## 3 Results and Discussion
To provide a deeper understanding of the electrocatalytic activity of the rGO catalyst, capacitance studies were performed using a rotating disk electrode (RDE) system, as shown in Figure 4. A catalyst ink comprising rGO was drop-casted on to the glassy carbon (GC) tip. The significant rise in the current density at the same potential for rGO on GC compared to that of the bare GC can be attribute to the electrochemically active catalyst (rGO), which can further improve the water splitting efficiency towards HER. When considering the resistance between electrolyte and the electrode interface, it is necessary to consider internal resistance (iR) compensation. Following iR compensation, the onset potential of rGO decreased significantly. To study the catalytic effects of rGO, Tafel plots for rGO-SiNW were obtained, as shown in Figure 4b. The Tafel slope provides the appropriate pathways suitable for HER [26]:  
\[\mathrm{H_{3}O^{+}+e^{-}\to H_{ads}+H_{2}O}\] \[\mathrm{Volmer\ reaction} \tag{4}\] \[\mathrm{H_{ads}+H_{3}O\to H_{2}+H_{2}O}\] \[\mathrm{H_{ads}+H_{ads}\to H_{2}}\] \[\mathrm{Tafel\ reaction} \tag{6}\]  
In the above process, \(\mathrm{H_{ads}}\) is the absorbed H, (4) is a discharge step (the Volmer reaction), (5) is the desorption of the H-atom (the Heyrovsky reaction), and (6) is the recombination step (the Tafel reaction). If the recombination of \(\mathrm{H_{ads}}\) is the rate determining step for the hydrogen evolution (the Tafel reaction), the measured Tafel slope was 30 mV per decade. In the Tafel reaction, the \(\mathrm{H_{ads}}\) coverage on the surface is almost 1 (\(\mathrm{\theta_{H}=1}\)). If the electrochemical desorption step (the Heyrovsky reaction) can be regarded as the rate-determining step here, a Tafel slope of 40-118 mV per decade is measured, which is dependent on the value observed for \(\mathrm{\theta_{H}}\) (\(\mathrm{\theta_{H}=0}\) to 1). The Tafel slope of 45 mV. per decade in our work indicates that the kinetics of the HER on the rGO catalyst was found to follow the Heyrovsky reaction. Comprehensive electrochemical measurements on the rGO-SiNW system also show the efficient roles of the rGO catalyst (as shown in Figure 5). The transient photoelectrochemical behavior of the rGO-SiNW is also dependent on limiting the illumination time range. Figure 5a shows the transient photoresponse of the rGO-SiNW and bare SiNW electrode. With a 10-s interval, the light was turned on and off, while the potential was maintained at open circuit potential (OCPT) vs RHE. As shown in Figure 5a, the initial rise in the photocurrent density (\(J_{in}\)) was 0.70 mA/cm\({}^{2}\) for rGO-SiNW and 0.45 mA/cm\({}^{2}\) for SiNW, respectively, caused by the abrupt turning on of the light source. After taking the peak, the current density saturated at a certain point called the saturated photocurrent density (\(J_{st}\)). Under illumination, the value of \(J_{st}\) was 0.19 mA/cm\({}^{2}\) for the rGO-SiNW, which is higher than that of bare SiNW. This shows that fewer carriers were trapped and that less recombination occurred at the surface state of the electrode surface. Simultaneously, when the light was turned off, the current  
Figure 4: **Electrochemical activity of bare GC electrodes on a rotating disk electrode system. ** (**a**) Cyclic voltammograms for bare GC, rGO on carbon electrodes and the CV data ohmic drop (iR) compensation are also plotted (dash curves). (**b**) Tafel plots derived from CV data in (**a**). density showed a shift in the direction opposite to \(J_{in}\), which was designated as \(J_{off}\). The \(J_{off}\) values of the rGO-SiNW and SiNW electrodes were 0.18 mA/cm\({}^{2}\) and 0.16 mA/cm\({}^{2}\), respectively. Capacitance measurements also showed that the rGO catalyst is much more effective for electrochemical hydrogen production.