*2.1. Structural Study*

Figure 1 shows the XRD pattern for the catalysts synthesized in this study. The hexagonal wurtzite phase (JCPDS No. 36−1451) is identified as a crystalline structure for ZnO thin films, with the signals of the diffraction pattern corresponding to those reported by other authors [46]. The doping process did not affect the main signals in the diffraction patterns, as the XRD patterns for ZnO:Cu and ZnO:Co showed signals of a wurtzite ZnO structure. However, the XRD patterns showed a change in the intensity of the signals, suggesting that metal ions could substitute Zn2<sup>+</sup> ions after the doping process. Lima et al. suggested that the change in the intensities could be associated with changes in both (i) grain size due to network defects and (ii) oxygen vacancies [47,48].

We used the Debye–Scherrer equation to calculate the crystalline domain size of the catalysts, using the full width at half maximum (FWHM) for the highest peak (101), with θ being the Scherrer diffraction angle [49]. Although there is no clear tendency between the intensity of the signal and metal doping load (see Figure 1b,c), all samples reduced the grain size of catalysis after the doping process (see Table 1). This could be explained by the incorporation of Co2<sup>+</sup> and Cu2<sup>+</sup> ions as dopants into the ZnO after the doping process [48,49]. Finally, the structural results suggest that ZnO films incorporated metallic ions. This observation was verified by Raman spectroscopy and diffuse reflectance, as described in the next sections.

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 13

**Figure 1.** (**a**) X-ray diffraction patterns for the catalysts synthesized in this study. (**b**) Comparison of the highest peak (101) for ZnO:Cu. (**c**) Comparison of the highest peak (101) for ZnO:Co. **Figure 1.** (**a**) X-ray diffraction patterns for the catalysts synthesized in this study. (**b**) Comparison of the highest peak (101) for ZnO:Cu. (**c**) Comparison of the highest peak (101) for ZnO:Co.


**Table 1.** Crystallographic results for the X-ray characterization of both undoped ZnO and metaldoped ZnO thin films. **Table 1.** Crystallographic results for the X-ray characterization of both undoped ZnO and metal-doped ZnO thin films.

ZnO:Cu 1% 0.2601 11,649 32.1 \* FWHM: full width at half maximum.

ZnO:Cu 3% 0.2803 9202 29.8 ZnO:Cu 5% 0.3075 5300 27.2
