Redox Reactions and Pattern Formation by Using Photoexcited Semiconductor-Electrolyte Interfaces

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Photochromic materials, whose optical absorption properties change in response to light, are important for a number of technological applications. A stable, reusable photochromic thin-film could be used, for example, in optical displays and high-density memories. Thin films of some transition-metal oxides can be colored blue by band-gap excitation using ultraviolet light. For use with common semiconductor laser sources, however, photochromic materials are required that respond to visible light. Here we have investigated the preparation of visible-light-sensitive, reversibly photochromic films of MoO_3. Pretreated, vacuum-evaporated MoO_3 thin films were slightly blued by cathodic polarization in a non-aqueous electrolyte ; subsequent irradiation with visible light in air produced a strong colour enhancement. The photochromism could be erased by anodic polarization, and the coloration-decoloration process was repeatable over at least five cycles. The behaviour of these films contrasts with those blued by band-gap irradiation, which were not responsive to visible light.