Photoelectrochemistry of nickel hydroxide thin films

The photoe lect rochemica l behav ior of e lectrochromic nickel hydrox ide fi lms in 1M KOH is reported. Cathodic photocur rent was obta ined from reduced films wh ich behave as p-type semiconductors. The effective bandgap of the re-duced phase is est imated to be 3.6-3.9 eV, a l though optical absorbance and photoact ion data indicate a s ignif icant ai l ing of states into the bandgap. Anodic photocur rent from oxidized fi lms wh ich behave as n-type semiconductors can be gen-erated by vis ible l ight of energies as low as 1.5 eV. Trans ient behaviors and eff iciencies of both the cathodic and anodic photocur rents are reported. An unusua l t rans ient photocur rent response was found in the potent ia l region in wh ich the nickel hydrox ide redox react ion occurs. I l luminat ion produced a cathodic cur rent spike fol lowed by a small, steady-state anodic photocurrent, whi le interrupt ion of the l ight caused an anodic cur rent ~pike. The cathodic photoresponse was found only at potent ia ls where both oxidized and reduced forms of the film were present. Poss ib le exp lanat ions for the t rans ients inc lude double- layer effects, accelerat ion of surface redox reactions, and the exc i tat ion of midgap states. The structure and e lectrochemistry of n ickel hydrox ide has been studied extensively due to its impor tance as the act ive mater ia l in nickel battery systems (1-2). Nickel hy-drox ide is also a promis ing e lectrochromic mater ia l wi t