## 3 Results and Discussion
### Gas Sensing Properties  
Due to the semiconducting character of the thin films, their gas sensing properties towards 3.5% hydrogen diluted in argon were examined. Changes in resistance after gas exposure as a function of time are shown in Figure 12. The gas-sensing mechanism of chemoresistance gas-sensing layers is correlated with oxygen vacancies created at the surface of the sensing film during exposure to the temperature. When exposed to air, oxygen is physisorbed on free states on the sensor surface at room temperature. As the temperature increases, O\({}_{2}\) molecules chemisorb in the previously physisorbed layer and trap conduction charges from the deeper part of the thin film, which causes the formation of oxygen ions on the surface (the oxidation state and reactivity of which depend on the temperature) and the resistance of the film increases or decreases depending on the type of semiconductor. When the sensing film is placed in the gas atmosphere, the resistance behaviour reverses after the adsorption of the gas molecules. The resistance changes are related to the bending of the Fermi level as a result of the formation of a depletion or enhancement region. After exposure to the ambient atmosphere, the resistance returns to the starting level before exposure to the gas. The sensing performance may be improved by adding noble dopants such as Pd or Au [52; 53]. Figure 12: Gas response upon exposure to the 3.5% of hydrogen of as-deposited V\({}_{\mathrm{x}}\)O\({}_{\mathrm{y}}\) thin films prepared using various ArO\({}_{2}\) gas ratios during sputtering: (**a**) 8.1, (**b**) 7:1, (**c**) 6:1, (**d**) 5:1. According to the literature, a better sensing response toward hydrogen is exhibited by _n_-type semiconductors. The sensor response of the _p_-type semiconducting thin film is usually lower and is defined as the square root of the same n-type thin film (3) [54; 55; 56]. H\({}_{2}\) molecules interact with the surface as protons, then desorb as water vapour and again interact as hydroxyl ions. The detection of hydrogen using a _p_-type semiconductor is described by Equations (4) and (5) [57; 58; 59]. \[S_{p}=\sqrt{S_{n}} \tag{3}\]  
\[2H_{2}+O_{2(adsorbel)}^{-}\leftrightarrow 2H_{2}O+e^{-} \tag{4}\]  
\[H_{2}O\leftrightarrow OH^{-}+H^{+} \tag{5}\]  
The sensor response was investigated for all deposited thin films, but only the results are shown for those prepared with 8:1, 7:1, 6:1 and 5:1 Ar:O\({}_{2}\) gas ratios during magnetron sputtering. A better sensor response was obtained for the _p_-type thin films (Figure 10) compared with the _n_-type, which were difficult to determine. The investigations were carried out at two operating temperatures: 120 \({}^{\circ}\)C and 150 \({}^{\circ}\)C. The characteristics of resistance changes as a function of time resemble a typical gas response graph for _p_-type semiconducting oxides [60; 61]. For other _n_-type semiconducting vanadium oxide thin films, i.e., prepared with Ar:O\({}_{2}\) gas ratio of 5:1 and lower, the gas response characteristics were analogous to those in. It was impossible to determine gas response or response/recovery times due to the low operating temperature and the amorphous structure of the _n_-type thin films that results in a very low active surface area. The summary of the gas sensing properties is presented in Table 2. The sensor response (\(S_{R}\)) was calculated as the maximum resistance of thin films after exposure to hydrogen (\(R_{gas}\)) to the resistance in pure air (\(R_{air}\)), after gas desorption according to the equation:  
\[S_{R}=\frac{R_{gas}}{R_{0}}\,100\% \tag{6}\]  
The response time is the time necessary to reach 90% of the resistance change from resistance in the air to maximum resistance after exposure to gas. In turn, the recovery time is defined in the same way but on the desorption curve. The lowest response time was obtained for the thin film prepared with the 8:1 Ar:O\({}_{2}\) gas ratio.