Electrochromic behavior of NiO–TiO2 films prepared with sodium dodecyl sulfonate added to the sol

• Electrochromic NiO–TiO2 films were prepared with SDS added to the sol.
• The films showed a reversible brown color varying between 80% and 33% (at 550 nm).
• The ΔT and ΔOD of the films became stable in fewer cycles than that without SDS.
• The ΔT and ΔOD of the films remained stable up to 15,000 cycles.
• The life time of NiO–TiO2 films with SDS is far longer than that without SDS.

The effects of the surfactant sodium dodecyl sulfonate (SDS) on NiO–TiO2 sols, xerogels and films have been studied. Xerogels and films with Ni concentration of 80 mol% have been prepared using sols containing 3, 6, 9 wt% of SDS added to nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O) and tetra-n-butyl titanate (TNBT) as precursors in a mixture of ethanol and n-butanol. The thermal analysis of dried xerogels has been measured by thermogravimetry (TG) and differential thermal analysis (DTA) in air up to 500 °C. The structural properties of xerogels sintered at 300 °C have been determined by XRD and the xerogels consist predominantly of bunsenite nanoparticles embedded in an amorphous TiO2 phase. Single layers have been deposited by dip coating on FTO (fluorine doped tin oxide, SnO2:F) coated glasses and sintered at 300 °C. Their thickness is found to increase with the SDS amount while the roughness of the surface is reduced. The electrochromic properties of films deposited on FTO coated glasses have been studied in 1 M KOH aqueous electrolyte via cyclic voltammetry (CV), chronoamperometry (CA) and visible light transmission. The anodic and cathodic peaks in CV cycles and the charge intercalated during CA cycles increase with the SDS content. The transmission in the bleached and colored states as well as the change in optical density remain stable for the film prepared with 9 wt% SDS up to almost 18,000 CA cycles, a great improvement compared to films prepared without surfactant. The coloration efficiency of the film prepared with 9 wt% SDS is about 30 cm2/C, a value comparable to that obtained for pure NiO film.