For the substrate immersed vertically into the precursor solution, branched ZnO nanowires with wurtzite crystal structure grow radially and form a flower shape check details on each of the Si backbones. The morphology of the product prepared by immersing

the substrate facedown into the reaction solution is the same as that of the former case, and both seem to possess an identical growth speed as the length of ZnO nanowires is similar. Nevertheless, for the third case with a faceup direction, the ZnO nanowire arrays disappear on the Si backbones. The Si nanowires tend to bundle up and their surface becomes much rougher in contrast to the Si nanowires with seed layer in Figure 1f. It is well known that water molecules run violently at high temperature, which may cause deformation

of adjacent nanowire tips into clusters for reducing the total energy. Meanwhile, the condensation of the ZnO nanoparticles from the growth solution results in the rough surface of the Si nanowires. The observation indicates that the presence of gravity gradient is a key issue for the growth of ZnO nanowire arrays. Otherwise, only the condensation of the ZnO nanoparticles takes place in a form of film on the seed layer. The intrinsic mechanism possibly lies in the specific BGB324 mw character of chemical reactions in the aqueous solution as well as the thermodynamics and kinetics of ZnO growth, which is under further

exploration. Figure 5 SEM images of products prepared in different substrate directions in solution: (a) vertical, (b) facedown, and (c) faceup. The Si nanowire arrays were capped with ZnO seed layer before hydrothermal growth. It is worthwhile to point out that the seed layer is another important factor in the growth of branched ZnO nanowires. Figure 6 shows the SEM images of the products prepared by 30-min etching and 2-h hydrothermal growth but without the seed layer deposition. The substrates were also soaked in different directions relative to the solution surface during the hydrothermal growth. It is found that after hydrothermal growth, all the Si nanowire arrays exhibit original morphologies except the Cobimetinib supplier bending of the nanowires to form sheaf-like structures in some specimens. The ZnO nanowires or nanorods are also created but disperse randomly on the Si nanowire arrays surface and are removed easily by subsequent cleaning. The sheaf-like structures in Figure 6 are due to the surface tension force presence in the high-temperature solution as well as in the drying process that deforms adjacent nanowire tips into clusters. For the disappearance of ZnO nanowire branches, it is well known that the crystal structure and chemical bonds of ZnO substance are different from those of Si substance.