# Effect of Platinum Doping on the Morphology and Sensing Performance for CuO-Based Gas Sensor
## 3 Results and Discussions
### Structure and Morphology of Sensing Materials
The EDX element mapping was employed to evaluate the uniformity of element distribution in a 1.25 Pt-CuO specimen. Figure 4 shows the distribution of the O, Cu, and Pt, respectively, and it is clearly demonstrated that all three elements are homogeneously distributed. Meanwhile, the concentrations of the 1.00 Pt-CuO, 1.25 Pt-CuO, and 1.50 Pt-CuO specimens were also determined by inductively coupled plasma mass spectroscopy, the Pt concentrations were 0.76 at%, 0.85 at%, and 0.91 at%, respectively. The XPS spectral measurements were used to analyze the chemical states of the 1.25 Pt-CuO sample. The full spectra of 1.25 Pt-CuO are shown in Figure 5a, which proved the existence of Cu, Pt, and O. The appearance of the C 1s peaks can be ascribed to the adsorbed or bonded organic species [32]. In Figure 5b, two peaks at 933.4 eV and 953.2 eV, corresponding to Cu 2p\({}_{3/2}\) and Cu 2p\({}_{1/2}\), denote the existence of Cu\({}^{2+}\) in the form of CuO. In addition, the satellite peaks of Cu 2p\({}_{3/2}\) and Cu 2p\({}_{1/2}\), denoted as peaks S1 and S2, respectively, are characteristic for a partially filled d-orbital (3 d\({}_{9}\) in the case of Cu\({}^{2+}\)) [33]. The Pt 4f spectra of 1.25 Pt-CuO are shown in Figure 5c. After the Pt 4f curve fitting (Figure 5c), the spectrum consists of two pairs of peaks at 74.7 and 76.5 eV and at 75.6 and 77.9 eV, which are assigned to Pt\({}^{0}\) and Pt\({}^{2+}\), respectively [34]. The above measurements clearly proved that the Pt\({}^{2+}\) ions were successfully doped into the CuO matrix, which is in good accordance with the results of EDX and ICP-AES. Figure 5d shows the high-resolution O 1s XPS profile of Pt-doped CuO, in which the two peaks at binding energies of 529.4 eV and 531.1 eV correspond to the typical surface  
Figure 3: Field emission scanning electron microscope (FESEM)images of (**a**) 0.00 Pt-CuO, (**b**) 0.75 Pt-CuO, (**c**) 1.25 Pt-CuO, and (**d**) 1.50 Pt-CuO. The inset is the magnification of each image. lattice oxygen and the surface absorbed oxygen species (O\({}_{2}^{-}\)ads and O\({}^{-}\)ads) [35]. The surface absorbed oxygen species play a positive role in the enhancement of gas sensing performance, as they are capable of reacting with test gas molecules [36]. Figure 4: Energy Dispersive X-Ray (EDX) element mapping of 1.25 wt % Pt-doped CuO structure, as follows: (**a**) 1.25 Pt-CuO, (**b**) O, (**c**) Cu, and (**d**) Pt. Figure 5: (**a**) X-ray photoelectron spectroscopy (XPS) spectrum of the 1.25 wt % Pt-doped CuO; (**b**–**d**) high-resolution spectra of Cu, Pt, and O peaks, respectively, (red line, blue line, purple line and green line are fitting curves). The BET surface areas of the pure CuO and 1.25 Pt-CuO were investigated via a BET method using N\({}_{2}\) adsorption and desorption. As shown in Figure 6, it has been shown that the BET surface area of the 1.25 Pt-CuO is 24.3 m\({}^{2}\)/g, which is 1.9 times larger than pure CuO (13.1 m\({}^{2}\)/g). However, none of such two samples showed a porous structure based on the BET results. The larger surface area of the 1.25 Pt-CuO can be beneficial for the enhancement of gas sensing properties.