Photoelectrochemistry of Nickel Hydroxide Thin Films

The photoelectrochemical behavior of electrochromic nickel hydroxide films in is reported. Cathodic photocurrent was obtained from reduced films which behave as p‐type semiconductors. The effective bandgap of the reduced phase is estimated to be 3.6–3.9 eV, although optical absorbance and photoaction data indicate a significant tailing of states into the bandgap. Anodic photocurrent from oxidized films which behave as n‐type semiconductors can be generated by visible light of energies as low as 1.5 eV. Transient behaviors and efficiencies of both the cathodic and anodic photocurrents are reported. An unusual transient photocurrent response was found in the potential region in which the nickel hydroxide redox reaction occurs. Illumination produced a cathodic current spike followed by a small, steady‐state anodic photocurrent, while interruption of the light caused an anodic current spike. The cathodic photoresponse was found only at potentials where both oxidized and reduced forms of the film were present. Possible explanations for the transients include double‐layer effects, acceleration of surface redox reactions, and the excitation of midgap states.