## 3 Results and Discussion
The generated electron charge carriers in the depletion region of Si by incident light can relative easily move to the Co\({}_{3}\)Se\({}_{4}\) co-catalyst to participate in the proton reduction to hydrogen production through the possible tunneling mechanism [36]. In addition, the remaining thin native oxide layer might be acted as the passivation layer during photoelectrochemical process combined with electrodeposited Co\({}_{3}\)Se\({}_{4}\) film [37]. Similarly, band bending effect is the other possible effect contributed by SiOx layer formation. The photons absorbed by the silicon wafer generates minority carriers that can be diffused onto the Co\({}_{3}\)Se\({}_{4}\) and band-bending effect can be accelerated by the native SiOx barrier layer. The formation of possible additional surface trap sites can be also suppressed by the native oxide formation at the interface. To study the exact effect of SiOx layer, we are under investigation to study these concepts in detail through in-depth studies such as Mott-Schottky plot by capacitance measurement or the change of carrier densities by the work function measure from UPS analysis in our next project. To evaluate the electrocatalytic properties of the material, the HER electrocatalytic activity was examined using the customary three-electrode system. To understand better the electrocatalytic activity with respect to the substrate, the Co\({}_{3}\)Se\({}_{4}\) material was deposited on GC and on FTO substrate. In Figure 4a, the HER polarization curves of Co\({}_{3}\)Se\({}_{4}\) on FTO and GC electrodes are compared, along with those of bare FTO and bare GC. In all the polarization curves, the reductive current density started to increase negatively according to the cathodic potential applied and promoted the hydrogen evolution reaction (HER). The HER curves indicate that Co\({}_{3}\)Se\({}_{4}\)(10) on the FTO electrode exhibited superior onset potential (\(-\)0.189 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)) than did Co\({}_{3}\)Se\({}_{4}\)(10) deposited on GC (\(-\)0.192 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)) and better onset potential than did bare FTO (\(-\)0.552 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)) and GC (\(-\)0.644 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)) electrodes. Moreover, at a similar current density of 5 mA/cm\({}^{2}\), the Co\({}_{3}\)Se\({}_{4}\) deposited on the FTO conductive electrode showed enhanced electrocatalytic activity resulting in a lower overpotential of 239 mV compared to Co\({}_{3}\)Se\({}_{4}\) on the GC electrode (292 mV). This indicated that FTO is a more suitable substrate and improves catalytic activity. Therefore, in-depth analyses were carried out at various deposition times of the Co\({}_{3}\)Se\({}_{4}\) electrocatalysts on the FTO substrate. Figure 4b shows that the HER activity of the Co\({}_{3}\)Se\({}_{4}\) electrocatalysts with respect to the deposition time (5-25 s). The electrocatalytic activity of the time dependent Co\({}_{3}\)Se\({}_{4}\) catalysts was observed to increase gradually with deposition time from 5 to 20 s, exhibiting a very low onset potential (\(-\)0.205 V to \(-\)0.119 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)). Subsequently, the sample deposited for 25 s abruptly declined due to an increase in onset potential (\(-\)0.152 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)) compared to the sample deposited for 20 s. Figure 4c displays the chronoamperometry (CA) curve of the Co\({}_{3}\)Se\({}_{4}\) electrocatalyst carried out at a steady potential of \(-\)0.2 V (vs. RHE) for 7200 s. The evolution of H\({}_{2}\) gas bubbles from the surface of the electrode was noticed all over the CA analysis. Hence, the steady-state curve obtained from the CA scrutiny substantiates the highly stable nature of the prepared Co\({}_{3}\)Se\({}_{4}\) electrocatalyst for HER activity in acidic HClO\({}_{4}\) medium. The inset of Figure 4c sorts out the essential overpotentials demanded by prepared Co\({}_{3}\)Se\({}_{4}\) electrocatalysts deposited on the FTO for different times.