Technique means that titanium dioxide could be better applied

It is claimed the development should make titanium dioxide more efficient in a range of applications, including photovoltaic cells, hydrogen production, anti-microbial coatings, smart sensors and optical communication technologies.

According to a statement, the new technique allows researchers to control the phase of the titanium dioxide by modifying the structure of the titanium trioxide and sapphire substrate.

Titanium dioxide most commonly comes in one of two major phases, meaning that its atoms arrange themselves in one of two crystalline structures.

These phases are anatase or rutile and the arrangement of atoms dictates the material’s optical, chemical and electronic properties. Consequently, each phase has different characteristics.

The anatase phase has characteristics that make it better suited for use as an anti-bacterial agent and for applications such as hydrogen production. The rutile phase is better suited for use in other applications, such as photovoltaic cells, smart sensors and optical communication technologies.

‘Traditionally, it has been a challenge to stabilise titanium dioxide in the desired phase,’ said Dr Jay Narayan, John C Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and co-author of a paper describing the work. ‘The material tends to transform into the anatase phase below 500^oC, and transform into the rutile phase at temperatures above 500^oC.

‘We have now developed a technique that precisely controls the phase, or crystalline structure, of titanium dioxide at room temperature — and stabilises that phase, so it won’t change when the temperature fluctuates. This process, called phase tuning, allows us to fine-tune the structure of the titanium dioxide, so that it has the optimal structure for a desired application.’

The university said the process begins by using a widely available sapphire substrate that has the desired crystalline structure. Researchers then grow a template layer of titanium trioxide on the substrate. The structure of the titanium trioxide mimics the structure of the sapphire substrate. The titanium dioxide is then grown on top of the titanium trioxide template layer.

The structure of the titanium dioxide differs from the titanium trioxide — but is dictated by the structure of that template layer. Titanium dioxide can be created in any phase, simply by modifying the structure of the titanium trioxide and sapphire substrate.

This works due to domain matching epitaxy (DME). In DME, the lattice planes in the template layer line up with the lattice planes of the material being grown on that template.

The paper, ‘Domain epitaxy in TiO2/[alpha]-Al2O3 thin film heterostructures with Ti2O3 transient layer,’ was published online on 20 June in Applied Physics Letters.

The researchers have also demonstrated how this technique can be used with silicon computer chip substrates, which can be integrated into electronics such as smart sensors.