# Photoconductive TiO\({}_{2}\) Dielectrics Prepared by Plasma Spraying
## 3 Results and Discussion
### Photoconductivity  
Run of the volume DC resistivity was observed in time, Figure 9. Light was first switched on after 3.0 h and switched off after 6.2 h after the beginning of the experiment. We saw a rather fast decrease in resistivity under illumination, which was later saturated. This corresponds to the activation of charge carriers and their subsequent depletion. Photoconductivity at room temperature could be mostly ascribed to the increment in carrier density under illumination. The new \"stable\" value induced by light was at about 20 M\(\Omega\)m, i.e., a decrease to about 42.5% of the initial value (i.e., the value before enlightenment being considered 100%). After switching the light off, the recovery started. In our experiment, this process was similarly fast as the decay (approximately 3.2 h). This interval can be called a persistent photoconductivity period. The initial horizontal trend of the \"dark\" resistivity was followed further. The experiment showed a relatively large, but slow, fully recoverable photo-induced change in resistivity (or, inversely, conductivity). In principle, similar transient photo-current responses have been detected at electrochemical measurements by other authors [25]. High and persistent photoconductivity of an oxide hetero-structure has been described [22] but for a system consisting of a thin LaFeO\({}_{3}\) film grown on a SrTiO\({}_{3}\) single crystal. The reproducibility of the photoconductivity phenomenon was monitored, Figure 9, for a total duration of 45 h. The \"light-off\" periods were characterized by a resistivity mean value 32.83 \(\pm\) 0.73 M\(\Omega\)m, and the \"light-on\" periods by a resistivity mean value 19.46 \(\pm\) 0.12 M\(\Omega\)m. The narrow \"secondary\" peaks, indicated in the figure by arrows, present because of fluctuations (various source instabilities and ripples), were abandoned for the calculations.  
Figure 8: Loss tangent of plasma-sprayed n-TiO\({}_{2}\).  
The same coating, but annealed at 750 \({}^{\circ}\)C, had a stationary (\"dark\") resistivity value of about 1300 M\(\Omega\)m, as published earlier [20]. The higher the annealing temperature, the stronger the re-oxidation back to the exact TiO\({}_{2}\) form, and the higher the \"dark\" resistivity. Material CaCu\({}_{3}\)Ti\({}_{4}\)O\({}_{12}\) (CCTO) was exposed to 200 mW \(\times\) cm\({}^{-2}\) (i.e., 10-times higher than in our case) of light with the intensity maximum at 405 nm (i.e., violet, whereas our light was much closer to red) [28]. Upon violet light exposure, the CCTO resistance showed an initial rapid decrease ascribed to oxygen-vacancy states induced by the light [28].  
The DC volt-ampere characteristics (VAC) of an n-TiO\({}_{2}\) sample were recorded, Figure 10. The signal was received at sweep from 0 V to 100 V DC, with a voltage step of 5 V and a time step of 10 s. Two dependencies are shown--VAC for the dark conditions (label \"off\"), and VAC recorded after turning the light on (label \"on\"). The graph shows that the stronger the voltage, the more pronounced the light-induced photo-conductance. This phenomenon is strictly linear for voltages over 20 V. The value of about 15 V represents a threshold value, under which the current flow is negligible. This character would be prospective for the sensor function of the n-TiO\({}_{2}\) coating.  
Figure 10: Volt-ampere characteristics (VAC) of the n-TiO\({}_{2}\) coating.  
Figure 9: Photoconductivity (expressed as resistivity variation in time).