## 3 Results and Discussion
### Effects on Photoelectrochemical Performance
#### 3.4.2 Mott-Schottky Plots  
In order to explain why the PEC activity of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) was enhanced by Co or Ni doping, M-S plots of both doped and undoped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) samples were measured and shown in Figure 8. All the samples exhibited a positive slope, indicating they are all n-type semiconductors with electrons as majority charge carriers. The slope of the linear part of the M-S plots could be used to estimate the concentration of charge carriers (N\({}_{d}\)) according to Equation (6). \[1/C^{2}=(2/\mathrm{e}_{0}\varepsilon\varepsilon_{0}\mathrm{N}_{d})\ [(\mathrm{E}- \mathrm{E}_{\mathrm{fb}})-\mathrm{kT}/\mathrm{e}_{0}] \tag{6}\]  
In this equation, C is the space charge capacitance per m\({}^{2}\) active area, \(\mathrm{e}_{0}\) is the elementary charge (\(1.6\times 10^{-19}\) C), \(\varepsilon\) is the dielectric constant of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) (14.2), \(\varepsilon_{0}\) is the permittivity of vacuum (\(8.85\times 10^{-12}\) F/m), E is the applied potential, \(\mathrm{E}_{\mathrm{fb}}\) is the potential of flat bands, k is Boltzmann's constant (\(1.3806505\times 10^{-23}\) J/K), and T is the absolute temperature (298 K). \begin{table}
\begin{tabular}{c c c c} \hline
**Samples** & **Photocurrent (mA/cm\({}^{2}\))** & **Potential (V vs. RHE)** & **Reference No.** \\ \hline undoped & 0.00498 & 0.85 & 16 \\ Cr-doped & 0.00903 & 0.85 & 16 \\ undoped & 0.72 & 1.23 & 4 \\ S-doped & 1.42 & 1.23 & 5 \\ undoped & 0.58 & 1.23 & 5 \\ Sn-doped & 1.54 & 1.23 & 6 \\ Y-doped & 0.0022 & 0.442 & 30 \\ P-doped & 3 & 1.23 & 44 \\ Sn-doped & 1.86 & 1.23 & 39 \\ undoped & 0.523 & 0 & 17 \\ Undoped & 0.61 & 1.27 & Present work \\ Ni-doped & 0.79 & 1.27 & Present work \\ Co-doped & 1.28 & 1.27 & Present work \\ \hline \end{tabular}
\end{table} 
Table 1: Photocurrent densities of doped and undoped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) at a certain potential versus reversible hydrogen electrode (RHE) reported in the literature. Figure 7: Current-voltage (I-V) plots of the \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) nanorod arrays: (**a**) Co-doped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) under solar irradiation; (**b**) Ni-doped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) under solar irradiation; (**c**) undoped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) under solar irradiation; (**d**) undoped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) in dark. The concentration of charge carriers derived from the slope and the E\({}_{\text{fb}}\) derived from the intercept of the M-S plot are listed in Table 2. The E\({}_{\text{fb}}\) of both doped and undoped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) samples is around \(-0.22\) V, indicating the doping has little influence on the E\({}_{\text{fb}}\). This conclusion is consistent with that obtained from I-V plots (see Figure 7). The charge carrier concentration of the undoped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) is \(1.65\times 10^{25}\) m\({}^{-3}\), which is in the same order of magnitude as those reported in the literature [5; 6; 39; 44]. After Co or Ni doping, the charge carrier concentration increased to \(3.74\times 10^{25}\) m\({}^{-3}\) and \(2.50\times 10^{25}\) m\({}^{-3}\), respectively, which are 2.3 times and 1.5 times that of the undoped sample. The increase in the charge carrier concentration means the improvement of the electrical conductivity, which may contribute to the enhanced PEC activity of the doped samples. [PERSON] et al. [45] reported that the heterogeneous interface of nickel and iron oxide exhibited a high electrocatalytic activity for hydrogen evolution reaction in alkaline media. In the present work, whether the Co or Ni doping promotes the electrocatalytic performance of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) nanorods has not been investigated. It is necessary to figure this question out in future because an improved electrocatalytic activity may be one of the potential reasons for the enhanced photocurrents observed for the doped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) samples.