Low temperature TiO 2 based gas sensors for CO 2

Abstract Monitoring the level of CO 2 , especially in closed spaces, is more and more required in technological applications, or in human activities. Since most of the literature data reveal CO 2 detection materials with high sensitivities over 300 °C, here we have concentrated on the gas sensing abilities of Cr doped TiO 2 thin films in front of CO 2 , close to the room temperature and at atmospheric pressure. The films were obtained by RF reactive sputtering. The undoped films contain a mixture of anatase and rutile phases. With the increase of Cr content, the crystallites size decreases, and the films become pure rutile for a 4 at% Cr concentration. We found out that these material based sensors are more sensitive to CO 2 for higher Cr concentration, the optimum operating temperature approaching to the room temperature, determining in fact low energy consumption. The explanation is related to the observed increase of oxygen vacancies number (which we have evidenced and clarified), and also to the presence of the rutile phase, whose higher dielectric constant (compared to anatase), and its finer crystallites, determine a better gas sensing. More, the surface active area in front of CO 2 increases, as the films become rougher for higher Cr contents. The increase of Cr 3+ percentage enhance the power of interaction with the adsorbed species (O 2 and/or CO 2 ). A grain boundary model was proposed for the thermal activation of the electrical conductivity. The energy barrier height at the grain boundary, the impurities concentration (characteristic parameters of this model) were calculated and found to agree well with the data in the literature.