## 3 Results and Discussion
In principle, same photoelectrode materials, despite the difference in surface structures, show similar open circuit potential (\(\rm V_{oc}\)). Bare Si and SiNW showed an almost same value for \(\rm V_{oc}\), indicative that no other metal residue participated during the photoelectrochemical measurements. Consequently, the SiNW electrode system exhibited 46 times higher half STH compared to bare planar Si electrode, which mainly contributes to the enhanced current density (Table 1). The enhanced current density, including limiting current density, is better explained by increased surface area by surface nanostructuring. The nanowire structure in turn increases the current density of the Si photocathode, thus increasing the number of effective reaction sites on the illuminated surface. Moreover, the reduced reflection also increases the possibility of enhanced incident light collection. It is well known that the bare planar Si reflects about 25% of the incident light [24]; however, the nanostructured surface decreases photon absorption significantly due to the its low reflectance. Thus, the SiNW structure electrode exhibits a higher light-trapping effect, which contributes to a higher photon-to-current conversion efficiency than that of bare planar Si electrode. Moreover, orthogonalization reduces the recombination rate between the electron-hole pair, generated at the photoelectrode. The nanowire structure has a relatively shorter horizontal length compared with its length in the vertical direction, and this structure enables the generated minority carrier (electrons) to move faster in the lateral direction of the electrode and to collide with a proton in the electrolyte to produce hydrogen. Consequently, the potential for the water splitting reaction approaches the theoretical value with a decreased kinetic barrier for hydrogen generation, resulting in the increased onset potential, as shown in Figure 2a. Under dark conditions, the onset potential for the bare planar Si electrode was \(-\)0.656 V vs. RHE and SiNW electrode exhibits \(-\)0.448 V vs. RHE. It is noteworthy to mention that the degree of onset potential shift from bare Si to SiNW in both light and dark conditions is almost the same (approximately 0.2 V vs. RHE). From the results, the photovoltage of bare Si and SiNW electrodes are 0.55 V vs. RHE and 0.62 V vs. RHE respectively, which indicate that with the nanowire structure, an increased photovoltage under both light and dark conditions and a decrease in the kinetic barrier for hydrogen production is observed. rGO catalyst further enhances the PEC property of the Si photocathode. Figure 2 shows the photoelectron-catalytic performance of rGO on SiNW toward HER. The onset potential of rGO deposited on the SiNW photocathode is 0.326 V vs. RHE, which is significantly higher than that of the bare SiNW photoelectrode (0.147 V vs. RHE). When the rGO co-catalyst was deposited on the Si photoelectrode, \(\rm V_{oc}\) slightly increased up to 0.04 V for rGO-bare planar Si and 0.08 V for rGO-SiNW, respectively. In the dark condition, the difference in the overpotential between rGO-SiNW and SiNW was 66 mV. The saturation current density of rGO-SiNW somewhat decreased (25 mA/cm\({}^{2}\)), compared to the bare SiNW, which is attributed to the light hindrance by opaque rGO. To further study the effect of loading amount on rGO, we varied the loading amount of rGO and found that 0.8 mg of rGO showed the best efficiency for photoelectrochemical response. (Figure 2b). It is also important to mention that as the loading amount of rGO increased, the saturation current density invariably decreased. To gather more details on the optical reflection property of SiNW and rGO functionalized SiNW, we measured optical reflectance in the wavelength range of 200-2000 nm with SiNW as control. As displayed in Figure 3, when SiNW is fabricated on rGO-SiNW, the light reflectance is much lesser than compared to bare SiNW. The reflectance of rGO on SiNW and bare SiNW is almost 2% and 6% in the visible light region.