Structural and electrochromic properties of sol-gel derived Ni(Si)-oxide films

In our recent publication we reported the FT-IR spectroscopic analysis of Ni-oxide films [A. Surca, B. Orel, B. Pihlar, P. Bukovec, J. Electroanal. Chem. 408 (1996) 83] prepared from aqueous sols containing a nickel sulphate precursor and polyvinylalcohol as an additive to improve the optical quality of the films. Films with improved adherence on ITO coated glass were deposited via the sol-gel route using a dip-coating technique from sols containing in addition a nickel sulphate precursor 3-aminopropyltrimethoxysilane (3-APMS), Heat treatment at 300 degrees C produced Ni(Si)-oxide films whose IR spectra resemble the alpha(II)-Ni(OH)(2) phase with NiO (bunsenite) nanocrystalline inclusions as was shown by X-ray diffraction analysis. The electrochromic effect with colouring/bleaching changes up to 55% was noted in 0.1 M LiOH. The colouration efficiency (eta) which is defined as the change in optical density per unit of inserted charge (Delta OD/Q) was up to 26 cm(2)C(-1). Evolution of newly formed phases appearing during extensive (up to 200 cycles) potential cycling revealed the transformation of the as-deposited alpha(II)-Ni(OH)(2) phase to beta(III)-NiOOH and beta(II)-NiOOH2 phases. In films cycled potentially more than 150 times in 0.1 M LiOH an irreversible transformation of the beta(III)-NiOOH phase into the gamma(III)-NiOOH phase was observed. From these results it was concluded that Ni(Si)-oxide films follow the Bode scheme [H. Bode, K. Dehmelt, J. Witte, Electrochim, Acta 11 (1966) 1079] when cycled potentially in protic electrolyte. (C) 1997 Elsevier Science S.A.