Humidity sensors have potential in many fields, including health care, industrial processes, and flexible electronics. The high sensitivity of humidity sensors with high performance and stability are super important in the sensing process. To fulfill these needs, significant efforts are needed to create various types of humidity sensors. Therefore, in this chapter, a variety of materials-based humidity sensors and their sensing mechanisms such as surface acoustic wave, quartz crystal microbalance, optical fiber humidity sensors, as well as field-effect transistors are discussed. Moreover, it covers a wide range of humidity sensor applications.

The photo-degradation of methylene blue (MB) dye solution is studied under UV light irradiation using a TiO₂-Mica composite calcined at 450 °C (TiO₂-M 450). X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FTIR), and UV spectroscopy are used to study the photocatalyst after it is made. XRD shows TiO₂ nanoparticles (NPs) are in the anatase and rutile phases. MB degradation is evaluated by different parameters, including the concentration of MB dye, pH, the amount of photocatalyst used, and the effect of the oxidant (hydrogen peroxide, H₂O₂). Under moderate exposure to UV light, the TiO₂-M 450 composite can remove approximately 99.2% of MB dye (1.0 × 10⁻⁴ M) for 80 min. The optimal amount of TiO₂-M 450 composite is 5.0 g L⁻¹, corresponding to 2.0 g L⁻¹ of TiO₂ NPs. MB dyes can be treated with a simple, successful, and effective process. The nanocomposite is thought to be a good candidate for a photocatalyst that could be used to clean up the environment.