**Abstract**

Population growth and urbanization have led to water scarcity and pollution, which is a health hazard not only to humans but also to the ecosystem in general. This has necessitated coming up with ways of treating water before consumption. Photocatalysis has proved to be one of the most promising cheap techniques that involve chemical utilization of solar energy. TiO2 widely used in photocatalysis absorbs a narrow range of the solar spectrum compared to *ZnO*. In this regard, this study aimed at preparing and optimizing cobalt-pigmented *ZnO,* which is applicable in photocatalytic water treatment. The objectives in this study were to fabricate zinc oxide (*ZnO*) thin films by anodization, pigment the fabricated films with varying cobalt concentrations, characterize the fabricated films optically, and investigate the cobalt-pigmented *ZnO*'s performance in the methylene blue degradation under UV light irradiation. Mirror-polished zinc plates were sonicated in ethanol and rinsed. Anodization was done at room temperature in 0.5 M oxalic acid at a constant voltage of 10 V for 60 min, and cobalt electrodeposited in the films. Post-deposition treatment was done at 250°C. Optical properties of the films were studied using a UV-VIS-NIR spectrophotometer in the solar range of 300–2500 nm. The photocatalytic activity of the fabricated films was studied in methylene blue solution degradation in the presence of UV light irradiation for 5 h. Cobalt pigmenting was observed to reduce reflectance and optical band gap from 3.34 to 3.10 eV indicating good photocatalytic properties. In this study, *ZnO* film pigmented with cobalt for 20 s was found to be the most photocatalytic with a rate constant of 0.0317 h<sup>1</sup> and hence had the optimum cobalt concentration for photocatalytic water treatment. This can be applied in small-scale water purification.

**Keywords:** photocatalysis, anodization, zinc oxide, pigmenting, optical properties

## **1. Introduction**

Contaminated waste water from industries, refineries, agriculture run-off, domestic and sewage water contain organic pollutants that are not only harmful to human but also the ecosystem in general. This calls for the need to treat water before discharge to the environment. Some of the water treatment methods used are reverse osmosis, sedimentation, filtration, distillation, coagulation and flocculation, chlorination, photocatalysis and aeration. According to Ref. [1], photocatalytic

**Figure 1.** *Photocatalyst e and h+ generation and their possible reactions in aqueous solutions [2].*

water treatment has proved to be one of the most promising ways of purifying waste water. This technique degrades the hazardous organic pollutants in water by chemically utilizing solar energy. It involves the excitation of electrons from the valence band to the conduction band of a semiconductor by a radiation with sufficient energy. The photogenerated electrons and holes react with oxygen and hydrogen in water respectively to form superoxide and hydroxyl radicals as illustrated in **Figure 1**.

Photocatalytic water treatment is preferred because it is efficient, simple, possible to use solar light thus reusable and also capable to mineralize pollutants into products which are environment friendly [3]. Photocatalysis is also applied in air purification, in antibacterial action, hydrogen evolution through water splitting, designing self-cleaning surfaces, sterilization and photo electrochemical conversion. In these applications, semiconductors are used as photocatalysts.

Some of the attractive photocatalysts applied in photocatalytic degradation are Titanium dioxide (TiO2), zinc oxide (ZnO), tin oxide (SnO2). Iron (iii) oxide (Fe2O3), Vanadium (v) oxide and niobium pentoxide (Nb2O5) have also been reported by [4]. TiO2 is the most studied and applied in photocatalysis because of its structural stability, nontoxicity and effectiveness according to [5, 6]. Its only drawback as compared with ZnO is that it absorbs a narrow range of the solar spectrum yet they have similar band gap energies, [7]. In addition, ZnO is readily available and cheaper than TiO2.

### **2. Zinc oxide (ZnO) properties and applications**

ZnO is an n-type direct wide band gap semiconductor whose crystals exist in hexagonal wurtzite, zincblende and rock salt structures but the most common structure is hexagonal wurtzite because it is stable. Its band gap energy is about 3.37 eV at room temperature, and it has a high exciton energy of 60 meV.

According to [8], *ZnO* photocatalysis is influenced by its direct band gap and large free exciton binding energy (60 meV). ZnO also possess unique properties like photosensitivity, nontoxicity, environmental stability and good optical and electrical properties. These properties according to [9] have attracted much interest in varied applications. It also has high electron mobility, good thermal conductivity and transparency. All these properties make *ZnO* a promising material in several semiconductor applications including designing of transparent thin film transistors, *Fabrication and Characterization of Cobalt-Pigmented Anodized Zinc for Photocatalytic… DOI: http://dx.doi.org/10.5772/intechopen.93790*

light emitting diodes (LEDs), heat protecting windows and transparent electrodes in liquid crystal displays. They are also vital in applications like antibacterial use as reported by [10], dye sensitized solar cells by [11] gas sensors by [12] and photocatalysis as reported by [13].

#### **2.1 ZnO photocatalytic water treatment**

In photocatalytic water treatment, light energy greater than that of *ZnO* band gap illuminated on its surface excites electrons from its valence band to the conduction band leaving behind holes in the valence band. The excited electrons then interact with the surrounding oxygen to form superoxide radicals *O*� <sup>∗</sup> 2 while holes interact with water to form hydroxyl radicals (*HO*<sup>∗</sup> ). These radicals react with the organic compounds dissolved in water decomposing them to water and carbon (IV) oxide. Photocatalytic water treatment process can be summarized using the equations, [14]:

$$\text{ZnO} + hv \rightarrow \text{e}\_{cb}^{-} + h\_{vb}^{+} \tag{1}$$

$$h\_{vb}^{+} + H\_2O \to H^{+} + HO^{\*} \tag{2}$$

$$
\mathfrak{e}\_{cb}^- + \mathrm{O}\_2 \to \mathrm{O}\_2^{-\*} \tag{3}
$$

$$O\_2^{-\*} + H^+ \to HO\_2^\* \tag{4}$$

$$\text{HO}\_2^\* + \text{HO}\_2^\* \to \text{H}\_2\text{O}\_2 + \text{O}\_2 \tag{5}$$

$$H\_2O\_2 + e\_{cb}^- \rightarrow HO^\* + HO^- \tag{6}$$

$$H\_2O\_2 + hv \rightarrow 2HO^\* \tag{7}$$

$$R + HO^\* \rightarrow CO\_2 + H\_2O \tag{8}$$

**Figure 2** illustrates how *ZnO* photocatalysis degrades organic pollutants.

ZnO photocatalytic activity is limited by its wide band gap property which makes it active only in the UV region of the solar spectrum [15]. This can be enhanced by doping or pigmenting it with a transition metal or nonmetal where the impurity added reduce the band gap consequently expanding its response to solar radiation. The preferred metal for pigmenting ZnO is Cobalt which has an ionic radius (0.745 Å) close to that of Zinc (0.74 Å) as stated by [16].

**Figure 2.** *Diagram showing* ZnO *photocatalytic mechanism [13].*
