Preparation of electrochromic thin films by transformation of manganese(II) carbonate

•A new chemical bath method for deposition of MnCO3 thin film is designed.•The deposition has been performed from aqueous solution containing urea and MnCl2.•The thin films with thickness in the range of 70–500 nm are deposited.•The as-deposited films were electrochemically transformed into electrochromic films.•These films exhibit electrochromism changing the color from brown to pale yellow.

A new chemical bath method for deposition of manganese(II) carbonate thin film on electroconductive FTO glass substrates is designed. The homogeneous thin films with thickness in the range of 70 to 500 nm are deposited at about 98 °C from aqueous solution containing urea and MnCl2. The chemical process is based on a low temperature hydrolysis of the manganese complexes with urea. Three types of films are under consideration: as-deposited, annealed and electrochemically transformed thin films. The structure of the films is studied by XRD, IR and Raman spectroscopy. Electrochemical and optical properties are examined in eight different electrolytes (neutral and alkaline) and the best results are achieved in two component aqueous solution of 0.1 M KNO3 and 0.01 M KOH. It is established that the as-deposited MnCO3 film undergoes electrochemically transformation into birnessite-type manganese(IV) oxide films, which exhibit electrochromic color changes (from bright brown to pale yellow and vice versa) with 30% difference in the transmittance of the colored and bleached state at 400 nm.