The Characterization of Doped Iron Oxide Electrodes for the Photodissociation of Water: Stability, Optical, and Electronic Properties

A characterization of optical and electronic properties is presented for p‐type (Mg‐doped) and n‐type (Si‐doped) iron oxides used in the photoelectrolysis of water. Photocurrent vs. wavelength spectra for these electrodes indicate that is the active optical component for both p‐type and n‐type materials. Band‐edge locations for p‐type and n‐type iron oxides in sodium hydroxide aqueous solution are determined from differential capacitance measurements. The thermodynamic feasibility of the catalytic photodissociation of water without external potential is demonstrated for a short‐circuited p/n diode assembly on an energy level diagram of the electrode/electrolyte interfaces. The open‐circuit voltage and short‐circuit current generated by the p/n assembly as a function of the intensity of laser irradiation indicate that these doped iron oxides are low mobility, high carrier density semiconductors. Photo‐oxidation of water at the n‐type anode is verified through oxygen detection. Gas evolution is monitored from an operating diode assembly using mass spectrometry and isotopically labeled water . Photocurrents from these p/n assemblies show excellent long‐term stability in aqueous solution and Auger analysis of the semiconductor surfaces indicates no evidence of electrode dissolution.