Growth and characterization of vertically aligned self-assembled IrO2 nanotubes on oxide substrates

The self-assembled vertically aligned iridium dioxide nanotubes (IrO2 NTs) have been successfully grown on sapphire (SA)(100) and LiNbO3 (LNO)(100) 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 [001] direction. Analysis of the growth pattern indicated that the IrO2 NTs on SA(100) and LNO(100) grow with the orientation relationship given by IrO2(001)// SA(100), IrO2[100]// SA[010] and IrO2(001)// LNO(100), IrO2[100]// LNO[010], 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.