where kUV þ US, kUV and kUS standing for the sonophotocatalytic, photocatalytic, and sonocatalytic rate constants, respectively al. studied the production of hydrogen in water-ethanol solutions through heterogeneous photocatalysis, sonocatalysis, and sonophotocatalysis. Using CdS nanorod arrays as a catalyst, Zhao et al. investigated the production of hydrogen using an ultrasonic-assisted photocatalytic process at various ultrasonic frequencies (21, 27, 50, and 68 kHz). Since 27 kHz is quite close to the resonance frequency of the nanorods utilized in the studies, it is likely that this is the reason why the maximum amount of hydrogen was observed at this frequency. According to the same study, the use of a catalyst in photoacoustic hydrogen production is a synergistic process. This is because the photogenerated electrons and holes are forced to move in opposite directions by the ultrasound source, which increases the density of free electrons on the catalyst surface.