Anomalous Sensor Response of TiO2 Films: Electrochemical Impedance Spectroscopy and ab Initio Studies

In this paper, we report the elaboration of TiO2 thin films (∼0.25 μm) from sol−gel solutions by dip-coating and annealing techniques as well as the annealing temperature-dependent response of these films when sensing ammonia. Electrochemical impedance spectroscopy studies revealed that films annealed at 400 °C show an anomalous p-type behavior manifested by an increase in resistance and capacitance upon ammonia adsorption, an electron donor molecule, while a typical n-type behavior is found in films annealed at 600 °C where the decrease in resistance is in accord with the ammonia present. To understand the possible physical origin of the measured data, we perform ab initio pseudopotential density functional theory (DFT) calculations to analyze the energetics, structural properties, and electronic behavior of NH3 molecules adsorbed on small-model Ti x O y (x = 4, y = 6−8) clusters. We also use the nudged elastic band (NEB) method to analyze possible reaction pathways and transition states that could be present in the here-considered Ti x O y + NH3 systems. We found that dissociative NH3 adsorption leads to a reversed Ti x O y → adsorbate direction for the charge transfer, which is a fact that is expected to increase the resistance in electron-conducting systems, as observed in our TiO2 samples annealed at 400 °C. The energy barriers that need to be overcome to achieve the here-predicted dissociation reaction strongly depend on the local atomic environment around the adsorption site being particularly reduced when poorly coordinated Ti sites are present in the clusters.