Titanium dioxide (TiO2) is perhaps one of the most widely used transition metal oxides in diverse applications due to its variety of unique and application-worthy properties. With the growing emphasis of the current science and technology on nanomaterials due to their unique and novel property domains, considerable efforts have been expended over the past decade to synthesize various metal oxides (including TiO2) in the form of different phases, shapes, and functions using a variety of soft chemical and physical synthesis techniques. Depending on the use of a particular process, specific precursors/radicals, capping agents, temperature, pressure etc. a particular morphology of the nano system evolves. In the context of different applications such as photovoltaic, catalysis, electro-optics etc., controlled nanocrystal growth is intensely researched. In addition to the size and composition, the shape control of nanomaterials is an important variable to adapt to the properties for various applications. However, different oxides have their specific symmetry-dependent crystal growth habits which make the proposition of developing specific desired (shape) morphology a non-trivial proposition. For instance, ZnO can grow more easily into anisotropic structures while TiO2 does not, unless efforts are made for facet control via selective capping.
Literature survey shows that the chemical transformation of inorganic nanocrystalline solids via diffusion or exchange of atoms is emerging as an attractive approach for nanostructure engineering in recent years. In particular, for transforming one ionic nanocrystal into another hetero-interfaced nanostructure, cation exchange reaction is shown to be a very useful process. It is generally assumed that the anionic structure of the crystal is conserved, while the cations undergo replacement during the exchange reaction due to their relatively smaller size and higher mobility. For instance, the morphology composed of a CdSe nanocrystal embedded in a CdS rod (CdSe/CdS) was exchanged to a PbSe/PbS nanorod via a Cu2Se/Cu2S structure keeping the seed size and position within the nanorod preserved. The morphology change in the cation exchange reactions of metal chalcogenide nanocrystals, CdE to MxEy (E=S, Se, Te and M=Pd, Pt) has been investigated by Son et al. Brock et al. have synthesized Ag2Se wet gel monoliths by an ion exchange reaction of a monolithic CdSe wet gel and converted the same to an aerogel by drying under supercritical conditions.
TiCl4 treatment of nanoparticulate TiO2 films has been researched by several groups, especially by O'Regan and Bakker, in the context of the dye sensitized solar cell (DSSC) application, and a significant improvement in cell efficiency has been demonstrated following such a treatment. However, there have not been many studies on the possible beneficial use of such a treatment for other oxides.
ZnO has attracted considerable interest of the DSSC community due to its unique set of optoelectronic properties; however, the corresponding DSSC efficiencies are quite low. The pioneering work by Yang and coworkers showed that DSSCs based on ZnO nanowire/TiO2 core-shell structures have higher charge separation yields. It is now known that TiO2 coating of ZnO nanostructure improves the DSSC efficiency, and in most cases such coating is applied by the expensive atomic layer deposition method. Only recently, Atienzar et al. reported a simple TiCl4 treatment that led the surface coating of TiO2 on ZnO core (equivalent of the TiCl4 post treatment of TiO2 structured materials) leading to improved DSSC performance. However, no details were provided about the effects of TiCl4 on ZnO morphology. Recently, the effect of TiCl4 treatment on porous ZnO photoelectrode has also been examined by Murakami et al.
An article titled “Materials “Alchemy”: Shape Preserving Chemical Transformation of Micro-to-Macroscopic 3-D Structures” by Kenneth H. Sandhage, published in TMS Vol. 62, No. 6 (2010) pp. 32-43 gives an overview on Shape Preserving Chemical Transformations. The article states;                “The scalable fabrication of nanostructured materials with complex morphologies and tailorable chemistries remains a significant challenge. One strategy for such synthesis consists of the generation of a solid structure with a desired morphology (a “preform”), followed by reactive conversion of the preform into a new chemistry. Several gas/solid and liquid/solid reaction processes that are capable of such chemical conversion into new micro-to-nanostructured materials, while preserving the macroscopic-to-microscopic preform morphologies, are described in this overview.”        
An article titled “Synergistic effects of ZnO compact layer and TiCl4 post-treatment for dye-sensitized solar cells” by NiuHuanga et al., published in Journal of Power Sources, Volume 204, 15 Apr. 2012, Pages 257-264 discloses the interaction between ZnO compact layer and TiCl4 post-treatment on TiO2 photo electrode for dye sensitized solar cell (DSSC). Photo electrode combined the two modifications is designated as ZnO+2 l+TiCl4. It is further disclosed that after the TiCl4 treatment the ZnO compact layer transforms to a bi-functional layer, which suppresses back electrons transfer from FTO to electrolyte and reduces the FTO/TiO2 interfacial resistance. In addition, the newly formed TiO2 coating generated by TiCl4 post-treatment contains abundant and well dispersed Zn element, which further facilitates electron transfer at TiO2 layer. Meanwhile, the electron lifetime in ZnO+2 l+TiCl4 is the longest. Consequently, the overall energy conversion efficiency of the cell with ZnO+2 l+TiCl4 is significantly enhanced to 8.9%, which is 8.8% higher than that with pure TiCl4 post-treatment and 17.7% higher than that without any treatment.
An article titled “Fabrication of TiO2 nanotube film by well-aligned ZnO nanorod array film and sol-gel process” by J Qiu et al. published in Thin Solid Films (2007), Volume: 515, Issue: 5, Pages: 2897-2902 discloses high density TiO2 nanotube film with hexagonal shape and narrow size distribution which was fabricated by templating ZnO nanorod array film and sol-gel process. Well-aligned ZnO nanorod array films obtained by aqueous solution method were used as template to synthesize ZnO/TiO2 core-shell structure through sol-gel process. Subsequently, TiO2 nanotube array films survived by removing the ZnO nanorod cores using wet-chemical etching. Polycrystalline anatase TiO2 nanotube films were similar to 1.5 μm long and similar to 100 nm in inter diameter with a wall thickness of similar to 10 nm.
Inspite of the above disclosures in the art for chemical conversion into micro-to-nanostructured materials, while preserving the macroscopic-to-microscopic preform morphologies, chemical transformation of ZnO mesostructures to TiO2 mesostructures using simple chemical treatment is however not explored hitherto.
In the context of the importance of applications of shape controlled metal oxides in electro-optics, photovoltaics etc. the present invention lays emphasis in providing shape preserving chemical transformation of ZnO mesostructures to TiO2 mesostructures using simple chemical treatment.