Flowerlike ZnO nanostructures (FZNs) have been deposited on Si substrate from aqueous solution by the hexamethylenetetramine (HMT)-assisted thermolysis of zinc-ethylenediamine (en) complex at low temperature (95 degrees C) and in a short time (60 min). Obtained FZNs exhibit well-defined flowerlike morphology, hexagonal wurtzite structure, and strong UV photoluminescence. The flower petals constructed by many well-aligned nanorods possess the typical tapering feature with tip sizes of 30-50 nm. Effects of en, zinc-en molar ratio, HMT, and reaction time were investigated. Results show that en is determinative to the formation of FZNs, and the partial capping of NH(3).(CH(2))(2).NH(3) molecules on the side surface of the ZnO crystal is responsible for the tapering feature of petals. HMT can step into the nucleation process of ZnO and inhibit the formation of nanorods on the substrate by preventing heterogeneous precipitation. Moreover, the formation of twin crystal ZnO nuclei at low precursor concentrations and their further evolution into spindle crystals with clear middle interfaces are also vitally important for the development of FZNs.

A TiO(2) nanotube array with a large surface area is fabricated on a glass substrate using a ZnO nanorod array and sol-gel process, and the structural characteristics of the TiO(2) nanotube array are investigated. The well-aligned ZnO nanorod array, which is deposited on ZnO seed layer coated glass substrates by the wet-chemical route, is used as a template to synthesize TiO(2)/ZnO composite nanostructures through the sol-gel process. Then, by selectively removing the ZnO template, a TiO(2) nanotube with contours of the ZnO nanorods is fabricated on the ZnO seed layer coated glass. The resultant TiO(2) nanotubes are 1.5 µm long and 100-120 nm in inner diameter, with a wall thickness of ∼10 nm. In addition, by adjusting the experimental parameters, such as the dip-coating cycle number or heating rate, porous TiO(2) thick films can also be obtained.

Dye-sensitized solar cells (DSCs) have been considered as one of the low cost alternatives for conventional silicon solar cells since the fi rst application of transparent TiO 2 nanocrystalline fi lms in 1990s. [ 1 ] Light-harvesting ability and internal carrier collection effi ciency via the sensitized photoanode constitute the key factors that determine the performances of DSCs. Efficiencies exceeding 11% have already been achieved in laboratories with the sensitizers of N719 and C101, [ 2 ] whereas the overall performances are still lower than the predicted values due to poor utilization of near infrared photons and recombination losses in the random network of TiO 2 nanocrystallines. [ 3 ] In this regard, intense efforts have been devoted to ordered 1D structures, pursuing the profi ts on the internal light trapping effect, fast electron transport, and fl uent redox penetration. [ 4 ] TiO 2 nanotube arrays are one of the representatives and yield effi ciencies of over 7% with well-defi ned anodized titanium foils, [ 5 ] which, however, generally suffer severe energy losses in the translucent counter electrode and electrolyte in the backilluminated geometry. [ 6 ] Accordingly, transferring processes of peeling-off and sputtering techniques are progressively developed to prepare aligned TiO 2 nanotube arrays directly on transparent conductive oxides (TCO) as the photoanode. [ 7 ] Although effi ciencies of ≈ 7-9% have been demonstrated with pure and hybrid nanotube structures, [ 7 b, 8 ] these processes encounter signifi cant challenges when scaled up due to crack and cost effectiveness problems. On the other hand, the template-assisted fabrication has emerged as an alternative solution to ordered TiO 2 nanotube arrays on TCO, i.e., by taking ZnO nanowires as the template. [ 9 ] A typical liquid-phase deposition has been widely accepted for the surface coating of the TiO 2 shell, with controlled hydrolysis of Ti complexes in boric acid for simultaneous coating and acid etching of the inner ZnO core. [ 9 b] However, the prepared nanotube arrays generally exhibit low aspect ratios and poor adhesion to the substrate due to elusive etching of the ZnO seed layer. [ 10 ] So far, the template derived TiO 2 nanotube arrays still exhibit relatively low surface area and restricted dye loading capability when applied in DSCs, which result in limited performances.Herein, we present a facile template process for long vertically aligned TiO 2 nanotube arrays (up to 40 μ m) on TCO and demonstrate the hierarchical derivation for effective surface area enhancement in DSCs. The overall strategy is illustrated in Figure 1 , involving ZnO nanowire as the template and the double-shell architecture for fi nal hierarchical TiO 2 nanotube with an outer nanocrystalline shell. In particular, the inner shell is introduced as the skeleton to maintain the nanotube array during the derivation and it also acts as the highway for fast electron transport. Signifi cantly enhanced dye loading capability is generally observed in hierarchical na...