ABSTRACT
Synthesis of advanced nanomaterials using acoustic cavitation, often referred to as sonochemical route, has received considerable interest in the past few decades. In this chapter, a brief description of the preparation of some of the newly emerging nanomaterials has been presented. The sonochemical route is by far the most reliable and efficient preparative technique, primarily owing to its ability to concentrate the acoustic energy in a smaller volume. The implosion of cavitation microbubbles results in the formation of millions of short-lived, localized hot spots that have an extreme temperature of
thousands of Kelvin and pressures of few thousands of atmospheres. Consequently, the fabrication of an array of nanostructured materials that differ distinctly from their bulk counterparts has been promising with this powerful cavitation approach. There are also other forms of nanomaterials such as nanowires and nanospheres that have been successfully prepared by this sonochemical approach. A proper selection of ultrasonic irradiation parameters influences the satisfactory formation of nanomaterials. In short, this chapter will therefore be confined to discussing the recent developments of the ultrasound-mediated synthesis of various multifunctional nanomaterials and the involved preparative strategies. 1.1 IntroductionRecent advancements in the area of nanomaterials have led to many promising applications in the fields extending from solid oxide fuel, superconductors and lithium batteries, catalysis, cosmetics to pharmaceuticals. Materials in the nanometer scale often demonstrate distinctive properties that are different from their bulky counterparts, predominantly owing to their unusual and extremely large specific surface area and surface-to-volume ratio. The magnetic, electronic, optical, and chemical properties vary significantly once the size of materials becomes smaller and smaller, and reduced to nanosized regime. The morphology also differs since the shapes of the nanoparticles alter. As far as the impact of innovations in the nanotechnology is concerned, an exponential growth of research interest and endeavors in the design and preparation of novel nanomaterials with notable physical and chemical properties has been observed in the past decade. Nanostructured materials have been synthesized through a diverse known chemical methods, including sol-gel, micelle and inverse micelle, hydrothermal, solvothermal, chemical precipitation, direct oxidation, chemical vapor deposition, physical vapor deposition, electrodeposition, and microwave-assisted hydrothermal techniques. Nonetheless, the design and manufacture of nanomaterials together with tunable particle size and morphology toward a specific application yet remain an onerous and challenging task to most of the scientists working in the area of nanotechnology. Furthermore, it is the selection of an appropriate synthetic route that determines the success rate of
nanomaterial synthesis, where the physical and chemical properties of nanostructured materials are strongly dependent upon their preparation techniques. Among a wide range of approaches, the utilization of high intensity ultrasound for the synthesis of materials has been considerably investigated in the past decade and emerged to be one of the most efficient, powerful, and versatile synthetic tool for the nanosized compounds that are often not accessible by the conventional methods. The first application of ultrasound irradiation to nanomaterial synthesis dates back to 1966 where CdS thin film deposition as solar cell was reported (Chamberlin and Skarman, 1966). Nanostructured metals were first prepared by Suslick and his coworkers where they successfully produced amorphous nanostructured iron metal and colloidal iron nanoparticles with the size ranging from 10 to 20 nm by treating a nonaqueous solution of Fe(CO)5 in the presence of ultrasound (Suslick et al., 1983). The synthesized iron nanoparticles showed narrow size distribution and were found to be superparamagnetic. Ever since rigorous efforts have been made with ultrasound, which has set off novel research prospects and agenda for the synthesis of myriad types of nanostructured materials, an array of induced effects of ultrasound on the nanomaterial synthesis can be ascribed to the acoustic cavitation. Cavitation is the growth of pre-existing gas nuclei to micron-sized bubbles and the implosive collapse of these bubbles in a liquid. Acoustic cavitation creates severe physical and chemical conditions within the collapsing bubble and serves as the impetus of a vast majority of sonochemical reactions in the liquid-liquid or liquid-solid systems. Cavitation induced by ultrasound waves imparts a unique molecular level interaction between energy and chemical species via the generation of millions of short-lived, localized hot spots, which have equivalent temperatures of approximately 5000 K, pressures of about 1000 bar, and heating and cooling rates above 1010 K/s, during the inertial bubble collapse phase. These extreme conditions can result in highly concentrated energy to accelerate and augment the chemical and biological reactions that are generally not viable to be achieved by the traditional methods, enabling the synthesis of a wide range of unusual nanomaterials (Suslick, 2001). When the intensity of ultrasound is higher than the threshold intensity, the primary physical phenomena associated with the
cavitational implosion of the bubbles that are relevant to material synthesis are shock waves and microstreaming. It is well recognized that the violent and implosive collapse provides a couple of strong physical effects outside the microbubbles: disruptive forces, liquid microjets, and shock waves that significantly lead to an enhanced mass transport, improved chemical reaction rates, decrease in the diffusion layer thickness and effective cleaning, and degassing action in the case of an electrode surface. In addition, the generation of free radicals from different species, and in particular the formation of free radicals by the sonolysis of water, has also been explored. This chapter strives to summarize the preparation techniques and recent developments in the applications of ultrasound in the synthesis of nanostructured materials. We focus on the recent progress in the synthesis, properties, modifications, and applications of nanomaterials. The synthesis of nanomaterials, including nanoparticles, nanorods, nanowires, and nanopowders, is primarily categorized with the synthetic routes. For detailed information, the readers are advised to refer to the corresponding literature. 1.2 Bimetallic Nanoparticles
1.2.1 Copper-Silver Core-Shell Nanoparticles Cu-Ag core-shell nanoparticles were prepared by a sonoelectrochemical route followed by a displacement reaction (Mancier et al., 2010). In this case, the experimental setup essentially composed of a homemade galvanostat/potentiostat and an ultrasonic horn with tunable power from 7 W to 100 W. The generation of nanopowder was controlled by three electrodes electrochemical setup using the sonotrode as working electrode. The reference electrode was a saturated mercury sulfate electrode, while copper wire acted as a soluble anode. The reference electrode was placed in a separate beaker to avoid any deterioration by ultrasonic waves (Fig. 1.1). TEM observations revealed that the sonoelectrochemically produced powder consists mainly of isolated particles with different nanometric diameters but also with some small agglomerates (Fig. 1.2). However, these agglomerates were avoided by using surfactants.
