Special Section: Nanoscience and Nanotechnology

WE are all aware of the use of ultrasound radiation in medicine, where it is being used mostly for diagnosis, and where more recently, focused ultrasound radiation is being used to destroy cancer cells. Less is known of its application in chemistry, despite the fact that it has applications across almost the entire breadth of that field. One of the main advantages in conducting sonochemical experiments is that it is very inexpensive to get started in the field. For people willing to learn more about sonochemistry, I suggest refs 1–4. Let us first address the question of how 20 kHz radiation can rupture chemical bonds, and try to explain the role of a few parameters in determining the yield of a sonochemical reaction and the unique products obtained when ultrasound radiation is used in materials science. To a spectroscopist using 10–10 Hz radiation to break chemical bonds, this is a puzzle. How is it that 20 kHz ultrasound radiation can do the same job? A number of theories were developed in order to explain how a 20 kHz sonic radiation could break chemical bonds. They all agree that the main event in sonochemistry is the creation, growth and collapse of a bubble that is formed in the liquid. The first puzzle is how such a bubble can be formed, considering the fact that the forces required to separate water molecules into a distance of two van-der Waals radii, would require a power of 10 W/cm. On the other hand, it is well known that in a sonication bath, with a power of 0.3 W/cm, water is already converted into hydrogen peroxide. Different explanations have been offered; they are all based on the existence of unseen particles, or gas bubbles, that decrease the intermolecular forces, enabling the creation of the bubble. These theories are supported by the experimental evidence, that when the solution undergoes ultrafiltration, before the application of the ultrasonic power, there is no sonochemistry. The second stage is the growth of the bubble, which occurs through the diffusion of solute vapour to the volume of the bubble. The third stage is the collapse of the bubble, that takes place when the bubble size reaches its maximum value. According to the hot-spot mechanism, this implosive collapse raises the local temperature to 5000 K and the pressures to a few hundred atmospheres. These extreme conditions cause the rupture of chemical bonds. From the time we entered the field in 1993, we were intrigued by the fact that the products of many sonochemical reactions were in the form of amorphous nanoparticles. For example, K. Suslick, who was one of the initiators of the field, has demonstrated that the sonication of Fe(CO)5 as a neat liquid, or its solution in decalin, yielded 5–20 nanometer-sized amorphous iron particles. The reason for the amorphicity of the products is related to the high cooling rates (> 10 K/s) obtained during the collapse of the bubble, which does not allow the products to organize and crystallize. These high cooling rates result from the fast collapse that takes place in less than a nanosecond. For this reason, a sonicated solution containing a volatile solute will always lead to amorphous products. However, the reason for the nanometer-sized particles is not yet clear. The estimated size of the collapsing bubble varies from ten to a few hundred microns. In addition to the region inside the bubble, where a gas phase reaction takes place upon its collapse, a second important region is of great significance. This is the interfacial region, which surrounds the collapsing bubble. Its width is calculated to be 200 nm, and the temperature reached after collapse is 1900 K. Sonochemical reactions of nonvolatile compounds such as salts will occur in this region. In this case, the sonochemical reactions occur in the liquid phase. The products are either amorphous or crystalline nanoparticles, depending on the temperature in the ring region in which the reaction takes place. We cannot mention here all the parameters (frequency, power, gas under which the sonication takes place, pressure of the gas, etc.) that affect the sonochemical yield and rate, and so we will address ourselves to that one important parameter, of temperature. The equation of an adiabatic implosion is