# 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  
Figure 2 plots the XRD patterns of pure CuO structures and Pt-doped CuO structures. The patterns of pure CuO are specified as monoclinic CuO (JCPDS No. 72-0629), and reveal a series of well-defined diffraction peaks at 20 of 32.53\({}^{\circ}\), 35.56\({}^{\circ}\), 38.75\({}^{\circ}\), 48.75\({}^{\circ}\), 53.50\({}^{\circ}\), 58.36\({}^{\circ}\), 61.57\({}^{\circ}\), 66.28\({}^{\circ}\), and 67.97\({}^{\circ}\), corresponding to the (110), (\(-\)111), (111), (\(-\)202), (020), (202), (\(-\)113), (\(-\)311), and (113) planes of monoclinic CuO. All XRD patterns look very similar at first sight, as shown in Figure 2a. However, the difference can be found at the specific region (35-40\({}^{\circ}\)) of XRD patterns in Figure 2b. After Cu\({}^{2+}\) ions had been replaced by Pt\({}^{2+}\) ions in the CuO matrix, the lattice parameter of CuO crystal is changed because of the different ionic radius between Cu\({}^{2+}\) (0.73 A) [28] and Pt\({}^{2+}\) (0.86 A) [29]. As a result, with the increase of the Pt dopant from 0.75 wt % to 1.50 wt %, the (\(-\)111) and (111) peaks were slightly shifted to a lower angle [30]. What is more, we used the least square refinement method in the unit cell program to determine the lattice constants. The obtained parameters of the 1.25 wt % Pt-doped CuO are \(\alpha\) = 4.6837 A, \(b\) = 3.4226 A, and \(c\) = 5.1288 A, and the unit cell volume is 82.2169 A\({}^{3}\). The lattice constants of the doped sample are the monoclinic data of undoped CuO (\(\alpha\) = 4.6504 A, \(b\) = 3.3903 A, \(c\) = 5.0997 A, and unit cell volume = 80.4032 A\({}^{3}\)) obtained under the same conditions. It can be seen that the unit cell expands with the Pt\({}^{2+}\) ion doping in CuO. This clearly proved that the Pt\({}^{2+}\) ions were successfully replaced by Cu\({}^{2+}\) in the CuO matrix. Field emission scanning electron microscope (FESEM) images of pristine CuO and Pt-doped CuO with various doping concentrations are shown in the Figure 3. As shown in Figure 3a, the morphology of the pristine CuO was olive-like, exhibiting particle diameters ranging from 200 nm to 4 \(\upmu\)m. Once the Pt\({}^{2+}\) ions were introduced, the morphology of the CuO structure transformed into being flower-like, as illustrated in Figure 3b,c. At a Pt doping concentration of 1.25 wt % (Figure 3c), a three-dimensional (3D) flower-like CuO with hierarchical nanostructures was obtained. The flower-like CuO is ca. 1.5 \(\upmu\)m in diameter and homogeneously distributed, and the hierarchical flower-like CuO consists of nanosheets with sizes around 200 nm. The resulting growth-mechanism for CuO after Pt doping can be explained as follows: A copper nitrate solution provides Cu\({}^{2+}\) ions to form a blue precipitate of copper hydroxide Cu(OH)\({}_{2}\). The color of the Cu(OH)\({}_{2}\) precipitate gradually turned into dark brown, which was caused by the dehydration of Cu(OH)\({}_{2}\) and finally formed the nano-CuO along with the  
Figure 2: X-ray diffraction (XRD) pattern of the Pt-doped CuO specimens: (**a**) 0.00 Pt-CuO, (**b**) 0.75 Pt-CuO, (**c**) 1.25 Pt-CuO, and (**d**) 1.50 Pt-CuO. increase of the water bath temperature. Once the Pt was introduced, it did not affect the precipitate color, but seemed to have a consecutive influence on the growth rate of the nanoflower CuO [31]. The Pt ions tend to be adsorbed on the basal planes of the CuO, and hence modify the chemical surface properties of CuO, such as the relative surface free energy on the facets. It was also observed that the presence of higher amounts of Pt influenced the dehydration of Cu(OH)\({}_{2}\) and retarded the phase transformation of the Cu(OH)\({}_{2}\) to CuO. In addition, the morphologies of the CuO were not significantly altered after increasing the Pt doping concentration to 1.50 wt %, instead, the uniformity of the CuO declined again (Figure 3d). Thus, it was clearly demonstrated that the morphology of CuO can be adjusted by the Pt doping.