Preparation of nanosheet thin films by the mist deposition method and investigation of the formation mechanism of their structures

Niobate nanosheets were mist-deposited onto substrates to form thin films, and their formation mechanism was investigated with respect to the nanosheets’ dispersibility in dispersion media and evaporation behavior. Three nanosheet dispersions were prepared by interlayer surface modification of layered potassium hexaniobate using phosphoric acid and subsequent exfoliation in water, ethanol and 1-propanol. Nanosheets bearing hydrophilic groups exhibited the highest dispersibility in water, followed by ethanol and, finally, by 1-propanol with the lowest. In the initial stage of film formation using low-concentration dispersions with about 92 mmHg vapor pressure of dispersion media, the nanosheets maintained their dispersion state during the mist deposition process with an aqueous dispersion but formed agglomerates with ethanol and 1-propanol dispersions, with the larger agglomerates formed when using 1-propanol. For long-time mist deposition at high-concentration dispersions with about 92 mmHg vapor pressure of dispersion media, fine porous films with thin pore walls were formed using an aqueous dispersion. In contrast, the use of 1-propanol provided films with thick, dense pore walls, while the films prepared using an ethanol dispersion showed intermediate characteristics. The mist-depositions with the ethanol dispersion at vapor pressures of 78 mmHg and 134 mmHg resulted in dense films. Thus, fine porous structures were likely achieved by maintaining a dispersion state with high dispersibility of nanosheets and appropriate evaporation behavior of dispersion media by suppressing aggregation and restacking of nanosheets in the mist transportation process and/or on the substrate. The advantage of mist deposition over drop-casting for porous film formation was also demonstrated. • Nanosheet dispersions were prepared using three types of dispersion media. • Nanosheet films were successfully fabricated by mist deposition. • Fine porous films can be formed from high dispersed nanosheet dispersions. • Dense films can be formed from nanosheet dispersions with low dispersibility. • Dense films were formed when the dispersion media evaporated too quickly or slowly.