Coloration and bleaching phenomena of amorphous WO₃ films due to the electrochemical insertion of divalent cations

It was confirmed that the divalent cations, Zn²⁺ and Cu ²⁺ were electrochemically inserted into amorphous WO₃ films prepared by r.f.-magnetron sputtering method. Coloration of the films was also observed, and the films were, however, bleached gradually. The bleaching mechanism was discussed based on the ab-initio molecular orbital calculations, in which Li⁺ or Zn²⁺ ions were introduced into the WO ₃ frameworks. In the case of Zn insertion, electrons were trapped in a non-bonding Zn 4s orbital just below the conduction band. It was finally concluded that in the films with distorted WO₃ frameworks, Zn 4s orbital was mingled in the conduction band and electrons were free to be transferred between W⁶⁺ and W⁵⁺. As the structural relaxation progressed, electrons were gradually trapped in Zn 4s orbital, and the films were finally bleached.