Jan 13, 2009
Photoinduced applications of titania nanotubes
The development of three dimensional nano-structures based on elements other than carbon presents an increasing interest for numerous applications. Recently, TiO2 nanotubes have emerged as very promising functional materials of superior performance in environmental cleaning and renewable energy. These innovative morphologies finely tuned at the nanoscale can be endowed with unique optoelectronic properties. Optimization of the interaction of the vertically oriented titania nanotubular materials with light is therefore the key element for efficient solar energy conversion to electricity as well for the decomposition and/or removal of pollutants.
Special research effort concerns self-assembled titania nanotube arrays developed by low cost electrochemical engineering. The work aims at tailoring and controlling the materials nanostructure via effective choice of the anodization conditions including electrolyte composition, porosification agent, pH, applied potential (or current density), anodization time etc. and involves technologies for the nanotubes production, characterisation and processing. These requirements have a major potential in three main areas: fundamental research on the nanoporous TiO2 structures (self-organisation effects together with their mechanisms); approaches to designing new titanium-based systems that have enhanced properties in terms of their application and use in the field of nano-technology; and accessibility of the produced systems for a direct investigation of their technological importance.
In a very recent study, which was published in Nanotechnology, the authors reported on self-organized TiO2 nanotubes with packed, vertically aligned morphology and different lateral characteristics, grown on Ti metal substrates by controlled electrochemical anodization in phosphate/HF and ethylene glycol/HF electrolytes. The materials present very interesting self-cleaning properties under ultraviolet irradiation, in terms of enhanced superhydrophilicity and azo-dye degradation activity. It has been demonstrated that both photo-induced wetting and catalytic effects strongly depend on the morphological characteristics of the TiO2 nanotube arrays. The wetting and the UV-induced superhydrophilicity was found to be very sensitive to the large scale roughness of the nanotubular arrays as well as to the small scale roughness induced by ridges at the outer tube surface. The researchers further discovered that the photocatalytic activity of the TiO2 nanotube arrays correlates quantitatively with the variation of the geometric roughness factor confirming the strong impact of morphology on the photo-induced properties. Current work at Demokritos focuses on the nanotubes post modification (e.g. silver nanoparticles deposition, non-metal doping, magnetic nanoparticles,…). This is expected to endow the materials with multifunctionality and great potential for a variety of photoinduced applications including biochemical ones, such us antibacterial effect and killing of cancer cells, and all that in the presence of visible light.
In another recent investigation, which was also published in Nanotechnology, the same group of researchers reported on dye-sensitized solar cells (DSSCs) prepared using TiO2 nanotubes, grown by controlled Ti anodic oxidation in non-aqueous media. The as grown nanotubes arrays present a high degree of self-organization, have a length of 20 μm and exhibit an amorphous structure, which transforms to the anatase TiO2 crystalline phase upon post-annealing in air at 450 0C. The annealed tubes were efficiently sensitized by the standard N719 ruthenium dye and were further used as active photoelectrodes after incorporation in sandwitch-type DSSCs using both liquid and solidified electrolytes. The efficiencies obtained under 1 sun (AM 1.5) conditions, using a back-side illumination geometry, were very promising. Tuning of the titania nanotube substrates morphology and optimization of the cell configuration, now underway at Demokritos and Erlangen, are expected to cause a significant increase in the device performance and attract interest on commercial applications, due to use of titanium (metal foil) flexible substrate.
About the author
The work was performed at the National Center of Scientific Research "Demokritos"-NCSRD (Greece) and the University of Erlangen-Nuremberg (Germany) and supported by the NMP/FP6/EU program under the Ti-nanotubes project, which also comprises the University of Manchester (UK). Dr. Athanassios G. Kontos is a Researcher, Athanassios I. Kontos and Dimitrios S. Tsoukleris are PhD students, Dr. Vlassis Likodimos and Dr. Thomas Stergiopoulos are postdoctoral researchers, Dr. Polycarpos Falaras is a Research Director in the Institute of Physical Chemistry of NCSR "Demokritos" and he is also the head of the Institute. Dr. Julia Kunze is a Senior Scientist, Andrei Ghicov is a PhD student and Prof. Patrik Schmuki is the head of the Institute "Chair for Surface Science and Corrosion" at the Department of Materials Science and Engineering of the University of Erlangen-Nuremberg.