Effect of formaldehyde gas adsorption on the electrical conductivity of Pd-doped TiO₂ thin films

0.5 wt% Pd-doped titanium oxide thin films were obtained by dip-coating on silicon substrates. The films were compacted by annealing in air at 300 and 500 °C. Temperature dependent electrical conductivity measurements were performed in the temperature range 373–623 K, in different environments (air, methane, acetone, ethanol, formaldehyde and liquefied petroleum gas), to test the films sensing gas properties. Formaldehyde was found to be the test gas that produces the most significant changes in the electrical conductivity of the studied films. This was the reason why it was chosen to investigate its effect on their electrical conductivity. A model was proposed, the model of the potential fluctuations at grain boundaries. A comparison between some parameters obtained in the proposed model was performed as a function of annealing temperature, and as a function of gas atmosphere. The values of the mean barrier height and the standard deviation were estimated to range between 0.336–0.588 eV and 0.175–0.199 eV, respectively. It was found that formaldehyde leads to a rather sharp decrease in the values of the barrier height and the standard deviation, and to an increase in the conductivity. We have observed the best sensing gas performance for the films annealed at 300 °C, comparing to their counterparts annealed at 500 °C, explained by the lowest values of the barrier energy height and the standard deviation.