Characterisation of redox states of Ni(La)-hydroxide films prepared via the sol-gel route by ex situ IR spectroscopy

Ni(La)-hydroxide films were prepared from aqueous colloidal solutions containing nickel sulfate and lanthanum acetate in the molar ratio 10:1. Two types of film were made by heating for 15 and 60 min at 300 degrees C. Thermogravimetry (TG) and X-ray diffraction (XRD) reveal that both films consist of NiO (bunsenite 40%) nanoparticles (particle size similar to 30 Angstrom), the remainder being amorphous. IR spectroscopy showed that the amorphous phase comprised the alpha(II)-Ni(OH)(2) phase incorporating SO42-, carboxylate and water species. Cyclic voltammetry (CV) in a 0.1 M LiOH electrolyte combined with in situ (UV-VIS) spectroscopy revealed that the colouring/bleaching changes, as a function of applied potential, differed considerably for the two types of film. Ex situ IR spectroelectrochemical measurements at near-grazing incidence angle conditions using P-polarised light (NGIA IR) were performed for films heated for 60 min in 0.1 M LiOH and 0.1 M tetramethylammonium hydroxide (TMAH) electrolytes and cycled 1402 and 1802 times. During the oxidation/reduction cycles the alpha(II)-Ni(OH)(2) phase transforms to the gamma(III)-NiOOH phase, while the beta(II)-Ni(OH)(2) did not develop. This explains the high cycling stability of Ni(La)-hydroxide films. The incorporation of TMA(+) ions was observed from the v(CH3) stretching band intensities in the IR spectra of cycled films.