Synthesizing metal nanoparticles

Pioneering studies by Gardea-Torresdey et al. [28,29] reported for the first time the formation of gold and silver nanoparticles by living plants. Their study demonstrated that alfalfa plants can form gold and silver nanoparticles. Furthermore, these researchers reported that nucleation/ growth of the metallic nanoparticles took place inside the plants. This study opened new and exciting ways to synthesize metallic nanoparticles [30,31]. 

To synthesize metal nanoparticles in an aqueous solution, the reduction reactions of the corresponding metal ions are generally performed. Gutierrez et al. [21] reported the reduction of A11CI4 and Ag+ ions in an aqueous solution by ultrasonic irradiation under H2-Ar mixed atmosphere. They found that the optimum condition of these reductions was under the 20 vol% H2 and 80 vol% Ar atmosphere. Following this study, many papers reported the sonochemical reduction of noble metal ions under pure Ar atmosphere to produce the corresponding metal nanoparticles [22-28],... 

Abstract A convenient method to synthesize metal nanoparticles with unique properties is highly desirable for many applications. The sonochemical reduction of metal ions has been found to be useful for synthesizing nanoparticles of desired size range. In addition, bimetallic alloys or particles with core-shell morphology can also be synthesized depending upon the experimental conditions used during the sonochemical preparation process. The photocatalytic efficiency of semiconductor particles can be improved by simultaneous reduction and loading of metal nanoparticles on the surface of semiconductor particles. The current review focuses on the recent developments in the sonochemical synthesis of monometallic and bimetallic metal nanoparticles and metal-loaded semiconductor nanoparticles. 

Figure 7.2 Involvement of pre-synthesized metallic nanoparticles in the formation of CP-based composites...

The production of nanoparticles was done by mixing of salt and various plant sources of extracts to maintain the exact size, shape, and morphological features by varying several conditions like pH, temperature, concentration of metal salt, reaction time, reaction medium, etc. by maintaining these parameters one can synthesize metal nanoparticles in a required morphological formation (Madhumitha and Roopan, 2013). 

Solvothermal processes have been also used to synthesize metal nanoparticles other than metal oxide nanoparticles. Palladium nanoparticles of about 1 pm have been synthesized by a solvothermal route, as shown by following reaction ... 

These films can be also used to synthesize metal nanoparticles with Ag (Figure 1.296) (Krylova et al. 2009) or Au, for example, for catalytic applications. Both metal particle sizes and distributions through the films can be varied depending on reaction and treatment conditions. 

In laser pyrolysis, a precursor in the gaseous form is mixed with an inert gas and heated with CO2 infrared laser (continuous or pulsed), whose energy is either absorbed by the precursor or by an inert photosensitizer such as SFs. Swihart [84], Ledoux et al. [116,117], and Ehbrecht and Huisken [118] prepared Si nanoparticles by laser pyrolysis of silane. By using a fast-spinning molecular beam chopper, Si nanoparticles in the size range of 2.5-8 nm were deposited on quartz substrates to study quantum confinement effects [116]. Li et al. [119] improved the stability of the Si nanoparticles ( 5 nm) by surface functionalization and obtained persistent bright visible photoluminescence. Hofmeister et al. [120] have studied lattice contraction in nanosized Si particles produced by laser pyrolysis. The method has been used to synthesize metal nanoparticles as well (see Table 2.1). Zhao et al. [121] obtained Co nanoparticles by laser pyrolysis of Co2(CO)s vapor at a relatively low temperature of 44° C. Ethylene was used as a photosensitizer for CO2 laser emission. Nanoparticles... 

Dendrimer-templated synthesis of metal nanoparticles is a powerful technique and many metal DENs can be synthesized. However, the approach cannot be used to synthesize metal nanoparticles whose precursors (metal ions) have weak interactions with the dendrimers. For certain metal ions, such as Ag% another technique was developed based on galvanic redox displacement [68]. Using the galvanic redox displacement technique, Cu DENs were mixed with Ag. Since the standard reduction potential of Ag+ is more positive than Cu, Ag will be reduced to form Ag(0), and Cu(0) will be oxidized to Cu. Therefore, Cu DENs will be converted to Ag DENs using this technique. The conversion of Cu DENs to Au, Pt, or Pd DENs has also been demonstrated using the galvanic redox displacement technique [68]. 

Besides the electrochemistry route, wet chemistry methods of shape-control synthesis have also been developed in some Chinese laboratories, and the synthesized metal nanoparticles with high-energy surface were apphed mainly as electrocatalysts for alcohol oxidations. 

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