Controlling the structural and optical properties of tungsten oxide films synthesized under environmentally friendly conditions

Abstract

Semiconductor oxides applied for light harvesting present special optical characteristics, such as high refractive index, high absorption coefficient, and low photoemission yield. This work reports an optimized, new, and easy route to obtain tungsten oxide films by anodization of tungsten foils, with controlled optical properties by using sodium chloride as an active electrolyte. Different from anodization processes, which are carried out in strong acidic media and/or with considerable concentrations of neutral electrolytes, we used small concentrations of sodium chloride in aqueous media to ensure the environmentally friendly conditions. A factorial design 23 showed the effects of electrolyte concentration, applied voltage, and thermal annealing (300 °C, 60 min) on the structural and optical properties of the films. The structure of films was characterized by atomic force microscopy, Raman spectroscopy, X-ray diffraction and photoluminescence (PL) spectroscopy. Real and imaginary refractive indexes, as well as the absorption curves, were obtained by ellipsometry. Two competitive reactions occurred on the anode surface: oxidation of tungsten and chlorine evolution. Continuous growth and dissolution of the oxide layer contribute to the production of porous films. The applied voltage directly affects thickness and structure of films, which originate different crystal structures, after thermal annealing. Thermal annealing is the main factor that modifies particle size and texture of the oxide layers. The PL bands allowed characterizing the main transitions, between 300 and 630 nm, when excited at 266 nm. Crystal growth contributed to the annihilation of luminescent centers during the annealing process.