R&D Profile: Synthesis of one-dimensional titanium dioxide nanostructures
Finely structured titanium dioxide is a technological material of long-standing importance for many applications including pigments and catalysis.
Overview courtesy of Milo Shaffer, Imperial College, London, UK
There is growing interest in smaller, truly nano-sized titanium dioxide particles with well-defined crystallinity and a range of geometries from spheres to rods and tubes, that are relevant to applications in composites, photovoltaics, sensors, and catalysis. High aspect ratios, in particular, introduce high surface to volume ratios, network forming abilities, and opportunities to control anisotropic properties.
At NSTI Nanotech 2008, we reported a number of different synthetic strategies for producing high aspect ratio titanium dioxide nanostructures. Titanium dioxide is commonly obtained via hydrolysis of metal alkoxides or halides; however, enhanced control over the reaction can be achieved in non-hydrous conditions. Dimensions of the resulting nanorods, and even their crystal phase, can be adjusted using different structure directing agents to adsorb to the growing surfaces. Typical products are small, single crystal nanorods of anatase (~ 3 25 nm), although aging reactions under suitable conditions yield single crystal rutile nanorods (15 x 135 nm). The more conventional hydrolytic synthesis can be dramatically accelerated (can around an order of magnitude) when performed on a microfluidic chip, as compared to the conventional bulk reaction. This first example of an on-chip synthesis of nanorods showed that the rapid mixing and controlled environment provides useful benefits, even though monodispersity was not significantly affected.
TEM images of rutile (top) and anatase (nanorods) grown using soft methods
More recently, we have developed the products of the non-hydrolytic further. Using a novel, phase transfer process, in suspension, we exploit the natural photocatalytic properties of titania to remove the structure directing ligands without causing agglomeration. After the reaction, the nanorods can be fully dispersed in aqueous solution, or functionalised as desired. The nanorods can be dispersed in a range of polymers to create optically clear nanocomposites with UV-filtering characteristics; by functionalising the titania with a monolayer of silane, the degradation of the matrix can be suppressed. The nanorods also offer valuable advantages in photovoltaic devices. We have demonstrated improved charge separation in hybrid systems based on titania nanorods in poly(3-hexylthiophene) (P3HT
References
- Self-cleaning anatase nanorods: Photocatalytic removal of structure directing agents and subsequent surface modification, U. Vukicevic, S. (Chy1a) Ziemian, A. Bismarck, and M. Shaffer, J. Mat. Chem., 18, 29, 2008
- Hybrid solar cells from the blend of poly(3-hexylthiophene) and ligand-capped TiO2 nanorods, J. Bouclé, S. Chyla, M. Shaffer, J. Durrant, D. Bradley, J. Nelson, Adv. Func. Mat., 18 (4), 622-633, 2008 (see also CR Physique doi:10.1016/j.crhy.2007.10.005)
- Synthesis of oriented arrays of TiO2 nanorods via a high temperature conversion of carbon nanotubes, B. Cottam, M. Shaffer, Chem. Commun., 4378-4380, 2007
- Accelerated synthesis of titanium oxide nanostructures using microfluidic chips, B.F. Cottam, S. Krishnadasan, A.J. deMello, J.C. deMello and M.S.P. Shaffer, Lab on a chip, 7, 167-169, 2007
