Unexpected hygroscopic behaviors of individual sub-50 nm NaNO3 nanoparticles observed by in situ atomic force microscopy

Hygroscopicity is one of the most important physicochemical properties of salt nanoparticles, greatly influencing the environment, climate and human health. However, the hygroscopic properties of salt nanoparticles are poorly understood owing to the great challenges of the preparation, preservation and in situ characterization. Here we show the unexpected shape- and size-dependent hygroscopic behaviors of NaNO3 nanoparticles prepared from molten salts using in situ environment-controlled atomic force microscopy. During the humidifying process, the angular and round sub-50 nm NaNO3 particles display anisotropic and isotropic water adsorption behaviors, respectively. The sub-10 nm NaNO3 nanoparticles abnormally shrink and disappear. The growth factors of the NaNO3 nanoparticles are highly sensitive to their sizes and shapes, and quite different from those of NaNO3 microparticles. These findings show that the hygroscopic behaviors of salt nanoparticles may not be comprehensively described by the traditional growth factors, and open up a new pathway to study the hygroscopic behaviors of salt nanoparticles.