Controllable optical transitions of amorphous Mg and Mg–Ni films via electrochemical methods

Amorphous Mg and MgNix (0.03 ≤ x ≤ 0.30) thin films capped with Pd were prepared by magnetron co-sputtering, and their hydrogen-induced optical transitions were investigated via electrochemical charging and discharging in KOH electrolyte solution. Repetitive transitions, up to dozens of times between the mirror state and transparent state, are achieved in these amorphous Mg and MgNix thin films even though some performance degeneration occurs during cycling. These deteriorations are mainly attributed to the breakdown of the film structure, which is caused by both a large change in film volume during cycling and the corrosive attack of the KOH electrolyte. In addition, calculations based on the electrochemical stripping method indicate that the hydrogen diffusion coefficient is significantly increased by amorphization; however, it is only slightly improved by the addition of Ni. Among the prepared amorphous films, MgNi0.09 film shows the largest hydrogen diffusion coefficient, namely, 2.64 × 10−13 cm2 s−1. More importantly, the optical properties of the amorphous Mg and MgNix films are readily manipulated in the charging process, especially under a small charging current density, where there is a linear correlation between charging capacity and transmittance. The tunable optical properties obtained in the present study will greatly expand the application fields of Mg-based thin films.