Growth and characterization of vertically aligned self-assembled IrO ₂ nanotubes on oxide substrates

The self-assembled vertically aligned iridium dioxide nanotubes (IrO ₂ NTs) have been successfully grown on sapphire (SA)(1 0 0) and LiNbO₃ (LNO)(1 0 0) substrates, via metalorganic chemical vapor deposition (MOCVD), using (MeCp)Ir(COD) as the source reagent. The surface morphology and structural properties of the as-grown NTs were characterized in detail using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected area electron diffractometry (SAD), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). FESEM images and XRD patterns revealed that the well-aligned and single-crystalline NTs were grown normal to the substrates. TEM and SAD measurements showed that the hollow IrO₂ NTs with square cross section have open-end morphology and long axis directed along the [0 0 1] direction. Analysis of the growth pattern indicated that the IrO₂ NTs on SA(1 0 0) and LNO(1 0 0) grow with the orientation relationship given by IrO₂(0 0 1)// SA(1 0 0), IrO ₂[1 0 0]// SA[0 1 0] and IrO₂(0 0 1)// LNO(1 0 0), IrO₂[1 0 0]// LNO[0 1 0], respectively. XPS spectra show the existence of a higher oxidation state of iridium in IrO₂ NTs. The probable mechanism for the formation of the vertically aligned NTs is discussed.