Dispersed TiO₂ particles form an adsorption equilibrium on the surface of a cathode in the electrolyte. Equilibrium is strongly affected by the surfactant concentration, which is related to the zeta potential of dispersed TiO₂ particles. Particles having a large zeta potential disperse stably without aggregation, because the repulsive force between particles is large. Particles having a small zeta potential aggregate and produce rough particles, decreasing the number of particles, that collide with and are adsorbed on the cathode. Rough particles easily desorbed from the cathode because of their weight, decreasing the amount of codeposited particles.