*2.2. Raman Study*

*2.2. Raman Study* 

\*FWHM: full width at half maximum. The Raman spectra of the catalysts are shown in Figure 2. All the peaks correspond with wurtzite–ZnO (C<sup>4</sup> 6v): (i) 97.4 cm−<sup>1</sup> (vibrational mode E2L), (ii) 340 cm (E2H–E2L), and (iii) 437.0 cm−<sup>1</sup> and 581 cm−<sup>1</sup> (A<sup>1</sup> vibrational mode) [50,51]. Figure 2a shows the Raman spectra for Cu-doped ZnO thin films. Signals E2L (~99 cm−<sup>1</sup> ) and E2H (~437 cm−<sup>1</sup> ) widen and decrease after the doping process—a behavior that can be explained by the incorporation of Cu2<sup>+</sup> into the ZnO lattice. Additionally, this behavior has been associated with reduction of ZnO crystallinity by the formation of nanocomposites [52]. For greater Cu loads, Figure 2a shows two new signals, the first one located at 298 cm−<sup>1</sup> and a second weak one at 614 cm−<sup>1</sup> . These two signals can be attributed to modes A1g and B2g for CuO, respectively, and this result suggests the formation of a ZnO–CuO heterojunction during the synthesis process. The hydrodynamic stability of the suspension is affected by the concentration of reagents; so for obtaining greater Cu loads, the CuO generation is feasible. This result is in line with previous reports [39]. 581 cm−1 (A1 vibrational mode) [50,51]. Figure 2a shows the Raman spectra for Cu-doped ZnO thin films. Signals E2L (~99 cm−1) and E2H (~437 cm−1) widen and decrease after the doping process**—**a behavior that can be explained by the incorporation of Cu2+ into the ZnO lattice. Additionally, this behavior has been associated with reduction of ZnO crystallinity by the formation of nanocomposites [52]. For greater Cu loads, Figure 2a shows two new signals, the first one located at 298 cm−1 and a second weak one at 614 cm−1. These two signals can be attributed to modes A1g and B2g for CuO, respectively, and this result suggests the formation of a ZnO**–**CuO heterojunction during the synthesis process. The hydrodynamic stability of the suspension is affected by the concentration of reagents; so for obtaining greater Cu loads, the CuO generation is feasible. This result is in line with previous reports [39].

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 4 of 13

wurtzite**–**ZnO (C46v): (i) 97.4 cm−1 (vibrational mode E2L), (ii) 340 cm (E2H**–**E2L), and (iii) 437.0 cm−1 and

**Figure 2.** Raman spectra for: (**a**) ZnO:Cu and (**b**) ZnO:Co thin films. Inside the figures are the Raman vibration modes, where (\*) corresponds to defects inside the ZnO structure. **Figure 2.** Raman spectra for: (**a**) ZnO:Cu and (**b**) ZnO:Co thin films. Inside the figures are the Raman vibration modes, where (\*) corresponds to defects inside the ZnO structure.

Figure 2b shows the Raman spectrum for ZnO:Co; the intensity of A1LO, E2L, and E2H modes decreases for these films. The ZnO:Co thin films (doping load 5%) show four new signals at 490 cm−1, 526 cm−1, 626 cm−1, and 725 cm−1. These signals could be attributed to the possible presence of Co3O4, and these results confirm both the doping process and the formation of a heterojunction of ZnO**–** Co3O4 [53,54]. Figure 2b shows the Raman spectrum for ZnO:Co; the intensity of A1LO, E2L, and E2H modes decreases for these films. The ZnO:Co thin films (doping load 5%) show four new signals at 490 cm−<sup>1</sup> , 526 cm−<sup>1</sup> , 626 cm−<sup>1</sup> , and 725 cm−<sup>1</sup> . These signals could be attributed to the possible presence of Co3O4, and these results confirm both the doping process and the formation of a heterojunction of ZnO–Co3O<sup>4</sup> [53,54].

