QCM-based humidity sensor and sensing properties employing colloidal SnO2 nanowires

In this paper, a quartz crystal microbalance (QCM)-based humidity sensor has been fabricated by colloidal tin oxide (SnO 2 ) nanowires. The SnO 2 nanowires were synthesized via a colloidal process that employing SnCl 4 ethanol solutions mixed with oleic acid (OA) and oleylamine (OLA). The spin-coating method was chosen to deposit the sensitive film at room temperature without high-temperature sintering. Furthermore, the Cu(NO 3 ) 2 methanol solution was used to remove the organic long chains coated on the surface of the colloidal SnO 2 nanowires and expose more adsorption sites. The characterizations of the material were studied by means of FTIR, XRD, and FE-SEM. The saturated salt solution method was used to investigate the humidity sensing performance (response, recovery, repeatability, etc.) of the QCM sensor in the range of 11%–97%. The results showed that the sample with 12 h synthesis has the best humidity response. This may be attributed to the larger specific surface area which makes the material provide more adsorption sites for water molecules, and more (110) planes formed which could absorb more water molecules proved by the Density Functional Theory (DFT) calculation.