) under a nitrogen atmosphere at 250°C resulted in the formation of 5-20-nm-diameter silver particles. Monolayers of the capped particles were spread from toluene and transferred onto TEM grids. An ordered two-dimensional array of particles was observed. The oleic acid-capped particle arrays had some void regions not present for the other two fatty acids. 

FIG. 12 TEM micrographs of wirelike assemblies of silver nanoparticles, (a) Octanethiol-capped silver nanoparticles of average diameter 3.4 nm deposited from hexane solution, (b) The same particles deposited from heptane solution, (c) Octanethiol-capped silver particles of average diameter 4.4 nm deposited from heptane solution. (Reproduced with permission from Ref. 130. Copyright 1998 American Chemical Society.)... 

Let us come back to the sample preparation A drop of solution containing silver nanoparticles dispersed in hexane is deposited on the substrate. The nanocrystals can be removed by washing the substrate and collected in hexane. The absorption spectrum of silver particles recorded before and after deposition remains the same. This indicates that coalescence does not take place. Similar behavior was observed by using HOPG as a substrate [6,35]. 

A specific example where heterogeneous supports provide nanoparticle size-control is the immobilization of homogeneous silver nanoparticles on polystyrene [366]. This work was extended later to the development of a one-pot method for the size-selective precipitation of silver nanoparticles on PVP-protected thiol-functionalized silica. During the immobilization of very small silver nanoclusters both the size of the silver nanoclusters and the thickness of the silver layer on the support could be controlled directly by the reaction parameters applied (Fi re 16) [367]. 

Competitive reduction of Au(III) and Ag(I) ions occurs simultaneously in solution during exposure to neem leaf extract leads to the preparation of bimetallic Au-core/Ag-shell nanoparticles in solution. TEM revealed that the silver nanoparticles are adsorbed onto the gold nanoparticles, forming a core/sheU structure. Panigrahi et al. [121] reported that sugar-assisted stable Au-core/Ag-shell nanoparticles with particles size of ca. 10 nm were prepared by a wet chemical method. Fructose was found to be the best suited sugar for the preparation of smallest particles. 

In Section 2 the general features of the electronic structure of supported metal nanoparticles are reviewed from both experimental and theoretical point of view. Section 3 gives an introduction to sample preparation. In Section 4 the size-dependent electronic properties of silver nanoparticles are presented as an illustrative example, while in Section 5 correlation is sought between the electronic structure and the catalytic properties of gold nanoparticles, with special emphasis on substrate-related issues. 

In Figure 8 [146] we present the valence band XPS and UPS spectra of the silver nanoparticles at different stages of the size reduction process. The contribution of the substrate was subtracted. The parameter at each spectrum is the measured Ag/Si ratio. 

Remarkably the position of the final plasmon peak of the alloy particles is dependent on the molar ratio of gold to silver nanoparticles. When the ratio is shifted favoring either metal, an alloy of any desired composition can be formed. This alloying phenomenon indicates that it is possible for true tuneability of the properties of a set of nanoparticles. 

In this chapter, photoelectrochemical control of size and color of silver nanoparticles, i.e., multicolor photo-chromism [1], is described. Silver nanoparticles are deposited on UV-irradiated Ti02 by photocatal5dic means [2]. Size of the nanoparticles can be roughly controlled in the photocatalytic deposition process. However, it is rather important that this method provides nanoparticles with broadly distributed sizes. The deposited silver nanoparticles are able to be dissolved partially and reduced in size by plasmon-induced photoelectrochemical oxidation in the presence of an appropriate electron acceptor such as oxygen. If a monochromatic visible light is used, only the particles that are resonant with the light are dissolved. That is, size-selective dissolution is possible [3]. This is the principle of the multicolor photochromism. 

The present technique enables light-induced redox reaction UV light-induced oxidative dissolution and visible light-induced reductive deposition of silver nanoparticles. Reversible control of the particle size is therefore possible in principle. The reversible redox process can be applied to surface patterning and a photoelectrochemical actuator, besides the multicolor photochromism. 

Thus, silver nanoparticles grow gradually during UV light irradiation (processes al-a3 in Figure 2). Nanoparticles of other noble metals such as gold, copper, platinum, and palladium can also be deposited by this method. 

Visible Light-Induced Dissolution of Silver Nanoparticles... 

Figure 1. Mechanisms of photoelectrochemical deposition and dissolution of silver nanoparticles (Ag NPs) (a) UV light-induced deposition and (b) visible light-induced dissolution.

Figure 2. UV light-induced deposition of silver nanoparticles (al-a3) and wavelength-selective visible light-induced dissolution of silver nanoparticles (bl-b3).

This process occurs only at the silver nanoparticles of which resonance wavelength is in accordance with the incident light wavelength. Therefore, size-specific dissolution is possible (processes bl-b3 in Figure 2). 

Instead of TiOg, ZnO can also be used [6]. Commercially available silver nanoparticles cast on Ti02 can be used instead of the photocatalytically deposited silver nanoparticles [6]. 

As a rutile TiOg single crystal is irradiated with UV light in a AgNOg solution, silver nanoparticles are deposited and grown gradually (Figure 3). 

When a nanoporous Ti02 film consisting of Ti02 nanoparticles is used instead of the single crystal, the extinction band of silver nanoparticles deposited by UV-irradiation is much broader. This is probably because the nanopores in the Ti02 film mold the silver nanoparticles into various anisotropic shapes [9], although direct observation of the particles in the nanopores is difficult. 

Upon irradiation with a monochromatic visible light, extinction of the silver nanoparticles deposited on a... 

Figure 3. AFM images of the silver nanoparticles on the Xi02(l 00) single crystal at the deposition times of (a) 15s and (b) 180 s. The images were recorded in a tapping mode with driving frequency of 110-150 kHz at a scan rate of 1 Hz by using a silicon cantilever with a normal spring constant of 15Nm (SI-DF20, Seiko instruments).

When the polydisperse silver nanoparticles are irradiated with a monochromatic light, only the nanoparticles that are resonant with the incident light are excited and the excited electrons are transferred to Ti02, giving rise to liberation of Ag. The resonant particles are thus reduced in size until they become non-resonant. Some of the electrons... 

Figure 5. Height of the silver nanoparticles plotted as a function of their lateral diameter, determined by extended particle analysis for the AFM image (silver deposition time was 180 s). The broken line is for perfect spheres (height/diameter = 1).

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