The solar light and heat allowed into a building can be controlled by WO₃-film ‘smart’ windows. Optical modulation is used to ‘switch’ such films to control the frequencies transmitted. Switching can be done in different ways. WO₃ films prepared by sputtering have fast colouring/bleaching kinetics, but textured surfaces. Sol-gel-made films have higher visible transmittance in the bleached state and thicker films are easier to make.

Now work from the National Institute of Chemistry, Ljubljana, Slovenia, and the Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany, combines sputtering and sol-gel methods: to make WO₃ film, with platinum catalyst sputtered on the surface and atomic hydrogen (gasochromism) to ‘switch’ them (U. Opara Krašovec, B. Orel, A. Georg and V. Wittwer, Solar Energy, 2000, 68, (6), 541–551).

Sol-gel WO₃ films were made by dip-coating. Adding an ormosil gave thicker, less brittle films of improved coloration. The sol-gel/Pt(sputtered)WO₃ films change colour as quickly as Pt/WO₃ sputtered films, and faster than WO₃ sol-gel films with Pd. In H₂/Ar gas mixtures the films colour in H₂ concentrations as low as 0.002 per cent. This may give simpler switchable ‘smart’ windows.