## 3 Results and Discussion
Co\({}_{3}\)Se\({}_{4}\)/NSi (15 s) at the potential 0.2 V vs. RHE (pink color) and Co\({}_{3}\)Se\({}_{4}\)/NSi (15 s) at the potential 0.1 V vs. RHE (purple color). (**c**) Dark current density-potential curves of Co\({}_{3}\)Se\({}_{4}\) film on highly P-doped n\({}^{+}\)-type Si electrodes with increasing electrodeposition time of Co\({}_{3}\)Se\({}_{4}\). shifted positively (anodically) while the limiting current density continuously decreased, induced by deposition of much thicker Co\({}_{3}\)Se\({}_{4}\) film. Overall, the conversion efficiency increased and conversion became saturated at 15 s of electrodeposition time. Combined with the surface structure of Co\({}_{3}\)Se\({}_{4}\) film, the very similar structures also resulted in the almost same light current density among samples of 15 s, 20 s and 25 s of electrodeposition time, which means that saturated porosity from the 3D structure showed the saturated light current densities according to the electrodeposition time. Consequently, the efficiency of the Co\({}_{3}\)Se\({}_{4}\)/nanostructured Si with 15 s of electrodeposition was 2.71%, a value 136 times higher than that of bare planar Si and 68 times higher than bare nanostructured Si without co-catalyst (Table 1). Moreover, Chronoamperometry test was implemented to measure the stability of selected Co\({}_{3}\)Se\({}_{4}\)/NSi (10 s) and Co\({}_{3}\)Se\({}_{4}\)/NSi (15 s) photocathode samples. The potential was applied approximately at its maximum current point (0.1 V vs. RHE for the 10 s sample and 0.2 V vs. RHE for the 15 s sample) during long-term operation. As a result, both samples showed a current density of approximately \(-\)19 mA cm\({}^{-2}\) and \(-\)13 mA cm\({}^{-2}\), respectively, which also maintained more than 80% of the initial current even after 10 h. From these results, it was shown that an optimal thickness of Co\({}_{3}\)Se\({}_{4}\) co-catalyst film exists for efficient hydrogen production. Possible native oxide formation at the interface between Si and Co\({}_{3}\)Se\({}_{4}\) film might affect the photoelectrochemical performance. In terms of band structure theory, previous study and our measurement have shown that conduction band and valence band edges for bare Si electrode are approximately +0.5 V and \(-\)0.6 V versus RHE, respectively. In this system, Co\({}_{3}\)Se\({}_{4}\) acts as the \begin{table}
\begin{tabular}{c c} \hline
**Type of Si Photocathode** & **Efficiency (\%)** \\ \hline Planar Si & 0.02 \\ Co\({}_{3}\)Se\({}_{4}\) on planar Si & 0.20 \\ Nanostructured Si & 0.04 \\ Co\({}_{3}\)Se\({}_{4}\) on nanostructured Si (5 s) & 0.47 \\ Co\({}_{3}\)Se\({}_{4}\) on nanostructured Si (10 s) & 1.72 \\ Co\({}_{3}\)Se\({}_{4}\) on nanostructured Si (15 s) & 2.71 \\ \hline \end{tabular}
\end{table} 
Table 1: Half solar-to-hydrogen conversion efficiency of various photocathodes. Figure 3: FESEM images Si photocathodes with variation of etching time (**a**) 10 min, (**b**) 20 min, (**c**) 40 min and (**d**) 150 min. co-catalyst, not acts as the photon-absorbing materials because we also measured photoresponse test with Co\({}_{3}\)Se\({}_{4}\) film on FTO under light condition and confirmed that there was no any photocurrent density. The native oxide with a thickness of less than 5 A can be formed on to the Si surface and can affect the photoelectrochemical property [35], however, the removal of the native oxide was also tried through HF treatment prior to electrodeposition process of Co\({}_{3}\)Se\({}_{4}\) film. Consequently, the possibility of the formation of a very thin native oxide layer might be much minimized and a thickness of much less than 5 A between Si and Co\({}_{3}\)Se\({}_{4}\) is able to be insignificant to produce an affective energy barrier. Nevertheless, the formation of native oxide cannot be removed perfectly.