Enhancement of Photochromism and Electrochromism in MoO₃/Au and MoO₃/Pt Thin Films

Freshly vacuum-evaporated, neat amorphous MoO3 thin films exhibit only UV photochromism. Visible-light photochromism is induced in these films when they are cathodically polarized for a short duration in a nonaqueous electrolyte. The efficiencies of the UV and visible-light photochromism of the MoO3 thin films increase at least 2-fold when they are coated with a thin layer (∼20-nm thickness) of Au or Pt. The enhancement in photochromism is attributed to the presence of a large Schottky barrier at the metal−semiconductor interface, which facilitates electron capture by Au or Pt. The electron capture by the metal produces a longer electron−hole separation lifetime, thereby enhancing the photochromic process. For the electrochromic process, the coloring and bleaching current densities for the MoO3/Au thin-film electrode are about 1.5 and 2.0 times larger than the respective current densities for the MoO3 thin-film electrode. The enhancement in electrochromism is ascribed to the electrocatalytic effect of the gold overlayer. The benefits of surface modifications are (1) inhibition of electron−hole recombination, which enhances photochromism, (2) enhancement of electrochromism, and (3) inhibition of corrosion reaction of surface MoO3.

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