**2.2 Thin film formation**

Prepare a coating of ZnO particles with zinc complex (zinc acetylacetonate (Zn(AcAc)), propanol (2-propanol) and a small amount of surfactant (thiol). See **Table 1** for the weight ratio; the ZnO particles were agglomerated and thiol was used to disperse them. The ZnO particles were agglomerated and thiol was used to disperse them. For the mixed solution of ZnO particles and Zn(AcAc) prepared in this study, the location of Zn(AcAc) in the residual solution after filtration of ZnO particles was not confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Zn(AcAc) was adsorbed on the zinc oxide surface. It is expected that Zn(AcAc) is electrostatically adsorbed on the surface of particles in the paint and the complexes serve as seeds for cross-linking between ZnO particles and promote the formation of ZnO thin films in two dimensions [19]. No solvent is added to act as a binder, and the coating is used to form a thin film. Plastic films with high transmittance, such as polyethylene terephthalate (PET) or polycarbonate film (PC), can be used as substrates to support the thin film. For thin film formation, a spray method was used, and a thin film with a thickness of 80 μm was applied before drying. The thin film was then dried using dry air at 60 °C to volatilize the solvent (propanol) and form a foil with only solids. At this point, the foil formed on the substrate is an aggregate of fine particles and almost no adhesion has occurred.
