**4. Conclusions**

We synthesized and characterized ZnO thin films doped with Co and Cu. Raman results corroborated the doping process, which suggested the generation of a heterostructure. For the doped ZnO catalysts, the results show a reduction in the *E<sup>g</sup>* values (from 3.22 to 2.42 eV for the best catalyst). This behavior is associated with a reduction of the Fermi level of ZnO by the presence of intragap states. The photocatalytic test indicated that doped catalysts had greater photocatalytic activity than unmodified catalysts, which could be attributed to (i) the generation of intraband states for the insertion of Co and Cu into ZnO, and (ii) the generation of ZnO–CuO and ZnO–Co3O<sup>4</sup> heterojunctions. Furthermore, the rate of the photodegradation process (ZnO:Co 5%) was 36 times greater than the rate for unmodified ZnO, and the k*app* values for the catalysts synthesized in this study had a suitable photocatalytic activity compared to other reports.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/5/528/s1. Figure S1: ICP calibration curve for: (a) Co and (b) Cu content. Table S1: Results ICP for catalysts. Fitting curves of kinetic model.

**Author Contributions:** Conceptualization, W.V.; methodology, W.V., C.D.-U., B.S., A.C., W.R., E.R. and M.H.; validation, W.V., C.D.-U., B.S., A.C.; formal analysis, W.V., C.D.-U., B.S., A.C.; investigation, W.V., C.D.-U., B.S., A.C.; resources, W.V., C.D.-U., B.S., A.C., W.R., E.R. and M.H.; data curation, W.V., C.D.-U., B.S., A.C.; writing—original draft preparation, W.V., C.D.-U., B.S., A.C., writing—review and editing, W.V., C.D.-U., B.S., A.C., W.R., E.R., M.H.; visualization, W.V., C.D.-U.; supervision, W.V., C.D.-U.; project administration, W.V., C.D.-U.; funding acquisition, W.V., C.D.-U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Universidad del Atlántico.

**Acknowledgments:** W.V., C.D.-U., B.S., and A.C. would like to thank Universidad del Atlántico.

**Conflicts of Interest:** The authors declare that there is no conflict of interest.
