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Water Photolysis by Antimony-Doped Gallium Nitride

September 1, 2011

I reviewed the photolysis of water (splitting water into hydrogen and oxygen using light energy) in two previous articles, Manganese Photolysis of Water (June 1, 2011) and Titania Photocatalysis (February 16, 2011). This would be one method of using solar energy to provide vehicular fuel . The other, of course, is photovoltaics for generating electricity .

Research in water-splitting using light accelerated after the 1972 publication of an article in Nature by Akira Fujishima and Kenichi Honda.[1] They described a photoelectrochemical cell that used titania as a photoanode .[1-2] This cell was capable of water photolysis .

One problem with the Fujishima-Honda photolysis cell is that it was inefficient in conversion of solar energy into hydrogen. Since titania, in its rutile crystal form, has a bandgap of 3.0 eV , it will only split water when illuminated by light shorter in wavelength than 413.75 nm . Less than 10% of solar energy appears in those wavelengths. Others have been researching ways to reduce the titania bandgap to increase efficiency.[3-4]

I reviewed gallium nitride (GaN), an important semiconductor material for high speed and high power electronics, in a previous article (Gallium Nitride Crystals, July 29, 2010). Since gallium nitride in its zinc blende crystal structure has a bandgap of 3.2 eV, which is higher than titania, you wouldn't expect it to be a candidate as a photolysis material. Also, two-inch diameter single crystal wafers of GaN are priced at about a thousand dollars, although one company is working on a process to make less expensive GaN wafers.

A recent paper in Physical Review B by a broadly-based research team with members at the University of Kentucky (Lexington, Kentucky), Daimler AG GR/PSS (Ulm, Germany), the Greek Foundation for Research and Technology, FORTH (Heraklio, Crete, Greece), the University of South Florida (Tampa, Florida) and the University of Louisville (Louisville, Kentucky) looks at the consequences of doping gallium nitride with antimony (Sb).[5-6]

They found that a 2% doping of Sb in GaN will significantly reduce its band gap. Since GaN is a high temperature semiconductor, there's a possibly for efficient operation in solar concentrator arrays. Unfortunately, this is a computation and not an experiment , so I'll reserve judgment until I see the real thing .

Water photolysis using Sb-doped gallium nitride

Water photolysis using Sb-doped gallium nitride.

(Image via University of Kentucky).

The Sb-GaN team used a first-principles density functional theory based on the Hubbard Model , a conventional starting place for semiconductor calculations. The Hubbard model uses a Hamiltonian that includes a term for electron hopping between sites and another for the potential energy of the electron while on site. In their model, they used a local density approximation and a generalized gradient approximation to refine the calculations.[6]

The calculations indicate that antimony in small concentrations will significantly narrow the band gap of gallium nitride to 2 eV, equivalent to a wavelength of 620 nm, thus allowing a greater fraction of sunlight to be harvested for hydrogen production. The authors state that Sb-GaN would be easy to produce and inexpensive.

If we consider ceramic , rather than single crystal, material, "inexpensive" is still a relative term. Gallium now sells for about $500,000 per metric ton , or fifty cents per gram .[7] As a reference value, iron sells for about $400 per ton. Photolysis of water with iron? Materials scientists take note, that's a worthy stretch goal! You have the example of Fritz Haber , who used iron as a catalyst to make ammonia .

Fritz Haber, 1905

Fritz Haber , 1905.

Haber's discovery of a cataytic process for the production of ammonia earned him a Nobel Prize in Chemistry (1918).
His work also generated some controversy .

(Deutsches Bundesarchiv (German Federal Archive), Photograph No. 183-S13651, via Wikimedia Commons).

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