Vertically aligned ZnO nanowires on FTO substrates. (a) FESEM image (tilted at 15°) of a directly synthesized ZnO nanowire array on a FTO substrate. The inset is a higher-magnification image. (b) XRD pattern of the ZnO nanowires grown on the FTO substrate. The inset shows the transmission electron microscopy image of a single wire and the corresponding selected area electron diffraction pattern.