**5.3 Optical band gap**

*ZnO* is a direct band gap semiconductor whose band gap can be obtained from a plot of ð Þ *αhν* <sup>2</sup> versus energy in eV. **Figure 8** shows the band gaps of the fabricated films.

The band gaps of the films ranged from 3.34 to 3.10 eV for pure ZnO and ZnO: Co 60s films respectively. The fabricated pure *ZnO* films had a band gap of 3.34 eV which is similar to the preceding work done in Ref. [24] who also found 3.34 eV. The gradual decrease observed as Cobalt was introduced into the *ZnO* films in different amounts is associated with the red shift of the absorption edge as observed in the reflectance spectra.

**Figure 8.** *Graph showing the band gap analysis of the fabricated films.*

*Fabrication and Characterization of Cobalt-Pigmented Anodized Zinc for Photocatalytic… DOI: http://dx.doi.org/10.5772/intechopen.93790*

In Refs. [25–27], a gradual decrease in ZnO band gap with Cobalt pigmenting is also reported. They attributed the decrease to sp-d exchange interactions between the d electrons of Cobalt and *ZnO* conduction band electrons once the Cobalt ions substitute the Zinc ions in the crystal lattice. It was also stated that High Cobalt concentration led to the wavefunctions of the electrons in the Cobalt atoms overlapping as the Cobalt density increases resulting in the formation of an energy band by the overlapping forces consequently reducing the gap.

#### **5.4 Photocatalytic degradation of methylene blue**

In the investigation of photocatalytic degradation of methylene blue solution, the pseudo-first order kinetic model was used which according to [28] enables quantification of the photocatalytic activity of samples. This model involves the presentation of raw data as integral data. The performance of the pure and some of the Cobalt pigmented *ZnO* films was as shown in **Figure 9**. Methylene blue degradation without catalyst was used the control experiment.

From the figure, it is observed that the rate at which methylene blue with catalyst degraded was faster than that without the catalyst. Also, Cobalt pigmenting increased the degradation rate which may be attributed to the decrease in the optical band gap resulting from the red shift which was observed in the reflectance spectra. This shift resulted in more electrons gaining kinetic energy consequently moving to the conduction band to take part in the degradation process.

ZnO:Co 20s is had a higher rate of degradation with its degradation rate of 0.0317 h�<sup>1</sup> obtained using the relation [28]:

$$-\ln\left(\frac{C}{C\_0}\right) = kt\tag{16}$$

where k is the degradation rate constant given by the slope of the graph.

Heavy Cobalt content however lowered the methylene blue degradation rate. This may be attributed to the fact that excess metal pigment covered the active sites

**Figure 9.** *A graph of ln (C/C0) versus time in minutes for sampled films.*

of the *ZnO* catalyst lowering its activity. It may also be due to the generation of the impurity levels deep in *ZnO* band gap which acted as recombination centers for the photogenerated electrons.
