NEW HORIZON IN TITANIUM DIOXIDE

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 (Nano Science and Technology Institute) Nanotech 2008, different synthetic strategies were reported 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.

More recently using a novel, phase transfer process, in suspension, natural photocatalytic properties have been exploited 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.