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Researchers from North
Carolina State University have developed a new
technique for controlling the crystalline structure of titanium dioxide at room
temperature. The development should make titanium dioxide more efficient in a
range of applications, including photovoltaic cells, hydrogen production,
antimicrobial coatings, smart sensors, and optical communication technologies.
Titanium dioxide most commonly comes in one on of two major “phases,”
meaning that its atoms arrange themselves in one of two crystalline structures.
These phases are “anatase” or “rutile.” The arrangement of atoms dictates the
material’s optical, chemical, and electronic properties. As a result, each
phase has different characteristics. The anatase phase has characteristics that
make it better suited for use as an antibacterial 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 stabilize titanium dioxide in the
desired phase,” says Jay Narayan, PhD, 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 Celsius, and transform into the rutile phase at temperatures
above 500 C.
“We have now developed a technique that precisely controls the phase, or
crystalline structure, of titanium dioxide at room temperature—and stabilizes
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 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. This means that you can
create the titanium dioxide in any phase, simply by modifying the structure of
the titanium trioxide and sapphire substrate.
This works because of a process called 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. Lattice planes are the lines, or
walls, which constitute a crystal.
The paper, “Domain epitaxy in TiO2/[alpha]-Al2O3 thin film heterostructures
with Ti2O3 transient layer,” was published online 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.
Source: North Carolina State University
