Nanostructured cobalt oxide and cobalt titanate thin films as optical humidity sensor: A new approach

In the present work, multilayered sensing films were prepared by sol–gel spin coating process using precursor solutions of cobalt oxide and cobalt titanate. Cobalt oxide films were annealed at 150, 300 and 450 °C respectively; however, the films of cobalt titanate were annealed at 450 °C. XRD reveals the formation of crystalline Co3O4 and CoTiO3 nanocomposites at various annealing temperatures. UV–visible spectra exhibit the dependency of optical energy gap on annealing temperature and thickness of the film. Further the optical humidity sensing properties based on modulation in transmitted intensity due to variation in %RH were investigated. SEM micrographs confirm the dependency of sensitivity of the sensor on porosity and thickness of the film. Double layered sensing film of cobalt oxide annealed at 450 °C showed linear behavior having average sensitivity 0.76 μW/%RH, while single layered film of cobalt titanate is more sensitive to humidity over others having sensitivity 1.84 μW/%RH.

► Opto-electronic humidity sensing properties were investigated.
► Minimum crystallite size for Co3O4 was found 9 nm.
► Particle size increases with film thickness.
► Sensitivity of single layered film of cobalt titanate was found 1.84 μW/%RH.