**3. Fabrication of zinc oxide thin films and nanostructures**

Several techniques have been adopted in ZnO synthesis into different morphologies. These include spray pyrolysis, hydrothermal method, Radio Frequency (RF) and Direct Current (DC) magnetron sputtering, sol**-**gel and electrochemical anodization. The technique used determines the ZnO nanostructure formed. These nanostructures can be nanowires, nanoflowers, nanobelts and nanoparticles. ZnO thin films can also be fabricated using these techniques.

The choice of the technique in designing ZnO thin films is determined by the quality of the films required, simplicity, fabrication time and cost among others. In the synthesis of ZnO thin films, [17] acknowledged electrochemical anodization as a simpler route to design thin films. This technique is cheap. Simple and affordable.

#### **3.1 Anodization**

This is a simple two electrode configuration which involves a working electrode (anode) and a counter electrode (cathode) dipped in an electrolyte. A constant direct current passed through the electrodes results in the formation of a thin oxide layer on the surface of certain metals. The schematic diagram is as illustrated in **Figure 3**.

In this method of fabrication, several parameters like electrolyte temperature, inter-electrode spacing, electrolyte concentration, applied voltage and anodizing time should be controlled since they not only affect the nanostructure formation but also the surface density. While the type of electrolyte used determine the shape of the nanostructure, electrolyte concentration, anodizing time and the applied voltage influence the nanostructure density.

In the formation of ZnO thin films by anodization, Zinc metal is the working electrode (anode) and any inert material is the counter electrode (cathode). A constant current is passed from a power supply through the electrodes which results in redox reactions. These reactions are expressed as:

At the cathode:

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_2 \tag{9}$$

At the anode:

$$\text{Zn} \rightarrow \text{Zn}^{2+} + \text{2e}^- \tag{10}$$

**Figure 3.** *Schematic diagram for the experimental setup for anodization [18].*

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

$$\text{Zn}^{2+} + 2\text{OH}^- \rightarrow \text{Zn}(\text{OH})\_2 \tag{11}$$

$$\text{Zn}(\text{OH})\_2 \rightarrow \text{ZnO} + \text{H}\_2\text{ O} \tag{12}$$

### **4. Experiment**

ZnO thin films were fabricated by anodizing about 99% pure ultrasonically cleaned Zinc metal in 0.5 M magnetic stirred Oxalic acid electrolyte at room temperature (300 K) for 60 min. Electric current was passed through the zinc metal (working electrode) and graphite (counter electrode) at a constant voltage of 10 V. The anodized films were rinsed in distilled water and then left to dry in air. This led to the formation of a white anodic film on the surface of Zinc metal. The experiment setup for anodizing Zinc is shown in **Figure 4**.

Varying Cobalt concentrations were incorporated into ZnO films by electrodeposition method using 0.5 M Cobaltous (II) sulphate solution as the Cobalt source. This was done by passing alternating current from a 20 V source through the Cobaltous (II) sulphate solution containing ZnO working electrode and graphite counter electrode for 10, 20, 30, 40, 50 and 60s. The films were then rinsed and left to dry in air. Heat treatment was done in a Carbolite 301 temperature controller at 523 K for 2 h.

The amount of Cobalt deposited in ZnO films at different deposition times was obtained using the Faraday's law of electrochemistry which is expressed mathematically as [19]:

$$m = \left(\frac{Q}{F}\right)\left(\frac{M}{z}\right) \tag{13}$$

where *m* is the mass of Cobalt deposited expressed in grams, *Q* is the charge in coulombs passed through ZnO, *F* is the Faraday constant (96485.33289 Cmol�<sup>1</sup> ), *M* represents the molar mass of Cobalt expressed in grams mol�<sup>1</sup> and *z* is the valency number of Cobalt. *Q* is given by the product of current passed and time in seconds (Q = It) where the current passed is 2.18 A.

Optical characterization was done using PERKIN ELMER UV/VIS/NIR Lambda 19 spectrophotometer equipped with an integrating sphere which can be used to measure the absorbance, transmittance and reflectance of the films.

