ABSTRACT
In general, the polar crystals of ZnO nanostructures grow preferentially along the [0001] direction (+c axis terminated by Zn) because of the lowest surface energy of the (0002) facet, and the growth velocity along other directions is relatively low. Recently, it has been reported that the etching direction of the ZnO nanostructure is also preferential, in which the etching rate along the c axis is faster than that in the radial direction, and thus the tubular nanostructure could be formed easily. This result inspires us to evaluate whether such an etching rate difference can be used to synthesize a tubular nanostructure with nanoholes on its side walls for easy gas/dye molecule diffusion into the nanotubes. We applied a two-step strategy to fabricate the porous nanotubes: growth of ZnO nanorods at a relatively higher temperature (90°C) and subsenquent preferential etching at lower temperature (75°C) [33]. Figure 3.11a shows a typical morphology of the final product. One can see that the products have tubular structures, and the tubes show a nearly homogeneous size of about 250 nm in diameter and 500 nm in length. Careful examination reveals that there are always some scattered nanoholes on the side walls of each nanotube, which makes the nanotube presents a porous feature. The nanoholes with diameters ranging from tens to hundreds of nanometers on the side walls can be clearly seen in the inset in Fig. 3.11a. Figure 3.11b is the corresponding XRD pattern. As indexed in the pattern, all diffraction peaks match well with the wurtzite ZnO structure with the lattice constants of a = 3.250 Å and c = 5.207 Å. The much weaker intensity of the (0002) peak as compared with that in the standard Joint Committee on Powder Diffraction Standards (JCPDS) card (36-1451) provides further evidence of the tubular structure. Figure 3.11c shows a typical TEM image of the porous ZnO nanotubes, which demonstrates that the nanotubes have an outer diameter of about 250 nm and a wall thickness of about 40 nm. The SAED pattern of an individual nanotube (the inset in Fig. 3.11c) confirms that the ZnO nanotube is single crystalline. Figure 3.11d shows a typical TEM image of a nanotube wall; a nanohole (the bright area) with the size of about 40 nm in width and 60 nm in length can
be clearly seen, which further confirms the porous feature of the nanotubes. The clear lattice fringe of the HRTEM image shown in the inset of Fig. 3.1d taken from the bright area is considered to come from the edge of a nanohole because of the coexistence of the lattice fringe and the amorphous carbon film, which indicates that there is another nanohole on the other side wall of the nanotube, and the two nanoholes form a channel for the two sides of the nanotube.
