Growth of Ultrathin Single-Crystalline IrO2(110) Films on a TiO2(110) Single Crystal.

The growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness on a single-crystalline TiO2(110) substrate is reported. The preparation starts with a deposition of metallic Ir at room temperature followed by a post-oxidation step performed in an oxygen atmosphere of 10-4 mbar at 700 K. On this surface, additional Ir can be deposited at 700 K in an oxygen atmosphere of 10-6 mbar to produce a IrO2(110) layer with variable thicknesses. To improve the crystallinity of the resulting IrO2(110) layer, the final film was post-oxidized in 10-4 mbar of O2 at 700 K for 5 min. The surface-sensitive techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO2(110) films and how these films decompose upon annealing under ultrahigh vacuum (UHV) conditions. STM provides evidence that the IrO2(110) films start already to reduce at 465 K under UHV conditions. Upon annealing to 605 K under UHV the reduction of IrO2 intensifies (XPS), but the oxide film can readily be restored by re-oxidation in 10-4 mbar of O2 at 700 K. Thermal decomposition at 725 K leads, however, to severe reduction of the IrO2(110) layer (XPS, STM) that cannot be restored by a subsequent re-oxidation step. The utility of the IrO2(110)-TiO2(110) system as model electrodes is exemplified with the electrochemical oxygen evolution reaction in an acidic environment.