Preparation of Ca2MnO4 films using Bayesian optimization and their black color properties owing to Ti4+-doping

Ca2MnO4-based films were prepared by using a sol–gel method and Ti4+ ions were doped into the Mn4+ site to control their color properties. For non-doped Ca2MnO4 films, several preparation conditions were screened by using Bayesian optimization. In the Ca2(Mn,Ti)O4 film samples that were prepared under suitable conditions, the target phase was obtained as a main phase and the solid solutions were formed. Scanning electron microscopy–energy-dispersive x-ray spectroscopy analysis elucidated that an intermediate layer of CaSiO3 with 1 µm was formed between the film and substrate, and hence the Ca/Mn ratio became Ca-poor closer to the intermediate layer. The film samples showed strong optical absorption in the visible-light region simultaneously with weak absorption in the near-infrared (NIR) region. The bandgap energy of the film sample was also found to decrease by doping with Ti4+, resulting in a color change from blackish brown to black. Accordingly, the films had high NIR-reflectivity/transmittivity and the deep black coloration was controlled by the Ti4+-doping.