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

Abstract

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 °C. Thermogravimetry (TG) and X-ray diffraction (XRD) reveal that both films consist of NiO (bunsenite 40%) nanoparticles (particle size ∼30 Å), the remainder being amorphous. IR spectroscopy showed that the amorphous phase comprised the α(II)-Ni(OH)₂ phase incorporating SO₄ ²⁻, 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 α(II)-Ni(OH)₂ phase transforms to the γ(III)-NiOOH phase, while the β(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 ν(CH₃) stretching band intensities in the IR spectra of cycled films.