## 3 Results and Discussion
The potential versus current plot under dark condition shows a 1 \(\mu\)A/cm\({}^{2}\) scale of current density, which indicates that the photoresponse of the Co\({}_{3}\)Se\({}_{4}\)/Si cell operated well with and without illumination. However, when Co\({}_{3}\)Se\({}_{4}\) was deposited on the Si electrode, the saturation current density decreased by approximately 23% from the initial current density (from \(-\)34.1 to \(-\)26.4 mA/cm\({}^{2}\)). This was attributed to the reduced light collection caused by deposition of the semi-transparent Co\({}_{3}\)Se\({}_{4}\) film. To evaluate the photocathodic efficiency, a half solar-to-hydrogen conversion efficiency was introduced. It was defined as [(photocurrent density at the reversible potential (0 V vs. RHE), J\({}_{\rm V=0}\) (mA/cm\({}^{2}\)) \(\times\) (open circuit potential, V\({}_{\rm oc}\) (V) \(\times\) Fill Factor/100 (mW/cm\({}^{2}\)) \(\times\) 100 (%)] [2; 34]. The equation is also equal to a maximum value of [J\({}_{\rm ph}\) (mA/cm\({}^{2}\)) \(\times\) (V\({}_{\rm redox}\) \(-\) V\({}_{\rm b}\)) (V)/(J\({}_{\rm light}\) (mW/cm\({}^{2}\)) \(\times\) 100%)] [2; 34]. Herein, J\({}_{\rm ph}\) is the photocurrent density obtained under the applied bias V\({}_{\rm b}\) and V\({}_{\rm redox}\) is the redox potential for hydrogen production (0 V vs. RHE). V\({}_{\rm bias}\) is an externally applied bias potential often necessary to achieve reasonable photocurrents and P\({}_{\rm light}\) is the intensity of the incident light under AM 1.5 G conditions. In this work, the incident light intensity was maintained at 100 mW/cm\({}^{2}\). From calculation, the half solar-to-hydrogen conversion efficiency increased to 10-times higher than for bare planar Si. Consequently, it was determined that Co\({}_{3}\)Se\({}_{4}\) is a highly active film for photoelectrochemical hydrogen production in terms of photocathodic efficiency. Treatment of the nanostructured Si surface using a metal-catalyzed electroless etching process further increased the light-to-hydrogen conversion efficiency. The FE-SEM images in Figure 3 portrays that the growth of nanostructured Si increases with respect to the electroless etching time. Optimized nanostructuring of the Si electrode surface (the sample with etching time of 20 min shown in Figure 3b) increased the limiting current density (the saturation current density at high negative potential) to 44.0 mA/cm\({}^{2}\) when deposited with Co\({}_{3}\)Se\({}_{4}\) film. This is 1.29 times higher than that of bare planar Si (34.1 mA/cm\({}^{2}\)), as shown in Figure 2a,b. The V\({}_{\rm os}\) of the nanostructured Si (\(-\)0.007 V vs. RHE) also shifted by a positive 80 mV (V\({}_{\rm os}\) of bare planar Si: \(-\)0.087 V vs. RHE). Consequently, the conversion efficiency of the nanostructured Si photocathode (0.04%) increased two fold compared to that of bare planar Si (0.02%). The enhanced efficiency of the nanostructured photoelectrode surface is mainly attributed to increased effective surface area and reduced reflectance of incident light. To increase the photocathodic activity even more, Co\({}_{3}\)Se\({}_{4}\) was deposited electrochemically as a function of time, maintaining the applied potential of \(-\)0.8 V versus RHE. Figure 2b shows representative photoresponse results with various electrodeposition times. With increasing deposition time, the onset potential  
Figure 2: Cyclic voltammograms of electrodeposited Co\({}_{3}\)Se\({}_{4}\) on Si photocathodes: (**a**) Current density-potential curves of bare planar Si photoelectrode and electrodeposited Co\({}_{3}\)Se\({}_{4}\) on planar Si photoelectrode under light condition (solid lines). For comparison, Co\({}_{3}\)Se\({}_{4}\) on Si under dark condition was also displayed (dashed line). (**b**) Photocurrent density-potential curves according to electrodeposition time of Co\({}_{3}\)Se\({}_{4}\) under light illumination condition. Inset image indicates chronoamperometry results from selected photocathode samples for the stability test.