## 3 Results and Discussion
### Optical Measurements  
The optical properties of the photoelectrode samples were analyzed using a UV-Vis-NIR spectrophotometer. The optical absorption of the prepared photoelectrode samples and their corresponding Kubelka-Munk plots are shown in Figure 5, in which the pre-annealed samples showed an absorption region at 400-600 nm. Accordingly, the corresponding K-M plots exhibited a band gap value of 4.30 eV (Figure 5b). The pre-annealed sample exhibited absorption at much lower wavelengths and a large band gap, because the pre-annealed sample was not activated. For the sample prepared at 450 \({}^{\circ}\)C, the intensity of the light absorption was greater than that of the pre-annealed sample, which covered a wider range of the light spectrum, ranging from 200 to 900 nm (Figure 5a). Within this light spectrum range, two main broad peaks were identified, between 200-500 and 580-900 nm. Correspondingly, two main broad peaks were identified with two band gaps of 2.00 and 2.50 eV (Figure 5b). This observation suggested that a higher temperature was required to affect the phase transformation of the pre-annealed sample. This suggestion was supported by the XRD analysis result, in which an identical phase of Co\({}_{3}\)O\({}_{4}\), as identified in the  
Figure 3: FESEM images of (**a1–a3**) pre-annealed sample, (**b1–b3**) 450 \({}^{\circ}\)C sample, and (**c1–c3**) 550 \({}^{\circ}\)C sample.  
pre-annealed sample, was also detected in the sample prepared at 450 \({}^{\circ}\)C. However, after annealing, the band gap values significantly shifted to smaller values. This shifting pattern of the band gap value could be an indication of the intermediate phase transformation. In general, Co\({}_{3}\)O\({}_{4}\) is known as \"Co\({}_{2}\)O\({}_{3}\)/CoO,\" which represents two constituents in the material. Therefore, these constituents might contribute to the creation of two optical absorption peaks. The first band (500-700 nm) can be referred to as the O\({}^{2-}\)\(\rightarrow\) Co\({}^{2+}\) charge transfer process of CoO, whereas the second absorption band (250-500 nm) can be attributed to the O\({}^{2-}\)\(\rightarrow\) Co\({}^{2+}\) charge transfer of Co\({}_{2}\)O\({}_{3}\)[35; 36].  
As the annealing temperature was increased to 550 \({}^{\circ}\)C, the intensity of absorption significantly decreased; only one absorption peak that signaled between 200 and 450 nm remained. Extrapolation of K-M plots on the \(x\)-axis found that the band gap value was narrowed to a single band gap value of 3.1 eV (Figure 5b). When the as-prepared Co\({}_{3}\)O\({}_{4}\) was annealed with a nitrogen gas flow, and the temperature was increased, Co\({}^{2+}\) was protected against oxidation, thereby ensuring the production of CoO nanowire. Oxygen vacancies occurred during annealing in an oxygen-deficient atmosphere, and this formation  
Figure 4: HRTEM images of (**a1**–**a3**) 450 \({}^{\circ}\)C and (**b1**–**b3**) 550 \({}^{\circ}\)C samples inserted with SAED and lattice fringes.  
Figure 5: (a) UV-Vis absorption, and (b) curves band gap determination by Kubelka-Munk plot of samples pre-annealed and annealed at 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C.  
was associated with the energy band gap. A narrowing of the band gap was the result of the increase in oxygen vacancies, and hence, the increase in the visible light absorption.