Effect of surface properties of ZnO rods on the formation of anatase-phase TiO2 tubes prepared by liquid deposition method

The effects of the surface properties of ZnO rods on the formation of anatase-phase TiO2 tubes prepared by liquid deposition method is reported. A template made of ZnO rods was prepared on FTO-coated glass by chemical bath deposition and this was then treated under different experimental conditions in an aqueous solution of ammonium hexafluorotitanate and boric acid to investigate the role of the neutral and polar (zinc/oxygen terminated) surfaces of ZnO rods on the formation of TiO2. A calcination step at 550 °C in air was applied to convert the developed Ti-based complexes on the ZnO rods into anatase-phase TiO2 nanostructures. It was observed that a 10-min deposition process led to the development of a mixture of end-capped, open-ended and perforated TiO2 tubes with dimensions resembling those of the ZnO template. Further chemical modification of the surfaces of ZnO rod templates led to the development of anatase-phase TiO2 tubes with open-ends. The surfaces of ZnO rods act as reaction sites for the synthesis of Ti-based complexes. The production of these complexes on the ZnO surface takes place on the neutral side facets of the ZnO rods, while etching of the ZnO rods occurs preferentially along the c-axis (i.e. on the Zn-(O) terminated faces. Consequently, the deposition of TiO2 is considered to happen in two stages: an early and a late stage. The early stage is characterised by the development of Ti-based complexes (Ti hydroxide monomers) on the ZnO surfaces, while the late stage is characterised by polymerisation of these monomers into Ti hydroxide polymers. At the early stage, the deposition process starts on lateral surfaces constituting a foundation for more Ti hydroxide development. Upon calcination at 550 °C in air, these tubular amorphous structures are converted into end-capped anatase-phase TiO2 tubes.