### *2.3. Morphological Study 2.3. Morphological Study*

Figure 3 shows SEM images for the catalysts. Figure 3a shows that the ZnO films formed microaggregates (~220 nm) composed of quasi-spherical ZnO nanoparticles (around 40 nm in diameter), and this is a typical result for this material sensitized by the sol**–**gel method. Figure 3b,c shows that the morphological properties changed significantly after the doping process. Regarding the ZnO:Cu thin films, Figure 3b shows the formation of nanorods. Meanwhile, Figure 3c shows the formation of nanosized elongated particles of various shapes (~100 nm) from the ZnO:Co thin films. Likewise, Figure 3c shows that the agglomeration on the catalyst surface reduced and the microaggregates disappeared. Different nanostructures have been reported for ZnO (e.g., nanorods, nanotubes, nanobelts, nanosprings, nanospirals, nanorings) [55]. It is known that ZnO's morphological properties rely on synthesis conditions, and in our case, it is clear that the metal ions used during synthesis reduced the agglomeration and changed the thin films' morphology [56]. Figure 3 shows SEM images for the catalysts. Figure 3a shows that the ZnO films formed microaggregates (~220 nm) composed of quasi-spherical ZnO nanoparticles (around 40 nm in diameter), and this is a typical result for this material sensitized by the sol–gel method. Figure 3b,c shows that the morphological properties changed significantly after the doping process. Regarding the ZnO:Cu thin films, Figure 3b shows the formation of nanorods. Meanwhile, Figure 3c shows the formation of nanosized elongated particles of various shapes (~100 nm) from the ZnO:Co thin films. Likewise, Figure 3c shows that the agglomeration on the catalyst surface reduced and the microaggregates disappeared. Different nanostructures have been reported for ZnO (e.g., nanorods, nanotubes, nanobelts, nanosprings, nanospirals, nanorings) [55]. It is known that ZnO's morphological properties rely on synthesis conditions, and in our case, it is clear that the metal ions used during synthesis reduced the agglomeration and changed the thin films' morphology [56].

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 13

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 13

**Figure 3.** SEM images: (**a**) ZnO, (**b**) ZnO:Cu 5% and (**c**) ZnO:Co 5% thin films. **Figure 3.** SEM images: (**a**) ZnO, (**b**) ZnO:Cu 5% and (**c**) ZnO:Co 5% thin films.

### *2.4. Optical Study 2.4. Optical Study* **Figure 3.** SEM images: (**a**) ZnO, (**b**) ZnO:Cu 5% and (**c**) ZnO:Co 5% thin films.

The diffuse reflectance spectra for the catalysts are shown in Figure 4. We used the Kubelka**–** Munk (KM) remission function for determining the bandgap energy value of the catalysts [57]. The use of the KM remission function makes it possible to obtain an analog to Tauc plots [58,59]: The diffuse reflectance spectra for the catalysts are shown in Figure 4. We used the Kubelka–Munk (KM) remission function for determining the bandgap energy value of the catalysts [57]. The use of the KM remission function makes it possible to obtain an analog to Tauc plots [58,59]: *2.4. Optical Study*  The diffuse reflectance spectra for the catalysts are shown in Figure 4. We used the Kubelka**–** Munk (KM) remission function for determining the bandgap energy value of the catalysts [57]. The

((∝ሺሺ

$$(F(\mathbb{R}\_{\approx})hv)^{\frac{1}{2}} = A(hv - E\_{\mathfrak{J}}) \tag{1}$$

**Figure 4.** Reflectance diffuse spectra for both catalysts. **Figure 4.** Reflectance diffuse spectra for both catalysts. **Figure 4.** Reflectance diffuse spectra for both catalysts.

