October 4, 2007

Solar Energy

Turning Water Into Fuel

Silicide compound and sunlight convert water to H₂ and O₂

Mitch Jacoby

In a two-for-one deal that may give the solar energy field a shot in the arm, researchers have discovered a semiconducting silicide that functions effectively as a water-splitting photocatalyst and doubles as a gas separator (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200701626).

Courtesy of Martin Demuth

SPLIT UP Sunlight and residual oxygen form active domains of oxide species (purple and green regions) on the surface of TiSi₂ particles. These species catalyze formation of H₂ and O₂ from water

Using sunlight to liberate hydrogen from water is an appealing way to generate a clean-burning fuel from a renewable energy source. As a result, scientists have examined a variety of materials over the years in search of a suitable catalyst to accelerate the water-splitting reaction. Several candidates show some level of promise, yet each material suffers from shortcomings that would limit its applications. For example, some catalysts absorb solar radiation inefficiently, exhibit low activity, or are unstable or costly.

Now, a team of researchers at the Max Planck Institutes for Bioinorganic Chemistry and for Coal Research, in Germany, report that titanium disilicide (TiSi₂)—an abundant and inexpensive semiconductor not known previously to be a water-splitting catalyst—separates water into hydrogen and oxygen when reactors containing the powdered catalyst are illuminated with simulated sunlight.

"Titanium disilicide has very unusual optoelectronic properties that are ideal for use in solar technology," says research group leader Martin Demuth. Specifically, the material absorbs light over a wide range of the solar spectrum and exhibits a bandgap???an important determinant of semiconductor properties???that varies by nearly 2 eV across that range. Semiconductors typically exhibit a much narrower variation in bandgap.

Another key observation reported by the team, which includes Demuth, Peter Ritterskamp, Andriy Kuklya, and their coworkers, is that hydrogen evolves readily during experiments, but oxygen adsorbs reversibly on the catalyst surface. Raising the temperature above 100 °C rapidly releases the stored oxygen, they say, which provides a convenient way to separate the gases.

On the basis of control experiments, isotope-labeling tests, and other measurements, the researchers propose that exposing commercial TiSi₂ to light in the presence of a small amount of oxygen (as found in water that has not been degassed) leads to formation of catalytically active sites. These nanometer-sized domains of oxidized species catalyze the water-splitting and gas-forming reactions, they say.

As news of the findings begins to spread, some scientists are puzzling over the unusual properties reported for TiSi₂. At the National Renewable Energy Laboratory, for example, senior research fellow Arthur J. Nozik notes that the "curiously" varying bandgap implies that the material is neither pure nor homogeneous. It is unclear, he says, whether this range represents individual particles with distinct chemical composition or a graded composition for individual particles, which suggests that the mechanism is not well-understood.

Demuth agrees that the behavior is "atypical" but adds that he has founded a start-up company to further study, develop, and possibly commercialize the technology.

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