Optical humidity sensors based on titania films fabricated by sol–gel and thermal evaporation methods

Abstract

This paper reports a comparative study of an optical humidity sensor based on titania films fabricated by sol–gel and thermal evaporation methods. As semiconducting oxides are known for their n-type conduction because of the presence of oxygen vacancies, therefore they prove to be very good sensors for humidity. Sensing elements of the optical humidity sensor presented here consist of a rutile structured one-layered TiO2 thin film deposited on the base of an isosceles glass prism of thickness 1000 Å. This TiO2 film is porous and sensitive to humidity. The other sensing element consists of a film of the same material deposited by the thermal evaporation method on the base of a prism of the same thickness. Light from a He–Ne laser enters the prism from one of the isosceles faces of the prism and gets reflected from the glass–film interface, before emerging out from its other isosceles face. The emergent beam is collected through an optical fibre, which is connected to an optical power meter for measurement. Variations in the intensity of light caused by changes in humidity lying in the range of 5% RH to 95% RH have been recorded. A sensor fabricated by the thermal evaporation method shows better sensitivity than the sol–gel method. Scanning electron micrographs of both the films show that the film prepared by the thermal evaporation method is more porous and continuous than the film prepared by the sol–gel method, resulting in more sensitivity to humidity.