Sonochemical reduction

Other one-pot preparations of bimetallic nanoparticles include NOct4(BHEt3) reduction of platinum and ruthenium chlorides to provide Pto.sRuo.s nanoparticles by Bonnemann et al. [65-67] sonochemical reduction of gold and palladium ions to provide AuPd nanoparticles by Mizukoshi et al. [68,69] and NaBH4 reduction of dend-rimer—PtCl4 and -PtCl " complexes to provide dend-rimer-stabilized PdPt nanoparticles by Crooks et al. [70]. 

Vinodgopal et al. prepared Pt/Ru bimetallic nanoparticles by sonochemical reduction of Pt(II) and Ru(III) in aqueous solutions. TEM images indicated that sequential reduction of the Pt(II) followed by the Ru(III) produced Pt-core/Ru-shell bimetallic nanoparticles. In the presence of sodium dodecyl sulfate (SDS), as a stabilizer, the nanoparticles had diameters between 5 and 10 nm. When PVP was used as the stabilizer, the rate of reduction is much faster, giving ultrasmall bimetallic nanoparticles of ca. 5nm diameter [141]. 

Although various techniques have been reported, sonochemical reduction technique for the synthesis of metal nanoparticles in an aqueous solution are reviewed in this chapter. 

Sonochemical reduction technique using reductants formed from hot cavitation bubbles Au, Pd, Pt, Ag, Ru, Au/Pd [9]... 

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],... 

It has been reported that the sonochemical reduction of Au(III) reduction in an aqueous solution is strongly affected by the types and concentration of organic additives. Nagata et al. reported that organic additives with an appropriate hydro-phobic property enhance the rate of Au(III) reduction. For example, alcohols, ketones, surfactants and water-soluble polymers act as accelerators for the reduction of Au(III) under ultrasonic irradiation [24]. Grieser and coworkers [25] also reported the effects of alcohol additives on the reduction of Au(III). They suggested that the rate of the sonochemical reduction of Au(III) is related to the Gibbs surface excess concentration of the alcohol additives. 

It is also observed in Fig. 5.3 that Pd(II) ions are partly adsorbed on AI2O3 before ultrasonic irradiation the concentration of Pd(II) just before irradiation becomes ca. 0.8 mM, although 1 mM Pd(II) was added in the sample solution. From a preliminary adsorption experiment, the rate of Pd(II) adsorption on A1203 was found to be slow compared with those of Pd(II) reduction in the presence of alcohols. Therefore, it is suggested that the sonochemical reduction of Pd(II) in the presence of alcohols mainly proceeds in the bulk solution. The mechanism of the Pd/Al203 formation is also described in the section of sonochemical synthesis of supported metal nanoparticles. 

Figure 5.7 shows the effects of the distance from the oscillator to the bottom of the reaction vessel on the rate of Au(III) reduction, where the distance is changed from 3.5 to 4.5 mm [29]. It is clear that the rates of reduction are affected by the position of the reaction vessel. The rate of reduction became the maximum at a distance of ca. 3.8 mm. This was almost the same as the half-wavelength of the ultrasound (3.71 mm) used in this study. It is suggested that ultrasound is effectively transmitted into the reaction vessel at 3.8 mm distance. It should be noted that the position of the reaction vessel sensitively affects the efficiency of the sonochemical reduction. 

The sonochemical reduction of Au(III) has been investigated under Ar in the presence of 20 mM 1-propanol at different frequencies, where two types of ultrasound irradiation systems were used one is a horn type sonicator (Branson 450-D, frequency 20 kHz, diameter of Ti tip 19 mm) and the other is a standing wave sonication system with a series of transducers operating at different ultrasound frequencies (L-3 Communication ELAK Nautik GmbH, frequency 213, 358, 647, and 1,062 kHz, diameter of oscillator 55mm) [33]. All experiments were performed at a constant ultrasound intensity ((0.1+/—0.01 W mL-1), which was determined by calorimetry. 

Figure 5.8 shows the rates of sonochemical reduction of Au(III) at five different frequencies [33]. It can be seen that the rate of reduction increases from 20 to 213 kHz and then decreases with increasing frequency (213 > 358 > 647 > 1,062 kHz). 

The rate of Au(ffl) reduction should have a correlation with the cavitation efficiency at these frequencies. Therefore, the result of Fig. 5.8 suggests that maximum amounts of reductants are sonochemically formed at 213 kHz in the presence of 1-propanol. The existence of an optimum frequency in the sonochemical reduction efficiency would be explained as follows. As the frequency is increased, the number of cavitation bubbles can be expected to increase. This would result in an increase in the amount of primary and secondary radicals generated and an increase in the rate of Au(HI) reduction. On the other hand, at higher frequencies there may not be enough time for the accumulation of 1-propanol at the bubble/solution interface and for the evaporation of water and 1 -propanol molecules to occur during the expansion cycle of the bubble. This would result in a decrease in the amount of active radicals. Furthermore, the size of the bubbles also decreases with increasing frequency. These multiple effects would result in a very complex frequency effect. 

The effects of various parameters on the rates of sonochemical reduction of metal ions were described in the previous sections. From this section, the effects of such parameters on the properties of metal nanoparticles are described in relation to the rates of reduction. 

To investigate how cavitation bubbles affect the sizes of the formed metal nanopartilces, sonochemical reduction of Au(III) was carried out under various irradiation conditions in an aqueous solution containing only a small amount of... 

By using sonochemical reduction processes, supported metal nanoparticles on metal oxides such as Au/Si02, Au/Fe203, Pd/Fe203, Pt/Ti02, etc. can be synthesized [38 -1],... 

