Frequency effects on the surface coverage of nitrophenyl films ultrasonically grafted onto indium tin oxide

The covalent attachment of various organic molecules on conductive supports including indium tin oxide (ITO) using diazonium chemistry has been known for many years. A commonly used method to achieve this is the electrochemical reduction of diazonium compounds leading to radicals, followed by binding of the radicals to the support. In the present report, an alternative method using ultrasound at different frequencies (20, 582, 863, and 1142 kHz) to induce the surface grafting of nitrobenzene diazonium onto an ITO surface is described. It is shown that the grafting on ITO is more efficient in the lower ultrasonic frequency range that is ascribed to changes in the balance between the physical and chemical effects of cavitation with frequency. Both the physical and chemical effects of cavitation play important roles at all frequencies, but at high ultrasound frequencies, the physical effects are relatively small. At 20 kHz, the physical component, including mass transport, is larger than at higher frequencies, and mechanisms based on these observations have been proposed. Ultrasonic grafting has an advantage over electrografting in that it may provide more control over surface coverage, thus it is suggested that the ultrasonic method is used where the surface concentration of the organic layer must be controlled.