Laser-induced coloration of WO3

It has long been recognized that WO3 can be colored through electro-, photo-, and thermo-chromism processes. We report here that WO3 turned to a deep blue color upon just one pulse irradiation of the XeCl excimer laser (lambda = 308 nm, pulse duration = 36 ns, output energy = 100 mJ/pulse). Scanning electron microscopy (SEM) images showed that a melted layer was formed on the surface of the sample. The color change was demonstrated by UV-visible diffuse reflectance. X-ray photoelectron spectroscopy (XPS) showed the partial reduction of WO3 and the appearance of conduction band electrons. Calorimetry measurement results indicated that reoxidation happened at 400-440 degrees C in air. From these experimental results combining with the calculation, it was concluded that the laser-induced coloration is different from ordinary photochromism induced by UV light, and may be attributed to thermochromism. (C) 1997 American Institute of Physics.