Size effect of one-dimensional nanostructures on bubble nucleation in water splitting

One-dimensional (1D) nanostructures have wide applications in photocatalysts for water splitting. Their surfaces may impact the gas bubble nucleation rate, thus influencing the efficiency of gas evolution. However, the effects of the 1D nanostructured surfaces on the bubble nucleation have not been studied to date. Herein, these effects are theoretically analyzed, based on the changes of free energy for a bubble nucleus forming inside/outside a nanotube. The results show that compared to flat surfaces, the inner tube wall favors the bubble nucleation, while the outer wall has an opposite effect. These differences become increasingly significant with tube radius R_t reducing when R_t < 10r (r^∗-bubble equilibrium radius). The size effect is further verified experimentally for bubble nucleation on TiO₂ nanotube arrays. The sensitivity of bubble nucleation to nanostructure dimensions should be considered in designing high-efficient photocatalysts.