## 3 Results and Discussion
### Photoelectrochemical Testing
#### 3.4.3 Mott-Schottky (M-S) Plot  
M-S Analysis was done to study the charge carrier concentration of samples. The implementation of M-S analysis on annealed samples of 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C was performed in H\({}_{2}\)SO\({}_{4}\) (pH 1). The charge carrier was analyzed from the flat band, wherein flat band is the potential at which the potential drop between the electrode surface and the bulk is zero. This can establish the position of semiconductor energy band with respect to the redox potential. The intrinsic value of flat band (\(V_{FB}\)) and carrier density (\(N_{A}\)) for both samples can be extracted from the x-intercept and slope of the plot between the reciprocal of the square \(1/C^{2}\) and the bias potential (Figure 7c), in accordance with the Mott-Schottky equation, as follows (Equation (1)) [37]  
\[\frac{1}{C^{2}}=\ \bigg{(}\frac{2}{e\varepsilon_{o}N_{A}}\bigg{)}\bigg{[}V-V_{ FB}-\frac{K_{b}T}{e}\bigg{]} \tag{1}\]  
where \(e\) is the electronic charge, \(\varepsilon\) is the relative permittivity of CoO (12.9), \(\varepsilon_{o}\) is the permittivity of vacuum, \(K_{b}\) is Boltzmann's constant, and \(T\) is the absolute temperature [38]. A negative slope from all samples implied a p-type semiconductor. The extrapolation of slopes from the M-S plot (Figure 7) when integrated into the gradient of Equation (1) gives the \(N_{A}\). The interception of slopes at \(x\)-axis represents the \(V_{FB}\). In this case, the calculated value of \(N_{A}\) was 1.24 \(\times\) 10\({}^{11}\) and 3.99 \(\times\) 10\({}^{16}\) for 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C samples. However, the valence band (\(V_{B}\)) of the samples was determined using the following equation (Equation (2))  
\[E_{FB}-E_{VB}=\ k_{B}T\ln\frac{N_{V}}{N_{A}} \tag{2}\]  
where \(N_{V}\) is the effective density of states in the valence band (\(V_{B}\)). The \(N_{V}\) can be expressed as follows (Equation (3))  
\[N_{V}=2\bigg{(}\frac{2\pi m^{*}k_{B}T}{h^{2}}\bigg{)}^{\frac{3}{2}} \tag{3}\]  
Incorporating the band gap energy (\(Eg\)) determined earlier (Figure 7), the conduction band (\(C_{B}\)) of the samples can be determined [39]. The analyzed data of \(V_{FB}\), \(V_{B}\), \(Eg\), and \(C_{B}\) are arranged as depicted in Figure 8. For this purpose, the potential measurements in Ag/AgCl were converted to the RHE using the following equation (Equation (4))  
\[E_{\text{RHE}}=E_{\text{Ag/AgCl}}+\ E_{\text{Ag/AgCl}}^{0}+\ 0.059\text{pH} \tag{4}\]  
where \(\ E_{\text{Ag/AgCl}}^{0}=0.197\) V at 25 \({}^{\circ}\)C.  
To electrochemically split the water molecule, the band edges of the photoelectrode need to be in straddle position to the water molecule redox potential, which means the \(C_{B}\) of the material should lie at a more negative potential than the water reduction potential. Meanwhile, the \(V_{B}\) position should lie at a more positive potential to the water oxidation potential. Based on the calculation, the resultant band edges of \(C_{B}\) and \(V_{B}\) of both samples are as illustrated in Figure 8. The energy band structure of sample prepared at 450 \({}^{\circ}\)C (Co\({}_{3}\)O\({}_{4}\)) is shown in Figure 8; the \(C_{B}\) and \(V_{B}\) are \(-1.04\) eV and \(0.98\) eV. The band edges in this study were found at the more negative potential of our previous study; the \(C_{B}\) and \(V_{B}\) of Co\({}_{3}\)O\({}_{4}\) are estimated to be \(-0.55\) and \(1.10\) eV, respectively [12]. Both the \(C_{B}\) and \(V_{B}\) located were at a more negative potential of H\({}^{+}\) to H\({}_{2}\) and oxidation potential of H\({}_{2}\)O to O\({}_{2}\), respectively [35]. A similar trend was also found in the sample prepared at 550 \({}^{\circ}\)C. Given that the bandgap of the sample prepared at 550 \({}^{\circ}\)C was bigger than the sample prepared at 450 \({}^{\circ}\)C, the \(C_{B}\) position falls at more negative potential than the 450 \({}^{\circ}\)C sample. Meanwhile, the \(V_{B}\) position is not much different.