# Effect of Platinum Doping on the Morphology and Sensing Performance for CuO-Based Gas Sensor
## 3 Results and Discussions
### The Sensing Mechanism  
The gas-sensing mechanism of the p-type semiconducting oxides has been previously reported, and it can be summarized as follows: Adsorption/desorption of gas molecules on the surface of semiconductors and reactions cause the change of resistance [11; 24]. When the p-type CuO semiconductor is in contact with air, oxygen molecules are adsorbed on the surface of CuO and are ionized by electrons from the valence band, thereby generating negatively charged chemisorbed oxygen species (O\({}^{2-}\), O\({}^{-}\), and O\({}_{2}\)\({}^{-}\)), the processes are shown in Equations (1)-(4). The types of absorbed oxygen species depend on the operating temperature. Oxygen is absorbed on the surface as O\({}_{2}\)\({}^{-}\) ions below 147 \({}^{\circ}\)C or as O\({}^{-}\) ions between 147-397 \({}^{\circ}\)C. Above 397 \({}^{\circ}\)C, O\({}^{2-}\) ions occur [39]. Figure 11: (**a**) Dynamic response transient of the gas sensor based on 1.25 Pt-CuO and 1.00 Pt-CuO to different concentrations of H\({}_{2}\)S gas at 40 \({}^{\circ}\)C. (**b**) gas response of 1.25 Pt-CuO and 1.00 Pt-CuO sensor to different concentrations of H\({}_{2}\)S at 40 \({}^{\circ}\)C.  
Figure 12: Long-term stability of 1.25 Pt-CuO sensor to 10 ppm H\({}_{2}\)S at 40 \({}^{\circ}\)C.  
According to the above conclusions, the major oxygen adsorption species on the surface of CuO is O2\({}^{-}\) when the operating temperature is 40 \({}^{\circ}\)C.  
\[\mathrm{O_{2}}\left(\mathrm{gas}\right)\rightarrow\mathrm{O_{2}}\left(\mathrm{ ads}\right) \tag{1}\]  
\[\mathrm{O_{2}}\left(\mathrm{ads}\right)+\mathrm{e^{-}}\rightarrow\mathrm{O_{2}}^ {-}\left(\mathrm{ads}\right)\left(<\)147 \({}^{\circ}\)C\(\right) \tag{2}\]  
\[\mathrm{O_{2}}^{-}\left(\mathrm{ads}\right)+\mathrm{e^{-}}\rightarrow\mathrm{2O ^{-}}\left(\mathrm{ads}\right)\left(147\ ^{\circ}\)C-397 \({}^{\circ}\)C\(\right) \tag{3}\]  
\[\mathrm{O^{-}}\left(\mathrm{ads}\right)+\mathrm{e^{-}}\rightarrow\mathrm{O^{2 -}}\left(\mathrm{ads}\right)\left(>\)397 \({}^{\circ}\)C\(\right) \tag{4}\]  
Once the sensor is exposed to an H2S (reducing gas) atmosphere operating at 40 \({}^{\circ}\)C, the H2S molecules are oxidized by O2\({}_{-}\) to form SO2 and H2O, while releasing electrons [40], and the sensing reaction is as shown in Equation (5). \[\mathrm{H_{2}S}\left(\mathrm{g}\right)+3\mathrm{O_{2}}^{-}\left(\mathrm{ads} \right)\rightarrow\mathrm{H_{2}O}\left(\mathrm{g}\right)+\mathrm{SO_{2}}\left( \mathrm{g}\right)+6\mathrm{e^{-}} \tag{5}\]  
The released electrons during this process will recombine with the holes on the surface of CuO and cause the decrease of hole concentration, which will lead to an increased resistance of the gas sensor. According to some previous reports [41, 42], a CuS layer might form on the surface of the CuO, when the sensor is exposed to high H2S concentrations, the spontaneous chemical reaction is as shown in Equation (6), as follows:  
\[\mathrm{CuO}\left(\mathrm{s}\right)+\mathrm{H_{2}S}\left(\mathrm{g}\right) \rightarrow\mathrm{CuS}\left(\mathrm{s}\right)+\mathrm{H_{2}O}\left(\mathrm{ g}\right) \tag{6}\]  
In order to confirm the formation of CuS, we have carried out XPS studies on a 10 ppm H2S exposed 1.25 Pt-CuO sensor. The recorded Cu 2p and S 2p spectra are shown in Figure 13. When the sensor was exposed to H2S gas, the Cu 2p spectrum confirmed the presence of CuO and CuS, and the corresponding peak of 169.2 eV in the S 2p spectrum also confirmed the presence of CuS [11]. Once H2S is removed by introducing air, the CuS is unstable, and is able to be slowly oxidized by oxygen in the air, as shown in Equation (7). As a result, recovery of the resistance will occur. \[\mathrm{CuS}\left(\mathrm{s}\right)+\mathrm{O_{2}}\left(\mathrm{g}\right) \rightarrow\mathrm{2 CuO}\left(\mathrm{s}\right)+\mathrm{2S}\left(\mathrm{s}\right) \tag{7}\]  
The influence of Pt dopant on the gas sensing properties of the sensor can be attributed to the spillover effect [43].