The epitaxial deposition of an oxide semiconductor ZnO thin film was demonstrated on a Pt(1 1 1)/sapphire(0 0 0 1) cathode by local pH control in the vicinity of the cathode. The local pH near the cathode was increased by the selective reduction of NO3- ions, resulting in the deposition of a ZnO thin film only on the cathode surface by hydrolysis. The pH distribution near the cathode during electrolysis was calculated using a 1-dimensional discrete model, which is simple but concise and beneficial to expeditiously estimate an appropriate deposition condition in combination with the obtained potential-pH diagram of a Zn-H2O system. An investigation undertaken using a quartz crystal microbalance revealed that ZnO started to deposit immediately after the application of electrical current at 100 mu A cm(-2), and that it grew linearly to ca. 350 nm over 5000 s, suggesting that the film thickness can be easily controlled by electrolysis time. (C) 2011 Elsevier Ltd. All rights reserved.