CHARACTERIZATION 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 deg rees 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 spectrosco py showed that the amorphous phase comprised the alpha(II)-Ni(OH)(2) p hase incorporating SO42-, carboxylate and water species. Cyclic voltam metry (CV) in a 0.1 M LiOH electrolyte combined with in situ (UV-VIS) spectroscopy revealed that the colouring/bleaching changes, as a funct ion of applied potential, differed considerably for the two types of f ilm. Ex situ IR spectroelectrochemical measurements at near-grazing in cidence angle conditions using P-polarised light (NGIA IR) were perfor med for films heated for 60 min in 0.1 M LiOH and 0.1 M tetramethylamm onium hydroxide (TMAH) electrolytes and cycled 1402 and 1802 times. Du ring the oxidation/reduction cycles the alpha(II)-Ni(OH)(2) phase tran sforms 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)-hydro xide films. The incorporation of TMA(+) ions was observed from the v(C H3) stretching band intensities in the IR spectra of cycled films.