**Figure 4.** *Plate showing the experimental setup for anodizing zinc metal.*

This spectrophotometer was set to measure the reflectance of the films in the solar range 300 nm < λ < 2500 nm against a barium sulphate reference standard.

The photocatalytic activity of unpigmented and Cobalt-pigmented *ZnO* thin films was studied in the degradation of aqueous methylene blue solution which is the simulated pollutant under UV light. The UV source composed of a UV cabinet with a UV lamp of wavelength 366 nm, 2 6 W irradiation power and methylene blue absorbance was measured using Optima SP-3000nano UV-VIS spectrophotometer. 60 ml of methylene blue with 10 ppm concentration was put in a petri dish and placed in the dark for 60 min to allow for adsorption-desorption equilibrium.

The petri dish and its contents were then transferred into the UV cabinet and illuminated for 5 h at ambient temperature. 1 ml of the degrading solution was drawn at 30 min intervals and its absorbance directly measured at 664 nm using the UV-VIS spectrophotometer. The absorbance recorded in this case was directly proportional to methylene blue concentration. This was revealed by a calibration curve plotted from standard methylene blue solutions of known concentrations prepared prior to the measurement. The calibration curve was used to obtain the concentration of methylene blue in the experiment with respect to absorbance.

#### **4.1 Data analysis**

Data was analyzed using SCOUT software which allows for analysis and simulation of optical spectra such as reflectance, transmittance and absorbance. This was done by fitting the measured experimental data into the simulated data in the software with the aid of different models. In this case, the models used are the Drude model for free carriers, harmonic oscillator to describe the atomic microscopic vibrations and Tauc Lorentz model to determine the band gaps of the films.

#### **5. Results**

A white ZnO thin film formed on Zinc electrode surface as a result of the redox reactions at the anode and cathode. The average film thickness as obtained from fitting of the experimental measured spectra into the simulated spectra using SCOUT software was 110 nm. **Figure 5** shows one of the obtained fitted spectra.

#### **5.1 Reflectance spectra**

The reflectance spectra of the fabricated films obtained from the spectrophotometer is shown in **Figure 6**.

As seen in the **Figure 6**, ZnO reflectance was affected by Cobalt pigmenting because Cobalt pigmented ZnO films had a lower reflectance than unpigmented ZnO. This decrease in reflectance may be attributed to darkening of the films when Cobalt concentration was increased. Lowered reflectance may also be as a result of the films becoming rough as Cobalt is deposited according to Ref. [20]. The more the Cobalt concentration, the rougher the films hence the decrease in the quantity of reflected light implying increased light absorption (absorbance).

#### **5.2 Absorption coefficient**

The absorption coefficient of the fabricated films was obtained from reflectance data of the films using the relation [21]:

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

$$R + T = e^{-ad} \tag{14}$$

which yields

$$a = \frac{1}{d} \ln \left[ \frac{1}{R(\lambda)} \right] \tag{15}$$

where *α* is the absorption coefficient, *d* is the film thickness and *R*ð Þ*λ* is reflectance as a function of wavelength. **Figure 7** shows the variation of the absorption coefficient of the films with wavelength.

From the figure, it was observed that there is a sudden increase in the absorption coefficient at shorter wavelengths about 348 nm. This peak corresponds to ZnO absorption edge indicating that ZnO absorbs at short wavelengths in the UV region of the solar spectrum. It was also observed that Cobalt pigmenting affected the absorption coefficient since an increase in the Cobalt concentration led to an increase in the absorption coefficient. This may be attributed to the decrease in

**Figure 5.** *Illustration of fitting of experimental to simulated spectra.*

**Figure 6.** *Measured reflectance for zinc and the* ZnO *films with different cobalt concentrations.*

#### **Figure 7.** *Variation of absorption coefficient of the films with wavelength.*

reflectance of the films as Cobalt concentration was increased. According to Ref. [22], the content of pigment in a film affects its absorption. Another peak was observed at about 1000 nm which shows absorption resulting from interband transition in the Zinc substrate [23].
