## 3 Results and Discussion  
### Phase and Crystal Structure Analysis  
The cobalt oxide thin film on FTO was fabricated using a sequential hydrothermal method and annealed under inert conditions. The product from the hydrothermal process that was not annealed (named as a pre-annealed sample) was also analyzed to confirm the first product produced by the hydrothermal method. XRD analysis was utilized in this study to fully understand the phase of cobalt oxide produced according to the annealing temperature. The XRD chromatogram in Figure 1 shows that cobalt (III) oxide (Co\({}_{3}\)O\({}_{4}\)) was formed on the FTO after the hydrothermal reaction (pre-annealed), as indicated by the diffracted peaks of (111), (220), (311), (511), and (440), which corresponded to the plane of the cubic crystal structure with the spinel Co\({}_{3}\)O\({}_{4}\) phase (JSPD NO 00-042-1467). In addition, as the sample was later subjected to an annealing treatment of 450 \({}^{\circ}\)C, the intensity of the peaks belonging to the Co\({}_{3}\)O\({}_{4}\) phase markedly increased, indicating an increase in crystallinity. Interestingly, at 550 \({}^{\circ}\)C, a mixed phase of Co\({}_{3}\)O\({}_{4}\) and CoO was observed at approximately 36\({}^{\circ}\), 45\({}^{\circ}\), and 59\({}^{\circ}\), indicating that oxidation of Co\({}_{3}\)O\({}_{4}\) to CoO occurred. According to previous studies [28], increasing the annealing temperature decreases the amount of oxygen in the films, which might be explained by the increase in purity and crystallinity of the as-deposited Co\({}_{3}\)O\({}_{4}\) (pre-annealed) and after annealing at 450 \({}^{\circ}\)C, and by the loss of oxygen in the form of water vapor during the annealing treatment. To confirm the composition of pre-annealed 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C samples, X-ray photoelectron spectroscopy (XPS) analysis was performed as displayed in Figure 2. The pre-annealed and the annealed 450 \({}^{\circ}\)C samples have similar full survey spectra. The Co 2p spectra for the pre-annealed appeared at the value of binding energy for 778 \(\pm\) 0.5 and 800 \(\pm\) 0.5 eV, whereas the annealed 450 \({}^{\circ}\)C appeared at the value of binding energy for 778 \(\pm\) 0.5 and 793 \(\pm\) 0.5 eV. The spectra of the pre-annealed are present in a broad area compared to the Co 2p spectra for the annealed 450 \({}^{\circ}\)C. The broad value of the pre-annealed perhaps is caused by water or solvent contaminant. A similar trend was also reported by [PERSON] et al. (2020) on the annealing effect on MoS\({}_{2}\)[29]. The 2p\({}^{3/2}\) and 2p\({}^{1/2}\) peaks separation for both pre-annealed and annealed 450 \({}^{\circ}\)C samples are around \(\pm\) 10 eV. According to [PERSON] et al., between the 2p\({}^{3/2}\) and 2p\({}^{1/2}\) peaks, an energy difference of \(\sim\)15 eV is characteristic of the Co\({}_{3}\)O\({}_{4}\) phase [30]. The major peak in Co 2p\({}^{3/2}\) for the sample annealed at 450 \({}^{\circ}\)C de-convoluted into two peaks centered at 776.96 and 778.90 eV, which corresponded to Co\({}^{3+}\) and Co\({}^{2+}\) species, respectively. For the Co 2p spectrum of the 550 \({}^{\circ}\)C thin film, the Co\({}^{3+}\) and Co\({}^{2+}\) species were located at 777.07 and 778.95 eV, respectively. In addition, the presence of CoO was confirmed by the indication of a low appearance of satellites [31]. The Co 2p spectrum of single-phase Co\({}_{3}\)O\({}_{4}\) showed a clear image of weaker peaks compared with that associated with CoO in the mixed phase of Co\({}_{3}\)O\({}_{4}\)/CoO [32]. Figure 2c shows the O 1s XPS spectra of the pre-annealed, annealed 450 \({}^{\circ}\)C, and annealed 550 \({}^{\circ}\)C samples. All samples produce oxygen peaks in different binding energy and height. The oxygen peaks of pre-anneal sample present at 529 and 530.6 eV. Meanwhile, the position oxygen peaks of annealed 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C samples are in lower binding energy, that is, at 527 and 528.6 eV.