Analysis of the photocatalytic efficiency of ZnO–ZnO nanorods films deposited by two-step chemical methods in hydrogen generation

Thin films of ZnO nanorods were grown by chemical bath deposition over a ZnO seed layer deposited by the sol–gel method. ZnO seed layer films showed a preferred crystal orientation in the (100) plane, so nanorods also grew in this plane. In photocatalytic hydrogen reaction tests of the thin films, ZnO nanorod films yielded 3.5 times as much hydrogen (0.14 µmol µm⁻²) as ZnO seed layer films (0.04 µmol µm⁻²). The hydrogen production of each film was related to its surface roughness and specific surface area, with larger specific surface areas improving photocatalytic activity by providing more adsorption sites for redox reactions. Atomic force microscopy showed that the roughnesses of ZnO seed layer films and ZnO nanorod films were 42 and 109 nm, respectively, and that their specific surface areas were 260 and 507 µm², respectively. Moreover, the higher ratio of surface area to volume in ZnO nanorod films improved their response to light radiation relative to ZnO seed layer films by reducing the charge transfer resistance. The current density was much higher in the photoresponse of ZnO nanorod films (~0.02 mA cm⁻²) than in that of ZnO seed layer films (~0.01 mA cm⁻²).