Figure 5 shows plots for ሺܨሺܴ∝)݄ݒ( <sup>ଵ</sup> ଶൗ versus *(hv)* and table 2 lists the optical properties of the catalysts. Figure 5 shows that ZnO had a bandgap value (Eg) of 3.22 eV, a value that corresponds with that reported by Srikant el al. (3.1 eV and 3.2 eV) [60,61]. For doped ZnO catalysts, the Eg value was lower, and this behavior is associated with the reduction of the Fermi level of ZnO by the generation of intragap states. For ZnO:Cu, the modification of the bandgap can be attributed to the induction of 3d states of Cu located inside the bandgap of ZnO [37]. Additionally, the visible light absorption observed for doped ZnO can be attributed to intragap transitions between Cu 3d and Zn 4s states. Furthermore, the ZnO:Co 5% catalyst has a lower bandgap value compared to other catalysts. This reduction is attributed to s-d and p-d exchange interactions between ZnO and Co2+ ions [62]. The 3d levels of Co2+ are located within the bandgap of ZnO, which can create new bands at larger wavelengths [63]. Some photoluminescence studies of the transition metal doping ZnO nanoparticles suggest that this important reduction in Eg value is due to oxygen deficiency [64]. Finally, the Figure 5 shows plots for ሺܨሺܴ∝)݄ݒ( <sup>ଵ</sup> ଶൗ versus *(hv)* and table 2 lists the optical properties of the catalysts. Figure 5 shows that ZnO had a bandgap value (Eg) of 3.22 eV, a value that corresponds with that reported by Srikant el al. (3.1 eV and 3.2 eV) [60,61]. For doped ZnO catalysts, the Eg value was lower, and this behavior is associated with the reduction of the Fermi level of ZnO by the generation of intragap states. For ZnO:Cu, the modification of the bandgap can be attributed to the induction of 3d states of Cu located inside the bandgap of ZnO [37]. Additionally, the visible light absorption observed for doped ZnO can be attributed to intragap transitions between Cu 3d and Zn 4s states. Furthermore, the ZnO:Co 5% catalyst has a lower bandgap value compared to other catalysts. This reduction is attributed to s-d and p-d exchange interactions between ZnO and Co2+ ions [62]. The 3d levels of Co2+ are located within the bandgap of ZnO, which can create new bands at larger wavelengths [63]. Some photoluminescence studies of the transition metal doping ZnO nanoparticles Figure 5 shows plots for (*F*(*R*∝)*hv*) 1 <sup>2</sup> versus (*hv*) and Table 2 lists the optical properties of the catalysts. Figure 5 shows that ZnO had a bandgap value (*Eg*) of 3.22 eV, a value that corresponds with that reported by Srikant el al. (3.1 eV and 3.2 eV) [60,61]. For doped ZnO catalysts, the *E<sup>g</sup>* value was lower, and this behavior is associated with the reduction of the Fermi level of ZnO by the generation of intragap states. For ZnO:Cu, the modification of the bandgap can be attributed to the induction of 3d states of Cu located inside the bandgap of ZnO [37]. Additionally, the visible light absorption observed for doped ZnO can be attributed to intragap transitions between Cu 3d and Zn 4s states. Furthermore, the ZnO:Co 5% catalyst has a lower bandgap value compared to other catalysts. This reduction is attributed to s-d and p-d exchange interactions between ZnO and Co2<sup>+</sup> ions [62]. The 3d levels of Co2<sup>+</sup> are located within the bandgap of ZnO, which can create new bands at larger wavelengths [63]. Some photoluminescence studies of the transition metal doping ZnO nanoparticles suggest that this important reduction in *E<sup>g</sup>* value is due to oxygen deficiency [64]. Finally,

formation of nanoheterojunctions in the catalyst surface leads to an enhanced separation of charge

suggest that this important reduction in Eg value is due to oxygen deficiency [64]. Finally, the

the formation of nanoheterojunctions in the catalyst surface leads to an enhanced separation of charge carriers, increasing photocatalytic efficiency in addition to the doping process. The generation of these heterostructures has been reported for photocatalytic applications [65]. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 6 of 13 carriers, increasing photocatalytic efficiency in addition to the doping process. The generation of

these heterostructures has been reported for photocatalytic applications [65].

**Figure 5.** Kubelka**–**Munk (KM) fitting for the catalysts synthesized in this study. **Figure 5.** Kubelka–Munk (KM) fitting for the catalysts synthesized in this study.


**Table 2.** Band gap and results of pseudo-first-order model fitting.

### the Co-doped ZnO films. Compared to the Co-doped ZnO films, the bandgap values of this catalyst did not change; however, the best photodegradation result for ZnO:Cu was 42.5%, a value greater *2.5. Photocatalytic Study*

catalysts.

than that obtained for the ZnO thin films. The combined effect of the doping process and the heterojunction can explain this behavior. The photodegradation kinetics of MB on catalysts were studied by using the pseudo-first-order model [66]: ݒሾெሿ ൌ ሾܤܯሿ݁ିೌ௧ (2) Figure 6 shows the decrease of MB as a function of time for all tests performed under visible irradiation. The MB concentration did not change after 140 min under visible irradiation, verifying the stability of MB dye. Furthermore, ZnO films did not show photocatalytic activity under visible irradiation (<3%). This result is in accordance with the ZnO bandgap energy value, and this photocatalyst is active only under UV irradiation. The ZnO:Co 5% catalyst reported the highest photocatalytic activity. This result can be explained by the lower bandgap value of the ZnO:Co 5% catalyst compared to other catalysts. The ZnO:Cu catalysts showed less photocatalytic activity than the Co-doped ZnO films. Compared to the Co-doped ZnO films, the bandgap values of this catalyst did not change; however, the best photodegradation result for ZnO:Cu was 42.5%, a value greater than that obtained for the ZnO thin films. The combined effect of the doping process and the heterojunction can explain this behavior. The photodegradation kinetics of MB on catalysts were studied by using the pseudo-first-order model [66]:

$$w\_{[MB]} = [MB]\_o e^{-k\_{ap}t} \tag{2}$$

*2.5. Photocatalytic Study* 

carriers, increasing photocatalytic efficiency in addition to the doping process. The generation of

