My research is focus on the understanding and development of new photocatalytic materials for the production of hydrogen from water using solar energy. Since Fujishima and Honda first reported direct photoelectrochemical water splitting on a semiconductor/electrolyte interface in 1972, researchers have been investigating semiconductor systems for the production of hydrogen from water[1]. Using renewable resources for the production of hydrogen by photoelectrolysis is attractive as a sustainable non-polluting, long term contribution to supplying energy. Some progress has been made toward this goal; however, even after 30 years we are still searching for an appropriate material/system that has all the necessary characteristics for efficient and cost effective photocatalysis. Theoretically, such material exists-it remains to be synthesized.
Hematite (a-Fe2O3) has many potential advantages for hydrogen photoproduction. It has a bandgap of 2-2.2 eV (absorbs approximately 40% of the solar spectrum), it is stable in electrolytes over a wide range of pH’s and is abundant, inexpensive and non toxic[2-4]. Several qualities have limited the use of this material as an efficient photocatalyst, including high electrical resistance and recombination rates of photogenerated electrons[5, 6]. The undesirable properties are partially due to the hopping mechanism of charge transfer via oxygen vacancies, grain boundaries, and surface traps, which results in relatively low quantum yields. Efforts have been devoted to reducing the resistivity of thin films by increasing the amount of charge carriers or transferring electrons along the (001) planes of the hematite, which has 4 orders or magnitude lower resistance than transport perpendicular to this plane. This last approach has shown some improvement of the hematite[7,8] however there is still much work to be done to increase the record IPCE of 42%. Our hypothesis is that by deliberately doping hematite with selected heteroatoms and being able to control the growth dimensions, crystal orientation and facets exposed these limitations may be overcome. There is much diversity to explore in searching for the appropriate combination of dopant species to improve photocatalytic performance,[6,9-11] as well as to understand and engineer the crystal structure, orientation and size of the crystalline domains of hematite.
The research that I am working on incorporates the idea of building nanostructures in which the conduction properties of the material can be tailored by growing structures with specific crystallographic orientations, control of the nanostructure morphology and size, deposition of an appropriate Oxygen evolution catalyst to improve the performance of the hematite nanorods and incorporation of heteroatoms into the hematite structure as dopants such that the conduction band of the hematite can be increased to make the Hydrogen evolution reaction favorable and achieve higher performance of this electrodes. A second step that is being taken is the reduction of the nanorods to ~6nm where they should be quantum confined giving a band widening of ~0.3ev, this effect should raise the conduction band of the hematite rods to make favorable the Hydrogen evolution reaction.
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[5] Ahmed S.M., Leduc J, Haller S.F., J. Phys. Chem. B 1988, 92,
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[6] Arutyunyan V.M., Arakelyan V.M., Sarkisyan A.G., Shakhnazaryan
G.E., Stepanyan G.M., Turner J. A, Russ. J. Electrochem. 1998, 38,
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[7] Vayssieres L., Beermann N., Lindquist S E, Hagfeldt A, Chem.
Mater., 13,2, 2001, 233.
[8] Kay A, Cesar I, and Grätzel M, Am. Chem. Soc., 128, 49, 2006,
15714
[9] Berry J. F., Greaves C., Helgason O., McManus J., J. Mater. Chem.
1999, 9, 223.
[10] Prasad N. V, Srinivas K, Kumar S. G., J. A.R., Appl. Phys.
A-Mater. 2001, 72, 341.
[11] Aroutuaunian V.M., Arakelyan V.M., Shahnazaryan G.E., Stepanyan
G.M., Turner J. A., Khaselev O, Int. J. Hydrogen Energ. 2002, 27, 33.
Contact Information:
Alan Kleiman
Stucky/McFarland Group, Graduate Student
University of California
Santa Barbara, CA 93106