Formation of Lithium Phthalocyanine Nanotubes by Size Reduction Using Low- and High-Frequency Ultrasound

High-power ultrasound is commonly used at relatively low frequencies (e.g., 20 kHz) to disperse and modify micrometer and nanosized particles in liquids. However, our a priori hypothesis was that relatively high frequency ultrasound is capable of modifying solid particles in aqueous solutions, because cavitation bubbles have a smaller resonance size at high frequency and would be more likely to violently collapse on the interface of submicrometer-sized particles. A combination of low-frequency (22.5 kHz) followed by high-frequency (70, 354, 803, and 1024 kHz) ultrasound was used to disperse and miniaturize a microcrystalline powder of lithium phthalocyanine (LiPc), an electron paramagnetic resonance oxygen-sensitive probe, in aqueous solution. In the absence of a stabilizing agent, high-frequency sonolysis produced nanosized particles that tended to agglomerate into clusters that were larger in size than the original particles. Furthermore, all of the particles sonicated exhibited some degree of sonochemical degradation, as evidenced by color changes of the sonicated solutions. The addition of sodium dodecyl sulfate (SDS) prior to high-frequency sonolysis of LiPc suspensions had a profound effect on stabilizing individual particles in solution, thereby creating relatively monodispersed, nanosized particles in water. These particles retained their EPR activity; however, unlike the micrometer-sized LiPc particles, the nanosized LiPc particles were almost insensitive to oxygen. High-frequency ultrasound creates interesting modifications to the LiPc particles, resulting in extremely thin, rod-shaped nanotubes that are not observed following high-power, low-frequency ultrasound exposure.