## 4 Conclusions  
In this article, we report for the reduced Ag catalysts on TiO\({}_{2}\)/_p_-Si photocathodes for PEC CO\({}_{2}\) reduction to produce syngas. The R-Ag/TiO\({}_{2}\)/_p_-Si photocathode has been synthesized by using the combination of electron-beam deposition method, electrodeposition, and electrochemical anodization/reduction treatment. The morphology, active sites, and electronic structure of Ag catalysts are modified by the RA treatment to achieve the high catalytic activity for the CO\({}_{2}\) reduction reaction. The new model using a patterned SiO\({}_{2}\) layer can allow the light absorption onto light absorber and facilitate the CO\({}_{2}\) reduction reaction on Ag catalysts. The best performing photocathode provided the lowest onset potential of \(-\)0.16 V vs. RHE, with a large saturated photocurrent density of \(-\)9 mA/cm\({}^{2}\) at \(-\)1.23 V vs. RHE. Furthermore, the faradaic efficiency for CO of 47% was achieved at \(-\)0.6 V vs. RHE. At this potential, the photocathode could produce the syngas ratio of 1:1 with the rate in 18.6 mol/s\(\cdot\)cm\({}^{2}\), which is a suitable proportion for the Fischer-Tropsch synthesis. Although this study focused on _p_-Si photocathode with Ag catalysts for PEC CO\({}_{2}\) reduction, our proposed new model for a photocathode could be useful in various PEC applications, such as hydrogen evolution reaction, oxygen evolution reaction, or N\({}_{2}\) reduction reaction.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/10/3487/s1](http://www.mdpi.com/2076-3417/10/10/3487/s1)]([http://www.mdpi.com/2076-3417/10/10/3487/s1](http://www.mdpi.com/2076-3417/10/10/3487/s1)), Figure S1: (a) Patterning mask used for deposition of SiO\({}_{2}\) layer. (b,c) SEM images and EDS spectrum of partially electrodeposited Ag particles on SiO\({}_{2}\)-free region on Ag/TiO\({}_{2}\)/_p_-Si photocathode., Figure S2: Scanning electron microscopic (SEM) images of anodized Ag catalysts on TiO\({}_{2}\)/_p_-Si photocathodes in (a) low and (b) high magnification., Figure S3: CV curves for (a) R-Ag/TiO\({}_{2}\)/_p_-Si and (b) Ag/TiO\({}_{2}\)/_p_-Si photocathodes at scan rate of 10, 20, 40, 60, 80, and 100 mV s\({}^{-1}\) in 0.1 M KHCO\({}_{3}\) electrolytes. (c) Corresponding capacitive currents at 0.7 V vs. RHE plotted against the scan rate for R-Ag/TiO\({}_{2}\)/_p_-Si and Ag/TiO\({}_{2}\)/_p_-Si photocathodes., Figure S4: XPS survey spectrum for (a) Ag/TiO\({}_{2}\)/_p_-Si and (b) R-Ag/TiO\({}_{2}\)/_p_-Si photocathodes., Figure S5: The faradaic efficiency of the _p_-Si and TiO\({}_{2}\)/_p_-Si photocathodes under 1 Sun illumination (AM 1.5 G) in CO\({}_{2}\)-saturated 0.1 M KHCO\({}_{3}\) electrolytes., Figure S6: Stability test of current density and CO faradaic efficiency for R-Ag/TiO\({}_{2}\)/_p_-Si photocathode at \(-\)0.4 V vs. RHE under 1 sun illumination (AM 1.5 G) in CO\({}_{2}\)-saturated 0.1 M KHCO\({}_{3}\) electrolytes., Figure S7: Flat band diagram of (a) _p_-Si, (b) _n_-TiO\({}_{2}\), and (c) SiO\({}_{2}\).  
Conceptualization, H.W.J. and C.K; methodology, C.K.; validation, C.K.; formal analysis, C.K. and S.C.; investigation, C.K.; resources, C.K., M.-J.C., S.A.L.; data curation, C.K.; writing--original draft preparation, C.K.; writing--review and editing, C.K.; visualization, C.K.; supervision, H.W.J.; project administration, H.W.J.; funding acquisition, H.W.J., S.H.A., S.Y.K. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the Basic Research Laboratory (2018R1A4A1022647) and the National Research Foundation of Korea (NRF) grant funded by the Korean government MSIT (2019M3E6A1103818).  
The authors declare no conflict of interest.