Local pH control by electrolysis for ZnO epitaxial deposition on a Pt cathode

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 μA 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.