## 3 Results and Discussion
After the onset potential, the photocurrent density of the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode gradually rises with the applied potential and is saturated to \(-\)9 mA/cm\({}^{2}\) at \(-\)1.23 V vs. RHE, which is superior to that of the Ag/TiO\({}_{2}\)/_p_-Si photocathode. Compared to the _p_-Si and TiO\({}_{2}\)/_p_-Si photocathodes, the photocurrent density of Ag catalysts electrodeposited photocathodes is reduced due to the Ag particles that inhibited the light absorption by the light absorber, but an enhancement of the catalytic activity occurs on the Ag catalysts. Figure 3: Performances of photoelectrochemical CO\({}_{2}\) reduction of Ag and R-Ag catalysts on SiO\({}_{2}\) patterned TiO\({}_{2}\)/_p_-Si photocathodes under 1 sun illumination (AM 1.5 G) in CO\({}_{2}\)-saturated 0.1 M KHCO\({}_{3}\) electrolytes. (**a**) Polarization curves and (**b**) Faradaic efficiency of the fabricated photocathodes. The current density (— left axis) and faradaic efficiency for CO (\(\bullet\) right axis) vs. time on the Ag catalysts (blue) and the R-Ag catalysts (red) on SiO\({}_{2}\) patterned TiO\({}_{2}\)/_p_-Si photocathodes at (**c**) \(-\)0.2 V, (**d**) \(-\)0.4 V, (**e**) \(-\)0.6 V, and (**f**) \(-\)0.9 V.  
To obtain faradaic efficiency, chronoamperometric measurements were conducted at potentials between \(-0.4\) and \(-0.9\) V vs. RHE while gas products are analyzed three times at each potential using gas chromatography (GC). The faradaic efficiency is calculated based on Equations (2)-(5) using the corresponding chronoamperometry (_i-t_) curves in Figure 3c-f. The average values of calculated faradaic efficiency for CO were plotted against the potential, as shown in Figure 3b. The _p_-Si and TiO\({}_{2}\)/_p_-Si photocathodes have a very low faradaic efficiency of less than 5% at all the measured potentials in Supplementary Figure S5, because Si has poor CO\({}_{2}\) reduction reaction catalytic activity and TiO\({}_{2}\) has catalytic properties for hydrogen evolution reaction [18, 44]. Although Ag catalysts were introduced, it is confirmed that the Ag/TiO\({}_{2}\)/_p_-Si photocathode still exhibit a low selectivity. However, with the RA treatment of Ag catalysts, the selectivity for CO is considerably improved at R-Ag/TiO\({}_{2}\)/_p_-Si photocathode, as shown in Figure 3b. At the low potential of \(-0.2\) V vs. RHE, the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode has a faradaic efficiency of 27%. With the increasing applied potential, the CO selectivity gradually rose and reached 47% at \(-0.6\) V vs. RHE, and decreased beyond the peak potential. Before the potential region of \(-0.6\) V vs. RHE, the hydrogen evolution reaction is suppressed by CO adsorbed on Ag catalysts, causing the faradaic efficiency for CO to increase [31]. However, beyond the \(-0.6\) V vs. RHE, the faradaic efficiency for CO decreased. This is due to reaching the mass transport limitation of CO\({}_{2}\) by the low concentration of CO\({}_{2}\) in the aqueous electrolyte at higher over-potential [45]. In Figure 3c-f, the photocurrent density on the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode is larger than that on the Ag/TiO\({}_{2}\)/_p_-Si photocathode over all measured potential. This is due to the enlarged active sites by morphology changes. Furthermore, the enhancement of faradaic efficiency for CO is attributed to the increased (110) planes and lower binding energy of R-Ag catalysts as mentioned before. The undercoordinated surface atom on (110) planes and lowered binding energy of R-Ag could facilitate the stabilization of the key reaction intermediate (COOH*). The best-performing R-Ag/TiO\({}_{2}\)/_p_-Si photocathode can synthesis syngas with ratios from 1:3 to 1:1 depends on the applied potentials, as shown in Figure 4a. At \(-0.4\) V vs. RHE, the photocathode produces 10.1 mol/s-cm\({}^{2}\) of syngas at a ratio of 1:2, which is suitable feedstock for the process of methanol production [46].