Mar 6, 2009
In situ doped titanium dioxide nanotubes come out on top
Titanium dioxide (TiO2) is one of the most important transition metal oxides for sustainable energy and other environmental applications. Its remarkable chemical and physical properties, biological inertness, non-toxicity, photostability and cost effectiveness commend it for use for photocatalysts as well as for gas sensors, photochromic devices and dye-sensitized TiO2 solar cells. Generally, a large specific surface area is crucial to achieve high photocatalytic activities. Nanotubes and nanofibrils have a particular advantage in the way they achieve high surface areas with three-dimensional mechanically coherent architectures that provide gas and radiation access.
Currently, the use of TiO2 as a photocatalyst is limited by its bandgap and the rather quick recombination of excited electrons and holes. TiO2 only absorbs about 3% of the solar light and thus requires near-UV light to operate as an efficient photocatalyst. It is crucial to extend the absorption range into the visible light region and hinder the electron-hole recombination, both of which can be best achieved by doping with low-mass ions such as nitrogen and sulphur or transition metal ions such as iron and cobalt. However, many of the chosen doping methods either require post-annealing, which often creates defects or multiphase structures that act as recombination centres, or are limited to low surface-area thin films as in the case of ion implantation.
In a recent study, which was published in Nanotechnology, the authors demonstrated a unique and effective doping route applicable in principle to a wide range of inorganic nanomaterials. They have utilized the encapsulated iron catalyst residues within as-grown carbon nanotubes (CNTs) as the dopant source. The process does not require post-annealing and allows a uniform distribution of iron at high concentrations as a solute in anatase or rutile nanotubes deposited onto the CNTs, which act as a template, apparently without any phase separation or iron segregation.
These new functional materials showed up to two orders of magnitude higher activities for the photocatalytic splitting of water per unit surface area compared with commercial TiO2, due to their higher illumination area, extended absorption range (bandgap reduced to 2.8 eV from 3.23 eV [anatase[ and 3.02 eV [rutile]) as well as a reduced electron-hole recombination rate. The deposition of very small (<2 nm) platinum nanoparticles further increased the hydrogen evolution rate in comparison with the pristine material, thus proving great potential for commercial application.
About the author
The work was performed in the Department of Materials Science and Metallurgy at the University of Cambridge. Dr Dominik Eder is an APART Advanced Research Fellow, supported by the Austrian Academy of Science. Until recently, Dr Marcelo Motta was a postdoctoral researcher in the Macromolecular Materials Laboratory. He is currently heading the carbon nanotube division at Thomas Swan & Co. Ltd, UK. Prof. Dr Alan H Windle FRS is the head of the Macromolecular Materials Laboratory of the Department of Materials Science and Metallurgy and a Fellow of Trinity College.