It is well known that the photoelectrochemical (PEC) hydrolysis of water via solar energy can directly produce both gaseous hydrogen and oxygen. The PEC process is, however, very inefficient when using semiconducting n-type photoanode materials that are chemically stable in the highly basic aqueous environment required for PEC cell operation. Solving this problem will require the identification of materials that: are chemically stable in basic solutions, that have a sufficiently small band gap to convert a reasonable part of the solar spectrum, and that have band edges located at potentials which allow the evolution of both H₂ and O_2. Here we describe the photoelectrochemical properties of n-type potassium tantalate (KTaO₃) and zinc oxide (ZnO) single-crystal PEC cell photoanodes. Although the band gaps of these two materials are not ideal solar collectors (the band edges are 3.64 and 3.34 eV respectively), they do not spontaneously disassociate in high pH aqueous solutions, and they are well-known indirect and direct gap semiconductors. Variations of these pure materials are obtained by doping KTaO₃ with alkaline-earth elements (Ca²⁺ and Ba²⁺) or other metallic impurities and by doping ZnO with Ga, Li, In, Gd, Er, or Mg. Both materials can form diodes when one face is in contact with a high-pH electrolyte (such as 8.5M NaOH) and can act as a photochemical cell, converting the photons above their respective band gaps into electricity and/or oxygen and hydrogen. The PEC characteristics include current-voltage (I-V) curves under illumination and spectral quantum efficiency (QE) measurements. Our results show that the maximum QE of KTaO₃ PEC cells occurs at 260 nm – with some photo-response for all wavelengths below 340 nm. ZnO PEC cells disassociate slowly in the electrolyte solution, but show significantly higher QE’s for wavelengths shorter than ~365 nm. Research sponsored by the Division of Materials Sciences and Engineering, U.S.D.O.E.

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