## 3 Results and Discussion
This low reflectance can be attributed to the increased loading amount of rGO; thus, we believe that rGO absorbs a part of the incident light and reduces the absorption of light on the Si photoelectrode, which is followed by a decrease in current density with an increase in the loading amount of rGO. To clarify the absorption property, we also measured absorbance (Figure 3b) on applying the following equation [25]:  
\[\text{Absorbance (\%)}=1-\text{Transmittance }-\text{Reflectance} \tag{3}\]  
assuming that transmittance is almost zero in the visible region, since rGO-SiNW happens to be a non-transparent wafer type sample. Nevertheless, the catalytic activity of rGO co-catalyst highly increased to a positive onset potential; eventually, the overall solar-to-current conversion efficiency increased on the introduction of rGO co-catalyst. rGO-deposited SiNW showed better photon to current conversion efficiency of 3.16%, which is 3.47 times higher than that of bare SiNW (Table 1). As a result, rGO works as an effective HER co-catalyst on the Si electrode, owing to the synergistic effects between rGO and SiNW, and is solely responsible for the improved photocatalytic performance. Figure 3: **Optical measurements of bare SiNW and rGO-SiNW samples. ** (**a**) reflectance and (**b**) absorbance measurements of bare SiNW and representative rGO functionalized SiNW with different loading amounts. Figure 2: **Photocurrent density-voltage characteristics of bare Si, SiNW and rGO-SiNW photocathodes. ** (**a**) Photo electrochemical responses for bare Si, SiNW, rGO functionalized SiNW and rGO-bare planar Si electrode. Different lines of the same color represent repeat measurements at the same condition. (**b**) Mass activity results according to the loading amount of rGO on SiNW. (**c**) Polarization curves for bare Si, rGO-bare Si, SiNW and rGO functionalized SiNW under dark condition. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & \multirow{2}{*}{**Electrode**} & \multirow{2}{*}{\begin{tabular}{c} **Onset Potential** \\ **[EV) at** \\ **[EV) at** \\ **[1 mV/cm]** \\ **[1 mV/cm]** \\ **[1 mV/cm]** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Potential at the** \\ **[EV) at** \\ **[EV) at** \\ **[1 mV/cm]** \\ **[1 mV/cm]** \\ **[1 mV/cm]** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Current Density at** \\ **[1 mV/cm]** \\ **[1 mV/cm]** **[1 mV/cm]** \\ **[1 mV/cm]** \\ **[1 mV/cm]** \\ \end{tabular} } & \multicolumn{2}{c}{\
\begin{tabular}{c} **Half** \\ **[1 mV/cm]** \\ **[1 mV/cm]** \\ \end{tabular} } \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Under \\ **illumination** \\ **(p type S)** \\ **[1 mV/cm]** \\ \end{tabular} } & Planar Si & \(-0.106\) & \(-0.295\) & \(-0.401\) & \(0.40\) & \(-0.293\) & \(0.17\) & \(0.02\) \\ \cline{2-8}  & Si NW & \(0.179\) & \(0.133\) & \(0.09\) & \(0.42\) & \(-22.0\) & \(0.10\) & \(0.91\) \\ \cline{2-8}  & rCO-planar Si & \(-0.018\) & \(-0.217\) & \(-0.354\) & \(0.44\) & \(-0.619\) & \(0.11\) & \(0.03\) \\ \cline{2-8}  & rCO-SNNW & \(0.326\) & \(0.275\) & \(0.239\) & \(0.50\) & \(-23.152\) & \(0.27\) & \(3.16\) \\ \cline{2-8}  & Planar Si & \(-0.656\) & \(-0.771\) & \(-0.843\) & & & & \\ \cline{2-8}  & Si NW & \(-0.448\) & \(-0.527\) & \(-0.571\) & & & & \\ \cline{2-8} (n\({}^{\prime}\) type S) & rCO-planar Si & \(-0.579\) & \(-0.663\) & \(-10.04\) & & & & \\ \cline{2-8}  & rCO-SNNW & \(-0.382\) & \(-0.434\) & \(-0.47\) & & & & \\ \cline{2-8}  & Planar Si & \(0.58\) & & & & & \\ \cline{2-8}  & s NW & \(0.627\) & & & & & \\ \cline{2-8}  & rCO-planar Si & \(0.561\) & & & & & \\ \cline{2-8}  & rCO-SNNW & \(0.708\) & & & & & \\ \cline{2-8}  & Bar GC & \(-0.397\) & \(-0.778\) & & & & \\ \cline{2-8}  & rCO-GC & \(-0.25\) & \(-0.345\) & \(-0.389\) & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of the experimental data of co-catalyst on Si and Glassy Carbon (GC) electrode.