# Study on the Ozone Gas Sensing Properties of rf-Sputtered Al-Doped NiO Films
## 3 Results and Discussion
### Gas Sensing
Moreover, films synthesized with 2.8% O\({}_{2}\) in plasma had a lowest detection limit of 400 ppb [27] (sensitivity 0.84%), 200 ppb (sensitivity 6.25%), 200 ppb (sensitivity 6.94%) and 60 ppb (sensitivity 6.22%) when the operating temperature equals to RT, 80 \({}^{\circ}\)C, 110 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C, respectively. Finally, the NiO:Al prepared with 4% O\({}_{2}\) in plasma appeared to have a lowest detection limit of 60 ppb (sensitivity 3.18%), 10 ppb (sensitivity 2.54%), 60 ppb (sensitivity 2.44%) and 400 ppb (sensitivity 9.02%) when the operating temperature equals to RT, 80 \({}^{\circ}\)C, 110 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C. The detection limit of 10 ppb at 80 \({}^{\circ}\)C is one order of magnitude less than the one that has been reported [32] for Co\({}_{3}\)O\({}_{4}\) nanobricks at almost the same working temperature (85 \({}^{\circ}\)C). The above mentioned values for all NiO:Al films are presented in Table 2.  
Furthermore, in Figure 7 the effect of the operating temperature on gas sensing performance of the rf-sputtered NiO:Al films is shown. The films tested against 400 ppb ozone at various operating temperatures, namely RT, 80 \({}^{\circ}\)C, 110 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C. It can be seen that films prepared with 4% O\({}_{2}\) in plasma appeared to have the best sensitivity at 80 \({}^{\circ}\)C, while the rest of them showed a maximum sensitivity at 150 \({}^{\circ}\)C. This can be attributed to both less Al content and smaller crystallite size of the films grown with 4% O\({}_{2}\) in plasma compared to the others, which also leads to a further decrease of the defect states that can reduce the adsorption energy of the O\({}_{3}\) gas molecules [34]. In addition, the operating temperature of 80 \({}^{\circ}\)C is one of the lowest that has been reported for ultra-low ozone concentration (10 ppb) [4; 32]. Finally, the response and recovery time of the NiO:Al films for each operating temperature were calculated and presented in Figure 8a-c. Films grown with 2% and 2.8% O\({}_{2}\) in plasma had a response time between 400 s and 800 s, depending on both operating temperature and ozone concentration, while for those prepared with 4% O\({}_{2}\) in plasma the response time was ranging from 150 s to 500 s. In the same way, the recovery time for the former films was between 400 s and 900 s, while it was varied from 200 s to 600 s for films grown with 4% O\({}_{2}\) in plasma. It should be noticed that the Al: NiO films grown with 4% O\({}_{2}\) in plasma showed the best response and the recovery time equals to 189.6 s and 243.6 s, respectively, against 10 ppb ozone at an operating temperature of 80 \({}^{\circ}\)C. The response time values of Al: NiO films are higher than those in other reported works, while the recovery time values are comparable to those reported in the literature [4; 32; 34; 35; 36; 37; 38]. However, most of these sensors work at higher temperatures and/or have a higher concentration detection limit than those presented in this work. Figure 6: Sensitivity on different ozone concentrations at various operating temperatures of rf-sputtered NiO:Al with (**a**) 2.0%, (**b**) 2.8%, and (**c**) 4.0% O\({}_{2}\) in plasma. A linear fitting plot has been applied. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample No.** & \begin{tabular}{c} **\% O\({}_{2}\)** \\ **in Plasma** \\ \end{tabular} & \begin{tabular}{c} **T\({}_{\text{oper}}\) (\({}^{\circ}\)C)** \\ **Concentration (ppb)** \\ \end{tabular} & \begin{tabular}{c} **Minimum Detected** \\ **S (\%)** \\ \end{tabular} \\ \hline S1749 & 2.0 & \begin{tabular}{c} RT \\ 80 \\ 110 \\ 150 \\ 100 \\ 150 \\ 150 \\ \end{tabular} & \
\begin{tabular}{c} 213 \\ 393 \\ 38.66 \\ 212 \\ 6.94 \\ 6.22 \\ 3.18 \\ 2.54 \\ 2.44 \\ 9.02 \\ \end{tabular} &
\begin{tabular}{c} 1.88 \\ 5.17 \\ 11.31 \\ 38.66 \\ 0.84 \\ 6.25 \\ 6.94 \\ 6.22 \\ 3.18 \\ 2.54 \\ 150 \\ \end{tabular} \\ S1743 & 2.44 \\ \hline \hline \end{tabular}
\end{table}