Fabrication of nanoporous and hetero structure thin film via a layer-by-layer self assembly method for a gas sensor

Nanoporous and hetero structure thin film consisted of weak polyelectrolytes and TiO₂ nanoparticles was fabricated by the sequential depositions of oppositely charged solutions via a layer-by-layer self assembly (LBL-SA) method on a quartz crystal microbalance (QCM) at room temperature and then by an immersion in acidic water (pH 1.8) for 2min and neutral water for 10s subsequently. Because in this process, the ionic bonds of weak polyelectrolytes between poly(allylamine hydrochloride) and poly(acrylic acid) were broken, which contributed to separate the agglomerated TiO ₂ nanoparticles in thin film. In order to measure the gas sensitivity of hetero structure thin film with a high surface area, the frequency shifts of a QCM were monitored as functions of the concentration of ammonia gas and relative humidity. The QCM immersed in acidic water showed larger frequency shifts than the QCM, without an immersion. As the concentration of gas and relative humidity were increased, the frequency shifts of a QCM were also increased gradually.