A sol-gel derived AgCl photochromic coating on glasses

Abstract

A new sol-gel synthesis route for the preparation of a transparent photochromic coating based on silver halide crystals is described. Therefore a sodium alumo borosilicate sol synthesized for dip coatings glass surfaces is used. Porous layers of about 2 µm in thickness have been obtained after drying. Silver ions were introduced into the layer by an infiltration process with an aqueous silver nitrate solution, forming small silver colloids that can be converted into silver chloride particles by HCl vapour. The so prepared coatings show a reversible photochromic behaviour. After 15 minutes of UV illumination transmittance decreases from about 86 % to 36 %. The darkend coatings recover completely after thermal bleaching at about 400 °C. The reversible formation of silver colloids after UV irradiation could be followed directly by x-ray diffraction and electron microscopy. Surprisely the photo efficiency is much higher in the prepared coating than in molten photochromic glasses.