Another investigation by Singh et al. examined the use of ultrasound in conjunction with photocatalysis. The researchers used a rGO-supported CdS catalyst, operated at a 20 kHz frequency with input powers ranging from 10 to 40 W, in aqueous solutions containing 4 mM and 10 mM of Na2SO3 and Na2S, respectively. Adding acoustic power to the photocatalysis system improved mass transfer of hydrogen from the catalyst surface to the bulk, leading to a 40% improvement in hydrogen generation under 20 kHz and 35 W. Consequently, this improvement was thought to be caused by the mechanical energy of ultrasound, which separates hydrogen bubbles from the photocatalyst's surface, creating more free space for reactants and reactive sites to be adsorbed. In the same study, the influence of temperature was also examined . They found that increasing the temperature from 299 K to 318 K improved sonophotocatalysis of water by 70%. Hydrogen generation increased, according to the authors , because of shockwaves and mechanical energy produced when acoustic cavitation bubbles burst.