## 3 Results and Discussion
Furthermore, the electronic structures of the Ag and R-Ag on TiO\({}_{2}\)/_p_-Si photocathode were analyzed through X-ray photoelectron spectroscopy (XPS). XPS survey spectra for Ag and R-Ag on TiO\({}_{2}\)/_p_-Si photocathode was shown in Supplementary Figure S4. In Figure 2b, for Ag/TiO\({}_{2}\)/_p_-Si photocathode, the Ag 3\(d_{5/2}\) peak and Ag 3\(d_{3/2}\) peak were observed at 368.2 eV and 374.2 eV, respectively. Also, R-Ag/TiO\({}_{2}\)/_p_-Si photocathode has Ag 3\(d\) peaks at 368 eV and 374 eV, which are assigned to Ag 3\(d_{5/2}\) and Ag 3\(d_{3/2}\) peaks, respectively. The Ag 3d peaks could be deconvoluted into one main peak, which shows there is only metallic Ag. However, the Ag 3d peaks of the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode were slightly shifted to lower binding energy than that of the Ag/TiO\({}_{2}\)/_p_-Si photocathode. The negative shifts of core-level binding energy are induced by increased electron density on the Ag surface by intermetallic electron transfer, which is reported to improve the catalytic activity for electrochemical CO\({}_{2}\) reduction to CO on Ag catalysts [36; 41]. [PERSON] et al. have shown that a decrease in core-level binding energy causes strong chemisorption of CO because the high electron density of host metal facilitates \(\pi\)-backdonation into adsorbed CO [42]. As a result, the negative shift in core-level binding  
Figure 2: Characterization of Ag and R-Ag catalysts on TiO\({}_{2}\)/_p_-Si photocathodes. (**a**) XRD patterns and (**b**) XPS spectrum of the Ag catalysts (blue) and R-Ag catalysts (red) on TiO\({}_{2}\)/_p_-Si photocathode. energy stabilizes the most important reaction intermediate (COOH*) on Ag catalysts due to the scaling relation. Therefore, due to the downshift of the binding energy by the RA treatment, the performance improvement of electrochemical CO\({}_{2}\) reduction to CO can occur on the R-Ag catalysts compared to Ag catalysts on TiO\({}_{2}\)/_p_-Si photocathodes. For PEC measurement, the 500 nm thick patterned SiO\({}_{2}\) layer was deposited on TiO\({}_{2}\)/_p_-Si substrates using a patterning mask (Supplementary Figure S1a), followed by electrodeposition of Ag catalysts in the same procedure mentioned earlier. In Supplementary Figure S1b,c, Ag catalysts were electrodeposited only on the SiO\({}_{2}\)-free surface of the TiO\({}_{2}\)/_p_-Si substrate, allowing light absorption to Si through the SiO\({}_{2}\) portion. The high electronic potential barrier is formed at the TiO\({}_{2}\)/SiO\({}_{2}\) interface due to the high band gap of SiO\({}_{2}\), so electrons for electrodeposition of Ag\({}^{+}\) ion to Ag cannot be move to the electrolyte. Therefore, Ag catalysts are only deposited on the SiO\({}_{2}\)-free surface. The PEC properties of the R-Ag on TiO\({}_{2}\)/_p_-Si, Ag on TiO\({}_{2}\)/_p_-Si, TiO\({}_{2}\)/_p_-Si, and _p_-Si photocathodes were measured with a three-electrode system in CO\({}_{2}\) saturated 0.1 M KHCO\({}_{3}\) electrolytes under AM 1.5 G illumination. Figure 3a displays the PEC current densities of the photocathodes plotted as a function of potential vs. RHE. The black lines indicate the dark current, which show negligible currents flows in the dark condition. As in the previous report [43], when the onset potential means a voltage generating a photocurrent of \(-\)0.6 mA/cm\({}^{2}\), the _p_-Si photocathode is found to have the largest onset potential of \(-\)0.54 V vs. RHE. The TiO\({}_{2}\)/_p_-Si photocathode has a slightly enhanced onset potential of \(-\)0.42 V vs. RHE and larger photocurrent, which is attributed to a heterojunction formed with _p_-Si and _n_-TiO\({}_{2}\)[44]. On the other hand, the two Ag-electrodeposited photocathodes exhibit the significantly lowered onset potential, and the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode exhibits the lowest onset potential of \(-\)0.16 V vs. RHE, which is 100 mV lower than that of the Ag/TiO\({}_{2}\)/_p_-Si photocathode.