A research team from Northeastern University and the National
Institute of Standards and Technology (NIST) has discovered, serendipitously,
that a residue of a process used to build arrays of titania nanotubes-a
residue that wasn't even noticed before this-plays an important
role in improving the performance of the nanotubes in solar cells that produce
hydrogen gas from water. Their recently published results* indicate that by
controlling the deposition of potassium on the surface of the nanotubes, engineers
can achieve significant energy savings in a promising new alternate energy system.
 | | Scanning electron microscope image of typical titania nanotubes for a photocatalytic cell to produce hydrogen gas from water. Nanotubes average roughly 90-100 nanometers in diameter. Credit: Menon, Northeastern University. |
Titania (or titanium dioxide) is a versatile chemical compound best known as
a white pigment. It’s found in everything from paint to toothpastes and
sunscreen lotions. Thirty-five years ago Akira Fujishima startled the electrochemical
world by demonstrating that it also functioned as a photocatalyst, producing
hydrogen gas from water, electricity and sunlight. In recent years, researchers
have been exploring different ways to optimize the process and create a commercially
viable technology that, essentially, transforms cheap sunlight into hydrogen,
a pollution-free fuel that can be stored and shipped.
Increasing the available surface area is one way to boost a catalyst’s
performance, so a team at Northeastern has been studying techniques to build
tightly packed arrays of titania nanotubes, which have a very high surface to
volume ratio. They also were interested in how best to incorporate carbon into
the nanotubes, because carbon helps titania absorb light in the visible spectrum.
(Pure titania absorbs in the ultraviolet region, and much of the ultraviolet
is filtered by the atmosphere.)
This brought them to the NIST X-ray spectroscopy beamline at the National Synchrotron
Light Source (NSLS)**. The NIST facility uses X-rays that can be precisely tuned
to measure chemical bonds of specific elements, and is at least 10 times more
sensitive than commonly available laboratory instruments, allowing researchers
to detect elements at extremely low concentrations. While making measurements
of the carbon atoms, the team noticed spectroscopic data indicating that the
titania nanotubes had small amounts of potassium ions strongly bound to the
surface, evidently left by the fabrication process, which used potassium salts.
This was the first time the potassium has ever been observed on titania nanotubes;
previous measurements were not sensitive enough to detect it.
The result was mildly interesting, but became much more so when the research
team compared the performance of the potassium-bearing nanotubes to similar
arrays deliberately prepared without potassium. The former required only about
one-third the electrical energy to produce the same amount of hydrogen as an
equivalent array of potassium-free nanotubes. “The result was so exciting,”
recalls Northeastern physicist Latika Menon, “that we got sidetracked
from the carbon research.” Because it has such a strong effect at nearly
undetectable concentrations, Menon says, potassium probably has played an unrecognized
role in many experimental water-splitting cells that use titania nanotubes,
because potassium hydroxide is commonly used in the cells. By controlling it,
she says, hydrogen solar cell designers could use it to optimize performance.
* C. Richter, C. Jaye, E. Panaitescu, D.A. Fischer, L.H. Lewis, R.J. Willey
and L. Menon. Effect of potassium adsorption on the photochemical properties
of titania nanotube arrays. J. Mater. Chem., published online as an Advanced
Article, March 27, 2009. DOI: 10.1039/b822501j
** The NSLS is part of the Department of Energy’s Brookhaven National
Laboratory.
Posted April 21st, 2009
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