Extracting the H₂ from H₂O has tantalized generations of chemists since it was shown two centuries ago that passing an electric current through water would split it into its component elements. Of course if electrical energy has to be used to extract H₂, then the net benefit in energy terms will be nil, but there is a source of energy that is equally unlimited, and that is light from the sun. Paper #5 in the current Hot Ten brings closer the day when sunlight may make all the H₂ we need.

Titanium dioxide (TiO₂) is the key to decomposing water into its elements, and it has been known for more than 30 years that this will catalyze the process when ultraviolet light is used. Unfortunately, relatively little of this radiation reaches the Earth’s surface. In 2003, this column highlighted a paper from a group of Japanese scientists who reported that indium-tantalum-oxide was capable of decomposing water when exposed to blue light, albeit with an efficiency of only 0.7% (see Science Watch, 14(6): 7, November/December 2003).

To make the activated oxide they used a torch of natural gas to heat and oxidize titanium metal sheet, and in the presence of the combustion products of the natural gas (carbon dioxide and steam). They kept the temperature at 850° C by adjusting the flow rate of oxygen. The best material was produced when the oxidation process took around 13 minutes, and the CM-TiO₂ obtained was dark grey. Khan examined the material using a scanning electron microscope and showed it was a mixture of the two forms of TiO₂ (rutile and anatase) but more porous than either. X-ray photoelectron spectroscopy showed that the chemical composition was TiO_2-xC_x where x was around 0.15. The new material absorbed light particularly at wavelengths of 535 nm (green) and 440 nm (violet), which is quite different absorption profile to that of n-TiO₂ itself.

When water containing CM-TiO₂ was exposed to the blue light from a xenon arc lamp, hydrogen and oxygen gases were released in abundance and in the exact 2:1 ratio to prove that water was being decomposed. The photoconversion efficiency was a remarkable 8.35%, and tantalizingly close to the 10% efficiency that is the U.S. Departments of Energy’s benchmark for commercially viable solar H₂ production. Another important aspect of CM-TiO₂ semiconductor was its high stability.

Recently Khan has published further work on the splitting of water, although this time he used p-type iron oxide semiconductor doped with zinc and showed that this too has the potential to work efficiently when combined with n-type iron oxide (see W.B. Ingler, et al., J. Amer. Chem. Soc., 126[33]: 10238-9, 2004). He claims that, in work not yet published, "we have shown that carbon-modified CM-TiO₂ can split water with a photoconversion efficiency close to 10% when it is combined with another p-type semiconductor of lower band gap energy and exposed to artificial sunlight." Moreover he says that they are now working to synthesize CM-TiO₂ by other methods, including a wet chemical method for making both nanoparticles and thin films. As he tells Science Watch: "There is every indication that we can do this, and that it will lead to a highly stable, inexpensive, self-driven, photoelectrochemical cell able to exceed the critical efficiency barrier of 10%."