It has been reported that bimetallic nanoparticles with core/shell structure can be prepared by ultrasonic irradiation. Mizukoshi et al. reported the formation of bimetallic nanoparticles of Au core/Pd shell structure [42,43] from the sonochemical reduction of Au(III) and Pd(II), where the stepwise reduction of metal ions was observed to proceed during ultrasonic irradiation. That is, the reduction of Pd(II) started after the reduction of Au(III) finished. Vinodgopal et al. reported... 

To understand a heterogeneous synthesis process, the following preliminary experiments were performed in a homogeneous solution (1) In the absence of zeolite, the sonochemical reduction of [Pd(NH3)4]2+ to Pd° was found to occur, but the rate of [Pd(NH3)4]2+reduction was much slower than that of [PdCL(]2 reduc-tion. (2) It was confirmed that 2-propanol acted as a precursor to form reductants under ultrasonic irradiation. 

By using other templates, the size of metal nanoparticles can be also controlled. Chen et al. reported the sonochemical reduction of Au(III), Ag(I) and Pd(II) and synthesis of Au, Ag and Pd nanoparticles loaded within mesoporous silica [48,49]. Zhu et al. also reported the sonochemical reduction of Mn04 to Mn02 and synthesis of Mn02 nanoparticles inside the pore channels of ordered mesoporous cabon [50]. Taking into account these reports, the rigid pore of inorganic materials can be used as a template for the size controlled metal nanoparticle synthesis even in the presence of ultrasound. 

Compared to inorganic materials, organic materials such as polymers, surfactant molecules and micelles also act as a capping material or soft template. Figure 5.15 shows TEM images of gold nanorods and nanoparticles synthesized by sonochemical reduction of Au(I) in the presence of cetyltrimethylammonium bromide,... 

As described in this chapter, the sonochemical reduction technique appears to be a promising method for the preparation of various types of metal nanoparticles in an aqueous solution. By choosing more efficient organic additives, easily-reducible metal precursors, supports and templates with an appropriate role, more advanced functional nanoparticles could be synthesized successfully using the sonochemical reduction technique. In future, it is also possible to develop effective synthetic methods by combining the sonochemical method with other chemical methods. 

Grieser L, Hobson R, Sostaric J, Mulvaney P (1996) Sonochemical reduction processes in aqueous colloidal systems. Ultrasonics 34 547-550... 

Mizukoshi Y, Takagi E, Okuno H, Oshima R, Maeda Y, Nagata Y (2001) Preparation of platinum nanoparticles by sonochemical reduction of the Pt(IV) ions role of surfactants. Ultrason Sonochem 8 1-6... 

Mizukoshi Y, Sato K, Konno TJ, Masahashi N, Tanabe S (2008) Magnetically retrievable palladium/maghemite nanocomposite catalysts prepared by sonochemical reduction method. Chem Lett 37 922-923... 

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. 

Sonochemical reduction processes of Pt(IV) ions in the presence of anionic, cationic or non-ionic surfactants was investigated by Mizukoshi et al. [38]. During the processes, the color of the aqueous solution containing H2PtCl6 and surfactants... 

Fig. 6.11 Absorption spectra showing the simultaneous sonochemical reduction of an aqueous solution of 1 mM RUCI3 and 1 mM K PtCLi containing 0.1 M HC104,...

Nemamcha A, Rehspringer JL, Kharmi D (2006) Synthesis of palladium nanoparticles by sonochemical reduction of palladium(II) nitrate in aqueous solution. J Phys Chem B 110 ... 

Salkar et al. [51] reported the formation of amorphous silver nanoparticles of approximately 20 nm size by the sonochemical reduction of an aqueous solution of silver nitrate in an atmosphere of argon-hydrogen [in the ratio of 95 5] at 10°C. Formation of silver nanoparticles, according to them was through the generation of radical species - as a primary reaction. 

Lippitt B, McCord JM, Fridovitch IJ (1972) The sonochemical reduction of cytochrome c and its inhibition by superoxide dismutase. Biol Chem 247 4688 1690... 

Sonochemical reduction of aqueous solutions containing ammonium dichromate and potassium permanganate produced ultrafine powders of Cr203 and... 

Sonochemical reduction of permanganate to manganese dioxide the effect of H202 formed in the sonolysis of water on the rates of reduction Kenji O, Masaki I, Ben Nishimura Rokura N, Yasuaki M (2009) Ultrason Sonochem 16(3) 387—391... 

Although the single bubble experiment in Fig. 14.10b and the aforementioned multi-bubble work of Didenko et al. does support the hypothesis that thermal conductivity is a defining parameter of SL emission intensity, an alternative explanation attributes the trend in multi-bubble systems to the gas solubility, rather than the thermal conductivity. If the SL data from Fig. 14.9 is re-plotted as a function of the gas solubility, as shown below in Fig. 14.11, a very good correlation is found. This explanation is supported by several studies by Okitsu et al. [42, 59]. They found sonochemical activity to obey the same trend for the rare gases as for thermal conductivity, SL luminosity and temperature, as described above. This is evident in Fig. 14.12, which shows the sonochemical reduction of Au(III) to colloidal gold as a function of sonication time for different gas atmospheres. 

This mechanism is the same as that implicated in the sonochemical reduction of metal salts to metal colloids. 

Sostaric et al. [67] also found that dissolution of CdS could be achieved through sonochemical reduction of the sulphur by hydroxyl radicals, hydrogen peroxide and superoxide ... 

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