Humidity sensors based on mesoporous silica thin films synthesised by block copolymers

The application of mesostructured thin films to fabricate electrochemical sensors requires the control of dimension, shape and distribution of pores in the material. Silica mesoporous thin films were deposited via dip coating on silicon and alumina substrates with interdigitated electrodes. Mesostructured films were obtained by sol-gel self-assembled process using di-block, tri-block or star-block copolymers: 2-D hexagonal mesoporous phases in silica were formed. After deposition the films were calcined in air to remove the surfactant and were characterised by Fourier transform infrared spectroscopy and low angle X-ray diffraction. Current variations with relative humidity were measured using different applied d.c. voltage; I/V characteristics were performed at various relative humidity values. Moreover the dependence of response from temperature and behaviour during cyclic test in dry-wet conditions was studied. The electrical response was found to be dependent on dimension of pores and their surface. Electrical characterisation upon exposure to humidity shows that the mesoporous structure is easily accessible by external environment, and the films prepared by non-ionic surfactants exhibit good performances in comparison with commercial humidity sensors.