The Research On Preparation And Growth Mechanism Of Branched ZnO-TiO₂ Nanorod Arrays

ZnO is a novel direct and wide band-gap II-IV compound semiconductor material. One-dimensional ZnO nano-materials have a wide range of applications in the field of nano-devices due to its unique physical and chemical properties. One dimensional ZnO nanarod array can substitute traditional TiO₂ nanocrystalline as photo anodes applicant in dye sensitized solar cells, and improve its light harvesting and module efficiency. This paper is focus on the preparation of branched ZnO-TiO₂ nanorod array based on one dimensional ZnO nanorod array. The morphologies and microstructures are characterized with X-ray diffraction (XRD) and field emission scanning electron microscopy (SEM). The growth mechanism is also studied.ZnO seed layer film is prepared on FTO substrates using vertical pulling method. ZnO nanorod array is prepared by chemical bath deposition, branched TiO₂ nano structure is prepared by reactive magnetron sputtering. The morphologies and microstructures is characterized, and the conclusion are as follow:(1) The crystalline of seed layer under 300℃annealing is random-orientation network structure. Under 400℃and 450℃annealing, the oriented crystalline has formed, which reduce the lattice mismatching, lead to higher density of nuclei on the seed surface and thus the preferred crystal growth of ZnO nanorod array perpendicular onto seed layer.(2) ZnO nanorod array prepared by chemical bath deposition method is ZnO wurtzite structure. Diffraction peak at (002) plane shown the nanorod is preferred orientation along c-axis. With the increasing of annealing temperature, growth duration and concentration of solution, the orientation of ZnO nanorod become ordered and concentrated, the diameter of nanorod also increased.(3) Branched TiO₂ were prepared by magnetron sputtering on ZnO nanorod array. rutile structure of TiO₂ is likely founded in XRD pattern, with the decrease of O2/Ar ratio. Under O2/Ar ratio was 1:1, the deposition is lower, formed core-shell structure ZnO-TiO₂ nanorod array. Under O2/Ar ratio was 1:3, branched TiO₂ crystalline is nuclear onto the side surface of ZnO nanorod.(4) The formation mechanism of branched ZnO-TiO₂ nanorod array should be dominated by a vapor-solid (V-S) mechanism. The growth model of the branched TiO₂ crystalline includes two stages. The first stage is that the initial nucleation of TiO₂ nanocrystals on the ZnO nanorods surfaces. The second stage is the TiO₂ crystalline growth along the prefer [110] direction with rutile phase after the nucleation. As the deposition continued, the vapor-like Ti⁴⁺ and O^2- species were deposited on the lower index plane of the seeds and formed a branched rutile structure TiO₂ crystalline.