**Figure 5.** Kubelka**–**Munk (KM) fitting for the catalysts synthesized in this study.

Figure 6 shows the decrease of MB as a function of time for all tests performed under visible irradiation. The MB concentration did not change after 140 min under visible irradiation, verifying the stability of MB dye. Furthermore, ZnO films did not show photocatalytic activity under visible irradiation (<3%). This result is in accordance with the ZnO bandgap energy value, and this photocatalyst is active only under UV irradiation. The ZnO:Co 5% catalyst reported the highest photocatalytic activity. This result can be explained by the lower bandgap value of the ZnO:Co 5% catalyst compared to other catalysts. The ZnO:Cu catalysts showed less photocatalytic activity than the Co-doped ZnO films. Compared to the Co-doped ZnO films, the bandgap values of this catalyst did not change; however, the best photodegradation result for ZnO:Cu was 42.5%, a value greater than that obtained for the ZnO thin films. The combined effect of the doping process and the heterojunction can explain this behavior. The photodegradation kinetics of MB on catalysts were

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

Photon energy (eV)

ݒሾெሿ ൌ ሾܤܯሿ݁ିೌ௧ (2)

these heterostructures has been reported for photocatalytic applications [65].

 ZnO ZnO:Co 1% ZnO:Co 3% ZnO:Co 5% ZnO:Cu 1% ZnO:Cu 3% ZnO:Co 5%

0

5

10

15

20

**(**F(R)hv**)**1/2

25

30

35

40

**Figure 6.** Methylene blue (MB) concentration vs. time of visible irradiation on the synthesized **Figure 6.** Methylene blue (MB) concentration vs. time of visible irradiation on the synthesized catalysts.

catalysts. Time (*t*) is expressed in minutes and *<sup>k</sup>app* is the apparent reaction rate constant (min−<sup>1</sup> ). Table 2 lists the kinetic parameters for the studied catalysts. Among all the catalysts, the ZnO thin films (*kapp* <sup>=</sup> 0.2 <sup>×</sup> <sup>10</sup>−<sup>4</sup> min−<sup>1</sup> ) showed the lowest *kap* value, while the best results were obtained for ZnO:Co 5% (*kapp* <sup>=</sup> 7.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> ) and ZnO:Cu 5% (*kapp* <sup>=</sup> <sup>4</sup> <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> ). In the best case, the kinetic rate constant was 36 times higher than the ZnO thin films. A combined effect could be present: (i) Cu doping in ZnO and (ii) the formation of a nanoheterojunction (ZnO–CuO). This synergic effect could be a reason for the increase in photocatalytic yields. The heterostructure generation for the methyl orange photodegradation under visible light irradiation has been reported before [67]. Table 3 lists other reports for the use of doped ZnO with different metals as catalysts. Our results indicate that the catalysts produced in this study are suitable options for solar photocatalytic applications.

**Table 3.** *kapp* values for different catalysts (ZnO doped with different metals) under visible irradiation.


The ZnO films did not show photocatalytic activity under visible irradiation. However, two different processes can contribute to photocatalytic degradation under visible irradiation: (i) the intraband transitions as dopants allow the doped ZnO thin films to absorb visible light, generating charge pairs; (ii) CuO and Co3O<sup>4</sup> can absorb visible light after the formation of ZnO–CuO and ZnO–Co3O<sup>4</sup> heterojunctions, generating charge pairs. In this case, the electron can be transferred to the conduction band of ZnO. After electrons are located at the conduction band of ZnO, different reactive oxygen species (ROS) can be generated (e.g., *O*<sup>−</sup> 2 ; *OH*), starting the MB photodegradation. Scheme 1 shows the general scheme of energetic levels for doped ZnO thin films and the ROS generation.

**Scheme 1.** Hypothetical scheme of energetic levels for metal-doped ZnO thin films: (**a**) Metal doping process. (**b**) Metal doping process and the generation of a heterojunction [64,70,71]*.* After charge pairs **Scheme 1.** Hypothetical scheme of energetic levels for metal-doped ZnO thin films: (**a**) Metal doping process. (**b**) Metal doping process and the generation of a heterojunction [64,70,71]. After charge pairs generation, the ROS can be yielded on the catalyst surface and MB degradation starts.

generation, the ROS can be yielded on the catalyst surface and MB degradation starts.
