Titanium oxide takes shape

Scientists at the US Department of Energy's

have developed new ways to make or modify nanorods and nanotubes of titanium oxide, a material used in a variety of industrial and medical applications.

The methods and new titanium oxide materials may lead to improved catalysts for hydrogen production, more efficient solar cells, and more protective sunscreens.

In one study, the scientists enhanced the ability of titanium oxide to absorb light.

‘Titanium dioxide's ability to absorb light is one the main reasons it is so useful in industrial and medical applications,’ said Wei-Qiang Han, a scientist at Brookhaven's Center for Functional Nanomaterials (CFN). It is used as a photocatalyst for converting sunlight to electricity in solar cells and also has applications in the production of hydrogen, in gas sensors, in batteries, and in using sunlight to degrade some environmental contaminants. It is also an ingredient in sunscreen.

Scientists have explored ways to improve the light-absorbing capability of titanium oxide by ‘doping’ the material with added metals. Han and his colleagues took a new approach. They enhanced the material's light-absorption capability by introducing nanocavities, which are enclosed pockets within the 100nm diameter solid titanium oxide rods.

The resulting nanocavity-filled titanium oxide nanorods were said to be 25 percent more efficient at absorbing certain wavelengths of ultraviolet A (UVA) and ultraviolet B (UVB) solar radiation than titanium oxide without nanocavities.

The cavity-filled nanorods could also improve the efficiency of photovoltaic solar cells and be used as catalysts for splitting water and also in the water-gas-shift reaction to produce pure hydrogen gas from carbon monoxide and water.

The method for making the cavity-filled rods is simple, said Han. ‘We simply heat titanate nanorods in air. This process evaporates water, transforming titanate to titanium oxide, leaving very densely spaced, regular, polyhedral nanoholes inside the titanium oxide.’

In a second study, Han and his colleagues described a new synthesis method to make iron-doped titanate nanotubes, hollow tubes measuring approximately 10 nanometres in diameter and up to one micrometer long. These experiments were also aimed at improving the material's photoreactivity. The scientists claim to have demonstrated that the resulting nanotubes exhibited noticeable reactivity in the water-gas-shift reaction.

‘Although the activity of the iron-doped nanotubes was not as good as that of titanium oxide loaded with metals such as platinum and palladium, the activity we observed is still remarkable considering that iron is a much less expensive metal and its concentration in our samples was less than one percent,’ Han said.

The scientists also observed interesting magnetic properties in the iron-doped nanotubes, and will follow up with future studies aimed at understanding this phenomenon.